| Summary: | Abstract Aqueous rechargeable mixed‐ion batteries (ARMBs), where two types of ions shuttle between the cathode and anode, are an important alternative to conventional non‐aqueous electrolyte‐based rechargeable batteries. Herein, we present fundamental insights into the function of an ARMB comprising of NASICON‐based sodium titanium phosphate (NaTi2(PO4)3/NTP) and olivine‐based lithium iron phosphate (LiFePO4/LFP) employed as the anode and cathode respectively in combination with mixed‐ion electrolytes, x‐M Li2SO4: y‐M Na2SO4 (x+y=1). Electrochemical and ex situ structural studies interestingly reveal a preferential Na+‐ion insertion into NTP, despite the presence of two different cations in the electrolyte. This is strongly supported by molecular dynamics simulations, which show a 1–2 orders higher diffusion coefficient for Na+‐ion than Li+‐ion in NTP. In contrast, co‐insertion of Li+ and Na+‐ions into LFP takes place when cycled in mixed‐ion electrolytes. We also show that batteries with mixed‐ion electrolytes perform better than electrolytes with individual cations.
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