Understanding the failure causes or mechanisms of lithium iron phosphate batteries is very important for improving battery performance and its large-scale production and use.

1. Failure in the production process

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

Failure of batteries caused by impurities in electrode active materials

During the synthesis of LiFePO4, there will be a small amount of impurities such as Fe2O3 and Fe. These impurities will be reduced on the surface of the negative electrode, which may pierce the diaphragm and cause an internal short circuit. When LiFePO4 is exposed to the air for a long time, the moisture will deteriorate the battery. In the early stage of aging, amorphous iron phosphate is formed on the surface of the material, and its local composition and structure are similar to LiFePO4 (OH). With the insertion of OH, LiFePO4 is continuously consumed. , manifested as an increase in volume; then recrystallization slowly formed LiFePO4(OH). The Li3PO4 impurity in LiFePO4 is electrochemically inert. The higher the impurity content of the graphite anode, the greater the irreversible capacity loss.

The failure of the battery caused by the chemical method

The irreversible loss of active lithium ions is first reflected in the lithium ions consumed during the formation of solid electrolyte interfacial films. The study found that increasing the formation temperature will cause more irreversible loss of lithium ions, because the proportion of inorganic components in the SEI film will increase when the formation temperature is increased, and the gas released during the transformation of the organic component ROCO2Li to the inorganic component Li2CO3 It will cause more defects in the SEI film, and the solvated lithium ions through these defects will intercalate into the graphite negative electrode in large quantities.

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

Battery failure due to moisture in the production environment

In actual production, the battery will inevitably come into contact with air. Since most of the positive and negative materials are micron or nanoscale particles, and the solvent molecules in the electrolyte have electronegative carbonyl groups and metastable carbon-carbon double bonds, are easy to absorb moisture in the air.

Water molecules react with lithium salts (especially LiPF6) in the electrolyte, which not only decomposes and consumes the electrolyte (decomposes to form PF5), but also produces an acidic substance HF. However, both PF5 and HF will destroy the SEI film, and HF will also promote the corrosion of the LiFePO4 active material. The water molecules also delithiate the lithium-intercalated graphite anode, forming lithium hydroxide at the bottom of the SEI film. In addition, the dissolved O2 in the electrolyte will also accelerate the aging of LiFePO4 batteries.

In the production process, in addition to the production process affecting the performance of the battery, the main factors that cause the failure of LiFePO4 power batteries 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 method of formation, etc. factors appear to be crucial.

2. Failure in Shelving

In the service life of the power battery, most of the time is in a state of shelving. Generally, after a long period of shelving, the performance of the battery 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 the degradation of battery performance, among which temperature, state of charge and time are the most obvious factors.

Kassema et al. analyzed the aging of LiFePO4 power batteries under different shelving states, and believed that the aging mechanism was mainly the side reactions of the positive and negative electrodes and the electrolyte (relative to the side reactions of the positive electrode, the side reactions of the graphite negative electrode were heavier, mainly due to the solvent decomposition, the growth of SEI film) consumes active lithium ions, and at the same time the overall impedance of the battery increases, the loss of active lithium ions leads to the aging of battery shelving; and the capacity loss of LiFePO4 power battery increases seriously with the increase of storage temperature, In contrast, as the stored state of charge increases, there is a lesser degree of capacity loss.

Grolleau et al. also came to the same conclusion: the storage temperature has a greater impact on the aging of LiFePO4 power batteries, followed by the storage state of charge; and a simple model is proposed. The capacity loss of LiFePO4 power batteries can be predicted based on factors related to storage time (temperature and state of charge). In a certain SOC state, with the increase of the shelf time, 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 proportion of irreversible lithium ions, SEI thickening and conductivity. The increase in impedance caused by the decrease (inorganic components increase, some have a chance to re-dissolve) and decrease in the activity of the electrode surface together contribute to the aging of the battery.

Differential scanning calorimetry did not find any reaction between LiFePO4 and different electrolytes (the electrolytes were LiBF4, LiAsF6 or LiPF6), regardless of whether it was charged or discharged, in the temperature range from room temperature to 85 °C. However, when LiFePO4 is immersed in the electrolyte of LiPF6 for a long time, it will still show a certain reactivity: because the rate of reaction to form the interface is very slow, there is still no passivation film on the surface of LiFePO4 after immersion for one month to prevent it from further reacting 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 aging of the battery more obvious.

3. Failure in recycling

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

The capacity loss of LiFePO4 power batteries during cycling is generally considered to be caused by the loss of active lithium ions. The research of Dubarry et al. shows that the aging of LiFePO4 power battery during cycling is mainly through a complex growth process that consumes the active Li-ion SEI film. In this process, the loss of active lithium ions directly reduces the capacity retention rate of the battery; the continuous growth of the SEI film, on the one hand, increases the polarization resistance of the battery, and at the same time, the thickness of the SEI film is too thick, and the electrochemical performance of the graphite negative electrode is reduced. The activity is also partially inactivated.

During high temperature cycling, Fe2+ in LiFePO4 will dissolve to a certain extent. 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 on the graphite negative electrode will play a catalytic role in the growth of the SEI film. . Tan quantitatively analyzed where and in which steps the active lithium ions were lost, and found that most of the loss of active lithium ions occurred on the surface of the graphite negative electrode, especially during high temperature cycling, that is, the high temperature cycling capacity loss was faster; and summed up the SEI film. There are three different mechanisms of damage and repair: (1) electrons in the graphite anode reduce lithium ions through the SEI film; (2) the dissolution and regeneration of some components of the SEI film; (3) due to the volume change of the graphite anode caused the rupture of the SEI membrane.

In addition to the loss of active lithium ions, both positive and negative electrode materials deteriorate during cycling. The appearance of cracks in LiFePO4 electrodes during cycling can lead to an increase in electrode polarization and a decrease in the conductivity between the active material and the conductive agent or current collector. Nagpure used scanning extended resistance microscopy (SSRM) to study the changes of LiFePO4 after aging semi-quantitatively, and found that the coarsening of LiFePO4 nanoparticles and the surface deposits produced by certain chemical reactions jointly led to the increase of LiFePO4 cathode impedance. In addition, the reduction of active surface and exfoliation of graphite electrodes caused by the loss of graphite active materials are also considered to be the reasons for battery aging. The instability of graphite negative electrodes will lead to the instability of 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. The results of Kim et al. show that the aging mechanisms of LiFePO4 cathode and graphite anode are different: with the increase of discharge rate, the capacity loss of cathode increases more than that of anode. The loss of battery capacity during low-rate cycling is mainly caused by the consumption of active lithium ions at the negative electrode, while the power loss of the battery during high-rate cycling is caused by the increase in the impedance of the positive electrode.

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 related to the increase of the impedance of the SEI film and the increase of the impedance of the whole battery. directly related. Although the effect of the upper limit of charging voltage on battery failure is not obvious relative to the loss of active lithium ions, a too low or too high upper limit of charging voltage will increase the interface impedance of LiFePO4 electrodes: it is not very good at a low upper limit voltage. A passivation film is formed on the ground, and a too high upper voltage limit will lead to the oxidative decomposition of the electrolyte, resulting in 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. Li studied the LiFePO4 cathode and the graphite anode respectively, and found that the main controlling factors limiting the low temperature performance of the cathode and anode are different. The decrease of ionic conductivity in the LiFePO4 cathode dominates, while the increase in the interface impedance of the graphite anode is the main reason.

During use, the degradation of LiFePO4 electrode and 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 very important, including appropriate The charging voltage, suitable depth of discharge, etc.

4. Failure during charging and discharging

The battery is often overcharged in the process of use. Relatively speaking, the overdischarge situation is less. The heat released during the overcharge or overdischarge process is easy to accumulate inside the battery, which will further increase the battery temperature. , affecting the service life of the battery and increasing the possibility of the battery catching fire or exploding. Even under normal charge-discharge conditions, as the number of cycles increases, the capacity inconsistency of the single cells inside the battery system will increase, and the battery with the lowest capacity will experience the process of overcharge and overdischarge.

Although the thermal stability of LiFePO4 is the best compared to other cathode materials under different charging states, overcharging will also cause unsafe hidden dangers 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 ethylene carbonate (EC) will preferentially undergo oxidative decomposition on the surface of the positive electrode in common organic solvents. Since the lithium intercalation potential (to lithium 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 reasons for battery failure under overcharged conditions is the internal short circuit caused by lithium dendrites piercing the separator. Lu et al. analyzed the failure mechanism of lithium plating on the surface of graphite anode due to overcharge. The results show that there is no change in the overall structure of the graphite negative electrode, but there are lithium dendrites and surface films. The reaction between lithium and the electrolyte causes the continuous increase of the surface film, which not only consumes more active lithium, but also allows lithium to diffuse into the graphite. The anode becomes more difficult, which in turn further promotes the deposition of lithium on the anode surface, resulting in a further decrease in capacity and Coulombic efficiency.

In addition to this, metal impurities (especially Fe) are generally considered to be one of the main reasons for battery overcharge failure. Xu et al. systematically studied the failure mechanism of LiFePO4 power batteries under overcharged conditions. The results show that the redox of Fe is theoretically possible during overcharge/discharge cycles, and the reaction mechanism is given: when overcharge occurs, Fe is first oxidized to Fe2+, Fe2+ is further oxidized to Fe3+, and then Fe2+ and Fe3+ are removed from the positive electrode. One side diffuses to the negative side, Fe3+ is finally reduced to Fe2+, and Fe2+ is further reduced to form Fe; during the overcharge/discharge cycle, Fe crystal dendrites will be formed on the positive and negative electrodes at the same time, which will pierce the diaphragm to form Fe bridges, resulting in microscopic changes in the battery. Short circuit, the obvious phenomenon that accompanies the micro-short circuit of the battery is the continuous increase of temperature after overcharging.

During overdischarge, the potential of the negative electrode will increase rapidly, and the increase of the potential will cause the destruction of the SEI film on the surface of the negative electrode (the part rich in inorganic compounds in the SEI film is more easily oxidized), which in turn will cause additional decomposition of the electrolyte , resulting in a loss of capacity. 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 would increase the internal resistance of the battery and cause the capacity loss of the battery.

He et al. studied the overdischarge process of LiFePO4 power battery in detail. The results show that the negative current collector Cu foil can be oxidized to Cu+ during overdischarge, and Cu+ is further oxidized to Cu2+. After that, they diffuse to the positive electrode and can undergo a reduction reaction at the positive electrode. In this way, Cu dendrites will form on the positive electrode side, which will pierce the separator and cause a micro-short circuit inside the battery. Also due to overdischarge, the battery temperature will continue to rise.

Overcharging of LiFePO4 power batteries may lead to oxidative decomposition of electrolyte, lithium precipitation, and formation of Fe crystal dendrites; while overdischarge may cause SEI damage, resulting in capacity attenuation, Cu foil oxidation, and even the formation of Cu crystal dendrites.

5. Failure in other aspects

Due to the low intrinsic conductivity of LiFePO4, the morphology and size of the material itself, as well as the influence of conductive agents and binders, are easily manifested. Gaberscek et al. discussed the two contradictory factors of size and carbon coating, and found that the impedance of LiFePO4 electrode is only related to the average particle size. The anti-site defect inside LiFePO4 (Fe occupies Li site) will have a certain impact on the performance of the battery: because the transport of lithium ions in LiFePO4 is one-dimensional, this defect will hinder the transport of lithium ions; due to the introduction of high valence states This defect also causes the instability of the LiFePO4 structure due to the additional electrostatic repulsion.

The large-sized LiFePO4 cannot fully delithiate at the end of the charge; the nanostructured LiFePO4 can reduce the anti-site defects, but can cause self-discharge due to its high surface energy. At present, the most commonly used binder is PVDF, which may react at high temperature, dissolve in non-aqueous electrolyte, and is not flexible enough, which has a certain impact on the capacity loss and shortened cycle life of 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.