Melt Growth of LiFePO 4 Crystals from Carbon-decorated LiFePO 4 Powder for Recycling Purpose

Melt Growth of LiFePO 4 Crystals from Carbon-decorated LiFePO 4 Powder for Recycling Purpose | 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 Melt Growth of LiFePO 4 Crystals from Carbon-decorated LiFePO 4 Powder for Recycling Purpose Chenxu Fang, Yiwen Dai, Chengming Hao, Handong Li This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6780549/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract The melt growth of LiFePO 4 crystals from carbon-decorated LiFePO 4 amorphous powder precursor via Bridgman approach is studied. Ball-like ingot composed of aggregates of LiFePO 4 polycrystals, crystalline Fe 2 PO 5 inclusions, and amorphous contents are obtained, depicting incomplete crystal growth in presence of carbon impurity. Chemical reactions involving Fe elements and crucible materials in presence of carbon reducing agent are evidenced to accompany with the crystal growth process of LiFePO 4 , which lead to Fe deficiency in the obtained ingot. The electrochemical performance of the regenerated LiFePO 4 materials is significantly degraded which should be attributed to the presence of Fe-related defects. The results suggest decarbonization as a necessary step for achieving phase-pure crystalline LiFePO 4 from wasted LiFePO 4 batteries. LiFePO4 Crystal Carbon Powder Bridgman method XRD Recycling Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction LiFePO 4 (lithium iron phosphate) is an important cathode material for lithium-ion batteries and is widely used in electric vehicles, energy storage systems, etc., owing to its high safety, long cycle life, and low cost [ 1 – 3 ]. To improve the performance of LiFePO 4 batteries, carbon coating is one of the most popularly employed methods [ 4 , 5 ]. By coating a conductive carbon layer on the surface of the LiFePO 4 particles, the internal resistance of the material can be reduced and the transmission rates of electrons and lithium ions can be increased, thereby improving the electrochemical performance of the material [ 6 – 8 ]. However, as an increasing number of lithium-ion batteries are being used, the problems of handling and recycling carbon-coated LiFePO 4 batteries have become increasingly prominent [ 9 ]. In the recycling process of LiFePO 4 powder, vacuum sintering technology is expected to remove impurities from the material surface and promote grain growth and structural optimization, thereby improving the electrochemical performance of the recycled materials. In smashed cathodes, calcination at 400°C removes polyvinylidene fluoride (PVDF). When the temperature reaches 800°C, the spent cathode powder excludes acetylene black from the cathode matrix [ 10 ]. In addition, direct solid-phase sintering at different temperatures can prevent the organizational structure of the powder from dissolving while maintaining the component or phase stability [ 11 ]. With a full-solid thermal treatment route, regenerated LiFePO 4 cathode materials have the characteristics of high tap density and high purity after purification and homogenization. The considerable capacity retention of the full cell makes the cathode materials comparable to those of commercial LiFePO 4 [ 12 ]. Not only can this sintering method avoid secondary contamination through conventional wet recovery routes, but it is also beneficial for lower energy consumption [ 13 ]. In the absence of additional impurities, the direct regeneration process involving solid-state sintering effectively restores the electrochemical performance of the recovered cathode scrap [ 14 ]. On the other hand, if the impurities cannot be effectively removed as the temperature increases during the sintering process, either undesired reaction products are produced after impurities-LiFePO 4 reactions, or the impurities would significantly hinder the grain growth of LiFePO 4 , which probably leads to recycling failure. Therefore, to develop a proper sintering technology for recycling spent LiFePO 4 batteries containing different impurities, it is vital to understand the crystal growth kinetics of LiFePO 4 and chemical reactions in the presence of specific impurities. In this work, we attempt to study the crystal growth of LiFePO 4 from carbon-incorporated LiFePO 4 powder using the Bridgman method. This is motivated by the purpose of achieving crystalline LiFePO 4 with improved performance and understanding the high-temperature reactivity between carbon impurities and LiFePO 4 . It is demonstrated that incomplete crystal growth of LiFePO 4 from the melt, which leads to polycrystalline LiFePO 4 of poor crystallinity, is achieved in the presence of carbon impurities using the Bridgman method. The crystalline mixed-valence compound Fe 2 PO 5 , consisting of equivalent Fe 2+ and Fe 3+ cations, is also observed in the obtained LiFePO 4 polycrystals, which are depicted as delithiation reactions involving LiFePO 4 and carbon. A significant Fe deficiency with a decrease in carbon content is observed in the as-grown ingot, demonstrating non-negligible reactions between the melt and crucible materials to consume Fe and C, which accompanies the melt growth process of LiFePO 4 . Lithium-ion diffusion coefficient of the regenerated LiFePO 4 is revealed to be significantly lower than that of the pure-phase material, which should be associated with the presence of Fe defects, the Fe 2 PO 5 impurity phase, and the amorphous phase. 2. Experimental detail Commercially available carbon-encapsulated LiFePO 4 (C-LiFePO 4 ) powder is used as a precursor for crystal growth. The precursor sources are sealed in a quartz crucible with an inner diameter of 20 mm under vacuum of ~ 10 − 6 mbar. The conical pocket at the bottom of the crucible is designed to facilitate the seed crystal growth, as shown in Fig. 1 (a). The precursor is melted at ~ 1100°C, which is slightly higher than the melting point of pure LiFePO 4 [ 15 ], for 10 hours (h) in a home-built two-zone vertical tube furnace, which has been employed for growing II-VI semiconductor crystals [ 16 ]. The crucible is then slowly descended to the adiabatic zone, where an axial temperature gradient in the range of ~ 40°C/cm is applied for LiFePO 4 crystal growth. The descent rate of the crucible is controlled at ~ 1 mm/h using a step motor. After growth, the temperature is reduced to room temperature at a rate of approximately 5°C/h. The as-grown LiFePO 4 ingot and other unreacted crucible contents are mechanically ground into powder for high-resolution X-ray diffraction (HRXRD) and energy-dispersive X-ray spectroscopy (EDS) measurements. The electrochemical performance of the achieved LiFePO 4 polycrystals is assessed using CR2025 coin-type half-cells. The working electrode is fabricated by combining 80 wt% active material, 10 wt% Super P, and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP). The resultant slurry is applied to an aluminum foil current collector and subsequently vacuum-dried at 80°C for 12 hours. Lithium metal foil serves as the counter electrode, while a polypropylene (PP) separator is utilized to separate the electrodes. The electrolyte comprises a mixture of 1 M LiPF6 in FEC/TTE/EMC/MA (in a volumetric ratio of 1:5:2:2) with an addition of 1 wt% LiPO2F2. Constant current charge-discharge tests are performed using the LAND battery tester within a voltage range of 2.5–3.8 V at a rate of 0.5C (where 1C = 170 mA/g). Electrochemical impedance spectroscopy (EIS) is conducted using the CHI604E electrochemical workstation, spanning a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. 3. Results and discussion Figure 1 (b) shows an optical photograph of the as-grown crystalline LiFePO 4 ingot taken from the crucible after growth. The surface of the centimeter-sized ball-like ingot is tiled with small flat smooth areas that are irregularly separated by pits, implying that the ingot ball might actually consist of randomly oriented small crystals. The dashed circle in Fig. 1 (a) indicates the position of the crystalline ingot in the sealed crucible before it is removed for further measurements. It can be clearly observed that the crystalline ingot is located in the upper part of the crucible, below which is the residual powdered slag. In addition, there is no wetting between the ball-type ingot and the crucible wall. Thus, one can rule out a complete crystal growth process of LiFePO 4 in the presence of carbon, which should be characterized by vertical movement of the horizontal liquid-solid interface as the crucible descends during vertical Bridgman growth [ 16 ] or the ingot moves during floating zone growth [ 17 ]. Based on the experimental observations, we speculate that the carbon-LiFePO 4 reactions produce undesired phases that hinder the crystallization kinetics of LiFePO 4 from the melt. To prove this hypothesis, the crystalline contents of the obtained ingot and residue slags are analyzed by HRXRD. As shown in Fig. 2 (a)–(c) are the XRD θ-2θ scans of the C-LiFePO 4 source powder, residue slag powder, and powder ground from the ingot, respectively. The powder diffraction pattern of LiFePO 4 (PDF#81-1173) is also shown in Fig. 2 for comparison. All peaks are well indexed to LiFePO 4 without any diffusive background in Fig. 2 (a), suggesting the good crystallinity of the LiFePO 4 source materials. In comparison, broad bumps arise at 15–30 degrees in the XRD curves of Fig. 2 (b) and (c), which are attributed to contributions from the amorphous contents of the residue slags and ingot, respectively. Both the integrated intensity (i.e., areas under a diffusive background) and peak positions of the bumps in curves (b) and (c) are quite different, indicating different amorphous contents in the residue slags and the obtained LiFePO 4 ingot. Except for the diffusive background, the peaks in curve (b) can be precisely indexed to those of pure LiFePO 4 , indicating that the crystallites in the residue slags after crystal growth are LiFePO 4 . This implies that the LiFePO 4 source materials are incompletely consumed during the melt-growth process. To better determine the phase structures of the obtained ingots, the XRD pattern with Rietveld refinement is shown in Fig. 3 (a). Although all observed peaks can be indexed to that of LiFePO 4 , the calculated data do not exactly match the experimentally observed data, which suggests a distorted and/or strained lattice of the as-grown LiFePO 4 crystals. In addition to the pattern of LiFePO 4 , an accompanying peak located near 27 ° is also observed in the XRD curve of the ingot, which is indexed to Fe 2 PO 5 consisting of equivalent mixed Fe 2+ and Fe 3+ cations, as illustrated in Fig. 3 (b). Both the revealed Fe 2 PO 5 impurity phases in the ingot and lattice imperfections of the achieved LiFePO 4 crystals suggest inferior growth during the crucible descending process. Chemical composition inspections of the C-LiFePO 4 source powder, residue slag powder, and powder ground from the ingot are further carried out by EDS. Because EDS technology cannot detect Li [ 18 ], only C, Fe, P, and O are of interest. The atomic percentages of C, Fe, P, and O in the source powder, slag, and ingots are listed in Table 1 . The measured atomic ratio of Fe to P to O (Fe: P: O) in the initial source powder is 1:1:4 with 17.4% carbon dopants identified, reflecting the phase purity of the LiFePO 4 precursor materials prior to melt growth. The Fe: P: O atomic ratio of the slag powder is 1:1:5, suggesting slight cation deficiency. Such a cation deficiency might lead to lattice disordering of LiFePO 4 , which is in accordance with the XRD observations. Fe: P: O atomic ratio of the ingot powder is revealed to be 1:2:8, which deviates significantly from that of pure LiFePO 4 , indicating the chemical decomposition of the LiFePO 4 . It lattice with a significant loss of Fe cations. It should be noted that crystalline Fe 2 PO 5 phases are detected in the ingot powder by XRD measurements, which indicates that the LiFePO 4 to Fe 2 PO 5 phase transition is triggered by delithiation reactions in the presence of carbon as the temperature increases. The excessive P and O elements, which maintain a 1:4 atomic proportion in the ingot powder, might suggest amorphous phosphate phases as products of a series of unidentified reactions that cannot be detected by XRD. Regarding the observed Fe deficiency in either the obtained ingot or residue slags, one might expect Fe cations to diffuse out of the melt region to react with the quartz crucible wall. This is plausible because the reactions between ferric compounds and SiO 2 are activated at high temperatures in the presence of a carbon-reducing agent [ 19 ]. The carbon content is found to be much lower in the ingot powder, while it is more abundant in the slag powder than in the initial source powder, manifesting carbon mass transfer from the melt region towards the crucible wall during the melt growth process, thus further supporting Fe-C-SiO 2 reactions to consume Fe and carbon near the crucible wall. Table 1 Atomic percentages (at%) of C, Fe, P, and O elements in different samples measured by EDS. Composition Fe P O C Initial powder 14.44 13.42 54.77 17.37 Ingot 8.01 16.47 64.55 10.97 Slag 8.92 8.28 40.71 42.09 To evaluate the electrochemical properties of the regenerated LiFePO 4 cathode material by the melt growth process, comprehensive electrochemical impedance spectroscopy (EIS) analysis coupled with lithium-ion diffusion kinetics investigation is performed. As shown in Fig. 4 , the Nyquist spectrum exhibits two distinct regions: a compressed semicircle in the high-frequency domain representing charge transfer resistance (R ct ), and a Warburg-type sloping line in the low-frequency range corresponding to solid-state lithium-ion diffusion. Notably, the relatively low R ct value could be associated with residual carbon impurities that potentially enhance electronic conductivity, as evidenced by EDS mapping. However, XRD Rietveld refinement reveales lattice distortions stemming from Fe vacancies, which might hinder the charge transfer process despite the carbon presence. The lithium-ion diffusion coefficient (D Li+ ) is determined to be 9.68×10⁻ 16 cm²/s through alternating current impedance method (Fig. 5 ), which is lower than that of pure-phase LiFePO 4 . This significant disparity suggests that while carbon residues may improve electronic percolation, their heterogeneous distribution fails to establish continuous ion transport pathways, thereby compromising ionic conductivity. This structure-property relationship implies that simultaneous optimization of defect engineering and carbon coating homogeneity is crucial for achieving balanced electron/ion transport in regenerated cathode materials. 4. Conclusions In summary, we explore the melt growth of LiFePO 4 mixed with elemental carbon using a quartz crucible via the Bridgman crystal growth method. By analyzing the structural and compositional properties of the obtained products after crystal growth, a series of complex reactions involving delithiation reactions of LiFePO 4 and melt-to-crucible reactions are expected to occur at the melt-to-solid and melt-to-crucible interface as the temperature is increased to 1100°C. Owing to the nucleation of undesired phases, the crystal growth kinetics of LiFePO 4 from the melt are seriously hindered, which accounts for the obtained polycrystalline LiFePO 4 products with poor crystallinity. Electrochemical tests indicate that while the charge transfer resistance of regenerated LiFePO 4 is marginally improved due to residual carbon impurities, its lithium-ion diffusion coefficient is significantly lower than that of the pure-phase material. This observation is directly associated with the presence of Fe defects, the Fe 2 PO 5 impurity phase, and the amorphous phase. Our work suggests that proper decarbonization processes should be carried out prior to subsequent recycling steps to achieve phase-pure crystalline LiFePO 4 from spent LiFePO 4 batteries. Declarations Author Contributions Handong Li; methodology, validation, writing—review and editing, supervision, project administration, funding acquisition. Chenxu Fang; writing—original draft preparation, data curation. Yiwen Dai; investigation. Chengming Hao; investigation. All authors have read and agreed to the published version of the manuscript. Funding This work is financially supported by the Open Project of Fujian College Application Technology & Engineering Center of Clean Energy for Automobiles (Grant No. CQJNY22-03). Conflict of interest The authors declare no conflict of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data Availability Statement The raw data supporting the conclusions of this article will be made available on request. References Konarov A, Myung S-T, Sun Y-K (2017) Cathode Materials for Future Electric Vehicles and Energy Storage Systems. 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J Mater Sci: Mater Electron 29:17661–17669. https://doi.org/10.1007/s10854-018-9870-x Meng X, Hao J, Cao H, et al (2019) Recycling of LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode materials from spent lithium-ion batteries using mechanochemical activation and solid-state sintering. Waste Management 84:54–63. https://doi.org/10.1016/j.wasman.2018.11.034 Gauthier M, Michot C, Ravet N, et al (2010) Melt Casting LiFePO 4 : I. Synthsis and Characterization. J Electrochem Soc 157:A453. https://doi.org/10.1149/1.3284505 Chen Z, Fang C, Dai Y, et al. (2024) Growth and Properties of Cadmium Zinc Telluride Selenide Single Crystals using Vertical Bridgman Method[J]. Semiconductor Optoelectronics 45(1): 105-110. DOI ：10.16818/j.issn1001-5868.2023102701 Chen DP, Maljuk A, Lin CT (2005) Floating zone growth of lithium iron (II) phosphate single crystals. Journal of Crystal Growth 284:86–90. https://doi.org/10.1016/j.jcrysgro.2005.06.024 Toprakci O, Ji L, Lin Z, et al (2011) Fabrication and electrochemical characteristics of electrospun LiFePO 4 /carbon composite fibers for lithium-ion batteries. Journal of Power Sources 196:7692–7699. https://doi.org/10.1016/j.jpowsour.2011.04.031 Liang Z, Yi L, Huang Z, et al (2019) Effect of Silica on Reduction Behaviors of Hematite-carbon Composite Compact at 1223–1373 K. ISIJ International 59:227–234. https://doi.org/10.2355/isijinternational.ISIJINT-2018-613 Additional Declarations No competing interests reported. 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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-6780549","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":475828813,"identity":"9cda61b7-79b2-499b-8839-17aa3d636888","order_by":0,"name":"Chenxu Fang","email":"","orcid":"","institution":"Quanzhou Vocational and Technical University","correspondingAuthor":false,"prefix":"","firstName":"Chenxu","middleName":"","lastName":"Fang","suffix":""},{"id":475828814,"identity":"dfcef1fe-cc07-4a1d-b871-78be12519f8b","order_by":1,"name":"Yiwen Dai","email":"","orcid":"","institution":"University of Electronic Science and Technology of China","correspondingAuthor":false,"prefix":"","firstName":"Yiwen","middleName":"","lastName":"Dai","suffix":""},{"id":475828815,"identity":"17e76122-28a0-4dc9-a5b7-cb22e3c47420","order_by":2,"name":"Chengming Hao","email":"","orcid":"","institution":"Quanzhou Vocational and Technical University","correspondingAuthor":false,"prefix":"","firstName":"Chengming","middleName":"","lastName":"Hao","suffix":""},{"id":475828816,"identity":"573c45f8-b51e-44ca-903c-b147b597937e","order_by":3,"name":"Handong Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuUlEQVRIiWNgGAWjYBACA4bkhgMfYDwe4rQkNhycAVNNtBZmHpK0mLMnNh62bbOxt2c/wPjgbRuDvDkhLZY9DxsO57alJfbwJDAbzm1jMNzZQMhhNxJBWg4n8EgwsEnztjEkGBwgRotl2397oBb238RrYWw7wNgDtIWZOC1nHjYc7DmXnNhzJrFZcs45CcMNBLUcTz784UeZnT17++GDH96U2cgTtAUJMDYACQni1Y+CUTAKRsEowA0A3JJAy7w16jgAAAAASUVORK5CYII=","orcid":"","institution":"University of Electronic Science and Technology of China","correspondingAuthor":true,"prefix":"","firstName":"Handong","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2025-05-30 03:23:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6780549/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6780549/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85557106,"identity":"4e428297-2e3a-4f7e-9bd4-cab991cce581","added_by":"auto","created_at":"2025-06-27 11:28:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":232625,"visible":true,"origin":"","legend":"\u003cp\u003eOptical photos of (a) the quartz crucible just taken out of the growth furnace after crystal growth, and (b) the as-grown LiFePO\u003csub\u003e4\u003c/sub\u003e ingot taken out from (a).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/013b7d516fc273056a44d312.png"},{"id":85557105,"identity":"19155ac9-5750-47d2-aa86-ff315ace8fbd","added_by":"auto","created_at":"2025-06-27 11:28:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":55432,"visible":true,"origin":"","legend":"\u003cp\u003eXRD θ-2θ scans from (a) LiFePO\u003csub\u003e4\u003c/sub\u003e source powder, (b) residue slags powder, and (c) powder ground from ingot. The referenced powder diffraction of pure LiFePO\u003csub\u003e4\u003c/sub\u003e (PDF#81-1173) is listed below (c).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/b992827ad2b9244206b94996.png"},{"id":85557104,"identity":"01260bd7-ff94-42dc-9357-03b05992927c","added_by":"auto","created_at":"2025-06-27 11:28:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":102231,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Rietveld refinement of powder-pattern XRD data of obtained LiFePO\u003csub\u003e4\u003c/sub\u003e ingot, (b) XRD pattern of obtained LiFePO\u003csub\u003e4\u003c/sub\u003e and simulated Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e XRD patterns. The dashed rectangular frame indicates excellent alignment of the diffraction peaks of the measured Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e inclusion phase to that of the referenced Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e phases.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/7dae7cf5e783d0afb7fe0274.png"},{"id":85557108,"identity":"16e02695-18a7-47e3-a1c3-a18f0065fe98","added_by":"auto","created_at":"2025-06-27 11:28:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":347884,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical impedance spectroscopy of LiFePO₄\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/a789e72993aa0d7286a84431.png"},{"id":85557110,"identity":"dc4903ab-ead5-403e-bb92-fcda87daddd5","added_by":"auto","created_at":"2025-06-27 11:28:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":424473,"visible":true,"origin":"","legend":"\u003cp\u003eLithium ion diffusion coefficient\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/0141c8a7da5f438231e883dd.png"},{"id":85557956,"identity":"137ec085-1936-4872-8e81-08676c22883e","added_by":"auto","created_at":"2025-06-27 11:52:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1302125,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6780549/v1/d4a6848f-7345-46be-8b3b-966556364e12.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Melt Growth of LiFePO 4 Crystals from Carbon-decorated LiFePO 4 Powder for Recycling Purpose","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eLiFePO\u003csub\u003e4\u003c/sub\u003e (lithium iron phosphate) is an important cathode material for lithium-ion batteries and is widely used in electric vehicles, energy storage systems, etc., owing to its high safety, long cycle life, and low cost [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. To improve the performance of LiFePO\u003csub\u003e4\u003c/sub\u003e batteries, carbon coating is one of the most popularly employed methods [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. By coating a conductive carbon layer on the surface of the LiFePO\u003csub\u003e4\u003c/sub\u003e particles, the internal resistance of the material can be reduced and the transmission rates of electrons and lithium ions can be increased, thereby improving the electrochemical performance of the material [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, as an increasing number of lithium-ion batteries are being used, the problems of handling and recycling carbon-coated LiFePO\u003csub\u003e4\u003c/sub\u003e batteries have become increasingly prominent [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the recycling process of LiFePO\u003csub\u003e4\u003c/sub\u003e powder, vacuum sintering technology is expected to remove impurities from the material surface and promote grain growth and structural optimization, thereby improving the electrochemical performance of the recycled materials. In smashed cathodes, calcination at 400\u0026deg;C removes polyvinylidene fluoride (PVDF). When the temperature reaches 800\u0026deg;C, the spent cathode powder excludes acetylene black from the cathode matrix [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. In addition, direct solid-phase sintering at different temperatures can prevent the organizational structure of the powder from dissolving while maintaining the component or phase stability [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. With a full-solid thermal treatment route, regenerated LiFePO\u003csub\u003e4\u003c/sub\u003e cathode materials have the characteristics of high tap density and high purity after purification and homogenization. The considerable capacity retention of the full cell makes the cathode materials comparable to those of commercial LiFePO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Not only can this sintering method avoid secondary contamination through conventional wet recovery routes, but it is also beneficial for lower energy consumption [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In the absence of additional impurities, the direct regeneration process involving solid-state sintering effectively restores the electrochemical performance of the recovered cathode scrap [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. On the other hand, if the impurities cannot be effectively removed as the temperature increases during the sintering process, either undesired reaction products are produced after impurities-LiFePO\u003csub\u003e4\u003c/sub\u003e reactions, or the impurities would significantly hinder the grain growth of LiFePO\u003csub\u003e4\u003c/sub\u003e, which probably leads to recycling failure. Therefore, to develop a proper sintering technology for recycling spent LiFePO\u003csub\u003e4\u003c/sub\u003e batteries containing different impurities, it is vital to understand the crystal growth kinetics of LiFePO\u003csub\u003e4\u003c/sub\u003e and chemical reactions in the presence of specific impurities.\u003c/p\u003e \u003cp\u003eIn this work, we attempt to study the crystal growth of LiFePO\u003csub\u003e4\u003c/sub\u003e from carbon-incorporated LiFePO\u003csub\u003e4\u003c/sub\u003e powder using the Bridgman method. This is motivated by the purpose of achieving crystalline LiFePO\u003csub\u003e4\u003c/sub\u003e with improved performance and understanding the high-temperature reactivity between carbon impurities and LiFePO\u003csub\u003e4\u003c/sub\u003e. It is demonstrated that incomplete crystal growth of LiFePO\u003csub\u003e4\u003c/sub\u003e from the melt, which leads to polycrystalline LiFePO\u003csub\u003e4\u003c/sub\u003e of poor crystallinity, is achieved in the presence of carbon impurities using the Bridgman method. The crystalline mixed-valence compound Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e, consisting of equivalent Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e cations, is also observed in the obtained LiFePO\u003csub\u003e4\u003c/sub\u003e polycrystals, which are depicted as delithiation reactions involving LiFePO\u003csub\u003e4\u003c/sub\u003e and carbon. A significant Fe deficiency with a decrease in carbon content is observed in the as-grown ingot, demonstrating non-negligible reactions between the melt and crucible materials to consume Fe and C, which accompanies the melt growth process of LiFePO\u003csub\u003e4\u003c/sub\u003e. Lithium-ion diffusion coefficient of the regenerated LiFePO\u003csub\u003e4\u003c/sub\u003e is revealed to be significantly lower than that of the pure-phase material, which should be associated with the presence of Fe defects, the Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e impurity phase, and the amorphous phase.\u003c/p\u003e"},{"header":"2. Experimental detail","content":"\u003cp\u003eCommercially available carbon-encapsulated LiFePO\u003csub\u003e4\u003c/sub\u003e (C-LiFePO\u003csub\u003e4\u003c/sub\u003e) powder is used as a precursor for crystal growth. The precursor sources are sealed in a quartz crucible with an inner diameter of 20 mm under vacuum of ~\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar. The conical pocket at the bottom of the crucible is designed to facilitate the seed crystal growth, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a). The precursor is melted at ~\u0026thinsp;1100\u0026deg;C, which is slightly higher than the melting point of pure LiFePO\u003csub\u003e4\u003c/sub\u003e [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], for 10 hours (h) in a home-built two-zone vertical tube furnace, which has been employed for growing II-VI semiconductor crystals [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The crucible is then slowly descended to the adiabatic zone, where an axial temperature gradient in the range of ~\u0026thinsp;40\u0026deg;C/cm is applied for LiFePO\u003csub\u003e4\u003c/sub\u003e crystal growth. The descent rate of the crucible is controlled at ~\u0026thinsp;1 mm/h using a step motor. After growth, the temperature is reduced to room temperature at a rate of approximately 5\u0026deg;C/h. The as-grown LiFePO\u003csub\u003e4\u003c/sub\u003e ingot and other unreacted crucible contents are mechanically ground into powder for high-resolution X-ray diffraction (HRXRD) and energy-dispersive X-ray spectroscopy (EDS) measurements. The electrochemical performance of the achieved LiFePO\u003csub\u003e4\u003c/sub\u003e polycrystals is assessed using CR2025 coin-type half-cells. The working electrode is fabricated by combining 80 wt% active material, 10 wt% Super P, and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP). The resultant slurry is applied to an aluminum foil current collector and subsequently vacuum-dried at 80\u0026deg;C for 12 hours. Lithium metal foil serves as the counter electrode, while a polypropylene (PP) separator is utilized to separate the electrodes. The electrolyte comprises a mixture of 1 M LiPF6 in FEC/TTE/EMC/MA (in a volumetric ratio of 1:5:2:2) with an addition of 1 wt% LiPO2F2. Constant current charge-discharge tests are performed using the LAND battery tester within a voltage range of 2.5\u0026ndash;3.8 V at a rate of 0.5C (where 1C\u0026thinsp;=\u0026thinsp;170 mA/g). Electrochemical impedance spectroscopy (EIS) is conducted using the CHI604E electrochemical workstation, spanning a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b) shows an optical photograph of the as-grown crystalline LiFePO\u003csub\u003e4\u003c/sub\u003e ingot taken from the crucible after growth. The surface of the centimeter-sized ball-like ingot is tiled with small flat smooth areas that are irregularly separated by pits, implying that the ingot ball might actually consist of randomly oriented small crystals. The dashed circle in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) indicates the position of the crystalline ingot in the sealed crucible before it is removed for further measurements. It can be clearly observed that the crystalline ingot is located in the upper part of the crucible, below which is the residual powdered slag. In addition, there is no wetting between the ball-type ingot and the crucible wall. Thus, one can rule out a complete crystal growth process of LiFePO\u003csub\u003e4\u003c/sub\u003e in the presence of carbon, which should be characterized by vertical movement of the horizontal liquid-solid interface as the crucible descends during vertical Bridgman growth [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] or the ingot moves during floating zone growth [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Based on the experimental observations, we speculate that the carbon-LiFePO\u003csub\u003e4\u003c/sub\u003e reactions produce undesired phases that hinder the crystallization kinetics of LiFePO\u003csub\u003e4\u003c/sub\u003e from the melt.\u003c/p\u003e \u003cp\u003eTo prove this hypothesis, the crystalline contents of the obtained ingot and residue slags are analyzed by HRXRD. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a)\u0026ndash;(c) are the XRD θ-2θ scans of the C-LiFePO\u003csub\u003e4\u003c/sub\u003e source powder, residue slag powder, and powder ground from the ingot, respectively. The powder diffraction pattern of LiFePO\u003csub\u003e4\u003c/sub\u003e (PDF#81-1173) is also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e for comparison. All peaks are well indexed to LiFePO\u003csub\u003e4\u003c/sub\u003e without any diffusive background in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a), suggesting the good crystallinity of the LiFePO\u003csub\u003e4\u003c/sub\u003e source materials. In comparison, broad bumps arise at 15\u0026ndash;30 degrees in the XRD curves of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (b) and (c), which are attributed to contributions from the amorphous contents of the residue slags and ingot, respectively. Both the integrated intensity (i.e., areas under a diffusive background) and peak positions of the bumps in curves (b) and (c) are quite different, indicating different amorphous contents in the residue slags and the obtained LiFePO\u003csub\u003e4\u003c/sub\u003e ingot. Except for the diffusive background, the peaks in curve (b) can be precisely indexed to those of pure LiFePO\u003csub\u003e4\u003c/sub\u003e, indicating that the crystallites in the residue slags after crystal growth are LiFePO\u003csub\u003e4\u003c/sub\u003e. This implies that the LiFePO\u003csub\u003e4\u003c/sub\u003e source materials are incompletely consumed during the melt-growth process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo better determine the phase structures of the obtained ingots, the XRD pattern with Rietveld refinement is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (a). Although all observed peaks can be indexed to that of LiFePO\u003csub\u003e4\u003c/sub\u003e, the calculated data do not exactly match the experimentally observed data, which suggests a distorted and/or strained lattice of the as-grown LiFePO\u003csub\u003e4\u003c/sub\u003e crystals. In addition to the pattern of LiFePO\u003csub\u003e4\u003c/sub\u003e, an accompanying peak located near 27 \u0026deg; is also observed in the XRD curve of the ingot, which is indexed to Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e consisting of equivalent mixed Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e cations, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (b). Both the revealed Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e impurity phases in the ingot and lattice imperfections of the achieved LiFePO\u003csub\u003e4\u003c/sub\u003e crystals suggest inferior growth during the crucible descending process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eChemical composition inspections of the C-LiFePO\u003csub\u003e4\u003c/sub\u003e source powder, residue slag powder, and powder ground from the ingot are further carried out by EDS. Because EDS technology cannot detect Li [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], only C, Fe, P, and O are of interest. The atomic percentages of C, Fe, P, and O in the source powder, slag, and ingots are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The measured atomic ratio of Fe to P to O (Fe: P: O) in the initial source powder is 1:1:4 with 17.4% carbon dopants identified, reflecting the phase purity of the LiFePO\u003csub\u003e4\u003c/sub\u003e precursor materials prior to melt growth. The Fe: P: O atomic ratio of the slag powder is 1:1:5, suggesting slight cation deficiency. Such a cation deficiency might lead to lattice disordering of LiFePO\u003csub\u003e4\u003c/sub\u003e, which is in accordance with the XRD observations. Fe: P: O atomic ratio of the ingot powder is revealed to be 1:2:8, which deviates significantly from that of pure LiFePO\u003csub\u003e4\u003c/sub\u003e, indicating the chemical decomposition of the LiFePO\u003csub\u003e4\u003c/sub\u003e. It lattice with a significant loss of Fe cations. It should be noted that crystalline Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e phases are detected in the ingot powder by XRD measurements, which indicates that the LiFePO\u003csub\u003e4\u003c/sub\u003e to Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e phase transition is triggered by delithiation reactions in the presence of carbon as the temperature increases. The excessive P and O elements, which maintain a 1:4 atomic proportion in the ingot powder, might suggest amorphous phosphate phases as products of a series of unidentified reactions that cannot be detected by XRD. Regarding the observed Fe deficiency in either the obtained ingot or residue slags, one might expect Fe cations to diffuse out of the melt region to react with the quartz crucible wall. This is plausible because the reactions between ferric compounds and SiO\u003csub\u003e2\u003c/sub\u003e are activated at high temperatures in the presence of a carbon-reducing agent [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The carbon content is found to be much lower in the ingot powder, while it is more abundant in the slag powder than in the initial source powder, manifesting carbon mass transfer from the melt region towards the crucible wall during the melt growth process, thus further supporting Fe-C-SiO\u003csub\u003e2\u003c/sub\u003e reactions to consume Fe and carbon near the crucible wall.\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\u003eAtomic percentages (at%) of C, Fe, P, and O elements in different samples measured by EDS.\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\u003eComposition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInitial powder\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e14.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e13.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e54.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e17.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIngot\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e16.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10.97\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSlag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e40.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e42.09\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\u003eTo evaluate the electrochemical properties of the regenerated LiFePO\u003csub\u003e4\u003c/sub\u003e cathode material by the melt growth process, comprehensive electrochemical impedance spectroscopy (EIS) analysis coupled with lithium-ion diffusion kinetics investigation is performed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the Nyquist spectrum exhibits two distinct regions: a compressed semicircle in the high-frequency domain representing charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e), and a Warburg-type sloping line in the low-frequency range corresponding to solid-state lithium-ion diffusion. Notably, the relatively low R\u003csub\u003ect\u003c/sub\u003e value could be associated with residual carbon impurities that potentially enhance electronic conductivity, as evidenced by EDS mapping. However, XRD Rietveld refinement reveales lattice distortions stemming from Fe vacancies, which might hinder the charge transfer process despite the carbon presence.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe lithium-ion diffusion coefficient (D\u003csub\u003eLi+\u003c/sub\u003e) is determined to be 9.68\u0026times;10⁻\u003csup\u003e16\u003c/sup\u003e cm\u0026sup2;/s through alternating current impedance method (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which is lower than that of pure-phase LiFePO\u003csub\u003e4\u003c/sub\u003e. This significant disparity suggests that while carbon residues may improve electronic percolation, their heterogeneous distribution fails to establish continuous ion transport pathways, thereby compromising ionic conductivity. This structure-property relationship implies that simultaneous optimization of defect engineering and carbon coating homogeneity is crucial for achieving balanced electron/ion transport in regenerated cathode materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn summary, we explore the melt growth of LiFePO\u003csub\u003e4\u003c/sub\u003e mixed with elemental carbon using a quartz crucible via the Bridgman crystal growth method. By analyzing the structural and compositional properties of the obtained products after crystal growth, a series of complex reactions involving delithiation reactions of LiFePO\u003csub\u003e4\u003c/sub\u003e and melt-to-crucible reactions are expected to occur at the melt-to-solid and melt-to-crucible interface as the temperature is increased to 1100\u0026deg;C. Owing to the nucleation of undesired phases, the crystal growth kinetics of LiFePO\u003csub\u003e4\u003c/sub\u003e from the melt are seriously hindered, which accounts for the obtained polycrystalline LiFePO\u003csub\u003e4\u003c/sub\u003e products with poor crystallinity. Electrochemical tests indicate that while the charge transfer resistance of regenerated LiFePO\u003csub\u003e4\u003c/sub\u003e is marginally improved due to residual carbon impurities, its lithium-ion diffusion coefficient is significantly lower than that of the pure-phase material. This observation is directly associated with the presence of Fe defects, the Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e impurity phase, and the amorphous phase. Our work suggests that proper decarbonization processes should be carried out prior to subsequent recycling steps to achieve phase-pure crystalline LiFePO\u003csub\u003e4\u003c/sub\u003e from spent LiFePO\u003csub\u003e4\u003c/sub\u003e batteries.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAuthor Contributions\u003c/p\u003e\n\u003cp\u003eHandong Li; methodology, validation, writing\u0026mdash;review and editing, supervision, project administration, funding acquisition. Chenxu Fang; writing\u0026mdash;original draft preparation, data curation. Yiwen Dai; investigation. Chengming Hao; investigation. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis work is financially supported by the Open Project of Fujian College Application Technology \u0026amp; Engineering Center of Clean Energy for Automobiles (Grant No. CQJNY22-03).\u003c/p\u003e\n\u003cp\u003eConflict of interest\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003eData Availability Statement\u003c/p\u003e\n\u003cp\u003eThe raw data supporting the conclusions of this article will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKonarov A, Myung S-T, Sun Y-K (2017) Cathode Materials for Future Electric Vehicles and Energy Storage Systems. ACS Energy Lett 2:703\u0026ndash;708. https://doi.org/10.1021/acsenergylett.7b00130\u003c/li\u003e\n\u003cli\u003eRauhala T, Jalkanen K, Romann T, et al (2018) Low-temperature aging mechanisms of commercial graphite/LiFePO\u003csub\u003e4\u003c/sub\u003e cells cycled with a simulated electric vehicle load profile\u0026mdash;A post-mortem study. Journal of Energy Storage 20:344\u0026ndash;356. https://doi.org/10.1016/j.est.2018.10.007\u003c/li\u003e\n\u003cli\u003eZhang K, Xiong R, Li Q, et al (2023) A novel pseudo-open-circuit voltage modeling method for accurate state-of-charge estimation of LiFePO\u003csub\u003e4\u003c/sub\u003e batteries. Applied Energy 347:121406. https://doi.org/10.1016/j.apenergy.2023.121406\u003c/li\u003e\n\u003cli\u003eChen Z, Zhang Q, Liang Q (2022) Carbon-Coatings Improve Performance of Li-Ion Battery. 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DOI ：10.16818/j.issn1001-5868.2023102701\u003c/li\u003e\n\u003cli\u003eChen DP, Maljuk A, Lin CT (2005) Floating zone growth of lithium iron (II) phosphate single crystals. Journal of Crystal Growth 284:86\u0026ndash;90. https://doi.org/10.1016/j.jcrysgro.2005.06.024\u003c/li\u003e\n\u003cli\u003eToprakci O, Ji L, Lin Z, et al (2011) Fabrication and electrochemical characteristics of electrospun LiFePO\u003csub\u003e4\u003c/sub\u003e/carbon composite fibers for lithium-ion batteries. Journal of Power Sources 196:7692\u0026ndash;7699. https://doi.org/10.1016/j.jpowsour.2011.04.031\u003c/li\u003e\n\u003cli\u003eLiang Z, Yi L, Huang Z, et al (2019) Effect of Silica on Reduction Behaviors of Hematite-carbon Composite Compact at 1223\u0026ndash;1373 K. ISIJ International 59:227\u0026ndash;234. https://doi.org/10.2355/isijinternational.ISIJINT-2018-613\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":"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":"LiFePO4 Crystal, Carbon, Powder, Bridgman method, XRD, Recycling","lastPublishedDoi":"10.21203/rs.3.rs-6780549/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6780549/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe melt growth of LiFePO\u003csub\u003e4\u003c/sub\u003e crystals from carbon-decorated LiFePO\u003csub\u003e4\u003c/sub\u003e amorphous powder precursor via Bridgman approach is studied. Ball-like ingot composed of aggregates of LiFePO\u003csub\u003e4\u003c/sub\u003e polycrystals, crystalline Fe\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e5\u003c/sub\u003e inclusions, and amorphous contents are obtained, depicting incomplete crystal growth in presence of carbon impurity. Chemical reactions involving Fe elements and crucible materials in presence of carbon reducing agent are evidenced to accompany with the crystal growth process of LiFePO\u003csub\u003e4\u003c/sub\u003e, which lead to Fe deficiency in the obtained ingot. The electrochemical performance of the regenerated LiFePO\u003csub\u003e4\u003c/sub\u003e materials is significantly degraded which should be attributed to the presence of Fe-related defects. The results suggest decarbonization as a necessary step for achieving phase-pure crystalline LiFePO\u003csub\u003e4\u003c/sub\u003e from wasted LiFePO\u003csub\u003e4\u003c/sub\u003e batteries.\u003c/p\u003e","manuscriptTitle":"Melt Growth of LiFePO 4 Crystals from Carbon-decorated LiFePO 4 Powder for Recycling Purpose","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-27 11:28:02","doi":"10.21203/rs.3.rs-6780549/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-22T15:25:21+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-21T12:31:33+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-20T14:02:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"164051156667327120311416568419774631656","date":"2025-07-17T02:52:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"259433983779506342560859340284113703836","date":"2025-07-12T21:05:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-11T13:12:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"64888478606172528425453891527437677796","date":"2025-07-11T13:00:02+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"96486067254841491360363434165980433132","date":"2025-07-10T17:16:10+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-24T14:29:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-02T03:05:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-02T03:03:35+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2025-05-30T03:17:00+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":"98799458-4c90-460d-aea9-4f155e2f4019","owner":[],"postedDate":"June 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-10-16T12:08:21+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-27 11:28:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6780549","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6780549","identity":"rs-6780549","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}