Introduction and synthesis of lithium ion battery anode materials
The anode materials being explored are nitride, PAS, tin-based oxide, tin oxide, etc. The following properties are required as lithium ion battery anode materials.
Table of Contents
Currently, lithium ion battery anode materials are generally carbon materials, such as graphite, soft carbon (e.g., coke, etc.), and hard carbon. The anode materials being explored are nitride, PAS, tin-based oxide, tin oxide, tin alloy, and nanoanode materials. The following properties are required as lithium ion battery anode materials.
(1) The redox potential of lithium ion insertion in the negative matrix is as low as possible, close to that of lithium metal, resulting in a high output voltage of the cell.
(2) A large amount of lithium in the matrix can undergo reversible insertion and deinsertion to obtain high capacity density, i.e., the reversible x-value is as large as possible.
(3) The insertion and de-insertion of lithium should be reversible and with no or little change in the body structure during the insertion/de-insertion process so that it is as large as possible.
(4) The variation of redox potential with x should be as little as possible, so that the voltage of the cell does not change significantly and a smoother charge and discharge can be maintained.
(5) The insert compound should have good conductivity and ionic conductivity so that polarization can be reduced and high current charging and discharging can be performed.
(6) The main material has a good surface structure and can form a good SEI film with the liquid electrolyte.
(7) The inserted compound has good chemical stability throughout the voltage range and does not react with the electrolyte, etc. after the formation of the SEI film.
(8) The lithium ion has a large diffusion coefficient in the main material, which facilitates rapid charging and discharging.
(9) From the practical point of view, the main lithium ion battery anode material should be cheap and non-polluting to the environment.
Carbon lithium ion battery anode material
Carbon anode lithium-ion batteries show better performance in terms of safety and cycle life, and carbon lithium ion battery anode materials are inexpensive and non-toxic, so carbon anode materials are widely used in commercial lithium-ion batteries. In recent years, with the continuous research work on carbon materials, it has been found that by surface modification and structural adjustment of graphite and various carbon materials.
Or make graphite partly disordered, or in various carbon materials to form nanoscale pores, holes and channels and other structures, the embedding-de-embedding of lithium in which not only can be carried out according to stoichiometric LiC6, but also can have non-stoichiometric embedding-de-embedding, its specific capacity is greatly increased, from the theoretical value of LiC6 372mAh/g to 700mAh/g ~ 1000mAh/g, so that the specific energy of lithium-ion battery is greatly increased.
At present, the lithium ion battery anode material that has been researched and developed mainly includes: graphite, petroleum coke, carbon fiber, pyrolysis carbon, intermediate phase pitch-based carbon microspheres (MCMB), carbon black, glass carbon, etc., among which graphite and petroleum coke are the most valuable applications.
The lithium insertion characteristics of graphite-based carbon lithium ion battery anode materials are:
(1) The low and flat lithium insertion potential can provide high and smooth operating voltage for Li-ion batteries. Most of the lithium insertion capacity is distributed between 0.00 and 0.20 V (vs. Li+/Li).
(2) High lithium insertion capacity, with a theoretical capacity of 372 mAh.g-1 for LiC6;
(3) Poor compatibility with organic solvents, prone to solvent co-insertion and reduced lithium insertion performance.
The properties of petroleum coke-based carbon materials for lithium insertion and removal are: (1) No obvious potential plateau appears in the starting lithium insertion process.
(2) The composition of the intercalation compound LixC6 with x=0.5 or so, and the lithium insertion capacity is related to the heat treatment temperature and surface state.
(3) Good compatibility with solvent and cycling performance.
According to the degree of graphitization, the general carbon lithium ion battery anode material is divided into graphite, soft carbon, hard carbon.
Graphite
Graphite lithium ion battery anode material has good electrical conductivity, high crystallinity with good laminar structure, suitable for lithium embedding-de-embedding, forming lithium-graphite interlayer compound, charge/discharge capacity up to 300mAh.g-1 or more, charge/discharge efficiency above 90%, irreversible capacity below 50mAh.g-1.
Lithium de-embedding reaction in graphite is around 0~0.25V, with a good charge/discharge platform, can be matched with the cathode materials providing Lithium source of cathode materials such as lithium cobaltate, lithium manganate, lithium nickelate, etc. Matching the average output voltage of the composed battery, it is the most used anode material for lithium ion batteries at present. Graphite includes two categories of artificial graphite and natural graphite.
(1) Artificial graphite
Artificial graphite is produced by high temperature graphitization of easily graphitized carbon (such as pitch coke) in N2 atmosphere at 1900~2800℃. Common artificial graphites include intermediate phase carbon microspheres (MCMB) and graphite fibers.
MCMB are highly ordered layered stacked structures that can be made from coal tar (asphalt) or petroleum residue oil. The embedded capacity of lithium can be more than 600 mAh.g-1 at pyrolytic carbonization treatment below 700°C, but the irreversible capacity is higher.
When heat treatment above 1000℃, the graphitization of MCMB increases and the reversible capacity increases. Usually the graphitization temperature is controlled above 2800°C, the reversible capacity can reach 300mAh.g-1 and the irreversible capacity is less than 10%.
Vapor deposited graphite fiber is a tubular hollow structure with more than 320mAh.g-1 discharge specific capacity and 93% first charge/discharge efficiency, which can be discharged with high current and long cycle life, but the preparation process is complicated and the cost is high.
(2) Natural graphite
Natural graphite is a better lithim ion battery anode material with a theoretical capacity of 372Amh/g, forming a structure of LiC6 with high reversible capacity, charging and discharging efficiency and operating voltage. Graphite material has obvious charging and discharging platform, and the discharging platform is very low for lithium voltage, and the battery output voltage is high.
There are two types of natural graphite, amorphous graphite and phosphor flake graphite. Amorphous graphite has low purity. The reversible specific capacity is only 260mAh.g-1, while the irreversible specific capacity is above 100mAh.g-1. The reversible specific capacity of phosphor flake graphite is only 300~350mAh.g-1, and the irreversible specific capacity is less than 50mAh.g-1 or more.
Natural graphite is a very ideal lithium ion battery anode material because of its high capacity due to its complete structure and many embedded lithium positions. Its main drawback is its sensitivity to electrolyte and poor performance in high current charging and discharging.
During the discharge process, a Solid Electrolyte Interface (SEI) film will be formed on the surface of the cathode due to the chemical reaction of electrolyte or organic solvent, and the volume expansion and contraction of the graphite flake layer caused by the insertion and de-insertion of lithium ions will easily cause graphite pulverization. The irreversible capacity of natural graphite is high and the cycle life needs to be further improved.
(3) Modified graphite
By graphite modification, such as oxidizing and coating polymer pyrolysis carbon on graphite surface to form composite graphite with core-shell structure, the charging and discharging performance and cycling performance of graphite can be improved.
By oxidizing the graphite surface, the irreversible capacity of Li/LiC6 battery can be reduced and the cycle life of the battery can be improved, and the reversible capacity can reach 446 mAh.g-1 (Li1.2C6). For the oxidizing agent of graphite material, HNO3,O3,H2O2,NO+,NO2+ can be chosen. Graphite fluorination can be done at high temperature by direct reaction of fluorine vapor with graphite to obtain (CF)n and (C2F)n, or at 100°C in the presence of Lewis acid (e.g. HF) to obtain CxFn. The capacity of carbon lithium ion battery anode materials will be increased after oxidation or fluorination.
(4) Graphitized carbon fiber
Vapor phase grown carbon fiber VGCF is a lithium ion battery anode material prepared from hydrocarbons. 2800℃ treated VGCF has high capacity and stable structure.
Intermediate phase bituminous carbon fiber (MCF). 3000℃ treated MCF has a radial crystal structure with a laminar organization in the center, which is a disordered layer graphite structure like rock tar, and it has high specific capacity and coulombic efficiency.
The carbon fibers have different structures and different lithium-embedded performance, among which carbon fibers with meridional structure have the best charge/discharge performance, and carbon fibers with concentric structure are prone to co-embedding with solvent molecules. Therefore, the performance of graphitized pitch-based carbon fibers is better than that of natural scaled graphite.
The volume of graphite increases only about 10% when reaching the maximum lithium embedding limit (LiC6). Therefore, graphite can keep the electrode size stable during repeated lithium embedding-removal, which gives good cycling performance of the carbon electrode.
Graphite also has some shortcomings, such as strong selectivity to electrolyte, good electrode performance only in certain electrolytes; poor resistance to overcharge and overdischarge, small diffusion coefficient of Li+ in graphite, which is not conducive to fast charging and discharging, etc.
Therefore, it is necessary to modify graphite, and intermediate phase carbon microspheres (MCMB), amorphous carbon (organic matter thermal carbon) and encapsulated graphite have been synthesized, and their charging and discharging performance has been significantly improved compared with graphite.
Soft carbon
Soft carbon, i.e. easily graphitized carbon, is amorphous carbon that can be graphitized at a high temperature above 2500°C. Soft carbon has low crystallinity (i.e. graphitization), small grain size, large crystal surface spacing, good compatibility with electrolyte, but higher irreversible capacity for first charge/discharge, lower output voltage, and no obvious charge/discharge plateau potential. Common soft carbons include petroleum coke, needle coke, carbon fiber, carbon microspheres, etc.
Hard carbon
Hard carbon anode refers to the difficult graphitization carbon, is the polymer pyrolysis carbon. This kind of carbon is difficult to graphitize even at high temperature above 2500℃, common hard carbon are resin carbon (phenolic resin, epoxy resin, polyfurfuryl alcohol PFA-C, etc.), organic polymer pyrolysis carbon (PVA, PVC, PVDF, PAN, etc.), carbon black (acetylene black).
Hard carbon has a very large lithium capacity (500~1000mAh.g-1), but they also have obvious disadvantages, such as low first charge and discharge efficiency, no obvious charge and discharge platform and a large potential hysteresis caused by the presence of impurity atom H.
Non-carbon lithium ion battery anode material
Nitride
Lithium transition metal nitride has very good ionic conductivity, electronic conductivity and chemical stability, used as lithium ion battery anode material, and its discharge voltage is usually above 1.0V. The discharge specific capacity, cycling performance and smoothness of charge and discharge curves of electrodes vary greatly depending on the type of material.
For example, when Li3FeN2 is used as LIB cathode, the discharge capacity is 150mAh/g and the discharge potential is around 1.3V (vs Li/Li+), the charging and discharging curves are very flat and there is no discharge hysteresis, but the capacity has obvious decay. But the charging and discharging curves are not very smooth, with obvious potential hysteresis and capacity decay. At present, these materials need to be studied in depth to reach practical applications.
Nitride system is a compound of anti-fluorite (CaF2) or Li3N structure, which has good ionic conductivity and electrode potential close to lithium metal, and can be used as the negative electrode of lithium ion electrode.
Li-M-N (M is transition metal) compounds of anti-fluorite structure such as Li7MnN4 and Li3FeN2 can be synthesized by ceramic method. That is, the transition metal oxide and lithium nitride (MxNx+Li3N) are reacted directly in 1% H2+99% N2 atmosphere, and also by reacting Li3N with metal powder.Both Li7MnN4 and Li3FeN2 have good reversibility and high specific capacity (210 and 150 mAh.g-1, respectively).
During charging and discharging of Li7MnN4, the transition metal valence state changes to maintain the electrical neutrality, the material has relatively low specific capacity, about 200mAh/g, but good cycling performance, flat charge and discharge voltage, no irreversible capacity, especially when this material is used as lithium ion battery anode material, the anode material that cannot provide lithium source can be used to match with it for battery.
Li3-xCoxN belongs to Li3N structure lithium transition metal nitride (its general formula is Li3-xMxN, M is Co, Ni, Cu), the material has high specific capacity, can reach 900mAh/g, no irreversible capacity, charge and discharge voltage is about 0.6V on average, also can match with the cathode material that can not provide lithium source to form a battery, at present this material embedded lithium, de-lithium The mechanism of lithium embedding and de-lithiuming and its charge/discharge performance need to be further studied.
Tin-based lithium ion battery anode material
(1) Tin Oxide
Tin oxides, including stannous oxide, tin oxide and their mixtures, have a certain reversible electrolithium capacity, which is higher than that of graphite materials, up to 500 mAh/g or more, but the first irreversible capacity is also larger. SnO/SnO2 has the advantages of high specific capacity and relatively low discharge potential (around 0.4-0.6 V vs Li/Li+) when used as the anode, but its first irreversible capacity loss is large, the capacity decays quickly and the curve is not very smooth.
However, its first irreversible capacity loss is large, capacity decay is faster, and the discharge potential curve is less smooth. snO/SnO2 has very different electrochemical properties depending on the preparation method. For example, the reversible capacity of SnO2 prepared by low-pressure chemical vapor deposition method is more than 500 mAh/g, and the cycle life is more desirable, and there is no decay after 100 cycles.
While SnO and the cycle performance of SnO2 prepared by sol-gel method with simple heating are not ideal. The introduction of some non-metal and metal oxides such as B, Al, Ge, Ti, Mn, Fe, etc. in SnO(SnO2) and heat treatment can result in an amorphous composite oxide called amorphous tin-based composite oxide (abbreviated as ATCO), whose reversible capacity can reach more than 600 mAh/g and the volume specific capacity is greater than 2200 mAh/cm3.
Which is the current carbon material The negative electrode (500~1200mAh/cm3) is more than two times of the current carbon material, showing a promising application. The current problem of this material is the high irreversible capacity for the first time, and the charge/discharge cycle performance also needs further improvement.
(2) Tin composite oxide
Tin-based composite oxides for lithium-ion battery anodes are prepared by mixing SnO,B2O3,P2O5 in a certain stoichiometric ratio, sintering with oxygen at 1000°C, and rapidly condensing to form an amorphous compound whose composition can be expressed as SnBxPyOz(x=0.4~0.6,y=0.6~0.4,z=(2+3x-5y)/2), where tin is Sn2+. Compared with tin oxide (SnO/SnO2) the cycle life of tin-based composite oxide has been greatly improved, but it is still difficult to meet industrial standards. (3) Tin alloys
Certain metals such as Sn, Si, and Al form lithium-metal alloys with high lithium content when they are embedded in lithium. For example, the theoretical capacity of Sn is 990 mAh/cm3, which is close to 10 times the theoretical volumetric specific capacity of graphite. In order to reduce the irreversible capacity of the electrode and maintain the stability of the negative electrode structure, a tin alloy can be used as the negative electrode of lithium ion electrode, which is composed of 25% Sn2Fe + 75% SnFe3C.
Sn2Fe is the active particle, which can form an alloy with lithium metal, and SnFe3C is the inactive particle, which can maintain the basic skeleton of the electrode during the electrode cycle. The volumetric specific capacity of this tin alloy is twice that of the graphite material. An electrode composed of 25% Sn2Fe+75% SnFe3C can obtain a reversible capacity of 1600 mAh.g-1 and exhibit good cycling performance.
The main problem of alloy anode material is the low first time efficiency and cycle stability problem, and the electrode structure damage caused by the volume effect of anode material during repeated charging and discharging must be solved. The cycle performance of pure metal material anode is very poor and the safety is not good. The use of alloy anode compounded with other flexible materials is expected to solve these problems.
Lithium-titanium composite oxide
The lithium titanium composite oxide used as lithium ion battery anode is mainly Li4Ti5O12, and its preparation methods are mainly: high temperature solid phase synthesis method, sol-gel method, etc.
(1) High temperature solid phase synthesis method
Mix and grind TiO2, LiCO3 in a certain amount, then cool to room temperature at 1000℃ for 26h under air atmosphere to get Li4Ti5O12. Mix and grind TiO2, LiOH.H2O, then cool to room temperature at 700℃ for 24h under air atmosphere to get the target product.
Carbon nanotubes
Carbon nanotubes are a new type of carbon crystal material discovered in recent years, which is a hollow tube with a diameter of a few nanometers to tens of nanometers and a length of tens of nanometers to tens of micrometers, with the following properties.
Electrical properties of carbon nanotubes
Specific surface area/m2
First charge capacity (mAh.g-1)
First discharge capacity (mAh.g-1)
Irreversible capacity (mAh.g-1)
Irreversible capacity (mAh.g-1)
170.4
1049
223.1
825.9
21.2
Nanotubes are prepared by DC arc method and catalytic pyrolysis method.
The catalytic thermal method was performed by pyrolysis of 20% H2+80% CH4 mixture on catalyst particles of Ni+Al2O3 at 500°C. The pyrolyzed samples were ground and soaked in hot nitric acid (80°C) for 48 h to remove the catalyst from the carbon tubes, washed and filtered repeatedly with water until the pH=6 of the washing solution, and the filtered samples were dried at 160°C.
The DC arc method is to beat the arc in a closed arc furnace under the protection of argon using high purity graphite rods as electrodes, and the resulting product is carbon nanotubes containing C60 series products. The carbon nanotubes can be separated by chemical oxidation method.
The main purpose of nanoanode materials is to improve the cycling performance by reducing the effect of volume expansion and contraction on the structure during charging and discharging by taking advantage of the nano-properties of the materials. Practical applications show that the effective use of nano properties can improve the cycling performance of these lithium ion battery anode materials, however, there is still a long way to go before practical applications.
The key reason is that nanoparticles gradually bind with cycling, thus losing the unique properties of nanoparticles again, leading to structural destruction and reversible capacity decay. In addition, the high cost of nanomaterials has become a major obstacle limiting their application. In conclusion, among lithium ion battery anode materiales, graphite-based carbon anode material has been the main type of anode material due to its wide source and cheap price. Except for graphitized mesophase carbon microspheres (MCMB) and low-end artificial graphite which occupy a small market share, modified natural graphite is gaining more and more market share.
Non-carbon anode materials have high bulk energy density and are increasingly attracting the interest of scientific researchers, but they also suffer from poor cycling stability, large irreversible capacity, and high material preparation cost, and have failed to achieve industrialization so far.
The development trend of anode materials is to improve the capacity and cycle stability as the goal, through various methods to compound carbon materials with various high-capacity non-carbon anode materials to research and develop new applicable high-capacity, non-carbon composite anode materials.
Introduction and synthesis of lithium ion battery anode materials
The anode materials being explored are nitride, PAS, tin-based oxide, tin oxide, etc. The following properties are required as lithium ion battery anode materials.
Carbon lithium ion battery anode material
Carbon anode lithium-ion batteries show better performance in terms of safety and cycle life, and carbon lithium ion battery anode materials are inexpensive and non-toxic, so carbon anode materials are widely used in commercial lithium-ion batteries. In recent years, with the continuous research work on carbon materials, it has been found that by surface modification and structural adjustment of graphite and various carbon materials.
Or make graphite partly disordered, or in various carbon materials to form nanoscale pores, holes and channels and other structures, the embedding-de-embedding of lithium in which not only can be carried out according to stoichiometric LiC6, but also can have non-stoichiometric embedding-de-embedding, its specific capacity is greatly increased, from the theoretical value of LiC6 372mAh/g to 700mAh/g ~ 1000mAh/g, so that the specific energy of lithium-ion battery is greatly increased.
At present, the lithium ion battery anode material that has been researched and developed mainly includes: graphite, petroleum coke, carbon fiber, pyrolysis carbon, intermediate phase pitch-based carbon microspheres (MCMB), carbon black, glass carbon, etc., among which graphite and petroleum coke are the most valuable applications.
The lithium insertion characteristics of graphite-based carbon lithium ion battery anode materials are:
(1) The low and flat lithium insertion potential can provide high and smooth operating voltage for Li-ion batteries. Most of the lithium insertion capacity is distributed between 0.00 and 0.20 V (vs. Li+/Li).
(2) High lithium insertion capacity, with a theoretical capacity of 372 mAh.g-1 for LiC6;
(3) Poor compatibility with organic solvents, prone to solvent co-insertion and reduced lithium insertion performance.
The properties of petroleum coke-based carbon materials for lithium insertion and removal are:
(1) No obvious potential plateau appears in the starting lithium insertion process.
(2) The composition of the intercalation compound LixC6 with x=0.5 or so, and the lithium insertion capacity is related to the heat treatment temperature and surface state.
(3) Good compatibility with solvent and cycling performance.
According to the degree of graphitization, the general carbon lithium ion battery anode material is divided into graphite, soft carbon, hard carbon.
Graphite
Graphite lithium ion battery anode material has good electrical conductivity, high crystallinity with good laminar structure, suitable for lithium embedding-de-embedding, forming lithium-graphite interlayer compound, charge/discharge capacity up to 300mAh.g-1 or more, charge/discharge efficiency above 90%, irreversible capacity below 50mAh.g-1.
Lithium de-embedding reaction in graphite is around 0~0.25V, with a good charge/discharge platform, can be matched with the cathode materials providing Lithium source of cathode materials such as lithium cobaltate, lithium manganate, lithium nickelate, etc. Matching the average output voltage of the composed battery, it is the most used anode material for lithium ion batteries at present. Graphite includes two categories of artificial graphite and natural graphite.
(1) Artificial graphite
Artificial graphite is produced by high temperature graphitization of easily graphitized carbon (such as pitch coke) in N2 atmosphere at 1900~2800℃. Common artificial graphites include intermediate phase carbon microspheres (MCMB) and graphite fibers.
MCMB are highly ordered layered stacked structures that can be made from coal tar (asphalt) or petroleum residue oil. The embedded capacity of lithium can be more than 600 mAh.g-1 at pyrolytic carbonization treatment below 700°C, but the irreversible capacity is higher.
When heat treatment above 1000℃, the graphitization of MCMB increases and the reversible capacity increases. Usually the graphitization temperature is controlled above 2800°C, the reversible capacity can reach 300mAh.g-1 and the irreversible capacity is less than 10%.
Vapor deposited graphite fiber is a tubular hollow structure with more than 320mAh.g-1 discharge specific capacity and 93% first charge/discharge efficiency, which can be discharged with high current and long cycle life, but the preparation process is complicated and the cost is high.
(2) Natural graphite
Natural graphite is a better lithim ion battery anode material with a theoretical capacity of 372Amh/g, forming a structure of LiC6 with high reversible capacity, charging and discharging efficiency and operating voltage. Graphite material has obvious charging and discharging platform, and the discharging platform is very low for lithium voltage, and the battery output voltage is high.
There are two types of natural graphite, amorphous graphite and phosphor flake graphite. Amorphous graphite has low purity. The reversible specific capacity is only 260mAh.g-1, while the irreversible specific capacity is above 100mAh.g-1. The reversible specific capacity of phosphor flake graphite is only 300~350mAh.g-1, and the irreversible specific capacity is less than 50mAh.g-1 or more.
Natural graphite is a very ideal lithium ion battery anode material because of its high capacity due to its complete structure and many embedded lithium positions. Its main drawback is its sensitivity to electrolyte and poor performance in high current charging and discharging.
During the discharge process, a Solid Electrolyte Interface (SEI) film will be formed on the surface of the cathode due to the chemical reaction of electrolyte or organic solvent, and the volume expansion and contraction of the graphite flake layer caused by the insertion and de-insertion of lithium ions will easily cause graphite pulverization. The irreversible capacity of natural graphite is high and the cycle life needs to be further improved.
(3) Modified graphite
By graphite modification, such as oxidizing and coating polymer pyrolysis carbon on graphite surface to form composite graphite with core-shell structure, the charging and discharging performance and cycling performance of graphite can be improved.
By oxidizing the graphite surface, the irreversible capacity of Li/LiC6 battery can be reduced and the cycle life of the battery can be improved, and the reversible capacity can reach 446 mAh.g-1 (Li1.2C6). For the oxidizing agent of graphite material, HNO3,O3,H2O2,NO+,NO2+ can be chosen. Graphite fluorination can be done at high temperature by direct reaction of fluorine vapor with graphite to obtain (CF)n and (C2F)n, or at 100°C in the presence of Lewis acid (e.g. HF) to obtain CxFn. The capacity of carbon lithium ion battery anode materials will be increased after oxidation or fluorination.
(4) Graphitized carbon fiber
Vapor phase grown carbon fiber VGCF is a lithium ion battery anode material prepared from hydrocarbons. 2800℃ treated VGCF has high capacity and stable structure.
Intermediate phase bituminous carbon fiber (MCF). 3000℃ treated MCF has a radial crystal structure with a laminar organization in the center, which is a disordered layer graphite structure like rock tar, and it has high specific capacity and coulombic efficiency.
The carbon fibers have different structures and different lithium-embedded performance, among which carbon fibers with meridional structure have the best charge/discharge performance, and carbon fibers with concentric structure are prone to co-embedding with solvent molecules. Therefore, the performance of graphitized pitch-based carbon fibers is better than that of natural scaled graphite.
The volume of graphite increases only about 10% when reaching the maximum lithium embedding limit (LiC6). Therefore, graphite can keep the electrode size stable during repeated lithium embedding-removal, which gives good cycling performance of the carbon electrode.
Graphite also has some shortcomings, such as strong selectivity to electrolyte, good electrode performance only in certain electrolytes; poor resistance to overcharge and overdischarge, small diffusion coefficient of Li+ in graphite, which is not conducive to fast charging and discharging, etc.
Therefore, it is necessary to modify graphite, and intermediate phase carbon microspheres (MCMB), amorphous carbon (organic matter thermal carbon) and encapsulated graphite have been synthesized, and their charging and discharging performance has been significantly improved compared with graphite.
Soft carbon
Soft carbon, i.e. easily graphitized carbon, is amorphous carbon that can be graphitized at a high temperature above 2500°C. Soft carbon has low crystallinity (i.e. graphitization), small grain size, large crystal surface spacing, good compatibility with electrolyte, but higher irreversible capacity for first charge/discharge, lower output voltage, and no obvious charge/discharge plateau potential. Common soft carbons include petroleum coke, needle coke, carbon fiber, carbon microspheres, etc.
Hard carbon
Hard carbon anode refers to the difficult graphitization carbon, is the polymer pyrolysis carbon. This kind of carbon is difficult to graphitize even at high temperature above 2500℃, common hard carbon are resin carbon (phenolic resin, epoxy resin, polyfurfuryl alcohol PFA-C, etc.), organic polymer pyrolysis carbon (PVA, PVC, PVDF, PAN, etc.), carbon black (acetylene black).
Hard carbon has a very large lithium capacity (500~1000mAh.g-1), but they also have obvious disadvantages, such as low first charge and discharge efficiency, no obvious charge and discharge platform and a large potential hysteresis caused by the presence of impurity atom H.
Non-carbon lithium ion battery anode material
Nitride
Lithium transition metal nitride has very good ionic conductivity, electronic conductivity and chemical stability, used as lithium ion battery anode material, and its discharge voltage is usually above 1.0V. The discharge specific capacity, cycling performance and smoothness of charge and discharge curves of electrodes vary greatly depending on the type of material.
For example, when Li3FeN2 is used as LIB cathode, the discharge capacity is 150mAh/g and the discharge potential is around 1.3V (vs Li/Li+), the charging and discharging curves are very flat and there is no discharge hysteresis, but the capacity has obvious decay. But the charging and discharging curves are not very smooth, with obvious potential hysteresis and capacity decay. At present, these materials need to be studied in depth to reach practical applications.
Nitride system is a compound of anti-fluorite (CaF2) or Li3N structure, which has good ionic conductivity and electrode potential close to lithium metal, and can be used as the negative electrode of lithium ion electrode.
Li-M-N (M is transition metal) compounds of anti-fluorite structure such as Li7MnN4 and Li3FeN2 can be synthesized by ceramic method. That is, the transition metal oxide and lithium nitride (MxNx+Li3N) are reacted directly in 1% H2+99% N2 atmosphere, and also by reacting Li3N with metal powder.Both Li7MnN4 and Li3FeN2 have good reversibility and high specific capacity (210 and 150 mAh.g-1, respectively).
During charging and discharging of Li7MnN4, the transition metal valence state changes to maintain the electrical neutrality, the material has relatively low specific capacity, about 200mAh/g, but good cycling performance, flat charge and discharge voltage, no irreversible capacity, especially when this material is used as lithium ion battery anode material, the anode material that cannot provide lithium source can be used to match with it for battery.
Li3-xCoxN belongs to Li3N structure lithium transition metal nitride (its general formula is Li3-xMxN, M is Co, Ni, Cu), the material has high specific capacity, can reach 900mAh/g, no irreversible capacity, charge and discharge voltage is about 0.6V on average, also can match with the cathode material that can not provide lithium source to form a battery, at present this material embedded lithium, de-lithium The mechanism of lithium embedding and de-lithiuming and its charge/discharge performance need to be further studied.
Tin-based lithium ion battery anode material
(1) Tin Oxide
Tin oxides, including stannous oxide, tin oxide and their mixtures, have a certain reversible electrolithium capacity, which is higher than that of graphite materials, up to 500 mAh/g or more, but the first irreversible capacity is also larger. SnO/SnO2 has the advantages of high specific capacity and relatively low discharge potential (around 0.4-0.6 V vs Li/Li+) when used as the anode, but its first irreversible capacity loss is large, the capacity decays quickly and the curve is not very smooth.
However, its first irreversible capacity loss is large, capacity decay is faster, and the discharge potential curve is less smooth. snO/SnO2 has very different electrochemical properties depending on the preparation method. For example, the reversible capacity of SnO2 prepared by low-pressure chemical vapor deposition method is more than 500 mAh/g, and the cycle life is more desirable, and there is no decay after 100 cycles.
While SnO and the cycle performance of SnO2 prepared by sol-gel method with simple heating are not ideal. The introduction of some non-metal and metal oxides such as B, Al, Ge, Ti, Mn, Fe, etc. in SnO(SnO2) and heat treatment can result in an amorphous composite oxide called amorphous tin-based composite oxide (abbreviated as ATCO), whose reversible capacity can reach more than 600 mAh/g and the volume specific capacity is greater than 2200 mAh/cm3.
Which is the current carbon material The negative electrode (500~1200mAh/cm3) is more than two times of the current carbon material, showing a promising application. The current problem of this material is the high irreversible capacity for the first time, and the charge/discharge cycle performance also needs further improvement.
(2) Tin composite oxide
Tin-based composite oxides for lithium-ion battery anodes are prepared by mixing SnO,B2O3,P2O5 in a certain stoichiometric ratio, sintering with oxygen at 1000°C, and rapidly condensing to form an amorphous compound whose composition can be expressed as SnBxPyOz(x=0.4~0.6,y=0.6~0.4,z=(2+3x-5y)/2), where tin is Sn2+. Compared with tin oxide (SnO/SnO2) the cycle life of tin-based composite oxide has been greatly improved, but it is still difficult to meet industrial standards.
(3) Tin alloys
Certain metals such as Sn, Si, and Al form lithium-metal alloys with high lithium content when they are embedded in lithium. For example, the theoretical capacity of Sn is 990 mAh/cm3, which is close to 10 times the theoretical volumetric specific capacity of graphite. In order to reduce the irreversible capacity of the electrode and maintain the stability of the negative electrode structure, a tin alloy can be used as the negative electrode of lithium ion electrode, which is composed of 25% Sn2Fe + 75% SnFe3C.
Sn2Fe is the active particle, which can form an alloy with lithium metal, and SnFe3C is the inactive particle, which can maintain the basic skeleton of the electrode during the electrode cycle. The volumetric specific capacity of this tin alloy is twice that of the graphite material. An electrode composed of 25% Sn2Fe+75% SnFe3C can obtain a reversible capacity of 1600 mAh.g-1 and exhibit good cycling performance.
The main problem of alloy anode material is the low first time efficiency and cycle stability problem, and the electrode structure damage caused by the volume effect of anode material during repeated charging and discharging must be solved. The cycle performance of pure metal material anode is very poor and the safety is not good. The use of alloy anode compounded with other flexible materials is expected to solve these problems.
Lithium-titanium composite oxide
The lithium titanium composite oxide used as lithium ion battery anode is mainly Li4Ti5O12, and its preparation methods are mainly: high temperature solid phase synthesis method, sol-gel method, etc.
(1) High temperature solid phase synthesis method
Mix and grind TiO2, LiCO3 in a certain amount, then cool to room temperature at 1000℃ for 26h under air atmosphere to get Li4Ti5O12. Mix and grind TiO2, LiOH.H2O, then cool to room temperature at 700℃ for 24h under air atmosphere to get the target product.
Carbon nanotubes
Carbon nanotubes are a new type of carbon crystal material discovered in recent years, which is a hollow tube with a diameter of a few nanometers to tens of nanometers and a length of tens of nanometers to tens of micrometers, with the following properties.
Electrical properties of carbon nanotubes
Nanotubes are prepared by DC arc method and catalytic pyrolysis method.
The catalytic thermal method was performed by pyrolysis of 20% H2+80% CH4 mixture on catalyst particles of Ni+Al2O3 at 500°C. The pyrolyzed samples were ground and soaked in hot nitric acid (80°C) for 48 h to remove the catalyst from the carbon tubes, washed and filtered repeatedly with water until the pH=6 of the washing solution, and the filtered samples were dried at 160°C.
The DC arc method is to beat the arc in a closed arc furnace under the protection of argon using high purity graphite rods as electrodes, and the resulting product is carbon nanotubes containing C60 series products. The carbon nanotubes can be separated by chemical oxidation method.
The main purpose of nanoanode materials is to improve the cycling performance by reducing the effect of volume expansion and contraction on the structure during charging and discharging by taking advantage of the nano-properties of the materials. Practical applications show that the effective use of nano properties can improve the cycling performance of these lithium ion battery anode materials, however, there is still a long way to go before practical applications.
The key reason is that nanoparticles gradually bind with cycling, thus losing the unique properties of nanoparticles again, leading to structural destruction and reversible capacity decay. In addition, the high cost of nanomaterials has become a major obstacle limiting their application.
In conclusion, among lithium ion battery anode materiales, graphite-based carbon anode material has been the main type of anode material due to its wide source and cheap price. Except for graphitized mesophase carbon microspheres (MCMB) and low-end artificial graphite which occupy a small market share, modified natural graphite is gaining more and more market share.
Non-carbon anode materials have high bulk energy density and are increasingly attracting the interest of scientific researchers, but they also suffer from poor cycling stability, large irreversible capacity, and high material preparation cost, and have failed to achieve industrialization so far.
The development trend of anode materials is to improve the capacity and cycle stability as the goal, through various methods to compound carbon materials with various high-capacity non-carbon anode materials to research and develop new applicable high-capacity, non-carbon composite anode materials.
For more articles about anode material, please refer to silicon based anode, top 10 silicon-based anode material companies.