Study of Lithium Ion Battery Manufacturing Process Using Quasi-Solid Electrolyte Sheet

Abstract

IntroductionIn conventional lithium ion battery (LIB), organic electrolyte exhibiting a high volatility and flammability is applied. Therefore, energy storage system with the LIB possesses a lot of auxiliary components for ensuring its safety, which do not directory contribute to electricity storage and lower its volumetric and gravitic energy density. Hence, the high safety of the LIB is the key to increase the volume energy density and the LIBs using several types of electrolytes such as a gel [1], a solid-state [2] and a quasi-solid-state [3] have been investigated. In particular, 100 Wh-class LIB using quasi-solid-state electrolyte (QSE) as a highly safe electrolyte demonstrated incombustibility in the nail-penetration test [3]. QSE is prepared by pseudo solidifying of solvate ionic liquid which consists of an equimolar mixture of lithium salt and tetraglyme(G4) at oxide particle surfaces. In this research, LIB manufacturing process using QSE that is highly compatible with conventional manufacturing processes was examined.Experiment Solvate ionic liquid, Li(G4)TFSA, was prepared by mixing an equimolar of lithium bis(trifluoromethanesulfonyl)amide, LiTFSA, and G4 [4]. Li(G4)TFSA, silica nano particle, and fluorine-based binder were mixed in a prescribed ratio. QSE sheet was prepared by coating and drying the mixture on the substrate. Li(G4)TFSA has a refractory characteristics. To prepare electrode containing Li(G4)TFSA, the electrode slurry including Li(G4)TFSA was coated and dried on the current collector foil. Li(Ni,Co,Mn)O2 pseudo-ternary oxide and graphite were used as the active material of the positive and negative electrode, respectively. The volatile mixed solution of propylene carbonate (PC) as low viscosity solvent [1] and vinylene carbonate (VC) for suppress the SEI formation was directly coated on the surface of the pressed electrode (coating method). In addition, QSE sheets and the electrode were stacked and heat-sealed by the four sides in order to suppress the volatilization of additive mixed solution (edge sealing structure). NMR analysis of the electrolyte components extracted by immersing in deuterated chloroform solvent after standing under manufacturing environment ( dew point : about -35℃) at various time ( for 0 to 24 hours) was carried out to examine the volatile behavior of PC and VC. Moreover, charge-discharge performance of LIB using QSE was evaluated by a model cell (diameter of electrode was less than 18 mm).Result and discussion In the previous research [5], the same charge-discharge performance as one of the conventional injection method was obtained by the coating method in which a small amount (5ml/cm2) of PC and VC mixed solution was directly coated on the electrode. It indicated that the injection process which had been a bottleneck in the LIB manufacturing process could be omitted. In order to apply the coating method to the mass production process, it is necessary to suppress concentration fluctuations of PC and VC by volatilization. In particular, the vapor pressure of VC is higher than the other electrolyte components. VC is an important component for SEI formation at the negative electrode interface. Concentration fluctuation of VC can be a factor for lowering LIB performance. Therefore, the edge sealing structure as shown in Figure 1 was prepared to seal the additive mixed solution. Figure 2 shows the relation between VC residual ratio and time. In the reference sample without applying the edge sealing structure, VC vapored quickly and the residual ratio decreased to about 60% after 30 minutes. On the other hands, VC residual ratio remained 93% even after 24 hours in the edge sealing structure, which proves the edge sealing can suppress VC volatilization significantly. Figure 3 shows the charge-discharge performance of LIB using QSE with the edge sealing structure. The charge-discharge performance of 24hours after coating the additive mixed solution did not deteriorate compared to the performance of immediately after coating. In this presentation, details of the manufacturing process using the coating method and the edge sealing structure will be discussed.