1. The composition of lithium-ion batteries
The cathode of lithium-ion batteries is generally ternary (NCM), lithium iron phosphate (LFP) and lithium cobalt oxide (LCO), and the cathode graphite (Gr). The charging temperature is also a significant effect to be taken in consideration. The process of charging and discharging includes electrochemical reactions and mass transfer with charged particles. Among them, we take the lifepo4 batteries as the typical.
When charging, lithium ions come out of the anode lattice through the electrolyte separator to the cathode, and are embedded between the graphite layers. When discharging, it can be understood as coming out from the graphite cathode layer and returning to the anode crystal grid.
2. The effect of charging temperature
Due to the lattice shrinkage of the anode and cathode materials at low charging temperature, charge transfer and solid-phase diffusion become slower. Li ions are more difficult to de-intercalate and embed, and diffusion in the electrolyte becomes more difficult, resulting in fewer Li ions in the electrode surface area and in the electrolyte.
This makes the polarization of the electrode larger. In addition, at low charging temperature, the ohmic internal resistance of lithium-ion batteries becomes larger.
In charging, the voltage V measurement read out by the test terminal includes V real, V polarization and VΩ, and in the battery management system, the V test is used as a benchmark to determine whether the battery is full (that is, after the V measurement reaches a certain value, the charging ends).
At low charging temperature, VΩ becomes larger and V-polarization increases. So that when the V measurement reaches the end voltage value, V is really still at a small value. After the end of charging, the voltage of the battery will have a large drop, at this time the tested voltage is V real, in actual use it is manifested as low charging temperature at full charge.
After cycling at low charging temperature, irreversible capacity loss occurs The permanent decrease in capacity is considered to be the irreversible structural destruction of the material and the permanent loss of the active material, especially circulating lithium.
3. Analysis of the consequences of charging temperature
● Growth of lithium precipitation and lithium dendrites
When charging at a low charging temperature, on the one hand, the electrochemical reaction and solid diffusion slow down. On the other hand, the material lattice shrinks, and lithium ions are too late and can not squeeze into the graphite layer.
And electrons will be directly obtained on the surface of the cathode to become metal lithium, which becomes a conversion reaction (the reaction potential is lower than the intercalation reaction, which can be understood as more difficult to occur, but the diffusion of the substance of the intercalation reaction is difficult, making the conversion reaction easy to occur at low charging temperature).
The uneven growth of lithium evolution at low charging temperature can easily form lithium dendrites, and large lithium dendrites will pierce the diaphragm and even cause functional failure.
During the discharge process, the reaction rate between the metal lithium deposited on the surface of the cathode and the electrolyte will also decrease, and the closer to the current collector, the more the lithium element will be the first to dissolve, leaving the top lithium to lose the connection with the cathode, resulting in "dead lithium", which will be permanently irreversible loss.
● Thickening of the SEI membrane
The lithium potential of lithium-ion battery anode materials is often lower than the reduction and decomposition potential of organic electrolytes, so a passivation layer, that is, an SEI film, is formed.
The formation of the SEI film is carried out throughout the use of the battery. At low charging temperature, the resistance of SEI film becomes larger, resulting in the shift of cathode potential to low potential, resulting in greater polarization, making lithium easier to precipitate.
Additionally, precipitation of lithium metal will make the electrode surface potential maintain a low level, so that the organic electrolyte continues to decompose to form SEI film, and the decomposition of lithium precipitation and electrolyte forms a vicious circle, making the active Li ions in the battery less and less.
● Local lattice destruction of the electrode material
The shrinking lattice at low charging temperature is strongly embedded, which can easily lead to local lattice damage inside the anode and cathode materials, which cannot be repaired by itself. That is what top 10 lithium ion battery anode material companies is worried because of the current lack of battery technology.
● Polarization decomposition of electrolyte
Under the condition of low charging temperature, electrochemical polarization and concentration polarization are serious, and side reactions are easy to occur at the electrode/electrolyte interface, resulting in the decomposition of electrolyte; In addition, during the thickening of the SEI film, the decomposition of the organic electrolyte is also irreversible damage.
4. How to avoid the influence of charging temperature
Since lithium-ion batteries are used at low charging temperature, they will have a greater impact on lithium-ion batteries. So what should be done to avoid the effect of the charging temperature.
Keeping the lithium-ion battery standing at a low charging temperature can avoid irreversible damage to the battery. There are two main aging mechanisms of lithium-ion batteries: calendar aging and cycle aging.
● Cyclic aging
Cyclic aging is a dynamic charge-discharge process, and calendar aging is aging during static non-use storage.
● Calendar aging
Calendar aging is mainly affected by temperature and SOC (how much lithium ions are stored in cathode graphite): at high temperature and high SOC, the stability of the electrode/electrolyte interface decreases, and the side reactions increase - anode metal ion dissolution, oxygen evolution, electrolyte decomposition, and thickening of the SEI film on the cathode surface. Therefore, the low charging temperature can inhibit the aging of the calendar to some extent.
That is, during non-use, if the mechanical damage caused by cold stress (thermal expansion and contraction) is not discussed, the low charging temperature condition itself will not cause irreversible losses of lithium-ion batteries.
5. Recommendations on charging temperature
What should we pay attention to when using lithium-ion batteries at low charging temperature? Here are some tips on the charging temperature.
When charging lithium-ion batteries at low charging temperature, due to poor kinetic conditions, it will not only lead to a decrease in battery capacity, but also because the speed of graphite lithium insertion decreases.
And metal lithium is precipitated on the surface of the cathode, and the degree of lithium evolution can be reduced by charging with a small current. In addition, after a period of shelving, the precipitated metal lithium can be embedded in the graphite again. So under the circumstances of battery temperature like this, it is better to stand for a period of time after charging.
6. ConclusionAt low charging temperature, irreversible structural damage of materials and permanent loss of active materials (especially circulating lithium) occur inside the battery, resulting in the inability to recover the capacity of lithium-ion batteries even when the battery is used in a suitable environment, even when charging and discharging with a small current.