What new technologies support the high performance of lithium
batteries?
In addition to increasing the diameter of lithium-ion batteries, another
effective way to increase the specific energy of lithium-ion batteries is to
reduce the thickness of the separator. The thickness of the currently common
PP-PE-PP three-layer composite separator generally reaches more than 30um,
reaching the positive and negative electrodes. About 20% of the thickness of the
film, which also results in serious waste of space. In order to reduce the space
occupied by the separator, currently the majority of lithium-ion battery
manufacturers generally use thin separators with coatings. The thickness of
these separators can reach less than 20um. On the premise of ensuring the safety
of lithium-ion batteries, it can significantly reduce the volume proportion of
the separator, increase the proportion of active materials, and increase the
specific energy of lithium-ion batteries.
Another way to increase the proportion of active materials is to start from
the perspective of the battery production process. First, increase the
proportion of active materials in the electrode. Generally, the electrode of a
lithium-ion battery is mainly composed of four parts, active material,
conductive agent, binder and current collector. In order to increase the
proportion of active materials, it is necessary to reduce the proportion of
other parts. By using new conductive agents and binders Thereby reducing the
proportion of conductive agent and binder, and using thinner current collectors
to reduce the proportion of inactive substances. Secondly, it is necessary to
increase the coating amount of the positive and negative electrodes, but there
is also a problem when increasing the coating amount of the electrodes: when the
electrode is too thick, the Li+ diffusion kinetic conditions of the electrode
will become worse, affecting the rate and cycle of the lithium-ion battery.
Performance, in order to solve this problem, BorisBitsch et al. [1] of Karlsruhe
Institute of Technology in Germany used capillary suspension and multi-layer
electrode processes to prepare high-performance thick electrodes with gradient
porosity. In the lower layer close to the copper foil, BorisBitsch and others
used ordinary slurry, which makes it have lower porosity and good conductivity.
On the surface layer far away from the copper foil, BorisBitsch used capillary
suspension slurry and applied it to the lower layer. The addition of 1-octanol
significantly increases the porosity and improves the kinetic conditions of the
electrode. As a result, the porosity of the electrode gradually increases from
bottom to top, significantly improving the kinetics of thick electrodes.
Conditions improve the electrochemical performance of thick electrodes, thereby
achieving an increase in the weight and volume specific energy of the battery
without reducing the cycle performance of the battery.
Another important method to improve the specific energy of lithium-ion
batteries is to control the amount of electrolyte. Reducing the amount of
electrolyte can effectively increase the energy density of lithium-ion
batteries. The electrolyte plays a role as a medium inside the lithium-ion
battery. Li+ in the positive and negative electrodes diffuses through the
electrolyte. Therefore, the electrolyte is theoretically a "non-consumable
product". As long as there is a small amount of electrolyte, the Li+ in the
positive and negative electrodes can be ensured. Free diffusion between the
negative electrodes is enough, but in fact due to the formation of the SEI film
during the formation process, the electrolyte decomposes, and the electrolyte
decomposition caused by SEI film damage and positive electrode oxidation during
the cycle causes the electrolyte to actually It is continuously consumed, so the
electrolyte in the battery is generally excessive, which is also an important
reason for the low specific energy of lithium-ion batteries. In order to reduce
the amount of electrolyte and ensure the performance of the battery, we need to
control the electrolyte The solvent system and electrolyte additive system are
improved to improve the stability of the electrolyte. In order to improve the
stability of the electrolyte in the ternary material NMC battery, Yunxian Qian
et al. [2] from the University of Münster in Germany added a small amount of
electrolyte to the electrolyte using traditional EC and EMC (weight ratio 3:7)
as solvents. FEC additive, it is found that FEC additive can effectively reduce
the decomposition of electrolyte, improve the first Coulombic efficiency of NMC
battery, and significantly improve the cycle stability of the battery.
2. Selection of positive and negative active materials
As our requirements for the energy density of lithium-ion batteries
continue to increase, traditional LiCoO2 materials have physically met the needs
of high-specific-energy lithium-ion batteries. In order to further improve the
energy density of lithium-ion batteries, we have two general directions to
choose from: 1) Increase the working voltage of lithium-ion batteries; 2)
Increase the capacity of positive and negative electrode materials. First, let’s
analyze the feasibility of improving the energy density of lithium-ion batteries
from aspect 1). The operating voltage of lithium-ion batteries depends on the
voltage difference between the positive and negative electrode materials. At
present, the voltage of the negative electrode material graphite is already very
low (about 0.1 VvsLi+/Li), there is not much room for further reduction.
Improving the voltage of lithium-ion batteries mainly starts from the cathode
material. Currently, the high-voltage materials available for selection mainly
include the following categories:
(1) The high-voltage spinel lithium nickel manganate LiNi0.5Mn1.5O4
material has an operating voltage of up to 5.0V, a voltage platform of around
4.7V, a theoretical capacity of 147mAh/g, and an actual capacity of more than
138mAh/g. . (2) Olivine-based high-voltage materials, such as LiMnPO4 and
LiCoPO4 materials, among which the voltage platform of LiMnPO4 material can
reach about 4.1V, and that of LiCoPO4 material can reach about 4.8V. (3)
Lithium-rich materials. The theoretical capacity of lithium-rich materials can
reach more than 200mAh/g, or even 300mAh/g. However, in order to take advantage
of the high capacity of lithium-rich materials, its working voltage needs to be
increased, even to about 4.8V. .
In addition to solving their own problems, these high-voltage materials
also face the same problem: the instability of the electrolyte at high voltage.
Currently, commercial lithium-ion battery electrolytes are generally based on
carbonate-based organic electrolytes. The solvent salt is LiPF6, which also
causes the electrolyte to be easily oxidized and decomposed at high potentials,
resulting in serious degradation of battery performance and even safety issues.
In order to overcome this problem, we can proceed from two aspects. The first is
the electrolyte solvent system. In order to improve the electrochemical
stability window of the electrolyte, more stable ionic liquid electrolytes and
new electrolyte salts can be used. On the other hand, in order to reduce the
oxidation of the electrolyte by the high-voltage material, the surface of the
high-voltage material can be coated to isolate the electrolyte and the active
material. DongruiChen et al. [3] of South China Normal University used Li3PO4 to
surface-coat lithium-rich layered materials. Li3PO4 coating significantly
improved the cycle performance of lithium-rich materials, reduced the
dissolution of transition metal elements, and inhibited the layering process.
transformation from a spinel-like structure to a spinel structure. Another
important aspect of improving the specific energy of lithium-ion batteries is to
increase the specific capacity of positive and negative active materials, which
requires both positive and negative electrode materials. In terms of cathode
materials, the high-capacity cathode materials that we can choose from mainly
fall into the following two categories: 1) ternary materials NCM and NCA; 2)
lithium-rich materials.
Ternary materials are currently the most mature high-capacity cathode
materials. As the Ni content increases, the specific capacity of ternary
materials will also increase accordingly. For example, the high-nickel NCM811
material has a specific capacity of about 200mAh/g. The specific capacity of
high-nickel NCA materials can also reach about 190mAh/g, which is much higher
than that of LiCoO2 materials. Lithium-rich materials are newly developed
high-capacity cathode materials in recent years. Their specific capacity can
reach more than 200mAh/g, or even 300mAh/g. However, lithium-rich materials are
currently relatively rare in the market. The main reasons are as follows.
Points: 1. High irreversible capacity; 2. Voltage decay; 3. Poor cycle
performance. Improving its performance requires aspects such as element doping
and surface coating, as well as material structure design.
In terms of high-capacity anode materials, we mainly have the following
options: 1) silicon-based materials; 2) N-doped graphite materials; 3)
transition metal Side materials; 4) metallic lithium anodes. Needless to say,
silicon anode material is the most mature and reliable high-capacity anode
material on the market. The specific capacity of crystalline Si can reach more
than 4200mAh/g, but it has large expansion and poor cycle performance. Although
SiOX has a slightly lower capacity (1500mAh/g) g), but the cycle performance is
excellent, but the disadvantage is low first efficiency. N-doped graphite
materials have been a research hotspot for high-capacity anode materials in
recent years. The electronegativity of N atoms is about 3.5. After incorporating
N elements into graphite, the specific capacity of the graphite anode can be
significantly improved. According to research from Wuhan University KaifuHuo et
al. [4] used the template method to prepare N-doped mesoporous carbon hollow
sphere materials. At a current density of 0.1A/g, the specific capacity can
reach 931mAh/g. At a current density of 0.5A/g, the cycle time is 1100.
Therefore, the specific capacity of 485.7mAh/g can still be maintained.
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