Bottleneck analysis and suggestions of power battery technology development

By investigating the data of power battery supporting industry of new energy vehicles in 2019, this paper studies the bottleneck of battery technology in the development of new energy vehicles, summarizes and analyzes the root causes of vehicle safety accidents, and then from the aspects of battery system R & D and design, cell production and manufacturing, production quality control, etc, The technical and management requirements for the safety protection of the power battery system are put forward, which is expected to provide good suggestions for the sustainable development of the power battery technology of new energy vehicles in the future.
The development of new energy vehicles is the only way for China from a big automobile country to a powerful automobile country. Since the State Council issued and implemented the energy-saving and new energy automobile industry development plan (2012-2020) in 2012, China's new energy automobile industry has made remarkable achievements and become an important force leading the transformation of the world automobile industry. On October 27, 2020, the technology roadmap 2.0 for energy-saving and new energy vehicles (hereinafter referred to as "technology roadmap 2.0") was officially released in Shanghai under the guidance of the Ministry of industry and information technology and organized by the China Society of automotive engineering to revise and compile by experts of the whole industry. It further studies and confirms the development direction of "low-carbon, information, and intelligent" global automotive technology, "New energy vehicles will gradually become mainstream products, and the automobile industry will realize the transformation of electrification" has become one of the six major goals of China's automobile industry development in 2035. To better implement the planning objective of "technology roadmap 2.0", the technical bottlenecks of power batteries in the development of new energy vehicles are analyzed from three aspects, and countermeasures and suggestions are put forward for reference.

1. Technical status of China's automotive power battery industry from 2019 to 2020

1.1 power battery industry trend

In terms of industrial development, the problem of structural overcapacity of domestic power battery still exists. The market performance of new energy vehicles in 2019 is not satisfactory, with negative growth for six consecutive months, which harms the development of the power battery industry.

In terms of the product cost, the pressure and challenge for enterprises to reduce cost and increase efficiency are still great: first, the price fluctuation of upstream raw materials has a great impact on battery cost; second, the path to achieving cost reduction by relying on technology iteration is still groping.

In terms of policy orientation, after the substantial decline of subsidies in 2019, vehicle enterprises transmit capital pressure to battery enterprises. Some enterprises have weak bargaining power in the market, and their profitability has been greatly reduced. In terms of revenue, they rely too much on subsidies, which has made it difficult for them to continue their business.

In terms of technology development, the R & D of key technologies of power battery still needs to be strengthened, the R & D investment in key materials and new materials, solid-state battery and other aspects is still insufficient, the core technology patents are relatively lacking, the level of complete sets of automation equipment is not high, and the consistency and reliability of power battery products still need to be improved.

From the perspective of the domestic market, with the rapid development of the new energy vehicle industry, China's power battery production scale and product performance have been greatly improved, and several competitive enterprises have emerged. However, on the whole, China is facing problems such as weak market growth, high cost, consistency of product safety and quality, and urgent technological innovation and breakthrough. Many lithium battery-related accidents are caused by the quality defects of lithium batteries, such as the introduction of burr in the production process, the uneven coating of active material, the defects of electrode lug welding process, and so on.

In practice, based on the standardized implementation of systematic quality control, if there are quality problems in battery products, the main reason is still related to the design of battery products. For example, to improve the energy density of the power battery, the use of a thin separator will make the manufacturing quality control of the product more difficult, and the product is easy to produce quality problems such as defects.

Therefore, the design of the cell is still fundamental to improve the safety of the cell, and the quality defect is the secondary reason.

1.2 main features of new energy and power battery industry in 2019

1.2.1 the output of new energy vehicles declined for the first time

According to the statistics of the industry association, a total of 1.1768 million new energy vehicles were produced in the whole year, a year-on-year decrease of 3.56%; Among them, there are 1007300 new energy passenger cars, 99100 new energy buses and 113900 new energy special vehicles, with passenger cars as the main market. Among the new energy passenger cars, 751800 are pure electric vehicles and 255500 are plug-in hybrid vehicles. In the whole year, the output of new energy vehicles declined, which was mainly affected by the decline of subsidies.

1.2.2 the loading capacity of power battery continues to show positive growth

In 2019, although the sales volume of new energy vehicles declined for the first time, with the increase of the average charged capacity of a single vehicle, the annual loading capacity of power batteries still showed positive growth, with a total loading capacity of 62.2gwh, a year-on-year growth of 9.2%. Among them, the ternary battery is 40.5gwh, the lithium iron phosphate battery is 20.2gwh, the lithium manganate battery is 0.5gwh, and the lithium titanate battery is 0.4gwh.

From the aspect of appearance and technology, the proportion of square batteries will further increase in 2019, while the market share of cylindrical batteries will continue to narrow. In 2019, the loading capacity of square, soft and cylindrical power batteries in China will be 51.8gwh, 6.3gwh, and 4.1gwh respectively, accounting for 83.2%, 10.1%, and 6.7% of the total power battery loading capacity respectively.

The packaging route is an important factor affecting energy density, and the competition pattern changes with the trend of energy density. The battery packaging routes can be divided into three categories: square, cylinder, and soft package. The upper limit of theoretical energy density from high to low is soft package, square, and cylinder. In 2019, the installed capacity of the square battery is 34.32gwh (a year-on-year increase of 45.04%), the installed capacity of soft pack battery is 5.86gwh (a year-on-year increase of 16.55%), and the installed capacity of the cylindrical battery is 2.06gwh (a year-on-year decrease of 53.03%). The installed capacity and growth rate of the square battery are higher than that of the soft pack and cylindrical battery, and the replacement trend of the cylindrical battery is obvious.

1.2.3 further improvement of power battery industry concentration

In 2019, with the substantial decline of subsidies and the further improvement of market requirements for battery product quality, a total of 79 power battery enterprises will provide complete vehicle support in 2019, 14 less than in 2018.

Among the top 3, 5, and 10 power battery enterprises, the loading capacity of power battery was 45.6gwh, 49.2gwh, and 54.7gwh respectively, accounting for 73.4%, 79.1%, and 87.9% of the total loading capacity, and the industry concentration increased significantly.

1.2.4 further improvement of power battery technology (driving down cost)

According to the new subsidy standard implemented on June 26, 2019, vehicles above 160wh / kg can get a double subsidy. The improvement of power battery energy density of new energy vehicles will be an important direction of industry development, and the subsidy policy may help to promote the proportion of high energy density batteries. The distribution of energy density of power battery systems in 2019 further proves the survival of the fittest of battery products. It is expected that the proportion of 160wh / kg batteries will increase significantly in 2020.

In 2019, the installed capacity of power battery is mainly concentrated in the range above 140wh / kg, among which the installed capacity of 140 ~ 150wh / kg products which can obtain 1.1 times of subsidy coefficient according to 2018 standard is the most.

In 2019, the installed capacity of batteries above 160wh / kg is 9.86gwh, and the main suppliers are Ningde times (6.26gwh), BYD (1.51gwh), and Funding Technology (0.37gwh).

In 2019, the energy density of the ternary battery and lithium iron phosphate battery system will reach 182wh / kg and 145wh / kg respectively, which will further reduce the battery cost. With Ningde times and Tianjin Lishen as representatives, the technological development of 300wh / kg single-cell products has made remarkable progress; Represented by the Institute of Physics, Chinese Academy of Sciences, and Wei Lan, solid-state battery technology research and development have also made some progress.

1.2.5 power battery safety has become the focus of the whole industry

According to incomplete statistics, there are more than seven fire accidents in 2019, with a high proportion of accidents.

Although vehicle safety involves many aspects and can not be attributed to a single technical link, vehicle safety control technology, battery safety technology, thermal diffusion control technology, and charging safety warning technology are the key to the high-quality development of the industry.
 

2. New energy vehicle power battery technology bottleneck

2.1 the safety of lithium cell is the root of the technical bottleneck

From the technical practice since the release of technology roadmap 1.0, we can see that the safety problem of lithium cells is the root of the technical bottleneck. In the research of the battery industry in recent ten years, it has been confirmed that the safety of lithium cells is closely related to the material. For example, the melting temperature of PE / PP / PE composite membrane is about 150 ℃, but the melting temperature of PE membrane coated with double-sided ceramic can reach more than 180 ℃. Choosing the latter can effectively improve the high-temperature resistance of the cell and reduce the probability of thermal runaway.

The safety of lithium batteries with different anode and cathode materials is also different. LiFePO4 cell has low energy density, but its thermal stability is better. The ternary cell has high energy density, but its safety is poor. Choosing the former is undoubtedly conducive to the improvement of battery safety.

The most important thing is that the electrolyte of the lithium battery itself is a combustible material. Many lithium battery research institutions around the world are studying the use of flame retardant electrolytes or flame retardant additives to improve the safety of lithium batteries. However, most of these technologies are still not mature enough, which usually affects the battery performance or life, so they are still not accepted by the market. Based on the practical experience of the promotion and application of new energy vehicles at this stage, the risk of lithium battery fire is always an unavoidable safety issue.

2.2 the lightweight technology of battery system is the engineering problem of technical bottleneck

Industry data show that the group efficiency of cylindrical cell module is about 90%, and the group efficiency of a system is about 70%; The group efficiency of the flexible battery module is about 89%, and the group efficiency of the system is about 65%; The group efficiency of the square shell battery module is about 92%, and the group efficiency of the system is about 75%.

According to the current group efficiency calculation, it is difficult to achieve the energy density of 260wh / kg for the new lithium-ion battery system proposed in the action for promoting the development of the automotive power battery industry in 2020, which is 370wh / kg for the cylindrical cell, 400wh / kg for the flexible cell and 347wh / kg for the square shell cell.

Based on the analysis of industry component materials, improving group efficiency is another way to improve the energy density of the power battery system. The lightweight design of the power battery system is one of the key technologies to improve the efficiency of the system, which plays a very important role in improving the mass-energy density of the power battery system.

The structure optimization of the power battery module and box is to reduce the use of nonenergy storage materials based on ensuring the structural strength of the battery pack. Through CAD / CAE / CAM integration technology to optimize and analyze the structure of the battery box, realize the simplification, integration, and lightweight of the internal parts of the box, which is the main method in the process of the battery box design.

The use of lightweight new materials, such as aluminum alloy materials, has been initially realized; At the same time, the strength of high-strength steel is greatly improved, which can help to realize the thin-walled design of the box, to realize the lightweight design of the power battery pack. Composites have excellent properties such as lightweight and high strength, such as SMC, BMC, and other thermosetting composites can play an important role in the development direction of the lightweight power battery box. Due to the limitation of high cost, the application of carbon fiber composites is less. However, with the reduction of the cost of composite materials, the composite materials are expected to achieve large-scale application in the battery box in the future.
 

3. Safety protection of power battery system as a breakthrough

The power battery system is a complete system composed of the cell, module, control system, auxiliary function module, and shell. In the driving process of the whole vehicle, the battery system is constantly subjected to various severe mechanical environment conditions, such as high and low-temperature impact, vibration and mechanical impact, road stone impact, traffic vehicle collision, and other abnormal conditions. The electrical connection part gradually or suddenly loses, resulting in a significant increase in the impedance of the connection, generating a lot of heat, which is transmitted to the battery, Finally, the local battery thermal runaway and quickly spread to the whole battery, causing the whole vehicle fire.

Many cases of a vehicle fire or battery fire in the market are related to the loose electrical connection. For example, in February 2008, the fire of Toyota Prius plug-in hybrid vehicle, due to the wrong assembly process of the connection point at the output end of the battery module, the connection can not be effectively locked, and there is no fastening device.

Therefore, safety protection measures of power battery systems should be considered from three aspects: design, quality control, and abuse protection. In particular, the design link is the primary condition to realize the safety of the power battery system. In the aspect of battery system safety design, it is suggested to pay attention to the mechanical strength of the battery pack shell, the insulation attenuation of the internal plastic insulation, the cable fixation inside the battery pack, the electrical connection locking device, the protection of high-voltage circuit, the high-voltage interlock of power battery, the thermal management of the battery system and other key factors, and carry out the forward design of the power battery system.

In addition, the design and development of multi-material and lightweight power battery pack boxes is one of the future development trends. While applying different materials in different parts of the box to achieve the optimal box structure design, the industrialization application of key lightweight materials such as high-strength steel, high-performance aluminum alloy, carbon fiber reinforced composites, engineering plastics, synthetic rubber, and products will be carried out. It should also be emphasized that the safety standard of power battery can not cover all cases of abuse, especially the extreme abuse conditions, such as high-speed collision, falling into water after the collision, foreign body puncture, etc. the power battery manufacturer should formulate the safety scheme and test verification specification of the battery according to the performance and safety requirements of the whole vehicle and the failure risk analysis, Control the relevant risks at the acceptable level of the whole vehicle industry.
 

4. Conclusion

With the development of new energy vehicles in the next 15 years, the technological progress of the automobile industry is still changing with each passing day, and enterprises will continue to face the severe challenges of continuous changes in the internal and external environment of the automobile industry. The current situation, characteristics, development trend, existing problems, suggestions, and solutions of the new energy vehicle power battery industry introduced in this paper are only one's own words due to the limitations of research and collection materials. It is also hoped that the enterprise entities can use them for reference. 
To ensure the scientificity, timeliness, and guidance of new energy vehicle technology development, we hope that under the guidance of the Ministry of industry and information technology, colleagues in the industry will continue to carry out the power cell technology research project by the guidance of new energy vehicle industry planning (2021-2035) and new energy vehicle technology roadmap 2.0, to provide effective solutions for the overall development of the industry.