Note: Descriptions are shown in the official language in which they were submitted.
W~ 93~095~2 2 1 2 2 3 ~ 5 i PCr/l~Sg2/09~
- 1-
~ELEcTRoLyTlc DOUBLE I~AYER CAP~CITO:E~
Thi~ applica'do~ is a ContiDuat~oIl-in-Part application of copending
U.S.S.N. 07/960,251 filed October 13,1992, which iB a Contil~u~tion of
application U.S.S.N. 07/783,8~0, filed October 29, 1991, now abandoned.
Ba~Du~d of the I~v~nt~on
This in~e~tioll relate6 to double layer capac~tors, arld more
par~cularly rel~tes to high-energy, high-power electrolyliic capacitors.
Conventional electroly~ic capac~tors store energy by accommodating
a so-called double lay~r of charge at the interface of each capacitor
electrode ~urface and ~he ele~olytic ~olu~on between ~he electrode~. The
electrode surface ar~a thu6 li~ts ~he ener~y storage capacity of ~uch
capacitor~; the larger the electrode ~urface area, t~e larger the ~ouble
layer of charge whi~ may b~ generated, and hence the greater the energy
~torage o~ ~e capacitor. Typic~l applications restrict 1~e prac~cal li~t
1~ of a capacitor~ phy~ical size, however, and thereby limit t;he achievable
e~ergy ~torage capacity provided by the maClrO8COpiC ~urfaces of the
capac~tor.
O~e doulble layer capacitor design w~i~ overcomes the macro8copic
capacitor surfiace area limitat;ion employs powdered electrode I~lterials9
e.g., ~ rea, actîvated c~rbon par~c~es, to microsco~cally increa~e ~he
~urfiace ~a of the capacitor elec~rode~ uch a ~pa~îtor, ~he carbo~
par~de~ are bo~d together to form a porous electrode ~trucl;ure iIl w~
the eazpo~ed ~u~face~ of t~e ~artîcles contribute to the overall electrode
s~ace area. ~he ~llte~ res~s~ce alld c~pacitance of the porous
el~ctrode st;ruct~e î~ a complicated func1;îon of the carbon par~cles'
6~cture and coDfi~1;îon~
$ummar~ o tlle Inv~ do~
In ge~eral, in one aspect, 1he i~ventîon pr~vide~ a dou~le layer
electrolyt~c capacitor of two electrodes ea~h în co~tact with a ~mmon
liqwd elect;rolyte~ At lea~t one of the electrode~ c~mp~e~ a c~stalline
ma~erial charaGterized by 1~he presence of ~an der Waal~ ch~ els in ~he
materîaL The~e van der Waals channels are adapted to accommodate the
wv93~0g~52 2122~ PC~/US92/Og~
- 2-
electrolyte w~hin the channels, such that a double la~rer of charge is
formed at inter~aces of the van der Waal8 ch~nels aIld the electrolyte
when a voltage is applied between the two electrodes. This abilit~y to
utilize 1~he va~ der Waals cha~els a~ extension~ of the elecl~rode's
5 macro~copic surface pro~ides a drama~c increa~e in capacitance over
co~ventional double layer capacitors. Further detailed desGriptions of
compounds b~ g vaIl der Waals channels and deYices w~ich u~lize these
compounds are prDvided in t~e following application~, all filed of equal
date as ~Electrolytic Double Layer Capacitor", and hereby incorporated by
10 reference: "~apacitive The~moelecttic Device" and "Energy Storage
De~ice~.
In prefe~red embodiment~, one of the electrode~ compri~es the
crystalline material and the other electrode i~ an electrically conducting
container in w~ the electrolyte and the cFystalline electrode are
1~ poE itio~ed; more preferably, both ele~odes of 1~he capacitor ~re compo~ed
of the crystalline mate~ial. Preferably9 t~e electrodes are each composed
of a monoGry~tal of the cry~talli~e material. In other preferred
embo~e~ts, the two electrodes each a~e composed of moIIocry~talline
powder par~cles of the s:rystalli~e material, the p~es beirlg
20 aplE~ro~ately 70 misron~ a l~ge6t &en~ion. Preferably, the
crys~e materiAl is a bismu1h ~lcogeI~ide, of Bi~,Chz, where Ch is
~ele~d firom the group consis~g of Te aIld Se, y i8 1 or 2, a~d z ifi iIl the
ra~g~ ~f i t~ 3.
In o~e pre~erred ~mbodiment, the ele~lyte is a 1.0 M ~ 104
26 801ution iIl proI~ylene carbona~e; in o1 her embodiment~, the electrolyte i~
a 1.2 M solu~o~ of organic cation of perchlorate in a mi~ture of propylene
carbonate w~ich i8 di~lved in dimetho~yel~ , or an aqueous ~olution
of potassium hydroxide.
Preferably, ~e elect~odes' van der Waals channels are adap~ed to
30 accommod~te the electrolyte by a tra~ning process co~prising ~ntercalation
of i~Ils f~om t~e elec~rolyte into the van der Waals channel~, t;he voltage
being ~ufficiently high to ac~ieve electrolyte penetration of the channels.
WO 93/09~52 2 1 2 2 3 5 5 PC~/US92/09244
- 3-
Preferably, the voltage i~ peliodically rever~ed in polarity between the
electrode~ during the Lntercalation process. Most preferably, the voltage
i~ applied to the electrode~ for appro~imately 600 minutes, and the voltage
polarity is re~erfied appro~imately every 30 minu~es. Thi~ training process
5 allow~ the electrolyte to penetrate the elect~odes' van der Waal~ chaImels
and form a double layer of charge at the channel surface~, thereby
dr~natically iIlcreasing the total surface area of the electrode. C)t~er
feature~ d advantage~ ofthe ~ven~on will be de~cribed in the ~ollowing
descript;ioll and in the claim~.
Brie De~ tion o$ ~e Drawi~
Fig. ~ a ~chematic illu~tration of oIle embodiment of the
capacit:or of t he i~ven~on;
Fig. lB i8 a schematic illu~tra~on of a seco~d embodiment of the
capacitor of th~ invention;
Fig. 2A i~ a ~chematic illu~tration of the capacitor of Fig. lB at a
fir~t s~age of ~g,
Fig. 2B ifi a ~chema1ic illu~tration of ~he capacîtor of Eig. 2A a~ a
later ~tage of 1~aining,
Fig. 2C i6 a ~ohematic il~ustrat;ion of t~e capacitor of Fig. 2A at a
20 final 6tage of tr~ng;
Fig. 2D i~ a ~hema1;ic illu~ on of ~e capacitor of Fig. 2A
i~uding ~e fo~natio~ of a double layer of charge; and
,
Fig. 3 is a diag~am of an ~qui~alent circuit for repre~e~ng 1~he
capacitor~ of Fig~. lA a~d lB.
2~
WO 93/09552 2 1 2 2 3 S ~ PCI`/US92/09244
De0~p1 ;on of the Pr~fexred Embodiment
A~ an e2~ample of limita~ion of capacitors, the capacitance of a
typical par~llel plate capacitor i8 giYen by:
C = ~o~S/d,
5 where ~ the peImitJdvity in vacuum (a cons~ant), F i~3 t~e dielectric
con~ta~t of the medium between the capacitor electrodes, S is the su~face
area of ~he capacitor electrode~, ~nd d i~ the width of the medium
separa~g the electrode~,. The capa~tance, and corre~pondixlgly, the
ener~y storage, o a ~ve~ capacitor are thus limited by the g~ome~, i.e.,
10 the surface area, ~he electrode ~pacing, t~e material propert;ies of the
eleGtrodes, and the ~edium ~epara1in g them.
The defini~on of capacit~ce for a double layer capa~tor is filrther
spe~ied by the ~tructure of the cha~ged double layer a~d its geometry.
T~iB double layer comprise~ charge accumulatioIl on the electrode surface
15 aIld accumula~on of ion~ at the electrode surface-0~ecl;rolyte ~terface.
Thus, for double layer electrolytic capas:itors, the wid~h d in the
a~pacita~ce egu~o~ iB gi~en by the dista~ce between the center~ of t~e
two region~ co~sl~itu~g t;he double Iayer.
The capacitor of t;he in~ention provide~ a ~ramatic increase in
20 capacitance and energy ~t~rag~ ~y providing a coITesp~ding increase ~
s~ace area oft~e capacitor electrodes and t~rough proper selectio~-ofthe
~lec~ode~ aDd t~e electrolyte. Most ~otable of ~e inve~on's adva~tages
i~ that t~e increa~ed surface area does nst ~ly on increasing ~he
macros~opic dime~io~s OI 1~he electrode~, and fi~ er, does ~ot rely on
25 p~d~l~ surface area~, a8 ~ ~pical caxbon electrodes. Rather, the
increa~ed electrode ~urface area is o~ ed u~ing a particular ~a~s of
mL~terial~, namely iIltercalal;ion compound~ w~ich are c}laracterized by a
layered c~etalliDe ~;~Ct~ he cry~tal layers OI iIltercalation
compouIlds comp~se plane~ of molecules or atoms which ~re weaklyboland
30 together ~d 6eparat~d ~om each other by van der Waal~ regions. The~e
van der WaU1B regio~s ~orm ~ otropic ~hannel~ in the c~8tal lattice
between ~he pla~e~ of mole~ules or atoms, resulting, i~ ef~ect, in a "two
WO 93/09552 2 1 2 2 3 ~ 5 i Pcr/U~92/O9244
dimensioIlal ~ystal s1;ructure. Inte~calation mate~ typically e~hibit
on the o~der of 106-107 layers per millimeter of material thickness. Due
to the weak van der Waile force between the crystal layers, the lattice
channels c~n accommodate the phy~ical introduction, or ~o-called
5 interc~la1ion, of a guest i~ erc~l~t species into them.
In the capacitor electrodes of the invention, the van der Waals
region~ of the electrode material are ~pulated ~uch that the ~ aces
of the cry~tal lattice channel~, alt;hough ~temal to the electrode material,
contribute to t~e overall electrode ~urface area, and thereby iIl~ease the
~O e~ec~ve electlode su~face area beyond that of its macro~copic ~urface. ~A~
desc~ibed in detail below, ~uFaces OI the valll der Waals channel~ ~ ~he
electrode material are capable OI fo~ning a double layer with an
electrolyte iIl e~ac~ly ~he same manner a~ t~e electrode macloscopic
surface fo~ a double layer. ~cog~ n and e~ itation of thi~ physical
15 process ha8 enabled th~ i~Yen~or~ herein to achieve the dramatic energy
storage capabiLi~y ~f the capacitor o~ t~e invention.
The iD:velltor6 herein have reco~zed that a particular type of
~ter~lation compou~d, Ilamelybismuth ~halcongenide~, L~cluding Bi2Te3
and B~2Se3, are p~cularly well-suited for pro~ g va~ der Waals
20 cha~nels a~ an e~n~ion of electrode ~urface ar~a. Elecl rodes co~po~ed
of these material8, whe~ used iIl combination wit;h a suitable elect~olyte,
ge~erate a highly ~o~ double layer of a de~irable structure. ~ is
well-know~ to 1 ho~e ~killed i~ art, bismu~h chalcongemde~ e~ibit a
layered cry~talline ~tice wbich i~ layered at the molecular level, each
25 layer being ~eparated by a van der Waals ~nel havi~g a widl h on the
order o 34 ~. :IFur~er matenal proper~ie~ of bismu1;h c~alcogenides are
given in 1~e copending Ul~ited State~ Patent Applic~on en1;i1~ed aLayered
C~ lline Material Capable of High Guest Loading," herein in¢orporated
by reference, being filed on ~he same day as $he present ~pplica~;ion. Of
30 the mate~ials ~urveyed, the inventors have found th~ of the bis~ut;~
chalcogenides, Bi2Te3 e~ibits 1~e best elect;rical conductivity, and i& t~lU~;
most preferable as an electrode material, while Bi2Se3 e~ibits a lower
wo g3,095~2 2 1 2 2 3 ~ ~ PCI/US92/û9244
conducti~ity, and thu8 i~ le88 pr~ferable as an electrode mate~ial.
The ability to manipulate ~he bi~mut;h chalcogenide~, and indeed
any layered intercalatioIl materi~, for employing their vaIl der Waal~
channel~ to increase electrode surface area, i8 dramatically ~mpacted by
the puri1~y and defect density of'Lhe cho~en material. Impurities and
cry~tal lattice defect~ distort the g00metry of the van der Waals channels,
re~dexing them le~ accessible to i~tercalati~g species, degrading the
chaDnel 6u~face ~tructure and thu~ degrading 1~he electric~ d
mechanical properties of ~he channel~. Accordi~gly, it i~ ideally preferred
tha~ t~e mate~i~l cho~en ~or the capacitor elec~rodes be prepared U~ g
~que processe~, developed by the inventors herein, yielding a bighly pure
and as defect~ee as po~sible monocrystalline mater~al. To th~t end, the
following siDgle ~y~tal gro~th proces~ i~ preferred for bismu~h
chalcogeDide matel'i~B. Alte~native processes, prov~dingles~ than ideally
pure and d~fe~-f~ee material, may nonethele~s be acoeptable for pa~cular
capacitor applicat;ion~. Tho~e sk;lled i~ e art will recogI~ize critieal
matenal paramet~r~ ~nd correspondiDg perfor~nce re~ult~.
II1 t he p~d inter~alat;ion compound preparation proce~,
stoi~me1;ric qu~ti1;ie~ of hig~ly purified ~99.9999% pure) bismu~ d
tellunum (or ot~er ~elected cllalcogenide) ar~ t charged irl~o a ~uartz
ampoule. IiP neceasary, the maten~6 are zQne ~efined be~ore use. Of~-
~toi~iomet~ re~ in ~ n- or p~oped ~terial wi1h the resultant
degradatioIi of the latt i~ ~t~ucture and t;he associated per~o~ ce. The
ampoule i8 evacuated to 10 7 mmHg aIld ba~filled to ~ pres~ure of 10
mmHg wit~ a small amou~it of iDert gas, ~uch a6 argorl, or a redu ing gas,
~ueh a~ hydrogsn (3-10 c~rcles), and ~hen ~ealed. Hydrogen i8 partiCll~ y
pre~e~ed because it rea~ with o~rgen duri~g proCeB8ing to prevellt
o~:idatiorl and decrease tbLe segregation of ~halcogeDide by r2duci~g its
vapor pre~ure.
A highly homoge~eou:~ polycr~rstalline material iB prepared iIl a fir~t
prl)Ce88iIlg 8t¢p. The ~ealed ampoule is placed in a fi~rnace ~ room
temper~ure and heated to a temperature ~-10C above its mel~ng point.
WO 93/09552 2 1 2 2 3 5 ~ P~/l~S92/09244
- 7-
The ramp rate, temperature and reaction time are dependent upon the
f;nal compound. The reaction condition~ are listed in Table I for l~he
prepara~on of polycry~t~line Bi2Sa, Bi2Se3, and Bi2Te3. The temperature
of the filrnace over 1~he entire length of ~he ampoule is coIltro~led to within
5 ~0.5C. Careful and accurate control of ~he temperature is important
because of the high volal~lity of chalcogenides. Temperature varia~ons
along the ampoule length causes segrega1 ion of chalcogenide which leads
to off-~toichiomet~. To optimize the temperature control along the length
of the ampou~e, a long furnace c~n be used. Additional heating coil~ can
10 be u~ed at fu~ace ends to reduce the temperature gradient at the furnace
e~
Table I. Proce88ing condit;ions for polycry~talline mat~rial.
Processin~ condi~on~ Bi?T~ Bi?Se~ Bi~S~
hea~grate to Tliq (C/h) 30 20 ~
exposuretime (h) 10 16 20
at Tljg ~ 10C
cooliIlg rate (~/O 50 40 36
2~
During the last hour of reaclion ~me, ~he ampoule is agitated or
vibrated to in~ure complete ~g of the ampoule compone~ts. The
ampoule vibration is in t;he r~ange of 26-100 Hz and is accompli~hed by
2~ g one e~d of ~e ampoule to an oscilla~on 80urce. AI1Y coIlve~tional
vibral;ioll mea~ is c~ntemplated by the ple~ent inYen~on. Afl;er react;ion
is ~omplete, ~e ampoule is cooled at a ~low controlled rate.
Once a homogeneous polyGrystalline material is obtained, it ca be
fur~er proce~ed illtO a highly defec~ee bismuth chalcogPnide single
30 cry~tal. Any known me~od ~ growi~g single crystal~ caD be u~ed, ~uch
as Bridg~man tec~ques, Czolchralski process and zonP refiDement
~recrystalliza~on). In particular zone refineme~t ha~ provea to be ~ghly
ef3Eeclive in obtaining high purity single CI~BtalB.
Zone refinement is preferably ca~ied out in a quartz boat
35 conWning a seed cTystal of the desired lattice 6tructure, e.g., the
W093/0~!iS2 21223~ PCr/US92~09244
- 8-
hexagonal lat~ce ~t~cture.. It is recommended that clea~ rooms levels of
Class lûO be maintained. The seed crgstal is o~ented in the boat ~uch
that ~rystal layer~ are horizo~tal. The e~tire apparatus ~hould be shock-
mounted to i~ulate against environmerltal vibrations. The boule of
polycrystalline material i~ positioned in ~urface contact with the seed
crys~.
The furnace comp~ises two parts, an outer fu~ace ~or ~ntaining
an e~evated temperat;ure along the entire boule length and a narrow zone
movable in a direction for heat~g a small porl~ion of the polycrystalline
10 mate~al. The ouhr fu~nace is maintained at 35C below the melting
poin~ and ~he zone, which is 2-3 cm in leng~h, is held at 10C above ~he
mel~ng point OI the polycrystalline material. U~like for the preparation
of 1~he polycrg~talline materia}~ i~ the fir~t processing ~tep desclibed above,
the boule can be rapidly heated to the opera~ng temperature. The zone
15 is initi~ly positiolled at ~he seed cry~oule interfiace and this region is
heated to the mel~g point of the mate ial. The zo~e is moved 810wly
down the len~ of the: boule. Zone travel rate vane~ with composition,
and exemplary ra~es are shown, aloIlg with other processing parameters,
in Table II. ZoIle tr~rel rate i~ an important proceBslI~g parameter. If it
20 is too great, c~y~zati~n is ~ncomplete and defects are ~ormed. If it is
too ~low, layer di~ on~ result. The low~r portio:n of the heat-treated
boule ~ conta~ with the quartz boat is preferably removed before use.
~able II. Pro~8~111g condition~ Ior h~ago~ ~ingle c3rystal
gro~
proce~sin~ ~onditions ~3 ~2~3 B~S~
boule te~pera~are M - 35C Mp - 3~C Mp - 35C
zone temperature Mp ~10C Mp ~ 10C Mp + 10C
zo~e t~avel rate 8 mm/hr 6 mm/hr 3 mm~hr
coolirlg rate 50 C/hr 40C/hr 35C/hr
The abo~e process can be modified ~lightly to produce crystals of
35 ~hombohedral st~ucture, in which case a rhombohedra~ seed crgst~ is used
wo 93/og552 2 1 2 ~ ~ 5 ~ PCr/US~2~092~4
in the zone refillement proce~s. In addition, to obtain rhombohedral
crystals, the fu~ace temperature is held at 30C below the melting point
and ~he zone is main1;ained at the mel~ing point of 1~he poly~talline
material.
Prefer~bly, a monocrystallirle intercalation compound, and most
preferably, bismu~} chalcogenide, is grown u6ing the process described
abo~e to produce monocrystalline electrode structure~. For example, as
one embodiment OI the invention, monocrsrstalline bismuth chalcogenide
electrodes are produced having a rectangular geometry with side~ of 4
millim~ter~-long and 5 millimeters-long, and having a tbickness of
between 0.5-1 millimeter~. It is preferable to n~etalize one of the faces of
the moIrocry~t~e material which is perpendicular to the plane OI the
van der Waals ch~els within the crystal. T~is metaliza~on may consist
of, for e2~ample, a nickel pa~te, whi~h i6 ~pread on the cry~tal to fonn a 10-
20 microIl-thick metal layer. ~he metaliza~on provides both a good
electrical contact to the crystalline pie~e and eDhanoe~ the rigidi1 y of 'che
~ys~e piece.
Alte~nat;iYely, the monocry~talline bi~muth chalcogeDide material
m~y be ground illtO a powder for fo~g the electrodes; ~uch a powdered
m~te ial is more ea~ily manipulated than the 6ingle c~sW m~terial. The
~y~t~l ~i~di~g process may be ~ed out u~ing, for e:l~ample9 a ball
millillg device, or other grinding device, to produce single crystal p~ticles
ea~ llaving a diameter o~ preferably ~ppro~imately 70 micron~. Other
par~le diameters may be more preferable in ~pecific in~tance~. ~he
cry~tal par~c~e~ are then mi~ed with an app~op~iate sompouIld to bind
them toget;her. ~e the~ bi~der acts, in e~ect, to ~glue" ~he par~cles
~ge~er~ ;t must not compl~tQly elect~ically in~ulat0 the par~c~e~ firom
ea~h o~her. The binder material i~ selected according ~o ~he elect;rolyte.
When an apro1 ic elec1 rolyte ~olvent is used, the binder pre~erably co~ ts
of a 3% aqueous solu~ion of car~o~ymethylcellulo~e, in which the particles
are ~ed; ifior other electrolytes~ alteInative bindiDg agents, e.g., a ~%
poly~t~ylene dispersion in normal he~ane, may be used. The resulting
wo 93/os~s2 212 2 3 S S Pcr/uss2/os244
- 10-
powder-binder mi~ture is placed into an electrode mold and then dried at
room temperature. The electrode geometry, as determined by the mold,
may be, ~or example, di~c-shaped, as is conventional for capacitors, with
an electrode thickness of between 0.3-1 millimeters. Altemative electrode
5 geometrie~ are also feasible.
The grir~ding process described above produces some amount of
crystal damage, and corresponding crystal defects. However~ because of
the weakness of the van der Waals att~active force between the cry~tal
l~yers of intercalation compou~ds, these compounds cleave readily along
10 the a~is of the channels w~thout much dangeI of lattice damage or
distortion.
Electrodes formed u~ing the process described above may b~
employed in any of a ~ariety of capac~tor configurations. ReferIing to Fig.
~A (not ~hown to scale), in one configur~l;ion, a capacitor 10 including a
1~ bismuth chalcogenide electrode 20 is constructed as follows. The capacitor
electr3de 20, whether COI16iStiIlg of a monoc2gstalliIle piece OI a molded
cry~tal powder, iæ located in contact with a ~elected electrolyte 30,
~upported by aIl ele~ri~ally conducting container 35. Ideally, ~his
eoIlducting co~tainer is co~po~ed of a~ ideally rlonpola~izable material.
20 A pow~r ~upply 40, ~u~h a~ a batte~, is electrically co~nected to t~e
electrode 20 ~ia a con~u~r 45, ~uch as a wire, and colTe~pondingly i~
~o~ec~ed to the condu~g container 35 via a ~imîlar ~du~or ~0.
The electrolyte 30 suita~y consists ~ an aqueous ~olution of, e.g.,
~9 or preferably, 1.0 M of L~Cl04 ~ propyle~e ~r~onate. A ~eparator,
2~ consi~ g of 2 layers of non wov~n polypropyleIle, each layer 100 Flm-
thick, ~d ~aturated wi1~h the electrolyte, pIo~ide~ mechanical support of
the electrolyte. Alte~atively, for various ele~rode materials, ~e
electrolyte ma~ COlIlpl'i8e8 a 1.2 M ~olution of orga~ic ca~on of perchlorate
in a mi~ture of propylene carbonate iIl dimetho~yethane, aIl aqueous
30 601ution of po~assium hydroa~de, an aqueous sol~tion of single valen~
metal sulphates, or other aqueous ~olution.
Referring also $o Fig. lB, in a second capacitor configur~tion 60, two
WO 93/Og552 212 2 3 S 5 - PCr/US92/092M
- 11-
identical bi~muth chalcogenide electrodes 20 are ~eparated by the
electrolyte 30. Using the LiCl04 propylene carbonate electrolyte di~cussed
above, a p~ypropylene separator is ~uitably impregnated wi~h the
electrolyte fiolu~io~ and i8 positioned between the electrodes 20. The
5 electrodes, held apart by the separator, are 'Lhen inserted into a supporting
frame (not ~hown) and ~ealed in a pressing foIm. The power supply 40 is
electrically connec~ed to each of the electrodes 20 ~ia similar conductors
45, e.g., good co~ducting wires.
Before an elect~olytic capacitor ha~ring electrodes of an intercalation
10 compouDd, preferably bismut~ chalcogeI~ide, csn provide increased ~urface
area when operated a~ a capacitor, the ~ismuth chalcogeDide va~ der
Waals ~hannels mu~t be ~pulated, or Utrained", to provide the e~tended
~urfaces. As e~ ed in the discus~ion above, the van der Waals chAnnel
surfaces, after b~ing trained, can form a double layer with the electrolyte
16 in a manner similar to ~Lhat in which the electrode macroscopic surface
fonns a double layer. Acco~gly, ~traiDingn i8 aprocess, described beloYv,
whereby electrolyte (and ion~) are driveIl wi~ the van der W~s
cha}~el~ to facilitate flow of electrolyte iIltO and out of the c~aImels.
Referring to Fig. 2A, 1here is shown a cap~citor 60 hav~g two
20 bi~u~ alcogenide electrodes 20a, 20b at the gtart of ~e trai~ing
proces~. The dimen~ion~ of the elec~rodes' van der Wa~s ~hannels 70a,
70b ~e greatly e~aggerated for clarity, and it must be recalled t~at each
electIode is compxi~ed of on the order of 106-107 8uch ~bannels. Between
t~e two electrodes i8 po~itioned a I~Cl04-based elecl;rolyte 3Q. During the
25 tr~g prOCefiS, 1 he power ~upply 40 is set to provide a voltage w~ich is
greater than 1 he faraday potential for cat~on i~terc~lation, ~d t;hus the
voltage depend~ &ec~ly on the p~rlicular combin~tioIl of capacitor
electrode mater~al and electrolyte employed. Given a par~cularly chosen
electrode-electrolyte combiIlation, those skilled in the art will recogni~e
30 ~hat the correspo~ding faraday potential may be determiDed iD a standard
table of m~terial sy~tems and faraday voltages.
At ~he start of the electrode training, when a voltage above the
wo 93/095s2 2 1 2 2 3 ~ 5 Pcrtusg2/og~
- 12-
faraday voltage i8 applied to the capacitor, the electrode 20b coImected to
the positive terminal of the power ~upply accumulate~ a positive sur~ace
~harge. The surfaces of the van der Waals channels 70b of the electrode
likewi~e accum~late this positive sur~ace ch~rge. Co~espondingly, both
the macro~copic ~ ace aIld the su~aces of the van der Waals channels
70a of t~e electrode 2ûa coImected to the negative terminal of t~e power
supply accumulate a negative surface charge.
In re~pon~e to thi~ sur~ace charge configuration, ~ee Li+ ions 72
readily intercalate the negatively charged electrode 20a, because of the
10 favorable charge and energy co~ ration, and becau~e their io~c radius
i8 relat;ively smaller than the width of the van der Waal~ c~annele. In
addition, solvated Li~ complexes 74 move toward the nega1;ively charged
electrode ~ ace and solvated Gl04- comple:~es 76 move toward the
po~itively charged electrode 6urface. The po~itively eharged electrQde's
15 van der Waal~ channel~ 70b, bsi~g 3-4 ~-wide (as o~g before the
tra~ning proces~ too ~all for the C104- comple~:e~ to penetrate wit~in
th~m, the ~olvated Li' comple:~es, however, do to a small deg~ee penetrate
the 3~ ~-wide cha~els 70a of ~e negatiYely charged electrode 20a,
e~eetively beiIlg traIl~po~d along wi~h the firee Li~ ioIls to the electrode
20 s~ace ~d wi~ t;~e electrode cha~els. As a re~ult, the ~olvated Li+
eomplexe~ ~lightly wide~ the channels that th~y partially e:~lter in the
negatively ~arged elect;rode.
I~ order to cau~e the ~olvated Li+ comple~eæ to peIletrate the
opposite electrode 2~b, ~he polan1 y of the power supply i5 rever~ed. TheIl,
26 $he accumulated ~urfiace charge di~ ution re~erses; the previous~y
positiYely charged electrode now a~ula~es negative ~u~face charge, and
attractæ the firee Li~ io~ 72 and ~olvated comple~es 74. The firee Li' ions
72 ~adily intercalate the ~hannels and the solvated comple~es 74 again
par~ally enter the co~respondi~g vaIl der Waals channels, and thereby
30 ~lightly widen ~he cha~els.
Refemng to Fig. 2B, repetition of tbis process of volt~ge polarity
switc~ing progres~ively widens the van der Waals channels of each of the
WO 93/0~52 2 1 2 2 3 S ~ PCr/~1~92/092A4
- 13-
electrodes 20a, 20b. Throughout the proce~s, the voltage may be
inc~eased, depending on the init;ially applied voltage, to thereby increa~e
the a~traction of the ions and electrolyte to the van der Waal~ cha~els.
At an interInediate point in the training proce~s, as depicted in the figure,
the ~olvated Li~ co~plexe8 74, as well as the free Li' ions, will be able to
completely penetrate the widened channels 70b of the electrode 70b which
is currently negakively charged. The solvated Cl04- complexes, being of a
larger ~ize than the solvated Li~ complexes, will not yet be able to
completely penetrate t~e channels of the cuITently po~itively charged
10 electrode 70a, however.
A~ ~he end of the ~g process pe~iod, refemng to Fig. 2C, both
the solvated ClO;comple:~es 76 ~nd t~e solvated Li' comple~ces 74 are able
to completely penetrate the van der Waals channels 70a, 70b, of bo~h
electrodes, 20a, 20b. As ~hot1vn in Fig. 2D, at t~is time, electlically neutral
lF~ electro~yte (inclu~g both ClOi comple~es 76 and Li' comple~es 74) is
thereby able t~ completely penetrate 1he va~ der Waals channels and
create an elec~Iic double layer of charge 80, 82 and 84, 86 at 1~he electrode-
electrolyte interface l~roughout the Yan der Waals chamlels of each
electrode, iD a ma~ner f~nilar to t~at which occur~ at ~he macrv~copic
20 surface of the elec~odeæ. This pe~etration of elec1;rolyte thr~ughout t~e
crystal channel~ fo~ns the bagis for achieving the ~ignificant capacitance
aIld energy ~tora~e in~ea~e~ provided by the inveIlt;ionO
The extent~vf 1~imng re~ired to achieve penetratio~ of ~e
electrolyte and its ~o~vated ioDic specie~ within t~e electrQde~' van der
2~ Waal~ chaImel0 i~ ~itically depe~dent on the par~cular combination of
electrode ~terial and electrolyte employed. The width of the ele~trode
van der Waals ch~el~ before undergoi~g aI~y trai~ing proceæs and the
radiu~ of t~e ~olvated complexe~ i~ the electrolyte determine the training
required~ the larger ~he radius of 1he comple:~es aIld the ~maller the van
30 derWaals channels' width, the longerthe training ~me requirement. For
the electrode material Bi2Te3 and an electrolyte based o~ LiCl04, the
~raining preferably COIlSi8tS of about 20 training cycles of appro~imately
WO 93/09552 PCI`/US92/092~4
21223~5
- 14-
30 minute~ each, where the polarity of ~he power ~upply is reveræed with
each cycle. For ~pecif;c capacitance requirements, ~is t~ ~ing may be
adjusted, howeYer. With less training, a lower degree of electrolyte
peIletration wi1~hin the channels would be achieved, and a correspo~dingly
lower double layer capa~tance would re~ult. Thu~, for achie~ing the
maximum po~sible capacitance of a given electrode, the traiI~ing should be
maDmized. Tho~e skilled in the art will recognize 1~hat a preferable
training procedure may be empirically determined for a given electrode-
electrolyte comb~nation aIld capacitance goal.
Alternative tr~g processes are within the intellded ~cope of the
in~0ntioll. For e~ample, t~e voltage polarity may be ~t~ined constant
in the above prooegS, or a charge-discharge process may be employed to
widen t he vaIl der Waal6 channels. In such a process, a voltage above the
faraday pote~tial iB applied between the electrode~ in t~e manner
15 discussed above, for a period of time, and then t~e capa~itor is discharged
acro~ an approp iate load. If t he voltage pol~t y is ~tained constant
during thi~ procea~, or if t~e ~oltage polarity is not ~witch~d d~g t;he
unC proces~ t de~cribed, one ~f the elect;rodes may not achie~e
widened ~el~, depending o~ the electrode ma~enal ~nd electrolyte
20 compo~ition~ For example, u~ing Bi2Te3 electrodes and a ~(~104-based
electrolyte in a 1~ing procedure in w~ich the voltage polarity is
~nstant, t~e el~ctrode having the nega'dve pola i~y will be intercalated
~th iEi ee and s~lvated I~ io~s (and thereby accommodate electrolyte), but
~e electrode of po~i~ve polarity will not have the benefit of fiee and
~5 solva~ea I~ ions be~g to open its latlice ~ha~els, and t}lUS the
solv~ted ClOi ions will not widen t~ose chamlels to accommodate
elec~rolyte; as a re~ult, the electrode of positive polari~y will not provide
the ea~ ded vaIl der Waals ~urfaces. It must be noted that a capacitor
o~ ~e de~ign U~iIlg a ~ingle intercalation compound-electrode (Fig. lA) is
30 also trained using the techIliques described above. A process of voltage
application and voltage polarity reversal will intercalate firee and solvated
Lif ions aIId solvated Cl04- ions in the elec~rode, thereby providing the
WO 93/09552 ~ 1Z23 S ~ Pcr/US92/û9244
- 15-
ability to accommodate electrol~e wit~in the electrode and achieve ~e
desired electrode surface e~tension.
Of p~icular importance is the fact that the training proceæs does
not defo~rn or distort ~e crystal planeæ of the layered cryfitalline electrode
material to any si~icant extent. The extent of crystal plane deformation
i6 related to the starting purity arld de~ect density of the electrode
material, as well as other propertie~ resulting from the grow~h proces~;
fewer initial de~ects in the crystal result in fewer cryætal plane
defo~nakion ~ites caused by the ~raining. With lit~e or ~o cry~tall~e
10 plane distortion at ~e end of trai~g, the electrodes' van der Waals
channels ret~ t he ability to be easily penetrated by ~he electrolyte ions,
and can correspon~gly develop a double layer in a short ~me period.
Also of importance is the act that the tr~g process widens the van der
Wasl6 cha~els beyond ~eir elastic limit; the channels thus do not later
15 shri~ to a smaller dimension.
ReiEemng now to Eig. 3, the capacitors descn~ed a~ove are
elect~ically modelled a~ a cir~it sa with t;he appli~d voltage 40, L~cluding
a firBt capacitor 92 and a ~econd capacitor 94, ~eparated by a resi~tance
96. The resistance 96 i8 that of the electrolyte, and i8 1~7pical1y about
20 0.003 Q. For the ~gle interc~lat;ion compound-electrode capacitor (Fig.
lA) the fir~t capacitor 92 corr~ponds to the double layer capac:itancR of
that etectrode 209 w~ile the seco~d capacitor 94 corre~pond~ to the
capaci~r~ce of ~he elect~ lly conducting co~t~iner 35. In pract i~e, as a
re~ult of the co~t~iner material, thi~ capacitance is many orders of
2~ ma~tude lower ~han 1hàt of the electrode 20. A~ a re~ult, the ~eries
capaMtance of ~he two capacitors is ~wamped by the smaller capacitor 9~.
Accordi~gly, the dou~le inter~alation compou~d ca~acitor ~Fig. lB~ is ~he
more preferable scheme; here 1;he t~vo capaGitor~ 92, 94 represent ~e
double layer capa~itances of the t~;vo electrodes 20. If each electrode is
30 idelltically cons~ructed, thereby e~hibiti~g th~ same capacitance, the
overall ~eries capacitallce of ~he capacitor is magimized.
Double electrode capacitors of the design and mate~ials described
WO 93/09552 PC~/US92/09244
2122355 16-
above have been made and e~hibit betwe~n 30-100 farads per cubic
cen~meter and an in~ernal resistance of appro~imately 0.02 Q/cm2. This
e~tremely low illternal resistance provides t~e ability to achieve high
power in ~he capacitor discharge. Theoretically, a monocrystalline
5 capacitor stl ucture of plLre aIld defect free bismuth chalcogenide would
exhibit 1000 farads per cuWc centimeter. Double layer capacitor~ having
Bi2Te3 electrodes haYe been charged to 2.6 volts and observed to exhibit no
specific energy degradation for up to 1000 cycles. Table 3 below tabulates
the ~pecific energy of ~his capacitor for corresponding elecl~rolytic solu1 ions.
TABLE 3
Electrolyte Solution ~oncentration Specific Energy
Mole Part J/cm3
0.~ M LiCl04 in PC 70
1 M LiCl04 in PC 105
1.5 M LiCI0,~ in PC 98
Other embodiments of capacitor mS~terial8 and trainiDg schemes are
intended a~ ~led witbin the ~pirit and ~cope of the invention.
What is claimed i8:
,
:~:
