CA1036535A - Preparation of intercalated chalcogenides - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Ions are intercalated in chalcogenides by flowing a current through a system comprising a cathode which contains the chalcogenides (e.g., TaS2 is a suitable chalcogenide), an anode containing electronically conductive material which is not a source of the intercalating ions (a suitable anode material would be gold) and an electrolyte comprising an ionically con-ducting material which will electrochemically release ions of the species which are to be intercalated in the chalcogenide (e.g., dilute HCl, in which case hydrogen ions would be intercalated in the chalcogenide). The voltage is applied at a level sufficient to permit electrochemical decomposition of the electrolyte to thereby release the ions to be intercalated into the chalcogenide.
Alternatively, the anode may act as the source of the intercalating ions and the electrolyte would comprise or contain a compound of the same species as that of the ions to be intercalated (e.g., a suitable anode would be lithium metal and a suitable electrolyte would be LiI in propylene carbonate). In this alternate method, an electrical circuit is provided within the system and the voltage within the system is controlled such that ions are released from the anode to intercalate into the chalcogenide. The first method is the subject of claims in parent application S.N.
204,611 whereas the second method is the subject of the claims herein.

Description

1~36S35 This application is a divisional of S.N. 204,611 filed 11 July 1974 and the claims are directed to the method of preparation of intercalated chalcogenides designated herein as ~ethod B whereas the claims of parent application S.N. 204,611 are directed to the method designated as A.
BACKGROUND OF THE INVENTION
This invention relates to methods for intercalating chalcogenides.
In the past, chalcogenides have been intercalated under condltions such that the quantities of the reactants must be controlled and generally past methods of preparation required elevated temperatures and significant periods of time (ranging from several hours to several months) for preparation. In contrast thereto, the present methods o intercalation permit the degree of intercala-tion to be controlled very readily by controlling the current pa~sed into the system without the nced to control the quantitLes of the reactnntfl. ~urther-more, the present methods Oe intercalation permlt controL and monl.torlng oE
the thermodynamlc actlvlty oE the lntercalfl~ed species as w~Ll as permlttlng the reactlon to be performed quite rapidly at room temperature, thus eliminat-ing the problem of lntercalatlng those chalcogenldes which might otherwise be decomposed at elevated temperatures. The resultant intercalated compounds -are known to be useful as lubricants, x-ray diffraction grating crystals, superconductors, and thermo-electric elements, e.g., see published German patent application 2,061,162 Australian Journal of Chemistry, Volume ll, page ~:~
471 et seq ~1958) and the Journal of Chemical Physics, Volume 58, page 697 et seq (1973).
SUMMARY OF THE INVENTION
The present invention involves the preparation of intercalated chalcogenides by controlling the flow of current through a system comprising a cathode, an anode and an electrolyte. The components o~ this system ns well as the controlled current flow to be utilized in the two types o~ methods (denoted herein as Methed A and Method B) for intercalating the chalcogenldes `
wlll now be described herein in

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greater detail. ~3~535 ~
I. M~T~OD A
.
The Cathode ' The cathode contains as the cathode-acti~e material the chalcogenides to be intercalated. The cathode itself need not necessarily consist solely of the cathode-active material but may be a structure or material such as carbon, copper, nickel, zinc, silver, etc., with which the chalcogenide is depoaited or mixed. Preferably, the cathode consists entirely of the chalcogenide to be intercalated. -The chalcogenide to be intercalated may be any of those di-chalcogenides ( or lower chalcogen:ldes) which are capable of being lntercalated. Elther the pure chalcogenides or alloys (oE both the catlon and the anion) of chalcogenldes may be used. The0e c'haLcogen.Lcles are well known ln the prlor ~rt and nlay be represented by the genera:l ~ormu'La ~Zx wherein M ls a metal selected fro~ the group conslstlng of titanlum, zinconium, hafnium, vanadium, niobium, tantalum and molybdenum; Z is at least one chalcogen from the group consisting of sulfur, selenium and tellurium; and x has a numeric'al value between about 1 and about 2.05. ';
Preferably, the metal is tantalum or titanium and the chalcogen ~i.e. Z) is sulfur or selenium. The subscript x preferably has a value in the range of 1.95 to 2.02 (a value of 2.00 is most preferred). Especially useful chalcogenides are those having a layered structure analogous to ''' that of graphite ~e.g., TaS2).
The Anode The anode is one which contains as the anode-active material an element or compound which is an electronic conductor, but is one whlch does not release ions into the

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electrolyte that are subsequently intercalated into the chalcogenide, such as noble metals, Group IVb metals, Group Vb metals, Group VIb metals, Group VIIb metals, Group VIII
metals and compounds of the aforesaid metals. Examples of such compounds include the nitrides, borides, carbides, etc.
As in the case of the cathode, the anode may be fabricated entirely ~rom the element or compound to be used a~ the anode~active material (which is preferred) or it may consist of an underlying structure (fabricated of an elec-tronically conductive material such as copper, nickel,platinum, etc.) upon which the anode-acti~e material is deposited. Furthermore, the anode-active material may consist of alloys or mixtures of any o~ the above mat~rials rather than a single material.
I'he ~lectrol~te The electrolyte useul in Method A may be con-veniently represented by the general formula LX wherein L
is a cationic moiety selected from the group consisting of Group Ia elements, Group Ib elements, Group IIa elements, Group IIb elements, ammonium (or substituted ammonium such as pyridinium~, aluminum, gallium, indium and thallium, and X is an anionic moiety or moieties selected from the group consisting of halides, sulfates, nitrates, beta-aluminas, phosphofluorides, thiocyanates, perchlorates and rubidium halide.
Especially useful electrolyte materials include LiC1~4, LiPF6, sodium beta-alumina, ammonium iodide, silver nitrate, copper sulfate, HCl, KCNS, KCl and MgC12. The electrolyte may be present in a pure state ~in the form of a solid or li~uid) or it may be conveniently dissol~ed in a suitable solvent such as water, alcohols, ketones, esters, ethers, organic carbonates, organic lactones, amides, ~036535 sulfoxides, nitrohydrocarbons and mixtures of such solvents.
Where the solvent is utilized, the electrolyte salt may be present in a range of concentrations determined by the electrolyte conductivity and chemical reactivity.
The Current After the system described above has been assem- `~
bled, intercalation proceeds by applying a current to the system at a voltage level such that the electrolyte will electrochemically decompose to thereby release the ions to intercalate into the chalcogenide. Preferably, the current is applied such that the cathode is placed at a potential of at least 1 millivolt more positive than that required for electrochemical decomposition o~ the electrolyte in the presence of an inert cathode material; this prevents the formation o~ an electro-deposited coatin~ on the sur~ace o~
the cha~cogenide.
In determining the preferable level of current, all that is necessary i~ that, in place of the cathode con-taining the chalcogenide, the system utilizes a reference cathode which contains as the cathode-active material an electronically conductive material (such as platinum) which will not electrochemically react with the electrolyte or the products of decomposition of the electrolyte and thereafter determine the potential required ~or electrochemical decom-position of the electrolyte utilizin~ the anode and the reference cathode. Once the determination of this potential has been made, the reference cathode is removed from the system and replaced with the cathode containin~ as the cathode-active material the chalcogenide to be intercalated and the current which is applied is such that the cathode is maintained at a potential of at least one millivolt more positive than that which was determined utilizin~ the ~ " ~ . . : ,. . ,. ;; . . :
: , ~03G53S
reference cathode.
Alternatively, the preferable le~el of currentcan be readily determined by use o~ a third electrode, a reference electrode, which is used to monitor the cathode potential. By the use of a potentiostat, the current can be automatically controlled to maintain a pre-determined cathode potential. When the reference electrode comprises the pure material to be intercalated, then the cathode potential is maintained at a positive value of at least 10 one millivolt. / -The potential to be applied to the system in Method A depends on the selected intexcalating species as well as the selected chalcogenide to be intercalated. ~9 a general rule, the potential should be at a level at least sufficient to provide e~ouyh driviny Eorce Eor ele~trolyti-cally decomposing the electrolyte so as to provide the source of the intercalating ions. At the opposite extreme, the potential should preferably not be so high such that ions from the electrolyte are deposited on the surface of the cathode-active material rather than intercalated into the cathode-active material (in the case of materials wherein the ions are, for example, hydrogen, ammonium, etc., the term "deposited" would mean that a bub~ling away of the gases at the surface of the chalcogenide rather than inter-calation of the ions into the chalcogenide would occur).
That intercalation of the chalcogenide proceeded at a satisfac-tory rate rather than deposition at the surface oE the chalco-genide may be readily determined by use of a third, reEerence electrode and a coulometer to monitor the total curr0nt passed and hence the number of charged species discharged, X-ray analysis of the chalcogenide, and visual inspection.
In a typical system which utilizes a chalcogenide . . .
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~L~3~i35 such as tantalum disulfide in an amount of 10 mg. for the cathode and cobalt wire in an amount of 100 mg. for the anode in an electrolyte consisting of NH4I in acetone, substantially full intercalation of the chalcogenide by the ammonium ions may take place over a period of about 3 hours utilizing a voltage of about 2 volts. Thus, a suit-able voltage would vary from 1 volt to 2 volts, and the resulting current would be 1-2 ma./10 mg. of tantalum disulfide.
II. METHOD B
The Cathode The cathode to be used in Method B will contain as the cathode-active material any o~ those materials des-cribed as being suitable for the cathode-active material in Method A above.
The Anode In Method B, the anode (rather than the electro-lyte in Method A) is employed as the source of the ions which are to be intercalated into the chalcogenide. Accord-ingly, the anode should contain as the anode-active material any electronically conductive material which will electro-chemically release ions which are to be intercalated into -the chalcogenide. Thus, suitable anode-active materials may be those selected from the group consisting of Group Ia metals, Group Ib metals, Group IIa metals, Group IIb metals, Group IIIa metals and mixtures of the aforesaid metals with one another or with other substances such that the aforesaid metals can be electrochemicall~ released from the mixture.
Other suitable anode-active materials include materials which are capable of releasing hydrogen and ammonium ions (e.g., LaNi5Hx or Hg (NH4)X). Preferably, the anode-active material is a Group Ia metal such as lithium, sodium or potassium.

.

~36535 Here again, the anode may consist entirely of the anode-active material (which is preferred) or the anode-active material may be deposited on a supporting structure which in turn may be constructed of materials such as copper, platinum, etc., which are preferably electronically conductive, but which are not the source of the intercalating ions. Further-more, the anode-active material may also consist of alloys, compounds or solutions of the above materials provided that the alloys, compounds or solutions meet the requirement that they are electronically conductive and are capable of electrochemically releasing ions which are to be inter-calated into the chalcogenide.
The El _trolyte .
The electrolyte comprises (or ma~ contain) a compound of the same species as that o e the ions to be intercalated into the chalcogenide (i.e. the cationic moiety of the electrolyte compound must be identical to the element employed as the anode-active material). Thus, the electro-lyte may be conveniently represented by the formula LX
wherein ~ is a cationic moiety selected from the group con-sisting of Group Ia elements, Group Ib elements, Group IIa elements, Group IIb elements, ammonium, aluminum, gallium, indium and thallium; and X is an anionic moiety or moieties -selected from the group consisting of halides, sulfates, nitrates, beta-aluminas, phosphofluorides, thiocyanates, ;
perchlorates and rubidium halide. As in the case of Method A, the electrolyte may be present in the form of a solid, molten or liquid pure compound or may be present in the form of a solution in a suitable solvent. For the ;~
purposes of Method B, the term "suitable solvent" should be understood as one which would not result in a chemical reaction with the selected anode-active material. Thus, . ~. , . . :

although the same electrolyte solvents which are indicated hereinabove as being ~ossibly useful in practicing Method A
may also be utilized for Method B, certain combinations of the selected anode-active material and electrolyte solvent are not possible (for example, pure lithium could not be used as the anode-active material in the presence of an electrolyte solvent consisting of or comprising water or an alcohol).
The Current ~.
10In Method B, an external current need not neces-sarily be applied to the system since the system of itself ~ -is a source of current. Rather, the current flow which is generated by the system should preferably be controlled such that the ions released from the anode are lntercalated into the chalcogenide rather than deposited on the surface o~ the chalcogenide. The control o~ the current developed by this system may be readily accomplished by utilizing a resistor in the circuit (variable resistors are also quite useful) or by providing an opposing potential ~such as with the use of a potentiostat) to insure intercalation. Here again, as in the case of Method A, the voltage across the system should be such that there is sufficient driving force to result in release of the ions from the anode-active material to intercalate into the chalcogenide, but prefer-ably should not be such that the ions are significantly deposited on the surface of the chalcogenide. The particular flow of current with the system in the case of Method B will depend on the selected anode-active materlal which is to form the intercalating ions as well as the chalcogenide to be intercalated. However, a satisfactory rate of intercala-tion of the chalcogenide using Method ~ may be determined by the same method described hereinabove with reference to _g . . , ~.: ;, ~ . .; , . ~
.. . .. ~
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Method A.
In a typical system for practicing Method B which utilizes a chalcogenide such as TaS2 in an amount of 20 mg.
for the cathode and copper wire in an amount of 50 mg. for the anode in an electrolyte consisting of copper sulfate in water, substantially full,intercalation of the chalcogenide by the copper ions may take place over a period of about ~, 3 hours in which the initial current flow is about 1 ma.
Thus, a suitable current flow would vary from 1 ma. to 0.1 ma. per 20mg/crystal where the cathode is at +50 mv.
potential relative to a copper wixe reference electrode.
The term "Group" as applied to one or more elements or compounds refers to a particular Group of the Periodic Table of the Elements oE the type set forth on the inside cover of The Merck Index (7th ed.). The following ex~mples shall serve to illustrate the intercalation of chalcogenides by Method A and Method B.
EXAMPI.E 1 - Method A
A single crystal of tantalum disulfide was indium soldered to a copper wire. The crystal weighed about 10 mg.
This was the cathode. A cobalt wire of diameter 0.020 inches served as the anode. A saturated solution of ammonium -iodide in acetone served as the electrolyte. This was con-tained in a beaker and the anode and cathode were dipped into the solution. A slowly increasing voltage was then applied to the cell, and a current began to flow above 0.8 volts. The potential was then held steady at 2 volts ~or about five hours, during which time the cathode crystal turned from a dark blue to a black color, which is indicative of intercalation, and the electrolyte solution turned from colorless to a very deep red color due to the formation of iodine at the anode. A current in the range 1 to 2 ma.

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1~36535 flowed during the period; X-ray analysis indicated that inter-calation had taken place.
EXAMPLE 2 - Method A
A gold wire anode and a gold wire cathode of dia-meter 0.05 inches were immersed in an N/10 aqueous hydro-chloric acid solution. The current flowing through the cell was measured as a function of the applied voltage; from this a value of the decomposition potential of the electro-lyte was determined. The gold wire cathode was then replaced by a tantalum disulfide crystal mounted as in example 1.
The drop in decomposition potential, which was shown by the use of a third, reference electrode to be entirely due to a decrease in the cathodic potential, is an indication that intercalation o~ hydrogen is taking place. Below 1.5 volts, the decomposition potential of the electrolyte, no gaseous evolution was observed at the sul~ide cathode. The system was then left under an applied potential of 1.26 volts and the current fell to zero (initially about 1 ma.); X-ray analysis indicated that intercalation had taken place.
EXAMPLE 3 - Method B
A silver wire anode and a 2.3 mg. single crystal of tantalum disulfide, mounted as above examples, were dipped into an almost saturated solution of silver nitrate in water.- The open circuit voltage, i.e. no current flow, of this cell was -0.14 volts, the TaS2 being more positive than the silver. On changing this to -0.10 v by application of an external potential, a current of 0.37 ma. Elowed; this decayed with time indicative oE a difEusion controlled process, namely silver diffusion into the tantalum sulfide lattice. The cell was then shorted to speed up intercala-tion; this cau~ed some deposition of silver metal on the crystal which was subsequently removed electrolytically by :. . ;

103653s ~
1 appl~iny a reversc potential~ After one hour, the cr~stal 2 was removed, weighed and X-rayed. rrhe change in ~ei~ht ~-3 indicated a composition o~ Ac30 6TaS2.

4 EX~PL]? ~ - ~ie~llod B
. .
A copper ~ire anode and a 20 m~. ~ingle crystal 6 o~ tantalum disu].~ide were dipped in~o a s~urated solution 7 o~ coppe~ sul~tc in ~7atcr. A copp~r ref~ence electrode 8 was al50 dippcd into this solution. The initial open cir-9 cuit voltage o~ the cell ~7as -0. 2 volts, the TaS2 bein~
10 positi.ve relative to th~ copper anod~. ~n external po~en~
11 tial ~7a~ applied to holcl th~ TAS2 c~lthocle at 50 mv positiv~
12 xel~tiv~ to the re~renc~ el~ctrocle. t~ curr~nt: t~len 1c~ 7~d 13 ~hrou~Jll the ~3y~ m, initiall~ 1 ma clecAyincJ {:o O.:L m~ aEt:cx 1~ thxee hours. ~ coulorn~er mea~uro~nent o~ tll~ curreJIt rlo~7 inclicate~ a compo~i~ion CuO 2O~r~S2.
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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of intercalated chalcogenides which comprises controlling the voltage in a system comprising:
(a) a cathode containing as the cathode-active material the chalcogenide to be intercalated, said chalcogenide having a layered structure and having the formula MZx wherein M is at least one metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum and molybdenum; Z is at least one chalcogen selected from the group consisting of sulfur, selenium and tellurium; and x has a numerical value between about 1 and about 2.05;
(b) an anode containing as the anode-active material an electronically conductive material which will provide ions which are to be intercalated into said chalcogenide, said anode-active material being at least one member selected from the group consisting of Group Ia metals, Group Ib metals, Group IIa metals, Group IIb metals, Group IIIa metals, mixtures of the aforesaid metals with other substances, materials containing hydrogen, and materials containing ammonium, such that the aforesaid metals, hydrogen or ammonium can be electrochemically released therefrom; and (c) an electrolyte comprising at least one salt having the formula LX wherein L is at least one cationic moiety selected from the group consisting of Group Ia elements, Group Ib elements, Group IIa elements, Group IIb elements, Group IIIa elements and ammonium, aluminum, gallium, indium and thallium and X is an anionic moiety or moieties selected from the group consisting of halides, sulfates, nitrates, beta-aluminas, phosphofluorides, thiocyanates and perchlorates, provided that at least one salt is a salt of the anode active material;

the voltage within said system being controlled to be sufficiently high such that the electrolyte is provided with ions of the species which are to intercalate the chalcogenide and such that the chalcogenide is intercalated and to be sufficiently low to avoid deposition onto the chalcogenide of the species being intercalated into the chalco-genide.

2. The process of claim 1 in which the chalcogenide is one which has a layered structure and M is at least one metal selected from the group consisting of titanium, zirconium, hafnium, niobium, and tantalum; Z is selected from the group consisting of sulfur and selenium; and x has a numerical value between 1.95 and 2.02.

3. The process of claim 2 wherein x has a numerical value of 2.00.

4. The process of claim 2 in which the chalcogenide is TaSx.

5. The process of claim 2 in which the chalcogenide is TaS2.

6. The process of claim 2 in which the chalcogenide is TiSx.

7. The process of claim 1 in which the anode active material is selected from the group consisting of lithium, sodium, potassium, copper and silver.

8. The process of claim 1 in which the anode active material is lithium.

9. The process of claim 1 in which the electrolyte is present as a solution in a solvent selected from the group consisting of water, alcohols, ketones, esters, ethers, organic carbonates, organic lactones, amides, sulfoxides, nitrohydrocarbons and mixtures of such solvents.

10. The process of claim 1 in which the current flow 19 controlled such that the cathode is at a potential of at least about one millivolt more positive than that required for deposition of the species being intercalated on an inert cathode.