Understanding pillar chemistry in potassium-containing polyanion materials for long-lasting sodium-ion batteries

K-containing polyanion compounds hold great potential as anodes for sodium-ion batteries considering their large ion transport channels and stable open frameworks; however, sodium storage behavior has rarely been studied, and the mechanism remains unclear. Here, using a noninterference KTiOPO 4 thin-film model, the Na + storage mechanism is comprehensively revealed by in situ/operando spectroscopy, aberration-corrected electron microscopy and density functional theory calculations. We find that incomplete K + /Na + ion exchange occurs and eventually 0.15 K + remains as a pillar to stabilize the tunnel structure. The pillar effect substantially maintains the volume change within 3.9%, much smaller than that of K + (Na + ) insertion into KTiOPO 4 (NaTiOPO 4 ) (9.5%; 5%), thus enabling 10,000 cycles. The powder electrode demonstrates comparable capacity and can work efficiently at commercial-level areal capacity of 2.47 mAh cm −2 . The quasi-solid-state pouch cell with high safety under extreme abuse also manifests long-term cycling stability. This pillar chemistry will inspire alkali metal ion storage in hosts containing heterogeneous cations. The sodium storage mechanism of K-containing polyanion compounds is intricate and unclear. Here, the authors reveal that the residual K + pillars uphold K-containing polyanion structure upon sodium storage, enabling long-term cycling stability.