LiFePO₄ with enhanced electrical conductivity via nitrogen-doped carbon coating layer

LiFePO₄ with enhanced electrical conductivity via nitrogen-doped carbon coating layer | 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 LiFePO₄ with enhanced electrical conductivity via nitrogen-doped carbon coating layer S. H. Lee, Yun Hyeok Choi, Min Seo Choi, J. M. One, Jeong H. Cho, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7405892/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Feb, 2026 Read the published version in Journal of Electroceramics → Version 1 posted 13 You are reading this latest preprint version Abstract In this comprehensive study, the significant impact of nitrogen-doped carbon coating layer (NC) addition on enhancing the electrochemical performance of LiFePO₄ cathode materials was systematically investigated. The electrical conductivity and electron transfer pathways within the electrode structure were substantially enhanced by strategically introducing NC onto the LiFePO₄ surface through advanced coating techniques. Comprehensive experimental results demonstrated that the NC coating layer remarkably improved the overall electrochemical performance, exhibiting an impressive discharge capacity of 104.43 mAh/g after 100 cycles at 1C rate and maintaining an exceptional capacity retention of 105.0%, which indicates superior cycling stability. The differential capacity analysis (dQ/dV) revealed significantly sharper and more well-defined peaks after 100 cycles, clearly indicating more efficient and rapid lithium ion insertion and extraction processes within the electrode matrix. Most importantly, detailed electrochemical impedance spectroscopy (EIS) analysis showed a substantially reduced charge transfer resistance (Rct) value of 52.18 Ω compared to the conventional LFP/C sample, definitively confirming that the electrical conductivity and overall electrochemical kinetics of the nitrogen-doped carbon coating layer sample were significantly enhanced compared to the conventional carbon coating layer approach. These findings demonstrate the promising potential of nitrogen-doped carbon coating strategies for advanced lithium-ion battery applications. electron conductivity hydrothermal synthesis lithium iron phosphate nitrogen-doped carbon coating Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Recent expansion of the secondary battery market has led to active research and development in this field. Lithium-ion batteries (LIBs) are used extensively in electric vehicles and energy storage devices owing to their high energy density and long cycle performance [ 1 – 2 ]. LiFePO 4 (LFP), a cathode material for LIBs, has an orthorhombic olivine structure belonging to the Pnma space group and offers advantages such as a theoretical capacity of 170 mAh/g, low cost, and stable cycle characteristics, making it the subject of extensive current research [ 3 – 4 ]. In particular, it exhibits high thermal and electrochemical stability owing to its strong P–O covalent bonds. However, it has the disadvantages of poor high-rate charge–discharge characteristics owing to its heavy weight, resulting in lower energy density compared with other materials and low electronic conductivity (10 –10 S/cm) and lithium ion diffusion coefficient (10 –14 cm 2 /s) [ 5 – 6 ]. To address these disadvantages, recent research has focused on the formation of carbon coating layers on particle surfaces or doping with transition metals such as Mn at Fe sites to increase the electronic and ionic conductivity, thereby expanding lithium-ion diffusion channels within the structure and significantly improving the rate and cycle performance. In addition, many studies have reported that reducing the particle size to the nanometer scale decreases the lithium-ion diffusion distance, leading to enhanced lithium charge–discharge characteristics [ 7 – 12 ]. In this study, a nitrogen-doped carbon (NC) coating was introduced onto the LiFePO 4 surface using melamine as the nitrogen source and glucose as the carbon source to improve electrical conductivity and enhance the electrochemical performance. 2. Experiments 2.1 Synthesis of the LiFePO 4 Precursor In this experiment, the precursor was synthesized using lithium hydroxide (LiOH·H 2 O, 99.9%, Sigma-Aldrich), iron(II) sulfate heptahydrate (FeSO 4 ·7H 2 O, 98.0%-102.0%, SAMCHUN), phosphoric acid (H 3 PO 4 , 85 wt%, Sigma-Aldrich), and ascorbic acid (C₆H₈O₆, 99.9%, SAMCHUN) as starting materials, with distilled water and ethylene glycol (EG, 99.9%, Sigma-Aldrich) as solvents. First, 90 mmol of LiOH·H2O and 30 mmol of H 3 PO 4 were dissolved in a mixed solution of 50 ml DI water (Deionized Water) and 50 ml EG by stirring at 80°C and 300 rpm for 30 min. Then, 30 mmol of FeSO 4 ·7H 2 O and a certain amount of C₆H₈O₆ (to prevent the oxidation of Fe 2+ to Fe 3+ ) were dissolved in a mixed solution of DI water and EG for 30 min. After complete dissolution, the two solutions were mixed and allowed to react for 30 min. The obtained suspension was placed in a Teflon-lined autoclave and subjected to hydrothermal synthesis at 180°C for 10 h. Following synthesis, the product was washed several times with DI water and ethanol and then dried in a vacuum oven at 80°C [ 13 – 18 ]. 2.2 Cathode Material The following procedure was used to form a coating layer on the dried LFP precursor. Glucose (C 6 H 12 O 6 , Sigma-Aldrich) was used as the carbon source and melamine (C 3 H 6 N 6 , Sigma-Aldrich) as the nitrogen source [ 19 – 20 ]. The coating layer materials were set to 10 wt% of the precursor, and the NC coating layer sample had a C:N molar ratio of 20:1. For mixing, 0.9 g of the precursor, 0.1 g of glucose, and 0.02 g of melamine were ball milled using ethanol as a solvent. Ball milling was performed at 300 rpm for 3 h at a ball-to-material ratio of 20:1. Following mixing, the sample was dried in a vacuum oven and then heat-treated in a nitrogen (N 2 ) atmosphere by calcination at 350°C for 2 h, followed by 700°C for 5 h. 2.3 Material Characterization The crystal structure and crystallinity of the cathode materials were analyzed using x-ray diffraction (XRD, D8 Advance, Bruker) with Cu radiation in the range of 10°–70° at a scan rate of 3°/min. The surfaces of the synthesized samples were analyzed using field-emission scanning electron microscopy (FE–SEM, FEI QUANTA 400). 2.4 Electrode Preparation and Evaluation of electrochemical Properties Coi n cells were fabricated to measure the electrochemical properties of the LFP/C. A slurry was prepared by mixing the active material (LiFePO 4 /C), conductive agent (Super P), and binder (6% polyvinylidene fluoride) at a ratio of 8:1:1 in an N-methyl pyrrolidone (NMP, Aldrich) solvent. The slurry was coated on Al foil to a thickness of 25m using a doctor blade and dried in a vacuum oven at 120°C. The dried electrode was rolled using a roll press and punched (14∅) to form discs. Li metal was used as the counter electrode for the coin cell fabrication (CR2032), and the electrolyte consisted of 1 M LiPF 6 dissolved in a solvent with EC/DEC = 3:7. The coin cells were assembled in a glove box filled with Ar gas according to the manufacturing process. The electrochemical properties of the fabricated coin cells were evaluated using a galvanostatic charge–discharge tester from PNE in the voltage range of 2.5–4.2 V at room temperature (25°C). To analyze the high-rate characteristics, charge–discharge tests were performed at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C. The internal resistance of the coin cells and diffusion coefficient of the lithium ions were analyzed using an impedance analyzer (ivium stat., HS technology). A schematic of the material synthesis and electrochemical property evaluation process is shown in Fig. 1 . 3. Results and Discussion Fig. 2 shows the XRD analysis results of the LFP/C and LFP/NC cathode materials. All samples exhibited typical LFP spectra with the characteristic orthorhombic olivine structure of the Pmna crystal structure. Both samples maintained their LFP structures without any changes in the crystalline phase. This confirms that the C and NC coating layers did not create impurity phases or affect the LFP crystal structure. The crystal lattice dimensions of the samples were calculated from the XRD patterns, and the results are listed in Table 1 . LFP/NC showed a slight expansion of the a-axis compared with LFP/C, whereas the b- and c-axes were slightly decreased. The cell volume increased by approximately 0.97 ų. These lattice structural changes reflect the subtle influence of the NC layer on the crystal structure and may have a positive effect on lithium-ion diffusion pathways. In addition, when considered together with the XRD results, LFP/NC exhibited enhanced crystallinity, which is expected to contribute to its improved electrochemical performance. Table 1 Lattice parameters of all samples Sample a (Å) b (Å) c (Å) Cell volume (Å ³ ) LFP/C 10.27255 (±0.0001) 6.0261 (±0.0001) 4.75535 (±0.0001) 286.427 LFP/NC 10.32702 (±0.0001) 6.00849 (±0.0001) 4.69206 (±0.0001) 287.394 Subsequently, the electrochemical properties of cells fabricated using cathode-active materials were compared. Fig. 3 (a) shows the initial cycle results of the electrodes conducted at 0.1 C. All samples showed a flat voltage plateau in the range of 3.4–3.5 V. LFP/C exhibited a discharge capacity of 105.47 mAh/g, whereas LFP/NC showed 128.70 mAh/g. The NC coating layer increased the discharge capacity. Furthermore, the charge–discharge curves of LFP/NC were relatively more linear because of the improved conductivity of the NC coating, which resulted in more uniform characteristics. Fig. 3 (b) and (c) compare the dQ/dV curves for each sample after 1 and 100 cycles. For the LFP/C electrode, the oxidation peak where Fe 2+ was oxidized to Fe 3+ decreased from 3.55 to 3.52 V, while for LFP/NC, it decreased from 3.50 to 3.49 V. Both electrodes showed a downward shift in oxidation peaks, indicating reduced overpotential. Both electrodes demonstrated increased lithium deintercalation efficiency after cycling, with LFP/NC showing a smaller downward shift than LFP/C, indicating more stable performance. The reduction peak where Fe 3+ was reduced to Fe 2+ remained at 3.37 V for the LFP/C electrode, while for LFP/NC, it decreased from 3.38 to 3.37 V, suggesting that LFP/NC intercalates lithium after cycling more efficiently. In addition, the difference between the oxidation and reduction peaks for LFP/C decreased from 0.18 to 0.15 V (a 16.67% reduction), whereas that for LFP/NC remained constant at 0.12 V. LFP/C showed improved electrochemical performance with reduced overpotential after cycling and increased lithium deintercalation efficiency, while LFP/NC exhibited excellent performance from the initial stage and maintained stable performance, suggesting superior cycling performance. Fig. 4-(a) compares the cycling performances of all electrodes. After 100 cycles at 1 C, LFP/C and LFP/NC exhibited capacity retentions of 102.6% and 105.0%, respectively, indicating highly reversible lithium-ion insertion/extraction. Compared with LFP/C, LFP/NC exhibited stable performance without rapid capacity degradation during the initial 20 cycles. This indicates that the NC coating layer is effective in improving the conductivity and surface stabilization within the electrode and is expected to enhance the long-term cycling performance. In addition, the discharge capacity after 100 cycles increased compared with the initial discharge capacity, which can be attributed to the electrode surface not being fully opened as a lithium-ion insertion/extraction pathway during the initial charge–discharge cycles. As the cycling progressed, the formation of the NC coating layer improved the electrochemical reactivity, resulting in an increased capacity. Fig. 4- (b) shows the discharge capacity at various current densities according to the charge–discharge rates. Each sample was measured from 0.1 to 10 C. The LFP/C sample showed a discharge capacity of 102.7 mAh/g at 0.1 C and 105.6 mAh/g at 0.1 C after 10 C, demonstrating excellent recovery. The LFP/NC sample showed an average discharge capacity of 130.1 mAh/g at 0.1 C, an excellent discharge capacity of 58.7 mAh/g at 10 C, and a discharge capacity of 134.8 mAh/g at 0.1 C, indicating excellent recovery. The electrode with the NC coating layer exhibited a higher discharge capacity, suggesting that the NC layer exhibited high electrical conductivity, enabling reversible lithium-ion insertion/extraction. Furthermore, the discharge capacity at 0.1 C after 10 C was higher than the initial 0.1 C, indicating that electrodes composed only of carbon layers undergo activation relatively later than the NC layers. Fig. 5 shows the results of comparing the electrode resistance characteristics of LFP/C and LFP/NC using electrochemical impedance spectroscopy (EIS). The values are listed in Table 2. R s represents the electronic charge-transfer resistance between the electrode and electrolyte. R ct is the charge transfer resistance of the cathode material. LFP/NC showed a lower resistance than LFP/C because the introduction of nitrogen into the carbon coating layer increased the electronic conductivity. In addition, both electrodes showed significantly reduced R ct values after 100 cycles because repeated charge–discharge cycles activated inactive sites within the electrode and formed pathways for easy electron movement. The lithium-ion diffusion coefficients (DLi + ) of LFP/C and LFP/NC increased by 331.49% and 152.67 %, respectively. Although LFP/C showed a higher increase in the lithium-ion diffusion coefficient, the LFP/NC value remained higher, indicating high electrical conductivity even after cycling and suggesting that the NC coating layer contributed to improved electrochemical performance. Table 2 Lithium-ion diffusion coefficients of all samples Sample Cycle (n) R s (Ω) R ct (Ω) D Li+ (cm 2 /s) LFP/C 0 10.21 451.12 1.01436×10 -14 100 8.23 65.61 3.36254×10 -14 LFP/NC 0 7.21 402.31 9.82267×10 -14 100 5.19 52.18 1.57839×10 -13 4. Conclusions In this study, the effects of an NC layer on the electrochemical performance enhancement of LFP cathode materials were investigated. XRD analysis confirmed that the NC coating did not generate impurities and did not affect the crystal structure, forming a pure crystalline phase. In terms of electrochemical characteristics, the formation of the NC coating layer resulted in no change in the overpotential between the oxidation/reduction peaks in dQ/dV, enabling the prediction of electrode stability and excellent performance retention, and demonstrating an outstanding capacity retention of 105% even after 100 cycles. In addition, the EIS analysis showed that LFP/NC exhibited lower resistance values than LFP/C, with R ct decreasing to 12.9% of its initial value after 100 cycles. The lithium-ion diffusion coefficient increased by 160%, indicating the successful synthesis of LiFePO4 with excellent electrical conductivity using the NC coating layer. Declarations Author Contribution S.H. Lee: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft. Yun Hyeok Choi: Formal analysis, Visualization, Validation. Min Seo Choi: Investigation, Resources. J.M. One: Data analysis, Visualization. Jeong H. Cho: Methodology, Review & editing. J. Hyuk Kim: Writing – review & editing.San Kang: Project administration. Jong-Tae Son: Funding acquisition, Supervision, Project administration, Writing – review & editing.All authors have read and agreed to the published version of the manuscript. Acknowledgments This research was supported by a Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS-2022-KI002562, HRD Program for Industrial Innovation), and by the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Institute for Advancement of Technology (KIAT) through the “Support for Middle Market Enterprises and Regional Innovation Alliances" program (R&D, RS-2025-02633071). References Padhi, A. K., Nanjundaswamy, K. S., & Goodenough, J. B. (1997). Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. Journal of the Electrochemical Society, 144, 1188-1194. Yuan, L. X., Wang, Z. H., Zhang, W. X., Hu, X. L., Chen, J. T., Huang, Y. H., & Goodenough, J. B. (2011). Development and challenges of LiFePO 4 cathode material for lithium-ion batteries. Energy & Environmental Science, 4(2), 269-284. Hasan, M. K., Mahmud, M., Habib, A. A., Motakabber, S. M. A., & Islam, S. (2021). 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International Journal of Electrochemical Science, 16, 210331. Ghafarian-Zahmatkesh, H., Javanbakht, M., & Ghaemi, M. (2015). Ethylene glycol-assisted hydrothermal synthesis and characterization of bow-tie-like lithium iron phosphate nanocrystals for lithium-ion batteries. Journal of Power Sources, 284, 339-348. Oh, J., Lee, J., Hwang, T., Kim, J. M., Seoung, K. D., & Piao, Y. (2017). Dual layer coating strategy utilizing N-doped carbon and reduced graphene oxide for high-performance LiFePO4 cathode material. Electrochemical Acta, 231, 85-93. Vernardou, D. (2022). Recent report on the hydrothermal growth of LiFePO4 as a cathode material. Coatings, 12(10), 1543. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 24 Feb, 2026 Read the published version in Journal of Electroceramics → Version 1 posted Editorial decision: Revision requested 21 Sep, 2025 Reviews received at journal 19 Sep, 2025 Reviews received at journal 17 Sep, 2025 Reviewers agreed at journal 14 Sep, 2025 Reviewers agreed at journal 11 Sep, 2025 Reviewers agreed at journal 09 Sep, 2025 Reviewers agreed at journal 08 Sep, 2025 Reviewers agreed at journal 08 Sep, 2025 Reviewers agreed at journal 08 Sep, 2025 Reviewers invited by journal 06 Sep, 2025 Editor assigned by journal 20 Aug, 2025 Submission checks completed at journal 20 Aug, 2025 First submitted to journal 19 Aug, 2025 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-7405892","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":513801905,"identity":"0445838c-5c02-44d0-b408-d670e4201597","order_by":0,"name":"S. H. Lee","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6klEQVRIie3RsWrDMBCA4TMCZTmjVcGDX+FMIHQI6asoGPwMhg6xKThLIGuGvkVeQObAWdQ9o7NnSLZOpW6G0qWKxw76Nwk+TuIAQqF/mIK27q+lFurnCh+Qac2c7d1iMq2Gkx1DiLsiiZtCkR1LoHOUoORkdrQyucIyBXzvvSLaOsrekGdzZ6S2kGdVvCEvEdqRuWjO5/ZOhAEl/e+S6YUsEq8Pu/6brB8ThG5VoykE6fsUNhA3fqKBOdrbhdCn8+uTo2PWYOcnz7bdfNw+h1Xu8vZUli+pwsJPfhVVw5qG340GoVAoFPqzL7fHRIFivlYSAAAAAElFTkSuQmCC","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":true,"prefix":"","firstName":"S.","middleName":"H.","lastName":"Lee","suffix":""},{"id":513801906,"identity":"6ca4965f-908d-45dc-aa8f-2690da70d59c","order_by":1,"name":"Yun Hyeok Choi","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"Yun","middleName":"Hyeok","lastName":"Choi","suffix":""},{"id":513801907,"identity":"0d0e3474-6a22-4b6f-a354-730f2108aa1d","order_by":2,"name":"Min Seo Choi","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"Seo","lastName":"Choi","suffix":""},{"id":513801908,"identity":"00c5ece2-210d-404b-8ecc-4d8e6376b3c7","order_by":3,"name":"J. M. One","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"J.","middleName":"M.","lastName":"One","suffix":""},{"id":513801909,"identity":"cbfbfce8-921a-45e6-a852-20d4c217d057","order_by":4,"name":"Jeong H. Cho","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"Jeong","middleName":"H.","lastName":"Cho","suffix":""},{"id":513801910,"identity":"fb9208c3-1711-42e4-a7d8-b53332f014cc","order_by":5,"name":"J. Hyuk Kim","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"J.","middleName":"Hyuk","lastName":"Kim","suffix":""},{"id":513801911,"identity":"bbdbe752-2c2c-4a53-8a0b-71e8cb74f1b8","order_by":6,"name":"San Kang","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"San","middleName":"","lastName":"Kang","suffix":""},{"id":513801912,"identity":"139188c9-79ff-463a-8e72-4fac58d43965","order_by":7,"name":"Jong-Tae Son","email":"","orcid":"","institution":"Korea National University of Transportation","correspondingAuthor":false,"prefix":"","firstName":"Jong-Tae","middleName":"","lastName":"Son","suffix":""}],"badges":[],"createdAt":"2025-08-19 07:53:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7405892/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7405892/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10832-025-00452-7","type":"published","date":"2026-02-24T15:58:42+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":91170915,"identity":"3077ed2f-53f0-4a65-95f7-fed07511733b","added_by":"auto","created_at":"2025-09-12 11:35:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":243190,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the experimental process.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/d83f40b171d056e5e56b915e.png"},{"id":91169718,"identity":"34748aab-df5d-4705-82c5-6fa8a69b4da7","added_by":"auto","created_at":"2025-09-12 11:19:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":60844,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of LFP/C and LFP/NC.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/36882b5e200a684e7cacf6dd.png"},{"id":91169719,"identity":"0b72e97b-17c7-4aa3-8079-503779017bd7","added_by":"auto","created_at":"2025-09-12 11:19:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":149297,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Initial charge–discharge curves of samples from 2.5–4.2 V at a 0.1 C rate, and dQ/dV curves for (b) LFP/C and (c) LFP/NC cathodes at 1st and 100th cycles with 1 C current in the range of 2.5–4.2 V.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/c332ee5cd173c4eb346a754e.png"},{"id":91170916,"identity":"5a687a7a-7c91-4dd6-811c-2cdf41e7f2f1","added_by":"auto","created_at":"2025-09-12 11:35:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":162849,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cycle stability of electrodes prepared with LFP/C and LFP/NC, and (b) discharge capacities of samples at different rates.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/bc275fb309e5728e58a631a9.png"},{"id":91169724,"identity":"49a23851-25e2-441d-b736-ae1557444321","added_by":"auto","created_at":"2025-09-12 11:19:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":91238,"visible":true,"origin":"","legend":"\u003cp\u003eEIS spectra of samples (a) before and (b) after cycling for R\u003csub\u003ect\u003c/sub\u003e evaluation.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/6ee5216c45b97828b274bf30.png"},{"id":91169730,"identity":"9b619914-f600-4b16-acd9-01a32e3993ce","added_by":"auto","created_at":"2025-09-12 11:19:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":35937,"visible":true,"origin":"","legend":"\u003cp\u003eEquivalent circuit model used to fit the EIS data of the cell.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/d4c14be2f415a6da76c45cd8.png"},{"id":103766730,"identity":"d86006da-b717-48b8-8516-a64027a1a639","added_by":"auto","created_at":"2026-03-02 16:15:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1178174,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7405892/v1/64f9e344-7320-4044-b3cf-951ae8de173b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"LiFePO₄ with enhanced electrical conductivity via nitrogen-doped carbon coating layer","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRecent expansion of the secondary battery market has led to active research and development in this field. Lithium-ion batteries (LIBs) are used extensively in electric vehicles and energy storage devices owing to their high energy density and long cycle performance [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eLiFePO\u003csub\u003e4\u003c/sub\u003e (LFP), a cathode material for LIBs, has an orthorhombic olivine structure belonging to the Pnma space group and offers advantages such as a theoretical capacity of 170 mAh/g, low cost, and stable cycle characteristics, making it the subject of extensive current research [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. In particular, it exhibits high thermal and electrochemical stability owing to its strong P\u0026ndash;O covalent bonds. However, it has the disadvantages of poor high-rate charge\u0026ndash;discharge characteristics owing to its heavy weight, resulting in lower energy density compared with other materials and low electronic conductivity (10\u003csup\u003e\u0026ndash;10\u003c/sup\u003e S/cm) and lithium ion diffusion coefficient (10\u003csup\u003e\u0026ndash;14\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e/s) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo address these disadvantages, recent research has focused on the formation of carbon coating layers on particle surfaces or doping with transition metals such as Mn at Fe sites to increase the electronic and ionic conductivity, thereby expanding lithium-ion diffusion channels within the structure and significantly improving the rate and cycle performance. In addition, many studies have reported that reducing the particle size to the nanometer scale decreases the lithium-ion diffusion distance, leading to enhanced lithium charge\u0026ndash;discharge characteristics [\u003cspan additionalcitationids=\"CR8 CR9 CR10 CR11\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this study, a nitrogen-doped carbon (NC) coating was introduced onto the LiFePO\u003csub\u003e4\u003c/sub\u003e surface using melamine as the nitrogen source and glucose as the carbon source to improve electrical conductivity and enhance the electrochemical performance.\u003c/p\u003e"},{"header":"2. Experiments","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Synthesis of the LiFePO\u003csub\u003e4\u003c/sub\u003e Precursor\u003c/h2\u003e\u003cp\u003eIn this experiment, the precursor was synthesized using lithium hydroxide (LiOH\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO, 99.9%, Sigma-Aldrich), iron(II) sulfate heptahydrate (FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO, 98.0%-102.0%, SAMCHUN), phosphoric acid (H\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, 85 wt%, Sigma-Aldrich), and ascorbic acid (C₆H₈O₆, 99.9%, SAMCHUN) as starting materials, with distilled water and ethylene glycol (EG, 99.9%, Sigma-Aldrich) as solvents.\u003c/p\u003e\u003cp\u003eFirst, 90 mmol of LiOH\u0026middot;H2O and 30 mmol of H\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003ewere dissolved in a mixed solution of 50 ml DI water (Deionized Water) and 50 ml EG by stirring at 80\u0026deg;C and 300 rpm for 30 min. Then, 30 mmol of FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO and a certain amount of C₆H₈O₆ (to prevent the oxidation of Fe\u003csup\u003e2+\u003c/sup\u003e to Fe\u003csup\u003e3+\u003c/sup\u003e) were dissolved in a mixed solution of DI water and EG for 30 min. After complete dissolution, the two solutions were mixed and allowed to react for 30 min. The obtained suspension was placed in a Teflon-lined autoclave and subjected to hydrothermal synthesis at 180\u0026deg;C for 10 h. Following synthesis, the product was washed several times with DI water and ethanol and then dried in a vacuum oven at 80\u0026deg;C [\u003cspan additionalcitationids=\"CR14 CR15 CR16 CR17\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Cathode Material\u003c/h2\u003e\u003cp\u003eThe following procedure was used to form a coating layer on the dried LFP precursor. Glucose (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e12\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e, Sigma-Aldrich) was used as the carbon source and melamine (C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eN\u003csub\u003e6\u003c/sub\u003e, Sigma-Aldrich) as the nitrogen source [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The coating layer materials were set to 10 wt% of the precursor, and the NC coating layer sample had a C:N molar ratio of 20:1. For mixing, 0.9 g of the precursor, 0.1 g of glucose, and 0.02 g of melamine were ball milled using ethanol as a solvent. Ball milling was performed at 300 rpm for 3 h at a ball-to-material ratio of 20:1. Following mixing, the sample was dried in a vacuum oven and then heat-treated in a nitrogen (N\u003csub\u003e2\u003c/sub\u003e) atmosphere by calcination at 350\u0026deg;C for 2 h, followed by 700\u0026deg;C for 5 h.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Material Characterization\u003c/h2\u003e\u003cp\u003eThe crystal structure and crystallinity of the cathode materials were analyzed using x-ray diffraction (XRD, D8 Advance, Bruker) with Cu radiation in the range of 10\u0026deg;\u0026ndash;70\u0026deg; at a scan rate of 3\u0026deg;/min. The surfaces of the synthesized samples were analyzed using field-emission scanning electron microscopy (FE\u0026ndash;SEM, FEI QUANTA 400).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Electrode Preparation and Evaluation of electrochemical Properties\u003c/h2\u003e\u003cp\u003e\u003cstrong\u003eCoi\u003c/strong\u003e\u003cp\u003en cells were fabricated to measure the electrochemical properties of the LFP/C. A slurry was prepared by mixing the active material (LiFePO\u003csub\u003e4\u003c/sub\u003e/C), conductive agent (Super P), and binder (6% polyvinylidene fluoride) at a ratio of 8:1:1 in an N-methyl pyrrolidone (NMP, Aldrich) solvent. The slurry was coated on Al foil to a thickness of 25m using a doctor blade and dried in a vacuum oven at 120\u0026deg;C. The dried electrode was rolled using a roll press and punched (14\u0026empty;) to form discs. Li metal was used as the counter electrode for the coin cell fabrication (CR2032), and the electrolyte consisted of 1 M LiPF\u003csub\u003e6\u003c/sub\u003e dissolved in a solvent with EC/DEC\u0026thinsp;=\u0026thinsp;3:7. The coin cells were assembled in a glove box filled with Ar gas according to the manufacturing process. The electrochemical properties of the fabricated coin cells were evaluated using a galvanostatic charge\u0026ndash;discharge tester from PNE in the voltage range of 2.5\u0026ndash;4.2 V at room temperature (25\u0026deg;C). To analyze the high-rate characteristics, charge\u0026ndash;discharge tests were performed at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C. The internal resistance of the coin cells and diffusion coefficient of the lithium ions were analyzed using an impedance analyzer (ivium stat., HS technology). A schematic of the material synthesis and electrochemical property evaluation process is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003eFig. 2\u003c/strong\u003e shows the XRD analysis results of the LFP/C and LFP/NC cathode materials. All samples exhibited typical LFP spectra with the characteristic orthorhombic olivine structure of the Pmna crystal structure. Both samples maintained their LFP structures without any changes in the crystalline phase. This confirms that the C and NC coating layers did not create impurity phases or affect the LFP crystal structure. The crystal lattice dimensions of the samples were calculated from the XRD patterns, and the results are listed in \u003cstrong\u003eTable 1\u003c/strong\u003e. LFP/NC showed a slight expansion of the a-axis compared with LFP/C, whereas the b- and c-axes were slightly decreased. The cell volume increased by approximately 0.97 \u0026Aring;\u0026sup3;. These lattice structural changes reflect the subtle influence of the NC layer on the crystal structure and may have a positive effect on lithium-ion diffusion pathways. In addition, when considered together with the XRD results, LFP/NC exhibited enhanced crystallinity, which is expected to contribute to its improved electrochemical performance.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1\u0026nbsp;\u003c/strong\u003eLattice parameters of all samples\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" align=\"\" width=\"287\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eSample\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ea (\u0026Aring;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eb (\u0026Aring;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ec (\u0026Aring;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCell\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003evolume (Å\u003csup\u003e\u0026sup3;\u003c/sup\u003e)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003eLFP/C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e10.27255\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e6.0261\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e4.75535\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003e286.427\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003eLFP/NC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e10.32702\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e6.00849\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e4.69206\u003c/p\u003e\n \u003cp\u003e(\u0026plusmn;0.0001)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 62px;\"\u003e\n \u003cp\u003e287.394\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eSubsequently, the electrochemical properties of cells fabricated using cathode-active materials were compared. \u003cstrong\u003eFig. 3\u003c/strong\u003e\u003cstrong\u003e(a)\u003c/strong\u003e shows the initial cycle results of the electrodes conducted at 0.1 C. All samples showed a flat voltage plateau in the range of 3.4\u0026ndash;3.5 V. LFP/C exhibited a discharge capacity of 105.47 mAh/g, whereas LFP/NC showed 128.70 mAh/g. The NC coating layer increased the discharge capacity. Furthermore, the charge\u0026ndash;discharge curves of LFP/NC were relatively more linear because of the improved conductivity of the NC coating, which resulted in more uniform characteristics. \u003cstrong\u003eFig. 3\u003c/strong\u003e(b) and (c) compare the dQ/dV curves for each sample after 1 and 100 cycles. For the LFP/C electrode, the oxidation peak where Fe\u003csup\u003e2+\u003c/sup\u003e was oxidized to Fe\u003csup\u003e3+\u003c/sup\u003e decreased from 3.55 to 3.52 V, while for LFP/NC, it decreased from 3.50 to 3.49 V. Both electrodes showed a downward shift in oxidation peaks, indicating reduced overpotential. Both electrodes demonstrated increased lithium deintercalation efficiency after cycling, with LFP/NC showing a smaller downward shift than LFP/C, indicating more stable performance. The reduction peak where Fe\u003csup\u003e3+\u003c/sup\u003e was reduced to Fe\u003csup\u003e2+\u003c/sup\u003e remained at 3.37 V for the LFP/C electrode, while for LFP/NC, it decreased from 3.38 to 3.37 V, suggesting that LFP/NC intercalates lithium after cycling more efficiently. In addition, the difference between the oxidation and reduction peaks for LFP/C decreased from 0.18 to 0.15 V (a 16.67% reduction), whereas that for LFP/NC remained constant at 0.12 V. LFP/C showed improved electrochemical performance with reduced overpotential after cycling and increased lithium deintercalation efficiency, while LFP/NC exhibited excellent performance from the initial stage and maintained stable performance, suggesting superior cycling performance.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 4-(a)\u003c/strong\u003e compares the cycling performances of all electrodes. After 100 cycles at 1 C, LFP/C and LFP/NC exhibited capacity retentions of 102.6% and 105.0%, respectively, indicating highly reversible lithium-ion insertion/extraction. Compared with LFP/C, LFP/NC exhibited stable performance without rapid capacity degradation during the initial 20 cycles. This indicates that the NC coating layer is effective in improving the conductivity and surface stabilization within the electrode and is expected to enhance the long-term cycling performance. In addition, the discharge capacity after 100 cycles increased compared with the initial discharge capacity, which can be attributed to the electrode surface not being fully opened as a lithium-ion insertion/extraction pathway during the initial charge\u0026ndash;discharge cycles. As the cycling progressed, the formation of the NC coating layer improved the electrochemical reactivity, resulting in an increased capacity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 4-\u003c/strong\u003e(b) shows the discharge capacity at various current densities according to the charge\u0026ndash;discharge rates. Each sample was measured from 0.1 to 10 C. The LFP/C sample showed a discharge capacity of 102.7 mAh/g at 0.1 C and 105.6 mAh/g at 0.1 C after 10 C, demonstrating excellent recovery. The LFP/NC sample showed an average discharge capacity of 130.1 mAh/g at 0.1 C, an excellent discharge capacity of 58.7 mAh/g at 10 C, and a discharge capacity of 134.8 mAh/g at 0.1 C, indicating excellent recovery. The electrode with the NC coating layer exhibited a higher discharge capacity, suggesting that the NC layer exhibited high electrical conductivity, enabling reversible lithium-ion insertion/extraction. Furthermore, the discharge capacity at 0.1 C after 10 C was higher than the initial 0.1 C, indicating that electrodes composed only of carbon layers undergo activation relatively later than the NC layers.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 5\u003c/strong\u003e shows the results of comparing the electrode resistance characteristics of LFP/C and LFP/NC using electrochemical impedance spectroscopy (EIS). The values are listed in \u003cstrong\u003eTable 2.\u0026nbsp;\u003c/strong\u003eR\u003csub\u003es\u003c/sub\u003e represents the electronic charge-transfer resistance between the electrode and electrolyte. R\u003csub\u003ect\u003c/sub\u003e is the charge transfer resistance of the cathode material. LFP/NC showed a lower resistance than LFP/C because the introduction of nitrogen into the carbon coating layer increased the electronic conductivity. In addition, both electrodes showed significantly reduced R\u003csub\u003ect\u003c/sub\u003e values after 100 cycles because repeated charge\u0026ndash;discharge cycles activated inactive sites within the electrode and formed pathways for easy electron movement. The lithium-ion diffusion coefficients (DLi\u003csup\u003e+\u003c/sup\u003e) of LFP/C and LFP/NC increased by 331.49% and 152.67 %, respectively. Although LFP/C showed a higher increase in the lithium-ion diffusion coefficient, the LFP/NC value remained higher, indicating high electrical conductivity even after cycling and suggesting that the NC coating layer contributed to improved electrochemical performance.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2\u0026nbsp;\u003c/strong\u003eLithium-ion diffusion coefficients of all samples\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" align=\"\" width=\"293\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eSample\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCycle (n)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 47px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eR\u003csub\u003es\u0026nbsp;\u003c/sub\u003e(\u0026Omega;)\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eR\u003csub\u003ect\u0026nbsp;\u003c/sub\u003e(\u0026Omega;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eD\u003csub\u003eLi+\u0026nbsp;\u003c/sub\u003e(cm\u003csup\u003e2\u003c/sup\u003e/s)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eLFP/C\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 47px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e10.21\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e451.12\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.01436\u0026times;10\u003csup\u003e-14\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e100\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 47px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e8.23\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e65.61\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e3.36254\u0026times;10\u003csup\u003e-14\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd rowspan=\"2\" style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eLFP/NC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e0\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 47px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e7.21\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e402.31\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e9.82267\u0026times;10\u003csup\u003e-14\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e100\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 47px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e5.19\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 57px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e52.18\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 76px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.57839\u0026times;10\u003csup\u003e-13\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, the effects of an NC layer on the electrochemical performance enhancement of LFP cathode materials were investigated. XRD analysis confirmed that the NC coating did not generate impurities and did not affect the crystal structure, forming a pure crystalline phase. In terms of electrochemical characteristics, the formation of the NC coating layer resulted in no change in the overpotential between the oxidation/reduction peaks in dQ/dV, enabling the prediction of electrode stability and excellent performance retention, and demonstrating an outstanding capacity retention of 105% even after 100 cycles. In addition, the EIS analysis showed that LFP/NC exhibited lower resistance values than LFP/C, with R\u003csub\u003ect\u003c/sub\u003e decreasing to 12.9% of its initial value after 100 cycles. The lithium-ion diffusion coefficient increased by 160%, indicating the successful synthesis of LiFePO4 with excellent electrical conductivity using the NC coating layer.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eS.H. Lee: Conceptualization, Methodology, Investigation, Data curation, Writing \u0026ndash; original draft. Yun Hyeok Choi: Formal analysis, Visualization, Validation. Min Seo Choi: Investigation, Resources. J.M. One: Data analysis, Visualization. Jeong H. Cho: Methodology, Review \u0026amp; editing. J. Hyuk Kim: Writing \u0026ndash; review \u0026amp; editing.San Kang: Project administration. Jong-Tae Son: Funding acquisition, Supervision, Project administration, Writing \u0026ndash; review \u0026amp; editing.All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e\u003cp\u003eThis research was supported by a Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS-2022-KI002562, HRD Program for Industrial Innovation), and by the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Institute for Advancement of Technology (KIAT) through the \u0026ldquo;Support for Middle Market Enterprises and Regional Innovation Alliances\" program (R\u0026amp;D, RS-2025-02633071).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003ePadhi, A. K., Nanjundaswamy, K. S., \u0026amp; Goodenough, J. B. (1997). Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. Journal of the Electrochemical Society, 144, 1188-1194.\u003c/li\u003e\n\u003cli\u003eYuan, L. X., Wang, Z. H., Zhang, W. X., Hu, X. L., Chen, J. T., Huang, Y. H., \u0026amp; Goodenough, J. B. (2011). Development and challenges of LiFePO\u003csub\u003e4\u003c/sub\u003e cathode material for lithium-ion batteries. Energy \u0026amp; Environmental Science, 4(2), 269-284.\u003c/li\u003e\n\u003cli\u003eHasan, M. K., Mahmud, M., Habib, A. A., Motakabber, S. M. A., \u0026amp; Islam, S. (2021). Review of electric vehicle energy storage and management system: Standards, issues, and challenges. Journal of Energy Storage, 41, 102940.\u003c/li\u003e\n\u003cli\u003eXu, B., et al. (2012). Recent progress in cathode materials research for advanced lithium ion batteries. Materials Science and Engineering: R: Reports, 73(5-6),51-65.\u003c/li\u003e\n\u003cli\u003eChung, S. Y., Bloking, J. T., \u0026amp; Chiang, Y. M. (2002). Electronically conductive phospho-olivines as lithium storage electrodes. Nature Materials, 1(2), 123-128.\u003c/li\u003e\n\u003cli\u003eNishimura, S. I., Kobayashi, G., Ohoyama, K., Kanno, R., Yashima, M., \u0026amp; Yamada, A. (2008). Experimental visualization of lithium diffusion in Li x FePO4. Nature Materials, 7(9), 707-711.\u003c/li\u003e\n\u003cli\u003eLi, H., \u0026amp; Zhou, H. (2012). Enhancing the performances of Li-ion batteries by carbon-coating: present and future. Chemical Communications, 48(9), 1201-1217.\u003c/li\u003e\n\u003cli\u003eQi, X., Blizanac, B., DuPasquier, A., Oljaca, M., Li, J., \u0026amp; Winter, M. (2013). Understanding the influence of conductive carbon additives surface area on the rate performance of LiFePO4 cathodes for lithium ion batteries. Carbon, 64, 334-340.\u003c/li\u003e\n\u003cli\u003eRaj, H., \u0026amp; Sil, A. (2018). Effect of carbon coating on electrochemical performance of LiFePO\u003csub\u003e4\u003c/sub\u003e cathode material for Li-ion battery. Ionics, 24, 2543-2553.\u003c/li\u003e\n\u003cli\u003eKim, D.-S., Kim, J.-K., \u0026amp; An.J.-H. (2013). Manganese Doped LiFePO4 as a Cathode for High Energy Density Lithium Batteries. Journal of the Korean Electrochemical Society, 16(3), 157-161.\u003c/li\u003e\n\u003cli\u003eChen, M., Shao, L. L., Yang, H. B., Ren, T. Z., Du, G., \u0026amp; Yuan, Z. Y. (2015). Vanadium-doping of LiFePO4/carbon composite cathode materials synthesized with organophosphorus source. Electrochimica Acta, 167, 278-286.\u003c/li\u003e\n\u003cli\u003eCho, M. Y., Kim, H., Kim, H., Lim, Y. S., Kim, K. B., Lee, J. W., ... \u0026amp; Roh, K. C. (2014). Size-selective synthesis of mesoporous LiFePO 4/C microspheres based on nucleation and growth rate control of primary particles. Journal of Materials Chemistry A, 2(16), 5922-5927.\u003c/li\u003e\n\u003cli\u003eKanagaraj, A. B., Al Shibli, H., Alkindi, T. S., et al. (2018). Hydrothermal synthesis of LiFePO4 microparticles for fabrication of cathode materials based on LiFePO4/carbon nanotubes nanocomposites for Li-ion batteries. Ionics, 24, 3685-3690.\u003c/li\u003e\n\u003cli\u003ePei, B., Yao, H., Zhang, W., \u0026amp; Yang, Z. (2012). Hydrothermal synthesis of morphology-controlled LiFePO4 cathode material for lithium-ion batteries. Journal of Power Sources, 220, 317-323.\u003c/li\u003e\n\u003cli\u003eMeng, D., Duan, H., Wu, S., Ren, X., \u0026amp; Yuan, S. (2023). Lithium iron phosphate with high-rate capability synthesized through hydrothermal reaction in low Li concentration solution. Journal of Alloys and Compounds, 967, 171570.\u003c/li\u003e\n\u003cli\u003eMa, Y., Li, T., Jiang, F., Jiang, Y., Gao, F., Liu, L., ... \u0026amp; Zi, Z. (2022). Effect of particle size of Li3PO4 on LiFePO4 cathode material properties prepared by hydrothermal method. International Journal of Electrochemical Science, 17(4), 220453.\u003c/li\u003e\n\u003cli\u003eZhu, H. T., Miao, C., Guo, R. T., Liu, Y., \u0026amp; Wang, X. Y. (2021). International Journal of Electrochemical Science, 16, 210331.\u003c/li\u003e\n\u003cli\u003eGhafarian-Zahmatkesh, H., Javanbakht, M., \u0026amp; Ghaemi, M. (2015). Ethylene glycol-assisted hydrothermal synthesis and characterization of bow-tie-like lithium iron phosphate nanocrystals for lithium-ion batteries. Journal of Power Sources, 284, 339-348.\u003c/li\u003e\n\u003cli\u003eOh, J., Lee, J., Hwang, T., Kim, J. M., Seoung, K. D., \u0026amp; Piao, Y. (2017). Dual layer coating strategy utilizing N-doped carbon and reduced graphene oxide for high-performance LiFePO4 cathode material. Electrochemical Acta, 231, 85-93. \u003c/li\u003e\n\u003cli\u003eVernardou, D. (2022). Recent report on the hydrothermal growth of LiFePO4 as a cathode material. Coatings, 12(10), 1543.\u003c/li\u003e\n\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":"journal-of-electroceramics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jecr","sideBox":"Learn more about [Journal of Electroceramics](https://link.springer.com/journal/10832)","snPcode":"10832","submissionUrl":"https://submission.nature.com/new-submission/10832/3","title":"Journal of Electroceramics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"electron conductivity, hydrothermal synthesis, lithium iron phosphate, nitrogen-doped carbon coating","lastPublishedDoi":"10.21203/rs.3.rs-7405892/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7405892/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this comprehensive study, the significant impact of nitrogen-doped carbon coating layer (NC) addition on enhancing the electrochemical performance of LiFePO₄ cathode materials was systematically investigated. The electrical conductivity and electron transfer pathways within the electrode structure were substantially enhanced by strategically introducing NC onto the LiFePO₄ surface through advanced coating techniques. Comprehensive experimental results demonstrated that the NC coating layer remarkably improved the overall electrochemical performance, exhibiting an impressive discharge capacity of 104.43 mAh/g after 100 cycles at 1C rate and maintaining an exceptional capacity retention of 105.0%, which indicates superior cycling stability. The differential capacity analysis (dQ/dV) revealed significantly sharper and more well-defined peaks after 100 cycles, clearly indicating more efficient and rapid lithium ion insertion and extraction processes within the electrode matrix. Most importantly, detailed electrochemical impedance spectroscopy (EIS) analysis showed a substantially reduced charge transfer resistance (Rct) value of 52.18 Ω compared to the conventional LFP/C sample, definitively confirming that the electrical conductivity and overall electrochemical kinetics of the nitrogen-doped carbon coating layer sample were significantly enhanced compared to the conventional carbon coating layer approach. These findings demonstrate the promising potential of nitrogen-doped carbon coating strategies for advanced lithium-ion battery applications.\u003c/p\u003e","manuscriptTitle":"LiFePO₄ with enhanced electrical conductivity via nitrogen-doped carbon coating layer","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-12 11:19:11","doi":"10.21203/rs.3.rs-7405892/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-21T12:37:21+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-19T15:05:15+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-17T07:13:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"195853808835950080326412902087277173011","date":"2025-09-14T08:44:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"295548219867984365690329961457722272651","date":"2025-09-11T17:19:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"234689389033284282094316517859981590125","date":"2025-09-09T06:05:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"171873652501757715552235943029953731142","date":"2025-09-09T00:14:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"319722904006981960881986291523603291492","date":"2025-09-08T17:17:02+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"223976979183655671138756167837346359455","date":"2025-09-08T06:22:27+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-06T16:56:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-20T06:42:07+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-20T06:41:33+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Electroceramics","date":"2025-08-19T07:49:54+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-electroceramics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jecr","sideBox":"Learn more about [Journal of Electroceramics](https://link.springer.com/journal/10832)","snPcode":"10832","submissionUrl":"https://submission.nature.com/new-submission/10832/3","title":"Journal of Electroceramics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"256ba5d0-1169-4581-b60c-da4c724665c0","owner":[],"postedDate":"September 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-02T16:14:22+00:00","versionOfRecord":{"articleIdentity":"rs-7405892","link":"https://doi.org/10.1007/s10832-025-00452-7","journal":{"identity":"journal-of-electroceramics","isVorOnly":false,"title":"Journal of Electroceramics"},"publishedOn":"2026-02-24 15:58:42","publishedOnDateReadable":"February 24th, 2026"},"versionCreatedAt":"2025-09-12 11:19:11","video":"","vorDoi":"10.1007/s10832-025-00452-7","vorDoiUrl":"https://doi.org/10.1007/s10832-025-00452-7","workflowStages":[]},"version":"v1","identity":"rs-7405892","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7405892","identity":"rs-7405892","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}