At present, the lithium salt of commercial lithium-ion battery electrolyte is mainly LiPF6, and LiPF6 confers the excellent electrochemical performance of the electrolyte. However, the thermal stability and chemical stability of LiPF6 are relatively poor and very sensitive to moisture. Under the action of a small amount of H2O Decomposition of HF and other acidic substances, and then corrode the cathode material leading to the dissolution of transition metal elements, and migrate to the negative electrode surface, destroy the SEI film, resulting SEI film continued growth, resulting in lithium-ion battery capacity continued decline.
In order to overcome these problems, there have been hopes for lithium imide salts such as lithium salts such as LiTFSI, LiFSI and LiFTFSI, which are more stable to H 2 O and have better thermal and chemical stability, but are subject to cost constraints , As well as the inability to solve the problems such as the corrosion of the Al foil by the anions of lithium salts such as LiTFSI and the like, LiTFSI lithium salts have not been used in practice Recently, Varvara Sharova of HIU Laboratory in Germany found new applications for imide lithium salts The way out - as an electrolyte additive.
Lithium-ion battery graphite anode potential is relatively low, will lead to decomposition of the electrolyte in the surface, the formation of the passivation layer, which is commonly known as the SEI film. SEI membrane to prevent the electrolyte continues to decompose on the anode surface, the SEI membrane Stability is of crucial importance for the cycling stability of lithium-ion batteries.Although lithium salts such as LiTFSI are temporarily unavailable as solutes for commercial electrolytes, they provide very good results when used as additives. Varvara Sharova's It was found that the addition of 2wt% LiTFSI to the electrolyte can effectively improve the cycle performance of LiFePO4 / graphite battery: 600 cycles at 20 ℃, the capacity decline less than 2%, while the control group added 2wt% VC additive electrolysis Liquid, the same conditions, the battery capacity decline reached 20% or so.
In order to verify the effect of different additives on the performance of Li-ion batteries, Varvara Sharova respectively prepared blank group LP30 (EC: DMC = 1: 1) without additives and experimental group electrolytes with addition of VC, LiTFSI, LiFSI and LiFTFSI, The performance of these electrolytes was evaluated in half and full cells.
The graph above shows the voltammetric curve of the electrolyte in the blank control group and the experimental group. During the reduction process, we noticed a significant current peak at about 0.65V in the blank group electrolyte, corresponding to the reduction and decomposition of the EC solvent, In the experiment group with VC additive, the decomposition current peak of the electrolyte shifted to the high potential, mainly because the decomposition voltage of VC additive was higher than that of EC, so the decomposition was first performed and the protection of EC was formed.When LiTFSI was added, The voltammograms of LiFSI and LiFTFSI electrolyte did not show any significant difference from the blank group, indicating that the imide additives did not reduce the EC solvent decomposition.
The above graph shows the electrochemical performance of the graphite negative electrode in different electrolytes. From the first charge-discharge efficiency, the first charge-discharge coulombic efficiency of the blank group was 93.3%. The first efficiency of LiTFSI, LiFSI and LiFTFSI electrolytes 93.3%, 93.6% and 93.8%, respectively, but the first efficiency of electrolyte added with VC additive was only 91.5%, mainly because VC decomposed more on the surface of graphite negative electrode during the first lithiation of graphite Li.
The composition of SEI membrane will have a greater impact on the ionic conductivity, thereby affecting the rate performance of lithium ion batteries, the rate performance test found that the use of LiFSI and LiFTFSI additive electrolyte, the discharge capacity in the large current play slightly lower than Other Electrolytes. In the C / 2 cycle test, all of the imide-based electrolyte cycles were very stable, whereas the VC additive added electrolyte experienced a decay in capacity.
In order to evaluate the stability of the electrolyte in the long-term cycle of Li-ion batteries, Varvara Sharova also used a button cell to prepare a LiFePO4 / graphite whole cell. The cycling performance of the electrolyte with different additives at 20 ℃ and 40 ℃ was evaluated , The following table is the evaluation results.From the table, we can see that the electrolyte with LiTFSI additive is not only the first efficiency is significantly higher than the addition of VC additive electrolyte, the cycle performance at 20 ℃ is more overwhelming advantage, the cycle 600 The capacity retention of LiTFSI with secondary addition of LiTFSI was 98.1%, while the capacity retention with addition of VC additive was only 79.6%. However, this advantage disappeared when cycled at 40 ° C and all of the electrolyte With similar cycle performance.
From the above analysis, it is not difficult to see that the imide lithium salt as a electrolyte additive can significantly improve the cycling performance of lithium-ion batteries.In order to study the mechanism of LiTFSI and other additives in lithium-ion batteries, Varvara Sharova using XPS Graphite negative SEI membrane components formed in different electrolyte were analyzed, the following picture shows the XPS analysis of the SEI film formed on the surface of the graphite negative electrode after the first and the 50th cycle.It can be seen in the addition of LiTFSI additive The content of LiF in the SEI film formed in the electrolyte was significantly higher than that of the electrolyte added with the VC additive.Further quantitative analysis of the composition of the SEI film showed that the order of the LiF content in the SEI film after the first cycle The LiFSI> LiFTFSI> LiTFSI> VC> blank group, but the SEI film is not constant after the first charge is formed.With the cycling of the battery, the composition of the SEI film is constantly changing, and after 50 cycles The LiF content in SEF films in LiFSI and LiFTFSI electrolytes decreased by 12% and 43% respectively, while the LiF content in LiTFSI-added electrolyte increased by 9% instead.
Generally, we think that the structure of SEI membrane is divided into two layers: the inner inorganic layer and the outer organic layer. The inorganic layer mainly consists of inorganic components such as LiF and Li2CO3. The electrochemical properties of the SEI film are better and the ionic conductivity is higher , The outer organic layer is mainly composed of porous electrolyte decomposition and polymerization products, such as ROCO2Li, PEO composition, the protection of the electrolyte is not strong, so we hope that the SEI film inorganic components more. Class additives can bring more inorganic LiF components to the SEI film, so that the structure of the SEI film is more stable, which can better prevent the electrolyte from decomposing during the cycle of the battery and reduce the consumption of Li, so as to significantly improve the performance of the battery Cyclic performance.
Lithium imide as an electrolyte additive, especially LiTFSI additives can significantly improve the battery cycle performance, mainly due to the addition of LiTFSI, graphite surface of the SEI film formed LiF more, SEI film more Thinner and more stable, thus reducing the decomposition of the electrolyte, reducing the interface resistance.But from the current experimental data, LiTFSI additive is more suitable for use at room temperature, LiTFSI additive at 40 ℃ high temperature compared to the VC additive No obvious advantage.
