Self-Adaptive Electrochemistry of Phosphate Cathodes toward Improved Calcium Storage.

Abstract

Polyanion phosphates exhibit great potential as calcium-ion battery (CIB) cathodes, boasting high working voltage and rapid ion diffusion. Nevertheless, they frequently suffer from capacity decay with irreversible phase transitions; the underlying mechanisms remain elusive. Herein, we report an adaptively layerized structure evolution from discrete NaV₂O₂(PO₄)₂F nanoparticles (NPs) to interconnected VOPO₄ nanosheets (NSs), triggered by electrochemical (de)calcification, leading to an improvement in Ca²⁺ storage performance. This electrochemistry-driven self-adapted layerization occurs over approximately 200 cycles, during which NPs undergo a "deform/merge-layerization" process, transitioning from a three-dimensional to a two-dimensional atomic structure, with a distinct 0.68 nm lattice spacing. The transition mechanism is demonstrated to be linked to the gradual separation of structural Na⁺ and F^-. The resultant VOPO₄ NSs exhibit exceptional Ca²⁺ diffusion kinetics (3.19 × 10^-9 cm² s^-1, currently the optimal value among inorganic cathode materials for CIBs), enhanced capacity (∼100 mA h g^-1), longevity (over 1000 cycles at 50 mA g^-1), and high rate (84% retention rates when increasing current density from 50 to 200 mA g^-1). Employing advanced electron microscopy, this study reveals an electrochemical activation-induced structure evolution at the atomic level, providing valuable insights into the design of high-performance CIB cathodes.