1 Composition and reaction process of lithium-ion batteries

The positive electrode of lithium-ion batteries is generally composed of ternary (NCM), lithium iron phosphate (LFP), and lithium cobalt oxide (LCO), while the negative electrode is graphite (Gr). The process of charging and discharging involves electrochemical reactions and mass transfer of charged particles.

During charging, lithium ions come out of the positive electrode lattice, pass through the electrolyte separator to the negative electrode, and are embedded between the graphite layers. During discharge, it can be understood as coming out of the graphite negative electrode layer and returning to the positive electrode lattice.

2 Why is it difficult to charge lithium ions at low temperatures?

Due to the lattice contraction of positive and negative electrode materials at low temperatures, charge transfer and solid-phase diffusion become slower, making it more difficult for Li ions to be deintercalated and intercalated. At the same time, diffusion in the electrolyte becomes more difficult, resulting in a decrease in Li ions on the electrode surface area and in the electrolyte, leading to greater polarization of the electrode. In addition, under low temperature conditions, the ohmic resistance of lithium-ion batteries will increase.

During charging, the voltage V measured by the testing end includes V true, V polarization, and V Ω. In the battery management system, V measurement is used as a reference to determine whether the battery is fully charged (i.e., after V measurement reaches a certain determined value, charging ends). Under low temperature conditions, V Ω increases and V polarization increases, so that when V reaches the final voltage value, V is still at a relatively small value. After the end of charging, there will be a significant drop in the voltage of the battery, and the tested voltage is V, which is true. In actual use, it is manifested as insufficient charging at low temperatures.

3 Why does lithium-ion battery experience irreversible capacity loss after cycling at low temperatures? 

The permanent decrease in capacity is believed to be the irreversible structural damage of the material and the permanent loss of active substances (especially cyclic lithium).

The sources of capacity loss in batteries at low temperatures include:

(1) Lithium deposition and growth of lithium dendrites:

When charging at low temperatures, on the one hand, the electrochemical reaction and solid diffusion slow down, and on the other hand, the material lattice shrinks.

Lithium ions cannot be squeezed into the graphite layer in time and will directly obtain electrons on the negative electrode surface to become metallic lithium, which becomes a conversion reaction (the reaction potential is lower than that of intercalation reaction, which can be understood as more difficult to occur, but the diffusion of substances in intercalation reaction is difficult, making the conversion reaction more likely to occur at low temperatures).

The uneven growth of low-temperature lithium precipitation can easily form lithium dendrites, and large lithium dendrites can pierce the membrane and even cause functional failure.

During the discharge process, the reaction rate between the metal lithium deposited on the negative electrode surface and the electrolyte will also decrease. The closer the lithium element is to the current collector, the more it will dissolve first, leaving the top lithium disconnected from the negative electrode, resulting in “dead lithium”, which will be permanently and irreversibly lost.

(2) Thickening of SEI film:

The lithiation potential of negative electrode materials in lithium-ion batteries is often lower than the reduction decomposition potential of organic electrolytes, resulting in the formation of a passivation layer, namely SEI film. The formation of SEI film runs through the entire process of battery use.

At low temperatures, the resistance of the SEI film increases, causing the negative electrode potential to shift towards a lower potential, resulting in greater polarization and making it easier for lithium to precipitate. The precipitation of lithium metal also maintains the electrode surface potential at a lower level, allowing the organic electrolyte to continue to decompose and form the SEI film. The decomposition of lithium and electrolyte forms a vicious cycle, resulting in a decrease in active Li ions in the battery.

(3) Localized lattice damage of electrode materials.

The lattice that shrinks at low temperatures and is strongly embedded can easily cause local lattice damage inside the positive and negative electrode materials, which cannot be repaired by itself.

(4) Polarization decomposition of electrolyte.

Under low temperature conditions, electrochemical polarization and concentration polarization are severe, and side reactions are prone to occur at the electrode/electrolyte interface, leading to the decomposition of the electrolyte; In addition, during the thickening process of SEI film, the decomposition of organic electrolyte is also irreversibly damaged.

4 Summary

At low temperatures, irreversible structural damage of materials and permanent loss of active materials (especially cyclic lithium) occur inside the battery, resulting in the inability to restore the capacity of lithium-ion batteries even when they are used in suitable environments, such as charging and discharging with low currents.

Next, there are two more questions:

Can keeping lithium-ion batteries idle at low temperatures avoid irreversible damage to the battery?

There are two main aging mechanisms for lithium-ion batteries: calendar aging and cyclic aging. Cyclic aging is a dynamic charging and discharging process, while calendar aging refers to the aging during static non use storage periods.

Calendar aging is mainly influenced by temperature and SOC (the amount of lithium ions stored in the negative electrode graphite): under high temperature and high SOC, the stability of the electrode/electrolyte interface decreases, and side reactions increase – dissolution of positive electrode metal ions, oxygen evolution, electrolyte decomposition, and thickening of the SEI film on the negative electrode surface.

Therefore, low temperature can suppress the aging of calendars to some extent. During non use, if mechanical damage caused by cold stress (thermal expansion and contraction) is not discussed, low temperature conditions themselves will not cause irreversible losses to lithium-ion batteries.

What should be noted when using lithium-ion batteries at low temperatures?

When charging lithium-ion batteries at low temperatures, the deterioration of dynamic conditions not only leads to a decrease in battery capacity, but also results in the deposition of metallic lithium on the negative electrode surface due to a decrease in graphite lithium insertion rate. By charging with low current, the degree of lithium deposition can be reduced.

In addition, after a period of storage, the precipitated lithium metal can be re embedded into the graphite, so it is better to let it sit for a period of time after charging.