Deeply understand the voltage of lithium ion battery

　　Voltage is an important parameter of lithium-ion battery, which directly determines the energy of the battery and the grouping mode of battery packs. This article summarizes the voltage of lithium ion battery, which is also the process of sorting out and learning my knowledge. Please criticize and correct any misunderstanding.

　　During the charge discharge test or actual use of lithium ion battery, the voltage parameters mainly include platform voltage, median voltage, average voltage, cut-off voltage, etc. The typical discharge curve is shown in Figure 1.

　　The platform voltage refers to the voltage value corresponding to the minimum voltage change and large capacity change. Lithium iron phosphate and lithium titanate batteries have obvious platform voltage, which can be clearly identified in the charge discharge curve. The voltage platform of most batteries is not obvious. During charging and discharging test, data are collected through voltage interval, then the voltage curve is differentiated, and the platform voltage is determined through the peak value of dQ/dV.

　　The median voltage is the voltage value corresponding to half of the battery capacity. For materials with obvious platform, such as lithium iron phosphate and lithium titanate, the median voltage is generally the platform voltage.

　　The average voltage is the effective area of the voltage capacity curve (i.e. the battery charge/discharge energy) divided by the capacity, and the calculation formula is |=π U (t) * I (t) dt/π I (t) dt. In the charge discharge test data, the average voltage is obtained by dividing the charge or discharge energy by the capacity data. Conversely, the battery energy density is also estimated based on the average battery voltage, that is, energy=capacity * average voltage/battery mass (or volume).

　　The cut-off voltage refers to the minimum voltage allowed when the battery is discharged and the maximum voltage allowed when the battery is charged. If the voltage is lower than the discharge cut-off voltage and the discharge continues, the positive electrode potential of the battery will continue to decrease, while the negative electrode potential will rapidly rise, forming excessive discharge. Overdischarge may cause damage to the active substance of the electrode, loss of reaction ability, and shorten the battery life; It will also lead to the decomposition of the negative copper foil and precipitation in the positive electrode, with the risk of short circuit. If the charging voltage is higher than the charging cut-off voltage, the potential of the battery positive electrode will continue to rise, causing excessive delithiation of the positive electrode material, crystal structure damage and failure, and the electrolyte will decompose and lose lithium ions. However, the negative electrode potential will continue to decline, excessive lithium insertion, graphite layer disintegration, lithium precipitation on the surface of the electrode plate and other problems.

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