In the production process, personnel, equipment, raw materials, methods, and environment are the main factors that affect product quality. In the production process of LiFePO4 power batteries, personnel and equipment belong to the scope of management, so we mainly discuss the last three effects factor.
1) Battery failure caused by impurities in the electrode active material
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 and may pierce the diaphragm and cause an internal short circuit. When LiFePO4 is exposed to the air for a long time, moisture will deteriorate the battery. In the early stage of aging, amorphous iron phosphate is formed on the surface of the material. 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; later recrystallized slowly to form 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.
2) The 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 interfacial membrane. Studies have found that increasing the formation temperature will cause more irreversible loss of lithium ions, because when the formation temperature is increased, the proportion of inorganic components in the SEI film will increase, and the gas released during the transformation from the organic component ROCO2Li to the inorganic component Li2CO3 It will cause more defects in the SEI film, and a large amount of lithium ions solvated by these defects will be embedded in the graphite negative electrode.
During formation, the composition and thickness of the SEI film formed by low-current charging are uniform, but time-consuming; high-current charging will cause more side reactions to occur, resulting in increased irreversible lithium ion loss, and the negative electrode interface impedance will also increase, but it saves time. At present, the formation mode of small current constant current-high current constant current and constant voltage is used more frequently, so that the advantages of both can be taken into account.
3) Battery failure caused by moisture in the production environment
In actual production, the battery will inevitably come into contact with the air, because the positive and negative materials are mostly micron or nano-sized particles, and the solvent molecules in the electrolyte have large electronegative carbonyl groups and metastable carbon-carbon double bonds. All easily absorb moisture in the air.
The water molecules react with the lithium salt (especially LiPF6) in the electrolyte, which not only decomposes and consumes the electrolyte (decomposes to form PF5), but also produces the acidic substance HF. Both PF5 and HF will destroy the SEI film, and HF will also promote the corrosion of the LiFePO4 active material. Water molecules will also delithiate the lithium-intercalated graphite negative electrode, forming lithium hydroxide at the bottom of the SEI film. In addition, O2 dissolved in the electrolyte will also accelerate the aging of LiFePO4 batteries.
In the production process, in addition to the production process that affects the battery performance, the main factors that cause the failure of the LiFePO4 power battery include the impurities in the raw materials (including water) and the formation process, so the purity of the material, the control of the environmental humidity, the formation method, etc. Factors are crucial.
2. Failure in shelving
During the service life of a power battery, most of its time is in a state of shelving. Generally, after a long time of shelving, 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 the degradation of battery performance, of which temperature, state of charge and time are the most obvious influencing factors.
Kassema et al. analyzed the aging of LiFePO4 power batteries under different storage conditions, and believed that the aging mechanism is mainly the side reaction of the positive and negative electrodes and the electrolyte (compared to the side reaction of the positive electrode, the side reaction of the graphite negative electrode is heavier, mainly caused by the solvent. Decomposition, the growth of the SEI film) consumes active lithium ions, and at the same time the entire impedance of the battery increases, the loss of active lithium ions leads to the aging of the battery when it is left; and the capacity loss of LiFePO4 power batteries increases with the increase of storage temperature. In contrast, as the storage state of charge increases, the capacity loss is smaller.
Grolleau et al. also reached 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 battery can be predicted based on factors related to storage time (temperature and state of charge). In a certain SOC state, as the shelf time increases, the lithium in the graphite will diffuse to the edge, forming a complex compound with the electrolyte and electrons, resulting in an increase in the proportion of irreversible lithium ions, thickening of the SEI and conductivity The increase in impedance caused by the decrease (inorganic components increase and some have a chance to re-dissolve) and the decrease in the activity of the electrode surface together cause the aging of the battery.
Regardless of the charging state or the discharging state, the differential scanning calorimetry did not find any reaction between LiFePO4 and different electrolytes (the electrolyte is LiBF4, LiAsF6 or LiPF6) 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 exhibit certain reactivity: because the reaction to form the interface is very slow, there is still no passivation film on the surface of LiFePO4 to prevent further reaction with the electrolyte after immersing for one month.
In the shelving state, poor 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
Batteries generally emit heat during use, so the influence of temperature is very important. In addition, road conditions, usage, and ambient temperature will all have different effects.
The capacity loss of LiFePO4 power battery during cycling is generally considered to be caused by the loss of active lithium ions. The study by Dubarry et al. showed that the aging of LiFePO4 power battery during cycling is mainly due to a complex growth process that consumes active lithium ion SEI film. 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 in the polarization resistance of the battery. At the same time, the thickness of the SEI film is too thick, and the electrochemical performance of the graphite anode The activity will also be partially inactivated.
During high temperature cycling, Fe2+ in LiFePO4 will dissolve to a certain extent. Although the amount of Fe2+ dissolved has no significant 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 where 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 is faster; and summarized the SEI film There are three different mechanisms of damage and repair: (1) The electrons in the graphite anode pass through the SEI film to reduce lithium ions; (2) The dissolution and regeneration of some components of the SEI film; (3) due to the volume change of the graphite anode The SEI membrane caused by rupture.
In addition to the loss of active lithium ions, both positive and negative materials will deteriorate during recycling. The occurrence of cracks in the LiFePO4 electrode during recycling will cause the electrode polarization to increase and the conductivity between the active material and the conductive agent or current collector to decrease. 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 surface deposits produced by certain chemical reactions together led to an increase in the impedance of LiFePO4 cathodes. In addition, the reduction of active surface and the exfoliation of graphite electrode caused by the loss of graphite active material are also considered to be the cause of battery aging. The instability of graphite anode will cause the instability of the SEI film and promote the consumption of active lithium ions. .
The high-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 research results of Kim et al. showed that the aging mechanism of LiFePO4 positive electrode and graphite negative electrode is different: with the increase of discharge rate, 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 in the impedance of the positive electrode.
Although the depth of discharge of the power battery in use will 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. This is due to the increase in the impedance of the SEI film and the increase in the impedance of the entire battery. Directly related. Although relative to the loss of active lithium ions, the upper limit of the charging voltage has no obvious influence on battery failure, but a too low or too high upper limit of the charging voltage will increase the interface impedance of the LiFePO4 electrode: a low upper limit voltage will not work well. The passivation film is formed on the ground, and a too high upper voltage limit will cause the oxidative decomposition of the electrolyte, and a product with low conductivity will be formed on the surface of the LiFePO4 electrode.
The discharge capacity of LiFePO4 power battery will drop rapidly when the temperature decreases, mainly due to the decrease of ion conductivity and the increase of interface impedance. Li studied LiFePO4 cathode and graphite anode separately, and found that the main control factors that limit the low temperature performance of anode and anode are different. The decrease in ionic conductivity of LiFePO4 cathode is dominant, and the increase in the interface impedance of graphite anode is the main reason.
During use, the degradation of LiFePO4 electrode and graphite anode and the continuous growth of SEI film will 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, the appropriate depth of discharge, etc.
4. Failure during charging and discharging
The battery is often inevitably overcharged during use. Relatively speaking, there is less overdischarge. The heat released during overcharge or overdischarge is likely to accumulate inside the battery, which will further increase the battery temperature. , It affects the service life of the battery and increases the possibility of fire or explosion of the battery. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the single cells in the battery system will increase, and the battery with the lowest capacity will undergo the process of charging and overdischarging.
Although the thermal stability of LiFePO4 is the best compared to other cathode materials under different charging conditions, overcharging can also cause unsafe hidden dangers in the use of LiFePO4 power batteries. In the overcharged state, the solvent in the organic electrolyte is more prone to oxidative decomposition. Among the commonly used organic solvents, ethylene carbonate (EC) will preferentially undergo oxidative decomposition on the surface of the positive electrode. Since the lithium insertion potential (versus 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 crystal branches piercing the diaphragm. Lu et al. analyzed the failure mechanism of lithium plating on the graphite negative electrode surface caused by overcharge. The results show that the overall structure of the graphite negative electrode has not changed, but there are lithium crystal branches and surface film. The reaction of lithium and electrolyte causes the surface film to increase continuously, which not only consumes more active lithium, but also causes lithium to diffuse into graphite. The negative electrode becomes more difficult, which in turn will further promote the deposition of lithium on the surface of the negative electrode, 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 overcharge failure. Xu et al. systematically studied the failure mechanism of LiFePO4 power batteries under overcharge conditions. The results show that the redox of Fe during the overcharge/discharge cycle is theoretically possible, 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 electrode side, Fe3+ is finally reduced to Fe2+, and Fe2+ is further reduced to form Fe; when overcharge/discharge cycles, Fe crystal branches will form at the positive and negative electrodes at the same time, piercing the separator to form Fe bridges, resulting in micro battery Short circuit, the obvious phenomenon that accompanies the battery's micro short circuit is the continuous increase in temperature after overcharging.
During overdischarge, the potential of the negative electrode will rise 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 likely to be oxidized), which will cause additional decomposition of the electrolyte , Resulting in a loss of capacity. More importantly, the negative current collector Cu foil will be 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, and the results showed that the negative current collector Cu foil can be oxidized to Cu+ during overdischarge, and Cu+ is further oxidized to Cu2+, after which they diffuse to the positive electrode, and a reduction reaction can occur at the positive electrode. In this way, Cu crystal branches will be formed on the positive electrode side, which will pierce the separator and cause a micro short circuit inside the battery. Also, due to over-discharge, the battery temperature will continue to rise.
Overcharge of LiFePO4 power battery may cause electrolyte oxidative decomposition, lithium evolution, and formation of Fe crystal branches; overdischarge may cause SEI damage, resulting in capacity degradation, Cu foil oxidation, and even formation of Cu crystal branches.
5. other failures
Due to the inherent low conductivity of 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 coating, and found that the electrode impedance of LiFePO4 is only related to the average particle size. The anti-site defects in LiFePO4 (Fe occupies Li sites) will have a certain impact on the performance of the battery: because the transmission of lithium ions inside LiFePO4 is one-dimensional, this defect will hinder the transmission of lithium ions; due to the introduction of high valence states Due to the additional electrostatic repulsion, this defect can also cause the instability of the LiFePO4 structure.
The large particles of LiFePO4 cannot be completely delithiated at the end of charging; the nano-structured LiFePO4 can reduce inversion defects, but due to its high surface energy, it will cause self-discharge. PVDF is the most commonly used binder at present, which has disadvantages such as reaction at high temperature, dissolution in non-aqueous electrolyte, and insufficient flexibility. It has a certain impact on the capacity loss and cycle life of LiFePO4. In addition, the current collector, diaphragm, electrolyte composition, production process, human factors, external vibration and shock, etc. will affect the performance of the battery to varying degrees.
