摘要/Abstract
采用低温固相燃烧法快速制备了一种具有{111}、{110}和{100}晶面的去顶角八面体LiNi0.08Mn1.92O4 (LNMO)正极材料, 其高暴露{111}晶面可以减少充放电过程中Mn的溶解, 面积相对较小的{110}和{100}晶面可增加Li+快速扩散的通道. 测试结果表明, 所合成的LNMO具有LiMn2O4特有的立方晶系结构, 其颗粒尺寸为亚微米级. LNMO的高温电化学性能优异, 在55 ℃, 1和5 C的首次放电比容量分别为109.9和98.0 mAh/g, 分别循环300次后容量保持率为75.8%和80.5%; 即使在55 ℃, 10和15 C下分别循环1000次后仍具有48.4%和49.4%的容量保持率, 而未掺杂的LiMn2O4于15 C循环1000次后容量损失高达98%. LNMO在55 ℃有较高的Li+扩散系数(D=3.86×10-15 cm2/s)和较小的电荷转移阻抗(循环前、后Rct=158.0和279.8 Ω)以及较低的表观活化能(Ea=17.63 kJ/mol), 说明Ni掺杂能够提高Li+在尖晶石型LiMn2O4内的扩散速率及减小锂离子在脱嵌过程中的能垒, 从而提高锂离子的扩散速率和倍率性能. 对LNMO于55 ℃循环1000次后的极片进行X射线衍射(XRD)分析, 发现LNMO电极材料的晶体结构基本保持不变, 表明Ni掺杂提高了锰酸锂材料在55 ℃长循环过程中的晶体结构稳定性, 有效抑制了Jahn-Teller效应及Mn的溶解, 显著提升了其高温电化学性能. 本工作为尖晶石LiMn2O4电极材料在高温方面的应用提供了借鉴.
关键词: LiMn2O4, Ni掺杂, 正极材料, 高温电化学性能, 去顶角八面体, 固相燃烧法, Jahn-Teller效应, Mn溶解
The truncated octahedral LiNi0.08Mn1.92O4 (LNMO) cathode material with dominant {111}, truncated {110} and {100} crystal planes was prepared by a low temperature solid-state combustion method. The dominant {111} crystal plane of the unique truncated octahedron can form firm solid electrolyte interphase (SEI) layer and alleviate the manganese dissolution during the discharge-charge process, and a small part of {110} and {100} crystal planes can increase the rapid diffusion channels of Li+ ions. The field emission scanning electron microscope (SEM) and X-ray diffractometer (XRD) results show that LNMO has cubic spinel structure with submicron particle size. The electrochemical performances of LNMO are also outstanding at high temperature of 55 ℃, the initial discharge capacities are 109.9 and 98.0 mAh/g with capacity retentions of 75.8% and 80.5% after 300 cycles at 1 and 5 C, respectively. Even at high current rates of 10 and 15 C, the capacity retentions of 48.4% and 49.4% have been maintained after 1000 cycles, while the capacity loss of undoped LiMn2O4 is as high as 98% after 1000 cycles at 15 C. Moreover, the dynamic performance tests show that LNMO owns larger Li+ diffusion coefficient (D=3.86×10-15 cm2/s), smaller charge transfer resistances (before cycle and after cycles, the Rct=158.0 and 279.8 Ω) and lower apparent activation energy (Ea=17.63 kJ/mol) than LiMn2O4 (LMO), demostrate its enhanced Li+ transport dynamic. The XRD tests of two electrode slices after 1000 cycles at 10 C show that the crystal structure of LNMO electrode material is almost unchanged, which indicated that the Ni doping and the truncated octahedral structure of particles could improve the structural stability of material, effectively inhibit the Jahn-Teller effect and Mn dissolution, remarkably improve its high temperature electrochemical performance. Therefore, this work provides a reference for the application of spinel LiMn2O4 electrode material at high temperature.
Key words: LiMn2O4, Ni doping, cathode material, high temperature electrochemical performance, truncated octahedron, solid-state combustion method, Jahn-Teller effect, Mn dissolution
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