We demonstrate here a simple alternative strategy of developing a stable and long-lived aqueous Zn-ion battery. The battery comprises a redox-active anthraquinone-based covalent organic framework (COF) and a graphene oxide composite (COF-GOPH) as the cathode, zinc metal as the anode, and a mixed-ion electrolyte with varying proportions of zinc and lithium ions. This cell configuration contrasts with those of conventional organic batteries with aqueous electrolytes having a single type of cation. Our findings convincingly show that an optimal Li + to Zn 2+ ion ratio is beneficial for Zn 2+ -ion diffusion into the COF. The energy storage mechanism is found to be due to the Zn 2+ -ion intercalation/ deintercalation into the COF with simultaneous reversible redox activity of the framework carbonyl and imine moieties. Additionally, a theoretical analysis of the radial distribution function reveals the preferential insertion of Zn 2+ -ions along with its partial solvation shell into the framework, leading to an optimal coordination of Zn 2+ with oxygen and nitrogen moieties of the COF network. On the other hand, the Li + ions preferentially reside in solution. Irrespective of the electrolyte composition, the composite electrode COF-GOPH performs better than the COF. The best battery performance is obtained with the COF-GOPH in the presence of 0.5 M ZnSO 4 and 0.5 M Li 2 SO 4 electrolyte. The cell shows excellent cyclability and superior capacity with 82% retention even after 500 cycles (from the second cycle onwards). Our studies also reveal a Li + -ion-assisted pseudocapacitance mechanism that is partially responsible for the enhancement in the electrochemical performance in the mixed-ion electrolytes.
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 electrolytebased rechargeable batteries. Herein, we present fundamental insights into the function of an ARMB comprising of NASICONbased sodium titanium phosphate (NaTi 2 (PO 4 ) 3 /NTP) and olivine-based lithium iron phosphate (LiFePO 4 /LFP) employed as the anode and cathode respectively in combination with mixed-ion electrolytes, x-M Li 2 SO 4 : y-M Na 2 SO 4 (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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