First principles studies on sodium-ion battery cathode materials

Abstract

We investigated on the thermodynamics and kinetics of sodium-ion battery (SIB) cathode materials using the first-principles calculations. The overall working mechanism of SIB is identical to that of lithium-ion battery (LIB), but the difference in atomic size provides dissimilar electrochemical characteristics in specific capacity, voltage, diffusivity, and occupation site. By connecting the thermodynamics of battery systems to the host-guest chemistry, we present detailed interpretations on the inductive effect and its limitation for predicting cell voltages. Also, we introduce the calculation scheme for averaged physical properties utilizing the sampling methods. Based on above aspects, we performed the theoretical calculations and interpretations on various cathode materials combined with experiments as follows. We reveal the origin of sodium site splitting in Na0.44MnO2 and identify the intermediate phases that are difficult to observe in experimental measurements. We also show that the lattice parameters evolution of this material for electrochemical cycles is attributed to the Jahn-Teller distortion, suggesting that the transition metal (TM) substitution can suppress volume expansions. Next we performed structural and electrochemical interpretations for two-sodium pyrophosphate materi-als, Na2FeP2O7 and Na2MnP2O7. Comparing those materials to the lithium counterpart, we reveal that the kinetic improvements in two-sodium pyrophosphates are attributed to the structural similarity between discharged and charged phases. We further interpret that the previously suggested mechanisms on the inactive nature of manganese compounds can be described by the “atomic reorganization” as a key factor. Lastly, we reveal the unique structural evolutions in Na7V4(P2O7)4PO4 during electrochemical cycles em-ploying the first principles calculations. We also suggest a method for evaluating the lattice mismatch energy between charged and discharged phases occurring for two-ph...