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Introduction to Car Inverters

A car inverter is a device that converts a vehicle’s DC power (typically supplied by the vehicle’s battery) into AC power for use by the vehicle’s electronic devices.

A car inverter typically connects to the vehicle’s 12V or 24V DC power system and provides a standard 110V or 220V AC power output. This allows the driver and passengers to use household appliances, charge electronic devices, or access other devices requiring AC power while in the vehicle, making it convenient for activities such as long trips or camping. Power inverter play an important role in charging vehicle electronics, during outdoor activities, or in emergency situations.

A car inverter draws power from a single 12V battery (preferably a deep-cycle 12V lithium battery) or from several batteries connected in parallel. The battery needs to be charged because the car inverter draws power from the battery. The battery can be charged by running the vehicle’s motor, a gas generator, solar panels, or wind. Alternatively, you can charge the battery using a battery charger plugged into an AC outlet.

The Role of an On-Board Inverter

Electric vehicles are powered by batteries, which output direct current (DC). However, the power system we use in our daily lives uses alternating current (AC), so DC needs to be converted to AC for use. The inverter is a key component in this conversion process.

Voltage and Frequency Adjustment

An inverter automatically adjusts the voltage and frequency of the AC output based on load demand to meet the requirements of different devices.

Battery and Motor Protection

An inverter monitors battery voltage, current, and other parameters in real time to ensure that the battery pack operates within safe ranges. Furthermore, it adjusts the frequency and voltage of the AC output based on the motor’s operating status to protect the motor from damage.

Energy Recovery

In certain situations, such as braking, electric vehicles generate significant amounts of regenerative energy. The inverter converts this regenerative energy into AC and feeds it back into the grid, thereby recycling the energy.

The Role of an Inverter in a Vehicle

In a pure electric vehicle, the inverter and controller together form the motor controller (MCU), which serves as the command center for the entire power system. When the vehicle needs to accelerate or brake, the controller adjusts the inverter’s frequency based on the drive motor’s demand signal, thereby controlling the vehicle’s movement.

The inverter converts the DC power output from the power battery into three-phase AC power, providing the motor with the necessary power. Furthermore, during braking, the inverter can recover energy. As shown in the figure, the inverter internally consists of six IGBTs arranged in an X-shape, Sa-Sc. Each phase output line (Ia, Ib, and Ic) and the positive and negative DC lines are connected to an IGBT.

It is important to note that the IGBTs in the inverter will not function properly if the temperature exceeds 150°C, so air or water cooling is required to dissipate heat. If a vehicle’s drive motor system malfunctions, such as overheating or excessive coolant temperature, a diagnostic tool should be used to read the specific fault code, as the fault information displayed on the instrument panel may not be specific enough.

Does a car inverter harm the battery?

Generally speaking, properly using a car inverter with appropriate power will not cause direct damage to the vehicle battery. However, if the vehicle inverter’s power is too high or it’s overloaded, the battery may drain too quickly, affecting the vehicle’s starting and normal operation. Furthermore, continuous high power output may shorten the battery life. Therefore, when selecting and using a vehicle inverter, it’s recommended to properly match the vehicle’s battery capacity with the inverter’s power rating, avoiding overloading to protect the battery and the vehicle’s electrical system for safe and stable operation.

In short, the inverter plays a vital role in electric vehicles. It not only converts DC to AC power but also regulates voltage and frequency, protects the battery and motor, and performs energy recovery. With the continuous advancement of electric vehicle technology, the performance and efficiency of inverters will continue to improve, providing strong support for the popularization and development of electric vehicles.

Related links

the amazing competition between solid-state batteries and lithium batteries

Analysis of the lithium battery market for new energy vehicles

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Lithium battery safety testing: 6 major standards

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The boom in electric vehicles has driven research into battery technology, especially the differences between solid-state and lithium batteries.

After analyzing the new energy vehicle lithium battery market analysis, we can understand many key points. Although they use lithium ions as charge carriers, their structures and performance are very different. This article will take a deep look at the working principles of solid-state batteries and lithium batteries, as well as their respective advantages and challenges.

Introduction to lithium batteries

Lithium batteries, the most commonly used type of battery in electric vehicles today, consist of a positive electrode, a negative electrode, and an electrolyte. The positive and negative electrodes are usually made of materials such as metal oxides or phosphates, while the electrolyte is a liquid or gel containing lithium salts that can transport lithium ions between the two electrodes.

When the battery is charged, lithium ions flow from the positive electrode to the negative electrode and are stored in the negative electrode, while when discharged, they flow from the negative electrode to the positive electrode, releasing electrical energy. Lithium batteries have high energy and power densities, which means they can store and output large amounts of electrical energy.

However, lithium batteries also have some disadvantages. First, they require a thermal management system to control the temperature of the battery, since being too hot or too cold can affect the performance and life of lithium batteries. Secondly, despite the strict lithium battery safety standards, lithium batteries still have safety hazards because the liquid electrolyte can easily leak, burn, or explode.

Introduction to solid-state batteries

Solid-state batteries are a new type of Battery Technology. The main difference from lithium batteries is that they use solid electrolytes instead of liquid electrolytes. A solid electrolyte is a solid material with high ionic conductivity, such as ceramics, polymers, or glass.

The advantages of solid-state batteries are many. First, they do not require a thermal management system because solid electrolytes can operate over a wider temperature range without thermal runaway. Second, solid-state batteries have higher energy density, which means they offer longer range and longer life.

Furthermore, they have a charging cycle of 8,000 to 10,000 times, much higher than the 1,500 to 2,000 times of lithium batteries. Most importantly, solid-state batteries are safer. Due to the use of solid electrolytes, they will not leak, burn or explode, and are more resistant to impact and puncture.

Challenges in the development of Battery Technology

However, solid-state batteries still face some challenges. Although they have many advantages, the current Battery Technology is still in the development stage and has not yet been commercialized on a large scale. Only a few companies are making prototypes of solid-state batteries, such as Bioenno Power and Solid Energies.

The manufacturing process of solid-state batteries is more complicated than that of lithium batteries, requiring higher temperatures and pressures and more precise control. In addition, the standardization and scale-up of solid-state batteries is also a problem, because different electric vehicle platforms and battery manufacturers have different designs and patents. If the industry reaches a consensus on this, the Battery Technology of solid-state batteries will be promoted faster.

In addition to the Battery Technology challenge, the cost of solid-state batteries is also a problem. Currently, the cost of solid-state batteries is twice that of lithium batteries. However, with the advancement of battery technology and the scale effect, the cost of solid-state batteries is expected to decrease, making the price of electric vehicles more competitive.

Solid-state batteries and lithium batteries are two different battery technologies. Although they use lithium ions as charge carriers, their structures and performance are very different. Lithium batteries have some “lithium battery safety” issues due to their liquid electrolytes. Solid-state batteries use solid electrolytes to have higher energy density, longer life, higher safety, and wider temperature adaptability. However, to realize its potential, some battery technology and cost challenges need to be overcome.

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Analysis of the lithium battery market for new energy vehicles in 2024. In the past two years, the new energy vehicle and energy storage industries have been highly prosperous, the demand for batteries has proliferated, and the proportion of batteries installed in new energy vehicles has declined. Due to the high nickel and cobalt prices, the differentiated growth of ternary lithium batteries and lithium iron phosphate batteries has been formed. China dominates the lithium battery market, and the global supply chain relies closely on its production capacity.

With the growth of long-range products, ternary batteries have recovered, and the total proportion of lithium iron phosphate batteries has dropped from 67% to 61%. In battery technology, the improvement of electrolytes is particularly important, which directly affects the performance and safety of batteries. With the promotion of policies, extended-range, and plug-in hybrids continue to strengthen, while pure electric vehicles are weak. Especially under the demand for long-range and high energy density, the selection and optimization of electrolyte materials have become one of the key research directions. It is expected that the growth in demand for battery installation of electric vehicles will continue to be slower than the growth in the total number of vehicles.

Lithium battery market analysis: the proportion of power battery installation

At present, the proportion of power batteries installed in the lithium battery market is constantly decreasing. In 2020, the installed capacity of power batteries reached 76%, 70% in 2021, 54% in 2022, 50% in 2023, and 46% in January-February 2024. The lithium battery market is growing rapidly and has become an important part of the global energy industry.

With the development of industries such as energy storage, especially the world energy crisis brought about by the Russian-Ukrainian crisis, the demand for batteries in industries such as energy storage has grown rapidly, resulting in a significant decline in the proportion of batteries installed. Both power batteries and energy storage batteries are overproduced and have relatively high pressure on inventory.

The growth rate of power batteries in 2021 and 2022 is lower than that of complete vehicles. The installation of power batteries in February 2024 is low, and the battery output is higher than the installation growth rate. Innovations in the lithium battery market continue to emerge, driving the continuous advancement of new energy technologies.

Lithium battery market analysis: the proportion of ternary batteries installed in domestic sales models has gradually recovered

The growth in demand for power battery installation is volatile. Demand increased by 10% in 2019; in 2020, 64GWh of power batteries were installed in domestic sales models, and demand increased by 2%; in 2021, 155GWh of power batteries were installed, and demand increased by 143%; in 2022, 295GWh were installed, and demand increased by 91%; in 2023, 388GWh were installed.

Among them, ternary batteries increased by 21% in 2019; decreased by 7% in 2020; increased by 91% in 2021; increased by 49% in 2022; and increased by 11% year-on-year from January to December 2023. Lithium iron phosphate batteries have grown significantly, increasing by 20% in 2020; 227% in 2021; 130% in 2022; and increased by 44% from January to December 2023. With the advancement of technology, the lithium battery market has ushered in more innovative products with high efficiency and long life.

In 2022, the installed capacity of power batteries was 295GWh, a year-on-year increase of 91%. Among them, the installed capacity of ternary batteries was 110GWh, a year-on-year increase of 49%. The installed capacity of lithium iron phosphate batteries was 184GWh, a year-on-year increase of 130%.

In 2023, the installed capacity of power batteries was 388GWh, a year-on-year increase of 32%. Among them, the installed capacity of ternary batteries was 126GWh, a year-on-year increase of 14%. The installed capacity of lithium iron phosphate batteries was 261GWh, a year-on-year increase of 42%.

In February 2024, the installed capacity of ternary batteries was 6.9 GWh, accounting for 38%, an increase of 6 points; while the installed capacity of lithium iron phosphate batteries was 11GWh, accounting for 61%, and the growth of ternary batteries improved.

Lithium battery market analysis: the growth of demand for automotive batteries continues to be strong

In recent years, the lithium battery market has been driven by environmental protection policies and has shown good development prospects. With the popularization of electric vehicles, due to the excellent life of lithium batteries, the demand for lithium batteries has shown explosive growth. The demand for passenger car batteries continues to grow strongly. In 2024, the battery demand for pure electric passenger cars increased by 15%, while the battery demand for plug-in hybrid passenger cars increased by 86%, which continued to grow strongly. Due to the low base factor of subsidy withdrawal, the battery demand for buses has relatively recovered, and the battery demand for special vehicles has also increased significantly.

Judging from the proportion of battery installation, the demand structure for power batteries has been changing rapidly in recent years. In 2020, pure electric passenger cars are still ranked first, pure electric buses are ranked second, and pure electric special vehicles are ranked third, while plug-in hybrid passenger cars are only ranked fourth.

This year, pure electric passenger cars still maintain the first place, while plug-in hybrid passenger cars rise to the second place, pure electric special vehicles rise to the third place, and pure electric buses fall to the fourth level.

In recent years, the pure electric bus market has declined sharply, while plug-in hybrid passenger cars have shown a rapid upward trend. Pure electric special vehicles have maintained a relatively stable battery consumption of about 7-8%.

At present, pure electric buses have dropped from 18.5% to a cumulative level of 1.1% in 2024, a decrease of 17 percentage points. The battery usage of plug-in hybrid passenger cars has grown relatively rapidly and has now risen from 7% to 23.4%, an increase of 16%, while pure electric has dropped to 67%, maintaining the absolute core battery demand characteristics of more than 90% for passenger cars.

Lithium battery market analysis: automobile certificate output

According to the certificate battery volume, the output of certificate products in February 2024 is 363,000 units, a year-on-year decrease of 22%, of which pure electric passenger cars decreased by 30% and plug-in hybrid passenger cars decreased by 5%. Pure electric special vehicles decreased by 31% year-on-year, and such production data is still relatively low.

Lithium battery market analysis: supporting battery companies are far from fully competitive

In the past few years, the competitive landscape of the battery market has not changed significantly. Due to the relatively slow technological progress in the power battery market and the relatively obvious scale growth characteristics, battery companies have obtained strong production and installation quantity growth characteristics.

The original battery pattern has not changed significantly. Whoever invests more will get a larger market share, so the main battery companies have continued to expand strongly, and small and medium-sized battery companies also have the opportunity to achieve certain growth through technological or other breakthroughs. Therefore, the battery pattern should be said to be relatively stable in the high-speed growth.

However, the opportunities for changes in the battery industry in the future are relatively large. In the future, the trend of vehicle manufacturers making batteries or vehicle manufacturers jointly making batteries with related companies is becoming increasingly obvious, and battery companies will gradually form core supporting products for vehicles.

At present, the demand for the high-end electric vehicle market is not very strong, but it is similar to the demand for upgrading “Lao Toule” to small and micro cars and low-end family transportation. Especially affected by the epidemic, the market demand for A0 and A00 vehicles is relatively high.

In terms of supply chain issues, vehicle manufacturers will become increasingly powerful in the future, and their control over battery companies and upstream industrial chains will be further strengthened, while their control over downstream brand marketing capabilities will also be further strengthened. Under the new energy system, the characteristic of “the whole vehicle is king” will continue to be reflected.

Lithium battery market analysis: lithium iron phosphate batteries need to decline

The current energy density range of the main battery of pure electric vehicles is between 125 and 160. In particular, the performance of batteries with a density of 125 to 140 in February 2024 reached 43%, an increase of 12 percentage points year-on-year.

In February 2024, the proportion of models with a battery energy density of more than 160Wh/kg was 16%, a significant decrease from 31% in 2020. This is mainly due to the decrease in energy density caused by the replacement of lithium iron phosphate batteries with ternary lithium batteries.

At the same time, as the market pays more and more attention to the safety of electric vehicles, lithium battery safety testing has become an important part of evaluating battery performance to ensure that new batteries find a balance between energy density and safety. Products with an energy density of less than 125Wh/kg increased from 9% in 2023 to 13% in 2024. This change also reflects the industry’s efforts to optimize battery safety and cost control.

Lithium battery market analysis: battery enterprise landscape

Competition in the lithium battery market is becoming increasingly fierce, and major manufacturers have increased their investment in research and development. The competitive landscape of battery companies has formed relatively strong characteristics of CATL and BYD. At present, the gap between BYD and CATL is still large. BYD’s share increased from 15% in 2020 to 17.8% in 2024, an increase of 2.9 percentage points.

CATL’s share increased by about 5.2 percentage points, and the share of other battery companies also showed a clear differentiation trend. Battery companies have formed the characteristics of slowing down the agglomeration effect of head companies. From the 72% share of the top two companies in 2022, it still maintains a proportion of 73% this year, and the space for other companies is only about 27%.

The product difference advantage of lithium iron phosphate batteries is obvious. BYD is relatively excellent, but it entered an adjustment period at the beginning of this year. CATL’s share of iron-lithium batteries has surpassed BYD since the beginning of this year. EVE Energy, Zhongxinhang, and Zhengli New Energy have improved significantly.

Due to BYD’s comprehensive transformation to lithium iron phosphate batteries, the advantages of ternary batteries of the top three companies such as CATL are more obvious.

Factors affecting the life of lithium batteries

Number of charge and discharge

Lithium batteries have a limited number of charges and discharges, generally 500-1000 times. This is because during the charging and discharging process, lithium ions migrate between the positive and negative electrodes and are gradually lost. Lithium batteries of different brands and qualities may have different charge and discharge times.

temperature

Temperature also has a great impact on the life of lithium batteries. At high temperatures, the chemical reactions inside the battery will accelerate, resulting in a decrease in battery performance; while at low temperatures, the battery’s charge and discharge performance will deteriorate.

state of charge

State of charge (SOC) refers to the ratio of the battery’s remaining capacity to its initial capacity. When the SOC is 0, the battery is empty; when the SOC is 100, the battery is fully charged. Keeping the battery in a high SOC state for a long time will accelerate battery aging.

Lithium battery life expectancy

cycle life

Cycle life refers to the number of charges and discharges experienced by the battery during the charge and discharge process before the energy it can provide drops to a certain proportion (for example, 80%) of the initial value. Generally speaking, the cycle life of lithium batteries is between 500-1000 times.

Storage life

Storage life refers to how long a battery can maintain its performance when not in use. When environmental factors such as temperature and humidity are appropriate, the storage life of lithium batteries can reach more than 2 years. However, if the temperature is too high or too low, and the humidity is too high or too low, the storage life will be significantly shortened.

Methods to improve the life of lithium battery

Reasonable charge and discharge

In order to extend the life of lithium batteries, deep charging and discharging should be avoided as much as possible. Charging when the battery is used between 20% and 80% can effectively reduce the number of charges and discharges and extend battery life. At the same time, avoid keeping the battery in a high temperature state for a long time to slow down the degradation of battery performance.

temperature control

Temperature has a great impact on the life of lithium batteries, therefore, controlling battery temperature is an important measure to extend its life. The battery temperature can be reduced by strengthening the heat dissipation design of the battery pack and installing heat sinks. At the same time, avoid exposing the battery to high temperatures to extend its service life.

in conclusion

Generally speaking, the life of lithium batteries is affected by many factors, including the number of charge and discharge times, temperature and state of charge. In actual use, attention should be paid to measures such as reasonable charging and discharging and temperature control to extend the life of lithium batteries. At the same time, there are also differences in the lifespan of different lithium-ion battery manufacturers and lithium batteries of different qualities. When choosing to use lithium batteries, you should reasonably choose a battery pack that meets your requirements based on actual needs.

Every lithium-ion battery developed needs to meet certain requirements called battery testing standards, which explain its behavior in terms of safe use, even if it is implemented as a component of another technology. The lithium battery testing standard is a standard for the battery manufacturing industry to promote the safe development of enterprises. Through these processes of testing development in the early stages, it is safe for both consumers to play in different environments.

The lithium battery is put through testing machines, exposing it to different environmental conditions. The reaction of the lithium battery to the influence of environmental conditions in the test machine was recorded. The recorded information will be used to ensure compliance with all lithium battery safety standards.

What are the testing standards for lithium-ion batteries?

Lithium battery safety testing standards are developed to test lithium-ion batteries in the development stage to ensure that they meet global safety requirements. These lithium-ion battery testing standards are developed by well-known international organizations such as Underwriters Laboratories (UL), Japan Standards Association (JSA), etc., and are therefore recognized globally.

For lithium-ion batteries, this selected standard serves to ensure adequate safety in all working processes.

For lithium battery safety testing, we most commonly use the following 6 standards:

IEC 62133
United Nations 38.3
ECE R100
IEC 62619
UL 1642
UL 2580

International Electrotechnical Commission (IEC) 62133

IEC 62133 is a safety requirement for testing secondary cells and batteries containing alkaline or non-acidic electrolytes. For safety testing of portable sealed secondary lithium-ion batteries. IEC 62133 ensures that lithium-ion batteries meet the safety requirements required for portable electronics and other applications.

With this standard, lithium battery cells are differentiated based on sufficient functionality. IEC 62133 was introduced to safeguard and eliminate chemical and electrical hazards such as vibration and mechanical shock that pose a threat to consumers and the environment.

United Nations Transportation Test (UN/DOT) 38.3

UN 38.3 standard testing ensures that lithium-ion batteries meet safe transportation requirements for air, sea, and land transportation. The requirements of UN 38.3 apply to all lithium battery cells and batteries. The United Nations (UN) and the U.S. Department of Transportation (DOT) both play a role in ensuring the safe transportation of lithium batteries.

Lithium battery is quite hazardous and require UN 38.3 standard transport testing and other regulations before being transported from one location to another. Any lithium battery is tested to UN 38.3 to ensure the battery complies with international rules and regulations for battery transport.

UN ECE Regulation R100

The ECE R100 standard test is conducted on electric vehicle batteries to ensure adequate safety. ECE R100 provides safety protection when charging electric vehicle batteries. To ensure this rule is met, electric vehicles should not be moved or driven while the battery is charging, and direct contact should be avoided.

The ECE R100 also ensures that electric vehicles maintain accurate positioning without cracks while driving. ECE R100 is only suitable for M+N electric vehicles with a top speed of 25km/hr. Therefore, this standard also applies to voltage conversion of electric vehicles.

International Electrotechnical Commission (IEC) 62619

International Electrotechnical Commission 62619 specifies the requirements required for the safe use of secondary lithium batteries and battery packs. It ensures that all lithium batteries are safe for use in electronics and other applications. International Electrotechnical Commission 62619 standard test requirements apply to stationary and moving applications.

IEC 62619 is also ideal for testing the safety of energy storage batteries. Energy storage batteries include secondary lithium batteries and batteries used in simple electronic devices such as cell phones to keep them stationary.

According to IEC 62619, batteries should withstand certain temperatures in the testing machine room. IEC 62619 specifies testing requirements to be performed on a series of testing machines at a temperature of 25±5°C.

Underwriters Laboratories (UL) 1642

UL 1642 standard requirements cover primary and secondary lithium batteries used in electronic product applications. It is suitable for different kinds of lithium batteries, whether they are single cells, double cells or multi-cell electrochemical cells connected in parallel or in series. Lithium batteries contain components such as metallic lithium, alloys, and lithium ions, which help convert chemical energy into electrical energy through a chemical reaction process. UL 1642 ensures the safety of lithium batteries in applications by reducing risks related to fire, explosion, etc. UL 1642 requirements apply to the use of lithium batteries as product power supplies.

Underwriters Laboratories (UL) 2580

Underwriters Laboratories 2580 standard requirements ensure correct evaluation of electrical energy storage. It ensures that electrical energy storage meets its safety requirements to withstand environmental impacts under any conditions and conditions in which consumers handle it. UL 2580 protects consumers from hazards caused by improper use of lithium batteries. Electrical energy storage evaluation process based on UL 2580, including design and module and manufacturer recommended initials for specified charge and discharge values. The importance of lithium-ion battery testing standards.

Lithium-ion battery testing standards improve the use of this type of battery in different products due to its advantages. Unlike other types of batteries, lithium-ion batteries take the use of batteries to power electronic devices to another level. Most consumers of lithium-ion batteries consider this a technological breakthrough.

There are two core benefits to having different international testing standards for lithium-ion batteries, including

Performance

International testing standards for batteries have improved the performance of lithium-ion batteries, and most people refer to international testing standards as a tool to push the performance of lithium batteries to the top while ensuring safety. Today’s lithium batteries are more stable than ever and can be recharged a hundred times without any defects. Lithium-ion batteries have higher density, higher voltage capacity, and low charge rates that beat other battery types. All of this improves the performance of lithium-ion batteries, which attract the attention of potential consumers due to their impressive power efficiency of maintaining a single charge for long periods of time.

Security

Safety is always the reason why lithium batteries must meet the requirements of international testing standards. Lithium battery has passed international testing standards to ensure safe transportation and use by consumers and reduce the risk of exposure to hazards. Lithium batteries are considered safe when they meet international testing standards. Almost every lithium battery manufacturer hopes to develop battery products that can attract the attention of potential consumers by passing the safety and improved performance requirements of standard testing regulations.The lithium ion battery manufacturer has implemented stringent quality control measures to ensure the safety and reliability of their products.

Test

It sounds difficult. Without international testing standards, it is difficult for lithium batteries to pass adequate safety tests. International testing standards ensure that all manufacturers’ lithium batteries are free of defects by providing requirements for comprehensive testing of lithium batteries. Testing of lithium batteries improves their performance and ensures the safety of electronic product applications.

Why are safety considerations needed for lithium-ion batteries?

Lithium batteries are dangerous when defective and can cause explosions or fires, causing harm to human health and the environment, so safety considerations must be taken. Taking the safety of lithium batteries into consideration at the early stages of development takes its efficiency to another level by preventing future risks.

Although, even if lithium batteries have passed international safety tests, consumers still need to comply with other safety requirements when handling, storing and disposing of lithium batteries, including:

Disposal of lithium batteries needs to be done by qualified experts as most lithium batteries can be recycled while others cannot. Therefore, in order to avoid harm caused by incorrect handling of lithium batteries, it is necessary to send lithium batteries to disposable battery institutions.

Unlike other types of batteries, lithium batteries require special care. Even though it is affected by electronic devices, the manufacturer always ensures that it clicks perfectly in a stable position to prevent unnecessary movement while using the device and thus avoid risks.

Storing lithium batteries is necessary to promote safe use. There are endless stories of lithium battery explosions and fires due to poor battery management. Although there are international testing standards for lithium batteries, it is important to always follow the manufacturer’s storage rules to avoid risks and ensure safety.​

Why is lithium-ion safety testing needed?

Lithium-ion safety testing is necessary because it ensures that the battery can be used safely under specific environmental conditions without any risk. Lithium batteries have become very popular in the market as lithium-ion safety tests prevent usage risks associated with lithium batteries from affecting users.

① At the end of charging, the redox potential is slightly higher than the apparent potential of the positive electrode (0.1 ~ 0.2V);

② During overcharging, the redox reaction between the positive and negative electrodes should be kinetically reversible (the electrochemical reaction rate constant is higher than 10-5cm/s);

③ Redox substances should be chemically stable and not react with other components;

④ The diffusion coefficient and solubility of redox substances should be as high as possible.

Researchers have recommended the use of various metal complexes [132,133] and aromatic compounds [132-139] as overcharge protectors, however, their redox potentials are too low for 4V Li-ion batteries. Their onset oxidation potentials are clearly shown in Figure 1 (measured values ​​are not in good agreement due to different measurement conditions) [132,136–139]. It is clear from these examples that the π-electron conjugation system is necessary for the redox reaction.

The equation for the limiting current density iim of a single-electron-transferred redox shuttle is as follows:

In the formula, F is the Ferrari constant; Co is the initial concentration; L is the thickness of the diaphragm; D is the diffusion coefficient [136]. Because the diffusion coefficient of cationic groups produced by oxidation reactions is usually an order of magnitude smaller than that of the original neutral compound, the limiting current density depends on the diffusion coefficient of cationic groups [135,136]. This is because charged particles are solvated and cationic groups tend to interact with a neutral compound. Sex molecules interact to form dimers. The diffusion coefficient of neutral molecules is inversely proportional to the viscosity of the electrolyte. In the electrolyte (1mol/L LiPF6/EC+2DMC) [136], their diffusion coefficient is 10-6 ~ 10-5cmz/s, which is different from that of solvated lithium ions. The diffusion coefficient values ​​are similar. For example: Assuming Co = 0.2moI/dL, L = 25mm, D = 10_6cm2/s, 3% (mass fraction) of 2,4-difluoroanisole (DFA)[138] can withstand iiim = 8mA/cm2 the maximum limiting current density.

Experiments show that in Li/LiCoO2 2025 coin cell (25mA h), the redox strobilide compound 4-bromo-1,2-dimethoxy soluble in electrolyte (1mol/L LiPF6/PC+DMC) Benzene (0.1mol/L) achieves overcharge protection at a rate of 0.02C, as shown in Figure 2 [132]. When there is no additive, the voltage of the battery continues to rise, and the electrolyte decomposes and releases heat at 4.6V measured by a C80 calorimeter. On the other hand, when there is an additive, the battery voltage stops rising at 4.3V, and the input energy W = 4.3VX0. 15mA = 0.65mW is all converted into thermal energy and consumed by the redox shuttle reaction.

Another one added 2,7-dibromophosphorus (0.05mol/L) to the electrolyte of 1mol/L LiPF6/EC+PC+DMC+DEC (35:10:20:35, volume ratio) of prismatic battery (900mA h) reported [139]. The heat release during overcharging was measured with an acceleration calorimeter (ARC). When the battery under test is overcharged at a rate of 1C, the voltage of the battery rises to the preset 12V after 1.1h, and then keeps the voltage until the end of charging, as shown in Figure 2. Although the temperature T of the battery reached about 110°C, no thermal runaway occurred. However, at rates above 2C, no battery can pass similar tests. It should be emphasized that other methods (using safety devices) must be used to avoid overcharging at high current.

We found that alkylbenzene derivatives without H atoms in the benzyl position can act as mediators for the decomposition of carbonate solvents, as hypothesized in Figure 3. Because these derivatives undergo a reversible redox reaction, which increases the amount of CO2 released without being consumed by themselves. After the overcharge test at 60 °C, tert-butylbenzene and 1,3-di-tert-butylbenzene are more than toluene and ethylbenzene. and cumene have low open circuit voltage, they are very good overcharge protection additives.

As shown in Fig. 1, it was confirmed by in situ normalized interfacial Fourier transform infrared spectroscopy (SNIFTIR) that VC in LiCoO2 started to form polymers at a potential of 4.3 V to Li/Li+; below 4.3 V, Upward peaks are formed at 1830 cm-1, 1805 cm-1 and 1750 cm-1, which correspond to the consumption of VC, EC and EMC, respectively. The polymer film formed at 4.3V suppressed the further decomposition of EC and EMC above 4.3V, where downward peaks were formed at 1850~1800cm-1 and 1650~1550cm-1 due to the decomposition caused by the product. The chemical structure of the polymer was determined by XPS to be polyethylene carbonate.

However, the reaction of VC with Lio.5CoO2 at 85°C cannot account for the presence of polymers, and these results show that the passivation film formed by VC on Li0.5CoO2 has poor thermal stability. The amount of SEI film in C/LiCoC2 cells has been measured by ion chromatography (IC), but pure VC has not been observed to significantly inhibit LiF deposition on the cathode. In situ electrochemical impedance spectroscopy also indicated that the addition of VC could increase the impedance of the membrane during discharge.

We also successfully characterized for the first time by X-ray absorption near-band-edge structure spectroscopy (XANES) the formation of the cathode in the electrolyte of Imol/L LiPF6/PC+DMC (50:50, volume ratio) + 5% (mass fraction) ES SE1 membrane [65]. The compounds deposited on the positive electrode are different from those deposited on the negative electrode, and the instrument can detect Li2SO4 and other organic sulfides, as shown in Figure 2. When Li2SO3 is formed on the negative electrode, ES on the positive electrode is oxidized to SO2. In situ electrochemical impedance spectroscopy indicated that the impedance of the SEI film formed by ES was higher than that of the SEI film formed by VC, which was the same as the case on the negative electrode.

The SEI film formed on the positive electrode by GBL solvent alleviates the exothermic reaction between the electrolyte and the positive electrode, which is the basis for the high safety of the battery. These phenomena can illustrate that it is possible to reduce the reaction between the cathode surface and the liquid electrolyte by passivating the active sites [20]. Adding a very small amount (0.1%~0.2%) of aromatic compounds, such as biphenyl (BP), o-terphenyl (o-TP), and meta-terphenyl (m-TP), can improve the cycling performance of the cathode, which is due to The electrolyte polymerized to form a Li+ conductive film covering the surface of the positive electrode.

We also found that adding some sulfur-containing compounds can reduce the reaction of the positive electrode without affecting the performance of the battery. Figure 3 shows the additive effect of cathode protection additives. The addition of 1% (mass fraction) sulfides (1,3-PS and disulfonate X) with higher oxidation potential can significantly reduce the amount of evolved gas and maintain the original after storage at high temperature and low current charging test. some positive capacity. The open circuit voltage (OCV) was higher after the test, indicating that these additives maintained the state of charge of the positive electrode, and the film was formed due to the presence of positive electrode additives, which suppressed the generation of CO2 gas and increased the interfacial impedance.

Read more: What is the charge-discharge mechanism of cathode materials?

The reductive decomposition of EC and chain carbonates at room temperature can effectively promote the formation of SEI films, but problems arise at high temperatures. It has been verified that a variety of compounds can be used as additives to improve the properties of the passivation film on the graphite anode.

1. Unsaturated carbon compounds

Ethylene carbonate (VC), the first electrolyte solvent to be developed, has an extremely high relative permittivity (ℇr = 127) and provides good electrical conductivity. It is found that adding a small amount of VC can suppress gas generation and obtain high cycle efficiency at the first charge, and VC can also inhibit the decomposition of easily reducing solvents such as trimethyl phosphate (TMP), so VC is a typical anode film formation. additive. Adding 1% (mass fraction) VC to lmol/L LiPF6/EC+DMC+DEC (33:33:33, mass ratio) can improve the cycle life of Li-ion polymer batteries, which shows that this passivation layer has excellent stability.

Because VC has a double bond structure, its lowest unoccupied molecular orbital (LUMO) energy is lower, so it is generally believed that VC is more easily reduced than other carbonates such as EC and DMC. The reduction potential of VC can be tested by the gold electrode in tetrahydrofuran (THF) solvent, and its reduction potential is higher than that of other carbonate solvents, indicating that the decomposition of VC precedes other carbonates and can form on the surface of the negative electrode. A good SEI film while suppressing further solvent decomposition and graphite exfoliation due to solvent co-intercalation.

We have detected the composition of the gas escaping at different potentials during the first charge, as shown in Figure 3. When VC was not added to 1mol/L LiPF6 EC+DMC (50:50 volume ratio), the main decomposition products of EC, ethylene and carbon monoxide, gradually accumulated. On the other hand, when 2% (mass fraction) of VC was added, carbon dioxide was the main decomposition product. This shows that carbon dioxide is the reductive decomposition product of VC, and the reductive decomposition of VC can significantly inhibit the decomposition of the solvent. It was observed by scanning electron microscopy that when the voltage to Li/Li+ was 1.0 V, granular products existed on the surface of the graphite negative electrode. With the intercalation of Li+, the gel organic film further covered the graphite surface, as shown in Figure 4.

There are various ways to characterize the films formed with the participation of VC. CHz=CHOCHO2Li can be detected by infrared reflection absorption spectroscopy (IRRAS) [30], and it is also speculated that the formation of a polymer with an OCOLi functional group improves the viscosity and toughness of the passivation film. When the potential to Li/Li+ is 1.3 V, VC starts to deposit by reductive deposition, which can be reduced by in situ to a film thickness of about 10 nm at 0.8 V [30,33]. Ex situ AFM revealed an ultrathin film (less than 1 nm thick) on the laminated base of the highly pyrolytic graphite (HOPG) anode [34] o AC impedance analysis found that the film impedance decreased, which was due to LiF on the graphite anode. formation is inhibited.

The presence of polymers on SEI films has been first confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and nuclear magnetic resonance spectroscopy (1H-NMR). . FTIR and XPS (O18) spectra indicated the presence of C=C double bonds and polymers, respectively. TOF-SIMS showed the presence of polyacetylene on the outer surface of the SEI membrane. To identify the structure of this polymer, the SEI film formed in 1 mol/L LiPF6/VC was extracted with dimethyl sulfoxide (DMSO) solvent, and the polycarbonate was successfully identified by two-dimensional 1 H-NMR. Presence of vinylidene esters and other ring-opening compounds.

The relative molecular mass of these polymers formed on the lithium metal anode was between 1000 and 3000 as measured by gel permeation chromatography (GPC), and the addition of VC could enhance the lithium cycling efficiency. The SEI film could be determined by temperature-programmed mass spectrometry (TPAMS). of thermal stability. Adding VC can make the decomposition temperature of the SEI film become higher. The improvement of thermal stability can inhibit the reaction between the anode and the electrolyte when the SEI film is damaged and repaired. According to the observations, although the quality of the SEI film is largely dependent on the charcoal species and charging conditions, the formation of a thinner and uniform SEI film by adding VC is also the reason for the improved battery performance.

The reaction mechanism of VC has not yet been determined. The possibility of EC splitting due to intermolecular electron transfer from VC to EC was proposed by density functional theory (DFT) calculations of supramolecular (EC)nLi+(VC). Although the polymer reaction of the one-electron reduction product is likely to proceed simultaneously with the establishment of the SEI film base, among cyclic carbonates, VC is the most prone to two-electron reduction, which may be the reason for its ability to form a SEI film. One of the properties of additives.

In addition to VC, ethylene ethyl carbonate (VEC) [44~49], styrene carbonate (PhEC) [50], vinylene carbonate (PhVC) [51], catechol carbonate (CC) [51 , 52], amino methyl carbonate (AMC) [53, 54], AEC (amino ethyl carbonate) [55], vinyl acetate (VA), other vinyl compounds [53, 54, 56], acrylonitrile (AAN) [57, 58] and 2-cyanofuran (CN-F) [59], whose chemical structures are shown in Fig. 4.10, have similar effects on graphite exfoliation inhibition in PC solvent systems.

2. Sulfur-containing organic compounds

Sulfite compounds, including vinyl sulfite (ES), propylene sulfite (PS), dimethyl sulfite (DMS), and diethyl sulfite (DES), have been validated as film formers in PC. Adding 5% (volume fraction) of additives to the electrolyte can make the graphite electrode perform normal charge and discharge. These compounds are consumed when the voltage to Li/Li+ is about 2V, forming a passivation layer, the ability of which hinders the co-intercalation of PC electrolyte in the graphite layer is in the following order: ES>PS>DMS>DES. Besides sulfite compounds, it has been reported that cyclic sulfite compounds such as 1,3-propanesultone (1,3-PS) can also form good SEI films, which can improve the hard carbon/LiMn2 04 cylindrical shape. Cycling and storage performance of batteries.

The researchers speculate that the reduction of ring-opened ES via single-electron transfer is more beneficial than carbonates such as EC and VC. In situ AFM observations indicate that when the voltage to Li/Li+ is IV, the SEI film formed is about 30 nm thick, which is thicker than that of VC, due to the HOPG (highly oriented pyrolytic graphite) expansion due to PC co-intercalation.

The SEI films formed in Imol/L LiPF6/PC+5% (mass fraction) or 10% (mass fraction) ES were also characterized by various analytical means such as SEM, XPS, TPD-GC/MS, TOF-SIMS, etc. At a voltage of 1.8 V for li/Li+, ES is more easily reduced than PC, and the composition of the formed SEI film depends largely on the current density. At high current densities, the inorganic Li2SO3 is the first precipitation on which an organic layer composed of CH3CH(OSO2Li)CH2OCO2Li is formed. In situ electrochemical impedance spectroscopy revealed that the impedance of the SEI film formed by ES was higher than that of the SEI film formed by VC.

3. Halogen-containing organic compounds

It is easy to know from molecular orbital calculations that the introduction of halogen atoms into ring-opened carbonates can improve their reducibility. Vinyl chloride carbonate (CIEC) is reduced at a potential of about 1.8 V to Li/Li+ to form a passivation film and release CO2 at the same time. It can be speculated that the LiCl generated by reductive cleavage becomes an internal chemical shuttle, which leads to low current efficiency. This deficiency can be compensated by using fluoroethylene carbonate (FEC), which produces less soluble LiF. In situ AFM showed that the thickness of FEC-generated films was intermediate between VC and ES-generated films, but degraded significantly during cycling. Trifluoropropylene carbonate (TFPC) is also reduced at a potential of about 1.8V to Li/Li+ to form a passivation layer with an interfacial impedance greater than CIEC. Halogen-containing GBL (BrGBL or FGBL and N,N-dimethyltrifluoroacetamide (DTA) also have similar effects.

4. Other organic compounds

Although some other organic compounds such as asymmetric dihydrocarbyl carbonates, dihydrocarbyl pyrocarbonates, and trans-2,3-butene carbonate (t-BC) are considered co-solvents, they have also been shown to be beneficial additive. The alkoxy carbon releases CO2 through natural decomposition, and the CO2 continues to react on the surface of the active material to form Li2CO3. For solvated Li+, t-BC is too large to even intercalate into graphite, thus suppressing graphite exfoliation. The researchers also detected a series of GBL derivatives with different side chains at the 5-position carbon, some of which reduced the number of PC molecules due to the incorporation of Li+, thereby effectively inhibiting the decomposition of PC; also reported that succinic anhydride and succinic anhydride Imide derivatives have the same effect. Tetrakis(ethylene glycol) methyl ether (TEGME), whose molecular composition is similar to that of organic SEI compounds, forms a stable non-porous passivation layer. ) phosphite (TTFP) can also effectively inhibit PC decomposition.

5. Inorganic compounds

Inorganic compounds include CO2, N2O, SO2, CS2. Gate and Sx2- (the electrochemical reduction product of S8) play an important role in the stability of Li-graphite and Li-metal SEI films. When the above additives are contained, Li2CO3, Li20, Li2S, Li2S2O4 will be formed, forming a good passivation Floor.

6. Ionic compounds

It is well known that the type of lithium salt also affects the composition and quality of SEI films, however, the superposition effect of lithium salts is not fully understood. It has been confirmed that organoboron complexes, such as lithium bis(salicylate)borate and lithium bisoxalatoborate (LiBOB), can form stable SEI films on graphite anodes because their organic groups can serve as the constituents of SEI films. point. Lil, LiBr and NH4 are commonly used to inhibit the reduction of Mn(II) on the negative electrode of C/LiMn2O4 batteries. It has been reported that the addition of Na+ can reduce the irreversible capacity loss of graphite anodes during the first charge. It has been confirmed by testing that AgPF6 and Cu(CF3S()3)2 can form a metal protective layer.

Read more: Current status of cathode materials

In lithium-ion batteries, the liquid electrolyte acts as an ion conductor, transporting lithium ions back and forth between the positive and negative electrodes during charging and discharging. Since the electrode of the lithium-ion battery is a porous composite electrode, it is composed of active materials [carbon in the negative electrode and lithium transition metal (Co, Ni, Mn) oxides in the positive electrode, respectively], a conductive agent (carbon black) and a polymer binder. The liquid electrolyte must be able to penetrate into the porous electrode and allow unobstructed transport of lithium ions at the liquid-solid interface. Most of the lithium-ion batteries in the market use non-aqueous electrolytes in which lithium salts are dissolved in aprotic organic solvents. Gel-type electrolytes used in polymer batteries are generally considered to be solidified from liquid electrolytes and high molecular weight polymers. Therefore, liquid and gel electrolytes are required to have the same function to some extent.

Many literatures describe liquid electrolytes in lithium or lithium-ion batteries, describing the different properties of aprotic solvents, lithium salts, and other mixtures. The researchers also commented on the above substances from the perspective of solution chemistry. Recently, however, research on liquid electrolytes has mainly focused on electrolyte additives, which can play other roles in Li-ion batteries in addition to their basic functions as ionic conductors.

2. Electrolytes with specific functions

The electrolytes of commercial lithium-ion batteries are mostly non-aqueous, and LiPF6 is dissolved in a mixed solvent of cyclic carbonate and chain carbonate to form a solution with a concentration of about 1 mol/L. Optional cyclic carbonates include ethylene carbonate (DEC), propylene carbonate (PC); chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) . The chemical structures of these carbonates are shown in Figure 1. Another liquid electrolyte, 1.5mol/L LiBF4/γ-butyrolactone (GBL) + ethylene carbonate, has recently been designed for high-safety thin-plate batteries on the market. Although researchers are working hard to develop new materials, the applications of many solvents and lithium salts are still limited. Electrolytes in which a small amount of additives are added to the above basic electrolytes are called “functional electrolytes”.

Now a variety of new additives have been developed, each with its own specific role. Adding additives to the electrolyte in the optimal proportion can make the electrolyte obtain a series of special functions, which can be called “electrolyte with specific functions”. According to the working principle, additives can be classified as follows: negative electrode film-forming additives; positive electrode protection additives; overcharge protection additives; wettability additives; flame retardant additives; other additives.

Although it is academically incorrect to use the terms “anode” and “cathode” to refer to the negative and positive electrodes in a battery, respectively, when charging, it is customary to use them that way. For the sake of convenience, the additives that affect the negative electrode are called type A (type 1), the additives that affect the positive electrode are called type C (type 2), and the additives that affect the bulk solution are called type B (types 3 to 5). It is also sometimes difficult to clearly define additives and co-solvents (or salts), especially when the additives are aprotic organic reagents (or lithium salts).

Read more: What are stable spinel compounds?

1. Natural graphite

Currently, natural graphite is the most promising material for lithium-ion battery anode materials due to its low price, low potential and smooth curve, high Coulombic efficiency in suitable electrolytes, and relatively high reversible capacity (330~350 mA h/g). one. On the other hand, it suffers from two major disadvantages: low rate capacity and incompatibility with PC-based electrolytes.

The low rate capacity of natural graphite actually stems from its high anisotropy. As shown in Fig. 1, the graphite flakes exhibit a typical disk-like pattern with greatly shortened dimensions in the direction of the c-axis and greatly widened dimensions in the direction perpendicular to the c-axis. After coating on the current collector (copper foil) and rolling, these particles are perpendicular to the current collector along the C axis [39]. The above inference of the directionality of the graphite fine particles means that the intercalation of lithium ions occurs in the vertical direction of the current. In addition, the resistivity of graphite varies with the direction of the graphite crystals. In the c-axis direction, the resistivity is 1O-²Ωcm, while in the a-axis direction the resistivity is 4X10-5Ωcm[165].

Therefore, the unfavorable orientation of the graphite particles can lead to delayed lithium ion intercalation and insufficient electrical contact between the graphite particles and the copper foil. These factors lead to the low rate capacity of natural graphite, especially at low temperature. To solve this problem, the natural graphite flakes can be ground into small pieces by mechanical grinding method [166]. In this way, the crystallographic orientation in the natural graphite flake particles is distorted to some extent by the individual graphite fragments, as shown in Fig. 1. Many research groups have also attempted to combine small particles of different orientations into larger graphite particles.

The incompatibility of graphite with PC-based electrolytes has been extensively studied. There are usually two ways to solve this problem: one is to modify the electrolyte with additives; the other is to modify the graphite by coating. Saga University and Mitsui Mining have applied superheated steam decomposition (TVD) technology to completely and uniformly coat the surface of natural graphite particles with carbon.

High-quality anode materials have been developed and commercialized. Before TVD carbon coating, small natural graphite fragments have been stacked into large fusiform particles, while the edge layer surface is mostly exposed, as shown in Fig. 2.

This morphology is easy for Li+ intercalation, but easily decomposes the PC electrolyte. Electrolyte decomposition can be substantially suppressed after TVD carbon coating. In addition, the fusiform structure disperses the graphite particles on the copper foil into various directions, which can improve the rate performance of natural graphite.

TVD-coated carbon and fusiform natural graphite have many advantages, but there is still a lot of room to improve their electrochemical performance. For example, the density of TVD-coated carbon (1.86 g/cm3) is smaller than that of natural graphite (2.27 g/cm3), which reduces the energy density of the negative electrode material. In addition, the TVD process increases the preparation cost of the negative electrode material, so carbon-coated natural graphite not only needs to suppress the decomposition of the electrolyte, but also how to reduce the amount of carbon coating. In order to meet the above requirements, it is necessary to reduce the surface activity of the core part (natural graphite). Before TVD carbon coating, the graphite flakes were packed into spherical shape, as shown in Fig. 2.

Since most of the surface of the end faces is hidden in the sphere, the surface of the inert basal plane occupies the outer surface before TVD carbon coating, so only a small amount of carbon is sufficient to coat the remaining end faces. The electrochemical performance of TVD carbon-coated spherical natural graphite is better than that of TVD carbon-coated fusiform natural graphite [172]. In addition, in each spherical natural graphite particle, the high orientation of the graphite layer is largely destroyed, and this morphology is very beneficial to improve the rate capability.

2. Artificial graphite

Artificial graphite has many of the same properties as natural graphite. In addition, artificial graphite has many remarkable advantages, such as high purity, and its structure is suitable for smooth intercalation and deintercalation of Li+. However, due to the high temperature (>2800 °C) required to process the soft carbon precursor, its cost is higher, and its reversible capacity is slightly lower than that of natural graphite. Graphitized MCMB, mesophase pitch carbon fiber (MCF), and vapor grown carbon fiber (VGCF) are typical representatives of synthetic graphite anode materials for lithium-ion battery applications in the market today.

MCMB precursors are usually isolated from hot pitches containing mesophase microspheres before graphitization. MCMB is available in different types of fibers such as Brooks-Taylor type, Honda type, Kovac-Lewis type and Huttinger type. In Japan, two major companies, Osaka Gas and Kawasaki Steel Co., Ltd., manufacture MCMB in large quantities, and the product is of the Brooks-Taylor type.

Graphitized MCMB has many advantages:
①High bulk density ensures high energy density;
② Small surface area reduces the irreversible capacity generated by electrolyte decomposition;
③ Most MCMB spherical surfaces are composed of end faces, so it is easier to intercalate lithium ions and improve rate performance;
④MCMB is easy to coat on copper foil.

MCF supplied by Petoca Co., Ltd. is produced from naphthalene spherical mesophase pitch by melt blast method. Figure 3 shows an SEM image of a typical cross-section of a graphitized MCF. MCF has a radial structure on the surface and a layered structure inside. The surface radial structure can smoothly intercalate lithium ions and improve the rate performance. On the other hand, the layered structure in the core seems to keep the carbon fiber stable and avoid the volume change during Li ion intercalation and deintercalation. In addition, the carbon fibers are easily disintegrated into fragments after several charge-discharge cycles, and the cycle performance becomes poor.

VGCF is formed by the decomposition of smoke compounds at 1000~3000 °C using transition metals as catalysts. The characteristic of this type of carbon fiber is that the graphite layers are arranged along the axis of the fiber. Figure 4 shows an SEM image of VGCF (Grasker) produced by Nikkiso. Since the outer surface of the long fibers is usually composed of basal planes, they are usually cut to about 10 μm long so that the cross-section exposes more of the end face. Graphitized short VGCFs can be prepared by two methods: truncation followed by graphitization and graphitization followed by truncation. The preparation method and the length and diameter of the fibers are the key factors affecting the electrochemical performance.

So far, MAG (mass artificial graphite) produced by Japan’s Hitachi Chemical Co., Ltd. has occupied 70% of the Japanese mobile phone market and has become one of the most commonly used artificial graphites for lithium-ion batteries. The rich pores (thin channels) inside the MAG particles can be filled with electrolyte, thus facilitating the migration of lithium ions in the electrode. If the electrolyte contains functional additives, the same stable and safe interfacial layer is formed inside the MAG particles as elsewhere.

3. Hard carbon

If hard carbons can adequately intercalate lithium, they can release high capacities in the low potential range (<0.2V, vs. Li/Li+). In general, it is very difficult to achieve this under normal circumstances. The random arrangement of the graphitic layers of hard carbon provides many spaces to accommodate lithium, however, the diffusion of lithium ions within hard carbon is like a labyrinth, so the diffusion of lithium ions becomes very slow, and the rate capability of hard carbon is usually poor. The space inside the hard carbon also occupies a certain volume. Although the mass specific capacity of hard carbon appears to be much higher than that of graphite, the volumetric specific capacity is indeed much lower than expected. Of course, hard carbon still has many advantages over graphite. For example, at the end of the discharge, the remaining capacity can be displayed by a sloped voltage and capacity graph, which is very valuable.

Kureha Chemical prepared hard carbon (Carbotron® P) from phenol resin [189,190]. This hard carbon has a charge capacity of up to 600mA h/g and discharge capacity of up to 500mA h/go The average distance (d002) between two adjacent graphite layers in Carbotron ® P is up to 0.38nm (compared to graphite d002 = 0.3354nm). After full lithium intercalation, the d002 value of Carbotron® P increased by only 1% (about 10% of the volume expansion of graphite). This shows that the crystal structure of Carbotron® P is very stable during the lithium ion deintercalation process, so its cycle performance is very good. In addition, Carbotron® P is relatively stable in PC electrolyte and is difficult to fall off.

Read more: What is the capacity of spinel compounds?