Lattice strengthening enables reversible anionic redox chemistry in sodium-ion batteries

Triggering anionic redox reaction (ARR) in layered oxide cathodes has emerged as an effective approach to overcoming the energy density limitations of conventional sodium-ion batteries (SIBs) solely based on cationic redox. However, the local structural deterioration and lattice oxygen loss associated with ARR remain challenging and unsolved, resulting in severe capacity and voltage decay. To address these issues, we herein present a lattice-strengthened P2-Na_0.66Ca_0.03[Li_0.24Mn_0.76]O₂ (NCLMO) cathode. The introduction of Ca into the Na layers enables the compressed TMO₂ slabs and reinforced TM–O bonds (TM = Li/Mn). Moreover, the incorporation of Ca into the Na layers effectively mitigates the out-of-plane migration of Li and impedes the in-plane migration of Mn during the anionic redox. The reduction of ion migration reduces the variation of the local environment surrounding O and hinders the formation of TM vacancy clusters, significantly mitigating the loss of lattice oxygen. Consequently, NCLMO delivers an impressive capacity retention of 76.04% at 1 C after 200 cycles. Our findings highlight the significance of maintaining local structural stability and offer novel insights towards achieving highly reversible ARR in layered oxide cathodes for high-energy SIBs.