Power lithium battery life is affected by what factors

Lithium ion power battery monomer can not meet the power demand of electric cars, so electric cars must use multiple batteries in series and parallel to form the power battery pack to provide energy for electric cars. From the current level of lithium ion power battery monomer manufacturing process, various factors in the production process so that the same type of single battery will be in the voltage, capacity, internal resistance and other aspects of deviation. The performance of a power battery depends on the performance of a single battery, but it is by no means a simple accumulation of the performance of a single battery. Due to the performance inconsistency of single battery, when the power battery pack is used repeatedly in electric vehicles, various problems will occur in the power battery pack, which will shorten the service life.

Limited by the current development bottleneck of lithium ion power battery technology, it is very important to study the factors affecting the service life of power battery and solve relevant problems to extend the service life of power battery. This will contribute to the development of li-ion power batteries in electric vehicles and improve the efficiency of li-ion power batteries.

Factors affecting the service life of single battery

For power batteries used in electric vehicles, the end-of-life is defined as the battery attenuation reaching 20% of the initial capacity. When the service life of power battery is repeatedly charged and discharged in electric vehicle, the material properties of the battery will decline due to the continuous side reactions inside the lithium ion battery. This decline is due to the following aspects: the change of electrode material lattice structure; The degradation, peeling or corrosion of electrode materials results in the decrease of active materials. Decrease of conductivity and increase of impedance due to electrolyte decomposition consumption; Lithium ions that can be detachable are consumed due to negative electrode segregation or side reactions. Increase in impedance due to side reactions of gases insoluble substances and binder modification and collector corrosion.

From the point of view of practical environmental conditions, the factors affecting the service life of power battery mainly include cut-off voltage of charge and discharge, charge and discharge ratio, service temperature and shelving condition.

Many literatures have shown that the cycle life of different cut-off voltage of charging is shorter with the higher charging voltage within a certain range. This indicates that the cut-off voltage of charging has a great influence on the service life of the battery. High cut-off voltage will aggravate the battery side reaction and shorten the battery life. When the power battery is used on the whole vehicle, the battery is prone to decline due to various driving conditions of the electric vehicle, and the performance of the battery declines seriously when the battery is charged and discharged in a high potential area.

In order to meet different driving conditions in the use of electric vehicles, power batteries adopt different charging and discharging rates. The study on the charging and discharging rate of power battery shows that the higher the charging and discharging rate is, the faster the battery capacity attenuation will be. This is mainly due to the change of the structure and properties of the anode material and the thickening of the anode surface film, which leads to the difficulty of lithium ion diffusion. If the charge and discharge ratio is too large, it may cause the single battery overheating, short circuit and explosion.

Different power batteries have different optimal operating temperature, too high or too low temperature will affect the service life of the battery. As the temperature decreases, the discharge capacity of li-ion battery will decrease. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, which causes the internal resistance of the battery to increase rapidly, resulting in poor output performance of the battery at low temperature.

Power batteries in the use of the condition, because the nature of the battery itself will occur self-discharge, positive and negative material passivity, electrolyte decomposition, etc. Some experimental results show that the instability of the negative SEI will lead to the rapid decay of the negative active material, and

Lithium metal is easy to precipitate, and the lithium battery with stable SEI film can be stored at high temperature for more than 4 years.

The impact of monomer inconsistency on the power battery pack

The inconsistency of battery monomer is mainly produced in the manufacturing process. Due to the technological level, there are slight differences in the thickness of battery plate, microporosity and activation degree of active substances. Such inconsistency in the internal structure of the battery will make it impossible for the voltage, capacity and internal resistance of the same batch of the same type of battery to be completely consistent. The impact of inconsistency of single battery on the service life of power battery pack is divided into inconsistency of voltage, inconsistency of capacity and inconsistency of internal resistance.

In the process of forming a single battery group, if the voltage inconsistency is large, the low-voltage battery will become the load of the battery group when used together with the normal battery. Because when there is a low voltage battery in the two batteries in parallel, mutual charging will occur and other batteries will charge the battery. This kind of connection mode will make the capacity of low-voltage battery increase slightly while that of high-voltage battery decrease greatly, and the energy loss cannot reach the ideal external output on mutual charging.

The inconsistency in initial capacity has been greatly reduced after screening before the battery is put into a group, although the difference in the initial capacity of a single battery can be balanced by charging the single battery separately in the use process. However, the continuous charging and discharging cycle process of electric vehicles amplifies this inconsistency to some extent. The capacity varies with the attenuation speed of the cycle, and with the increase of the number of battery cycles, the difference in capacity will become larger and larger. This will make the capacity attenuation of the single battery aggravate and lead to the capacity attenuation of the whole battery pack.

The inconsistency of internal resistance results in uneven distribution of voltage and current in the battery pack, and partial overvoltage charging or under voltage discharge. The inconsistency of internal resistance will also lead to the unequal heat loss of the single battery in the process of discharge. The greater the internal resistance is, the faster the temperature will rise, which may eventually cause thermal runaway.

Inconsistencies in voltage, capacity and internal resistance lead to differences in battery life, mainly in temperature, charge and discharge rate, discharge depth and available capacity. For example, due to the difference in the initial capacity of the battery, most of the batteries are still in the condition of shallow discharge, and the battery with low capacity has been deeply discharged.

Conclusion

There are many factors affecting the life of power battery pack and their interactions make the performance of the battery pack prone to serious decline, especially in the conditions of high temperature and low temperature or overcharge and discharge and the lack of effective management and control, the performance of the battery pack will further decline. In addition, the series and parallel connection of a large number of single cells makes a single cell with poor capacity and performance limit the performance of the whole battery pack, and thus restrict the full use of the battery pack.

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