Exploring the factors affecting the fast charging ability of 18650 battery lithium ion 2200mah

When a new car is launched, of course I am talking about electric cars, there is often such an introduction: "Fast charging, charging 80% in half an hour, 200 kilometers of range, completely solving your mileage anxiety!" Fast charging is used by commercial vehicles to improve equipment efficiency and passenger cars to solve mileage anxiety, constantly approaching the time of "filling a tank of gas". It is likely to become a standard feature. Today, let's dig into the fast charging method and the origin of the method.

How fast can be called "fast charging"?

Our basic charging demands:

1) Charging must be fast;

2) Do not affect the life of my battery cell;

3) Try to save money, and try to charge as much electricity as the charger discharges into my battery.

How fast can be called fast charging? There is no standard document that gives specific values. Let's refer to the numerical threshold mentioned in the most well-known subsidy policy. The following table is the subsidy standard for new energy buses in 2017. As you can see, 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, we can see that people generally believe that 80% full charge in 30 minutes can be used as a gimmick for fast charging and promoted. So let's assume that 1.6C for passenger cars can be the reference value for entry-level fast charging. According to this idea, the promotion of 80% full charge in 15 minutes is equivalent to 3.2C.

Where is the bottleneck of fast charging?

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

When we discuss fast charging, we directly think of whether there will be problems with the battery. In fact, before the battery has problems, the first problem is the charger and the distribution line. We mentioned Tesla's charging pile, which is called a super charging pile, and its power is 120kW. According to the parameters of Tesla Model S85D, 96s75p, 232.5Ah, and the highest 403V, its 1.6C corresponds to a maximum demand power of 149.9kW. From here, we can see that for long-range pure electric vehicles, 1.6C or 80% full in 30 minutes has already posed a test to the charging pile.

In the national standard, it is not allowed to directly set up charging stations in the original residential power network. The power consumption of one fast charging pile has exceeded the power consumption of dozens of households. Therefore, charging stations need to be equipped with 10kV transformers separately, and the distribution network in a region does not have the margin to add more 10kV substations.

Then let's talk about batteries. Whether the battery can bear 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 the title of this section is called "Macroscopic fast charging theory" is that the direct determinant of the battery's fast charging ability is the properties of the positive and negative electrode materials inside the lithium battery, the microstructure, the electrolyte composition, additives, the properties of the diaphragm, etc. These microscopic contents are temporarily put aside, standing outside the battery, and looking at the method of fast charging of lithium batteries.

Lithium batteries have an optimal charging current

In 1972, American scientist J.A.Mas proposed that batteries have an optimal charging curve and his three laws 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 generation of a small amount of side reaction gas. Obviously, this condition is related to the specific reaction type.

However, the idea that the system has an optimal solution is universal. Specifically for lithium batteries, the boundary conditions for defining their maximum acceptable current can be redefined. Based on the conclusions of some research literature, its optimal value is still a curve trend similar to Mas's law.

It is worth noting that the boundary conditions for the maximum acceptable charging current of lithium batteries, in addition to considering the factors of lithium battery cells, also need to consider system-level factors, such as different heat dissipation capabilities, and the maximum acceptable charging current of the system is different. Then we will continue to discuss on this basis.

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 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, it will not only fail to increase the charging rate, but also increase the gassing of the battery; if it is less 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 explanation of this theory includes three levels, which are the three laws of Mars:

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

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

③ After the battery is discharged at different discharge rates, its final allowable charging current It (acceptance) is the sum of the allowable charging currents at each discharge rate.

The above theorem is also the source of the concept of charging acceptance. First understand what charging acceptance is. After searching for a while, I didn't see a unified official definition. According to my own understanding, the charging acceptance is the maximum current of a rechargeable battery with a certain charge under specific environmental conditions. Acceptable means that there will be no unnecessary side reactions and no adverse effects on the life and performance of the battery.

Then understand the three laws. The first law is that after the battery discharges a certain amount of electricity, its charging acceptance is related to the current charge. The lower the charge, the higher its charging acceptance. The second law is that during the charging process, pulse discharge helps the battery to improve the real-time acceptable current value; the third law is that the charging acceptance will be affected by the superposition of the charging and discharging conditions before the charging moment.

If the Maas theory is also applicable to lithium batteries, then the reverse pulse charging (specifically named Reflex fast charging method below) can not only explain its help in suppressing temperature rise from the perspective of depolarization, but also the Maas theory supports the pulse method. And further, the real full application of Maas theory is the intelligent charging method, that is, tracking battery parameters so that the charging current value always follows the Maas curve of the lithium battery, so that the charging efficiency is maximized within the safety boundary.

Common fast charging methods

There are many ways to charge lithium batteries. For the requirements of fast charging, the main methods include pulse charging, reflex charging, and smart charging. Different battery types have different applicable charging methods, so we will not make a specific distinction in this method section.

Pulse charging

This is a pulse charging method from the literature. Its pulse stage is set after the charging reaches the upper limit voltage of 4.2V and continues above 4.2V. Without mentioning the rationality of its specific parameter settings, there are differences between different types of batteries. Let's focus on the pulse implementation process.

Below is the pulse charging curve, which mainly includes three stages: pre-charging, constant current charging, and pulse charging. During the constant current charging process, the battery is charged with a constant current, 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), it enters the pulse charging mode: the battery is intermittently charged with a pulse current of 1C. The battery voltage will continue to rise during the constant charging time Tc, 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 begins. The charging cycle is repeated until the battery is fully charged.

During the pulse charging process, the battery voltage drop rate will gradually slow down, and the charging stop time T0 will become longer. When the constant current charging duty cycle is as low as 5% to 10%, the battery is considered to be fully charged and the charging is terminated. Compared with conventional charging methods, pulse charging can be charged with a larger current. During the charging stop 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 the battery life is small. Therefore, it is currently widely used. However, its disadvantages are obvious: a power supply with a current limiting function is required, which increases the cost of the pulse charging method.

Intermittent charging method

Lithium battery intermittent charging methods 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 of Xiamen University. Its characteristic is that the constant current charging is changed to voltage-limited 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 used to charge the battery first, and the charging is stopped when the battery voltage reaches the cut-off voltage V0. At this time, the battery voltage drops sharply. After a period of charging suspension, the charging is continued with a reduced charging current. When the battery voltage rises to the cut-off voltage V0 again, the charging is stopped. After several reciprocating times (generally about 3 to 4 times), the charging current will be reduced by the set cut-off current value. Then the constant voltage charging stage is entered, and the battery is charged at a constant voltage until the charging current is reduced to the lower limit value, and the charging is completed.

In the main charging stage of the variable current intermittent charging method, the charging current is increased by an intermittent method with a gradually decreasing current under the condition of a limited charging voltage, that is, the charging process is accelerated and the charging time is shortened. However, this charging mode circuit is relatively complex and expensive, and is generally only considered for high-power fast charging.

2) Variable voltage intermittent charging

Based on the variable current intermittent charging method, some people have studied the variable voltage intermittent charging method. The difference between the two lies in the first stage of the charging process, where the intermittent constant current is replaced by an intermittent constant voltage. Comparing the above figure (a) and figure (b), it can be seen that constant voltage intermittent charging is more in line with the charging curve of optimal charging. In each constant voltage charging stage, due to the constant voltage, the charging current naturally decreases according to the exponential law, which is in line with the characteristic that the battery current acceptability gradually decreases as the charging progresses.

Reflex fast charging method

The Reflex fast charging method is also known as the reflective charging method or the "hiccup" charging method. Each working cycle of this method includes three stages: forward charging, reverse instantaneous discharge, and stop charging. It solves the battery polarization phenomenon to a large extent and speeds up the charging speed. However, reverse discharge will shorten the life of lithium batteries.

As shown in the above figure, in each charging cycle, a 2C current charging time of 10s Tc is first used, followed by a stop charging time of 0.5s Tr1, a reverse discharge time of 1s Td, and a stop charging time of 0.5s Tr2. Each charging cycle time is 12s. As the charging progresses, the charging current will gradually decrease.

Intelligent charging method

Intelligent charging is currently the most advanced charging method, as shown in the figure below. Its main principle is to apply du/dt and di/dt control technology, judge the battery charging state by checking the increment of battery voltage and current, dynamically track the acceptable charging current of the battery, and make the charging current close to the maximum charging curve acceptable to the battery from beginning to end. This type of intelligent method generally combines advanced algorithm technologies such as neural network and fuzzy control to achieve automatic optimization of the system.

Experimental data on the influence of charging method on charging rate

The literature compares the constant current charging method and a reverse pulse charging method. Constant current charging is to charge the battery with a constant current throughout the charging process. In the early stage of constant current charging, large current charging can be used, but as time goes by, the polarization resistance gradually appears and increases, causing more energy to be converted into heat, which is consumed and causes the battery temperature to gradually rise.

Comparison of constant current charging and pulse charging

The pulse charging method is a short reverse charging current after a period of charging. Its basic form is shown in the figure below. The short discharge pulse mixed in the charging process plays a depolarization role and reduces the influence of polarization resistance in the charging process.

Some studies have specifically compared the effects of pulse charging and constant current charging. Taking the average current as 1C, 2C, 3C and 4C (C is the rated capacity value of the battery), 4 groups of comparative experiments were conducted respectively, and the actual amount of electricity charged was measured by the amount of electricity discharged after the battery was fully charged. The figure shows the current and battery terminal voltage waveform of pulse charging when the charging current is 2C. Table 1 is the experimental data of constant current pulse charging. The pulse period is 1s, the positive pulse time is 0.9s, and the negative pulse time is 0.1s.

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

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

The influence of different pulse duty ratios on pulse charging

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

Therefore, pulse charging is better than continuous constant current charging. When choosing a specific duty cycle, it is necessary to focus on the battery temperature rise and charging time requirements.

Each lithium battery has an optimal charging current value under different state parameters 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 batteries are called "rocking chair" batteries. Charged ions move between the positive and negative electrodes to achieve charge transfer, power the external circuit or charge from an external power source. In the specific charging process, the external voltage is applied to the two poles of the battery, and lithium ions are deintercalated from the positive electrode material and enter the electrolyte. At the same time, excess electrons are generated through the positive electrode current collector and move to the negative electrode through the external circuit; lithium ions move from the positive electrode to the negative electrode in the electrolyte, pass through the diaphragm to the negative electrode; through the SEI film on the surface of the negative electrode, they are embedded in the negative electrode graphite layered structure and combined with electrons.

During the entire process of ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will affect the fast charging performance.

Fast charging, requirements for various parts of the battery

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

Positive electrode

In fact, almost all kinds of positive electrode materials can be used to manufacture fast-charging batteries. The main performances that need to be guaranteed include conductivity (reducing internal resistance), diffusion (ensuring reaction kinetics), life (no need to explain), safety (no need to explain), and appropriate processing performance (specific surface area should not be too large, reduce side reactions, and serve safety). Of course, the problems to be solved for each specific material may be different, but our common positive electrode materials can meet these requirements through a series of optimizations, but different materials are also different:

A. Lithium iron phosphate may focus more on solving problems in conductivity and low temperature. Carbon coating, moderate nano-sizing (note that it is moderate, and it is definitely not a simple logic that the finer the better), and surface treatment of particles to form ion conductors are the most typicalStrategy.

B. The conductivity of ternary materials is relatively good, but their reactivity is too high. Therefore, there is little work on nano-scale ternary materials (nano-scale is not a panacea for improving material performance, especially in the field of batteries, there are sometimes many adverse effects). More attention is paid to safety and inhibiting side reactions (with electrolytes). After all, one of the key factors of ternary materials is safety. The recent frequent battery safety accidents have also put forward higher requirements in this regard.

C. Lithium manganese oxide is more concerned about lifespan. There are also many lithium manganese oxide fast-charging batteries on the market.

Negative electrode

When 18650 battery lithium ion 2200mah are charged, lithium migrates to the negative electrode. The excessive potential caused by the high current of fast charging will cause the negative electrode potential to be more negative. At this time, the pressure on the negative electrode to quickly accept lithium will increase, and the tendency to generate 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 generation. Therefore, the main technical difficulty of fast charging cells is actually the embedding of lithium ions in the negative electrode.

A. 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 cheap (you complain about the high price of batteries every day, exclamation mark!), and graphite has excellent comprehensive processing performance and energy density, and relatively few disadvantages. Of course, there are also problems with graphite negative electrodes. Its surface is more sensitive to electrolytes, and the lithium embedding reaction has a strong directionality. Therefore, the main direction of efforts is to treat the graphite surface, improve its structural stability, and promote the diffusion of lithium ions on the base.

B. There have been many developments in hard carbon and soft carbon materials in recent years: hard carbon materials have high lithium embedding potentials, and there are micropores in the materials, so the reaction kinetics are good; while soft carbon materials have good compatibility with electrolytes, and MCMB materials are also very representative, but hard and soft carbon materials are generally inefficient and costly (and it is unlikely to be as cheap as graphite from an industrial perspective), so the current usage is far less than that of graphite, and it is more used in some special batteries.

C. Some people will ask the author how lithium titanate is. To put it simply: the advantages of lithium titanate are high power density and safety, but the disadvantages are also obvious, the energy density is very low, and the cost is very high when calculated by Wh. Therefore, the author's view on lithium titanate batteries has always been: it is a useful technology with advantages in specific occasions, but it is not very suitable for many occasions with high requirements for cost and mileage.

D. Silicon negative electrode materials are an important development direction. Panasonic's new 18650 battery has begun the commercialization process of such materials. However, how to achieve a balance between the pursuit of performance in nano-scale and the general micron-level requirements of the battery industry for materials is still a relatively challenging task.

Diaphragm

For power batteries, high current operation provides higher requirements for their safety and life. Diaphragm coating technology is inevitable. Ceramic coating diaphragms are rapidly being promoted because of their high safety and ability to consume impurities in the electrolyte, especially for ternary batteries. The effect of improving the safety is particularly significant. The main system currently used for ceramic diaphragms is to coat alumina particles on the surface of traditional diaphragms. A more novel approach is to coat solid electrolyte fibers on the diaphragms. Such diaphragms have lower internal resistance, better mechanical support for the diaphragms, and lower tendency to block the diaphragm pores during service. The diaphragms after coating have good stability. Even at high temperatures, they are not easy to shrink and deform to cause short circuits. Jiangsu Qingtao Energy Company, which is technically supported by the research group of Academician Nan Cewen of the School of Materials Science and Engineering of Tsinghua University, has some representative work in this regard. The diaphragm is shown in the figure below.

Diaphragm coated with solid electrolyte fibers

Electrolyte

The electrolyte has a great influence on the performance of fast-charge 18650 battery lithium ion 2200mah. To ensure the stability and safety of the battery under fast-charge and high current, the electrolyte must meet the following characteristics: A) cannot be decomposed, B) high conductivity, and C) inert to the positive and negative electrode materials and cannot react or dissolve. If these requirements are to be met, additives and functional electrolytes are key. For example, the safety of ternary fast-charging batteries is greatly affected by it. Various high-temperature resistant, flame-retardant, and anti-overcharge additives must be added to protect it in order to improve its safety to a certain extent. The long-standing problem of lithium titanate batteries, high-temperature flatulence, must also be improved by high-temperature functional electrolytes.

Battery structure design

A typical optimization strategy is stacking vs. winding. The electrodes of stacking batteries are equivalent to a parallel relationship, while the winding type is equivalent to a series relationship. Therefore, the former has a much smaller internal resistance and is more suitable for power-type occasions. In addition, you can also work on the number of tabs to solve the internal resistance and heat dissipation problems. 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 embedding rate of the electrode holes in the battery will affect the fast charging ability of lithium batteries.

Overview of fast charging technology routes of mainstream manufacturers

CATL

For the positive electrode, CATL has developed the "super electron network" technology, which makes lithium iron phosphate have excellent electronic conductivity; on the surface of the negative electrode graphite, the "fast ion ring" technology is used for modification. The modified graphite takes into account the characteristics of super fast charging and high energy density. During fast charging, there is no excessive by-product in the negative electrode, which enables it to have 4-5C fast charging capability, achieve 10-15 minutes of fast charging, and can ensure an energy density of more than 70wh/kg at the system level, and achieve a cycle life of 10,000 times (this life 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 SOCs, greatly broadening the operating temperature of lithium batteries.

Watma

Watma has not been doing well recently, let's just talk about technology. Wattma uses lithium iron phosphate with a smaller particle size. The common lithium iron phosphate particle size on the market is between 300 and 600nm, while Wattma only uses lithium iron phosphate with a particle size of 100 to 300nm. In this way, lithium ions will have a faster migration speed and can be charged and discharged at a higher rate of current. In systems other than batteries, strengthen the thermal management system and system safety design.

Microvast Power

In the early days, Microvast Power chose lithium titanate + porous composite carbon with spinel structure that can withstand fast charging and large current as the negative electrode material; in order to avoid the threat of high-power current to battery safety during fast charging, Microvast 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 when fast charging is achieved.

In 2017, it released a new generation of high-energy density batteries, using high-capacity and high-power lithium manganese oxide positive electrode materials, with a single energy density of 170wh/kg, achieving 15-minute fast charging, and targeting both life and safety issues.

Zhuhai Yinlong

Lithium titanate negative electrode is known for its wide operating temperature range and large charge and discharge rate. There is no clear information on the specific technical solution. Talking with the staff at the exhibition, it is said that its fast charging can already achieve 10C and the life span is 20,000 times.

The future of fast charging technology

Is the fast charging technology of electric vehicles a historical direction or a flash in the pan? In fact, there are many different opinions now, and there is no conclusion. As an alternative solution to mileage anxiety, it is considered on the same platform as battery energy density and overall vehicle cost.

Energy density and fast charging performance can be said to be two incompatible directions in the same battery, and they cannot be achieved at the same time. The pursuit of battery energy density is currently the mainstream. When the energy density is high enough, a car is loaded with enough power to avoid the so-called "mileage anxiety", and the demand for battery rate charging performance will be reduced; at the same time, if the battery cost per kilowatt-hour is not low enough, then whether to buy enough power to "not worry" requires consumers to make a choice. In this way, fast charging has its value. Another perspective is the cost of fast-charging supporting facilities mentioned yesterday, which is of course part of the cost of promoting electrification in the whole society.

In conclusion, whether fast-charging technology can be widely promoted, which of the energy density and fast-charging technologies develops faster, and which of the two technologies reduces costs more sharply may play a decisive role in its future prospects.

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