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The new car has been launched, and of course, I am referring to electric vehicles. This introduction often appears: fast charging, 80% charging in half an hour, and a range of 200 kilometers, completely solving your range anxiety! Fast charging is used by commercial vehicles to improve equipment efficiency, and by passenger cars to solve range anxiety, constantly approaching the time to fill a tank of fuel. There is a trend towards becoming standard equipment. Today, let's explore the fast charging method together, along with the origin of the mining method.

1、 How fast can it be called fast charging?

Our basic demand for charging:

1) Charging should be fast;

2) Don't affect the lifespan of my battery cells;

3) Try to save money by charging as much electricity as possible into my battery.

How fast can it be called fast charging? There is no standard literature that provides specific numerical values, so we will temporarily refer to the numerical threshold mentioned in the most well-known subsidy policies. The following table shows the subsidy standards for new energy buses in 2017. As can be seen, the entry-level of fast charging is 3C. In fact, there is no mention of fast charging requirements in the subsidy standards for passenger cars. From the promotional materials of general passenger cars, it can be seen that people generally believe that filling 80% in 30 minutes can already be used as a gimmick for fast charging. If it is promoted, then let's assume that the 1.6C of passenger cars can be the reference value for entry-level fast charging. According to this idea, promoting for 15 minutes is 80% full, equivalent to 3.2C.

2、 Where is the bottleneck of fast charging?

In the context of fast charging, stakeholders are divided according to physical entities, including batteries, chargers, and distribution facilities.

When we discuss fast charging, we immediately think about whether there will be any problems with the battery. Actually, before there is a problem with the battery, the first issue is with the charger and distribution circuit. We mentioned Tesla's charging station, called the Supercharging Station, which has a power of 120kW. According to the parameters of Tesla Model S85D, 96s75p, 232.5Ah, and a maximum voltage of 403V, the maximum required power corresponding to 1.6C is 149.9kW. From here, it can be seen that for long-range pure electric vehicle models, a 1.6C or 30 minute full charge of 80% already poses a challenge to the charging station.

In national standards, it is not allowed to directly set up charging stations in the original residential electricity network. The electricity consumption of one fast charging pile has already exceeded the electricity consumption of dozens of households. Therefore, charging stations need to install separate 10kV transformers, and not all distribution networks in a region have enough capacity to add more 10kV substations.

Then talk about batteries. Whether the battery can withstand the charging requirements of 1.6C or 3.2C can be viewed from both macroscopic and microscopic perspectives.

Macro fast charging theory

The reason why the title of this section is called Macro Fast Charging Theory is because the properties and microstructure of the positive and negative electrode materials inside the lithium-ion battery, electrolyte composition, additives, separator properties, and so on directly determine the fast charging ability of the battery. We will temporarily set aside these micro level contents and stand outside the battery to see the methods of fast charging for lithium-ion batteries.

There is an optimal charging current for lithium-ion batteries

In 1972, American scientist J A. Mas proposed that there is an optimal charging curve and his Mas's Three Laws during the charging process of batteries. It should be noted that this theory is proposed for lead-acid batteries, and the boundary condition for defining the maximum acceptable charging current is the presence of a small amount of side reaction gases. Obviously, this condition is related to the specific reaction type.

But the idea that there is an optimal solution for the system is universally applicable. Specifically for lithium-ion batteries, the boundary conditions for defining their maximum acceptable current can be redefined. Based on the conclusions of some research literature, its optimal value still follows a curve trend similar to Mas' law.

It is worth noting that the boundary conditions for the maximum acceptable charging current of lithium-ion batteries need to consider not only the individual factors of lithium-ion batteries, but also system level factors, such as different heat dissipation capabilities and the maximum acceptable charging current of the system. Then let's continue our discussion on this basis for now.

The formula description of Mas's theorem:

I=I0*e^αt

In the formula; I0 is the initial charging current of the battery& alpha; For charging acceptance rate; T is the charging time. I0 and alpha; The value of is related to the type, structure, and age of the battery.

At present, the research on battery charging methods is mainly based on the optimal charging curve. As shown in the figure below, if the charging current exceeds this optimal charging curve, not only will it not increase the charging rate, but it will also increase the amount of gas evolution in the battery; If it is smaller than this optimal charging curve, although it will not cause damage to the battery, it will prolong the charging time and reduce the charging efficiency.

The exposition of this theory includes three levels, which are known as Mas's Three Laws:

① Regarding any given discharge current, the current acceptance ratio during battery charging is alpha; Inversely proportional to the square root of the capacity released by the battery;

② Regarding any given discharge amount,α Directly proportional to the logarithm of the discharge current Id;

③ The final allowable charging current It (acceptance capacity) of a battery after discharging at different discharge rates is the sum of the allowable charging currents at each discharge rate.

The above theorem is also the origin of the concept of charging acceptance ability. First, let's understand what charging acceptance capability is. I searched around but couldn't see the official meaning of unification. According to one's own understanding, charging acceptance ability refers to the maximum current that a rechargeable battery with a certain charge can charge under specific environmental conditions. The acceptable meaning is that there will be no unnecessary side reactions and no adverse effects on the lifespan and performance of the battery cell.

Further understand the three laws. The first law states that after a battery has discharged a certain amount of electricity, its charging acceptance capacity is related to the current charge level. The lower the charge level, the higher its charging acceptance capacity. The second law states that during the charging process, pulse discharge can help improve the real-time acceptable current value of the battery; The third law states that the charging acceptance ability will be affected by the superposition of charging and discharging situations before the charging time.

If the Mas theory is also applicable to lithium-ion batteries, then reverse pulse charging (referred to as Reflex fast charging method in the following text) can not only be explained from the perspective of depolarization to help suppress temperature rise, but also serve as a support for the pulse method. Furthermore, what truly applies the Mas theory comprehensively is the intelligent charging method, which tracks battery parameters to ensure that the charging current value always follows the Mas curve of the lithium-ion battery, maximizing charging efficiency within the safety boundary.

3、 Common fast charging methods

There are many charging methods for lithium-ion batteries, and for the requirement of fast charging, important methods include pulse charging, Reflex charging, and intelligent charging. The charging methods applicable to different types of batteries are not entirely the same, and no specific distinction will be made in this section on methods.

Pulse charging

This is a pulse charging method from the literature, whose pulse phase is set after the charging reaches the upper limit voltage of 4.2V and continues above 4.2V. Leaving aside the rationality of its specific parameter settings, there are differences among different types of battery cells. Let's focus on the pulse execution process.

The following is the pulse charging curve, which includes three important stages: pre charging, constant current charging, and pulse charging. During the constant current charging process, the battery is charged with a constant current, and some of the energy is transferred to the inside of the battery. When the battery voltage rises to the upper limit voltage (4.2V), it enters the pulse charging mode: intermittently charging the battery with a 1C pulse current. The battery voltage will continuously increase within a constant charging time Tc, and the voltage will slowly decrease when charging stops. When the battery voltage drops to the upper limit voltage (4.2V), charge the battery with the same current value and start the next charging cycle. Repeat the charging process until the battery is fully charged.

During the pulse charging process, the rate of voltage drop in the battery will gradually slow down, and the stop charging time T0 will become longer. When the constant current charging duty cycle drops to 5% to 10%, it is considered that the battery is fully charged and the charging will be terminated. Compared with conventional charging methods, pulse charging can charge with a larger current. During the shutdown period, the concentration polarization and Ohmic polarization of the battery will be eliminated, making the next round of charging smoother. The charging speed is fast, the temperature change is small, and the impact on battery life is small. Therefore, it is widely used nowadays. But its drawbacks are obvious: it requires a power supply with limited current function, which adds cost to the pulse charging method.

Intermittent charging method

Intermittent charging methods for lithium-ion batteries include variable current intermittent charging method and variable voltage intermittent charging method.

1) Variable current intermittent charging method

The variable current intermittent charging method was proposed by Professor Chen Tixian from Xiamen University. Its characteristic is to change constant current charging to limited voltage variable current intermittent charging. As shown in the figure below, in the first stage of the variable current intermittent charging method, a larger current value is first used to charge the battery, and charging is stopped when the battery voltage reaches the cut-off voltage V0, at which point the battery voltage drops sharply. After a certain period of charging stoppage, continue charging with a reduced charging current. When the battery voltage rises again to the cut-off voltage V0, stop charging. Repeat this process several times (usually about 3-4 times), and the charging current will decrease by the set cut-off current value. Then enter the constant voltage charging stage, charging the battery with a constant voltage until the charging current decreases to the lower limit value, and the charging is completed.

The main charging stage of the variable current intermittent charging method adopts an intermittent method with gradually decreasing current to increase the charging current under limited charging voltage conditions, which accelerates the charging process and shortens the charging time. However, this charging mode has a complex circuit and high cost, and is generally only considered for high-power fast charging.

2) Variable voltage intermittent charging

On the basis of the variable current intermittent charging method, some people have also studied the variable voltage intermittent charging method. The difference between the two lies in the charging process in the first stage, which replaces intermittent constant current with intermittent constant voltage. Comparing Figure (a) and Figure (b) above, it can be seen that constant voltage intermittent charging is more in line with the optimal charging curve. At each constant voltage charging stage, due to the constant voltage, the charging electricity

The flow naturally decreases exponentially, which is consistent with the characteristic that the acceptable rate of battery current gradually decreases with the progress of charging.

Reflex fast charging method

Reflex fast charging method, also known as reflex charging method or hiccup charging method. Each working cycle of this method includes three stages: forward charging, reverse instantaneous discharge, and stop charging. It largely solves the problem of battery polarization and accelerates the charging speed. However, reverse discharge can shorten the lifespan of lithium-ion batteries.

As shown in the above figure, in each charging cycle, a current of 2C is used to charge Tc for 10 seconds, followed by a charging stop time of 0.5 seconds for Trl, a reverse discharge time of 1 second for Td, and a charging stop time of 0.5 seconds for Tr2. Each charging cycle lasts for 12 seconds. As charging progresses, the charging current will gradually decrease.

Intelligent charging method

Intelligent charging is currently a more advanced charging method, as shown in the following figure. Its important principle is to apply du/dt and di/dt control technology to determine the battery charging status by checking the increment of battery voltage and current, dynamically tracking the acceptable charging current of the battery, and making the charging current always near the maximum acceptable charging curve of the battery. This type of intelligent method generally combines advanced algorithm technologies such as neural networks and fuzzy control to achieve automatic optimization of the system.

Experimental data on the influence of charging method on charging rate

The literature compared the constant current charging method with a reverse pulse charging method. Constant current charging is the process of charging a battery with a constant current throughout the entire charging process. In the initial stage of constant current charging, there can be high current charging, but over time, the polarization resistance gradually appears and increases, causing more energy to be converted into heat, consumed, and causing the battery temperature to gradually rise.

Comparison between constant current charging and pulse charging

The pulse charging method involves a brief reverse charging current after a period of charging. Its basic form is shown in the following figure. During the charging process, there are brief discharge pulses mixed in, which serve the purpose of depolarization and reduce the impact of polarization resistance during the charging process.

There is a study specifically comparing the differences in effectiveness between pulse charging and constant current charging. Four comparative experiments were conducted with average currents of 1C, 2C, 3C, and 4C (where C represents the rated capacity of the battery). The actual amount of charge was measured by the amount of electricity released after the battery was fully charged. The figure shows the waveform of pulse charging current and battery terminal voltage when the charging current is 2C. Table 1 shows the experimental data of constant current pulse charging. The pulse period is 1 second, the positive pulse time is 0.9 seconds, and the negative pulse time is 0.1 seconds.

Ichav is the average charging current, Qin is the amount of charged electricity; Qo is the amount of discharged electricity,η For efficiency.

From the experimental results in the table above, it can be seen that the efficiency of constant current charging is similar to that of pulse charging, with pulse charging slightly lower than constant current charging. However, the total amount of electricity charged into the battery is significantly higher in pulse charging than in constant current charging.

The influence of different pulse duty cycles on pulse charging

The negative current discharge time in pulse charging has a certain impact on the charging speed, and the longer the discharge time, the slower the charging; When charging with the same average current, the longer the discharge time. From the table below, it can be seen that different duty cycles have a clear impact trend on efficiency and charging capacity, but the numerical differences are not significant. There are two important parameters related to this, charging time and temperature, which are not displayed.

Therefore, choosing pulse charging is superior to continuous constant current charging, and the specific duty cycle should focus on considering the battery temperature rise and charging time requirements.

Each lithium-ion battery has an optimal charging current value under different state and environmental parameters. So, from the perspective of battery structure, what are the factors that affect this optimal charging value.

Microscopic process of charging

Lithium ion batteries are known as rocking chair type batteries, where charged ions move between the positive and negative electrodes to achieve charge transfer, providing power to external circuits or charging from external power sources. During the specific charging process, an external voltage is applied to the two poles of the battery, causing lithium ions to detach from the positive electrode material and enter the electrolyte. At the same time, excess electrons pass through the positive electrode current collector and move towards the negative electrode through the external circuit; Lithium ions move from the positive electrode to the negative electrode in the electrolyte, pass through the separator, and reach the negative electrode; The SEI film on the negative electrode surface is embedded into the graphite layered structure of the negative electrode and combines with electrons.

The battery structure that affects charge transfer during the entire operation of ions and electrons, whether electrochemical or physical, will have an impact on fast charging performance.

4、 Fast charging, requirements for various parts of the battery

For batteries, to improve power performance, efforts need to be made in every aspect of the battery, including the positive and negative electrodes, electrolyte, separator, and structural design.

positive electrode

In fact, almost all types of positive electrode materials can be used to manufacture fast charging batteries, and important performance requirements include conductivity (reducing internal resistance), diffusion (ensuring reaction kinetics), lifespan (not to be explained), safety (not to be explained), and appropriate processing performance (the specific surface area should not be too large to reduce side reactions and serve safety). Of course, there may be differences in the specific problems that each material needs to solve, but the commonly used positive electrode materials can meet these requirements through a series of optimizations, but different materials also have differences:

A、 Lithium iron phosphate may focus more on addressing issues related to conductivity and low temperature. Carbon coating, moderate nanomaterialization (note that it is moderate, definitely not the simple logic of finer is better), and surface treatment of particles to form ion conductors are the most typical strategies.

B、 The conductivity of ternary materials is already relatively good, but their reactivity is too high. Therefore, there is little work on nanomaterialization of ternary materials (nanomaterialization is not a panacea for improving material performance, especially in the field of batteries where there are sometimes many counter reactions). More attention is paid to safety and the suppression of side reactions (with electrolytes). After all, safety is currently a major lifeline of ternary materials, and recent frequent battery safety accidents have put forward higher requirements in this regard.

C、 Lithium manganese oxide places greater emphasis on lifespan, and there are currently many lithium manganese oxide based fast charging batteries available on the market.

negative pole

When charging a lithium-ion battery, lithium migrates towards the negative electrode. The high potential caused by fast charging high current can lead to a more negative negative negative electrode potential. At this time, the pressure for the negative electrode to quickly accept lithium will increase, and the tendency to generate lithium dendrites will also increase. Therefore, during fast charging, the negative electrode not only needs to meet the dynamic requirements of lithium diffusion, but also needs to solve the safety problems caused by the increased tendency of lithium dendrite generation. Therefore, the important technical difficulty of fast charging cores is the insertion of lithium ions into the negative electrode.

A、 At present, the dominant negative electrode material in the market is still graphite (accounting for about 90% of the market share). The fundamental reason is that it is not cheap (you complain about the high cost of batteries every day, sigh!), and graphite has excellent processing performance and energy density, with relatively few disadvantages. Of course, graphite negative electrodes also have problems. Their surface is sensitive to the electrolyte, and the insertion reaction of lithium has strong directionality. Therefore, surface treatment of graphite to improve its structural stability and promote the diffusion of lithium ions on the substrate is an important direction to strive for.

B、 In recent years, there have also been many developments in hard carbon and soft carbon materials: hard carbon materials have a high lithium insertion potential, and there are micropores in the material, resulting in good reaction kinetics; Soft carbon materials have good compatibility with electrolytes, and MCMB materials are also representative. However, hard and soft carbon materials generally have lower efficiency and higher costs (and it is unlikely that they will be as cheap as graphite from an industrial perspective). Therefore, their current usage is far less than graphite and they are more commonly used in some special batteries.

C、 Someone may ask me how lithium titanate works. Simply put, the advantages of lithium titanate are high power density, safety, and obvious disadvantages, such as low energy density and high cost calculated in Wh. Therefore, the author's viewpoint on lithium-ion batteries has always been that they are a useful technology with advantages in specific situations, but they are not very suitable for many fields that require high cost and range.

D、 Silicon negative electrode materials are an important development direction, and Panasonic's new 18650 battery has begun the commercial process of using such materials. However, achieving a balance between the pursuit of performance in nanotechnology and the general micron level requirements of materials in the battery industry remains a challenging task.

the diaphragm

Regarding power type batteries, high current operation provides higher requirements for their safety and lifespan. The membrane coating technology cannot be bypassed, and ceramic coated membranes are rapidly being pushed forward due to their high safety and ability to consume impurities in the electrolyte, especially in terms of improving the safety of ternary batteries. The current important system used for ceramic diaphragms is to coat alumina particles on the surface of traditional diaphragms. A relatively novel approach is to coat solid electrolyte fibers on the diaphragm, which has lower internal resistance, better mechanical support effect of fibers on the diaphragm, and a lower tendency to block diaphragm holes during service. After coating, the diaphragm has good stability and is not prone to shrinkage deformation and short circuit even at high temperatures. Jiangsu Qingtao Energy Company, supported by the research group of Academician Nan Ce Wen from the School of Materials Science and Technology at Tsinghua University, has some representative works in this regard. The diaphragm is shown in the following figure.

Diaphragm coated with solid electrolyte fibers

electrolyte

The electrolyte has a significant impact on the performance of fast charging lithium-ion batteries. To ensure the stability and safety of the battery under fast charging and high current, the electrolyte must meet the following characteristics: A) non decomposition, B) high conductivity, and C) inert to the positive and negative electrode materials, unable to react or dissolve. If you want to meet these requirements, the key is to use additives and functional electrolytes. For example, the safety of ternary fast charging batteries is greatly affected, and various high-temperature resistant, flame-retardant, and overcharge resistant additives must be added to them for protection in order to improve their safety to a certain extent. The long-standing problem of high temperature swelling in lithium-ion batteries also needs to be improved by high-temperature functional electrolytes.

Battery structure design

A typical optimization strategy is the stacked vs. wound battery. The electrodes of the stacked battery are in parallel, while the wound battery is in series. Therefore, the former has much lower internal resistance and is more suitable for power applications. Additionally, efforts can be made to increase the number of polar ears to address internal resistance and heat dissipation issues. In addition, using high conductivity electrode materials, using more conductive agents, and coating thinner electrodes are also strategies that can be considered.

In short, factors that affect the internal charge movement and insertion rate into electrode holes in batteries can also affect the fast charging ability of lithium-ion batteries.

5、 Overview of mainstream manufacturers' fast charging technology routes

CATL

Regarding the positive electrode, CATL has developed super electronic mesh technology, which endows lithium iron phosphate with excellent electronic conductivity; On the surface of the negative graphite, fast ion ring technology is used for modification. The modified graphite combines the characteristics of super fast charging and high energy density. During fast charging, there are no excessive by-products in the negative electrode, making it capable of 4-5C fast charging, achieving 10-15 minutes of fast charging, and ensuring a system level energy density of over 70wh/kg, achieving a cycle life of 10000 times (which is quite high). In terms of thermal management, its thermal management system fully identifies the healthy charging range of fixed chemical systems at different temperatures and SOC, greatly expanding the operating temperature of lithium-ion batteries.

Waltmal 

Watma hasn't been doing well lately, let's just talk about technology. Waterma uses smaller particle size lithium iron phosphate. Currently, the common particle size of lithium iron phosphate on the market is between 300-600nm, while Waterma only uses 100-300nm lithium iron phosphate. This way, lithium ions will have a faster migration speed and can charge and discharge at a higher rate of current. Strengthen the thermal management system and system safety design on systems other than batteries.

Micro Macro Power

In the early days, Micro Macro Power chose lithium titanate with spinel structure and porous composite carbon as the negative electrode material, which could withstand fast charging and high current; In order to prevent the threat of high-power current to battery safety during fast charging, Micro Macro Power combines non combustible electrolyte, high porosity and high permeability diaphragm technology, and STL intelligent thermal control fluid technology to ensure the safety of the battery during fast charging.

In 2017, it announced a new generation of high-energy density batteries, using high-capacity and high-power lithium manganese oxide cathode materials, with a single energy density of 170Wh/kg, achieving 15 minute fast charging, with the goal of balancing lifespan and safety issues.

Zhuhai Yinlong

Lithium titanate negative electrode is known for its wide operating temperature range and high charge discharge rate, but there is no clear information on the specific technical method. At the exhibition, I talked to the staff and it was reported that its fast charging can achieve 10C with a lifespan of 20000 times.

6、 The future of fast charging technology

Whether fast charging technology for electric vehicles is a historical direction or a fleeting phenomenon, there is currently no consensus. As an alternative method to address range anxiety, it is considered on the same platform as battery energy density and overall vehicle costs.

Energy density and fast charging performance are two incompatible directions in the same battery, and cannot be achieved simultaneously. The pursuit of battery energy density is currently mainstream. When the energy density is high enough and a car is loaded with enough electricity to prevent so-called range anxiety, the demand for battery rate charging performance will decrease; At the same time, with a large battery capacity, if the cost per kilowatt hour of the battery is not low enough, consumers need to make a choice whether to purchase enough electricity without anxiety. With this in mind, fast charging has value. Another perspective is the cost of fast charging supporting facilities mentioned yesterday, which is certainly a part of the overall cost of promoting electrification in society.

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