CA2122369A1 - Layered crystalline material capable of high guest loading - Google Patents

Abstract

A crystalline material capable of high guest loading greater than three and exhibiting independence of the Gibbs free energy function from guest concentration and a method for its manufacture is provided. The crystalline material may belong to the class of MeyChz, where Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, y is 1 or 2 and z is in the range of 1 to 3, such that the material has a defect density which is sufficiently low to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of said material without significant distortion of the lattice. An intercalated material with guest loading up to 10 mole/mole host is provided.

Description

WO 93/08g81 PCI`/US92/09243 - 1- 2 1 2 23 fig 5 Layered Crystalline Material Capable of High Gue~t Loading This application is a continuation-in-part application of U.S.S.N.
07/784,525 filed October 29, 1991.

Back~round of the Invention l~e present invention relates to highly defect-free mon~
crystalline materials capable of intercalating high levels of guest species :~
and a method of mal~g the same. The present invention further reaates to a bigbly interc~ated layered crystalline material.
Layeret ~y~ne mate~s (hosts) can accommodate foreign species (guests) i~ the channels between the layer~ thereby form~ng interullated mat~rials. Interc~ation compounds, such as graphite and -`
transition ;met~l ~halcoge~ides, have been il~sively investigated over ; the last two decades and have found ulility in a number of fields, ~cludinl~ batteries, photovoltaic ceals, superconductivib and hydrogen `:
accumulation.
The hosts of interest to this application have aDisotropic lamellar structures ~ep~ated by van der Waals ~hannels ~hich are cap~ble of ~: - acoommodating small guest parti~les, such a lithium or hydrogen.Because of the wea1~es~ of a van der Waals attractive force, the van der Waal~ Ch~l~lB CaD read;ly accept guest l~pede8, whi~h can be neutral or ~harged. l~he guest species fill 1 he chamlel until no more sites are energetically available. Ihe amount of guest ~pecie~ whicll occupy sites in 1 he van der Waals channel i8 the loading capacity of the guest. nLoadi~g" or ~oading capacity" iB defined herein a6 moles guest specie~ per mole host material; "Bpecies iB defined as ions or un~harged atoms of elements. It is recognized that for many applications, increased loading capacity of the guest into the layered material would ~gnific~Dtb impro~e denoe perfonna~ce.
Di~rtion of a ia~ered crystalline material has been u~ed to incre~e the channel width and thereby increase guest loading. U.S.

WO 93/08981 PCI`/US92/09243 212236~ 2-4,288,508 reports a catbode active material for a battery having the preferred formula I~NayTiS2, where y is in the range of 0.15-0.20 and z can be as high as 3.25. However, increased loading in these materials is accomplished by using the larger sodium atom to pry open the van 5 der Waals channel6. This results in significant distortion of the TiS2 lattice.
U.S. 4,309,491 reports a solid solution contaiDing a bismuth halcogenide for use as a cathode active material. The guest loading process in the bismuth chalcogenide i8 reported to require up to six 10 Faradays of electrons, suggesting that sis moles of guest are intsrc~ated. Ho~rever, it is also reported that 1 here is substantial dependence of the discharge voltage on the guest conc~tration.
Additionally, there is no indication that the bismuth chalcogenide has been prepar~ed in a highly defect-free form with controlled lattioe 15 parameters.
Accordingly, it is the object of the present invention to provide a mono-crystalline material capable of intercalating estremely high loads of a guest species in which a ~ange in the Gibbs free energy (~G) of the material is s~b~tially independent of intercalant conoentration.
20 Under these conditions, properties dependent upon ~G, suc~ as working potential of an electrochemical cell, remain unchanged.
It iB a further object of the invent ion, to provide a hig~ly i~tercalated crystalline material having controlled lattice parameters.
It iB yet a further object of 1 he invention to provide a method of 26 making a hig~ly defect-free mon~crystalline material with controlled lattice parameters capable of high gue~t loading.
Summar ,r of the Invention The present invention overcomes the limitations of the prior art by providing a highly defect-free material with controlled lattice 30 characteristics. When used in devioes such as a battery it provides superior perfonnanoe over prior art devices.

WO g3/08g81 PCI ~US92/09243 -3- 2122:369 In one aspect of the invention, a layered mono-crystalline material i6 provided having a defect density whicb iB sufficiently low to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of the matenal without significant distortion of 5 the lattice. The material iB further charactenzed in that its change in Gibbs free energy is substantially independent of the lithium intercalation concentration.
In another aspect of the present invention, a mono-crystalline material having the formula, Me"Ch~, is provided where Me is selected 10 from the group consisting of Bi and Sb, Ch i8 selected from the group consisting of Te, Se and S, y is 1 or 2 and z iB 1, 2, or 3. The material has a defect den~ity which u sufficiently low to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of the material without sigl~ificant distortion of the lattice. The 15 material is further characterized in that ~G is substantially independent of the lithium intercalation conoentration. FuIther detail conoerDing intercalation compounds and devices using these compounds are given in the follo~nn;g applications, filed on equal date as "Layered Crystalline Material Capable of High Gusst Loading", and incorporated 20 herein by reference: '13nergy Storage Device~, ~Electrolytic Double Layer Capacitor, and "Capacitive The~moelectric Device.
In preferred embodiments, t~e material has a hexagonal crystal lattice structure, a defect density of less than 10'2/cm~ and a gradient of impurity defect disttibution in the van der Waals channel inversely 25 proporlional to the direction of lithium intercalation. The mono-crystalline mate~als can be further characterized in that a condensation of lithium occurs at a loading capacity of approsimatQly three. The condensation is from a lattice ga6 to a quasi-liquid. The lattice ~trucl~re may be rhombohedral or hegagonal. The mono-30 crystalline material is preferably a single crystal. In preferredembodiments, the defect density is minimized and perfection of the lafflce crystal iB accomplished by using careful processing controls. In WO 93/08g81 ~ 1 2 2 3 6 9 PCI /US92/0924?~

other preferred embodiments, the mono-crystalline material is used in a battery, a capacitor or a thermal electric device; although the mono-crystalline material may be ground into a powder for some device applications.
In another aspect of the invention, a method of preparing a highly purified bismuth chalcogenide consists of the following steps: an ampoule is charged with stoichiometric quantities of a chalcogeDide element and bismuth; the ampoule is provided with an atmosphere selected to prevent osidation and sealed; the sealed ampoule is heated at a temperature in the range of 5C to 10C above T,j9 of the bismuth ~halcogenide, the temperature i8 controlled to within a range of ~ 0.5 along the length of the ampoule and for a time sufficient to melt the component materials and react to form the bismuth chalcogenide, whereby the~ ampoule iB agitated during heating to ensure homogeneous mi~ing of the component mate~ials; the material i8 cooled to room temperature at a controlled rate to form a homogeneous polycry~talline bismuth chalcogenide; the polycrystalline bismuth chalcogenide is placed in surface contact with a seed crystal of a specific.crystal lattice structure; the seed crystal and polycrystalline bismuth chalcogenide are heated to a temperature which is in the range of 30C to 40C below T,j9 of the bismuth chalcogenide; a zone of the polycrystalline bismuth chalcogenide adjaceIlt to the seed cry~tal i8 heated to a temperature which is in the range of 0C to 15C above T~jq of t~e bismuth chalcogenide; the zone is moved along the length of the polycrystalline chalcogenide at rate in the rangs of 2 to 10 mm/hr, whereby a single cry~tal highly defect-free bismuth chalcogenide is fo~med; and the single cry~tal bismuth chalcogenide is cooled to room temperature at the controlled cooling rste.
In another aspect of the present invention, a highly intercalated cIy~talline material i8 provided having a guest capacity in the range of three to ten moles within van der Waals channels per one mole of said intercalated matenal without significant distortion of the lattice. The wo 93/08981 2 1 2 2 3 6 9 PC~/US92/0924~

guest selected from the group consiæting of Group IA and Group IIA
metals. The intercalated material is further characterized in that ~G is substantially independent of the guest concentration. The intercalated material may bave the formula, G~MeyChz, where G is selected from the 5 group consi6ting of Group IA and Group IIA metals, Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S"c is in the range of 3 to 10, y is 1 or 2 and z i8 in the range of l to 3.
Brief DescriPtion of the DrawinP
In the Drawing, Figure 1 is a schematic illustration of the localized sites in the van der Waals ~hannels for the crystal lattice structures used in the present invention;
Figure 2 iB a cross-sectional schematic illustration of the zone-refinement apparatus used in preparing single crystals of the present invention; and Figure 3 is a discharge curve of a battery using the crystalline màterial of the present invention.
DescriDtion of the Preferred Embodiment The present invention has identified that highly defect~free crystalline layered materials with appropriate impunty distribution are capable of intercalating high loads of guest species into van der Wa~s channels of the mate~aL
Structure. A family of highly defect-free compounds of the fonnula, BiyCh~, where Ch i~ Te, Se or S, y is 1 or 2 and z is in the range of 1 to 3, have been identified which permit e~ctremely high loading capacity of the guest species, well beyond the loading capacity conventionally predicted by the lattice structure of the crystal and the model of the lattice gas. Solid solutions of these compounds, i.e., Bi2(Te"~Se,~)3, are also within the scope of the invention. Importantly, high loading i8 achieved without significant distortion of the crystal lattice and without significant dependence of ~G of the material on the WO 93/OX981 ;~ 1 2 ~ 3 b 9 6- PCl /IJS92/0924 intercalant concentration. ~G has been correlated with the opera~ng performance of the crystalline material when used in devices such as a cathode material in a galvanic cell.
Although discussions are directed to a bismuth chalcogenide, it is contemplated that any layered material of requisite crystalline lattice parameters, guest loading capacity and thermodynamic behavior is within the scope of the present invention.
The bismuth chalcogenide family is known to clystallize in rhombohedral and he~agonal lattices. The hexagonal and rhombohedral crystal lattices possess two types of energetically accessible sites which per~t localization of the guest species ~ithin the van der Waals ~hannel. The basis for this observation has been presented in a co-pending application U.S.S.N. 07/784,525 of which this applicatio~ is a continuation-in-part and which is herein incorporated by referenoe.
Figure 1 is a schematic representation of the two types of guest sites. A first site 22 i8 in the plane of the center of the channel, while a remaining site 24 localizes th~ guest species along walls 26 of the channel. Total loading of the channel by guest is predicted to be three.
Occupation of sites 22 is more energe~cally favorable at the begiDning of intercalation than of sites 24. However, relative energy levels change dwing tlle intercalation process. All sites are sufflcien1 Iy ~lose in energy that "hopping" of guests from one to another site of diffenng energy is possible. The guest species behave as a 'lattice gasn.
The conventional model would seem to suggest, 1 herefore, that the upper limit to guest loading without distortion of the latt;ice is three. However, we have discovered that much higher loading is possible. For the specified lattice types shown in Fig. 1, orbital interaction in a filled van der Waals channel results in increased guest-guest interaction and a decrease in the average guest-guest interatomic distance. This conversion ~om occupation of localized energy minima to free movement tbroughout the van der Waals channel is equivalent to a WO 93/08981 7 2 1 2 2 3 6 9 PCr/US92/0924~

phase change. The lattice gas condenses into a high density state, which is defined herein as a "quasi-liquid phase".
Because the new phase has a smaller average interatomic distance, additional guests can be introduced without distortion of the crystal lattice. Hence, a loading capacity of three is no longer a li~tation to the system and rapid and high levels of guest loading is now possible. Loading capacity of lithium of up to eight and nine have been observed in the bismuth chalcogenide compounds of this invention.
We estimate that the loading capacity in this system can be even higher, in particular, capacity of up to ten is considered possible.
The class of compounds of the present invention bas stable crystalline phases of hesagonal or rhombohedral symmetry which can be prepared ~mth minimal defect densities and the appropriate impurity distribution. It is known in the prior art that the Bi2Ch3 class of compounds may cry~tallize in a rhombohedral unit cell of space group D6aa (R3m, aO=9.83 ~; a=24.4 for bismuth selenide) containing five atoms. This crystal structure consists of layer~ formed by equal atoms in he~agonal arrangement. It is also given in prior art that a hesagonal unit cell for bismuth selenide (a~,=4.14 A; co=28.55 A) also has been identified.
In a battery, the discharge curve is directly correlated to ~G of the cathode material (a bismuth chalcogenide). The following theImodynamic parameters which are related to ~G must be considered when evaluating a guest~host combination: the entropy of distribution of the host/guest atoms, the energy of guest-guest and guest-host interactions, the change in the Fermi energy (~Fe)~ and the lattice distortion (LD)-The lattice gas to quasi-liquid condensation of the system prevents significant distortion of the lattice. In the present case, loading over the range of intercalation from 0 to 8 or 9 results in `
distortion only in t}le range of 2-3%. Such distortion does not contribute sig~ficantly to the Gibbs free energy of the system. In the wo 93/08g8l 2 1 2 2 3 6 9 PCI /US92/0924~

invention, distortion i8 held not exceed 10%. In contrast, in prior art TiS2 intercalation of Lithium, the c-axis of the intercalated LiTiS2, i.e., the axis perpendicular to the intercalated van der Waals channel, has been shown to increase by 10% in response to as little intercalation as x=0 to x=1. The change in entropy (~S) is significant only in the early stages of the intercalation process. ~S is therefore very small over the course of the process and need not be considered in the Gibbs free energy equation.
Tbe characteristics of the crystal lattice have a great effect on the lemaining two thermody~c parameters, however. The energy of interaction, E~", is a mea~urement of guest-guest and guest-host interactions. Both of these are greatly affected by lattice crystalline characte~istics. If the crystal lattice contain6 significant levels of defects and/~or dislocations or has suff{ciently uneven distribution of defects, the energy miDima associated with the localized sites will be disrupted and filling is not umform along the length of the channel.
Fermi energy level of the crystal is also effected by the interstitial and lattice site impurities and the lattice structure The defect or impurity distribution is important in identifying acceptable crystalline purity. If all defects are clu~tered near the entrance to van der Waals channels, no guests can enter and guest capacil~y is low even though crystalline lattice punt~y is high. It is apparent therefore, that carefi~l crystal growth is important to preparing layered cry6talline materials capable of the high loading of the present invention.
Process. The following detailed description is presented to provide details of the crystal growth process providing bighly defect-free layered crystalline materials with the specific lattice characteristics, such as defect distribution, of the present invention. The description which follows is for the bismuth chalcogenide family; however, it is contemplated t~at any layered material of requisite crystalline lattice parameters, guest loa&g capacity and the~nodynamic behavior is wit;hin the scope of the present invention.

wo 93/0898l 9 2 ~ 2 ~ 3 ~ 9 Pcr/US92,0924~

Stoichiomet~ic quantities of highly purified (99.9999% pure) bismuth and chalcogenide are charged into a quartz ampoule. If necessary, the materials are zone refined before use. Off-stoichiometrg results in an n- or p-doped material with characteristic degradation of 5 the lattice structure and the associated performance. The ampoule is evacuated to 10 7 mm~g and backfilled to a pressure of 10 3 mmHg with a small amount of inert ga~, such as argon, or a reducing gas, such as hydrogen (3-10 cycles), and then sealed. Hydrogen is particularly preferred because it reacts with oxygen during processing to prevent 10 o~idation and decrease the ~egregation of chalcogenide by reducing its vapor pressure.
A highly homogeneous polycrystalline material is prepared in a f rst processing step. The sealed ampoule is placed in a furnace at room temperature and heated to a temperature ~-10C above its melting 15 - point. The ramp rate, t2mperature and reaction time are selected for the finàl compound. The reaction conditions are listed in Table I for the preparation of polycrystalline Bi2S3, Bi2Se3, and Bi2Te3. The temperature of the fu~ace o~er the entire length of the ampoule is controlled to wi~hin :1:0.5C. Careful and accurate control of the -- 20 temperature is important because of the hi~h volatility of chalcogenides.
Temperature var~ation along the ampoule length cau6es segregation of chalcogenide which leads to off-stoichiome~. To optim~ze the temperature co~trol along the length of the ampoule, a long furnace f~n be used. Additional heating coils can be used at furnace ends to reduce the temperature gradient at t;he furnace e~its. -2 1 2 2 3 6 9 PCl /US92/09243 Table I. Prooel~8ing condition~ for polycry~talline material.
processing conditions Bi2Te3 Bi2Se3 Bi2S3 _ heatingrate tOT,jq (C/h) 30 20 15 e~posure ti~ne (h) 10 15 20 at T~jq 1 10C
coolingrate (Clh) 50 40 35 .
During the last hour of reaction time, the ampoule i8 agitated or 15 vibrated to insure complete mising of the ampoule components. The ampoule vibration preferably is in the rallge of 2~100 Hz and is accomplished by fi iDg one end of the ampoule to an oscillation source.
Any conventional nbration means is contemplated by the present invention. After reaction is complete, the ampoule is cooled at a slow 20 controlled rate.
Onoe a homogeneous polycrystalline material is obtained, it can be fi~rther processed into a highly defect-free bismuth chalcogenide single crystal. Any known method of growing single crystals can be u~ed, such as Bridgeman techniques, Czolchralski process and zone 25 refinement techniques (recry~tallization). In particular zone refinement has proved to be highly effective in obtaiDing high purity ~ngle crystals.
Refer~ng to Fig 2, zone refinement is carried out in a qtz boat 40 containing a seed crystal 42 of the desired lattice structure. It 30 is recommended t hat c3ean rooms levels of Class 100 be maintained.
The seed crystal 42 i~ oriented in the boat such that c~8tal layers 43 are horizontal. The entire apparatus should be shock-mounted to insulate against environmental vibrations. The boule 44 of polycryst~11ine material is positioned in surface contact with the seed 35 crystal.
The furnaoe comprise two parts, an outer furnace 46 for maintaining an elevated temperature along the entire boule length and wo 93/08981 2 1 2 2 3 6 9 PCrtUS92/Og24~

a narrow zone 47 movable in the direction of arrow 48 for heating a small portion of the polycrystalline material. For production of hexagonal structure, the outer furnace 46 is maintained at 35C below the melting point, and the zone 47, which is 2-3 cm in length, is held at 5 10C above the melting point of the polycrystalline material. Unlike for the preparation of the polycrystalline material int he first processing step, the boule can in this step be rapidly heated to the operating temperature. The zone is initially positioned at the seed crystallboule inOerface and this region is heated to the melting point of the material.
10 The zone 47 is then moved slowly down the length of the boule. Zone travel rate is selected according to the particular composition and recommended rates are shown, along with other processing parameters, in Table II. Zone travel rate is an important processing parameter. If the rate is too great, crystallization is incomplete and defects are 15 formed. If the rate is too slow, layer distortions result. The lower portion of the heat-treated boule in contact with the quartz boat is preferably removed before use. The process produces a single crystalline matenal having less than 10l2/cm3 defect density ~nd an impurity distribution which is inversely proportional to the intended 20 direction of intercalation. The single crystal typically contains 106 layers/mm with a spacing of 3-4 A/layer.
Table II. PrOOe88iIlg conditions for hesagonal ~ingle crystal growth.
processing conditions Bi2Te3 Bi2Se3 Bi2S3 boule temperature Mp - 35C Mp - 35C Mp -zone temperature Mp + 10C Mp ~ 10C Mp +

zone travel rate 8 mm/hr 6 mm/hr 3 mm/hr 3~ cooling rate 60 C/hr 40C~r 35C/hr The above process can be modified slight Iy to produce crystals of rhombohedral structure, in which case a rhom~ohedral seed crystal is WO g3/089X1 2 1 2 2 3 6 9 PCI/US92/0924~s employed in the zone refinement process. In addition, to produce rhombohedral crystals, the furnace temperature is held at 30C below the melting point and the zone is maintained at the melting point of the polycrystalline material.
Depending on the composition of the material, there is a preference for either hesagonal or rhombohedral lattice structure. This is summarized below in Table III.
Table III. Pr~ferred lattice ~tructure for bismuth chalcogeDide.

lattice structure Bi2Te3 Bi2Se3 Bi2S3 rhoml~ohedral - - X
hesagonal X X

The above process provides a highly defect-free single crystalline material. The crystal can be further ground into particles for use in devices and each 6uch particle is a mono-crystal. - A grinding technique is selected so as not to introduce many defects and dislocations into the crystal. However, because of the weakness -of the van der Waals attractive force, the crystal cleaves readily along the length of the --25 channel without much danger of lattice distortion.
Once formed, the material is tested by introducing it as the cathode-active material into a galYaDic cell. The use of highly intercalated crystalline materials in an energy storage cell is disclosed in a co-pending U.S. application entitled HEnergy Storage System"
which is being filed this date. A standard battery is constructed using a lithium anode, a non-aqueous LiCl04 electrolyte solution and the test material as the cathode. The amount of li~ium that can be introduced into the van der Waals layer is determined by monitoring the moles of Faraday electrons passed through an esternal circuit during intercalation. A typical dis~barge curve 50 using a bismuth chalcogeDide cathode prepared according to the invention is shown in Fig. 3. An acceptable crystalline material is capable of intercalating at WO 93/08981 P~/US92/09243 - 13- 21223~!) least three mole of lithium per mole of bismuth chalcogenide and has discharge curve that is essentially flat, that is, a change of no more than 0.1-0.3 V is observed over an intercalation capacity range of 0.4 to 8 moles litbium. The flatness of the curve iB an indication of the substantial independence of Gibbs free energy change on guest concentration.
Condensation of a lattice gas to a quasi-liquid manifests itself in the di~charge ve as a sudden change in the discharge voltage. The actual change in voltage iB quite small, however, and is not significant to operation of the cell. Eig. 3 illustrates the smooth, flat discharge curve. In an eDlarged por~ion 52 of the curve at appro~imately a guest load of three it i8 poBBible to observe a "blip" in the curve. This is observable under carefully controlled conditions.
Once a highly defect free crystalline material is prepared as described above, it can be intercalated to obtain the highly intercalated crystalline mate~ial of the present invention. Inter~ can be canied out using conventional methods, such as e~posing the crystalline material to the vapor phase of the interc~ant or placing the crystalline material in a liquid that contains the intercalant or passing a current through an electrochemical cell where t~e crystalline matenal i8 an -electroactive material in one of the electrodes. The preferred method for ac~ieving high load o~intercalant is the electro~hemical method.
What i8 claimed is:

Claims (37)

1. A layered mono-crystalline material, wherein said material has a sufficiently low defect density and an appropriate impurity distribution, together sufficient to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of said material without significant distortion of the lattice, said material further characterized in that .DELTA.G is substantially independent of the lithium intercalation concentration.

2. A layered mono-crystalline material having the formula, MeyChz, where Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, y is 1 or 2 and z is 1, 2, 3, wherein said material has a sufficiently low defect density and an appropriate impurity distribution, together sufficient to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of said material without significant distortion of the lattice, said material further characterized in that .DELTA.G is substantially independent of the lithium intercalation concentration.

3. A layered mono-crystalline material having the formula, MeyChz, where Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, y is 1 or 2 and z is 1 to 3, wherein said material has a sufficiently low defect density and an appropriate impurity distribution, together sufficient to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of said material without significant distortion of the lattice, and wherein a discharge curve from an electrochemical cell in which said material is the cathode has a voltage change of no more than 0.3 volts over an intercalation capacity range of 0.4 to 8.

4. The mono-crystalline material of claim 1, 2 or 3, wherein said material has a hexagonal lattice structure, defect density less than 1012/cm3 and a gradient of impurity distribution in the van der Waals channel inversely proportional to the direction of lithium intercalation.

5. The mono-crystalline material of claim 1, 2 or 3, said material further characterized in that a condensation of lithium lattice gas occurs at approximately a loading capacity of three.

6. The mono-crystalline material of claim 2 or 3, wherein said material comprises a solid solution of MeyChz, where Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, y is 1 or 2 and z is 1, 2, 3.

7. The mono-crystalline material of claim 1, 2 or 3, wherein said material is ground into a powder, such that each powder particle comprises a mono-crystal.

8. The crystalline material of claim 4 wherein said condensation is from a lithium lattice gas to a quasi-liquid.

9. A battery comprising the crystalline material of claim 1, 2 or 3.

10. A capacitor comprising the crystalline material of claim 1, 2 or 3.

11. A thermoelectric device comprising the crystalline material of claim 1, 2 or 3.

12. A method of preparing a highly purified bismuth chalcogenide, comprising the steps of:
charging an ampoule with stoichiometric quantities of a chalcogenide element and bismuth;
providing the ampoule with an atmosphere selected to prevent oxidation;
sealing the ampoule;
heating the sealed ampoule at a temperature in the range of 5°C
to 10°C above Tliq of the bismuth chalcogenide, said temperature controlled to within a range of ?0.5 along the length of the ampoule and for a time sufficient to melt the component materials and react to form the desired bismuth chalcogenide, whereby the ampoule is agitated during heating to ensure homogeneous mixing of the component materials;

cooling the material to room temperature at a controlled rate to form a homogeneous polycrystalline bismuth chalcogenide;
placing the polycrystalline bismuth chalcogenide in surface contact with a seed crystal of a specific crystal lattice structure;
heating the seed crystal and polycrystalline bismuth chalcogenide to a temperature which is in the range of 30°C to 40°C below Tliq of the bismuth chalcogenide;
heating a zone of the polycrystalline bismuth chalcogenide adjacent to the seed crystal to a temperature which is in the range of 5°C to 15°C above Tliq of the bismuth chalcogenide;
moving said zone along the length of the polycrystalline chalcogenide at rate in the range of 2 to 10 mm/hr, whereby a single crystal highly defect free bismuth chalcogenide is formed; and cooling the single crystal bismuth chalcogenide to room temperature at said controlled cooling rate.

13. The method of claim 12, wherein said single crystal bismuth chalcogenide has a hexagonal lattice structure, a defect density of less than 1012/cm3 and a gradient of impurity defect distribution in the van der Waals channel inversely proportional to the direction of lithium intercalation.

14. The method of claim 12, wherein said selected atmosphere is argon.

15. The method of claim 12, wherein said selected atmosphere is hydrogen.

16. The method of claim 12, wherein said bismuth and chalcogenide element are of 99.9999% purity.

17. The method of claim 12, wherein said zone is in the range of 2-5 cm wide.

18. The method of claim 12, wherein said agitation comprises vibration.

19. The method of claim 12, wherein said agitation comprises oscillation from a fixed point.

20. The method of claim 19, wherein said oscillation has a frequency in the range of 25 to 100 Hz.

21. The method of claim 12, wherein the agitation occurs in the last hour of heating.

22. The method of claim 12, wherein controlled cooling has a rate in the range of 30 to 50°C/hr.

23. The method of claim 12, wherein said heating temperature is 10°C above Tliq of the bismuth chalcogenide.

24. The method of claim 12, wherein the chalcogenide is tellurium and the heating of said sealed ampoule occurs for 10 hours, said controlled cooling rate is 50°C/h, and said zone travel rate is 8 mm/h.

25. The method of claim 12, wherein the chalcogenide is selenium and said heating of said sealed ampoule occurs for 15 h, said controlled cooling rate is 40°C/h, and said zone travel rate is 6 mm/h.

26. The method of claim 12, wherein the chalcogenide is sulfur and said heating of said sealed ampoule occurs for 20 hours, said controlled cooling rate is 35°C/h, and said zone travel rate is 3 mm/h.

27. The method of claim 12, wherein said heating of said seed crystal and said polycrystalline bismuth chalcogenide is 35 °C below Tliqand said zone temperature is 10°C above Tliq for a hexagonal structure.

28. The method of claim 12, wherein said heating of said seed crystal and said polycrystalline bismuth chalcogenide is 30°C below Tliq and said zone temperature is at Tliq for a rhombohedral structure.

29. A capacitor comprising the bismuth chalcogenide prepared according to the method of claim 12.

30. A thermoelectric device comprising the bismuth chalcogenide prepared according to the method of claim 12.

31. A highly defect-free mono-crystalline material prepared according to the method of claim 12.

32. An intercalated crystalline material comprising a guest in the range of three to ten moles within van der Waals channels per one mole of said intercalated material without significant distortion of the lattice, said guest selected from the group consisting of Group IA and Group IIA metals, said intercalated material further characterized in that .DELTA.G is substantially independent of the guest concentration.

33. An intercalated crystalline material having the formula, GxBiyChz, where G is selected from the group consisting of Group IA
and Group IIA metals, Ch is selected from the group consisting of Te, Se and S, x is in the range of 3 to 10, y is 1, 2 or 3 and z is in the range of 1 to 3, such that there is no significant lattice distortion, said material further characterized in that .DELTA.G is substantially independent upon the lithium intercalation concentration.

34. The intercalated crystalline material of claim 32 or 33, wherein said intercalant is lithium.

35 . An energy storage system comprising the intercalated crystalline material of claim 32 or 33.

36. A capacitor comprising the intercalated crystalline material of claim 32 or 33.

37. A thermoelectric device comprising the intercalated crystalline material of claim 32 or 33.