Principles of charging and discharging lithium-ion batteries

The change around the electrode during discharge is a schematic diagram of lithium ion insertion and migration during discharge. In the negative electrode, there are lithium ions between the carbon layers, and the energy of the negative electrode is higher than that of the positive electrode. When there is an external load, the lithium ions of the negative electrode release electrons and migrate to the positive electrode with low energy. Lithium ions extracted from the negative electrode migrate to the positive electrode through the electrolyte and the pores of the diaphragm, and are embedded in the positive electrode active material of the layered structure. At the same time, electrons are received, and lithium ions are fixed and become stable. If over-discharged, lithium ions accumulate in the positive electrode too much, which will increase the internal resistance, heat the battery, and cause rapid deterioration. It can be seen from Figure 1 that the load current (battery capacity) is almost determined by the number of movable lithium ions. The electrons pass through the active material of the current collector and reach the external terminal. The current collector of the positive electrode is aluminum, and the current collector of the negative electrode is copper. The reason for this is: under the respective potentials of the positive and negative electrodes, aluminum and copper are metals that will not be doped (infiltrated) by lithium ions. When charging, an external voltage is applied to the external terminals, forcing a reaction opposite to the discharge reaction. As a result, the lithium ions of the positive electrode release electrons, which migrate to the negative electrode through the electrolyte under the use of an electric field, and are embedded in the active material of the negative electrode. At the same time, electrons are received, and lithium ions are fixed by the negative electrode active material. Lithium ions quickly migrate in the electrolyte, slow down on the surface of the negative electrode, and diffuse very slowly inside the negative electrode active material. This is similar to the process of a car leaving the highway, entering an ordinary highway, and then driving into the street near your home. When charging, lithium ions are congested on the surface of the negative electrode. When the battery is degraded during charging, the organic solvent used as the electrolyte is decomposed in the positive electrode and reacts with lithium ions on the surface of the negative electrode to form a solid electrolyte interface membrane (SEI). Therefore, the number of lithium ions that migrate is reduced, resulting in a decrease in battery capacity. When charging, deliberately create this state on the surface of the negative electrode that allows chemical reactions to easily occur. This is also related to the battery degradation related content mentioned later. In addition, overcharging causes excessive accumulation of lithium ions in the negative electrode, increasing internal resistance and heating the battery, which will cause rapid deterioration. The relationship between SOC and voltage OCV is determined by the materials constituting the battery. The relationship between the charge-discharge curve and OCV when charging and discharging with a weak current of 0.02C is shown. The horizontal axis represents SOC, and the vertical axis represents voltage. If charging and discharging with a weak current, the terminal voltage is only I×R higher than the OCV (charging current×battery internal resistance). When discharging, the terminal voltage is lower than OCV by I×R (discharging current×battery internal resistance). From the SOC point of view, the average value of the charging voltage and the discharging voltage is almost the same as the OCV. It can be seen that where the SOC is high, the OCV is also high; where the SOC is low, the OCV is also low. Moreover, the relationship between OCV and SOC is almost irrelevant to temperature changes and is stable. Therefore, to manage the upper and lower limits of SOC is to manage the upper and lower limits of OCV. Control method of overcharge and overdischarge When charging, the terminal voltage is higher than OCV. If the terminal voltage is controlled below the upper limit voltage, overcharge can be prevented. When discharging, the terminal voltage is lower than OCV. If the terminal voltage is controlled above the lower limit voltage, over-discharge can be prevented. The upper and lower limits of OCV and the slope of the OCV-SOC curve vary according to the different cathode materials used. Generally speaking, a positive electrode material containing nickel has a large capacity, a positive electrode material containing manganese has a high voltage, and a positive electrode material using iron phosphate has a low voltage and a low slope (nearly flat) change trend. Electronic circuit modeling of internal resistance. The current inside the battery originates from the migration of lithium ions: when it migrates quickly in the electrolyte, the resistance of the electrolyte; when it migrates on the electrode surface, the surface resistance and surface capacitance of the millisecond reaction time, and time-consuming Diffusion resistance and diffusion capacitance that react slowly from 30 minutes to several hours. The surface capacitance of the electrode is also called electric double layer capacitance, and the principle is the same as that of an electric double layer capacitor (EDLC). Figure 4 is an equivalent circuit model that fits each electrochemical reaction to the transient response of the circuit, and fits the parameters of R and C to match the response time of the battery. This is an abstract model of an electrochemical reaction, which can be easily converted into an electrochemical reaction equation, so it is often used in battery control. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Analysis of common causes and types of explosions in lithium-ion batteries