Toward Optimization of Polymer Electrolytes by Electrochemical Characterization— Poly(pentyl malonate) Versus Poly(ethylene oxide)

Demand for high-performance rechargeable batteries continues to grow as electric vehicle production increases. The energy density of lithium-ion batteries, used in this application, has not significantly improved in the past decade. Electrodes such as silicon and lithium metal have the potential to increase energy density, but electrolytes that are stable against these electrodes are not yet been identified. Polymer electrolytes have the potential to stabilize these electrodes. Lithium salts dissolved in poly(ethylene oxide) (PEO) were first hypothesized for battery electrolyte applications in 1979 by M. Armand. It is not clear if any of the polymer electrolytes developed in subsequent years is better for battery applications.<br/>For practical applications, electrolytes must support large currents. The limiting current density, <i>i</i><i>_lim</i>, is the maximum current density which can be stably applied across an electrolyte. Salt concentration gradients develop when current passes through an electrolyte, resulting in high concentration near the positive electrode and low concentration near the negative electrode. At the limiting current, either the concentration at the negative electrode approaches zero, or the concentration at the positive electrode approaches the solubility limit. The limiting current is a convenient metric for comparing different electrolytes. To our knowledge, there is no polymer electrolyte present in the published literatures that exhibits a higher limiting current than PEO. Another metric for comparing different electrolytes is the potential drop needed to sustain the limiting current. An ideal electrolyte will exhibit the lowest potential drop and the highest limiting current.<br/>The limiting current and potential drop through an electrolyte can be predicted by Newman’s concentrated solution theory. Applying this theory requires knowledge of four concentration-dependent parameters. In these previous studies, it was shown that the limiting current can be predicted using conductivity, <i>κ</i>, salt diffusion coefficient, salt diffusion coefficient, <i>D</i>, current fraction measured in Bruce-Vincent experiment, <i> ρ</i>₊, and the open circuit potential of concentration cells, <i>U</i>. We present values for these four parameters for poly(pentyl malonate) (PPM) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte. The predicted values of limiting current and potential drop are in good agreement with experimental measurements, which were conducted in symmetric cells using lithium-indium alloy electrodes. The maximum limiting current value of PPM/LiTFSI is about 1.7 times higher than that of PEO/LiTFSI, and the potential drop at the maximum limiting current density through PPM/LiTFSI electrolyte is about 1.6 times smaller than that of PEO/LiTFSI. We present a new plot for comparing the performance of different electrolytes.