During the first discharge/charge process, a passivation film will be formed on the carbon electrode. This film is generally called the passivation layer or solid electrolyte interface (SEI) film. The SEI membrane is generally composed of the decomposition products of the electrolyte. Here, we focus on the composition and physical and electrochemical properties of the SEI membrane, without involving the decomposition process and mechanism of the electrolyte.
The SEI film is conductive to lithium ions and insulative to electrons. The perfect SEI film can prevent the co-intercalation of solvent molecules and/or the delamination of the graphite surface and the further decomposition of the electrolyte. At present, it is generally believed that the SEI film is a "mosaic" mixture composed of several different organic and inorganic substances such as LiF, Li2CO3, and electrolyte reduction products such as ROCO2Li. After EC reductive decomposition, (CH2OCO2Li)2 may be formed, and the reduction products of γ-butyrolactone may be CH3OLi and CH3OCO2 Li. Another view is that the side of the SEI film closest to the graphite is composed of inorganic materials such as LiF and Li2CO3, while the outermost layer is composed of polymer-based organic materials.
Online mass spectrometry was used to detect the production of ethylene and propylene gas in EC and PC-based electrolytes. It is found that the gas is generated on graphite rather than on the lithium storage alloy, and the solvent co-intercalation reaction only occurs when the graphite anode is used. Due to the different side reactions that occur on graphite and lithium storage alloys, the composition of the SEI film on each and the requirements for the SEI film are also different. When studying the decomposition of different electrolytes in graphite/LiCoO2 batteries, the solvents used include EC, DMC, EMC and DEC, and the lithium salt is LiPF6. Carbon monoxide and ethane were detected in the one to three-component solvent electrolyte containing EC, and Li2CO3, RCOOLi and (CH2OLi)2 were detected on the surface of the graphite anode. It can be considered that the formation of these substances is due to the reduction reaction of EC during the first charging process. Further analysis of the electrolyte residue showed that carboxylic acid esters were formed after the initial lithium intercalation, indicating that super-esterification occurred.
However, due to the incomplete Coulomb transformation, high irreversible capacity, low diffusion rate, and increase in electrode ohmic resistance, the SEI film formed on these thick electrodes after the first cycle is incomplete. The electrochemical scanning tunneling microscope (STM) was used to observe the topological changes of the basal surface of highly oriented pyrolytic graphite (HOPG) in several electrolytes under polarization conditions, and it was found that in the 1mol/L LiClO4 electrolyte with EC/DEC and EC/Dimethoxyethane (DME) as solvents, the irregular blister-like structure on the surface of HOPG gradually grows as the discharge voltage decreases. This is due to the intercalation of lithium ions between the graphite layers and the accumulation of decomposition products on the surface of the graphite electrode. Therefore, the formation of a complete SEI film often requires several charge and discharge cycles to complete. Adding γ-butyrolactone as a co-solvent can inhibit some side reactions of gas production. When discharging in 1mol/L LiClO4/PC electrolyte, only the rapid exfoliation and fracture of the graphite layer can be observed, but the formation of blister-like structure can not be seen. According to the morphological changes on the surface of HOPG observed in different electrolytes, it was suggested that the intercalation of solvated lithium ions is a necessary step to form a stable surface layer on the graphite electrode.
Graphite carbon and hard carbon materials were prepared by chemical vapor deposition of ethylene and pyrolysis of dehydrated sucrose, using this carbon material and lithium metal as electrodes, and LiPF6 and LiAsF6 EC/DEC solutions were used as electrolytes to make batteries and cycled at 0~2.00V (relative to Li/Li+). It was found that the composition and thickness of the SEI film firstly depend on the properties of the carbon material used, especially the chemical groups on the surface of the carbon material and the structure of the carbon material, and the composition of the electrolyte has only a secondary effect on the properties of the SEI film. In the two electrolytes, the SEI film on the graphite-like soft carbon has no carbonate, and its inorganic part is almost only LiF; the SEI film on the hard carbon material is much thicker and contains compounds of phosphorus or arsenic. In all cases, there is a large amount of polymer structure (ie, the product of solvent polymerization) inside the SEI film, and when the electrolyte contains LiAsF6, only carbonates are present on the hard carbon surface. However, in the case of graphitic carbon electrodes, the electrolyte composition only affects the potential of the SEI film formed by the passivation of the electrode on the upper surface of the carbon electrode. The SEI membrane is mainly composed of the reduction product of solvent molecules, and the structure of the SEI is the determinant of the compatibility of the electrolyte and the carbon electrode.
Lithium bis (oxalato) borate [LiBOB] is an electrolyte salt newly synthesized in recent years and has many good properties. Through the continuous removal of hemicarbonate by Art bombardment, it is found that the main components of the SEI film come from two competing parts: the reduction product of cyclic carbonate and BOB- anion. Due to the higher reduction potential of EC molecules, the initial surface chemistry is determined by the EC in the electrolyte. In the EC-free electrolyte, the reduction products are mainly derived from BOB- anions. To ensure the good performance of the electrolyte at high temperatures, the coexistence of EC and LiBOB is required. When studying the electrochemical stability of boron anion receptor tris (pentafluorophenyl) borane (TPFPB) on MCMB electrode and its compatibility with SEI membrane on carbon electrode, the capacity loss of MCMB electrode before and after TPFPB addition in the first constant current cycle shows that TPFPB has very good electrochemical stability on carbon electrode. Cyclic voltammetry studies have shown that when TFPPB is present, a stable SEI film can be formed on the surface of the carbon electrode after EC is decomposed. Even after multiple cycles, the SEI film on the carbon electrode using the electrolyte additive TFPPB can still maintain long-term stability. Even after the heat treatment that can dissolve LiF, the SEI film will not be dissolved by TFPPB. This indicates that the SEI film on the carbon electrode forms a cross-linked structure. Under the same conditions, lithium-ion batteries made with electrolytes containing TFPPB have better cycle performance than those without TFPPB.
Since lithium ion batteries usually work in a relatively wide temperature range, and the internal temperature difference between lithium ion batteries in working and non-working conditions is also very large, the thermal stability of SEI film is also an important aspect of the properties of SEI film. Research on the thermal stability of the graphite anode shows that the properties of the lithium salt have a critical effect on the thermal stability of the electrolyte on the anode. For the electrolyte using LiBF6, the irreversible exothermic reaction of the SEI film occurs at 50-60℃. In the electrolyte using LiPF6 as the lithium salt, the same exothermic reaction can only occur at a temperature above 100℃. These results have been verified by DSC and high temperature storage experiments and accelerated calorimetry (ARC) experiments. One explanation for this difference is that the graphite surface catalyzes a low-temperature exothermic reaction. Similar reactions do not occur on other electrode materials (such as lithium metal) and cathode materials (such as LiMn2O4). On the contrary, compared with LiBF6, using LiBF4 as the metal lithium/LiMn2O4), the electrolyte of the battery can alleviate the dissolution of Mn2+ from the cathode material to the electrolyte.
Using in-situ AFM to track the relationship between the morphology of the HOPG/electrolyte interface and the temperature, using cyclic voltammetry to study HOPG crystals covered by 1mol/L LiBF4-EC/γ-BL(2:1) electrolyte containing 0.5% water, it was observed that water was reductively decomposed at 1.4V, and organic solvents decomposed at 0.8V to form SEI film. After the first discharge reaction, the surface of the HOPG electrode is covered by the SEI film, which contains many small island-like structures, indicating that the formation of a complete SEI film requires several cycles to complete. What is interesting is that the SEI film on the basal surface of HOPG is thin, mainly organic compounds generated by the reduction of EC/DEC, while the film on the edge surface of HOPG is thicker and is composed of inorganic substances generated by the reaction of organic solvents and lithium salts.
The morphology changes of the SEI film on the HOPG and graphite composite electrodes were observed by in-situ AFM at 25~70℃. It was found that the morphology of the SEI film basically did not change with the heating temperature below 40℃. When the temperature continues to rise, the morphology of the SEI film begins to change. At 50°C, blister-like protrusions (DSC shows an exothermic peak at 58℃; this is true regardless of whether the electrolyte contains water or not) were observed on the surface of the electrode HOPG due to the destruction or melting of the SEI film. At 60℃, the blisters grew and merged into larger blisters. At 70℃, there are basically no small blisters visible, and the surface of the HOPG is exposed to the electrolyte again. When the temperature is higher, the reaction products will accumulate on the edge surface of the HOPG. The explanation for this phenomenon is that the lithium ions that have been embedded in the surface graphite layer of HOPG participate in the exothermic reaction. In-situ XRD studies have shown that after lithium-intercalated graphite is stored at 60°C for 12 hours, 50% of the lithium ions will be deintercalated from the graphite to form Li0.5C6 compounds. The SEI film on the graphite composite electrode will also undergo a similar thermal collapse, and a thick film containing carbon and oxygen will be formed on the electrode surface when the temperature is above 60°C. The reaction mechanism is:
BF4+xe+2xLi+→xLiF+ LixBF4-x
BF3 (gas)+EC (or DMC)→polymer
The highly reactive Lewis acid gas BF3 immediately reacts with organic or inorganic molecules. The reaction between BF3 and EC is mainly through the boron atom and carbonate oxygen, which causes the ring of EC to be broken to form a polymer. XPS measurement of the graphite electrode after bombardment with argon ions shows that the SEI film formed on the graphite electrode after heating is "macroscopic". That is, the SEI film covers the entire electrode instead of wrapping on the surface of each graphite particle. Therefore, the formation and destruction of SEI membranes involves a series of complex chemical reactions, which depend not only on the water content in the electrolyte, but also on the nature of the anion in the electrolyte and the structure of the carbon material.
At 2.2~2.1V (relative to Li/Li+), vinyl chloride carbonate is reduced and decomposed into CO2 on the graphite electrode. Through the secondary reaction, CO2 participates in the formation of an effective SEI film. Inserting or filling a small amount of Li2CO3 into the micropores of the amorphous carbon negative electrode material can significantly reduce the amount of CO2 adsorbed by the amorphous carbon material and greatly reduce the irreversible capacity of the carbon material. This is due to the filling of the micropores by Li2CO3. Adjusting the amount of Li2CO3 and the carbonization temperature can increase the reversible capacity of the material.
