Analysis of the cause of failure of lithium iron phosphate battery

As a competitive positive electrode material, lithium iron phosphate has attracted a lot of attention. Understanding the cause or mechanism of failure of lithium iron phosphate batteries is important for improving battery performance and its large-scale production and use.

1. The failure in the production process

In the production process, personnel, equipment, raw materials, methods, and environment are the main factors affecting product quality. In the production process of LiFePO4 power battery is no exception, personnel and equipment belong to the management category, so we mainly discuss the last three effects. factor.

1. Failure of impurities in the electrode active material on the battery

2. The failure of the formation method on the battery

3. Failure of the water in the production environment to the battery

In the production process, in addition to the production process affecting battery performance, the main influencing factors of LiFePO4 power battery failure include impurities (including water) in the raw materials and the formation process, so the purity of the material, the control of the environmental humidity, the formation method, etc. The factorsare crucial.

2. The failure in the shelving

During the life of the battery, most of the time is on hold. Generally, after a long period of suspension, the battery performance will decrease, generally showing an increase in internal resistance, a decrease in voltage, and a decrease in discharge capacity. There are many factors that cause battery performance degradation, among which temperature, state of charge and time are the most obvious factors.

1) The aging under different idle state consumes active lithium ions, and the whole impedance of the battery increases, and the capacity loss of the battery increases severely with the increase of the storage temperature, in comparison with the increase of the state of charge of the storage. The capacity loss is less.

2)The storage temperature has a great influence on the LiFePO4 battery. The lithium in the graphite will diffuse to the edge, forming a complex complex with the electrolyte and electrons, resulting in an increase in the irreversible lithium ion ratio, SEI thickening and conductivity. reduce.

3)In the idle state, the harsh storage conditions (high temperature and high state of charge) will increase the degree of self-discharge of the LiFePO4 power battery, making the battery aging more obvious.

3. Failure in recycling

The battery is generally exothermic during use, so the effects of temperature are important.

1. The capacity loss during the cycle of LiFePO4 power battery is generally considered to be caused by the loss of active lithium ions. The loss of active lithium ions directly reduces the retention of the battery capacity; the continuous growth of the SEI film causes an increase in the polarization resistance of the battery, while the thickness of the SEI film is too thick, and the electrochemical activity of the graphite negative electrode is partially lost. live.

2. During high temperature cycling, Fe2+ will dissolve in LiFePO4. Although the amount of Fe2+ dissolved has no obvious effect on the capacity of the positive electrode, the dissolution of Fe2+ and the precipitation of Fe in the graphite negative electrode will play a role in the growth of SEI film. Catalysis.

3. In addition to the loss of active lithium ions, the positive and negative materials will deteriorate during recycling. The presence of cracks in the LiFePO4 electrode during recycling causes an increase in electrode polarization and a decrease in conductivity between the active material and the conductive agent or current collector.

4. The discharge capacity of LiFePO4 power battery will decrease rapidly when the temperature is lowered, mainly due to the decrease of ionic conductivity and the increase of interface impedance.

During the process of use, the degradation of LiFePO4 electrode, graphite negative electrode and the continuous growth of SEI film cause battery failure to varying degrees; in addition, in addition to uncontrollable factors such as road conditions and ambient temperature, the normal use of the battery is also important, including appropriate Charging voltage, suitable depth of discharge, etc.

4. Failure during charging and discharging

In the process of using the battery, it is inevitable that overcharge will occur. Relatively speaking, the overdischarge will be less. The heat released during overcharge or overdischarge will easily accumulate inside the battery, which will further increase the battery temperature. It affects the service life of the battery and increases the possibility of the battery catching fire or exploding. Even under normal charge and discharge conditions, as the number of cycles increases, the capacity inconsistency of the cells in the battery system increases, and the battery with the lowest capacity experiences the process of overcharging and overdischarging.

1)In an overcharged state, the solvent in the organic electrolyte is more susceptible to oxidative decomposition. In a common organic solvent, ethylene carbonate (EC) preferentially oxidatively decomposes on the surface of the positive electrode. Since the lithium insertion potential of the graphite negative electrode (to the lithium potential) is very low, there is a great possibility that lithium is precipitated in the graphite negative electrode.

2)During overdischarge, the potential of the negative electrode rises rapidly, and the increase in potential causes destruction of the SEI film on the surface of the negative electrode (the inorganic compound-rich portion of the SEI film is more susceptible to oxidation), which in turn causes electrolyte solution. Additional decomposition, resulting in capacity loss. More importantly, the anode current collector Cu foil is oxidized. Yang et al. detected Cu2O, the oxidation product of Cu foil, in the SEI film of the negative electrode, which caused an increase in internal resistance of the battery and caused a loss in capacity of the battery.

5. The feature

In the normal use of the battery, the loss of active lithium ions is the most important cause of failure of the LiFePO4 power battery. Therefore, for the LiFePO4 power battery (graphite anode), the quality and stability of the SEI film is the key to improving the cycle life of the battery. The formation process of SEI film (including changes in its morphology and thickness), the mechanism of action of film-forming additives, and the diffusion mechanism of lithium ions in SEI film are increasingly used by various experimental methods and theoretical methods. Understand that this also provides favorable conditions for improving the service life of the LiFePO4 power battery.