| Conventional Li-ion batteries,which typically employ lithium transition-metal oxide as cathode materials have relatively low energy densities.Their theoretical maximum energy density is only 350?400 Wh·kg-1.Besides,lithium transition-metal oxides are usually expensive.These disadvantages severely limit their use in power-intensive applications such as electrical vehicles.Li-S battery has attracted tremendous attentions due to its high theoretical energy density of 2600 Wh·kg-1.In addition,sulfur is naturally abundant,inexpensive and environmentally friendly.As a result,Li-S battery has been considered as one of the most promising candidates for high energy density electrochemical systems.However,the commercialization of rechargeable Li-S battery is still hindered by problems including the low sulfur utilization,poor cycle life etc.These problems are mainly attributed to the electrically insulating nature of S and Li2S,the dissolution of the reaction intermediates of polysulfides,and the cathode volume variation during charge and discharge process,which leads to low active materials utilization and poor cycle life.Moreover,Li-S battery employs ether as the electrolyte solvent,which engenders safety vulnerability of the battery.Herein,we present two methods to optimize the battery design in the respect of sulfur cathode binder and electrolytes.The main research contents and results are as follows.In order to solve the sulfur cathode volume variation and sulfur shuttle problem in the process of charging and discharging,a new type of water-based sulfur cathode binder,poly(styrene-b-n-butyl acrylate-b-styrene)(SnBAS)having three different St/nBA weight ratios,was designed and synthesized by RAFT emulsion polymerization method.Binder with an appropriate St/nBA weight ratio gave the cathode a satisfactory mechanical strength and an appropriate swelling ratio in the electrolyte.Binder with strong mechanical strength could resist the electrode volume change,thus maintaining the integrity of the electrode structure.Binder with high swelling rate can improve the migration of lithium ion,resulting in improved battery capacity,and the utilization of active materials.In addition,polar functional groups in the binder such as carboxyl and ether oxygen radicals can interact with polysulfide,which could inhibit sulfur shuttle and thus improve the discharge specific capacity and coulomb efficiency.The sulfur cathode prepared from the binder showed a capacity of 781 mAh·g-1 at 0.2C after a series of charge and discharge cycles at 0.1C?0.2C?0.5C?1C?2C?0.1C.After additional 100 cycles at 0.2C,the specific capacity was still 740 mAh·g-1 corresponding to a small capacity decay of 0.05%per cycle and a capacity retention rate of 94.7%.To alleviate the sulfur shuttle problem and eliminate the battery potential safety hazard caused by flammable liquid electrolyte and battery leakage,a new type of solid polymer electrolytes(SPEs),PS-b-PmPEGA-b-PS(SnBAS)doped with LiClO4 was designed and synthesized by RAFT solution polymerization method.The PS block provided the mechanical strength and vastly improved SPEs film-forming to separate cathode and anode.The PmPEGA block allowed for ion conduction in the polymer electrolyte through coordination-dissociation interaction between the ether and lithium ion.Besides,the structure design of side chain increases EO chain segment motion ability and avoids the crystallization of EO chains at room temperature.The SPEs with a higher EO content exhibited higher ionic conductivity of 9.07×10-6S·cm-1 at 25?.Using the SPEs,a LiFePO4/SPEs/Li metal cell was constructed and tested.The prepared cell can't work due to the low ion transport rate at 25?.To increase the ion transport rate,the prepared cell exhibited a discharge capacity of 10 mAh·g-1 at 60?and 97 mAh g-1 at 80?,respectively.However,the battery cycle performance is still bad.As a result,the further research of employing these SPEs in Li-S batteries was not carried out. |
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