Batteries Made with Calcium Could be Better for Electric Cars or Storing Renewable Energy

Abstract

In coming decades, energy storage demand is projected to increase greatly, in part due to accelerated transition to renewable energy and greater demand for electric cars. Should demand keep pace with projections, dependence on cobalt and lithium poses a humanitarian risk, as high demand for cobalt has incentivized mining with unethical labor practices. Multivalent-ion batteries (MVIBs) may offer an alternative to lithium-ion batteries (LIBs), as materials are orders of magnitude more plentiful and MVIB voltage and capacity may be able to match LIBs. MVIBs may be well-suited for applications like electric cars or utility-scale storage, which lack the constraints of small consumer devices like cell phones. Building off earlier work suggesting CaMn2O4 spinel cathodes should have favorable voltage and volumetric expansion characteristics, the present work focuses on building a voltage profile for CaMn2O4. DFT calculations with VASP were used to calculate stable structures through stepwise intercalation and deintercalation of a Ca8Mn16O32 supercell followed by convex hull analysis. Results showed a lower-than-expected voltage with the Ca- and Mg-intercalated cathode. This may be because of differences in how calculations were performed, as previous work used a selective dynamics (SD) approach and included exchange-correlation corrections with DFT+U. It is assumed that the structures obtained approach the lowest possible energy, but this cannot be verified using the approach taken in this work. Even after many intercalation cycles of the cathode, it is still not clear whether lowest-energy structures were found. Future work should focus on whether it can be safely assumed that the spinel “backbone” does not change with intercalation, at least as far as voltage calculations are concerned. Alternatively, techniques like ab initio molecular dynamics (AIMD), quantum Monte Carlo (QMC), or machine learning (ML) could prove useful in finding lowest-energy structures from which to calculate voltage. Figure 1