When new cars are launched, especially electric vehicles, you often see descriptions like: "Fast charging, 80% charge in half an hour, 200km range, completely solving your range anxiety!" Fast charging is used in commercial vehicles to improve equipment efficiency, and in passenger cars to solve range anxiety, constantly approaching the time it takes to fill up a tank of gas. It seems to be becoming a standard feature. Today, let's explore fast charging methods and, incidentally, its origins.
1. How fast does charging have to be to be considered fast charging?
Our basic needs for charging:
1) Charging should be fast;
2) Do not affect the lifespan of my battery cells;
3) Try to save money as much as possible, and use as much of the electricity that the charger discharges as possible to charge my battery.
So, what constitutes "fast charging"? There's no standard document providing specific values, so we'll tentatively refer to the thresholds mentioned in the most well-known subsidy policies. The table below shows the 2017 subsidy standards for new energy buses. As you can see, the entry-level for fast charging is 3C. In fact, the subsidy standards for passenger vehicles don't mention fast charging requirements. From general passenger vehicle promotional materials, it's clear that charging to 80% in 30 minutes is generally considered a fast charging gimmick to advertise. Therefore, let's assume 1.6C for passenger vehicles is a good entry-level fast charging reference. Following this logic, advertising 80% charging in 15 minutes is equivalent to 3.2C.
II. Where are the bottlenecks in fast charging?
In the context of fast charging, the relevant parties are categorized by physical entity, including batteries, chargers, and power distribution facilities.
When we discuss fast charging, our immediate thought is whether there will be problems with the battery. Actually, before battery issues arise, the primary concern is the charger and the power distribution lines. We mentioned Tesla's Superchargers, which have a power output of 120kW. Based on the specifications of the Tesla Model S 85D (96s75p, 232.5Ah, maximum 403V), its maximum power demand at 1.6C is 149.9kW. This shows that for long-range pure electric vehicles, 1.6C, or charging to 80% in 30 minutes, already poses a challenge to charging stations.
National standards prohibit the direct installation of charging stations within existing residential power grids. The power consumption of a single fast-charging station exceeds the electricity consumption of dozens of households. Therefore, charging stations require dedicated 10kV transformers, and not every regional power distribution network has the capacity to add more 10kV substations.
Then we come to the battery. Whether a battery can withstand 1.6C or 3.2C charging requirements can be viewed from both macroscopic and microscopic perspectives.
Macroscopic fast charging theory
The reason this section is titled "Macroscopic Fast Charging Theory" is that the fast charging capability of a battery is directly determined by the properties and microstructure of the positive and negative electrode materials inside the lithium-ion battery, the composition of the electrolyte, additives, the properties of the separator, and so on. We will put these microscopic aspects aside for now and look at the methods of fast charging lithium-ion batteries from the outside.
Lithium-ion batteries have an optimal charging current.
In 1972, American scientist J.A. Mas proposed that there is an optimal charging curve for batteries during the charging process and his three laws of Mas. It should be noted that this theory was 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 gas. Obviously, this condition is related to the specific type of reaction.
However, the idea that an optimal solution exists in a system is universally applicable. Specifically for lithium-ion batteries, the boundary conditions defining their maximum acceptable current can be reinterpreted. Based on the conclusions of some research literature, the optimal value still follows a curve-like trend similar to Mas's Law.
It's worth noting that the boundary conditions for the maximum acceptable charging current of a lithium-ion battery must consider not only the factors of the individual lithium-ion battery cells but also system-level factors. For example, different heat dissipation capabilities will result in different maximum acceptable charging currents for the entire system. We will continue our discussion based on this premise.
Formulaic description of Mas's theorem:
I=I0*e^αt
In the formula, I0 is the initial charging current of the battery; α is the charge acceptance rate; and t is the charging time. The values of I0 and α are related to the battery type, structure, and condition.
Current research on battery charging methods is primarily based on the optimal charging curve. As shown in the figure below, if the charging current exceeds this optimal charging curve, it will not only fail to increase the charging rate but will also increase the amount of gas evolution in the battery; if it is less than this optimal charging curve, although it will not damage the battery, it will prolong the charging time and reduce the charging efficiency.
The explanation of this theory comprises three levels, known as Mas's Three Laws:
① For any given discharge current, the current acceptance ratio α during battery charging is inversely proportional to the square root of the battery's discharged capacity;
② For any given discharge quantity, α is proportional to the logarithm of the discharge current Id;
③After a battery is discharged at different discharge rates, its final allowable charging current It (acceptance capacity) is the sum of the allowable charging currents at each discharge rate.
The above theorem is also the origin of the concept of charge acceptance. Let's first understand what charge acceptance is. After searching around, I couldn't find a unified official definition. According to my understanding, charge acceptance is the maximum current that a rechargeable battery with a certain charge level can withstand under specific environmental conditions. Acceptable means that no undesirable side reactions will occur, and it will not adversely affect the lifespan and performance of the battery cell.
Let's further understand the three laws. The first law states that after a battery has discharged a certain amount of charge, its charging acceptance capability is related to its current charge level; the lower the charge level, the higher its charging acceptance capability. The second law states that during charging, pulsed discharge helps the battery increase its real-time acceptable current value. The third law states that charging acceptance capability is affected by the cumulative effect of charging and discharging events prior to the charging moment.
If Musk's theory also applies to lithium-ion batteries, then reverse pulse charging (hereinafter specifically called Reflex fast charging) not only helps suppress temperature rise from the perspective of depolarization, but Musk's theory also supports the pulse method. Furthermore, the method that truly applies Musk's theory in its entirety is the intelligent charging method, which tracks battery parameters to ensure that the charging current always follows the Musk curve of the lithium-ion battery, maximizing charging efficiency within safe limits.
III. Common Fast Charging Methods
There are many charging methods for lithium-ion batteries. For fast charging, important methods include pulse charging, Reflex charging, and smart charging. Different battery types require different charging methods, which will not be specifically distinguished in this section.
Pulse charging
This is a pulse charging method from the literature, where the 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, which vary between different types of battery cells, let's focus on the pulse execution process.
Below is the pulse charging curve, which mainly includes three stages: pre-charge, constant current charging, and pulse charging. During constant current charging, the battery is charged with a constant current, and some energy is transferred to the battery's internal structure. When the battery voltage rises to the upper limit voltage (4.2V), it enters pulse charging mode: the battery is intermittently charged with a 1C pulse current. Within a constant charging time Tc, the battery voltage will continuously rise, and when charging stops, the voltage will slowly decrease. When the battery voltage drops back to the upper limit voltage (4.2V), the battery is charged with the same current value, starting the next charging cycle. This cycle continues until the battery is fully charged.
During pulse charging, the rate of battery voltage drop gradually slows down, and the charging pause time T0 becomes longer. When the constant current charging duty cycle drops to 5%–10%, the battery is considered fully charged, and charging is terminated. Compared to conventional charging methods, pulse charging can charge with a larger current. During the charging pause period, concentration polarization and ohmic polarization in the battery are eliminated, allowing for a smoother subsequent charging cycle. It also offers faster charging speeds, smaller temperature variations, and less impact on battery life, making it widely used. However, its drawback is significant: it requires a power supply with current limiting capabilities, which increases the cost of pulse charging.
Intermittent charging method
Intermittent charging methods for lithium-ion batteries include variable current intermittent charging and variable voltage intermittent charging.
1) Variable current intermittent charging method
The variable current intermittent charging method was proposed by Professor Chen Tixian of Xiamen University. Its key feature is replacing constant current charging with voltage-limited variable current intermittent charging. As shown in the diagram, in the first stage of the variable current intermittent charging method, a relatively large current is initially used to charge the battery. Charging stops when the battery voltage reaches the cutoff voltage V0, at which point the battery voltage drops sharply. After a period of inactivity, charging resumes with a reduced charging current. When the battery voltage rises back to the cutoff voltage V0, charging stops. This process is repeated several times (typically about 3-4 times), gradually reducing the charging current to the set cutoff current value. Then, the constant voltage charging stage begins, charging the battery with a constant voltage until the charging current decreases to the lower limit, at which point charging ends.
In the main charging phase of the variable current intermittent charging method, under a limited charging voltage, the charging current is increased by gradually decreasing intermittently, thus accelerating the charging process and shortening the charging time. However, this charging mode has a relatively complex circuit and high cost, and is generally only considered for high-power fast charging.
2) Variable voltage intermittent charging
Building upon the variable current intermittent charging method, some researchers have also studied the variable voltage intermittent charging method. The difference lies in the first stage of the charging process, where intermittent constant current is replaced with intermittent constant voltage. Comparing Figures (a) and (b) above, it is clear that constant voltage intermittent charging better matches the optimal charging curve. In each constant voltage charging stage, because the voltage is constant, the charging current...
The current naturally decreases exponentially, which is consistent with the characteristic that the acceptable battery current gradually decreases as charging progresses.
Reflex fast charging method
The Reflex fast charging method, also known as reflective charging or burp charging, consists of three phases per cycle: forward charging, reverse instantaneous discharge, and a charging pause. It largely solves the battery polarization problem and accelerates charging. However, reverse discharge shortens the lifespan of lithium-ion batteries.
As shown in the diagram above, in each charging cycle, a 2C current is first used for charging for 10 seconds (Tc), followed by a 0.5s charging pause (Tr1), a 1s reverse discharge (Td), and a 0.5s charging pause (Tr2). Each charging cycle lasts for 12 seconds. As charging progresses, the charging current gradually decreases.
Smart charging method
Intelligent charging is a relatively advanced charging method. As shown in the figure below, its key principle is the application of du/dt and di/dt control techniques. By checking the increments of battery voltage and current, it determines the battery's charging state and dynamically tracks the acceptable charging current, ensuring that the charging current remains near the battery's maximum acceptable charging curve throughout the entire process. This type of intelligent method typically combines advanced algorithmic techniques such as neural networks and fuzzy control to achieve automatic system optimization.
Experimental data on the effect of charging method on charging rate
The literature compares constant current charging with a reverse pulse charging method. Constant current charging involves charging the battery with a constant current throughout the entire charging process. Initially, a large current can be applied during constant current charging, but as time progresses, polarization resistance gradually becomes apparent and increases, causing more energy to be converted into heat, which is then consumed and causes the battery temperature to gradually rise.
Comparison of constant current charging and pulse charging
The pulse charging method involves a brief reverse charging current appearing after a period of charging. Its basic form is shown in the figure below. The brief discharge pulses interspersed during charging serve to depolarize and reduce the impact of polarization resistance during charging.
Studies have specifically compared the effects of pulse charging and constant current charging. Four sets of comparative experiments were conducted with average currents of 1C, 2C, 3C, and 4C (C being the battery's rated capacity). The actual amount of charge received was measured by the amount of electricity discharged after the battery was fully charged. The figure shows the current and battery terminal voltage waveforms during pulse charging at a charging current of 2C. Table 1 shows the experimental data for constant current pulse charging. The pulse period was 1 s, the positive pulse duration was 0.9 s, and the negative pulse duration was 0.1 s.
Ichav is the average charging current, Qin is the amount of charge received, Qo is the amount of charge discharged, and η is the efficiency.
As can be seen from the experimental results in the table above, constant current charging and pulse charging have similar efficiencies, with pulse charging being slightly less efficient than constant current charging. However, the total amount of electricity charged into the battery is significantly greater with pulse charging than with constant current charging.
The effect of different pulse duty cycles on pulse charging
The negative current discharge time in pulse charging has a certain impact on the charging speed; the longer the discharge time, the slower the charging. Maintaining the same average charging current results in a longer discharge time. As shown in the table below, different duty cycles have a clear trend in their impact on efficiency and the amount of charge, but the numerical differences are not significant. Two other important parameters are also related: charging time and temperature, which are not displayed.
Therefore, pulse charging is better than continuous constant current charging. When choosing the duty cycle, the battery temperature rise and charging time requirements should be taken into account.
Each type of lithium-ion battery has an optimal charging current value under different state and environmental parameters. So, from the perspective of battery structure, what factors affect this optimal charging value?
Microscopic process of charging
Lithium-ion batteries are known as rocking chair batteries. Charged ions move between the positive and negative electrodes, transferring charge to power external circuits or be charged by an external power source. Specifically, during charging, an external voltage is applied to the battery's electrodes. Lithium ions are extracted from the positive electrode material and enter the electrolyte. Simultaneously, excess electrons pass through the positive electrode current collector and move towards the negative electrode via the external circuit. In the electrolyte, lithium ions move from the positive to the negative electrode, passing through the separator to reach the negative electrode. They then pass through the SEI film on the negative electrode surface and embed into the graphite layered structure of the negative electrode, where they combine with electrons.
Throughout the entire process of ion and electron transport, battery structures that affect charge transfer, whether electrochemical or physical, will impact fast-charging performance.
IV. Fast charging: Requirements for various battery components
To improve the power performance of batteries, efforts must be made in every aspect of the battery, including the positive electrode, negative electrode, electrolyte, separator, and structural design.
positive electrode
In fact, almost all cathode materials can be used to manufacture fast-charging batteries. Key performance requirements include conductivity (reducing internal resistance), diffusion (ensuring reaction kinetics), lifespan (no explanation needed), safety (no explanation needed), and appropriate processing performance (specific surface area should not be too large to reduce side reactions and ensure safety). Of course, the specific problems to be solved for each material may differ, but commonly used cathode materials can generally meet these requirements through a series of optimizations. However, there are differences between different materials:
A. Lithium iron phosphate may focus more on solving problems related to conductivity and low temperature. Carbon coating, moderate nano-sizing (note, moderate, not the simple logic of the finer the better), and surface treatment to form ionic conductors are the most typical strategies.
B. Ternary materials already have good electrical conductivity, but their reactivity is too high. Therefore, there is little work on nano-scale production of ternary materials (nano-scale production is not a panacea for improving material performance, especially in the field of batteries where it sometimes has many adverse applications). The focus is more on safety and suppressing side reactions (with electrolytes). After all, one of the biggest weaknesses of ternary materials is safety, and the recent frequent battery safety incidents have placed higher demands on this aspect.
C. Lithium manganese oxide batteries place more emphasis on lifespan, and there are currently many lithium manganese oxide fast-charging batteries on the market.
negative electrode
When a lithium-ion battery is charged, lithium migrates towards the negative electrode. However, the excessively high potential brought about by the high current of fast charging will cause the negative electrode potential to become even more negative. At this time, the pressure on the negative electrode to rapidly accept lithium will increase, and the tendency to form lithium dendrites will increase. Therefore, during fast charging, the negative electrode must not only meet the kinetic requirements of lithium diffusion, but also solve the safety problems caused by the increased tendency of lithium dendrite formation. Thus, the important technical challenge of fast charging cells is actually the embedding of lithium ions in the negative electrode.
A. Currently, graphite remains the dominant anode material in the market (accounting for about 90% of the market share). The fundamental reason is simply its low price (you all complain about how expensive batteries are, exclamation mark!), and graphite's overall processing performance and energy density are superior, with relatively fewer drawbacks. Of course, graphite anodes also have their problems. Their surface is quite sensitive to electrolytes, and the lithium intercalation reaction has strong directionality. Therefore, surface treatment of graphite to improve its structural stability and promote lithium-ion diffusion on the substrate is an important area of focus.
B. Hard carbon and soft carbon materials have also seen considerable development in recent years: hard carbon materials have high lithium intercalation potentials and micropores, resulting in good reaction kinetics; while soft carbon materials have good compatibility with electrolytes, and MCMB materials are also very representative. However, both hard and soft carbon materials generally have low efficiency and high cost (and it is unlikely that they will be as cheap as graphite from an industrial perspective). Therefore, their current usage is far less than that of graphite, and they are used more in some special batteries.
C. Some may ask about lithium titanate. In short: the advantages of lithium titanate are high power density and relatively high safety, but its disadvantages are also obvious: very low energy density and high cost per Wh. Therefore, the author's view on lithium titanate batteries has always been: it is a useful technology with advantages in specific situations, but it is not very suitable for many applications where cost and driving range are critical.
D. Silicon anode materials are an important area of development, and Panasonic's new 18650 battery has already begun the commercialization process using such materials. However, achieving a balance between pursuing performance through nano-scale technology and meeting the general micron-scale requirements of the battery industry for materials remains a challenging task.
diaphragm
For power batteries, high-current operation places higher demands on their safety and lifespan. Membrane coating technology is indispensable. Ceramic-coated membranes are rapidly gaining popularity due to their high safety and ability to absorb impurities in the electrolyte, especially significantly improving the safety of ternary lithium batteries. Currently, the main system used for ceramic membranes involves coating alumina particles onto the surface of traditional membranes. A more novel approach is to coat the membrane with solid electrolyte fibers. This results in membranes with lower internal resistance, better mechanical support from the fibers, and a lower tendency to clog membrane pores during service. Coated membranes exhibit good stability and are less prone to shrinkage and deformation leading to short circuits even at high temperatures. Jiangsu Qingtao Energy Co., Ltd., with technical support from Academician Nan Cewen's research group at the School of Materials Science and Engineering, Tsinghua University, has some representative work in this area. The membrane is shown in the figure below.
Membrane coated with solid electrolyte fibers
electrolyte
Electrolytes have a significant impact on the performance of fast-charging lithium-ion batteries. To ensure the stability and safety of the battery under the high current of fast charging, the electrolyte must meet the following characteristics: A) It cannot decompose; B) It must have high conductivity; C) It must be inert to the positive and negative electrode materials, and cannot react or dissolve them. Achieving these requirements hinges on the use of additives and functional electrolytes. For example, the safety of ternary lithium fast-charging batteries is greatly affected by electrolytes, and various high-temperature resistant, flame-retardant, and overcharge-protective additives must be added to protect them and improve their safety to some extent. The long-standing problem of high-temperature gas expansion in lithium titanate batteries also relies on high-temperature functional electrolytes for improvement.
Battery structure design
A typical optimization strategy is the difference between stacked and wound designs. In stacked batteries, the electrodes are essentially connected in parallel, while in wound batteries, they are connected in series. Therefore, the former has significantly lower internal resistance and is more suitable for power applications. Additionally, the number of tabs can be increased to address internal resistance and heat dissipation issues. Furthermore, using high-conductivity electrode materials, employing more conductive agents, and coating thinner electrodes are also strategies to consider.
In summary, factors affecting the movement of internal charges and the rate of insertion into electrode pores all influence the fast charging capability of lithium-ion batteries.
V. Overview of Fast Charging Technology Routes from Mainstream Manufacturers
CATL
Regarding the positive electrode, CATL has developed a super-electron mesh technology, which gives lithium iron phosphate excellent electronic conductivity. On the negative electrode graphite surface, a fast-ion ring technology is used for modification. This modified graphite combines super-fast charging and high energy density, eliminating excessive byproducts during fast charging and enabling 4-5C fast charging, achieving 10-15 minutes of fast charging while maintaining a system-level energy density of over 70Wh/kg and a cycle life of 10,000 cycles (which is quite high). In terms of thermal management, its thermal management system fully identifies the healthy charging range of the fixed chemical system at different temperatures and SOCs, greatly expanding the operating temperature range of lithium-ion batteries.
Watma
Wotema hasn't been doing well lately, so let's focus on the technology. Wotema uses lithium iron phosphate with a smaller particle size. Currently, the common lithium iron phosphate particle size on the market is between 300 and 600 nm, while Wotema only uses lithium iron phosphate with a particle size of 100 to 300 nm. This allows lithium ions to have a faster migration speed, enabling higher charge and discharge rates. In addition to the battery, they have strengthened the thermal management system and system safety design.
Microvast
In the early stages, Microvast Power chose lithium titanate with a spinel structure and porous composite carbon as the negative electrode material, which can withstand the high current of fast charging. In order to prevent the high power current during fast charging from threatening battery safety, Microvast Power combined non-flammable electrolyte, high porosity and high air permeability membrane technology and STL intelligent thermal control fluid technology to ensure battery safety during fast charging.
In 2017, it announced a new generation of high-energy-density batteries, which use high-capacity, high-power lithium manganese oxide cathode materials, with a single cell energy density of 170Wh/kg and can be fast-charged in 15 minutes. The goal is to balance lifespan and safety.
Zhuhai Yinlong
Lithium titanate anodes are known for their wide operating temperature range and high charge/discharge rates, but specific technical methods are not clearly documented. During a conversation with staff at the exhibition, it was claimed that their fast charging can already achieve 10C and their lifespan is 20,000 cycles.
VI. The Future of Fast Charging Technology
Whether fast charging technology for electric vehicles is the future or just a fleeting phenomenon is currently a matter of much debate and no definitive conclusion. As an alternative to address range anxiety, it should be considered alongside battery energy density and overall vehicle operating costs.
Energy density and fast charging performance are essentially incompatible goals within the same battery; they cannot be simultaneously achieved. Currently, the pursuit of higher battery energy density is the mainstream approach. When energy density is high enough, a vehicle can carry a large enough battery capacity to alleviate range anxiety, reducing the need for faster charging rates. However, with larger capacities, if the cost per kilowatt-hour (kWh) of the battery is not low enough, consumers must choose whether to purchase sufficient capacity to alleviate range anxiety. This perspective gives fast charging its value. Another aspect is the cost of fast charging infrastructure, as mentioned yesterday, which is undoubtedly part of the overall cost of promoting electrification in society.
