As a competitive cathode material for electric vehicles, lithium iron phosphate has attracted a lot of attention. Understanding the failure cause or mechanism of lithium iron phosphate batteries is very important for improving battery performance and its large-scale production and use. This paper reviews the research progress of lithium iron phosphate power battery failure in recent years. The effects of impurities, formation methods, storage conditions, recycling, overcharge and overdischarge on battery failure are discussed.

Failure in 1. production

In the production process, personnel, equipment, raw materials, methods, and the environment are the main factors that affect product quality. In the production process of LiFePO4 power batteries, there is no exception. Personnel and equipment belong to the category of management, so we mainly discuss the latter three influences. factor.

1. Failure of battery caused by impurities in electrode active materials

LiFePO4 in the process of synthesis, there will be a small amount of Fe2O3, Fe2P, Fe and other impurities, these impurities will be reduced on the surface of the negative electrode, it is possible to pierce the separator to cause an internal short circuit. LiFePO4 exposed to air for a long time, moisture can deteriorate the battery. Aging mechanism of lithium iron phosphate battery: amorphous iron phosphate is formed on the surface of the material at the initial stage of aging, and its local composition and structure are similar to LiFePO4(OH); With the embedding of OH, the LiFePO4 is continuously consumed, showing an increase in volume. After recrystallization, LiFePO4(OH) is slowly formed. The Li3PO4 impurity in the LiFePO4 is electrochemically inert. The higher the impurity content of the graphite negative electrode, the greater the irreversible capacity loss.

2. Failure of the battery caused by the formation method

The irreversible loss of active lithium ions is first reflected in the lithium ions consumed during the formation of the solid electrolyte interface film. It is found that increasing the formation temperature will cause more irreversible lithium ion loss, because when the formation temperature is increased, the proportion of inorganic components in the SEI film will increase, and the gas released during the transition from organic component ROCO2Li to inorganic component Li2CO3 will cause more defects in the SEI film, and a large number of lithium ions solvated by these defects will be embedded in the graphite anode.

During formation, the composition and thickness of SEI film formed by small current charging are uniform, but it is time-consuming. Large current charging will cause more side reactions, resulting in increased irreversible lithium ion loss and increased negative electrode interface impedance, but time-saving. At present, the formation mode of small current constant current-large current constant current and constant voltage is widely used, which can take into account the advantages of both.

3. The failure of the battery caused by the moisture in the production environment

In actual production, the battery will inevitably contact the air, because the positive and negative materials are mostly micron or nanometer particles, and the solvent molecules in the electrolyte have large electronegativity carbonyl and metastable carbon-carbon double bonds, which are easy to absorb moisture in the air.

The water molecules react with the lithium salt in the electrolyte, which not only decomposes and consumes the electrolyte, but also produces an acidic substance HF. will destroy the SEI film, HF will also promote the corrosion of LiFePO4 active species. The water molecules also partially delithiate the lithium-intercalated graphite negative electrode, forming lithium hydroxide at the bottom of the SEI film. In addition, the dissolved O2 in the electrolyte will also accelerate the aging of the LiFePO4 battery.

In the production process, in addition to the production process affecting the performance of the battery, the main factors causing the failure of the LiFePO4 power battery include impurities in the raw materials (including water) and the process of formation, so the purity of the material, the control of environmental humidity, the way of formation and other factors are very important.

Failure in 2. hold

In the service life of the power battery, most of the time is in a shelved state. Generally, after a long period of shelving, the battery performance will decline, 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.

Analyze the aging of LiFePO4 power battery in different shelving states. The aging mechanism is mainly the side reaction of positive and negative electrodes and electrolyte, which consumes active lithium ions. At the same time, the entire impedance of the battery increases, and the loss of active lithium ions leads to the aging of battery shelving. Moreover, the capacity loss of LiFePO4 power battery increases seriously with the increase of storage temperature. In contrast, with the increase of storage state of charge, the degree of capacity loss is smaller.

The storage temperature has a great influence on the aging of the LiFePO4 power battery, and the storage state of charge is the second. The capacity loss of the LiFePO4 power battery can be predicted based on factors related to the storage time (temperature and state of charge). Under a certain SOC state, with the increase of shelf time, lithium in graphite will diffuse to the edge and form a complex compound with electrolyte and electrons, resulting in an increase in the proportion of irreversible lithium ions, an increase in impedance caused by SEI thickening and a decrease in conductivity (an increase in inorganic components and a partial opportunity to redissolve), and a decrease in the activity of the electrode surface jointly cause the aging of the battery.

The differential scanning calorimetry method did not find any reaction between the LiFePO4 and different electrolytes in the temperature range from room temperature to 85°C, regardless of the charged state or the discharged state. However, LiFePO4 a long time immersed in LiPF6 electrolyte, it will still show certain reactivity: because the reaction to form the interface is very slow, after soaking for one month, there is still no passivation film on the surface of the LiFePO4 to prevent its further reaction with the electrolyte.

In the shelving state, 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.

Failure in 3. recycling

Batteries are generally exothermic during use, so the effect of temperature is important. In addition, road conditions, usage, ambient temperature, etc. will have different effects.

For the capacity loss of LiFePO4 power battery cycle, it is generally believed that the loss of active lithium ions is caused. The research shows that the aging of LiFePO4 power battery cycle is mainly a complex growth process of SEI film that consumes active lithium ions. In this process, the loss of active lithium ions directly reduces the retention rate of the battery capacity; the continuous growth of the SEI film, on the one hand, causes the increase of the polarization resistance of the battery, and at the same time, the thickness of the SEI film is too thick, and the electrochemical activity of the graphite anode will be partially deactivated.

In the high temperature cycle, Fe2 in the LiFePO4 will have a certain dissolution, although the amount of Fe2 dissolved on the capacity of the positive electrode has no obvious effect, but the dissolution of Fe2 and the precipitation of Fe in the graphite negative electrode will play a catalytic role in the growth of SEI film. The loss of most active lithium ions occurs on the surface of the graphite negative electrode, especially in high temperature cycling, I .e. the loss of capacity in high temperature cycling is faster. There are three different mechanisms for the destruction and repair of SEI film:(1) electrons in graphite negative electrode reduce lithium ions through SEI film;(2) dissolution and regeneration of some components of SEI film;(3) SEI film rupture caused by volume change of graphite negative electrode.

In addition to the loss of active lithium ions, both the positive and negative electrode materials deteriorate during recycling. LiFePO4 the electrode has cracks during cyclic use, the polarization of the electrode increases and the conductivity between the active material and the conductive agent or the current collector decreases. The coarsening of the LiFePO4 nanoparticles and the surface deposits produced by certain chemical reactions together lead to an increase in the impedance of the LiFePO4 positive electrode. In addition, the reduction of the active surface caused by the loss of the graphite active material and the exfoliation of the graphite electrode are also considered to be the reasons for the aging of the battery. The instability of the graphite negative electrode will lead to the instability of the SEI film, which will promote the consumption of active lithium ions.

The large rate discharge of the battery can provide large power for the electric vehicle, that is, the better the rate performance of the power battery, the better the acceleration performance of the electric vehicle. As the discharge rate increases, the capacity loss of the positive electrode increases more than that of the negative electrode. The loss of battery capacity during low rate cycling is mainly due to the consumption of active lithium ions in the negative electrode, while the power loss of the battery during high rate cycling is due to the increase of the positive electrode impedance.

Although the depth of discharge in the use of the power battery does not affect the capacity loss, it will affect its power loss: the speed of power loss increases with the increase of the depth of discharge, which is directly related to the increase of the impedance of the SEI film and the increase of the impedance of the whole battery. Too low or too high an upper limit of the charging voltage will increase the interface impedance of the LiFePO4 electrode: a low upper limit voltage cannot form a passivation film well, while a too high upper limit of the voltage will lead to oxidative decomposition of the electrolyte and the formation of products with low conductivity on the surface of the LiFePO4 electrode.

The discharge capacity of LiFePO4 power batteries decreases rapidly when the temperature decreases, mainly due to the decrease of ionic conductivity and the increase of interfacial impedance. The main controlling factors limiting the low-temperature performance of the positive and negative electrodes are different. The decrease of ionic conductivity in the LiFePO4 positive electrode is dominant, while the increase of interfacial impedance in the graphite negative electrode is the main reason.

During use, the degradation of the LiFePO4 electrode and the graphite negative electrode and the continuous growth of the 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 very important, including Appropriate charging voltage, appropriate discharge depth, etc.

Failure of 4. during charging and discharging

The battery is often inevitably overcharged during use. Relatively speaking, there are fewer cases of overdischarge. The heat released during overcharge or discharge is easy to accumulate inside the battery, which will further increase the battery temperature., Affect the service life of the battery and increase the possibility of battery fire or explosion. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the single battery inside the battery system will increase, and the battery with the lowest capacity will also experience the process of overcharge and overdischarge.

Although the thermal stability of the LiFePO4 is the best compared to other positive electrode materials under different charging states, overcharge can also cause unsafe risks in the use of LiFePO4 power batteries. In the overcharged state, the solvent in the organic electrolyte is more likely to undergo oxidative decomposition, and in the common organic solvent, ethylene carbonate will preferentially undergo oxidative decomposition on the surface of the positive electrode. Since the lithium intercalation potential of the graphite negative electrode is very low, there is a great possibility of lithium precipitation in the graphite negative electrode.

One of the main causes of battery failure under overcharge conditions is the internal short circuit caused by lithium crystal branches puncturing the separator. The appearance of lithium crystal branches and surface film of graphite negative electrode, the reaction of lithium and electrolyte causes the surface film to increase continuously, which not only consumes more active lithium, but also makes it more difficult for lithium to diffuse to graphite negative electrode, which in turn will further promote the deposition of lithium on the surface of negative electrode, resulting in further reduction of capacity and coulombic efficiency.

In addition, metal impurities are generally considered to be one of the main causes of battery overcharge failure. The oxidation and reduction of Fe during overcharge/discharge cycles is theoretically possible. Fe crystal branches will form at the positive and negative electrodes at the same time, piercing the separator to form Fe bridges, causing micro-short circuits in the battery. The obvious phenomenon accompanied by micro-short circuits in the battery is the continuous increase in temperature after overcharge.

During over-discharge, the potential of the negative electrode rises rapidly, and the increase in potential causes the destruction of the SEI film on the surface of the negative electrode (the part of the SEI film rich in inorganic compounds is more likely to be oxidized), which in turn causes additional decomposition of the electrolyte, resulting in capacity loss. More importantly, the Cu foil of the negative electrode current collector is oxidized. This causes the internal resistance of the battery to increase, causing the capacity loss of the battery.

The negative electrode current collector Cu foil can be oxidized to Cu during over-discharge, and Cu is further oxidized to Cu2, after which they diffuse to the positive electrode and can undergo reduction reaction at the positive electrode. In this way, Cu crystal branches will be formed on the side of the positive electrode, which will pierce the separator and cause micro-short circuit inside the battery. Similarly, due to over-discharge, the battery temperature will continue to rise.

The overcharge of LiFePO4 power battery may lead to the oxidation decomposition of electrolyte, lithium precipitation and the formation of Fe crystal branches. However, overdischarge may cause SEI damage, resulting in capacity attenuation, Cu foil oxidation, and even the formation of Cu crystal branches.

5. other aspects of failure

Due to the low conductivity inherent in the LiFePO4, the shape and size of the material itself, as well as the influence of the conductive agent and the binder, are easily manifested. Gaberscek et al. discussed the two contradictory factors of size and carbon coating, and found that the LiFePO4 electrode impedance is only related to the average particle size. The LiFePO4 internal anti-bit defects (Fe occupies the Li bit) will have a certain impact on the performance of the battery: because the transmission of lithium ions within the LiFePO4 is one-dimensional, this defect will hinder the transmission of lithium ions; due to the introduction of high-priced states With additional electrostatic repulsion, this defect will also cause instability of the LiFePO4 structure.

LiFePO4 with large particles cannot completely delithiate at the end of charging; the LiFePO4 of nanostructures can reduce anti-site defects, but will cause self-discharge due to its high surface energy. At present, PVDF is the most widely used binder, which has the disadvantages of high temperature reaction, solubility in non-aqueous electrolyte, and insufficient flexibility, which has a certain impact on the capacity loss and cycle life of the LiFePO4. In addition, the current collector, diaphragm, electrolyte composition, production process, human factors, external vibration and shock will affect the performance of the battery to varying degrees.

Reference: Miao Meng et al., Research Progress on Failure of Lithium Iron Phosphate Power Battery