Bifunctional LiI additive for poly(ethylene oxide) electrolyte with high ionic conductivity and stable interfacial chemistry

The development of polymer-based solid-state batteries is severely limited by the low ionic conductivity of solid polymer electrolyte and the instable interface between polymer electrolyte and Li-metal anode. In this work, lithium iodide (LiI) as a bifunctional additive was introduced into the poly(ethylene oxide) (PEO)-based electrolyte to improve the ionic conductivity and to construct a stable interphase at the Li/PEO interface. I− anions offer a strong electrostatic interaction with hydrogen atoms on PEO chains (HPEO) and forming massive I–H bonds that cross-link PEO chains, decrease crystallinity of PEO, and thus improve Li+ interchain transport. In addition, LiI participates in the formation of an inorganic-rich interphase layer, which decreases the energy barrier of Li+ transport across the interface and thus inhibits the growth of lithium dendrites. As a result, the composite PEO electrolyte with 2 wt% LiI (PEO-2LiI) presents a very high ionic conductivity of 2.1 × 10−4 S cm−1 and a critical current density of 2.0 mA cm−2 at 45 °C. Li symmetric cell with this PEO-2LiI electrolyte exhibits a long-term cyclability over 600 h at 0.2 mA cm−2. Furthermore, solid-state LiFePO4 and LiNi0.8Mn0.1Co0.1O2 batteries with the PEO-2LiI electrolyte show an impressive electrochemical performance with outstanding cycling stability and rate capability at 45 °C.