Tuning electrolyte-electrode interphases for low-temperature Li-ion batteries

Abstract: Lithium ion batteries (LIBs) are crucial for modern electronics and electric vehicles (EV). However,their electrochemical performance is facing challenges at low temperatures (e.g ≤ 0 °C) due to reducedLi+ kinetics and increased charge-transfer resistance. Given the growing dependence on LIBs for bothelectronics and EVs, especially in cold environments, it is imperative to address the low-temperaturelimitations. Thus, improving the low-temperature performance of LIBs is essential for the broaderadoption and further advancement of LIBs. To address these challenges, this thesis demonstrates thatsignificant improvement of electrochemical performance at low temperatures can be achieved by in-corporating Lithium difluoro(oxalato)borate (LiDFOB) as an additive into the baseline electrolyte forthe Li(Ni0.8Mn0.1Co0.1)O2(NMC811)∥Li cell.At a low temperature of -20 °C, the NMC811∥Li cell with the electrolyte containing 4 wt% LiDFOBexhibited an impressive discharge capacity of 125 mAh/g at 0.1C (1C = 2.0 mAh cm−2), representingabout 61.6% of the capacity delivered at 20 °C. In contrast, the cell with the baseline electrolyte de-livered negligible discharge capacity under the same conditions. This result emphasizes the functionsof LiDFOB as an electrolyte additive in enhancing the low-temperature performance of NMC811∥Licells. This work reveals the kinetics bottleneck of Li+ transport during charge/discharge processes atlow temperatures can be mitigated by tuning cathode-electrolyte interphase (CEI) through introducingadditive into the baseline electrolyte.To substantiate these findings, Electrochemical Impedance Spectroscopy (EIS) was employed to re-veal the significant decrease of interface resistance resulting from the addition of LiDFOB into thebase electrolyte. X-ray Photoelectron Spectroscopy (XPS) further confirmed the benefits of LiDFOB,indicating that a B-rich, more conductive and thinner CEI formed on the NMC811 cathode induced byLiDFOB. The results indicate that the inclusion of LiDFOB in the baseline electrolyte is advantageousin tuning CEI at the cathode for reducing charge-transfer resistance and enhancing electrochemicalperformance.In conclusion, the tuned CEI induced by LiDFOB additive plays an important role in improving thelow-temperature performance of the NMC811∥Li cells. This improvement in the capacity delivery at-20 °C can be attributed to the formation of a highly conductive and uniform and thinner CEI layer,which in turn facilitates reduced charge-transfer resistance at low temperatures. This work sheds newlight on the electrolyte design with additives to develop high-performance LIBs operating at extremeconditions.2