A lithium ion battery has the following structure and uses an electrolyte in which a lithium salt such as LiPF6 is dissolved in a solvent such as propylene carbonate (PC).
The active material of the negative electrode is LiC6 (Li covered with graphite), and the current collector is Cu.
The active material of the positive electrode is CoO2, and the current collector is immersed in Al.
A separator is an insulator that prevents fire caused by a short circuit between the positive and negative electrodes.
There is a hole through which Li can pass.
■Basic operation of lithium-ion batteries
＜Discharge＞
① Li at the negative electrode becomes Li+ and dissolves into the electrolyte.
② Electrons leaving Li pass through the external circuit and move to the positive electrode, and at the same time, Li crosses the separator and moves to the negative electrode.
③ Electrons and Li+ attach to CoO2 at the positive electrode.
The negative electrode loses electrons, resulting in an oxidation reaction; the positive electrode gains electrons, resulting in a reduction reaction. (Reference: Redox reaction)
＜Charge＞
When an external power source is connected as shown below, a reaction opposite to discharge occurs, and electrons flow to the negative electrode side.
This is charging. Voltaic batteries or Daniel batteries cannot be recharged and are called primary batteries, but rechargeable batteries such as lead batteries or lithium-ion batteries are called secondary batteries.
＜Electromotive force＞
The electromotive force can be calculated from the standard electrode potentials of LiC6 and LiCoO2. LiC6 is -2.9V and LiCoO2 is 0.9V, so the difference between the two, 3.8V, is the electromotive force.
■Charge/discharge characteristics
＜Effects of temperature＞
When the battery temperature is low, molecules move more slowly and reactions are less likely to occur, resulting in a lower output voltage. When the temperature becomes too low (below 0°C), Li cannot be incorporated into the graphite at the negative electrode and Li crystallizes alone, causing a decline in battery performance.
As the battery temperature increases, the molecules become more active and the output voltage increases. However, if the temperature becomes too high (approximately 45 degrees Celsius or higher), deterioration will be accelerated, as will be explained later.
Below are examples of battery discharge characteristics and current/voltage control during charging.
This charging method is called constant current-constant voltage (CCCV), and in order to charge efficiently, a large amount of current is applied initially, and the current is reduced as charging progresses.
If the current decreases even if the voltage is kept constant, it means that polarization is progressing, and if a large amount of current is passed in this state, the temperature will rise, causing deterioration or fire, so this is prevented.
＜Effects of overcharging and overdischarging＞
When overcharged, Li cannot be incorporated into the graphite at the negative electrode and Li crystallizes on its own.
When overcharging continues, Li crystals grow (called dendrites), break through the separator and reach the positive electrode, causing a short circuit (dendritic short) and ignition.
When overdischarged, the negative electrode becomes depleted of Li, and the Cu of the negative electrode current collector is decomposed, making it impossible to maintain Li even if the battery is charged.
■Factors causing battery heat generation
The main causes of heat generated when charging and discharging batteries are entropy heat generation and polarization heat generation.
＜Heat generation during polarization＞
Polarization heat generation refers to the heat loss caused by the increase in internal resistance of the battery due to polarization.
Factors that change internal resistance include temperature and battery capacity (SOC).
When a battery is discharged, it generates heat by the amount of internal resistance, but when it generates heat, the internal resistance decreases, so the amount of heat generated is reduced accordingly.
Also, when charging, the internal resistance increases as the SOC increases, and when discharging, the internal resistance increases as the SOC decreases.
＜Heat generation due to entropy＞
This is the amount of heat generated during chemical reactions during charging and discharging. The reaction is exothermic when discharging, and the reaction is endothermic when charging. The larger the amount of reaction, the more heat is generated. Although polarization generates heat during charging, the endothermic reaction can suppress the temperature rise.
■Cause of deterioration
Deterioration is caused by the following factors.
＜Storage deterioration＞
① If stored for a long time with a high charge, the total amount of active Li ions will decrease due to oxidation of Li in the negative electrode.
If the amount of charge is low, there will be a lot of Li on the positive electrode, but this is not a problem.
This is because the Li in the negative electrode is covered with graphite and exists as a single Li, so chemical reactions (oxidation) are likely to occur, but the Li in the positive electrode is a compound, so the Li in the negative electrode This is because it has a structure that is less susceptible to chemical reactions.
Storing at high temperatures is also not a good idea as it accelerates oxidation reactions. The characteristics of deterioration with respect to temperature can be expressed by the Arrhenius equation, and it is said that the rate of deterioration doubles when the temperature increases by 10°C.
② As explained above, storage in an over-discharged state also causes deterioration as Cu in the negative electrode is decomposed.
Based on the above, the storage method to suppress deterioration is to keep the charge level low (approximately 30% capacity) and store it in a low temperature place such as a refrigerator (if it is too low, it will freeze and destroy the structure, so it is not good) is good.
＜Cycle deterioration＞
Cycle deterioration is a condition in which Li ions decrease or cannot be exchanged due to the following phenomena when charging and discharging are repeated.
① Li ions receive electrons at the negative electrode, are reduced, and dissolve into the electrolyte, reducing the total amount of active Li ions.
② The film (SEI: Solid Electrolyte Interphase) formed by the reaction of the electrolyte on the negative electrode surface becomes thicker, making it difficult for Li ions to pass through.
The empirical rule used to express deterioration characteristics is called the root law, which states that battery capacity is proportional to the square root of the number of charge/discharge cycles.
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