Characterization Method for Blade Battery Expansion Behavior

With the continuous development of new energy vehicles, people have put forward higher requirements for the vehicle's cruising range and service life. How to arrange more batteries in a limited space, engineers follow the idea of "removing modules and beams and innovating space utilization", The cell itself is used both as an electrical energy storage body and as a load-bearing structural component. Through a series of efforts, a blade-shaped cell was finally produced.

Blade batteries are generally lithium-ion batteries made of lithium iron phosphate. What's unique about it is the shape and size of the battery, as well as its production process. Blade batteries are shaped like a razor blade, hence the name. This design allows the battery to be directly embedded into the battery pack, eliminating the need for traditional battery modules, thereby increasing the energy density and structural strength of the battery pack.

Figure 1. Comparison between blade battery and traditional battery assembly

Blade batteries can be roughly divided into two categories: long blade batteries, such as BYD's Long Knife battery; and short blade batteries, such as Honeycomb Energy's Short Knife battery. According to public information, BYD's Changdao battery is actually a square hard-shell battery, but it adopts a long and thin structure design. The overall dimensions are 960.0±10 mm × 90.0±1.0 mm × 13.5+2.5/-1.5 mm. Different models have slightly different sizes. For example, the thickness of the 138Ah blade battery is about 12mm, while the thickness of the 202Ah blade battery is about 13.5mm. After disassembly, it was found that the size of the electrode piece was approximately 944 mm × 83mm for the positive electrode and 946 mm × 85mm for the negative electrode. The length of the short knife battery also reaches more than 500mm, such as 573 mm × 117 mm × 21 mm. There are also some difficulties in the design and manufacturing of this long and thin cell: (1) The dimensional accuracy of long and thin cell is relatively high, because any slight dimensional deviation may affect the safety and electrochemical performance of the battery; (2) Long and thin batteries usually use a lamination process, which places higher requirements on manufacturing precision and quality control. During the lamination process, it is necessary to ensure the alignment of each layer of separators and electrode sheets, as well as the overall flatness. The longer electrode sheets are a challenge to production equipment and process control; (3) Due to the long and thin shape of the cell, Its mechanical strength and durability are also design difficulties. During long-term use, the cell will be subject to various forces, including internal stress caused by expansion. Therefore, a shell with a certain rigidity is required to ensure the structural stability of the cell.

In short, compared with traditional cylindrical and square batteries, the manufacturing process of blade batteries is more stringent and adopts a multi-layer "sandwich" structure, in which positive and negative electrode plates and separator layers are alternately stacked, however, bubbles are easily generated during the stacking process, and the cell are uneven, causing uneven pressure inside the battery and affecting its strength. Due to its special process and structure, there are currently few mechanical performance characterizations of the expansion of single blade cell. For this reason, IEST has been committed to developing high-precision equipment dedicated to characterizing the expansion performance of blade cell.

2. Equipment Functions and Parameters

The SWE3500 expansion equipment launched by IEST can be used to characterize the expansion behavior of short knife cell, as shown in Figure 2: The main structure is made of steel with high hardness and high wear resistance to ensure the stability and durability of the mechanism; the test pressure plate is made of high-hardness insulating material, which is resistant to deformation and provides insulation protection; the power system of the equipment adopts precise control of servo motors, combined with high-precision displacement sensors and pressure sensors for full closed-loop real-time monitoring and control, and combined with charge and discharge instruments to achieve in-situ characterization of the cell expansion force and expansion thickness. At the same time, it also integrates high and low temperature chamber design, which can simulate the test environment of -20~80℃.

Figure 2. Expansion characterization equipment SWE3500

2.1 Main Parameters of Equipment:

l Cell compatibility range (700-400mm) *(400~200mm) *100m (W*D*H).

l Maximum pressure 5T, pressure sensor accuracy 0.3%F.S.

l Thickness sensor resolution 0.1 μm.

l Temperature control range: -20°C~80°C.

l Size:1500x1700x2000mm.

2.2 Device Test Mode:

l Constant Pressure Mode: Constant pressure during the charging and discharging process of the cell, testing the changes in the expansion thickness of the cell.

l Constant Gap Mode: Constant gap during the charging and discharging process of the cell, testing the changes in the expansion force of the cell.

l Steady-state Compression Mode: Evaluation of static cell stress-strain curve.

3.Application Cases

3.1 Cell Information:

3.2 Charge and Discharge Process:

3.3 Characterization of Expansion Performance:

Set a constant pressure of 1000kg and a constant gap of 1000kg to test the expansion thickness and expansion force of the cell, as shown in Figure 3 below: the thickness and expansion force change curves show regular changes with the cell voltage curve, and the trends of the two curves are similar, and both have the unique "hump" phenomenon of the LFP/Gr system.

Figure 3. Cell expansion thickness and expansion force change curves with voltage

Apply different pressures to the static cells, as shown in the left picture of Figure 4 (initial pressure is 50kg, increase in steps of 500kg until it reaches 5000kg, and then reduce the pressure to 50kg with 500kg), at the same time, the thickness changes of the cell under different pressures were recorded, and the stress-strain curve of the cell was obtained, as shown in the right picture of Figure 4: the maximum deformation of this cell was 1.71% when the pressure was 5000kg, and it irreversibly rebounded by 0.54% after the pressure was re-released, therefore, it can be calculated that the reversible rebound of this process is 1.17%.

Figure 4. Pressure control curve (left picture) cell stress-strain curve (right picture)

This long and thin blade battery has significant advantages in battery cooling and battery pack assembly. However, it is also necessary to pay attention to the uniformity of the cell during the design and manufacturing process. Due to the long size of the blade battery, the manufacturing and assembly process of the electrode needs to be accurately controlled during the production process to ensure the consistency of the internal structure of the battery. Throughout the production process, from electrode manufacturing to liquid injection and other steps, strict quality control is required to avoid degradation of battery performance due to uneven production. Long and thin batteries are also prone to internal stress inconsistencies during expansion and contraction, resulting in uneven thickness. We should manage internal stress more effectively by improving the design of the cell. For example, optimizing housing materials can reduce stress concentrations due to expansion. Ensuring the quality and dimensional accuracy of electrode sheets and separators during the lamination process, as well as the uniform pressing between them, helps reduce internal stress inconsistencies. During the assembly process, a reasonably distributed pre-pressure is applied to the cell to compensate for the stress caused by expansion in advance and maintain the stability of the cell. Therefore, we need to more comprehensively and systematically study the expansion and contraction process of blade batteries and optimize the design of cells and battery packs. The high-precision equipment launched by IEST specifically for characterizing the expansion performance of blade-type batteries can provide characterization devices and methods for it, which can effectively improve the process and design rationality.

4. Conclusion

IEST has launched a large-size cell expansion force characterization equipment that can accurately characterize the expansion thickness, expansion force, stress-strain and other related properties of large-size cells in-situ. It helps engineers to screen the modification process of positive and negative electrode materials and determine the best modification process; accelerate the progress of cell research and development, and develop safer and more reliable cell. At the same time, you can also explore the best usage conditions and extend the service life of the cell.

5. References

[1] "New Energy Battery Pack Technology" public account: [The Origin of the Blade Battery Storm] BYD Blade Battery Analysis 3.0.

[2] The Analysis on the Principle and Advantages of Blade Battery of BYD -- A Domestic New Energy Manufacturer, DOI: 10.1051/shsconf/202214402003.