Why does the lithium iron phosphate battery fail?

Understanding the cause or mechanism of failure of lithium iron phosphate batteries is very important for improving battery performance and its large-scale production and use. This article discusses the effects of impurities, formation methods, storage conditions, recycling, overcharge, and over-discharge on battery failure.

1. Failure in the production process

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.

The impurity in the active electrode material causes the failure of the battery.

During the synthesis of LiFePO4, there will be a small number 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.

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 while forming the solid electrolyte interfacial membrane. Studies have found that increasing the formation temperature will cause a more irreversible loss of lithium ions. When the formation temperature is increased, the proportion of inorganic components in the SEI film will increase. The gas released during the transformation from the organic part ROCO2Li to the inorganic component Li2CO3 will cause more defects in the SEI film. A large number of lithium ions solvated by these defects will be embedded in the negative graphite electrode.

During the 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. Time; Nowadays, the formation mode of small current constant current-large current constant current and constant voltage is used more frequently so that it can take the advantages of both into account.

Battery failure caused by moisture in the production environment

In actual production, the battery will inevitably contact 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 decomposes and consumes the electrolyte (decomposes to form PF5) and 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 shelving time, the battery performance will decrease, usually showing an increase in internal resistance, a decrease in voltage, and a decrease in discharge capacity. Many factors cause the degradation of battery performance, of which temperature, state of charge, and time are the most apparent influencing factors.

Kassem et al. analyzed the aging of LiFePO4 power batteries under different storage conditions. They believed that the aging mechanism is mainly the side reaction of the positive and negative electrodes. The electrolyte (compared to the side reaction of the positive electrode, the side reaction of the negative graphite electrode is heavier, mainly caused by the solvent. Decomposition, the growth of the SEI film) consumes active lithium ions. At the same time, the total impedance of the battery increases, the loss of active lithium ions leads to the battery's aging when it is left. The capacity loss of LiFePO4 power batteries increases with the rise of storage temperature. In contrast, as the storage state of charge increases, the capacity loss is more minor.

Grolleau et al. also reached the same conclusion: the storage temperature has a more significant impact on the aging of LiFePO4 power batteries, followed by the storage state of charge, and a simple model is proposed. It can predict the capacity loss of the LiFePO4 power battery based on factors related to storage time (temperature and state of charge). In a specific 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 reduction in the electrode surface activity together cause the battery's aging.

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 specific reactivity. Because the reaction to form the interface is prolonged, 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 significant. In addition, road conditions, usage, and ambient temperature will all have different effects.

The loss of active lithium ions generally causes the capacity loss of LiFePO4 power batteries during cycling. Dubarry et al. showed that the aging of LiFePO4 power batteries during cycling is mainly due to a complex growth process that consumes functional 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. It will partially inactivate the activity.

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 negative graphite 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 negative graphite electrode, especially during high-temperature cycles, that is, the high-temperature cycle 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 was 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 specific 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 electrodes caused by the loss of active graphite 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 significant power for the electric vehicle; that is, the better the rate performance of the power battery, the better the acceleration performance of the electric car. 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. In contrast, 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 rise in the impedance of the SEI film and the increase in the impedance of the entire battery. It is directly related. Although relative to the loss of active lithium ions, the upper limit of the charging voltage has no apparent influence on battery failure, 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. It will create a product with low conductivity on the surface of the LiFePO4 electrode.

The discharge capacity of the LiFePO4 power battery will drop rapidly when the temperature decreases, mainly due to the reduction 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 regular use of the battery is also essential, including appropriate charging voltage, the appropriate depth of discharge, etc.

4. failure during charging and discharging

The battery is often inevitably overcharged during use. There is less over-discharge. The heat released during overcharge or over-discharge is likely to accumulate inside the battery, further increasing the battery temperature. It affects the battery's service life and raises the possibility of fire or explosion of the storm. Even under regular charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of the single cells in the battery system will increase. The battery with the lowest capacity will undergo the process of charging and over-discharging.

Although LiFePO4 has the best thermal stability compared to other positive electrode materials under different charging conditions, overcharging can also cause unsafe risks in using 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 negative graphite electrode is shallow, lithium precipitation is highly likely in the negative graphite 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 opposing electrode surface caused by overcharge. The results show that the overall structure of the negative graphite 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 consumes more active lithium and causes lithium to diffuse into graphite. The negative electrode becomes more complex, which will further promote lithium deposition 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 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+ further deteriorates 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 begin at the positive and negative electrodes at the same time, piercing the separator to create Fe bridges, resulting in micro battery Short circuit, the apparent phenomenon that accompanies the battery's micro short circuit is the continuous increase in temperature after overcharging.

During overcharge, the potential of the negative electrode will rise rapidly. The potential increase will destroy 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. In the SEI film of the negative electrode, Yang et al. detected Cu2O, the oxidation product of Cu foil, which would increase the battery's internal resistance and cause the capacity loss of the storm.

He et al. studied the over-discharge process of LiFePO4 power batteries in detail. The results showed that the negative current collector Cu foil could be oxidized to Cu+ during over-discharge, and Cu+ is further oxidized to Cu2+, after which they diffuse to the positive electrode. A reduction reaction can occur at the positive electrode. In this way, it will form crystal branches on the positive electrode side, 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 oxidative electrolyte decomposition, lithium evolution, and formation of Fe crystal branches; over-discharge may cause SEI damage, resulting in capacity degradation, Cu foil oxidation, and even appearance Cu crystal branches.

5. other failures

Due to the inherent low conductivity of LiFePO4, the morphology and size of the material itself and 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 particular impact on the performance of the battery: because the transmission of lithium ions inside LiFePO4 is one-dimensional, this defect will hinder the communication 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 utterly delighted at the end of charging; the nano-structured LiFePO4 can reduce inversion defects, but its high surface energy will cause self-discharge. PVDF is the most commonly used binder at present, which has disadvantages such as reaction at high temperature, dissolution in the non-aqueous electrolyte, and insufficient flexibility. It has a particular impact on the capacity loss and cycle life of LiFePO4. In addition, the current collector, diaphragm, electrolyte composition, production process, human factors, external vibration, shock, etc., will affect the battery's performance to varying degrees.

Reference: Miao Meng et al. "Research Progress on the Failure of Lithium Iron Phosphate Power Batteries."