High-performance Na-doped LiFePO 4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals

High-performance Na-doped LiFePO 4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article High-performance Na-doped LiFePO 4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals Jun Cong, Shao-hua Luo, Kun Li, Jiachen Wang, Ya feng Wang, Shengxue Yan, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3138757/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract To immobilizing hazardous elements of metallurgical waste and meet the demand for cathode materials in lithium-ion battery industries, LiFePO 4 /C cathode material was successfully prepared via a simple carbothermal reduction method using acid-pickled iron oxide red as raw material by orthogonal tests. To further improve electrochemical performance, Na-doped LiFePO 4 /C cathode material designed with the first-principles calculation verification was synthesized by solid-state method at the optimal experimental conditions including the ball-milling medium of 3 h, the reaction temperature at 750°C in heating rate of 5 ℃·min − 1 for 4 h. The results reveal that Na doping can effectively change the band gap structures and microstructure, which reduced the size of the particle and increased the electronic conductivity. The Li 0.75 Na 0.25 FePO 4 /C electrode showed a discharge specific capacity of 139.2 mAh·g − 1 at 0.5 C and an excellent capacity retention of 98.9% after 50 cycles. The synergy strategy was a sustainable solution for immobilizing hazardous heavy metal elements, which paves a novel facile and cost-effective way towards high-performance LiFePO 4 cathodes and promising markets for lithium-ion battery industries. Lithium-ion battery Cathode material Pickling iron oxide red Na-doped LiFePO4 Heavy metals immobilization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction In the process of industrial production and mining and steelmaking, wastes such as waste water and slag will be generated [ 1 , 2 ]. The heavy metals in it will be released into the environment with wastewater and eventually enter the food chain, causing harm to humans and nature [ 3 ]. Therefore, it is urgent to solidify the heavy metals in wastewater. When iron and steel is rolled at high temperature, iron oxide scale will be formed on the surface. The usual way to deal with iron oxide is pickling. However, a large amount of acid will be consumed in the process of pickling, and waste acid and iron salt solution will be produced at the same time [ 4 ], which will pollute the environment if discharged directly. The iron salt obtained in the process of treating and recycling these wastes is "pickling iron red". At present, the use of pickling iron red is not very thorough, the utilization rate of resources is not very high, most of them are used to make pigments. Therefore, it is of great significance to use pickled iron red as iron source to prepare battery cathode. On account of its low cost, non-toxic and high theoretical specific capacity, LiFePO 4 is the most anticipated cathode material in the next generation. However, in the process of practical production and application of LiFePO 4 batteries, its electronic and ion conductivity is very poor [ 5 , 6 ]. People have taken a variety of methods to weaken or even overcome these shortcomings. The commonly used methods are to reduce the size of the particles [ 7 ], wrap the carbon layer on the surface of the material [ 8 ], and add ions [ 9 ]. Chung [ 10 ] and his team doped multivalent ions (Al 3+ , Zr 4+ , etc.) with LiFePO 4 . The doping position is mainly in the Li 4a site. Subsequently, it was found that the electronic conductivity of the material was greatly increased by eight orders of magnitude. LiFePO 4 can also get good properties by replacing ions at Fe site [ 11 – 15 ]. In addition, doping anions such as F on the O site can effectively increase the capacity of the battery [ 16 , 17 ]. Meanwhile, a large number of theoretical studies on the electrical properties of pure LiFePO 4 have been carried out by using the first-principles method [ 18 – 27 ]. First-principles calculations are helpful for understanding the electronic structure of doped LiFePO 4 . Shi's team [ 23 ] used first-principles calculations to study the energy and electronic structure of the LiFePO 4 system, and it is found that the electronic conductivity of LiFePO 4 can be improved when Cr 3+ replaces Li + . Zhou [ 19 ] used DFT + U method to calculate the electronic structure and band gap of LiFePO 4 . The ~ 3.8 eV band gap calculated by DFT + U is consistent with the experimental results [ 16 ]. In this paper, LiFePO 4 /C were compounded by carbothermal reduction method via citric acid, pickled iron red, lithium hydroxide (LiOH) and NH 4 H 2 PO 4 as raw materials, and the influence of several control parameters LiFePO 4 /C cathode materials was investigated by orthogonal test. Then, we doped LiFePO 4 with single ion and doped Na + with Li + site to improve the charge-discharge performance of LiFePO 4 . Finally, we studied the electronic structure of Na doped LiFePO 4 by first principles calculation method. In addition, their electronic conductivity is also discussed. 2. Experimental 2.1 Experimental and optimization Using the method of orthogonal experiment to explore the optimal preparation conditions. And we discussed the influence of heating rate, sintering time, calcination temperature and ball milling time on the performance of LiFePO 4 /C. Table 1 lists the horizontal factors of the orthogonal experiment. Table 1 The factors and levels of orthogonal experimental No. Heating rate (℃/min) Calcinating time (h) Calcination temperature (℃) Milling time (h) 1 5 4 650 2 2 7.5 6 700 3 3 10 8 750 4 Table 2 Composition analysis results of pickling iron red Ingredient Fe 2 O 3 Cl SiO 2 NiO CaO Average value of steel powder 99.6% 795 ppm 74 ppm 27 ppm 167 ppm Ingredient SO 4 2- Al 2 O 3 P 2 O 5 MnO Average value of steel powder 530 ppm 27 ppm 25 ppm 2929 ppm 2.2 Preparation of Na + doping LiFePO 4 /C The Na + doping LiFePO 4 /C were prepared by a carbothermal reduction using Na 2 CO 3 , C 6 H 8 O 7 , LiOH, Pickled iron oxide red and NH 4 HPO 4 as raw materials. Among then, citric acid as carbon source and Na 2 CO 3 as modified material were added to the mixture. Then we user ethanol as the ball-milling medium for 3 h. Subsequently, the temperature was raised to 750 ℃ at a heating rate of 5 ℃·min − 1 for 4 h. In other words, the Li 1 − x Na x FePO 4 /C (0.25, 0.5, 0.75) cathode materials were obtained. The results of composition analysis of pickled iron red in raw materials are shown in Table 2 , in which the content of Fe 2 O 3 reaches 99.6%, the content of Fe reaches 69.72%, and other impurities are less. The purity of Fe 2 O 3 sample of Fuchen Chemical Reagent Co., Ltd. is 69.8–70.1%. There is little difference in purity between the two. Therefore, the effects of other impurity ions are not considered in this study. 2.3 Characterization and electrochemical measurements Using a powder X-ray diffraction (XRD) to characterize the crystal structure of the materials. In order to characterize the morphology and structure of the material, scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to realize it. Using X-ray photoelectron spectroscopy (XPS) to analyze the element types and valence states. The carbon content of the sample was measured by thermogravimetric analysis (TG, HTG-1) in air in the temperature range of 25 ℃ to 900 ℃. The electrochemical performance of the sample was tested by using the CR2032 coin cell. First of all, put the raw materials into the glass mixing bottle. The mixture was stirred on a magnetic stirrer for about 3 h to make it evenly mixed. After that, the slurry obtained was coated on the aluminum foil and placed in the oven for drying. The oven temperature was set at 60 ℃ and the time was 8 h. Cut the dried aluminum foil into a small round piece with a diameter of about 14 mm, which is the cathode material of the battery. Finally, the electrochemical test of the button battery was carried out. 2.4 The first principle calculation All the calculations in this study were performed using the Vienna Ab initio Simulation Program (VASP) [ 27 , 28 ]. The atomic configuration of the system and the corresponding electronic structure were calculated by density functional theory (DFT) and generalized gradient approximation (GGA) [ 29 ]. In order to reduce the amount of calculation, the situation of electron spin is ignored. The electron exchange correlations energy was calculated the Perdew-Wang exchange correlation functional (PW91) [ 30 ]. Energy cut-off for the plane waves is set to 340 eV. The Monkhorst-Pack [ 31 ] scheme with 3×4×5 k-point sets has been used for the integration in the irreducible Brillouin zone. The ultrasoft pseudopotential (USPP) was used to be the atomic pseudopotential function. 3. Results and discussion The TG-DSC curve of the mixture after ball milling is shown in Fig. 1 . It can be seen from the diagram that the pyrolysis of the precursor mixture is mainly divided into three stages, which are mainly corresponding to the reduction of ferric iron in Fe 2 O 3 and the formation of lithium iron phosphate crystals in the temperature range of 442.7 ℃~700.1 ℃. The weight loss in this stage is about 6.7 wt.%. When the temperature exceeds 700 ℃, the mass no longer changes obviously and tends to be stable,and the final weight is 69.1 wt.%. According to the thermogravimetric analysis of the precursor mixture, the precursor mixture will be calcined in the temperature range of 650 ℃~750 ℃. Figure S1 shows the SEM diagram of LiFePO 4 /C synthesized under different conditions of orthogonal experiment, It is found that all the samples have no special morphology. The irregular block particles can be observed in the scanning diagram of S2, S3, S4, and S7, and the size is different. On the other hand, the scanning images of other samples show that the calcined products have different degrees of agglomeration, and only one plane with holes can be observed under the same magnification, which may be caused by the alkaline LiOH in the raw materials, such as S1, S6, S8 and S9 samples. Even only one larger particle was observed in S5 sample. The S2 sample has uniform particles and small particle size, which maybe correspond to better electrochemical properties. In order to verify this conjecture, we assembled all the samples into CR2032 button semi-cells for electrochemical performance tests. Figure 2 a shows the electrochemical impedance spectra of LiFePO 4 /C synthetic materials. The illustration in the figure is a fitted equivalent circuit diagram. It can be seen that the curve is composed of semicircle and slope. The intercept of semicircle in high frequency region represents the resistance R E of electrolyte, R C is the contact resistance at the collector/cathode interface, The intersection of the semicircle and the X axis represents the charge transfer resistance R CT . Among them, the intersection point between the curve of S2 and S7 and the X axis is smaller, which means that the charge transfer polarization resistance of the two samples is smaller. Figure 2 b shows the first circle charge and discharge curve of all samples under 0.5 C. All samples have a discharge plateau at about 3.4 V. The longer the platform length, the smaller the voltage difference between the charging platform and the discharging platform, the less obvious the polarization of the sample during the charging and discharging process, and the better the electrochemical performance. The specific capacity of S2 and S7 is obviously higher than that of other samples, and above 130 mAh·g − 1 , they reach more than 76.5% of the standard specific capacity. Figure 2 ( c , d ) is a comparison of all samples cycled for 50 cycles at 0.5 C charge-discharge rate. We notice that the discharge specific capacity of S2 sample increases after 50 cycles, reaching 136 mAh·g − 1 , indicating that the battery will have a slight polarization phenomenon in the initial charge-discharge process, and the subsequent discharge specific capacity will increase after the completion of polarization. To select the optimal preparation conditions, we drew the horizontal factor diagram of the orthogonal experiment, as shown in Fig. 3 . The results show that the influence of each factor on LiFePO 4 is different, as follows: ball milling time > heating rate > calcination temperature > sintering time. To sum up, the optimum ball milling time for preparing LiFePO 4 was 3 h, the heating rate was 5 ℃·min − 1 , calcination temperature was 750 ℃, and the holding time was 4 h. We try to improve the cyclic performance of the material by doping Na + into LiFePO 4 . Figure 5 a shows the XRD patterns of Na-0, Na-0.25, Na-0.5, and Na-0.75 samples. The peak shape of each sample is complete, and the position and intensity of each diffraction peak are consistent with the XRD pattern of LiFePO 4 (PDF#40-1499). The sharp peak indicates that all the samples have good crystallinity and Na doping doesn’t change the structure. At the same time, the diffraction peaks of Na 2 CO 3 , LiOH, Fe 2 O 3 and impurities were not observed in the XRD spectrum, indicating that the raw materials reacted completely and no impurities were introduced under these conditions. For to explore the effect of different Na + doping content on cell parameters and cell volume, all samples were refined by Rietveld. The lattice parameters after refinement are shown in Table 3 . As the amount of doping increases, the a, c and v values of the samples decrease, which has also been confirmed by Liu et al [ 13 ]. The values of a, c and v of Na-0.25 are the largest of the three samples. And the lithium ion channel of this sample is the widest. The decrease of lattice constant b is beneficial to the intercalation / delamination of Li + and shortens the diffusion distance of Li + . Generally speaking, the lattice constant changes little with the increase of Na content. When the doping amount is too high, LiFePO 4 with different structure may be formed. Table 3 Lattice parameters of Li 1 − x Na x FePO 4 /C Sample a/Å b/Å c/Å V/Å 3 Na-0 10.3470 6.0189 4.7039 292.947 Na-0.25 10.3646 6.0068 4.7145 293.516 Na-0.5 10.3499 6.0176 4.7124 293.496 Na-0.75 10.3354 6.0198 4.7082 292.930 Figure 4 ( a, b, c ) shows the SEM images of three samples. There is no special appearance. The morphology and particle size of LiFePO 4 /C with different Na + doping amount are basically the same. For to further explore the microstructure of the materials, Na-0.25 samples were scanned by TEM. The result is shown in Fig. 4 d. Figure 4 e shows the Bragg lattice of the sample, and both of them can see the ordered bright spots corresponding to the (121) crystal plane of lithium iron phosphate crystal respectively. Figure 4 ( f , g ) is a high resolution transmission image of the sample. Na + doping LiFePO 4 /C particles have high crystallinity, the particle surface is covered with a uniform carbon layer, and carbon exists in amorphous form. Continuous and complete carbon coating can not only effectively restrain the growth of grains, but also ensure the full contact between electrons to realize the rapid transfer of electrons [ 7 , 32 , 33 ]. Figure 5 ( b , c , d ) and S2 show the XPS spectra of three samples, and further analyzes the surface element composition and valence information of LiFePO 4 /C cathode materials. The peaks of Na 1s, Fe 2p, O 1s, C 1s and P 2p can be observed from Figure S2 . The XPS spectrum of Na is shown in Fig. 5 b. The main peak of the three samples is approximately at 1071 eV, which is attributed to Na 1s. For to investigate the influence of doping on the oxidation state of Fe, the XPS spectrum of Fe 2p was studied, as shown in Fig. 5 c. All samples have two peaks with binding energies around 710.2 and 723.8 eV, corresponding to Fe 2p 1/2 and 2p 3/2 , respectively. This corresponds to Fe 2p in LiFePO 4 [ 34 ]. The binding energy of the main peak and subsidiary peak of each sample has no obvious change with the different doping amount, indicating that the doping of Na + has no obvious effect on the chemical valence of Fe (II). The lattice distortion is usually caused by the doping of other ions. This analysis shows that the charge difference caused by Na + doping may be balanced by cation vacancies, which is helpful to improve the electronic conductivity. Figure 5 d is the XPS spectrum of C1s. Three peaks approximately located at 283.8 eV, 284.5 eV, 287.2 eV were detected, corresponding to C-C, C-O, O-C = O [ 35 ]. Figure 6 shows the electrochemical performance diagram of four samples. We carried out electrochemical impedance spectroscopies (EIS) tests to explore the effect of Na doping on the kinetic behavior on the samples. The optimal equivalent circuit model is given in the figure, as shown in Fig. 6 a. The R CT of the three doped samples are 355, 407 and 482 Ω, respectively. It is generally believed that the charge transfer resistance is closely related to the electrode reaction kinetics. The smaller the charge transfer resistance, the better the kinetic performance of the electrode. It can be easily seen that Na-0.25 samples can provide better kinetic behavior, which is anastomose with the above electrochemical tests. For to further understand the structure of carbon, the Raman spectra of three samples were analyzed as shown in Fig. 6 b. Two prominent peaks can be seen from the diagram, one is the G band related to graphite (sp2), which is located in ~ 1600 cm − 1 , and the other is that the D band related to disordered carbon (sp3) is located in ~ 1360 cm − 1 [ 36 ]. In addition, according to the Raman analysis, the ID/IG values of the three samples are 0.480, 0.413 and 0.403 respectively. It shows that the LiFePO 4 /C cathode material with 0.25 doping content has higher degree of graphitization and better electrical conductivity. Figure 6 c shows the first charge-discharge curves of four samples at 0.5 C. The initial discharge specific capacities of the four samples are 135.9, 142.1, 130.8 and 123 mAh g − 1 . The initial discharge specific capacity of Na-0.25 is the highest. With the increase of the doping amount, the decrease of the first charge/discharge specific capacity may be due to the different insertion/deintercalation modes of Na + and Li + during the charge-discharge process. It will stay in position 4a [ 37 , 38 ]. With the decrease of Li + intercalation/deintercalation, the first discharge capacity of the sample will decrease. Figure 6 d shows the cycle performance curve of four samples after 50 cycles at 0.5 C. It can be seen that the first discharge specific capacity of the sample will change with the increase of Na + doping amount, which is 135.25, 139.35, 130.19 and 122.8 mAh·g − 1 . The discharge specific capacity after 50 cycles is 131.45, 137.65, 129.2and 118.1 mAh·g − 1 . This may be due to the fact that the activity of Na + doped Na + is lower than that of Li + . The Na + maintains the 4a position and supports the one-dimensional channel, which makes the crystal structure of the material stronger and the cycling performance improved [ 39 – 41 ]. We use the first principle calculation method to verify the correctness of the experiment. The lattice constants and Fermi energy obtained from the optimized Li 1 − x Na x FePO 4 system are shown in Table 4 . We can see from the table that with the increase of Na + content, the lattice constant increases and the Fermi energy decreases. This is because the radius of Na + is larger than that of Li + . After Na + occupies the Li site, the lattice expands slightly and the volume increases. The band structures of Li 1 − x Na x FePO 4 (x = 0, 0.25, 0.5, 0.75) are shown in Fig. 7 ( a - d ). It can be seen that the position of the conduction band gradually moves down with the increase of the doping amount x, the width of the band gap becomes narrower. It means that the shorter the path from the valence band to the conduction band is, the less energy is needed, which improves the electronic conductivity of the system to a certain extent. Table 4 Lattice constants and Fermi energies obtained after optimization of Li 1 − x Na x FePO 4 system Doping amout a/Å b/Å c/Å V/Å 3 Fermi energy/V 0 9.8527 5.7893 4.6635 266.010 4.40 0.25 9.8955 5.8430 4.7120 272.391 4.35 0.5 9.9116 5.9116 4.7701 279.397 4.26 0.75 9.9782 5.9528 4.8308 286.895 4.18 Figure 7 ( e - h ) shows the density of states (DOS) for all samples. By comparison, it is found that the peak near the Fermi energy of the system becomes slightly sharp after doping. This means that the number of energy levels near the Fermi energy increases, which may increase the electronic conductivity of the doped LiFePO 4 . The energy band near the − 45 eV range should be contributed by Li-2s orbital electrons. With the increase of the amount of Na + doping, the number of Li decreases, and the intensity of this peak decreases. At the same time, there is an energy band formed by the participation of Na-3s orbital electrons in -51 eV. Therefore, the gradual increase of the amount of doping will gradually reduce the band gap. But with too much doping, too many Na atoms will occupy more Li sites. In addition, its radius is larger than that of Li + , which leads to greater distortion of the lattice and decrease the electrochemical performance. It can be inferred that the Li + diffusion of Li 0.75 Na 0.25 PO 4 is least affected by the hindrance of Na, and the electrochemical performance should be relatively good. This is consistent with the experimental results. Figure 8 shows the total density of states of Li 0.75 Na 0.25 FePO 4 and the partial density of states of each element. By comparing the PDOS diagram of LFP, it can be found that the energy levels near the Fermi energy before and after doping are still mainly contributed by Fe-3d electrons. However, the band gap decreases after doping, indicating that the doping of Na is not directly involved in the formation of the energy level near the Fermi level, but indirectly changes the band gap of the system by affecting the electrons of the Fe-3d orbital. We compared the average length of Li-O bond before and after doping. It is found that before doping, the length is 2.10 Å, and after doping, the length is slightly increased to 2.14 Å. The deformation of the atomic position of Fe may lead to the increase of the length of Li-O bond, which slightly broadens the channel of Li ion migration. The conduction band in the range of 0 ~ 10 eV is mainly contributed by electrons on the Fe-3d and P-3p. The valence band in the range of -25 ~ 0 eV is also mainly contributed by the electron contribution of 2s, 2p of O and 3s, 2p of P. The peaks of the four are wide and the bonding is strong, so it is easy to form a [PO 4 ] tetrahedron. It is found from the diagram that the electrons of Li atoms still have strong delocalization after doping. The interaction force with other atoms is weak, mainly electrostatic Coulomb interaction, and Li + are relatively free, indicating that Li 0.75 Na 0.25 FePO 4 can be used as cathode materials. 4. Conclusion Firstly, using the orthogonal experiment to optimize the preparation of LiFePO 4 /C optimum conditions, and then using a high temperature solid state method to successfully synthesize Na + doping LiFePO 4 /C electrode materials. And the first principles calculation is used to verify the correctness of the experiment. The results show that Li 0.75 Na 0.25 FePO 4 material has better cycle rate performance. This may be due to the expansion of the lattice by a small amount of Na + doping, which deforms the positions of Fe atoms and reduces the band gap. At the same time, the Li + migration channel was slightly widened, which improved various properties of the material. With the increase of doping amount, Na + will occupy too many positions of Li + , which will narrow the channel and reduce various properties of the materials. The first discharge specific capacity of Li 0.75 Na 0.25 FePO 4 at 0.5C is 142.1 mAh g − 1 , and after 50 cycles, the discharge specific capacity is 139.35 mAh g − 1 . Declarations Author Statement Jun Cong: Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing - Original Draft; Shao-hua Luo:Conceptualization, Funding Acquisition, Resources, Supervision, Writing - Review & Editing. Kun Li: Data Curation, Writing-Original Draft; Supervision Jia-chen Wang: Visualization, Investigation; Validation Ya-feng Wang: Data Curation, Supervision; Sheng-xue Yan : Software, Validation; Review; Peng-wei Li:Writing-Review & Editing; All authors read and contributed to the manuscript. Funding This work was financially supported by the National Natural Science Foundation of China (NSFC) (52274295), Natural Science Foundation of Hebei Province (E2021501029, E2020501001, A2021501007, E2022501028, E2022501029), The Natural Science Foundation-Steel, the Iron Foundation of Hebei Province (No. E2022501030), The Fundamental Research Funds for the Central Universities (No. N2323025, N2302016), Performance subsidy fund for Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province (22567627H), the Science and Technology Project of Hebei Education Department (ZD2022158), the Central Guided Local Science and Technology Development Fund Project of Hebei province (226Z4401G), 2023 Hebei Provincial Postgraduate Student Innovation Ability training funding project (CXZZSS2023195) and The instrumental or data analysis from Analytical and Testing Center, Northeastern University. The authors would like to thank Yu-xin Jiang from Shiyanjia Lab (www.shiyanjia.com) for the XPS, Raman and HTEM analysis. Ethical approval No experiments about human or animals were conducted. Conflict of interest The authors declare no competing interests. Data availability The data underlying this article will be shared on reasonable request to the corresponding author. References H. Zhang, X. Hong, An overview for the utilization of wastes from stainless steel industries, Resour. Conserv. Recycl. 55(8) (2011) 745–754. Y. Tian, W. Zuo, D. Chen, Crystallization evolution, microstructure and properties of sewage sludge-based glass-ceramics prepared by microwave heating, J. Hazard. Mater. 196 (2011) 370–379. R. Bian, D. Chen, X. Liu, L. Cui, L. Li, G. Pan, D. Xie, J. Zheng, X. Zhang, J. Zheng, A. Chang, Biochar soil amendment as a solution to prevent Cd-tainted rice from China: Results from a cross-site field experiment, Ecol. Eng. 58 (2013) 378–383. M. Regel-Rosocka, A review on methods of regeneration of spent pickling solutions from steel processing, J. Hazard. Mater. 177(1–3) (2010) 57–69. M. Wang, W. Zhang, Y. Liu, Y. Yang, C. Wang, Y. Wang, Electrochemical performance of patterned LiFePO 4 nano-electrode with a pristine amorphous layer, Appl. Phys. Lett. 104(17) (2014). L. Dimesso, C. Spanheimer, W. Jaegermann, Y. Zhang, A.L. Yarin, LiFePO 4 -3D carbon nanofiber composites as cathode materials for Li-ions batteries, J. Appl. Phys. 111(6) (2012). D. Choi, P.N. Kumta, Surfactant based sol-gel approach to nanostructured LiFePO 4 for high rate Li-ion batteries, J. Power Sources 163(2) (2007) 1064–1069. G. Wang, H. Liu, J. Liu, S. Qiao, G.M. Lu, P. Munroe, H. Ahn, Mesoporous LiFePO 4 /C Nanocomposite Cathode Materials for High Power Lithium Ion Batteries with Superior Performance, Adv. Mater. 22(44) (2010) 4944-+. Y.-M. Chiang, N. Meethong, Y.-H. Kao, Reply to Comment on "Aliovalent Substitutions in Olivine Lithium Iron Phosphate and Impact on Structure and Properties", Adv. Funct. Mater. 20(2) (2010) 189–191. S.Y. Chung, J.T. Blocking, A.S. Andersson, Y.M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes, 8th Asian Conference on Solid State Ionics, Langkawi, Malaysia, 2002, pp. 85–85. D.Y. Wang, H. Li, S.Q. Shi, X.J. Huang, L.Q. Chen, Improving the rate performance of LiFePO 4 by Fe-site doping, Electrochim. Acta 50(14) (2005) 2955–2958. Y. Wen, L. Zeng, Z. Tong, L. Nong, x.W. Wei, Structure and properties of LiFe 0.9 V 0.1 PO 4 , J. Alloys Compd. 416(1–2) (2006) 206–208. H. Liu, Q. Cao, L.J. Fu, C. Li, Y.P. Wu, H.Q. Wu, Doping effects of zinc on LiFePO 4 cathode material for lithium ion batteries, Electrochem. Commun. 8(10) (2006) 1553–1557. T. Takeuchi, M. Tabuchi, A. Nakashima, T. Nakamura, Y. Miwa, H. Kageyama, K. Tatsumi, Preparation of dense LiFePO 4 /C composite positive electrodes using spark-plasma-sintering process, J. Power Sources 146(1–2) (2005) 575–579. D. Shanmukaraj, G.X. Wang, R. Murugan, H.K. Liu, Electrochemical studies on LiFe 1-x Co x PO 4 /carbon composite cathode materials synthesized by citrate gel technique for lithium-ion batteries, Mater. Sci. Eng. B-Adv. 149(1) (2008) 93–98. S.H. Kang, I. Belharouak, Y.K. Sun, K. Amine, Effect of fluorine on the electrochemical properties of layered Li(Ni 0.5 Mn 0.5 )O 2 cathode materials, J. Power Sources 146(1–2) (2005) 650–653. G.G. Amatucci, N. Pereira, Fluoride based electrode materials for advanced energy storage devices, J. Fluor. Chem. 128(4) (2007) 243–262. P. Tang, N.A.W. Holzwarth, Electronic structure of FePO 4 , LiFePO 4 , and related materials, Phys. Rev. B 68(16) (2003). F. Zhou, M. Cococcioni, C.A. Marianetti, D. Morgan, G. Ceder, First-principles prediction of redox potentials in transition-metal compounds with LDA + U, Phys. Rev. B 70(23) (2004). J.M. Osorio-Guillen, B. Holm, R. Ahuja, B. Johansson, A theoretical study of olivine LiMPO 4 cathodes, Solid State Ion. 167(3–4) (2004) 221–227. Y.N. Xu, W.Y. Ching, Y.M. Chiang, Comparative studies of the electronic structure of LiFePO 4 , FePO 4 , Li 3 PO 4 , LiMnPO 4 , LiCoPO 4 , and LiNiPO 4 , J. Appl. Phys. 95(11) (2004) 6583–6585. C.Y. Ouyang, S.Q. Shi, Z.X. Wang, X.J. Huang, L.Q. Chen, First-principles study of Li ion diffusion in LiFePO 4 , Phys. Rev. B 69(10) (2004). C.Y. Ouyang, D.Y. Wang, S.Q. Shi, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, First principles study on Na x Li 1-x FePO 4 as cathode material for rechargeable lithium batteries, Chinese Phys. Lett. 23(1) (2006) 61–64. M.E.A. Dompablo, J.M. Gallardo-Amores, U. Amador, Lithium insertion in the high-pressure polymorph of FePO 4 -Computational predictions and experimental findings, Electrochem. Solid St. 8(11) (2005) A564-A569. S.Q. Shi, C.Y. Ouyang, Z.H. Xiong, L.J. Liu, Z.X. Wang, H. Li, D.S. Wang, L.Q. Chen, X.J. Huang, First-principles investigation of the structural, magnetic, and electronic properties of olivine LiFePO 4 , Phys. Rev. B 71(14) (2005). T. Maxisch, F. Zhou, G. Ceder, Ab initio study of the migration of small polarons in olivine Li x FePO 4 and their association with lithium ions and vacancies, Phys. Rev. B 73(10) (2006). F. Shimojo, K. Hoshino, Y. Zempo, Ab initio molecular-dynamics simulation method for complex liquids, Comput. Phys. Commun. 142(1–3) (2001) 364–367. R. Stadler, W. Wolf, R. Podloucky, G. Kresse, J. Furthmuller, J. Hafner, Ab initio calculations of the cohesive, elastic, and dynamical properties of CoSi 2 by pseudopotential and all-electron techniques, Phys. Rev. B 54(3) (1996) 1729–1734. G. Kresse, J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6(1) (1996) 15–50. I.H. Lee, R.M. Martin, Applications of the generalized-gradient approximation to atoms, clusters, and solids, Phys. Rev. B 56(12) (1997) 7197–7205. E.L. Shirley, Optimal basis sets for detailed Brillouin-zone integrations, Phys. Rev. B 54(23) (1996) 16464–16469. L. Dimesso, S. Jacke, C. Spanheimer, W. Jaegermann, Investigation on 3-dimensional carbon foams/LiFePO 4 composites as function of the annealing time under inert atmosphere, J. Alloys Compd. 509(9) (2011) 3777–3782. Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, M. Liu, One-step microwave synthesis and characterization of carbon-modified nanocrystalline LiFePO 4 , Electrochim. Acta 54(11) (2009) 3206–3210. R. Dedryvere, M. Maccario, L. Croguennec, F. Le Cras, C. Delmas, D. Gonbeau, X-Ray Photoelectron Spectroscopy Investigations of Carbon-Coated Li x FePO 4 Materials, Chem. Mater. 20(22) (2008) 7164–7170. J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Reduction of graphene oxide via L-ascorbic acid, Chem. Commun. 46(7) (2010) 1112–1114. L. Wang, R. Zhang, U. Jansson, N. Nedfors, A near-wearless and extremely long lifetime amorphous carbon film under high vacuum, Sci. Rep. 5 (2015). X. Yin, K. Huang, S. Liu, H. Wang, H. Wang, Preparation and characterization of Na-doped LiFePO 4 /C composites as cathode materials for lithium-ion batteries, J. Power Sources 195(13) (2010) 4308–4312. A.T. Phan, A.E. Gheribi, P. Chartrand, Coherent and para-equilibrium phase transformations in Mn-doped- LiFePO 4 cathode materials: Implications for lithium ion battery performances, J. Alloys Compd. 838 (2020). D. Zhang, Y. Liu, L. Wu, L. Feng, S. Jin, R. Zhang, M. Jin, Effect of Ti ion doping on electrochemical performance of Ni-rich LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode material, Electrochim. Acta 328 (2019). W. Yao, Y. Liu, D. Li, Q. Zhang, S. Zhong, H. Cheng, Z. Yan, Synergistically Enhanced Electrochemical Performance of Ni-rich Cathode Materials for Lithium-ion Batteries by K and Ti Comodification, J. Phys. Chem. C 124(4) (2020) 2346–2356. Z. Cao, G. Zhu, R. Zhang, S. Chen, M. Sang, J. Jia, M. Yang, X. Li, S. Yang, Biological phytic acid guided formation of monodisperse large-sized carbon@LiFePO 4 /graphene composite microspheres for high-performance lithium-ion battery cathodes, Chem. Eng. J. 351 (2018) 382–390. Additional Declarations No competing interests reported. Supplementary Files GraphicalAbstract.docx Highlights.docx Supportinginformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 15 Jul, 2023 Reviews received at journal 09 Jul, 2023 Reviewers agreed at journal 06 Jul, 2023 Reviewers invited by journal 06 Jul, 2023 Submission checks completed at journal 05 Jul, 2023 Editor assigned by journal 05 Jul, 2023 First submitted to journal 04 Jul, 2023 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3138757","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":215980008,"identity":"3d308a86-c026-4df1-96da-847221f307dc","order_by":0,"name":"Jun Cong","email":"","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":false,"prefix":"","firstName":"Jun","middleName":"","lastName":"Cong","suffix":""},{"id":215980009,"identity":"2e4a9824-fe55-44f1-879e-709b8e401b6b","order_by":1,"name":"Shao-hua Luo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA40lEQVRIiWNgGAWjYDACZijNx8DD+CChooawDh6YFjYg0+DBmWNEaIExgFrYJB+2MONTDAH27MzPHvPU3LFr4z97rCKxgY2Bv707gYDD2MyNeY49S26TyEu7kbhDhkHizNkNBLQwmEnzsB1OZpPgMbuReIaNwUAil5AW9m/SPP+AWvjPmBUktjETo4XHTJq37bAdG0OOGQNxWg7zlEnO7TucwCaRYyyRcOYYD0G/sPcf3ybx5tthe37+M4Yff1TUyPG39+LXAgJMwMhJbIBZS1A5CDD+AMYPUSpHwSgYBaNgZAIA4oA/l4FJ6l0AAAAASUVORK5CYII=","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":true,"prefix":"","firstName":"Shao-hua","middleName":"","lastName":"Luo","suffix":""},{"id":215980010,"identity":"58826c5d-35ee-4ad0-9880-cfe7c4d43c17","order_by":2,"name":"Kun Li","email":"","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":false,"prefix":"","firstName":"Kun","middleName":"","lastName":"Li","suffix":""},{"id":215980011,"identity":"6614ebc7-3d25-44e3-b6bd-bb519ab342ef","order_by":3,"name":"Jiachen Wang","email":"","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":false,"prefix":"","firstName":"Jiachen","middleName":"","lastName":"Wang","suffix":""},{"id":215980012,"identity":"85808f1b-a3de-4682-87ce-9d94a2b9eef1","order_by":4,"name":"Ya feng Wang","email":"","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":false,"prefix":"","firstName":"Ya","middleName":"feng","lastName":"Wang","suffix":""},{"id":215980013,"identity":"8b6c29ed-7b2f-432d-b9b7-626c3d5281d9","order_by":5,"name":"Shengxue Yan","email":"","orcid":"","institution":"Northeastern University at Qinhuangdao","correspondingAuthor":false,"prefix":"","firstName":"Shengxue","middleName":"","lastName":"Yan","suffix":""},{"id":215980014,"identity":"0220ab3a-4018-4d52-977e-c881412bb8db","order_by":6,"name":"Pengwei Li","email":"","orcid":"","institution":"Aalborg University","correspondingAuthor":false,"prefix":"","firstName":"Pengwei","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2023-07-04 10:44:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3138757/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3138757/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":39855751,"identity":"2d3cf1e9-2a63-41bf-9eff-f5b17608a582","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":107862,"visible":true,"origin":"","legend":"\u003cp\u003eTG curves of precursor mixture.\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/397c242d63135627e65477ae.png"},{"id":39855750,"identity":"25baac0e-6526-436f-8b97-fe913bef9fcc","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":39780,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode materials synthesized under different conditions: (a) EIS curves, (b) Charge-discharge curves of the first cycle, (c, d) Cycle performance.\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/cf68fe4d93bab2cf77b1f3c0.png"},{"id":39857129,"identity":"733abb5c-6eaf-41bd-808f-62bb31c0127e","added_by":"auto","created_at":"2023-07-11 14:17:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":60948,"visible":true,"origin":"","legend":"\u003cp\u003eAnalysis diagram of the results of orthogonal experiment.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/59fdae8a37a9a4d49c4b5e88.png"},{"id":39855760,"identity":"3daff330-56b2-47e3-b8a4-64fe151af66d","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":285259,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) Na-0.25(b) Na-0.5 and (c) Na-0.75; TEM images of Na-0.25 sample (d), (f), (g); HRTEM of (e) Na-0.25.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/0e4b879a2796ada7a55c15a3.jpeg"},{"id":39855752,"identity":"55761b5d-e1d7-4c01-8869-fa122c6ce319","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":48120,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of the four samples. XPS of sample prepared with Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e as lithium source. (a) full-range spectrum; (b) Na 3d; (c) Fe 2p; (d) C 1s.\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/db9c6c96f93af16b4e127828.png"},{"id":39855756,"identity":"c4799703-2f4f-46d9-9f2e-8d3de8613e12","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":41100,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The EIS curves of four samples. (b) The Raman spectroscopy of Na\u003csup\u003e+\u003c/sup\u003e doping samples. (c) The first charge/discharge curves of four samples. (d) Cyclic performance of four samples.\u003c/p\u003e","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/0eba3703b81ec7f2feb4de27.png"},{"id":39855753,"identity":"75945859-8326-45ab-8510-b28feca0dbfd","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":57975,"visible":true,"origin":"","legend":"\u003cp\u003e(a-d): The band structure of Li\u003csub\u003e1-x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e (x= 0, 0.25, 0.5, 0.75) changes with the doping amount; (e-h): Total density of states of Li\u003csub\u003e1-x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e (x= 0, 0.25, 0.5, 0.75).\u003c/p\u003e","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/4a5766b3056cc4aef6f3e0d5.png"},{"id":39857130,"identity":"e79612e6-8346-4839-be93-a6b79957da84","added_by":"auto","created_at":"2023-07-11 14:17:42","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":40561,"visible":true,"origin":"","legend":"\u003cp\u003eTotal density of states of Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e and partial density of states of each element.\u003c/p\u003e","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/f643bee57b6c8d5d64e4f292.png"},{"id":39859758,"identity":"d84b0aa5-4a28-4a78-8afe-b8e3f9f6f06e","added_by":"auto","created_at":"2023-07-11 14:33:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1348142,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/6f2bcb16-e250-4cce-a892-f45f8427f8be.pdf"},{"id":39858264,"identity":"56bf5cac-57ac-4410-b997-ac6991b1a018","added_by":"auto","created_at":"2023-07-11 14:25:42","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":107164,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/21e644cc6f37cddb0167837a.docx"},{"id":39859757,"identity":"ebc787d4-0717-4d36-b84b-df832c18f5a2","added_by":"auto","created_at":"2023-07-11 14:33:42","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":12175,"visible":true,"origin":"","legend":"","description":"","filename":"Highlights.docx","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/40b97c2b9d61a178b2d0fc00.docx"},{"id":39855759,"identity":"c2c9a11f-6ed2-42a4-9f43-1dad9d04310d","added_by":"auto","created_at":"2023-07-11 14:09:42","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":518347,"visible":true,"origin":"","legend":"","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-3138757/v1/3b4256344394bb428ae3cb41.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"High-performance Na-doped LiFePO 4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn the process of industrial production and mining and steelmaking, wastes such as waste water and slag will be generated [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The heavy metals in it will be released into the environment with wastewater and eventually enter the food chain, causing harm to humans and nature [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Therefore, it is urgent to solidify the heavy metals in wastewater. When iron and steel is rolled at high temperature, iron oxide scale will be formed on the surface. The usual way to deal with iron oxide is pickling. However, a large amount of acid will be consumed in the process of pickling, and waste acid and iron salt solution will be produced at the same time [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], which will pollute the environment if discharged directly. The iron salt obtained in the process of treating and recycling these wastes is \"pickling iron red\". At present, the use of pickling iron red is not very thorough, the utilization rate of resources is not very high, most of them are used to make pigments. Therefore, it is of great significance to use pickled iron red as iron source to prepare battery cathode.\u003c/p\u003e \u003cp\u003eOn account of its low cost, non-toxic and high theoretical specific capacity, LiFePO\u003csub\u003e4\u003c/sub\u003e is the most anticipated cathode material in the next generation. However, in the process of practical production and application of LiFePO\u003csub\u003e4\u003c/sub\u003e batteries, its electronic and ion conductivity is very poor [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. People have taken a variety of methods to weaken or even overcome these shortcomings. The commonly used methods are to reduce the size of the particles [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], wrap the carbon layer on the surface of the material [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], and add ions [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Chung [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] and his team doped multivalent ions (Al\u003csup\u003e3+\u003c/sup\u003e, Zr\u003csup\u003e4+\u003c/sup\u003e, etc.) with LiFePO\u003csub\u003e4\u003c/sub\u003e. The doping position is mainly in the Li 4a site. Subsequently, it was found that the electronic conductivity of the material was greatly increased by eight orders of magnitude. LiFePO\u003csub\u003e4\u003c/sub\u003e can also get good properties by replacing ions at Fe site [\u003cspan additionalcitationids=\"CR12 CR13 CR14\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In addition, doping anions such as F on the O site can effectively increase the capacity of the battery [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Meanwhile, a large number of theoretical studies on the electrical properties of pure LiFePO\u003csub\u003e4\u003c/sub\u003e have been carried out by using the first-principles method [\u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22 CR23 CR24 CR25 CR26\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. First-principles calculations are helpful for understanding the electronic structure of doped LiFePO\u003csub\u003e4\u003c/sub\u003e. Shi's team [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] used first-principles calculations to study the energy and electronic structure of the LiFePO\u003csub\u003e4\u003c/sub\u003e system, and it is found that the electronic conductivity of LiFePO\u003csub\u003e4\u003c/sub\u003e can be improved when Cr\u003csup\u003e3+\u003c/sup\u003e replaces Li\u003csup\u003e+\u003c/sup\u003e. Zhou [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] used DFT\u0026thinsp;+\u0026thinsp;U method to calculate the electronic structure and band gap of LiFePO\u003csub\u003e4\u003c/sub\u003e. The ~\u0026thinsp;3.8 eV band gap calculated by DFT\u0026thinsp;+\u0026thinsp;U is consistent with the experimental results [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this paper, LiFePO\u003csub\u003e4\u003c/sub\u003e/C were compounded by carbothermal reduction method via citric acid, pickled iron red, lithium hydroxide (LiOH) and NH\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e as raw materials, and the influence of several control parameters LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode materials was investigated by orthogonal test. Then, we doped LiFePO\u003csub\u003e4\u003c/sub\u003e with single ion and doped Na\u003csup\u003e+\u003c/sup\u003e with Li\u003csup\u003e+\u003c/sup\u003e site to improve the charge-discharge performance of LiFePO\u003csub\u003e4\u003c/sub\u003e. Finally, we studied the electronic structure of Na doped LiFePO\u003csub\u003e4\u003c/sub\u003e by first principles calculation method. In addition, their electronic conductivity is also discussed.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Experimental and optimization\u003c/h2\u003e \u003cp\u003eUsing the method of orthogonal experiment to explore the optimal preparation conditions. And we discussed the influence of heating rate, sintering time, calcination temperature and ball milling time on the performance of LiFePO\u003csub\u003e4\u003c/sub\u003e/C. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e lists the horizontal factors of the orthogonal experiment.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe factors and levels of orthogonal experimental\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHeating rate\u003c/p\u003e \u003cp\u003e(℃/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCalcinating time\u003c/p\u003e \u003cp\u003e(h)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCalcination temperature\u003c/p\u003e \u003cp\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMilling time\u003c/p\u003e \u003cp\u003e(h)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e650\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e750\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComposition analysis results of pickling iron red\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIngredient\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNiO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage value of steel powder\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e795 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e74 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e27 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e167 ppm\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIngredient\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMnO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage value of steel powder\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e530 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e27 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2929 ppm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of Na\u003csup\u003e+\u003c/sup\u003e doping LiFePO\u003csub\u003e4\u003c/sub\u003e/C\u003c/h2\u003e \u003cp\u003eThe Na\u003csup\u003e+\u003c/sup\u003e doping LiFePO\u003csub\u003e4\u003c/sub\u003e/C were prepared by a carbothermal reduction using Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, LiOH, Pickled iron oxide red and NH\u003csub\u003e4\u003c/sub\u003eHPO\u003csub\u003e4\u003c/sub\u003e as raw materials. Among then, citric acid as carbon source and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e as modified material were added to the mixture. Then we user ethanol as the ball-milling medium for 3 h. Subsequently, the temperature was raised to 750 ℃ at a heating rate of 5 ℃\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 4 h. In other words, the Li\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e/C (0.25, 0.5, 0.75) cathode materials were obtained.\u003c/p\u003e \u003cp\u003eThe results of composition analysis of pickled iron red in raw materials are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, in which the content of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e reaches 99.6%, the content of Fe reaches 69.72%, and other impurities are less. The purity of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e sample of Fuchen Chemical Reagent Co., Ltd. is 69.8\u0026ndash;70.1%. There is little difference in purity between the two. Therefore, the effects of other impurity ions are not considered in this study.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization and electrochemical measurements\u003c/h2\u003e \u003cp\u003eUsing a powder X-ray diffraction (XRD) to characterize the crystal structure of the materials. In order to characterize the morphology and structure of the material, scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to realize it. Using X-ray photoelectron spectroscopy (XPS) to analyze the element types and valence states. The carbon content of the sample was measured by thermogravimetric analysis (TG, HTG-1) in air in the temperature range of 25 ℃ to 900 ℃.\u003c/p\u003e \u003cp\u003eThe electrochemical performance of the sample was tested by using the CR2032 coin cell. First of all, put the raw materials into the glass mixing bottle. The mixture was stirred on a magnetic stirrer for about 3 h to make it evenly mixed. After that, the slurry obtained was coated on the aluminum foil and placed in the oven for drying. The oven temperature was set at 60 ℃ and the time was 8 h. Cut the dried aluminum foil into a small round piece with a diameter of about 14 mm, which is the cathode material of the battery. Finally, the electrochemical test of the button battery was carried out.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 The first principle calculation\u003c/h2\u003e \u003cp\u003eAll the calculations in this study were performed using the Vienna Ab initio Simulation Program (VASP) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The atomic configuration of the system and the corresponding electronic structure were calculated by density functional theory (DFT) and generalized gradient approximation (GGA) [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In order to reduce the amount of calculation, the situation of electron spin is ignored. The electron exchange correlations energy was calculated the Perdew-Wang exchange correlation functional (PW91) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Energy cut-off for the plane waves is set to 340 eV. The Monkhorst-Pack [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] scheme with 3\u0026times;4\u0026times;5 k-point sets has been used for the integration in the irreducible Brillouin zone. The ultrasoft pseudopotential (USPP) was used to be the atomic pseudopotential function.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eThe TG-DSC curve of the mixture after ball milling is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. It can be seen from the diagram that the pyrolysis of the precursor mixture is mainly divided into three stages, which are mainly corresponding to the reduction of ferric iron in Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and the formation of lithium iron phosphate crystals in the temperature range of 442.7 ℃~700.1 ℃. The weight loss in this stage is about 6.7 wt.%. When the temperature exceeds 700 ℃, the mass no longer changes obviously and tends to be stable,and the final weight is 69.1 wt.%. According to the thermogravimetric analysis of the precursor mixture, the precursor mixture will be calcined in the temperature range of 650 ℃~750 ℃.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e shows the SEM diagram of LiFePO\u003csub\u003e4\u003c/sub\u003e/C synthesized under different conditions of orthogonal experiment, It is found that all the samples have no special morphology. The irregular block particles can be observed in the scanning diagram of S2, S3, S4, and S7, and the size is different. On the other hand, the scanning images of other samples show that the calcined products have different degrees of agglomeration, and only one plane with holes can be observed under the same magnification, which may be caused by the alkaline LiOH in the raw materials, such as S1, S6, S8 and S9 samples. Even only one larger particle was observed in S5 sample. The S2 sample has uniform particles and small particle size, which maybe correspond to better electrochemical properties. In order to verify this conjecture, we assembled all the samples into CR2032 button semi-cells for electrochemical performance tests.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the electrochemical impedance spectra of LiFePO\u003csub\u003e4\u003c/sub\u003e/C synthetic materials. The illustration in the figure is a fitted equivalent circuit diagram. It can be seen that the curve is composed of semicircle and slope. The intercept of semicircle in high frequency region represents the resistance R\u003csub\u003eE\u003c/sub\u003e of electrolyte, R\u003csub\u003eC\u003c/sub\u003e is the contact resistance at the collector/cathode interface, The intersection of the semicircle and the X axis represents the charge transfer resistance R\u003csub\u003eCT\u003c/sub\u003e. Among them, the intersection point between the curve of S2 and S7 and the X axis is smaller, which means that the charge transfer polarization resistance of the two samples is smaller.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows the first circle charge and discharge curve of all samples under 0.5 C. All samples have a discharge plateau at about 3.4 V. The longer the platform length, the smaller the voltage difference between the charging platform and the discharging platform, the less obvious the polarization of the sample during the charging and discharging process, and the better the electrochemical performance. The specific capacity of S2 and S7 is obviously higher than that of other samples, and above 130 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, they reach more than 76.5% of the standard specific capacity.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(\u003cb\u003ec\u003c/b\u003e, \u003cb\u003ed\u003c/b\u003e) is a comparison of all samples cycled for 50 cycles at 0.5 C charge-discharge rate. We notice that the discharge specific capacity of S2 sample increases after 50 cycles, reaching 136 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, indicating that the battery will have a slight polarization phenomenon in the initial charge-discharge process, and the subsequent discharge specific capacity will increase after the completion of polarization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo select the optimal preparation conditions, we drew the horizontal factor diagram of the orthogonal experiment, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The results show that the influence of each factor on LiFePO\u003csub\u003e4\u003c/sub\u003e is different, as follows: ball milling time\u0026thinsp;\u0026gt;\u0026thinsp;heating rate\u0026thinsp;\u0026gt;\u0026thinsp;calcination temperature\u0026thinsp;\u0026gt;\u0026thinsp;sintering time. To sum up, the optimum ball milling time for preparing LiFePO\u003csub\u003e4\u003c/sub\u003e was 3 h, the heating rate was 5 ℃\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, calcination temperature was 750 ℃, and the holding time was 4 h.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe try to improve the cyclic performance of the material by doping Na\u003csup\u003e+\u003c/sup\u003e into LiFePO\u003csub\u003e4\u003c/sub\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the XRD patterns of Na-0, Na-0.25, Na-0.5, and Na-0.75 samples. The peak shape of each sample is complete, and the position and intensity of each diffraction peak are consistent with the XRD pattern of LiFePO\u003csub\u003e4\u003c/sub\u003e (PDF#40-1499). The sharp peak indicates that all the samples have good crystallinity and Na doping doesn\u0026rsquo;t change the structure. At the same time, the diffraction peaks of Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, LiOH, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and impurities were not observed in the XRD spectrum, indicating that the raw materials reacted completely and no impurities were introduced under these conditions. For to explore the effect of different Na\u003csup\u003e+\u003c/sup\u003e doping content on cell parameters and cell volume, all samples were refined by Rietveld. The lattice parameters after refinement are shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. As the amount of doping increases, the a, c and v values of the samples decrease, which has also been confirmed by Liu et al [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The values of a, c and v of Na-0.25 are the largest of the three samples. And the lithium ion channel of this sample is the widest. The decrease of lattice constant b is beneficial to the intercalation / delamination of Li\u003csup\u003e+\u003c/sup\u003e and shortens the diffusion distance of Li\u003csup\u003e+\u003c/sup\u003e. Generally speaking, the lattice constant changes little with the increase of Na content. When the doping amount is too high, LiFePO\u003csub\u003e4\u003c/sub\u003e with different structure may be formed.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLattice parameters of Li\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e/C\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ea/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eb/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ec/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eV/\u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa-0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.3470\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.0189\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7039\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e292.947\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa-0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.3646\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.0068\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7145\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e293.516\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa-0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.3499\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.0176\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7124\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e293.496\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa-0.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.3354\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.0198\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7082\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e292.930\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(\u003cb\u003ea, b, c\u003c/b\u003e) shows the SEM images of three samples. There is no special appearance. The morphology and particle size of LiFePO\u003csub\u003e4\u003c/sub\u003e/C with different Na\u003csup\u003e+\u003c/sup\u003e doping amount are basically the same. For to further explore the microstructure of the materials, Na-0.25 samples were scanned by TEM. The result is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee shows the Bragg lattice of the sample, and both of them can see the ordered bright spots corresponding to the (121) crystal plane of lithium iron phosphate crystal respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(\u003cb\u003ef\u003c/b\u003e, \u003cb\u003eg\u003c/b\u003e) is a high resolution transmission image of the sample. Na\u003csup\u003e+\u003c/sup\u003e doping LiFePO\u003csub\u003e4\u003c/sub\u003e/C particles have high crystallinity, the particle surface is covered with a uniform carbon layer, and carbon exists in amorphous form. Continuous and complete carbon coating can not only effectively restrain the growth of grains, but also ensure the full contact between electrons to realize the rapid transfer of electrons [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(\u003cb\u003eb\u003c/b\u003e, \u003cb\u003ec\u003c/b\u003e, \u003cb\u003ed\u003c/b\u003e) and S2 show the XPS spectra of three samples, and further analyzes the surface element composition and valence information of LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode materials. The peaks of Na 1s, Fe 2p, O 1s, C 1s and P 2p can be observed from Figure \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e. The XPS spectrum of Na is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. The main peak of the three samples is approximately at 1071 eV, which is attributed to Na 1s. For to investigate the influence of doping on the oxidation state of Fe, the XPS spectrum of Fe 2p was studied, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. All samples have two peaks with binding energies around 710.2 and 723.8 eV, corresponding to Fe 2p\u003csub\u003e1/2\u003c/sub\u003e and 2p\u003csub\u003e3/2\u003c/sub\u003e, respectively. This corresponds to Fe 2p in LiFePO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The binding energy of the main peak and subsidiary peak of each sample has no obvious change with the different doping amount, indicating that the doping of Na\u003csup\u003e+\u003c/sup\u003e has no obvious effect on the chemical valence of Fe (II). The lattice distortion is usually caused by the doping of other ions. This analysis shows that the charge difference caused by Na\u003csup\u003e+\u003c/sup\u003e doping may be balanced by cation vacancies, which is helpful to improve the electronic conductivity. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed is the XPS spectrum of C1s. Three peaks approximately located at 283.8 eV, 284.5 eV, 287.2 eV were detected, corresponding to C-C, C-O, O-C\u0026thinsp;=\u0026thinsp;O [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the electrochemical performance diagram of four samples. We carried out electrochemical impedance spectroscopies (EIS) tests to explore the effect of Na doping on the kinetic behavior on the samples. The optimal equivalent circuit model is given in the figure, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea. The R\u003csub\u003eCT\u003c/sub\u003e of the three doped samples are 355, 407 and 482 Ω, respectively. It is generally believed that the charge transfer resistance is closely related to the electrode reaction kinetics. The smaller the charge transfer resistance, the better the kinetic performance of the electrode. It can be easily seen that Na-0.25 samples can provide better kinetic behavior, which is anastomose with the above electrochemical tests.\u003c/p\u003e \u003cp\u003eFor to further understand the structure of carbon, the Raman spectra of three samples were analyzed as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb. Two prominent peaks can be seen from the diagram, one is the G band related to graphite (sp2), which is located in ~\u0026thinsp;1600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the other is that the D band related to disordered carbon (sp3) is located in ~\u0026thinsp;1360 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. In addition, according to the Raman analysis, the ID/IG values of the three samples are 0.480, 0.413 and 0.403 respectively. It shows that the LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode material with 0.25 doping content has higher degree of graphitization and better electrical conductivity.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec shows the first charge-discharge curves of four samples at 0.5 C. The initial discharge specific capacities of the four samples are 135.9, 142.1, 130.8 and 123 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The initial discharge specific capacity of Na-0.25 is the highest. With the increase of the doping amount, the decrease of the first charge/discharge specific capacity may be due to the different insertion/deintercalation modes of Na\u003csup\u003e+\u003c/sup\u003e and Li\u003csup\u003e+\u003c/sup\u003e during the charge-discharge process. It will stay in position 4a [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. With the decrease of Li\u003csup\u003e+\u003c/sup\u003e intercalation/deintercalation, the first discharge capacity of the sample will decrease.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed shows the cycle performance curve of four samples after 50 cycles at 0.5 C. It can be seen that the first discharge specific capacity of the sample will change with the increase of Na\u003csup\u003e+\u003c/sup\u003e doping amount, which is 135.25, 139.35, 130.19 and 122.8 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The discharge specific capacity after 50 cycles is 131.45, 137.65, 129.2and 118.1 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This may be due to the fact that the activity of Na\u003csup\u003e+\u003c/sup\u003e doped Na\u003csup\u003e+\u003c/sup\u003e is lower than that of Li\u003csup\u003e+\u003c/sup\u003e. The Na\u003csup\u003e+\u003c/sup\u003e maintains the 4a position and supports the one-dimensional channel, which makes the crystal structure of the material stronger and the cycling performance improved [\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe use the first principle calculation method to verify the correctness of the experiment. The lattice constants and Fermi energy obtained from the optimized Li\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e system are shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. We can see from the table that with the increase of Na\u003csup\u003e+\u003c/sup\u003e content, the lattice constant increases and the Fermi energy decreases. This is because the radius of Na\u003csup\u003e+\u003c/sup\u003e is larger than that of Li\u003csup\u003e+\u003c/sup\u003e. After Na\u003csup\u003e+\u003c/sup\u003e occupies the Li site, the lattice expands slightly and the volume increases. The band structures of Li\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e(x\u0026thinsp;=\u0026thinsp;0, 0.25, 0.5, 0.75) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(\u003cb\u003ea\u003c/b\u003e-\u003cb\u003ed\u003c/b\u003e). It can be seen that the position of the conduction band gradually moves down with the increase of the doping amount x, the width of the band gap becomes narrower. It means that the shorter the path from the valence band to the conduction band is, the less energy is needed, which improves the electronic conductivity of the system to a certain extent.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLattice constants and Fermi energies obtained after optimization of Li\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNa\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e system\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDoping amout\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ea/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eb/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ec/\u0026Aring;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eV/\u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFermi energy/V\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.8527\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.7893\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.6635\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e266.010\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.8955\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.8430\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e272.391\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.35\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.9116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.9116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.7701\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e279.397\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.9782\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.9528\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.8308\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e286.895\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(\u003cb\u003ee\u003c/b\u003e-\u003cb\u003eh\u003c/b\u003e) shows the density of states (DOS) for all samples. By comparison, it is found that the peak near the Fermi energy of the system becomes slightly sharp after doping. This means that the number of energy levels near the Fermi energy increases, which may increase the electronic conductivity of the doped LiFePO\u003csub\u003e4\u003c/sub\u003e. The energy band near the \u0026minus;\u0026thinsp;45 eV range should be contributed by Li-2s orbital electrons. With the increase of the amount of Na\u003csup\u003e+\u003c/sup\u003e doping, the number of Li decreases, and the intensity of this peak decreases. At the same time, there is an energy band formed by the participation of Na-3s orbital electrons in -51 eV. Therefore, the gradual increase of the amount of doping will gradually reduce the band gap. But with too much doping, too many Na atoms will occupy more Li sites. In addition, its radius is larger than that of Li\u003csup\u003e+\u003c/sup\u003e, which leads to greater distortion of the lattice and decrease the electrochemical performance. It can be inferred that the Li\u003csup\u003e+\u003c/sup\u003e diffusion of Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e is least affected by the hindrance of Na, and the electrochemical performance should be relatively good. This is consistent with the experimental results.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the total density of states of Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e and the partial density of states of each element. By comparing the PDOS diagram of LFP, it can be found that the energy levels near the Fermi energy before and after doping are still mainly contributed by Fe-3d electrons. However, the band gap decreases after doping, indicating that the doping of Na is not directly involved in the formation of the energy level near the Fermi level, but indirectly changes the band gap of the system by affecting the electrons of the Fe-3d orbital. We compared the average length of Li-O bond before and after doping. It is found that before doping, the length is 2.10 \u0026Aring;, and after doping, the length is slightly increased to 2.14 \u0026Aring;. The deformation of the atomic position of Fe may lead to the increase of the length of Li-O bond, which slightly broadens the channel of Li ion migration. The conduction band in the range of 0\u0026thinsp;~\u0026thinsp;10 eV is mainly contributed by electrons on the Fe-3d and P-3p. The valence band in the range of -25\u0026thinsp;~\u0026thinsp;0 eV is also mainly contributed by the electron contribution of 2s, 2p of O and 3s, 2p of P. The peaks of the four are wide and the bonding is strong, so it is easy to form a [PO\u003csub\u003e4\u003c/sub\u003e] tetrahedron. It is found from the diagram that the electrons of Li atoms still have strong delocalization after doping. The interaction force with other atoms is weak, mainly electrostatic Coulomb interaction, and Li\u003csup\u003e+\u003c/sup\u003e are relatively free, indicating that Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e can be used as cathode materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eFirstly, using the orthogonal experiment to optimize the preparation of LiFePO\u003csub\u003e4\u003c/sub\u003e/C optimum conditions, and then using a high temperature solid state method to successfully synthesize Na\u003csup\u003e+\u003c/sup\u003e doping LiFePO\u003csub\u003e4\u003c/sub\u003e/C electrode materials. And the first principles calculation is used to verify the correctness of the experiment. The results show that Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e material has better cycle rate performance. This may be due to the expansion of the lattice by a small amount of Na\u003csup\u003e+\u003c/sup\u003e doping, which deforms the positions of Fe atoms and reduces the band gap. At the same time, the Li\u003csup\u003e+\u003c/sup\u003e migration channel was slightly widened, which improved various properties of the material. With the increase of doping amount, Na\u003csup\u003e+\u003c/sup\u003e will occupy too many positions of Li\u003csup\u003e+\u003c/sup\u003e, which will narrow the channel and reduce various properties of the materials. The first discharge specific capacity of Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e at 0.5C is 142.1 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and after 50 cycles, the discharge specific capacity is 139.35 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJun Cong:\u0026nbsp;Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing - Original Draft;\u003c/p\u003e\n\u003cp\u003eShao-hua Luo:Conceptualization, Funding Acquisition, Resources, Supervision, Writing - Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003eKun Li: Data Curation, Writing-Original Draft; Supervision\u003c/p\u003e\n\u003cp\u003eJia-chen Wang: Visualization, Investigation; Validation\u003c/p\u003e\n\u003cp\u003eYa-feng\u0026nbsp;Wang: Data Curation, Supervision;\u003c/p\u003e\n\u003cp\u003eSheng-xue Yan\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eSoftware, Validation;\u0026nbsp;Review;\u003c/p\u003e\n\u003cp\u003ePeng-wei Li:Writing-Review \u0026amp; Editing;\u003c/p\u003e\n\u003cp\u003eAll authors read and contributed to the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (NSFC) (52274295), Natural Science Foundation of Hebei Province (E2021501029, E2020501001, A2021501007, E2022501028, E2022501029), The Natural Science Foundation-Steel, the Iron Foundation of Hebei Province (No. E2022501030), The Fundamental Research Funds for the Central Universities (No. N2323025, N2302016), Performance subsidy fund for Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province (22567627H), the Science and Technology Project of Hebei Education Department (ZD2022158), the Central Guided Local Science and Technology Development Fund Project of Hebei province (226Z4401G), 2023 Hebei Provincial Postgraduate Student Innovation Ability training funding project (CXZZSS2023195) and The instrumental or data analysis from Analytical and Testing Center, Northeastern University. The authors would like to thank Yu-xin Jiang from Shiyanjia Lab (www.shiyanjia.com) for the XPS, Raman and HTEM analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNo experiments about human or animals were conducted.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data underlying this article will be shared on reasonable request to the corresponding author.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eH. Zhang, X. Hong, An overview for the utilization of wastes from stainless steel industries, Resour. Conserv. Recycl. 55(8) (2011) 745\u0026ndash;754.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Tian, W. Zuo, D. Chen, Crystallization evolution, microstructure and properties of sewage sludge-based glass-ceramics prepared by microwave heating, J. Hazard. Mater. 196 (2011) 370\u0026ndash;379.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. Bian, D. Chen, X. Liu, L. Cui, L. Li, G. Pan, D. Xie, J. Zheng, X. Zhang, J. Zheng, A. Chang, Biochar soil amendment as a solution to prevent Cd-tainted rice from China: Results from a cross-site field experiment, Ecol. Eng. 58 (2013) 378\u0026ndash;383.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Regel-Rosocka, A review on methods of regeneration of spent pickling solutions from steel processing, J. Hazard. Mater. 177(1\u0026ndash;3) (2010) 57\u0026ndash;69.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Wang, W. Zhang, Y. Liu, Y. Yang, C. Wang, Y. Wang, Electrochemical performance of patterned LiFePO\u003csub\u003e4\u003c/sub\u003e nano-electrode with a pristine amorphous layer, Appl. Phys. Lett. 104(17) (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eL. Dimesso, C. Spanheimer, W. Jaegermann, Y. Zhang, A.L. Yarin, LiFePO\u003csub\u003e4\u003c/sub\u003e-3D carbon nanofiber composites as cathode materials for Li-ions batteries, J. Appl. Phys. 111(6) (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Choi, P.N. Kumta, Surfactant based sol-gel approach to nanostructured LiFePO\u003csub\u003e4\u003c/sub\u003e for high rate Li-ion batteries, J. Power Sources 163(2) (2007) 1064\u0026ndash;1069.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG. Wang, H. Liu, J. Liu, S. Qiao, G.M. Lu, P. Munroe, H. Ahn, Mesoporous LiFePO\u003csub\u003e4\u003c/sub\u003e/C Nanocomposite Cathode Materials for High Power Lithium Ion Batteries with Superior Performance, Adv. Mater. 22(44) (2010) 4944-+.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY.-M. Chiang, N. Meethong, Y.-H. Kao, Reply to Comment on \"Aliovalent Substitutions in Olivine Lithium Iron Phosphate and Impact on Structure and Properties\", Adv. Funct. Mater. 20(2) (2010) 189\u0026ndash;191.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.Y. Chung, J.T. Blocking, A.S. Andersson, Y.M. Chiang, Electronically conductive phospho-olivines as lithium storage electrodes, 8th Asian Conference on Solid State Ionics, Langkawi, Malaysia, 2002, pp.\u0026nbsp;85\u0026ndash;85.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD.Y. Wang, H. Li, S.Q. Shi, X.J. Huang, L.Q. Chen, Improving the rate performance of LiFePO\u003csub\u003e4\u003c/sub\u003e by Fe-site doping, Electrochim. Acta 50(14) (2005) 2955\u0026ndash;2958.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Wen, L. Zeng, Z. Tong, L. Nong, x.W. Wei, Structure and properties of LiFe\u003csub\u003e0.9\u003c/sub\u003eV\u003csub\u003e0.1\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, J. Alloys Compd. 416(1\u0026ndash;2) (2006) 206\u0026ndash;208.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Liu, Q. Cao, L.J. Fu, C. Li, Y.P. Wu, H.Q. Wu, Doping effects of zinc on LiFePO\u003csub\u003e4\u003c/sub\u003e cathode material for lithium ion batteries, Electrochem. Commun. 8(10) (2006) 1553\u0026ndash;1557.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT. Takeuchi, M. Tabuchi, A. Nakashima, T. Nakamura, Y. Miwa, H. Kageyama, K. Tatsumi, Preparation of dense LiFePO\u003csub\u003e4\u003c/sub\u003e/C composite positive electrodes using spark-plasma-sintering process, J. Power Sources 146(1\u0026ndash;2) (2005) 575\u0026ndash;579.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Shanmukaraj, G.X. Wang, R. Murugan, H.K. Liu, Electrochemical studies on LiFe\u003csub\u003e1-x\u003c/sub\u003eCo\u003csub\u003ex\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e/carbon composite cathode materials synthesized by citrate gel technique for lithium-ion batteries, Mater. Sci. Eng. B-Adv. 149(1) (2008) 93\u0026ndash;98.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.H. Kang, I. Belharouak, Y.K. Sun, K. Amine, Effect of fluorine on the electrochemical properties of layered Li(Ni\u003csub\u003e0.5\u003c/sub\u003eMn\u003csub\u003e0.5\u003c/sub\u003e)O\u003csub\u003e2\u003c/sub\u003e cathode materials, J. Power Sources 146(1\u0026ndash;2) (2005) 650\u0026ndash;653.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG.G. Amatucci, N. Pereira, Fluoride based electrode materials for advanced energy storage devices, J. Fluor. Chem. 128(4) (2007) 243\u0026ndash;262.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eP. Tang, N.A.W. Holzwarth, Electronic structure of FePO\u003csub\u003e4\u003c/sub\u003e, LiFePO\u003csub\u003e4\u003c/sub\u003e, and related materials, Phys. Rev. B 68(16) (2003).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eF. Zhou, M. Cococcioni, C.A. Marianetti, D. Morgan, G. Ceder, First-principles prediction of redox potentials in transition-metal compounds with LDA + U, Phys. Rev. B 70(23) (2004).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ.M. Osorio-Guillen, B. Holm, R. Ahuja, B. Johansson, A theoretical study of olivine LiMPO\u003csub\u003e4\u003c/sub\u003e cathodes, Solid State Ion. 167(3\u0026ndash;4) (2004) 221\u0026ndash;227.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY.N. Xu, W.Y. Ching, Y.M. Chiang, Comparative studies of the electronic structure of LiFePO\u003csub\u003e4\u003c/sub\u003e, FePO\u003csub\u003e4\u003c/sub\u003e, Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, LiMnPO\u003csub\u003e4\u003c/sub\u003e, LiCoPO\u003csub\u003e4\u003c/sub\u003e, and LiNiPO\u003csub\u003e4\u003c/sub\u003e, J. Appl. Phys. 95(11) (2004) 6583\u0026ndash;6585.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eC.Y. Ouyang, S.Q. Shi, Z.X. Wang, X.J. Huang, L.Q. Chen, First-principles study of Li ion diffusion in LiFePO\u003csub\u003e4\u003c/sub\u003e, Phys. Rev. B 69(10) (2004).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eC.Y. Ouyang, D.Y. Wang, S.Q. Shi, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, First principles study on Na\u003csub\u003ex\u003c/sub\u003eLi\u003csub\u003e1-x\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e as cathode material for rechargeable lithium batteries, Chinese Phys. Lett. 23(1) (2006) 61\u0026ndash;64.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM.E.A. Dompablo, J.M. Gallardo-Amores, U. Amador, Lithium insertion in the high-pressure polymorph of FePO\u003csub\u003e4\u003c/sub\u003e-Computational predictions and experimental findings, Electrochem. Solid St. 8(11) (2005) A564-A569.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.Q. Shi, C.Y. Ouyang, Z.H. Xiong, L.J. Liu, Z.X. Wang, H. Li, D.S. Wang, L.Q. Chen, X.J. Huang, First-principles investigation of the structural, magnetic, and electronic properties of olivine LiFePO\u003csub\u003e4\u003c/sub\u003e, Phys. Rev. B 71(14) (2005).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT. Maxisch, F. Zhou, G. Ceder, Ab initio study of the migration of small polarons in olivine Li\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e and their association with lithium ions and vacancies, Phys. Rev. B 73(10) (2006).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eF. Shimojo, K. Hoshino, Y. Zempo, Ab initio molecular-dynamics simulation method for complex liquids, Comput. Phys. Commun. 142(1\u0026ndash;3) (2001) 364\u0026ndash;367.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. Stadler, W. Wolf, R. Podloucky, G. Kresse, J. Furthmuller, J. Hafner, Ab initio calculations of the cohesive, elastic, and dynamical properties of CoSi\u003csub\u003e2\u003c/sub\u003e by pseudopotential and all-electron techniques, Phys. Rev. B 54(3) (1996) 1729\u0026ndash;1734.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG. Kresse, J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6(1) (1996) 15\u0026ndash;50.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eI.H. Lee, R.M. Martin, Applications of the generalized-gradient approximation to atoms, clusters, and solids, Phys. Rev. B 56(12) (1997) 7197\u0026ndash;7205.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eE.L. Shirley, Optimal basis sets for detailed Brillouin-zone integrations, Phys. Rev. B 54(23) (1996) 16464\u0026ndash;16469.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eL. Dimesso, S. Jacke, C. Spanheimer, W. Jaegermann, Investigation on 3-dimensional carbon foams/LiFePO\u003csub\u003e4\u003c/sub\u003e composites as function of the annealing time under inert atmosphere, J. Alloys Compd. 509(9) (2011) 3777\u0026ndash;3782.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, M. Liu, One-step microwave synthesis and characterization of carbon-modified nanocrystalline LiFePO\u003csub\u003e4\u003c/sub\u003e, Electrochim. Acta 54(11) (2009) 3206\u0026ndash;3210.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eR. Dedryvere, M. Maccario, L. Croguennec, F. Le Cras, C. Delmas, D. Gonbeau, X-Ray Photoelectron Spectroscopy Investigations of Carbon-Coated Li\u003csub\u003ex\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e Materials, Chem. Mater. 20(22) (2008) 7164\u0026ndash;7170.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Reduction of graphene oxide via L-ascorbic acid, Chem. Commun. 46(7) (2010) 1112\u0026ndash;1114.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eL. Wang, R. Zhang, U. Jansson, N. Nedfors, A near-wearless and extremely long lifetime amorphous carbon film under high vacuum, Sci. Rep. 5 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eX. Yin, K. Huang, S. Liu, H. Wang, H. Wang, Preparation and characterization of Na-doped LiFePO\u003csub\u003e4\u003c/sub\u003e/C composites as cathode materials for lithium-ion batteries, J. Power Sources 195(13) (2010) 4308\u0026ndash;4312.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA.T. Phan, A.E. Gheribi, P. Chartrand, Coherent and para-equilibrium phase transformations in Mn-doped- LiFePO\u003csub\u003e4\u003c/sub\u003e cathode materials: Implications for lithium ion battery performances, J. Alloys Compd. 838 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Zhang, Y. Liu, L. Wu, L. Feng, S. Jin, R. Zhang, M. Jin, Effect of Ti ion doping on electrochemical performance of Ni-rich LiNi\u003csub\u003e0.8\u003c/sub\u003eCo\u003csub\u003e0.1\u003c/sub\u003eMn\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e cathode material, Electrochim. Acta 328 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. Yao, Y. Liu, D. Li, Q. Zhang, S. Zhong, H. Cheng, Z. Yan, Synergistically Enhanced Electrochemical Performance of Ni-rich Cathode Materials for Lithium-ion Batteries by K and Ti Comodification, J. Phys. Chem. C 124(4) (2020) 2346\u0026ndash;2356.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ. Cao, G. Zhu, R. Zhang, S. Chen, M. Sang, J. Jia, M. Yang, X. Li, S. Yang, Biological phytic acid guided formation of monodisperse large-sized carbon@LiFePO\u003csub\u003e4\u003c/sub\u003e/graphene composite microspheres for high-performance lithium-ion battery cathodes, Chem. Eng. J. 351 (2018) 382\u0026ndash;390.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Lithium-ion battery, Cathode material, Pickling iron oxide red, Na-doped LiFePO4, Heavy metals immobilization","lastPublishedDoi":"10.21203/rs.3.rs-3138757/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3138757/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo immobilizing hazardous elements of metallurgical waste and meet the demand for cathode materials in lithium-ion battery industries, LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode material was successfully prepared via a simple carbothermal reduction method using acid-pickled iron oxide red as raw material by orthogonal tests. To further improve electrochemical performance, Na-doped LiFePO\u003csub\u003e4\u003c/sub\u003e/C cathode material designed with the first-principles calculation verification was synthesized by solid-state method at the optimal experimental conditions including the ball-milling medium of 3 h, the reaction temperature at 750\u0026deg;C in heating rate of 5 ℃\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 4 h. The results reveal that Na doping can effectively change the band gap structures and microstructure, which reduced the size of the particle and increased the electronic conductivity. The Li\u003csub\u003e0.75\u003c/sub\u003eNa\u003csub\u003e0.25\u003c/sub\u003eFePO\u003csub\u003e4\u003c/sub\u003e/C electrode showed a discharge specific capacity of 139.2 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 0.5 C and an excellent capacity retention of 98.9% after 50 cycles. The synergy strategy was a sustainable solution for immobilizing hazardous heavy metal elements, which paves a novel facile and cost-effective way towards high-performance LiFePO\u003csub\u003e4\u003c/sub\u003e cathodes and promising markets for lithium-ion battery industries.\u003c/p\u003e","manuscriptTitle":"High-performance Na-doped LiFePO 4 cathode material derived from acid-washed iron red for the simultaneous immobilization of multi-metals","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-07-11 14:09:37","doi":"10.21203/rs.3.rs-3138757/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2023-07-15T14:58:28+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-07-09T14:03:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"bb03cbf0-050e-4fce-b874-807726672597","date":"2023-07-06T22:00:41+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-07-06T21:18:31+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-07-05T13:15:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-07-05T13:15:50+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2023-07-04T10:42:46+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9cabfd38-8c82-4b68-90dc-ccd30c6416cc","owner":[],"postedDate":"July 11th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2023-07-27T22:14:14+00:00","versionOfRecord":[],"versionCreatedAt":"2023-07-11 14:09:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3138757","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3138757","identity":"rs-3138757","version":["v1"]},"buildId":"J0_U0BvcaRcwD8yVFaRlm","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}