Direct Observation and Suppression Effect of Lithium Dendrite Growth for Polyphosphazene Based Polymer Electrolytes in Lithium Metal Cells

Dry solvent-free and gel polymer electrolytes were analyzed based on poly[bis(2-(2-methoxyethoxy) ethoxy) phosphazene] (=MEEP) with the salts LiBOB, LiPF6, and LiTFSI with regard to stability versus lithium deposition at lithium metal electrodes under dc current flow. Symmetrical cells were used with two lithium metal electrodes. The interfaces were monitored using direct optical microscopy and accompanying intermediate impedance measurements. The results were compared with measurements on a dry polymer electrolyte of dissolved LiTFSI in PEO (Li:EO -1:10). The PEO-based electrolyte and the saltin-MEEP electrolytes have shown a comparable ability of dendrite inhibition. The MEEP based gel polymer electrolytes containing similar to 50 wt% of a 1:1 mixture of EC/DMC, however, showed a much-enhanced ability of inhibition towards dendrite formation made evident by increased dendrite onset time (t0) and short-circuit time (ts) when observed in special visualization cells. This could be explained by an increased lithium ion conductivity, an increased lithium transference number and a lower interface resistance at the interface Li/MEEP gel polymer electrolyte. Among the three different salts investigated in the MEEP based polymer electrolytes, LiBOB and LiTFSI show much better stability at the lithium metal interface as compared to LiPF6 which hints to a more stable and conductive SEI at the Li/MEEP interface with dissolved LiBOB and LiTFSI. For MEEP/LiBOB polymer electrolytes, the dendrites grow directly towards the positive electrode with a fast velocity at the early stage which then decays with time in a later phase. This can be explained by the stress in electrolyte and the 'competitive growth' of dendrite tips.