A Reaction Model for Li Deposition at the Positive Electrode of the Braga-Goodenough Li-S Battery

Abstract

The Braga-Goodenough all-solid-state Li-S battery demonstrates Li deposition at the positive electrode during discharge. This Li deposition behavior is explained herein in terms of a newly proposed multi-stage mechanism that can be summarized by the shorthand formula E[(ECC)c]n in which E stands for an electrochemical step, C stands for a chemical step, c indicates steps that are catalytic, and n indicates a part of the process that is repeated n times. The catalytic part of the reaction, (ECC)c, cycles the deposition of Li. The n in the E[(ECC)c]n formula represents the number of cycles of the Li deposition step, (ECC)c. An intermediate radical S8 − formed during the first E step, the one-electron reduction of S8, plays an essential catalytic role in the process. The thermodynamics of the second E step were examined by taking account of the electrochemistry involving two consecutive one-electron steps and of the theory of generalized charge neutrality levels in respect of Schottky barriers. The thermodynamics relating to the Li deposition in the second E step was considered to result in a free energy change of at least ΔG < −2.34 eV. The thermodynamics relating to the overall steps of the mechanism were also examined.