Intrinsic Barrier to Electrochemically Decompose Li₂CO₃ and LiOH

Abstract

It is widely acknowledged that Li2CO3 and LiOH as side-products in the operation of a Li–air cell should be completely removed in the cycling to avoid cumulative negative effect on the cycling performance. However, the understanding of their electrochemical decomposition is limited. We report a mechanistic analysis of the intrinsic barrier to electrochemically decompose Li2CO3 and LiOH. Our first-principles study reveals that the decomposition is rate-limited by the electrochemical extraction of Li+, whereas the chemical release of anions is barrierless once the applied voltage overcomes the energy penalty to generate a Li-deficient surface. The voltage necessary for the decomposition of Li2CO3 is predicted to be in the range of 4.38–4.61 V, whereas for LiOH it is in the range of 4.67–5.02 V. The maximum charge efficiency to decompose Li2CO3 and LiOH in the operation of a Li–air cell is estimated to be 66% and 61%, respectively. The high intrinsic barrier originates from the energy cost of oxidizing redox-inert anions for the charge neutrality when lithium is extracted. Therefore, one strategy for lowering the barrier is incorporating redox-active species as a charge mediator to compensate the electron loss during the decomposition.