Estimation method for gas production internal pressure in battery cell constraint state

Abstract

The invention relates to a method for estimating gas production internal pressure in a battery cell constraint state, which comprises the following steps of: performing overcharge treatment on a battery cell to induce gas production; discharging the overcharged battery cell to a safe voltage; gas in the battery cell is extracted, and the mass change delta m of the battery cell is measured; analyzing the components of the extracted gas and calculating the average molar mass M of the extracted gas; calculating the total molar weight n of gas produced by the battery cell according to delta m and M; obtaining the void volume V capable of expanding the gas in the battery cell; and calculating the gas production internal pressure P according to n, V and the cell temperature T. Controllable induction, safe operation, accurate analysis and constraint simulation are taken as core ideas, the internal pressure data of the battery cell in different overcharge states can be accurately obtained, and the problems that a traditional method cannot simulate real constraint conditions, the safety risk is high, the air interference is large, and the design of a pressure release valve lacks a quantitative basis are solved.

Description

Technical Field

[0001] This invention relates to the field of battery safety design technology, specifically a method for estimating the internal pressure of gas generation under cell constraint. Background Technology

[0002] Overcharging, prolonged cycling, and high temperatures can cause electrolyte decomposition in batteries, producing gases (H2, CO, CO2, etc.) that increase internal pressure. Improperly designed pressure relief valves can lead to explosions or thermal runaway.

[0003] The existing technology has the following technical problems:

[0004] 1. Overcharging of the battery causes the electrolyte to decompose and produce gas, which is a key factor in thermal runaway. However, directly measuring the internal pressure requires the implantation of a sensor, which will damage the cell structure and cannot reflect the true composition of the gas produced.

[0005] 2. Existing methods are difficult to quantify the internal pressure of gas production under different overcharge levels, resulting in a lack of accurate basis for the design of pressure relief valve pressure thresholds;

[0006] 3. Ignoring constraints: Traditional methods do not simulate the mechanical constraints of the battery in the pack or fixture (limited void volume);

[0007] To address the aforementioned issues, there is an urgent need for a safe and feasible method for estimating the internal pressure of generated gas under constrained conditions, providing a quantitative basis for the design of pressure relief valves. Summary of the Invention

[0008] To address the problems existing in the prior art, the purpose of this invention is to provide a method for estimating the internal pressure of gas generated in a battery cell under constrained conditions. This method can safely and accurately estimate the internal pressure of gas generated in a battery cell under mechanically constrained internal volume conditions, thereby providing a quantitative basis for the design of pressure relief valves.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for estimating the internal pressure of gas generation under cell constraint conditions includes the following steps:

[0011] Overcharging the battery cells to induce gas production;

[0012] Discharge the overcharged battery cells to a safe voltage.

[0013] Extract gas from inside the battery cell and measure the change in cell mass Δm.

[0014] Analyze the composition of the extracted gas and calculate its average molar mass M;

[0015] Calculate the total molar amount of gas produced by the battery cell, n, using Δm and M.

[0016] Obtain the void volume V inside the battery cell that allows for gas expansion;

[0017] Calculate the internal pressure P of the generated gas based on n, V and cell temperature T.

[0018] Furthermore, the overcharge treatment includes first charging to 100% SOC in a constant current and constant voltage charging mode, and then continuing to charge to at least one state between 103% SOC and 105% SOC in a constant current charging mode.

[0019] Regarding the selection of charging cutoff capacity, overcharging can induce rapid decomposition of the battery electrolyte, resulting in rapid gas accumulation. As for the design of the upper limit of the battery pressure relief valve, since the degree of gas generation inside the battery varies under different overcharge SOCs, using different levels of overcharge SOC as cutoff capacity can estimate the internal pressure of gas generation inside the battery under different levels.

[0020] Regarding charging modes, to ensure that the battery cells can be charged to the required capacity, constant current and constant voltage charging is prioritized to ensure that the battery cells are fully charged first. Then, constant current charging is used. Since a certain amount of overcharging is required to induce gas production, constant current charging is used when the battery is fully charged, depending on the overcharge requirements. As for the selection of charging rate, using a low charging rate can reduce the polarization effect of the battery, so that the degree of overcharging of the battery cells can meet the required requirements as much as possible. Low-rate constant current charging can make the battery cells reach the charging capacity requirements more stably.

[0021] Furthermore, overcharge treatment is performed at a rate no greater than the rated rate of the battery cell.

[0022] Furthermore, after charging to 100% SOC, let it stand for half an hour before continuing to charge; before discharging and extracting gas, let the battery cell stand for no less than 2 hours each time.

[0023] Furthermore, the safety voltage is 2.5V. Since the battery will subsequently require gas extraction via needle puncture, performing gas extraction and needle puncture in an overcharged state could easily cause short circuits and thermal runaway risks to the battery. Therefore, discharging the battery to 0% charge allows for safer gas extraction.

[0024] Furthermore, the extraction is achieved by puncturing the pressure relief valve and collecting the gas.

[0025] Furthermore, the composition of the gas was determined using a gas chromatograph, and the influence of nitrogen and oxygen was eliminated before calculating M.

[0026] Furthermore, the void volume V is obtained through actual measurement after disassembly or through modeling. In order to simulate the gas production internal pressure state of the battery under the condition of restricted expansion direction in a constant displacement fixture or pack, ΔV is equal to the difference between the total volume of the battery and the volume of the internal filling material. The volume of a portion of the battery can be measured after disassembly to calculate ΔV, or it can be estimated through modeling.

[0027] Furthermore, the internal pressure P of the generated gas is calculated using the ideal gas law P = nRT / V, where r represents the ideal gas constant and T represents the battery temperature. Based on the gas generation mechanism caused by battery overcharging, the main gas-generating components of the battery are hydrogen, carbon dioxide, carbon monoxide, alkenes, and alkanes. Therefore, to minimize the influence of air, the effects of nitrogen and oxygen can be excluded when determining the component proportions.

[0028] Furthermore, the battery cell is a lithium-ion battery.

[0029] In summary, the present invention has the following advantages:

[0030] 1) By overcharging to different SOC levels through multi-node testing, the degree of gas production and internal pressure of gas production corresponding to different overcharge states can be obtained, which can provide a certain reference for the evolution law of the degree of gas production expansion of the battery cell.

[0031] 2) By allowing the gas to stand after charging and discharging, uniform gas distribution is ensured, and gas stabilization is achieved by avoiding gas extraction errors caused by local high pressure.

[0032] 3) By controlling the constant current charging at a low rate, overcharging deviation caused by polarization is avoided.

[0033] 4) Obtain the battery void volume through actual measurement or modeling to realize the internal pressure calculation under the mechanical constraint effect in the Pack or fixture. Attached Figure Description

[0034] Figure 1 This is a flowchart of the present invention.

[0035] Figure 2 This is an analysis of the gas composition of the battery cell at 103% SOC in this invention.

[0036] Figure 3 This invention relates to the analysis of gas composition at 105% SOC of the battery cell.

[0037] Figure 4 This refers to the percentage of gas-generating components in the battery cell at 103% SOC in this invention.

[0038] Figure 5 This refers to the percentage of gas-generating components in the battery cell at 105% SOC in this invention. Detailed Implementation

[0039] This invention induces gas generation in the battery cell through controllable overcharging, simulating the internal volume constraint under mechanical conditions. It safely and accurately estimates the internal gas generation pressure of the battery cell, thus providing a quantitative basis for pressure relief valve design. This is reflected in the following four aspects:

[0040] Controllable induced gas production: After charging to 100% SOC using a small-rate constant current and constant voltage, it is then overcharged to 103% or 105% SOC using a constant current, which precisely induces controllable electrolyte decomposition to produce gas, avoiding polarization interference and achieving phased control of gas production.

[0041] Safety operation design: After overcharging, the battery is first discharged to a safe voltage (2.5V) before needle puncture and gas extraction to avoid the risk of short circuit and thermal runaway; the static step ensures stable gas distribution and improves the safety and accuracy of testing.

[0042] Gas composition purification and interference removal: By combining needle puncture and gas chromatography analysis, interference from nitrogen and oxygen in the air is actively removed, retaining only the true gas-producing components (H2, CO2, CO, olefins / alkanes), thus improving the accuracy of internal pressure calculation.

[0043] Simulated mechanical constraint state: The internal void volume of the battery under restricted expansion in the pack or fixture is obtained through actual measurement or modeling. The actual internal pressure is calculated by combining the ideal gas law, which truly reflects the gas generation pressure of the cell under constraint state.

[0044] The present invention will now be described in further detail.

[0045] like Figure 1 As shown, a method for estimating the internal pressure of gas generation under cell constraint mainly includes the following steps:

[0046] (1) Select at least two freshly manufactured cells from the same series.

[0047] (2) Perform a series of pre-processing operations on the battery cell, including capacity calibration, DC internal resistance measurement and quality measurement.

[0048] (3) Charge the selected battery cell at a rate within the rated rate of the battery cell (choose the lowest possible rate).

[0049] (4) According to the selected rate (e.g., 0.33C), charge the two cells to 100% SOC in constant current and constant voltage charging mode and let them rest for half an hour. Then charge the two cells to 103% SOC and 105% SOC respectively in constant current charging mode.

[0050] (5) After the battery has been charged to the corresponding capacity according to the predetermined requirements, keep it resting for at least two hours, and then discharge the battery to 2.5V in a constant current manner according to the selected rate (such as 1C).

[0051] (6) After completing the above steps of the test, leave the battery for two hours to allow the battery gas to stabilize.

[0052] (7) Puncture the pressure relief valve of the battery cell with a needle and use a syringe to extract the air, thereby obtaining the composition and proportion of the gas inside the battery cell, which is used to calculate the internal air pressure.

[0053] (8) After the gas inside the battery is completely extracted, measure the mass of the battery and calculate the change in mass before and after the gas extraction.

[0054] (9) Based on the extracted gas, use a gas chromatograph to perform component analysis and percentage analysis of each gas.

[0055] (10) Calculate the total molar mass of the internal gas:

[0056] M = ∑X i %·n i (1)

[0057] In the above formula, M represents the total molar mass of the gas, and X i % represents the percentage of each gas in the total gas, n i This indicates the molar mass of each gas.

[0058] (11) Calculate the molar amount of the internal gas:

[0059] n 总 =Δm / M (2)

[0060] In the above formula, n 总 Δm represents the total molar amount of gas, and Δm represents the change in the mass of the battery before and after gas extraction.

[0061] (12) Calculate the internal pressure of the battery:

[0062]

[0063] In the above formula, R represents the ideal gas constant, with a value of 8.314 J / (mol·K), T represents the temperature of the cell, and X... i % represents the percentage of each gas in the total gas, n i The molar mass of each gas is represented by ΔV, which represents the void volume inside the battery, i.e., the volume of the gas inside the battery. ΔV is equal to the difference between the total battery volume and the volume of the internal filling material. The battery can be disassembled and its volume measured to calculate ΔV, or it can be estimated through modeling. Here, the ΔV of the battery is obtained as 239753 mm². 3 Calculations show that the battery internal pressure is 0.2 MPa at 103% SOC and 0.57 MPa at 105% SOC.

[0064] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for estimating the internal pressure of gas generation under cell constraint, characterized in that: Includes the following steps, Overcharging the battery cells to induce gas production; Discharge the overcharged battery cells to a safe voltage. Extract gas from inside the battery cell and measure the change in cell mass Δm. Analyze the composition of the extracted gas and calculate its average molar mass M; Calculate the total molar amount of gas produced by the battery cell, n, using Δm and M. Obtain the void volume V inside the battery cell that allows for gas expansion; Calculate the internal pressure P of the generated gas based on n, V and cell temperature T.

2. The method according to claim 1, characterized in that: Overcharge handling includes first charging to 100% SOC in constant current and constant voltage charging mode, and then continuing to charge to at least one state between 103% SOC and 105% SOC in constant current charging mode.

3. The method according to claim 1, characterized in that: Overcharge treatment is carried out at a rate no greater than the rated rate of the battery cell.

4. The method according to claim 2, characterized in that: After charging to 100% SOC, let it stand for half an hour before continuing to charge; before discharging and extracting gas, let the battery cells stand for at least 2 hours each time.

5. The method according to claim 1, characterized in that: The safety voltage is 2.5V.

6. The method according to claim 1, characterized in that: The extraction is achieved by puncturing the pressure relief valve and collecting the gas.

7. The method according to claim 1, characterized in that: The composition of the gas was determined using a gas chromatograph, and the effects of nitrogen and oxygen were eliminated before calculating M.

8. The method according to claim 1, characterized in that: The void volume V is obtained through actual measurement after disassembly or by modeling.

9. The method according to claim 1, characterized in that: The internal pressure P of the generated gas is calculated using the ideal gas law P = nRT / V, where R represents the ideal gas constant and T represents the temperature of the battery.

10. The method according to any one of claims 1 to 9, characterized in that: The battery cell is a lithium-ion battery.

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