鎂摻雜對磷酸釩鋰正極材料電化學性能影響之研究

Effect of Mg Doping on the Electrochemical Performance of Li3V2(PO4)3

磷酸鋰釩Li3V2（PO4）3（LVP）具高工作電壓與熱穩定性，但受限於電子導電率與鋰離子擴散速率。本研究以固態反應合成Li3V2-xMgx（PO4）3（x=0、0.05、0.1），在5%H2/95%Ar、800°C之還原氣氛下燒結並以葡萄糖裹碳處理，系統評估鎂取代之影響。XRD/Rietveld顯示三組樣品皆為單斜P21/c、無雜相；其中摻雜0.05 mol樣品具更細緻且均勻的顆粒分布。電化學方面，摻雜0.05 mol組在循環伏安中呈現較清晰之氧化還原峰與較小峰間距；恆電流充放電於0.2C的初始比容量153.46 mAh g-1，優於未摻雜（111.15 mAh g-1）與摻雜0.1 mol（138.95 mAh g-1）。EIS擬合結果顯示0.05 mol組Rct最低，反映更佳界面動力學。綜合結構、形貌與電化學證據，適量Mg（0.05 mol）摻雜可藉顆粒細化與V3+/ V4+價態補償降低極化與電荷轉移阻抗，強化電子/離子傳輸與循環穩定性；過量取代（0.1 mol）則可能因活性V位點減少與缺陷累積而使傳輸受限。結果顯示0.05 mol Mg為LVP的最佳取代量，進而提升該正極材料電化學性能。

Lithium vanadium phosphate, Li3V2(PO4)3 (LVP), offers a high operating voltage and good thermal stability, yet its practical use is limited by low electronic conductivity and sluggish Li-ion diffusion. In this work, Li3V2-xMgx(PO4)3 (x=0, 0.05, 0.1) was synthesized via a solid-state route and calcined at 800°C in a reducing atmosphere (5% H2/95% Ar), followed by glucose-derived carbon coating, to systematically evaluate the effect of Mg substitution. X-ray diffraction with Rietveld refinement confirms a monoclinic P21/c structure without secondary phases for all compositions; the x = 0.05 sample exhibits finer and more uniform particles. Electrochemically, x=0.05 shows sharper redox peaks and a smaller peak separation in cyclic voltammetry. At 0.2 C, it delivers an initial specific capacity of 153.46 mAh g-1, surpassing x= 0 (111.15 mAh g-1) and x = 0.1 (138.95 mAh g-1). Electrochemical impedance spectroscopy further reveals the lowest charge-transfer resistance (Rct) for x = 0.05, indicating improved interfacial kinetics. These results suggest that moderate Mg substitution (0.05 mol) promotes particle refinement and valence compensation (V3+/ V4+), thereby reducing polarization and Rct and enhancing electron/Li-ion transport and cycling stability, whereas excessive substitution (x=0.1) likely diminishes active V sites and accumulates lattice defects, leading to transport limitations. Overall, 0.05 mol Mg is identified as the optimal substitution level to improve the electrochemical performance of LVP.