Analyzing the water stability mechanism in O-type NaLi0.33Mn0.67O2 as a cathode material for sodium-ion batteries

Abstract

The moisture-induced instability of the sodium-layered transition metal oxides (NaxTMO2) presents a significant challenge in developing electrode materials for sodium-ion batteries (SIBs). Herein, via first-principles calculations, we investigate the impact of Li substitution on the water stability of O-type NaLi0.33Mn0.67O2 (NLMO). In particular, the processes of H2O decomposition, Na+/H+ exchange reaction, and hydrogen (H) diffusion on NLMO (101), are specifically compared with those on NMO (101). The results demonstrate that H2O can decompose into O and H species at the Mn–Mn bridge site, but into OH and O species at the Na–Na bridge site, suggesting H2O is unstable on both surfaces. Thereafter, Na+/H+ exchange reaction becomes more difficult on NLMO (101), with the values of −2.73/−2.25 eV and −3.45/−2.82 eV in P1/P2 sites for NLMO (101) and NMO (101). Meanwhile, H diffusion on NLMO (101) is also more difficult due to hydrogen resistance from the subsurface to the bulk. The corresponding barriers are 2.17 and 1.63 eV. However for NMO (101), H can penetrate from the surface to the subsurface and continue to the bulk, with the lowest barrier of 0.61 eV (“Path III-12”) and 0.83 eV (“Path I-23”), respectively. The Columbic interaction between H and metal (Li, Mn, and Na) atoms plays a key role in hydrogen resistance. Notably, Li doping can increase the difficulties in the Na+/H+ exchange reaction and H diffusion on NLMO (101). For this reason, NLMO shows stronger water stability compared to NMO. The in-depth understanding of the water stability mechanism of NLMO can facilitate the future development of high-stable cathodes for SIBs.