Effect of Positive Active Material De-Agglomeration on 350Wh/Kg Class High Energy Density Lithium Sulfur Laminated Batteries
Authors: Mikuriya Hitoshi , Nara Hiroki , Momma Toshiyuki 
Published: December 2024

In recent years, lithium-sulfur (Li-S) batteries have been widely developed as next generation secondary batteries for aircraft applications such as drones and HAPS (High Altitude Platform Station), which are requires high gravimetric energy density. In order to achieve the cell design of 350 Wh/kg class Li-S batteries, we should focus on these three steps.
First, we need to develop elemental and process technologies to achieve the capacity close to the theoretical sulfur capacity. Second, we need to use an electrolyte that is sparsely soluble or insoluble in polysulfide. Thirdly, we have to minimize the components other than the active material. The cathode is conventionally composed sulfur in Ketjen black (KB) on aluminum foil current collector. However, this limits the capacity of about 1250 mAh/g. Additionally, increasing the thickness of the cathode is an effective method for minimizing the number of cathode substrates, however, it is difficult to balance the high sulfur loading amounts and the high-rate performances, due to a difficulty to make adequate Li ion and electron paths.1) Micro-porous carbon has been reported as a supporting material that can exhibit a capacity close to the theoretical sulfur capacity2), In this study, we focused on activated carbon (AC) manufactured by Kuraray, which is commercially available and has a proven track record of mass synthesis. We used it in combination with ultra-thin aluminum sheet, successfully achieving a 350 Wh/kg class Li-S laminated battery.
S/AC composite was obtained by mixing sulfur and AC (YP-80F, Kuraray) with weight ratio of 60 : 40, followed by heating at 155 °C for 12 h. Afterward the of S/AC were de-agglomerated by using the energy ball milling (MM500nano, Retsch). S/AC cathode was fabricated by coating S/AC slurry (S/AC: carbon nanofiber (CNF) : binder = 92.5 :4 :3.5 by weight) on Al sheet. To assemble the cell, the cathode (S loading: 4.7 mg/cm2) was pressed to reduce its thickness by 34%. 5 Ah class laminated cell was prepared by stacking 10 sheets of the S/AC cathodes, 11 sheets of Li anodes, 20 sheets of a separator, and the electrolyte lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/lithium bis(fluorosulfonyl)imide (LiFSI) : sulfolane (SL) : hydrofluoroether (HFE) = 0.8/0.2: 1: 1.7 (molar ratio)). Electrochemical performances were tested with voltage range between 1.0 V – 3.3 V after activation process.
Focusing on the de-agglomeration of the positive electrode active material, we investigated a dispersion process of S/AC composite to achieve a capacity close to the theoretical sulfur capacity. After preparing S/AC composite, the A/AC was de-agglomerated using the ball milling. Figure 1 shows the comparison of the capacity of dispersion phase A and B at each cycle. It was found that the dispersion treatment increases the initial capacity close to the theoretical capacity of 1672 mAh/g. Also, the dispersion treatment (phase B) increased the capacities up to the 15th cycle, compared with those without the treatment (phase A). This suggest that the well dispersed S/ACimproved the diffusion of Li ions.
Furthermore, in order to increase the energy density of the battery, we adopted an ultra-thin Al current collector of 7 μm in thickness instead of the conventional 20 μm thick Al sheet, and using the above dispersion process, we fabricated a large 70 mm x 70mm Li-S laminated battery.
Fig.2 shows its charge-discharge curve of a new designed Li-S battery. The first plateau, which is generally observed around 2.3 V in a conventional S/KB system, is not confirmed. This is a feature of micro-porous carbon system. The Li-S battery delivered a discharge capacity of 6.7 Ah, which is equal to 1632 mAh/g-S.
Fig. 3 shows a comparison of battery weights by employing KB and AC. The capacity of S/KB is approximately 1150 mAh/g-S, which is smaller than that of the AC (1632 mAh/g-S). Therefore, the loading amount of the active material can be reduced by employing the AC instead of KB, resulting in reduction of the carbon constituting the complex at the same time. Consequently, a laminate-type battery with high capacity of 6.7 Ah, and energy density (351 Wh/kg, laminate film and tabs are excluded in this calculation) was achieved.
In the presentation, I would like to discuss the effect of de-agglomeration of the S/AC composite.

[References]

[1]. H. Nara, et.al., J. Electrochem. Soc., 164, A5026–A5030 (2017).
[2] S. Usuki, et. al., J. Electrochem. Soc., 85, 650–655 (2017).

[Acknowledgement]

This work was partly supported by Advanced Low Carbon Technology Research, Development Program Special Priority Research Area “Next-Generation Rechargeable Battery” (ALCA-Spring) from the Japan Science and Technology Agency (JST) (Grant Number JPMJAL1301).

Figure 1