The Importance of Li-Ion Nature and Stacking in Layered Cathode Materials for Aqueous Ion Batteries: From Bulk to Surface Models

Abstract

The performance of aqueous ion batteries 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. The use of aqueous electrolytes opens the possibility of proton co-insertion, one of the common assumptions to explain performance issues in aqueous electrolytes. Experimental and computational works compared the feasibility of several cathode materials to be intercalated by H+ concluding likewise that non-layered cathode materials are less prone to proton intercalation than layered materials.Therefore, layered compounds should not be good candidates as cathodes of aqueous batteries, although Li+ insertion/extraction using high concentrated LiNO3 aqueous solution as electrolyte, show results consistent with the Li (de)intercalation in organic electrolytes between the ranges from x equals 0.5-1.0 in layered LixCoO2. Thus, could layered materials have a good performance in aqueous environment despite of their tendency to accommodate protons?In this work, we use ab-initio computations to investigate the behavior of LixCoO2 in aqueous electrolytes analyzing the proton insertion to determine which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries by means of bulk and surface models. The investigations have concerned layered NaxCoO2 in order to investigate the role of stacking and ion material.Our simulations reveal the excellent performance of LixCoO2 in contrast to NaxCoO2. By means of DFT calculations, the H+/ion exchange have been studied and the grand potential phase diagram and voltage-composition curves have been built considering different phases of LixCoO2 and NaxCoO2. Our simulations predicts the CoO(OH) as the most stable insertion/extraction product at battery operation conditions, due to the formation of O-H-O bonds. However, LixCoO2 is not affected by the presence of protons since they co-existence is not stable in comparison to full lithiated and full protonated systems. Our computations reveal that H+ could have a secondary role as surface detrimental agent, without compromising the battery performance. Nevertheless, the nature of Na and the stacking of NaxCoO2 favors the accommodation of protons, showing different performance than LixCoO2, due to the different stacking and different behavior of Na.Our work suggests that to be layered is not enough justification to be discarded as cathode material for aqueous electrolytes. We suggest that the stacking, the alkali-proton repulsion, and specially the operation conditions (voltage window, pH...) are key factors to investigate the suitability or not of a layered material as positive electrode for ion aqueous batteries.