Research on the failure of lithium iron phosphate power lithuim ion battery 18650

　　Lithium iron phosphate has attracted a lot of attention as a competitive cathode material for electric vehicles. Understanding the failure reasons or mechanisms of lithium iron phosphate batteries is very important for improving battery performance and its mass production and use. This article reviews the research progress on the failure of lithium iron phosphate power batteries in recent years. The effects of impurities, formation methods, storage conditions, recycling, overcharge and over-discharge on battery failure are discussed.

　　1. Failure during the production process

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

　　Failure of batteries caused by impurities in electrode active materials

　　During the synthesis process of LiFePO4, 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 and may pierce the separator and cause an internal short circuit. When LiFePO4 is exposed to the air for a long time, moisture will cause the battery to deteriorate. Aging mechanism of lithium iron phosphate battery: Amorphous iron phosphate is formed on the surface of the material in the early stage of aging, and its local composition and structure are similar to LiFePO4(OH); with the embedding of OH, LiFePO4 is continuously consumed, showing an increase in volume; later Crystallization slowly forms LiFePO4(OH). The Li3PO4 impurity in LiFePO4 is electrochemically inert. The higher the impurity content of the graphite negative electrode, the greater the irreversible capacity loss.

　　Failure caused by formation method to battery

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

　　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 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 - large current constant current and constant voltage is more commonly used, which can take into account the advantages of both.

　　Failure of batteries caused by moisture in the production environment

　　In actual production, batteries will inevitably come into contact with air. Since most of the positive and negative electrode materials are micron or nanometer-sized particles, and the solvent molecules in the electrolyte contain highly electronegative carbonyl groups and metastable carbon-carbon double bonds, All easily absorb moisture in the air.

　　The reaction between water molecules and the lithium salt in the electrolyte not only decomposes and consumes the electrolyte, but also produces the acidic substance HF. will destroy the SEI film, and HF will also promote the corrosion of LiFePO4 active material. Water molecules will also partially delithiate the lithium-embedded 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 LiFePO4 batteries.

　　During the production process, in addition to the production process affecting battery performance, the main influencing factors causing 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, etc. factors appear to be crucial.

　　2. Failure on hold

　　During the service life of a power battery, most of the time it is in a state of shelving. Generally, after a long period of time, 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 to degrade, among which temperature, state of charge and time are the most obvious factors.

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

　　Storage temperature has a greater impact on the aging of LiFePO4 power batteries, followed by storage state of charge. The capacity loss of LiFePO4 power batteries can be predicted based on factors related to storage time (temperature and state of charge). As the storage time increases under a certain SOC state, 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, and the SEI will become thicker and less conductive. The increase in impedance caused by the decrease (increased inorganic components, some of which have the opportunity to be redissolved) and the decrease in activity on the electrode surface jointly cause the aging of the battery.

　　Regardless of the charging state or the discharge state, differential scanning calorimetry did not find any reaction between LiFePO4 and different electrolytes in the temperature range from room temperature to 85°C. However, if LiFePO4 is immersed in the LiPF6 electrolyte for a long time, it will still show a certain degree of reactivity: because the reaction to form an interface is very slow, 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 state of charge) will increase the degree of self-discharge of LiFePO4 power batteries, making the aging of the battery more obvious.

　　3. Failure during recycling

　　Batteries generally release heat during use, so the impact of temperature is important. In addition, road conditions, usage patterns, ambient temperature, etc. will all have different effects.

　　The capacity loss of LiFePO4 power batteries during cycling is generally believed to be caused by the loss of active lithium ions. Research shows that the aging of LiFePO4 power batteries during cycling is mainly due to a complex growth process that consumes active lithium ion SEI films. In this process, the loss of active lithium ions directly reduces the battery capacity retention rate; the continuous growth of the SEI film causes an increase in the polarization resistance of the battery. At the same time, the thickness of the SEI film is too thick, and the electrochemistry of the graphite negative electrode Activity will also be partially inactivated.

　　During high-temperature cycles, Fe2+ in LiFePO4 will dissolve to a certain extent. Although the amount of Fe2+ dissolved has no obvious impact on the capacity of the cathode, 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. . The loss of most active lithium ions occurs on the surface of the graphite anode, especially during high-temperature cycles, that is, the high-temperature cycle capacity is lost faster; there are three different mechanisms for the destruction and repair of the SEI film: (1) Electron penetration in the graphite anode Reduction of lithium ions through the SEI film; (2) dissolution and regeneration of some components of the SEI film; (3) rupture of the SEI film caused by volume changes of the graphite anode.

　　In addition to the loss of active lithium ions, both positive and negative electrode materials deteriorate during recycling. The occurrence of cracks in the LiFePO4 electrode during recycling will lead to an increase in electrode polarization and a decrease in conductivity between the active material and the conductive agent or current collector. The coarsening of LiFePO4 nanoparticles and surface deposits produced by certain chemical reactions together lead to an increase in the resistance of the LiFePO4 cathode. In addition, the reduction of the active surface caused by the loss of graphite active materials and the peeling of graphite electrodes are also considered to be the causes of battery aging. The instability of the graphite negative electrode will lead to the instability of the SEI film and promote the consumption of active lithium ions. .

　　The high-rate discharge of the battery can provide high power for electric vehicles. 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 caused by the consumption of active lithium ions in 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 during use of a power battery will not affect the capacity loss, it will affect its power loss: the speed of power loss increases with the increase in the depth of discharge, which is related to the increase in the impedance of the SEI film and the increase in the impedance of the entire battery. directly related. A charging voltage limit that is too low or too high will increase the interface impedance of the LiFePO4 electrode: a passivation film cannot be formed well at a low limit voltage, while a voltage limit that is too high will cause oxidation and decomposition of the electrolyte. In LiFePO4 Products with low conductivity form on the electrode surface.

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

　　During use, the degradation of LiFePO4 electrodes, graphite negative electrodes 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 The charging voltage, appropriate discharge depth, etc.

　　4. Failure during charging and discharging

　　During the use of the battery, overcharging is unavoidable. Over-discharging is relatively rare. The heat released during overcharging or over-discharging 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 battery fire or explosion. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the individual cells within the battery system will increase, and the battery with the lowest capacity will also experience overcharge and overdischarge.

　　Although LiFePO4 has the best thermal stability compared to other cathode materials under different charging states, overcharging can also cause unsafe risks during use of LiFePO4 power batteries. In an overcharged state, the solvent in the organic electrolyte is more likely to undergo oxidative decomposition. Among commonly used organic solvents, ethylene carbonate will preferentially undergo oxidative decomposition on the surface of the positive electrode. Since the lithium insertion potential of the graphite negative electrode is very low, there is a high possibility of lithium precipitation in the graphite negative electrode.

　　One of the main causes of battery failure under overcharge conditions is internal short circuit caused by lithium dendrites puncturing the separator. The emergence of lithium dendrites and surface films on the graphite negative electrode, and the reaction between lithium and the electrolyte cause the surface film to continue to increase, which not only consumes more active lithium, but also makes it more difficult for lithium to diffuse into the graphite negative electrode, which in turn will further promote The deposition of lithium on the surface of the negative electrode causes further reduction in capacity and Coulombic efficiency.

　　In addition, metal impurities are often considered to be one of the main causes of battery failure due to overcharge. The redox of Fe during overcharge/discharge cycles is theoretically possible. Fe dendrites will form on both the positive and negative electrodes at the same time, which will pierce the separator and form an Fe bridge, causing a micro-short circuit in the battery. The obvious phenomenon accompanying the micro-short circuit in the battery is The temperature continues to rise after overcharging.

　　During overdischarge, the potential of the negative electrode will rise rapidly. The increase in 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 susceptible to oxidation), which will in turn cause additional decomposition of the electrolyte. , resulting in capacity loss. More importantly, the negative electrode current collector Cu foil will be oxidized. This will cause the internal resistance of the battery to increase, causing battery capacity loss.

　　The Cu foil of the negative electrode current collector can be oxidized into Cu+ during overdischarge, and Cu+ is further oxidized into Cu2+. After that, they diffuse to the positive electrode, and a reduction reaction can occur at the positive electrode. In this way, Cu dendrites will form on the positive electrode side, which will pierce the separator and cause battery damage. Internal micro short circuit, also due to over-discharge, the battery temperature will continue to rise.

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

　　5. Failure in other aspects

　　Due to the low intrinsic conductivity of LiFePO4, the shape 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 conflicting factors of size and carbon coating and found that the LiFePO4 electrode impedance is only related to the average particle size. The anti-site defect inside LiFePO4 (Fe occupies the Li site) 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 In addition to additional electrostatic repulsion, this defect will also cause instability in the LiFePO4 structure.

　　Large particle LiFePO4 cannot completely delithiate at the end of charging; nanostructured LiFePO4 can reduce anti-site defects, but will cause self-discharge due to its high surface energy. The most commonly used binder at present is PVDF, which has the disadvantages of reacting at high temperatures, being soluble in non-aqueous electrolytes, and not being flexible enough. It has a certain impact on the capacity loss and cycle life shortening of LiFePO4. In addition, the current collector, separator, electrolyte composition, production process, human factors, external vibration and impact, etc. will affect the performance of the battery to varying degrees.

　　6. Outlook

　　In the normal use of batteries, the loss of active lithium ions is the main reason for the failure of LiFePO4 power batteries. Therefore, for LiFePO4 power batteries (graphite anode), the quality and stability of the SEI film are the key to improving the battery cycle life. The formation process of the SEI film (including changes in its morphology and thickness), the action mechanism of film-forming additives, and the diffusion mechanism of lithium ions in the SEI film have been increasingly understood through various experimental and theoretical methods. Understand that this also provides favorable conditions for increasing the service life of LiFePO4 power batteries.

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