1. the structure of lithium battery
Lithium battery is a kind of rechargeable battery, which generally uses lithium-containing materials as electrodes, and mainly relies on lithium ions to move between the anode and the cathode to work. The internal materials of the battery are positive and negative electrodes, diaphragm and electrolyte.
1.1 anode and cathode
In lithium battery, the positive electrode material (such as LFP and NCM) is coated on aluminum foil (current collector), and the negative electrode material (such as graphite and LTO) is coated on copper foil (current collector).
1.2 diaphragm
There is a Separator between the positive and negative electrodes, which isolates the positive and negative electrodes, prevents electrons from passing through, and enables lithium ions to pass through smoothly at the same time.
1.3 electrolyte
The electrolyte plays a role in conducting lithium ions in the battery. In the process of battery discharge, Li+ passes through the diaphragm from the negative electrode to the positive electrode, and electrons return from the negative electrode to the positive electrode through an external circuit to form a current. The charging process of the battery is just the opposite. The following figure shows the internal structure of lithium battery.
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2. Working principle and attenuation of lithium battery
If you compare the positive electrode to "factory", the negative electrode to "apartment" and Li+ (lithium ion) to "employee". Then discharge is the process in which employees go to work in the factory from the apartment to release energy, and charging is the process in which employees go back to the apartment after work to rest and replenish energy. From this metaphor, we can imagine that whether it is the reduction of factory jobs, or the disrepair of apartments, or the loss of employees, it will eventually lead to the overall decline.
The author puts forward another analogy model: if the battery is like a city, then the positive state of the battery reflects the jobs in the city, the negative state of the battery represents the residential apartment in the city, and Li+ is the employed person in the city. Then we can preliminarily understand the possible causes of battery attenuation through this analogy model:
2.1 capacity attenuation
It is equivalent to the decline of the GDP of the city, which may be due to the reduction of employment, the high living cost or the deterioration of the living environment, and the loss of employed people. Corresponding is the reduction of positive and negative active materials and the reduction of movable Li+.
2.2 internal resistance increases
It is equivalent to the low work efficiency in the city, which may be due to the great administrative resistance of the government, or the paralysis of the transportation system, which leads to the high commuting cost of employees, as well as the unreasonable urban planning, and the farther and farther away from the place of residence and work. That is, the ohmic resistance of the battery increases, the conductivity decreases, and the movement path of Li+ deteriorates.
2.3 large self-discharge
It is equivalent to an increase in the proportion of unemployed people in cities, which takes up the resources of cities but does not create benefits. That is, the abnormal loss of Li+ and the internal short circuit of the battery.
3. Cause analysis of attenuation
3.1 anode and cathode materials fall off and age
During the continuous charging and discharging process of the battery, the positive and negative electrodes will contract and expand continuously, which will inevitably lead to the shedding of the positive and negative electrode materials on the current collector, which will reduce the number of lattice that can be embedded with Li+, thus affecting the capacity of the battery. With the increase of usage, this is inevitable.
3.2 lithium precipitation (overcurrent, low temperature)
When the battery runs beyond the acceptable rate current, a large amount of Li+ can't be embedded in the electrode, which leads to a large amount of Li+ accumulation on the electrode surface, and finally forms metallic lithium dendrites on the electrode surface. Li+ decreases, and the battery capacity naturally decreases.
3.3 diaphragm damage (high temperature)
First of all, once there is metallic lithium dendrite on the surface of the battery electrode, it may pierce the diaphragm and cause the short circuit of the positive and negative electrodes. In addition, the diaphragm will decompose and shrink in high temperature environment, which will also cause short circuit. Once the anode and cathode are short-circuited, the electrons can reach the anode without external circuit, and the whole electrochemical reaction of the battery will be out of control, resulting in overcurrent and overtemperature, which will further damage the battery and cause more serious problems such as thermal runaway.
3.4 Loss and decomposition of electrolyte (overvoltage and undervoltage)
The formula of the electrolyte is determined according to the voltage working range of the battery, so unreasonable use of the battery (over-voltage and under-voltage) will also cause the decomposition of the electrolyte. For example, when the battery is overcharged, that is, the valence of the positive electrode material increases (too much electrons are lost). At this time, the positive electrode material has strong reducibility, and it is easy to get electrons by reacting with the electrolyte. The reaction not only consumes the electrolyte and the positive electrode material, but also generates the hidden danger of mechanical deformation and liquid leakage caused by gas.
The premise of ensuring the long life, safety and stability of the battery is to keep the electrochemical reaction controllable and orderly. However, once the reasonable conditions of use are exceeded, when over-voltage, under-voltage, over-temperature, over-current, mechanical damage and so on occur, the attenuation of the battery will be significantly aggravated, and more serious safety problems will be caused.
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