This paper focuses on the properties, advantages and disadvantages, and improvement methods of lithium-ion battery anode materials (Sn-based materials, Si-based materials), lithium titanate, carbon materials (carbon nanotubes, graphene, etc.), and these negative electrode materials. The application has made further prospects.
Lithium-ion batteries have been widely used in 3C electronic products (Computer, ConsumerElectronic and CommunicaTIon), energy storage equipment, electric vehicles due to their high energy density, high operating voltage, long cycle life, low self-discharge and environmental friendliness. Marine field.
The energy density (170Wh/kg) of lithium-ion batteries is about 3-4 times that of traditional lead-acid batteries, making it attractive in the power supply field.
The energy density of the anode material is one of the main factors affecting the energy density of lithium-ion batteries. It can be seen that the anode material plays a vital role in the lithium-ion battery chemical system. Among them, the lithium ion battery anode material is a metal base. A negative electrode material such as (Sn-based material, Si-based material), lithium titanate, carbon material (carbon nanotube, graphene, etc.).
Metal based material
1.1 tin-based materials
At present, tin-based anode materials mainly include tin oxide and tin alloy.
1.1.1 tin oxide
SnO2 has attracted much attention due to its high theoretical specific capacity (781 mAh/g). However, it also has some problems in the application process: the first irreversible capacity is large, and there is a large volume effect when the lithium is intercalated (volume expansion 250) %~300%), easy to agglomerate during the cycle.
Studies have shown that by preparing composite materials, the agglomeration of SnO2 particles can be effectively suppressed, and the volume effect during lithium insertion can be alleviated, and the electrochemical stability of SnO2 can be improved.
Zhou et al. prepared SnO2/graphite composites by chemical deposition and high-temperature sintering. The specific capacity was above 450 mAh/g at a current density of 100 mA/g, and the reversible specific capacity exceeded 230 mAh/g at a current density of 2400 mA/g. ,
Experiments show that graphite as a carrier can not only disperse SnO2 particles more uniformly, but also effectively inhibit particle agglomeration and improve the cycle stability of materials.
1.1.2 tin alloy
SnCoC is a kind of material which is successfully commercialized in the Sn alloy anode material. It is obtained by uniformly mixing the three elements of Sn, Co and C at the atomic level and amorphizing. The material can effectively suppress the electrode during charge and discharge. The volume change of the material increases the cycle life.
For example, in 2011, Japan's SONY Corporation announced the use of Sn-based amorphized material as the negative electrode of a 18650 cylindrical battery with a capacity of 3.5AH. The theoretical specific capacity of elemental tin is 994 mAh/g, which can form intermetallic compounds with other metals such as Li, Si, Co and the like.
For example, Xue et al. first prepared a three-dimensional porous Cu film carrier by electroless plating, and then loaded the Sn-Co alloy on the surface of the Cu film carrier by surface electrodeposition, thereby preparing a three-dimensional porous Sn-Co alloy.
The first discharge specific capacity of the material is 636.3 mAh / g, the first coulombic efficiency reaches 83.1%, and the specific capacity can still reach 511.0 mAh / g after 70 charge and discharge cycles.
Wang et al. used graphite as a dispersant and a mixture of SnO/SiO and lithium metal as the reactant. The high-energy mechanical ball milling method and post-heat treatment were used to prepare a uniformly dispersed Sn/Si alloy in the graphite matrix. The material was charged and discharged 200 times. After the cycle, its reversible capacity can still reach 574.1mAh/g, and the performance is better than that of the negative materials such as SnO or SiO alone.
1.2 silicon-based materials
As an ideal anode material for lithium-ion batteries, silicon has the following advantages: silicon can form Li4.4Si alloy with lithium, and the theoretical lithium storage capacity is as high as 4200 mAh/g (more than 10 times the specific capacity of graphite); lithium intercalation potential of silicon (0.5) V) is slightly higher than graphite, and it is difficult to form "lithium dendrites" during charging; silicon has low reactivity with electrolyte, and co-embedding of organic solvents does not occur.
However, the silicon electrode undergoes cycle performance degradation and capacity decay during charge and discharge, mainly for two major reasons: when silicon and lithium are formed into Li4.4Si alloy, the volume expansion is as high as 320%, and the large volume change easily leads to the active material from the set. Falling off in the fluid, thus reducing the electrical contact with the current collector, Graphang, the first domestic carbon-graphite industry chain e-commerce platform ---- want to exchange micro-signal: shimobang caused rapid decline in electrode cycle performance; The trace amount of HF generated by the decomposition of LiPF6 corrodes silicon, causing a decrease in the capacity of the silicon electrode.
In order to improve the electrochemical performance of silicon electrodes, there are generally the following ways: preparation of silicon nanomaterials, alloy materials and composite materials.
For example, Ge et al. prepared boron-doped silicon nanowires by chemical etching. Under the charge and discharge current of 2A/g, the capacity can still reach 2000mAh/g after 250 weeks of cycle, showing excellent electrochemical performance. The lithium deintercalation mechanism of silicon nanowires can effectively alleviate the volume expansion during the cycle.
Liu et al. prepared the Si-NiSi-Ni composite by high-energy ball milling, and then dissolved the Ni element in the composite by HNO3 to obtain a porous Si-NiSi composite.
It is known by XRD that there is a NiSi alloy in the system, which not only provides reversible capacity for the anode material, but also synergizes with the pores inside the particles, buffers the volume expansion of the silicon during the charge and discharge cycle, and improves the cycle performance of the silicon electrode.
Lee et al. prepared a core-shell Si/C composite by using a phenolic resin as a carbon source and pyrolysis at 700 ° C under an argon atmosphere. After 10 cycles, the reversible capacity of the composite still reached 1029 mAh/g, indicating that Na2CO3 forms a covalent bond between the silicon surface and the phenolic resin, and then undergoes pyrolysis, which improves the contact between the silicon and the cracked carbon, thereby improving the cycle property of the negative electrode material and reducing the irreversible capacity loss.
Lithium titanate
Spinel-type lithium titanate is used as a negative electrode material of great interest because of the following advantages:
1) Lithium titanate is almost "zero strain before and after deintercalation of lithium (the unit cell parameter before and after deintercalation of lithium) a from 0.836nm only to 0.837nm);
2) The lithium insertion potential is higher (1.55V), avoiding the occurrence of "lithium dendrites" and having higher safety;
3) has a very flat voltage platform;
4) High chemical diffusion coefficient and coulombic efficiency.
The advantages of lithium titanate determine its excellent cycle performance and high safety. However, its conductivity is not high, and capacity attenuation is severe when charging and discharging large currents. Surface modification or doping is usually used to improve its conductance. rate.
For example, Xiao et al. prepared Mg2+ doped lithium titanate by solid phase method with Mg(NO3)2 as the magnesium source, indicating that doping Mg2+ did not destroy the spinel crystal structure of lithium titanate, and the doped material It has better dispersibility, and its specific capacity at 10C discharge rate can reach 83.8mAh/g, which is 2.2 times that of undoped materials, and the capacity is not significantly attenuated after 10 cycles of charge and discharge. It is shown by AC impedance test. The charge transfer resistance of the post-wax material is significantly reduced.
Zheng et al. prepared pure phase lithium titanate and carbon-coated lithium titanate by high temperature solid phase method using Li2CO3 and lithium citrate as lithium sources.
Experiments show that the carbon coated lithium titanate has a smaller particle size and good dispersibility, showing better electrochemical performance, mainly due to the carbon coating improves the electronic conductivity of the surface of lithium titanate particles. At the same time, the smaller particle size shortens the diffusion path of Li+.
Carbon material
3.1 carbon nanotubes
Carbon nanotubes are carbon materials with a graphitized structure, which have excellent electrical conductivity. At the same time, due to their small depth and short stroke, the polarization of the negative electrode material is small when charging and discharging at a large rate. Large rate charge and discharge performance of the battery.
However, when the carbon nanotube is directly used as a negative electrode material for a lithium ion battery, there are problems such as high irreversible capacity, voltage hysteresis, and inconspicuous discharge platform.
Single-walled carbon nanotubes were prepared by simple filtration such as Ng, and used directly as a negative electrode material. The initial discharge capacity was 1700 mAh/g, and the reversible capacity was only 400 mAh/g.
Another application of carbon nanotubes in the negative electrode is to combine with other negative electrode materials (graphite, lithium titanate, tin-based, silicon-based, etc.), using its unique hollow structure, high electrical conductivity and large specific surface area as the carrier. Improve the electrical properties of other negative materials.
For example, Guo et al. used chemical vapor deposition to grow carbon nanotubes in situ in the pores of expanded graphite, and synthesized expanded graphite/carbon nanotube composites with a first reversible capacity of 443 mAh/g and a charge and discharge cycle of 50 times after 1 C rate. The reversible capacity can still reach 259mAh/g.
The hollow structure of the carbon nanotubes and the pores of the expanded graphite provide a large amount of lithium active sites, and this structure can buffer the volume effect of the material during charge and discharge.
3.2 graphene
In 2004, researchers at the University of Manchester in the United Kingdom first discovered graphene materials and won the Nobel Prize.
Graphene is a new carbon material formed from a carbon six-membered ring. It has many excellent properties, such as a large specific surface (about 2600m2g-1), high thermal conductivity (about 5300Wm-1K-1), and high electron conductivity. The electron mobility is 15000 cm 2 V -1 s -1 ) and good mechanical properties have been attracting attention as lithium ion battery materials.
Graphene has a very high electrochemical performance when used directly as a negative electrode material for lithium ion batteries.
Wang et al. used hydrazine hydrate as a reducing agent to prepare a graphene-like graphene sheet, which has both hard carbon and soft carbon characteristics, and exhibits characteristics of a capacitor in a voltage range higher than 0.5V.
The graphene anode material has a first reversible capacity of 650 mAh/g at a 1 C discharge rate, and the capacity can still reach 460 mAh/g after 100 charge and discharge cycles.
Graphene can also be used as a conductive agent to recombine with other negative electrode materials to improve the electrochemical performance of the negative electrode material.
For example, Zai et al. prepared Fe3O4/graphene composites by ultrasonic dispersion method and discharged at a current density of 200 mA/g. After 50 cycles, the capacity was 1235 mAh/g; at 5000 and 10000 mA/g current density, the discharge was passed. After 700 cycles, the capacities were 450 mAh/g and 315 mAh/g, respectively, showing high capacity and good cycle performance.
Prospect
In recent years, lithium ion battery anode materials have progressed toward high specific capacity, long cycle life, and low cost.
Metal-based (tin-based, silicon-based) materials are accompanied by volume changes while exhibiting high capacity. Since the capacity of metal-based alloy materials is proportional to volume change, the actual cell volume is not allowed to undergo large changes (generally less than 5%). Therefore, its capacity in practical applications is greatly limited, and solving or improving the volume change effect will become the direction of metal-based material research and development.
Lithium titanate has great potential in large-scale energy storage fields such as electric vehicles due to its small volume change, long cycle life and good safety. Due to its low energy density, LiMn1.5Ni0 is a high-voltage cathode material. .5O4 matching use is the development direction of high-safety power battery in the future.
Carbon nanomaterials (carbon nanotubes and graphene) have the advantages of specific surface area, high electrical conductivity, chemical stability, etc., and have potential applications in new lithium ion batteries. However, the carbon nanomaterial alone has disadvantages such as high irreversible capacity and voltage hysteresis as a negative electrode material, and it is a practical choice to use in combination with other negative electrode materials.
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