CA2186999C

CA2186999C - Carbonaceous electrode material for secondary battery and process for production thereof

Info

Publication number: CA2186999C
Application number: CA002186999A
Authority: CA
Inventors: Naohiro Sonobe; Jiro Masuko; Tomoyuki Aita; Minoru Ishikawa
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1995-10-03
Filing date: 1996-10-02
Publication date: 2002-10-29
Anticipated expiration: 2016-10-02
Also published as: CA2186999A1; DE69602405T2; DE69602405D1; KR100241194B1; EP0767505B1; KR970024354A; EP0767505A1

Abstract

A carbonaceous electrode having improved capacities for doping and dedoping of a cell active substance, such as lithium, and suitable for a non-aqueous solvent-type secondary battery, is constituted by a carbonaceous material obtained by carbonizing an organic material of plant origin, and having a pore volume of at least 0.55 ml/g of pores having a pore diameter of at most 5 µm as measured by mercury injection method and a specific surface area of at most 100 m2/g as measured by the nitrogen adsorption BET method. The carbonaceous material may preferably be produced by carbonizing an organic material of plant origin in contact with a flowing inert gas at a temperature of 700 - 1500 °C under a pressure exceeding 10 kPa.

Description

CARBONACEOUS ELECTRODE MATERIAL FOR ~Oh~ARY BAll~K~
AND PROCESS FOR ~KO~U~-lION .~K~OF

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a carbonaceous material obtaine~ by carbonizing an organic material of plant origin and suitable as an electrode material for a non-aqueous solvent-type ~econ~ry battery, and a process for production 10 thereof. The present invention also relates to an electrode structure comprising such a carhQnaceous electrode material, and a non-aqueous solvent-type secondary battery having such an electrode structure.
Non-aqueous solvent-type lithium S~r,on~ry 15 batteries having a negative electr~de comprising a carbonaceous material have been proposed (Japanese Laid-Open Patent Application (JP-A) 62-90863, JP-A 62-122066, etc.). Such a secon~ry battery utilizes a phenomenon that a carbon intercalation compound of 20 lithium as a (cell) active substance is easily formed electrochemically. When the battery is charged, lithium in a positive electrode comprising a chalcogenide, such as LiCoO2, is introAllce~ between layers of negative eleCtrode carbon (i.e., dopes the 25 carbon) electrochemically. The carbon thus doped with lithium functions as a lithium electrode, from which the lithium is released (i.e., de-doped) during 21 86qq9 discharge to return to the positive electrode. Thus, a sPcQ~dary battery capable of repetitive charge-discharge is formed.
As carbonAceous materials capable of 5 providing non-aqueous solvent-type lithium sPconAAry batteries of high energy density, there have been known so-called ~hard carbon~ obtAined by calcining phenolic resin or furan resin at a relatively low temperature (e.g., ca. 800 - 1500 C), so-called "soft 10 carbon~ obtaine~ by carbonizing pitch or tar; and activated carbon having a large specific surface area on the order of 900 - 2000 m2/g.
However, the above-mentioned known carbo~ceous materials are accompanied with a problem 15 that a large amount of active substance, such as lithium, remains in the carbon (i.e., the carbon shows a large non-dedoping capacity) during the dedoping step, so that the active substance is wasted uselessly, and also a problem that the dedoping 20 capacity per se determining the battery performance (discharge capacity) is relatively small.

SUMMARY OF THE INVENTION
In the course of our study for obt~ining 25 high-performance carbon~ceous electrode materials more suitably used for non-aqueous solvent-type s~cQn~Ary batteries, we have examined organic materials of plant _3_ 21 86q~q origin characterized by the presence of vessel, sieve tube, plant fiber, etc., as new carbon sources. As a result, it has been llneY~sctedly found that the carbonization of such an organic material of plant 5 origin under appropriate conditions provides an excellent carbon~ceous electrode material having well-balanced performances including a small non-dedoping capacity and a large dedoping c~racity in combination, and the resultant carbon~ceous material has a large 10 pore volume suitable for doping with a cell active substance not found in ~o~ Lional carh~nAC~ous electrode materials.
Thus, a principal object of the present invention is to provide such a carbon~ceous electrode 15 material having well-balanced high p~rformances as described above, a process for pro~lci n~ such a carbon~ceous material, and also a non-aqueous solvent-type ~econ~ry battery having an electrode comprising such a carhonac~ous material.
Another object of the present invention is to provide a carbonac~ous material having advantageous features in production thereof, such as easy pulverization by means of a jet mill, etc., and easy handling of feedstock for calcination.
According to the present invention, there is provided a c~rh~n~ceous electrode material for a non-aqueous solvent-type secon~ry battery, obt~in~d by 2 1 8 6 j 9 ~

carbonizing an organic material of plant origin, and having a pore volume of at least 0.55 ml/g of pores having a pore diameter of at most 5 ~m as measured by mercury injection and a specific surface area of at 5 most 100 m2/g as measured by the nitrogen adsorption BET method.
The carbonaceous material may preferably be a non-graphitic material as represented by an average (002)-plane spacing of at least 0.365 nm as measured 10 by X-ray diffraction method.
According to another aspect of the present invention, there is provided a process for producing a carbonaceous electrode material for a non-aqueous solvent-type seco~A~ry battery, comprising:
15 carbonizing an organic material of plant origin in contact with a flowing inert gas at a temperature of 700 - 1500 C under a pressure exceeAing 10 kPa (ca.
0.1 atm), or carbonizing an organic material of plant origin except for coconut shell at a temperature of 20 700 - 1500 C under a reA-lcPA pressure of at most 10 kPa (ca. 0.1 atm).
According to another aspect of the present invention, there is provided an electrode structure for a non-aqueous solvent-type seconA~ry battery, 25 comprising: an electrocon~-~ctive substrate and a composite electrode layer disposed on at least one surface of the electroconA~Ictive substrate; the _5_ 2 1 86~99 composite electrode layer comprising a carbo~c~ous electrode material as described above in a particulate form, and a binder.
According to a further aspect of the present 5 invention, there is provided a non-aqueous solvent-type ~econd~ry battery, comprising, a positive electrode, a negative electrode, and a separator and a non-aqueous electrolytic solution disposed beL.7_cn the positive and negative electrodes; at least one of the 10 positive and negative electrodes comprising an electrode structure as described above.
The carbon~ceQus material according to the present invention is practically so-called non-graphitizable carbon capable of storing a large amount 15 f active substance and accordingly has an essentially large capacity for doping with an active substance.
In addition, the carbon~ceous material according to the present invention has many pores of a relatively large diameter represented by a pore volume of at 20 least 0.55 ml/g of pores having a pore diameter of at most 5 ~m as measured by mercury injection method and is characterized by relatively large penetrating or open pores originated from structures, such as vessel, sieve tube and plant fiber, attributable to the 25 starting material.
Accordingly, the electrolytic solution is allowed to easily penetrate into the interior of the 21 ?~6't99 carbon through pores, and the active substance is allowed to easily move between the inæide and outside of the carbon. As a result, it is possible to provide a carbon~ceous electrode material having a small non-5 dedoping capacity and capable of effectively utilizingan active substance.
Incidentally, we have already proposed a carbon~ceous electrode material for a non-aqueous solvent-type R~con~ry battery obtained by carbonizing 10 coconut shell at a temperature of 900 - 1500 C under a re~llced pressure of at most 10 kPa (EP-A 0,700,105).
The present invention provideæ a further development of the proposal based on a knowledge that even an organic material of plant origin other than ~v~O~
15 shell can provide an excellent carbonaceous electrode material for a non-aqueous solvent-type RecQn~ry battery if treated under appropriate carbonization conditions, and the excellent performance thereof is attributable to its pore structure originated from 20 natural plant materials.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the 25 present invention taken in conjunction with the accompanying drawing.

_7_ 2 1 86't9q BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a partially exploded perspective view of a non-aqueous solvent-type secQndAry battery 5 which can be constituted according to the invention.
Figure 2 is a partial sectional view of an electrode structure adopted in the secondary battery.

DETAILED DESCRIPTION OF THE INVENTION
The carbor~cçous material according to the present invention is characterized by a specific surface area of at most 100 m2/g as measured by the BET method using nitrogen as adsorbate gas, and a pore volume of at least 0.55 ml/g of pores having a 15 diameter of at most 5 ~m as measured by mercury injection method. A carhonAc~ous material having a specific surface area in excess of lOO m2/g as represented by activated carbon or a carbon~ceous material having a pore volume below 0.55 ml/g provides 20 a large non-dedoping capacity, i.e., a large amount of active substance left within the carbonaceous material without dedoping. The specific surface area may preferably be 0.5 - 10 m2/g, more preferably 0.5 - 5 m2/g .
The large pore volume is a characteristic of the cArhQn~ceous material. A smaller pore volume makes it difficult for the electrolytic solution to -8- 2186t~9 penetrate into the inside of the carhon and hi nAers free movement of the active substance within the cArhonAceous material, thereby resulting in a remarkable increase in non-dedoping cAr~city defined 5 as a difference (A-B) between a doping cApAcity (A) and a dedoping capacity (B) and a lowering in rate of effective utilization of active substance. On the other hand, an extremely large pore volume results in a lowering in packing density of the carbonAceous 10 material for preparing a seconA~ry battery.
Accordingly, the pore volume may preferably be 0.55 -1.00 ml/g, further preferably 0.55 - 0.70 ml/g.
The carbonAceous material according to the present invention having a developed pore structure has a characteristic that it is readily pulverized and c~ es little wearing of a pulverizer, such as a jet mill. Further, as for micropores relating to the production process described hereinafter, the characteristic micropore and fibrous structures of an organic material of plant origin are believed to allow easy dissipation or removal of a decomposition by-product formed during the carbonization, and contribute to an increased pore volume in the finally obtAineA carbonAceous material.

In the present invention, the carbon-^eous material should be construed as a term covering a graphitic material having a developed graphite 9 21 86, ~9 structure as obtAin~ through heat treatment at a temperature of 2000 C or higher. ~ever, a high-temperature heat treatment cA~ c a shrinkage of carbon structure and is liable to deprive the pore structures, such as vessel, sieve tube and plant fiber, originated from the starting material.
Accordingly, the carbon~ceous material according to the present invention is practically advantageously realized as a non-graphitic carbonAceous material having an average (002)-plane spacing as measured by X-ray diffraction method (hereinafter sometimes denoted by "doo2~) of at least 0.365 nm. doo2 is more preferably 0.365 - 0.390 nm, further preferably 0.370 - O.390 nm. If doo2 is below 0.365 nm, the carbon~ceous material is liable to exhibit a small capacity for doping with active substance.
The carhonaceous material may preferably have a hydrogen-to-cArbon atomic ratio H/C of at most 0.1.
A carbonAceous material having an H/C excee~ing 0.1 is insufficient in carbonization and is not suitable as a carbo~ceous electrode material for a non-aqueous solvent-type secon~Ary battery.
Now, the process for pro~ncing a carbonAc~ous material according to the present invention will be described~
The production process according to the present invention comprises: carbonizing an organic -lo- 21 86t99 material of plant origin in contact with a flowing inert gas at a temperature of 700 - 1500 C under a pressure eyceedin~ 10 kPa (first process), or carbonizing an organic material of plant origin other than coconut shell at a temperature of 700 - 1500 C
under a re~llced pressure of at most 10 kPa (ca. 0.1 atm) (second process).
Preferred examples of the organic material of plant origin used in the present invention as carbon sources of the carbo~acçous material may generally include: coconut shell, coffee bean, chaffs, broad-leaf tree wood, conifer wood, and bamboo.
The c~rhonization may preferably be performed while taking care so that tar or decomposition products, such as hydrogen and methane, will not hi n~çr the pore formation in the organic material.
In case where the organic material is carbonized in an environment rich or dense in decomposition product, the formation of minute pores is liable to be insufficient, thus resulting in a carhon~c~ous material having a lower capacity for doping with active substance.
As the organic material of plant origin is inherently porous h~r~ e of the pr~senr,~ of vessel, sieve tube, etc., the dissipation or removal of decomposition products during the c~rhonization is facilitated to result in a large volume of pores -11- 2 1 86't99 having a relatively large diameter.
According to the first production process of the present invention, the organic material is carbonized while flowing an inert gas in contact with the organic material under a pressure of atmospheric pressure (1 atm = ca. lO0 kPa) or higher, or a sub-atmospheric pressure eYcçe~ing 10 kPa (ca. 0.1 atm).
In this instance, the material to be carbonized (organic material as it is or after pre-calcination as desired) may be disposed in a piled layer within a reactor and is carbonized while flowing the inert gas in a space outside but in contact with the layer (outside-layer flow scheme), or the material to be carbonized (organic material) is disposed in a layer or bed and is cArhQnized while flowing the inert gas through within the layer or bed (intra-layer flow scheme).
In a batch-wise outside-layer flow scheme, it is preferred to suppress the piled layer thickne~s of the material to be carbonized as thin as possible so as to increase the area of contact of the material layer with the inert gas and allow quick removal of the decomposition product from the material out of the system. The piled layer thicknPcc of the material to be ~rhQnized may preferably be at most 50 mm, more preferably at most 30 mm. The inert gas may be supplied or flowed at a vacant reactor-basis speed of at least 1 mm/sec, more preferably at least 5 mm/sec.
It is preferred to adopt an intra-layer flow scheme of a continuous-type or a batch-type using a fluidized bed, a fixed bed, etc. In this case, the 5 inert gaæ may preferably be æupplied or flowed at a rate of at leaæt lO ml/min., more preferably at least 50 ml/min., further preferably at least lOO ml/min., per gram of the material to be carbonized, while it can depend on the amount of the material to be 10 carbonized per unit time. A higher inert gas supply rate may be preferred in view of the properties of the product carhon~ceous material, but practically the supply rate may be at most 500 ml/min. per gram of the material to be carbonized.
In the first production process, th~ inert gas may preferably be nitrogen or argon, and the above-mentioned inert gas supply rate is calculated based on the volume of the inert gas under the standard state (O C and 1 atm). The inert gas can 20 contain up to 40 mol. % of halogen gas, such as chlorine.
In the ~con~ production process, the carbonization is performed at a re~llc~ pressure of at most lO kPa, preferably at most 1 kPa, further 25 preferably at most O.l kPa. If the pressure at the time of carbonization exceeds lO kPa in the absence of a flowing inert gas, the withdrawal of decomposition -13- 2 1 86'j9~

products from the material to be carbonized becomes insufficient. The whole course of carbonization can be performed under a rPAll~eA pressure of at most 10 kPa, but the object of the present invention may be 5 sufficiently achieved if the carbonization in a temperature range of 700 & or higher is performed under the reAllcPd pressure.
The carbonization may be performed at a tPmperature of 700 - 1500 C in either of the first 10 and second processes. Carbonization at a temperature below 700 C results in an increased non-dedoping active substance cAr~city of the product carhonAceous material. Carbonization at a temperature higher than 1500 C results in a decrease in capacity for doping 15 with active substance. The carbonization temperature is 700 - 1500 C, preferably 1000 - 1400 C, further preferably 1100 - 1400 C.
In either of the first and sPco~A production processes, the organic material of plant origin may 20 preferably be subjected to pre-calcination at 300 -1000 C in an inert gas atmosphere or under a rPA~ceA
pressure so as to remove tar and other volatile matters in advance. Further, it is preferred to pulverize the pre-calcined organic material into fine 25 particles of at most 100 ~m in average diameter so as to promote the dissipation of decomposition products generated from the material to be carbonized during -14- 21 86i~9 the cArhonization.
The pre-calcined organic material before carbonization shows a lower hardness than the carbonAceQus material after the final cArhonization, 5 thus being easily pulverized. Accordingly, in case of requiring a powder carhQnAceous electrode material, pulverization after the pre-calcination is accompanied with less abrasion of the pulverizer, and is advantageous from the viewpoint of production process.
Next, the non-aqueous solvent-type ~c~n~ry battery of the present invention will be described.
Figure 1 is a partially exploded perspective view of a lithium secon~Ary battery as an embodiment of a non-aqueous solvent-type c~Con~ry battery 15 according to the present invention.
More specifically, the ~cQn~Ary battery basically includes a laminate structure including a positive electrode 1, a negative electrode 2 and a separator 3 disposed between the positive and negative 20 electrodes 1 and 2 and comprising a fine porous film of a polymeric material, such as polyethylene or polypropylene, impregnated with an electrolytic solution. The laminate structure is wound in a vortex shape to form an electricity-generating element which 25 is housed within a metal casing 5 having a bottom constituting a negative electrode terminal 5a. In the c~con~Ary battery, the negative electrode 2 is electrically connected to the negative electrode terminal 5a, and the uppermost portion of the battery is constituted by disposing a gasket 6 and a safety valve 7 covered with a top plate 8 having a projection 5 constituting a positive electrode terminal 8a electrically connected to the positive electrode.
Further, the uppermost rim 5b of the casing 5 is crimped toward the inner side to form an entirely sealed cell structure enclosing the electricity-10 generating element.
Herein, the positive electrode 1 or negativeelectrode 2 may be constituted by an electrode structure 10 having a sectional structure as partially shown in Figure 2. More specifically, the electrode 15 structure 10 includes an electrocon~llctive substrate 11 comprising a foil or wire net of a metal, such as iron, stainless steel, copper, aluminum, nickel or titanium and having a thicknec-c of, e.g., 5 - 100 pm, or 5 - 20 ~m for a small-sized battery, and a composite 20 electrode layer (12a, 12b) of, e.g., 10 - 1000 pm, preferably 10 - 200 ~m, in thickne-c-c for a small-sized battery, on at least one surface, preferably on both surfaces as shown in Figure 2, of the electrocon~-lctive substrate 11.
The composite electrode layers 12a and 12b are respectively a layer comprising a particulate carbon~ceous material according to the present -16- 2 ~ 86`~9q invention, an electrocon~llctive material such as electrocon~llctive carbon, optionally included, and a binder such as a vinylidene fluoride resin.
More specifically, in case of using the 5 carbon~r~ous material according to the present invention for producing an electrode 10 (1 or 2) of a non-aqueous solvent-type secon~-ry battery as described above, the carbonaceous material may be optionally formed into fine particles having an 10 average particle size of 5 - 100 ~m and then mixed with a binder stable against a non-aqueous solvent, such as polyvinylidene fluoride, polytetrafluoro-ethylene or polyethylene, to be applied onto an electroconductive substrate 11, such as a circular or 15 rectangular metal plate, to form, e.g., a 10 - 200 ~m-thick layer. The binder may preferably be added in a proportion of 1 - 20 wt. % of the carbonaceous material. If the amount of the binder is excessive, the resultant electrode is liable to have too large an 20 electric res-istance and provide the battery with a large internal resistance. On the other hand, if the amount of the binder is too small, the adhesion of the carbonAceous material particles with each other and with the electrocon~llrtive substrate 11 is liable to 25 be insufficient. The above described formulation and values have been set forth with respect to production of a ~eron~ry battery of a relatively small size, -17- 2 1 86 ~9 whereas, for production of a s~c~n~ry battery of a larger size, it is also possible to form the above-mentioned mixture of the c~rhQn~ceous material fine particles and the binder into a thicker shaped 5 product, e.g., by press-forming, and electrically connect the shaped product to the electroconductive substrate.
The carbon~c~.ous material of the present invention can also be used as a positive electrode 10 material for a non-a~ueous solvent-type ~econ~ry battery by utilizing its good doping characteristic but may preferably be used as a negative electrode material of a non-aqueous solvent-~ype secon~ry battery, particularly for constituting a negative 15 electrode to be doped with lithium as an active substance of a lithium ~econ~ry battery.
In the latter case, the positive electrode material may comprise a complex metal chalcogenide, particularly a complex metal oxide, such as LiCoO2, 20 LiNiO2 or LiMn204. Such a positive electrode material may be formed alone or in combination with an appro-priate binder into a layer on an electrocon~llctive substrate.
The non-aqueous solvent-type electrolytic 25 solution used in combination with the positive electrode and the negative electrode described above may generally be formed by dissolving an electrolyte -18- 2~6~q~

in a non-aqueous solvent. The non-aqueous solvent may comprise one or two or more species of organic solvents, such as propylene carbonate, ethylene carbonate, dimethyl c~rhQn~te~ diethyl carbonate, 5 dimethoxyethane, diethoxyethane, ~-~uLylolactone, tetrahydrofuran, 2-methyl-tetrahydrofuran, sulfolane, and l,3-dioxolane. Examples of the electrolyte may include LiC104, LiPF6, LiBF4, LiCF3S03, LiAsF6, LiCl, LiBr, LiB(C6H5)4, and LiN(S02CF3)2.
As described above, a ~ecQn~ry battery of the present invention may generally be formed by disposing the above-formed positive electrode 1 and negative electrode 2 opposite to each other, optionally with a liquid-permeable separator 3 15 composed of, e.g., I ~ov~l cloth or other porous materials, disposed therebeL.reen, and dipping the positive and negative electrodes together with an intermediate permeable separator in an electrolytic solution as described above.
In the above, a cylindrical battery has been described as an embodiment of the non-aqueous solvent-type ~Qn~ry battery according to the present inven-tion. However, the non-aqueous solvent-type S~con~ry battery acFording to the present invention can basi-25 cally have any other shapes, such as those of a coin, a rectangular parallelepiped, or a paper or sheet.
Incidentally, the measurement of various -19- 21 86q9~

parameters of carbonaceous materials described herein, i.e., the pore volume by mercury injection method, the specific surface area by nitrogen adsorption, doo2 and the hydrogen/carbon atomic ratio (H/C), was performed in the following manner.
[Pore volume by mercury injection method]
In case where mercury is injected (pressurized) into a cylindrical pore having a diameter D under a pressure P, the following equation is given based on a balance between a surface tension and a pressure acting on a sectional area of the pore:
-~D~-cosH = ~(D/2)2P, wherein ~ represents a surface tension of the mercury, and ~ denotes a contact angle between the mercury and the pore well. Accordingly, D = (-4~-cosO)/P.
Herein, the surface tension (~) of mercury was assumed to be 484 dyn/cm, the contact angle (0) between mercury and carbon was assumed to be 130 deg;
and the pressure P and the diameter D were expressed in the units of MPa and ~m, respectively, whereby the above equation was reduced to D = 1.27/P.
Based on the formula, a relationship between the pore diameter (D) and the mercury pressure P was derived.
More specifically, the pore volume was 2 1 86Y'~9 measured by nAUTOPORE 9200" (available from Micromeritics Instrument Corp.) in the following m~nner .
A sample c~rhon~ceous material in the form of 5 particles having an average diameter of 10 - 30 ~m was placed in a sample vessel, which was then evacuated for 30 min. at room temperature and a pressure of at most 2.67 Pa. Then, mercury was intro~llce~ into the sample vessel and gradually injected into pores under 10 a gradually increasing pressure (up to a maximum pressure of 414 MPa). From the measurement, a relationship between pressure P and injected volume of mercury was derived and converted into a relationship between pore diameter D and the injected volume. From 15 the relationship, a pore volume distribution was derived versus pore diameter as a variant. Thus, the volume of mercury injected from a pressure (0.25 MPa) correspon~i ng to a pore diameter of 5 ~m to the maximum pressure (414 MPa; corresponding to a pore 20 diameter of 3 nm) was measured as a pore volume of pores having a diameter of at most 5 ~m.
[Specific surface area by nitrogen adsorption]
An approximate equation Vm = l/(V ( l-x) ) 25 derived from the BET equation was used to obtain vm (amount (cm3/g-sample)) of adsorbed nitrogen required to form a mono-molecular layer of nitrogen on the 2 1 86q99 sample surface) from a measured nitrogen volume v at a relative pressure x (= 0.3) according to the BET
single-point method using nitrogen adsorption. From the thus-obt~ine~ vm-value, a specific surface area 5 SBET was calculated h~e~ on the following equation:
SBET = 4.35 x vm (m2/g).
More specifically, the nitrogen adsorption onto a carbon~ceous material was performed at liquid nitrogen temperature by using "Flow Sorb II 2300"
lO (available from Micromeritics Instrument Corp.) in the following manner.
A sample carhon~ ous material pulverized into an average diameter of 5 - 50 ~m was packed in a sample tube, and the sample tube was cooled to -196 C
15 while flcwing helium gas cont~ining nitrogen at a conc~ntration of 30 mol. %, thereby to cause the carbon~ceous material to adsorb nitrogen. Then, the sample tube was restored to room temperature to measure the amount of nitrogen desorbed from the 20 sample by a thermal ~on~llctivity-type detector, thereby to obtain the adsorbed nitrogen amount v (cm3/g-sample).
[doo2 of carbon~ceous material]
A powdery sample of a carb~n~ceous material 25 was packed in an aluminum-made sample cell and irradiated with monochromatic CuKa rays (wavelength ~ = 0.15418 nm) through a graphite monochromator to -22- 2 ~ 86't99 obtain an X-ray diffraction pattern. The peak position of the diffraction pattern is determined by the center of gravity method (i.e., a method wherein the position of a gravity center of diffraction lines 5 is obt~ine~ to determine a peak position as a 20 value correspon~ing to the gravity center) and calibrated by the diffraction peak of (lll) plane of high-purity silicon powder as the standard substance. The doo2 value is calculated from the Bragg's formula shown 10 below.
doo2 = ~/(2-sinO) (Bragg's formula) [Hydrogen/carbon (H/C) atomic ratio]
A sample of carbon~cPous material was subjected to elementary analysis by using a CNH
15 analyzer, and a hydrogen/carbon(H/C) atomic ratio was calculated as a ratio of numbers of atoms of hydrogen/carbon based on the weight proportions of hydrogen and carbon in the sample.
[Examples]
Hereinbelow, the present invention will be described more specifically based on Examples and Comparative Examples. All the volumes or flow rates of inert gases described hereinafter are values calculated under the st~n~rd state (0 C, l atm).
25 ExamPle l CGconu~ shell char (available from M.C.
Carbon K.K.) was heated to 600 C and held at 600 C

for 1 hour in a nitrogen gas atmosphere (normal pressure) to be pre-calcined, whereby a carbon precursor having a volatile content of at most 2 %.
The carbon precursor was pulverized into a powdery 5 carbon precursor having an average particle size (diameter) of 25 ~m. Then, 20 g of the carbon precursor was piled in a ca. 2 mm-thick layer in an alumina-made boat and then placed in a horizontal tubular fllrn~ce- of 100 mm in diameter to be heated to 10 1200 C at a temperature-raising rate of 250 C/hr and held for 1 hour at 1200 C for carbonization while flowing nitrogen gas at a rate of 10 liter/min.
The properties of the resultant carbon~ceous material are shown in Table 1 appearing hereinafter 15 together with those of other Examples and Comparative Examples.
Example 2 A carbon~ceous material was prepared in the same manner as in Example 1 except that the 20 carbonization temperature was changed to 1300 C.
Example 3 A carbonaceous material was prepared in the same m~nner as in Example 1 except that the nitrogen flow rate during the carbonization was decreased to 1 25 liter/min.
Example 4 Milled and extracted coffee bean (produce of -24- 2 1 &6~9 Brazil) in particle size of 2 - 3 mm was dried at 120 C for 1 hour and then pre-calcined by heating up to 600 C and holding at 600 C for 1 hour in a nitrogen atmosphere (normal pressure), followed by 5 pulverization into a powdery carbon precursor having an average particle size of 25 ~m. The carbon precursor was carbonized in the same manner as in Example 1.
Example 5 Mohsoh bamboo trunk (produce of Fukushima-ken, Japan; age: 3, diameter: ca. 70 mm) was dried at 120 C for 3 hours and pre-calcined by heating up to 600 C and holding at 600 C for 1 hour in a nitrogen atmosphere (normal pressure), followed by 15 pulverization into a powdery carbon precursor having an average particle size of 25 pm. Then, 20 g of the carbon precursor was placed on an alumina-made boat and charged in a vacuum furnace, which was then sucked by a vacuum pump to an internal pressure of 0.01 -20 0.03 Pa. While keeping the internal pressure, the vacuum furnace was heated up to 1200 C and held at 1200 C to effect carbonization, thereby obtAinin~ a carhonAceous material.
Example 6 A carbonAceous material was prepared by r-Arhonization of cherry wood (produce of Fuknchi ken, Japan, age: 10, diameter: ca. 50 mm), otherwise -25- 21 ~6999 in the same manner as in E~ample 5.
Example 7 A carbonaceous material was prepared by c~rho~ization of oak wood (produce of Fukushima-ken, 5 Japan, age: 10, diameter: ca. 50 mm), otherwise in the same manner as in Example 5.
Example 8 Milled and extracted coffee bean (produce of Brazil) in particle size of 2 - 3 mm was dried at 120 10 C for 1 hour and then pre-calcined by heating up to 600 C and holding at 600 C for 1 hour in a nitrogen atmosphere (normal pressure), followed by pulverization into a powdery carbon precursor having an average particle size of 25 ~m.
Then, 30 g of the powdery carbon precursor was placed on a porous filter plate disposed at a middle of a reaction tube of a vertical tubular furnace (diameter = 50 mm) and heated at a rate of 250 C/hour while supplying nitrogen gas downward at a 2~ rate of 2400 ml/min from an upper part of the reaction tube. When the fllrnace temperature r~ac-h~A 1100 C, the temperature was ret~ineA for 1 hour to effect carbonization while keeping the nitrogen supply rate.
After cooling, a carhon~ceous material was recovered.
25 ExamPle 9 A carbonaceous material was prepared in the same manner as in Example 8 except that the nitrogen 21 86q~

gas was supplied upward from a lower part of the reaction tube and the nitrogen gas supply rate was decreased to 100 ml/min.
Comparative Example 1 A phenolic resin ("Bellpearl C-800", available from Kanebo K.K.) was pre-cured at 170 C
for 3 min., and then cured at 130 C for 8 hours, followed by pulverization into an average particle size of 25 pm. Then, 20 g of the pulverizate was 10 placed in a horizontal tubular furnace identical to the one used in Example 1 and carbonized by heating up to 1200 C at a rate of 250 C/hr, holding at 1200 C
for 1 hour and cooling, while flowing nitrogen gas at a rate of 10 liter/min., whereby a phenolic resin-15 calcined carbon was prepared.comParative Example 2 A furan resin ("Hitafuran VF-303", available from Hitachi Kasei K.K.) was cured at 100 C for 14 hours, followed by pulverization into an average 20 particle size of 25 pm. Then, 20 g of the pulverizate was placed in a horizontal tubula~ furnace identical to the one used in Example 1 and carbonized by heating up to 1200 C at a rate of 250 C/hr, holding at 1200 C for 1 hour and cooling, while flowing nitrogen gas 25 at a rate of 10 liter/min., whereby a furan resin-calcined carbon was prepared.
Comparative Example 3 2 1 86~9q A petroleum pitch having a softening point of 210 C, a quinoline-insoluble content of 1 wt. ~ and an H/C atomic ratio of 0.63 was pre-calcined by heating up to 600 C and holding at 600 C for 1 hour 5 in a nitrogen gas atmosphere (normal pressure), followed by pulverization into a powdery carbon precursor having an average particle size of 20 pm.
The carbon precursor was cArhonized at 1200 C for 1 hour under a reAllc~A pressure of 0.01 - 0.03 Pa to 10 prepare a carhonAc~ous material.
Comparative Example 4 Coconut shell-based activated carbon (available from Kuraray Chemical K.K.) was pulverized to an average particle size of 25 pm and treated at 15 1200 C for 1 hour in a nitrogen atmosphere, to prepare a carbonAc~ous material.
The properties of the carbon~ceous materials prepared in the above Examples and Comparative Examples are inclusively shown in Table 1 appearing 20 hereinafter.
[Doping/de-doping capacity for active substance]
The carbonAc~ous materials obtained in Examples and Comparative Examples were respectively used to prepare a non-aqueous solvent-type ~econ~ry 25 battery (cell) and the performances thereof were evaluated in the following manner.
The carbonAceous material is generally suited -28- 2 1 869~

for constituting a negative electrode of a non-aqueous solvent secon~ry battery. However, in order to accurately evaluate the performances of a carhon~ceous material inclusive of a doping capacity (A) and a de-5 doping capacity (B) and also a non-dedoping capacity (A-B) for a cell active substance without being affected by a fluctuation in performance of a counter electrode material, a large excess amount of lithium metal showing a stable performance was used as a 10 negative electrode, and each carbon~ceous material prepared above was used to constitute a positive electrode, thereby forming a lithium s~con~ry battery, of which the performances were evaluated.
More specifically, the positive electrode was 15 prepared as follows. That is, 9O wt. parts of the carbonAceous material thus formulated in the form of fine particles and 10 wt. parts of polyvinylidene fluoride were mixed together with N-methyl-2-pyrrolidone to form a paste composite, which was then 20 applied uniformly onto a copper foil. The composite, after being dried, was peeled off the copper foil and stamped into a 21 mm-dia. disk. The disk was then press-bonded onto a 21 mm-dia. circular shaped net of stainless steel to form a positive electrode 25 cont~inin~ about 40 mg of the carhon~c~ous material.
On the other hand, a negative electrode was prepared by stamping a 1 mm-thick sheet of lithium metal into a -29- 21 86~'~9 21 mm-dia. disk.
The thus-prepared positive and negative electrodes were disposed opposite to each other with a porous polypropylene film as a separator disposed 5 therebetween, and the resultant structure was dipped in an electrolytic solution comprising a 1:1 (by volume)-mixture solvent of propylene carbonate and dimethoxyethane and LiC104 dissolved therein at a rate of 1 mol/liter, thereby forming a non-aqueous solvent-10 type lithium s~con~ry battery.
In the lithium ~cQn~ry battery thusconstituted, the carbon~c~ous material in the positive electrode was subjected to doping and dedoping of lithium to evaluate capacities therefor.
More specifically, the doping was effected by repeating a cycle including 1 hour of current conduction at a current density of 0.5 mA/cm2 and 2 hours of pause until the equilibrium potential between the positive and negative electrodes r~che~ 5 mV.
20 The electricity thus flowed was divided by the weight of the carbon~ceous material to provide a doping capacity (A) in terms of Ah/kg. Then, in a similar manner, a current was flowed in a reverse direction to dedope the lithium from the doped carbQn~ceous 25 material. The de-doping was effected by repeating a cycle including 1 hour of current ron~l~tion at a current density of 0.5 mA/cm2 and 2 hours of pause, 2 î 86q~9 down to a cut-off voltage of 1.5 volts. The electricity thus flowed was divided by the weight of the carhQ~ ous material to provide a dedoping capacity (B) in terms of Ah/kg. Then, a non-dedoping 5 ç~r-^ity (A-B) was calculated as a difference between the doping capacity (A) and the dedoping c~city (B), and a discharge efficiency (%) was obtained by dividing the dedoping capacity (B) with the doping capacity (A) and multiplying the quotient (B/A) with 10 lOO. The discharge efficiency is a measure of effective utilization of the active substance.
The performances of the lithium seron~ry batteries using positive electrodes of the respective carbon~ceous materials measured in the above-described 15 ~nn~r are summarized in the following Table 2.
In view of Table 2, it is understood that the seÇon~ry batteries prepared by using the carbonaceous materials according to Examples of the present invention showed larger values in both doping capacity (A) and de-doping capacity (B) and also remarkably small non-dedoping c~cities (A-B) determined as differences therebetween, so that the carbon~reQus materials of the present invention allow effective utilization of cell active substance.
Secon~ry batteries obt~inefl by using carbon~ceous materials of Comparative Examples 1, 2 and ~ showed large doping ~p~cities but also showed 21 86C~99 very large non-dedoping c~p~cities~ thus having a disadvantage that lithium as the active substance was not effectively utilized.
A secon~ry battery obt~in~ by using the 5 carbon~ceous material of Comparative Example 3 showed doping capacity and dedoping capacities which were both very small.
As described above, according to the present invention, there is provided a carbon~ceous electrode 10 material having well-balanced performances including a small non-dedoping capacity and a large dedoping capacity in combination. Such a carbon~c~ous material can be easily produced by carbonizing an organic material of plant origin as a starting material under 15 appropriate conditions. By using an electrode, particularly a negative electrode, prepared by using the carbon~ ous material according to the present invention, it becomes possible to provide a non-aqueous solvent-type secQnd~ry battery having large 20 char~e-discharge capacities and a large active substance-utilization efficiency.

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Claims

WHAT IS CLAIMED IS:

1. A carbonaceous electrode material for a non-aqueous solvent-type secondary battery, obtained by carbonizing an organic material of plant origin, and having a pore volume of at least 0.55 ml/g of pores having a pore diameter of at most 5 µm as measured by mercury injection method and a specific surface area of at most 100 m2/g as measured by the nitrogen adsorption BET method.

2. A carbonaceous electrode material according to Claim 1, wherein the organic material is a member selected from the group consisting of coffee bean, chaffs, broadleaf tree wood, conifer wood, and bamboo.

3. A carbonaceous electrode material according to Claim 1 or 2, having an average (002)-plane spacing of at least 0.365 nm as measured by X-ray diffraction method.

4. A process for producing a carbonaceous electrode material for a non-aqueous solvent-type secondary battery, comprising: carbonizing an organic material of plant origin in contact with a flowing inert gas at a temperature of 700 - 1500 °C under a pressure exceeding 10 kPa.

5. A process according to Claim 4, wherein the organic material is piled in a layer and the inert gas is caused to flow in parallel with the layer.

6. A process according to Claim 5, wherein the organic material is caused to form a bed, and the inert gas is caused to flow through the bed.

7. A process according to Claim 6, the organic material forms a fluidized bed under the action of the flowing inert gas.

8. A process according to Claim 6 or 7, wherein the inert gas is caused to flow at a rate of at least 10 ml/min. per gram of the organic material to be carbonized.

9. A process according to any one of Claims 4 -7, wherein the organic material is pre-calcined and pulverized to an average diameter of at most 100 µm prior to the carbonization.

10. A process for producing a carbonaceous electrode material for a non-aqueous solvent-type secondary battery, comprising: carbonizing an organic material of plant origin except for coconut shell at a temperature of 700 - 1500 °C under a reduced pressure of at most 10 kPa.

11. An electrode structure for a non-aqueous solvent-type secondary battery, comprising: an electroconductive substrate and a composite electrode layer disposed on at least one surface of the electroconductive substrate;
said composite electrode layer comprising a carbonaceous electrode material according to any one of Claims 1 - 3 in a particulate form, and a binder.

12. A non-aqueous solvent-type secondary battery, comprising, a positive electrode, a negative electrode, and a separator and a non-aqueous electrolytic solution disposed between the positive and negative electrodes;
at least one of said positive and negative electrodes comprising an electrode structure according to Claim 11.

13. A secondary battery according to Claim 12, wherein the electrode structure constitutes the negative electrode.