LAUSR

Abstract Sluggish solid-state K+-migration kinetics and electrode material degradation represent two critical issues facing K-ion batteries. Here we report a new inorganic-open-framework (IOF) compound K0.76V0.55Nb0.45OPO4 (KVNP, synthesized through aliovalent substitution) which, as a bifunctional electrode material, simultaneously mitigates these issues on both anode and cathode sides. The distinctive IOF crystal structure enables a low K+-migration energy barrier (0.16 eV) even surpassing the Li+ counterpart in state-of-the-art LiCoO2/LiFePO4 (0.21/0.55 eV) and consequently achieves fast-charging capability and high energy efficiency. Equally important, KVNP undergoes exceptionally small lattice-volume changes upon cycling (7.1% on anode and 3.1% on cathode, versus 10% for graphite anode and 6.8% for LiFePO4 cathode in commercial Li-ion batteries). This intrinsic low-strain feature ensures high structural stability against K+-ion (de)intercalation and ultralong cycle lives of > 18 months for both KVNP anode and cathode (versus