Carbon negative electrode material is currently the most successful commercial application of lithium ion battery
negative electrode material, but the working voltage of carbon negative electrode is outside the stable electrochemical window of electrolyte components, so when the electrode is in a charging state, that is, in the cathodic polarization process, the reduction and decomposition of electrolyte will occur at the electrode electrolyte interface, accompanied by irreversible consumption of lithium ions.
The decomposition product covers the electrode surface and forms a protective layer.
This process mainly occurs at the beginning of the cycle, especially the first cycle.
The decomposition product layers formed on the graphite surface can be divided into two categories according to their functions.
At the end face and base surface defects of graphite, lithium ions pass in and out of graphite through deintercalation reaction. The protective layer formed on these surfaces is called solid electrolyte interface, namely SEI film.
SEI film has its special function, that is, allowing lithium ions to pass through to prevent solvent components and electrons from passing through, so SEI film has the effects of preventing electrolyte from further reduction and decomposition and inhibiting electrode corrosion in the charging state.
When the passivation effect of the protective layer is not good, on the one hand, co-embedding of solvent molecules and lithium ions may occur, resulting in peeling and amorphous graphite materials, the other may deactivate some graphite electrode active materials.
However, on other surfaces of graphite, where there is no lithium ion deblocking (Base)
The formed surface protective layer does not have the above functions, but the protective layer can also inhibit further reaction between graphite surface and electrolyte.
At the same time, due to the slight difference in the activity of carbon atoms on the end surface and the base surface, it is 5J/m2 and 0 respectively.
11J/m2, so the composition and thickness of the protective layer on the end face and the base surface are different.
Due to the different functions and compositions of the two protective layers, the failure process in the attenuation process is also different. In actual research, more attention is paid to the relationship between SEI film on the end face and battery failure.
The transmission of lithium ions at the electrolyte and electrode interface must pass through SEI film, so SEI film has many characteristics: SEI film impedance, passivation effect on electrode, flexibility of lithium ion in repeated deintercalation process, lithium ion diffusion rate, and these characteristics ultimately determine the kinetics of lithium ion deintercalation process [B2]
And the stability of the electrode/electrolyte interface, thus determining the cycle life, self-discharge and other properties of the battery.
SEI membrane is composed of electrolyte decomposition products, so the composition of electrolyte largely determines the characteristics of SEI membrane, good film-forming solvent and film-forming additive can not only support the formation process of SEI film in the first cycle, but also slow down the aging process of SEI film in the cycle.
In addition, the surface state and charging and discharging mechanism of the negative electrode material will also affect the performance of SEI film.
The structure of SEI membrane consists of two or more layers of substances with different components.
Among them, the first layer of material is close to the negative electrode material, which is relatively dense and consists of LizolB3-3], LizCOg/3637], LiOH38], Life9]And other inorganic products;
The second layer is attached to the first layer and consists of organic compounds such as lithium alkoxide and polymers with loose or porous structures.
At the beginning of the research on SEI film, the research on negative electrode was mainly focused.
As we all know, the increase of negative electrode interface impedance will lead to the attenuation of high energy battery capacity.
However, for high-power batteries, a significant increase in positive interface impedance will lead to power loss of high-power batteries.
Both high temperature and high charging cut-off voltage will accelerate the increase of positive electrode interface impedance.
It is pointed out that a layer of passivation film will be formed on the positive electrode surface due to oxidation and decomposition of electrolyte and decomposition of lipf6.
Taking Lithium nickel oxide and lithium drill acid as an example, the material itself will be used as the oxygen source of electrolyte oxidation reaction, and after oxidation reaction, a rock salt structure with oxygen deficiency will be generated.
Compared with the negative SEI film, the thickness of the positive electrode interface film is thinner, but it can effectively prevent further oxidation reaction of electrolyte components and improve the oxidation potential of electrolyte components;
The passivation film also increases the interfacial impedance and reduces the reversible cycle capacity of the battery.
Therefore, the electrochemical behavior of the anode material will depend to some extent on the surface chemical properties of the electrode.
The main factors affecting the formation of positive SEI film are temperature, storage potential and the composition of the original surface film on the electrode surface. The circulation process has no effect on the composition of the surface film.
At a temperature higher than normal temperature, no new surface reaction will be caused, but only the surface reaction will be accelerated.
The thickness of the surface film increases with the increase of storage time, number of cycles and temperature.
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