Electrochemical Properties of Lif-Coated Graphite Negative Electrode Modelized By Atomic Layer Deposition

Introduction In recent years, carbon dioxide emissions are rapidly increasing, and electric vehicles (EVs) are expected to play a significant role towards achieving a carbon-neutral society with most of them utilizing Lithium-Ion Batteries (LIBs). The negative electrode active material of LIB is typically made of graphite, and during initial charging, the electrolyte undergoes reductive decomposition at the graphite/electrolyte interface, forming a Solid-Electrolyte Interphase (SEI). SEI exhibits lithium-ion conductivity while being electronically insulating, facilitating highly efficient charging and discharging by suppressing further electrolyte decomposition. On the other hand, the formation of SEI consumes lithium ions, resulting in irreversible capacity loss [1] . Therefore, the nature of SEI has a significant impact on LIB performance. Regarding the composition of SEI, it is known to comprise clusters of inorganic compounds such as LiF, Li 2 CO 3 , Li 2 O and organic decomposition products like polyolefins. Each component has different properties such as LiF was reported to have lower lithium-ion diffusion barrier and higher chemical stability than other inorganic components [2] . However, many aspects of the individual electrochemical roles are still unclear, and forming an excellent SEI is an important issue. Objective In this work, we focused on LiF, which is considered to be a crucial component in SEI [3] , and modelized the LiF-coated graphite by employing atomic layer deposition (ALD) on highly oriented pyrolytic graphite (HOPG). We investigated the influence of LiF on the lithium-ion transfer reaction behavior at the graphite/electrolyte interface. Experimental Highly oriented pyrolytic graphite (HOPG) served as the graphite negative electrode, and atomic layer deposition (ALD) was used for the LiF coating. Regarding ALD, LiN(Si(CH 3 ) 3 ) 2 and WF 6 were used as the Li and F sources, respectively. The deposition temperature was set at 300 °C, and HOPG was deposited at various numbers of cycles (10, 30, 60, and 90 times). X-ray photoelectron spectroscopy (XPS) was performed to analyze the electrode after the ALD coating. Regarding electrochemical measurements, the setup utilized a three-electrode cell with HOPG as the working electrode, lithium metal as the counter and the reference electrodes. 1.0 mol dm −3 LiClO 4 /EC+DEC (1:1 vol.%) was used as the electrolyte. Cyclic voltammetry (CV) was performed at a scanning rate of 0.1 mV s −1 and a scanning range of 0.005 – 3 V (vs Li + /Li). Electrochemical impedance spectroscopy (EIS) was performed at an applied AC voltage of 5 mV and a frequency range of 100 kHz – 10 mHz. EIS measurement was also performed at different temperature with a holding potential of 0.2 V to obtain activation energy. Results and discussion For the HOPG electrode coated with LiF, XPS analysis was performed and it was confirmed that the LiF coating layer was thickening as the number of coating cycle increased. In the CV measurement results, redox peaks were observed in the range of 0 – 0.6 V, confirming that the intercalation-deintercalation reaction of lithium ion was progressing. From the Nyquist plot obtained by EIS measurements, semicircles derived from SEI were observed in the high frequency region, and semicircles derived from charge transfer resistance were observed in the medium and low frequency regions. The activation energy of the charge transfer resistance was calculated by fitting the Nyquist plot (Figure 1) obtained from the temperature-dependent measurement of EIS, and revealed that the values were 54.8 kJ mol −1 for the uncoated HOPG electrode, 55.1 kJ mol −1 for 10 coatings, and 52.8 kJ mol −1 for 30 coatings, respectively, showing no significant change. On the other hand, the activation energy slightly decreased with increasing number of depositions, which was found to be 49.7 kJ mol −1 and 50.0 kJ mol −1 in the case of 60 and 90 coatings, respectively. This suggests that when the LiF coating layer reaches a certain thickness, lithium-ion charge transfer between the electrolyte and the electrode becomes easier. Details will be discussed in the conference. References [1] Y. Tateyama, J. Comput. Chem. Jpn. , 18 (2019) 18-28. [2] M. Chen, J. Zheng, Y. Liu, O. Sheng, Z. Ju, G. Lu, T. Liu, Y. Wang, J. Nai, Q. Wang, X. Tao, Adv. Funct. Mater. , 31 , 2102228 (2021). [3] Y. Liu, X. Tao, Y. Wang, C. Jiang, C. Ma, O. Sheng, G. Lu, X.W. Lou, Science. , 375 (2022) 739-745. Figure 1