Power batteries are the power source for new energy vehicles. Power batteries are mainly divided into battery packs, modules, and cells.
1 Battery Pack
Battery packs are generally composed of battery modules, thermal management systems, battery management systems (BMS), electrical systems and structural parts
2 Modules
Battery modules can be understood as the intermediate product between cells and packs formed by combining lithium-ion cells in series and parallel and installing single-cell battery monitoring and management devices. Its structure must support, fix and protect the cells. Its basic components include: module control (commonly known as BMS slave board), battery cells, conductive connectors, plastic frames, cold plates, cooling pipes, pressure plates at both ends, and a set of fasteners that combine these components together. The pressure plates at both ends not only play the role of gathering single cells and providing a certain amount of pressure, but also often design the fixed structure of the module in the battery pack on them.
The module is designed to facilitate BMS to manage the battery cells, improve battery safety, and facilitate maintenance and repair, just like a country needs to be divided into several provinces for the convenience of governance.
3 Battery Cells
Battery cells are mainly composed of positive electrodes, negative electrodes, separators and electrolytes. The main working principle is to achieve charging and discharging by the migration of lithium ions between the positive and negative electrodes. The charging process requires external energy, that is, grid power, which is equivalent to storing grid power in the battery; the discharging process can be completed spontaneously, and this process releases the stored energy.
Lithium-ion battery is mainly divided into three categories according to the material system: lithium manganese oxide, ternary material lithium battery, and lithium iron ferrate. These three types of battery performance have their own advantages and disadvantages, and they also have different applications in the market. Comparing the characteristics of the three types of lithium battery materials, lithium manganese oxide has the lowest price.
From the above table, we can see that the price of lithium manganese oxide material is the lowest, 50,000-60,000 yuan per ton, and the corresponding battery cycle life and storage performance are also the most general, which are ≥300 times, and the monthly decay is more than 5%. The price of ternary material lithium battery material is 160,000-200,000 yuan per ton, the storage performance is the best, the monthly decay is 1-2%, and the battery cycle life is ≥600 times. The price of lithium iron ferrate material is 150,000-180,000 yuan per ton, the battery cycle life is the best ≥1500 times, the storage performance is medium among the three, and the monthly decay is 3%.
The values mentioned in the above table are the hard parameters of the three types of lithium battery performance. The safety, stability, low temperature resistance and other properties of lithium battery are also important indicators for comprehensive evaluation of lithium battery performance.
Lithium manganese oxide: high temperature performance, cycle performance, and storage performance are poor. Manganese is easily decomposed under high temperature conditions, and the battery pack has a short service life and is difficult to store.
Lithium battery: high and low temperature, cycle, safety, storage and individual electrical properties are relatively average. The volume specific energy is high, the material price is moderate and the performance is stable. According to the ratio of nickel, cobalt and manganese, the ternary material battery has a series of systems such as 532, 811, etc. In recent years, the 811 system battery has been more popular. The higher the proportion of nickel, the more unstable the power battery. At the same time, increasing the proportion of nickel can increase the energy density of the battery. Therefore, the design of power batteries is a balancing process, balancing practicality and safety.
Lithium iron phosphate: good safety performance, low conductivity, low volume specific energy, high material cost, poor low temperature performance, and cannot meet the use of electric vehicles in winter.
The positive electrode of the lithium battery is the positive electrode material (such as LFP, NCM) coated on the aluminum foil (current collector), and the negative electrode is the negative electrode material (such as graphite, LTO) coated on the copper foil (current collector). Generally speaking, batteries are named according to the positive electrode material, so they are generally called ternary batteries or lithium iron phosphate batteries; while LTO is the negative electrode material in lithium titanate batteries, so this is a special case of naming batteries after negative electrode materials. When reading foreign literature, I found that the positive electrode material is often called the cathode (Cathode) and the negative electrode material is called the anode (Anode). At first, I didn’t understand it very well, because we generally believe that the electrode that undergoes reduction reaction is the cathode, and the electrode that undergoes oxidation reaction is the anode; and the positive and negative electrodes of the battery also change during the process of switching between discharge and charge. Later, I gradually figured it out that this definition should refer to the situation under the condition without external energy influence, so the positive and negative electrodes of the battery are determined by the reaction situation under the discharge state.
Battery degradation can be analyzed in two aspects, one is performance, and the other is safety.
1) Performance degradation
After a certain period of use, the mileage of electric vehicles will decrease, and the degradation of acceleration performance may also be felt. This can be analyzed mainly from the aspects of capacity degradation, increase in internal resistance, and increase in battery self-discharge.
2) Safety degradation
Safety degradation is relatively difficult to detect. It is possible that the battery has already undergone mechanical deformation, or the probability of internal short circuit has increased, and there is a risk of leakage.
Therefore, we can find out what affects the reduction of capacity, what factors cause the increase of internal resistance, the process of battery deformation, and the factors that cause internal short circuit to understand the battery attenuation process.
In terms of safety, lithium manganese oxide batteries are much safer than ternary batteries. For example, domestic manufacturers now use Xinzheng's lithium manganese oxide LMA-30 to make 90 ampere-hour single cells, which can pass all the safety tests of 201. As for ternary materials, it is possible that domestic 20 ampere-hour single cells cannot pass the needle puncture test. This is mainly determined by the stability of the material structure. The structure of lithium manganese oxide itself is more stable than that of ternary materials. In addition, the development time of lithium manganese oxide as a material is relatively long, and its maturity is much higher. The aforementioned Xinzheng LMA-30 is lithium manganese oxide modified with Al, and the introduction of modified ternary materials is not ruled out in the future. In addition, due to the matching problem of electrolyte, ternary materials are more likely to produce gas than lithium manganese oxide, which is also one of the reasons why ternary batteries are not as safe as lithium manganese oxide. However, the energy density of ternary materials is much higher than that of lithium manganese oxide. Therefore, whether in Japan or South Korea, the most mature power products are mainly based on lithium manganese oxide, and the use of mixed ternary materials not only ensures safety, but also improves energy density. This is also a trend in the future development of power.
According to different structures, the battery cell is divided into cylindrical battery cell, soft pack battery cell and square battery cell.
Square battery cell
Cylindrical battery cell
The typical cylindrical battery cell structure includes: positive electrode sheet, negative electrode sheet, diaphragm, electrolyte, shell, cap/positive electrode cap, gasket, safety valve, etc. Cylindrical battery cells generally use the cap as the positive electrode of the battery and the shell as the negative electrode of the battery.
Cylindrical battery cells are highly standardized, and common models are: 14650, 14500 (No. 5 battery), 18650, 21700, etc. The first two digits of the model represent the diameter of the cylindrical battery (in mm), the third and fourth digits represent the height of the cylindrical battery (in mm), and 0 refers to the cylinder. Tesla currently uses 18650 and 21700 cylindrical batteries, and in the future, 4680 (a battery with a thicker waist and a taller body) will be put into mass application.
Soft-pack battery cells
Comparison of the three types of batteries, each has its own advantages. Combined with the convenience of the production process, domestic electric vehicles now mainly use square batteries. Tesla electric vehicles use cylindrical batteries.
4 Battery Management System (BMS)
BMS is the English name Battery Management System, and the Chinese name is Power Battery Management System. It is a system for monitoring and managing batteries. It collects and calculates parameters such as voltage, current, temperature, and SOC, and then controls the battery's charging and discharging process to protect the battery and improve the battery's comprehensive performance. It is an important link between on-board power batteries and electric vehicles.
There are three main functions of BMS (Battery Management System): by measuring the state of charge of the power battery, the driver is provided with the remaining power to remind the driver to charge the electric battery in time; secondly, the battery temperature is monitored and managed, the temperature of the battery is detected during operation, and a blower or heat sink is used to ensure that the battery is working in the best state; finally, the battery is balanced. Due to factory manufacturing errors, ventilation differences during use, and electrochemical performance conversion, the battery voltage and remaining power are detected to prevent overcharging.
During the development of BMS, the hazard analysis of BMS includes hazard events such as overvoltage (overcharge), undervoltage, overtemperature and overcurrent. For example, overvoltage may be a more serious event, especially long-term overcharging of the battery will lead to battery performance degradation and irreversible damage, and even battery deformation and leakage. Then, the goal of BMS system safety design is to be able to detect battery overcharge in time, and through reasonable hazard analysis and evaluation, consider the design of safety mechanisms from the aspects of single point failure and potential failure, and finally make appropriate and timely treatment.
5 Battery Development Trends
5.1 Cobalt-free Batteries
The full name of ternary lithium battery is "ternary polymer lithium battery", which refers to a lithium battery whose positive electrode material uses nickel cobalt manganese oxide (NCM) or nickel cobalt aluminum oxide (NCA) ternary positive electrode material. Among them, cobalt, which is mainly used to stabilize the layered structure of the material and improve the material cycle and rate performance, is an indispensable precious metal in ternary batteries.
For a long time, the price fluctuation of cobalt has greatly affected the price of ternary materials. But it should be noted that more than half of the world's cobalt is produced in the Democratic Republic of the Congo, and the excessive concentration of resources has also exacerbated the vulnerability of the global cobalt supply chain. Since the beginning of this year, with the continued deterioration of the overseas epidemic, the blockade measures and social unrest in the Congo have intensified concerns about cobalt mine production. At the same time, the border blockade policies of Zambia, South Africa and other countries have restricted the transportation of cobalt raw materials. It is expected that the export of cobalt raw materials from the Democratic Republic of the Congo and South Africa in the second quarter will decline significantly, which will have an adverse impact on the domestic import of cobalt raw materials in the third quarter.
Cost issues have always been a stumbling block to the development of the new energy vehicle market. As the core cost, "power batteries" have always been expected to reduce costs as soon as possible. After reducing the proportion and content of cobalt, ternary lithium batteries will correspondingly reduce the cost of the entire vehicle, and the impact of cobalt price fluctuations on enterprises will also be weakened. "Anxious" enterprises have begun to change from active to passive, which will be conducive to promoting the development of the new energy vehicle market.
The following figure shows the honeycomb cobalt-free battery:
5.2 Solid-state battery
Solid-state battery is a battery technology. Unlike the lithium-ion battery and lithium-ion polymer battery commonly used today, solid-state battery is a battery that uses solid electrodes and solid electrolytes.
Because the scientific community believes that lithium-ion batteries have reached their limits, solid-state batteries have been regarded as batteries that can inherit the status of lithium-ion batteries in recent years. Solid-state lithium battery technology uses glass compounds made of lithium and sodium as conductive materials to replace the electrolyte of previous lithium batteries, greatly improving the energy density of lithium batteries.
Solid electrolytes have a high electrochemical stability window and can be used with high-voltage electrode materials to increase the energy density of the battery; solid electrolytes have high mechanical strength and are willing to effectively inhibit the penetration of lithium dendrites during the battery cycle, making it possible to use metal lithium with high theoretical energy density as a negative electrode material. Disadvantages of solid electrolytes (problems encountered in current development): ultra-high solid-solid contact impedance between electrodes and electrolytes.
5.3 Blade battery
Blade battery is a new design concept. While using long battery cells, it eliminates the intermediate module link and directly installs the battery cells into the battery system. In this way, the weight and cost are effectively reduced, which is similar to CATL's CTP. At the same time, BYD's battery structure design draws on the principle of honeycomb aluminum plates. The battery cells are fixed between two layers of aluminum plates through structural glue, allowing the battery cells themselves to act as structural parts to increase the strength of the entire system.
The length of C Company's product is 148 mm, the thickness is 79 mm, and the height is 97 mm. The internal structure is winding and looks like a brick. The blade cell is 960 mm long, 13.5 mm thick, 90 mm high, and has a stacked internal structure. It is named blade battery because of its long and thin shape that resembles a blade.
5.4 Stacking process
The stacking process is a Li-ion cell manufacturing process that cuts the positive and negative electrodes into small pieces and stacks them with a separator to form a small cell monomer, and then stacks the small cell monomers in parallel to form a large cell.
For example, soft-pack lithium batteries rely on "stacking", such as "z"-shaped stacking. First, the positive and negative electrode raw materials are cut into rectangular pole pieces of the same size, and then stacked on the diaphragm respectively. The diaphragm "Z" runs through it to separate the two poles, and finally wrapped in aluminum-plastic packaging.
The stacking process is cumbersome, mainly cutting the pole pieces and diaphragms into pieces. However, the qualified rate of pole piece cutting is low, the quality (cross section, burrs, etc.) is difficult to maintain high consistency, and the alignment accuracy is not enough, which requires a relatively high quality of the manufacturing process. This is also the main reason why stacked batteries are not popular.
5.5 CTP/CTC
The full name of CTP technology is Cell To Pack. By eliminating the module design, the battery cells are directly integrated into the battery pack, and the battery pack is integrated into the body floor as part of the vehicle structure.
This method reduces the side panels and end panels (module structural parts) of the module itself and the beams and longitudinal beams (battery pack supporting structure) originally used to separate the modules and help connect the modules. The entire battery structure is greatly simplified, the space is released, the capacity of the battery pack of the same size can be expanded, and the weight of the battery pack can be reduced, thereby increasing the battery energy density and reducing the cost.
There are two different routes for CTP technology. The first is to completely eliminate the module, represented by BYD Blade Battery; the second is to integrate small modules into large modules, represented by CATL CTP technology
BYD Blade Battery vs CATL CTP
CTC technology is called Cell to Chassis. Zeng Yuqun, chairman of CATL, introduced it at the China Automotive Blue Book Forum: "This technology integrates the battery cell and chassis, and then integrates the motor, electronic control, and vehicle high voltage such as DC/DC, OBC, etc. through an innovative architecture, and optimizes power distribution and reduces energy consumption through an intelligent power domain controller. CTC will enable the cost of new energy vehicles to compete directly with fuel vehicles, with more riding space and better chassis passability."
CTC can be understood as a further extension of CTP in a sense. Its core is to eliminate the module and packaging process, integrate the battery cell directly into the vehicle chassis, and achieve a higher degree of integration.
Traditional technology vs CTP vs CTC
The emergence of CTC will break through the limitations of PACK and directly involve the chassis of the car, which is the most critical core component of the whole vehicle, the core advantage accumulated by the whole vehicle manufacturer after long-term development, and it is difficult for battery companies/professional PACK companies to develop independently. So now some battery suppliers are starting to plan chassis development.
At the Giga Fest event held by Tesla at its Berlin factory last year, the 4680 Structural Battery (CTC) solution was displayed-the 4680 battery pack canceled the module design, and the battery cells were densely arranged in the vehicle chassis. The battery cover was responsible for the two functions of sealing the battery and the body floor, and the seats could be directly installed on the battery pack.
