Fast charging method for lithium ion battery

The new car on the market I mean electric vehicles, often introduced in such way: "Fast charging, charging 80% in half an hour, 200 kilometers in battery life, completely solve your mileage anxiety!" Fast charging, commercial vehicles use it to improve equipment use efficiency, the passenger car use it to solve the mileage anxiety, and constantly approaching the "add a tank of oil" time. There is a tendency to become a standard. Today, let's explore quick charging methods and the origin of the method.

How fast can you charge "fast charge"?

The basic appeal of our charging:

1) Charging should be fast;

2) Do not affect the life of my battery;

3) Try to save money, how much electricity the charger puts out, and try to charge it into my battery.

So how fast can it be called fast charging? There is no standard literature to give specific values, let us refer to the numerical threshold mentioned in the most well-known subsidy policy for the time being. The following table shows the 2017 subsidy standards for new energy buses. As you can see, the entry level for fast charging is 3C. In fact, in the subsidy standard for passenger cars, there is no mention of the requirement for fast charging. From the publicity materials of general passenger cars, we can see that everyone generally thinks that 30% of the 30 minutes is enough to be used as a fast-filling gimmick, and it is publicized. Then I think that the 1.6C of the passenger car can be an entry-level fast. Fill in the reference value. According to this idea, propaganda for 15 minutes is full of 80%, which is equivalent to 3.2C.

Where is the bottleneck of fast charging?

In the context of fast charging, relevant parties are classified according to physical subjects, including batteries, chargers, and power distribution facilities.

We discuss fast charging and think directly about whether the battery will have problems. In fact, before the battery has a problem, the first problem is the charger and the distribution line. We refer to Tesla's charging pile, which is called the super-charged pile, which has a power of 120 kW. According to the parameters of Tesla ModelS85D, 96s75p, 232.5Ah, the highest 403V, its 1.6C corresponds to the maximum required power of 149.9kW. From here, we can see that for long-life pure electric vehicles, 1.6C or 30 minutes full of 80% has already tested the charging pile.

In the national standard, it is not allowed to directly set the charging station directly in the original residential electricity network. The power consumption of a fast-filled pile has exceeded the electricity consumption of dozens of households. Therefore, the charging station needs to set a separate 10kV transformer, and the distribution network of one area does not have the margin to add more 10kV substation.

Then say the battery. Whether the battery can carry the charging requirements of 1.6C or 3.2C can be viewed from both macro and micro perspectives.

Macroscopic fast charging theory

The reason why this section is called "the macroscopic fast charging theory" is because the direct determination of the battery's rapid charging capability is the nature of the positive and negative materials inside the lithium battery, the microstructure, electrolyte composition, additives, diaphragm properties, etc. At the micro level, we are temporarily set aside and stand outside the battery to see how the lithium battery can be quickly charged.

Lithium battery has optimal charging current

In 1972, American scientist J.A.Mas proposed that the battery has the best charging curve and his Maestro's law during charging. 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 production of a small amount of side reaction gas is obviously related to the specific reaction type.

But the system has the idea of an optimal solution, but it is universally applicable. Specific to lithium batteries, the boundary conditions defining their maximum acceptable current can be redefined. Based on the conclusions of some research literatures, the optimal value is still a curve trend similar to Maas's law.

It is worth noting that the maximum acceptable charging current boundary condition of a lithium battery requires not only the factors of the lithium battery cell but also the system level factors. For example, the heat dissipation capability is different, and the maximum acceptable charging current of the system is different. . Then we will continue to discuss it on this basis for the time being.

The formula description of Mas's theorem:

I=I0*e^αt

Where: I0 is the initial charging current of the battery; α is the charging acceptance rate; t is the charging time. The values of I0 and α are related to the type, structure and age of the battery.

At present, the research on the battery charging method is mainly based on the optimal charging curve. As shown in the figure below, if the charging current exceeds this optimal charging curve, it will not increase the charging rate, but also increase the gas outflow of the battery; if it is less than this optimal charging curve, it will not cause damage to the battery, but will prolong the charging. Time, reduce charging efficiency.

The elaboration of this theory consists of three levels, which are for the three laws of Mas:

1 For any given discharge current, the current acceptance ratio α of the battery is inversely proportional to the square root of the capacity discharged by the battery;

2 for any given amount of discharge, α is proportional to the logarithm of the discharge current Id;

3 After the batteries are discharged at different discharge rates, their final allowable charging current it (acceptance) is the sum of the allowable charging currents at the respective discharge rates.

The above theorem is also the source of the concept of charge acceptance. First, understand what charging receptivity is. There’s no definition of the unified official. According to my understanding, the charging capacity is the maximum current that can be charged by a rechargeable battery with a certain amount of charge under certain environmental conditions. The acceptable meaning is that it will not produce undesirable side reactions and will not adversely affect the life and performance of the battery.

Then understand the three laws. The first law, after the battery emits a certain amount of power, its charge acceptance is related to the current charge. The lower the charge is, the higher the charge acceptance. In the second law, during the charging process, a pulse discharge occurs, which helps the battery to increase the real-time acceptable current value. In the third law, the charging acceptance capability is affected by the superposition of the charging and discharging conditions before the charging time.

If Maas theory is also applicable to lithium batteries, reverse pulse charging (hereinafter specifically named Reflex fast charging method) can be used as a counterpoint to explain the temperature rise suppression in addition to the depolarization angle and support of the pulse method. Further, the true use of Mas's theory is the intelligent charging method, which tracks the battery parameters, so that the charging current value always changes according to the maser curve of the lithium battery, so that the charging efficiency is maximized within the safety margin.

Common fast charging method

There are many ways to charge lithium batteries. For fast charging requirements, the main methods include pulse charging, Reflex charging, and smart charging. Different battery types, the applicable charging methods are not exactly the same, and no specific distinction is made in the method section.

Pulse charging

This is a pulse charging method from the literature. The pulse phase is set after the charging reaches the upper limit voltage of 4.2V, and continues above 4.2V. For the time being, the rationality of its specific parameter setting is not mentioned, and there are differences between different types of batteries. We pay attention to the pulse implementation process.

The following is a pulse charging curve, which mainly includes three phases: precharge, constant current charge, and pulse charge. The battery is charged with a constant current during constant current charging, and part of the energy is transferred to the inside of the battery. When the battery voltage rises to the upper limit voltage (4.2V), the pulse charging mode is entered: the battery is intermittently charged with a pulse current of 1C. During a constant charging time Tc, the battery voltage will continue to rise, and the voltage will slowly drop when charging stops. When the battery voltage drops to the upper limit voltage (4.2V), the battery is charged with the same current value, and the next charging cycle is started, so that the battery is fully charged until the battery is fully charged.

During the pulse charging process, the battery voltage drop speed will gradually slow down, and the stop time T0 will become longer. When the constant current charge duty cycle is as low as 5% to 10%, the battery is considered to be full and the charging is terminated. Compared with the conventional charging method, the pulse charging can be charged at a large current, and the concentration polarization and the ohmic polarization of the battery are eliminated during the shutdown period, so that the charging of the next round is performed more smoothly, and the charging speed is fast. The change in temperature is small and has little effect on battery life, so it is currently widely used. But its shortcomings are obvious: a power supply with a limited current function is required, which increases the cost of the pulse charging method.

Intermittent charging method

The lithium battery intermittent charging method includes a variable current intermittent charging method and a variable voltage intermittent charging method.

1) Variable current intermittent charging method

The variable current intermittent charging method was proposed by Tixian Chen, a professor at Xiamen University. It is characterized by changing the constant current charging to limiting voltage and current intermittent charging. As shown in the figure below, in the first stage of the variable current intermittent charging method, the battery is charged with a larger current value, and the charging is stopped when the battery voltage reaches the cutoff voltage V0, at which time the battery voltage drops sharply. After maintaining a period of charge down, continue charging with a reduced charge current. When the battery voltage rises again to the cutoff voltage V0, the charging is stopped, and the reciprocating several times (generally about 3 to 4 times) the charging current will decrease the set off current value. Then enter the constant voltage charging phase, charge the battery at a constant voltage until the charging current decreases to the lower limit value, and the charging ends.

In the main charging phase of the variable current intermittent charging method, under the condition of limiting the charging voltage, the charging current is gradually increased by using the intermittent mode in which the current is gradually reduced, that is, the charging process is accelerated, and the charging time is shortened. However, this charging mode circuit is relatively complicated and expensive, and is generally considered only when it is charged at a high power.

2) Variable voltage intermittent charging

On the basis of the variable current intermittent charging method, a variable voltage intermittent charging method has been studied. The difference between the two is the charging process in the first stage, which replaces the intermittent constant current with intermittent constant pressure. Comparing the above figures (a) and (b), it can be seen that the constant voltage intermittent charging is more in line with the charging curve of the best charging. In each constant voltage charging phase, due to the constant voltage, charging electricity

The flow naturally decreases according to the exponential law, which is consistent with the fact that the acceptable rate of battery current gradually decreases with the progress of charging.

Reflex fast charging method

The Reflex fast charging method is also known as the reflective charging method or the "hiccup" charging method. Each duty cycle of the method includes three stages of forward charging, reverse instantaneous discharging, and shutdown. It solves the phenomenon of battery polarization to a large extent and speeds up the charging. But reverse discharge will shorten the life of the lithium battery.

As shown in the above figure, in each charging cycle, the Tc of 2C current charging time is 10s, then the charging time is 0.5s Tr1, the reverse discharging time is 1s Td, and the charging time is 0.5s. Tr2, each charging cycle time is 12s. As the charging progresses, the charging current will gradually become smaller.

Intelligent charging method

Smart charging is a more advanced charging method. As shown in the figure below, the main principle is to apply du/dt and di/dt control technology to check the battery charging status by checking the increment of battery voltage and current. The dynamic tracking battery is acceptable. The charging current is such that the charging current is always near the maximum acceptable charging curve of the battery. This kind of intelligent method generally combines advanced algorithm technologies such as neural network and fuzzy control to realize automatic optimization of the system.

Experimental data on the effect of charging method on charging rate

The literature compares the constant current charging method with a reverse pulse charging. Constant current charging is to charge the battery with a constant current during the entire charging process. At the beginning of constant current charging, there is a large current charging, but as time goes on, the polarization resistance gradually appears and increases, causing more energy to be converted into heat, which is consumed and the battery temperature is gradually increased.

Comparison of constant current charging and pulse charging

The pulse charging method is a short reverse charging current after a period of charging. The basic form is shown below. A short discharge pulse is mixed during the charging process to depolarize and reduce the influence of the polarization resistance during the charging process.

Studies have specifically compared the effects of pulse charging and constant current charging. The average currents were 1C, 2C, 3C and 4C (C is the rated capacity of the battery). Four sets of comparison experiments were performed. The amount of electricity discharged after the battery was charged was used to measure the actual charge. The figure shows the pulse charging current and the battery terminal voltage waveform when the charging current is 2C. Table 1 shows the experimental data of constant current pulse charging. The pulse period is 1 s, the positive pulse time is 0.9 s, and the negative pulse time is 0.1 s.

Ichav is the average charging current, Qin is the charging capacity; Qo is the discharging power, and η is the efficiency.

From the experimental results in the above table, it can be seen that the constant current charging is similar to the pulse charging efficiency, and the pulse is slightly lower than the constant current, but the total charge of the battery is charged, and the pulse mode is significantly more than the constant current mode.

Effect of different pulse duty cycles on pulse charging

The negative current discharge time in pulse charging has a certain influence on the charging speed. The longer the discharge time is, the slower the charging; the longer the discharge time is when the same flat current is charged. As can be seen from the table below, different duty cycles have a clear influence on efficiency and charge, but the numerical difference is not very large. Related to this, there are two important parameters: charging time and temperature are not displayed.

Therefore, the selection of pulse charging is better than continuous constant current charging. To select the duty cycle, it is necessary to consider the battery temperature rise and charging time requirements.

Each lithium battery has an optimal charging current value under different state parameters and environmental parameters. Then, from the perspective of the battery structure, what factors affect the optimal charging value.

Microscopic process of charging

Lithium batteries are called "rocking chair type" batteries, and charged ions move between the positive and negative electrodes to achieve charge transfer, to supply power to external circuits or to charge from an external power source. During the specific charging process, the external voltage is applied to the two poles of the battery, lithium ions are deintercalated from the positive electrode material, enter the electrolyte, and at the same time, excess electrons are generated to pass through the positive current collector, and move to the negative electrode through an external circuit; lithium ions are in the electrolyte. The positive electrode moves toward the negative electrode and passes through the separator to reach the negative electrode; the SEI film passing through the surface of the negative electrode is embedded in the negative graphite layered structure and bonded to the electron.

During the entire ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will have an impact on fast charge performance.

Fast charge, requirements for all parts of the battery

For the battery, if you want to improve the power performance, you need to work hard in all aspects of the battery, including the positive electrode, negative electrode, electrolyte, diaphragm and structural design.

Positive electrode

In fact, almost all kinds of cathode materials can be used to make fast-fill batteries. The main performances required to be guaranteed include conductance (reduction of internal resistance), diffusion (guaranteed reaction kinetics), longevity (no need to explain), and safety (not required). Explain), proper processing performance (the specific surface area cannot be too large, reduce side reactions, for safety services). Of course, the problems to be solved for each specific material may vary, but our common cathode materials can be optimized through a series of optimizations, but different materials are also different:

A. lithium iron phosphate may be more focused on solving problems of conductance and low temperature. Carbon coating, moderate nanocrystallization (note that it is moderate, definitely not as fine as the simple logic), the formation of ionic conductors on the surface of the particles is the most typical strategy.

B, the ternary material itself has a good conductance, but its reactivity is too high, so the ternary material has little work of nanocrystallization (It is not an antidote to the performance improvement of the metallurgical material, especially in the field of batteries. There are sometimes many reactions in the system. More attention is paid to safety and inhibition (and electrolyte) side effects. After all, the main goal of ternary materials is safety. The recent battery safety accidents are also frequent. Put forward higher requirements.

C, lithium manganate is more important for life. There are a lot of fast-charge batteries of lithium manganate on the market.

Negative electrode

When the lithium ion battery is charged, lithium migrates to the negative electrode. The excessively high potential caused by the fast charge and high current will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode rapidly accepting lithium will become larger, and the tendency to generate lithium dendrites will become larger. Therefore, the negative electrode must not only satisfy the lithium diffusion during fast charging. The kinetic requirements also solve the safety problems caused by the increased tendency of lithium dendrite formation, so the main technical difficulty of the fast charge core is the insertion of lithium ions in the negative electrode.

A．At present, the dominant anode material in the market is still graphite (about 90% of the market share), the root cause is no him - cheap (you are too expensive every day, exclamation mark!), and the comprehensive processing performance and energy of graphite The density is relatively good, and the disadvantages are relatively few. Of course, graphite anodes also have problems. The surface is sensitive to electrolytes, and the lithium intercalation reaction has strong directionality. Therefore, it is mainly necessary to work hard to carry out graphite surface treatment, improve its structural stability, and promote the diffusion of lithium ions on the substrate.

B. Hard carbon and soft carbon materials have also developed in recent years: hard carbon materials have high lithium insertion potential, micropores in the materials, and good reaction kinetics; and soft carbon materials have good compatibility with electrolytes, MCMB The materials are also very representative, but the hard and soft carbon materials are generally low in efficiency and high in cost (and Imagine that graphite is as cheap as I hope from an industrial point of view), so the amount is far less than graphite, and more used in some specialties on the battery .

C, someone will ask the author how to lithium titanate. To put it simply: lithium titanate has the advantages of high power density, safer, and obvious disadvantages. The energy density is very low, and the calculation cost is high according to Wh. Therefore, the author's view on lithium titanate batteries has always been: a useful technology that is advantageous in certain situations, but it is not suitable for many occasions where the cost and cruising range are high.

D, silicon anode material is an important development direction. Panasonic's new 18650 battery has begun commercial process for such materials. But how to achieve a balance between the pursuit of performance in nanotechnology and the battery industry's general micron-scale requirements for materials is still a challenging task.

Diaphragm

For power batteries, high current operation provides higher requirements for safety and longevity. Diaphragm coating technology is inseparable. Ceramic coated membranes are rapidly being pushed away due to their high safety and the ability to consume impurities in the electrolyte. Especially for the safety of ternary batteries, the safety effect is particularly remarkable. The main system currently used in ceramic diaphragms is to coat alumina particles on the surface of conventional diaphragms. A relatively novel approach is to coat solid electrolyte fibers on the membrane. Such membranes have lower internal resistance and the mechanical support effect of the fibers on the membrane is more Excellent, and it has a lower tendency to block the diaphragm hole during service. After the coating, the separator has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform, resulting in short circuit. Jiangsu Qingtao Energy Co., Ltd., technical support of the Academic Researcher of Tsinghua University School of Materials, has some representative aspects in this respect. Work, the diaphragm is shown below.

Separator coated with solid electrolyte fibers

Electrolyte

The electrolyte has a great influence on the performance of a fast-charged lithium ion battery. To ensure the stability and safety of the battery under fast charge and high current, the electrolyte should meet the following characteristics: A) cannot be decomposed, B) the conductivity is high, C) is inert to the positive and negative materials, cannot React or dissolve. If these requirements are to be met, the key is to use additives and functional electrolytes. For example, the safety of ternary fast-charged batteries is greatly affected by it. It is necessary to add various anti-high temperatures, flame-retardant and anti-overcharged additives to protect them to a certain extent. The problem of the old lithium titanate battery, the high temperature flatulence, also depends on the high temperature functional electrolyte.

Battery structure design

A typical optimization strategy is the stacked VS winding type. The electrodes of the laminated battery are equivalent to a parallel relationship, and the winding type is equivalent to a series connection. Therefore, the internal resistance of the former is much smaller, and it is more suitable for the power type. In addition, you can work hard on the number of poles to solve internal resistance and heat dissipation problems. In addition, the use of high-conductivity electrode materials, the use of more conductive agents, and the application of thinner electrodes are also strategies to consider.

In short, factors affecting the internal charge movement of the battery and the rate of embedding the electrode cavity will affect the rapid charging capability of the lithium battery.

Mainstream manufacturers fast charge technology route overview

CATL

For the positive electrode, Ningde era developed the "super-electronic network" technology, which makes lithium iron phosphate have excellent electronic conductivity; on the surface of the negative graphite, it is modified by "fast ion ring" technology, and the modified graphite has both super fast charge and high The characteristics of energy density, no excess by-products appear in the negative electrode during fast charging, so that it has 4-5C fast charge capacity, achieve fast charging and charging for 10-15 minutes, and can guarantee energy density above 70wh/kg of system level, achieving 10000 The cycle life (speaking that this life is quite high). In terms of thermal management, its thermal management system fully recognizes the “healthy charging range” of fixed chemical systems at different temperatures and SOCs, greatly expanding the operating temperature of lithium batteries.

Waterma

Waterma is not very good lately, let's just talk about technology. Waterstone uses lithium iron phosphate with a smaller particle size. Currently, the lithium iron phosphate particle size on the market is between 300 and 600 nm, and Waterma only uses 100 to 300 nm lithium-iron phosphate, so that lithium ions will have Faster migration speed, able to charge and discharge with higher current. On systems other than batteries, the design is enhanced with thermal management systems and systems.

Micro macro power

In the early days, Weihong Power chose lithium titanate + porous composite carbon which can withstand high current and has a spinel structure as the anode material; in order to avoid the threat of high power current to battery safety during fast charging, Weihong Power Combined with non-combustion electrolyte, high porosity and high permeability diaphragm technology and STL intelligent thermal control fluid technology, the battery safety is ensured when the battery is fast charged.

In 2017, it released a new generation of high-energy-density batteries, using high-capacity and high-power lithium manganate cathode materials. The monomer energy density reached 170wh/kg, achieving fast charging in 15 minutes, aiming at both life and safety issues.

Zhuhai Yinlong

Lithium titanate negative electrode, wide operating temperature range and large charge and discharge rate is known, the specific technical solutions, there is no clear data. Conversation with the staff at the show, it is said that its fast charge can already achieve 10C, life expectancy of 20,000 times.

The future of fast charging technology

The fast charging technology of electric vehicles is the direction of history or the glimpse of the past. In fact, there are many different opinions and there is no conclusion. As an alternative to solving mileage anxiety, it is considered on a platform with battery energy density and overall vehicle cost.

Energy density and fast charge performance, in the same battery, can be said to be incompatible in both directions, not both. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough, the load of one car is large enough to avoid the so-called "mileage anxiety", the demand for battery rate charging performance will be reduced; at the same time, the power is large, if the cost of battery power is not low enough, then whether it is necessary Ding Kezhen's purchase of electricity that is "not anxious" requires consumers to make choices. If you think about it, fast charge will have value. Another angle is the cost of fast-charging facilities mentioned yesterday, which is of course part of the cost of electrifying the entire society.

To sum up: whether fast charging technology can be promoted in a large area, energy density and fast charging technology who develops fast, the two technologies who reduce the cost, may play a considerable role in their future decisive role.

The page contains the contents of the machine translation.