With the continuous development of lithium-ion battery technology, the
pursuit of energy density is also getting higher and higher. In the latest
national guidelines for new energy vehicles, it is proposed that by 2020, the
specific energy of a single power battery will reach 300Wh/ kg, this indicator
is very difficult to achieve on the existing lithium-ion battery system. At the
just-convened Third New Battery Positive and Negative Materials Technology
Estimation Forum, researcher Liu Jun from the Pacific Northwest National
Laboratory in the United States proposed that there are two main methods for
developing batteries with a specific energy of 500Wh/kg in the future:
High-nickel NCM And the metallic lithium system, the other is the lithium-sulfur
battery system, no matter which method is used, we cannot avoid metallic
lithium. The biggest problem with lithium metal as a lithium-ion battery is the
generation and growth of lithium dendrites. Although current research shows that
ether solvent electrolytes can effectively inhibit the generation of lithium
dendrites, due to the low decomposition voltage of ether compounds, And it is
highly flammable, so it is difficult to apply in commercial lithium-ion
batteries. From the current point of view, for lithium-ion batteries using
metallic lithium anodes, all-solid electrolytes are more feasible. Solid
electrolytes have higher The elastic modulus can well inhibit the generation and
growth of lithium dendrites, so it can effectively improve the cycle life and
safety performance of metal lithium batteries.
At present, solid electrolytes are mainly divided into two categories:
inorganic ceramic electrolytes and organic polymer electrolytes. Among them,
sulfide solid electrolytes are the most attractive because of their high lithium
ion conductivity (10-2S/cm) and good flexibility. However, the sulfide solid
electrolyte easily reacts with polar solvents, and its particulate
characteristics also make it difficult to homogenize the positive and negative
electrodes. In order to solve this problem, Dong Hyeon Kim from the University
of Ulsan in South Korea proposed a new method for large-scale preparation of
all-solid-state batteries. Both the positive and negative electrodes of the
all-solid-state battery adopt the traditional lithium-ion battery electrode
structure, using Li6PS5Cl (LPSCl) ethanol or a methanol solution of
0.4LiI-0.6Li4SnS4 was used to infiltrate traditional lithium-ion battery
electrodes. The battery showed a high reversible capacity. The positive
electrode LiCoO2 reached 141mAh/g, and the negative electrode graphite material
reached 364mAh. /g (0.1C, 30℃). At the same time, the battery also showed good
electrochemical performance at 100℃, indicating that the battery has good
thermal stability and safety.
Traditional solid-state lithium-ion battery production requires a
relatively complicated dry mixing process to mix active materials, solid
electrolytes, conductive agents, and binders. However, in the actual process, we
prefer to use a wet mixing process to mix these electrode components. However,
Since the solid electrolyte has strong reactivity with polar solvents, the polar
solvents used in the traditional lithium-ion battery production process cannot
be used in the production of all-solid-state lithium-ion batteries. Therefore,
we need to develop a non-polar solvent for For the production of all-solid-state
lithium-ion batteries, such as toluene and xylene, and traditional binders such
as PVDF, CMC, SBR, etc. are not suitable for all-solid electrolytes, so it is
necessary to develop suitable binders. In addition, for lithium-ion batteries,
the homogenization process requires the uniform mixing of three materials
(active material, conductive agent, binder), while for all-solid-state
batteries, solid electrolytes must be added, not only the properties of the
electrodes must be considered Electronic conductivity must also take into
account the ionic conductivity of the electrode. In general, the preparation
process of all-solid electrolyte electrodes is far more complicated than that of
lithium-ion batteries.
In order to uniformly mix the various components of the all-solid-state
lithium-ion battery electrode, Dong Hyeon Kim and others first used traditional
processes to obtain lithium-ion battery electrode pieces, and then made the
solid electrolytes Li6PS5Cl and 0.4LiI-0.6Li4SnS4 into ethanol respectively. and
methanol solution, immerse the lithium-ion battery pole pieces into the above
solution, and then dry and roll them. This process ensures uniform mixing
between the solid electrolyte and the active material and ensures good
electrochemical performance of the battery. The reversible capacity of the
positive electrode LiCoO2 reached 141mAh/g, and the negative electrode graphite
material reached 364mAh/g (0.1C, 30℃, half cell). At the same time, the battery
also showed good electrochemical performance at 100℃. It shows that the battery
has good thermal stability and safety.
The experimental process is shown in the figure below. First, the
traditional production process is used to obtain the positive and negative
electrodes. Then Li2S, P2S5 and LiCl are mixed evenly by ball milling. Then the
uniformly mixed powder is dissolved in ethanol to form a uniform solution. The
initially coated electrode is used to absorb the solid electrolyte solution,
then dried in a vacuum environment to remove the solvent, and then heat treated
in a vacuum environment at 180°C. Finally, a cold rolling mill is used to roll
the above electrode under a pressure of 770MPa. , reduce the porosity of the
electrode and increase the ionic conductivity.
Field emission microscope SESEM and X-ray energy spectroscopy EDXS were
used to observe the element distribution of the pole pieces prepared above. The
results are shown in the figure below. It can be seen that the solid electrolyte
occupies the space between the active material particles very well.
Dong Hyeon Kim made the positive and negative electrodes prepared above
into half cells for electrochemical testing. The test results are shown in the
figure below. From the results, we can see that for the positive LCO material,
the experimental battery using solid electrolyte has a specific capacity of
141mAh /g, while the specific capacity of the control group using liquid
electrolyte is 154mAh/g, and for the negative electrode graphite, the specific
capacity of the experimental battery using solid electrolyte is 364mAh/g, and
the control group using liquid electrolyte is only 312mAh/g, but the solid The
first efficiency of electrolyte batteries is low (76.6% for the positive
electrode and 80.7% for the negative electrode). This is related to the
instability of the sulfide solid electrolyte at high and low voltages. During
the first charging process, some Li+ is irreversibly embedded into the solid
electrolyte. , but the subsequent formation of inert decomposition products can
protect the solid electrolyte from further decomposition.
The rate performance test of the solid electrolyte is shown in the figure
below. From the results, reducing the content of PVDF binder can effectively
improve the rate performance of the battery. This is mainly because the lower
PVDF binder content helps Improve the ionic conductivity between the solid
electrolyte and the active material, thereby improving the rate performance of
the battery. However, reducing the content of conductive agent SP will lead to a
decrease in rate performance, which may be due to a decrease in the electronic
conductivity of the electrode.
Cycling tests show that the cycling performance of solid electrolyte
electrodes has declined, with a capacity retention rate of 88.6% after 50 cycles
at 0.5C. In comparison, the capacity retention rate of traditional liquid
electrolytes after 50 cycles is 97.1%. This is mainly due to the solid
electrolyte. This problem is caused by the instability of the interface with
LCO. This problem can be effectively overcome by Al2O3. The capacity retention
rate of the coated electrode after 50 cycles can reach 98.1%.
After completing the above tests, Dong Hyeon Kim also conducted a full
battery test. The positive and negative electrodes used solid electrolyte
electrodes prepared by the above-mentioned wet mixing process. The separator
used a new type of flexible solid electrolyte-non-woven separator, with a
thickness of only 70um, 0.5 The capacity retention rate after C cycle is 95.9%.
The battery's capacity retention rate is 82% after 100 cycles at 6C at 100°C.
This temperature far exceeds the normal operating temperature of traditional
lithium-ion batteries.
The new solid-state battery preparation process developed by Dong Hyeon Kim
uses a solid electrolyte solution to obtain a uniform solid electrolyte
electrode, thus ensuring good ionic conductivity of the electrode. Compared with
the traditional dry mixing process to prepare solid electrolyte electrodes, the
battery's The rate performance and cycle performance have been greatly improved.
At the same time, the solid electrolyte battery can operate normally at 100°C
and shows excellent cycle performance. At the same time, the use of non-woven
fabric + solid electrolyte thin separator makes this solid electrolyte battery
technology more convenient to apply in the production of traditional wound
battery cells.
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