兼具高能量密度、高功率密度、长循环寿命性能的正极材料是当下电池储能材料研究的重点，也是储能市场的重要需求。富锂锰基正极材料(LRMO)因其极高的放电比容量(≥250 mAh/g)、较高的工作电压(4.2～4.5 V vs. Li/Li+)、低成本且环境友好等优点成为当下最具应用前景的正极材料之一。虽然金属阳离子和阴离子依次或同时进行的氧化还原反应使LRMO材料的容量超过了传统层状氧化物，但首次不可逆容量高、循环和倍率性能较差等一系列的问题阻碍了其工程化应用，这与材料中阴离子氧化还原反应紧密相关。本文首先介绍了LRMO材料的晶体结构，然后基于分子轨道理论，回顾了LRMO材料的能带结构与阴阳离子氧化还原反应的联系，总结了阴离子氧化还原反应对富锂锰基正极材料的影响，包括高容量、不可逆的氧流失、过渡金属离子迁移。同时，分别从过渡金属比例调节、表面修饰、离子掺杂三个方面总结了近些年国内外研究人员针对阴离子氧化还原反应造成的负面影响设计的改性策略。最后展望了LRMO材料理论研究与应用研究的大致方向。
关键词：锂离子电池
;
富锂锰基
;
正极材料
;
阴离子氧化还原
;
改性策略
Abstract
Cathode materials with high energy density, high power density, and long cycle life are the focus of current research on battery energy storage materials and are also in high demand in the energy storage market. Lithium-rich manganese-based oxide (LRMO) cathode materials are some of the most promising cathode materials owing to their high discharge specific capacity (≥250 mAh/g), high operating voltage (4.2～4.5 V vs. Li/Li+), low cost, and environmental friendliness. Although the sequential or simultaneous redox of cations and anions of LRMO materials results in their enhanced capacity compared with other conventional layered oxides, several problems such as high irreversible capacity for the first cycle and poor cycling and rate performance hinder their engineering applications, which are closely related to the anionic redox reactions in the materials. This paper introduces the crystal structure of LRMO materials and then reviews the relationship between the energy band structure of LRMO materials and anionic redox reactions based on molecular orbital theory. In addition, the effects of anionic redox reactions on LRMO cathode materials, including high capacity, irreversible oxygen loss, and transition metal ion migration, are summarized. Moreover, recent modification strategies for mitigating the negative effects of anionic redox reactions are summarized from three perspectives: transition metal ratio adjustment, surface modification, and ion doping. Finally, the paper discusses the future theoretical and application direction of LRMO materials.
Keywords：lithium-ion batteries
;
lithium-rich manganese based
;
cathode material
;
anionic redox
;
modification strategy
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随着电动汽车、5G基站、智能电网等新型储能领域的发展，对锂离子电池的能量密度、功率密度、循环寿命及安全性能都提出了更高的要求。正极材料作为锂离子电池的重要组成部分，开发高能量密度、长循环寿命和高安全性兼具的正极材料成为当下亟待解决的问题之一。目前商用的正极材料，如钴酸锂[1](LiCoO2，140 mAh/g)、磷酸铁锂(LiFePO4，150 mAh/g)、锰酸锂(LiMn2O4，120 mAh/g)和三元材料[2](LiNi x Co y Mn1-x-y O2，220 mAh/g)等尚且无法满足当下需求。1997年，Numata等[3]首次合成了一种锂锰钴氧化物Li(Li x/3Mn2x/3Co1-x )O2(0≤x≤1)，可以看成是LiCoO2和Li2MnO3组成的固溶体。在随后的研究中发现，在大于4.5 V的高电压下，锂离子可以从Li2MnO3中脱出 [4]，这种特性使得这一类富锂层状氧化物的容量高于传统的层状氧化物，理论容量可高达250 mAh/g成为目前报道中容量最高的正极材料之一[5]。一直以来，富锂正极材料在脱锂过程中过渡金属阳离子被认为会发生氧化还原反应参与电荷补偿来提供容量[6]。但近期研究者发现在Li2MnO3活化时，材料中的阴离子也发生氧化反应参与电荷补偿以提供容量[7]。
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... [11]The energy band structure of the transition metal ion (M) and the ligand (L)<sup>[<xref ref-type="bibr" rid="R11">11</xref>]</sup>Fig. 23 阴离子氧化还原反应对富锂锰基材料的影响<strong>3.1</strong> 高容量
... [14]<strong>Qualitative analysis of the relationship between the d-d Coulombic interaction term <i>U</i> and the charge transfer term</strong><i>Δ</i><sup>[<xref ref-type="bibr" rid="R14">14</xref>]</sup>Fig. 4<strong>3.2</strong> 不可逆的氧流失
... [39]Schematic diagram of the heterostructure O-shared bonding of LNCM (104 plane) and NFO (400 plane)<sup>[<xref ref-type="bibr" rid="R39">39</xref>]</sup>Fig. 5<strong>4.3</strong> 离子掺杂
