探討碳層包覆及改質對商用磷酸錳鐵鋰正極材料之性能影響

Abstract

隨著電動車與再生能源儲能系統的快速發展，鋰離子電池因具有高能量密度、長循環壽命與低自放電率等優點，已成為重要的能量儲存技術。為進一步提升電池能量密度與安全性，正極材料的開發與改良一直是研究焦點。 磷酸錳鐵鋰在磷酸鐵鋰的基礎上引入錳的高電位氧化還原反應，可提升工作電壓與能量密度，並保留橄欖石結構的穩定性，因此極具發展潛力。然而，磷酸錳鐵鋰仍受限於電子與鋰離子傳輸緩慢等問題，商用材料通常需藉由碳層包覆改善導電性與電化學表現。由於碳層的包覆程度、連續性與表面覆蓋狀態可能影響電子傳輸路徑與界面保護效果，本研究以商用磷酸錳鐵鋰材料為對象，探討表面碳層特性及後續表面改質對材料穩定性與電化學性能之影響。 本研究比較兩種商用磷酸錳鐵鋰材料之表面特性與電化學表現。兩者具有相似的橄欖石結構與接近的過渡金屬比例，但在顆粒形貌與表面碳層包覆特性上有所差異。碳含量分析、穿透式電子顯微鏡與酸蝕刻結果顯示，較完整的碳層包覆有助於維持顆粒結構，並降低空氣儲存後的界面阻抗增加與循環性能衰退，說明碳層連續性與表面保護能力對材料之儲存穩定性具有重要影響。基於此結果，本研究進一步採用六甲基二矽胺烷 (Hexamethyldisilazane, HMDS) 氣相處理作為溫和的後合成表面改質方法，以降低材料表面親水性與水分吸附，並期望減少材料表面與電解液之間可能發生的界面副反應。改質後材料之水接觸角提升，且儲存後熱重損失降低，顯示此方法可改善表面潤濕性與吸濕行為。儘管目前改質條件仍需進一步優化，改質層均勻性、表面針狀物種來源及其對界面反應之影響也有待釐清，但本研究顯示，透過調控商用磷酸錳鐵鋰粉末之表面化學與碳層保護特性，有機會提升其空氣儲存穩定性，並為後續商用磷酸鹽系正極材料之表面改質設計提供參考。

With the rapid development of electric vehicles and renewable energy storage systems, lithium-ion batteries (LIBs) have become one of the most important energy storage technologies owing to their high energy density, long cycle life, and low self-discharge rate. Cathode materials play a key role in determining battery energy density, cost, and safety; therefore, their design and modification are essential for further improving the energy density and safety of LIBs. Lithium manganese iron phosphate (LiMnxFe1-xPO4, LMFP) is a promising olivine-structured cathode material that incorporates the high-voltage redox activity of Mn into the LiFePO4 framework, thereby increasing the operating voltage and energy density while retaining structural stability. However, LMFP suffers from sluggish electron and Li-ion transport, and commercial materials typically rely on carbon coating to improve electronic conductivity and electrochemical performance. Since the degree of coverage, continuity, and surface uniformity of the carbon layer may affect electron transport pathways and interfacial protection, this study investigates how surface carbon layer characteristics and subsequent surface modification influence the stability and electrochemical performance of commercial LMFP materials. In this study, two commercial LMFP powders were compared to clarify how surface carbon-coating characteristics affect electrochemical behavior and ambient-storage stability. Although both materials showed similar olivine structures and transition-metal ratios, they differed in particle morphology and carbon-layer features. Elemental analysis, TEM, and acid etching indicated that a more complete carbon coating helped preserve particle integrity and might suppress the increase in interfacial resistance and capacity fading after ambient storage, highlighting the importance of carbon-layer continuity and protection. Based on this finding, hexamethyldisilazane (HMDS) vapor treatment was applied as a mild post-synthetic surface modification to reduce surface hydrophilicity and moisture adsorption, with the aim of potentially mitigating interfacial side reactions between the material surface and the electrolyte. The treated material showed a higher water contact angle and lower mass loss after storage, suggesting reduced wettability and moisture uptake. Although further optimization is needed, these results suggest that regulating surface chemistry and carbon-layer protection may improve the storage stability of commercial LMFP cathodes.