Are Protons Involve in the Fading Capacity in Li Ion Aqueous Batteries? Shedding Light By Means of First Principle Computations

Abstract

Commercial lithium ion batteries are composed of an anode based on carbon, a transition metal oxide cathode separated by an organic liquid electrolyte containing a lithium salt such as LiPF6. Nevertheless, operating with an organic electrolyte presents many drawbacks. The organic electrolyte is flammable, expensive and need to be kept away from any water contamination. On the other hand, working in aqueous electrolyte would therefore be extremely beneficial especially to lower production cost and obtain higher safety and lower toxicity. The performance of aqueous based LIBs has been demonstrated since the foundational work from Dahn et al. who combined an anode of VO2 and a cathode of LiMn2O4 in an aqueous electrolyte.In contrast to organic electrolytes, where only the inserted metal ions can (de)intercalate in the electrode material, the use of aqueous electrolytes opens the possibility of proton co-insertion. Proton intercalation has been one of the common assumptions to explain performance issues in aqueous electrolytes such as capacity fading and the increase in lithium diffusion barriers. Several (electro)chemical studies combined with first-principles computations have confirmed the easy protonation of layered electrodes at different ranges of pH values and this fact has been directly related with the detrimental in cathode materials. Nevertheless, several studies reported the excellent performance of layered LiCoO2 and LiCo1/3Ni1/3Mn1/3O2 as positive electrode -without coating- using neutral or basic salt in water electrolytes. Thus, it remains an open question: Why these layered cathode materials so prone to proton intercalation could perform as well in aqueous environment?In this work, we use density functional computations to investigate the lithium/proton exchange in some well-known cathode materials. The analysis of the proton insertion mechanism has been carried out in order to demonstrated which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries. To analyze the Li+/H+ exchange, several Li+/H+/vacancy orderings and arrangements were computed to study the phase stability at different pH values.Given that the importance of proton insertion for aqueous batteries, this study provides fundamental investigations about the role of H-intercalation in the performance of aqueous battery operations.