Date Published: January 02, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Hongtao Qu, Jianjun Zhang, Aobing Du, Bingbing Chen, Jingchao Chai, Nan Xue, Longlong Wang, Lixin Qiao, Chen Wang, Xiao Zang, Jinfeng Yang, Xiaogang Wang, Guanglei Cui.
Due to its high theoretical energy density (2600 Wh kg−1), low cost, and environmental benignity, the lithium–sulfur (Li‐S) battery is attracting strong interest among the various electrochemical energy storage systems. However, its practical application is seriously hampered by the so‐called shuttle effect of the highly soluble polysulfides. Herein, a novel design of multifunctional sandwich‐structured polymer electrolyte (polymer/cellulose nonwoven/nanocarbon) for high‐performance Li‐S batteries is demonstrated. It is verified that Li‐S battery with this sandwich‐structured polymer electrolyte delivers excellent cycling stability (only 0.039% capacity decay cycle−1 on average exceeding 1500 cycles at 0.5 C) and rate capability (with a reversible capacity of 594 mA h g−1 at 4 C). These electrochemical performances are attributed to the synergistic effect of each layer in this unique sandwich‐structured polymer electrolyte including steady lithium stripping/plating, strong polysulfide absorption ability, and increased redox reaction sites. More importantly, even with high sulfur loading of 4.9 mg cm−2, Li‐S battery with this sandwich‐structured polymer electrolyte can deliver high initial areal capacity of 5.1 mA h cm−2. This demonstrated strategy here may open up a new era of designing hierarchical structured polymer electrolytes for high‐performance Li‐S batteries.
The conventional Li‐ion batteries (LIBs) with theoretical gravimetric energy density lower than 400 W h kg−1 cannot well satisfy the ever‐growing demand of high energy storage systems, especially in the field of large‐scale electric transportation tools and renewable stationary energy storage systems.1, 2, 3 Therefore, there is a pressing need to exploit alternative energy storage systems with higher energy density. Lithium–sulfur (Li‐S) battery has attracted much attention due to its high theoretical gravimetric energy density up to 2600 Wh kg−1, which is 3–5 times higher than LIBs.4, 5, 6, 7 In addition, sulfur is naturally abundant, inexpensive, and environmentally friendly. However, the practical application of Li‐S battery is plagued by several problems: (1) the intrinsic poor ionic/electronic conductivity of both element sulfur (S) and discharge products (i.e., Li2S2, Li2S), causing low sulfur utilization; (2) large volume change between S and lithiation product Li2S can damage cathode structure, resulting in poor contact between active material and conductive matrix;8, 9 (3) the severe “shuttle effect” of high soluble long‐chain lithium polysulfides (Li2Sn 4 ≤ n ≤ 8).10, 11 The item (3) is the biggest roadblock blocking the commercialization process of Li‐S battery. Specifically, long‐chain lithium polysulfides formed by the reduction of sulfur at the cathode side can migrate to the anode side and are chemically reduced on Li metal. Then, some of the reduced lithium polysulfides diffuse back to the cathode side and are reoxidized to polysulfides with various valences. The so‐called “shuttle effect” of polysulfides has detrimental effect on Li metal anode that causes severe self‐discharge phenomenon and shortened lifetime.12, 13, 14
In summary, we have designed a novel sandwich‐structured polymer electrolyte (NCP‐CPE) to address the shuttle effect that severely hindered the practical application of Li‐S battery. Notably, Li‐S battery based on this multifunctional polymer electrolyte exhibits superior cycle performance for 1500 cycles and excellent rate capability up to 4 C with a pure sulfur cathode, which is comparable to the best reported value so far. On the cathode side, the nanocarbon coating can function as a second current collector, which facilitates the electron transfer and accelerates the conversion of polysulfides. In addition, the nanocarbon coating also employs as a physical barrier to mitigate the migration of polysulfides. Furthermore, the hydroxyl groups in cellulose backbone of NCP‐CPE have strong affinities with polysulfides which can chemically suppress the immigration of polysulfides. Finally, on anode side, PEG‐PPG‐PEG coating layer of NCP‐CPE can enable uniform lithium ion stripping/plating, which gives a well protection of Li metal anode. The demonstrated approaches here may guide us to go ahead to develop multifunctional polymer electrolyte for high‐performance Li‐S battery.
Preparation of Sulfur Cathode: Sulfur cathode with a content of 60% was prepared by grinding commercial sulfur powder, Super P, and PVDF at a ratio of 6:3:1 in N‐methyl‐2‐pyrrolidone (NMP) solvent to form a uniform slurry. Then, the slurry was casted on an aluminum foil and followed by drying at 60 °C before use. Cathodes with sulfur contents of 70 and 80% were prepared with the same method. The average loading of sulfur in cathode was ≈1.5 4.9 mg cm−2.
The authors declare no conflict of interest.