In the field of battery industry, the charge and discharge rate is usually used to describe the relationship between charging speed and current size, such as 1 hour to fill the battery rate is called 1C, only 30 minutes rate is called 2C, and so on, more than 1C can be called fast charging. Nowadays, the charging rate of lithium-ion batteries can generally do 1C-3C, the highest can probably go to 5C, but compared to the often 10C discharge rate is still very far from natural.
In addition to the maximum charge rate has a bottleneck, different SOC (State of Charge, that is, the state of charge, that is, the remaining power) under the battery can withstand the charging rate is also different. Generally speaking, the charging rate will follow the rhythm of slow-fast-slow. Generally when the SOC reaches 90% or more, the internal resistance of the battery will rise significantly, making the charging rate slow down.
So if you are an electric car user, if you want to save charging time as much as possible, try not to use the power to less than 10%, and charging does not have to be full, it is enough to reach 90% or more, or to meet the mileage you need for the next trip. In addition to the bottleneck of the battery itself, the peripheral charging equipment also has its own limitations.
In theory, it is true that you can increase the charging speed by increasing the current. But the current is too large, the battery internal lithium ion diffusion rate can not keep up with the electron diffusion rate, it will lead to electron-ion transport disconnect, affecting the battery performance, the ability to achieve the charging capacity is correspondingly reduced, the battery life is even more miserable, and even the risk of fire and explosion.
Therefore, in general, in the case of not in a hurry, we recommend using slow charging as much as possible, which is conducive to extending the life of the battery, and lithium batteries are safer to charge slowly.
When charging, the rate of diffusion of lithium ions inside the lithium battery, and temperature, cathode material and structure are closely related.
First of all, the temperature, generally speaking, the higher the temperature is naturally the faster the diffusion rate, but the temperature is too high will also lead to reduced battery life, charging safety and other issues. The same does not work if the temperature is too low, the temperature is too low, the deposition of lithium metal in the battery will occur, thus causing a short circuit inside the battery, especially lithium iron phosphate batteries. General 0 ℃ when the capacity of lithium iron phosphate battery is only about 60 to 70%, to -20 ℃ will be left with a poor 20 to 40%. So in the cold climate of the northern winter, electric vehicles must have to give the battery module heating function, thus power consumption is naturally faster.
Next is the material, different materials diffusion capacity gap is very large, like lithium cobaltate, lithium manganate, lithium iron phosphate, NCM, NCA, etc. are very good performance of the anode material, and the latter two are also the best performance, the application of the popularity of the two materials. This is also an important reason why today's lithium batteries are named after the cathode material.
The basic working principle and structure of lithium-ion battery
The basic principle of the battery: reduction reaction occurs at the positive electrode, gaining electrons; oxidation reaction occurs at the negative electrode, losing electrons. Electrons pass through the load and flow from the negative electrode to the positive electrode, forming a current in the direction from the positive electrode to the negative electrode.
1 (+1) valence lithium ion < ------ (1-x) (+1/(1-x)) valence lithium ion + x (+1) valence lithium ion + x electrons
Let x = 0.5 and obtain.
1 (+1) valence lithium ion < ------0.5 (+2) valence lithium ions + 0.5 (+1) valence lithium ions + 0.5 electrons
Multiplying both sides by 2 gives
2 (+1) valence lithium ions < ------1 (+2) valence lithium ions + 1 (+1) valence lithium ion + 1 electron
Further simplification.
1 (+1) valence lithium ion < ------1 (+2) valence lithium ion + 1 electron
This formula actually describes the overall reaction, not the individual reactions. In simple words.
The (+1/(1-x)) valence of the positive electrode (where, 0
The lithium atoms at the negative electrode lose electrons and are oxidized to (+1) valence lithium ions, which flow from the negative electrode into the load circuit, and the lithium ions flow through the electrolyte to the positive electrode;
Back to the basic principle of the battery, right? The core of the positive electrode is the (+1/(1-x)) valence lithium ion, and the core of the negative electrode is the lithium atom, which reacts to produce the (+1) valence lithium atom, and the electron flow in the redox reaction forms the current.
In reality, when making a battery, you always need a substance to carry the lithium ion of the positive electrode and the lithium atom of the negative electrode, just like goods always need shelves. Then the shelf of lithium ion is cobaltate ion, which together with lithium ion constitutes the positive electrode; the lithium atom of the negative electrode is composed of materials such as graphite with holes, so that the negative electrode is not reacted out after the reaction. Between the positive and negative electrodes are electrolytes and diaphragms, which are used for both lithium ion flow and for isolating the positive and negative electrodes to prevent internal short circuits.
Why talk about the basic working principle and structure of lithium-ion batteries? Later to talk about the lithium battery charging, discharging as of voltage and overcharge, overdischarge hazards will be used.
Lithium-ion battery characteristics
The most important characteristic of lithium-ion battery that users are concerned about is the electric capacity, such as the commonly used 2000mAh, which refers to the number of charges that can be discharged from a lithium battery under normal working conditions. Let's look at the specifications of a lithium-ion battery.
Some of the more important parameters of this battery.
Capacity : 2500 mAh
Charge cut-off voltage: 4.2 V
Discharge cut-off voltage: 2.5 V
Maximum charge current: 4000 mA
Maximum discharge current: 20,000 mA
In short, it's all about battery capacity and charging and discharging. Battery capacity depends on how many electrons can be released from the negative electrode and how many electrons can be absorbed by the positive electrode.
Why is there a charge cutoff voltage, in other words, what happens after overvoltage charging? As mentioned earlier when describing the structure of a lithium-ion battery, the negative electrode is composed of graphite and lithium atoms. In fact, lithium does not exist in atomic form, but coexists with graphite in the form of lithium ions. After over-voltage charging, lithium ions will precipitate as crystalline lithium, which cannot participate in charging and discharging, resulting in a reduction in battery capacity.
Why is there a discharge as of voltage, in other words, what is the problem after over-discharge? After over-discharging, a large amount of lithium ions in the negative electrode flow to the positive electrode, leading to graphite void and collapse in some areas, which can no longer store lithium ions, also leading to a reduction in battery capacity.
Specifically for a lithium battery, its capacity also varies at different discharge currents and temperatures, and decreases with the increasing number of charge and discharge cycles. The following is the relationship between temperature and battery capacity of a model of lithium-ion battery.
What exactly limits the charging rate of lithium batteries?
It boils down to materials and technology, as mentioned earlier. Lithium ions do not exist in the form of atoms and need to coexist with graphite. Lithium batteries are fully charged process is the lithium ion wandering between the positive and negative electrodes by carrying electric ions and releasing electric ions to achieve the function of the lithium battery storage electricity. These are the need for a certain reaction time, too fast charging will cause the lithium battery to react abnormally to produce crystallization, and in charging if more than the battery tolerance charging speed, it will make the lithium battery internal resistance increases, thus making the battery produce too high a temperature danger.
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