The effect of electrolyte additives on the performance of silicon carbon batteries!

battery Technology TOP+ November 11, 2024 09:01 Anhui

At present, the most studied electrolyte additives such as: The additives such as vinylidene carbonate (VC), fluorovinyl carbonate (FEC), allyl sulfite (PS), vinyl sulfate (DTD), allyl-1, 3-sulfonolide (PST), methyl methane disulfonate (MMDS) have been gradually used in many aspects in the silicon carbon negative electrode system. Thus, the comprehensive performance of the battery material is better improved. Thioorganic solvent is a good film forming additive for solid electrolyte phase interface film (SEI) of lithium ion battery, which can effectively improve the characteristics of SEI film and enhance the performance of battery.

In this paper, two additives, propenyl-1, 3-sulfonolide (PST) and methylenedisulfonate (MMDS), were compared and used together to investigate the improvement of high temperature cycling and high temperature storage properties of silicon carbon anode system.

1 Experiment 1.1 Preparation of positive electrode of soft coated electrode sheet: binder PVDF-S5130, composite conductive agent Super-P/KS-6, NCM811 terpolymer positive electrode material and solvent NMP were used to adjust the viscosity of positive electrode to 8000mPa ·s, and then mixed into positive electrode paste according to a certain proportion.

Negative electrode: Silicon carbon composite material, conductive agent Super-P, thickener CMC, solvent H2O and binder SBR are used as raw materials. The negative electrode adjusts the viscosity to 3000mPa ·s and is mixed into negative electrode paste according to a certain proportion.

The positive slurry is evenly coated on the surface of the aluminum foil, and the negative slurry is evenly coated on the surface of the copper foil, and then dried in the oven.

1.2 Production of soft pack battery

By slicing, rolling, slitting, drying, taping, coiling, drying at 80℃ for 48 hours, lithium-ion battery was sealed with liquid injection according to different electrolyte formulations in Table 1, shelving for 24 hours, forming, primary final sealing, aging and secondary final sealing, lithium-ion soft pack battery was prepared, and then the storage performance of the battery was tested at high temperature.

1.3 Theoretical calculation and performance test

1.3.1 In this paper, the Gaussian 09 package is used to calculate the density functional theory, taking 6-311++G(d, p) as the functional basis group. The molecular structures of vinyl carbonate (EC), methyl ethyl carbonate (EMC), vinyl fluorocarbonate (FEC), allyl-1, 3-sulfonolide (PST) and methylenedisulfonate (MMDS) were optimized at the B3LYP theoretical level.

1.3.2 Formation test silicon carbon soft pack battery formation process: stand 10min, 0.05C constant current charge 120min, 0.1C constant current charge 120min, 0.2C constant current charge to 3.85V cut-off.

1.3.3 High temperature cycle performance test High temperature cycle performance: charge and discharge voltage range is 2.75 ~ 4.20V, charge current is 1C(1.8A) to 4.20V, 4.20V constant voltage charge to cut-off current ≤0.05C(0.1A), after standing for 5min, 1C(1.8A) discharge to 2.75V, standing for 5min; So cycle charge and discharge, calculate the capacity retention rate of different cycles.

1.3.4 High temperature storage performance High Temperature storage performance: 0.2C constant current constant pressure Full capacity C1, and thickness d1. After standing at 60℃ for 7 days, take it out immediately to test the hot thickness d2, cool it to room temperature for 4 hours to test the cold thickness d3. The discharge capacity of 1C(1.80A) at room temperature was denoted as C2, and the average discharge capacity was denoted as C3 for continuous cycle calculation for 3 times. The capacity retention rate is C2/C1, the capacity recovery rate is C3/C1, the thermal expansion rate is d2/d1, and the cold expansion rate is d3/d1. And carry out the corresponding impedance test (such as ACIR, DCIR). When disassembling the battery analysis interface, the battery is stored at a high temperature for one month.

1.4 Test equipment software package high temperature pressure semi-automatic formation cabinet, capacity cycle performance test using battery test system (CT2001A), impedance test using electrochemical workstation (PARSTAT 4000).

2 Conclusion and Comments

2.1 Based on frontier molecular orbital theory, the ability of organic molecules to gain and lose electrons can be predicted by the highest occupied molecular orbital (HOMO) value and the lowest unoccupied orbital (LUMO) value. Generally, the higher the HOMO value is, the lower the molecular oxidation potential is. When the battery is charged, the higher the oxidation potential is on the surface of the positive active material, and the molecules with high HOMO energy give priority to electrochemical oxidation and decomposition reactions, and have poor oxidation resistance to high voltage. The lower the LUMO energy is, the higher the molecular reduction potential is, and the molecules are more likely to undergo electrochemical reduction decomposition on the surface of the negative electrode material.

The HOMO value of PST molecule is higher than that of vinyl carbonate (EC) molecule, which indicates that PST has the potential of oxidation reaction before that of EC solvent molecule to generate positive CEI protective film during battery charging. At the same time, the LUMO value of PST molecule is much lower than that of vinyl carbonate (EC) molecule and methyl ethyl carbonate (EMC) molecule, which indicates that PST can be reduced and decomposed preferentially than solvent molecules on the surface of negative electrode materials, and has the possibility of acting as a negative electrode film forming additive. Therefore, it can be theoretically inferred that PST has the potential to become a dual-function film forming additive, which occurs electrochemical oxidation and reduction decomposition respectively on the surface of the positive and negative electrode materials, blocking the direct contact between the electrolyte and the electrode sheet, inhibiting the continuous decomposition of the electrolyte and the solvent and structural damage of metal ions, and improving the stability of the high pressure cycle of the battery.

The HOMO value of MMDS molecule is lower than that of vinyl carbonate (EC) molecule and methyl ethyl carbonate (EMC) molecule, which indicates that MMDS has higher oxidation potential and better oxidation resistance. At the same time, the LUMO value of MMDS molecule is much lower than that of vinyl carbonate (EC) molecule and methyl ethyl carbonate (EMC) molecule, which indicates that MMDS can also undergo reduction decomposition on the surface of negative electrode materials before solvent molecules, and has the possibility of acting as a negative electrode film forming additive.

The HOMO and LUMO values of solvent molecules and additive molecules are shown in Table 2. Figure 1 shows the HOMO and LUMO molecular orbital energy levels of EC, EMC, FEC, PST and MMDS.

PST and MMDS used as additives can not only improve the oxidation stability of the electrolyte, but also facilitate the formation of SEI film on the negative electrode surface, which can be used as an ideal electrolyte additive.

2.2 Result and data analysis

By comparing formulations 3 and 4, 5 and 6 (Figure 2), it can be seen that formulations 3 and 7 with PST are decomposed preferently at about 2.4V compared with formulations 4 and 5, and the peak value is higher, and the formed protective layer can effectively inhibit the decomposition of the electrolyte on the negative electrode surface. It can be concluded that this is consistent with the calculated HOMO and LUMO values of PST and MMDS molecules, and PST has the characteristics of preferential film formation in the positive and negative poles. At the same time, this is consistent with the data analysis of ACIR and DCIR growth rate, thermal expansion rate and cold expansion rate, capacity retention rate and capacity recovery rate using the two additives.

2.3 High temperature cycle results and data analysis

By comparing formulations 1, 2,3, and 4 (Figure 3), it can be seen that the use of additive MMDS makes the battery have a lower capacity retention rate, which is not conducive to improving the high-temperature cycle performance. At the same time, compared with formulations 5 and 6,7 and 8, it can be seen that the use of additives PST and MMDS causes deterioration of the battery and is not conducive to improving the high-temperature cycle performance. Further, the deterioration of MMDS to the electrode is explained. This is consistent with the results of the following capacity differential curves for different cycle times.

As can be seen from Figure 4, when the addition of PST and MMDS starts the first charge-discharge cycle, the positions of the delithiation peak and the lithium insertion peak almost coincide without deviation. However, at the 900th cycle, the peak shift and peak area decrease occurred in the charge-discharge process. And the use of PST and MMDS formed a more obvious contrast.

With the use of MMDS additive during the charging process, the delithium peak shifts seriously to the direction of high potential, and the peak current decreases obviously. In the process of discharge, the lithium peak shifted to the low potential and the peak area gradually decreased, which was consistent with the attenuation of high temperature capacity in Figure 3. The results showed that compared with PST additive, MMDS additive increased electrode polarization more, which may be related to the different film forming characteristics of the two. PST has film forming characteristics in positive and negative electrodes, and the LUMO value is relatively small. However, the use of MMDS additive could not prevent the violent side reaction between electrode and electrolyte. This results in the instability of the electrode/electrolyte interface, serious damage to the positive electrode structure, and poor reversibility of lithium ion removal/insertion.

Similarly, it can be seen from the differential capacity curves of formula 5 and 6 under different cycle times in FIG. 5 that, with the increase of cycle times, the joint use of PST and MMDS causes a large shift in the positions of the delithium peak and the lithium embedding peak, and the peak area gradually decreases, which is consistent with the analysis results in FIG. 3 and FIG. 4.

2.4 High-temperature storage results and data analysis

Table 3 shows the ACIR and DCIR values of different electrolyte formulations. By comparing formula 1 and 2, it can be seen that the addition of MMDS makes the battery have higher ACIR and DCIR, and has a larger impedance value. Compared with formula 3 and 4, it can be seen that adding PST has a larger impedance value than adding MMDS. The results show that there are differences between the film thickness and the film density of the two additives, both additives will increase the impedance, but the addition of PST is relatively more obvious. Compared with formulations 5 and 6, 7 and 8, it can be seen that the ACIR and DCIR with PST and MMDS added at the same time is higher than that without PST and MMDS, so that it has a larger impedance value.

By comparing formula 1 and 2, it can be seen that adding MMDS can improve the capacity retention rate and capacity recovery rate to a certain extent (Figure 6). Compared with formula 3 and 4, it can be seen that adding PST has better capacity retention rate and capacity recovery rate than adding MMDS. These results show that the use of the two additives can form a dense and stable SEI film on the surface of the negative silicon carbon electrode, prevent the continuous occurrence of side reactions on the surface, ensure the integrity of the interface, and effectively improve the stability of the electrochemical performance of the electrode.

Compared with formulations 5 and 6, 7 and 8, it can be seen that adding PST and MMDS at the same time has greater improvement in capacity retention rate and capacity recovery rate than that without adding PST and MMDS. By comparing formula 1 and 2, it can be seen that the addition of MMDS can inhibit and reduce the thermal expansion rate and cold expansion rate of the battery to a certain extent (Figure 7). Compared with formulations 3 and 4, the addition of PST has a lower thermal expansion rate and a lower cold expansion rate than the addition of MMDS. These results indicate that the SEI film formed on the surface of the negative silicon carbon electrode by the use of the two additives can alleviate the gas production problem caused by the continuous generation of SEI film which consumes lithium salt and solvent due to the volume expansion of silicon carbon to a certain extent, and effectively reduce the electrode expansion rate. Compared with formulations 5 and 6, 7 and 8, it can be seen that the thermal expansion rate and cold expansion rate of the battery with PST and MMDS added at the same time are significantly improved compared with those without PST and MMDS added at the same time, which is significantly lower than that without addition.

By comparing formula 1 and 2, it can be seen that the formula with MMDS has a relatively low ACIR growth rate and DCIR growth rate (Figure 8), indicating that its positive and negative electrode film formation has a certain inhibition effect on the impedance growth of the battery. By comparing formula 5 and 6, and formula 7 and 8, it can be seen that the formula with PST and MMDS has a smaller ACIR growth rate and DCIR growth rate. It shows that the dense SEI film on the electrode surface reduces the side reaction between the electrolyte composition and the silicon carbon anode. Meanwhile, compared with formula 3 and 4, it can be seen that the addition of PST has a relatively lower ACIR and DCIR growth rate than the addition of MMDS.

By disassembling the batteries of formula 5 and 6 in the fully charged state after 28 days of high temperature storage, it can be seen from the analysis of the negative electrode interface (Figure 9) that PST and MMDS additives were not added to formula 5, and a large area of gray metal lithium was precipitated at the negative electrode interface. In stark contrast to the formula 6 battery with the addition of two additives, the negative interface of the formula 6 battery is still golden as a whole, that is, LiC6 in a fully charged state. This is consistent with the smaller ACIR and DCIR growth rate of the previous battery with two additives, and the use of both additives reduces the increase in impedance while reducing the polarization of the battery. The use of additives can maintain the relative integrity of the battery interface by forming a negative electrode film, and reduce the generation of interfacial lithium.

3 Conclusion

In this paper, the effect of PST and MMDS additives on the high temperature performance of silicon carbon anode with high energy density is compared and analyzed. Through the analysis of the DFT density functional theory calculation results of different additives, it can be seen that both additives have the characteristics of preferentially forming films in the negative electrode, thus inhibiting the decomposition of carbonate solvent.

1. In terms of high-temperature cycle performance testing, PST, due to its excellent film formation, has a smaller increase in polarization than MMDS, so that lithium ion removal/embedding reversibility is better.
2. In terms of high temperature storage performance test, compared with MMDS, the use of PST has relatively small ACIR and DCIR impedance growth rate, lower expansion rate, higher capacity retention rate and capacity recovery rate. At the same time, the combined use of PST and MMDS additives significantly reduces the growth rate of ACIR and DCIR impedance of the battery, inhibits the thermal expansion rate and cold expansion rate, improves the capacity retention rate and capacity recovery rate of the battery, and ensures the integrity of the interface, thus obtaining better high-temperature storage performance.

References: QIAO Shunpan, Yang Huan, Sun Chunsheng, GUO Yingjun. Effect of film forming additives on high temperature performance of Silicon carbon System [J]. Power Supply Technology,2023,47(3):298-301. (in Chinese

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