(Invited) Intercalation-Type Cathode Materials for Nonaqueous Calcium-Ion Batteries

Abstract

Calcium-ion batteries (CIBs) represent a promising alternative to the current lithium-ion batteries (LIBs). Calcium ions can transfer two electrons per ion. Thus, in principle, the capacity of a host material can double due to the divalency of calcium, provided the host material can release and accept the transferred electrons. The final battery product is expected to be cost-effective, owing to the earth-abundance of calcium reserves. The redox potential of calcium is close to that of lithium, enabling a high cell voltage. The larger ionic radius of calcium (1.0 Å) compared to those of other divalent ions (0.60–0.74 Å) could be a disadvantage because a narrow diffusion pathway sufficient for smaller ions will not allow the passage of larger Ca ions. However, the larger ionic radius ensures a lower effective intercalant-ion charge density, which can be rather advantageous for the diffusion in the host materials as well as in the 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 cathode materials have been reported so far, only to exhibit low energy-storage capability and poor cyclability. In this talk, the synthesis, structural and electrochemical properties of new cathode materials for nonaqueous CIBs will be presented, which were recently developed in our group. Some examples include NASICON-type NaV2(PO4)3, FeV3O9∙1.2H2O, Ca0.13MoO3·(H2O)0.41. These stimulating discoveries will lead to the development of new strategies for obtaining high energy density CIBs.