Single-layer and multilayer graphene, method of manufacturing the same, object including the same, and electric device including the same

Abstract

Graphene is formed with a practically uniform thickness on an uneven object. The object is immersed in a graphene oxide solution, and then taken out of the solution and dried; alternatively, the object and an electrode are immersed therein and voltage is applied between the electrode and the object used as an anode. Graphene oxide is negatively charged, and thus is drawn to and deposited on a surface of the object, with a practically uniform thickness. After that, the object is heated in vacuum or a reducing atmosphere, so that the graphene oxide is reduced to be graphene. In this manner, a graphene layer with a practically uniform thickness can be formed even on a surface of the uneven object.

Claims

1. A secondary battery comprising: a positive electrode; and a negative electrode, wherein the negative electrode comprises: a negative current collector; a negative active material layer over the negative current collector, the negative active material layer having an uneven surface; and a plurality of graphene layers over the uneven surface, wherein a first part of the plurality of graphene layers is in contact with the uneven surface, wherein a space is formed between a second part of the plurality of graphene layers and a depression of the uneven surface, wherein a number of layers of the plurality of graphene layers is 50 or less, wherein each of the plurality of graphene layers has at least one pore, and wherein the pore of a first layer of the plurality of graphene layers does not overlap the pore of a second layer adjacent to the first layer of the plurality of graphene layers.

2. The secondary battery according to claim 1, wherein the uneven surface comprises a plurality of silicon whiskers.

3. The secondary battery according to claim 2, wherein the plurality of silicon whiskers comprise a first whisker and a second whisker; and wherein the first part of the plurality of graphene layers connects the first whisker to the second whisker.

4. The secondary battery according to claim 1, wherein a thickness of the plurality of graphene layers over the uneven surface is substantially uniform.

5. The secondary battery according to claim 1, wherein a concentration of elements other than carbon and hydrogen contained in the plurality of graphene layers is lower than or equal to 15 atomic %, and wherein a concentration of oxygen contained in the plurality of graphene layers is higher than or equal to 5 atomic % and lower than or equal to 15 atomic %.

6. The secondary battery according to claim 1, wherein the secondary battery is an lithium-ion secondary battery.

7. A secondary battery comprising: a positive electrode; and a negative electrode, wherein the negative electrode comprises: a negative current collector; a negative active material layer over the negative current collector, the negative active material layer having an uneven surface; and a plurality of graphene layers over the uneven surface, wherein a part of the plurality of graphene layers is in contact with the uneven surface, wherein a number of layers of the plurality of graphene layers is 50 or less, wherein each of the plurality of graphene layers has at least one pore, and wherein the pore of a first layer of the plurality of graphene layers does not overlap the pore of a second layer adjacent to the first layer of the plurality of graphene layers.

8. The secondary battery according to claim 7, wherein the uneven surface comprises a plurality of silicon whiskers.

9. The secondary battery according to claim 8, wherein the plurality of silicon whiskers comprise a first whisker and a second whisker; and wherein the part of the plurality of graphene layers connects the first whisker to the second whisker.

10. The secondary battery according to claim 7, wherein the number of layers of the plurality of graphene layers over the uneven surface is substantially uniform.

11. The secondary battery according to claim 7, wherein a concentration of elements other than carbon and hydrogen contained in the plurality of graphene layers is lower than or equal to 15 atomic %, and wherein a concentration of oxygen contained in the plurality of graphene layers is higher than or equal to 5 atomic % and lower than or equal to 15 atomic %.

12. The secondary battery according to claim 7, wherein the secondary battery is an lithium-ion secondary battery.

13. A secondary battery comprising: a positive electrode; and a negative electrode, wherein the negative electrode comprises: a negative current collector; a negative active material layer over the negative current collector, the negative active material layer having an uneven surface; and a plurality of graphene layers over the uneven surface, wherein a number of layers of the plurality of graphene layers is 20 or less, wherein each of the plurality of graphene layers has at least one pore, and wherein the pore of a first layer of the plurality of graphene layers does not overlap the pore of a second layer adjacent to the first layer of the plurality of graphene layers.

14. The secondary battery according to claim 13, wherein the uneven surface comprises a plurality of silicon whiskers.

15. The secondary battery according to claim 14, wherein the plurality of silicon whiskers comprise a first whisker and a second whisker; and wherein a part of the plurality of graphene layers connects the first whisker to the second whisker.

16. The secondary battery according to claim 13, wherein the number of layers of the plurality of graphene layers over the uneven surface is substantially uniform.

17. The secondary battery according to claim 13, wherein a thickness of the plurality of graphene layers over the uneven surface is substantially uniform.

18. The secondary battery according to claim 13, wherein the secondary battery is an lithium-ion secondary battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates an apparatus used in Embodiment 1.

(2) FIG. 2 is a view relating to one embodiment of the present invention.

(3) FIG. 3 is a view relating to one embodiment of the present invention.

(4) FIG. 4 is a view relating to one embodiment of the present invention.

(5) FIGS. 5A to 5C are images showing silicon whiskers and a graphene layer formed on surfaces of the silicon whiskers.

(6) FIGS. 6A to 6C are images relating to Example 1.

(7) FIGS. 7A and 7B are graphs relating to Example 2.

(8) FIGS. 8A and 8B are images relating to Example 3.

DETAILED DESCRIPTION OF THE INVENTION

(9) Embodiments of the present invention will be described below. Note that the embodiments can be implemented in various modes, and it is easily understood by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below.

Embodiment 1

(10) In this embodiment, a method of forming graphene on an object will be described. Graphene oxide is obtained in such a manner that graphite is oxidised to form graphite oxide and the graphite oxide is subjected to ultrasonic vibrations. Patent Document 3 may be referred to for the details. Alternatively, commercial graphene oxide may be used.

(11) Graphene oxide is dispersed in a solvent such as water or N-methylpyrrolidone (NMP). The solvent is preferably a polar solvent. The graphene oxide may be contained at a concentration of 0.1 g to 10 g per liter.

(12) FIG. 1 illustrates an apparatus used in this embodiment. A solution 102 in which graphene oxide is dispersed is put in a container 101, and an object 103 on which a graphene layer is to be formed is put therein and is used as an anode. In addition, a conductor 104 serving as a cathode is put in the solution, and appropriate voltage (e.g., 5 V to 20 V) is applied between the anode and the cathode. Note that the voltage is not necessarily constant. By measuring the amount of charge flowing between the anode and the cathode, the thickness of a graphene oxide layer deposited on the object can be estimated.

(13) When graphene oxide with a required thickness is obtained, the object is taken out of the solution and dried. Furthermore, the object is heated at 150° C. or higher, preferably 200° C. or higher, in vacuum or a reducing atmosphere such as an inert gas (e.g., nitrogen or a rare gas) atmosphere. The higher the heating temperature is and the longer the heating time is, the higher the degree of reduction of graphene oxide can be and the higher the purity of graphene can be (i.e., the concentration of elements other than carbon can be low); however, the heating temperature should be determined in consideration of the reactivity with the object. Note that it is known that graphene oxide is reduced at 150° C.

(14) Note that the treatment is preferably performed at high temperatures in order to improve electron conductivity of graphene to be obtained. For example, although the resistivity of multilayer graphene is about 240 MΩcm at a heating temperature of 100° C. (1 hour), the resistivity is about 4 kΩcm at a heating temperature of 200° C. (1 hour) and is about 2.8 Ωcm at a heating temperature of 300° C. (1 hour) (each resistivity is an average value of 8 samples).

(15) As described above, the graphene oxide formed on a surface of the object is reduced to be graphene. At this time, adjacent graphenes are bonded to form a larger net-like or sheet-like network. Even in the case where the object has depressions and projections, the graphene formed in such a manner has a substantially uniform thickness on the depressions and the projections.

Embodiment 2

(16) In this embodiment, an example in which 1 to 50 layers of graphene are formed on surfaces of silicon whiskers formed on one surface of a negative electrode current collector of a lithium-ion secondary battery will be described. In this embodiment, graphene is formed by an immersion method.

(17) First, graphite is oxidized to form graphite oxide, and the graphite oxide is subjected to ultrasonic vibrations to obtain graphene oxide. The graphene oxide is dispersed in a solvent such as water or NMP.

(18) A current collector with a silicon active material layer including silicon whiskers is immersed in the solution, taken out of the solution, and then dried. In addition, the current collector is heated at 150° C. or higher, preferably 200° C. or higher, in vacuum or a reducing atmosphere such as an inert gas (e.g., nitrogen or a rare gas) atmosphere. Through the above steps, 1 to 50 layers of graphene can be formed on a surface of the silicon active material layer.

(19) The current collector including the silicon whiskers whose surfaces are provided with the graphene layer may be further subjected to treatment as described above, so that another graphene layer is formed to overlap with the graphene layer. This may be repeated three or more times. When multilayer graphene is formed in such a manner, graphene can have higher strength and can be prevented from being separated from the whiskers.

(20) In the case where a thick graphene layer is formed through one step, the directions of sp.sup.2 bonds in the graphene layer become random, and the strength of the graphene layer is not proportional to the thickness thereof. In contrast, in the case where the graphene layer is formed through several steps as described above, the directions of sp.sup.2 bonds in the graphene layer are substantially parallel to surfaces of whiskers; thus, the thicker the graphene layer is, the higher the strength of the graphene layer is.

(21) Further, in order to form a thick graphene layer, a layer of an appropriate material may be formed over a graphene layer, and another graphene layer may be formed over the layer of an appropriate material. As the material for the layer, a material which is flexible like copper and does not absorb lithium is preferably used.

(22) FIG. 2 schematically illustrates the above current collector and whiskers, and a graphene layer. A surface of a current collector 201 is processed to be provided with whiskers 202. In addition, a first layer of graphene 203_1, a second layer of graphene 203_2, and the like are formed on surfaces of the whiskers 202 by the above treatment. A graphene layer 203 has adequate pores 204. In the case where the graphene layer 203 is formed through plural steps, the positions of the pores 204 do not overlap in some cases.

(23) Note that the surface of the current collector 201 may be provided with a groove or a hole instead of the whiskers 202. In any ease, there is no limitation to such structures as long as the surface area of the current collector 201 is increased. Structures such as the current collector 201 and the whiskers 202 may be formed using different materials. For example, the current collector 201 may be formed using titanium and the whiskers 202 may be formed using silicon by a CVD method.

(24) FIG. 3 schematically illustrates a structure of a coin-type secondary battery. As illustrated in FIG. 3, the coin-type secondary battery includes a cathode 303, an anode 309, a separator 305, an electrolyte solution (not illustrated), a housing 304, and a housing 312. Besides, the coin-type secondary battery includes a ring-shaped insulator 306, a spacer 310, and a washer 311.

(25) In the cathode 303, a negative electrode active material layer 302 provided with whiskers is formed over a negative electrode current collector 301. As the negative electrode current collector 301, for example, copper or titanium is preferably used. After the negative electrode current collector 301 is immersed in the above-described graphene oxide solution, heat treatment is performed to reduce the graphene oxide, so that 1 to 50 layers of graphene are formed on surfaces of the whiskers.

(26) This process is performed plural times to obtain a graphene layer with a total thickness of 30 nm to 300 nm. The graphene layer serves as a negative electrode active material. The graphene layer formed on the whiskers has a very-large surface area, and thus is suitable for high-speed charging or high-capacity discharging. Note that the graphene layer is very thin, and thus is not illustrated in FIG. 3.

(27) As a material for a positive electrode current collector 307, aluminum is preferably used. A positive electrode active material layer 308 may be formed in such a manner that slurry in which positive electrode active material particles, a binder, and a conduction auxiliary agent are mixed is applied on the positive electrode current collector 307 and is dried.

(28) As the positive electrode active material, lithium cobaltate, lithium iron phosphate, lithium manganese phosphate, lithium manganese silicate, lithium iron silicate, or the like can be used; however, one embodiment of the present invention is not limited thereto. The size of the active material particle is preferably 20 nm to 100 nm. Further, a carbohydrate such as glucose may be mixed at the time of baking of the positive electrode active material particles so that the positive electrode active material particles are coated with carbon. This treatment can improve the conductivity.

(29) An electrolyte solution in which LiPF.sub.6 is dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) is preferably used; however, the present invention is not limited hereto.

(30) An insulator with pores (e.g., polypropylene) may be used for the separator 305. Alternatively, a solid electrolyte which can transmit lithium ions may be used.

(31) The housing 304, the housing 312, the spacer 310, and the washer 311 each of which is preferably made of metal (e.g., stainless steel). The housing 304 and the housing 312 have a function of electrically connecting the cathode 303 and the anode 309 to the outside.

(32) The cathode 303, the anode 309, and the separator 305 are immersed in an electrolyte solution. Then, as illustrated in FIG. 3, the cathode 303, the separator 305, the ring-shaped insulator 306, the anode 309, the spacer 310, the washer 311, and the housing 312 are stacked in this order with the housing 304 positioned at the bottom. The housing 304 and the housing 312 are subjected to pressure bonding. In such a manner, the coin-type secondary battery is manufactured.

Embodiment 3

(33) In this embodiment, another example of forming a graphene layer including 1 to 50 layers of graphene on surfaces of whiskers formed over a current collector will be described. In this embodiment, graphene is formed by an electrophoresis method. As in Embodiment 2, graphene oxide is dispersed in a solvent such as water or NMP. The graphene oxide may have a concentration of 0.1 g to 10 g per liter.

(34) A current collector provided with whiskers is put in a solution in which graphene oxide is dispersed and is used as an anode. A conductor serving as a cathode is also put in the solution, and appropriate voltage (e.g., 5 V to 20 V) is applied between the anode and the cathode. Since part of an edge of a graphene sheet with a certain size in graphene oxide is terminated with a carboxyl group (—COOH), hydrogen ions detach from the carboxyl group in a solvent such as water and the graphene oxide itself is negatively charged. Thus, the graphene oxide is drawn to and deposited on an anode. Note that the voltage in that case is not necessarily constant. By measuring the amount of charge flowing between the anode and the cathode, the thickness of a graphene oxide layer deposited on the whiskers can be estimated.

(35) When graphene oxide with a required thickness is obtained, the current collector is taken out of the solution and dried. In addition, the current collector is heated at 150° C. or higher, preferably 200° C. or higher, in vacuum or a reducing atmosphere such as an inert gas (e.g., nitrogen or a rare gas) atmosphere. As a result, the graphene oxide formed on surfaces of the whiskers is reduced to be graphene. At this time, adjacent graphenes are bonded to form a larger net-like or sheet-like network.

(36) The thus formed graphene has a substantially uniform thickness even on depressions and projections which are formed by the whiskers or the like. In such a manner, a graphene layer including 1 to 50 layers of graphene can be formed on a surface of a silicon active material layer.

(37) After the graphene layer is formed in the above-described manner, the formation of a graphene layer by the method described in this embodiment or the formation of a graphene layer by the method described in Embodiment 2 is performed plural times to obtain a graphene layer with a total thickness of 30 nm to 300 nm. The graphene layer serves us a negative electrode active material. After that, the process described in Embodiment 2 is performed. In such a manner, a secondary battery can be manufactured.

Embodiment 4

(38) A power storage device of one embodiment of the present invention can be used in, for example, electric vehicles, electric power tools, personal computers, mobile phones, and the like. Such electric devices are not always supplied with power through wires, and thus are provided with storage batteries inside. As negative electrode active materials for the storage batteries, for example, silicon whose surface is covered with 1 to 50 layers of graphene, which is described in Embodiment 2 or Embodiment 3, may be used.

(39) In particular, storage batteries with low internal resistance are needed for applications where a large amount of current needs to be fed momentarily or where a required current value varies greatly. Therefore, a sufficient effect can be obtained by using one embodiment of the present invention. Further, a storage battery with high electric capacity is needed for a moving object; a sufficient effect can be obtained by using one embodiment of the present invention.

(40) Besides, the following are given as specific examples of electronic and electric devices: the power storage device according to one embodiment of the present invention, display devices, lighting devices, image reproduction devices which reproduce a still image or a moving image stored in a recording medium such as a digital versatile disc (DVD), high-frequency heating apparatus such as microwaves, electric rice cookers, electric washing, machines, air-conditioning systems such as air conditioners, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, dialysis devices, and the like.

(41) In addition, moving objects driven by an electric motor using electric power from a power storage device are also included in the category of electronic and electric devices. As examples of the moving object, electric vehicles, hybrid vehicles which include both an internal-combustion engine and a motor, motorized bicycles including motor-assisted bicycles, and the like can be given.

(42) In the electronic and electric devices, the power storage device according to one embodiment of the present invention can be used as a power storage device for supplying enough electric power for almost the whole power consumption (referred to as a main power supply). Further, in the electronic and electric devices, the power storage device according to one embodiment of the present invention can be used as a power storage device which can supply electric power to the electronic and electric devices when the supply of power from a commercial power supply is stopped (such a power storage device is referred to as an uninterruptible power supply).

(43) Furthermore, in the electronic and electric devices, the power storage device according to one embodiment of the present invention can be used as a power storage device for supplying electric power to the electronic and electric devices at the same time as the electric power supply from the main power supply or a commercial power supply (such a power storage device is referred to as an auxiliary power supply).

(44) FIG. 4 illustrates specific structures of the electronic and electric devices. In FIG. 4, a display device 403 is an example of an electronic and electric device including a power storage device 405 according to one embodiment of the present invention. Specifically, the display device 401 corresponds to a display device for TV broadcast reception and includes a housing 402, a display portion 403, speaker portions 404, the power storage device 405, and the like. The power storage device 405 according to one embodiment of the present invention is provided inside the housing 402.

(45) The display device 401 can receive electric power from a commercial power supply. Alternatively, the display device 401 can use electric power stored in the power storage device 405. Thus, the display device 401 can be operated with the use of the power storage device 405 according to one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from the commercial power supply due to power failure or the like.

(46) A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a digital micromirror device (DMD), a plasma display panel (PDP), a field emission display (FED), and the like can be used in the display portion 403.

(47) Note that the display device includes, in its category, all of information display devices for personal computers, advertisement displays, and the like other than TV broadcast reception.

(48) In FIG. 4, an installation lighting device 411 is an example of an electric device including a power storage device 414 according to one embodiment of the present invention. Specifically, the lighting device 411 includes a housing 412, a light source 413, a power storage device 414, and the like. FIG. 4 illustrates the case where the power storage device 414 is provided in a ceiling 415 on which the housing 412 and the light source 413 are installed; alternatively, the power storage device 414 may be provided in the housing 412.

(49) The lighting device 411 can receive electric power from a commercial power supply. Alternatively, the lighting device 411 can use electric power stored in the power storage device 414. Thus, the lighting device 411 can be operated with the use of the power storage device 414 according to one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from the commercial power supply due to power failure or the like.

(50) Note that although the installation lighting device 411 provided in the ceiling 415 is illustrated in FIG. 4 as an example, the power storage device according to one embodiment of the present invention can be used in an installation lighting device provided in, for example, a wall 416, a floor 417, a window 418, or the like other than the ceiling 415. Alternatively, the power storage device can be used in a tabletop lighting device and the like.

(51) As the light source 413, an artificial light source which emits light artificially by using electric power can be used. Specifically, a discharge lamp such as an incandescent lamp and a fluorescent lamp, and a light-emitting element such as an LED and an organic EL element are given as examples of the artificial light source.

(52) In FIG. 4, an air conditioner including an indoor unit 421 and an outdoor unit 425 is an example of an electric device including a power storage device 424 according to one embodiment of the present invention. Specifically, the indoor unit 421 includes a housing 422, a ventilation duct 423, the power storage device 424, and the like. FIG. 4 illustrates the case where the power storage device 424 is provided in the indoor unit 421; alternatively, the power storage device 424 may be provided in the outdoor unit 425. Further alternatively, the power storage devices 424 may be provided 111 both the indoor unit 421 and the outdoor unit 425.

(53) The air conditioner can receive electric power from a commercial power supply. Alternatively, the air conditioner can use electric power stored in the power storage device 424. Specifically, in the case where the power storage devices 424 are provided in both the indoor unit 421 and the outdoor unit 425, the air conditioner can be operated with the use of the power storage device 424 according to one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from the commercial power supply due to power failure or the like.

(54) Note that although the separated air conditioner including the indoor unit and the outdoor unit is illustrated in FIG. 4 as an example, the power storage device according to one embodiment of the present invention can be used in on air conditioner in which functions of an indoor unit and an outdoor unit are integrated in one housing.

(55) In FIG. 4, an electric refrigerator-freezer 431 is an example of an electric device including a power storage device 435 according to one embodiment of the present invention. Specifically, the electric refrigerator-freezer 431 includes a housing 432, a door for a refrigerator 433, a door for a freezer 434, the power storage device 435, and the like. The power storage device 435 is provided in the housing 432 in FIG. 4. The electric refrigerator-freezer 431 can receive electric power from a commercial power supply. Alternatively, the electric refrigerator-freezer 431 can use electric power stored in the power storage device 435. Thus, the electric refrigerator-freezer 431 can be operated with the use of the power storage device 435 according to one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from the commercial power supply because of power failure or the like.

(56) Note that among the electronic and electric devices described above, a high-frequency heating apparatus such as a microwave, and an electric device such as an electric rice cooker require high electric power in a short time. The tripping of a circuit breaker of a commercial power supply in use of electric devices can be prevented by using the power storage device according to one embodiment of the present invention as an auxiliary power supply for supplying electric power which cannot be supplied sufficiently by a commercial power supply.

(57) In addition, in a time period when electronic and electric devices are not used, specifically when the proportion of the amount of electric power which is actually used in the total amount of electric power which can be supplied by a commercial power supply source (such a proportion is referred to as a usage rate of electric power) is low, electric power can be stored in the power storage device, whereby the usage rate of electric power can be reduced in a time period when the electronic and electric devices are used. For example, in the case of the electric refrigerator-freezer 431, electric power can be stored in the power storage device 435 at night when the temperature is low and the door for a refrigerator 433 and the door for a freezer 434 are not opened or closed. Meanwhile, the power storage device 435 is used as an auxiliary power supply in the day time when the temperature is high and the door for a refrigerator 433 and the door for a freezer 434 are opened and closed; thus, the usage rate of electric power in daytime can be reduced.

Example 1

(58) In this example, a sample in which graphene is formed on a surface of whisker-like silicon by the immersion method described in Embodiment 2 will be described. The whisker-like silicon is formed over a current collector (titanium sheet) by a reduced-pressure CVD method using a silane gas as a source material and has a surface shape illustrated in FIG. 5A.

(59) An aqueous solution in which graphene oxide is dispersed was manufactured as described below. Potassium permanganate was added to a mixture of graphite (flake carbon) and concentrated sulfuric acid, followed by stirring for 2 hours. After that, pure water was added to the mixture, the mixture was stirred for 15 minutes while being heated, and a hydrogen peroxide solution was added thereto, so that a yellow-brown solution containing graphite oxide was obtained. Furthermore, the obtained solution was filtered, hydrochloric acid was added, followed by washing with pure water. After that, ultrasonic treatment was performed for 2 hours, so that the graphite oxide was changed into graphene oxide. In such a manner, an aqueous solution in which graphene oxide was dispersed was obtained.

(60) After that, the titanium sheet with the above-described whisker-like silicon was immersed in the aqueous solution, and then taken out of the solution. The titanium sheet with the whisker-like silicon was dried and heated at 300° C. in vacuum (0.1 Pa or lower) for 10 hours. FIG. 5C shows an observed surface of the sample manufactured in this manner.

(61) As shown in FIG. 5C, a depression of the whisker-like silicon is covered with a graphene layer. FIG. 5C also shows that the graphene layer is formed so as to connect projections of the whisker-like silicon.

(62) Cross-sectional TEM observation was performed in order to see how thick the graphene covering the whisker-like silicon was. Cross sections of two portions were observed. In a portion shown in FIG. 6A, the thickness of the graphene was 6.8 nm; in a portion shown in FIG. 6B, the thickness of the graphene was 17.2 nm.

(63) In another portion, a graphene layer was not seen, which shows that the thickness of the graphene layer greatly varies even in one sample.

(64) Cross-sectional observation for checking the state of the depressions in FIG. 5C shows that, as shown in FIG. 6C, a space is formed between the graphene layer (layer including graphene) and the depression of the whisker-like silicon. Note that a deposited carbon film was formed over the graphene layer for the sample observation. It needs to be noted that the deposited carbon film is formed over the graphene layer in FIG. 6C.

Example 2

(65) In this example, a sample in which graphene is formed on a surface of whisker-like silicon by the electrophoresis method described in Embodiment 3 will be described. The whisker-like silicon used in this example is the same as that used in Example 1. The graphene oxide aqueous solution prepared in this example is the same as that used in Example 1.

(66) A titanium sheet with the whisker-like silicon was immersed in the aqueous solution in which graphene oxide was dispersed, and a stainless steel plate was immersed therein as a counter electrode. The distance between the titanium sheet and the stainless steel plate was 1 cm. Then, with the titanium sheet used as an anode and the stainless steel plate as a cathode, a voltage of 10 V was applied between the anode and the cathode for 5 minutes. The amount of charge flowing during the 5 minutes was 0.114 C.

(67) After that, the titanium sheet was taken out of the solution, dried, and then heated at 300° C. in vacuum (0.1 Pa or lower) for 10 hours. In such a manner, the sample was manufactured. FIG. 5B shows an observed surface of the obtained whisker-like silicon. Although a noticeable difference from FIG. 5A is not seen, a film-like object connecting whiskers is seen in the central part of the image. In addition, dark portions are seen in places on the surfaces of the whiskers. The dark portions are possibly graphene with a large thickness.

(68) Peaks of a D band and a G band, which are characteristics of graphene, were seen in any portion of the whiskers in measurements by Raman spectroscopy. This shows that substantially the entire surfaces of the whiskers are probably covered with graphene.

(69) As described above, in the electrophoresis method, the thickness of the graphene layer can be controlled with the amount of charge; thus, the electrophoresis method has extremely high reproducibility. As described above, the graphene layer formed by the electrophoresis method described in Embodiment 3 can be extremely uniform.

Example 3

(70) In this example, comparison is made between the case where whisker-like silicon whose surface was provided with graphene was used as a cathode of a lithium-ion secondary battery and the case where whisker-like silicon whose surface was not provided with anything was used as a cathode of a lithium-ion secondary battery. It is known that an electrolyte solution used in the lithium-ion secondary battery is reacted with an electrode (especially cathode), so that a film of a compound decomposed from the electrolyte solution is formed on a surface of the electrode.

(71) Such a film of a compound is referred to as a solid electrolyte interface (SEI) and thought necessary for relieving and stabilizing the reaction between an electrode and an electrolyte. However, since the thickness of the SEI depends on the combination of an electrode and an electrolyte, the thickness might be larger than necessary.

(72) As adverse effects caused when the SEI is formed, there are a reduction in coulombic efficiency, a reduction in lithium ion conductivity between an electrode and an electrolyte solution, wasting of an electrolyte solution, and the like. An attempt to cover a surface of an electrode by an evaporation method or a CVD method has been conventionally made in order to suppress the formation of such an SEI.

(73) The surface area of an electrode of a lithium-ion secondary battery is preferably large; therefore, for example, an object having a complex shape such as whisker-like silicon is preferably used. The surface of such an object cannot be covered by an evaporation method or a CVD method. However, the method described in Embodiment 1 or Example 1 makes it possible to adequately coyer even whisker-like silicon.

(74) In this example, two kinds of samples, Sample A and Sample B, were prepared. Sample A is whisker-like silicon whose surface was not provided with anything and had an initial state of the surface similar to that of the whisker-like silicon shown in FIG. 5A. Sample B is whisker-like silicon whose surface was provided with graphene by the method described in Example 1 and had an initial state of the surface similar to that of the whisker-like silicon shown in FIG. 5B.

(75) Then, cyclic voltammetry measurement (CV measurement) was performed once on each of Sample A and Sample B to observe the surface of each whisker-like silicon. The CV measurement was performed at a scan rate of 0.1 mV/sec using a three-electrode beaker cell (working electrode: Sample A or Sample B, reference electrode: metallic lithium, counter electrode: metallic lithium, electrolyte solution: mixed solution of ethylene carbonate (EC) solution (1 mol/L) of lithium hexafluorophosphate (LiPF.sub.6) and diethyl carbonate (DEC) (1:1 (volume ratio))).

(76) FIG. 8A shows the surface of Sample A after being subjected to one cycle of the above-described CV measurement (scan range: 0 V to 1 V (vs. Li/Li.sup.+)). FIG. 8B shows the surface of Sample B after being subjected to ten cycles of the above-described CV measurement (scan range: 0 V to 1 V (vs. Li/Li.sup.+)).

(77) As is clear from the comparison between FIG. 8A and FIG. 5A, a thick SEI is formed on the surface of Sample A and it is difficult to see the shape of the original whisker-like silicon. In contrast, as is clear from the comparison between FIG. 8B and FIG. 5B or between FIG. 8B and FIG. 8A, an SEI formed on the surface of Sample B was not as thick as the SEI formed on the surface of Sample A.

(78) A coin cell was manufactured using Sample A or Sample B as an anode, metallic lithium as a cathode, a mixed solution of an ethylene carbonate (EC) solution (1 mol/L) of lithium hexafluorophosphate (LiPF.sub.6) and diethyl carbonate (DEC) (1:1 (volume ratio)) as an electrolyte solution, and polypropylene with minute holes as a separator. Then, the coin cell was charged and discharged to measure the amount of change in capacity associated with desorption and absorption of lithium. The amount of current in the charging and discharging was set to 50 μA for the first cycle and 4 mA for the second cycle and the subsequent cycles.

(79) As shown in FIG. 7A, when desorption and absorption of lithium were repeated, the capacity of each of Sample A and Sample B was reduced; however, the capacity of Sample B was increased to be higher than that of Sample A in the tenth cycle and the subsequent cycles. FIG. 7B shows the relation between the capacity and a change in the potential which are associated with the desorption (or absorption) of lithium in the thirtieth cycle. FIG. 7B shows that Sample B can desorb and absorb a larger amount of lithium than Sample A. This is probably because the SEI formed on Sample B was not as thick as the SEI formed on Sample A.

(80) This application is based on Japanese Patent Application serial no. 2011-124952 filed with the Japan Patent Office on Jun. 3, 2011 and Japanese Patent Application serial no. 2011-125057 filed with the Japan Patent Office on Jun. 3, 2011, the entire contents of which are hereby incorporated by reference.