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1. Basic structure of square battery

A typical square lithium-ion battery consists of essential components such as a top cover, housing, positive and negative plates, laminated or wound separators, insulation components, safety components, etc. Among them, the two in the red circle are safety structures, NSD needle puncture safety devices; OSD overcharge protection device.

Needle safety protection device (NSD, NailSafetyDevice)。 This is achieved by adding a metal layer, such as copper foil, to the outermost part of the core. When acupuncture occurs, the local high current at the acupuncture site rapidly reduces the current per unit area through a large area of copper foil, which can prevent local overheating at the acupuncture site and slow down the occurrence of battery thermal runaway.

Overcharge safety protection device (OSD, OverchargeSafetyDevice)， At present, this safety design can be seen on many batteries. It is usually a thin metal sheet used in conjunction with a fuse. The fuse can be designed to be placed on the positive current collector. When overcharged, the pressure inside the battery triggers an internal short circuit in the OSD, resulting in a momentary high current and causing the fuse to melt, thereby cutting off the internal current circuit of the battery.

The shell is generally made of steel or aluminum, and with the market's pursuit of energy density and advances in processing technology, aluminum shells have gradually become mainstream.

2. Characteristics of Square Battery

Square battery is an early promoted form of power lithium-ion battery in China. According to data from 2016, the production of cylindrical, soft pack, and square lithium-ion batteries in China was 13.92GWh, 21.64GWh, and 28.14GWh, respectively, accounting for 21.85%, 33.97%, and 44.17% of the total production. Square batteries have regained market attention.

Advantages: high reliability of square battery packaging; High energy efficiency of the system; Relatively lightweight and high energy density; The structure is relatively simple and the expansion is relatively convenient, making it an urgent option to improve energy density by increasing the individual capacity; If the individual capacity is large, the system composition is relatively simple, making it possible to monitor each individual unit one by one; Another benefit of a simple system is relatively good stability.

The disadvantage is that square lithium-ion batteries can be customized according to the size of the product, so there are thousands of models on the market. However, due to the large number of models, it is difficult to unify the process; The level of processing automation is not high, and the individual differences are significant. In large-scale use, there is a problem that the system life is much lower than the individual life.

Speaking of which, we cannot ignore the national recommended standard "Gb/T34013-2017 Specification and Dimensions for Power Battery Products for Electric Vehicles" issued in July 2017 and officially implemented in February last year. For square batteries, eight series of dimensions are provided, as shown in the following figure and table.

In my personal opinion, guiding the specifications and dimensions of battery cells may not have a particularly clear effect in the short term. Some people even think that giving command opinions at this time will constrain the development of the industry, and changing product dimensions is not only a problem of tooling and molds for battery cell processing, but also has a significant impact. But as a recommendation standard, as long as it can give manufacturers a tendency to prepare for new processing capacity and make production line adjustments, in the long run, it will inevitably have a driving effect on the gradual development of specifications and dimensions towards serialization. The consistency between battery cells and modules is a prerequisite for truly achieving hierarchical utilization. As for the possibility of crossing the technological path in the future, it does not actually affect the efforts to move towards visible goals before crossing.

3. Main manufacturers

The main foreign manufacturer Samsung SDI uses NCA and NCM as positive electrode materials, with a square aluminum shell. Famous case BMW i3. The square battery cell displayed on Samsung's official website. The product includes high-energy bEV (pure electric) 60Ah and 94Ah batteries; PHEV (plug-in hybrid electric vehicle) 26Ah and 37Ah batteries (26Ah will gradually be replaced by 37Ah); HEV (Hybrid Electric Vehicle) 5.2Ah and 5.9Ah batteries; High power batteries (4.0Ah, 11Ah), a total of 4 series.

4. Typical square battery module

The following picture shows the i-MiEV battery module from Mitsubishi in 2011. The PCb board collects the voltage and temperature of the cell, and both ends are fastened with bolts. The most common way of connecting cells is through busbars and bolts.

Next is the 2012 MY Toyota Prius PHEV battery module, which uses a wiring harness (this type of wiring harness collection method seems to be very troublesome in some situations and has hidden dangers) to collect cell information. It also uses bolt connection, but adds an orange part for protection.

The following is the battery module of the 2014MY Volkswagen Jetta HEV, which is fastened with two side pressure strips and insulated with a plastic cover plate on the outer side of the end plate.

The Volkswagen eGolf2015MY battery module features a versatile end plate design that not only reduces weight but also meets structural strength requirements and assembly needs. It uses a PCB board to collect cell information and only requires a low-voltage connector at both ends of the module (more and more modules are using this method today).

The following diagram is a conceptual design of a PHEV2 module from Audi in 2014, matched with the design of a liquid cooled plate. From the exploded view, some internal structures that cannot be seen above can be seen.

The BMW i3 uses Samsung SDI square battery cells. The battery pack consists of 8 modules, each with 12 cells connected in series, for a total of 96 cells connected in series. The 183km range version uses 94Ah cells, as shown in the following figure. (To clarify, the following image is not the latest version as rumored today. The video circulating online shows that the latest version of the pack box is different from the previous version.) The aluminum welded module shell has mounting holes at the four corners fixed to the pack box body, with a simple structure that is conducive to automated manufacturing.

Square batteries have a larger capacity compared to cylindrical cells, and are less restricted in the process of increasing capacity. But with the increase of individual volume, some problems have also emerged, such as severe swelling on the side, difficulty in heat dissipation, and increased unevenness.

5. Typical Problems and Countermeasures of Square Batteries

Side swelling problem

During the charging and discharging process of lithium-ion batteries, there is a certain pressure inside the battery (relevant empirical data 0.3-0.6MPa). Under the same pressure, the larger the force area, the more severe the deformation of the battery shell wall.

The main cause of battery expansion is the presence of gas during the formation of SEI during formation, which increases the pressure inside the battery. Due to the poor pressure resistance of the square battery's planar structure, the shell is deformed; During charging, the lattice parameters of the electrode material change, causing electrode expansion. The electrode expansion force is applied to the shell, causing deformation of the battery shell;

During high-temperature storage, a small amount of electro-hydraulic analysis and an increase in gas pressure due to temperature effects can cause deformation of the battery casing. Among the three reasons mentioned above, the expansion of the shell caused by electrode expansion is the most critical one.

The swelling problem of square batteries is a common problem, especially for large capacity square lithium-ion batteries. Battery swelling can cause an increase in internal resistance, local electrolyte depletion, and even shell breakage, seriously affecting the safety and cycle life of the battery.

The method proposed by Zhang Chao et al. utilizes small structural forms to enhance the strength of the shell; Optimize the arrangement from two angles to address the issue of square battery swelling.

Enhance the strength of the shell, design the original flat shell as a reinforced structure, and detect the effect of the shell reinforcement structure design by pressing it inward. According to the different fixing methods (fixed length direction and fixed width direction), detect them separately.

The purpose of enhancing the structure can be clearly observed. Taking the case of fixed width as an example, under a pressure of 0.3 Mpa, the deformation of the shell with reinforced structure is 3.2 mm, while the deformation of the shell without reinforced structure reaches 4.1 mm, a decrease of more than 20% in deformation.

Pressing under fixed width conditions:

Pressing under fixed length conditions:

Optimizing the arrangement of battery cells in the module, researchers compared two types of arrangements, as shown in the following figure, and the deformation amount is shown in the table below. Comparison shows that the thickness direction deformation of arrangement II is significantly smaller than that of arrangement I.

The heat dissipation performance of large square batteries deteriorates

As the volume of the individual cell increases, the distance between the internal heating part of the battery and the shell becomes longer, and there are more and more conductive media and interfaces, making heat dissipation difficult. Moreover, the problem of uneven heat distribution on the individual cell becomes increasingly apparent.

Wu Weixiong et al. conducted a study using a 3.2V/12Ah square lithium-ion battery, with the base numbers shown in Table 1. The battery charging and discharging equipment is the Xinwei CT-3001W-50V120ANFT. The ambient temperature during the detection process is 31 ℃, and the heat dissipation method is air cooling. The temperature change of the battery is recorded using a temperature inspection instrument.

Experimental steps:

1) Voltage charging, charge the battery with a current of 12A until the charging cut-off voltage is 3.65V and the stopping current is 1.8A;

2) Set aside, after charging, set aside for 1 hour to stabilize the battery;

3) Constant current discharge, discharge at different rates until the discharge cut-off voltage is 2V. Among them, the discharge rates are set as 1C, 2C, 3C, 4C, 5C, and 6C respectively.

As shown in the figure below, the temperature changes on the surface of the battery under different discharge rates. It can be seen that as the rate increases, the temperature also increases. The highest surface temperatures of the battery corresponding to each discharge rate are 38.1, 48.3, 56.7, 64.4, 72.2, and 76.9 ℃, respectively. When discharging at 3C rate, the highest temperature has exceeded 50 ℃. At 6C, the temperature reached 76.9 ℃ and exceeded 50 ℃ for 470 seconds, accounting for two-thirds of the entire discharge process, which is very unfavorable for the safe continued operation of the battery.

By using phase change materials as thermal conductive media and attaching them to the surface of individual cells, the heat dissipation effect is greatly improved.

The temperature rise comparison after applying thermal conductive material is shown in the following figure:

In addition, there is also a way to combine thermal conductive materials with water cooling, allowing the water cooling system to transfer the heat absorbed by the thermal conductive materials to the outside of the system. The form is shown in the following figure:

The ideal solution for preventing thermal runaway in lithium-ion battery systems is to be able to directly test the parameters of each battery cell (basic temperature, voltage, current, etc.). This way, even without the emergence of new cost-effective sensors with good functionality, early warning and disposal of thermal runaway will become possible. The small number of battery cells in the system should be one of the key competitive advantages of square batteries.

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