The Study on the LiFePO4 Battery Failure

LiFePO4 Battery draws our eyes for its outstanding advantages. Of course, their performance will be influenced by many kinds of factors. Today we are going to talk about reasons why our LiFePO4 batteries fail.

Totally, we will study these reasons in four parts:

Failure in Production Process

Failure in Storage

Failure in the cycle of use

Failure in charging and discharging process

In this article, it is a little academic, but it is very useful to help you to further understand why LiFePO4

Now, lets back to our topic today.

The first part:

Failure in Production Process

In the production process, personnel, equipment, raw materials, producing methods and environment are the main factors affecting product quality.

No exception, In the production process of LiFePO4 battery is no, personnel and equipment belong to the category of management, so we mainly discuss the last three influencing factors:raw materials, producing methods and environment.

  • Impurities in electrode active material

In the synthesis process of LiFePO4, there will be a small amount of Fe2O3, Fe and other impurities existing which will react on the surface of the negative electrode and may puncture the diaphragm to trigger an internal short circuit.

That is,

When LiFePO4 is exposed to air for a long time and moisture will deteriorate the cell. The higher the impurity content of the graphite negative electrode, the greater the irreversible capacity loss caused.

  • Formation Method

The irreversible loss of reactive Li+ is reflected in the Li+ consumed during the SEI formation(SEI: Solid Electrolyte Interface). When formation temperature increases, there will be more irreversible Li+ loss because the inorganic components proportion in the SEI film increases and releasing gas will cause more “holes” in SEI film. And then much more Li+ will come to graphite negative electrode according to “holes”. Then the irreversible loss forms.

  • Moisture in producing environment

In the actual production, LiFePO4 battery will contact air inevitably. It is easily absorb moisture from the air because most materials in positive, negative electrode and electrolyte have large electric negativity.

Moisture will react with lithium salts in the electrolyte especially LiPF6. This reaction decomposes electrolyte and produces HF and PF5(both of them are strong acid) which will corrode LiFePO4 and SEI film.

Moisture will also react with lithium and then form Li-OH which will accelerate battery aging together with O2.

Therefore, factors such as the material purity, the control of the humidity of the environment, and the way of chemical formation are crucial.

Above consequences appears in manufacturing process. Thus manufacturers need to grasp their process level controlling in order to make qualified LiFePO4 batteries.

The remaining parts are factors about users daily operations. Let us break them one by one.

Failure in Storage

In the service life of a battery, it is in a shelved state most of the time. Generally, after a long storage period, the battery performance will decline, generally showing an increase in internal resistance, volt

There are many factors that cause battery performance degradation, among which temperature, charge state and time are the most obvious influencing factors.

Battery aging due to reactive Li+ loss. Battery capacity declines when storage temperature increases.

In the storage state, the harsh storage conditions (high temperature and high charge state) will increase the self-discharge of LiFePO4 power cells and make the aging of the cells more obvious.

age reduction and a decrease in discharge capacity.

Failure in the Cycle of Usage

Batteries are generally exothermic during use, so the temperature effect is important. In addition, the circuit condition, using methods, and the ambient temperature can all have different effects.

For LiFePO4 battery capacity loss of during cycling, it is generally believed that it is caused by the loss of active Li+. The aging of LiFePO4 battery during cycling mainly undergoes a complex SEI film growth process of consuming active Li+.

In this process, the loss of active Li+ directly reduces the battery capacity retention; the continuous growth of the SEI film causes an increase in the polarization impedance of the battery on the one hand, while at the same time, the SEI film is too thick and the electric chemical activity of the graphite anode is partially deactivated.

During high temperature cycling, there are some Fe2+ dissolute 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 play a catalytic role in the growth of the SEI film.

According to quantitative analysis: most of the active Li+ loss occurs on the graphite cathode surface, especially at high temperature cycling. That is, faster capacity loss at high temperature cycling.

Three different mechanisms for the destruction of SEI.

(1) Electrons in the graphite negative electrode go through the SEI membrane.

(2) Dissolution of some components of the SEI membrane.

(3) Rupture of the SEI membrane due to volume change of the graphite anode .

During cycling, in the LiFePO4 electrode, the appearance of cracks can lead to increased polarization of the electrode and decreased conductivity. The large discharge rate can provide large power to the electric vehicle.

That is,

The better the battery discharge rate, the better the acceleration performance of the electric vehicle.

The aging mechanism of LiFePO4 positive electrode and graphite negative electrode is different: as the discharge rate increases, the capacity loss of positive electrode increases to a greater extent than that of negative electrode.

The loss of battery capacity at low discharge rate is mainly due to the consumption of active Li+ at the negative electrode,

while the power loss of the battery at high-discharge-rate cycles is due to the increase in impedance of the positive electrode.

The last part.

Failure in Charging and Discharging Process

Inevitably, in the process of use, batteries are often overcharged.

Relatively speaking, there are fewer cases of over-discharging, but the heat released in the process of overcharging or over-discharging tends to gather inside the battery, which will further make the battery temperature rise and affect the service life of the battery.

Even it will increase the possibility of battery fire or explosion.

Even under normal charging and discharging conditions,

with the increasing number of cycles, the capacity inconsistency of internal cells will increases, and a cell with the least capacity will also experience the process of overcharge and over-discharge.

Although in different charging states, compared with other cathode materials, heat stability of LiFePO4 is the best. Nevertheless, overcharging still arises unsafe risk.

Under overcharge condition, one of the mainly reasons about batteries’ failure is internal short circuit caused by lithium crystal branches.

With the appearance of lithium crystal branches, the reaction between lithium and electrolyte generates an increasing surface film, which not only consumes more active lithium, but also makes it more difficult for lithium to diffuse into the graphite cathode.

In turn, it will further promote the deposition of lithium on the surface of the cathode, causing a further decrease in capacity.

In addition, metal impurities (especially Fe) are often considered to be one of the main causes of battery overcharge failure. During the overcharge/discharge cycle, Fe gets redox reaction. When the battery is overcharging or discharging, Fe crystal branches will be formed at both positive and negative electrodes, which will pierce the diaphragm and form Fe bridges, causing a micro-short circuit in the battery. And the obvious phenomenon accompanying the micro-short circuit in the battery is the continuous increase in temperature after overcharge.

In all, LiFePO4 batterys is delicate. We need to carefully use them and know right using and storing methods in order to save our life and money.

According to above four parts, we can conclude some tips about how to protect our LiFePO4 batteries.

Lets have a try:

  1. Do not put them into humid and high temperature places
  2. Do not use them in humid and high temperature places
  3. Do not over charge and over discharge
  4. Do not charge them with too high or too low voltage
  5. Choose good brand of battery
  6. ……

If you have any other ideas about how to protect our LiFePO4 batteries. We are welcome for your comments.