Although layered transition metal oxides are the most commercially available battery materials, they suffer from capacity fading over time due to transition metal (TM) and Li layer mixing that can lead to decreased Li diffusion and phase segregation due to oxygen loss. We have implemented ⁷Li and ²⁷Al NMR to study the structural changes in Li_xNi_0.8Co_0.15Al_0.05O₂ (NCA) during electrochemical cycling. Using an optimized DFT calculated structure, the predicted NMR shifts are calculated using the spin-flipping method of Middlemiss et al.¹ In the pristine materials, the ⁷Li and ²⁷Al spectra indicate a dynamic Jahn-Teller distortion, as expected for LiNiO_2, and the absence of Li in the TM layer. ²⁷Al NMR also indicates that Al has a preference to be surrounded by six Ni³⁺ compared to a random solution model structure. ⁷Li NMR has been widely used to monitor local changes in the Li environment arising from Li/ TM layer mixing and the formation of defects and/or other phases.  Here we show that the ²⁷Al NMR shift of the Al environment is highly sensitive to its proximity to Ni³⁺, allowing for the indirect monitoring of Ni migration out of the TM layer after multiple cycles. Both ⁷Li and ²⁷Al NMR can be utilized to indicate the mechanisms leading to capacity fade in NCA and other Al-substituted layered transition metal oxides over multiple cycles.

Reference

1.         D. S. Middlemiss, A. J. Ilott, R. J. Clément, F. C. Strobridge and C. P. Grey, Chem Mater, 2013, 25, 1723-1734.