Contact of lithium metal with PC solvent can produce a surface layer. In other words, the decomposition of PC can be caused by spontaneous chemical reactions even without electrochemical reactions. The main substance contained in the surface layer is Li2CO3 [14, 15], and ex-situ FTIR analysis of alkyl lithium carbonate (ROCO2Li) is the most important substance in the surface layer [16]. When in contact with lithium metal, the HOMO and LUMO of the electrolyte change. In lithium metal, electrons are freely transferred from the valence band to the conduction band, and then from the conduction band to the electrolyte LUMO. A positive charge is generated on the surface of the lithium metal, while a negative charge is released from the electrolyte. These chemical reactions create a surface layer on the lithium metal surface. When the lithium battery is charged, the lithium ions are reduced, and the electrons are transferred to the negative electrode through the external circuit. At this time, the energy level of the valence band increases and reaches the LUMO energy level. The transfer of electrons from the anode to the electrolyte is more likely to stimulate the reduction and decomposition reaction of the electrolyte. If you are also interested in lithium batteries, you can click Tycorun Lithium Battery to learn more.
Surface layers can also be formed when Li metal and graphite are placed in other organic solvents such as EC, DEC or EC+DEC. Taking EC as an example, chemical electron transfer occurs on the surface of the lithium metal anode, and a passivation layer is formed at the lithium metal and lithium-graphite interface. When DEC is used as a solvent, no surface layer is formed on the Li metal surface. Instead, lithium ions dissolve out into the electrolyte, and the organic solvent turns dark brown. FTIR analysis of the brown solution indicated the presence of a mixture of CH3CH2OCO2 and CH3CH2CO2Li [16]. By recognizing the reduction and decomposition reactions of the above electrolytes, we have a deeper understanding of the electrochemical interface reaction mechanism.
From a thermodynamic point of view, solvents with lower LUMO orbital levels have higher interfacial reactivity because they are better able to accept electrons transferred from lithium metal. Many cyclic carbonates have similar LUMO energy levels, and double bond or atom substitution will lower the LUMO energy level. Electrolyte additives containing many functional groups were commonly used in the formation and stabilization of early SEI films. The relatively low LUMO orbital energy levels of these additives allow electron transfer at high reduction potentials, which results in more stable SEI films.
When lithium metal is placed in the DMC electrolyte, a reductive decomposition reaction of the electrolyte occurs with electron transfer, as described below. The reduction reaction product forms a passivation layer on the surface of lithium metal [16].
As shown in Figure 1, FTIR analysis was performed on separately synthesized lithium alkyl carbonate and on the surface of lithium metal soaked in DMC electrolyte. Comparing the analysis results of the spectrum with the calculated spectrum results of Hartree-Fock, it can be seen that the passivation layer is composed of lithium carbonate, lithium oxalate and methyl lithium [17]. These results suggest that the interfacial layer is generated by the reaction of lithium metal with an organic solvent, and the reductive decomposition reaction is initiated by electron transfer from lithium metal to the organic solvent.
When lithium metal comes into contact with DMC, electrons are transferred from lithium metal to DMC, and the ester bond of DMC begins to break. The reaction process is shown in the chemical equation in the figure below. As a result, unstable CH3·radical and methylcarbonate were generated
salt ions. Ethane gas is released when the CH3·radicals come into contact with each other.
Methyl carbonate ions react with lithium ions to form CO2 and LiOCH3, along with gas production. At the same time, the acyl bond of the ester group is broken, resulting in a stable CH3O- ion and an acyl radical. Although lithium alkyl carbonates are considered to be thermodynamically stable compounds, they can generate oxalate and CH3O radicals and even methoxides when they come into contact with acyl radicals [16].
The resulting alkyl lithium carbonates are stable below 400 K. The following thermodynamic decomposition process begins at 400 K, which produces CO2 gas and Li2CO3 [18].
A similar phenomenon was observed for the graphite anode in 1M LiPF6/EC:DEC (2:1) electrolyte. According to the thermodynamic analysis results shown in Figure 2, the passivation layer was decomposed at 220 °C and converted into Li2CO3 [17].
EC further promotes the generation of alkyl lithium carbonate. From the FTIR spectroscopic analysis of Li metal immersed in LiAsF6/EC:DEC (1:1) electrolyte, (CH2OCO2Li)2 is the main component of the SEI film [16]. The EC electrolyte produces vinyl sodium bicarbonate lithium salt (CH2OCO2Li)2 and releases ethylene gas as the following two-electron reduction reaction proceeds:
Spectroscopic analysis of the as-synthesized sodium-lithium vinyl bicarbonate salt and metal surface composition. The comparison of the analytical results with the theoretically calculated structure further confirms the existence of sodium and lithium vinyl bicarbonate in the actual SEI film [18]. The SEI film containing vinyl sodium bicarbonate lithium salt is caused by the electrochemical reduction reaction of the electrolyte at a potential of 1.8–1.9 V against lithium and shows a large reduction peak. EC is generally believed to accept electrons from lithium metal and be reduced by the following steps:
Due to the strong interaction between O…Li-O in (CH2OCO2Li)2, vinyl sodium bicarbonate lithium salt is the main component in the surface functional groups. This intermediate radical ion is coordinated with the hydroxyl group, and the neutral ethylene gas is released. Lithium vinyl bicarbonate reacts with ppm water and is converted to Li2CO3.
According to the experimental thermodynamic analysis results, ethylene gas is also the product of EC reductive decomposition reaction. Ethylene and other gases are released during reductive decomposition reactions of all organic solvents. When electrolyte decomposition is accelerated in a lithium-ion battery, the rapid release of large amounts of gas can cause the battery to swell. A small amount of water exists in the electrolyte in the form of impurities or is adsorbed on the electrode surface, which will lead to the hydrolysis of lithium ions (such as BFA4-, PF6-) and generate HF gas [16], which will decompose the alkyl lithium carbonate into LiF .
As an unstable compound, the lithium alkyl carbonate may become another compound due to the continuous reduction reaction caused by prolonged contact with lithium metal.
The compound generated by the reduction reaction of the electrolyte solution forms an SEI film on the surface of the electrode. The composition of the SEI film can be detected by XPS analysis by immersing lithium metal in 1 M LiBF4-PC or γ-butyrolactone or after one electrochemical cycle. Alkyl lithium carbonate and other organic substances were found in the outer layer, and inorganic substances such as Li2O and Li2CO3 were detected near the metallic lithium layer. When LiBF4 is hydrolyzed to produce HF and HF is adsorbed on the inner layer of the SEI film, LiOH, Li2O and Li2CO3 are all converted into LiF. The Li-based inorganic compounds formed by the reaction coexist with the spontaneously formed LiOH, Li2O and Li2CO3 on the Li metal surface [19].
