Structural transformation from crystalline to amorphous states to boost sodium storage properties of NaVOPO4 cathode

Polyanionic materials are considered one of the most promising cathode materials for sodium-ion batteries because of the stable structure framework and high working voltage. However, most polyanionic materials possess limited sodium storage sites and have to undergo complex local structure evolution during charge/discharge. Herein, we conducted a systematic investigation into the impact of structural forms of NaVOPO₄ on the electrochemical properties. Amorphous and crystalline NaVOPO₄ are synthesized through a controlled reflux reduction method, and the amorphous NaVOPO₄ (a-NVOP) demonstrates much better electrochemical performance compared to the crystalline counterpart. Specifically, the a-NVOP electrode delivers high reversible capacity (142 mAh g⁻¹ at 14.5 mA g⁻¹, close to the theoretical capacity of 145 mAh g⁻¹), high energy density (497 Wh kg⁻¹ based on cathode material) and remarkable cyclability with capacity retention of 80% after 500 cycles. In situ and ex situ experimental analyses and theoretical calculations reveal that the superior performance is primarily due to the maintaining of the amorphous state during the charge/discharge process to endow high stability and accelerated intercalation/deintercalation of large-sized Na⁺ without lattice constraints. Furthermore, the amorphous cathode materials show promising electrochemical properties in lithium-, potassium- and zinc-ion batteries, highlighting their broad adaptability and potential across various battery systems.

Graphical abstract

A series of NaVOPO₄ with different crystal forms were synthesized through a controlled reflux reduction method. The amorphous NaVOPO₄ electrodes demonstrated boosted electrochemical performance compared to the crystalline counterpart. Specifically, the amorphous NaVOPO₄ electrodes deliver a high working voltage (3.5 V vs. Na⁺/Na), high reversible capacity (142 mAh g⁻¹ at 14.5 mA g⁻¹), high energy density (497 Wh kg⁻¹), and good cycle stability. Ex situ and in situ characterizations and theoretical calculations reveal the redox characteristics of the amorphous structures. This work provides an inspiring example that amorphous materials can serve as advanced cathode materials to achieve both high reversible capacities and stable cycling performance.