Stabilizing the High-Voltage Aqueous Battery Chemistry of LiMn2O4

Abstract

Energy security across the varied landscapes of a nation in the era of renewable energy will be determined by the reliability of large-scale grid storage solutions. Economic aspects, both in terms of materials processing and deployment, environmental impacts along with typical battery parameters such as energy and power density, cycling rate, etc., have championed the aqueous zinc-ion batteries (AZBs) on the forefront of decarbonisation of the grids. The possibility of harnessing the energy density exceeding 85-100 Wh kg-1 is closest to reality more than ever by the introduction of the Manganese-based cathodes, i.e. α-MnO2 and spinel-LiMn2O4, thus ensuring a sustainable energy transition without compromising the energy security of the countries.1,2 However, the practical scale deployment of AZBs is still facing hindrances beyond the controlled environment of the labs. Two crucial deterrence factors have emerged in the process of this scalability, with one being the corrosion of the zinc anodes at a high current rate and the stability of the oxide-type cathode active materials being the second. The latter issue particularly becomes more evident in terms of the realization of high capacity storage, especially at a low current rate when the dominance of proton intercalation in the oxide cathode materials subsequently leads to accumulation of electronically and ionically insulative ‘layered double hydroxide’ (LDH) type byproducts, e.g. Zn4SO4(OH)6.nH2O (ZHS) in the case of aqueous ZnSO4 electrolyte on the cathode interface during discharge and its dissolution with further progression of proton deintercalation.3,4 The accumulation of ZHS at the cathode interface limits the charge transport process by being an insulator, adding up to the reversibility woes associated with AZBs and its direct consequence of wattage loss. In the question of suppression of proton co-intercalation in such systems, existing literature only suggests efforts from the perspective of either conversion of the electrolyte essentially to water in salt type electrolyte or addition of non-aqueous solvents to the system with mass/volume ratio exceeding 50%. While these approaches effectively mitigate H+ co-intercalation in oxide cathodes, they compromise the safety advantages or cost-effectiveness of AZBs.5,6 Manganese-based oxide cathodes, especially the LiMn2O4 spinel, stand out for two compelling reasons. Firstly, they hold the potential for a high median discharge voltage of 1.8 V. Secondly, they have been assumed to intercalate Li+ exclusively in aqueous systems. However, this assumption regarding the charge storage mechanism in aqueous electrolytes has not been directly probed. Despite the widespread occurrence of proton intercalation among oxide cathode materials in aqueous systems, there has been a notable absence of in-depth investigations into the charge storage mechanism in aqueous electrolyte systems for LiMn2O4.7 To bridge the knowledge gap regarding the intercalation mechanism, we conducted experiments involving LiMn2O4 cathode materials in various electrolytic mediums, manipulating the Li+ to Zn2+ ratio in the aqueous sulphate electrolyte. Our electrochemical studies, complemented by in-operando X-ray Diffraction, ex-situ structural analysis through synchrotron PD, and DFT-based in-silico calculations, establish the thermodynamic factors governing the competitive intercalation of Zn2+, Li+, and H+, and provide a detailed account of the intercalation chemistry of LiMn2O4. Our investigation reveals that the charge storage mechanism of the LiMn2O4 system in an aqueous medium involves a competitive scenario of Li+-H+ co-intercalation, with Li+ intercalation favoured at higher Li+: Zn2+ ratios in the electrolyte.Furthermore, we identified the optimal ionic composition of the sulphate-based aqueous electrolyte to effectively mitigate detrimental H+ co-intercalation and its consequential impact on median voltage decay. We propose that the integration of LiMn2O4 as a cathode with optimal electrolyte concentration in AZBs represents a notable stride in advancing the feasibility of deploying AZBs for storage applications on a grid scale.