Studies of aluminum-doped LiNi_0.5Co_0.2Mn_0.3O₂: Electrochemical behavior, aging, structural transformations, and thermal characteristics

This paper is dedicated to studies of the electrochemical behavior, the structural and thermal features of the Ni-rich LiNi_0.5Co_0.2Mn_0.3O₂ undoped and Al-doped (0.01 at.%) materials for positive electrodes of lithium batteries. We have found that structural characteristics of these materials are quite similar from the crystallographic point of view. It was demonstrated that Al substitution in the doped LiNi_0.5Co_0.2Mn_0.3O₂ is preferred at Ni sites over Co sites, and the thermodynamic preference for Al³⁺ substitutions follows the order: Ni>Co>Mn. The lower capacity fading of the Al-doped electrodes upon cycling and aging of the cells in a charged state (4.3 V) at 60°C, as well as more stable mean voltage behavior, are likely due to the chemical and structural modifications of the electrode/solution interface. The Al-doped LiNi_0.5Co_0.2Mn_0.3O₂ electrodes demonstrate also lower resistances of the surface film and charge-transfer as well as lower activation energies for the discharge process. From XPS studies we conclude that the modified stable and less resistive interface on the Al-doped particles comprises the Li⁺-ion conducting nano-sized centers like LiAlO₂, AlF₃, etc., which promote, to some extent, the Li⁺ ionic transport to the bulk. A partial layered-to-spinel transformation was established upon cycling of LiNi_0.5Co_0.2Mn_0.3O₂ cathodes.