Enhanced Performance in Lithium–Polymer Batteries Using Surface-Functionalized Si Nanoparticle Anodes and Self-Assembled Block Copolymer Electrolytes

We have explored new Li–polymer batteries composed of surface functionalized Si nanoparticles (SiNPs) as anode active materials and nanostructured block copolymers as solid electrolytes. Surface protection of SiNPs with poly(ethylene oxide) chains successfully prevents aggregation of SiNPs during cycling and also helps fast Li+ transport to the active centers in the anodes. The self-assembly nature of block copolymer electrolytes in ca. 50 nm periodicity is aimed to restrain the formation of macroscopic ionic clusters during Li-insertion/desertion. To decouple the electrical and mechanical properties of polymer electrolytes, two different nonvolatile additives (ionic liquid and non ionic plasticizer) were incorporated and remarkably different cycle performances have been observed. The incorporation of ionic liquid yields the utmost ionic conductivity and distinctly large first lithium insertion capacity of 2380 mA h/g was seen. However, the formation of solid electrolyte interphase (SEI) was responsible for highly irreversible lithium desertion capacity and the system indicate fast capacity fading during cycling. With the use of non ionic plasticizer, in contrast, the SiNPs anode can store lithium up to a reversible capacity of ∼1850 mA h/g under aggressive test profiles of 80 °C and voltage window between 0–4.5 V. The focused ion beam technique was successfully used to obtain ex-situ transmission electron microscopy images of cycled polymer electrolytes and anode materials to underpin the origin of capacity retention or fading upon cycling. The results suggest that the structural retention of both polymer electrolytes and SiNPs during cycling attributes to the improved battery performance.