Lithium doped N,N-dimethyl pyrrolidinium tetrafluoroborate organic ionic plastic crystal electrolytes for solid state lithium batteries

The organic ionic plastic crystal material N,N-dimethyl pyrrolidinium tetrafluoroborate ([C1mpyr][BF4]) has been mixed with LiBF4 from 0 to 8 wt% and shown to exhibit enhanced ionic conductivity, especially in the higher temperature plastic crystal phases (phases II and I). The materials retain their solid state well above 100 [degree]C with the melt not being observed up to 300 [degree]C. Interestingly the conductivity enhancement is highest with the lowest level of LiBF4 addition in phase, but then the order of enhancement is reversed in phase I. In all cases, a conductivity drop is observed at the II [rightward arrow] I phase transition (105 [degree]C) which is associated with increased order in the pure matrix, as previously reported, although the conductivity drop is least for the highest LiBF4 amount (8 wt%). The 8 wt% sample displays different conductivity behaviours compared to the lower LiBF4 concentrations, with a sharp increase above 50 [degree]C, which is apparently not related to the formation of an amorphous phase, based on XRD data up to 120 [degree]C. Symmetric cells, Li/OIPC/Li, were prepared and cycled at 50 [degree]C and showed evidence of significant preconditioning with continued cycling, leading to a lower over-potential and a concomitant decrease in the cell resistivity as measured by EIS. An SEM investigation of the Li/OIPC interfaces before and after cycling suggested significant grain refinement was responsible for the decrease in cell resistance upon cycling, possibly as a result of an increased grain boundary phase.