Potential Solid Electrolytes for Multivalent-Ions Based on the Anti-Perovskite Structure
Authors: Kim Kwangnam , Siegel Donald J 
Published: May 2020

Li-ion batteries (LIBs) are the most widely used energy storage devices. Beyond Li, Na and K ion systems are being explored for grid energy storage. Additionally, multivalent (MV) ions like Mg, Ca, and Al have the potential to increase the volumetric capacity of batteries. Currently, additional improvements in battery technologies are being driven by demands for longer-lasting batteries and safety concerns. Safety hazards in LIBs originate from the use of liquid electrolytes, which are volatile and flammable. In principle, these safety problems can be circumvented by the use of a solid electrolyte (SE). Furthermore, SEs may enable the use of metallic anodes which significantly increase the energy density.

Recently, Li and Na anti-perovskites (AP) have been suggested as SEs. APs adopt the formula X3AB, where X is a monovalent cation and A and B are anions with respective charges of -2 and -1. Using a series of model APs with X = Li, Na, or K, A = O, S, or Se, and B = F, Cl, Br, or I, our previous study revealed that ‘lattice distortion’ can enhance ion mobility. We found that symmetry-lowering lattice distortions correlate with lower percolating migration barriers and reduced thermodynamic stability. Therefore, ‘distortion tuning’ can result in SE materials by balancing a mobility/stability tradeoff.

Thus far, few MV SEs have been reported, and the discovery of MV SEs remains an important pursuit to advance the prospects for MV solid-state batteries. Previous literature showed that most alkaline earth metals, 3d transition metals, and lanthanides can reside in the X site of the AP system. These materials exhibit a wide range of electronic, magnetic, and thermal properties, and have been exploited in applications ranging from thermoelectrics, memory devices, sensors, and optical devices.

Among the many possible MV APs, here we focus on ten non-metallic Mg and Ca APs for MV SEs, i.e., Mg3NB, Ca3NB, Ca3PSb, and Ca3AsSb (where B = P, As, Sb, or Bi). First-principles calculations were used to evaluate suitability for SE applications by predicting equilibrium structures, band gaps, elastic moduli, percolating migration barriers, defect formation energies, and thermodynamic (electro)chemical stability. All compounds are predicted to be thermodynamically stable. Ca and Mg APs follow a similar trend to that observed in monovalent APs, where lattice distortions decrease the barriers for percolating ion migration. These MV APs are also predicted to have high defect formation energies, implying that defect concentrations need to be frozen in during synthesis at high temperatures.

We found that Mg3NAs, Ca3NAs, and Ca3PSb have relatively low percolating barriers of less than 500 meV for vacancy migration and less than 200 meV for dumbbell migration. Band gaps are predicted to be larger than 2 eV. Also, these systems are stable against the respective metal anode and expected to have moderate oxidative stability up to 1.2, 1.7, and 1.4 V, respectively, implying that coating materials may be needed to prevent oxidation by the cathode and increase the operating voltage.