Enhancing cycle stability of lithium iron phosphate in aqueous electrolytes by increasing electrolyte molarity

Aqueous lithium ion batteries (ALIBs) exhibit great potential to reduce the cost and improve the safety of rechargeable energy storage technologies. Lithium iron phosphate (LFP) cathodes have become a material of choice for many conventional, high power LIBs. However, experimental studies on LFP in aqueous lithium (Li) ion electrolytes are limited. Here, results of systematic studies are shown where it is demonstrated that the Li salt concentration of the aqueous electrolyte can significantly improve discharge capacity retention while minimally impacting rate capability, for electrodes made with a typical commercial sub-micron sized LFP powder. Based on the postmortem analysis and the results of electrochemical characterization it is proposed that undesirable side reactions of aqueous electrolytes with LFP induce electrochemical separation of individual particles within the electrode, leading to the observed capacity fading. Increasing the salt concentration in aqueous solutions effectively reduces the concentration of water molecules in the electrolyte, which are mostly responsible for these undesirable side reactions. Similar trends observed with other cathode materials suggest that the use of concentrated aqueous electrolyte solutions offers an effective route to improve stability of aqueous Li ion batteries. Systematic studies on the charge-discharge behavior of lithium iron phosphate (LFP) in aqueous electrolytes reveal the beneficial impact of higher electrolyte salt concentrations on discharge capacity retention. Higher salt concentrations for either Li₂SO₄ or LiNO₃-based electrolytes reduce the extent of undesirable side reactions of water molecules with LFP that lead to the electrochemical separation of individual particles and capacity fading.