磷酸铁锂废料中FePO4·2H2O提取及其杂质形成机理

采用化学共沉淀方法从磷酸铁锂废料中提取FePO₄·2H₂O，并研究了回收过程中杂质形成的机理。在热力学计算基础上绘制了298和363 K时Fe-P-Li-H₂O体系的电势(φ)-pH图，结果表明当pH≤5.0时，Fe(OH)₃相可以自发地转成FePO₄·2H₂O相，从而得到高纯的FePO₄·2H₂O。但实验结果发现当溶液中铁、磷的物质的量之比(n_Fe∶n_P)为1∶1，合成pH为1.5～2.2时得到的FePO₄·2H₂O中存在Fe(OH)₃杂质，这是因为在共沉淀过程中少量Fe³⁺以Fe(OH)₃快速沉淀，而陈化时Fe(OH)₃相转化速率慢，因此FePO₄·2H₂O中含有Fe(OH)₃

Recycling FePO4·2H2O from waste LiFePO4 powders and formation mechanisms of the impurities during precipitation process

Applying waste LiFePO₄ powders as raw material, the FePO₄·2H₂O precursor was effectively recycled through chemical precipitation. Thereafter, the formation mechanisms of the impurities were discussed. The potential (φ)-pH diagram of the Fe-P-Li-H₂O system was investigated and the result demonstrated that FePO₄·2H₂O could form at a temperature of 298-363 K under a pH value of 0-5.0 by precipitation. The experiment results indicated that the Fe₃PO₇ phase started to form through the reaction of Fe(OH)₃ and FePO₄·2H₂O during the sintering process. When the molar ratio of Fe and P (n_Fe:n_P) was 1:1, pH=1.5-2.2, some Fe³⁺ ions will form Fe(OH)₃, and the yield coefficient increased with the pH and temperature. This is because the solubility product constant (K_sp) of Fe(OH)₃ was much less than the one of FePO₄·2H₂O, suggesting that the precipitate rate of Fe(OH)₃ was faster than FePO₄·2H₂O. Based on thermodynamic principles, aging may be an effective way to convert Fe(OH)₃ into FePO₄ ·2H₂O according to the φ-pH diagram of the Fe-P-Li-H₂O system. Unfortunately, the rate of Fe(OH)₃ conversion was slowed, resulting in some Fe(OH)₃ in precipitation. Therefore, low pH value and temperature are essential to avoid Fe(OH)₃ generation during the co-precipitation process. Also, when n_Fe:n_P=1:2, some H₃PO₄ could react with NaOH to form NaH₂PO₄, which would further react with FePO₄·2H₂O to produce NaFeP₂O₇ during the sintering process. At 333 K, the equimolar ratio of co-precipitation precursor FePO₄·2H₂O can be obtained by adjusting the pH value at 1.5, matching the molar ratio of 1:1 of Fe to P. The purity of this as-prepared FePO₄·2H₂O was 99.97%. Besides, the synthesized LiFePO₄ which used this FePO₄·2H₂O as a precursor exhibited a reversible capacity of 154.1 mAh·g^-1 and excellent capacity retention of 96.79% after 100 cycles at 0.2C (1C=180 mA·g^-1). FePO₄·2H₂O obtained from waste LiFePO₄ powders can be used as precursor to synthesize LiFePO₄ cathode material, which greatly improves the economic effi-ciency of recycling the spent LiFePO₄ battery.