Three-dimensional electrodes in hybrid electrolytes for high-loading and long-lasting calcium-ion batteries

Abstract

Aqueous calcium-ion batteries (CIBs) exhibit a reliable long-term cycling and rate performance, but they have a low energy density owing to the narrow electrochemical stability window (ESW) of aqueous electrolytes. Highly concentrated water-in-salt electrolytes offer enhanced stability by limiting the number of free water molecules present within the system. However, CIB electrolytes still suffer from low conductivity and high viscosity due to the high concentration of calcium salt. Hence, to resolve this problem, a series of hybrid aqueous/organic calcium electrolytes were formulated to improve the ESW and long-term reversibility while maintaining a low electrolyte concentration. The best hybrid-electrolyte candidate, which had a calcium perchlorate/water/acetonitrile molar ratio of 1.0:4.0:4.8 (i.e., Ca(ClO4)2/(H2O)4.0(AN)4.8), exhibited a wide ESW (≈3.0 V) and a facile insertion of calcium ions into Prussian green host materials, leading to an impressive 90% capacity retention after 1000 cycles. Nickel foam (NF) was also employed as a cathode current collector to investigate the feasibility of the drop-coated three-dimensional electrodes. Owing to its high-porosity architecture, the NF substrate could accommodate for four to eight times higher mass loading than traditional foil-type collectors, resulting in full cells with much higher energy densities.