China team reports high-rate, high-capacity, long lifecycle Li-sulfur cell using nitrogen-doped graphene cathode material
Researchers in China, with colleagues from Lawrence Berkeley National Laboratory, have synthesized an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside nitrogen-doped graphene sheets (S@NG). Used as a cathode material for a Li-sulfur battery, the Li/S@NG can deliver high specific discharge capacities at high rates: 1167 mAh g–1 at 0.2 C; 1058 mAh g–1 at 0.5 C; 971 mAh g–1 at 1 C; 802 mAh g–1 at 2 C; and 606 mAh g–1 at 5 C.
The cells also exhibited an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle)—among the best performance demonstrated so far for Li/S cells, according to the researchers. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles.
The achieved best performance for earlier nitrogen-doped graphene-S nanocomposites was about 671 mA h g−1 after 200 cycles at a current rate of 1500 mA g−1 (∼0.9 C), the team noted.
X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to a well-restored C–C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The researchers suggested that their results indicate that the S@NG nanocomposite based Li/S cells have a good potential to replace current Li-ion batteries.
Lithium/sulfur (Li/S) batteries have attracted considerable attention due to its great potential to deliver two to three times the energy density of current lithium-ion batteries. Because the electrochemical redox reactions in a Li/S cell are based on a two electron reaction (S + 2Li+ + 2e− ↔ Li2S), the corresponding theoretical specific capacity and energy density are as high as 1675 mA h g−1 and 2600 Wh kg−1, respectively. However, the poor intrinsic electrical conductivity of the active material, the high solubility of intermediate polysulfides into the electrolyte, and the large volumetric expansion of approximately 76% based on full transformation of sulfur to Li2S result in the low utilization of sulfur and the rapid capacity fading on cell cycling, which has hindered the further development of Li/S battery technology and its application.
To address these key issues, chemists and materials scientists have been trying to design and construct novel micro-structured/nanostructured S cathode materials to improve cell performance. … Despite … progresses, achieving Li/S batteries with a high rate and a long cycling life is still difficult.
… Recently, our group found that cetyltrimethylammonium bromide (CTAB) modified graphene oxide (GO)-S nanocomposite cathode could significantly improve the cycle life of the Li/S cells to 1,500 cycles. However, low-temperature reduced GO in the previous work was not able to achieve high enough electrical conductivity, and it was necessary to add 10−20 wt % carbon black additives in the cathode to improve its conductivity that led to a lower S content percentage in the final cathode. We also find that it is necessary to develop a more simple and scalable approach to preparing highly conductive S/C composite materials for achieving high capacity over long-term cycling.—Qiu et al.
In this current study, the team prepared a cathode comprising N-doped graphene wrapped S and PVDF binder only, without additional carbon black additives using a simple, low-cost, and scalable method.
The cathode with 60% of sulfur in the total electrode weight exhibited high specific capacity and excellent rate performance up to 5 C. They further demonstrated an ultralong cycle life up to 2000 cycles at 2 C with a decay rate of as low as 0.028% per cycle.
Yongcai Qiu, Wanfei Li, Wen Zhao, Guizhu Li, Yuan Hou, Meinan Liu, Lisha Zhou, Fangmin Ye, Hongfei Li, Zhanhua Wei, Shihe Yang, Wenhui Duan, Yifan Ye, Jinghua Guo, and Yuegang Zhang (2014) “High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene,” Nano Letters doi: 10.1021/nl5020475