(Invited) Nasicon-Type NaV2(PO4)3 As a Cathode Material for Calcium-Ion Batteries

Abstract

Li-ion batteries (LIBs) have been the most prevailing rechargeable batteries for the last three decades. However, the present LIBs have energy densities close to the theoretical limits, and the Li resources are scarce. Calcium-ion batteries (CIBs) are one of the promising candidates to overcome such limitations of LIBs. The divalency of calcium as the carrier ion can increase the capacity of a host material because two electrons are transferred per carrier ion. The earth-abundance of calcium resources provides potential cost-effectiveness to the products. Calcium has the redox potential (Ca/Ca2+ = −2.87 V vs. SHE) close to Li, potentially leading to a higher cell operating voltage. Calcium ion also has a relatively low effective intercalant-ion charge density (0.49 e/Å3), compared to other multivalent cations such as Mg2+ (1.28 e/Å3), Zn2+ (1.18 e/Å3), and Al3+ (4.55 e/Å3), enabling fast diffusion kinetics for Ca ions in solids as well as in electrolytes. Recent discoveries of reversible plating or alloying of calcium provoked considerable interest in calcium-based rechargeable batteries. Theoretical calculations also predict that the energy barriers for Ca diffusion are lower than other multivalent ions in some structures, and the development of positive materials are anticipated. However, only a few positive materials have been reported so far, only to exhibit low energy-storage capability and poor cyclability. In this work, we report NASICON-type NaV2(PO4)3 as a cathode material for nonaqueous CIBs, with high capacity and voltage with a good cyclability. This work demonstrates experimentally that an oxide material can be a good host structure for Ca diffusion at room temperature with high energy-storage capability.