Recently, with increasing the supplement of portable device like smart phone and growth of electrical vehicle market, the needs for high energy density battery has continuously increased. However, commercialized lithium ion battery (LIB), which is composed of graphite anode and transition metal oxide as cathode, cannot meet the demands for high energy battery due to the limited theoretical capacity. To overcome this capacity limitation, next generation battery such as lithium-sulfur (Li-S) and lithium-air (Li-air) battery has been studied because of its high energy density..
Among the next generation batteries, Li-S battery is very promising candidate for beyond LIBs. Its high theoretical capacity (1,675 mAh $g^{-1}$) facilitate the development of high energy density (2,500 Wh $kg^{-1}$) battery for electrical vehicles. In addition, low price of active materials and non-toxic nature give big merits for commercialization of Li-S battery. However, even though there are many advantages, Li-S battery has not been commercialized yet due to the different chemistry compared to conventional Li ion battery. During redox reaction, elemental sulfur ($S_8$) transformed to soluble polysulfides (PSs) and it deposited again on carbon surface at the end of discharge. Because $Li_2S$ is insulator, the fast carbon passivation decrease the sulfur utilization, leading to low discharge capacity. Therefore, the suppression of carbon passivation is a big challenge for high performance in Li-S battery. To address this issue, redox mediator [ref] for discharge was reported to induce three-dimensional $Li_2S$ deposition. It dramatically enhanced the sulfur utilization by suppressing the carbon passivation by $Li_2S$. In addition, few studies for improving the sulfur utilization has been reported recently. They commonly suggest that 3D growth of $Li_2S$ plays key role for high performance Li-S battery. However, the factors for controlling the deposition morphology of $Li_2S$ has not been clearly understood.
To alleviate the issues from the low sulfur utilization, in this work, various factors for controlling $Li_2S$ deposition were investigated by the electrode and electrolyte modification. First, in chapter 2, to investigate the particle size effect of porous carbon affecting the sulfur utilization, zeolite-templated carbon was used. Because of its unique properties that particle size can be tuned without any change of other structural properties, it clearly demonstrated that decreased Li ion diffusion is responsible for low sulfur utilization. Moreover, by decreasing particle size, the uniform $Li_2S$ deposition was obtained, leading to dramatic increase of sulfur utilization.
Chapter 3 studied the effect of salt anion on three-dimensional $Li_2S$ growth. By changing the salt with high donor number (DN), the solubility of $Li_2S$ increased, triggering 3D growth of $Li_2S$. It can be explained the competition between salt anion and $S^{2-}$. The strong interaction of anion with Li ion weaken the interaction between Li ion and $S^{2-}$, leading to increase of solubility of $Li_2S$. Because of the effectively suppressed carbon passivation, almost theoretical capacity can be obtained. However, the salt with high DN is reactive with Li metal, leading to poor cycle performance. To alleviate this issue, chapter 4 deals with the control of $Li_2S$ in highly concentrated electrolyte. Even though a highly concentrated electrolyte has been considered as a promising approach, it cannot be directly applicable to Li-S battery due to the limited PS solubility. For suppression of carbon passivation and Li metal stabilization, 3 M dual-salt (2 M LiFSI and 1 M LiTf) was introduced. As a results, it greatly enhanced Li metal stability, as well as effectively suppress the carbon passivation because of the synergetic effect of salt combination. Furthermore, we found that the balancing between the solubility of $Li_2S$ and ionic conductivity plays key role le for 3D growth of $Li_2S$. In this work, we will discussed in detail the design factors for controlling $Li_2S$ growth for high performance Li-S battery. The understanding about these factors provide the new concepts for high performance Li-S battery.
