WO2017029692A1

WO2017029692A1 - Porous Graphene Coated Oxygen-Containing Carbon Material for High Capacity and Fast Chargeable Anode of Lithium Ion Battery

Info

Publication number: WO2017029692A1
Application number: PCT/JP2015/004102
Authority: WO
Inventors: Qian CHENG; Noriyuki Tamura
Original assignee: Nec Corporation
Priority date: 2015-08-18
Filing date: 2015-08-18
Publication date: 2017-02-23

Abstract

An anode material for a lithium ion battery, comprising an oxygen-containing carbon where oxygen is in the form of functional groups, the oxygen being distributed from the surface to the inside of the carbon, and the carbon having an interlayer space d₀₀₂ larger than 0.335 nm and less than 0.45 nm; and a porous graphene layer covering the oxygen-containing carbon, the graphene being in the form of monolayer or few-layer graphene.

Description

Porous Graphene Coated Oxygen-Containing Carbon Material for High Capacity and Fast Chargeable Anode of Lithium Ion Battery

The present invention relates to an anode material for a lithium ion battery, in particular, an oxygen-containing carbon material with porous graphene coating which can provide a high capacity and fast chargeable anode material of a lithium-ion battery.

Lithium-ion (Li-ion) batteries have been widely used for portable electronics, and they are being intensively pursued for hybrid vehicles (HVs), plug-in hybrid vehicles (PHVs), electric vehicles (EVs), and stationary power source applications for smarter energy management systems.　The greatest challenges in adopting the technology for large-scale applications are the energy density, power density, cost, safety and cycle life of current electrode materials. Energy density as well as the capacity of both anode and cathode is the most important factor for the energy storage systems.　The state of the art portable electronics devices, like smart phone which has quad-core processor, 3GB RAM, 20 mega pixels camera and 4G wireless communication capability, but the battery can only last for one day.　It is also the same for the electric vehicles, most car could have a limited cursing range after a single charge.　On the other hands, the charging time as well as the power density is another important characteristic for the battery, especially as the application targets of Li-ion batteries move from small mobile devices to transportation.　This is because electrical vehicle (EV) users are hardly to wait more than half an hour to charge their vehicles compared with a refueling period of less than 5 minutes for gasoline cars.　The speed of charge greatly depends on the lithiation rate capability of anode materials.

At present, graphite is the most popular and practical anode material for Li-ion batteries because of its low cost, relatively long cycle life, and ease of processing.　However, the relative small capacity (<372mAh/g) and poor rate capability limited their application in high energy and power energy storage systems.　CN103708437 and US8691442 are using amorphous carbon based materials such as soft carbon and hard carbon usually have larger interlayer spaces than graphite, offering a faster lithium input rate than graphite.　However, soft carbon usually has an even smaller capacity (around 250 mAh/g) than graphite and higher average potential while charging and discharging; it is difficult to be used in Li-ion batteries with high energy density.　Hard carbon has a capacity around 400 mAh/g, but its low density, low coulombic efficiency, and high cost, which make it difficult to use in batteries for EVs and plug-in hybrid vehicles (PHVs) at a low enough cost.　Other high capacity anode materials, proposed in WO2013/142287, US2012/0129054, WO2008/139157, US2010/0190061, WO2014/083135, CN101914703 and US7687201, are using silicon and tin alloys have even worse lithiation rate capabilities because of the low kinetics of lithium alloying and the accessibility of lithium ion through thick solid-electrolyte interface (SEI).　There are some attempts such as JP2014-130821A, JP2001-302225A and JPH10-188958A, in which some additional elements such as boron are added to increase the capacity of the carbon materials.

CN103708437 US8691442 WO2013/142287 US2012/0129054 WO2008/139157 US2010/0190061 WO2014/083135 CN101914703 US7687201 JP2014-130821A JP2001-302225A JPH10-188958A,

However, prior art did not provide anode materials having fast charging capability and high capacity as well as long cyclability.　In summary, there is no anode material that can simultaneously satisfy the high capacity, fast chargeable capability and good cyclability for lithium ion battery up to now.

In order to solve these problems, a new material is proposed to improve the capacity and rate capability of anode materials by means of porous graphene surface coating, oxygen adding for higher capacity and better rate capability.

That is, one aspect of the present invention provides an anode material for a lithium ion battery, including: an oxygen-containing carbon where oxygen is in the form of functional groups, the oxygen being distributed from the surface to the inside of the carbon, and the carbon having an interlayer space d₀₀₂ larger than 0.335 nm and less than 0.45 nm; and a porous graphene layer covering the oxygen-containing carbon, the graphene being in the form of monolayer or few-layer graphene.

Another aspect of the present invention is to provide a process for fabricating this anode material, which the process includes: (a) preparing a starting carbon material; (b) wet oxidizing the starting carbon material by acid treatment and alkaline compound treatment; (c) heat treating the carbon material oxidized in (b) at 500-1000℃ for 1-24 hours under an inert atmosphere; and (d) mixing the carbon material heat treated in (c) with porous graphenes to obtain a porous graphene coated carbon material.

Still another aspect of the present invention is to provide a lithium ion battery including the above anode material.

According to the aspect of the present invention, a high capacity and fast chargeable anode material for a lithium ion battery can be provided.

Fig. 1 shows a schematic diagram of material fabrication process. Fig. 2 shows a mechanism for high capacity anode materials including an oxygen-containing carbon. Figs. 3(A) and 3(B) show SEM images of a surface of a carbon material according to Comparative Example 1. Figs. 4(A) and 4(B) show SEM images of a surface of a carbon material according to Example 1. Fig. 5 shows a SEM image of a surface of a carbon material according to Example 2. Fig. 6 shows an XRD pattern of an oxygen-containing carbon (Example 1) and standard XRD pattern of silicon and graphite (Comparative Example 1). Fig. 7 shows an XPS analysis. Fig. 8 shows results of RBS analysis of Comparative Example 1 (left) and Example 1 (right). Fig. 9 shows half-cell rate capability in Comparative Example 1 and Example 2. Fig. 10 shows rate capabilities of full cells in Comparative Example 1 and Example 2.　The inserted image is the rate cycle ability of example 2.

The present invention proposes a new structure of carbon based materials for an anode material of a lithium ion battery.　Generally, graphite has a graphene A-B stacking structure with the interlayer space d₀₀₂ of 0.335 nm, which can store one lithium ion every six carbon atoms (LiC₆).　That is, it is a reason why the theoretical capacity of graphite is 372 mAh/g.　Our strategy is to add up some oxygen-containing functional groups or heterogeneity atoms to the interlayer of graphite: for one thing, these functional groups or heterogeneity atoms could increase the interlayer spaces, that lithium ions can intercalate more easily; and for another thing, the functional groups, heterogeneity atoms and interlayer defects could be supposed to have a reversible reaction with lithium ion while charging and discharging, so as to have a much larger capacity than non-treated graphite material.　Finally, these oxygen-containing carbon materials can be coated with porous graphene for better conductivity to be used as high capacity, fast chargeable anode materials for lithium ion battery.

The anode material includes a carbon material containing 2 wt% to 10 wt% of oxygen.　The form of oxygen can be functional groups, such as carboxylic residue, hydroxyl, carbonyl, etc.　The oxygen is distributed not only the surface of the carbon material, but also the inside of the carbon material.

Because of containing oxygen, the interlay space becomes larger than the non-treated graphite.　The interlayer space d₀₀₂ can be larger than 0.335 nm and less than 0.45 nm.

The oxygen-containing carbon materials may have much lower reversible capacity than the natural graphite due to the low electron conductivity of the materials.　However, in the present invention, porous graphene can increase the electron conductivity between carbon particles by coating on the surface of the oxygen-containing carbon.　The graphene can be monolayer or few-layer graphene, and it preferably includes at least one single layer graphene.　The thickness of the coating layer of graphene can be from 1 nm to 50 nm.　The porous on graphene can help the intercalation of lithium ions.　The number of the pores can be in the range of 5 to 500 pores per μm².　The size of pores can be 5 nm to 500 nm.

The carbon materials can be doped with boron for a larger capacity.　The boron atom or boron-containing functional groups can provide a site for a reversible reaction with lithium ions as an additional capacity besides lithium ion intercalation.　As a result, the boron doping can increase the capacity of the anode materials.

Regarding to the quantity of the doped boron, it is preferred to have the weight percentage of boron more than 0.5 wt%, more preferably more than 1.5 wt%, most preferably more than 2.5 wt%.

The status of the boron atom can be exotic atom, or boron-containing functional groups, like as C-B, -B(OH)₂, -B-N, et al, or both of them.

The anode materials can further include anode active particles which are capable of absorbing and desorbing lithium ions.　Examples of the anode active particles include: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements, wherein the alloys or intermetallic compounds are stoichiometric or nonstoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Ni, Co, or Cd, and their mixtures or composites; and (d) combinations thereof.　There is essentially no constraint on the type and nature of the anode active particles that can be used in practicing the present invention.　Among them, metal or semi-metal particles or compound particles of at least one element selected from a group consisting of Si, Sn, Al, Ge and Pb are preferable.

The anode materials may be further coated with a thin layer of carbon after coated with the porous graphenes or combining with other active particles, such as Si, Sn, etc.　For instance, micron-, sub-micron-, or nano-scaled particles or rods, such as SnO₂ nano particles, may be decorated on the surface of the graphene coated oxygen-containing carbon material to form a composite material.　Then the composite material may be coated with carbon by pyrolysis of hydrocarbons such as sugar or using CVD method.　The thickness of the thin layer of carbon can be 2 to 15 nm.

FABRICATION METHOD
The fabrication procedure of the carbon materials for the present embodiment is described as below with referring to Fig. 1:
(a). Raw carbon 1A such as natural graphite, artificial graphite, soft carbon or hard carbon can be used as starting materials.
(b). The raw carbon is oxidized by wet oxidation (S1).　The wet oxidation includes acid treatment and alkaline compound treatment.　Examples of acids include nitric acid, sulfuric acid, For example, the raw carbon is firstly mixed with fuming nitric acid (HNO₃) on an ice bath.　Then potassium chlorate (KClO₃) was added thereto.　The resultant mixture is stirred for 1 to 24h on the ice bath and then heated at 30 to 60℃ for 1 to 24h.　After that, the thick paste is diluted with deionized water and filtrated in vacuum.　Alternatively, the raw carbon can be mixed with 98% concentrated sulfuric acid and sodium nitrate.　Then potassium permanganate (KMnO₄) is added and brown mixture is stirred for 10 to 120 min. at 30 to 45℃.　After addition of deionized water, temperature is raised to 100℃.　The resultant slurry is diluted with water and H₂O₂ is added to reduce manganese, consequently color of the mixture is turned to bright yellow.　The product is filtrated in vacuum and washed with plenty of water to obtain an oxygen-containing carbon 1B.
(c). The oxygen-containing carbon 1B by step (b) is then heat treated at 500 to 1000℃ for 1 to 24 hours　under an inert atmosphere such as N₂ atmosphere to obtain a heat treated carbon material 1C (S2).
(d). The heat treated oxygen-containing carbon 1C is coated with porous graphene sheets 2 by using a mechanical mixer or the like to obtain a final material 3 (S3).
(e). The final material 3 can be used as anode materials for high capacity lithium ion batteries.

The Synthesis Method for Porous Graphene
Natural graphite is used as the raw material, followed by a Hummer`s method to make graphite into graphite oxide (GO).　For example, graphite and NaNO₃ are first mixed together in a flask.　After that, H₂SO₄ (95% conc.) is added to the flask with keeping a suspension at a low temperature, e.g., on an ice bath while being stirred.　Potassium permanganate is slowly added to the suspension to avoid overheating.　The suspension is then stirred at room temperature for a few hours.　The color of the suspension will become bright brown.　Then, distilled water is added to the flask with stirring.　The temperature of the suspension will quickly raise and the color of the suspension will change to yellow.　The diluted suspension is then stirred at 98 ℃ for 12 h.　H₂O₂ (30%) is then added to the suspension.　For purification, the solid content can be washed by rinsing with 5% HCl and then deionized water several times.　After that the suspension is centrifuged at 4000 rpm for 6 min.　After filtration and drying in a vacuum, the graphene oxide can be obtained as black powders.　Thus synthesized graphite oxide is then subjected to thermal shock in 200-500℃ within 20-60 min in N₂ atmosphere and followed by a mild oxidation in dry air in 500-800℃ for 30-120 min to activate the graphene surface to make a porous graphene precursor.　In the next step, the precursor is heated in a reducing atmosphere such as N₂ atmosphere to 700-1500℃ with slow temperature raising, e.g., 5℃/min, and the temperature is kept for 1-24h for a completely reduction of activated porous graphene precursor to porous graphene.

Fig. 2 shows a mechanism for high capacity applicability in the oxygen-containing carbon according to the present embodiment.　The raw carbon (graphite) 1A can store lithium ions 5 by converting to LiC₆ in theoretical.　The heat treated oxygen-containing carbon 1C has wider interlayer space and reversible reaction sites due to oxygen content with lithium ions so as to store more lithium ions than LiC₆.

LITHIUM ION BATTERY
A lithium ion battery of one exemplary embodiment of the present invention includes positive and negative electrodes, and the negative electrode includes the anode material of the above exemplary embodiment.

As for the positive electrode active material, but there is also no particular restriction on the type or nature thereof, known cathode materials can be used for practicing the present invention.　The cathode materials may be at least one material selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphates, metal sulfides, and combinations thereof.　The positive electrode active material may also be at least one compound selected from chalcogen compounds, such as titanium disulfate or molybdenum disulfate.　More preferred are lithium cobalt oxide (e.g., Li_xCoO₂ where 0.8≦x≦1), lithium nickel oxide (e.g., LiNiO₂) and lithium manganese oxide (e.g., LiMn₂O₄ and LiMnO₂) because these oxides provide a high cell voltage.　Lithium iron phosphate is also preferred due to its safety feature and low cost.　All these cathode materials can be prepared in the form of a fine powder, nano-wire, nano-rod, nano-fiber, or nano-tube.　They can be readily mixed with an additional conductor such as acetylene black, carbon black, and ultra-fine graphite particles.

For the preparation of an electrode, a binder can be used.　Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene propylenediene copolymer (EPDM), or styrene-butadiene rubber (SBR).　The positive and negative electrodes can be formed on a current collector such as copper foil for the negative electrode and aluminum or nickel foil for the positive electrode.　However, there is no particularly significant restriction on the type of the current collector, provided that the collector can smoothly path current and have relatively high corrosion resistance.　The positive and negative electrodes can be stacked with interposing a separator there between.　The separator can be selected from a synthetic resin nonwoven fabric, porous polyethylene film, porous polypropylene film, or porous PTFE film.

A wide range of electrolytes can be used for manufacturing the cell.　Most preferred are non-aqueous and polymer gel electrolytes although other types can be used.　The non-aqueous electrolyte to be employed herein may be produced by dissolving an electrolyte (salt) in a non-aqueous solvent.　Any known non-aqueous solvent which has been employed as a solvent for a lithium secondary battery can be employed.　A mixed solvent comprising ethylene carbonate (EC) and at least one kind of non-aqueous solvent whose melting point is lower than that of ethylene carbonate and whose donor number is 18 or less (hereinafter referred to as a second solvent) may be preferably employed as the non-aqueous solvent.　This non-aqueous solvent is advantageous in that it is (a) stable against a negative electrode containing a carbonaceous material well developed in graphite structure; (b) effective in suppressing the reductive or oxidative decomposition of electrolyte; and (c) high in conductivity.　A non-aqueous solvent solely composed of ethylene carbonate (EC) is advantageous in that it is relatively stable against decomposition through a reduction by a graphitized carbonaceous material.　However, the melting point of EC is relatively high, 39-40℃, and the viscosity thereof is relatively high, so that the conductivity thereof is low, thus making EC alone unsuited for use as a secondary battery electrolyte to be operated at room temperature or lower.　The second solvent to be used in the mixed solvent with EC functions to make the viscosity of the mixed solvent lowering than that of which EC is used alone, thereby improving an ion conductivity of the mixed solvent.　Furthermore, when the second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is employed, the aforementioned ethylene carbonate can be easily and selectively solvated with lithium ion, so that the reduction reaction of the second solvent with the carbonaceous material well developed in graphitization is assumed to be suppressed.　Further, when the donor number of the second solvent is controlled to not more than 18, the oxidative decomposition potential to the lithium electrode can be easily increased to 4 V or more, so that it is possible to manufacture a lithium secondary battery of high voltage.　Preferable second solvents are dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), γ- butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA).　These second solvents may be employed singly or in a combination of two or more.　More desirably, this second solvent should be selected from those having a donor number of 16.5 or less.　The viscosity of this second solvent should preferably be 28 cps or less at 25℃.　The mixing ratio of the aforementioned ethylene carbonate in the mixed solvent should preferably be 10 to 80% by volume.　If the mixing ratio of the ethylene carbonate falls outside this range, the conductivity of the solvent may be lowered or the solvent tends to be more easily decomposed, thereby deteriorating the charge/discharge efficiency.　More preferable mixing ratio of the ethylene carbonate is 20 to 75% by volume.　When the mixing ratio of ethylene carbonate in a non-aqueous solvent is increased to 20% by volume or more, the solvating effect of ethylene carbonate to lithium ions will be facilitated and the solvent decomposition-inhibiting effect thereof can be improved.

Comparative Example 1 (Graphite)
10g of granulated graphite having diameter of about 20 μm were used without any treatment is used as Comparative Example 1.　Figs. 3(A) and 3(B) show SEM images of a surface of the carbon material according to Comparative Example 1.

Example 1
10g of granulated graphite (comparative example 1) was added to 100 ml fuming HNO₃ in a beaker with magnetically stirring on an ice bath.　Then, 85 g of KClO₃ was added thereto.　The resultant mixture was further stirred for 48h on the ice bath and then heated at 60℃ for 10h.　After that, the thick paste was diluted with deionized water and filtrated off in vacuum.　Thus treated graphite was then heat treated under N₂ atmosphere at 800℃ for 4h.　This material is used as Example 1.　Figs. 4(A) and 4(B) show SEM images of a surface of the carbon material according to Example 1.

Synthesis Example of porous graphenes
Natural graphite and NaNO₃ were first mixed together in a flask.　After that, H₂SO₄ (100 ml, 95% conc.) was added to the flask on an ice bath while being stirred.　Potassium permanganate (8 g) was slowly added to the suspension to avoid overheating.　The suspension was then stirred at room temperature for 2 h.　The color of the suspension became bright brown.　Then, distilled water (90 ml) was added to the flask with stirring.　The temperature of the suspension quickly reached to 90℃ and the color changed to yellow.　The diluted suspension was then stirred at 98℃ for 12 h.　H₂O₂ (30 ml of 30%) was then added to the mixture.　The liquid medium was substituted with 5% HCl and then deionized water several times for rinsing.　After that the suspension was centrifuged at 4000 rpm for 6 min.　After filtration and drying in a vacuum, the graphene oxide was obtained as black powders.　Thus synthesized graphite oxide was then subjected to a thermal shock at 400℃ within 20 min in N₂ atmosphere and followed by a mild oxidation in dry air at 500℃ for 30min to activate the graphene surface to make porous graphene precursor.　In the next step, the precursor was heated in N2 atmosphere to 1000℃ with 5℃/min and kept the temperature for 6h for a complete reduction of the activated porous graphene.
Thus obtained porous graphenes included a single-layer graphene, and had 100 pores perμm² and the pore size was 46 nm.

Example 2
The carbon material obtained by Example 1 was mixed with the porous graphenes obtain in the above synthesis example in a mixer.　The mixing ratio of the carbon material and the porous graphenes was 98:2 in terms of mass.　Thus obtained porous graphene coated oxygen-containing carbon material was used as Example 2.　The thickness of the porous graphene layer was about 5 nm.　Fig. 5 shows a SEM image of the porous graphene coated carbon material.

Table 1 shows results of the elemental analysis for each carbon material.　Table 2 shows interlayer spaces and crystal sizes of Comparative Example 1 and Example 1.

The data are the raw data of elemental analysis without any corrections.　It may happen that the sum of the appearance elements is not 100%.

The example 1 showed enhanced interlayer spaces and reduced crystal size than pristine graphite of Comparative Example 1.　The interlayer space of d₀₀₂ is larger than 0.336 nm and the crystal size can be reduced 30% or more with respect to the original crystal size.
Fig. 5 shows an XRD (X-ray diffraction) pattern of an oxygen-containing carbon (Example 1) and standard XRD pattern of silicon and graphite (Comparative Example 1).　The peaks in Example 1 are shifted to left compare with Comparative Example 1 (PDF #41-1487), which means that the interlayer space is enlarged after adding oxygen into the carbon materials.
Table 3 shows results of XPS (X-ray photoelectron spectroscopy) analysis.　As can be seen from XPS results, the C-O is the main contribution of the oxygen containing instead of C=O or O=C-O.　Fig. 7 is result of XPS in Example 1.

Fig. 8 shows results of RBS (Rutherford Backscattering Spectrometry) analysis of comparative example 1 (left) and example 1 (right).　Example 1 has a uniform distribution of oxygen to the depth direction, however, Comparative Example 1 just have oxygen on the surface.

FABRICATION OF TEST CELL
Each sample of the carbon materials, carbon black, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were mixed in a weight ratio of 91:3:4:2 and the resultant mixture were dispersed in pure water to prepare negative slurry.

The negative slurry was coated on a Cu foil as a current collector, dried at 120℃ for 15 min, pressed to 45 μm thick with a load of 80 g/m² and cut into 22×25 mm to prepare a negative electrode.　The negative electrode as a working electrode and a metal lithium foil as a counter electrode were stacked by interposing porous polypropylene film there between as a separator.　The resultant stack and an electrolyte prepared by dissolving 1M LiPF₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 were sealed into an aluminum laminate container to fabricate a test cell.　The anode materials were also characterized in full cell.　The cathode slurry which coated on Al foil is made of lithium iron phosphate, carbon black, PVDF with the weight ratio of 87: 6:7.

The test cell was evaluated in initial charge capacity, coulombic efficiency, rate capability and cyclability.
Table 4 shows capacity, coulombic efficiency and rate capability in each full cell.

Example 2 showed excellent capacity and much enhanced rate capability than the Comparative Example 1 and Example 1.　The 1C/0.1C, 6C/0.1C, 8C/0.1C and 10C/0.1C mean the ratio of capacity charging in 1C, 6C, 8C and 10C, respectively to the capacity charging in 0.1C.　Examples 1 and 2 were worse in rate capability in low rate such as 1C due to the decay of the high oxygen-containing materials.　However, they showed much better rate capability in high rate, such as 6C, 8C and 10C than Comparative Example 1.

Fig. 9 shows half-cell rate capability.　Obviously, Example 2 shows better rate capability than Comparative Example 1, especially in high rate.
Further, Fig. 10 shows rate capability of full cell.　Example 2 shows a worst rate capability in low rate, but better rate capability in high rate.　The inserted image is the rate cycle ability of Example 2.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments.　It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.

1A: Raw carbon
1B: Oxygen-containing carbon
1C: Heat treated oxygen-containing carbon
2: Porous graphene
3: Final material (Anode material)
5: Lithium ions

Claims

An anode material for a lithium ion battery, comprising:
　　 an oxygen-containing carbon where oxygen is in the form of functional groups, the oxygen being distributed from the surface to the inside of the carbon, and the carbon having an interlayer space d₀₀₂ larger than 0.335 nm and less than 0.45 nm; and
　　 a porous graphene layer covering the oxygen-containing carbon, the graphene being in the form of monolayer or few-layer graphene.
The anode material according to claim 1, wherein the thickness of the graphene layer is from 1 nm to 50 nm, the graphene has 5 to 500 pores per μm² and the size of the pore is 5 nm to 500 nm.
The anode material according to claim 1, wherein the oxygen-containing carbon coated with the porous graphene layer has a particle size of from 10μm to 25μm.
The anode material according to claim 1, wherein the oxygen is uniformly distributed to a depth direction of the carbon and the oxygen is contained with a weight ratio of 2% to 10%.
The anode material according any one of claims 1 to 4, wherein the carbon material is coated with a thin layer of carbon outside of the porous graphene layer.
A lithium ion battery comprising positive and negative electrodes, the negative electrode comprises the anode material according to any one of claims 1 to 5.
The lithium ion battery according to claim 6, wherein the anode material has a capacity of at least 400 mAh/g.
A process for fabricating an anode material for a lithium ion battery, which the process comprises:
　　 (a) preparing a starting carbon material;
　　 (b) wet oxidizing the starting carbon material by acid treatment and alkaline compound treatment;
　　 (c) heat treating the carbon material oxidized in (b) at 500-1000℃ for 1-24 hours under an inert atmosphere; and
　　 (d) mixing the carbon material heat treated in (c) with porous graphenes to obtain a porous graphene coated carbon material.
The process according to claim 8, wherein the acid treatment is using fuming nitric acid or a mixture of 98% concentrated sulfuric acid and sodium nitrate, and the alkaline compound treatment is using potassium chlorate or potassium permanganate.
The process according to claim 8 or 9, wherein the starting carbon material is selected from the group consisting of natural graphite, artificial graphite, soft carbon and hard carbon.