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Analysis of over-charge additive on gas production behavior of cell

Author:INITIAL ENERGY SCIENCE&TECHNOLOGY Co.,Ltd(IEST) Click: Time:2022-03-21 21:23:36

When lithium ion batteries are overcharged, too many lithium ions take out from the cathode material, and the voltage and temperature of the battery rise rapidly, releasing a large amount of oxygen and heat. When a certain potential is reached, the electrolyte will undergo oxidation decomposition, violent chemical reaction, produce a large amount of heat, and then may be dangerous.The electrolyte against overcharge additive, can early warning in the battery, prevent the failure of active materials, including biphenyl (B P) as a type of polymerization against overcharge additive, can be charged to 4.5V, form a layer of polymer film on the positive surface, produce a large amount of gas, increase the battery resistance, the function of early warning battery overcharge.

In this paper, the in-situ volume monitor (GVM) was used to test the N CM523 / graphite cell (theoretical capacity of 1000m Ah) in the electrolyte system with different contents of B P, the gas production behavior was relatively analyzed, and the influence of B P addition on gas production composition was analyzed with gas chromatography (G C).

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Figure 1. Schematic diagram of the gas production behavior of different electrolyte decomposition and the electropolymerization reaction of B P1

Experimental equipment and test methods

1. Experimental equipment: model GVM2200 (I EST 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.

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Figure 2. Appearance diagram of the GVM2200 equipment

2. Test Parameters: 25℃, 1C CC to 5V.

3. Test method: initial weighing m0Put the cell to be tested into the corresponding channel of the device, open the MISG software, set the corresponding cell number and sampling frequency parameters of each channel, and the software automatically read the volume change, test temperature, current, voltage, capacity and other data.The gas composition test uses G C-2014C gas chromatography, with 1m L of gas removed in the glove box, and two detectors with T CD and F ID, respectively. The measurable gas types are shown in Figure 3.

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Figure 3. teable gas composition of F I D and T CD detectors

 

Analysis of in situ overcharge gas production of the cell

1. Charge and discharge curve and volume change curve analysis

The volume and voltage changes of the cell are shown in Figure 4 (a) and (b), and the B P content in the electrolyte is 0%, 1%, 2%, 3% and 5%, respectively.It can be seen from the curve that, with the increase of the additive content, the volume change of the cell becomes larger and larger, indicating that the cell is bulging due to the gas production reaction of the additive.When the content of additives reaches 5%, it can be seen from the voltage curve that the voltage of the cell is difficult to reach the upper limit of 5V.At this time, the additive of the NCM positive electrode first occurs electrochemical polymerization reaction, forming a layer of polymer deposition on the particle surface, and then as the continuous reaction of the additive in the electrolyte gradually grows, the reaction process stably maintains the battery voltage at the polymerization potential of the additive, the voltage platform shown in FIG. 4a.As overcharging continues, large amounts of polymeric deposits aggregate and penetrate the diaphragm, eventually forming a direct connection between the two electrodes.Because the polymerization product of biphenyl (such as poly (biphenyl)) is electronically conductive, the conductive bridge formed by the polymerization sediment in the two electrodes will lead to an internal short circuit in the battery, which continues to consume the charging current, thus preventing the battery voltage out of control and the voltage cannot reach 5V.The internal short circuit caused by electropolymer growth is a slow process and occurs only on some part of the particle surface, which produces moderate self-discharge to automatically discharge the battery to a safe state, and this process does not lead to intense thermal runaway of the battery.As shown in FIG. 4a, the battery voltage gradually decreases with the self-discharge inside the battery.

Cell total gas production and gas starting potential analysis, when no B P, the cell gas starting point almost close to 5V, the overcharge voltage, is likely to cause the cathode material structure, after adding B P, the cell between 4.5~4.6V voltage, the voltage range of BP polymerization, and with the increase of B P concentration, the total gas production increased from about 2mL to about 10mL, can be combined with explosion-proof safety valve to achieve the effect of early warning.

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Figure 4 Charge and discharge and volume changes for different amounts of B P

2. Analysis of the gas production composition of the electric cell

After the overcharged gas cells, 1m L of gas, gas chromatography, as shown in Figure 5, we found that with the increase of B P concentration, the concentration of methane C H4 cell gas, and the concentration of hydrogen H 2 gradually increased, other components such as oxygen O 2, ethylene C 2H4, carbon monoxide C O have no obvious trend.

Further combined with the reaction mechanism analysis of B P, biphenyl (B P) as a type of electric polymerization against overcharge additive, can be charged to battery above 4.5V, monomer molecules in electrolyte oxidation into radical ions, these radical ions even synthesis polymer in the electrolyte, and deposited on the positive and near the positive electrode diaphragm surface, electrode isolation, increase battery resistance, at the same time accompanied by a large number of hydrogen release, lead to cell gas, the function of early warning battery overcharge.

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Figure 5. Changes of methane and hydrogen concentration in the cell gas with different additive content

sum up

This paper adopts a controlled temperature dual-channel gas production volume monitor, combined with gas chromatography, the qualitative quantitative analysis of gas production behavior of overcharge additive B P, not only provides the electrolyte gas potential of BP, but also proved the function of the additive is a lot of hydrogen in the process of overcharge, thus can realize the function of early warning cell overcharge.

reference documentation

1. Kazuma Kumai , Hajime Miyashiro.Gas generation mechanism due to electrolyte decomposition in  commercial lithium-ion cell .Journal of Power Sources.81–82(1999):715–719.

2. Zeng Biao, Research on lithium ion battery overadditive [M], master's thesis, Central South University, 2000.

3. Xiao L ,  Ai X ,  Cao Y , et al.Electrochemical behavior of biphenyl as polymerizable additive for overcharge protection of lithium ion batteries[J].Electrochimica Acta, 2004, 49(24):4189-4196.

4. Li Yunfeng, Effect of biphenyl on ion battery charging performance [J], Henan Chemical, 2018,35 (8): 27-29.

5. Cui Shengyun, electrochemical oxidation polymerization of biphenyl and bitriphenylbenzene in organic solvents [J], electrochemistry, 2000,6 (4): 428-433.


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