Ni Migration Arising from Inter-transition Metal Charge Transfer in the NCM811 System

Abstract

Ni-rich materials (LiNixCoyMn1-x-yO2, x ≥ 0.6) are highly desirable for use in lithium-ion batteries (LIBs) due to their high energy density. However, voltage decay caused by heavy migration of transition metals (TMs), particularly Ni, remains a major issue. While Ni2+ is the dominant form of Ni migration into the Li layer, the high content of Ni4+ and the low content of Ni2+ during the late stages of de-lithiation leave the underlying cause of Ni redox during migration unclear. This knowledge gap hinders the design of a better cathode material. To address this, we investigated the migration of Ni in the classic material NCM811 (LiNi0.8Co0.1Mn0.1O2) and found that Ni migrates to the lithium layer when x ≥ 0.6 (Li1-xNi0.8Co0.1Mn0.1O2). We discovered that Ni4+ can be reduced to Ni2+ by charge transfer with the surrounding Ni, changing its valence state at x = 0.7. Ni2+ then continues to migrate to the lithium layer, revealing the charge-transfer migration mechanism between TM Ni. In addition, variable-valence Ni3+ cations around migrating Ni atoms were replaced by constant-valence cations, which did not provide charge to the migrating Ni atoms. This reduced the amount of low-valence Ni available to provide charge for Ni migration and ultimately inhibited Ni migration. Overall, our study provides a novel approach to inhibiting Ni migration in Ni-rich cathode materials by considering the charge-transfer mechanism between TM Ni, which may lead to the development of cathode materials with better structural properties.