Safe Electrolytes For Lithium Metal Batteries

The demand for batteries with high energy density grows with the rapid development of society.Li metal battery with metallic lithium as anode material has attracted tremendous attention for its extremely high energy density.The development of Li metal batteries,however,has been hindered by the parasitic reactions between metallic lithium and organic electrolytes and the uncontrolled growth of dendritic lithium,which brings serious safety hazards to Li metal batteries.The electrolyte is a key component of a lithium metal battery,which plays a significant role in improving the interfacial stability of Li metal anode and inhibiting the Li dendrite growth.Nevertheless,the traditional organic liquid electrolytes（such as an electrolyte containing diethyl carbonate（DEC）,ethylene carbonate（EC）,and lithium hexafluorophosphate（Li PF₆）that is commonly used in lithium-ion batteries）are insufficient in improving the interfacial stability of Li metal anode and inhibiting the Li dendrite growth.Besides,these organic electrolytes are inflammable.In this thesis,we aim to improve the safety and electrochemical performance of lithium metal batteries by studying the influence of different electrolyte components on the safety and electrochemical performance of lithium metal batteries.The effectiveness of electrolytes optimization in improving the safety and electrochemical performance of lithium metal batteries are confirmed by various characterizations.1.We study the gas evolution during Li plating/stripping on a Cu substrate in concentrated electrolytes by employing in-situ differential electrochemical mass spectrometry（DEMS）.As evident from our DEMS measurement,the electrochemical reduction of solvent molecules（propylene carbonate（PC）,tetraethylene glycol dimethyl ether（G4）,dimethyl sulfoxide（DMSO））and the corresponding gaseous products could be effectively suppressed in the concentrated electrolyte.It is because almost all of the solvent molecules coordinate with Li+and there are no free solvent molecules,thus improving the reduction stability of the solvent.Gas evolution is related to the safety of the battery,so the improved concentration can improve safety.The chemical reactions between solvent molecules and the deposited Li,however,still exist in the concentrated electrolyte.In this concern,ether solvents（e.g.G4 solvent,etc.）,which are generally inert to Li metal,may be more suitable for Li metal batteries.2.A safe electrolyte（Li HFDF/G4+Li NO₃）was prepared by combining high-safety solvent（G4）and high-performance conducting lithium salt（lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide,Li HFDF）and applied to lithium-sulfur（Li-S）battery.The designed electrolyte has low flammability and can inhibit the growth of Li dendrites and protect the Li metal anode and sulfur cathode simultaneously.The Li-S battery based on Li HFDF/G4+Li NO₃electrolyte demonstrates high initial capacity of 1143 m Ah g^-1 at a rate of 0.5 C.After 300cycles,the Li-S battery still delivers a discharge capacity of above 700 m Ah g^-1.After 600cycles at a high rate of 1.0 C,the Li-S battery based on Li HFDF/G4+Li NO₃ electrolyte still maintains a discharge capacity of above 500 m Ah g^-1,which is much higher than that of the Li-S battery based on Li TFSI/G4+Li NO₃ electrolyte.These results demonstrate the effectiveness of the designed electrolyte in improving the cycling stability of the Li-S battery.