Research progress on alloy anode materials for lithium-ion batteries
Abstract: This review summarizes the research progress of alloy anode
materials for lithium-ion batteries, including aluminum-based, tin-based and
silicon-based alloy anode materials. The problems existing in the development of
alloy anode materials and the lithium storage mechanism are analyzed.
Nanonization of active particles, multiple buffer structures and multi-component
composite methods are the development directions of alloy anode materials.
Keywords: lithium-ion battery; active components; inert components; cycle
performance
At present, commercial lithium-ion batteries widely use graphite and
modified graphite as negative electrode materials, and their theoretical
capacity is low [1-2].
The research and application of high-capacity lithium-ion battery anode
materials have become the key to improving battery performance. In theory, some
metals or metalloids that can form an alloy system with lithium can be used as
negative electrode materials for lithium-ion batteries. These negative electrode
materials are collectively called alloy negative electrode materials. Compared
with graphite, alloy anode materials have a large theoretical lithium storage
capacity and a low lithium storage potential. The author of this article
summarizes the research progress of different alloy anode materials in recent
years, points out the existing problems in research and development, and looks
forward to the development direction of alloy anode materials.
1Lithium storage mechanism
During the lithium insertion process, Li+ reaches the active material
surface of the negative electrode through the electrolyte, obtains free
electrons on the negative electrode, forms lithium ions, and is deposited on the
surface of the negative electrode [3]. Afterwards, the lithium ions diffuse from
the surface of the negative electrode material to the negative electrode
material. Inside, an alloying reaction occurs. This process corresponds to the
discharge process of the external circuit; conversely, at high potential,
lithium ions lose electrons on the surface of the negative electrode due to
their active chemical properties, forming Li+, which migrates to the positive
electrode under the action of the electric field; the lithium ions inside the
negative electrode diffuse To the surface of the negative electrode, the
decomposition of the alloy occurs for the negative electrode. This process
corresponds to the charging process of the external circuit [4].
For graphite electrodes, during the battery charging and discharging
process, lithium realizes the intercalation and deintercalation process between
graphite sheets, and forms the lithium carbon intercalation compound LiC6. The
intercalation and extraction reaction of Li+ in layered graphite will only cause
changes in the interlayer spacing and will not destroy the crystal structure
[4].
Table 1 gives the performance parameters of active lithium storage elements
that are currently being studied more. Obviously, alloy anode materials have
higher theoretical capacity than graphite.
2 Problems with alloy anode materials
The disadvantages of the alloy anode material are its large first
irreversible capacity and poor ring performance. In particular, its ring
performance is far different from that of graphite electrodes. The fundamental
reason is that the reaction mechanism of alloy anode materials is different from
that of graphite anode materials [4]. The graphite anode material has a special
layered open structure. During the lithium insertion and removal reaction, the
structure does not undergo reconstruction, but only shrinks and expands in
volume. Most metal elements that can form alloys with lithium will form a
lithium-containing intermetallic compound LixMy when the lithium insertion and
removal reaction occurs. The crystal structures formed by different elements
vary greatly. In this way, when the compound is formed, the structure of the
component crystals will be reconstructed, accompanied by large volume expansion;
at the same time, in crystal materials, the formation of intermetallic compounds
will also lead to uneven volume changes in the boundary area between the two
phases. Causes cracking or pulverization of active particles. This cracking and
pulverization will cause the active particles to lose contact with the
electrode, and even cause the negative electrode to collapse. The lithium
embedded in the active material cannot be released in the subsequent process;
even if the lithium can be released from the active material, the active
material will not be released at room temperature. It does not have the ability
to recrystallize and will become loose amorphous material [5,9-10]. In amorphous
materials, although the volume expansion caused by lithium intercalation is
uniform, even if the expansion and contraction are reversible, the volume change
will produce a large irreversible capacity.
3 Research status of alloy anode materials
Based on the above problems, most research on alloy anode materials focuses
on how to reduce the irreversible capacity of the material or improve the ring
performance. At present, the form of A-M" intermetallic compounds is widely used
to buffer the volume change of alloy anode materials [11], where A is an active
lithium storage element such as Al, Sn, Si, and Sb, and M" is an element that
cannot form an alloy with lithium. The salient feature of this system is that
the active particles are evenly distributed on the inert matrix. The inert
components can buffer the volume deformation caused by the lithium intercalation
and extraction reaction, which improves the ring performance of the alloy anode
material to a certain extent.
3.1 Tin-based alloys and oxides
Tin has a higher theoretical capacity. When Li22Sn5 intermetallic compound
is generated, the theoretical capacity is 990mAh/g. Since the Li-Sn compound
will produce a large volume expansion, the performance of the Ñ ring of
tin-based materials will deteriorate.
The intermetallic compound Cu6Sn5 is a typical AM' type anode material
[12]. Conventional Cu6Sn5 preparation methods include mechanical alloying, gas
atomization and melt rapid quenching [13]. J.Wolfenstine et al. [14] prepared a
Cu6Sn5 alloy. When the particle size reaches the nanometer level, the capacity
attenuation is significantly reduced. The volume capacity after 100 N rings is
1450mAh/ml. The Ñ ring performance of Cu6Sn5 still needs to be improved. The
capacity retention rate of nanocrystalline Cu6Sn5 prepared by high-energy ball
milling after 20 Ñ rings is only 60% [15]. The research results of K.D. Kepler
et al. [12] show that increasing the content of the inert element Cu can
increase the stability of the Ñ ring of the alloy.
J. Yang et al. [16] studied the lithium storage mechanism of tin-based
oxides: lithium reacts with tin-based oxides to first generate an amorphous Li2O
inert phase and a metallic tin phase, and as the lithium intercalation reaction
proceeds further , lithium and tin combine to form intermetallic compounds.
Multi-component composite methods are currently widely used to further
improve the Ñ-ring performance of Cu-Sn alloys. D.C. Kim et al. [17] used
chemical reduction method to prepare nano-Cu6Sn5 alloy powder doped with 0.3% B.
The capacity after 80 cycles was about 200mAh/g.
C.
Among various types of alloy anode materials, thin film materials occupy an
important position. The thin film negative electrode and the current collector
are tightly bonded, which reduces the impact of the binder on the electrode
performance. Y.L. Kim et al. [11] used electron beam evaporation to prepare
Ag-doped Sn-Zr films. As the amount of Zr increases, the stability of the Ñ ring
of the Sn-Zr film increases. After 200 Ñ rings, the capacity of Sn57Zr33Ag10 is
around 1700mAh/cm3.
3.2 Aluminum-based alloy
The Al-Li binary phase diagram shows that lithium and aluminum can combine
to form AlLi, Al2Li3, Al4Li9, etc. When Al4Li9 is generated, the mass specific
capacity of aluminum is as high as 2234mAh/g, which is about 7 times that of the
graphite anode [19].
Ultrafine aluminum powder will cause strong electrode passivation after
lithium is intercalated, making it unsuitable as anode material. Researchers'
research on pure aluminum mostly focuses on thin film samples. Y. Hamon et al.
[19] used thermal evaporation method to prepare aluminum films of different
thicknesses, and used lithium metal as the counter electrode to conduct constant
current charge and discharge tests. When lithium is intercalated, an obvious
voltage platform appears at 0.26V (vs. Li), but the Al4Li9 phase is not
generated. Among them, the first reversible capacity of the film with a
thickness of 0.1μm reaches 800mAh/g; the film with a thickness of 1.0μm
decreases to 420mAh/g. The thinner the film, the smaller the volume effect
caused in the direction perpendicular to the film, the corresponding greater
reversible capacity, and the higher the charge and discharge efficiency.
How to reduce the volume deformation during the N-ring process is also a
problem that must be solved for aluminum-based alloy materials. Multi-component
composite method is an effective method to solve this problem. Currently, the
most studied binary aluminum-based alloys include Al2Cu, Al6Mn, and AlNi [19],
but the lithium storage mechanism is relatively complex. M.J. Lindsay et al.
[20] prepared Fe2Al5 powder. The reversible capacity of the sample with a
particle size of 0.5μm reached 485mAh/g, which is close to the theoretical
capacity (543mAh/g) when generating AlLi. However, the capacity retention rate
after two N rings was only 30%.
3.3 Silicon-based alloy
The combination of silicon and lithium can produce Li12Si7, Li7Si3, Li13Si4
and Li22Si5, etc. Among them, the corresponding lithium storage capacity of
Li22Si5 reaches 4200mAh/g, but the capacity decays quickly. As an anode material
for lithium-ion batteries, elemental silicon occupies an important position in
early electrode material research, mostly focusing on thin film materials. The
N-ring characteristics of various types of amorphous silicon films are higher
than those of crystalline silicon. For example, the discharge capacity of the
N-ring in the first three times of the amorphous silicon film prepared by vapor
deposition method [21] reaches 1000mAh/g, but the performance of the N-ring
needs to be improved. improve.
3.3.1 Silicon-metal composite anode material
The introduction of Mg, Mn, Ca and Cr elements into silicon-based materials
can alleviate the volume changes caused by the lithium insertion and extraction
process. Among them, Mg2Si has been studied extensively [22]. Junmei et al. [23]
synthesized Mg2Si and MnSi intermetallic compounds using mechanical alloy
annealing. The first discharge capacity of Mg2Si is close to 600mAh/g, but the
capacity retention rate after three Ñ rings is only 9%; the capacity decay of
MnSi intermetallic compounds is smaller than that of Mg2Si, and the capacity
after 60 Ñ rings remains above 200mAh/g.
J.Wolfenstlne[24] studied the charge and discharge properties of CaSi2.
After 50 N rings, CaSi2 with a particle size of 30 μm has a reversible capacity
of 150 mAh/g; while CaSi2 with a particle size of 1-3 μm has a reversible
capacity of 220 mAh/g. .
3.3.2 Silicon-nonmetal composite anode material
J.Yang[25] studied the charge and discharge properties of different
silicon-based oxides and conducted constant current charge and discharge tests
at 0.2mA/cm2. The first reversible capacity of SiO0.8 was 1600mAh/g.
At present, Si/C binary composite materials have also been widely studied
[26]. The Si/C binary composite anode material [m(MCMB):m(Si)=7:3] is controlled
at a specific capacity of 500mAh/g. , can perform Ñ ring 90 times, and has
better Ñ ring characteristics.
3.4 Other alloy anode materials
Other alloy anode materials, such as Ag, SB-based, etc., have also been
widely studied. Due to the high price and large volume and mass of Ag, there are
few studies on Ag as a single substance. Instead, Ag-based negative electrode
materials are mostly prepared by adding other active elements and inert
elements. J.T. Yin et al. [27] prepared Ag-Fe-Sn alloy using mechanical alloying
method. The addition of Fe improved the performance of the negative electrode
material; the alloy composed of Ag36.4Fe15.6Sn48 maintained reversible capacity
after 300 N rings. At 280mh/g, it is much higher than Ag52Sn48.
As for the active element Sb, β-Zn4Sb3, CoSb3 and InSb are the most
studied. Currently, Sb has attracted widespread attention as an added element.
E. Ronnebro et al. [28] studied Ag36.4Sb15.6Sn48, which passed through the N
ring 300 times. The specific capacity was maintained at 410mAh/g, which was
higher than the graphite anode material.
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