To explore the factors that affect quick charge capacity of lithium battery

New electric cars are often introduced: "quick charge, half an hour charging can get 80%. The range is 200 km, completely solve your range anxiety!" Commercial vehicle used quick charge to improve equipment efficiency, while passenger vehicles used it to solve the range anxiety, looming "add a tank of gas" time which becomes the trend of the standard. Today let's come to know more about fast charge method.

How fast charging can be called "quick charge"?

We charge the basic demands of:

1) charging faster;

2) don't affect my life of batteries;

3) as far as possible to save money, put out how much electricity charger, as far as possible all charged to my battery.

So how fast can call quick charge? There is no standard document specific values, we have mentioned the most famous subsidy policy for reference value threshold. The table below is the new energy bus subsidy standard in 2017.As you can see, fast filling entry-level is 3 c. In fact, in the passenger car subsidy standard, made no mention of the requirement of quick charge. From general passenger car promotional materials, you can see, people generally think that 30 minutes of 80% has can be used as a quick charge stunt, take out publicity, so let's think of passenger car 1.6 C can be entry-level quick charge reference value. According to this train of thought, promote 15 minutes with 80%, equivalent to 3.2 C.

The bottleneck of quick charge in where?

In quick charge this context, the parties shall, in accordance with the physical body points, including battery, charger, power distribution facilities.

We discuss a quick charge, will there be direct thought of batteries. In fact, before the battery has a problem, first is charging machine, the problem of distribution line. We noted tesla's charging pile, known as super charging pile, its power is 120 kw. According to the tesla ModelS85D parameters, 96 s75p, 232.5 Ah, up to 403 v, the 1.6 C corresponding to maximum demand power is 149.9 kW. From here you can see, for a long range of pure electric vehicles, 1.6 C or 30 minutes with 80% have charging pile test.

In the national standards, does not allow directly in the original residential electricity network Settings charging stations. One is beyond the power capacity of the filling pile quickly dozens of residents of power consumption. Charging station, therefore, requires a separate set of 10 kv transformer, and a regional distribution network is not have allowance to add more 10 kv substation.

He said the battery. Batteries will be able to carry 1.6 C and 3.2 C charging requirements, from the macroscopic and microscopic two angles to look at.

On the macroscopic theory of quick charge

Is the topic of this section is called "quick charge on the macroscopic theory", because directly decide the battery fast charge capacity is lithium battery anode materials are internal properties, microstructure, electrolyte composition, additive, membrane properties and so on, the content of the micro level, we temporarily put aside, standing outside the battery, the lithium battery quick charging method.

The lithium battery is optimal charging current

1972 American scientists J.A.M as proposed in the process of charging battery is the best charging curve and his three laws of mas, it is important to note that this theory is for lead-acid batteries, the definition of maximum acceptable charging current boundary conditions is the production of a small amount of reaction gas, obviously this condition associated with specific reaction type.

But, the idea of system exists the optimum solution is universal. Specific to the lithium battery, define the boundary conditions of maximum acceptable current can be redefined. Based on some research conclusions, and its optimal value is still the trend curve of similar laws of mas.

It is worth noting that the maximum acceptable charging current of lithium-ion batteries boundary condition, in addition to the need to consider the factor of lithium battery monomer, also need to consider the factor of system level, such as heat capacity, the system's maximum acceptable charging current is different. And then we'll to continue down the basis of this discussion.

Mas theorem formula describing:

I = I0 * e ^ alpha t

Type;I0 for initial battery charging current; Alpha accepted by charging rate; T for the charging time.I0 and alpha values associated with cell types, structure, and degree of old and new.

At this stage the study of battery charging method is mainly based on the best charging curve in order to develop. As shown in the figure below, if the charging current is over the best charging curve not only cannot improve the charging rate, and will increase the analysis of cell volume; If less than the best charging curve, although will not harm the cells, but will extend the charging time, reduce the charging efficiency.

On this theory this paper contains three levels, is for the mas three laws:

(1) for any given discharge current, battery charging current accept than alpha and out of your battery capacity is inversely proportional to the square root;

(2) for any given discharge, alpha and discharge current is directly proportional to the logarithm of Id;

(3) battery at different discharge rates after discharge, the ultimate (accept) allow the charging current of It are all under the discharge rate of allowing the sum total of charging current.

The above theorem is the source of charging ability to accept this concept. Understand first what is the charge to accept ability. Found a ring saw no unified definition of official. According to own understanding, charging accept ability is in a specific environment conditions, have a certain load capacity of the maximum current of rechargeable batteries. Acceptable means will not produce should not have side effects, won't cause bad effects to the life of the batteries and performance.

Understand the three laws. First law, released after a certain power, the battery charging accept ability is associated with the current load capacity, load capacity is lower, the higher charge acceptance. The second law is in the process of charging pulse discharge, help battery improve real-time acceptable current value; Third law, charging will accept ability is affected by the superposition of charging and discharging of charging time ago.

If the mas theory applies to lithium batteries, the reverse pulse charging (below the specific name for Reflex fast charging method) to polarization Angle can be used in addition to explain its helpful to inhibit temperature, mas theory as support for the pulse method. And further, really will full use of mas theory, is an intelligent charging method, namely tracking battery parameters, the charging current value is always followed the mas curve of lithium battery, made within the secure boundary, charging to maximize efficiency.

Common fast charging method

There are many kinds of lithium battery charging method, according to the requirement of quick charge, its main methods include pulse charging, Reflex, and intelligent charging. Different cell types, and its applicable charging ways is not identical also, do not do in this section method to distinguish.

Pulse charging

This is a pulse charging ways of literature, the pulse phase set after charging touch the ceiling voltage of 4.2 V, and above 4.2 V continues. Aside the rationality of its specific parameter Settings, the differences are between different types of batteries. We focus on pulse implementation process.

Below is the pulse charging curve, mainly includes three phases: preliminary charge, constant current charging and pulse charging. In the process of constant current charging with constant current to charge batteries, part of the energy is transferred to the internal battery. When the battery voltage rises to the maximum voltage (4.2 V), into pulse charging mode, the pulse current with 1 c intermittently for the battery. In constant charging time Tc battery voltage will continue to rise, stop charging voltage will gradually decline. When the battery voltage drops to limit the voltage (4.2 V), with the same current value of the battery, begin the next charge cycle, the cycle charge until the battery is full.

In the process of pulse charging, battery voltage drop speed will gradually slow down, stop filling time T0 get longer, when the constant current charging duty ratio as low as 5% ~ 10%, think the battery is fully charged, the charging termination. Compared with the conventional charging method, pulse charging in large current charging, in rechargeable battery of concentration polarization and ohm polarization can be eliminated, make the next round of charging more smoothly, charging speed, small temperature changes, the impact on battery life, and therefore is widely used at present. But its drawback is obvious: need a limited flow function of power supply, this increases the cost of pulse charging mode.

Intermittent charging method

Lithium battery intermittent charging method includes variable current intermittent charging method and variable voltage intermittent charging method.

1) The variable current intermittent charging method

Variable current intermittent charging method was put forward by professor Chen body bit by Xiamen university, its characteristic is the constant current charging pressure limiting variable current intermittent charging instead. As shown in the figure below, the first stage of the variable current intermittent charging method to adopt larger current value of the battery, the battery voltage reaches cutoff voltage where V0 stop charging, the battery voltage has fallen sharply. Keep at a time, by reducing the charging current continue to charge. When the battery voltage is rising again to cut-off voltage where V0 stop charging, so several times (generally is about 3 ~ 4 times) set by charging current will decrease as the current value. Then entered the stage of constant voltage charging, with constant voltage to reduce the battery until the charging current to the lower limit value, the charging is over.

Variable current intermittent charging method of the filling stage under the condition of limited charging voltage, the current decreases the intermittent mode increased the charging current, which speeds up the process, shorten the charging time. But the charging mode circuit is more complex, high cost, generally only consider adopting in high-power quick charge.

2) variable voltage intermittent charging

On the basis of variable current intermittent charging method, and variable voltage intermittent charging method was studied. Differences of the two are that the first stage of the process, will be intermittent constant current with intermittent constant pressure. Compare the above figure (a) and (b), visible constant pressure intermittent charging more in line with the best charging curve. At every stage of constant voltage charging, the constant voltage, charging power

Flow naturally in accordance with the law of index decline, in line with the battery with the charging current acceptable rate on the characteristics of the gradually decline.

Reflex fast charging method

Reflex fast charging method, also called reflection charging method or "hiccups" charging method. Each work cycle of the method include the positive charge, reverse instantaneous discharge and at three stages. It to a large extent, solve the battery polarization phenomenon, to speed up the charge. But back discharge will shorten the lithium battery life.

As shown in the above, in each of the charging cycle, first using 2 c current charging time is 10 s of Tc, then stop filling time is 0.5 s Tr1, reverse discharge time is 1 s of Td, at time of 0.5 s Tr2, the cycle time of each charge for 12 s. As the charging, charging current will gradually become smaller.

Intelligent charging method

Intelligent charging is one of the more advanced charging method, as shown in the figure below, the main principle is applied the du/dt and di/dt control technology, through the check of the battery voltage and current increment to judge the battery charging status, dynamic tracking acceptable charging current of battery, the charging current all the near the battery acceptable maximum charging curve. This kind of intelligent method, the general combined with advanced algorithms such as neural networks and fuzzy control technology, realize the automatic optimization of the system.

The influence of charging way of charging rate experiment data

Literature comparison of the constant current charging method and a reverse pulse charge. Constant current charging is the entire charging process with constant charging current of battery charge. Constant current charging early, can have a large current charging, but with the passage of time, the polarization resistance appeared and increased gradually, causing more energy into heat, consume and make the battery temperature rise gradually.

The comparison of constant current charging and pulse charging

Pulse charging method, charge is for a period of time, after a brief reverse charging current. Its basic form as shown in the figure below. With short discharge pulse charging process, have the effect of depolarization, reduce the impact of polarization resistance in the process of charging.

Specialized research has compared the pulse charging and the effect of constant current charging. Take the average current for 1 C, 2 C, 3 C and 4 C (C for battery rated capacity values), four groups contrast experiment was done respectively, measured through after the battery is out of power under the power of the actual filling the. The graph is when the charging current is 2 c pulse charging current and battery voltage waveform. For constant current charging pulse experiment data in table 1.Pulse duration is 1 s, is the pulse time of 0.9 s, the negative pulse time is 0.1 s.

Ichav for average charging current, Qin for filling the electricity; Qo to release energy, eta for efficiency.

From the experimental results can be seen in the table above, constant current charging and approximate pulse charging efficiency, pulse slightly under constant current, but always filling the battery power, pulse mode significantly more than the constant current mode.

The influence of different pulse duty ratio pulse charging

Electric pulse charging the negatively charged exile time of charging speed, impact, the longer the discharge time, charging the slower;保持相同平均电流充电时，放电时间越长。Can be seen from the table below, different duty ratio on the efficiency and the influence of filling the power there is a clear trend, but the numerical difference is not very big. Related to this, there are two important parameters, charging time and temperature are not shown.

So, choose the pulse charging is better than that of continuous constant current charging, specific duty ratio choice, you will need to consider the battery temperature rise and charging time.

Each kind of lithium battery parameters and environmental parameters under different states there is an optimum charging current value, so, look from the cell structure, the factors affecting the optimal charge value what are.

The micro process of charging

Referred to as "rocking chair" type batteries, lithium-ion batteries charged ions movement between is negative, to realize the charge transfer, recharge external circuit power supply or from an external power source. Concrete in the process of charging voltage load in poles of the battery, lithium ion from the anode materials embedded, into the electrolyte, excess electrons through the positive set fluid at the same time, the external circuit to the cathode movement; Lithium ions in the electrolyte from the positive to the negative movement, through the diaphragm to the cathode; Through the SEI film on the surface of the cathode is embedded into the anode graphite layer structure, and combine with electrons.

In the process of the whole operation of the ions and electrons, impact on the charge transfer of cell structure, electrochemical and physical, will have an effect on the fast charging performance.

Quick charge, the battery requirements of each part

For battery, if you want to improve power performance, need the battery every link in the overall effort, mainly including the anode, cathode, electrolyte and the diaphragm and structure design, etc.

The anode

In fact, all kinds of the anode materials can be used to make almost all quick charge battery, need to ensure that the performance of the main including conductance (reduce the internal resistance), diffusion reaction kinetics (), (no need to explain life, security (no need to explain), the appropriate processing performance, specific surface area is too big, reduce side effects, for the security services)., of course, for each of the specific material to solve the problem may differ, but we usually common anode material can be through a series of optimization to meet these requirements, but also different from different materials:

A, lithium iron phosphate may be more focus on the problems of conductivity and low temperature. Nano carbon coated, moderation (note that is moderate, is definitely not the thinner the better simple logic), formed in the particle surface treatment ion conductor is the most typical strategy.

Conductance, ternary material itself has B is better, but its high reactivity, therefore the ternary material for nano rare work (nano is not all of the antidote of material performance, especially in the field of battery and sometimes there are a lot of reaction), more pay attention to safety and restrain adverse event (with electrolyte), after all, a curse of the ternary material is safe, recently in battery frequent safety accidents has put forward higher requirements.

C, manganese acid lithium is more value for life, also have on market now quite a few quick charge battery lithium manganese acid system.

The cathode

Lithium ion batteries when charging, lithium to negative transfer. And quick charge large current of high potential will lead to negative electrode potential is more negative, the negative pressure quickly accept lithium get bigger, tend to generate lithium dendrite get bigger, so quick charge when the cathode is not only to satisfy the requirement of the lithium diffusion dynamics, to solve more lithium dendrite formation tend to exacerbate brings security problems, so fast charging core and the main technical difficulties for lithium ion embedded in the cathode.

A, currently on the market dominance of the anode materials is still graphite (about 90% of the market share), the root cause without him - cheaper (you every day too expensive batteries, exclamation point!, as well as graphite composite processing is better performance, energy density, relatively few disadvantages. Graphite anode, of course, also have a question, its surface is very sensitive to electrolyte, lithium embedded reaction with strong directivity, therefore carries on the graphite surface treatment, improve its structure stability, promote the lithium ion on the base of the direction of diffusion is the major requires hard work.

B, hard and soft carbon materials in recent years, there are many: the development of hard carbon materials intercalated-li potential is high, the material of microporous so reaction kinetics performance good; And soft carbon material has good compatibility with electrolyte, MCMB material is also very representative, just hard soft carbon material is generally low efficiency, high cost (and imagine graphite as cheap as I'm afraid from the industrial perspective unlikely), so the dosage is far less than graphite, multi-purpose in some more special battery on.

C, someone will ask how the author lithium titanate. Simple said once: lithium titanate has the advantage of high power density, safer, shortcomings are obvious, the energy density is low, according to Wh calculation cost is very high. Therefore, the author for Titanium acid lithium battery point of view has always been: it is a useful technology has an advantage in a particular situation, but for a lot of costs, requiring higher range are not applicable.

D, silicon anode materials is an important development direction, Panasonic new 18650 battery has already started the commercial process of this kind of material. But how do you pursuit performance and battery industry for the general micron grade of the material requirements to achieve a balance in nano? It is still a more challenging job.

The diaphragm

For power battery, large current work on its safety, life provides a higher request. Diaphragm is not open around coating technology, ceramic coating because of its high safety, can consume diaphragm character such as impurities in the electrolyte is rapidly away, especially for the ternary battery safety of the promotion effect is particularly significant. Ceramic membrane is mainly used in system is to put the alumina particles coated in conventional diaphragm surface, is new, it is a solid electrolyte fiber is coated on the diaphragm, so a lower internal resistance of the diaphragm, the mechanics of fiber for the diaphragm support effect is better, and in the process of serving the tendency of congestion on the diaphragm hole is lower. After coating of the diaphragm, good stability, even if the temperature is higher, it's not easy contraction deformation cause a short-circuit, Tsinghua university institute of materials member of policy, technical support team of Jiangsu south Qing Tao energy companies have some representative works in this aspect, the diaphragm as shown in the figure below.

The diaphragm coated solid electrolyte fiber

The electrolyte

Electrolyte for quick charge has great influence on the performance of lithium ion batteries. In quick charge battery to ensure the stability and security of the shed, the electrolyte should satisfy the following features: A) cannot be separated and B) higher conductivity, C) for the cathode material is inert, not response or dissolved. If you want to meet these requirements, the key is to use electrolyte additives and function. Such as ternary quick charge battery safety by its influence is very big, must to join all kinds of high temperature resistant, flame retardant, prevent overcharge additives to protect, to a certain extent, improve its security. And lithium titanate battery problem, the high temperature bilge gas, improved function, but also depends on the high temperature electrolyte.

Battery structure design

A optimization strategy is a typical laminated type VS coiling, rather then parallel relationship between laminated type battery electrodes, coiling are so series, and therefore the former internal resistance is much smaller, more suitable for power occasion. Also can be in very ear number fluctuation Kongfu, solve the problem of internal resistance and heat dissipation. Besides using high conductivity electrode materials, using more conductive agent, coating and thinner electrode are also can consider the strategy.

In short, affects the mobile and embedded inside the battery charge electrode cavity rate factor, will affect the lithium battery quick charging ability.

Mainstream manufacturers quick charge technology overview

CATL

Developed for the anode, Ningde era "super electronic network technology, makes the lithium iron phosphate has excellent electronic conductivity; In anode graphite surface, adopted the "fast ionic ring" technical modification, after modification of graphite/super quick charge and the characteristics of high energy density cathode no longer appear when quick charge excessive by-products, make its have 4 to 5 c fast filling capacity, achieve 10-15 minutes fast charge, and can guarantee the system level the energy density of 70 wh/kg above, to achieve the cycle life of 10000 times (saying that life is pretty high).Thermal management, its thermal management system, fully recognize the fixed chemical system under different temperature and SOC "health charge interval", greatly broaden the operating temperature of the lithium battery.

Walter m

Walter is not very good recently, let's just theory of technology. Walter the smaller particle size of the use of lithium iron phosphate, common on the market at present the lithium iron phosphate particle size between 300 ~ 600 nm, and walter m only 100 ~ 300 nm, lithium iron phosphate lithium ion will have faster migration, to a greater ratio of current charge and discharge. On the battery outside the system, we should strengthen the thermal management system design and system security.

Micro macro power

Early, micro macro power chose to withstand with spinel structure quick charge high current, lithium titanate + for porous carbon composite anode materials; In order to avoid a quick charge when high power electric current of battery safety threat, micro macro dynamic combination does not burn the electrolyte, high porosity, high permeability diaphragm technology and STL intelligent thermal control fluid technology, guarantee the safety of the battery when quick battery charge.

In 2017, it released a new generation of high energy density batteries, with high capacity high power lithium manganese acid anode materials, monomer energy density reached 170 wh/kg, 15 minutes to achieve fast charge, targeted at both life and safety.

Zhuhai silver long

Lithium titanate cathode, wide working temperature range and is famous for its large charge-discharge rate, specific technical scheme, there is no clear data shows. Show talking to staff, said its quick charge has been achieved 10 c, life 20000 times.

The future of quick charge technology

The electric car is quick charge technology, the direction of history or a flash in the pan mischiefs, actually now opinions vary, no conclusion. As an alternative to solve the range anxiety, it with battery energy density and the overall transport costs on a platform to consider.

Energy density and fast filling performance, at the same battery only, can be said to be the incompatibility of two direction, "she said. The pursuit of the battery energy density, now is the mainstream. When the energy density is high enough, a car loading capacity is large enough, enough to avoid the so-called "range anxiety", will reduce the demand of the battery charging properties; At the same time, the capacity is big, if the battery KWH cost is not low, so whether to KeDingKeMao buy enough to "anxiety", requires people to make a choice, such a thought, quick charge is the value of existence. Another point of view, is the fast charging facilities cost mentioned yesterday, this is, of course, is part of a push electric cost of the whole society.

Standing back pain is a summation, quick charge technology would be able to get large extension, energy density and fast filling technology who is developing fast, two technology who fall too malicious, cost reduction might play a fairly crucial role for its future prospects.

The page contains the contents of the machine translation.