As a competitive cathode material for electric vehicles, lithium iron phosphate has attracted a lot of attention. Understanding the failure causes or mechanisms of lithium iron phosphate batteries is very important for improving the performance of the battery and its mass production and use. This paper summarizes the research progress on the failure of lithium iron phosphate power battery in recent years. It discusses the effects of impurities, formation methods, storage conditions, cycling, overcharge and overdischarge on battery failure.
I. Failure in the production process
In the production process, personnel, equipment, raw materials, methods, and environment are the main factors affecting product quality, and there is no exception in the production process of LiFePO4 power batteries, personnel and equipment belong to the category of management, so we mainly discuss the latter three influencing factors .
Failure of battery caused by impurities in electrode active material
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, may puncture the diaphragm triggering an internal short-circuit. LiFePO4 exposed to air for a long time, the humidity will deteriorate the battery, Cuisinier et al. revealed its aging mechanism: the early stage of aging of the surface of the material to form an amorphous iron phosphate, the local composition and structure are similar to those of the battery. Its local composition and structure are similar to LiFePO4(OH); with the embedding of OH, LiFePO4 is continuously consumed, which is manifested as an increase in volume; after that, recrystallization slowly forms LiFePO4(OH). The Li3PO4 impurity in LiFePO4, on the other hand, shows electrochemical inertness. The higher the impurity content of the graphite anode (mainly Al and Fe), the greater the irreversible capacity loss caused (the smaller the reversible capacity loss).
Failure of the battery due to the mode of chemical formation
The irreversible loss of active lithium ions is first seen in the lithium ions consumed during the formation of the solid electrolyte interface film (SEI film). It has been found that increasing the formation temperature causes more irreversible loss of lithium ions, because the proportion of inorganic components in the SEI membrane increases when the formation temperature is increased, and the gases released during the transition from the organic component ROCO2Li to the inorganic component Li2CO3 cause more defects in the SEI membrane, and the lithium ions that are solvated through these defects become embedded in the graphite anode in large quantities.
The composition and thickness of the SEI film formed by small current charging is uniform, but time-consuming; high current charging will cause more side reactions to occur, resulting in an increase in the irreversible loss of lithium ions and an increase in the negative electrode interfacial impedance, but time-saving; nowadays, the more widely used is the small-current constant-current-large-current constant-current-constant-voltage mode of synthesis, which can take into account the advantages of the two.Zhong et al. further demonstrated that activation affects the SEI film by using the electrochemical method. Zhong et al. used electrochemical methods to further prove that the activation of the battery affects the stability of the SEI membrane, and the higher the stability of the SEI membrane, the lower the self-discharge rate of the battery.
Failure of batteries due to moisture in the production environment
In actual production, the battery will inevitably come into contact with air, because most of the positive and negative electrode materials are micron or nanometer particles, and the solvent molecules in the electrolyte have electronegative carbonyl groups and sub-stable carbon-carbon double bonds, which are prone to absorbing water in the air.
Water molecules and lithium salts in the electrolyte (especially LiPF6) react, not only decompose and consume the electrolyte (decomposition of the formation of PF5), but also produce acid HF, and PF5 and HF will destroy the SEI membrane, HF will also promote the corrosion of LiFePO4 active material. Water molecules will also make the graphite anode with embedded lithium partially delithiated, forming lithium hydroxide at the bottom of the SEI membrane. In addition, the dissolved O2 in the electrolyte will also accelerate the aging of LiFePO4 battery .
In the production process, in addition to the production process affects the performance of the battery, the main factors affecting 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 ambient humidity, the way of formation and other factors appear to be critical.
II. Failure in shelving
In the service life of the power battery, most of the time is in the shelving state, generally after a long time of shelving, the battery performance will decline, generally showing an increase in internal resistance, voltage reduction and discharge capacity decline. There are many factors that cause battery performance degradation, among which temperature, state of charge and time are the most obvious influencing factors.
Kassema et al. analyzed the aging of LiFePO4 power batteries in different shelving states, and concluded that the aging mechanism is mainly the side reaction of positive and negative electrodes and electrolyte (compared to the side reaction of positive electrode, the side reaction of graphite negative electrode is heavier, which is mainly the decomposition of solvents, and the growth of SEI film) consuming the active lithium ions, and at the same time, the whole impedance of the battery increases, and the loss of active lithium ions causes the aging of the battery shelving; and the capacity loss of LiFePO4 power batteries severely increases with increasing storage temperature, compared to a lesser degree of capacity loss with increasing storage state of charge.
The same conclusion was obtained by Grolleau et al. that the storage temperature has a greater effect on the aging of LiFePO4 power cells, with the storage state of charge having the second greatest effect; and a simple model was proposed. The capacity loss of LiFePO4 power cells can be predicted based on factors (temperature and state of charge) related to storage time. As the shelving time increases in a certain SOC state, lithium in graphite diffuses to the edges and forms a complex complex with the electrolyte and electrons, resulting in an irreversible proportion of lithium ions that also increases, and an increase in impedance caused by thickening of the SEI and a decrease in the electrical conductivity (an increase in the inorganic component, with a chance for some of them to be redissolved), as well as a decrease in the activity on the surface of the electrodes, combine to contribute to the aging of the battery.
Differential scanning calorimetry (DSC) did not reveal any reaction between LiFePO4 and different electrolytes (LiBF4, LiAsF6 or LiPF6) in both charging and discharging states in the temperature range from room temperature to 85 °C. However, LiFePO4 was not found to react with different electrolytes for a long period of time. However, LiFePO4 immersed in LiPF6 electrolyte for a long period of time still shows some reactivity: since the reaction forms an interface very slowly, after one month of immersion there is still no passivation film on the surface of LiFePO4 preventing further reaction with the electrolyte.
In the shelving state, poor storage conditions (high temperature and high state of charge) will increase the degree of self-discharge of LiFePO4 power battery, making the aging of the battery more obvious.
III. Failure in cyclic use
Batteries are generally exothermic during use, so the influence of temperature is important. In addition to this, road conditions, usage patterns, and ambient temperatures all have different effects.
For the capacity loss of LiFePO4 power batteries during cycling, it is generally believed that it is caused by the loss of active lithium ions.Dubarry et al. showed that the aging of LiFePO4 power batteries during cycling mainly undergoes a complex process of depleting the active lithium-ion SEI film growth. In this process, the loss of active lithium ions directly reduces the battery capacity retention; the continuous growth of the SEI film, on the one hand, causes an increase in the polarization impedance of the battery, while at the same time the thickness of the SEI film is too thick, and the electrochemical activity of the graphite negative electrode is partially inactivated.
During high temperature cycling, there will be some dissolution of Fe2+ in LiFePO4, although the amount of dissolved Fe2+ has no significant effect on the capacity of the anode, but the dissolution of Fe2+ and the precipitation of Fe in the graphite anode will play a catalytic role in the growth of the SEI film.Tan quantitatively analyzed the loss of active lithium ions in where and in which step, and found that most of the active lithium ions lost in the surface of the graphite anode, especially in the graphite anode, the loss of active lithium ions in the surface of the graphite anode. Tan found that most of the loss of active lithium ions occurs on the surface of the graphite anode, especially during high-temperature cycling, i.e., the loss of capacity is faster during high-temperature cycling. He also summarized three different mechanisms for the destruction and repair of the SEI membrane: (1) the reduction of lithium ions through the SEI membrane by electrons in the graphite anode; (2) the dissolution and re-generation of some of the components of the SEI membrane; and (3) rupture of the SEI membrane due to the change in the volume of the graphite anode.
In addition to the loss of active lithium ions, both positive and negative electrode materials deteriorate during cycling.The presence of cracks in LiFePO4 electrodes during cycling leads to an increase in the polarization of the electrodes and a decrease in the conductivity between the active material and the conductive agent or the collector.Nagpure semi-quantitatively investigated the changes of LiFePO4 after aging by using a scanning extended resistivity microscope (SSRM) and found that the LiFePO4 electrode is not as good as its predecessor. Nagpure used scanning extended resistance microscopy (SSRM) to semi-quantitatively study the changes in LiFePO4 after aging, and found that the coarsening of LiFePO4 nanoparticles and surface deposits from certain chemical reactions combined to cause an increase in the impedance of LiFePO4 anodes. In addition, the loss of graphite active material leads to the decrease of active surface and the flake peeling of graphite electrodes are also considered to be the cause of battery aging, and the instability of graphite anode leads to the instability of SEI film, which promotes the depletion of active lithium ions .
The large magnification discharge of the battery can provide large power for EVs, i.e., the better the magnification performance of the power battery, the better the acceleration performance of the EV.Kim et al. showed that the aging mechanisms of LiFePO4 positive electrode and graphite negative electrode are different: with the increase of the discharge magnification, the capacity loss of the positive electrode increases more than that of the negative electrode. The loss of battery capacity during low-multiplier cycling is mainly due to the depletion of active lithium electrons at the negative electrode, while the power loss of the battery during high-multiplier cycling is due to the increase in the impedance of the positive electrode.
Although the depth of discharge in power battery use does not affect the capacity loss, it does affect its power loss: the rate of power loss increases with the depth of discharge, which is directly related to the increase in impedance of the SEI membrane and the increase in impedance of the whole battery. Although the effect of the upper charge voltage limit on battery failure is not very obvious relative to the loss of active lithium ions, too low or too high an upper charge voltage limit will make the LiFePO4 electrode's interfacial impedance increase: low upper voltage does not form the passivation film well, while too high an upper voltage limit will lead to the 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 battery decreases rapidly when the temperature decreases, mainly due to the decrease of ionic conductivity and the increase of interfacial impedance.Li found that the main controlling factors limiting the low-temperature performance of the positive and negative electrodes are different by studying the LiFePO4 positive and graphite negative electrodes respectively, and that the decrease of ionic conductivity in LiFePO4 positive electrodes is dominant, and the increase of interfacial impedance in graphite negative electrodes is the main reason. The increase in impedance is the main reason.
The degradation of LiFePO4 electrode and graphite negative electrode and the continuous growth of SEI film in the process of use cause the battery failure in different degrees; in addition, besides the uncontrollable factors such as road condition and ambient temperature, the normal use of the battery is also very important, including the appropriate charging voltage and the appropriate depth of discharge.
IV. Failure in the process of charging and discharging
Battery in the use of the process will often inevitably appear overcharging, relatively speaking, less over-discharge, over-charging or over-discharge process of heat release is easy to gather inside the battery, which will further make the battery temperature rise, affecting the service life of the battery, increase the likelihood of battery fire or explosion. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the single cell within the battery system will also increase, and the lowest capacity battery will also experience the process of overcharging and over-discharging.
Although LiFePO4 has the best thermal stability compared to other cathode materials under different charging states, overcharging also triggers unsafe hidden dangers in the use of LiFePO4 power batteries. Under the state of overcharging, the solvents in the organic electrolyte are more prone to oxidative decomposition, and among the commonly used organic solvents, ethylene carbonate (EC) will be prioritized to oxidative decomposition on the surface of the anode. Since the embedded lithium potential (para-lithium potential) of graphite anode is very low, there is a high possibility of lithium precipitation in graphite anode.
One of the main reasons for battery failure under overcharging conditions is the internal short circuit caused by lithium crystal branches piercing the diaphragm.Lu et al. analyzed the failure mechanism of lithium plating on the surface of graphite negative electrode due to overcharging. The results show that the overall structure of the graphite anode has not changed much, but there are lithium dendrites and the appearance of the surface film, and the reaction between lithium and the electrolyte causes the surface film to increase, which not only consumes more active lithium, but also makes it more difficult for lithium diffusion to the graphite anode, which in turn further promotes the deposition of lithium on the surface of the negative electrode, resulting in a further reduction of the capacity and coulombic efficiency.
In addition, metal impurities (especially Fe) are usually considered as one of the main reasons for the overcharge failure of batteries.Xu et al. systematically investigated the failure mechanism of LiFePO4 power batteries under overcharge conditions. The results showed that the redox of Fe during the overcharge/discharge cycle is theoretically possible, and the reaction mechanism was given: when overcharge occurs, Fe is first oxidized to Fe2+, and Fe2+ is further oxidized to Fe3+, and then Fe2+ and Fe3+ diffuse from the anode side to the negative side, and Fe3+ is finally reduced to Fe2+, and Fe2+ is further reduced to form Fe; when overcharge/discharge cycle occurs, the Fe grains will be reduced to Fe2+, and Fe2+ will be further reduced to form Fe. When overcharging/discharging cycle, Fe dendrite will be formed at both positive and negative electrodes, which will puncture the diaphragm to form Fe bridge, resulting in micro-short-circuit of the battery, and the obvious phenomenon accompanying micro-short-circuit of the battery is the continuous increase of temperature after overcharging.
During over-discharge, the potential of the negative electrode will increase rapidly, and the increase in potential will cause the destruction of the SEI film on the surface of the negative electrode (the inorganic compound-rich part of the SEI film is easier to oxidize), which will cause additional decomposition of the electrolyte, resulting in capacity loss. More importantly, the negative electrode collector Cu foil is subject to oxidation, and Yang et al. detected the oxidation product Cu2O of Cu foil in the SEI film of the negative electrode, which causes an increase in the internal resistance of the battery, leading to a loss of capacity.
He et al. studied the overdischarge process of LiFePO4 power battery in detail, and the results showed that the negative electrode collector Cu foil can be oxidized to Cu+ during overdischarge, and Cu+ is further oxidized to Cu2+, and after that, they diffuse to the positive electrode, and can be reduced at the positive electrode, so that the Cu crystalline dendrites are formed at the positive electrode side, which will puncture the diaphragm, and cause the microshort-circuit inside the battery, and the battery temperature will continue to rise due to the overdischarge. battery temperature will also continue to rise.
Overcharging of LiFePO4 power battery may lead to oxidative decomposition of electrolyte, lithium precipitation, formation of Fe dendrite; while over-discharging may cause SEI damage leading to capacity degradation, oxidation of Cu foils, and even formation of Cu dendrite.
V. Other failures
Due to the low conductivity inherent in LiFePO4, the morphology and size of the material itself, as well as the effects of conductive agents and binders are easily manifested.Gaberscek et al. discussed the two contradictory factors of size and carbon cladding, and found that LiFePO4 electrode impedance is only related to the average particle size. In contrast, antisite defects (Fe occupying Li sites) within LiFePO4 can have an effect on the performance of the battery: since the transport of lithium ions within LiFePO4 is one-dimensional, such defects impede the transport of lithium ions; such defects can also cause destabilization of the structure of LiFePO4 due to the additional electrostatic repulsion introduced by the high valence state.
LiFePO4 with large particles does not completely delithiate at the end of charging; nanostructured LiFePO4 reduces the antisite defect, but causes self-discharge due to its high surface energy. The binder used more often is PVDF, which has the disadvantages of possible reaction at high temperatures, solubility in non-aqueous electrolyte, and also insufficient flexibility, which has a certain impact on the capacity loss and cycle life shortening of LiFePO4. In addition, the collector, diaphragm, electrolyte composition, production process, human factors, external vibration and shock, etc. will affect the performance of the battery in different degrees.
VI. Prospect
The loss of active lithium ions is the most important cause of LiFePO4 power battery failure during normal battery use. Therefore, for LiFePO4 power battery (graphite anode), the quality and stability of SEI film is the key to improve the cycle life of the battery. The formation process of SEI film (including the changes of its morphology and thickness), the mechanism of film-forming additives and the diffusion mechanism of lithium ions in the SEI film are increasingly understood by various experimental and theoretical methods, which also provides favorable conditions for improving the service life of LiFePO4 power battery.
