Investigation Of Discharge Process And Property Improvement Of Lithium Sulfur Batteries

The lithium-sulfur battery has many attractive properties, it has a high theoretical energy density and it is made with relatively cheap materials, it has special advantages in terms of batteries building for future. But the application and development of lithium sulfur battery is limited by discharge capability and cycle life. The morphology of cathode, the impedance change of battery during discharge and the effect of shuttle phenomenon on charge process of lithium sulfur were investigated to explore the causes which were responsible for poor discharge capability and cycle life of lithium sulfur battery. The effect of liquid transport factors such as temperature, viscosity and ionic conductivity of electrolyte on the discharge process of lithium sulfur were investigated in this dissertation. Cathode coating and electrolyte additive were adopted to restrain shuttle phenomenon and improve the electrochemical properties of lithium sulfur battery.The investigation of charge-discharge process showed that the dissolution of higher-order polysulfides and solid film formed by lower-order lithium sulfide caused the increase of diffusion and convection resistance of ion in electrolyte. Shuttle phenomenon formed by the dissolution of polysulfides caused poor charge-discharge efficiency of lithium sulfur battery. Temperature and concentration of lithium salt were factors which had vital effect on shuttle phenomenon. Shuttle phenomenon increased with higher temperature and concentration of lithium salt.The viscosity of electrolyte increased when the concentration of lithium salt in electrolyte was higher than 1M, then the diffusion coefficient of electrolyte was moved down. The increase of viscosity of electrolyte caused reduction of ionic mobilities and ionic conductivity. The viscosity was more than six times noumenon and the ionic conductivity was less than half of noumenon after elemental sulfur transforming into S₄^2-during the discharge process of lithium sulfur battery. It was showed that the discharge process of lithium sulfur battery was restrained by liquid transport process. The viscosity of electrolyte depressed after adding PC(propylene carbonate) who had high dielectric constant, then the ionic conductivity of electrolyte increased. The content of PC in the best cell was vol 6%.The dissolution and diffusion of polysulfides were prevented after the carbon-sulfur composite was coated by PEO, but low ionic conductivity of PEO resulted in high transfer resistances of cathode. The initial discharge capacity of lithium sulfur coated at 4%wt(PEO) is 1136 mAh·g^-1, a reversible capacity of 1023 mAh·g^-1 after 20 cycles. Carbon particle and conductive node were connected by PANI molecule and formed electronic conduction network, electronic conductivity and structural stabilization of cathode were improved. The initial discharge capacity of lithium sulfur coated at 5.8%wt(PANI) is 1273 mAh·g^-1, a reversible capacity of 1029 mAh·g^-1 after 20 cycles. LiNO3 adopted as additive of electrolyte reacted with Li electrode to form protective surface film. The film resulted in passivation at the surface of the electrode and prevented the shuttle phenomenon. Cycle life and capacity of lithium sulfur battery were improved because The film prevented form of irreversible Li₂S on Li electrode. The initial discharge capacity of lithium sulfur with LiNO3 is 1172 mAh·g^-1, a reversible capacity of 1017 mAh·g^-1 after 20 cycles. LiBOB(lithium bis(oxalato) borate) adopted as additive of electrolyte reacted with Li electrode to form protective surface film at a certain voltage to prevented the parasitic reaction between polysulfide and Li electrode. The film formed at 4 mol% LiBOB showed low transfer resistances and integrity. The initial discharge capacity of lithium sulfur with 4 mol% LiBOB is 1191 mAh·g^-1, a reversible capacity of 892mAh·g^-1 after 20 cycles.