300Wh/Kg Class High-Energy Li-S Laminated Batteries Using High-Sulfur Loading Cathode on 3D Mesh Structured Aluminum Foam

Abstract

[Introduction] In recent years, lithium-sulfur batteries can be expected as a promising candidate for next generation energy storage devices because of its high energy density. Generally, the cathode is fabricated by infiltration of sulfur into Ketjen black (KB) with aluminum foil current collector. However, it is not easy to increase the sulfur loading amounts and maintain the rate performances for electric vehicles which are required more than 3C rate, due to the lack of Li ion and electron paths. To solve this issue, we have reported the new type sulfur electrode prepared using 3D mesh structured Al foam, resulting in increased sulfur loading amounts which are 5 - 10 times or more compared to conventional sulfur cathode fabricated using 2D structured aluminum foil current collector [1], [2], [3]. In this study, we optimized the fabrication method of sulfur cathode for high sulfur loading on 3D mesh structured Al foam for a high energy density Li-S laminated battery.[Experimental] S/KB composite was obtained by mixing sulfur and KB with weight ratio of 6:4, by heating at 155 °C for 12 h under N2 atmosphere. Afterward obtained S/KB composite was dispersed by ball milling for 12h. S/KB cathode was fabricated by S/KB slurry (weight ratio of S/KB (6/4) : Binder = 96.5 : 3.5). The high sulfur loaded cathode (15.3 mg/cm2) was obtained using 3D mesh structured Al foam (1mm in thickness). To assemble the cell, the cathode was pressed (0.4 mm in thickness). 5 Ah class laminated cell (70 x 70 mm) was prepared using the S/KB cathodes (6 stacked), Li anodes (7 stacked), separators (7 stacked), and the electrolyte (LiTFSI: G1(monoglyme)/G3(triglyme): HFE(hydrofluoroether)= 1: 1: 4 (molar ratio)). Electrochemical performances were tested with voltage range between 1.0 V – 3.3 V after activation process.In addition, coin cells were prepared using above materials to evaluate the ionic conductivity by DC polarization method.[Results and Discussion] To design a high energy density cell such as laminated type cell, it is important to optimize the ratio between using amount of electrolyte and solid materials in electrode. Fig. 1 shows the relationship between the calculated E/S ratio (electrolyte amount (µℓ)/sulfur loading amounts (mg)), porosity filling rate of electrolyte (%), and the gravimetric energy density (Wh/kg). It was possible to reduce the electrolyte filling rate to 100% which is corresponding to E/S = 1.7, achieving the 340 Wh/kg theoretically. It is important to secure an ion path in whole electrode to optimize utilization of E/S ratio. In our previous report, we fabricated the S/KB cathode using 3D mesh structured Al foam regardless of the porosity, implying that it has bad ion conductivity. To improve the ion conductivity of S/KB cathode which has high sulfur loading amounts, we prepared a cathode that loaded S/KB very uniformly and retained the high porosity of the 3D mesh structured Al foam.As a result, we confirmed that the S/KB cathode/3D Al mesh which has good porosity shows improved ionic conductivity than control sample fabricated by conventional method.Fig. 2 shows the charge/discharge curve of Li-S laminated battery prepared by a new method with optimized E/S ratio of 2.3 (conventional method requires high E/S ratio of 3.6 or more). High capacity of 5.2 Ah, and energy density (301 Wh/kg and 435 Wh/ℓ, excluding laminate and tab) was achieved with high sulfur loaded laminated cell (sulfur loading amount of 15.4 mg/cm2, 1156 mAh/g-S).