Understanding the cation ordering transition in high-voltage spinel LiNi0.5Mn1.5O4 by doping Li instead of Ni

Abstract

We determined how Li doping affects the Ni/Mn ordering in high-voltage spinel LiNi0.5Mn1.5O4(LNMO) by using neutron diffraction, TEM image, electrochemical measurements, and NMR data. The doped Li occupies empty octahedral interstitials (16c site) before the ordering transition, and can move to normal octahedral sites (16d (4b) site) after the transition. This movement strongly affects the Ni/Mn ordering transition because Li at 16c sites blocks the ordering transition pathway and Li at 16d (4b) sites affects electrostatic interactions with transition metals. As a result, Li doping increases in the Ni/Mn disordering without the effect of Mn3+ ions even though the Li-doped LNMO undergoes order-disorder transition at 700 °C. Li doping can control the amount of Ni/Mn disordering in the spinel without the negative effect of Mn3+ ions on the electrochemical property.

Cation-Disordered O3-Na0.8Ni0.6Sb0.4O2 Cathode for High-Voltage Sodium-Ion Batteries

“…Compared to ordered distribution of cations, cation-disordered P3-Na 2/3 Ni 1/3 Mn 2/3 O 2 can construct stable TMO 2 layers and improve the cycling stability . In particular, increasing cationic disorder in the spinel structure has been demonstrated to be effective in improving phase transformation behavior for LIBs by extending the solid solution region . Therefore, the distribution of cations in the TMO 2 layers can effect electrostatic interaction between cations and the coordinated atoms and further affect the performance of the material.…”

“…22 However, the correlation between the ordered/disordered arrangement of cations and electrochemical performance in 24 In particular, increasing cationic disorder in the spinel structure has been demonstrated to be effective in improving phase transformation behavior for LIBs by extending the solid solution region. 25 Therefore, the distribution of cations in the TMO 2 layers can effect electrostatic interaction between cations and the coordinated atoms and further affect the performance of the material. These results inspire us to rationally design high-performance antimony-based O3-type compounds via cationic disorder by specifying a structural composition.…”

The role of metal substitutions in the development of Li batteries, part I: cathodes

“…The ordered P4 3 32 structure is formed near 700 1C and the disordered Fd% 3m structure is formed at higher temperatures. 354 Generally, the disordered structure demonstrates higher stability and energy density. Like other highvoltage cathodes, the nominal voltage of LNMO is outside the window of stability of contemporary electrolytes restricting its practical application.…”

Tuning Electrochemical Performance through Non-stoichiometric Compositions in High-voltage Spinel Cathode Materials

“…Key words: high voltage; spinel-structured; cathode materials; non-stoichiometric; cycling stability 近年来, 锂离子动力电池在纯电动汽车和混合 动力汽车领域得到了广泛的应用, 但是成本及续航 里程问题一直是锂离子动力电池迫切需要解决的问 题 [1] 。锂离子动力电池对高能量密度和高输出功率 的需求, 使得高电压正极材料的研发受到了人们的 广泛关注。LiNi 0.5 Mn 1.5 O 4 (LNMO)材料晶体结构稳 定, 可以提供三维锂离子传输通道, 具有 4.7 V(vs. Li + /Li 0 )高电压平台, 理论比容量达到 147 mAh/g, 且其拥有较好的倍率性能和循环稳定性 [2][3][4][5][6] 。同时, 与已经商业化的正极材料 LiMn 2 O 4 (~400 Wh/kg)、 LiFePO 4 (~495 Wh/kg)、 LiNi 1/3 Co 1/3 Mn 1/3 O 2 (~576 Wh/kg) 相比较 [7] , LiNi 0.5 Mn 1.5 O 4 材料的能量密度高达650 Wh/kg, 更加适用于混合动力和电动汽车用锂离子电池。但 是在循环过程中, LiNi 0.5 Mn 1.5 O 4 电极材料在高电压 下其材料颗粒表面结构发生改变, 从而引起材料表 面的物质从电极材料中溶解后进入电解液, 导致电 池库伦效率降低, 放电容量急剧衰减 [7][8][9][10][11][12][13][14] 。 LNMO 通常是立方相尖晶石结构, 存在两种不 同结构的空间群: 一种是 Ni/Mn 无序的 Fd-3m 空间 群, Mn 元素的价态为+3 和+4 价, 材料中存在部分 氧缺陷; 另一种是 Ni/Mn 有序 P4 3 32 空间群, Mn 元 素均为+4 价, 材料中不存在氧缺陷 [15] , 这两种结构 在一定的合成条件下可以相互转变 [7,16] 。有研究认 为, 因为 Fd-3m 结构锂离子扩散系数较高, 所以 Fd-3m 型材料比 P4 3 32 型材料具有更好的倍率性能。 其主要原因是 Fd-3m 结构正极材料在充放电过程中 仅进行单一相转变 [17][18] , 而 P4 3 32 结构材料在充放 电过程中还会出现一种类似 Fd-3m 结构的中间相, 需经过两步相转变过程, 所以 Fd-3m 结构的循环性 能更优。而两者最为显著的区别是在 Fd-3m 材料中 含有微量的 Mn 3+ 元素, 而在 P4 3 32 材料中 Mn 的化 合价完全为+4 价。因此, 研究者尝试通过各种方法 来实现高电压正极材料从 P4 3 32 结构转变到 Fd-3m 结构, 主要包括表面改性 [19] 元素掺杂 [15,20] 等。Xiao 等 [15] 通过掺杂部分 Cr 3+ 来调控 Mn 3+ 离子的浓度, 来 改变其结构的有序化程度, 改善其电化学性能。Jo 等 [19] 在高电压正极材料表面进行磷酸盐化处理, 加 剧其结构的无序化程度, 从而显著提高倍率和循环 性。最近, Lee 等 [20] [23] Fig. 3 Neutron diffraction patterns of (a) Non-stoichiometric-LNMO and (b) Stoichiometric-LNMO P4 3 32 结构转变成无序化的 Fd-3m 结构 [22] 。 对于 Ni/Mn 阳离子的有序性程度导致的晶体对 称性变化, 拉曼光谱是一个有力的考察手段。图 4 为实验样品的拉曼光谱。图中 638.8 cm -1 处的 A 1g 峰以及在 594.7 和 611.3 cm -1 的 F 2g(1) 峰对应于 MnO 键的对称性伸缩振动, 与此同时, 在 408.4 cm -1 处 的 E g 峰和 498.4 cm -1 处的 F 2g(2) 峰对应于 NiO 键的 伸 缩 振 动 。 相 比 于 Stoichiometric-LNMO 样 品 , Non-stoichiometric-LNMO 样品在 638.8、498.4 和 408.4 cm -1 处峰的强度显著降低, 这是由于 Li + 掺杂 导 致 Ni/Mn 阳 离 子 的 有 序 化 程 度 降 低 。 此 外 , Non-stoichiometric-LNMO 样品中 594.7 cm -1 处的 F 2g(1) 峰存在着更明显的劈裂, 这种现象主要是由于 Ni/Mn 的 无 序 化 程 度 增 加 导 致 其 结 构 从 有 序 的 P4 3 32 结构转变成无序的 Fd-3m 结构…”