Pentafluorobenzene boronic acid with strong Lewis acidity for the modification of PEO-based polymer electrolytes

The practical application of polyethylene oxide (PEO) based solid-state electrolytes is severely limited by their low room-temperature ion transport, narrow electrochemical window, and unstable Li/PEO interfaces. This work introduces pentafluorobenzene boronic acid (PFBBA), with –C6F5 and –B(OH)2 ligands, as an advanced additive to resolve these problems. At 30 °C, the electrolyte containing 1.0% PFBBA shows the best performance with an ionic conductivity of 3.09 × 10−5 S cm−1, an Li+ ion transference number of 0.342, and an electrochemical window up to 5.0 V (vs. Li/Li+). The Li//Li battery can stably deposit/peel Li for 1000 h without a short circuit. The reversible capacity of Li//LiFePO4 full batteries at 0.5C after 500 cycles is 124.8 mA h g−1. The modification mechanism is studied by experimental and computational methods. The electron-withdrawing –C6F5 endows B in PFBBA with stronger Lewis acidity and better affinity with F in LiTFSI by forming π–π coordination. The hydrogen bonding interaction between B–OH in PFBBA and –O– in PEO changes the crystallinity of PEO. These two aspects enhanced Li+ ion transport and widened the electrochemical stability window. More importantly, PFBBA participates in in situ forming a LiF/Li–B–O rich SEI layer on the Li anode surface, enhancing the SEI film's mechanical strength and Li+ ion flux.