Addressing the High-Voltage Structural and Electrochemical Instability of Ni-Containing Layered Transition Metal (TM) Oxide Cathodes by "Blocking" the "TM-Migration" Pathway in the Lattice.

Abstract

"Layered"/"cation-ordered"/O3-type Li-TM-oxides (TM: transition metal) suffer from structural instability due to "TM migration" from the TM layer to the Li layer upon Li removal (viz., "cation disordering"). This phenomenon gets exacerbated upon excessive Li removal, with Ni ions being particularly prone to migration. When used as cathode material in Li-ion batteries, the "TM migration" and associated structural changes cause rapid impedance buildup and capacity fade, thus limiting the cell voltages to ≤4.3 V for stable operation and lowering the usable Li-storage capacity (concomitantly, energy density). Looking closely at the "TM migration" pathway, one realizes that the tetrahedral site (t-site) of the Li layer forms an intermediate site. Accordingly, the present work explores a new idea concerning suppression of "Ni migration" by "blocking" the intermediate crystallographic site (viz., the t-site) with a dopant, which is the most stable at that site. In this regard, density functional theory (DFT)-based simulations indicate that the concerned t-site is energetically the most preferred and stable site for d10 Zn2+. Detailed analysis of crystallographic data (including bond valence sum) obtained with the as-prepared Zn-doped Li-NMC supports the same. Furthermore, the simulations also predict that Zn doping is likely to suppress "Ni migration" upon Li removal. Supporting these predictions, galvanostatic delithiation/lithiation studies with Zn-doped and undoped Li-NMCs demonstrate significantly improved cyclic stability, near-complete suppression of "cation mixing", and negligible buildup of impedance (as well as potential hysteresis) for the former, even upon being subjected to long-term cycling using a high upper cut-off potential of 4.7 V (vs Li/Li+). Accordingly, such subtle tuning of the composition and structure, in the light of electronic configuration of the dopant and specific crystallographic site occupancy, is likely to pave the way toward the development of Ni-containing stable high voltage O3-type Li-TM-oxide cathodes for the next-generation Li-ion batteries.