button cell battery cr1620

Internal Mechanism and External Factors of Life Attenuation of button cell battery cr1620

Analysis of Internal Mechanisms Affecting Life of Lithium Batteries

Lithium batteries convert chemical energy into electrical energy through normal chemical reactions. In theory, the reaction inside the battery is the redox reaction between the positive and negative electrodes. According to this reaction, the ion deintercalation and current are generated, so the lithium ion concentration usually does not change. However, in actual battery cycles, lithium ions undergo many side reactions in addition to normal reactions, such as the formation and growth of SEI membranes and the decomposition of electrolytes. Any reaction that can produce or consume lithium ions will destroy the balance inside the battery. Once the balance changes, it will have a serious impact on the battery. The internal factors of the battery that cause the capacity of lithium batteries to decrease and the life of lithium batteries to decrease are as follows:

1. Changes in positive electrode materials

The dissolution and structural changes of positive electrode materials are important reasons for the changes in positive electrode materials. As the number of battery cycles increases, lithium ions are constantly deintercalated in the positive electrode, causing the lattice volume of the positive electrode material to expand and shrink, causing the structure of the positive electrode material to change, and reducing the ability of the positive electrode material to deintercalate lithium ions. The dissolution of positive electrode materials generally occurs under deep discharge conditions. The dissolution of the positive electrode material will generate a solid film attached to the surface of the positive electrode material, which hinders the insertion and extraction of lithium ions, which will also cause the battery capacity to decay.

2. Electrolyte decomposition

The positive electrode will generally decompose into insoluble products such as lithium fluoride. These insoluble products will block the pores, consume active substances, and decay the battery capacity. The decomposition voltage of the positive electrode is usually greater than 4.5V, so the electrolyte is not easy to decompose at the positive electrode. The electrolyte is not stable in the graphite layer, and side reactions are prone to occur, which reduces the lithium ion concentration inside the battery and causes the battery capacity to decay. Under charging conditions, the electrolyte is prone to reduction reactions at the negative electrode, which will cause the decomposition of the electrolyte and the reduction of the solvent. During the initial charge and discharge, a passivation film (SEI film) will be formed inside the battery to prevent further oxidation of the electrolyte and the negative electrode. These will have an adverse effect on the battery capacity.

3. SEI film formation and growth

SEI film is formed during the initial charge and discharge process of the battery. Its formation will consume a part of the active material. SEI film can protect the negative electrode of the battery, prevent the direct contact reaction between the electrolyte and the negative electrode of the battery, and prevent the loss of active lithium, thereby increasing the cycle life of the battery. However, in subsequent cycles, due to the continuous expansion and contraction of the electrode material, new positioning points are exposed. The SEI film will continue to grow with the exposure of the new positioning points, which will cause continuous loss of lithium ions. The macroscopic manifestation is a decrease in capacity.

4. Lithium dendrite formation

If the battery is charged at a current higher than its rated current for a long time or charged and discharged at low temperature, its negative electrode is prone to form metal lithium dendrites. This metal lithium dendrite can easily pierce the diaphragm, causing the positive and negative electrodes of the battery to directly contact each other, thereby causing an internal short circuit in the battery. This is a destructive failure for the battery, and lithium dendrites are difficult to detect before the battery short circuits.

5. Influence of inactive components

As the number of battery cycles increases, the binder will decompose inside the battery, which will cause the active materials inside the battery to fall off continuously, and the active materials participating in the positive and negative electrode reactions will continue to decrease. The current collector will also corrode after multiple charge and discharge cycles. The corrosion products will passivate the active materials and increase the internal resistance of the battery. The failure mechanism inside the lithium battery is mostly caused by the formation of lithium dendrites, changes in positive electrode materials and decomposition of electrolytes. In particular, the formation of lithium dendrites is easy to cause short circuits and thermal runaway of the battery cells. If it is not well controlled, it will cause the battery cells to explode.

The failure research of lithium batteries is to study the failure modes and mechanisms of batteries, optimize the batteries and improve the safety of batteries. Therefore, the failure research of batteries can not only have important guiding significance for actual production and operation, but also have vital significance for improving battery life, the safety and reliability of electric vehicles, and reducing the cost of electric vehicles.

Analysis of external factors affecting the life of lithium batteries

Studies have shown that the external factors that affect the capacity decay and life decay of lithium batteries include temperature, charge and discharge rate, etc., which are all determined by the user's usage conditions and actual working conditions. The following external factors that affect battery aging are the most common.

1. Depth of discharge DOD

The depth of discharge refers to the percentage of the amount of electricity discharged from the full state of the battery to the rated capacity of the battery. When the battery is discharged to the cut-off voltage, the battery discharge rate is 100% DOD, and 60% DOD means that the battery is between 100% SOC and 40% SOC. The greater the depth of battery discharge, the greater the amount of electricity discharged. Studies have shown that when the battery is used under the conditions of DOD (20%~80%), the increase in the AC internal resistance of the battery during the charging and discharging process is relatively small, and deep discharge will increase the internal resistance of the battery, thereby reducing the service life of the battery.

2. Overcharge

When the battery is overcharged, the negative electrode lithium ions have already reached saturation, but continuing to embed lithium ions will cause lithium ions to deposit on the surface of the negative electrode, blocking the porous material of the negative electrode, making it more difficult for lithium ions to be deintercalated, and also reducing the concentration of lithium ions in the battery. The macroscopic manifestation is the loss of battery capacity. Overcharging will cause the battery voltage to rise. When it is higher than a critical value, the electrolyte will oxidize to generate insoluble substances and gas. The insoluble substances will block the pores of the porous material and reduce the ion transfer rate. Generally, the battery is prevented from overcharging by setting the charge cut-off voltage and charge cut-off current. Regardless of whether it is a nickel-hydrogen battery or a lithium battery, when overcharging occurs, the heat energy converted by the current will be dissipated in large quantities, resulting in many reactions inside the battery, such as reactions between the positive and negative electrodes and the electrolyte, which reduces the maximum capacity of the lithium battery. When the heat accumulates and is not easy to dissipate, it may even cause fires and explosions.

3. Self-discharge

Lithium-ion button cell battery cr1620 will self-discharge. Usually, self-discharge manifests itself as battery capacity loss. Most self-discharges are reversible, but there is still irreversible self-discharge. There are many reasons for irreversible self-discharge, such as loss caused by irreversible reactions of lithium ions, insoluble substances generated by oxidation reactions of the electrolyte blocking micropores to increase internal resistance, and the rise of SEI membranes. Chemical reactions such as these will reduce the lithium ion concentration inside the battery, thereby causing capacity decay.

4. Ambient temperature

Lithium batteries will cause changes in battery performance at too high or too low temperatures. Too low temperature will affect the activity of the electrolyte inside the battery, reducing the battery's charging and discharging efficiency. Too high temperature will destroy the chemical balance system inside the battery. The battery will also undergo many irreversible side reactions at high temperatures, causing the battery's electrode structure to deform, reducing the battery capacity and reducing the number of battery cycles.

5. Pressure

Pressure. In order to facilitate the diffusion of lithium ions inside the battery, the diaphragm and positive and negative electrodes of the lithium battery usually have a porous structure, and pressure will have a certain effect on the porosity and tortuosity of the porous material. Therefore, mechanical pressure will indirectly affect the diffusion rate of lithium ions between the positive and negative electrodes and the diaphragm, thereby affecting the discharge performance of the lithium battery. If no pressure is applied, the battery will be difficult to fix, but too much pressure will make the distance between the negative electrode graphite layers too small, resulting in an increase in the interlayer van der Waals force, an increase in the insertion resistance of lithium ions, and a corresponding decrease in the number of embedded lithium ions, which will cause a decrease in capacity. Therefore, research on battery pressure is very necessary.

References:

Zeng Yunlu. Hunan University. Study on the influence of pressure on the performance and life of soft-pack lithium batteries

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