Method for measuring maximum expansive force of large surface of square battery cell

Abstract

The invention relates to a method for measuring the maximum expansive force of a large surface of a square battery cell, which adopts a non-contact measurement principle, changes the traditional direct constraint on the large surface into constraint on an area outside the large surface, and enables the expansive force of the battery cell to act on a specific measurement surface in a concentrated manner. The core technology innovation comprises the following steps: 1) capturing the expansion displacement of the battery cell through a displacement sensor, and replacing a direct measurement mode of a traditional force sensor; 2) establishing a displacement-force value conversion model, and calibrating mechanical parameters by using a false cell inflation experiment; and 3) displacement of a large surface is not limited, produced gas is concentrated to the large surface, and an experimental basis is established for research between reversible expansion and irreversible expansion. The clamp can accurately quantify the maximum expansion force and the limit displacement of large-surface free expansion of the square battery cell in charge and discharge cycles, provides key data support for battery safety design and performance evaluation, and also provides data support for a reserved gap in the module lamination direction; and expansion force accumulation caused by too small gaps or space utilization rate reduction caused by too large gaps are avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery performance evaluation, in particular to a method for measuring the maximum expansion force of a large face of a square battery cell. BACKGROUND

[0002] The expansion force of a battery cell has always been one of the main problems of concern in the electric vehicle industry, and detailed research on the expansion force of a battery cell can greatly reduce the safety of driving. Current research on the expansion force of a battery cell is mostly at the particle level and the unit level, and there is less research at the battery cell level. At the battery cell level, we can divide the expansion force generated by the battery cell during the charging and discharging cycle into four parts: lithium intercalation expansion force, thermal stress, SEI film-induced force, and gas production-induced stress.

[0003] In the experiment of studying the effect of gas production on the expansion force of the battery cell, we found that the gas production stress is difficult to measure. The experiment achieves faster gas production by overcharging, and the specific experimental scheme is as follows:

[0004] 1. One square LF253 battery cell is charged to full capacity under a pre-tightening force of 5000N at 1C constant current and constant voltage, and then overcharged to 110% SOC using a small current of 0.33C to observe the gas production effect.

[0005] 2. One square LF253 battery cell is charged to full capacity without pre-tightening force at 1C constant current and constant voltage, and then overcharged to 110% SOC using a small current of 0.33C to observe the gas production effect.

[0006] The results are as follows: when overcharged to 110% SOC under the condition of applying a pre-tightening force of 5000N, the long direction expands from 173.9mm to 184.2mm, with an expansion of 5.9%, the high direction changes from 207.2mm to 207.5mm, with an expansion of 0.1%, and the width direction has almost no displacement due to the influence of the pre-tightening force. Under the condition of not applying a pre-tightening force, the long direction expands from 173.4mm to 174.6mm, with an expansion of 0.7%, the high direction shrinks from 204mm to 201.9mm, with a shrinkage rate of 1.2%, and the width direction changes from 53.88mm to 99.88mm, with an expansion of 85.4%.

[0007] Through analysis, it can be seen that the traditional measurement method of the maximum expansion force of the large face of the square battery cell has the following technical problems:

[0008] The traditional tooling fixture can cause the gas expansion force generated by the battery cell during the charging and discharging process to diffuse to the surrounding area, rather than concentrating on the large face, resulting in a smaller measured expansion force and an inability to accurately reflect the actual maximum expansion force on the large face;

[0009] Directly using a force sensor to press the large face poses a risk of crushing the battery cell and causing a short circuit and fire.

[0010] The free expansion limit displacement cannot be quantified, and it is difficult to provide accurate data for module lamination gap design;

[0011] There is no effective means to separate reversible expansion from irreversible (gas production) expansion, which restricts the research on the safety mechanism of the battery cell. SUMMARY

[0012] In view of the problems in the prior art, the purpose of the present application is to provide a method for measuring the maximum expansion force of the large face of a square battery cell, which can accurately obtain the maximum expansion force of the large face of the square battery cell caused by gas production factors under charging and discharging conditions at the battery cell level, the measurement result is accurate, the experiment is safe, and reversible expansion and irreversible expansion can be separated.

[0013] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0014] A method for measuring the maximum expansion force of the large face of a square battery cell, comprising the following steps:

[0015] Constraining the expansion of the square battery cell in the non-large face direction by using a clamp, and only releasing the displacement degree of freedom of the large face;

[0016] Charging and discharging the square battery cell, and collecting the maximum free expansion displacement of the large face of the square battery cell during the charging and discharging process;

[0017] Constraining the expansion of the non-large face direction of the battery cell model by using a clamp, and only releasing the displacement degree of freedom of the large face, wherein the battery cell model is a sealed shell with the same shape as the square battery cell;

[0018] Inflating the battery cell model, and collecting the expansion displacement of the large face of the battery cell model in real time during the inflation process;

[0019] When the expansion displacement of the large face of the battery cell model is equal to the maximum free expansion displacement of the large face of the square battery cell, the corresponding gas pressure of the battery cell model and the area of the large face of the battery cell model after expansion are collected;

[0020] Based on the collected gas pressure and the area of the large face of the battery cell model after expansion, the maximum expansion force of the large face of the square battery cell is calculated.

[0021] Further, the realization mode of constraining the expansion of the non-large face direction of the square battery cell or the battery cell model by using a clamp is that a clamp frame containing the square battery cell or the battery cell model is composed of a bottom steel plate, two opposite side plates and a top insulating cover plate; adjustable pre-tightening force is applied to the side plates and the top cover plate to limit the displacement of the side face and the top and bottom face of the square battery cell or the battery cell model, while ensuring that the large face direction is zero constraint.

[0022] Further, the height of the side plate is not greater than the height of the square battery cell or the battery cell model.

[0023] Further, the top insulating cover plate is made of insulating wood material and has a charging tab hole to prevent short circuit during charging and discharging and not to hinder normal wiring.

[0024] Further, the implementation of collecting the expansion displacement of the large face of the square battery cell or the battery cell model is that two opposite fixed steel plates are installed on the bottom steel plate, the fixed steel plates are arranged corresponding to the large face of the square battery cell or the battery cell model, and a displacement sensor is installed on each of the two fixed steel plates, the displacement sensors directly contact the center of the large face of the square battery cell or the battery cell model to collect the expansion displacement of the large face in real time.

[0025] Further, the displacement sensors are connected to a data display, the clamp and the square battery cell are put into a charging cabinet to charge and discharge according to a required charging and discharging strategy, the readings of the displacement sensors are recorded, and the sum of the readings of the two displacement sensors is the expansion displacement of the large face of the square battery cell.

[0026] Further, the battery cell model is provided with a vent hole and is connected with an air pipe.

[0027] Further, the collection of the area of the large face of the battery cell model after expansion is that a laser ring scanner is used to measure the area of the large face of the battery cell model after expansion.

[0028] Further, the collection of the area of the large face of the battery cell model after expansion is that a contact type profilometer is used to measure the area of the large face of the battery cell model after expansion.

[0029] Further, the collection of the area of the large face of the battery cell model after expansion is that the original area of the large face of the battery cell model is taken as the area of the large face of the battery cell model after expansion.

[0030] Overall, the present application has the following advantages:

[0031] 1) The displacement sensor is used to capture the expansion displacement of the battery cell, and the traditional direct measurement method of the force sensor is replaced, so that the strength of the large face does not need to be considered, and the safety is improved.

[0032] 2) The displacement of the large face is not limited, and the gas production is concentrated to the large face, which establishes an experimental basis for the research between reversible expansion and irreversible expansion.

[0033] 3) The clamp can accurately quantify the maximum expansion force of the square battery cell in the charging and discharging cycle and the limit displacement of the free expansion of the large face, provides key data support for battery safety design and performance evaluation, and provides data support for the reserved gap in the lamination direction of the module, avoids the accumulation of expansion force caused by too small gap, or reduces the space utilization caused by too large gap. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is a schematic diagram of the overall structure of the tool clamp.

[0035] Figure 2 Figure 1 is a schematic diagram of the open side plate structure.

[0036] Figure 3 Figure 2 is a schematic diagram of the clamp side plate and sensor installation steel plate installation structure.

[0037] Figure 4 Figure 3 is a schematic diagram of the displacement sensor installation.

[0038] Figure 5 Figure 4 is a schematic diagram of the method flow of the present application.

[0039] In the figure:

[0040] 1 - bottom steel plate; 2 - side plate; 3 - top insulation cover plate; 4 - fixed steel plate; 5 - displacement sensor; 6 - square cell. DETAILED DESCRIPTION

[0041] The present application will be further described in detail below.

[0042] As Figure 5 shown, a square cell large surface maximum expansion force measurement method, comprising the following steps:

[0043] 1) Two side plates 2 are fixed on the bottom steel plate 1 with 6 M10-L25 bolts, and two fixed steel plates 4 for fixing the displacement sensor 5 are fixed on the bottom steel plate 1 with four M10-L25 bolts, as shown in Figures 1-3 In this embodiment, the width of the fixing hole of the side plate 2 and the bottom steel plate 1 is designed to be 21 mm. If there is a certain margin for the constraint of different width cells (LF253 cell model is used in the model), the width difference of the square cell 6 is small, and the width of the hole can be designed according to the specific cell width. If the width difference of the square cell 6 is large, the width of the bottom steel plate 1 needs to be changed

[0044] 2) The square cell 6 to be measured is placed in the assembled clamp, the position of the side plate 2 is adjusted to ensure that both side plates 2 can fit the left and right side surfaces of the square cell 6 to be measured.

[0045] The side plate 2 and the fixed steel plate 4 of the displacement sensor 5 are connected together with 8 M10-L85 bolts, and at the same time a certain pre-tightening force is applied to the left and right sides of the square cell 6 through the bolts to ensure that it will not bulge on both sides. The height of the side plate 2 should be less than or equal to the height of the cell (excluding the aluminum bar). The clamp needs to apply a certain pre-tightening force to the top. If the rigidity of the side plate 2 is higher than the height of the cell, the pre-tightening force of the top cover plate will act on the side plate 2, resulting in that the top of the cell is not constrained, so that part of the gas force acts on the top.

[0046] 3) Fix two displacement sensors 5 on the fixed steel plate 4 of the displacement sensor 5 through two mounting seats, ensure that the contact points of the displacement sensor 5 are in the center of the large face of the battery cell, as shown in Figure 4 , facilitate the collection of subsequent displacement. In this embodiment, the range of the displacement sensor 5 is ±5 mm, and different ranges of displacement sensors 5 can be selected according to the actual inflation amount of the square battery cell 6 under the charging condition without the clamp. The positioning and installation of the contact type displacement sensor 5 require the use of a non-magnetic clamp with a small temperature expansion coefficient for fixation. If a metal iron block clamp is used for installation, it will affect the displacement sensor 5.

[0047] 4) The top insulating cover plate 3 is provided with bolt holes, and the top insulating cover plate 3 is fixed on the side plate 2 through 8 M8-L20 bolts passing through the bolt holes, and at the same time, the B plate is determined in the opening of the top insulating cover plate 3, which facilitates subsequent charging. The bolts are fixed and a part of the pre-tightening force is applied to ensure that the top of the battery cell will not be inflated and deformed. If the position of the side plate 2 needs to be adjusted in step 2), the position of the bolt hole of the top insulating cover plate 3 is adjusted accordingly, which is low in cost and convenient to operate.

[0048] In this embodiment, the top insulating cover plate 3 is a top wooden cover plate, and the use of the top wooden cover plate for top restraint can prevent short circuit during charging and discharging and avoid safety problems. At the same time, the top wooden cover plate will reserve enough space to ensure the normal charging. The bottom steel plate 1 and the top wooden cover plate will ensure that the force of gas production will not escape to the surrounding and ensure that the force of gas production will act on the large face. In order to meet the charging and discharging demand, enough charging position needs to be reserved on the top wooden cover plate according to the design of the aluminum B plate to avoid the failure to install the charging and discharging equipment. The top wooden cover plate will try to ensure the coverage area under the condition of not causing short circuit and reserving the charging port.

[0049] 5) Connect the displacement sensor 5 to the data display, put the clamp and the measured square battery cell 6 into the charging cabinet and charge according to the demand charging strategy, record the reading of the displacement sensor 5, and the sum of the readings of the two displacement sensors 5 under the target working condition is the maximum free expansion displacement of the large face of the square battery cell 6.

[0050] 6) Design a battery cell model (sealed empty battery cell) with the same shape as the square battery cell 6, open a hole on the side of the battery cell model, insert an air inflation pipe, and at the same time, smear glue around the hole to prevent air leakage. The area of the glue needs to be smaller than the area of the opening.

[0051] 7) After removing the top insulating cover plate 3, take out the square battery cell 6 that has been tested, put the corresponding battery cell model into the clamp, and re-fix the battery cell model according to step 4). The side plate 2 is provided with a circular hole with a diameter of 30 mm at a distance of 50 mm from the bottom, the air inflation pipe is stretched out of the circular hole of the side plate 2, and is connected to the air inflator.

[0052] 8) Inflator inflates, inflates the cell model to the maximum free expansion displacement measured in step 6), and records the gas pressure filled in the cell model at this time.

[0053] 9) If you have a laser ring scanner or a contact profilometer, you can measure the large area of the cell model after expansion, and calculate the maximum expansion force on the large area according to F=PxA; if the area after expansion of the large area cannot be obtained, use the area of the large area before expansion as A.

[0054] The method comprises the following core technologies:

[0055] 1) Capture the cell expansion displacement by displacement sensor 5, instead of traditional direct measurement method of force sensor;

[0056] 2) Establish displacement-force value conversion model, calibrate mechanical parameters by cell model inflation experiment;

[0057] 3) Without limiting the displacement of the large area and concentrating the gas to the large area, establish an experimental basis for the study between reversible expansion and irreversible expansion. The fixture can accurately quantify the maximum expansion force of the square cell 6 in the charge and discharge cycle and the limit displacement of the free expansion of the large area, provide key data support for battery safety design and performance evaluation, also provide data support for reserving gap in the module lamination direction, avoid the accumulation of expansion force caused by too small gap or reduce the space utilization caused by too large gap.

[0058] The present application has the following technical advantages:

[0059] 1. Replace the traditional measurement method and improve safety: indirect measurement by displacement sensor 5 instead of traditional direct measurement by force sensor, without considering the influence of large area strength on sensor, avoiding sensor damage or measurement failure caused by excessive cell expansion force, improving experimental safety.

[0060] 2. Concentrate expansion force to ensure measurement accuracy: by restricting the expansion of the left and right sides, bottom and top of the cell, the force generated by gas production and structural expansion is concentrated on the large area, solving the problem of small measurement value caused by the diffusion of expansion force in the traditional fixture, ensuring the measurement accuracy of the maximum expansion force of the large area.

[0061] 3. Support reversible and irreversible expansion research: the design of not limiting the displacement of the large area provides an experimental basis for distinguishing reversible expansion (such as lithium intercalation) and irreversible expansion (such as SEI film or gas production) in the charge and discharge process of the cell, and helps the in-depth research of the expansion mechanism of the cell.

[0062] 4. Provide key data for battery design: accurate quantification of large surface maximum expansion force and limit displacement, which can provide direct data support for square battery cell 6 shell strength design, module lamination direction gap optimization (avoiding too small gap leading to force accumulation or too large reducing space utilization), improving battery safety performance and structural rationality.

[0063] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, which are all included in the protection scope of the present application.

Claims

1. A method for measuring the maximum expansion force of a square battery cell on its large surface, characterized in that, Includes the following steps: By using clamps to constrain the expansion of the square battery cell in directions other than the large surface area, only the displacement degree of freedom of the large surface area is released; Charge and discharge the square battery cell and collect the maximum free expansion displacement of the large surface of the square battery cell during the charging and discharging process; The expansion of the battery cell model in directions other than the large surface area is constrained by a clamp, while only the displacement degree of freedom of the large surface area is released. The battery cell model is a sealed shell with the same shape as a square battery cell. Inflate the battery cell model and collect the expansion displacement of the large surface of the battery cell model in real time during the inflation process. When the expansion displacement of the large surface of the battery cell model is equal to the maximum free expansion displacement of the large surface of the square battery cell, the gas pressure corresponding to the battery cell model and the area of ​​the large surface of the battery cell model after expansion are collected. Based on the collected gas pressure and the area of ​​the large surface of the battery cell model after expansion, the maximum expansion force of the large surface of the square battery cell was calculated.

2. The method according to claim 1, characterized in that: The method of using a clamp to constrain the expansion of a square battery cell or battery cell model in the non-major direction is as follows: a clamp frame for accommodating the square battery cell or battery cell model is composed of a bottom steel plate, two opposing side plates, and a top insulating cover plate; an adjustable preload is applied to the side plates and the top cover plate to limit the displacement of the sides and top and bottom surfaces of the square battery cell or battery cell model, while ensuring zero constraint in the major direction.

3. The method according to claim 2, characterized in that: The height of the side plate shall not exceed the height of the square battery cell or the battery cell model.

4. The method according to claim 2, characterized in that: The top insulating cover is made of insulating wood and has a pre-drilled hole for the charging pad to prevent short circuits during charging and discharging without interfering with normal wiring.

5. The method according to claim 1, characterized in that: The method for collecting the expansion displacement of a square battery cell or battery cell model in the large-area direction is as follows: two opposing fixed steel plates are installed on the bottom steel plate, and the fixed steel plates are arranged corresponding to the large-area of ​​the square battery cell or battery cell model. Displacement sensors are installed on the two fixed steel plates respectively, and the displacement sensors directly contact the center of the large-area of ​​the square battery cell or battery cell model to collect the expansion displacement of the large-area in real time.

6. The method according to claim 5, characterized in that: Connect the displacement sensor to the data display, place the fixture and the square battery cell into the charging cabinet and charge and discharge according to the required charging and discharging strategy, record the readings of the displacement sensor, and the sum of the readings of the two displacement sensors is the amount of expansion displacement of the large surface of the square battery cell.

7. The method according to claim 1, characterized in that: The battery cell model has a vent hole on its side, which is connected to an inflation tube.

8. The method according to claim 1, characterized in that: The method for acquiring the area of ​​the expanded battery cell model is to use a laser ring scanner to measure the area of ​​the expanded battery cell model.

9. The method according to claim 1, characterized in that: The method for collecting the area of ​​the expanded surface of the battery cell model is to use a contact profilometer to measure the area of ​​the expanded surface of the battery cell model.

10. The method according to claim 1, characterized in that: The method for collecting the area of ​​the expanded surface of the battery cell model is to use the original large surface area of ​​the battery cell model as the area of ​​the expanded large surface of the battery cell model.

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