Advanced electrode materials for calcium rechargeable batteries

Abstract

Calcium-ion batteries (CIBs) have garnered increasing attention due to the abundance of calcium, its high capacity, and low reduction potential. However, the electrochemical performance of CIBs is constrained by the scarcity of high-performance electrode materials, which stems from the sizable ionic radius and bivalent character of Ca2+ ions. These factors result in slow ion diffusion kinetics and, thereby leading to low capacity and power output. This research aims to develop advanced electrode materials and elucidate their Ca ion storage mechanisms for sustainable CIBs. Firstly, tetracarboxylic diimide (PTCDI), a small molecular crystal organic material, has been developed as an anode material for aqueous CIBs. PTCDI employs carbonyl enolization (C=O↔C-O−), circumventing diffusion issues in intercalation-type materials and preventing capacity loss in organic polymer materials, resulting in rapid and high calcium storage capacities. PTCDI demonstrates a reversible capacity of 112 mAh g−1, retains 80% of its capacity after 1000 cycles, and exhibits high power capability at 5 Ag−1,outperforming current state-of-the-art anode materials in CIBs. Both experimental and simulation results indicate that Ca ions diffuse along the a-axis tunnel, enolizing carbonyl groups without being trapped in aromatic carbon layers. The coupling of PTCDI anodes with cost-effective Prussian blue analogous cathodes in aqueous CaCl2 electrolytes yields a prototype aqueous CIB. The significant calcium storage performance of small molecular crystals is anticipated to advance the development of eco-friendly organic CIBs. Secondly, a polyanionic K-vacant KxVPO4F (x~0) material is explored as a cathode for non-aqueous CIBs, addressing the difficulty of seeking high-voltage cathodes. KxVPO4F (x~0) demonstrates remarkable Ca storage capacities of 75 and 40 mAh g–1 at 10 and 500 mA g –1, respectively, and 84% cyclability retention after 1000 cycles. Small volume change and low cation diffusion barriers contribute to exceptional stability and high-power capabilities. This material achieves the highest average voltage of 3.85 V for CIBs to date, which is attributed to the strong inductive effect of the F-rich structure. When KxVPO4F (x~0) cathodes are coupled with Ca metal anodes and a compatible electrolyte, full cells can exhibit an initial energy density of 300 Wh kg−1, which is among the highest recorded for CIBs. This work provides a feasible approach for high-voltage and high-energy Ca metal rechargeable batteries. Thirdly, a polyanionic NaV1.5Cr0.5(PO4)3 material is investigated as a high-capacity and ultrastable cathode for CIBs. This work addresses the challenge of limited capacities for polyanionic cathodes, which are restricted to no more than one electron per redox center. NaV1.5Cr0.5(PO4)3 exhibits multielectron reactions of V2+/V3+, V3+/V4+, and V4+/V5+ redox couples, which are validated by synchrotron X-ray absorption spectroscopy. This material exhibits a calcium storage capacity of 162 mAh g–1 at a current density of 10 mA g–1, achieving a record-high energy density of ~400 Wh kg−1. Low volume change and fast diffusion kinetics result in excellent cycling stability with 98.2%/80.8% retention over 600/5000 cycles at 50/500 mA g–1, respectively. Ca metal full cells displayed a capacity of 122 mAh g–1 at 50 mA g–1, with a capacity retention of 60% over 50 cycles. This research highlights the potential for rationally designed polyanionic cathode materials that utilize multi-electron redox reactions for high-energy CIBs.