Improvement in Energy and Power Density of Lithium Ion Battery Employing a Less Volatile Electrolyte Sheet
Authors: Nishimura Katsunori , Kawaji Jun 
Published: December 2020

Introduction
Lithium-ion batteries (LIBs) are now used in the variety of fields including vehicles, stationary energy storage, and portable devices. Since the conventional LIBs consist of anode, cathode and volatile and flammable electrolyte liquid, they potentially have a risk on safety relevant to the electrolyte, e.g. ignition of electrolyte leaked from the mechanically crashed or short-circuited batteries. To overcome the safety issue, we recently developed a novel electrolyte with a volatilization temperature higher than 100 °C, and incorporated the electrolyte into the cells consisting of Si based anode and Ni-Mn-Co (NMC) oxide cathode for realizing the high energy density of 300 Wh/kg[1]. In this research, we employed a thin and less volatile electrolyte sheets as a separator to improve power density of the high energy density cell.
Experimental
Solvated ionic liquid, Li(G4)TFSI, was prepared by mixing an equimolar of lithium bis(trifluoromethanesulfonyl)imide(LiTFSI) and tetraglyme(G4) [2]. Mixture of the solvated ionic liquid and silica nanoparticle was incorporated in cellulose-based porous fiber sheets, and the composite sheets of 20 μm in thickness were used as separators. Less volatile electrolyte was composed of Li(G4)TFSI, propylene carbonate as low-viscos solvent to enhance lithium ion conductivity [3] and a small amount of vinylene carbonate as electrolyte additive. Test cells were fabricated by adding the less volatile electrolyte to a stack of Si-graphite anode, the composite sheet and a Ni-Mn-Co (NMC) oxide cathode. The apparent area of the both electrodes was 2 cm2, and the designed energy amount was ca. 50 mWh.
Charge-discharge tests were conducted for evaluating initial capacities and energy densities of the cells. Constant current charge was applied for the cell from the open circuit voltage to 4.2V, while constant voltage charge denoted as CV was successively conducted until the current decreased to 0.005 mA/cm2. The fully-charged cell was discharged at 0.1 CA to 2 V. Capacity was obtained from discharge curve where from 4.2 V to 2 V at the discharge current of 0.1 CA. Maximum power density was estimated by multiplication of the discharge cutoff voltage (2 V) and the maximum current which was obtained from extrapolation of current-voltage relationship in discharge for 10 s.
Energy- and power density were estimated under the assumption that the anode, cathode and electrolyte sheet used in the test cell was applied to the large-formatted laminated cell for ca 100 Wh.
Results and discussions
Figure 1 shows a scanning electron micrograph (SEM) image of surface morphology of the composite sheet. The solvent ionic liquid and SiO2 mixture denoted as (a) were seen in the porous fiber sheet and was entangled by the fibers (b). Figure 2 shows charge- and discharge curves of a cell using the less volatile electrolyte and the composite sheet at the first cycle. The charge capacity, the discharge capacity and the initial coulombic efficiency were obtained to be 5.0 mAh/cm2, 3.85 mAh/cm2 and 77%, respectively. The authors designed laminated lithium ion battery of 90 Wh and estimated its energy density of 300 Wh/kg.
Power density of the cell of Fig. 2 was evaluated from data of the current-voltage relationship during discharge after 10 s. A cell with the conventional separator was tested as comparison. The maximum current densities of those cells were 120 mA/cm2 (open circles) and 90 mA/cm2 (closed circles) by extrapolation of the voltage to 2.0 V. Power density of the laminated cell applying the same cell design of Fig. 2 was 2,771 W/kg, whereas that of the battery using the commercial separator was decreased to 2,086 W/kg. Ionic conductivity of the composite sheet was higher than that of the commercial separator, which probably attributes to increase in power density.
Acknowledgement
This work was financially supported by Innovative Science and Technology Initiative for Security, ATLA, Japan.
References
[1] A. Unemoto el al., Abstract for 236th ECS meeting 322 (2019).
[2] K. Yoshida et al., J. Am. Chem. Soc., 133, 13121 (2011).
[3] A. Unemoto et al., Electrochemistry, 87(1), 100 (2019).

Figure 1