Universal Relationship between Limiting Current and Electrochemical Transport Properties in Malonate-Based Polymer Electrolytes

There is considerable interest in developing high-performance electrolytes for rechargeable lithium batteries. For practical applications, the electrolyte must support large dc currents. However, the parameters most often reported in the literature, conductivity, κ, and current fraction, ρ +, reflect ion transport in the limit of infinitesimal currents. In this limit, the efficacy of an electrolyte is given by the product κρ + . The limiting current density, i lim, is the maximum current density that can be applied across an electrolyte; the cell voltage diverges if the applied current density exceeds i lim . This parameter reflects ion transport in the limit of large dc currents and is therefore of practical interest. It would therefore be convenient if i lim could be predicted from measurements of κρ + . In order to explore this possibility, we studied six malonate-based polymers and PEO at a fixed salt concentration ( r = 0.08) and temperature (90°C) using symmetric cells with planar electrodes. Unfortunately, there is no correlation between i lim and κρ + . When the applied current density, i, is less than i lim, the cell voltage approaches a stable plateau, ϕ plateau . We found a linear dependence between i and thickness-normalized plateau potential, ϕ plateau L –1, irrespective of the magnitude of the applied current. In all seven polymer electrolytes, we found a linear correlation between i lim and the slopes of these lines, σ. In other words, measurements of σ can be used to predict the limiting current.