CA1237767A - Coatings for electrochemical electrodes and methods of making the same - Google Patents

Abstract

ABSTRACT

The present invention provides a coating for electrodes for use in electrochemical cells having an electrochemically active species and an elec-trolyte. The coating includes a selectively per-meable material which allows for the diffusion of the active species through the coating during op-eration of the cell while providing a substan-tially impervious barrier to the electrolyte.
Electrodes utilizing the coatings described herein may be used in primary and secondary cells over a wide range of operating temperatures to deliver better electrochemical performance even at room temperature. Methods of making the coating and an apparatus for performing these methods on a con-tinuous basis are included in the present inven-tion. A novel composition of matter also is con-templated containing lithium, silicon, and fluo-rine prepared by exposing lithium metal to SiF4.

Description

643 lZ37''7~i7 The present invention is directed generally to electrochemical cells having an electrolyte and at least one electrode. More particularly, a coating is provided for the electrode which allows for the diffusion of an electrochemically active species through the coating during electrochemical release of the active species from the electrode or deposition of the active species onto the elect trove while providing a substantially impervious barrier to the electrolyte. Electrodes utilizing the coatings described herein may be made with various active materials and used in cells over a wide range of operating temperatures to deliver better electrochemical performance compared to the prior art.
In an electrochemical cell, chemical energy is converted to electrical energy with a reduction in the free energy of the system. In the course of the cell reaction, negative charge leaves the anode, travels through the external driven air-cult, and repenters the cell at the cathode.
Thus, the cathode is the positive electrode and the anode is the negative electrode. By virtue of the established electromotive series, it is posse-bye to select suitable cathodes and anodes to ox-lain a desired theoretical voltage. The ideal cell would give the theoretical voltage under con-tinted, constant load and the loss of free energy would manifest itself entirely as electrical en-orgy outside the cell. However, this ideal is never attained in practice, because the internal resistance of the cell is not zero and the react . 1 _ I

3LZ3~7t7~7 lions within the cell never occur in a completelyreverslble manner. Moreover, incompatibility of the electrodes with each other or with the elect trolyte, polarization, and other well-known probe lets prevent performance at theoretical values.
In particular, lithium has several properties which prove advantageous when used as an anode.
Lithium in equilibrium with lithium ions in van-ions solvent systems affords very negative potent trials, and thus it is readily possible to achieve cell voltages greater than three volts, using van-ions oxide cathodic reactants, i.e., Noah. Be-cause lithium has the lowest equivalent weight of any metal, the high cell voltages result in high energy per unit weight of cell. Lithium is fee-son ably stable in many non aqueous electrolyte soys-terms. This stability arises as a consequence of a chemical reaction between the lithium anode and the electrolyte, forming a passivating film at the interface which limits further reaction. Such films cause a number of problems which are disk cussed below.
Over the past decade, many battery companies and government laboratories have worked on the development of ambient temperature lithium bat-tories. Several primary lithium cells are now commercially available, although no secondary lithium cells have yet reached this stage.
In the field of primary cells, the Li-LiAlC14/SOC12-C system has been extensively in-vestigated. This system has received particular attention because of its relatively high open-~LZ~7J'7~,~

circuit voltage of 3.6 volts and high energy den-sty of 5~0 W-hr/Xg. Such lithium primary systems demonstrate very high energy densities. For exam-pie, the Li/SOCl2 system exceeds 16-20 Winnie, which represents an energy density ten times greater than the Lechanche primary typo In add-lion to the questionable safety of the cell, how-ever, there are other problems which hinder the ability of these systems to achieve their great potential for commercial success.
Using lithium as an anode material poses significant problems. Although lithium has been used successfully in aqueous electrolytes for very high drain rate batteries in military application, more conventional applications require the use of aprotic solvents to achieve reasonable shelf life and coulombic efficiency at low discharge rates.
Use of these solvents cause handling and safety problems for such cells.
Lithium cells using a non aqueous electrolyte have ruptured during use even though the cells were built to contain the gas generated by the electrochemical reaction between these combo-newts. Excessive internal heating, sometimes as-situated with inadvertent short circuits or over-discharging, is one reported cause of such rut-lure.
High drain-rate lithium cells have encounter-Ed problems with thermal runaway initiated by the exothermic reaction between lithium and the elect trolyte, particularly an aqueous electrolyte.
Since lithium has a relatively low melting point ~;23~767 and its reactivity greatly increases upon melting, high operating temperatures must be avoided.
Lithium cells with liquid cathodes based on sulfur-containing electrolytes have a particular problem with gas generated from the degradation of the electrolyte. Since the cells are sealed to prevent leakage of the electrolyte, the potential for the cell to rupture is dramatically increased.
Lithium cells operating at ambient tempera-lure using liquid cathodes based on organic or inorganic electrolytes tend to suffer from a shortcoming known as voltage delay. This term describes a temporary voltage depression on load when cells are tested after extended periods of storage, especially at high temperature. This phenomenon results from lithium directly contact-in a soluble depolarizer to form a passivating film at the anode surface. The factors control-lying this delay in activation are not well under-stood. The passivation film is at least partially responsible for the chronic low rate capability that nearly all lithium battery systems suffer from.
The voltage delay phenomenon also plagues other types of cells. For example, in a magnesium dry cell a passivation film forms on the magnesium anode to limit corrosion. The cell is then unable to deliver full operating voltage after it is placed under load.
Attempts to solve the voltage delay phoneme-non have generally concentrated upon additives to the electrolyte. Although some additives have no- -123'7'7~

duped the effect of the voltage delay phenomenon, the voltage and overall electrochemical perform mange of the cell is significantly decreased.
An inconvenient and expensive method of avoiding the voltage delay phenomenon is to place a small rechargeable cell i.e. nickel cadmium cell, in parallel circuit with the passivated cell. The rechargeable cell provides the operate in voltage until the passivated cell is capable of doing so.
Passivation films often form before the cell is assembled to limit the operating voltage of a cell unless current densities are used above a barrier value. For example, this problem is ox-habited by titanium anodes used in electrolytic manganese dioxide processes. Sandblasting and chemical washes are treatments used in the attempt to remove the passivation film before the anode is used in the process.
Anode materials like lithium also are inhere entry rechargeable, i.e., lithium can be electron deposited from lithium ion-containing electron lyres. One of the major problems limiting the successful development of rechargeable versions of lithium cells is the nature of the lithium deposit during the recharging of the cells. Past invest-gallons indicate that lithium plating can occur in dendritic form lowering the cell's utilization efficiency and ultimately shorting the cell.
Attempts to prevent deleterious dendrite formation include alloying the lithium with other metals like aluminum. Electrolyte additives also ~Z37~7~

have been used to promote surface alloy format lion. Although cycle life of the cell improves, the power density of the cell significantly de-creases. Other attempts employ cell separators, such as permeable membranes, to act as physical barriers to dendritic growth. Although cell sepal rotors are initially effective, lithium dendrites can eventually penetrate the cell separators and establish transient or permanent electronic shorts.
In order to increase the voltage generated by devices utilizing lithium metal, coatings or layers of lithium compound compositions have been used on the lithium metal contained in these de-vices. For example, US. Patent No. 3,528,856, discloses a high temperature voltage and current generating device including a layer of lithium metal which is coated by various lithium compound compositions, i.e., lithium oxide, lithium nitride, etc. The lithium metal, as coated, is useful for generating voltage and/or electrical power in response to the application of heat.
Open circuit voltages of the order of 1.5 to 2.5 volts were observed when the device was exposed to high temperature.
An example of a lithium-air device which gent crates voltage by utilizing lithium compounds as a coating is disclosed in US. Patent No.
3,615,835. Various lithium compounds were used as a solid coating over lithium metal. The device operated at room temperature and was activated by exposure to water moisture which penetrated the coating to contact the lithium metal layer.

~37 7tj~

In accordance with the present invention, primary and secondary electrochemical cells having an electrolyte are fabricated using at least one electrode containing an electrochemically active species such as lithium. A coating is provided on the electrode which is particularly useful because it allows for diffusion of the active species through the coating to the electrolyte upon elect trochemical release of the active species from the electrode or deposition of the active species onto the electrode and provides a substantially impel-virus barrier to the electrolyte.
The electrochemical cells described herein exhibit a wide operating temperature range with improved cell capacity at various discharge rates even at ambient temperatures. Improved rate gape-ability for several cathodic reactants has also been achieved. With improved cell voltage and capacity, the cell is more efficient. In a more efficient cell, there is less power and heat disk sipation. Thus, thermal runaway and rupturing of the cell is prevented.
The coating of the present invention provides protection against degradation of the electrode by the electrolyte during storage and increases the shelf life of the electrode. Cells are less prone to rupture because electrolyte degradation is de-creased and less gas is generated during cycling.
We have found that the above discussed disk advantages can be overcome by employing the pros-en invention which includes a new and improved coating for an electrode for use in an electron I

~237~767 chemical cell having an electrochemically active species and an electrolyte. The coating includes a selectively permeable material which allows for the diffusion of the active species through the coating during operation of the cell while provide in a substantially impervious barrier to the electrolyte. The problems in prior primary and secondary cells as discussed above are obviated by the present invention, all with improved electron chemical performance of the cells.
The selectively permeable material includes an electrochemically active species and at least one electronegative element. Another component of the coating may be carbon or at least one amp ho-tonic element.
In one embodiment, an electrode for use in an electrochemical cell having an electrochemically active species and an electrolyte includes means for storing said active species and subsequently discharging at least a portion of such stored active species to provide a supply of electrons.
The electrode has a surface adapted to be opera-lively associated with the electrolyte in the cell. The electrode also includes a coating, as described above, on the electrode surface.
In another embodiment, an electrochemical cell is provided which includes an electrochemi-gaily active species and anode means for storing the active species and subsequently discharging at least a portion of such stored active species to provide a supply of electrons, the anode means has an operative surface. A coating, as described ~f~37t7~7 above, is provided on the surface. The cell fur-then includes cathode means for providing Defoe-soon of the active species between said anode means and said cathode means during operation of the cell. The cathode means operatively contacts the surface through the coating. The cathode means includes an electrolyte. Lithium is one preferred active species. A second preferred active species is hydrogen wherein the electrode includes means for storing hydrogen and subset quaintly discharging at least a portion of the stored hydrogen to provide a supply of electrons.
Other suitable active species include sodium, potassium, magnesium, and calcium. The present invention also contemplates applications of the inventive concept to both primary and secondary cells, and batteries having a plurality of such cells.
A method of making an electrode for use in an electrochemical cell having an electrochemically active species and an electrode includes the steps of providing an electrode having means for storing said active species and subsequently discharging at least a portion of such stored active species to provide a supply of electrons. The electrode includes a surface adapted to be operatively also-elated with the electrolyte in the cell. The method further includes applying a coating, as described above, to the electrode surface. Prey-drably, application of the coating to the elect trove surface takes place by either solution depot session, vapor deposition, or rapid quench tech-piques.

go _ ~3'7~76 I' The solution techniques include physically coating the selectively permeable material onto the electrode surface, chemically reacting the coating to the electrode surface, and reactively electroplating the coating onto the electrode sun-face. The vapor deposition techniques include sputtering, reactive sputtering, co-sputtering, reactive co-sputtering, glow deposition, vacuum evaporation, chemical vapor deposition, and no-active chemical vapor deposition. The rapid quench techniques include thermal spraying and plasma spraying.
The present invention also provides an Papa-fetus for performing the methods of making the coated electrodes of the invention on a continuous basis. The electrode is made from a roll of a web of substrate with one or more electrode-forming regions thereon. The apparatus includes means for unrolling the substrate substantially continuously into an application chamber, the chamber including at least one depositing region therein. The Papa-fetus further includes means for depositing a coating onto at least some of said one or more electrode-forming regions. The depositing means is positioned in the application chamber. Prefer-ably, the application chamber may be a solution bath for the solution techniques described above;
a reactive chemical vapor deposition chamber; a vacuum chamber for the vapor deposition techniques described above; or, means for melting the select lively permeable material for subsequent deposit-in and quenching onto a substrate such as with thermal or plasma spraying.

~;~37'7~7 A composition of matter including the eye-mints lithium, fluorine, and silicon also is con-template by the present invention. The compost-lion of matter exhibits characteristic absorption bands in the infrared region of the spectrum at the frequencies expressed in reciprocal genii-meters and the X-ray diffract lion spectrum described below. The composition of matter is prepared by exposing lithium metal to a mixture of 18 percent Sift in argon at one atom-sphere at a temperature of 125C. Another combo-session of matter is prepared by exposing potassium or sodium to a mixture of I percent Sift in argon at one atmosphere at a temperature of 125C.
Fig. 1 is a block diagram of one illustrative apparatus for performing the methods of making the coated electrodes of the invention on a continuous basis;
Fig. 2 is a graph of cell voltage vs. kapok-fly for a cell having a coated electrode of the invention compared to conventional, uncoated elect troves of the prior art;
Fig. 3 is a discharge curve plotting cell voltage vs. time for a coated lithium anode of the invention in Suckle electrolyte compared to a con-ventional, uncoated anode of the prior art demon-striating the elimination of the voltage delay phenomenon; and Fig. 4 is a discharge curve plotting cell voltage vs. time for a coated lithium anode of the invention made by a reactive chemical vapor depot session technique compared to a conventional, us-coated anode of the prior art.

,~;

~23~7~7 Fig. 5 presents the infrared spectrum for a coating of the present invention formed by the no-action of lithium metal and Sift.
Fig. 6 presents the X-ray diffraction specs trump of a coating of the present invention formed by the reaction of lithium metal and Sift.
Generally, the present invention provides a coating for an electrode which is used in electron chemical cells having an electrochemically active species and an electrolyte. The coating allows diffusion of the active species away from the electrode and into the electrolyte during the disk charge stage of the electrochemical reaction. In secondary electrochemical cells, the diffusion of the active species through the coating is fevers-isle. The coating also provides a substantially impervious barrier to the electrolyte. The combo-session of the coating includes the active species and at least one electronegative element. The coating may also be a compositional disordered material with single or multiple phases. The coating may be applied through a variety of soul-lion or vapor deposition techniques. Various cathode systems, and more particularly, electron lyres, may be used in an electrochemical cell con-twining an electrode having the inventive coat-in. The coatings of the invention provide great-lye improved specific capacities, cell voltages, rate capabilities, and other electrochemical cell characteristics.
In particular, the coatings of the present invention provide a selectively permeable material ~237t76~

which allows an active species to diffuse through the coating during operation of the cell. The term electrochemically active species is used to generally denote the form which the active mate-fiat of the electrode takes upon electrochemical operation of the the electrode in a cell. Somali-tonsil, the coating functions as a barrier to prevent diffusion of the electrolyte through the coating so that the electrode does not come in direct physical contact with the electrolyte.
Thus, the coating is selectively permeable to the diffusion of the particular active species and imp previous to the diffusion of the electrolyte.
The diffusion of the active species need only be in one direction in a primary cell, i.e., from the electrode. The inventive coating may also be used on electrodes in secondary cells. Upon disk charge of the electrode, the active species Defoe fuses away from the electrode and into the elect trolyte. upon charging of the electrode, the active species diffuses from the electrolyte to the electrode. Thus, the diffusion of the active species through the coating in a secondary system is reversible.
For prolonged operational life, the coatings of the invention also must be substantially impel-virus to degradation by the electrolyte Although the active species diffuses through the coating during the electrochemical reaction, the coating itself does not react with the electrolyte so that the structure of the coating remains intact.
Thus, the coating interposes a physical barrier between the electrode and the electrolyte.

~23~

The composition of the inventive coating is a selectively permeable material which includes the active species and at least one electronegative element. The term electronegativity refers to the attraction of nuclei for electrons and comparisons between the electronegativity of elements are made with Pylons electronegativity series on elect tronegative element has a Pausing electronegative fly value greater than 2.2. Suitable electro~ega-live elements are fluorine, iodine, bromide, Shelley-fine, sulfur, nitrogen, and oxygen. Compounds of these elements also may be suitable. Preferably, a coating of the invention incorporates lithium with either nitrogen or iodine.
The coatings of the present invention may in-elude carbon or at least one amphoteric element.
The term amphoteric is used to denote an element with a Pausing electronegativity value ranging be-tweet 1.5 and 2.2. Carbon need not be in element tat form; organic compounds such as polymers are suitable. Silicon and aluminum are preferred am-footwork elements. Other suitable amphoteric eye-mints include phosphorus, boron, gallium, arsenic and zinc. The amphoteric element need net nieces-sanity be in elemental form, and thus, compounds of these elements also may be used.
Preferably, a coating of the invention in-corporate fluorine as the electronegative eye-mint, ? lithium as the active species, and silicon as the amphoteric element.
The coating also may contain an electroposi-live element incorporated in the selectively per-~Z37~6 'I

Mobil material. An electropositive element has a Pausing electronegativity value less than 1.5.
The electropositive element is a different element from the active species. Suitable electropositive elements include lithium, sodium, potassium, mug-noisome and calcium. Adding an electropositive element increases the ionic diffusion rate of the active species through the coating. Other means for increasing the diffusion rate of the active species through the coating are also contemplated.
To enhance the electrochemical character-is tics of the electrode, the diffusion coefficient of the active species through the coating is prey-drably greater than about 10~1cm2/sec.. More imp portent, the coatings of the invention prefer-ably have an ionic conductivity greater than about 10-6 ohm~1cm~1.
The coating of the invention may have a structure which is either a single phase crystal-line material, or a compositional disordered material. Such disordered materials have tailor made local chemical environments which are design-Ed to improve the electrochemical performance of a coated electrode by increasing the density and/or reducing the barrier height of the diffusivity sites for the active species of the electrode.
The diffusivity sites must be selective, so that the electrolyte will not be diffused through the coating.
Disordered materials are ideally suited for manipulation since they are not constrained by the symmetry of a single phase crystalline lattice or ~3'7~67 by stoichiometry. By moving away from materials having such restrictive single phase crystalline symmetry, it is possible to accomplish a signify-cant alteration of the local structural chemical environments to selectively enhance and control the active species diffusion coefficient of the inventive materials.
The types of disordered structures which pro-vise the local structural chemical environments for the enhanced selective diffusion character-is tics contemplated by the invention include mull ticomponent polycrystalline materials lacking long-range compositional order, microcrystalline materials, amorphous materials or multi phase mate-fiats containing both amorphous and crystalline phases. The following list provides a classifica-lion of the spectrum of disordered structures de-scribed by the present invention:
1. Multi component polycrystalline materials lacking long-range compositional order.

2. Microcrystalline materials.

3. Mixtures of polycrystalline and micro-crystalline phases.

4. Mixtures of polycrystalline or micro-crystalline and amorphous phases.

5. Amorphous materials containing one or more amorphous phases.
In carrying the present invention into pray-lice, the coatings described herein may be used on the surface of an electrode adapted to be opera-lively associated with an electrolyte in an elect trochemical cell. The coated electrode may in-lZ37~

elude the active species as part of the coating before operation of the cell diffuses the active species through the coating.
The electrode includes means, described be-low, for storing the active species and subset quaintly discharging at least a portion of the stored active species to provide a supply of elect irons. The electrode may initially be in a charge Ed or uncharged state. In a charged state, the active species is initially stored with the coated electrode. In an uncharged state, the active species is initially released from a counter elect trove or from the electrolyte upon operation of the cell and diffuses through the coating to de-posit onto the coated electrode.
The active material of an electrode that can be used with the coatings of the present invention include at least one element selected from the group consisting of Group IA, Group IBM Group IDA, Group JIB, and mixtures of the aforesaid elements with other substances such that the aforesaid eye-mints can be electrochemically released from the mixture. Preferably, the active material of the electrode is the metal lithium. Advantageously, other Group IA elements such as sodium, and poles-slum are suitable. Suitable Group IDA elements are magnesium and calcium. A second preferred active material is a hydrogen storage material;
other means for charging by absorbing and storing hydrogen and subsequently discharging at least a portion of the stored hydrogen to provide a supply of electrons also are contemplated.

~Z37'76~

The electrode also may consist entirely of the active material or the active material can be deposited on a supporting structure which, in turn, can be constructed of materials such as copper, steel, nickel, carbon, etc., which are advantageously electronically conductive but which are not the source of the active species. In some instances, the electrode may incorporate alloys, compounds or solutions containing the active mate-fiat provided that the alloys, compounds or soul-lions meet the requirement that they are electron-icily conductive and are capable of electrochemi-gaily releasing the active species which is to be transported into the electrolyte.
A general concept of the invention is to sub-statute a designed barrier with the characters-tics described herein in place of the passivation layer spontaneously formed by the reaction of the electrode and the electrolyte or environment which deleteriously affects many electrode systems. As applied to the electrode, the coating should be of an effective thickness and uniformity to reduce or prevent the formation of a passivation layer on the electrode surface which limits the electron chemical release of the active species.
The coating should be uniform so that areas of the electrode surface are not exposed directly to the electrolyte. Otherwise a passivation layer may form over such uncoated areas and decrease the electrochemical performance of the electrode.
Furthermore, direct contact between the electron lyre and electrode provides an area for degrade-lion of the electrode.

~Z3 7 76~

The thickness of the coating must be suffix client to act as a substantially impervious barrier to the diffusion of the electrolyte to the elect trove. The invention contemplates, but is not limited to, a coating with a thickness not less than about AYE and not more than about Lowe. Advantageously, the thickness of the coating is not less than about AYE and not more than about OWE. Preferably, the thickness of the coating is about AYE. The coating prefer-ably have ionic conductivity greater than 10-6 ohm ~1cm~1 and/or a diffusion coefficient greater than 1 o-1 0cm2/sec.
The coated electrode described above may be used as anode means in an electrochemical cell.
Such anode means store the active species and sub-sequently discharge at least a portion of the stored active species to provide a supply of elect irons. The anode means include an operative sun-face for the coating.
Using the coated electrode in an electron chemical cell requires cathode means for providing diffusion of the active species between the anode means and the cathode means during operation of the cell. The cathode means operatively contacts the surface through the coating. The present in-mention is not limited to a particular structure nor material used for the cathode means.
The cathode structure itself need not nieces-sanity consist of a cathodic reactant. The struck lure may be made of materials such as carbon, copper, nickel, zinc, silver, etc., upon which or - ~3776~

in which the cathodic reactant is deposited or imp pregnated. Advantageously, the cathode structure consists entirely of the cathodic reactant when it displays significant electrical conductivity.
Preferably, the cathodic reactant is not admixed or diluted with an electrochemically inactive material.
The cathodic reactant may itself be dissolved or suspended directly in the electrolyte. In these embodiments, the structure of the cathode means is a current collector spaced from the anode. Often, the use of a separator is unneces-spry when the electrolyte itself functions as a separator The cathode means includes any type or kind of cathodic reactant. The cathodic reactant may be a pure compound or any combination of come pounds; a liquid or solid; and used with a liquid or solid electrolyte. Preferably, the cathodic reactants used with a lithium anode coated by the invention include: Phase, Tess, (CFX)m~ and Suckle. Other suitable cathodic reactants include, but are not limited to: Noah, Moo, VOW, VOW, Cut, Ag2CrO4, Cuss SO, Suckle, It Bra, PbI2~ Pus and S.
n electrolyte useful in practicing the press en invention must be chemically impervious to the inventive coating and cathode materials. The electrolyte must permit migration of ions from the coated anode to the cathodic reactants during the discharge of a primary or secondary cell, and vice versa, during the charge cycle of a secondary ~LZ377~

cell. The term electrolyte as used in the apply-cation herein refers individually to either an ionic conductive component or a solvent or the combination of both.
Suitable electrolytes include both aqueous and non aqueous, alkaline and acidic, organic and inorganic materials. A wide range of aprotic sol-vents and their mixtures can be used as non aqueous electrolytes with the above-described anodes.
Preferably, these non aqueous solvents include dioxolane, dim ethyl formamide, methyl format, and propylene carbonate. Other suitable solvents in-elude water, butyrolactone, dimethoxyethane, elk-ylene carbonate, sulfolane, and tetrahydrofuran.
Other suitable electrolytes include an ionize able salt having at least one cation moiety so-looted from the group of elements previously list-Ed above and at least one anionic moiety selected from the group consisting of halides, hydroxides, sulfates, nitrates, fluoroarsenates, barfly-rides, borochlorides, phosphofluorides, thus-notes, and per chlorates.
For use with a lithium anode, preferred elect trolytes include salts of lithium per chlorate, lithium tetrafluoroborate, lithium aluminum twitter-chloride, and lithium hexafluorarsenate.
The concentration of the salt in the electron lyre is determined by the electrolyte conductivity and chemical reactivity. However, in most in-stances, concentrations between about 0.1 molester liter and 5 moles per liter of the ionizable salt in the solvent are preferred. In addition to ~Z3~ 7 the foregoing electrolytes, it should be noted that some electrolytes may be used in the pure state as in the form of a solid.
The present invention contemplates several methods of applying the coating to the electrode surface described herein. These methods may be used to successively deposit several layers of the coating. Coatings of the selectively permeable material described above may be formed by atoms-tic, molecular, particulate, or bulk deposition techniques. Although particulate and bulk tech-piques are often inexpensive by comparison, atop-fistic or molecular deposition techniques allow for greater control over the local chemical environ-mints to structure a selectively permeable mute-fiat with the characteristics described above.
Generally, three classes of methods for applying the coating can be described. This classifica-lion, however, is presented to better understand the contemplated invention and is not meant to be a limitation. These classes are solution deposit lion, vapor deposition and rapid quench deposit lion. As used in the following discussion, the term selectively permeable material refers goner-ally to some or all of the individual elements, compounds, or components comprising the coating.
The first class of methods for applying the coating to an electrode surface includes preparing a solution of a solute and the selectively Perle-able material. This solution is applied to the electrode surface. The selectively permeable material is then deposited onto the electrode sun-face to form the coating.

37~6~

The step of depositing the selectively Perle-able material onto the electrode surface may be done with or without an electrical bias. In one embodiment, the solute may be dried from the soul-lion to leave the selectively permeable material as a coating on the electrode surface. In a second embodiment, subjecting the electrode to an electrical bias plates certain components of the selectively permeable material onto the electrode surface where they may react. Thus, the coating is reactively electroplated. In a third embody-mint, the certain components of selectively Perle-able material chemically reacts with the electrode surface to form the coating.
The first class of methods for applying the coating also includes adding the selectively per-Mobil material directly to the electrolyte con-twined in the electrochemical cell with the elect trove. Once the electrode surface is placed in operative contact with the electrolyte, the select lively permeable material reactively deposits onto the electrode surface to form the coating. The deposition of the selectively permeable material onto the electrode surface once it is placed in operative contact with the electrolyte may take place by either a chemical reaction between the selectively permeable material and the electrode surface; or by plating certain components of the selectively permeable material onto the electrode surface by subjecting the electrode to an electric eel bias so as to reactively electroplate the coating.

377~7 A second class of methods of applying the coating to the electrode can be generally defined as vapor deposition The depositing of the select lively permeable material is accomplished by a number of conventional techniques which are car-fled out in a vapor environment. These techniques include sputtering, reactive sputtering, co-sput-toning, reactive co-sputtering, glow discharge, vacuum evaporation, chemical vapor deposition, and reactive chemical vapor deposition.
A third class of methods of applying the coating to the electrode includes rapid quench techniques where the selectively permeable mate-fiat is melted and deposited onto the electrode surface. As the selectively permeable material is deposited, it is rapidly quenched. A composition-ally disordered material may be formed. Prefer-ably, thermal or plasma spraying is used.
The methods of preparing the coatings de-scribed herein include reactively electroplating and reactive chemical vapor deposition. It is be-lived that such reactions are advantageously pro-muted by using the high mobility of the active species in the coating. Thus, the active species is diffused through the coating during its format lion to react with other components of the select lively permeable material at the interface between the growing coating and solution.
Referring to Figure 1, a block diagram of a system apparatus is illustrated for making coated electrodes of the invention, as described above.
Although these processes could be performed on a ~23'77~i~

batch basis, continuous processing is preferred.
Thus, the vapor deposition, solution deposition, and rapid quench processes described above may be performed in an application chamber 10. A roll of a web of substrate material 12 is fed off of a payout reel 14 into and through the chamber 10 where the coating is deposited onto at least some of the one or more electrode-forming regions on the substrate 12. After the electrode-forming regions have been coated, the web moves to a take-up reel 16. The deposition process may be observed through a viewing port 18 or through monitoring and control instrumentation.
Other means may be provided for unrolling the substrate substantially continuously into the apt placation chamber 10. At least one depositing region is located in the application chamber 10.
The means for depositing said selectively permeable material may include filling the apply-cation chamber 10 with a solution to operatively contact the electrode-forming regions as the sub-striate 12 moves through the chamber 10. The soul-lion includes some or all of the components of the selectively permeable material and a solute. If all of the components are present in the solute, then the coating may be deposited on said elect trode-forming regions by providing means for dry-in the solute from the solution. In a second em-bodime~t, means for subjecting the electrode-form-in regions to an electrical bias may plate con-lain components of the selectively permeable mate-fiat onto the substrate 12 where they react with 31 Z3'~'~6~

the active species on the surface of the electrode so as to reactively electroplate the coating. In a preferred third embodiment, means are provided for chemically reacting dissolved components of the selectively permeable material with the active species at the electrode surface to form the coat-in.
The application chamber 10 may also be evoke-axed so that the means for depositing the select lively permeable material on the electrode-forming regions may include vapor deposition methods. The vapor deposition techniques contemplated by the invention are referred to by those skilled in the art as: sputtering, reactive sputtering, co-sput-toning, reactive co-spu~tering, glow discharge, vacuum evaporation, chemical vapor deposition, and reactive chemical vapor deposition.
The means for depositing said selectively permeable material on said electrode-forming regions may also include means for melting the selectively permeable material, means for deposit-in the selectively permeable material onto the electrode-forming regions, and means for quenching the selectively permeable material. Thus, the substrate 12 and application chamber 10 can be used in a fashion similar to a thermal or plasma spraying apparatus.
EXAMPLE I
comparative tests were made between cells using coated lithium anodes of the invention and conventional, uncoated lithium anodes. A coating was prepared on a scraped lithium foil mechanical-1237'7t~7 lye positioned on an anode platter by reactively co-sputtering an alloy of lithium and silicon (about 50 weight percent of each) in an atmosphere of about I fluorine/98% argon using a Maths OF Sputtering Unit. The anode platter was cooled by a liquid nitrogen bath during the sputa toning. The lithium foil was supplied by Foote Mineral Company of the United States of America, and was 99.9% pure. The area and thickness of the 10 foil were about .1875 so. in. and .10 in., respect lively. The thickness of the coating on the anode was about AYE.
A cathode for the cell was made with about 100 my. of finely ground natural iron pyrites ore (Phase). Twenty percent XC-72 carbon from the Cabot Corporation of the United States of America, and 3% dry powdered Teflon were blended with the Phase powder. The mix was pressed at about 1,200 psi into a nickel grid with tabs for electrical contact. The cathode formed was then cured at 190C. to bond the mix. The active materials were balanced in such à way that a stoichiometric ox-cuss of lithium was present compared to the disk charge capacity of the Phase.
Filter paper was used as a separator in the cell. The cell was assembled with the separator sandwiched between the anode and cathode. The electrolyte, EM Luke in 1, 3 dioxolane, was added and the test cell mounted in a glass test vessel equipped with a standard taper joint and with glass to metal throughput for electrical contact.

* Trade Mark I

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A second cell was constructed in an identical manner as described above, except that the anode was conventional, uncoated lithium foil A
Figure 2 compares the test results from these two cells, plotting cell voltage vs. discharge capacity at room temperature. A third set of data, published in the article entitled "Ambient Temperature Secondary Luffs Cells" Advanced Secondary Batteries Session, pp. 201-204, by Newman et at., also is presented in Fig. 2 for comparison. The third set of data is for a cell constructed in a similar manner as described above, except that a conventional, uncoated lithe I'm anode is used with LiB(C6H5)4-dioxolane-dimethoxyethane as the electrolyte.
The data clearly demonstrate that the cell capacity nearly doubled. The theoretical capacity of the cell was calculated at about 880-900 Meg of Phase. The capacity of the cell achieved using the coated anode was close to the theoretical capacity thus indicating much more effective Utah-ligation of the active materials. The open air-cult voltage also was considerably improved over the conventional, uncoated anodes.
EXAMPLE II
A comparison of jells was made using a lithe I'm anode coated in accordance with the present invention against a conventional, uncoated lithium in various electrolytes. Two cells were prepared as described in Example I, except for using a dip-fervent electrolyte. Two additional cells were prepared as described in Example I, except that a Tao lithium aluminum alloy (50 percent by weight) was used as the substrate foil. The following table presents the highest discharge capacity for each cell tested:
TABLE I
Electrolyte Discharge Capacity (Meg) Coated node Conventional Anode H Liar H Liar LCl04 in dioxolane 967 884 492 178 LiAsF6 in MeTHF 227 ~185 As demonstrated by these results, the coating of the present invention consistently and dramatic gaily increases the capacity of the cells, ire-spective of which electrolyte or anode composition is used.
EXAMPLE III
A comparison of rechargeable cells was made using a lithium anode prepared with a coating of the present invention and a conventional, uncoated lithium anode. Two cells were constructed ides-tidal to the cells described in Example I, except that Tess was used instead of Phase for the cathode to reactant. The following table presents the discharge capacity for the first three cycles of each cell:
TABLE II
Cycle Discharge Capacity Meg Coated Anode 125.4 93.1 88.5 Conventional Anode 91.7 72 ~Z37'7~

These results demonstrate that the present invention improves the cycling efficiency of the cells. Nearly two-thirds of the initial cell capacity was rechargeable.
EXAMPLE IV
Comparative tests were made between cells using coated lithium anodes of the invention and conventional uncoated lithium anodes in an Suckle electrolyte. One cell contained a coated lithium anode prepared in a manner described in Example I. The area and thickness of the foil was about .60 square inches and .10 inches, respectively.
The thickness of the coating on the anode was about AYE. A cathode for the cell was made with XC-72*carbon and 3% dry powdered Teflon*. The mix was pressed at about 1200 psi into a nickel grid with tabs for electrical contact. The cathode formed was then cured at 100C to bond the mix.
Filter paper was used as a separator in the cell. The electrolyte, 1.8 M LiAlCl4-SOCl2 was obtained from the Lithium Corporation of America and added to the test cell.
A second cell was constructed in an identical manner as described above, except that the anode was a conventional uncoated lithium foil.
Fig. 3 compares the discharge of each cell at 20 ma As the data clearly demonstrate, the volt-age delay phenomena is eliminated by using the lithium anode coated with the present invention.
This is accomplished with an improvement in open-cling voltage at the same drain rate.

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EXAMPLE V
Lithium foil (99.9% pure) reacted with 18%
Sift gas mixed with argon in a sealed glass con-trainer at slightly less than one atmosphere total pressure At 125C. the lithium foil immediately started reacting gently and turned black in color, forming a coating within about 5 minutes.
Fig. 4 compares the discharge at 20 ma of a conventional, uncoated lithium anode with the anode described above. The data illustrate the improved electrochemical performance of the cell having the inventive coating made by the reactive chemical vapor deposition technique.
The infrared spectrum of the resultant coat-in appears as Fig. 5 and does not agree with the infrared spectra of Li2SiF6, Sift, and Na2SiF6 presented in the literature. The sample was pro-pared with finely ground KBr as a pressed pellet.
The X-ray diffraction spectrum of the coating on a glass substrate appears as Fig. 6 and does not agree with the X-ray diffraction spectra of the above-identified reference compounds found in the literature.
As illustrated by these examples, a number of advantages result from substituting a designed coating for the passivating layer otherwise formed on electrodes. The coatings of the invention allow for diffusion of the active species through the costing to the electrolyte upon electrochemi-eel release of the active species from the elect trove. Simultaneously, the coating provides a substantially impervious barrier to the electron lZ;~77~7 lyre. Because the electrolyte can neither diffuse through the coating nor degrade the coating to come in direct contact with the electrode, a past sivation layer is not formed directly on the elect trove. The phenomena of voltage delay is come pletely eliminated without deleterious cons-quinces to the electrochemical performance of the cell. In fact, the electrochemical cells describe Ed herein exhibit a wide operating temperature range with improved cell capacity at various disk charge rates.
Improved rate capability for several cathodic reactants has also been achieved by the invent lion. Improved cell voltage and capacity demon-striate greater electrochemical efficiency with a concomitant reduction in internal heat genera-lion. Furthermore, the coatings provide long shelf lives for the electrodes because they are protected against degradation by the electrolyte.
Secondary cells embodying the inventive con-crept described herein demonstrate an improved no-chargeability and decreased dendrite formation.
The invention avoids excessive solvent degrade-lion. Consequently, less gassing was observed during cycling of the cells. This provides a safer environment because the cells are less prone to rupture because of pressure buildup. All of the advantages described herein have been achieved without rigorous purification of the electrolyte.
Thus, cells produced in accordance with the invent lion are less expensive and easier to manufacture.

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Modifications and variations of the present invention are possible in light of the above teachings and therefore, it is to be understood that the invention may be practiced otherwise than as specifically described.

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Claims (14)

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

1. A coated electrode which is capable of electrochemically releasing an active species to an electrolyte in an electrochemical cell, said electrode comprising:
an electrode body comprising active material for storing said active species and subsequently discharging at least a portion of such stored active species to provide a supply of electrons; and a non-metallic, inorganic coating for reducing passivation of said electrode, said coating consisting essentially of lithium, silicon and fluorine.

2. The coated electrode of claim 1, wherein the ionic conductivity of said coating is greater than about 10-6ohm_1cm_1.

3. The coated electrode of claim 1, wherein the diffusion coefficient of said active species through said coating is greater than about 10-10cm2/sec.

4. The coated electrode of claim 1, wherein said permeable material further consists essentially of at least one amphoteric element in addition to silicon.

5. The coated electrode of claim 1, wherein said coating is compositionally disordered.

6. The coated electrode of claim 1, wherein said coating is amorphous.

7. A coated lithium anode for use in an electrochemical cell having an electrolyte, said anode comprising:
an anode body of lithium or an alloy thereof having an anode surface adapted to be operatively associated with said electrolyte in the cell; and a non-metallic, inorganic coating on said lithium anode surface for reducing passivation of said anode, said coating consisting essentially of lithium, silicon and fluorine.

8. The coated anode of claim 7, wherein the ionic conductivity of said coating is greater than about 10-6ohm_1cm_1.

9. The coated anode of claim 7, wherein the diffusion coefficient of said lithium through said coating is greater than about 10-10cm2/sec.

10. The coated anode of claim 7, wherein said coating is compositionally disordered.

11. The coated anode of claim 7, wherein said coating is amorphous.

12. An electrochemical cell comprising:
an electrochemically active species, said active species selected from a group consisting of lithium, sodium, and calcium;
an anode for storing said active species and subsequently discharging at least a portion of such stored active species to provide a supply of electrons, said anode having an anode surface;
a non-metallic, inorganic coating on said anode surface for reducing passivation thereof, said coating consisting essentially of lithium, silicon and fluorine; and a cathode for providing diffusion of said active species between said anode and said cathode during operation of the cell, said cathode operatively contacting said anode surface through said coating, said cathode comprising a cathodic reactant for said anode.

13. The electrochemical cell of claim 12, wherein said coating on said anode is compositionally disordered.

14. The electrochemical cell of claim 12, wherein said coating on said anode is amorphous.