Enhancement of Charge Transfer Reaction at Electrode/Electrolyte Interface in Acetonitrile Solvent for Lithium-Ion Battery Cathode
Authors: Suzuki Tatsumi , Zhong Chengchao , Shimoda Keiji , Okazaki Ken-ichi , Orikasa Yuki 
Published: December 2024

High rate capability of Li-ion batteries for automotive battery applications is desired, which requires a reduction in internal resistance. Among the elementary reactions on Li-ion battery, the charge transfer resistance at electrode/electrolyte interface is known to be the rate-determined reaction1), which needs to be reduced to achieve the high-rate performance. The activation energy of the charge transfer at electrode/electrolyte interface depends on the donor number2) and the pre-exponential factor on the viscosity of electrolyte3). The use of acetonitrile (AN) solvent, which has relatively low viscosity and low donor number, as electrolyte solvent is expected to improve the high-rate performance. In this study, the effect of AN solvent on the charge transfer reaction at electrode/electrolyte interface is analyzed by electrochemical reaction analysis using the three-electrode cell with solid electrolyte, which enables AN to be used as the electrolyte solvent.

Composite electrodes with LiCoO2: polyvinylidene fluoride: acetylene black in weight ratio of 8:1:1 was used as the working electrode, and lithium metal was used as the counter electrode and the reference electrode. In order to use AN as the electrolyte solvent, which is highly reactive with lithium metal, NASICON-type solid electrolyte Li1+x+y
Al
x
(Ti2−y
Ge
y
)P3−z
Si
z
O12 was inserted between the working and counter electrodes, enabling the use of different liquid electrolytes on the working electrode side and the counter electrode side. Liquid electrolyte 1 M LiTFSI/Propylene Carbonate (PC) was used as the electrolyte on the counter electrode side, which is in contact with lithium metal, and three types of liquid electrolyte 1 M LiTFSI/AN, 1 M LiTFSI/PC and 1 M LiTFSI/ethylene carbonate: ethyl methyl carbonate (3:7 v/v%) (EC:EMC) were used as the electrolyte on the working electrode side, which is not in contact with lithium metal. After 10 hours from the cell preparation, two cycles of galvanostatic charge/discharge measurements at current rate of 0.1 C (cut off potential : 3.2 V-4.2 V vs. Li/Li+) were performed, followed by galvanostatic charge/discharge measurements at current rate of 0.1 C-0.5 C (cut off potential : 3.2 V-4.2 V vs. Li/Li+) and AC impedance measurements in measurement temperature range 268 K-298 K (measurement frequency :1 MHz-10 mHz, measurement potential : 4.0 V vs. Li/Li+, potential amplitude : 10 mV).

The galvanostatic charge/discharge tests showed that the cells with PC and EC:EMC both exhibited a charge/discharge capacity of approximately 60 mAh g-1 at current rate of 0.25 C. On the other hand, the cell with AN exhibited a high charge/discharge capacity of 105 mAh g-1, indicating that the cell with AN maintains better reversible capacity under high current density conditions. AC impedance analysis showed no significant difference in the activation energy of the charge transfer at electrode/electrolyte interface among the three cells, although the reduction in the interfacial charge transfer resistance due to the increase in the pre-exponential factor was observed in the cell with AN. This can be explained by the use of low viscosity acetonitrile solvent, which increases the frequency of reorientation of solvent molecules during the solvation/desolvation processes3). This study confirms the high-rate performance of the cell with AN due to the increase in the pre-exponential factor rather than the reduction in the activation energy of the charge transfer at electrode/electrolyte interface.

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