CA2122355A1 - Electrolytic double layer capacitor - Google Patents

Abstract

A double layer electrolytic capacitor of two electrodes each in contact with a common electrolyte. At least one of the electrodes is comprised of a crystalline material characterized by the presence of van der Waals channels in the material. These van der Waals channels are adapted to accommodate the electrolyte within the channels, such that a double layer of charge is formed at interfaces of the van der Waals channels and the electrolyte when a voltage is applied between the two electrodes.

Description

W~ 93~095~2 2 1 2 2 3 ~ 5 i PCr/l~Sg2/09~

~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~

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 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 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 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 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

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~

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 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 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 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 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.

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:

, :~:

Claims (22)

1. A double layer electrolytic capacitor comprising two electrodes each in contact with a common liquid electrolyte, at least one of said electrodes comprising a crystalline material characterized by the presence of van der Waals channels, the van der Waals channels being adapted to accommodate the electrolyte within the channels, whereby a double layer of charge is formed at interfaces of the van der Waals channels and the electrolyte when a voltage is applied between the two electrodes.

2. The capacitor of claim 1 wherein both electrodes comprise said crystalline material.

3. The capacitor of claim 2 wherein said two electrodes each comprises a monocrystal of said crystalline material.

4. The capacitor of claim 2 wherein said two electrodes each comprise monocrystalline powder particles of said crystalline material.

5. The capacitor of claim 2 wherein said crystalline material is a bismuth chalcogenide.

6. The capacitor of claim 2 wherein said crystalline material comprises a solid solution of Bi(Te3ySey), where x is 1 or 2, and y is 0-3.

7. The capacitor of either of claims 2 or 5 wherein said electrolyte comprises a 1.0 M LiClO4 solution in propylene carbonate.

8. The capacitor of either of claims 2 or 5 wherein said electrolyte comprises a 1.2 M solution of organic cation of perchlorate in a mixture of propylene carbonate in dimethoxyethane.

9. The capacitor of claim 2 wherein said electrolyte comprises an aqueous solution of potassium hydroxide.

10. The capacitor of claim 2 where aid electrolyte comprises an aqueous solution of single valence metal sulphates.

11. The capacitor of claim 1 wherein one of said electrodes comprises an electrically conducting container in which said electrolyte and said other electrode are positioned.

12. The capacitor of claim 4 wherein said monocrystalline powder comprises of monocrystalline particles of approximately 70 microns in a largest dimension.

13. The capacitor of claim 12 further comprising a binding agent for binding together said monocrystalline powder particles.

14. The capacitor of claim 13 wherein said binding agent comprises 5% polyethylene dispersed in acetone.

15. The capacitor of claim 13 wherein said binding agent comprises a 3% carboxymethylcellulose solution in water.

16. The capacitor of either of claims 2 or 5 wherein said van der Waals channels are adapted to accommodate the electrolyte by a training process comprising intercalation of the electrolyte into the van der Waals channels.

17. The capacitor of claim 16 wherein said intercalation is produced by the application of a voltage between said electrodes, said voltage being sufficiently high to achieve solvated ionic complex penetration of said channels.

18. The capacitor of claim 17 wherein said voltage is periodically reversed in polarity between the electrodes.

19. The capacitor of claim 18 wherein said voltage is increased over time from a first voltage sufficient to produce faradaic processes in the electrolyte to a second voltage sufficient to achieve electrolyte penetration of said channels.

20. The capacitor of claim 19 wherein said voltage is applied to said electrodes for approximately 600 minutes.

21. The capacitor of claim 20 wherein said voltage is reversed in polarity approximately every 30 minutes.

22. The capacitor of claim 18 wherein said capacitor is periodically discharge across a resistive load.