1. Basic structure of a square battery
A typical square lithium-ion battery's key components include: a top cover, a casing, stacked or wound positive and negative electrode plates and a separator, insulating components, and safety devices. The two components circled in red are safety structures: the NSD (Needle Penetration Safety Device) and the OSD (Overcharge Protection Device).
Nail Safety Device (NSD) is a metal layer, such as a copper sheet, added to the outermost layer of the core. When a puncture occurs, the large local current at the puncture site is quickly reduced per unit area by the large copper sheet, thus preventing localized overheating at the puncture site and mitigating battery thermal runaway.
Overcharge safety devices (OSDs) are now commonly found in many batteries. They are typically thin metal sheets used in conjunction with fuses, which can be integrated into the positive current collector. During overcharging, the internal pressure of the battery triggers an internal short circuit in the OSD, resulting in a large instantaneous current. This causes the fuse to melt, thus cutting off the battery's internal current circuit.
The casing is generally made of steel or aluminum, but with the market's pursuit of energy density and the advancement of processing technology, aluminum casing has gradually become the mainstream.
2. Characteristics of square batteries
Square batteries were among the earliest types of power lithium-ion batteries promoted in China. Data from 2016 shows that the domestic production of cylindrical, pouch, and square lithium-ion batteries were 13.92 GWh, 21.64 GWh, and 28.14 GWh, respectively, accounting for 21.85%, 33.97%, and 44.17% of the market. Square batteries have regained market attention.
Advantages: Square battery packaging has high reliability; high system energy efficiency; relatively light weight and high energy density; relatively simple structure and relatively convenient capacity expansion, making it a key option for increasing energy density by increasing the capacity of individual cells; large cell capacity results in a relatively simple system structure, making it possible to monitor each cell individually; another benefit of the simple system is relatively good stability.
The disadvantages are that, since square lithium-ion batteries can be customized according to the size of the product, there are thousands of models on the market. Because there are so many models, it is difficult to standardize the process. The level of automation in the process is not high, and the individual cells have large differences. In large-scale use, there is a problem that the system life is much shorter than the individual cell life.
Speaking of this, we must mention the national recommended standard "GB/T34013-2017 Specifications and Dimensions of Power Batteries for Electric Vehicles", which was promulgated in July 2017 and officially implemented in February of last year. It provides eight series of dimensions for square batteries, as shown in the figure and table below.
Personally, I believe that providing guidance on cell specifications and dimensions may not have a significant effect in the short term. Some even think that giving instructions at this time will hinder industry development. Changing product dimensions has far-reaching consequences for cell manufacturing, going beyond just tooling and molds. However, as a recommended standard, as long as it provides a direction for manufacturers preparing new processing capacity and adjusting production lines, it will inevitably promote the gradual standardization of specifications and dimensions in the long run. The consistency between cells and modules is a prerequisite for truly achieving tiered utilization. As for the possibility of future technological leaps, this will not affect efforts to move towards the visible goal before such leaps occur.
3. Major Manufacturers
Samsung SDI, a major international manufacturer, primarily uses NCA and NCM as cathode materials, employing square aluminum casings. A notable example is the BMW i3. Samsung's official website displays square battery cells. Their products include high-energy bEV (pure electric vehicle) 60Ah and 94Ah batteries; PHEV (plug-in hybrid electric vehicle) 26Ah and 37Ah batteries (the 26Ah will gradually be replaced by the 37Ah); HEV (hybrid electric vehicle) 5.2Ah and 5.9Ah batteries; and high-power batteries (4.0Ah and 11Ah), totaling four series.
4. Typical square battery module
The image below shows Mitsubishi's i-MiEV battery module from 2011. The PCB board collects the voltage and temperature of the cells, and the two ends are fastened with bolts. The most common connection method between cells is busbar and bolt.
Next is the 2012MY Toyota Prius PHEV battery module, which uses a wiring harness (nowadays, this method of data collection seems quite troublesome in some situations and poses potential risks) to collect cell information. It also uses a bolt connection method, but an orange section has been added for protection.
Below is the battery module of the 2014MY Volkswagen Jetta HEV. The module is secured by two pressure strips on the side, and a plastic cover is added to the outside of the end plate for insulation.
The Volkswagen eGolf 2015MY battery module features a well-designed end plate that reduces weight while meeting structural strength requirements and assembly needs. It uses a PCB board to collect cell information, and only low-voltage connectors are needed at both ends of the module (more and more modules are adopting this method nowadays).
The image below is a modular design concept of Audi's PHEV2 from 2014, featuring a liquid cooling plate design. The exploded view reveals some internal structures that are not visible from the top.
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 picture below. (Note: the picture below is not the latest version that has been rumored. Videos circulating online show that the latest pack casing is different from previous versions.) The aluminum welded module casing has mounting holes at the four corners to fix it into the pack casing. The structure is simple and conducive to automated manufacturing.
It's easier to increase the capacity of prismatic batteries compared to cylindrical cells, and there are fewer restrictions in the process of increasing capacity. However, with the increase in the size of the individual cells, some problems have also emerged, such as severe lateral bulging, difficulty in heat dissipation, and increased unevenness.
5. Typical problems and solutions for square batteries
Side bulging problem
During the charging and discharging process, lithium-ion batteries experience a certain internal pressure (relevant empirical data: 0.3–0.6 MPa). Under the same pressure, the larger the area subjected to force, the more severe the deformation of the battery casing.
The main reasons for battery swelling are: gas is generated during the formation of the SEI (Sediment Injection), increasing the internal pressure of the battery. Due to the poor pressure resistance of the planar structure of the prismatic battery, the casing deforms; during charging, the lattice parameters of the electrode material change, causing the electrodes to expand. The force of the electrode expansion is applied to the casing, causing the battery casing to deform.
During high-temperature storage, a small amount of electrohydraulic analysis and increased gas pressure due to temperature effects can cause deformation of the battery casing. Among these three reasons, casing expansion caused by electrode expansion is the most important.
The swelling problem of square batteries is a common issue, especially for large-capacity square lithium-ion batteries. Battery swelling can cause increased internal resistance, localized electrolyte depletion, and even casing breakage, seriously affecting battery safety and cycle life.
The solution proposed by Zhang Chao and others involves using a small structural form to enhance the strength of the casing and optimizing the arrangement at two angles to address the problem of bulging in square batteries.
The shell strength was enhanced by redesigning the original planar shell into a reinforced structure. The effect of the reinforced structure design was tested by applying pressure to the inside of the shell, and the test was conducted according to different fixing methods (fixed length direction and fixed width direction).
The purpose of the reinforcement structure is clearly visible. Taking a fixed width as an example, under a pressure of 0.3 MPa, the deformation with the reinforcement structure is 3.2 mm, while the deformation without the reinforcement structure reaches 4.1 mm, a reduction of more than 20%.
Pressure testing under fixed width conditions:
Pressure testing under fixed length conditions:
To optimize the cell arrangement in the module, researchers compared two arrangement types, as shown in the figure below, with the deformation amounts shown in the table below. The comparison revealed that the deformation in the thickness direction of arrangement type II was significantly less than that of arrangement type I.
Large square batteries have poor heat dissipation performance
As the volume of individual cells increases, the distance between the heat-generating parts inside the battery and the casing becomes longer and longer, and there are more and more conductive media and interfaces, making heat dissipation more difficult. Furthermore, the problem of uneven heat distribution on the individual cells becomes more and more obvious.
Wu Weixiong et al. conducted a study using 3.2V/12Ah square lithium-ion batteries, the baseline values of which are shown in Table 1. The battery charging and discharging equipment was a Xinwei CT-3001W-50V120ANTF. The ambient temperature during the detection process was 31℃, and the heat dissipation method was air cooling. A temperature monitoring instrument was used to record the temperature changes of the battery.
Experimental steps:
1) Charge the battery with a 12A current until the charging cutoff voltage of 3.65V is reached and the current drops to 1.8A.
2) Let it rest: After charging, let it rest for 1 hour to allow the battery to stabilize;
3) Constant current discharge, discharging at different rates to the discharge cutoff voltage of 2V. The discharge rates are set as 1C, 2C, 3C, 4C, 5C, and 6C respectively.
The figure below shows the temperature changes on the battery surface at different discharge rates. It can be seen that the temperature increases with the discharge rate, with the highest surface temperatures corresponding to each discharge rate being 38.1, 48.3, 56.7, 64.4, 72.2, and 76.9℃, respectively. At 3C discharge rate, the highest temperature already exceeds 50℃. At 6C, the temperature reaches 76.9℃ and remains above 50℃ for 470 seconds, accounting for two-thirds of the entire discharge process. This is highly detrimental to the battery's continued safe operation.
By using phase change materials as a heat-conducting medium and attaching them to the surface of individual battery cells, the heat dissipation effect is greatly improved.
The temperature rise after applying thermally conductive material is shown in the following figure:
Alternatively, there is a method to combine thermally conductive materials with water cooling, allowing the water cooling system to transfer the heat absorbed by the thermally conductive materials to the outside of the system, as shown in the figure below:
In lithium-ion battery systems, the ideal way to prevent thermal runaway is to be able to directly test the parameters of each individual cell (basic parameters such as temperature, voltage, and current). This would make early warning and handling of thermal runaway possible even without the emergence of new, cost-effective, and high-performance sensors. The small number of cells within the system should be one of the key competitive advantages of prismatic batteries.
