Small voltage hysteresis and flat plateaus in Ca-metal batteries using a Ti-based NASICON cathode

Abstract

Ca-metal batteries have emerged as promising candidates for post-Li-ion batteries owing to their high energy density, cost-effectiveness, and natural abundance. However, few cathode materials have been evaluated for full-cell configurations with Ca-metal anodes, and most of these materials exhibit large voltage hysteresis and sloping voltage profiles. In this study, we propose polyanionic Na superionic conduction (NASICON)-type NaTi₂(PO₄)₃ (NTP) as a cathode material for Ca-metal batteries, leveraging its relatively large lattice size among NASICON compounds to accelerate Ca²⁺ diffusion. Nanoparticulate NTP/carbon nanotube (CNT) composites were synthesised via solvothermal synthesis. The composite structure consisted of NTP nanoparticles uniformly dispersed within a conductive CNT network, which provided high electronic conductivity and short Ca²⁺ diffusion pathways, enabling fast electrochemical reactions. By combining NTP cathodes with a Ca-metal anode and a hydride-based electrolyte, i.e. Ca(CB₁₁H₁₂)₂ in 1,2-dimethoxyethane/tetrahydrofuran, capable of supporting reversible Ca plating/stripping at room temperature, we fabricated Ca-metal batteries that achieved a reversible Ca storage capacity of 40.6 mA h g⁻¹ at 5 mA g⁻¹. Notably, these cells exhibited small voltage hysteresis and flat voltage profiles, indicating rapid Ca²⁺ diffusion. Ex situ analysis, including X-ray diffraction and X-ray photoelectron spectroscopy, confirmed the reversible insertion and extraction of Ca²⁺ within the NTP framework. This study provides valuable insights into the design of practical Ca cathodes with enhanced ion-transport properties, paving the way for the development of high-performance Ca-metal batteries.