Improvement of Sulfur Cathode Reversibility by Specific Chemical Lithium Pre-doping Method

1.Intoroduction Recently, lithium-ion batteries (LIBs) have been required in terms of a high energy density for large-scale applications such as power supplies for electric vehicles. Lithium-sulfur (Li-S) batteries are, therefore, expected as next generation batteries because of their high energy density. Sulfur is utilized as cathode material for Li-S batteries because of a high specific capacity. However, sulfur has a low electric conductivity and a risk of dissolution into an electrolyte during a charge-discharge process [1]. L. Nazar et al. applied activated carbon as substrate for sulfur to help electric conduction and prevent dissolution of sulfur [2]. The activated carbon allows Li-S batteries to show high specific capacity with high reversibility. Even if these problems are solved, however, Li-S batteries have a further problem of excessive initial irreversible capacity during the first discharge process. Li pre-doping is a useful technique to cancel the large irreversible capacity in actual LIBs. Abe et al. reported a chemical lithium pre-doping method for a graphite using lithium naphthalenide [3]. Studies have been reported on the use of such pre-doping to improve the performance of Li-S batteries, for instance, with a sulfur-Ketjenblack composite cathode [4]. In our previous study, we have applied that method to a sulfur cathode composed of microporous activated carbon and sulfur before cell assembly and developed a Li 2 S cathode that suppressed the initial irreversible capacity during the first discharge process. However, the Li 2 S cathode showed an unignorable initial irreversible capacity and poor cycle life [5]. This work attempts to improve the reversible capacity of the Li 2 S cathode by a specialized chemical lithium pre-doping method. Our report would lead to the proposal of novel cathode and anode options in Li-S batteries. Method 2-1. Fabrication of lithium naphthalenide solution As Li metal and Li 2 S are both sensitive to moisture in air, all the following synthesis processes were carried out in an Ar-filled glove box. Lithium naphthalenide, a dark green solution was prepared by mixing an equal mol amount of Li metal with naphthalene in a cyclic ether solvent. 2-2. Fabrication of Li 2 S-AC cathodes Each sulfur cathode was prepared by mixing the S-AC (a composite of microporous activated carbon and sulfur), acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 93: 3: 2: 2 and coating