Liberating Ca²⁺ Storage from Lattices: Amorphous FePOx Unveiling an Inside-Out Adaptive Cathode Paradigm

Calcium-ion batteries (CIBs) offer a promising candidate within multivalent-ion batteries (MVIBs), but their advancement is impeded by the lack of cathode materials capable of efficiently accommodating large Ca²⁺ with rapid kinetics. Here, this study demonstrates how amorphous FePO_x effectively liberates Ca²⁺ storage from such lattice restrictions by virtue of its inherently disordered and flexible framework, unveiling an adaptive storage mechanism in two distinct yet correlated aspects. First, its amorphous network not only revives electrochemical activity but also provides more open and isotropic ion transport pathways compared to rigid crystalline structures, enabling superior internal Ca²⁺ accommodation and yielding the optimal Ca²⁺ diffusion coefficient (3.24 × 10^-9 cm² s^-1) among the current CIBs inorganic cathode materials. Then, this inherent structural flexibility within the amorphous network further enables dynamic surface self-optimization process of amorphous FePO_x via void migration from Ca²⁺ extraction. The evolving surface morphology provides more Ca²⁺ adsorption sites, enhancing decalciation/calciation kinetics. This synergistic adaptation yields a high capacity (124.3 mAh g^-1 at 20 mA g^-1), exceptional cyclability (92.1 mAh g^-1 at 100 mA g^-1 after 1000 cycles), and high rate (≈76% retention rate when increasing from 20 to 300 mA g^-1), demonstrating the broad advantages of amorphous architectures for advanced MVIBs.