What factors Influenced the Life of Lithium Battery

Lithium-ion power cell monomer can not meet the power demand of electric vehicles. Therefore, electric vehicles must use multiple cells for series and parallel power batteries to provide power for electric vehicles. Judging from the current level of lithium ion power battery monomer manufacturing process, various factors in the production process have caused the same type of monomer battery to have deviations in voltage, capacity, and internal resistance. The performance of a power battery depends on the performance of the cell monomer, but it is by no means a simple accumulation of the performance of a single cell. Because of the inconsistent performance of individual cells, power batteries are used repeatedly in electric vehicles, and power batteries cause various problems, resulting in a shorter service life.

Due to the current bottleneck of lithium-ion power cell technology, it is very important to study the factors that affect the service life of power cells and solve related problems to extend the service life of power cells. This will help the development of lithium-ion power batteries in electric vehicles and improve the efficiency of lithium-ion power batteries.

Factors affecting the service life of power cell monomer

For power batteries used in electric vehicles, the end of their useful life is defined as the battery's decay of 20 % of its initial capacity. When the power cell's service life is repeatedly charged and discharged on electric vehicles, the nature of the battery body material will decline due to the continuous occurrence of the side reaction inside the lithium ion battery. This decline is due to the following aspects: changes in the lattice structure of electrode materials; Decomposition, peeling or corrosion of electrode material causes the reduction of active material; The decrease of conductivity and increase of impedance caused by the decomposition consumption of electrolyte; Deem-bedded lithium ions are consumed due to negative polar lithium or side reactions; Impedance increases due to the modification of gases, insoluble substances and binders produced by side reactions and the corrosion of concentrated fluids.

From the actual use of environmental conditions, the main factors that affect the service life of power cell monomers include charge-discharge cut-off voltage, charge-discharge doubling rate, operating temperature, and shelving conditions.

There has been a lot of literature to show that the cycle life of different charging cut-off voltage is shorter with the higher charging voltage. This shows that the impact of charging off voltage on battery life is very large. High charging cut-off voltage will aggravate the occurrence of the battery side reaction and shorten the battery life. When the power battery is used in the entire vehicle, the performance of the battery is seriously reduced when charging and discharging in the higher potential area due to the various driving conditions of the electric vehicle.

The power battery uses different charge and discharge ratio to meet different driving conditions during the use of electric vehicles. The study of the power cell ploidy charge and discharge shows that the high-ploidy charge and discharge will accelerate the attenuation of the battery capacity, and the larger the charge and discharge ratio, the faster the battery capacity decay. This is mainly due to the changes in the structure and properties of the positive electrode material and the thickening of the negative surface film resulting in the difficulty of lithium ion diffusion. If the charge and discharge ratio is too large, it may also cause the monomer battery to overheat and short-circuit to cause an explosion.

Different power cells have different optimal operating temperatures, and excessive or low temperatures have an impact on the battery's service life. With the decrease of temperature, the discharge capacity of lithium-ion power cells will be reduced. This is because with the decrease of temperature, the Ionic conductivity of the electrolyte decreases, causing the internal resistance of the battery to increase rapidly, resulting in poor output performance of the battery at low temperatures.

Under the condition that the power battery is not in use, self-discharge, passivation of positive and negative electrode materials, and electrolyte decomposition will occur due to the nature of the battery itself. The experimental results show that the unstable performance of negative electrode SEI will lead to the rapid decline of negative electrode active materials.

Lithium metal precipitation is easy to produce, and lithium batteries that form stable SEI membranes can be stored at high temperatures for more than 4 years. At the same time, different electrolyte components have different effects on the degradation of electrode materials.

Effects of monomer incoherence on power batteries

The inconsistency of battery monomers is mainly generated during the manufacturing process. Due to the level of the process, there are slight differences in the thickness of the battery plates, the micro-pore ratio, and the activation degree of the active substances. Inconsistency in the internal structure of the battery makes it impossible for the voltage, capacity, and internal resistance of the same type of battery to be manufactured in the same batch to be completely consistent. The effect of the incoherence of the single cell on the service life of the power battery is divided into the incoherence of voltage, the incoherence of capacity, and the incoherence of internal resistance.

In the process of forming a single cell, if the voltage is inconsistent, the low-voltage cell will become a load of the battery when used together with the normal cell. Because when there are low-voltage batteries in two parallel batteries, cross-charging occurs, and other batteries will charge the battery. This connection method will make the capacity of the low-voltage battery increase slightly and the capacity of the high-voltage battery decrease significantly, and the energy loss will not achieve the ideal external output in mutual charging.

The initial capacity inconsistency has been greatly reduced before the battery is grouped, although the initial capacity of the single battery can be balanced by the individual charging method of the battery. However, the continuous charging and discharging cycle of electric vehicles makes this inconsistency magnify to some extent. The capacity varies with the rate of decay of the cycle. As the number of battery cycles increases, the difference in capacity will increase. This will cause the capacity of the single cell to increase the attenuation of the entire battery capacity.

Inconsistent internal resistance causes the voltage current distribution of the monomer cell in the battery to be uneven, and local over-voltage charging or under-voltage discharge occurs. The inconsistency of internal resistance will also cause the loss of heat in the discharge process of the monomer battery. The greater the internal resistance, the faster the temperature will increase, and it may eventually cause thermal runaway.

Voltage, capacity, internal resistance and other types of inconsistencies lead to differences in cell monomer life and battery life, mainly manifested in differences in temperature, charge and discharge ratio, discharge depth and available capacity. For example, due to the difference in the initial capacity of battery cells, most batteries are still under shallow discharge conditions, and low-capacity battery cells have been deeply discharged.

summary

There are many factors that affect the life of power batteries and interact with each other, resulting in a relatively serious decline in the performance of batteries. In particular, the performance of batteries will further decline in the absence of effective management and control under conditions such as high temperature, low temperature or overcharge and discharge. In addition, the series and parallel of a large number of monomer cells makes a single monomer with poor capacity and performance limit the performance of the entire battery pack, which in turn limits the full use of the battery pack.

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