Anionic redox reaction in Na-deficient layered oxide cathodes: Role of Sn/Zr substituents and in-depth local structural transformation revealed by solid-state NMR

Anionic redox reaction (ARR) offers a complementary electron reservoir that affords the egress/ingress of additional Na beyond the conventional cation redox reaction. However, oxygen-related ARR in sodium-ion batteries is universally confronted with several roadblocks such as insufficient reversibility and poor understanding on the functioning mechanisms. We herein attempt to improve the reversibility of oxygen-related ARR in a Na-deficient Li/Mn(IV)-based layered oxide cathode by Sn and Zr substitutions. The as-designed P2-Na0.66Li0.22Mn0.775Sn0.005O2 and P2-Na0.66Li0.22Mn0.775Zr0.005O2 demonstrate much enhanced cycle stability and rate capability (72% capacity retention at 10 mA g−1 after 100 cycles and 92 mAh g−1 reversible capacity at 1 A g−1 for the former). The role of Sn/Zr substituents was meticulously studied, manifesting the effective inhibition of the anisotropic O2−α−Mn4+coupling (0 < α < 2) and the irreversible lattice oxygen loss. More impressively, the local structural transformations studied via 23Na/7Li solid-state NMR substantiate that (i) Li-ions migrate from TM (transition metal) layer to AM (alkali metal) layer upon charge, and do not return to the original sites in TM layer upon discharge; (ii) the egress/ingress of Na follows a solid-solution reaction within 4.5–1.5 V, but the local structural transition is not fully reversible; (iii) Li/Mn migrations continue beyond the initial cycle, bringing about irreversible Li/Mn losses and Na-site local disorder upon prolonged cycling. Altogether, this study provides deep insights into the ARR mechanism of Na-deficient layered oxide cathodes and will pave a new avenue to fully address the intrinsic shortcomings of oxygen-related ARR.