The cause and analysis of SEI produced by lithium ion battery
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2023/10/13 11:54:01
The generation of battery SEI has a significant effect on the electrochemical performance of lithium-ion batteries. On the one hand, the formation of battery SEI consumes part of lithium ions, increases the irreversible capacity of the first charge and discharge, and reduces the charge and discharge efficiency of the electrode material.
Battery SEI is insoluble in organic solvents. So, what exactly is such an important battery SEI? Why does the negative surface of lithium-ion batteries produce battery SEI? 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
When the liquid lithium-ion battery is charged and discharged for the first time, the electrolyte reacts at the solid-liquid interface of the electrode to form a passivation layer covering the surface of the electrode material.
The passivation layer is an interface layer, which has the characteristics of solid electrolyte and electronic insulator, but is also an excellent conductor of lithium ions. Li ions can be freely embedded and removed through the passivation layer, so the passivation layer is called the solid electrolyte interface, referred to as the battery SEI.
2. Why does the negative battery form SEI
We use molecular orbital theory to explain the formation of battery SEI. So let's first figure out what the HOMO, LUMO and Fermi levels are. HOMO and LUMO are the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals, respectively.
According to the front-line orbital theory, the two are collectively called front-line orbitals, and the electrons in the front-line orbit are called front-line electrons.
● Frontline orbit theory
There are electrons in a molecule that are similar to the "valence electrons" of a single atom, and the valence electrons of a molecule are frontier electrons. Therefore, in the process of intermolecular chemical reactions, the first molecular orbital is the frontline orbital, and the electrons that play a key role are the frontline electrons.
This is because the HOMO of the molecule is more relaxed in electron binding and has the properties of an electron donor, while the LUMO has a stronger affinity for electrons and has the properties of an electron acceptor, and these two orbitals are most likely to interact. They interact with each other and play an extremely important role in the chemical reaction process.
In other words, LUMO means that it can provide empty orbitals for foreign electrons, and the lower the LUMO, the stronger the force it exerts on foreign electrons, the easier it is to trap electrons.
HOMO represents the highest energy orbital occupied by its own electrons, and the higher the HOMO, the weaker its binding force to its own electrons, the more likely it is to lose electrons.
● Fermi level
The Fermi level is the highest energy level that electrons can occupy at absolute zero, and each energy level can place two electrons with opposite spins. Now suppose we remove all fermions from these quantum states.
These fermions then fill the individual occupied quantum states according to certain rules, and each fermion occupies the lowest occupied quantum state in this filling process. The quantum state occupied by the last fermion can be roughly understood as the Fermi level.
That is, imagine you have a bag of apples (electrons), you have a long staircase in front of you (energy band), and you go up from the lowest step (energy level). For each step (energy level), place two apples (electrons) on top of that step and continue until completed. The phase you're in is called the Fermi level.
3. Explanation of molecular orbital theory
The energy difference between the Fermi level at the negative electrode and the lowest unoccupied molecular orbital (LUMO) of the lithium-ion battery electrolyte determines the thermodynamic stability of the electrolyte at the negative electrode, which is the possibility of forming an SEI battery.
Specifically, if the LUMO level of the electrolyte solution is lower than the Fermi level at the negative electrode, the electrolyte solution will accept electrons from the negative electrode, triggering a reduction reaction and being reduced.
Similarly, if the highest occupied molecular orbital (HOMO) level of an electrolyte solution is higher than the Fermi level of the positive electrode, the electrolyte solution loses electrons, initiating an oxidation reaction and being oxidized. The electrolyte is thermodynamically stable only when both the negative Fermi level and the positive Fermi level are within the electrochemical potential stability window of the electrolyte.
In the case of the graphite negative pole of the lithium iron phosphate battery, before the formation begins, the potential of the graphite lies between the electrochemical stability window of the electrolyte, so the negative pole does not produce the battery SEI.
At the beginning of formation, lithium ions are driven to the negative surface by an external voltage. At this point, the Li ion potential is very negative and outside the electrochemical stability window of the electrolyte, so the reaction of the battery SEI will begin to be generated.
4. Specific steps for generating battery SEI
The SEI battery formation process consists of four steps:
Electrons are transferred from the collector - conductive - negative material particles to the SEI cell to form.
The solvated lithium ions are diffused from the positive electrode to the surface of the resulting battery SEI under the solvent encapsulation.
Electrons diffuse through electron tunneling.
The transition electrons react with lithium salts, solvated lithium ions, and reagents to form the battery SEI.
● Electronic tunneling effect
Electron tunneling is the diffusion of free electrons in the conductor into the insulating layer, thereby increasing the energy states of valence electrons in the insulating layer.
A phenomenon that changes from a bound state (local state) to a free state (common state) and thus participates in the current-carrying phenomenon. Due to the coulomb repulsion, the energy states of the valence electrons in the lattice potential field in the insulation layer increase and the height of the barrier decreases.
At the same time, the Hall electric field generated by the directional motion of the carrier also increases the energy state of the valence electron due to the work done by the valence electron and the Joule heat generated by the current. Under the influence of three factors, the valence electron energy state of the insulation layer increases, and the local state changes to a free state, thus participating in current carrying.
Throughout the formation process, the inner inorganic layer continues to grow and maintain a rough interface, while the outer organic layer maintains a porous structural feature. Therefore, the initial formation of the SEI battery is divided into two processes:
The electrolyte is decomposed on the electrode surface to form a double-layer porous SEI battery with an inner layer of inorganic matter and an outer layer of organic matter.
The electrolyte penetrates into the pores of the battery SEI and continues to decompose, making the battery SEI grow until the inner layer becomes uniform and dense, and enough organic components appear in the outer layer. It can effectively block electrons and prevent further breakdown of the electrolyte.
5. Battery SEI structure
● Mosaic model
There are many SEI battery models, the most accepted of which 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 inorganically enriched inner layer (in contact with lithium) and an organically enriched outer layer (in contact with the electrolyte).
On the other hand, it is assumed that each component constitutes a pure microphase and that the battery SEI is a Mosaic assembly of different microphases. As shown in the figure below, the inner layer is mainly high-density inorganic layer, and the outer layer is mainly low-density organic layer.
When the complete battery SEI is generated, for the Mosaic model, we believe that the negative charging process can be divided into four successive steps at the microscopic level.
Dissolved lithium ions diffuse into the electrolyte.
Lithium ions are de-solubilized by breaking the solvated shell.
Lithium ions diffuse onto the battery SEI.
The diffusion of Li ions in the anode material is accompanied by electron transfer and the lattice rearrangement of the anode material.
Vi. Conclusion
The passivation layer covering the surface of the electrode material is the battery SEI. It can be stabilized in the organic electrolyte solution, and the solvent molecules cannot be passivated through the layer, which can effectively prevent the co-embedding of solvent molecules. It avoids the damage caused by the co-embedding of solvent molecules into the electrode material, thus greatly improving the cycle performance and service life of lithium-ion batteries.
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