High Conductivity in a Fluorine-Free K-Ion Polymer Electrolyte

Abstract

Solid polymer electrolytes (SPEs) could play a major role in the transition to safer and high-energy-density potassium-based batteries. However, most polymeric K-ion electrolytes are based on fluorine-containing anions and flammable organic solvents, whose safety is nowadays in question. Herein, we report a facile solvent-free synthesis of a series of several poly(ethylene oxide) (PEO)-based SPE solid solutions with KBPh 4 salt as potassium-ion source, including the formation of two crystalline (PEO) n /KBPh 4 complexes. The ionic conductivity of these novel K-ion SPEs above and below the melting point of PEO is rationalized in light of their glass-transition temperature and chemical composition. We highlight that below the melting point of PEO, the crystalline complexes may not be intrinsically ionically conducting, but they act as ion sinks preventing the polymer cross-linking and the formation of contact ion pairs and lower the glass-transition temperature leading to a conductivity of 1.1 × 10 −4 S cm −1 at 55 °C. A high ionic conductivity of 1.8 × 10 −3 S cm −1 is achieved at 80 °C for the optimum (PEO) 30 /KBPh 4 composition (3.2 mol % KBPh 4 ). Besides, the SPE compositions were found to be stable up to 4 V vs K 0 metal electrode at 60 °C. The (PEO) 30 /KBPh 4 composite electrolyte employed in an all-solidstate symmetric cell with Prussian Blue electrodes showed reversible K + -ion (de)intercalation, where a reversible capacity of 20 mAh g −1 and low voltage hysteresis were achieved for 20 cycles. Considering the material availability, ease of synthesis, and promising electrochemical properties, this work may encourage future research on fluorine-free polymer electrolytes for K-ion batteries.

Building Stable Solid‐State Potassium Metal Batteries

“…Notably, this work achieves, for the first time, SPBMs with ultra‐high average coulombic efficiency of 99.94% (Figure 5d; Figure S36a, Supporting Information) stable cycles over 3000 cycles (≈5100 h), and the cell does not fail during the entire test, and this work is the longest cycle of SPMBs at present (Figure S36b, Supporting Information). [ 66,73–79 ] Corresponding cyclic voltammetry tests, cross‐section SEM, and FTIR spectroscopy of the cell demonstrate that ISPE remains electrochemically and structurally stable after a long period of cycling (Figures S37 and S38, Supporting Information). These results indicates that the ISPE has an excellent high voltage resistance and long cycle life.…”

Advancements in cathode materials for potassium-ion batteries: current landscape, obstacles, and prospects

“…439 Despite the existence of polymer electrolytes exhibiting high K + conductivity, the literature on solid-state potassium-ion conductors remains relatively scarce. 443–476 As a result, the current focus of research revolves around enhancing the ionic conductivity of solid-state potassium-ion conductors. However, it is essential to note that other pivotal electrochemical properties often remain insufficiently characterised, including electrochemical stability, ion selectivity, mechanical properties, and assembly techniques.…”

Li, Na, K, Mg, Zn, Al, and Ca Anode Interface Chemistries Developed by Solid‐State Electrolytes

“…[ 469 ] The (PEO) 30 /KBPh 4 SPEs with Prussian blue electrodes show reversible K + ‐ion de‐/intercalation with a reversible 20 mAh g–1 capacity and lower‐voltage hysteresis. [ 470 ] Rayung et al. [ 471 ] reported polyurethane acrylate‐based SPE due to inspiration of coordination of characteristic of N─H, C═O, and C‐O‐C groups for K + stabilization; however, it limits EW ≈ 2 V. PEO‐based SPEs with K‐salts (such as KCl, KFSI, KBr, CH 3 COOK), polyester‐based ([‐O‐ (C═O) –O‐]) and polyurethane (─[NH─(C═O)─O]n─) SPEs and others (PVP, PVA) have been reported for K + interface kinetics; however, their poor ion‐conductivity (10 −5 –10 −8 S cm −1 ) and operating temperatures, and EWs inhibits the chemical and electrochemical stabilities against K‐metals.…”