Although the application of high-capacity Si anode materials is gradually
becoming more and more popular, graphite anodes are still the mainstream anode
materials for lithium-ion batteries due to their excellent electrochemical
properties. During the charging process, Li+ escapes from the positive electrode
and diffuses through the electrolyte to the surface of the negative electrode
and is embedded inside the graphite negative electrode. The discharge process is
exactly the opposite. The lithium insertion potential of the graphite material
is close to that of metal Li. This aspect can effectively improve the
performance of lithium-ion batteries. Voltage, thereby increasing energy
density, but on the other hand, it also causes the current conventional
carbonate electrolyte to undergo reductive decomposition on the surface of the
graphite negative electrode, resulting in the consumption of active Li. Numerous
studies have shown that the decomposition of the electrolyte on the surface of
the negative electrode is responsible for It is an important reason for the
capacity decline of lithium-ion batteries, so the selection of graphite anode
materials is of great significance for improving the life characteristics of
lithium-ion batteries.
Recently, Chengyu Mao (first author) and Zhijia Du (corresponding author)
of Oak Ridge National Laboratory in the United States analyzed the impact of six
mainstream artificial and natural graphite materials on the cycle performance of
NCM811 batteries. The analysis showed that the specific surface area is more
Smaller materials result in higher first Coulombic efficiencies and perform
better over long cycles.
In the experiment, Chengyu Mao used the NCM811 material from Targray
Company of Canada as the positive electrode, and the six graphite negative
electrodes were A12 from ConcoPhillips Company, APS19 from GrafTech Company,
SCMG-BH from Showa Denko, MAGE and MAGE3 from Hitachi Chemical, and SLC from
Superior Company. 1520T (six material information is shown in the table
below).
The picture below shows the morphology of several graphite materials. It
can be seen from the picture that SCMG-BH, MAGE and MAGE3 materials are
basically "potato" shaped, A12 and APS19 have a flake structure, and SLC 1520T
material is closer to a spherical structure. , the surface is relatively smooth,
so SLC 1520T also obtains the smallest specific surface area (as shown in the
table above).
The graphite crystal size can be obtained from the cross-sectional view of
the particles. From the figure below, you can see that the SCMG-BH material has
the smallest graphite crystal particles. Since the electrolyte is easier to
decompose at the edges of the graphite crystal sheets, SCMG-BH has smaller
crystal particles. The material will cause more electrolyte decomposition,
causing the battery's Coulombic efficiency to be low and affecting the cycle
life of the lithium-ion battery.
The picture below shows the reversible capacity of 6 negative electrode
materials discharged at C/3 rate in button batteries. From the picture, it can
be seen that the reversible capacity of most graphite can reach more than
350mAh/g. Only Showa Denko’s SCMG-BH material The reversible capacity is
322mAh/g.
As the potential of the graphite anode decreases during the first lithium
insertion process, the electrolyte will decompose on the surface of the graphite
anode. Therefore, the specific surface area of graphite will have a significant
impact on the Coulombic efficiency of the battery for the first time. The figure
below shows the difference between NCM811 materials and different The charge and
discharge curve of a full battery (soft pack) composed of graphite negative
electrode during the formation process. The capacity of the NCM811 material in
the first charge and discharge is shown in the table below. Among them, the MAGE
material of Hitachi Chemical with a smaller specific surface area has the
highest first efficiency, reaching It was 86.1%, followed by SLC 1520T. The
MAGE3 material had the lowest Coulombic efficiency for the first time, only
82.2%, which is also related to its large specific surface area of 4.97m2/g.
The picture below shows the cycle curves of batteries using several
different graphite anodes (3.0-4.2V, C/3 charging and C/3 discharging). In order
to accelerate the decline of the battery, the author also added a 3-hour cycle
to each charge. Constant voltage process, as can be seen from the figure, the
addition of the 3-hour constant voltage process greatly accelerates the decline
rate of lithium-ion batteries. After 300 cycles, only the battery capacity
retention rates of MAGE and SLC 1520T materials exceed 80 %, MAGE3 and SCMG-BH
materials have the worst battery cycle performance and reach the end of life
first. Batteries using different anodes also show different degradation
characteristics. For example, batteries using A12 and APS19 materials begin to
decline faster after 200 cycles, while MAGE3 and SCMG-BH materials show faster
degradation in the early stages. At the same time, we can also notice from the
table below that the initial capacity of batteries using MAGE3 and SCMG-BH is
also lower than batteries using other materials. This is mainly due to the
relatively low formation Coulombic efficiency of these two materials.
caused.
In order to analyze the degradation mechanism of several lithium-ion
batteries with different negative electrodes during cycling, the author
dissected the cycled battery and made button batteries using the positive and
negative electrodes respectively. From Figure c below, you can see the positive
electrode after cycling Not only the capacity has been significantly reduced,
but the rate performance has also been significantly reduced. On the other hand,
the negative electrode (d in the figure below) has only a slight reduction in
reversible capacity (less than 3%) after cycling, but the rate performance has
improved. Declining, the SCMG-BH, A12 and MAGE3 materials after cycling have
relatively low capacities at high rates.
In order to analyze the reasons for the decline in rate performance after
aging of positive and negative electrode materials, the author also used the AC
impedance method to analyze the button battery. The figure below shows the AC
impedance diagram of the NCM811 material button battery matched with different
negative electrode materials. From In the figure, we can notice that in addition
to a semicircle in the high-frequency region, the recycled NCM811 material also
has a new semicircle in the mid-frequency region. This may be due to a
relatively slow charge exchange process in the recycled NCM811 material, such as
It is possible that new phases are generated on the surface of NCM811 particles,
resulting in an increase in charge exchange resistance.
The figure below shows the AC impedance spectrum of a button-type half-cell
made from the formed negative electrode and the recycled negative electrode.
Compared with the positive electrode, the impedance of the negative electrode is
smaller, indicating that the increase in internal resistance of the full battery
after cycling is more due to the impedance of the positive electrode. increase.
Moreover, the AC impedance changes of different negative electrodes after
cycling are also different. The impedance of A12, SCMG-BH and MAGE3 materials
almost doubled after cycling. This is mainly related to their larger comparative
area, which leads to more decomposition of the electrolyte, which also leads to
A large amount of active Li is consumed, resulting in poor cycle performance of
the full battery of these materials. MAGE and SLC 1520T materials with better
cycle performance also have relatively less increase in impedance after cycling.
This is mainly due to these two materials. The smaller specific surface area of
this material reduces the decomposition of the electrolyte.
The figure below shows the relationship between the specific surface area,
first-time efficiency and negative electrode particle size of several negative
electrode materials and the capacity retention rate of the full battery. From
the figure, we can see that the MAGE and SLC 1520T materials with the smallest
specific surface areas not only have the highest first-time Coulombic
efficiency, but also have the highest first-time Coulombic efficiency. It also
showed the highest capacity retention rate in long-term cycling, while MAGE3 and
APS19 materials with larger specific surface areas showed lower first Coulombic
efficiency and poor cycle performance.
Generally speaking, the specific surface of the graphite anode has a
crucial impact on its Coulombic efficiency and long-term cycle stability.
Materials with a smaller specific surface can reduce the decomposition of the
electrolyte, thus improving the first Coulombic efficiency and long-term cycle
stability of the battery. Therefore, for lithium-ion batteries with higher
requirements on life characteristics, graphite materials with smaller specific
surface areas should be selected.
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