Electrolyte is one of the four main materials of lithium ion battery, known as the "blood" of lithium ion battery, the electrolyte is mainly composed of organic solvents, electrolyte lithium salts and different types of additives. Organic solvents are the main part of the electrolyte, and the solvents commonly used in lithium ion batteries are vinyl carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl carbonate (EMC), etc. The mixed solvent of EC and a chain carbonate is the excellent electrolyte of lithium ion batteries, such as EC + DMC, EC + DEC, etc.LiPF6 Is the most commonly used electrolyte lithium salt, its high conductivity, small internal resistance, fast charge and discharge speed, but extremely sensitive to water and HF, easy to react, and not high temperature resistance, 80℃ ~100℃ will decompose, generate phosphorus pentafluoride and lithium fluoride, and appropriate additives can effectively reduce the trace water and HF in the electrolyte, and then effectively inhibit LiPF6 The occurrence of hydrolysis reaction, many research on the kinds of additives, different manufacturers have differences in battery performance and requirements, the choice of additives type is different, usually the effect of additives in addition to reduce the water in the electrolyte and H F, and improve the interface stability, high and low temperature performance and prevent overcharge, excessive application. Figure 1 shows the calculation results of the reduction potential of several common solvents, additives, and desolubilizing ions【1】
graph 1a. Electrote open circuit energy diagram;
b. The results of the reduction potential of several common solvents, additives and desolubilizing ions¹
The electrochemical window of conventional carbonate solvents is narrow and generally below 4.3V. After the voltage increases, the solvent will decompose, and the solvent will also side-react with the positive electrode of the high voltage state, resulting in the transition metal in the ternary material dissolution, and produce a large amount of gas, greatly reducing the capacity and even the safety of the battery. For the NCM ternary positive pole, the oxidation states of Ni, Co, and Mn are + 2, + 3, and + 4 valence, respectively. When charging, lithium ion from the positive electrode, so Ni from + 2 to + 3 and + 4 price, while the positive electrode and electrolyte reaction generates a solid electrolyte interface (CEI layer), CEI layer to protect the positive electrode, hinder the further reaction of the positive electrode and electrolyte, improve the stability of the positive electrode in the state of lithium. However, the formation of CEI membrane will also cause the increase of battery impedance, fold ratio attenuation, capacity attenuation and other problems. Therefore, the impact of the electrolyte system is very important on the gas production of lithium-ion battery. Internal gas production of the battery directly increases the safety risk of battery use, so the battery gas production is one of the important indicators to investigate the battery quality and reliability. At present, the research on the gas production behavior of lithium-ion batteries at home and abroad mainly focuses on the cathode and the electrolyte, and the action efficiency of the additives is closely related to the type and composition of the cathode materials. This paper analyzes the influence of different electrolyte systems on the gas production behavior and composition of L i.
Experimental equipment and test methods
1.Experimental equipment：model GVM2200 (I EST Yuan energy technology), test temperature range of 20℃ ~85℃, support dual-channel (2 cells) synchronous test, the appearance of the equipment as shown in Figure 2.
Figure 2. GVM2200 Equipment appearance diagram
2.Test parameters：0.3C CC to 4.4V at 70℃ temperature.
3.Test method:Select different electrolyte systems (Electr ol yte 1 & Electr ol y te2, in which Electr ol y te2 adds some additives on the basis of Electr ol y te1) in the glove box to assemble them into a single-layer laminated cell, and the cell is initially weighed by m0Put the cell to be tested into the corresponding channel of the equipment, open the MISG software, set the corresponding cell number and sampling frequency parameters of each channel, and automatically read the volume change, test temperature, current, voltage, capacity and other data. The gas composition test uses G C-2014C gas chromatography, removing 1m L of gas from the overcharged cell in the glove box, and testing different types of gas concentrations using T CD and F ID detectors, respectively. The measurable gas types are shown in Figure 3.
Figure 3. teable gas composition of F I D and T CD detectors
In-situ gas production and composition analysis of different electrolyte systems
1.Analysis of charging voltage curve and unit volume change curve
The change curve of the voltage and unit volume of the cells of the two different electrolyte systems is shown in Figure 4. As can be seen from the curves of the different electrolyte Electrolyte1 and Electrolyte2 cells, the charging voltage curves and the volume change curves of the two different electrolyte systems are obviously different.From the unit volume change curve, Electrolyte1 system cells always maintain a high rate of volume change throughout the charging stage, While the Electrolyte2 system cells maintain a low rate of volume change in the initial stage of charging, Charging voltage reaches about 4.2V, After the cell S OC reaches about 80%, The rate of volume change only increases significantly, This shows that the addition of additives in Electrolyte2 system can effectively reduce the unit gas production rate of N CM to Li cells; From the voltage curve, Compared to the Electrolyte1 system, The average charging voltage of the Electrolyte2 system cell is lower, If the differences in the assembly of the cell itself are ignored, The addition of additives to the Electrolyte2 system, May cause the cell NCM positive electrode and the electrolyte additive reaction to form the interface membrane, Preventing the continuous reaction of the electrolyte with the cathode material, The average voltage of the cell is also low.
Figure 4. Change curve of charging voltage and unit volume of two electrolyte systems
2.Analysis of gas production composition of different electrolyte systems
Gas chromatography method respectively for the two kinds of electrolyte system after charging cell gas composition analysis, take out 1m L of gas, gas chromatography for qualitative analysis, as shown in figure 5 for the different electrolyte system gas composition analysis, comparative analysis found that compared to Electrolyte1 system, Electrolyte2 system after adding the additive gas category CO2 This decreased significantly, while the CO increased significantly.
Figure 5 Differences in gas production composition between different electrolyte systems: T CD & FID
In order to further clarify the difference of gas production between the two electrolyte systems, the type and concentration of gas production were relatively analyzed. As shown in Table 1 and Figure 6, the Electrolyte1 system was CO2 after charging2The concentration was 6.949%, while after the addition of the additive, the Electrolyte2 system was CO2 The concentration was almost 0. The CO2 was reported according to related studies2 It is the main gas of the cathode reaction【2】 And the cathode gas production is mainly produced by the side reaction between the cathode material and the electrolyte. On the one hand, it comes from the decomposition of Li2CO3 on the cathode surface; on the other hand, the surface of the NCM material when the battery is charged to a relatively high voltage begins to release atomic oxygen. The strong oxidized atomic oxygen will lead to the oxidation decomposition of the electrolyte and produce CO2 And CO; the resulting H2 Mainly from the residual water in the electrolyte; this shows that the additive in Electrolyte2 electrolyte may be an effective positive electrode film-forming additive, can form a stable protective film on the cathode surface, and then effectively reduce the positive electrode and electrolyte side reaction, the same C2H4 And C2H2 The reduction in concentration also correlated with the reaction changes in the positive pole under this system. CO gas breaks down the EC solvent in the electrolyte on the surface of the negative electrode. For the change of CO concentration related to the negative electrode reaction, the C O concentration of Electrolyte2 system is 0.097%, compared with the CO concentration of 0.097%. In 870%, because the cathode material is greatly affected by the temperature in the gas production reaction of the electric cell system, the addition of additives in the Electrolyte2 system will reduce the thermal stability of the negative electrode and accelerate the occurrence of the side reaction at high temperature.In addition, several other types of gas concentrations will vary, which may be related to differences in cell assembly.
Figure 6 Comparison of the concentration of gas production types in different electrolyte systems
Table 1 Comparison table of gas production type concentration in different electrolyte systems
This paper adopts a controlled temperature dual channel in situ gas volume monitoring instrument, and combined with gas chromatography, compare NCM of L i battery gas behavior and gas composition difference, further clarify the additive to cell internal electrochemical reaction, and confirmed the analysis system can be used as an effective means of electrolyte formula optimization and additive performance evaluation and optimization and screening.
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