After the liquid injection of the lithium battery is completed, during the
first charge and discharge process, the electrode material and the electrolyte
will undergo an electrochemical reaction at the solid-liquid interface to form a
solid electrolyte interface film (SEI film) covering the surface of the
electrode material. SEI The quality of the membrane directly determines the
cycle performance of the battery. SEI is composed of Li2O, LiF, LiCl, Li2CO3,
LiCO2-R, alkoxide and non-conductive polymer. It has a multi-layer structure.
The side close to the electrolyte is porous and the side close to the electrode
is dense. On the one hand, the formation of the SEI film consumes part of the
lithium ions, which increases the irreversible capacity of the first charge and
discharge and reduces the charge and discharge efficiency of the electrode
material; on the other hand, the SEI film is insoluble in organic solvents and
can exist stably in organic electrolyte solutions. Can improve the cycle life of
lithium batteries.
The formation process has an important impact on the formation of the SEI
film of lithium batteries and directly affects the performance of the battery.
Usually the formation potential of the SEI film is in the range of 0.6V-0.8V, so
the current in the initial stage of formation is often kept at a very small
state to ensure that the SEI film is formed more densely and is conducive to
improving the cycle life.
Although the traditional low-current precharging method helps to form a
stable SEI film, long-term low-current charging will increase the resistance of
the formed SEI film, thus affecting the battery cycle, rate performance, etc.;
the length of the formation time will also Affects the formation of the SEI film
of the battery, because the formation of the lithium-ion battery is a first
activation process. As charging proceeds, the internal voltage of the battery
increases and is accompanied by the generation of gas. Once the gas production
rate is higher than the exhaust of the liquid injection hole rate, the gas will
accumulate between the separators inside the battery, which will affect the
formation of the SEI film on the surface of the negative electrode. Therefore,
it is necessary to select the appropriate current and formation time.
At present, the formation process is mainly divided into two categories,
the multi-step stepped formation process and the constant flow formation
process. So which formation process is more suitable?
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Group A multi-step formation process:
Charging (0.05CC/4h→0.1CC/2h→0.2CC/1h→0.4CCCV/4.2V→0.02Ccut)→Station
0.5h→Discharge (0.5C to cutoff voltage)→Station 0.5h, cycle three times and then
0.2C /2h, charge to 4.0V.
Group B adopts constant current formation process:
Charging (0.2C/2.5h)→Standing for 12h→Discharging (0.2C cut-off
voltage)→Standing for 0.5h→Charging (0.2C/4.2V, 0.02Ccut-off)→Standing for
0.5h→Discharging (0.2C cut-off voltage)→ Rest for 0.5h → charge (0.2C/4.0V).
As you can see, Group A uses slow charging with a small current, gradually
increasing the current, and reducing the charging time at the same time. Group B
uses a 0.2C charge/discharge current, and there are not many parameters that
change. So what are the differences between these two formation processes on the
SEI and electrochemical performance of the battery?
1. SEI film
Through SEM analysis of the negative electrode surfaces of both groups of
batteries, it can be found that the electrode surface is covered by the SEI
film, but it is impossible to see any difference in thickness or coverage area
between the two.
2. Electrochemical performance
By analyzing the basic electrochemical properties of the two sets of
batteries, it can be concluded that the capacity of the lithium battery cathode
material using the stepped formation process is 3mAh/g higher than that of the
constant current formation process, and the charge and discharge efficiency of
the entire battery is higher. After 50 cycles, the specific capacity decay rate
of the constant flow formation process is slower than that of the step formation
process.
In terms of first effect, the constant current type is lower than the step
type, but the second cycle is higher than the step type. This also explains that
the degree of reversible reaction of the battery formed by the constant current
type is higher than that of the step type formation process, that is, Less
irreversible capacity loss.
3. SEI component analysis
By analyzing the SEI of the two groups of batteries, the following
conclusions can be drawn:
1. The lithium ion content on the CMS surface of the negative electrode of
the lithium-ion battery after step-type formation is higher than that of
constant-current formation. This is because a variety of lithium-containing
compounds are formed under different current densities, resulting in excessive
lithium content.
2. XPS results show that Li2CO3 or LiCO2-R exists in the SEI films of both
groups of batteries.
3. The SEI thickness formed by two chemical formation processes is greater
than 3nm.
Through the analysis of the above two sets of batteries with different
formation processes, it can be concluded that different current sizes and times
have different effects on battery performance. The composition and properties of
the SEI film are also different, which will inevitably affect the performance of
the battery.
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