Deciphering the dynamic interfacial chemistry of calcium metal anodes

Calcium (Ca) metal batteries, due to the high crustal abundance and potential for dendrite-free cycling of Ca, are promising alternatives to current lithium batteries. Ca deposition in aprotic organic electrolytes had been stalled by ion-blocking passivation layers on the Ca metal. This limitation has recently been overcome by using borate-based electrolyte solutions, but the electrode/electrolyte interfacial chemistry enabling reversible Ca metal deposition remains unclear. This study elucidates the formation and dynamic evolution of passivation layers upon immersion of Ca metal electrodes and during electrochemical Ca deposition/dissolution processes in a representative calcium tetrakis(hexafluoroisopropyloxy)-borate (Ca[B(hfip)₄]₂) and glyme electrolyte solution. Upon ageing, a native passivation layer comprising porous Ca metal and a Ca ion conducting solid-electrolyte interphase is formed. In subsequent electrochemical cycles, the pre-passivated Ca metal shows superior activities compared to fresh Ca electrodes. Nevertheless, the electrolyte solution can penetrate the passivating layer to further corrode the Ca metal to form secondary passivation layers, compromising cyclic stability. The native passivation layer, on the other hand, can facilitate Ca metal reversibility in otherwise incompatible electrolyte solutions, such as Ca(TFSI)₂ in glyme. New insights related to the interfacial chemistry of the Ca metal can spur the advancement of anticorrosion interphases or electrolyte systems for rechargeable Ca metal batteries. © 2024 The Royal Society of Chemistry.