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. As personnel and equipment belong to the category of management, we will focus on the last three factors.
Battery Failure Caused by Impurities in the Active Material of the Electrode
During the synthesis of LiFePO4, there will be a small number of impurities, such as Fe2O3 and Fe. They will be reduced on the surface of the anode, which may pierce the diaphragm to trigger 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 it slowly forms LiFePO4(OH) by recrystallisation. The Li3PO4 impurity in LiFePO4, on the other hand, is electrochemically inert. The higher the impurity content of the graphite anode, the greater the irreversible capacity loss.
Failure of Batteries Caused by Formation
The irreversible loss of active Li-ions is firstly reflected in the Li-ions consumed during the formation of Solid Electrolyte Interphase (SEI). It has been found that increasing the formation temperature will cause more irreversible loss of Li-ions, because the proportion of inorganic components in the SEI will increase at higher formation temperatures, and the gas released during the transition from the organic component ROCO2Li to the inorganic component Li2CO3. It will cause more defects in the SEI, through which the solvated Li-ions intercalate into the graphite anode in large quantities.
During formation, low-current charging results in a uniform composition and thickness of the SEI, but it is time-consuming, while high-current charging results in more side reactions, leading to increased irreversible Li-ion losses and negative interface impedance, but it is time-saving. At present, the low current constant current – high current constant voltage mode of formation is used more, which takes into account both advantages.
Failure of Batteries Caused by Moisture in the Production Environment
In practice, the battery will inevitably come into contact with air. Since most of the positive and negative materials are micron or nanoscale particles, and there are solvent molecules in the electrolyte with electronegative carbonyl groups and metastable carbon-carbon double bonds, both of which tend to absorb moisture in the air.
The reaction between the water molecules and the lithium salts in the electrolyte (especially LiPF6) not only decomposes and consumes the electrolyte (decomposes to form PF5), but also produces the HF acid. However, both PF5 and HF will destroy the SEI, and HF will also promote the corrosion of the LiFePO4 active material. The water molecules will also delithiate the lithium-intercalated graphite anode, forming lithium hydroxide at the bottom of the SEI. In addition, the dissolved O2 in the electrolyte accelerates the aging of the LiFePO4 batteries.
In addition to the production process, which affects 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 formation process, so the purity of the material, the control of environmental humidity, the formation method and other factors are crucial.
Failure in Abeyance
In the service life of the power battery, most of the time it is in a state of shelving. Generally, after a long period of shelving, the battery performance will decline, showing an increase in internal resistance, voltage reduction and discharge capacity decline. Many factors cause the degradation of battery performance, among which temperature, charge state and time are the most noticeable factors.
Kassema et al. analyzed the aging of LiFePO4 power batteries under different shelving states and concluded that the aging mechanism was mainly the side reactions of the positive and negative electrodes and the electrolyte (the graphite negative side reactions are heavier compared to those of the positive electrode, mainly due to the solvent decomposition and the growth of the SEI) consume active Li-ions, and at the same time the overall impedance of the battery increases. The loss of active Li-ions leads to the aging of battery shelving, and the capacity loss of LiFePO4 power battery increases greatly with the increase of storage temperature. In contrast, as the stored state of charge increases, there is a lesser degree of capacity loss.
The same conclusion was reached by Grolleau et al. that storage temperature has a great 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 shelving time, the lithium in the graphite diffuses towards the edges, forming a complex complex with electrolytes and electrons, and the resulting irreversible Li-ion ratio increases. The thickened SEI and reduced conductivity (inorganic components increase, part of which can re-dissolve) combined with decreased activity at the electrode surface contribute to the aging of the battery.
Differential scanning calorimetry does not find any reaction between LiFePO4 and different electrolytes (LiBF4, LiAsF6, or LiPF6), either in the charged or discharged state and at temperatures ranging from room temperature to 85°C. However, when LiFePO4 is immersed in the electrolyte of LiPF6 for a long time, it still shows reactivity, because the reaction forms the interface very slowly. After one month of immersion, there is still no passivation film on the surface of LiFePO4 to prevent further reaction with the electrolyte.
In the shelving state, harsh storage conditions (high temperature and high charge state) increase the degree of self-discharge of the LiFePO4 power battery, making the aging of the battery more pronounced.
Failure in Recycling
Batteries are generally exothermic during use, so the effect of temperature is important. In addition, road conditions, usage, ambient temperature, etc. have different effects.
The capacity loss of LiFePO4 power batteries during cycling is considered to be caused by the loss of active Li-ions. The research of Dubarry et al. shows that the aging of LiFePO4 power battery during cycling is through a complex growth process that consumes the active Li-ion SEI. In this process, the loss of active Li-ions directly reduces the capacity retention rate of the battery. The continuous growth of the SEI causes an increase in the polarization impedance of the battery on the one hand, while at the same time the SEI is too thick and the electrochemical activity of the graphite anode is partially deactivated.
During high-temperature cycling, there is a certain amount of Fe2+ dissolution in LiFePO4. Although the amount of Fe2+ dissolution has no significant effect on the capacity of the cathode, the dissolution of Fe2+ and the precipitation of Fe in the graphite cathode will catalyze the growth of the SEI. Tan quantitatively analyzed where and in which steps the active Li-ions are lost, and found that most of the active Li-ion loss occurs on the surface of the graphite anode, especially during high-temperature cycling, i.e., a faster loss of capacity occurs during high-temperature cycling. Three different mechanisms for the destruction and repair of the SEI were also summarized: (1) reduction of Li-ions through the SEI by electrons in the graphite anode; (2) dissolution and regeneration of some components of the SEI; (3) rupture of the SEI due to volume changes in the graphite anode.
In addition to the loss of active Li-ions, both positive and negative electrode materials deteriorate during cycling. The appearance of cracks in LiFePO4 electrodes during cycling leads 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 semi-quantitatively study the changes of LiFePO4 after aging, 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 leads to the instability of SEI, which promotes the consumption of active Li-ions.
The high-rate discharge of the battery can provide great power for the electric vehicle, i.e., 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 in discharge rate, the capacity loss of the cathode increases more than that of the anode. The loss of battery capacity during low-rate cycling is caused by the depletion of active Li-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 does affect its power loss. The speed of power loss increases with the increase of the depth of discharge, which is directly the increased impedance of the SEI and the entire battery. Although the effect of the upper limit of charging voltage on battery failure is not relative to the loss of active Li-ions, too low or too high an upper charge voltage limit can make the interfacial impedance of the LiFePO4 electrode increase. Specifically, a low upper voltage limit does not allow for good passivation film formation, while a too high voltage limit can 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. 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 electrodes and graphite negative electrodes and the continuous growth of SEI cause battery failure to varying degrees. In addition, apart from uncontrollable factors, such as road conditions and ambient temperature, the normal use of the battery is also very important, including appropriate charging voltage, suitable depth of discharge, etc.
Failure during Charging and Discharging
The battery is often overcharged in the process of use. Relatively speaking, the over-discharge situation is less. The heat released during the overcharge or over-discharge process tends 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 fire or explosion. Even under normal charge-discharge conditions, as the number of cycles increases, the capacity inconsistency of the single cells within the battery system increases. The battery with the lowest capacity will also experience overcharge and over-discharge.
Although the thermal stability of LiFePO4 is the best compared to other cathode materials under different charging states, overcharging can also lead to unsafe hazards during 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 (para-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 little 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 makes it more difficult for lithium to diffuse into 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, metal impurities (especially Fe) are generally considered to be one of the main reasons for battery failure under overcharge conditions. 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, i.e., when overcharge occurs, Fe is first oxidized to Fe2+, Fe2+ is further oxidized to Fe3+, and then Fe2+ and Fe3+ diffuse from the positive side to the negative side. Fe3+ is finally reduced to Fe2+, and Fe2+ is further reduced to form Fe. In 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 a micro-short circuit in the battery. The obvious phenomenon accompanying the micro-short circuit in the battery is the continuous increase in temperature after overcharging.
During over-discharge, the potential of the negative electrode rises rapidly, which causes damage to the SEI on the surface of the negative electrode (the part rich in inorganic compounds in the SEI is more easily oxidized), which in turn causes additional decomposition of the electrolyte, resulting in a loss of capacity. More importantly, the anode current collector Cu foil is subject to oxidation. Yang et al. detected Cu2O, the oxidation product of Cu foil, in the SEI 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 over-discharge process of LiFePO4 power batteries in detail. The results show that the negative current collector Cu foil can be oxidized to Cu+ during over-discharge, 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. Besides, due to over-discharge, the battery temperature will continue to rise.
Overcharging of LiFePO4 power batteries may lead to oxidative decomposition of electrolytes, lithium precipitation, and formation of Fe crystal dendrites, while over-discharge may cause SEI damage, resulting in capacity decay, Cu foil oxidation, and even the formation of Cu crystal dendrites.
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 the conductive agent and binder, are easily manifested. Gaberscek et al. discussed the contradictory factors of size and carbon cladding and found that the LiFePO4 electrode impedance is only related to the average particle size. In contrast, anti-site defects within LiFePO4 (Fe occupies Li site) can have an impact on the performance of the battery. As the transport of Li-ions within LiFePO4 is one-dimensional, such defects can hinder the transport of Li-ions. Such defects can also cause instability in the LiFePO4 structure due to the additional electrostatic repulsion introduced by the high valence states.
The large-sized LiFePO4 cannot fully delithiate at the end of charging, while the nano-structured LiFePO4 can reduce the anti-site defects, but it can cause self-discharge due to its high surface. At present, the most commonly used binder is PVDF, which may react at high temperature, dissolve in the non-aqueous electrolyte, and is insufficiently flexible, which has an 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.
