The in situ impedance spectroscopy was used to study the interface behavior of the LiFePO4 cathode with IL and compared to the standard electrolyte. It was found that interfacial “Ri” decreased when LiFePO4–FePO4 phases coexist (90%>DoD>10%). On the other hand, the diffusion resistance “Rd” was found higher when the iron phosphate was present in the monophase state; LiFePO4 (0% DoD) or FePO4 (100% DoD).
When a small amount of polymer (1%) is mixed with IL, the interfacial resistance improves by forming a stable SEI at the electrodes. By increasing the polymer content at 5%, Ri and Rd increase, independently of the state of discharge. In all cases, Li-cells have shown a good cycling resulting from the stability of the permeable (to Li+ ions) layer formed at the electrode surfaces. For the full Li-ion cell, a low first-coulombic efficiency was found at 68.4%, related to the low first CE of the graphite anode. The rate capability is good until 2 C rate, and above this rate the capacity drops to 54% of the rated capacity. This behavior is probably associated with the high population of ions causing some ion pairing [52, 53] which limits their freedom of mobility and prevents them from the full contribution to conduction in the battery. Further improvement should be focused to improve the CE of the anode side and technical optimization is needed; such as pores control in the electrodes and the separator, improving the wetting process in the electrodes, and the attainment of high rate performance before this technology is considered for commercialization.
Acknowledgment This work was financially supported by Hydro-Québec.
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