Research on Gas Production of High Nickel/Silicon Carbon Batteries under High Temperature Conditions
Aug,07,24
At present, research on the high nickel LiNi0.8Co0.1Mn0.1O2 (NCM811)/silicon carbon (Si-C) battery system mainly focuses on the cycling capacity,
and there is relatively little research on the gas generation that affects battery reliability,
especially the systematic study of gas generation behavior and mechanism during storage and cycling under different SOC (state of charge) states has not been reported yet.
This article uses drainage method to measure gas production, gas chromatography-mass spectrometry (GC-MS) technology to measure gas composition,
and unipolar storage method to analyze the source of gas production.
The study systematically investigates the storage of commercial flexible packaging lithium-ion batteries under high temperature and different SOC,
with a focus on analyzing the storage of 100% SOC and 0% SOC, as well as the gas production behavior during cycling.
Research has shown that in the high temperature range of 0-100% SOC, the gas production curve exhibits a bathtub shaped variation pattern.
At 100% SOC, gas production is significant and continues to increase over time, with the main gas components being CO2 and CO;
FEC and injection coefficient have a significant impact on gas production;
The main source of gas production is the reaction between EC and NCM811 in the electrolyte,
followed by the reaction between FEC and NCM811, and Si-C. At 0% SOC, gas production is gentle and stable with no change over time, mainly consisting of CO2, CO, and H2;
The main source of gas production comes from the reaction between electrolyte and Si-C.
The main gases during high-temperature cycling are CO2, CO, and H2. In addition, the gas production of the battery cells during high-temperature storage after cycling will intensify;
The gas generated by high-temperature storage will be partially absorbed during circulation.
Keywords: lithium-ion battery; NCM811； Silicon carbon; High temperature gas production; storage
Lithium ion batteries have the advantages of high energy density, fast charging speed, long service life,
and wide operating temperature range, and have been widely used in fields such as electric vehicles and energy storage systems.
In order to meet the increasing demand for battery energy density and range in the market,
batteries composed of high nickel NCM811 positive electrode
and silicon carbon composite negative electrode with high specific capacity are gradually receiving more and more attention.
Due to the serious volume expansion problem in lithium embedded silicon carbon negative electrodes,
current research on high nickel/silicon carbon batteries mainly focuses on capacity decay and expansion force during cycling.
FEC, as an excellent film-forming additive, also plays an important role in improving the cycling performance of silicon carbon.
Lithium ion batteries will produce gas during actual storage and application, causing the battery to expand, deform, or even break and leak, which may lead to safety accidents in severe cases.
The increase in battery volume due to gas generation is a very important indicator for reliability testing of flexible packaging lithium-ion batteries.
At present, there is a relatively comprehensive understanding of the gas production phenomenon
and mechanism of traditional graphite carbon negative electrode lithium-ion batteries under high temperature conditions,
while there are relatively few reports on the gas production behavior of silicon carbon negative electrodes during storage and cycling processes.
Schiele et al. studied the differences in gas production components of silicon-based negative electrodes in electrolytes containing EC and FEC,
indicating a close relationship between gas composition and electrolyte.
Among them, the gas production components of FEC as electrolyte are mainly H2 and CO2,
while those of EC as electrolyte are mainly H2, C2H4, and CO. Lin et al. investigated the promoting effect of Si OH on electrolyte decomposition on the surface of silicon-based negative electrodes.
Si OH reacted with LiPF6 to generate HF and POF3, which can further catalyze the decomposition of solvent EC to produce CO2.
In this article, a flexible packaging lithium-ion battery assembled from commercial high specific energy NCM811 positive electrode material
and silicon carbon composite negative electrode material is taken as the research object.
The drainage method, GC-MS technology, and single pole storage method are combined to systematically study the storage under different SOC storage conditions at 60 ℃,
with a focus on 100% SOC and 0% SOC, as well as the gas production and gas composition in the cycle.
The influencing factors and main sources of gas production are analyzed, and the gas production behavior under cross cycle storage conditions is further studied.
The research results provide a reference for the better application of high specific energy batteries in high nickel/silicon carbon systems.
1、experiment
1.1 Preparation of Soft Pack Batteries
Uniformly coat the mixed slurry of NCM811, PVDF binder, and conductive carbon black with a mass ratio of 96.5:1.5:2 on aluminum foil,
and uniformly coat the mixed slurry of silicon carbon, PAA binder, CMC binder, and conductive carbon black with a mass ratio of 93:4:1:2 on copper foil.
After drying, cold pressing, cutting, and other processes, obtain positive and negative electrode plates respectively;
Wrap the positive and negative electrodes according to the designed size into a dry battery, inject 1 mol · L-1 LiPF6/(EC+EMC+DEC, volume ratio 1:1:1)+15% FEC electrolyte,
vacuum package and make a flexible packaging lithium-ion power battery.
After disassembling the soft pack battery adjusted to the target SOC storage state in the drying room,
the positive and negative pole pieces are taken out and placed separately in different aluminum-plastic film packaging bags for vacuum sealing,
thus completing the preparation of positive and negative pole single pole piece storage test samples.
1.2 Gas production testing and analysis
Adjust the battery (voltage range: 2.5~4.25 V) to different SOC states, adjust the constant temperature furnace to a storage temperature of 60 ℃,
and place the battery on a plastic tray to avoid short circuits between different batteries.
During the storage process of the battery, it should be taken out every few days, cooled to room temperature, tested and recorded for volume changes using the drainage method,
and then placed back in the constant temperature furnace for storage until the target time or when the battery shows obvious swelling, then stop storage and testing.
Drainage method for measuring volume: Use a balance to weigh the mass m of the sample (battery or positive and negative pole unipolar) to be measured;
Then immerse the test sample completely into the container filled with water, while ensuring that it does not touch the bottom or the wall.
Place a tension meter above the container and record the reading F of the tension meter after the sample is completely immersed in water.
Using the formula ρ Vg=mg - F, the overall volume V of the test sample can be calculated. Utilize GC-MS technology to test and study the gas production components of the sample.
2、Results and Discussion
2.1 Analysis of Gas Production from 100% SOC Storage
The soft pack battery with high nickel NCM811 and silicon carbon negative electrode was stored at a high temperature of 60 ℃ and 100% SOC.
The gas production volume was measured every 5 days, and the gas composition was measured every 15 days.
The gas production changes were continuously monitored for 30 days.
With the increase of storage time, the gas production volume expansion rate of soft pack batteries shows a linear growth trend, reaching a maximum value of 30% at 30 days;
At 15 days, there was a decrease in gas production, mainly due to capacity testing conducted at this time,
and the charging and discharging reactions consumed some of the gas generated during the high-temperature storage process.
The changes in gas production components during high-temperature storage over time: it is evident that the main components of gas production are CO2 and CO,
with a total content of over 80%, with CO2 accounting for the largest proportion;
And as the storage time increased from 15 days to 30 days, the total content of the two further increased, reaching as high as 96% after 30 days of storage;
Relatively speaking, as the CO2 content increases, the CO content decreases.
In addition to the main CO2 and CO, it also contains a small amount of alkanes CH4, C2H6, and alkenes C2H4, with a total content of less than 2%.
The content changes little with the increase of storage time. In addition, it also contains a small amount of O2 and N2.
When stored for 15 days, the volume ratio of O2 to N2 is 1:4, which is speculated to be due to the incomplete removal of residual air inside the battery during vacuum packaging;
After 30 days of storage, the content of O2 and N2 decreased, indicating their involvement in the chemical reaction during high-temperature storage.
Different FEC contents in silicon-based negative electrodes can have a significant impact on the cycling performance of the battery.
FEC is sensitive to water and easily decomposes, thereby deteriorating gas production.
The influence of two main electrolyte factors, additive FEC and injection coefficient, on gas production under high temperature and 100% SOC storage conditions at 60 ℃.
As the concentration of FEC gradually increases from 4%, 7%, 10%, and 15%,
the gas production volume expansion rate of soft pack batteries with high nickel/silicon carbon system also shows a significant increasing trend under high-temperature storage conditions;
The gas production rate under the same FEC content shows a linear growth trend with increasing storage time.
The injection amount of electrolyte in soft pack batteries should be maintained within an appropriate concentration range.
Insufficient infiltration of the electrodes can affect the electrochemical reaction;
Excessive liquid swelling can affect the packaging process of the battery.
The gas production during high-temperature storage with FEC content of 7%
and electrolyte injection coefficients of 2.0 g · Ah-1 and 2.8 g · Ah-1 shows a significant decrease with the increase of injection coefficient.
This may be mainly due to the solubility of CO2 generated during storage in the electrolyte.
Compared to 2.0 g · Ah-1, 2.8 g · Ah-1 has a higher total amount of electrolyte and can dissolve and absorb more CO2, resulting in a significant decrease in gas production.
The main sources of gas production under high-temperature 100% SOC storage conditions were analyzed using the unipolar storage method.
After fully charging the prepared fresh soft pack battery cells, disassemble them in a drying room.
At this time, there is very little residual electrolyte in the soft pack bag, and the positive and negative electrode pieces have been fully soaked by the electrolyte.
Remove the fully charged positive and negative electrode pieces and package them separately, and store them at 60 ℃.
After 35 days of storage, the gas volume expansion rate of fully charged NCM811 unipolar chip is 90%, and that of fully charged silicon carbon negative electrode is 35%.
The reaction gas production between fully charged positive electrode and electrolyte is more than twice that of fully charged negative electrode and electrolyte,
indicating that the gas production in high-temperature 100% SOC storage mainly comes from the reaction between NCM811 positive electrode and electrolyte.
The gas components involved in the reaction between the fully charged positive electrode, negative electrode, and electrolyte are mainly CO2 and CO,
with very small amounts of CH4, C2H6, and C2H4.
The positive and negative electrode plates were placed in electrolyte components (EC, FEC, EC+FEC, EMC, DEC) and thoroughly soaked and cleaned before being stored separately.
The main components causing gas production in the electrolyte were further analyzed.
Comparison of gas production of various components in the electrolyte on NCM811: EC+FEC>EC>FEC>DEC>EMC.
The gas production mainly comes from the reaction of EC, FEC, and positive electrode, and the mixture of the two further intensifies the gas production at 100% SOC.
DEC and EMC also produce a small amount of gas;
Comparison of gas production of each component on silicon carbon: FEC> EC+FEC > DEC > EC > EMC，
The total gas production is significantly reduced compared to NCM811, and the gas production comes from the reaction between FEC and the negative electrode.
EC and DEC produce very little gas, while EMC produces almost no gas.
The mixture of EC has an inhibitory effect on the gas production of FEC at the negative electrode.
The gas production of the battery under high temperature 100% SOC storage conditions mainly comes from the reaction between EC and the positive electrode,
followed by the reaction between FEC and the positive and negative electrodes.
The gas production of FEC on the NCM811 positive electrode is about 2.4 times that on the silicon carbon negative electrode.
The proportion of CO2/CO gas produced by EC on the positive electrode is higher than that on the negative electrode,
while the proportion of CO2/CO gas produced by FEC on the positive and negative electrodes is basically the same;
This indicates that the gas production reaction process of EC on the positive electrode is significantly affected by the oxidation of the positive electrode at high temperature and high SOC,
and is significantly affected by the reduction of the negative electrode on the negative electrode.
The reaction process of FEC is less affected by the electrode and mainly depends on its own high-temperature stability.
During the high-temperature 100% SOC storage process, the sources of gas production in the silicon carbon negative electrode and graphite negative electrode are similar:
mainly the oxidation decomposition of the positive electrode side electrolyte,
the reduction decomposition of the negative electrode side electrolyte, and the decomposition reaction of the electrolyte under Lewis acid PF5.
It is generally believed that the generation of CO2 comes from the decomposition of carbonate solvents and residual Li2CO3 on the positive electrode surface,
as well as the oxidation of conductive carbon black;
The oxidative decomposition of carbonate solvents is the main source of CO2, and the solvent directly loses electrons on the positive electrode surface to generate CO2.
The generation of CO comes from the reduction reaction of electrolyte solvent and CO2 on the negative electrode surface,
as well as the insufficient oxidation reaction of electrolyte solvent on the positive electrode.
CH4 and C2H6 mainly come from the reduction decomposition of EMC and DEC, while C2H4 mainly comes from the reduction decomposition of EC.
The most likely decomposition pathways for FEC on silicon electrodes are defluorination and ring opening.
At high temperatures, FEC decomposes into VC and LiF, and VC is further reduced to Li2CO3, ultimately decomposing to CO2.
2.2 0% SOC storage gas production analysis
High nickel NCM811 and silicon carbon soft pack batteries were stored at a high temperature of 60 ℃ and 0% SOC.
The gas production volume was measured every 5 days, and the gas production changes were continuously monitored for 60 days.
In the first 5 days of initial storage, the gas production volume expansion rate reaches a maximum value of about 7%,
and the total amount of gas produced during subsequent storage remains basically unchanged.
The main components of the gas produced after 60 days of storage are 38% CO2, 48% CO, and 11% H2, as well as a small amount of CH4, C2H6, and C2H4.
Study the storage gas production of full batteries and positive and negative unipolar plates under different low SOC potentials,
and analyze the main sources of gas production under high temperature 0% SOC storage conditions.
The positive and negative unipolar plates at different potentials are prepared by discharging the battery 0.33C to the corresponding research potential, disassembling and packaging.
It can be seen that the gas production patterns of the full cell, NCM811, and silicon carbon are consistent at different potentials.
The lower the potential, the greater the gas production (2.0 V>2.5 V>2.8 V>3.0 V).
The pattern of gas production for the same number of days of storage is: single negative electrode>full battery>single positive electrode;
This indicates that the main source of gas production during storage at 0% SOC at high temperatures is the reaction between the negative electrode and the electrolyte.
Shows that after being stored at low SOC for 180 days,
the silicon carbon negative electrode and electrolyte produce gas components of more than 50% H2, about 20% CO2, a small amount of CO,
and extremely small amounts of CH4, C2H6, and C2H4. The generation of H2 comes from the reduction of residual water in the electrolyte,
and the diffusion of proton electrolyte oxide (R-H) from the positive electrode to the negative electrode surface caused by high temperature or high potential reduction.
Except for 2.0 V, the lower the potential, the stronger the negative electrode reducibility, resulting in higher H2 content and lower CO2 and CO content.
At a potential of 2.0 V, the over discharge of the battery causes the decomposition of the SEI film, producing a significant amount of CO2.
2.3 Analysis of Gas Interaction Effects in Cycle and Storage
In practical applications, the cycling and storage conditions of batteries are intertwined, and the gas production behavior of cycling after storage and cycling after storage.
The gas production volume expansion rate and gas composition of cells with fresh and cycling capacity decay to 80% SOH under high-temperature storage.
When the cycled battery cells are stored again, the increase in gas production volume expansion rate with increasing days is more significant,
and the gas production at 25 days is twice that of fresh battery cells.
The CO2/CO volume of fresh battery cells after storage is 3.1, and the volume ratio of the two increases to 5.2 after storage of battery cells at the end of the cycle;
This may be due to the appearance of some defect sites in the coating structure on the surface of the positive electrode material during the cycling process,
which enhances the material's oxidation ability and causes the electrolyte to be fully oxidized.
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The gas production of the battery cells after 30 days of high temperature 100% SOC storage at 60 ℃ and 50 cycles at 25 ℃ after 30 days of high temperature 100% SOC storage at 60 ℃.
After 50 cycles at room temperature, the volume expansion rate of the gas generated by high-temperature storage decreased by 4.8%;
After high-temperature storage, the CO2/CO volume ratio was 3.1, and after 50 cycles, the volume ratio decreased to 1;
Indicating that the main reaction during the cycling process absorbed CO2.
This provides guidance for improving the gas production in the actual use of battery cells.
By optimizing materials and electrolytes, the composition of high-temperature stored gas can be guided towards a direction more conducive to the generation of CO2,
which helps to reduce the consumption of gas in the actual circulation process and lower the risk of gas production.
3、conclusion
This article combines drainage method and GC-MS technology to measure and analyze the gas production and gas composition of batteries under high temperature conditions.
The high-temperature storage gas production behavior under different SOC is systematically studied, with a focus on storage at 100% SOC and 0% SOC.
The main sources of gas production under different conditions are analyzed through unipolar storage method.
The results showed that under high temperature 100% SOC storage, the battery produced significant gas,
and the volume expansion rate showed a linear growth trend with increasing storage time;
The main gas components are CO2 and CO; The additive FEC content and injection coefficient of the electrolyte have a significant impact on gas production.
The higher the FEC content, the lower the injection coefficient and the higher the gas production;
The main source of gas production is the reaction between EC and NCM811 in the electrolyte, followed by the reaction between FEC and NCM811, SiC.
Under high temperature 0% SOC storage, the battery produces relatively less gas, and there is basically no change after reaching the maximum value after 5 days of storage;
The main gas components are CO2, CO, and H2; The gas production mainly comes from the reaction between the electrolyte and the negative electrode.
Within the high temperature range of 0-100% SOC, the gas production follows a bathtub curve shape, with the lowest at 30% SOC and the highest at 100% SOC;
As the storage SOC increases, the proportion of CO2 volume gradually increases, while CO and H2 gradually decrease.
The main components of gas produced during high-temperature cycling are CO2 and CO, with a small amount of H2.
The analysis of gas production under the cross working condition of circulation and storage shows that circulation will intensify the gas production of high-temperature storage,
and some of the gas produced by storage will be absorbed during the circulation process.
This research work helps to systematically understand the gas production behavior of high nickel/silicon carbon systems,
understand the gas production mechanism of silicon-based systems, and provide guidance
and assistance for finding measures to improve high-temperature gas production and better apply high capacity battery systems.
