As the energy density of lithium-ion batteries continues to increase,
especially high-energy lithium-ion batteries using silicon-containing negative
electrodes, the application of lithium replenishment technology is particularly
urgent due to low first efficiency. At present, the most common lithium
replenishment process is the negative electrode lithium replenishment method,
which uses lithium powder, lithium foil and other processes to supplement the
irreversible capacity loss of the negative electrode during the first charging
process. In addition, another lithium replenishment method being studied is the
positive electrode lithium replenishment process, both Add a small amount of
high-capacity lithium-containing oxide, such as Li5FeO4 material, to the
positive electrode, and use the positive electrode to store additional Li to
supplement the Li loss during the first discharge process. These two lithium
supplementation methods have their own advantages. Today we will discuss and
compare the two lithium supplementation methods.
Expansion: The first effect problem of silicon anode
In the fully lithium-embedded state, the specific capacity of pure Si can
reach 4200mAh/g (Li4.4Si), but it is also accompanied by a volume expansion of
up to 300%, which will cause particle fragmentation and differentiation of pure
silicon materials during the lithium-embedded process. , the negative electrode
material falls off, resulting in serious capacity decline during material
circulation. In order to overcome the problem of silicon anode materials, people
have tried to make pure silicon into nanoparticles to suppress the expansion of
Si particles. However, in fact, this strategy is not successful. Related
calculations show that only when the particle size of pure Si particles is
smaller than the unit cell It is only possible to completely suppress the volume
expansion of Si particles when the size is larger, which is obviously impossible
to achieve. Therefore, nanoparticles only reduce the volume expansion of Si
anode particles. At the same time, the larger specific surface area of
nanoparticles will also cause negative electrode and electrolysis Side reactions
between liquids increased significantly. In addition, another strategy is to
make the Si material into a raisin bread structure, that is, to disperse the
nano-Si particles in the graphite ocean, and use the graphite to absorb the
volume expansion of the Si particles during the charge and discharge process,
but this method is not perfect. , First of all, the specific capacity of the
material is very low. Due to the high graphite content, the specific capacity of
most such silicon carbon anodes is only 400-500mAh/g. At the same time, the
cycle life of this type of silicon carbon material has not been much improved.
improve.
Due to the above-mentioned problems in pure Si materials, people began to
try to use another silicon oxide-SiOX as the negative electrode material. The
bond energy of the Si-O bond is twice that of the Si-Si bond. At the same time,
during the process of lithium insertion In the process, Li will react with the O
element in the material to generate LiXO. These Li oxides then lose their
activity and become a buffer layer inside the silicon oxide particles, which can
inhibit the material during the charge and discharge process. Volume expansion
improves the material's cycle performance. Since the metallic lithium oxide LiXO
is generated during the first lithium insertion process in SiOX, the first
Coulombic efficiency of the silicon oxide material is only about 70%. In recent
years, after many technical improvements, the first time efficiency has also
been increased by about 80%. , which is still far behind the 90% of graphite
materials. Therefore, in order to take advantage of the high specific capacity
of SiOX materials, it is necessary to use a lithium replenishment process to
supplement the irreversible capacity loss during the first lithium insertion
process.
Comparison of positive electrode lithium replenishment process and negative
electrode lithium replenishment process
At present, the lithium replenishment process is mainly divided into two
categories; 1) negative electrode lithium replenishment process; 2) positive
electrode lithium replenishment process. Among them, the negative electrode
lithium replenishment process is our most common lithium replenishment method,
such as lithium powder lithium replenishment and lithium foil lithium
replenishment. These are all lithium supplementation processes that major
manufacturers are currently focusing on developing. The lithium powder
replenishment process was first proposed by FMC Company. FMC Company developed
inert lithium powder for this purpose, and added an appropriate amount of
lithium powder to the negative electrode through processes such as spraying and
homogenization. Lithium foil replenishment is also an emerging lithium
replenishment process in recent years. The metal lithium foil is rolled to a
thickness of several microns, and then compounded and rolled with the negative
electrode. After the battery is filled with liquid, these metal Li react quickly
with the negative electrode and are embedded in the negative electrode material,
thus improving the first efficiency of the material. However, these methods have
to face a problem - the safety of metallic lithium. Metal lithium is a highly
reactive alkali metal that can react violently with water, making metallic
lithium very demanding on the environment. This makes these two Each negative
electrode lithium replenishment process requires huge investment in the
transformation of the production line and the purchase of expensive lithium
replenishment equipment. At the same time, in order to ensure the lithium
replenishment effect, the existing production process also needs to be
adjusted.
Compared with the difficult and high-investment negative electrode lithium
supplementation process, the positive electrode lithium supplementation process
is much simpler. The typical positive electrode lithium supplementation process
is to add a small amount of high-capacity positive electrode material to the
positive electrode during the homogenization process. During the charging
process, excess Li elements are extracted from these high-capacity cathode
materials and embedded into the negative electrode to supplement the
irreversible capacity of the first charge and discharge. For example, Xin Su and
others from Argonne National Laboratory in the United States added 7% Li5FeO4
(LFO) material to the LiCoO2 cathode, which increased the battery's first
efficiency by 14% and significantly improved the battery's cycle performance.
The theoretical specific capacity of Li5FeO4 material can reach 700mAh/g, and
almost all of the capacity is irreversible. After delithiation is completed, the
material quickly deactivates and no longer participates in the charge and
discharge reaction. The delithiation equation is as follows.
Li5FeO4®4Li++4e-+LiFeO2+O2
Giulio Gabrielli and others from Germany adopted a method of mixing two
positive electrode active materials: LiNi0.5Mn1.5O4 and Li1+XNi0.5Mn1.5O4.
Li1+XNi0.5Mn1.5O4 can be used during the first charging process of the battery.
Provide additional Li to make up for the loss of Li during the first lithium
insertion process of the negative electrode. After complete delithiation, Li1+
Impact, Li1+XNi0.5Mn1.5O4 can be regarded as a cathode material that temporarily
stores excess Li. By changing the ratio of Li1+XNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4,
the amount of additional Li that the cathode can provide can be Control is
performed to accommodate negative electrodes with different first
efficiency.
Through the above analysis, it is not difficult to find that the biggest
advantage of the positive electrode lithium replenishment process is that the
process is simple. There is no need to change the existing lithium-ion battery
production process, nor to transform the existing production workshop, nor to
purchase. Expensive lithium replenishment equipment, and more importantly, the
positive electrode lithium replenishment greatly improves the safety of the
lithium replenishment process. However, the positive electrode lithium
replenishment process may cause the proportion of the active material of the
positive electrode to decrease. For example, when using Li5FeO4, it needs to
reach 7 % content, and these products after lithium supplementation are
inactive, thus affecting the further improvement of the energy density of
lithium-ion batteries.
Comparing the two lithium supplementation methods, the author is more
optimistic about cathode lithium supplementation. The negative electrode lithium
replenishment process has strict conditions, high investment, and the use of
metallic lithium poses greater safety risks. In contrast, the positive electrode
lithium replenishment process is simple, does not require the modification of
existing production lines and processes, and has a small investment. There is no
safety risk. The cathode lithium supplementation process developed by Giulio
Gabrielli and others solves the problem of lithium supplementation products
affecting the composition of the cathode. Although this technology is currently
only applied to LiNi0.5Mn1.5O4 materials, through the development of related
technologies, this supplementation It is believed that the lithium process can
also be applied to ternary materials such as NCM and NCA to improve the first
efficiency of the battery.
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