Synthesis And Performance Of High Energy Li-S Pouch Cells And PVDF Based Hybrid Membranes For Secondary Lithium Batteries

As a key component in lithium ion batteries,the separator plays important roles,which is significant beneficial to enhance electrochemical properties of lithium-ion batteries.However,the energy density of lithium ion battery can not exceed 200 Wh kg^-1,resulting in it cannot meet the demand of society for large energy storage devices.Therefore,the researchers turn to study the lithium-sulfur（Li-S）battery with high theoretical energy density(2600 Wh kg^-1),low cost and good environmental benignity.In this work,poly（vinylidene fluoride）（PVDF）as matrix material and silica nanoparticles（SiO₂）as inorganic fillers prepared the PVDF/SiO₂/PP hybrid membranes and PVDF/SiO₂ hybrid membranes via surface coating menthod combined with phase inversion.After that,the basic physical properties and electrochemical performance of these membranes were investigated.A simple strategy combined mechanical ball milling method and melt-diffusion technique was developed to fabricate sulfur/carbon composites based on active carbon（AC）with porous and conductive carbon black（SP）as sulfur hosts.By controlling variables,the compacted density of cathode materials was evaluated and excellent electrochemical performance of Li-S pouch cell under appropriate compacted density was obtained.The main research contents are as follows:（1）The PVDF/SiO₂ slurry got by the incorporation of inorganic fillers SiO₂ into polymer matrix PVDF was applied to the commercial polypropylene（PP）separator.Then the PP/PVDF/SiO₂ hybrid membranes were obtained via phase inversion on the basis of aforementioned membrane.The influence of solid content,SiO₂ content,coating thickness on the physical properties of PP/PVDF/SiO₂ hybrid membranes was investigated,including porosity,ionic conductivity,etc.The PP/PVDF/SiO₂ hybrid membranes exhibited high electrolyte uptake（340%）,excellent ionic conductivity(1.4mS cm^-1@25°C),better transverse and longitudinal shrinkage rates（<1.0,<4.75）when solid content,SiO₂ content,coating thickness were 25 wt.%,5 wt.%,15?m,respectively.Compared to the PP separator,420 mAh LiNi_0.8Co_0.1Mn_0.1O₂/graphite pouch cells demonstrate that PP/PVDF/SiO₂ hybrid membranes endow enhanced cycling stability of cells.Therefore,PP/PVDF/SiO₂ hybrid membranes exhibit the great potential in large-scale battery applications.（2）A series of rationally designed hybrid membranes composed of PVDF as polymer matrix and SiO₂ as inorganic fillers were prepared by combining slurry coating method and phase inversion method.The effects of the added SiO₂ nanoparticles on the porosity,electrolyte wettability,thermal stability,and ionic conductivity of PVDF/SiO₂hybrid membranes were investigated systematically.Compared to the PP membrane,PVDF/SiO₂ hybrid membranes present enhanced physical and electrochemical performance.Particularly,the incorporation of 5 wt.%SiO₂ to PVDF polymer matrix（PVDF5 hybrid membrane）shows the highest ionic conductivity of 1.0 mS cm^-1 at25°C among all the samples.The electrochemical tests demonstrate that the LiNi_0.8Co_0.1Mn_0.1O₂/Li coin cell assembled with PVDF5 hybrid membrane exhibits high reversible discharge capacity(179 mAh g^-1 at 0.05 C),excellent cyclic stability（169 mAh g^-11 after 100 cycles at 0.1 C）,and superior rate performance,which are much better than other counterparts and PP separator.Moreover,as for the large capacity battery application,1.1 Ah LiNi_0.8Co_0.1Mn_0.1O₂/graphite pouch cell with PVDF5 hybrid membrane can deliver a high discharge capacity of 992 mAh and good Coulombic efficiency of 99.5%.Evidently,the optimized PVDF/SiO₂ hybrid membrane will be a very promising alternative to the commercial PP separator for advanced lithium ion batteries.（3）A facile strategy for the rational design of S/C cathode in Li-S pouch cell has been successfully synthesized under the mechanical ball milling method with the assistance of melt-diffusion technology.The porous AC and SP were deemed as carbon matrixs and lithium copper composites were used as the anode in order to explore the influence of compacted density of cathode on electrochemical properties.The physical barrier layer developed as single-walled carbon nanotubes and graphene（SWCNTs/graphene,thickness=30μm）was covered on cathode electrode to capture polysulfide,increasing the sulfur utilization.Then it revealed that sulfur has integrated well into the carbon holes and sulfur content in AC/SP was 66.2%（theoretical value70%）.Additionally,SWCNTs/graphene barrier layer is embedded between the sulfur particles in implantable way.The electrode surface with compacted density of 1.2 g cm^-33 becomes flat and does not occur structural collapse or crack after 10 cycles compared with before cycling（at charged state）,corresponding excellent electrochemical properties.It delivers higher specific capacities(1296 mAh g_s^-1 at 0.05 C)、superior energy density(295 Wh kg^-1)、remarkable first high platform capacity（C_H）and low platform capacity（C_L）,attributing to the better contact among electrode particles and the appropriate storage of electrolyte under the rational compacted density of cathode.As a result,the electrochemical properties of battries are enhanced.Herein,choosing cathode with proper compacted density provides a novel strategy in synthesizing high energy density for large-scale Li-S pouch cells.