Nanosized LiFePO4 Reveals the Electrochemical Kinetics of Electrolytes in Porous Li-Ion Battery Electrodes

Abstract

Electrified transportation is critical for a decarbonized future, but high cost and range anxiety hamper the adoption of electric vehicles (EV). For low-cost EVs, extreme fast charging of lithium-ion batteries (LIB) is an ideal solution to range anxiety, but this requires significant improvements to rate performance. The measurement of electrochemical kinetics in LIBs is central to understanding and optimizing their rate performance. However, while LIB rate performance is typically limited by the electrolyte, the behaviors of electrolytes during high-rate (dis)charge remain elusive to electrochemical characterization. It is commonly assumed that the effects of ion transport in the electrolyte are negligible in sufficiently thin electrodes (e.g., ≤ 5 μm), allowing measurement of the intrinsic electrochemical kinetics of LIB active materials (AM). On the contrary, we suggest that when the AM is not rate limiting (i.e., with nanosized AMs), electrochemical characterization of LIB electrodes reveals only the kinetics of the electrolyte, making the AM kinetics impossible to measure. In this work, we use LiFePO4 (LFP) as a model system to investigate the fundamental electrochemical kinetics of porous battery electrodes in which the electrolyte is the sole rate limiting component.We first synthesize a nanosized, carbon-coated LFP active material (LFP-C) and fabricate electrodes with both a practical composition (95% AM) and extremely low electronic resistance. Electrochemical impedance spectroscopy shows that charge-transfer resistance is immeasurably small compared to pore resistance, while cyclic voltammetry proves that semi-infinite Li+ diffusion in the electrolyte is the rate limiting process. We develop a novel pseudo-steady-state extrapolation (S3E) technique for quantification of charge-transfer kinetics in porous battery electrodes, which shows that LFP obeys Butler-Volmer kinetics, even at a current density of > 100 mA cm-2. Unexpectedly, we find that the apparent transfer coefficient of LFP-C is ~ 1.5, rather than the commonly assumed value of 0.5. Furthermore, the remarkably low electrode-level exchange current (< 1 mA cm-2) suggests that only a small fraction of particles (~ 1%) are actively reacting, even at extreme rates. These results show that a moving reaction front resulting from rapid interfacial kinetics is the root cause of the widely observed electrode-level reaction heterogeneity of phase-separating AMs. Using LFP three-electrode cells, we conducted rate performance testing on the electrolyte, comparing the conventional LP57 electrolyte to highly conductive 2M LiFSI in acetonitrile (LiFSI-AN). LiFSI-AN achieves up to 70% charge capacity at a 600C rate and 35% discharge capacity at a 3000C rate, exceeding all other reports of LIB rate performance. A novel resistance vs. capacity analysis of the rate performance data enables direct measurement of electrolyte resistance growth, revealing how the complex interaction between a moving reaction front (i.e., increase in conduction path length) and concentration polarization (i.e., decrease in electrolyte conductivity) controls electrolyte rate performance in LFP electrodes.Overall, this work shows that the electrochemical kinetics of nanosized AMs in porous battery electrodes can only be understood by explicitly accounting for the behaviors of the electrolyte. Consequently, the common a priori assumption that the effects of the electrolyte are negligible for electrochemical kinetic measurements in thin electrodes is shown to be, in many cases, fundamentally misleading. Nanosized AMs are, therefore, a powerful tool to investigate the electrochemical kinetics of electrolytes in porous battery electrodes. Particularly, all-LFP three-electrode cells were developed as an ideal platform for rate performance testing of electrolytes, enabling direct measurement of electrolyte resistance growth during high-rate battery operation.