In addition to lithium metal, the highest capacity of the current anode material is silicon-based material, with a theoretical specific capacity of up to 4200mAh/g. The silicon-based anode is unstable in the existing electrolyte system, and the volume of the silicon particle continues to expand and contract during the charging and discharging process, resulting in the maximum volume expansion rate of silicon particles reaching 300%, which is easy to cause the rupture and pulverization of the anode interface, bringing a series of performance deterioration and safety problems.
As a transition material, silicon-oxygen anode can not only ensure a certain high capacity, but also reduce the volume expansion caused by pure silicon, and improve the stability of pure silicon anode. There are still inevitable problems in the use of such materials, such as poor cycling stability, high temperature failure and more gas production compared with graphite systems.
In order to solve these problems, the authors applied the common basic electrolyte additives for graphite anodes, such as vinylene carbonate (VC), 1,3-propanesulfonolactone (PS), fluoroethylene carbonate (FEC) and vinyl sulfate (DTD), to NCM811|SiOx/graphite flexible packaging batteries and Li|In SiOx/graphite button batteries, the effects of each additive on the silicon-oxygen anode were analyzed by the initial charge-discharge performance, electrochemical impedance spectroscopy (EIS), formation gas production, DC internal resistance (DCIR), rate discharge, cycling performance and anode interface morphology.
1 Experiment
1.1 Preparation of electrolyte and preparation of battery
The benchmark electrolyte composition was 1mol/L LiPF6/ EC+EMC (mass ratio 3 ∶7), which was recorded as a blank group. VC, PS, FEC and DTD with 1% and 2% mass fractions were added to the reference electrolyte for comparative testing.
NCM811/SiOx flexible packaging battery is a 575166 type battery with a rated capacity of 1Ah, of which the positive active material is NCM811 and the negative active material is SiOx/graphite (the mass fraction of SiOx is 10%).
The negative electrode piece of CR2032 button battery is derived from the whole battery of flexible packaging, which is disassembled and cut into a circular piece with a diameter of 12mm, the diameter of the circular lithium metal sheet used is 15mm, and the diameter of the circular polyethylene (PE) separator is 17mm. The assembly method is as follows: in the argon protected glove box, according to the order of the negative electrode shell, gasket, negative electrode sheet (half cell is lithium sheet), separator, positive electrode piece (half cell is SiOx/graphite electrode piece) and positive electrode shell, put it from bottom to top in turn, inject 1~2ml of electrolyte, and seal. After sealing, the battery is left at room temperature for 24 hours, and then the follow-up performance test is carried out. The NCM811/SiOx flexible packaging battery was cut in the glove box, injected with an appropriate amount of electrolyte, resealed, and left at room temperature for 24h for aging. The battery is precharged with a battery test system, set aside at 45°C for 24 hours, aged, and then sealed for the second time and formed, and the normal battery performance test can be carried out.
Pre-charge: Charge to 3.7V at 0.20C, and charge to 0.05C at constant voltage. Formation: (1) Charge to 4.2V at 0.20C, charge to 0.05C at constant voltage, and discharge to 3.0V at 0.20C;(2) Charge to 4.2V at 0.50C, charge to 0.05C at constant voltage, and discharge to 3.0V at 0.50C;(3) Cycle 3 steps (2).
1.2 Test Methods
Formation gas production: The volume of flexible packaging batteries before and after formation is tested by the drainage method, and the difference obtained is the formation gas production.
DCIR: After the flexible packaging battery is formed, it is charged to 50% of the rated capacity at 0.50C constant current, charged and discharged at 0.20C and 0.50C for 10s respectively, and the ratio of voltage difference to current under the same current is calculated, which is DCIR.
Rate discharge: After the battery is formed, it is charged to 4.2V at a constant current of 0.20C at room temperature of 25 °C, and then placed in a high and low temperature cabinet at a constant temperature, and then put it in a high and low temperature cabinet at a constant temperature, and then put it for 4 hours, and the discharge test of different currents is the ratio of the discharge capacity to the capacity of 0.20C at room temperature, which is the rate discharge performance of the battery at a certain temperature.
High-temperature storage: After the battery is formed, it is charged to 4.2V at a constant current of 0.20C at room temperature of 25 °C, the open-circuit voltage (OCV) is tested with a digital multimeter, the internal resistance is tested by the internal resistance tester, and the volume before and after storage is tested by the Archimedes drainage method, and then placed in a high-temperature box, taken out at intervals, discharged to 3.0V at 0.50C, and then cycled 3 times at 3.0~4.2V at 0.50C, and the ratio of the first discharge capacity to the capacity before storage is the capacity retention rate after shelving. The ratio of the capacity after three cycles to the capacity before storage is the capacity recovery rate. After cycling, the OCV, internal resistance, and volume were tested to analyze the changes.
Cycle performance: After the battery is formed, it is charged and discharged in a high and low temperature box at a certain temperature. The charging and discharging system is as follows: charge to 4.2V with 0.50C (or 1.00C) constant current, charge to 0.02C with constant voltage, and discharge to 3.0V at 1.00C.
Button battery formation: After liquid injection and sealing, it is left at room temperature 25°C for 24h, discharged to 0.01V with a constant current of 0.05C, and then charged to 2.00V at the same current.
EIS test: The button battery is charged and discharged 3 times at 0.50C constant current, and then charged to 50% of the rated capacity at 0.50C constant current. Tested with an amperostat.
2 Results and Discussion
2.1 Initial charge-discharge performance
The additives VC, PS, FEC and DTD were effective for Li|The effect of the initial charge-discharge performance of SiOx/graphite button batteries is shown in Figure 1.
As can be seen from Figure 1, the first lithium insertion and delithiumization capacity of the blank button battery without any additives are low. Batteries with additives, except for those containing PS, have higher lithium intercalation and delithiumization capacity than those containing VC, FEC and DTD, which are about 30% higher than those of blank button batteries, among which the button batteries containing DTD have the highest delithiumization capacity due to their better film-forming properties and solid electrolyte phase interface (SEI) film composition. Button batteries containing DTD have the highest lithium intercalation capacity, but some of the active lithium will be lost during delithiumization, mainly because the decomposition products of FEC are easy to cause the SEI film to dissolve, and lithium needs to be continuously consumed to generate the SEI film. The lithium intercalation and delithiumization capacity of PS-containing button batteries are about 40% lower than those of blank button batteries, because although the electrolyte containing PS will also form a SEI film at the negative electrode during the first lithium intercalation process, the film impedance of the formation is too large, resulting in the obstruction of Li+ transmission, which affects the capacity play.
2.2 EIS comparison
The additives VC, PS, FEC and DTD were effective for Li|The effects of the EIS of SiOx/graphite button cells are shown in Figure 2.
In Figure 2, the intersection of the impedance spectrum and the transverse axis is considered to be the impedance of the electrolyte itself, and the transverse and vertical axes corresponding to the semicircle are the negative SEI film impedance (RSEI) and the charge transfer impedance (Rct), respectively. As can be seen from Figure 2, the impedance of the electrolyte itself with several additives is not much different. The RSEI and Rct of the PS-containing electrolytes were the largest, which was attributed to the difference in film-forming composition, resulting in a large Li+ insertion resistance, which was consistent with the initial charge-discharge performance. In addition to PS, the VC-containing electrolyte also has a large RSEI and Rct, because there are many inorganic components in the SEI film, which affects the conductivity of the film. The RSEI and Rct of the electrolytes containing FEC and DTD were low, the EIS curves of the electrolytes containing FEC were almost coincident with the blank group, the film formation was thin and dense, and the composition of the electrolyte containing DTD had more organic components. According to the EIS data, FEC and DTD are more suitable for silicon-oxygen anode systems to reduce the impedance of the battery, thereby improving the rate and long-term cycling performance.
2.3 Gas production of flexible packaging batteries
During the formation phase of the battery, as the voltage rises, some of the solvents in the electrolyte will continue to decompose, producing gas. In the case of different VC, PS, FEC and DTD addition amounts, NCM811|The results of the gas production analysis of the SiOx/graphite flexible packaging battery charging to the 4.25V stage are shown in Table 1.
It can be seen from Table 1 that VC, PS and FEC can significantly inhibit the formation of gas, and the effect of FEC is the most obvious, when the content is 1%, the formation gas production of flexible packaging batteries is about 70% lower than that of blank batteries, and as the FEC content increases to 2%, the formation gas production increases. VC and PS can also inhibit the formation of gas production, the blank battery formation gas production is 17.82ml, and the battery containing 1% VC and PS electrolyte has the formation gas production of 5.74ml and 10.16ml, respectively, a decrease of 67.8% and 43.0%, respectively. The reason for this phenomenon is that the film-forming potential of VC, PS and FEC is higher than that of carbonate solvent, and during the first charge of the battery, it is preferentially preferentially film-forming at the negative electrode, which reduces the decomposition and gas production of carbonate solvent. Among the batteries containing additives, the gas production is the highest when DTD is used, and the gas production is 28.12ml and 38.58ml when the content is 1% and 2%, respectively, because DTD is easy to transesterify with carbonate to form bisulfate, and in this reaction process, the carbon chain of carbonate is broken to form olefin compounds, resulting in an increase in gas production.
The composition of blank battery formation gas is mainly methane (CH4), ethane (C2H6) and H2. CH4 and C2H6 are mainly carbonate decomposition, and the olefins produced are further reduced to alkanes. H2 mainly comes from the decomposition of H2O in the separator, electrode piece and electrolyte. Cells using DTD produce the most C2H6 during formation because DTD decomposes into ethylene (C2H4) during the first film formation process, and C2H4 is further reduced to C2H6. The electrolyte containing 2% FEC obviously generated CO2 during the first formation process, and the FEC decomposed under the action of lithium salt, resulting in the formation of LiF and CO2, of which LiF is an important component of the SEI film.
2.4 Charge and discharge DCIR
At different temperatures, NCM811|SiOx/graphite flexible packaging batteries were charged and discharged by DCIR, and the results are shown in Figure 3.
A: Charging B: Charging
Figure 3 NCM811|DCIR of SiOx/graphite flexible packaging batteries at room temperature and low temperature
As can be seen from Figure 3(a), the additives cause NCM811|The impedance difference of SiOx/graphite flexible packaging batteries is obvious, and the discharged DCIR is smaller than that of the charged battery. The impedance of the battery can be divided into three types: concentration impedance, ohmic impedance and interfacial impedance, and the interfacial impedance becomes the dominant factor at room temperature.
As can be seen from Figure 3(b), the impedance difference between the batteries with different additives is very small at low temperature (0°C), almost on the same level, and the discharge DCIR is higher than that at normal temperature. This is because at low temperatures, the impedance of the battery is mainly derived from the kinetics of the electrolyte itself, which is mainly related to the solvent system.
The interfacial impedance plays a dominant role in DCIR at room temperature, which in turn is related to the composition of the film. From the comparison of several additives, PS has the highest membrane impedance, followed by VC and DTD, and FEC has the lowest membrane impedance. From the perspective of the film-forming mechanism of FEC, in addition to the formation of LiF, the formation of organic polymers may also reduce the impedance of SEI films.
2.5 rate discharge performance
NCM811|The discharge performance of SiOx/graphite flexible packaging batteries at room temperature and low temperature rates is shown in Figure 4.
1：空白 2：1%VC 3：1%PS 4：1%FEC 5：1%DTD
A：0.5C B：2.0C
Figure 4 NCM811|Normal and low temperature rate performance of SiOx/graphite flexible packaging batteries
It can be seen from Figure 4 that when the discharge current is 0.5C, the discharge capacity of the flexible packaging battery with different additives is not much different, but when the discharge current is increased to 2.0C, the discharge capacity of the flexible packaging battery with different additives is significantly different. At room temperature, the discharge capacity of the battery containing FEC is the highest at 2.0C, close to 80% of the rated capacity; PS Due to the obvious increase in the impedance of the battery after film formation, the discharge performance at high rate is the worst, and the capacity is only 47% of the rated capacity, which is even lower than the blank electrolyte without any additives; At 2.0C, the discharge capacity of batteries containing VC and DTD is about 65% of the rated capacity, which is higher than that of blank. This indicates that VC, FEC and DTD are beneficial to the discharge performance at room temperature rate and are consistent with the data of DCIR at room temperature. At low temperatures (-20°C), batteries containing FEC still have a high discharge capacity, which at 0.5C is about 70% of the rated capacity. The fluidity of the electrolyte deteriorates at low temperatures, and the kinetic properties are reduced, resulting in poor discharge performance at high rates, even for batteries containing FEC, the discharge capacity of 2.0C at low temperatures is only about 20% of the rated capacity. In order to increase the low high-rate discharge capacitance, it is necessary to adjust the solvent composition of the electrolyte, reduce the content of high freezing point EC, and increase the use of low boiling point solvents.
2.6 High temperature storage performance
High-temperature storage performance is an important evaluation index of flexible packaging batteries. NCM811|A comparison of the performance of SiOx/graphite flexible packaging batteries stored at 60°C for 14 days is shown in Figure 5.
Figure 5 NCM811|High temperature 60°C storage (14d) performance of SiOx/graphite flexible packaging batteries
As can be seen from Figure 5(a), VC, PS and DTD can improve the capacity retention rate and recovery rate of the battery after high-temperature storage, among which DTD has the best effect, compared with the blank battery, the capacity retention rate after high-temperature storage is increased by 7% and the capacity recovery rate is increased by 10%, mainly because of the better film-forming performance of the anode and its own oxidation stability. Under high temperature conditions, FEC is easy to oxidize and decompose due to its instability, resulting in low capacity recovery and retention rates. FEC has a low internal resistance due to the thin initial film formation, and the SEI film continues to thicken and the growth rate of internal resistance increases due to continuous rupture and repair in the later stage.
As can be seen from Figure 5(b), FEC increases gas production during high-temperature storage, and the gas generation will cause the contact inside the battery to deteriorate, affecting the capacity retention rate and recovery rate. In summary, FEC is used in silicon-oxygen anode systems, although it can reduce impedance, improve magnification and low-temperature performance, but there are shortcomings in high-temperature performance.
2.7 Cycle performance
For commercial flexible packaging batteries, cycle performance is also an important evaluation index. NCM811|The normal and high-temperature cycling performance of SiOx/graphite flexible packaging batteries is shown in Figure 6.
1：空白 2：1%VC 3：1%PS 4：1%FEC 5：1%DTD
Figure 6 NCM811|Normal and high temperature cycling performance of SiOx/graphite flexible packaging batteries
As can be seen from Figure 6(a), several additives can effectively form films, and have a stable effect on the interface after film formation, so they can improve the cycling performance at room temperature. FEC greatly improves the battery performance at room temperature, and the capacity retention rate of 800 cycles is 70%. There was no significant difference between VC and DTD-containing cells, and the capacity retention rate for the 700th cycle was 14% lower than that of FEC-containing cells, but about 15% higher than that of blank groups. PS Due to the large film-forming impedance, the kinetics at room temperature deteriorated, resulting in poor cycling performance at room temperature, which was slightly better than that of the blank group. In general, VC, FEC and DTD are all conducive to improving the normal temperature cycling performance of silicon-oxygen anode batteries, and PS has almost no effect.
As can be seen from Figure 6(b), FEC dissolves the SEI film due to its thin film formation and decomposition into HF at high temperature, resulting in poor high-temperature cycling performance. DTD showed a significant improvement in performance for high-temperature cycling, with a capacity retention rate of 70% for 450 cycles at 45°C with DTD alone, while it was reduced to 60% at 400 cycles in the blank group. The effects of PS and DTD on high-temperature cycling performance are similar, and it can be seen that chalcogenide additives, such as sulfate esters and sulfonate compounds, which have a protective effect on the cathode, are conducive to improving high-temperature cycling performance.
2.8 SEM Analysis
The electrochemical performance test results show that FEC and DTD are more effective additives for silicon-oxygen anode, but the effect is different. In terms of high-temperature battery performance, DTD has obvious advantages over FEC. The interface morphology of the silicon-oxygen anode sheet after high temperature cycling was observed by SEM analysis, and the effects of the two additives on the anode interface were compared, and the results are shown in Figure 7.
Figure 7 SEM image of the negative electrode after high temperature cycling of an electrolyte containing FEC and DTD
It can be seen from Fig. 7(a) that after high temperature cycling of the electrolyte containing FEC, there is obviously a thick layer of covering on the surface of the negative electrode, and the edge of the interface of the silicon particles is not clear, indicating that the SEI film is continuously broken and repaired after dissolution at high temperature, and more electrolyte reduction reaction products are generated at the negative electrode interface and covered on the surface. As can be seen from Fig. 7(b), the interface of silicon particles is relatively clear and the covering is less after high temperature cycling of the electrolyte containing DTD, indicating that the stability of the SEI film at high temperature is relatively good.
3 Conclusion
In this paper, the authors prepared a lithium-ion battery with high nickel cathode material, and evaluated the effects of four commonly used additives VC, PS, FEC and DTD on the silicon-oxygen anode. In addition to the decomposition of DTD itself, other additives can reduce the formation of gas. VC and PS significantly increase the DCIR at room temperature, both of which can improve the normal temperature and high temperature cycle performance of the battery, but PS reduces the rate discharge ability of the battery. The comprehensive performance of DTD is good, the DCIR at room temperature and low temperature does not increase significantly, the rate performance is excellent, and the cycle performance is improved, especially the high temperature cycle performance, but the disadvantage is that the formation gas production is large, which leads to the unsatisfactory interface condition after formation and affects the performance of the battery in the later stage. FEC can significantly reduce the formation of gas production, reduce DCIR, improve the rate discharge at room temperature and low temperature, and have outstanding advantages in normal temperature cycle performance, but the disadvantage is poor performance at high temperature.
Source: Battery Technology TOP+
Note: Most of the articles reproduced on this site are collected on the Internet, and the copyright of the article belongs to the original author and the original source. The views in the article are only for sharing and exchange, if it involves copyright and other issues, please let us know, and I will deal with it in a timely manner.