What causes a battery failure? LiFePO4 battery failure reasons
It is very important to understand the battery failure reasons and mechanism of lithium iron phosphate batteries for improving the performance of batteries and their large-scale production and use. In addition to the battery failure reasons that we as users cannot influence or change, the effects of storage conditions, recycling, overcharge and overdischarge as battery failure reasons are discussed in this paper.
Battery failure reasons of shelving process
In the service life of the power battery, most of its time is in the state of shelving, generally after a long time of shelving, battery performance will decline, generally showing an increase in internal resistance, voltage reduction and discharge capacity decline.
There are many factors that cause the degradation of battery performance, among which temperature, state of charge and time are the most obvious influencing factors.
Kassema et al. Analyzed the aging of LiFePO4 power battery under different shelving conditions, its aging mechanism was thought to be mainly positive and negative electrodes and electrolyte side effects (relative to the anode side effects, graphite anode side effect is heavier, and mainly is the solvent of decomposition, the growth of the SEI film) consumes reactive lithium ion, the whole battery impedance increase at the same time.
The loss of active lithium ions leads to the aging of batteries. Moreover, the capacity loss of LiFePO4 power battery increases significantly with the increase of storage temperature. In contrast, the capacity loss degree is smaller with the increase of storage charge state.
Grolleau et al. also reached the same conclusion: Storage temperature is one of the LiFePO4 battery failure reasons, and it has a greater impact on the aging of LiFePO4 power battery, followed by the storage state of charge. And proposed a simple model that 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 storage time, the lithium in graphite will diffuse to the edge and form a complex complex with electrolyte and electrons, resulting in an increase in the proportion of irreversible lithium ions, thicker SEI and reduced electrical conductivity. The increased resistance and reduced activity of the electrode surface combine to be the battery failure reasons.
Differential scanning calorimetry did not detect any reaction between LiFePO4 and different electrolytes (LiBF4, LiAsF6 or LiPF6), either in charge or discharge state, within the temperature range from room temperature to 85℃.
However, LiFePO4 still shows some reactivity after being immersed in LiPF6 electrolyte for a long time: due to the very slow rate of interface formation, there is still no passivation film on the surface of LiFePO4 to prevent further reaction with the electrolyte after being immersed for a month.
In shelvingidle state, bad storage conditions (high temperature and high charge state) will increase the degree of self-discharge of LiFePO4 power battery, becoming the important battery failure reasons.
Battery failure reasons of cycling use
Batteries are usually exothermic during use, so temperature is important. In addition, road conditions, use methode, ambient temperature and so on all could be the battery failure reasons.
As for the capacity loss of LiFePO4 power battery during the cycle, it is generally believed to be caused by the loss of active lithium ions. The study of Dubarry et al. shows that the aging of LiFePO4 power battery during its cycle mainly goes through a complex growth process of consuming active lithium ion SEI film.
In this process, the loss of active lithium ions directly reduces the battery capacity retention rate. The continuous growth of SEI film, on the one hand, causes the increase of the polarization impedance of the battery, and at the same time, the electrochemical activity of the graphite anode will be partially inactivated if the SEI film thickness is too thick.
During the high temperature cycle, Fe2+ in LiFePO4 will dissolve to a certain extent. Although the amount of Fe2+ dissolution has no obvious effect on the capacity of the positive electrode, the dissolution of Fe2+ and the precipitation of Fe in the graphite negative electrode will play a catalytic role in the growth of SEI film.
Tan quantitatively analyzed where the loss of active lithium ion occurred and which step, and found that most of the loss of active lithium ion occurred on the graphite anode surface, especially in the high temperature cycle, that is, the high temperature cycle capacity loss is faster.
Three different mechanisms for the destruction and repair of SEI film are summarized :(1) the electron in graphite anode reduces lithium ions through SEI film; (2) Dissolution and regeneration of some components of SEI film; (3) SEI film rupture caused by volume change of graphite anode.
In addition to the loss of reactive lithium ions, both cathode and anode materials deteriorate during cycling use. Cracks occur in the LiFePO4 electrode during cycling use, resulting in increased electrode polarization and decreased electrical conductivity between the active material and the conductive agent or collector.
Nagpure used scanning extended resistance microscopy (SSRM) to semi-quantitatively study the changes of LiFePO4 after aging. It was found that the coarsening of LiFePO4 nanoparticles and the surface sediments generated by certain chemical reactions together led to the increase of the positive electrode impedance of LiFePO4.
In addition, the decrease of active surface caused by the loss of graphite active material and the lamellar stripping of graphite electrode are also considered to be the battery failure reasons. The instability of graphite anode will lead to the instability of SEI film and promote the consumption of active lithium ion.
The large discharge rate 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 results of Kim et al. show that the aging mechanism of the positive electrode of LiFePO4 is different from that of the graphite negative electrode: with the increase of discharge rate, the capacity loss of the positive electrode increases greatly than that of the negative electrode.
The loss of battery capacity at low power cycle is mainly due to the consumption of reactive lithium ions at the negative electrode, while the power loss at high power cycle is due to the increase of the positive electrode impedance.
Although the discharge depth in the use of power battery does not affect the capacity loss, it will affect the power loss: the speed of power loss increases with the increase of discharge depth, which is directly related to the increase of impedance of SEI film and the impedance of the whole battery.
Although the influence of charging voltage upper limit on battery failure is not obvious compared with the loss of active lithium ion, too low or too high charging voltage upper limit will increase the interfacial impedance of LiFePO4 electrode:
The passivation film cannot be formed well at a low upper voltage limit, while a high upper voltage limit will lead to oxidative decomposition of the electrolyte and the formation of low conductivity products on the surface of the LiFePO4 electrode.
The discharge capacity of the LiFePO4 power battery decreases rapidly as the temperature decreases, mainly due to the decrease in ionic conductivity and the increase in interfacial impedance. By studying the positive electrode of LiFePO4 and the negative electrode of graphite respectively, Li found that the main control factors limiting the low temperature performance of the positive electrode and the negative electrode are different.
The reduction of ionic conductivity of the positive electrode of LiFePO4 is dominant, while the increase of the interface impedance of the graphite cathode is the main reason.
The degradation of LiFePO4 electrode and graphite anode and the continuous growth of SEI film are battery failure reasons in different degrees. In addition to road conditions, ambient temperature and other uncontrollable factors that may become the battery failure reasons, the normal use of the battery is also very important, including appropriate charging voltage, appropriate discharge depth, etc.
Battery failure reasons of charging and discharging process
In the process of using the battery, it is inevitable that there will be overcharge, and relatively less overdischarge. The heat released in the process of overcharge or overdischarge tends to accumulate in the battery, which further increases the temperature of the battery, affects the service life of the battery, and increases the possibility of fire or explosion of the battery.
Even under normal charge and discharge conditions, as the number of cycles increases, the capacity inconsistency of individual cells in the battery system will increase, and the battery with the lowest capacity will also go through the charge and overdischarge process.
Although the thermal stability of LiFePO4 battery is the best compared with other types of lithium batteries under different charging conditions, overcharging can also cause unsafe risks in the use of LiFePO4 power battery.
In the state of overcharge, the solvent in organic electrolyte is more prone to oxidative decomposition, and the commonly used organic solvent ethylene carbonate (EC) will preferentially undergo oxidative decomposition on the positive electrode surface. Due to the very low lithium embedding potential (to lithium potential) of graphite anode, there is a great possibility of lithium precipitation in graphite anode.
One of the main battery failure reasons under the condition of overcharge is the internal short circuit caused by lithium dendrites piercing the diaphragm. Lu et al. analyzed the failure mechanism of lithium plating on graphite anode surface caused by overcharge.
Results show that the overall structure of the graphite anode is no change, but there are lithium dendrites and the emergence of surface film, lithium and surface film caused by the reaction of the electrolyte increases, not only consumes more active lithium, also makes it more difficult for lithium diffusion to the graphite anode, in turn, it will further promote the lithium in the cathode surface deposition, which reduces the capacity and the coulomb efficiency further.
In addition, metal impurities (especially Fe) are generally considered to be one of the main battery failure reasons. Xu et al. systematically studied the failure mechanism of LiFePO4 power battery under the condition of overcharge. The results show that the REDOX of Fe in overcharge/discharge cycle is theoretically possible, and the reaction mechanism is given:
When overcharging occurs, Fe is first oxidized to Fe2+, which is further oxidized to Fe3+, 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.
During the overcharge/discharge cycle, Fe dendrites will form at the positive and negative poles at the same time, which will Pierce the diaphragm and form Fe bridge, resulting in a micro-short circuit of the battery. The obvious phenomenon of the micro-short circuit of the battery is the continuous rise of the temperature after overcharge.
During overdischarge, the potential of the negative electrode will increase rapidly, which will lead to the destruction of SEI film on the surface of the negative electrode (the part of SEI film rich in inorganic compounds is more easily oxidized), which will lead to the additional decomposition of electrolyte, resulting in capacity loss.
More importantly, oxidation of the negative collector Cu foil occurs. 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 lead to 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 collector Cu foil could be oxidized to Cu+ during overdischarge, and Cu+ could be further oxidized to Cu2+.
After that, they diffused to the positive electrode, where reduction reaction could occur, so that The Cu crystal dendritic would form on the positive electrode side and puncture the diaphragm. Cause the battery internal micro short circuit, also due to overdischarge, the battery temperature will continue to rise.
Overcharge of LiFePO4 power battery may lead to electrolytic liquid oxygen decomposition, lithium and Fe dendritic formation. Overdischarge may cause SEI damage, lead to capacity attenuation, oxidation of Cu foil, and even the formation of Cu crystal branches.
Battery failure reasons of 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 and binder are easy to show. Gaberscek et al. discussed the two contradictory factors of size and carbon coating and found that the impedance of LiFePO4 electrode was only related to the average particle size.
The anti-position defect (Fe occupies Li position) in LiFePO4 will have certain influence on the performance of battery: because the transmission of lithium ion in LiFePO4 is one-dimensional, this defect will hinder the transmission of lithium ion; This defect also causes the instability of the LiFePO4 structure due to the extra electrostatic repulsion introduced by the high state.
LiFePO4 with large particles cannot be completely delithium at the end of charging; Nanostructured LiFePO4 can reduce the disorientation defect, but can cause self-discharge due to its high surface energy.
At present, PVDF is the most commonly used binder, which has some disadvantages such as possible reaction at high temperature, dissolving in non-hydrolyzed solution and insufficient flexibility, which has certain influence on capacity loss and cycle life shortening of LiFePO4.
In addition, the collector fluid, diaphragm, electrolyte composition, production process, human factors, external vibration are all battery failure reasons and will affect the performance of the battery to varying degrees.