Structural and Chemical Evolution of Li- and Mn-Rich Layered Cathode Material

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• 作者：Jianming Zheng ; Pinghong Xu ; Meng Gu ; Jie Xiao ; Nigel D. Browning ; Pengfei Yan ; Chongmin Wang ; Ji-Guang Zhang
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：February 24, 2015
• 年：2015
• 卷：27
• 期：4
• 页码：1381-1390
• 全文大小：682K
• ISSN：1520-5002

文摘

Lithium (Li)- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, the voltage fading mechanism in these materials as well as its relationships to fundamental structural changes is far from being sufficiently understood. Here we report the detailed phase transformation pathway in the LMR cathode (Li[Li_0.2Ni_0.2Mn_0.6]O₂) during cycling for samples prepared by the hydrothermal assisted (HA) method. It is found that the transformation pathway of the LMR cathode is closely correlated to its initial structure and preparation conditions. The results reveal that the LMR cathode prepared by the HA approach experiences a phase transformation from the layered structure (initial C2/m phase transforms to R3虆m phase after activation) to a LT-LiCoO₂ type defect spinel-like structure (with the Fd3虆m space group) and then to a disordered rock-salt structure (with the Fm3虆m space group). The voltage fade can be well correlated with Li ion insertion into octahedral sites, rather than tetrahedral sites, in both defect spinel-like and disordered rock-salt structures. The reversible Li insertion/removal into/from the disordered rock-salt structure is ascribed to the Li excess environment that permits Li percolation in the disordered rock-salt structure despite the increased kinetic barrier. Meanwhile, because of the presence of a large quantity of oxygen vacancies, a significant decrease in the Mn valence is detected in the cycled particle, which is below that anticipated for a potentially damaging Jahn鈥揟eller distortion (+3.5). Clarification of the phase transformation pathway, cation redistribution, oxygen vacancy and Mn valence change provides unique understanding of the voltage fade and capacity degradation mechanisms in the LMR cathode. The results also inspire us to further enhance the reversibility of the LMR cathode via improved surface structural stability.