## 1 Introduction  
The demand for sustainable energy storage is becoming more important owing to the emergence of applications such as electric vehicles and portable electronic devices [1]. Among the various types of rechargeable batteries, lithium-sulfur (Li-S) batteries have been considered one of most promising next-generation storage systems. Sulfur (S) has a theoretical specific capacity that is 3-5 times higher (1672 mAh g\({}^{-1}\)) than that of commercialized Li battery cathode materials such as transition metal oxides and iron phosphates [2], and the resultant Li-S battery yields very high energy density (2567 Wh kg\({}^{-1}\)). However, there are still several major issues facing Li-S batteries that impede their practical use, namely: (a) The low electronic conductivity of S (5 \(\times\) 10\({}^{-30}\) S cm\({}^{-1}\) at 25 \({}^{\circ}\)C) significantly increases the overpotential and the insolubility of the discharge products (e.g., Li\({}_{2}\)S\({}_{2}\)/Li\({}_{2}\)S), leading to low utilization of S. This results in a reduction of the practical specific capacity to far less than the theoretical capacity [3]. This necessitates the use of carbon as a conducting additive in the sulfur cathode of Li-S batteries. (b) Soluble lithium polysulfide (PS, S\({}_{4-8}\)\({}^{2-}\)) intermediates formed during the cycling process can diffuse in the organic liquid electrolytes, which causes the PS shuttle phenomenon [3,4]. This phenomenon results in active mass loss from the cathode, the reduction of coulombic efficiency, and capacity decay upon cycling. To overcome these limitations, some strategies such as physical confinement (sulfur-carbon composites), chemical adsorption (N and/or S doping) [5], electrolyte design [6; 7], and separator design [4] have already been proposed. Among the physical confinement methods, one approach is to incorporate sulfur into nano-sized hosts such as mesoporous carbon. Recently, extensive research has been carried out on composites of sulfur and carbon matrix technologies such as carbon nano-tubes [8], graphene [9], and activated carbon [10] due to their excellent accessibility, producibility, electrochemical inertness, and wide temperature range. However, the production of these carbon matrices is usually very complicated and expensive [11]. Therefore, we attempted to reduce the time and cost by shortening the carbonization process. Calcium citrate is considered one of the most attractive carbon sources since a simple and effective carbonization method was reported for preparing porous carbon without any activation process, using calcium citrate as carbon source [11].  
To make low-cost, high-efficiency Li-S batteries, carbon materials were made and tested using calcium citrate [12]. However, since calcium citrate alone cannot help with electrical conductivity, we improved the electrical conductivity through N-doping [13]. Thiourea (TU) is a very attractive substance used for N doping. It has the chemical formula CH\({}_{4}\)N\({}_{2}\)S and is a planar molecule [14]. It has nitrogen elements and sulfur elements that ensure high electrical conductivity when combined with sulfur. Several studies have shown that doping a carbon material with nitrogen, sulfur, and phosphorous can enhance both electrochemical activity and electrical conductivity [15]. N-doped carbons support the interaction of sulfur atoms with the carbon matrix [16]. Sulfur doping can also develop the affinity for polysulfides in the carbon matrix for better cycling performance [17]. The combined force of the sulfur is very high, producing a good adsorption effect to control the polysulfide during charge and discharge processes [18]. In this experiment, we synthesized calcium citrate and sulfur to conduct a charging-discharging test and compared them by adding thiourea and sulfur in the calcium citrate.