Effect of Electrolyte Salt Concentration on the Electrochemical Performance of K-Ion Batteries Using Diglyme-Based Electrolytes

Abstract

The increased amount of studies has been performed in recent years to attain the capacity and energy characteristics of potassium-ion batteries being competitive to the lithium-ion one. However, the electrolyte choice has been observed to be the bottleneck for the stable electrochemical performance of the potassium-ion batteries in terms of the formed solid electrolyte interface (SEI) and cathode electrolyte interface (CEI). In the presented research the drawbacks of the conventional carbonate-based electrolyte usage are shown. Moreover, the advantages provided by the diethylene glycol dimethyl ether (so-called diglyme) as a solvent is considered. The effect of salt concentration in diglyme-based electrolytes on cycling performance of promising KVOPO4 and K1.69Mn[Fe(CN)6]0.85×0.4H2O cathodes and hard carbon anode for next-generation potassium-ion (K-ion) batteries is investigated. A decrease in free solvent molecular number with increasing electrolyte concentration is found, which results in a better aluminum current collector stability, formation of thinner solid electrolyte interphase (SEI) passivation layers and further inhibition of solvent degradation redox processes occurring at the electrode surface upon cycling. The KVOPO4 and K1.69Mn[Fe(CN)6]0.85×0.4H2O cathodes exhibit an enhanced specific discharge capacity (54 and 105 mA·h·g-1 respectively) in K-ion cells at the highest electrolyte concentrations (2 M and 2.5 M KPF6 in diglyme, respectively) at a C/10 rate. However, the behavior of the hard carbon anode is noticeably affected by the salt concentration over the first few cycles, a phenomenon tentatively attributed to the SEI layer formation and the presence of irreversible intercalation sites for K+ ions in the hard carbon framework. Finally, electrochemical tests of K-ion full cells consisting of the K1.69Mn[Fe(CN)6]0.85×0.4H2O cathode, hard carbon anode and ether-based electrolyte show capacity retention of 86% over 300 cycles at a 0.6C rate (figure 1). The results clearly show the impact of solvent-salt choice on the K-ion battery performance. We believe the achieved results prove the feasibility of full K-ion battery implementation, however, the future study is required. Figure 1. Comparison of galvanostatic measurements of K full cells comprised of K1.69Mn[Fe(CN)6]0.85×0.4H2O cathode and hard carbon anode with carbonate-based and ether-based electrolytes. a) Variation in the specific discharge capacity with cycle number, and b) Dependence of the Coulombic efficiency on the cycle number. Figure 1