Self-discharge & over-discharge of lithium-ion batteries.
With the further increase of energy density and the further reduction of cost, lithium-ion batteries have been widely used in electric vehicles and energy storage fields. The consistency of self-discharge and over-discharge of lithium-ion batteries is very important for the life and reliability of electric vehicles and energy storage systems. The research results of self-discharge and over-discharge of lithium-ion batteries in recent years reviewed from the aspects of formation mechanism, influencing factors and detection methods.
Self-discharge is a phenomenon of natural loss of battery capacity during storage, which generally shows that the open circuit voltage drops after storage for times. The self-discharge of lithium-ion batteries divided into physical self-discharge and chemical self-discharge according to reaction types. From the influence of self-discharge on battery, some customers divide self-discharge into two categories: self-discharge with reversible compensation for lost capacity and self-discharge with irreversible compensation for lost capacity. Under normal circumstances, the capacity loss caused by physical self-discharge is reversible, while the capacity loss caused by chemical self-discharge is irreversible.
Lithium-ion batteries are widely used as energy systems in various instruments and electric vehicles because of their pollution-free, high specific energy and long cycle life. The existence of self-discharge of lithium-ion batteries not only causes the loss of energy of the batteries themselves, but also shortens the life of lithium-ion batteries due to the inconsistency of self-discharge. The rapid decline of capacity leads to a large error in the prediction of battery state of charge (SOC) by battery management system (BMS), and the control strategy of electric vehicles fails, resulting in over-discharge of electric vehicle battery systems.
Causes of self-discharge of lithium ion batteries
1. Causes of reversible capacity loss: The cause of reversible capacity loss is the reversible discharge reaction, and the principle is consistent with the normal discharge reaction of the battery. The difference is that the normal discharge electron path is an external circuit and the reaction speed is very fast; electron path of self-discharge is electrolyte, and the reaction speed is very slow.
2. Causes of irreversible capacity loss: When an irreversible reaction occurs inside the battery, the capacity loss caused is irreversible capacity loss. The types of irreversible reactions including irreversible reaction between positive electrode and electrolyte, irreversible reaction between negative electrode material and electrolyte, irreversible reaction caused by impurities in electrolyte itself, and irreversible reaction caused by micro-short circuit caused by impurities during manufacturing.
Self-discharge rate is an important parameter to measure the life of lithium-ion battery, and the self-discharge process occurs inside the battery, which related to the battery material and technology, and changes with the changes of environmental temperature, storage time and state of charge. Rapid detection of self-discharge of lithium-ion battery can reduce the times of self-discharge parameter measurement and improve its accuracy. Self-discharge detection also applicable to battery pack, which can provide new theoretical data for battery consistency research and sorting in practical application, thus improving the performance of lithium-ion battery.
After the battery has discharged the stored electricity, it will cause over-discharge if the voltage continues to discharge after reaching a certain value. Usually, the discharge cut-off voltage is determined according to the discharge current. Over-discharge may bring disastrous consequences to the battery; especially the heavy current over-discharge or repeated over-discharge has a greater impact on the battery. Generally, over-discharge will increase the internal pressure of the battery, destroy the reversibility of the positive and negative active materials, and even if it can only partially recover the capacity after charging, it will obviously decrease.
In order to ensure that some lithium ions remain in the graphite layer after discharge, it’s Necessary to limit the minimum voltage of discharge termination. Lithium ion batteries cannot be over-discharged. The discharge termination voltage is usually 3.0V/ knot, and the minimum voltage cannot be lower than 2.5V/ knot. The battery discharge time related to the battery capacity and discharge current And the battery discharge time (hours) = battery capacity/discharge current; discharge current of lithium-ion battery should not exceed 3 times of the battery capacity, otherwise the battery will be damaged.
Influence of over-discharge of lithium ion battery.
1. The termination voltage value specified in the battery standard is the voltage value reached when the battery is continuously discharged; However, in the actual use process, the discharge is mostly intermittent, so even if the discharge reaches the specified termination voltage value, overdischarge often occurs.
2. After the battery is discharged to the termination voltage, let it stand for several minutes to half an hour, and the battery voltage will automatically rise. This misleads the user to think that the battery can still continue to discharge, thus causing the battery to overdischarge.
3. Cyclic treatment charging and discharging can improve the battery capacity compared with the previous one, but continuous excessive deep discharge will not only fail to further activate the active substances that did not participate in the reaction, but also cause positive grid corrosion and the transformation of a part of α-PbO2 to β-PbO2, which will inevitably shorten the cycle life of the battery. The deeper the discharge depth, the faster the battery capacity decreases, the more obvious the side effects of overdischarge and periodic treatment, and the shorter the cycle life of the battery.
At present, most of the batteries of electronic products use lithium-ion batteries. Lithium-ion batteries have been rapidly developed and widely used in society since they came out in 1990, and lithium-ion battery manufacturers have also achieved the largest development. Don't charge the lithium-ion battery to 100% full charge, let alone use it up. If the situation permits, try to keep the battery charge near half full, and the smaller the range of charging and discharging, the better.
