Exploration of NaSICON Frameworks as Calcium-ion Battery Electrodes

The development of energy storage technologies that are alternative to the state-of-the-art lithium-ion batteries but exhibit similar energy densities, lower cost and better safety, is an important step in ensuring a sustainable energy future. Electrochemical systems based on Calcium (Ca)-intercalation form such an alternative energy storage technology, but require the development of intercalation electrode materials that exhibit reversible Ca-exchange with reasonable energy density and power density performance. To address this issue, we use first-principles calculations, screening over the wide chemical space of sodium superionic conductor (NaSICON) frameworks, with a chemical formula of CaxM2(ZO4)3 (where M = Ti, V, Cr, Mn, Fe, Co, or Ni, and Z = Si, P, or S) for Ca electrode materials. We calculate the average Ca2+ intercalation voltage, and the thermodynamic stability (at 0 K) of charged and discharged Ca-NaSICON compositions. We find CaxMn2(PO4)3 and CaxV2(PO4)3 NaSICONs to be promising as Ca-cathodes given their energy densities and thermodynamic (meta)stabilities, while CaxMn2(SO4)3 and CaxFe2(SO4)3 NaSICONs can also be explored as Ca-intercalation hosts. Additionally, we find all silicate Ca-NaSICONs to be thermodynamically unstable and hence unsuitable as Ca-cathodes. We report the overall trends in Ca-intercalation voltages, thermodynamic stabilities, and the ground state Ca-vacancy configurations in all the NaSICON compositions considered. Our work contributes to unearth strategies for developing practical calcium-ion batteries, involving polyanionic intercalation frameworks.