In Situ Preparation Of PVA-coated Sulfur Core-shell Particles And Its Application In Lithium-sulfur Batteries

As society develops rapidly,the requirement for energy storage in social life and production is also increasing.Lithium-ion batteries are widely used as a chemical energy storage system in various fields.Lithium-sulfur batteries are the most promising next-generation lithium-ion battery system due to their high theoretical specific capacity(1675 m Ah g^-1)and wide source of cathode materials compared with conventional lithium-ion batteries.However,the application of lithium-sulfur batteries in actual practice is limited by three main problems.Firstly,the shuttle effect of polysulfide during charging and discharging processes,leads to the loss of active material in the cathode and rapid battery capacity decay.Secondly,the relatively poor reaction kinetics of lithium-sulfur batteries.Because the charge and discharge of lithium-sulfur batteries are both complex multi-step chemical reactions,as the charge and discharge gradually deepen the reaction becomes more and more difficult.The rate performance of lithium-sulfur batteries will be decreased,and difficult to satisfy the application requirements.Thirdly,the volume change of the active materials during charging and discharging.About 80%volume expansion will happen during the discharging of sulfur,which will lead to the breakdown of the fluid collection structure and the quick decay of the battery capacity.The core-shell structure has become a hot research topic as a sulfur cathode structure that may solve these three problems once for all.However,there are many problems in the cathode preparation and working mechanism of the core-shell structure.Based on these,the paper here proposes an in-situ method to prepare core-shell structure particles.The novelty points and research results are listed as below:（1）PVA-coated sulfur particles（S@PVA）with Yolk-shell structure are prepared by a one-step forming self-emulsification process,which solve the challenge of forming Yolk-shell structure in situ for polymer coating sulfur structure.The shuttle effect is effectively suppressed,and the cycle stability of lithium-sulfur batteries is improved,which has provided a new method for the preparation of high-performance lithium-sulfur battery cathode materials.The decay rate of lithium-sulfur batteries prepared using S@PVA particles is 0.31%per cycle and the first cycle discharge capacity at 0.1 C is 687 m Ah g^-1.（2）The inner mechanism of the difficulty to achieve the balance between the cycle stability and rate performance of S@PVA particles based lithium-sulfur batteries is analyzed deeply.The modulation of the electrochemical performance of lithium-sulfur batteries is achieved by tuning the shell structure of S@PVA particles and the transport speed of electrons and ions in the electrode.And the mechanism of the shell structure on the diffusion behavior of electrons and ions is also revealed.The final prepared battery can be stably cycled for 800 cycles at a current density of 1 C,with a discharge capacity of 720 m Ah g^-1 in the first cycle,and a decay rate of only 0.036%per cycle.（3）The effects of the distribution state of Zn S particles（distributed inside or outside the Yolk-shell shell layer）on the battery cycling and rate performance are investigated.The activation energy of the reduction reaction between Zn S and polysulfide at different stages is accurately determined,which provides a strong experimental basis for clarifying the intrinsic mechanism of polysulfide conversion catalyzed by Zn S particles.By distributing the Zn S particles inside the shell layer,the lithium-sulfur battery achieves a 4C rate performance.Long cycle tests show a first cycle discharge capacity of 822 m Ah g^-1 at a current density of 1C and a capacity decay rate of 0.015% per cycle.（4）Core-shell particles of PVA-coated Li₂CS₃ are prepared.The barrier effect of the PVA shell layer is used to improve the deposition morphology of Li₂CS₃ during battery cycling and also to avoid the phenomenon of Li₂CS₃ deposits blocking the current collector.The cycle decay of Li₂CS₃ as the cathode of lithium-sulfur battery is suppressed,and the capacity retention of the battery is effectively improved.The lithium-sulfur battery using Li₂CS₃@PVA particles can reach a discharge specific capacity of 1305 m Ah g^-1 at a current density of 0.1 C,and it can cycle 100 times stably.The rate performance can achieve 5C and the discharge specific capacity is 349 m Ah g^-1.