The Interface Mechanism And Performance Researches Of Lithium Anode/Electrolyte Regulated By Additives

Lithium metal is considered as a promising anode material for the next generation of lithium batteries because of its high specific capacity and low redox potential.However,the practical application of lithium anode is limited by the growth of lithium dendrite,which leads to the safety problems and rapid capacity attenuation of lithium metal battery.In the efforts to inhibit the formation of lithium dendrites,the design of electrolyte may be the most critical,because the solid electrolyte interface（SEI）,which is responsible for the transmission of lithium ions and the regulation of rapid lithium growth,is spontaneously generated by the reaction between lithium metal and electrolyte.In this paper,from the perspective of additives used in carbonate electrolyte,combined with theoretical calculation and a series of characterization,the high voltage positive electrode Li Ni_0.8Mn_0.1Co_0.1O₂（NCM811）at 4.5 V and stable lithium metal negative electrode in nonflammable electrolyte are constructed respectively.It mainly includes the following contents:（1）A carbonate electrolyte containing Potassium Thienyl Trifluoroborate（PTF）additive with configuration of cation and anion was designed.Firstly,the boron element contained in the anion of PTF can absorb the HF acid in the electrolyte and PTF can react with the lithium anode to form an SEI film rich in inorganic substances（Li₂CO₃,Li₂S_x）.The rich Li₂CO₃ and Li₂S_x in the SEI can reduce the Li⁺diffusion barrier and improve the charge transfer ability of the electrode surface.Secondly,the cations of PTF will gather on the surface of lithium negative electrode to form electrostatic shielding,which can effectively avoid the rapid growth of lithium dendrites caused by tip effect.PTF additive can also affect the sheath structure of Li⁺solvation and reduce the desolvation energy barrier of Li⁺.The electrochemical performance results show that when the current density of 0.5 m A cm^-2(the deposition capacity is 0.25 m Ah cm^-2)is used for cyclic charge and discharge,the Li||Li cell achieves a stable cycle of more than 1000 hours in the electro lyte containing 1 wt%PTF.In addition,since PTF can absorb a small amount of HF in the electrolyte an d reduce the erosion of HF on the electrode surface,a thin and uniform cathode electrolyte interface（CEI）layer is formed on the surface of NCM811 in the electrolyte containing PTF,which effectively protects the cathode surface.As a result,the Li||NCM811 full battery remained stable after 150 cycles at 200 m A g^-1.（2）Triethyl phosphate based nonflammable electrolyte with Tris（hexafluoroisopropyl）phosphate（THFP）as electrolyte additive was designed to balance the nonflammable and electrochemical properties of the battery.First,THFP carries a large number of fluorine atoms with strong electronegativity,which can significantly reduce the charge density of P=O double bond,increase the binding strength between THF and anionic PF₆^-,dissociate Li PF₆ and increase the ionic conductivity of electrolyte.At the same time,the presence of fluorine atoms further reduces the lowest unoccupied molecular orbital and increases the highest occupied molecular orbital of THFP respectively,so that THFP can not only be preferentially reduced by lithium negative electrode to construct LIF rich SEI film,but also enhance the oxidation resistance of electrolyte.Third,THFP can reduce the HF content in the electrolyte and reduce the dissolution of transition metal ions on the cathode surface.The experimental results show that in the electrolyte containing 1 wt%THFP,the Li||Li battery obtains a stable cycle of about 250 h under the same test conditions.The average coulomb efficiency of Li||NCM622 full battery after 200 cycles at 100 m A g^-1current density is 99.3%,and the reversible capacity reaches 145 m Ah g^-1.