Lithium-rich layered oxides (LLOs) have been extensively studied as cathode materials for lithium-ion batteries (LIBs) by researchers all over the world in the past decades due to their high specific capacities and high charge-discharge voltages. However, as cathode materials LLOs have disadvantages of significant voltage and capacity decays during the charge-discharge cycling. It was shown in the past that fine-tuning of structures and compositions was critical to the performances of this kind of materials. In this report, LLOs with target composition of Li_1.17Mn_0.50Ni_0.24Co_0.09O₂ were prepared by carbonate co-precipitation method with different pH values. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and electrochemical impedance spectroscopies (EIS) were used to investigate the structures and morphologies of the materials and to understand the improvements of their electrochemical performances. With the pH values increased from 7.5 to 8.5, the Li/Ni ratios in the compositions decreased from 5.17 to 4.64, and the initial Coulombic efficiency, cycling stability and average discharge voltages were gained impressively. Especially, the material synthesized at pH = 8.5 delivered a reversible discharge capacity of 263 mAhg⁻¹ during the first cycle, with 79.0% initial Coulombic efficiency, at the rate of 0.1 C and a superior capacity retention of 94% after 100 cycles at the rate of 1 C. Furthermore, this material exhibited an initial average discharge voltage of 3.65 V, with a voltage decay of only 0.09 V after 50 charge-discharge cycles. The improved electrochemical performances by varying the pH values in the synthesis process can be explained by the mitigation of layered-to-spinel phase transformation and the reduction of solid-electrolyte interface (SEI) resistance. We hope this work can shed some light on the alleviation of voltage and capacity decay issues of the LLOs cathode materials.

中文翻译：

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缓解锂离子电池正极材料富锂层状氧化物中的电压衰减

过去数十年来，由于富锂层状氧化物（LLO）的高比容量和高充电-放电电压，它们已被全世界的研究人员广泛地用作锂离子电池（LIB）的阴极材料。然而，作为阴极材料，LLO具有在充电-放电循环期间显着的电压和容量衰减的缺点。过去表明，结构和组成的微调对于这种材料的性能至关重要。在本报告中，目标成分为Li _1.17 Mn _0.50 Ni _0.24 Co _0.09 O _2的LLO采用不同pH值的碳酸盐共沉淀法制备。使用X射线粉末衍射（XRD），扫描电子显微镜（SEM），透射电子显微镜（TEM）和电化学阻抗谱（EIS）来研究材料的结构和形态并了解其电化学性能的改进。随着pH值从7.5增加到8.5，组合物中的Li / Ni比值从5.17降低到4.64，初始库仑效率，循环稳定性和平均放电电压得到显着提高。特别是，在pH = 8.5时合成的材料的可逆放电容量为263 mAhg ^-1在第一个循环中，初始库仑效率为79.0％，在0.1 C的速率下，在以1 C的速度进行100次循环后，其卓越的容量保持率为94％。此外，该材料的初始平均放电电压为3.65 V，经过50次充放电循环后，电压衰减仅为0.09V。在合成过程中通过改变pH值可以改善电化学性能，这可以通过减轻层状至尖晶石相变和降低固体电解质界面（SEI）电阻来解释。我们希望这项工作可以为减轻LLO阴极材料的电压和容量衰减问题提供一些启示。