Note: Descriptions are shown in the official language in which they were submitted.
2 1 92429
SPECIFICATION
Title of the Invention
Lithium ion secondary cell
Field of the Invention
This invention relates to a lithium ion secondary cell.
More particularly, the present invention relates a negative
electrode material for the lithium ion seconda~y cell in an
aspect and to an inexpen~i~e lithium ion secondary cell in
~nother aspect The invention also relates to a process for
preparing h~gh purity graphite for the negative electrode
Back~round of the In~ention
In the lithi~m ion ~econdary cell, as negative electrode
active materials, natural graphite, fine glo~ular carbon
particles (c~ r~ially ~vailable under the trade name
"Mesocarbon Microbead~"), me~ophase pitch type car~on fiber,
amorphous carbon which i~ not easily graphitized, etc. have
been proposed and are actually u~ed. Plo~a,Lie6 and prepara-
tion met~ods of t~ese carbon materials are described in
Japanese Laid-Open Patent Specification~ No. 4-188559, 4-
190551, 7-223809, 7-249411, etc.
In any case, c~ho~ material~ used for negative elec-
trode acti~e materials must be highly pure.
Method~ for highly p~rifying carbon material~ are, for
instance, described in Japane~e Laid-Open Patent Specifica-
tion~ No. 63-79759 and 6-298510. In the methods described in
these patent documents, carbon is graphitized in a graphitiz-
ing furnace and the reculting secondary carbon material~ are
purified by heating them in the presence of chlorine ga~ or
hydrogen chloride ga~ in a reactor to remo~e impuritie~
contained therein an~ thus to enhance the purity.
However, the6e ~o~,vcntional graphitization methods
require a number of process ~teps such as formation of pri-
mary carbon materials by firing, formation of 6econdar~
~Arhon materials from graphitized carbon, removal of i~puri-
tie~ from the ~econdary carbon material6, etc. and equipment
therefor a~ well a~ treating material9. ~herefore, known
2 1 ~2429
purification method~ are not commercially practical
SummarY o~ the Invention
The object of the present invention is to provide a
negative electrode for high capacity lithium ion secondary
cell comprising an high purity artificial graphite which i6
obtained by thermally d~-~posing high purity 6ilicon carbide
at a temperature in excess o~ the sublimation temperature of
~ilicon in an aspect.
In a preferred embo~i t of this aspect, ~aid graphite
i~ obtained by proces~ ~teps~
(a) charging a furnace comprising a pair of electrodes facing
each other and a carbon core mounted ~etween the t~o elec-
trodes with a mixture of coke and 6ilica sand, applying
electric current between the two electrode6 in a reducing
atmosphere to heat the furnace and thu~ conducting reduction-
carbonization reaction to form an ingot of silicon carbide
surrounding the core; and
~b) f~rther applying electric c~rrent between the two elec-
trodes in a reducing atmosphere to heat the core rod to a
tF - ~ature higher than the ~ublimation temperature of sili-
con, whereby the formed silicon carbide is decomposed by the
heat generated by the core releasing silicon atoms.
In a preferred form of ~his :- ho~i - t, the furnace is
heated to 2,700 - 3,200 C in the A~po6ition step.
In another preferred form o~ this embo~i ~ t, said
reduction-carbonization ~tep i5 carried out at 2,000 to 2,50C
C for 20 - 40 hour~ and the decr _6ition 3tep i~ carried
out for not less than 1 hour.
~ n ~nother preferred embodiment of this aspect, the
gr~phite is prepared by the process step~:
~a) charging a crucible with high purity ~ilicon carbide,
and
(b) heating the crucible to Z,700 to 3,000 C in a reducing
atmosphere, whereby silicon carbide is de~ _sed rele~sing
6ilicon atoms to form high purity graphite.
In a prefe~red form of this embodiment, the temperature
2 1 92429
o~ 2,700 to 3,200 C i~ maintained for not le6~ than one hour
in the decomposition step.
The object of the present in~ention in the ~econd aspect
is to provide, in the lithium ion secondary cell comprising a
negative electrode comprising a negative electrode active
material and a positive electrode compri~ing a positive
electrode active materLal and a separator (partition~ sepa-
rating the negative electrode and the positive electrode, a
lithium ion 6econdary cell chactracterized in that the graph-
ite i~ a highly pure graphite which is obtained by thermally
decompoxing high purity ~llicon carbide at a temperature in
exce~6 o~ the sl~bli -tion t~- p_.ature o~ ~ilicon in a reduc-
ing atmosphere to release silicon atom~.
In a preferred embodiment of this aspect, said graphite
is obtained by process steps:
(a) charging a furnace comprising a pair of electrodes facing
each other and a car~on core mounted between the two elec-
trodes with a mixture of coke and silica cand, applying
electric current between the two electrodes in a reducing
atmosphere to heat the carbon core and thus conducting reduc-
tion-car~onization reaction to form an ingot of silicon car-
bide surrounding the carbon core;
(b) further applying electric current between the two elec-
trodes in a reducing atmosphere to heat the furnace to a
temperature higher tha~ the sublimation t~ rature of sili-
con, whereby the fo~med silicon carbide is decomposed by the
heat generated by the cArho~ core releasing ~ilicon atoms.
In ~ preferred form of thi6 emho~i - t, the decomposi-
tion step is carried out at 2,700 to 3,200 C .
In a pre~erred form o~ this emboAi~nt, said re~uction-
;z~tion step is carried out at 2,000 to 2,500 C for
20 - 40 hours and the decomposition step is carried out for
not less than 1 hour.
In another preferred : ho~; -nt of this aspect, the
graphite i~ prepared by 6teps:
(a) charging a crucible with high purity silicon carbide,
2 1 92429
(b) heating the cru~i~le to 2,700 to 3,200 C in a reducing
atmosphere, whereby ~ilicon carbide is decompo~ed releasing
silicon atom~ to form high purity graphite.
In a preferred form of thi~ ~hn~ i~nt~ the t ~ature
of 2,700 tO 3,000 C i~ ~aintained for not less than one hour
in the decompo~ition ~tep.
In another aspect, ~he pre~ent invention provides a
proces~ for preparation of high purity graphite compri~ing:
(a) charging a furn~ce compri~ing a pair of electrodes facing
each other and a carbon core mounted between the two elec-
trodes with a mixture of coke and silica ~and, applying
electric current between the two electrodes in a red~cing
atmosphere to heat the carbon core and thus conducting reduc-
tion-carbonization reaction to form an ingot silicon carbide
~urro~n~i ng the carbon core; and
(b) further applying electric current between the t~o elec-
trodes in a reducing atmosphere to heat the furnace to a
temperature higher than the sublimation t~ ~~ atu~e of sili-
con, where~y the formed ~ilicon carbide is decomposed by the
heat generated by the carbon core releasing ~ilicon atoms.
Brief Description of the Attached Drawina~
of the attached drawing~,
Fig. 1 i9 a schematic crocs-sectional view of an example
of lithium ion secondary cell;
Fig. 2 i~ an exploded view of another example of lithium
ion ~econdarr cell;
Fig. 3 is schematic cross-sectional views of a furnace
which represent steps of preparation of pure graphite for the
lithium ion secondary cell of the present invention;
Fig. 4 is a diagram illuRtrating an example of operation
of the furnace for preparation of highly pure graphite;
Fig. 5 is a chart of X-ray diffraction analy~is of the
produced graphite; and
Fig 6 is a schematic cross-~ectional view of another
ex~mple of the furnace for preparation of highly pure graph-
ite.
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2 1 92429
Descri~tion of Preferred Embodiments of the Invention
Now the in~ention is specifically described with refer-
ence to the attached drawing~, especially b~ way of preferred
embodiments.
cLithium Ion Secondary Cell~
The lithium ion secondary cell of the pre~ent in~ention
ba~ically compri~e~ a negative electrode comprising a nega-
tive electrode active material, a po6itive electrode compris-
ing a po6itive electrode active material and a separator
separating the two electrodes ax stated before.
A specific structure of lithium ion ~econdary cell i8
exemplarily sho~n in Fig. 1. The cell comprises a ca~ing 10,
which i8 a circular plate ~ith an upright peripheral wall and
hou~es an electricity collector 16 laid on the circular plane
of the ca~ing 10, a negative electrode 15 laid on the elec-
tricity collector 16, a separator 14 laid on the negative
electrode 15, a po~itive electrod~ 13 placed on the separator
14; and a gasket 12 which insulates the positive electrode 13
and the negati~e electrode 15 from the ~all. This cell is
generally like a disk or a button.
There i5 also a lithium ion secondary cell a~ shown in
Fig. 2 other than that of Fi. 1. The lithium ion ~econdary
cell 30 of Fig. 2 comprises a metal container 31 which house~
a positive electrode sheet 32 and a negative electrode sheet
33 which are rolled with a separator ~heet 34 in between into
a roll of a plurality of layers with an insulator 6heet 35
inside the container wall. The po6itive electrode 32 i6
electrically connected to a positive Qlectrode t~_ in~l 38
provided in a lid 37, which clo~e6 the opening of the con-
tainer 31 with a ga~ket 36 in6erted, and the negative elec-
trode 33 is elQctrically connected to the container 31.
of whatever structure the cell i~, the po6itive elec-
trode contains lithium cobaltate, lithium nickelate, lithium
manganate, etc. as a positive electrode active material. The
electrode i6 formed by shaping a mixture of one of the posi-
tive electrode acti~e material~ and a binder into a disk or
2 1 92429
sheet form, for in-~tance. A6 bi~ders, fluorine re~ins such
as polytetrafluoroethylene, tetrafluoroethylene-hexafluoroe
copolymer, poly~vinylidene fluoride~, etc can be re~erred
to.
The separator prevent~ short-c~rcuit of the po~itive
electrode and the negative electrode and may be made of
~arious material6 insofar a~ they are a~le to insulate the
po6itive electrode and the negative electrode even when the
cell temperature rise~ owing to a~no~mal discharge Suitable
separators are porous membrane~ of polyethylene, polypropyl-
ene, etc., laminated do~ble porous membrane of a polyethylene
sheet and a polypro~ylene sheet, a triple porous mem~rane
comprising a polyethylene sheet ~andwiched between t~o poly-
propylene sheets, etc.
The electrolyte enables transportation of lithium ion~
~etween the positi~e electrode and the negative electrode.
As to the electrolyte, a solvent cont~i n i ng at least one of
propylene carbonate and diethyl carbonate, a solvent contain-
ing ~t least one of ethylene c~h~n~te and diethyl carbonate,
a ~olvent cont~i n i ng at least one of propylene carbonate and
triethyleneglycol-dimethylether etc. ~an be u~ed to. The
solvent may contain a ~upporting electrolyte such a~ LiPF6,
LiBF4.
The negative electrode of the lithium ion secondary cell
of the pre~ent invention is made of a mixture of a powder of
a graphite prepared by a specific process and a binder.
The same binder a~ ~sed for preparing the positi~e
electrode can ~uitably be used.
The graphite, which is suitably used ~or preparation of
the negative electrode, 6hould be as pure as not les~ than 99
% and have an inter-crystallite di~tance of 3.353 ~ The
graphite 6uitably usable for formation of the negative elec-
trode has a number of munute void~ in the cry6tallites into
which lithium ions are doped a~ clu3ters in a large amount.
Thi~ high purity graphite 6hould preferably be in the form of
particles ha~ing a particle diameter of 5 - 100 ~m~ more
--6--
21 92429
preferably 10 - 30 ~m.
cpreparation of High Purity Graphite~
The process for preparing ~uitably u~able for lithium
ion secondary cell comprises a reduction-carbonization ~tep.
and a ~ec osition step.
(Reduction-carbonization Proce~s)
The reduction-cArh~nization step can ~uitably be car~ied
out using an indirect heating furnace represented by "Acheson
furnace". For instance, a~ ~hown in ~ig. 3 (a), a furnace 1
comprises furnace walls 2 and a pair of electrodes 3, 3. ~he
furnace wall comprise a pair of ~ide w~lls set 2, 2 on the
furnace bed on the right and on the left and a pair of wall~
removably supported on the bed in the front and in the rear.
The furnace is generally of a bathtub-like form long in the
right-and-left direction with the upper ~ide open. The
furnace bed and walls 2 are made of a refractory material.
The electrodes 3, 3 are re~pectively provided on a side ~all
3, 3 so as to fa~e each other s~itably ~paced apart. They
a~e made of graphite. A core 6 of carbon, preferably of high
purity graphite, in the form of a rod is mounted between the
two electrode~ 3, 3 electrically in con~act with them. The
furnace is constructed so that high electric current is
supplied to the electrodes 3, 3 from an electric ~ource (not
shown). The electric source ix freely regulatable with
respect to voltage in accordance with the monitored fluctua-
tion of electric current and power in operation of the fur-
nace.
~ hen the furnace a~ shown in Fig. 3 (a) is u~ed, a
mixture 5 of coke and silica sand iB loaded in the furnace 1
so that the ~hnn core 6 i6 imbedded therein.
To the mixture 5, which compri~es pul~erized coke and
~ilica sand, ~aw dust, a ~lux such as sodium chloride which
assistg rC v~l Of metal oxides, etc. can be added as de-
sired. The core 6 can ~e made of particulate or lump coke
although it can be ~ade o~ o~h0r carbon materials including
graphite.
21 92429
After the furnace 1 is charged with the mixture 5,
electric current i5 applied to the electrodes 3, 3 so that
the furnace is heated to 2,000 - 2,500 C by the produced
Joule heat. The electric current or the voltage to be ap-
plied ta the electrode pair is decided so that the furnace is
heated to the abo~e-mentioned te~perature. The values of the
electric current and voltage are exempli~ied in the working
example~ described below.
When the mixture S is heated in the furnace to the
a~ove-mentioned t~ _^rature range, the reduction-carboniz~-
tion reaction of silica sand proc~edx and an ingot of silicon
carbide 7 is ~ormed on the surface of the core rod 6 as
~hown in Fig. 3 ~b). The ~ilicon carbide ingot 7 formed on
the surface of the core rod comprises silicon carbide of
a crystal form and out~ide thereof thin layer~ of ~ crystal
~ilicon carbide are formed concentrically~
Detail of ~he reduction-carbonization step is as fol-
lo~s.
~ hen electric current is applied to the pair of elec-
trodes 3, 3, the carbon core 6 is heated and raise~ the
temperature in the furnace l The period until the tempora-
ture reaches 2,000 - 2,500 ~C is the preheating stage wherein
the charge i~ dried and preheated. Then the preli~inA~y
reaction stage follows, wherein the initial cry~tals of
silicon carbide are ~ormed. Afte~ the cor~ rod 6 has reached
a predet~ ined t~ , ~ature in a range of 2,000 - 2,500 C,
crystals of ~ilicon ~hi~ begin to grow on the ~urface of
the core 6 and cylindrical ingot of silicon carbide is
formed surrounding the core 6. Impurities are not taken in
the cry~tal~ and, therefore, the formed silicon carbide 7 is
highly pure.
(Decomposition Step)
A highly pure graphite i~ produced ~rom the thus formed
highly pure silicon carbidH.
The high purity graphitQ i~ obt~i n~ by h~ating this
~ilicon car~ide to a tempe~ature higher than the ~ublimation
2 1 9242q
tempe~ature of ~ilicon, practically to 2,700 - 3,200 C,
prefera~ly 2,800 - 3,000 C in a reducing atmosphere and
keeping it at that t- ~ratu~e for not les~ than one hour,
preferably fo~ 15 - 20 hours. B~ this treatment, 6ilicon
carbid~ is ther~ally decomposed releasing ~ilicon atoms.
The decomposition step i~ carried out in an indirect
heating furnace a~ ~hown in Fig. 3 which i~ represented ~y
"A~he~on furnace". When thiC furnace is used, the decompo~i-
tion step is continuously carried out follo~ing the reduc-
tion-c~h~nization step.
When the furnace 1 a~ shown in Fig 3 is used in the
decomposition ~tep, the furnace is heated by Joule heat
generated by the core 6 to 2,700 - 3,200 C, preferably 2,800
- 3,000 C by further applying electric current to the elec-
trodes 3, ~ and the furnace i~ maintained at that temperature
for not le~s than one hour, preferably 1~ - 20 hour~. By
this treatment, the silicon atoms which constitute the sili-
con carbide crystals are relea~ed and vaporize from the
ingot. As a result, only carbon atoms ~ ~i n in the ingot,
which turn to high purity graphite ~y ~ufficient heat treat-
ment.
The high purity graphite g~ow6 ~urrounding the core 6
and almost all of the ingot turns to high purity graphite 8
a~ fihown in Fig. 3 (c) and newly formed silicon car~ide 7 i~
precent outside of the graphite surrolln~i n~ it. There exi~t
clear de~inite interfaces between the core 6 and the graphite
8 and the graphite 8 and the silicon carbide 7 at the time
when the de~ ition step has finished. Therefore, the
high purity graphite can easily be isolated by disassembling
the furnace, that is, moving apart the front and rear wall~
and l~ ving the rr--ining unreacted mixture 5 and breaking
down the silicon carbide ingot 7 The i~olated graphite is
washed and crushed and ~urther pulverized to a desired parti-
cle size for use as a negative electrode material. The
desired particle diameter as DS0 is 5 - 100 ~m, more prefera-
bly 10 - 30 ~m.
2~ 92429
Not only the high purity silicon carbide formed by the
above-described reduction-car~onization ~tep but also any
high purity Rilicon carbide such as those formed by m; xi n~
powder of metallic silicon and carbon powder and heat-treat-
ing the mixture at a predetermined temperature in a graphite
cruci~le or by thermally decomposing an organic silicon
polymer or silicon carbide formed b~ gaseous pha~e reaction
can be used.
High purity graphite also can be prepared by loading the
above-described high purity silicon carbide in a crucible,
he~ting the crucible to 2,700 - 3,200 C, preferably 2,800 -
3,000 C and the temperat~re i~ ~aintained for not le~ than
1 hour, prefera~ly 15 - 20 hours and thu~ releasing silicon
atoms from ~ilicon carbide leaving only carbon in the cruci-
ble.
In thi~ cace, if an indirect heating furnace as ~hown in
Fig. 6, high purity ~ilicon carbide Z0 i9 placed in the
graphite crucible 21 with the upper opening closed with a
graphite lid 22 having through hole~ 24 and the crucible is
covered by coke and i~ in contact with the electrodes 25, 25
The crucible it~elf i~ heated by applying electric current to
the graphite electrodes 25 in a reducing atmosphere. When
the crucible 21 is heated to the above-mentioned temperature,
the silicon of the ~ilicon carbide is released and the sili-
con vapor escapes through the through holes 24. If a cruci-
~le of a refractory material other than graphite i~ used,
~ilicon carbide 6hould p~eferably be placed in a crucible
enveloped with carbon to ~event impuritie~ from entering the
~ormed graphite.
Whatever apparatus is used, when silicon carbide i~
heat-treated in the above-mentioned t~ -rature range in the
decomposition step, silicon carbide i6 th~rmAlly decompos~d
and the silicon ato~ are relea6ed from carbon ~toms and
vapori~es leaving carbon atomc only. The carbo~ i6 graphi-
tized by heat treatment of sufficient time.
The cry6tal structure of silicon carbide i~ triangular
--10--
2 1 92429
pyramidal tetrahedral. sut at the initial stage of formation
of silicon carbide, the crystal structure is polymorphized
and the crystal axis orientation~ are made random. When
elemental silicon is released ~rom 6ilicon carbide whose
crystal structure has been polymorphized and whose cry~tal
axi~ orientstions are random, the neighboring carbon atom~
come clo3er to each other to fill the po~ition~ from which
silicon atom6 have been relea~ed and, as a result, there are
formed a number of voids or interstice~ at the interface~
between the crystallites. While the charge i~ kept at the
temperature in the above-mentioned range, carbon crystals are
grad~ally graphitized. Thu~ highly graphitized graphite,
which have voids or interstice~ for lithium-ion~ to penetrate
as clusters, is formed by the time when the decomposition
step is finished.
(Wo~king Example)
A furnace as shown in Fiy. 3 was used. As described
abo~e, a core 6 wa~ mounted between a pair of electrode~ 3,
3, a mixture 5 of ~ilica sand and coke was loaded densely
into the furnace ~o as to imbed the core rod, voltage was
applied to the electrodes 3, 3 to cause electric current a~
shown in the operation diagram of Fig. 4.
The 4 hour period a~ter the operation was started was
the preheating stage. In thi~ preheating 6tage, 400 V was
applied at the 6tart of the operation to rai~e the t~mp~a-
ture of the furnace. A~ the temperature ro5e, the electric
re6i~tance of the core dropped and the electric current
increa~ed. The increase in the electric current was ob-
served, and the voltage wa~ gradually dropped until it fell
down to 20a volts at the end of the preheating stage and the
electric current was adjusted to 3 RA. The furnace tempera-
ture was 2,000 C at the end of the preheating ~tage.
Now the preliminary reaction stage began, which contin-
ued for about 16 hours. In the first 4 hours in the prelimi-
nary reaction ~tage, the voltage was d~y~d to 100 V as the
increa~e in the electric current ~as being monitored and the
2t 92429
furnace temperature was rai~ed to about 2,000 C. Even
thereafter, the electri~ resistance of the core 6 tended to
fall, and, therefore, the voltage ~a~ gradually dropped to 50
V as the increa~e in the electric current was being monitored
and the furnace temperature wa~ kept at 2,000 - 2,500. The
electric current at the end of the preliminary reaction stage
was 24RA.
The crystal growth sta~e followed the preliminary reae-
tion stage. The temperature at the end of ~he prel ;min~y
reactio~ stage wa~ maintained for a~out 26 hours 80 as to
allow growth of the ~ilicon c~rbide crystals. Also in the
crystal growth ~tage, the voltage was gradually dlo~ as
the increa6e in the electric current was being monitored, and
the electric potential finally fell to 40 V. During this
stage, the electric current rose from 24 KA to 30 RA.
At the time when the crystal growth stage finished,
generall~ ~ylindrical ingot o~ silicon carbide 7 had been
formed surrollnAin~ the core 6 as shown in Fig. 3 (b). Most
of this ingot comprised a-silicon car~ide and only the thin
~urface layer comprised ~-type silicon carbide.
Now the deco~.~G~ition stage followed. Electric poten-
tial of 50 V was applied to the electrodes 3, 3 and the
voltage wa~ gra~ually d~ a~ the increase in the electric
current was bein~ monitored and the furnace temperature was
rai~ed to 2,700 - 3,200 C, ~hich temperature was maintained
for 15 hours. At the end of the decomposition step, the
electric current was 37.5 RA and the voltage wa~ 40 V.
After the d~-- ~iti~n stage fini~hed, the furnace l
was cooled. After cooling, the furnace was dixa~embled and
the ingot was taken out. The ingot was broken and the gràph-
ite formed inside thereof ~as taken out.
The graphite wa~ crushed, ~ashed and pul~erized.
Of the obtained graphite, the puri~ was m~a~ured by
~emi-quantit~tive elemental analysi~ using a wavelength
dispersive fluorescent ~-ray spectromoter and the crystallin-
ity ~as measured u~ing an X-ray diffractometer. The re~ults
-12-
-
21 92429
of the purity measurement are shown in Table 1. No detect-
able impurities ~ere present in the graphite, which was
proved to be highly pure By the X-ray diffrato~etry, the
lattice con~tant and the ~ize of the crystallite were deter-
mined and the inter-crystallite distance wa~ calculated from
the te6t re~ult~. ~he inter-crystallite distance of the
obtained graphite was 3.353 ~ and it was ~r oved that ~aid
value almo6t accord~ with the inter-crystallite distan~e 3.35
of purified natural ~raphite. The result of the X-ray
dif~ractometry is xhown a~ Fig. 5, wherein m~ltiple peaks
appear. The peaks appeared at po~itions A (42.2550), B
~43.3317) and C ~44.4179) in all sample~. Thi3 peak pattern
completely accord~ ~ith that of natural graphite.
Table 1
Element Weiqht %
C Balance
Si Not detected
Ca Ditto
Fe Ditto
Ni Ditto
Cu Ditto
U6ing the obt~i n~ graphite, a lithium ion secondary
cell of the structure as 6hown in ~ig 1 wa~ made by the
u~ual procedure. Lithium cobaltate, poly(vinylidene fluo-
ride) and N-methylpyrolidone were mi~ed and a positive elec-
trode di~k was prepared therefrom. In the sam~ manner, a
negative electrode di~k was prepared from the graphite ob-
tainec~ by the abo~e-de~cribed process, poly(vinylidene fluo-
ride) and N-methylpyrolidone. In a casing 10, ~hich i~ a
circular plate with an upright peripheral wall, an annular
gasket 12 was fitted, a copper-made circular electricity
-
2 1 92429
collector 16 was laid on the bottom, the above-described
negati~e electrode disk wa-~ laid on the collector 16, a
separator 14, ~hich is a porous polypropylene sheet, was lald
on the collector so a~ to co~er it and the a~ove-described
positive electrode disk wa~ laid thereon. Ethylene carbonate
was injected into the expo~ed positive electrode disk so a~
to impregnate it, thus forming a positi~e electrode 13 and a
negative electrode 15. Finally the positive electrode 13 wa~
covered by a lid 11 so that t~e~~positive electrode 13 was
fixed by the gasket 12. Thus a lithium ion secondary cell
was prepared. ~he discharge capacity of this c~ll was 350
Ah/Rg
As has been described above in detail, a highly pure and
highly graphitized graphite i~ obtained by 6ynthesizing
silicon carbide, which is obt~in~hle in highly pure state,
and decompo~ing it. In the present invention, high purit~
graphite i~ easily obtained and, therefore, the high purity
graphite can be inexpensively produced in a large amount with
less proces~ steps in contrast ~ith the con~entional method,
which is started with impure carbon stock, which is purified
by complicated process steps. Especiall~, if a series of the
process step6 of from synthesis of silicon carbide to th~r~- 1
decomposition thereof is carried out using an indirect heat-
ing furnace represented by "Ache~on furnace", a continuous
through operation of synthe~is and decompo~ition of silicon
carbide i~ pos~ible without nece~ity of cooling the ~urnace
between the process ~teps. That is, highly pure graphite can
be produced with high heat efficiency and le~ power con~ump-
tion, which means reduction of m~nufacturing cost. Graphite
of excellent quality can be inexpensively manufactured in a
large amo~nt and, there~ore, the lithium ion secondary cell,
who~e negative elctrode is made of this graphite is inexpen-
sive and has a large discharge capacity.
