Calcium-ion batteries (CIBs) are promising candidates for post-lithium-ion batteries but face challenges including unsuitable cathodes and unclear failure mechanisms. A new study in SCIENCE CHINA Chemistry reported a reconstructed vanadium oxide (Li@MgVO) designed to probe interlayer ion evolution during Ca²⁺ storage. The team found that the previously overlooked non-electrochemical (de)intercalation processes are a key factor affecting Ca²⁺ storage. Specifically, although Ca²⁺ can intercalate into the host without an applied electrochemical driving force, it does not contribute to the discharge capacity and, more seriously, hinders complete Ca²⁺ deintercalation. Interestingly, the pre-intercalated Mg²⁺ and Li⁺ can undergo ion exchange with Ca²⁺ during cycling, and this “pre-intercalation-exchange” mechanism significantly suppresses the insertion of non-capacity-providing Ca²⁺ via non-electrochemical pathways, thereby greatly enhancing the Ca²⁺ storage performance of Li@MgVO during electrochemical processes.