Cathode reaction models for Braga-Goodenough Na-ferrocene and Li-MnO2 rechargeable batteries

Abstract

Braga-Goodenough all-solid-state Na-Fc and Li-MnO2 batteries demonstrate deposition of Na and Li on the cathode during discharge. These reaction mechanisms were investigated in light of the generalized charge neutrality level and the experimental results of Braga et al., and two new types of mechanisms were proposed. The Na-Fc mechanism is represented by a multi-step C[(CE)cC]n mechanism where C is the chemical step, E is the electrochemical step, c is the catalytic (CE) step, and n denotes the number of [(CE)cC] part cycles. The nth cycle corresponds to n moles of Na and Li deposition. For Li-MnO2, two mechanisms were considered. One is the C[(CE)cC]n mechanism which is the same as Na-Fc, and the other is the C[2(CE)cC]n mechanism, which involves two consecutive (CE)c steps. In the C step of (CE)c of both mechanisms, Fc and MnO2 reduce Na+(sf) and Li+(sf) (sf - surface states) to deposit Na and Li, respectively, which are intramolecular charge transfer reactions within the adsorbed molecules. Fc and MnO2 are oxidized to inter­mediates immediately reduced to Fc and MnO2 by their anodes in the subsequent E step. Based on these mechanisms, these batteries' discharge capacity and cathode alkali metal deposition were examined in detail.