Synergistic regulation of electrochemical and non-electrochemical processes: An often-neglected factor in calcium-ion batteries

Calcium-ion batteries (CIBs) have been recognized as one of the potential candidates for large-scale energy storage systems in the “post-lithium-ion battery era”, but their development remains in an initial exploration stage. One of the key challenges lies in identifying suitable cathode materials for Ca²⁺ storage. Layered vanadium oxides have attracted extensive attention due to their high theoretical capacity, diverse crystal structures, and variable oxidation states, and their Ca²⁺ storage capability can be further enhanced through a “pre-intercalation engineering” strategy. However, because of the extremely strong electrostatic interaction between Ca²⁺ and the host lattice, Ca²⁺ can compete with pre-intercalated ions for binding sites even without an applied electrochemical driving force, triggering ion exchange or redistribution of interlayer ions. Consequently, the behavior of pre-intercalated ions and Ca²⁺ during cycling remains unclear, and the influence of non-electrochemical processes (the general term for electrode processes in the electrolyte that proceed without an applied electrochemical driving force) on the Ca²⁺ storage mechanism has received little systematic investigation. Elucidating the evolution mechanism of interlayer ions in both electrochemical and non-electrochemical processes may provide a new perspective for understanding the Ca²⁺ storage mechanism.

Recently, the joint team of Fanghua Ning, Zhenyou Li, and Zhenghui Pan reported a reconstructed vanadium oxide (Li@MgVO) prepared through a “dual-ion-in-sequence” intercalation approach (hydrothermal pre-intercalation of Mg²⁺ followed by quenching to intercalate Li⁺). The team found that Ca²⁺ can be intercalated into the interlayers via both electrochemical and non-electrochemical processes. Ca²⁺ intercalated via the latter does not contribute to the discharge capacity and, owing to sluggish diffusion kinetics, is difficult to fully extract during charging; its accumulation in the interlayers continuously impairs the Ca²⁺ storage capability of the material. Accordingly, the team proposed a “pre-intercalation-exchange” mechanism—the pre-intercalated Mg²⁺ and Li⁺ occupy active sites in advance and undergo ion exchange with Ca²⁺ during cycling, thereby significantly weakening the ability of vanadium oxides to introduce Ca²⁺ via non-electrochemical routes. Benefiting from this mechanism, the assembled Li@MgVO||activated carbon calcium-ion hybrid capacitor delivers an initial capacity of 60 mAh g^-1 at 500 mA g^-1 and maintains a capacity retention of 97.13% after 300 cycles, markedly outperforming unmodified vanadium oxide (18 mAh g^-1 and 80.45% capacity retention). This work provides new insights into the Ca²⁺ storage mechanism and the failure origins of cathode materials for CIBs.

The first author of the paper is Huanxuan Lin from Tongji University, and the corresponding authors are Fanghua Ning (Shanghai University), Zhenyou Li (the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences), and Zhenghui Pan (Tongji University).