Ni-Rich Li[Ni0.9Co0.045Mn0.045Al0.01]O2 (NCMA) Cathode with Optimized Microstructure for Microstrain Control

Abstract

Layered Ni-rich cathode materials Li[Ni x Co y M 1-x-y ]O2 (M = Mn and Al are denoted as NCM and NCA, respectively) has been common cathodes for the batteries of electric vehicles (EVs). Recently, Li[Ni x Co y Mn z Al1-x-y-z ]O2 (NCMA), a hybrid of NCM and NCA cathodes, has received significant attention owing to the announcement by General Motors of its intention to deploy NCMA cathodes for Li-ion batteries (LIBs) in its next EVs.1 The NCMA cathode can provide high discharge capacity with stable cycle life owing to strong Al-O bonding which strengthens the host layered structure. However, an introduction of the high concentration of Al dopant for stabilization of Ni-rich cathodes is restricted as an electrochemically inactive Al lowers the capacity of cathode.For Ni-rich cathodes, it is well acknowledged that increasing the Ni composition in the cathodes for higher capacity leads to the deterioration in both cycling performance and thermal stability.2 The fast capacity fading of Ni-rich cathode is mostly attributed to the H2–H3 phase transition at the charged state, and the extent of H2–H3 phase transition increases with increasing Ni fraction in the cathodes. As the phase transition induces a lattice volume change generating microcracks in the particles, the alleviation of internal microstrain is required to suppress the microcrack formation in the cathode particles and to improve the cycling stability of Ni-rich cathodes.To reinforce the mechanical stability of Ni-rich NCMA cathodes, we proposed a hybrid structure cathode in which Li[Ni0.92Co0.04Mn0.03Al0.01]O2 in the interior of the particle is encapsulated by Li[Ni0.845Co0.067Mn0.078Al0.01]O2 that acts as a buffer against the microstrain. Similar to the tempered glass which has different states of stress in the surface and bulk material to increase the strength,3 the proposed cathode structure suppresses the formation of microcracks by creating a non-uniform spatial distribution of microstrain within the cathode particle. The electrochemical performance of the proposed cathode is correlated with the microstructure, and the mechanism suppressing the formation of microcracks in the Ni-rich NCMA cathode is demonstrated. Reference s : [1] General Motors Co. LYRIQ Show Car Leads Cadillac Into Electric Future, 2020; https://media.gm.com/media/us/en/cadillac/news.detail.html/content/Pages/news/us/en/2020/aug/0806-lyriq.html (accessed December 2020).[2] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121.[3] A. L. Zijlstra, A. J. Burggraaf, J. Non-Cryst. Solids, 1968, 1, 49.