Probing the Na + /Li + ‐ions Insertion Mechanism in an Aqueous Mixed‐Ion Rechargeable Batteries with NASICON‐NaTi 2 (PO 4 ) 3 Anode and Olivine‐LiFePO 4 Cathode

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.