Lithium-ion power battery can not meet the power demand of electric vehicle, so the electric vehicle must use multiple batteries in series and parallel to form a power battery group to provide the energy of electric vehicle. From the current level of lithium-ion power battery monomer manufacturing process, the influence of various factors in the production process makes the same type of monomer battery also produce deviation in voltage, capacity, internal resistance and other aspects. The performance of power battery depends on the performance of battery cell, but it is not a simple accumulation of cell performance. Because of the performance inconsistency of the single battery, when the power battery pack is used repeatedly on the electric vehicle, the power battery pack has various problems, which leads to the shortening of service life.

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

Factors affecting the service life of single power battery

For the end of service life of power battery used in electric vehicle, the definition is that the attenuation of battery reaches 20% of the initial capacity. When the service life of the power battery is charged and discharged repeatedly on the electric vehicle, the material properties of the battery body will decline due to the side reactions inside the lithium-ion battery. This kind of decline is caused by the following aspects: the change of lattice structure of electrode material; the decrease of active material caused by decomposition, peeling or corrosion of electrode material; the decrease of conductivity and increase of impedance caused by decomposition consumption of electrolyte; the consumption of detachable lithium ion caused by lithium evolution or side reaction of negative electrode; the modification and Increase of impedance caused by corrosion of collecting fluid.

From the actual use of environmental conditions, the factors that affect the service life of the single power battery mainly include the cut-off voltage of charge and discharge, the rate of charge and discharge, the use temperature and the use conditions.

There are many documents that show that in a certain range, the cycle life of different charging cut-off voltage is shorter with the higher charging voltage. This shows that the charging cut-off voltage has a great influence on the battery life. High charging cut-off voltage will aggravate the occurrence of battery side effects and shorten the battery life. When the power battery is used in the whole vehicle, the performance of the battery will decline seriously in the high potential area because of the various driving conditions of the electric vehicle.

In order to meet different driving conditions, the power battery uses different charge discharge ratio in the use of electric vehicles. The study of the power battery shows that the larger the rate of charge and discharge, the faster the capacity decay. This is mainly due to the change of the structure and properties of cathode materials and the difficulty of lithium diffusion due to the thickening of the surface of negative surface. If the rate of charge and discharge is too large, it may also cause overheating and short circuit of single battery, which may cause explosion.

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

When the power battery is not in use, it will have self discharge, passivation of positive and negative materials, decomposition of electrolyte and so on due to the nature of the battery itself. Some experimental results show that the unstable SEI performance of the negative electrode will lead to the rapid degradation of the negative active material.

It is easy to produce lithium metal precipitation, and the lithium battery with stable SEI film can be stored at high temperature for more than 4 years. At the same time, different electrolyte components have different effects on the degradation of electrode materials.

The influence of monomer inconsistency on power battery pack

The inconsistency of cell is mainly produced in the manufacturing process. Due to the technological level, there are small differences in the thickness of cell plate, microporosity, activation degree of active substances, etc. The inconsistency in the internal structure of the battery will make the voltage, capacity and internal resistance of the same type of battery from the same batch of factory impossible to be completely consistent. The influence of cell inconsistency on the service life of power battery pack is divided into voltage inconsistency, capacity inconsistency and internal resistance inconsistency.

In the process of single battery group, if the voltage inconsistency is large, the low-voltage battery will become the load of the battery group when it is used together with the normal battery. Because when there is a low-voltage battery in the two parallel batteries, mutual charging will occur, and other batteries will charge the battery. This connection mode will increase the capacity of the low-voltage battery and reduce the capacity of the high-voltage battery. The energy loss can not reach the ideal output in the mutual charging.

The initial capacity inconsistency has been greatly reduced by screening before the battery is grouped. Although the initial capacity of the single battery can be balanced by charging the single battery separately in the use process. However, the process of continuous charge discharge cycle of electric vehicle makes the inconsistency enlarge to some extent. The capacity varies with the decay speed of cycle, and with the increase of battery cycle times, the capacity difference will be more and more.