Probing the Degradation Mechanism of Li2MnO3 Cathode for Li-Ion Batteries

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• 作者：Pengfei Yan ; Liang Xiao ; Jianming Zheng ; Yungang Zhou ; Yang He ; Xiaotao Zu ; Scott X. Mao ; Jie Xiao ; Fei Gao ; Ji-Guang Zhang ; Chong-Min Wang
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：February 10, 2015
• 年：2015
• 卷：27
• 期：3
• 页码：975-982
• 全文大小：639K
• ISSN：1520-5002

文摘

Capacity and voltage fading of Li₂MnO₃ is a major challenge for the application of this category of material, which is believed to be associated with the structural and chemical evolution of the materials. This paper reports the detailed structural and chemical evolutions of Li₂MnO₃ cathode captured by using aberration corrected scanning/transmission electron microscopy (S/TEM) after certain numbers of charge鈥揹ischarge cycling of the batteries. It is found that structural degradation occurs from the very first cycle and is spatially initiated from the surface of the particle and propagates toward the inner bulk as the cyclic number increases, featuring the formation of the surface phase transformation layer and gradual thickening of this layer. The structure degradation is found to follow a sequential phase transformation: monoclinic C2/m 鈫?tetragonal I4₁ 鈫?cubic spinel, which is consistently supported by the decreasing lattice formation energy based on DFT calculations. For the first time, high spatial resolution quantitative chemical analysis reveals that 20% oxygen in the surface phase transformation layer is removed and such a newly developed surface layer is a Li-depleted layer with reduced Mn cations. This work demonstrates a direct correlation between structural degradation and the cell鈥檚 electrochemical degradation, which enhances our understanding of Li鈥揗n-rich (LMR) cathode materials.