Battery SEI generation has a significant impact on the electrochemical performance of lithium-ion batteries. On the one hand, the formation of the battery SEI consumes part of the lithium ions, which increases the irreversible capacity of the first charge and discharge and reduces the charging and discharging efficiency of the electrode material.
Battery SEI is insoluble in organic solvents. So, what exactly is such an important battery SEI? Why is battery SEI generated on the surface of lithium ion battery anodes? What are the specific steps for battery SEI generation? What exactly is the structure of the generated battery SEI? This article will help you understand.
1. What is battery SEI
During the first charge and discharge of a liquid lithium-ion battery, the electrolyte reacts at the solid-liquid interface of the electrode to form a passive layer covering the surface of the electrode material.
This passivation layer is an interface layer with the characteristics of a solid electrolyte, an electronic insulator, but an excellent conductor of Li ions. Li ions can be freely embedded and removed through this passivation layer, so this passivation layer is called the solid electrolyte interface, or battery SEI for short.
2. Why is battery SEI formed on the anode
We use molecular orbital theory to explain the formation of battery SEI, which the top 10 lithium ion battery anode material companies have been studying this for a long time. First figure out what are HOMO, LUMO and fermi levels HOMO and LUMO refer to the highest occupied molecular orbit and the lowest unoccupied molecular orbit, respectively.
According to the theory of forward-line orbits, the two are collectively called forward-line orbits, and electrons in forward-line orbits are called forward-line electrons.
● Front-line orbit theory
There are electrons similar to the "valence electrons" of individual atoms in molecules, and the valence electrons of molecules are front-line electrons. Therefore, in the process of chemical reactions between molecules, the first molecular orbital is the front-line orbital, and the electrons that play a key role are the front-line electrons.
This is because the molecule's HOMO is more relaxed in its electron binding and has the properties of an electron donor, while LUMO has a strong affinity for electrons and has the properties of an electron acceptor, and these two orbitals are the most likely to interact with each other and play an extremely important role in the process of chemical reactions.
That is, LUMO means that it can provide empty orbits for foreign electrons, and the lower LUMO, the stronger its force on foreign electrons, and the easier it is to capture electrons.
HOMO means the highest energy level orbit occupied by its own electrons, and the higher the HOMO, the weaker its binding force on its own electrons, and the easier it is to lose electrons.
● Fermi level
The Fermi level is the highest energy level that electrons can occupy at absolute zero, and two electrons with opposite spins can be placed at each energy level. Now suppose we remove all fermions from these quantum states.
These fermions are then filled into each occupied quantum state according to certain rules, and each fermion occupies the lowest occupied quantum state during this filling process. The quantum state occupied by the last fermion can be roughly understood as the Fermi level.
That is, imagine that you have a bag of apples (electrons), a long staircase in front of you (energy belt), and you go up from the bottom step (energy level). Each step (energy level), place two apples (electrons) on top of this step and continue until it is finished. The stage you are on at this time is the Fermi level.
3. Interpretation of the molecular orbital theory
The energy difference between the Fermi level at the anode and the lowest unoccupied molecular orbital (LUMO) of the lithium ion battery electrolyte determines the thermodynamic stability of the electrolyte solution at the anode, which is the possibility of forming SEI batteries.
Specifically, if the LUMO level of the electrolyte solution is lower than the Fermi level at the anode, the electrolyte solution will accept electrons from the anode, initiate a reduction reaction, and be reduced.
Similarly, if the highest occupied molecular orbital (HOMO) level of the electrolyte solution is higher than the fermi level at the cathode, the electrolyte solution loses electrons, triggers an oxidation reaction, and is oxidized. The electrolyte is thermodynamically stable only if both the fermi level at anode and the fermi level at cathode are within the electrolyte's electrochemical potential stabilization window.
Taking graphite anodes of lifepo4 batteries as an example, before the start of formation, the potential of graphite lies between the electrochemical stabilization windows of the electrolyte, so there will be no battery SEI generation at the anode.
At the beginning of formation, Li ions are driven by an external voltage to the negative surface. At this time, the Li ion potential is very negative and is outside the electrochemical stability window of the electrolyte, so the reaction to generate the battery SEI will begin.
4. The specific steps of battery SEI generation
The SEI battery formation process in the lithium ion battery formation process consists of four steps:
- Electrons are transferred from the current collector-conductive agent-anode material particles to the SEI cell to be formed.
- Solvated lithium ions diffuse from the cathode to the surface of the generated battery SEI under the encapsulation of the solvent.
- Electrons diffuse through the electron tunneling effect.
- Transition to the reaction of electrons with lithium salts, solvated lithium ions, and agents to generate battery SEI.
● Electron tunneling effect
The electron tunneling effect is the diffusion of free electrons in the conductor into the insulating layer, which increases the energy state of valence electrons in the insulating layer.
The phenomenon of changing from a bound state (local state) to a free state (public state) and thus participating in the phenomenon of current carrying. Free electrons in the conductor diffuse into the insulating layer, and due to coulomb repulsion, the energy state of valence electrons in the lattice potential field in the insulating layer increases, reducing the height of the barrier.
At the same time, the hall electric field generated by the directional movement of carriers also increases the energy state of valence electrons due to the work done by valence electrons and Joule heat generated by current. Under the influence of three factors, the valence electron energy state of the insulating layer increases, and the local state changes to the free state, thus participating in the current carrying.
Throughout the formation process, the inner inorganic layer continues to grow and maintain a rough interface, while the outer organic layer maintains porous structural characteristics. Therefore, the initial formation of SEI batteries is divided into two processes:
The electrolyte decomposes on the electrode surface to form a double layer porous SEI cell with an inner layer of inorganic matter and an outer layer of organic matter.
The electrolyte penetrates the pores of the battery SEI and continues to decompose, so that the battery SEI continues to grow until the inner layer becomes uniform and dense, and sufficient organic components appear in the outer layer. It can effectively block electrons and prevent further breakdown of the electrolyte.
5. The structure of the battery SEI
● Mosaic model
There are many SEI battery models, and the most accepted is the mosaic model. On the one hand, it inherits the hypothesis of the two-layer model, which holds that the battery SEI consists of an inorganic rich inner layer (in contact with lithium) and an organically rich outer layer (in contact with electrolyte).
On the other hand, it assumes that each component constitutes a pure microphase and that the battery SEI is a mosaic assembly of different microphases. As shown in the following figure, the inner layer is mainly a high density inorganic layer, and the outer layer is mainly a low density organic layer.
When the complete battery SEI is generated, for the mosaic model, it is believed that the anode charging process can be divided into four sequential steps at the microscopic level.
- Dissolved Li ions diffuse into the electrolyte.
- Li ions are desolubilized by breaking the solvated shell.
- Li ions are diffused over the battery SEI.
- The diffusion of Li ions in the anode material, accompanied by electron transfer and rearrangement of the crystal lattice of the anode material.
The passive layer covering the surface of the electrode material is battery SEI. It can be stable in the organic electrolyte solution, and solvent molecules cannot be passivated through this layer, which can effectively prevent co-embedding of solvent molecules. It avoids the damage caused by the co-embedding of solvent molecules in the electrode material, thus greatly improving the cycle performance and service life of lithium-ion batteries.