Comparison is made between the use of either a carbon powder or a polymer additive to the precursors in the synthesis of LiFePO₄ from the Fe(III) compound FePO₄(H₂O)₂ and Li₂CO₃. The evolution of the structural properties and phase purity with temperature and time have been monitored at all length scales by X-ray diffraction, Fourier transformed infrared spectroscopy, and magnetic susceptibility. The reactor temperature was decreased to 300 °C to investigate the early stages of the reaction. Formation of crystalline LiFePO₄ begins in the range 300−400 °C only if the polymer is used as the carbonaceous additive. This LiFePO₄ formation is made possible by the reduction of Fe(III) species by gases such as H₂ or gaseous hydrocarbons evolved during the calcination of the polymer. Moreover, decomposition of the polymer results in a carbonaceous deposit on the surface of the LiFePO₄ particles. An Li₃Fe₂(PO₄)₃ impurity found after sintering at 400 °C for 4 h was greatly reduced after sintering at 400 °C for 24 h, and phase-pure LiFePO₄ was attained at 700 °C. Where the solid carbon powder was used as the reducing agent, no Fe(II) species could be detected after sintering at 400 °C. Carbothermal reduction of Fe(III) is ruled out in this temperature range.

中文翻译：

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高分子添加剂合成LiFePO _4的低温Fe ³⁺还原机理

在由Fe（III）化合物FePO ₄（H ₂ O）₂和Li ₂ CO ₃合成LiFePO _4中，对前体使用碳粉或聚合物添加剂进行了比较。通过X射线衍射，傅立叶变换红外光谱和磁化率，可以在所有长度范围内监测结构性质和相纯度随温度和时间的变化。将反应器温度降至300°C，以研究反应的早期阶段。仅当使用聚合物作为碳质添加剂时，结晶LiFePO _4的形成才在300-400°C的范围内开始。该LiFePO ₄通过在聚合物煅烧过程中释放出的气体（例如H _2）或气态烃来还原Fe（III）物种，可以使其形成。而且，聚合物的分解导致在LiFePO ₄颗粒表面上的碳沉积物。在400℃下烧结24h后，发现的Li ₃ Fe ₂（PO ₄）₃杂质大大减少，在400℃下烧结了24h，在700℃下获得了纯LiFePO ₄。在使用固态碳粉作为还原剂的情况下，在400°C烧结后无法检测到Fe（II）物种。在此温度范围内排除了碳热还原Fe（III）的可能性。