Functional Cellulose Separator And The Application In High-performance Lithium Metal Battery

In recent years,various kinds of electronic products have been widely integrated into our life.Now,human beings heavily dependent on the energy supply of batteries.Lithium battery has the advantages of high energy density,long cycle life,environmental protection,good coulombic efficiency,etc.However,it still shows many shortcomings that cannot be ignored.For example,most metal lithium was used in the negative electrode,which can cause explosion and other risks.The most commonly used diaphragm is microporous diaphragm made of polyolefin materials.Although these diaphragm has stable electrochemical performance and high mechanical strength,due to the defects of the material itself,it has extremely poor wettability to electrolyte,serious lack of thermal stability,and high requirements on ambient temperature during operation.All these disadvantages seriously endanger the safety performance and development of lithium batteries.Therefore,it is very important to develop the new lithium battery diaphragm.As one biomass,cellulose is widely available,cheap,environmentally friendly and sustainable.Cellulose-based new lithium battery diaphragm has good electrolyte absorption performance and high thermal stability,which is of positive significance for improving the safety and electrochemical performance of lithium batteries.Therefore,this paper takes cellulose material as the research object,making cellulose-based lithium battery diaphragm with high safety performance and biomass modified lithium-sulfur batteries diaphragm with functional characteristics.The specific content of this research is mainly reflected in the following three parts:First,wood pulp cellulose(WPC)is used as the substrate while aluminum hydroxide as the flame retardant to prepare the wood pulp/aluminum hydroxide composite flame retardant diaphragm with flame retardant effect.The diaphragm has the structure of a large number of gaps and holes.It can absorb a large amount of electrolyte,with the liquid absorption rate of220%,which is 120%higher liquid absorption rate than polyolefin separator.When the mass ratio of wood pulp to aluminum hydroxide is 3:7,it shows excellent flame retardancy and thermal stability.After the heat treatment at 220?,the size of the diaphragm has no obvious change.Even by flame,the diaphragm has no open flame,which can significantly improves the safety performance of lithium battery.At the same time,the lithium battery device assembled with the diaphragm also shows excellent electrochemical performance,with ionic conductivity 5.7×10?⁴ S/cm,the first-cycle discharge specific capacity 141.7 m Ah·g^-1 at 1 C current density,135.1 m Ah·g^-1 remains after cycle,the capacity retention rate 95.34%in 0.2C rate cycle.The first-cycle discharge specific capacity is as high as 164.4 m Ah·g^-1,which is close to the theoretical specific capacity.It is better than the lithium battery assembled with the commercial diaphragm.Secondly,using bacterial cellulose(BC)as the substrate,hydrophilic nano-Si O₂ as appropriate pore-forming agent and flame retardant aluminum hydroxide,a high-performance lithium battery diaphragm(BSA composite flame retardant diaphragm)with flame retardant characteristics is manufactured by a simple suction filtration method.When the mass ratio of BC,Si O₂ and Al(OH)₃ is 1.5:1.5:7,the composite diaphragm has the best comprehensive performance.The diaphragm has a unique laminated sandwich structure,which is beneficial to a large amount of storage and absorption of electrolyte.The liquid absorption rate can reach to 240%.Besides having pore-forming function,hydrophilic nano Si O₂ improves the ionic conductivity(6.1×10?⁴ S/cm)of the diaphragm.BSA composite flame retardant diaphragm also has excellent thermal stability and flame retardancy.The lithium metal battery equipped with BSA composite flame retardant diaphragm has a first-turn discharge specific capacity of 166.5 m Ah·g^-1 at a current density of 0.2 C in the rate test,and has good capacity recovery performance when changing from high rate(5 C)to low rate(0.2 C).The retention rate can reach to 98.93%at the current density of 1 C,showing excellent cycle stability.Thirdly,wood pulp cellulose(WPC)is sulphurized and carbonized at high temperature in the nitrogen environment to produce a carbon-sulfur compound WPC-S,which does not contain zero-valent sulfur.Mixed with a conductive agent and binder for pulping,the WPC-S was coated on one side of a commercial diaphragm(Celgard 2400)to produce a WPC-S@pp composite diaphragm which can supply additional energy for the lithium-sulfur battery.The thickness of the coating layer is 11?m.For the same active substance mass loading,the specific discharge capacity of the positive electrode in the first cycle is 1342m Ah·g^-1 at a current density of 0.5 C,which is about twice than that of the commercial diaphragm.After 300 cycles of charging and discharging,the specific discharge capacity still remains 776.1 m Ah·g^-1,and the capacity retention rate is 57.83%.In the multiplying power performance test,the performance is excellent,especially at the current density of 2C,which is 7 times higher than the capacity of lithium-sulfur batteries.