U.S. patent application number 10/060704 was filed with the patent office on 2002-06-20 for method of producing a durable electrochemical cell.
Invention is credited to Cheong, Hyeonsik M., Lee, Se-Hee, Tracy, C. Edwin.
Application Number | 20020076616 10/060704 |
Document ID | / |
Family ID | 24120631 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020076616 |
Kind Code |
A1 |
Lee, Se-Hee ; et
al. |
June 20, 2002 |
Method of producing a durable electrochemical cell
Abstract
The invention provides a method of protecting an ion insertion
material from the degradative effects of a liquid or gel-type
electrolyte material by disposing a protective, solid ion
conducting, electrically insulating, layer between the ion
insertion layer and the liquid or gel-type electrolyte material.
The invention further provides liquid or gel-type electrochemical
cells having improved durability having a pair of electrodes, a
pair of ion insertion layers sandwiched between the pair of
electrodes, a pair of solid ion conducting layers sandwiched
between the ion insertion layers, and a liquid or gel-type
electrolyte material disposed between the solid ion conducting
layers, where the solid ion conducting layer minimizes or prevents
degradation of the faces of the ion insertion materials facing the
liquid or gel-type electrolyte material. Electrochemical cells of
this invention having increased durability include secondary
lithium batteries and electrochromic devices.
Inventors: |
Lee, Se-Hee; (Lakewood,
CO) ; Tracy, C. Edwin; (Golden, CO) ; Cheong,
Hyeonsik M.; (Seoul, KR) |
Correspondence
Address: |
PAUL J WHITE, SENIOR COUNSEL
NATIONAL RENEWABLE ENERGY LABORATORY (NREL)
1617 COLE BOULEVARD
GOLDEN
CO
80401-3393
US
|
Family ID: |
24120631 |
Appl. No.: |
10/060704 |
Filed: |
January 29, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10060704 |
Jan 29, 2002 |
|
|
|
09532168 |
Mar 21, 2000 |
|
|
|
Current U.S.
Class: |
429/300 |
Current CPC
Class: |
H01M 6/18 20130101; G02F
1/1533 20130101; G02F 2001/1536 20130101; H01M 50/46 20210101; H01M
4/485 20130101; Y02E 60/10 20130101; H01M 10/0525 20130101; H01M
4/66 20130101; H01M 6/183 20130101; Y10T 29/49108 20150115 |
Class at
Publication: |
429/300 |
International
Class: |
H01M 006/14 |
Goverment Interests
[0002] The United States Government has rights in this invention
pursuant to Contract No. DE-AC36-99G010337 between the United
States Department of Energy and the Midwest Research Institute.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of producing a durable electrochemical cell,
comprising: (a) preparing a first electrode-equipped section by the
method comprising: (i) depositing a first ion insertion material on
a first conducting material; and (ii) depositing a first solid ion
conducting material on said first ion insertion material; (b)
preparing a second electrode-equipped section by the method
comprising (i) depositing a second ion insertion material on a
second conducting material; and (ii) depositing a second solid ion
conducting material on said second ion insertion material; (c)
disposing said first electrode-equipped section on said second
electrode-equipped section, wherein said first solid ion conducting
material is parallel to and spaced apart from said second solid ion
conducting material by a plurality of spacers, wherein said first
solid ion conducting material, said second ion conducting material,
and said plurality of spacers define a void; and (d) inserting a
liquid or gel-type ion conducting material into said void.
2. The method of claim 1, wherein said first and second solid ion
conducting layers are lithium aluminum fluoride.
3. The method of claim 1, wherein said first and second solid ion
conducting layers are lithium phosphorous oxinitride.
4. The method of claim 1, wherein said first ion insertion layer is
a cathodic electrochromic material
5. The method of claim 1, wherein said second ion insertion layer
is an anodic electrochromic material.
6. The method of claim 1, wherein said first ion insertion layer is
selected from the group consisting of transition metal oxides,
transition metal sulfides, transition metal oxysulfides, transition
metal halides, selenides, tellurides, chromates, molybdates,
tungstates, vanadates, niobates, tantalates, titanates,
stannates.
7. The method of claim 6, wherein said first ion insertion layer is
tungsten oxide.
8. The method of claim 1, wherein said second ion insertion layer
is selected from the group consisting of V.sub.2O.sub.5, IrO.sub.2,
and NiO.sub.2.
9. The method of claim 8, wherein said second ion insertion layer
is V.sub.2O.sub.5.
10. The method of claim 1, wherein said first and second solid
electrolyte layers have a thickness of about 1000 to 5000
Angstroms.
11. The method of claim 1, further comprising, prior to step
(a)(i), depositing said first conducting material on a first
substrate, and prior to step (b)(i), depositing said second
conducting material on a second substrate.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This is a 35 U.S.C. 121 divisional application of the
co-pending 35 U.S.C. 111(a) application, Ser. No. 09/532,168, filed
Mar. 21, 2000.
BACKGROUND OF THE INVENTION
[0003] 1. Field of Invention
[0004] This invention relates generally to devices comprising ion
insertion materials and methods for manufacturing the same, and
more particularly to methods of protecting ion insertion or
intercalation materials in electrochemical cells, such as lithium
ion batteries or electrochromic devices, to improve durability of
such materials.
[0005] 2. Description of the State of the Art
[0006] Electrochemical cells find utility in numerous devices such
as lithium rechargeable batteries and electrochromic devices.
Small-sized lithium rechargeable (secondary) batteries have been
widely used as a power sources for portable electronic equipment in
the fields of office automation equipment, household electronic
equipment, communication equipment and the like. Electrochromic
devices are highly beneficial in a variety of practical
applications where light modulation is desirable. These include,
for example, alphanumeric displays for clocks, watches, computer
monitors, outdoor advertisement and announcement boards, and other
types of displays. In addition, an important application for the
electrochromic devices of the present invention is light modulation
in, for example, mirrors of variable reflectance (as are used in
some automotive rearview mirrors), sunglasses, automotive
windshields, sunroofs, and building windows. Both rechargeable
lithium batteries and electrochromic devices operate on the
principle of an electrochemical cell (also referred to as a
galvanic cell). An electrochemical cell is a composite structure
containing a negative electrode (the cathode), a positive electrode
(the anode) and an ion-conducting electrolyte interposed
therebetween.
[0007] A conventional lithium rechargeable battery has a negative
electrode (the cathode) comprising an active material which
releases lithium ions when discharging, and intercalates or absorbs
lithium ions when the battery is being charged. The negative active
materials commonly utilized in lithium ion batteries include
niobium pentoxide, carbon, and similar materials capable of
intercalating lithium ions. The positive electrode (the anode) of a
conventional lithium ion battery contains a substance capable of
reacting chemically or interstitially with lithium ions, such as
transition metal oxides, including vanadium oxides, cobalt oxides,
iron oxides, manganese oxide and the like. In general, the positive
active material comprised by the positive electrode will react with
lithium ions in the discharging step of the battery, and release
lithium ions in the charging step of the battery. Since both the
anode and cathode materials of lithium ion batteries can
intercalate lithium ions, the anode and cathode materials are often
referred to as "ion insertion materials" or "intercalation
materials." The external faces of the anode and cathode lithium ion
batteries are usually equipped with some structure or component to
collect the charge generated by the battery during discharge and to
permit connection to an external power source during recharging.
Conventional lithium ion batteries usually comprise a non-aqueous
liquid or a solid polymer electrolyte, which has dissolved lithium
salt that is capable of dissociating to lithium ion(s) and an
anions, such as for example lithium perchlorate, lithium
borohexafluoride, and other lithium salts that are soluble in the
electrolyte utilized. During discharge, lithium ions from the anode
pass through the liquid electrolyte to the electrochemically active
material of the cathode, where the ions are taken up or absorbed
with the simultaneous release of electrical energy. During
charging, the flow of ions is reversed so that lithium ions pass
from the electrochemically active cathode material through the
electrolyte and are plated back onto the anode.
[0008] Another example of an electrochemical cell is an
electrochromic device, such as those used on electrochromic
windows. Conventional electrochromic windows comprise multi-layered
devices, similar to a lithium secondary battery, comprising a pair
of transparent electrodes sandwiched between two transparent
substrates. A pair of ion-insertion materials, referred to as the
electrochromic layer and an ion storage layer, are sandwiched
between the pair of electrodes. The electrochromic layer of an
electrochromic device is an electrochromic ion insertion material,
which reversibly changes its color by the injection or extraction
of ions as a result of an application of an electric potential.
This reversible color change in a material caused by an applied
electric field or current is known as "electrochromism." The ion
storage layer of an electrochromic device is an ion insertion
material, which may or may not have electrochromic properties. An
ion-conducting material (also known as an electrolyte layer) is
disposed between the electrochromic layer and the ion storage
layer. Positive ions are induced by the voltage to move through the
ion conducting material, i.e., electrolyte, in the direction from
the ion storage layer and toward the electrochromic layer. Upon
application of a voltage across the electrochromic device,
electrons flow through an external circuit in a direction from the
electrode adjacent the ion storage layer to the electrode adjacent
the electrochromic layer. Simultaneously, a resulting current is
conducted by ions, such as lithium ions (Li.sup.+) or hydrogen ions
(H.sup.+). The positive ions are induced by the voltage to move
through the ion conducting layer in the direction from the ion
storage layer and toward the electrochromic layer.
[0009] An example of an electrochromic material used in an
electrochromic device is a tungsten oxide (WO.sub.3) film. To color
the WO.sub.3 film, a battery is connected between the pair of
transparent conductive electrodes. When a negative voltage is
applied to one of the electrodes (the negative electrode),
electrons from the negative electrode and lithium ions from the
lithium electrolyte are injected simultaneously into the WO.sub.3
film. This ion injection process continues until the colorless
WO.sub.3 is converted into the blue-colored Li.sub.xWO.sub.3. To
bleach the blue-colored Li.sub.xWO.sub.3 film, the polarity is
reversed so that the electrons and lithium ions are depleted from
the Li.sub.xWO.sub.3 film. Current flows until the entire film is
restored to its original WO.sub.3 (colorless) state. Thus, it is
convenient to think of the coloring and bleaching process of an
electrochromic device as the charging and discharging of a battery.
Typically, for maximum efficiency, electrochromic devices include
an electrochromic layer comprising an electrochromic material and
an ion storage layer comprising a "complementary" electrochromic
material, i.e., an electrochromic layer that becomes colored upon
positive ion insertion and an ion storage material that becomes
colored upon removal of positive ions. As a result of this type of
complementary system, the electrochromic and ion storage layers
change color simultaneously as a result of an applied voltage to
produces a more highly colored (darker) state.
[0010] Electrochemical devices such as lithium secondary batteries
and electrochromic devices can use either a solid, liquid, or
polymer gel-type electrolyte as the ion conducting layer, and
therefore are referred to as either solid-state, liquid or polymer
gel (also known as gel-type) devices, respectively. The ion
conducting layer must possess high ionic conductivity (i.e.,
conducts positive ions such as Li.sup.+ or H.sup.+) and low
electronic conductivity (does not conduct electrons).
[0011] Solid-state electrochemical devices have solid thin-film
electrolytes made of so-called fast-ion conductor materials, in
which either lithium or hydrogen ions diffuse readily. Examples of
such fast-ion conductor materials include Li.sub.3N, Li.sub.2NH,
Li.sub.1-xM.sub.xTi.sub.2-x(PO.sub.4).sub.3 and LiAlF.sub.4. During
the manufacture of solid-state electrochemical devices, the solid
electrolyte layer (which is disposed between the cathode and the
anode) is deposited in a manner which often results unavoidably in
the formation of "pinholes". Pinholes are defects in the solid
electrolyte layer which act as electron "channels" between the
cathode and the anode, such as the electrochromic layer and the ion
storage layer in an electrochromic device. Consequently, in an
electrochromic device, if a pinhole is present in the solid
electrolyte layer, electrons will flow from the electrochromic
layer, through the pinhole in the solid electrolyte layer, and back
to the ion storage layer. Under this condition, known as
"shorting", electrons do not remain in the electrochromic layer
during applied voltage; therefore, the electrochromic device cannot
remain colored. Due to the inherent pinhole defects in the
manufacture of solid state electrochromic devices, it is difficult
to scale up these devices for larger applications, such as for
electrochromic windows.
[0012] Liquid or gel-type electrochemical devices were developed to
alleviate the "shorting" problems associated with solid state
electrochemical devices. Liquid or gel-type electrochemical devices
have a liquid or gel material as the ion conducting layer, which is
typically formed by sandwiching the liquid or gel-type ion
conducting material between the cathode and the anode after the
electrochemical device has been assembled. Consequently, liquid
electrochemical devices do not suffer the drawback of pinholes as
in solid-state devices. Therefore, they are easier to scale up than
the solid state devices. However, liquid or gel-type
electrochemical devices are often less durable than solid state
devices, possibly due to degradation of the ion storage layer and
the electrochromic layer by the liquid electrolyte. As the
electrochromic and ion storage layers degrade, it becomes necessary
to apply increasing amounts of voltage or current to the device to
achieve the same degree of color intensity.
[0013] A need therefore exists for a liquid or gel-type
electrochemical device that has increased durability and wherein
the ion insertion materials do not suffer from the degradative
effects of being in contact with the liquid or polymer gel
electrolyte as in conventional liquid or gel-type electrochemical
devices.
SUMMARY OF THE INVENTION
[0014] Accordingly, objects, features and advantages of the present
invention are to provide an improved liquid or gel-type
electrochemical cell based, for example, on lithium, which
maintains its integrity over a prolonged life-cycle as compared to
conventional liquid or gel-type electrochemical cells, and to
provide a protective, solid ion conducting layer between the ion
insertion material(s) and the liquid or gel-type electrolyte,
wherein the protective layers prevent degradation of the ion
insertion materials. The protective layers are characterized by an
ability to conduct positively charged ions but are poor electronic
conductors. The protective layers are of a sufficient thickness to
restrict penetration of the liquid electrolyte layer and
consequently reduce or prevent degradation of the ion insertion
layer(s).
[0015] Accordingly, it is a general object of this invention to
provide for a method of protecting an ion insertion material having
a surface which faces a liquid or gel-type ion conducting
material.
[0016] A more specific object of this invention is to provide a
liquid or gel-type electrochemical device having increased
durability.
[0017] Another specific object of the present invention is to
provide a liquid or gel-type electrochemical device having improved
cycling lifetime.
[0018] Another specific object of the present invention is to
provide a liquid or gel-type electrochemical device having improved
durability comprising a solid ion conducting layer disposed between
a ion insertion layer and a liquid or gel-type ion conducting
layer.
[0019] Another specific object of the present invention is to
provide a liquid or gel-type electrochromic device which is able to
maintain a substantially constant color intensity over time with
repeated application of an electric current.
[0020] Another specific object of the present invention is to
provide a liquid or gel-type electrochromic device having improved
durability comprising a solid ion conducting layer disposed between
an ion-insertion layer and a liquid or gel-type ion conducting
layer.
[0021] Another specific object of the present invention is to
provide a method of manufacturing a liquid or gel-type
electrochemical cell having increased durability.
[0022] Additional objects, advantages and novel features of this
invention shall be set forth in part in the description that
follows, and in part will become apparent to those skilled in the
art upon examination of the following specification or may be
learned by the practice of the invention. The objects and
advantages of the invention may be realized and attained by means
of the instrumentalities, combinations, and methods particularly
pointed out in the appended claims.
[0023] To achieve the foregoing and other objects and in accordance
with the purposes of the present invention, as embodied and broadly
described therein, an electrochemical cell of this invention
comprises a pair of substrates, a pair of electrodes sandwiched
between the pair of substrates, a pair of ion-insertion layers
sandwiched between the pair of electrodes, a pair of solid ion
conducting layers sandwiched between the ion insertion layers, and
a liquid or gel-type ion conducting material disposed between the
solid ion conducting layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The accompanying drawings, which are incorporated in and
form a part of the specifications, illustrate the preferred
embodiments of the present invention, and together with the
description serve to explain the principles of the invention.
[0025] In the Drawings:
[0026] FIG. 1 is a diagrammatic cross-sectional view of a
conventional (prior art) electrochemical cell comprising separate
substrate, anode, and cathode layers;
[0027] FIG. 2 is a diagrammatic c cross-sectional view of an
alternative conventional (prior art) electrochemical cell
comprising a first substrate layer which serves as the anode, and a
second substrate layer which serves as the cathode;
[0028] FIG. 3 is a diagrammatic cross-sectional view showing one
embodiment of a liquid or gel-type electrochemical cell according
to this invention having a protective solid ion conducting layer
between each ion insertion layer and the liquid or gel-type
electrolyte, and further comprising separate substrate, anode, and
cathode layers;
[0029] FIG. 4 is a diagrammatic cross-sectional view showing an
alternative embodiment of a liquid or gel-type electrochemical cell
according to this invention having a protective solid ion
conducting layer between each ion insertion layer and the liquid or
gel-type electrolyte, and further comprising a first substrate
layer which serves as the anode, and a second substrate layer which
serves as the cathode;
[0030] FIG. 5 is a graph of capacity in .mu.Ah/cm.sup.2 versus
cycle number for a vanadium oxide electrode coated with a
protective solid ion conducting layer of lithium aluminum fluoride
(curve 100) and an uncoated vanadium oxide electrode (curve
102);
[0031] FIG. 6 is a graph of capacity in .mu.Ah/cm.sup.2 versus
cycle number for a vanadium oxide electrode coated with a
protective solid ion conducting layer of lithium phosphorous
oxinitride material (curve 104) and an uncoated vanadium oxide
electrode (curve 106);
[0032] FIG. 7 is a graph of capacity in .mu.Ah/cm.sup.2 versus
cycle number for a tungsten oxide electrode coated with a
protective solid ion conducting layer of lithium aluminum fluoride
(curve 108) and an uncoated tungsten oxide electrode (curve 110);
and
[0033] FIG. 8 is a graph of capacity in .mu.Ah/cm.sup.2 versus
cycle number for a tungsten oxide electrode coated with a
protective solid ion conducting layer of a lithium phosphorous
oxinitride material (curve 112) and an uncoated tungsten oxide
electrode (curve 114).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0034] This invention generally provides a novel method and
structure for inhibiting or preventing degradation of any ion
insertion material having a surface that faces a liquid or gel-type
electrolyte by inserting a protective solid ion conducting
(electrolyte) layer between the ion insertion material and the
liquid or gel electrolyte. This invention further provides liquid
or gel-type electrochemical devices having increased durability,
comprising a solid ion conducting (electrolyte) layer disposed
between the ion insertion material and the liquid or gel-type
electrolyte layers of the device. While the method and structures
of this invention are not limited to applications to
electrochemical cells, for convenience of explanation the preferred
embodiments of this invention will be described in reference to an
electrochemical cell, with the understanding that it applies to
electrochromic and other devices that comprise similar materials or
operate on similar principles.
[0035] As shown in FIG. 1, a conventional (prior art)
electrochemical cell 10 has a first electrode-equipped section 12,
a second electrode-equipped section 14, and an ion conducting layer
16 therebetween. The first electrode-equipped section 12 is the
anode during discharge of cell 10, and the second
electrode-equipped section 14 is the cathode during discharge of
cell 10. The first electrode-equipped section 12 includes substrate
17, electrode 18, typically of nickel, iron, aluminum, stainless
steel, and/or copper foil, and ion insertion material 20. When
electrochemical cell 10 is a lithium battery, the ion insertion
material 20 comprises lithium, or compounds and alloys of lithium,
and is often referred to in the art as the "negative electrode" or
"anode". When electrochemical cell 10 is an electrochromic device,
the ion insertion material 20 typically comprises an electrochromic
ion insertion material (discussed below) and is typically referred
to as the electrochromic layer.
[0036] The second electrode-equipped section 14 includes substrate
19, electrode 22, typically of aluminum, nickel, iron, stainless
steel, and/or copper, and ion insertion material 24 which is
usually different than ion insertion material 20 in the first
electrode-equipped section 12. When the electrochemical cell 10 is
a lithium battery, the ion insertion material 24 is often referred
to as the "positive electrode" or "cathode". When the
electrochemical cell 10 is an electrochromic device, the ion
insertion material 24 is typically referred to as the ion storage
layer. The ion insertion material 24 in an electrochromic device
may optionally be an electrochromic ion insertion material which is
complementary to the negative electrode (discussed below). The ion
conducting or electrolyte material 16 is positioned between the ion
insertion material 20 and the ion insertion material 24.
[0037] A variation of the above-described conventional (prior art)
electrochemical cell 30 is shown in FIG. 2 and comprises a first
electrode-equipped section 36, a second electrode-equipped section
38, and an ion conducting layer 16 therebetween. Section 36
comprises layer 32, which is both the first substrate and the anode
of device 30, and section 38 comprises layer 34 which is both the
second substrate and the cathode of device 30. Layers 20 and 24 of
device 30 are ion insertion materials as described above.
[0038] A common problem associated with conventional liquid or
gel-type electrochemical cells, such as cells 10 and 30 shown in
FIGS. 1 and 2 wherein the ion conducting or electrolyte material 16
is a liquid or a gel, is degradation of faces 26 and 28 of the ion
insertion materials 20 and 24, respectively. While not wishing to
be bound by theory, the inventors believe that degradation of
conventional liquid and gel-type electrochemical devices 10, in
which each ion insertion layer 20 and 24 is interfaced with the
liquid or gel-type ion conducting layer 16, may occur as a result
of chemical corrosion of the ion insertion materials 20 and 24,
respectively, by the liquid or gel ion conducting layer 16. Such
problems are obviated in accordance with this invention by the use
of protective, solid-state ion conducting layers interposed between
the ion insertion materials and the liquid or gel-type ion
conducting layer 16, wherein the solid-state ion insertion
materials act as protective layers between an ion insertion layer
and a liquid or gel-type ion conducting layer to prevent corrosion
or other degradative effects of the ion insertion layers.
[0039] One preferred embodiment of a liquid or gel-type
electrochemical cell 40 with protective solid state ion conductor
or electrolyte layers 50, 52 according to this invention is shown
in FIG. 3. This comprises a first electrode-equipped section 42, a
second electrode-equipped section 44, and a liquid or gel-type ion
conducting layer 16 sandwiched between the first electrode-equipped
section 42 and the second electrode-equipped section 44. First
electrode-equipped section 42 of liquid or gel-type electrochemical
cell 40 comprises first substrate 17, first electrode 18 disposed
on substrate 17, a first ion insertion material 20 disposed on
first electrode 18 and having a face 26 on a side opposite to the
first electrode 18, and a first protective solid ion conducting
layer 50 disposed on face 26 of first ion insertion material 20.
Second electrode-equipped section 44 of liquid or gel-type
electrochemical cell 40 comprises second substrate 19, second
electrode 22 disposed on substrate 19, a second ion insertion
material 24 disposed on second electrode 22 and having a face 28 on
a side opposite to the second electrode 22, and a second protective
solid ion conducting layer 52 disposed on face 28 of second ion
insertion material 24.
[0040] First electrode-equipped section 42 and second
electrode-equipped section 44 of electrochemical cell 40 of this
invention are disposed in spaced relation to one another such that
solid ion conducting layers 32 and 34 face each other and are
separated by one or more spacers 35. The solid ion conducting
layers 32 and 34 together with spacer(s) 35 define a void which is
filled with liquid or gel-type ion conducting material 16.
[0041] Electrochemical cell 40 of the present invention has
significantly improved durability compared to conventional liquid
or gel-type electrochemical cell 10. The term "durability" as used
herein refers to the ability to repeatedly cycle voltage through an
electrochemical cell while maintaining the integrity of the
electrochemical cell over a prolonged life-cycle. In reference to
an electrochromic device, durability refers to the ability to
repeatedly cycle voltage through an electrochromic device without
increased resistance to loss of optical properties (e.g., color
intensity). In reference to a lithium rechargeable battery,
durability refers to the ability to repeatedly cycle voltage
through a lithium rechargeable battery without a loss of
charge-discharge capacity. In practice, a cycle life of greater
than 10.sup.7 cycles is desirable for most applications. The term
"improved durability" as used herein means that a liquid or
gel-type electrochemical cell 40 of the invention has the ability
to cycle with voltage for a greater number of cycles with
significantly reduced loss of charge-discharge properties (when the
electrochemical cell is a lithium battery) or without loss of
optical properties (when the electrochemical cell is an
electrochromic device) when compared to a conventional liquid or
gel-type electrochemical cell 10.
[0042] A novel feature of the present invention, which improves the
durability of electrochemical cell 40, is the incorporation of
protective solid state ion conducting layers 50 and 52 in liquid or
gel-type electrochemical cell. It was discovered that solid ion
conducting layers 50 and 52 act as protective layers by preventing
liquid or gel-type ion conducting layer 16 from degrading first ion
insertion material and second ion insertion material 24,
respectively, while still allowing the flow of positive ions
through liquid or gel-type electrolyte layer 16 to either the first
or second ion insertion layers 50 and 52. Consequently, protective
solid-state ion conducting layers 50 and 52 increase the durability
of the electrochemical cell 40 (FIG. 3) of this invention by
minimizing or preventing degradation of ion insertion layers 20 and
24, which is a common problem with conventional liquid or gel-type
electrochemical cell 10 (FIG. 1).
[0043] Solid ion conducting layers 50 and 52 of electrochemical
cell 40 preferably comprise one or more solid electrolyte
material(s) (i.e., solid ion conducting material(s)) that is/are an
excellent conductor of a positively charged ion (e.g., Li.sup.+ or
H.sup.+) and a poor electric conductor (e.g., a poor conductor of
negatively charged electrons). As used throughout, the terms
"electrolyte material" and "ion conducting material" are
interchangeable and refer to a solid, liquid, or gel (polymer)-type
material which conducts positive ions but does not conduct negative
ions. Lithium ion (Li.sup.+) conductors are a preferred material
for solid ion conducting layers 50 and 52, since lithium ion
conductors generally have the combined features of high ionic
conductivity with greater chemical stability than do other solid
ion conductors, such as hydrogen ion conductors. When
electrochemical cell 40 is an electrochromic device, the solid ion
conducting material for use as protective solid ion conducting
layers 50 and 52 is preferably transparent. Suitable materials for
use as protective solid ion conducting layers 50 and 52 include
lithium aluminum fluoride (LiAlF.sub.4), and lithium phosphorous
oxinitride compounds including Li.sub.xPO.sub.yN.sub.z, where "x"
is approximately equal to 2.8, the sum of "2y+3z" is approximately
equal to 7.8, and "z" has a value between 0.16 and 0.46. Such
Li.sub.xPO.sub.yN.sub.z compounds are described in U.S. Pat. No.
5,597,660 to Bates, et al., which is incorporated herein by
reference. Other suitable materials for the solid ion conducting
layer include, but are not limited to, LiI, Li.sub.2WO.sub.4,
LiSO.sub.4, LiIO.sub.3, Li.sub.4SiO.sub.4, Li.sub.2Si.sub.2O.sub.5,
LiAlSiO.sub.4, Li.sub.4(Si.sub.0.7Ge.sub.0.3)O.sub.4,
Li.sub.4GeO.sub.4, LiAlCl.sub.4, Li.sub.3PO.sub.4, Li.sub.3N,
Li.sub.2S, Li.sub.2O, Li.sub.5AlO.sub.4, Li.sub.5GaO.sub.4,
Li.sub.6ZnO.sub.4, LiAr.sub.2(PO.sub.4).sub.3,
LiHf.sub.2(PO.sub.4).sub.3, LiInS.sub.2, LiMgF and LiAlMgF.sub.4.
Solid ion conducting layers 50 and 52 of liquid or gel-type
electrochromic device 40 are preferably between about 500-5000
Angstroms thick.
[0044] Similarly, liquid or gel-type ion conducting layer 16 of
electrochromic device 40 should also be an electrolyte material
that has high ionic conductivity and low electric conductivity.
Preferably, the liquid or gel-type ion conducting layer 16 is an
excellent conductor of lithium ions (Li.sup.+). When ion conducting
layer 16 is a liquid electrolyte layer, the liquid electrolyte may
be obtained by dissolving a lithium salt in a suitable
solvent--preferably a non-aqueous solvent. Suitable lithium salts
for preparing ion conducting liquid electrolyte material 16 include
LiClO.sub.4, LiBF.sub.4, LiAlCL.sub.4, LiCF.sub.3SO.sub.3,
LiAsF.sub.6, LiCl, and other compounds known in the art which
exhibit similar ion conducting properties. Suitable non-aqueous
solvents for use in preparing liquid electrolyte material 16
include propylene carbonate, tetrahydrofuran and its derivatives,
acetonitrile, 1,3-dioxalane -methyl-2-pyrrolidone, sulpholane
methylformate, dimethyl sulfate, .gamma.-butyrolactone,
1,2-dimethoxyethane, and other non-aqueous solvents which are known
in the art which exhibit similar properties. In one embodiment, ion
conducting layer 16 is a liquid electrolyte material comprising
LiClO.sub.4 dissolved in propylene carbonate to form a 1 molar
concentration. When electrolyte layer 16 is a gel-type electrolyte
material (also known in the art as a polymer gel electrolyte), the
gel-type electrolyte material may be obtained by adding a
conventional liquid electrolyte (e.g., lithium perchlorate
dissolved in propylene carbonate) to a cross-linkable polymer host
which functions as a container for the liquid electrolyte material.
Suitable polymer hosts include, but are not limited to,
polyacrylonitrile, poly(ethylene oxide), poly(methyl methacrylate),
poly(vinylidene fluoride), poly(vinylidene
fluoride-co-hexafluoropropylene), polyethylene glycol, diacrylate,
and trimethylolpropane triacrylate.
[0045] A necessary requirement in the selection of the solid
electrolyte material for use as solid ion conducting layers 50 and
52 and the selection of liquid or gel-type electrolyte material 16
is that both the solid ion conducting material and liquid or
gel-type electrolyte material has the same positive ion. That is,
if the positive ion in the liquid or gel-type electrolyte material
is a lithium ion (Li.sup.+), then the positive ion in the solid
electrolyte material should also be a lithium ion (Li.sup.+).
[0046] Referring again to electrochemical cell 40 of this invention
as illustrated in FIG. 3, solid ion conducting layers 50 and 52 are
sandwiched between ion insertion layers 20 and 24. As used herein,
the term "ion insertion material" refers to a mixed conductor
(i.e., ionic and electric) in which positive and negative ions can
be rapidly and reversibly inserted. That is, for charge neutrality
of the ion insertion material, an electron (i.e., a negative ion)
is inserted into the ion insertion material from the electrode 18
or 20 whenever a positive ion (e.g., Li.sup.+ or H.sup.+) is
inserted into the ion insertion material from the ion conducting
layer 16, and likewise an electron is extracted whenever a positive
ion is extracted. Examples of ion-insertion materials which are
suitable for ion insertion layers 20 and 24 in a lithium
rechargeable battery include, but are not limited to,
Li.sub.xTiS.sub.2, Li.sub.xV.sub.2O.sub.5, Li.sub.xCoO.sub.2,
Li.sub.xMn.sub.2O.sub.4, Li.sub.xTiO.sub.2, Li.sub.xSnO.sub.2 and
Li.sub.xNiO.sub.2.
[0047] An ion insertion material in an electrochromic device is an
ion insertion material whose optical properties (e.g., degree of
color change) depends strongly on the number of inserted ions.
Electrochromic ion insertion materials may thus be regarded as
storage batteries with a visible state of color change. Suitable
electrochromic materials for ion insertion layers 20 and 24 when
electrochemical cell 40 is an electrochromic device include
cathodic electrochromic materials, which take on color in a reduced
state, and anodic electrochromic materials, which take on color in
an oxidized state. Suitable materials for ion insertion layers 20
and 24 in an electrochromic device include, but are not limited to,
transition metal oxides, transition metal sulfides, transition
metal oxysulfides, transition metal halides, selenides, tellurides,
chromates, molybdates, tungstates, vanadates, niobates, tantalates,
titanates, stannates, etc. especially oxides, sulfides and
stannates of metals of Groups IV-B, V-B, and VI-B, and oxides and
sulfides of Lanthanide Series metals, and more particularly,
tungsten oxide, molybdenum oxide, titanium oxide, vanadium oxide,
niobium oxide, iridium oxide, rhodium oxide, nickel oxide, cerium
oxide, copper stannate, and cobalt tungstate. Ion insertion layers
20 and 24 are each preferably between about 1000 to 10,000
angstroms thick, and more preferably between 1000-5000 angstroms
thick. Preferably, when electrochemical cell 40 is an
electrochromic device, ion insertion layer 20 is tungsten oxide
(WO.sub.3) (which takes on color in a reduced state) and is about
5000 angstroms thick, and ion insertion layer 24 is a complementary
electrochromic ion insertion material, that is, an anodic
electrochromic ion insertion material which takes on color in an
oxidized state. Suitable anodic electrochromic ion insertion
materials include V.sub.2O.sub.5, IrO.sub.2, and NiO.sub.2.
Preferably ion insertion layer 24 is V.sub.2O.sub.5 and is about
5000 angstroms thick.
[0048] Referring again to FIG. 3, ion insertion materials 20 and 24
of electrochemical cell 40 are sandwiched between first and second
electrodes 18 and 22. Suitable materials for first and second
electrodes 18 and 22, for use in lithium rechargeable batteries
include highly conductive metals such as aluminum, copper, nickel,
platinum, and palladium. When electrochemical cell 40 is lithium
rechargeable battery, first and second electrodes 18 and 22 of
device 40 are preferably between 1000 .ANG. and 10,000 .ANG. thick.
Suitable materials for first and second electrodes 18 and 22 for
use in liquid or gel-type electrochromic devices include highly
conductive, transparent materials such as doped metal oxides,
including tin oxides (SnO.sub.2:F or SnO.sub.2:Sb), indium-tin
oxides (ITO) such as In.sub.2O.sub.3:Sn, and zinc oxides (ZnO:In,
ZnO.sub.2:Al), and ultrathin, transparent metallic films including,
but not limited to, gold (Au), aluminum (Al), silver (Ag), and
copper (Cu). However, other substrates of various kinds of
materials may be used depending on the use of electrochemical cell
40. When electrochemical cell 40 is an electrochromic device, first
and second electrodes 18 and 22 of electrochemical cell 40 are
preferably doped indium-tin oxide (ITO) and are between 1000 and
5000 angstroms thick. Alternatively, when electrochromic cell 40 is
an electrochromic mirror device, a metallic reflector layer
typically replaces one of the electrodes 18 or 22.
[0049] Referring again to FIG. 3, first and second electrodes 18
and 22 of electrochemical device 40 are sandwiched between
substrates 17 and 19. Suitable substrates 17 and 19 when
electrochemical device 40 is a lithium rechargeable battery may be
transparent or non-transparent and include glass, polymers, and
thin plastic substrates. Suitable substrates 17 and 19 when
electrochemical device 40 is an electrochromic device, one or both
substrates 17 and/or 19 is transparent, and substrates 17 and 19
are preferably glass or plastic substrates. Alternatively, resins
such as polymethylmethacrylate, polycarbonate, and the like may be
used.
[0050] Referring again to FIG. 3, the assembly of electrochemical
cell 40 of this invention will be described. For the sake of
simplicity, but meant to be limiting, the assembly will be
described for the assembly of an electrochromic device.
Electrochromic device 40 may be assembled by first separately
preparing sections 42 and 44 of the device 40. To assemble section
42, a film of transparent conducting material is deposited on
substrate 17 to form first electrode 18. The deposition method for
forming electrode 18 may be any film-forming methods known in the
art, such as vacuum deposition, sputtering, ion plating, chemical
vapor deposition, screen printing, sol-gel deposition, and the
like. Next, ion insertion layer 20 comprising an electrochromic
material is formed by depositing electrochromic material such as
vanadium oxide (V.sub.2O.sub.5) onto electrode 18 by any of the
film-forming techniques described above. Next, solid ion conducting
layer 50 is deposited onto ion insertion layer by depositing a
solid electrolyte such as LiAlF.sub.4, or Li.sub.xPO.sub.yN.sub.z
onto ion insertion layer 20 by known deposition techniques. For
example, a layer of Li.sub.xPO.sub.yN.sub.z may be deposited by
radio-frequency (rf) magnetron sputtering of a Li.sub.3PO.sub.4
target in a nitrogen atmosphere as described in U.S. Pat. No.
5,597,660 which is incorporated herein by reference, to complete
section 12 Due to the nature of the deposition process for forming
solid ion conducting layer 50, pinholes may occur in solid ion
conducting layer 50 of device 40. However, as will be described
below, such pinholes are not detrimental to device 40.
[0051] Section 44 of electrochromic device 40 is then assembled in
a manner similar to that described above for the assembly of
section 42, with the exception that ion insertion material 24 of
section 44 is either an electrochromic material that is
complementary to ion insertion material 20, or ion insertion
material 24 may be a non-electrochromic ion insertion material such
as magnesium fluoride (MgF.sub.2), tin oxide (SnO.sub.2), or
silicon dioxide (SiO.sub.2).
[0052] Once sections 42 and 44 of device 40 have been assembled,
sections 42 and 44 are positioned parallel to one another with
first and second protective solid ion conducting layers 50 and 52
facing each other and spaced apart by one or more spacers 35 around
the perimeter edges of layers 50 and 52. Spacer(s) 35 is typically
a polymer, and serves not only to separate sections 42 and 44, but
also to contain liquid or gel-type ion conducting material 16.
Other suitable materials for use as spacer(s)35 include tape,
polymers containing glass beads, and other materials known to those
skilled in the art. Spacer(s) 35, together with protective solid
ion conducting layers 32 and 34, define a space for containing
liquid or gel-type ion conducting material 16. After sections 42
and 44 are joined, a hole is drilled either through section 42 or
section 44 to the space defined by layers 50, 52, and spacer(s) 35.
A vacuum is applied to the hole to evacuate the defined void, and
then liquid or gel-type ion conducting material 16 is injected into
the voided space and the hole is sealed.
[0053] The above-described method of assembling electrochemical
device 40 is merely exemplary and is not intended to be limiting.
Thus, other methods known in the art may be utilized to assemble
electrochromic device 40 of this invention. In addition, the method
of assembly described above is not limited to the assembly of
electrochromic devices, but the above described method as well as
other methods known in the art, may be utilized to assembly
electrochemical devices 40 of this invention comprising protective
solid ion conducting layers 50 and 52. For example, the method
described by Lee et al. (Electrochemical and Solid-State Letters
(1999) 2:425), which is incorporated herein by reference, may also
be employed for assembling device 40.
[0054] To operate electrochromic device 40 described above, two
leads (not shown) are connected to transparent conducting electrode
layers 18 and 22 to provide an electric potential and circuit
across electrochromic device 40 which is necessary to cause
electrochromic layer 20 and ion insertion layer 24 to change color
during an applied voltage. The leads in turn may be connected to a
polarity reversing switch, which allows for the polarity of the
charge across the electrochromic device 40 to be reversed, thereby
changing the electrochromic layers from colored to colorless, or
vice versa.
[0055] As discussed above, since the solid protective ion
conducting layers 50 and 52 are formed in the same manner as a
solid electrolyte layer in a conventional solid-state
electrochemical cell, some pinholes may be present in the
protective solid electrolyte layers 50 and/or 52 of liquid or
gel-type electrochemical cell 40 of this invention. Surprisingly,
it was discovered by the inventors that solid protective ion
conducting layers 50 and 52 need not be free of pinholes to provide
sufficient protection to ion insertion layers 20 and 24. Any
pinholes present in the solid protective ion conducting layers 50
and/or 52 were discovered to be, at the most, minimally detrimental
to the performance of liquid or gel-type electrochemical cell 40.
Referring to FIG. 3, if for example a pinhole (not shown) was
present in solid ion conducting layer 50, only a pinhole-sized
amount of liquid electrolyte 16 would pass through the solid ion
conducting layer 50 and come in contact with face 26 of ion
insertion layer 20. Consequently, only a pinhole-sized area of
degradation may occur on face 26, which would not cause the
dramatic deterioration of liquid or gel-type electrochemical cell
40, which is a common occurrence in conventional liquid or gel-type
electrochemical cells 10 (FIG. 1). While not wishing to be bound by
any theory, the inventors believe that degradation of conventional
liquid or gel-type electrochemical cells, such as cell 10 shown in
FIG. 1, may occur as a result of chemical corrosion of faces 26 and
28 ion insertion layers 20 and 24, respectively, by the liquid or
gel-type ion conducting material 16. This is due to the fact that,
in conventional liquid or gel-type electrochemical cells 10, the
liquid or gel-type electrolyte layer 16 is in contact with the
entire surface of faces 26 and 28 of ion insertion layers 20 and
24, respectively. Therefore, whereas in conventional liquid or
gel-type electrochemical cells 10 the liquid or gel electrolyte
layer 16 can eventually degrade the entire face of ion insertion
layers 20 and 24 at the liquid electrolyte/ion insertion layer
interface, the novel design of electrochemical cell 40 avoids this
degradation problem observed with conventional electrochemical
cells 10 in that electrochemical cell 40 of this invention has
protective solid ion conducting layers 50 and 52 interposed between
ion insertion layers 20 and 24, respectively, and liquid or
gel-type electrolyte layer 16. Protective solid ion conducting
layers 50 and 52 are thick enough to prevent liquid or gel-type
electrolyte 16 from contacting ion insertion layers 20 and 24,
while still allowing the flow of positive ions such as Li.sup.+
from ion insertion layer 20 to ion insertion layer 24 during
discharge of cell 40, and similarly to allow the flow of the
positive ions in the reverse direction (from ion insertion layer 24
to ion insertion layer 20) when the cell 40 is being charged.
[0056] FIG. 4 illustrates an alternative embodiment of this
invention, which is an improvement over conventional
electrochemical cell 30 illustrated in FIG. 2. Electrochemical cell
70 of FIG. 4 comprises first electrode-equipped section 74, second
electrode-equipped section 78, and a liquid or gel-type ion
conducting layer 16 sandwiched therebetween. First and second
electrode-equipped sections 74 and 78 are separated by spacers 35
as described above. First electrode-equipped section 74 comprises a
first electrode 32, a first ion insertion material 20 disposed on
first electrode 32 and having a face 26 on a side opposite to the
first electrode 32, and a first protective solid ion conducting
layer 50 disposed on face 26 of first ion insertion material 20.
Second electrode-equipped section 78 comprises second electrode 34,
a second ion insertion material 24 disposed on second electrode 34
and having a face 28 on a side opposite to the second electrode 34,
and a second protective solid ion conducting layer 52 disposed on
face 28 of second ion insertion material 24. Thus, the primary
difference between electrochemical cell 40 of FIG. 3 and
electrochemical cell 70 of FIG. 4 is that electrode 32 of
electrochemical cell 70 functions both as the substrate and
electrode 17 and 18 of electrochemical cell 40 (FIG. 3), and
likewise electrode 34 of electrochemical cell 70 (FIG. 4) functions
both as the substrate and electrode 19 and 22 of electrochemical
cell 30. Electrochemical cell 70 of this invention has increased
durability compared to conventional electrochemical cell 30 due to
the presence of protective solid ion conducting layers 50 and 52
for the reasons explained above.
[0057] As discussed above, this invention in general provides a
method of protecting any ion insertion layer from the degradative
effects of a liquid or gel-type electrolyte by inserting a
protective solid ion conducting layer between the ion insertion
layer and the liquid or gel-type electrolyte layer. Consequently,
the scope of this invention includes methods of protecting ion
insertion layers in devices other that those described in FIGS. 3
and 4, and further includes any device having a protective solid
ion conducting layer disposed between an ion insertion layer and a
liquid or gel-type electrolyte layer.
[0058] FIG. 5 compares the charge/discharge capacity of a protected
vanadium oxide (V.sub.2O.sub.5) electrode of this invention with an
unprotected electrode. In FIG. 5, curve 100 represents the ion
insertion behavior of a V.sub.2O.sub.5 ion insertion layer coated
with a thin film of protective solid ion conducting layer lithium
aluminum fluoride (LiAlF.sub.4). Curve 102 represents the ion
insertion behavior of an uncoated V.sub.2O.sub.5 electrode. Both
electrodes were tested in a liquid or gel-type electrochemical cell
comprising 1 M LiCl0.sub.4 in propylene carbonate as the liquid or
gel-type electrolyte. Curve 100 shows that, for a V.sub.2O.sub.5
electrode coated with a protective solid electrolyte film, the
capacity remains substantially constant over repeated cycling of
the electrode. In contrast, the V.sub.2O.sub.5 electrode which does
not have a protective coating, represented by curve 102, loses
capacity with repeated cycling. FIG. 6 summarizes the results of a
similar experiment in which the durability of a V.sub.2O.sub.5
electrode of this invention coated with a protective film of a
lithium phosphorous oxinitride material (Li.sub.xPO.sub.yN.sub.z)
(curve 104) was compared to an unprotected V.sub.2O.sub.5 electrode
(curve 106). As shown in FIG. 6, the Li.sub.xPO.sub.yN.sub.z-coated
V.sub.2O.sub.5 electrode (curve 104) had substantially improved
durability compared to the unprotected V.sub.2O.sub.5 electrode
(curve 106).
[0059] As a further illustration of the increased durability of
liquid or gel-type electrochemical cell of the present invention,
FIG. 7 shows a comparison of the charge/discharge capacity for
lithium ions of two different tungsten oxide (WO.sub.3) electrodes.
Curve 108 represents the ion insertion behavior of a WO.sub.3
electrode of this invention coated with a protective solid
electrolyte thin film of LiAlF.sub.4, and curve 110 represents the
ion insertion behavior of an uncoated WO.sub.3 electrode. Again,
the capacity of the electrode coated with a protective solid
electrolyte film of LiAlF.sub.4 (curve 108) remained substantially
constant over repeated cycling of the electrode. In contrast, the
V.sub.2O.sub.5 electrode which did not have a protective coating,
represented by curve 110, lost capacity with repeated cycling. FIG.
8 shows a comparison of the charge/discharge capacity for lithium
ions of a tungsten oxide (WO.sub.3) electrode of this invention
coated with a thin film of the protective solid ion conducting
layer a lithium phosphorous oxinitride (Li.sub.xPO.sub.yN.sub.2)
(curve 112), and an uncoated WO.sub.3 electrode (curve 114).
EXAMPLES
[0060] 1. Preparation of Thin Films of V.sub.2O.sub.5 Protected
with LiAlF.sub.4 or Li.sub.xPO.sub.yN.sub.z
[0061] Thin films of V.sub.2O.sub.5 were deposited by thermal
evaporation of V.sub.2O.sub.5 powders. The base pressure for the
deposition process was typically 10.sup.-5 mbar. The solid
electrolyte, LiAlF.sub.4, was also deposited by conventional
thermal evaporation of the corresponding powder. The solid
electrolyte, Li.sub.xPO.sub.yN.sub.z, was deposited by RF magneton
sputtering of a Li.sub.3PO.sub.4 target in a nitrogen
atmosphere.
[0062] Each electrode was cycled at a constant current between
preset voltage limits. Glass beaker-type test cells were used to
evaluate the electrochemical properties of the vanadium oxide
half-cells. In these half-cell experiments, the setup consists of a
V.sub.2O.sub.5 film as the working electrode, lithium metal foil as
the counter electrode and reference electrode, and 1M LiClO.sub.4
propylene carbonate as the electrolyte. FIG. 5 compares the
charge/discharge capacity for lithium ions versus cycle number of a
vanadium oxide electrode of this invention coated with LiAlF.sub.4
(curve 100) to the capacity versus cycle number of an uncoated
vanadium oxide electrode (curve 102). Curve 100 shows that, for a
V.sub.2O.sub.5 electrode coated with a protective solid electrolyte
film of LiAlF.sub.4, the capacity remains substantially constant
over repeated cycling of the electrode. In contrast, the
V.sub.2O.sub.5 electrode which does not have a protective coating,
represented by curve 102, loses capacity with repeated cycling.
[0063] FIG. 6 compares the charge/discharge capacity for lithium
ions versus cycle number of a vanadium oxide electrode of this
invention coated with Lipon.TM. (curve 104) to the capacity versus
cycle number of an uncoated vanadium oxide electrode (curve 106).
Curve 104 shows that, for the V.sub.2O.sub.5 electrode coated with
a protective solid electrolyte film of Li.sub.xPO.sub.yN.sub.z, the
capacity remains substantially constant over repeated cycling of
the electrode. Again, the V.sub.2O.sub.5 electrode which does not
have a protective coating, represented by curve 106, loses capacity
with repeated cycling.
[0064] 2. Preparation of Thin Films of WO.sub.3 Protected with
LiAlF.sub.4 or Li.sub.xPO.sub.yN.sub.z
[0065] Thin films of WO.sub.3 were deposited by thermal evaporation
of WO.sub.3 powders. The base pressure for the deposition process
was typically 10.sup.-5 mbar. The solid electrolyte, LiAlF.sub.4,
was also deposited by conventional thermal evaporation of the
corresponding powder. The solid electrolyte,
Li.sub.xPO.sub.yN.sub.z, was deposited by RF magneton sputtering of
a Li.sub.3PO.sub.4 target in a nitrogen atmosphere.
[0066] Each electrode was cycled at a constant current between
preset voltage limits. Glass beaker-type test cells were used to
evaluate the electrochemical properties of the tungsten oxide
half-cells. In these half-cell experiments, the setup consists of a
WO.sub.3 film as the working electrode, lithium metal foil as the
counter electrode and reference electrode, and 1M LiClO.sub.4
propylene carbonate as the electrolyte. FIG. 7 shows a comparison
of the cycling capacity of two different tungsten oxide (WO.sub.3)
electrodes. Curve 108 represents a WO.sub.3 electrode of this
invention coated with a protective solid electrolyte thin film of
LiAlF.sub.4, and curve 110 represents an uncoated WO.sub.3
electrode. Again, the capacity of the electrode coated with a
protective solid electrolyte film of LiAlF.sub.4 (curve 108)
remained substantially constant over repeated cycling of the
electrode. In contrast, the V.sub.2O.sub.5 electrode which did not
have a protective coating, represented by curve 110, lost capacity
with repeated cycling.
[0067] FIG. 8 shows a comparison of the cycling capacity of a
tungsten oxide (WO.sub.3) electrode of this invention coated with a
protective solid electrolyte thin film of Li.sub.xPO.sub.yN.sub.z
(curve 112), and an uncoated WO.sub.3 electrode (curve 114). Again,
the capacity of the WO.sub.3 coated with the protective solid ion
conducting layer is improved over the uncoated WO.sub.3.
[0068] The foregoing description is considered as illustrative only
of the principles of the invention. Further, since numerous
modifications and changes will readily occur to those skilled in
the art, it is not desired to limit the invention to the exact
construction and process shown as described above. Accordingly, all
suitable modifications and equivalents may be resorted to falling
within the scope of the invention as defined by the claims which
follow.
[0069] The words "comprise," "comprising", "include," "including,"
and "includes" when used in this specification and in the following
claims are intended to specify the presence of stated features,
integers, components, or steps, but they do not preclude the
presence or addition of one or more other features, integers,
components, steps, or groups thereof.
* * * * *