Development of ultra-high-capacity lithium-sulfur battery electrolytes
Lithium-ion batteries have become essential parts of various industries from portable electronics to electric vehicles. Lithium-sulfur batteries have a theoretical energy density per active material about six to seven times higher than that of lithium-ion batteries, and sulfur, one of their raw materials, is cheap. Thus, lithium-sulfur batteries are gaining significant attention as the next-generation secondary battery that will replace lithium-ion batteries. However, during the operation of lithium-sulfur batteries, discharge byproducts, such as lithium sulfides, tend to accumulate on the surfaces of electrodes, blocking the transfer of electrons on the electrode surface. Thus, theoretical battery capacity cannot be realized in practice. To overcome this problem, this Center conducted research to develop a new-concept electrolyte by replacing the lithium salt anions used in the electrolyte of existing lithium-sulfur batteries with anion salts with high electron-donating ability.
Design of new electrolyte for lithium-sulfur batteries
Theoretically, lithium-sulfur batteries have high energy density. Also, they are cost-effective because low-cost raw materials, such as sulfur, are used. However, the electrochemical reaction of sulfur causes electrode passivation in which lithium sulfides, which are non-conductive, are formed as films on electrode surfaces. Therefore, electrode reversibility is limited during charging and discharging. This makes it impossible for the theoretical capacity of sulfur to be realized in practice, and the service life is reduced due to degradation of battery performance. To ease the problem of electrode passivation, an excessive amount of conductive agent was added to the electrodes, but the energy density of lithium-sulfur batteries happened to be greatly lowered, and the achieved capacity amounted to only up to 70% of the theoretical capacity.
To overcome this problem, this Center conducted research on developing a new-concept electrolyte by replacing the lithium salt anions used in the electrolyte of existing lithium-sulfur batteries with anion salts with high electron-donating ability. This electrolyte salt helped increase the solubility of lithium sulfides in the electrolyte and thus induce growth of lithium sulfide with a three-dimensional structure. Also, it effectively suppressed the occurrence of electrode passivation so that a high capacity could be realized in practice. Based on this electrolyte technology, the upper bound of existing lithium-sulfur battery technology was exceeded. More specifically, it was possible to achieve 92% of the theoretical capacity of a high-capacity sulfur electrode that has a capacity density per area (4mAh/cm2) equivalent to that of existing lithium-ion batteries.
Development of lithium-sulfur battery that can reach 92% of its theoretical capacity even after 100 cycles
The developed lithium-sulfur battery based on anion salts with high electron-donating ability not only suppressed the passivation of the cathode but also formed a stable passive film on the lithium-metal anode surface, thereby ensuring a stable service life, i.e., 92% of the theoretical capacity for 100 cycles. Some studies reported high utilization rates of more than 90% of theoretical capacity at the first cycle; however, at the next cycle, utilization rates decreased to 60-70%. The developed technology to control the structure of lithium sulfides through electrolyte design has high potential in industrial applications because it can be widely used for sulfur electrodes of different structures under various operation conditions. The major findings of the present study will set a milestone in the development of new lithium-sulfur batteries in that new physical and chemical principles to overcome the limitations of lithium-sulfur batteries are provided, and more than 90% of the theoretical capacity of lithium-sulfur batteries can be achieved in practice without any performance degradation, even after 100 cycles.
Prof. Kim, Heetak
2019 KI Annual Report