Phase Stability and Kinetics of Topotactic Dual Ca<sup>2+</sup>–Na<sup>+</sup> Ion Electrochemistry in NaSICON NaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>
Lauren Blanc, Yunyeong Choi, Abhinandan Shyamsunder, Baris Key, Saul H. Lapidus, Chang Li, Liang Yin, Xiang Li, Bharat Gwalani, Yihan Xiao, Christopher J. Bartel, Gerbrand Ceder, Linda F. Nazar
Abstract
Recent reports of reversible calcium plating and stripping have rekindled interest in the development of Ca-ion batteries (CIBs) as next-generation energy storage devices. This technology has the potential to overcome the limitations of conventional Li-ion batteries, but CIBs are plagued by a paucity of suitable cathode materials. To date, NaSICON-structured NaV2(PO4)3 has been demonstrated as a successful cathode candidate, exhibiting reversible (de)intercalation of 0.6 mol Ca2+ along with stable cycling performance. However, a complex multiphase mixture forms on discharge so the Ca-ion charge storage mechanism in the NaSICON framework is poorly understood. In this work, we report on an investigation of the structure and/or Na+/Ca2+ environment(s) of a variety of chemically prepared NaSICON CaxNayV2(PO4)3 phases which were characterized using synchrotron XRD, SEM-EDS, 23Na NMR, and TEM. Highly calciated CaV2(PO4)3, Ca1.5V2(PO4)3, and CaNaV2(PO4)3 phases can be prepared at high temperature, but─unlike Ca0.6NaV2(PO4)3─these materials are electrochemically inactive. To better understand the fundamental factors impacting successful Ca2+ electrochemistry in this system, DFT was employed to examine the CaxNayV2(PO4)3 phase diagram and Ca2+ diffusion mechanism. Theoretical insights show that phase separation into Na-rich and Ca-rich phases is a reason for the capacity limitation and demonstrate that Na+ ions in the host materials assist the migration of neighboring Ca2+ ions, enabling reversible electrochemistry in CaxNayV2(PO4)3. This investigation of fundamental principles affecting reversible Ca2+ (de)intercalation in CaxNayV2(PO4)3 allows for the development of design principles to enable the discovery of a variety of successful cathodes for CIBs.