Problems and Performance of Lithium-ion Batteries for EV

Currently, electric vehicle batteries are deficient in terms of range, charging efficiency and safety, so they cannot fully meet people's car needs and hinder the further development of the electric vehicle industry. In order to ensure the performance of electric vehicle batteries and the user's charging experience, batteries are required to have a high energy density to ensure their range, and at the same time have good high and low temperature charging and discharging performance to improve charging efficiency and charging time. In addition, in order to reduce safety issues such as short circuit, thermal shock, and explosion caused by traffic accidents, it is necessary to put forward higher requirements for battery safety. The author mainly combs through the problems and performance faced by electric vehicles and puts forward suggestions for the routine maintenance and care of electric vehicle batteries.

In recent years, with the introduction of low-carbon and environmental protection policies in China, the new energy vehicle industry has developed rapidly. According to the statistics of the Ministry of Public Security, the domestic ownership of new energy vehicles at the end of 2022 has increased significantly compared to 2021, and has reached 13.1 million vehicles, an increase of 67.13 % year-on-year. 5.35 million new energy vehicles were newly registered in 2022, an increase of 81.48 % year-on-year, which shows a very rapid momentum of development. As the leading new energy vehicle, electric vehicle has the advantages of low pollution, high efficiency, low noise, no dependence on petrol, and simpler structure compared with the traditional internal combustion engine vehicle. Battery is the key core component of pure electric vehicles. Lithium-ion power batteries have become the mainstream choice of power batteries for pure electric vehicles due to their advantages of high operating voltage, high specific energy, long cycle life, low self-discharge rate, no memory, no pollution, and diverse shapes, etc. However, there are still problems with range, charging time, and safety. The author mainly combs through the problems and performance faced by electric vehicles, and puts forward suggestions for the routine maintenance and care of electric vehicle batteries.

During charging, positively charged lithium ions travel from the positive electrode, lithium iron phosphate, across the diaphragm to the laminated graphite at the negative electrode, where they are stored. In this way, the battery completes its charge. When the battery is discharged, that is, when energy is released from the battery in the form of electrical energy, the lithium ions pass through the electrolyte, travelling from the negative electrode and returning across the diaphragm to the positive electrode. The motor converts the electrical energy into mechanical energy, which allows the car to move.

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EV Lithium-ion Battery Problems and Performance Research

Battery is the "heart system" of electric vehicles, so its performance plays a key role in the operation of the car, restricting the popularity and development of electric vehicles. Affected by the cost, production technology, manufacturing process, battery materials and other objective factors, the current electric vehicle battery still exists short range, low charging efficiency, safety and other shortcomings, can not be fully adapted to the needs of people's cars, hindering the further development of the electric vehicle industry. For this reason, electric vehicle manufacturers should increase the strength of scientific research investment, face up to the problems existing in the use of electric vehicles, and improve them, while reducing the cost of electricity, so that the battery becomes more affordable and widely used in the automotive drivetrain.

In order to ensure that the performance of electric vehicle batteries can meet the daily needs, so that electric vehicles have a better range, it is required that electric vehicle batteries must have a high energy density. At the same time, because the sales of electric vehicles across a wide range of geographical areas, the electric vehicle battery is required to have good high and low temperature charging and discharging performance. In order to ensure the user's charging experience, charging efficiency and charging time need to be improved. In the event of a traffic accident resulting in a collision, etc., the battery of an electric vehicle may cause an explosion and other safety issues due to a short circuit, thermal shock, etc., so the safety of the battery of an electric vehicle is required to be higher.

2.1 Range

The range of an electric vehicle is directly related to the energy density of the battery. Battery energy density can be divided into two types: volumetric energy density and mass energy density. Volumetric energy density generally refers to the size of the power output per unit volume of the battery, and the unit can be expressed in Wh/L. Mass energy density generally refers to the size of the electrical energy output per unit mass of the battery, and the unit can generally be expressed in Wh/kg.

Low energy density is the core problem of electric vehicle batteries. Low energy density means that the continuous range of the car is weak. Currently, for electric vehicles used in the market, under the condition of maintaining a suitable driving speed, the driving range after each charge is usually between 100 and 300 km, and the mass of the battery pack is about 150 to 500 kg. Obviously, there is still a lot of room for improving the energy density of the battery.

On the one hand, the size constraints of the battery pack are manifested by the fact that existing electric vehicles are modified from conventional fuel vehicles, so there is not a lot of space to increase the size of the battery, and the reusable space of the battery pack is very small. In order to improve space utilisation and increase convenience and practicality, battery companies are modularising their batteries. On the other hand, it is unrealistic to increase range simply by increasing the number of lithium-ion single cells. This is because as the number of single cells increases, the total mass of the vehicle increases, and thus the power consumed by the vehicle during driving increases. Although the energy reserve rises after adding batteries, the range can theoretically be increased, but with the increase in mass the energy consumption during driving also increases, and the driving range will be reduced. Of course, increasing the energy density of the battery will also bring problems in terms of cost and safety.

In addition, the driving range of electric vehicles is closely related to the life of the battery. Studies have shown that as the battery capacity declines, the driving range of the vehicle will be greatly reduced. The factors affecting the cycle life of lithium-ion batteries are manifold. Longer cycle life means less energy consumption, including the depth of discharge and rechargeable frequency, battery materials, charging habits, use of temperature, etc.. Batteries typically account for about 30% of the total cost of a vehicle, and in some cases up to 50%, so the cost of replacing batteries will have a significant impact on the widespread adoption and popularity of electric vehicles.

There are three charging modes for pure electric vehicles, which are battery replacement, wireless induction, and wired conduction. Pure electric vehicle charging technology has electromagnetic induction technology, drive motor technology, high-power charging pile, flexible charging technology, power exchange technology, radio wave type and so on. Wireless charging technology may become the mainstream charging method for pure electric vehicles in the future due to its advantages of convenient charging, short charging time, low cost and high operating efficiency. Therefore, how to reduce the charging time and improve the charging rate of electric vehicle charging pile has become an urgent problem to be solved.

Up to now, the charging methods commonly used in EV fast charging piles include constant voltage charging, constant current charging, high current pulse charging, intermittent constant current or constant voltage charging, and so on. In recent years, the pulse charging method has become a major fast charging method for lithium-ion batteries. The pulse technology can greatly improve the charging efficiency, shorten the charging time, and better reduce the polarisation effect and gas precipitation of the battery. Guan Lei through the discharge experiment, pulse charging technology and constant current charging technology, compared with the charging time has been reduced, charging efficiency is significantly improved, the battery polarisation effect is effectively reduced. Although pulse charging technology is an effective way of rapid battery charging, its energy conversion efficiency needs to be improved, and the control requirements are complex. Intermittent charging method due to its ultra-high energy efficiency, rapid development in recent years, but also put forward in the control of higher requirements.

South Korean battery manufacturer SK On has developed a new battery charging technology that takes only 18 minutes to charge compared to the current common charging technology for electric vehicles, which typically takes 30 min. The key to technology is understood to be a unique coating material that effectively reduces the anode resistance of the battery during the charging process.

The results of ANG X G et al. show that a thermally regulated lithium iron sulphate battery can have a range of about 250 km in just 10 min of charging, and that new energy vehicles equipped with thermally regulated lithium iron sulphate batteries can accelerate to 100 km in less than 3 s. The technology has been shown to be effective in reducing the resistance of the anode during the charging process. Thermally regulated lithium iron sulfate batteries can simultaneously meet the multiple needs of electric vehicles, combining low cost, high safety, high power, fast charging, all-weather, long life.

At present, the development trend of pure electric vehicle charging technology is mainly to improve the charging speed and efficiency, innovate the battery range of pure electric vehicles, promote the intelligent interconnection of charging equipment, standardise the unified transaction and settlement mode, and cooperate with new energy power generation.

2.3 Safety

In recent years, although China's new energy vehicle industry has continued to develop rapidly, the safety hazards of power batteries have been prominent, and fires caused by fuel explosions have attracted sustained attention. Thermal safety remains an important factor limiting the development of power batteries and even the electric vehicle industry. Lithium-ion batteries work in a relatively narrow range of safety, too high or too low a temperature, may lead to short-circuiting inside the battery, causing an explosion. Currently, thermal runaway of power batteries is the main cause of automotive fire accidents. When a short-circuit occurs in the battery, the temperature inside the battery rises sharply and rapidly emits heat to the outside, which leads to overheating and triggers an explosion and other accidents. Factors affecting thermal runaway can be divided into internal and external factors. Internal factors include the material safety and product quality of the materials used in the production of EV batteries (including electrolyte, etc.). External factors are mainly due to mechanical or electrical abuse, such as extrusion or collision, during the operation and use of electric vehicle batteries, resulting in deformation or cracking of the battery, which leads to internal short-circuits or temperature abnormalities, triggering a series of safety issues.

In order to ensure the safety of lithium batteries for electric vehicles, battery manufacturers must make further improvements to the materials, production and processing of batteries. In addition, the use of battery management system can achieve the electric vehicle battery thermal runaway prediction and alarm, real-time analysis of various data in the battery system operation process, so as to achieve the purpose of monitoring and early warning of the battery's operational safety. This is a practical measure to enhance the safety of lithium-ion battery operation. However, up to now, the safety of electric vehicle batteries has not been fundamentally resolved. Thermal runaway is a safety risk in the whole life cycle of electric vehicle batteries, and some existing methods for detecting the safety of electric vehicle batteries cannot be fully and effectively implemented on a large scale due to the high cost, destructive and other shortcomings. In the current EV battery industry, the principle of "prevention first, fire suppression as a supplement" is still the focus of thermal runaway safety protection.

Maintenance and Care Strategy of Pure Electric Vehicle Lithium Battery

For a series of problems in electric vehicle batteries, effective maintenance can improve the service life and safety of the battery. For example: regular daily inspection of the battery, to truly "prevent problems before they occur"; correctly grasp the charging time, to avoid improper control of the charging time affects battery performance (in general, to maintain the depth of discharge of the battery to maintain the best charging effect at 50 % ~ 80 %); maintain good driving habits, avoid emergency acceleration or deceleration during the driving process, and driving on uneven surfaces. Driving habits, try to avoid emergency acceleration or deceleration in the driving process, and in the uneven road surface when driving the car, try to maintain a slow and even speed, so as not to cause battery extrusion and deformation, which will cause irreversible damage to the physical properties of the battery; try to avoid sunlight exposure and the depth of the power feed occurs; to protect the car charger and so on.

Conclusion

In summary, the development and popularity of electric vehicles is an inevitable trend in the development of the automotive industry. Only by focusing on all aspects of electric vehicle battery performance improvement and use of safety, strengthening the research and development of new materials for lithium batteries, reducing the cost of electricity, and innovating new technologies and processes, so as to overcome the short range, long charging time, unstable performance, poor safety and other shortcomings, is it the key to take into account the needs of personal travelling, global emission reduction and the enhancement of air quality.