The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-ray Microscopy Study

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• 作者：Chih-Jung Chen ; Wei Kong Pang ; Tatsuhiro Mori ; Vanessa K. Peterson ; Neeraj Sharma ; Po-Han Lee ; She-huang Wu ; Chun-Chieh Wang ; Yen-Fang Song ; Ru-Shi Liu
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：July 20, 2016
• 年：2016
• 卷：138
• 期：28
• 页码：8824-8833
• 全文大小：676K
• 年卷期：0
• ISSN：1520-5126

文摘

The mechanism of capacity fade of the Li₂MnO₃·LiMO₂ (M = Li, Ni, Co, Mn) composite positive electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallographic, and morphological evolution of the material during electrochemical cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R3̅m LiMO₂ with the remaining 27(1) wt % C2/m Li₂MnO₃ likely existing as an intergrowth. Cracking in the Li₂MnO₃·LiMO₂ electrode particle under operando microscopy observation was revealed to be initiated by the solid-solution reaction of the LiMO₂ phase on charge to 4.55 V vs Li⁺/Li and intensified during further charge to 4.7 V vs Li⁺/Li during the concurrent two-phase reaction of the LiMO₂ phase, involving the largest lattice change of any phase, and oxygen evolution from the Li₂MnO₃ phase. Notably, significant healing of the generated cracks in the Li₂MnO₃·LiMO₂ electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally associated with the solid-solution reaction of the LiMO₂ phase. This work reveals that while it is the reduction of lattice size of electrode phases during charge that results in cracking of the Li₂MnO₃·LiMO₂ electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-solution reaction occurring during discharge. Importantly, it is the phase separation during the two-phase reaction of the LiMO₂ phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase separation, such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode.