Lithium-ion Batteries at Low Temperatures
In this post, we will introduce what will occur when lithium-ion batteries are at low temperature and analyse the reasons for the situation.
Now, let’s first learn about the composition and reaction process of lithium-ion batteries to make the following analysis more clear and easy to understand.
1. The composition and reaction process of lithium-ion batteries
The positive electrode of the lithium-ion battery is generally:
-Lithium iron phosphate (LFP) and
-Lithium cobalt oxide (LCO),
and the negative electrode is graphite (Gr).
The charging and discharging process includes electrochemical reactions and mass transfer of charged particles.
As shown in the figure below, when charging, lithium ions come out from the positive electrode lattice, pass through the electrolyte diaphragm to the negative electrode, and intercalate between the graphite layers.
When discharging, they come out from the graphite negative electrode layer and return to the positive electrode lattice.
2. Why can't lithium ions be charged at low temperature?
As shown below. Due to the contraction of the lattice of positive and negative electrode materials at low temperature, the charge transfer and solid phase diffusion become slower.
Besides, it is more difficult for Li ions to deintercalate and intercalate. At the same time, the diffusion in the electrolyte becomes difficult. And both jointly leads to the decrease of the electrode surface area and the Li ions in the electrolyte, which makes the polarization of the electrode larger.
In addition, under low temperature conditions, the ohmic internal resistance of lithium-ion batteries will increase.
As shown below. During charging, the voltage test read by the test terminal includes true voltage(Vt), voltage polarization(Vp) and voltage resistance(VΩ).
In the battery management system, voltage measurement is used as a benchmark to determine whether the battery is fully charged. That is, after V measurement reaches a certain value, the charging ends.
Under low temperature conditions, V-Ω becomes larger and V-p becomes larger. This makes Vt still at a smaller value when V measured reaches the end voltage value. After the end of charging, the voltage of the battery will have a large drop.
At this time, the tested voltage is V-t, and in actual use, it shows that the battery is not fully charged at low temperature.
3. Why do lithium-ion batteries experience irreversible capacity loss after cycling at low temperatures?
The permanent drop in capacity is considered to be the irreversible structural damage of the material and the permanent loss of active species (especially cycling lithium).
As shown in the figure below, at low temperatures, the sources of battery capacity loss are:
3.1 Growth of lithium precipitation and lithium dendrites
When charging at low temperature, on the one hand, the electrochemical reaction and solid diffusion slow down. And on the other hand, the material lattice shrinks.
Lithium ions will directly obtain electrons on the surface of the negative electrode and become metal lithium, which is a conversion reaction.
The reaction potential is lower than the intercalation reaction, which means it is more difficult to occur. However, the material diffusion of the intercalation reaction is difficult, making the conversion reaction easy to occur at low temperature. The uneven growth of low-temperature lithium precipitation can easily form lithium dendrites, and large lithium dendrites will pierce the separator and even cause functional failure.
During the discharge process, the reaction rate between the metal lithium deposited on the surface of the negative electrode and the electrolyte will also decrease.
The closer the lithium element is to the current collector, the more it will dissolve first. And that will leave the lithium on the top to lose its connection with the negative electrode, resulting in “dead lithium”. This part of lithium will be a permanent irreversible loss.
3.2 Thickening of SEI film
The lithiation potential of the negative electrode material of lithium-ion batteries is usually lower than the reduction and decomposition potential of the organic electrolyte, so a passivation layer, namely SEI film, will be formed.
The formation of the SEI film runs through the entire battery life.
At low temperature, the negative force of the SEI film becomes larger, causing the anode potential to shift to a lower potential. This creates greater polarization, making it easier for lithium to precipitate.
The precipitation of lithium metal will keep the electrode surface potential at a low level, allowing the organic electrolyte to continue to decompose to form an SEI film.
Lithium analysis and electrolyte decomposition form a vicious cycle, making the active Li ions in the battery less and less.
3.3 Partial lattice destruction of electrode materials
The lattice that shrinks at low temperature is strongly embedded, which can easily lead to local lattice damage inside the positive and negative electrode materials. This cannot be repaired by itself.
3.4 Polarized decomposition of electrolyte
Under low temperature conditions, the electrochemical polarization and concentration polarization are serious.
This makes it easy for side reactions to occur at the electrode/electrolyte interface, leading to the decomposition of the electrolyte.
In addition, during the thickening process of the SEI film, the decomposition of the organic electrolyte is also irreversible damage.
At low temperatures, irreversible structural damage of materials and permanent loss of active species (especially cycled lithium) occurs inside the battery.
As a result, even if the battery is used in a suitable environment, the capacity of the lithium-ion battery cannot be recovered when charging and discharging even with a small current.
(1) Since using lithium-ion batteries at low temperatures will have a greater impact on lithium-ion batteries. So can keeping the lithium-ion battery at a low temperature and not using it avoid irreversible damage to the battery?
There are two main aging mechanisms for lithium-ion batteries: calendar aging and cycle aging.
– Cyclic aging: it is a dynamic charging and discharging process.
– Time aging: aging during static non-use storage.
Time aging is mainly affected by temperature and SOC (how much lithium ions are stored in negative electrode graphite).
Under high temperature and high SOC, the stability of the electrode/electrolyte interface decreases, and side reactions increase, such as:
The positive metal ion dissolves, oxygen evolves, the electrolyte decomposes, and the SEI film on the surface of the negative electrode thickens.
Therefore, low temperature can inhibit the progress of time aging to some extent. That is, during the non-use period, if the mechanical damage caused by cold stress (thermal expansion and contraction) is not considered, the low temperature condition itself will not cause irreversible loss of lithium-ion batteries.
(2) What should be paid attention to when using lithium-ion batteries at low temperatures?
When charging a lithium-ion battery at low temperature, due to the deterioration of kinetic conditions, not only will the battery capacity decrease, but also metal lithium will be precipitated on the surface of the negative electrode due to the slowing of graphite intercalation speed.
Charging with a small current can reduce the degree of lithium precipitation.
In addition, after a period, the precipitated lithium metal can be re-intercalated into the graphite, so it is better to keep stay for a period of time after charging.