Reversible Calcium Alloying on Bulk and Indium (Sub)‐Micro Negative Electrodes for Calcium‐Based Batteries
Wouter Monnens, Robert Markowski, Zhenyu Zhou, Claude Poleunis, Livia Grgurić, Jan Bitenc, Pingping Tan, Han‐Hung Hsu, Alexandru Vlad, Koen Binnemans, Jan FransaerABSTRACT
Indium metal enables highly reversible calcium alloying in both Ca(BH 4 ) 2 /tetrahydrofuran and Ca[B(hfip) 4 ] 2 /dimethoxyethane‐based electrolytes, with substantially improved cycling stability in the former. Electrochemical measurements reveal stable calcium–indium alloying/dealloying at potentials well above that of calcium metal plating, with long‐term cycling exceeding 1000 cycles and Coulombic efficiencies >99% in Ca(BH 4 ) 2 ‐based systems. Structural characterization indicates the reversible formation of a surface‐localized Ca–In phase that likely consists predominantly of CaIn 2 . To overcome the limited areal capacity of planar bulk electrodes, a synthesis strategy for indium (sub‐)microparticles was developed, based on the disproportionation of electrochemically generated indium(I) species. While commercial indium powder improves the accessible capacity compared to planar electrodes, the synthesized (sub‐)microparticles provide a further increase due to their higher surface area and the formation of self‐welded metallic networks. Electrodes based on these materials maintain stable cycling behavior yet exhibit an extremely low utilization of the active material. These findings establish Ca–In alloying as a strategy for durable and stable negative electrodes in calcium‐based batteries.