Main content:
Although graphite, the traditional negative electrode material for lithium-ion batteries, has the advantages of low cost and good stability, the theoretical capacity limit is too low to meet the increasing technical requirements. Alloy materials, transition metal compounds and silicon-based compounds have been the focus of research on anode materials for lithium-ion batteries.
Although the relevant research has made remarkable progress, there are still some challenges and shortcomings. For example, the coulomb efficiency and capacity retention of alloy anode materials still need to be improved; The problems of volume expansion and electrolyte decomposition of transition metal compound anode materials have not been completely solved. The conductivity and cycle stability of silicon-based anode materials still need to be further optimized.
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Alloy materials are used as negative electrode materials for lithium-ion batteries
The graphite anode has the problem of low capacity, and the theoretical specific capacity is only 372 mAh/g (LiC6). People have tried to use aluminum, tin, magnesium, silver and antimony and other metals and their alloys for lithium-ion battery negative materials, and found that the upper limit of the capacity of aluminum, tin, magnesium, silver and antimony and their alloy negative electrode is 2 to 10 times higher than that of graphite negative electrode. In addition, the alloy negative electrode has a higher initial voltage than the graphite negative electrode, which reduces the possibility of lithium deposition.
Li+ reacts with active metal to form LinM compound (M is metal, n>1), which is the main reason that the upper limit of the negative electrode capacity of the alloy is higher than that of the graphite negative electrode. Although the alloy negative electrode has the advantages of higher capacity and less lithium deposition than the graphite negative electrode, there are also many disadvantages:
(1) the volume expansion of the alloy negative electrode is more serious than that of the graphite negative electrode, which will damage the performance of the lithium-ion battery; The volume change caused by Li+ impaction will lead to irreversible capacity loss, which will shorten the cycle life of lithium-ion batteries. These shortcomings restrict the wide range of use of alloy anode.
Transition metal compound
Transition metal compounds, including oxides, sulfides and phosphates, etc., as negative electrode materials have the advantage of high theoretical specific capacity and low industrial production cost, such as diabase (MnO2) and pyrite (FeS2) can be found everywhere in nature.
The lithium insertion potential of the negative electrode of the transition metal compound is lower than that of the graphite negative electrode, which does not appear in the lithium dendrite phenomenon that often occurs on the graphite negative electrode, and the safety is higher, but the disadvantages are also very obvious compared with the graphite negative electrode: obvious volume expansion, continuous electrolyte decomposition and low electrical conductivity. At present, many researchers use nanoengineering technology to improve the shortcomings of transition metal compounds negative electrode.
Transition metal oxides: The theoretical specific capacity of MOx(M for Zn, Cu, Ni, Co and Fe, etc.) exceeds 600 mAh/g, the capacity limit is 2 to 3 times that of the graphite anode, and the recovery is simple and the natural stock is rich, the disadvantage is low electronic conductivity, poor cycle stability and the capacity limit decline faster.
Transition metal sulfides: When transition metal sulfides such as cobalt sulfide, iron disulfide, molybdenum disulfide and tin sulfide are used as negative electrode materials, the theoretical capacity is also higher than that of graphite negative electrode, because Li+ is stored through the mechanism of electrochemical conversion.
Transition metal phosphide: Transition metal phosphide includes nickel, copper, cobalt, tin and iron phosphide, etc., compared with graphite anode advantages are higher thermal stability, higher specific capacity, higher safety and so on. The disadvantages of such materials are also obvious, such as low electronic conductivity, and the change in the volume of phosphorus during charge and discharge will cause cracking of the material. Other transition metal compounds Transition metal oxalates, transition metal carbides and transition metal nitrides are also the negative electrode materials of lithium ion batteries that have attracted more attention in recent years.
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Silicon-based compound
Transition metal compounds, including oxides, sulfides and phosphates, etc., as negative electrode materials have the advantage of high theoretical specific capacity and low industrial production cost, such as diabase (MnO2) and pyrite (FeS2) can be found everywhere in nature.
The lithium insertion potential of the negative electrode of the transition metal compound is lower than that of the graphite negative electrode, which does not appear in the lithium dendrite phenomenon that often occurs on the graphite negative electrode, and the safety is higher, but the disadvantages are also very obvious compared with the graphite negative electrode: obvious volume expansion, continuous electrolyte decomposition and low electrical conductivity. At present, many researchers use nanoengineering technology to improve the shortcomings of transition metal compounds negative electrode.
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Transition metal oxides: The theoretical specific capacity of MOx(M for Zn, Cu, Ni, Co and Fe, etc.) exceeds 600 mAh/g, the capacity limit is 2 to 3 times that of the graphite anode, and the recovery is simple and the natural stock is rich, the disadvantage is low electronic conductivity, poor cycle stability and the capacity limit decline faster.
Transition metal sulfides: When transition metal sulfides such as cobalt sulfide, iron disulfide, molybdenum disulfide and tin sulfide are used as negative electrode materials, the theoretical capacity is also higher than that of graphite negative electrode, because Li+ is stored through the mechanism of electrochemical conversion.
Transition metal phosphide: Transition metal phosphide includes nickel, copper, cobalt, tin and iron phosphide, etc., compared with graphite anode advantages are higher thermal stability, higher specific capacity, higher safety and so on. The disadvantages of such materials are also obvious, such as low electronic conductivity, and the change in the volume of phosphorus during charge and discharge will cause cracking of the material. Other transition metal compounds Transition metal oxalates, transition metal carbides and transition metal nitrides are also the negative electrode materials of lithium ion batteries that have attracted more attention in recent years.
Challenges faced
In view of the problems existing in the negative electrode materials of lithium-ion batteries, researchers have proposed many solutions, but there are still many obstacles to be solved in terms of the current commercialization of lithium-ion batteries.
The leading factor of negative electrode failure mechanism of lithium-ion battery is not clear, whether it is electrode material loss, structural deformation, dendrite growth, there is no recognized theory. Lithium has a high activity and is prone to chemical reactions in the air, which restricts the study of apparent structure. At present, the observation of the apparent structure of lithium metal is mostly carried out indirectly by cryo-electron microscopy in the glassy electrolyte state. This observation efficiency is too low, and more and more effective direct observation methods are needed.
At present, the negative electrode design of lithium-ion batteries is mostly designed for conventional scenarios. For some applications such as wide temperature range, stretchable, flexible and other special lithium-ion battery research is insufficient. This is a direction for future exploration. At present, the commercial cost of lithium-ion batteries is still high, and the gap between laboratory costs and commercial costs is still not open. New energy vehicle enterprises still rely on financial subsidies in the process of market-oriented operation, but financial subsidies are not sustainable for a long time, so it is necessary to research and develop lithium-ion batteries with lower costs.
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Non-graphite anode materials prospect
Alloy materials, transition metal compounds and silicon-based compounds are all promising anode materials, but they also have some disadvantages. At present, in view of these shortcomings, researchers have carried out a variety of modifications: alloy materials from the form, structure to improve; Transition metal compounds are improved by nanostructure engineering, mixed nanostructures based on metal oxides and carbon, and the introduction of catalytic elements. The silicon-based compounds are improved by micro-nano structure design, silicon alloy structure control and pre-lithium.
In the future, the research of non-graphite anode materials for lithium-ion batteries can be generally carried out from the following four directions: (1) To explore the failure mechanism of the negative electrode of lithium-ion batteries, and expand the observation technology, in order to reveal the real cause of performance attenuation; (2) Strengthen the research of negative electrode materials used in extreme environments to meet the needs of specific special scenarios; (3) Innovative material design process, so that the preparation process more simplified, lower cost, in order to meet the needs of large-scale commercial; (4) Strengthen the safety of negative electrode materials and the compatibility with other battery components.
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