Self-Adaptive Electrochemistry of Phosphate Cathodes toward Improved Calcium Storage

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 NaV 2 O 2 (PO 4 ) 2 F nanoparticles (NPs) to interconnected VOPO 4 nanosheets (NSs), triggered by electrochemical (de)calcification, leading to an improvement in Ca 2+ 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 VOPO 4 NSs exhibit exceptional Ca 2+ diffusion kinetics (3.19 × 10 –9 cm 2 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.