摘要/Abstract

本工作通过“碳酸盐共沉淀-沉淀转化-固相反应”方法，实现磷酸锂原位包覆和改性富锂锰基锂离子电池正极材料Li_1.2Mn_0.54Co_0.13Ni_0.13O₂，研究了磷酸锂包覆层的形成过程及其对电化学性能的影响.结果显示，碳酸盐前驱体经沉淀转化反应原位形成磷酸镍包覆层，与锂源混合煅烧，最终转化为厚度小于30 nm的磷酸锂包覆层.该材料组装的半电池在125 mAh·g^-1电流密度下循环175圈后容量达191.1 mAh·g^-1，容量保持率为81.8%，平均每圈电压衰减仅为1.09 mV.磷酸锂包覆层缓解了材料表面与电解液之间的副反应，抑制了不可逆相变和过渡金属溶出，同时磷酸锂作为锂离子导体促进锂离子传输.本工作表明沉淀转化法原位包覆磷酸锂是提升富锂锰基正极材料性能的有效途径.
关键词: 富锂锰基正极材料, 锂离子电池, 磷酸锂, 原位包覆, 沉淀转化
Lithium-rich manganese-based oxides (LRMO) are promising cathode materials to build next generation lithium-ion batteries because of high capacity and low cost. However, the severe capacity fade and voltage decay, which originate from surface oxygen loss, side reactions and irreversible phase transformation, restrict their practical application. Proposed approaches to address these issues include electrolyte modification, synthesis condition optimization, tuning elemental composition, bulk doping and surface coating. Surface coating has been proved to be an effective method to stabilize the interface between LRMO and electrolyte. Herein, we report a facile approach to synthesize Li₃PO₄-coated LRMO (LRMO@LPO) by in-situ carbonate-phosphate precipitate conversion reaction. The formation of Li₃PO₄ layer and its contribution to enhanced electrochemical performance are investigated in detail. Transmission electron microscopy (TEM) reveals that the surface of carbonate precursor converts to Ni₃(PO₄)₂ after reacting with Na₂HPO₄ solution, which finally transforms to Li₃PO₄ coating layer with thickness below 30 nm during calcination process. Quinoline phosphomolybdate gravimetric method gives the optimal Li₃PO₄ coating content of 0.56%. The modified LRMO@LPO sample exhibits improved cycling stability (191.1 mAh·g^-1 after 175 cycles at 0.5C between 2.0~4.8 V and 81.8% capacity retention) and suppressed voltage decay (1.09 mV per cycle), compared with bare LRMO material (72.9% capacity retention, 1.78 mV per cycle). The electrodes are studied by galvanostatic intermittent titration technique, electrochemical impedance spectroscopy, TEM and inductively coupled plasma atomic emission spectrometry. The results suggest efficient mitigation of phase transformation and dissolution of transition metal in LRMO@LPO. As a coating material with lithium-ion conductivity, Li₃PO₄ not only acts as a physical barrier to inhibit side reaction between the electrolyte and LRMO, but also promotes lithium ion transport at the surface region of cathode. The in-situ surface modification approach simplifies the traditional post coating process, and may provide new insight to build stable and low cost Li-rich cathode for lithium-ion batteries.
Key words: Li-rich Mn-based cathode materials, lithium ion battery, Li₃PO₄, in-situ coating, precipitate conversion


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