U.S. patent application number 12/267659 was filed with the patent office on 2009-05-14 for all-solid-state cell.
This patent application is currently assigned to Kyushu University. Invention is credited to Eiji Kobayashi, Shigeto Okada, Yosuke Sato, Kazuhiro Yamamoto, Toshihiro Yoshida.
Application Number | 20090123847 12/267659 |
Document ID | / |
Family ID | 40342231 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090123847 |
Kind Code |
A1 |
Okada; Shigeto ; et
al. |
May 14, 2009 |
ALL-SOLID-STATE CELL
Abstract
An all-solid-state cell has a fired solid electrolyte body, a
first electrode layer integrally formed on one surface of the fired
solid electrolyte body by mixing and firing an electrode active
material and a solid electrolyte, and a second electrode layer
integrally formed on the other surface of the fired solid
electrolyte body by mixing and firing an electrode active material
and a solid electrolyte. The first and the second electrode layers
are formed by mixing and firing the electrode active material and
the amorphous solid electrolyte, which satisfy the relation
Ty>Tz (wherein Ty is a temperature at which the capacity of the
electrode active material is lowered by reaction between the
electrode active material and the solid electrolyte material, and
Tz is a temperature at which the solid electrolyte material is
shrunk by firing).
Inventors: |
Okada; Shigeto;
(Fukuoka-City, JP) ; Kobayashi; Eiji;
(Fukuoka-City, JP) ; Yamamoto; Kazuhiro;
(Nagoya-City, JP) ; Yoshida; Toshihiro;
(Nagoya-City, JP) ; Sato; Yosuke; (Hashima-Gun,
JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Assignee: |
Kyushu University
Fukuoka-City
JP
NGK Insulators, Ltd.
Nagoya-City
JP
|
Family ID: |
40342231 |
Appl. No.: |
12/267659 |
Filed: |
November 10, 2008 |
Current U.S.
Class: |
429/319 ;
429/209; 429/221; 429/231.5; 429/231.95; 429/304 |
Current CPC
Class: |
H01M 4/5825 20130101;
H01M 10/0562 20130101; Y02E 60/10 20130101; H01M 10/052
20130101 |
Class at
Publication: |
429/319 ;
429/209; 429/304; 429/231.95; 429/231.5; 429/221 |
International
Class: |
H01M 6/18 20060101
H01M006/18; H01M 4/64 20060101 H01M004/64; H01M 4/58 20060101
H01M004/58 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 12, 2007 |
JP |
2007-293682 |
Oct 17, 2008 |
JP |
2008-268333 |
Claims
1. An all-solid-state cell comprising positive and negative
electrode portions containing an electrode active material, an
electrolyte portion containing a solid electrolyte, and positive
and negative collector portions, wherein one or both of the
positive and negative electrode portions are formed by mixing and
firing the electrode active material and an amorphous solid
electrolyte material, and the electrode active material and the
solid electrolyte material satisfy an inequality: Ty>Tz wherein
Ty is a temperature at which a capacity of the electrode active
material is lowered by a reaction between the electrode active
material and the solid electrolyte material, and Tz is a
temperature at which the solid electrolyte material is shrunk by
firing.
2. An all-solid-state cell according to claim 1, wherein Tz is a
temperature at which the relative density of the solid electrolyte
material is increased to 70% or more of the theoretical density
thereof due to the shrinkage by firing.
3. An all-solid-state cell according to claim 1, wherein the solid
electrolyte material comprises an amorphous polyanion compound, and
the one or both of the positive and negative electrode portions are
formed by mixing and firing the electrode active material and the
solid electrolyte material.
4. An all-solid-state cell according to claim 1, wherein the solid
electrolyte material comprises an amorphous phosphate compound, and
the one or both of the positive and negative electrode portions are
formed by mixing and firing the electrode active material and the
solid electrolyte material.
5. An all-solid-state cell according to claim 4, wherein the solid
electrolyte material comprising the phosphate compound is of
Nasicon type after the firing.
6. An all-solid-state cell according to claim 5, wherein the
phosphate compound of the solid electrolyte material is LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3
(0.ltoreq.x.ltoreq.1).
7. An all-solid-state cell according to claim 5, wherein the
phosphate compound of the solid electrolyte material is LATP
Li.sub.1+xAl.sub.xTi.sub.2-x(PO.sub.4).sub.3
(0.ltoreq.x.ltoreq.1).
8. An all-solid-state cell according to claim 1, wherein the
electrode active material is a Nasicon type material comprising a
phosphate compound.
9. An all-solid-state cell according to claim 8, wherein the
phosphate compound of the electrode active material is LVP
Li.sub.mV.sub.2(PO.sub.4).sub.3 (1.ltoreq.m.ltoreq.5).
10. An all-solid-state cell according to claim 1, wherein the
electrode active material for the positive electrode portion is an
olivine type positive electrode active material comprising a
phosphate compound.
11. An all-solid-state cell according to claim 10, wherein the
phosphate compound of the positive electrode active material is LNP
Li.sub.nNiPO.sub.4, LCP Li.sub.nCoPO.sub.4, LMP Li.sub.nMnPO.sub.4
or LFP Li.sub.nFePO.sub.4 (0.ltoreq.n.ltoreq.1).
12. An all-solid-state cell according to claim 2, wherein the solid
electrolyte material and the electrode active material are of
Nasicon type after the firing.
13. An all-solid-state cell according to claim 2, wherein the solid
electrolyte material and the electrode active material are of
Nasicon type, the solid electrolyte material comprises LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 (0.ltoreq.x.ltoreq.1),
and the electrode active material comprises LVP
Li.sub.mV.sub.2(PO.sub.4).sub.3 (1.ltoreq.m.ltoreq.5) in both the
positive and negative electrode portions, whereby the
all-solid-state cell has a symmetrical structure.
14. An all-solid-state cell according to claim 1, wherein one or
both of the positive and negative electrode portions are formed by
firing under an applied pressure.
15. An all-solid-state cell according to claim 1, wherein one or
both of the positive and negative electrode portions are formed
from a paste for printing by firing it under an inert atmosphere.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2007-293682 filed on
Nov. 12, 2007 and Japanese Patent Application No. 2008-268333 filed
on Oct. 17, 2008 in the Japanese Patent Office, of which the
contents are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an all-solid-state cell
utilizing a combination of an electrode active material and a solid
electrolyte material.
[0004] 2. Description of the Related Art
[0005] In recent years, with the advancement of portable devices
such as personal computers and mobile phones, there has been
rapidly increasing demand for batteries usable as a power source
thereof. In cells of the batteries for the purposes, a liquid
electrolyte (an electrolytic solution) containing a combustible
organic diluent solvent has been used as an ion transfer medium.
The cell using such an electrolytic solution can cause problems of
solution leakage, ignition, explosion, etc.
[0006] In view of solving the problems, all-solid-state cells,
which use a solid electrolyte instead of the liquid electrolyte and
contain only solid components to ensure intrinsic safety, have been
developing. The all-solid-state cell contains a sintered ceramic as
the solid electrolyte, and thereby does not cause the problems of
ignition and liquid leakage, and is hardly deteriorated in battery
performance by corrosion. Particularly all-solid-state lithium
secondary cells can achieve a high energy density easily, and thus
have been actively studied in various fields (see, for example,
Japanese Laid-Open Patent Publication Nos. 2000-311710 and
2005-063958, Yusuke Fukushima and four others, "Fabrication of
electrode-electrolyte interface in all-solid-state cell lithium
batteries using the thermal softening adhesion behavior of
Li.sub.2S--P.sub.2S.sub.5 glass electrolytes", Lecture Summary of
Chemical Battery Material Association Meeting, Vol. 9th, Pages
51-52, issued on Jun. 11, 2007)
[0007] Japanese Laid-Open Patent Publication No. 2005-063958
discloses a thin-film, solid, lithium ion secondary cell. The
secondary cell described in Japanese Laid-Open Patent Publication
No. 2005-063958 is a bendable thin-film cell having a flexible
solid electrolyte and thin layers of positive and negative
electrode active materials sputtered thereon. The electrodes of the
cell have to be thin, and the amounts of the electrode active
materials are limited. Thus, the cell is disadvantageous in that it
is difficult to achieve a high capacity.
[0008] The article of Fukushima et al. reports formation of an
electrode-electrolyte interface of a complex of a glass electrolyte
and an electrode active material, utilizing softening fusion of the
glass electrolyte. In this report, it is described that the
resistance between electrolyte particles is effectively lowered due
to the fusion of the glass electrolyte, and further a heterophase
is not formed in a reaction between the electrolyte and the active
material.
[0009] However, an all-solid-state cell having positive and
negative electrodes is not described in this report, and it is
unclear whether the reaction resistance can be lowered in the
electrolyte-electrode active material interface. Further the
relation between the electric properties and the fact that the
heterophase is not formed is not specifically described, and the
charge-discharge ability of the all-solid-state cell is unknown.
Furthermore, the electrolyte used in this report is a sulfide,
which is expected to be unstable in the atmosphere (air). The
electrolyte may generate a toxic gas when brought into contact with
the air due to breakage or the like. Thus, this technology is
disadvantageous in safety.
[0010] The internal resistance of a cell is partly due to an
interface between an electrode active material and an electrolyte.
The resistance against transfer of electrons and Li ions through
the interface during charge and discharge is hereinafter referred
to as the interface reaction resistance. The present invention
relates to a technology for lowering the interface reaction
resistance in an all-solid-state cell system using a solid
electrolyte.
[0011] For example, in the conventional lithium ion cell using the
electrolytic solution, the electrolyte is a liquid containing an
organic solvent, though the electrode active material is a solid.
Therefore, the electrolyte can readily penetrate between particles
of the electrode active material to form an electrolyte network in
the electrode layers, resulting in a low interface reaction
resistance.
[0012] In terms of the interface reaction resistance according to
the present invention, a reaction resistance per unit area of
connected particles largely depends on the combination of the
active material and the electrolyte to be used. As the connected
area between the particles is increased, the interface reaction
resistance of the entire cell is lowered and the internal
resistance is lowered such that resistances are parallel-connected
in an equivalent circuit. Thus, the interface reaction resistance
between the electrolyte and the active material can be lowered by
(1) selecting the material combination in view of smoothly
transferring the Li ions and (2) increasing the connection
interface area between the electrolyte and the active material per
an electrode capacity.
[0013] In the present invention, a combination of an electrode
active material and a solid electrolyte containing a common
polyanion or a combination of an electrode active material and a
solid electrolyte of phosphate compounds is used in view of the
process of (1), and a solid electrolyte is mixed with an electrode
active material to form a network in an electrode layer, whereby
the connection interface area between the electrode active material
and the solid electrolyte is remarkably increased to lower the
interface reaction resistance in view of the process of (2).
[0014] Japanese Laid-Open Patent Publication No. 2000-311710
discloses a solid electrolyte cell containing a solid electrolyte
material of an inorganic oxide, which forms a three-dimensional
network between particles of an electrode active material. Thus,
the inventors have selected the combination of the phosphate
compounds containing a common polyanion as the combination of the
electrode active material and the solid electrolyte suitable for
smoothly transferring the Li ions, and have produced an
all-solid-state cell having electrodes containing the solid
electrolyte between the electrode active material particles.
However, because the solid electrolyte was fired in the state of a
mixture with the electrode active material in the electrode layer,
the electrolyte was reacted with the active material, so that
reduction in the peak intensity of the active material and
formation of a heterophase were found in an XRD (X-ray diffraction)
observation. The active material in this state was subjected to a
charge-discharge ability measurement using an ideal system
containing an electrolytic solution. As a result, the
charge-discharge capacity of the active material was extremely
reduced, and the active material was incapable of charge and
discharge at its original theoretical capacity. Thus, the capacity
of the electrode active material was lowered.
[0015] Then, the inventors have lowered the firing temperature to
prevent the reaction between the electrode active material and the
solid electrolyte. However, the solid electrolyte particles were
not sufficiently sintered, the particle boundary resistance between
the solid electrolyte particles was increased, and the connection
interface area between the electrode active material and the solid
electrolyte was not increased. As a result, both the particle
boundary resistance of the solid electrolyte and the interface
reaction resistance of the electrode active material and the solid
electrolyte could not be lowered, whereby the resultant
all-solid-state cell had no charge-discharge capacity (no
charge-discharge ability).
SUMMARY OF THE INVENTION
[0016] In view of the above problems, an object of the present
invention is to provide such an all-solid-state cell that the
particle boundary resistance of a solid electrolyte can be lowered
in an electrode layer while preventing capacity reduction due to a
reaction of the solid electrolyte with an electrode active
material, a network of the solid electrolyte can be formed in the
electrode layer, the connection interface area between the solid
electrolyte and the electrode active material can be remarkably
increased to lower the interface reaction resistance, and thus
charge and discharge can be carried out even in the all solid
state.
[0017] In research of an all-solid-state cell having an electrode
layer composed of a mixture of a solid electrolyte material and an
electrode active material, the inventors have found that the
charge-discharge capacity of the electrode active material is
reduced below its original theoretical capacity due to reduction in
the crystallinity of the electrode active material and formation of
a heterophase by a reaction between the solid electrolyte material
and the electrode active material. Based on this finding, the
inventors have further found that when a combination of the
materials satisfies the inequality Ty>Tz (in which Ty is a
temperature at which the capacity of the electrode active material
is lowered by the reaction, and Tz is an initiation temperature at
which the solid electrolyte material is shrunk by firing), an
electrolyte network can be formed in the electrode layer to lower
the resistance within the temperature range of Tz to Ty, the
connection area between the materials can be increased while
preventing the reaction between the electrolyte material and the
electrode active material, and the interface reaction resistance at
the connection interface between the materials can be lowered,
whereby the resultant all-solid-state cell has a low internal
resistance.
[0018] In the present invention, a combination of phosphate
compounds containing a common polyanion may be selected as the
combination of an electrode active material and a solid electrolyte
material suitable for smoothly transferring Li ions, and the solid
electrolyte material comprising the phosphate compound may be
vitrified. In a specific example, a Nasicon type LAGP having a
relatively higher ion conductivity among the phosphate compounds
was vitrified, and the resultant solid electrolyte material had low
transition temperatures, Tg (glass transition point) of
approximately 480.degree. C. and Tx (crystallization temperature)
of approximately 590.degree. C. (see FIG. 10). This glass material
had a firing shrinkage initiation temperature of 550.degree. C. to
600.degree. C. Then, the reactivity between this vitrified solid
electrolyte material and the electrode active material was
evaluated, and crystallinity reduction and heterophase formation
were not observed even at a temperature sufficiently higher than
the firing shrinkage initiation temperature. Thus, the novel
combination of the phosphate compound materials containing a common
polyanion satisfied the relation of Ty>Tz.
[0019] As a result, the inventors found a condition for preventing
the deterioration in the charge-discharge ability of the electrode
active material due to the reaction between the electrode active
material and the solid electrolyte material while maintaining
sufficient connection of the solid electrolyte particles. The above
problems were solved based on this finding.
[0020] By using such materials for forming the mixture electrode
layer of the all-solid-state cell, the particle boundary resistance
between the solid electrolyte particles could be lowered while
preventing the reduction in the capacity of the electrode active
material, and the electrolyte network could be formed in the
electrode layer. Therefore, the connection interface area between
the electrode active material and the solid electrolyte material
could be remarkably increased to lower the interface reaction
resistance, and thus the resultant all-solid-state cell was capable
of charge and discharge operations even in the all solid state.
[0021] Thus, an all-solid-state cell according to the present
invention comprises positive and negative electrode portions
containing an electrode active material, an electrolyte portion
containing a solid electrolyte, and positive and negative collector
portions, and is characterized in that the one or both of the
positive and negative electrode portions are formed by mixing and
firing the electrode active material and an amorphous solid
electrolyte material, which satisfy the inequality:
Ty>Tz
wherein Ty is a temperature at which the capacity of the electrode
active material is lowered by a reaction between the electrode
active material and the solid electrolyte material, and Tz is a
temperature at which the solid electrolyte material is shrunk by
firing.
[0022] Specifically, Tz is a temperature at which the relative
density of the solid electrolyte material is increased to 70% or
more of the theoretical density thereof due to the firing
shrinkage. Tz is preferably a temperature at which the relative
density of the material is increased to 80% or more due to the
firing shrinkage within the temperature range of Ty>Tz.
[0023] Specifically, Ty is a temperature at which the
charge-discharge capacity of the electrode active material is
lowered below 50% of the original theoretical capacity thereof. Ty
is preferably a temperature at which the charge-discharge capacity
of the electrode active material is 80% or more of the theoretical
capacity within the temperature range of Ty>Tz.
[0024] In the present invention, the solid electrolyte material may
comprise an amorphous polyanion compound, and the one or both of
the positive and negative electrode portions may be formed by
mixing and firing the electrode active material and the solid
electrolyte material. Alternatively, the solid electrolyte material
may comprise an amorphous phosphate compound, and the one or both
of the positive and negative electrode portions may be formed by
mixing and firing the electrode active material and the solid
electrolyte material.
[0025] In the present invention, the solid electrolyte material
comprising the phosphate compound may be of Nasicon type after the
firing. In this case, the phosphate compound of the solid
electrolyte material may be LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 or LATP
Li.sub.1+xAl.sub.xTi.sub.2-x(PO.sub.4).sub.3
(0.ltoreq.x.ltoreq.1).
[0026] In the present invention, the electrode active material may
be a Nasicon type material comprising a phosphate compound. In this
case, the phosphate compound of the electrode active material may
be LVP Li.sub.mV.sub.2(PO.sub.4).sub.3 (1.ltoreq.m.ltoreq.5).
[0027] In the present invention, the electrode active material for
the positive electrode portion may be an olivine type positive
electrode active material comprising a phosphate compound. In this
case, the phosphate compound of the positive electrode active
material may be LNP Li.sub.nNiPO.sub.4, LCP Li.sub.nCoPO.sub.4, LMP
Li.sub.nMnPO.sub.4 or LFP Li.sub.nFePO.sub.4
(0.ltoreq.n.ltoreq.1).
[0028] In the present invention, the solid electrolyte material and
the electrode active material may be of Nasicon type after the
firing.
[0029] In the present invention, the all-solid-state cell may have
such a symmetrical structure that the solid electrolyte material
and the electrode active material are of Nasicon type, the solid
electrolyte material comprises LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 (0.ltoreq.x.ltoreq.1,
preferably 0.3.ltoreq.x.ltoreq.0.7), and the electrode active
material comprises LVP Li.sub.mV.sub.2(PO.sub.4).sub.3
(1.ltoreq.m.ltoreq.5) in both the positive and negative electrode
portions.
[0030] In the present invention, the one or both of the positive
and negative electrode portions may be formed by firing under an
applied pressure. In this case, by the firing under an applied
pressure, a dense microstructure can be formed in the one or both
of the positive and negative electrode portion, the interface area
between the electrode active material and the solid electrolyte
material can be increased, and the interface charge transfer
resistance can be lowered.
[0031] In the present invention, one or both of the positive and
negative electrode portions may be formed from a paste for printing
by firing it under an inert atmosphere. In this case, a binder
component can be carbonized to ensure the electron conductivity of
the electrode portion. Thus, the electron conductivity of the
electrode portion can be maintained without intentional addition of
a carbon component useful as an electron conducting aid.
[0032] As described above, in the all-solid-state cell of the
present invention, the particle boundary resistance between the
solid electrolyte particles can be lowered while preventing the
reduction in the capacity of the electrode active material in the
electrode layer.
[0033] Furthermore, in the present invention, since the electrolyte
network can be formed in the electrode layer, the connection
interface area between the electrode active material and the solid
electrolyte material can be remarkably increased, the interface
reaction resistance can be lowered, and thus the resultant
all-solid-state cell is capable of charge and discharge operations
even in the all solid state.
[0034] The above and other objects, features and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings in which a preferred embodiment of the present invention
is shown by way of illustrative example.
DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic cross-sectional view showing a
structure of an all-solid-state cell according to an embodiment of
the present invention;
[0036] FIG. 2 is a schematic cross-sectional view showing a
structure of an all-solid-state cell according to a modification
example of the embodiment;
[0037] FIG. 3A is an SEM photograph at .times.1000 magnification
showing a cross section of a crystalline LAGP solid electrolyte
sintered at a firing temperature (600.degree. C.) under an Ar
atmosphere;
[0038] FIG. 3B is an SEM photograph at .times.5000 magnification
showing a cross section of a crystalline LAGP solid electrolyte
sintered at a firing temperature (600.degree. C.) under an Ar
atmosphere;
[0039] FIG. 4A is an SEM photograph at .times.1000 magnification
showing a cross section of the crystalline LAGP solid electrolyte
sintered at a different firing temperature (700.degree. C.) under
the Ar atmosphere;
[0040] FIG. 4B is an SEM photograph at .times.5000 magnification
showing a cross section of the crystalline LAGP solid electrolyte
sintered at a different firing temperature (700.degree. C.) under
the Ar atmosphere;
[0041] FIG. 5A is an SEM photograph at .times.1000 magnification
showing a cross section of the crystalline LAGP solid electrolyte
sintered at a different firing temperature (800.degree. C.) under
the Ar atmosphere;
[0042] FIG. 5B is an SEM photograph at .times.5000 magnification
showing a cross section of the crystalline LAGP solid electrolyte
sintered at a different firing temperature (800.degree. C.) under
the Ar atmosphere;
[0043] FIG. 6A is an SEM photograph at .times.1000 magnification
showing a cross section of a vitrified LAGP solid electrolyte
sintered at a firing temperature (550.degree. C.) under an Ar
atmosphere;
[0044] FIG. 6B is an SEM photograph at .times.5000 magnification
showing a cross section of a vitrified LAGP solid electrolyte
sintered at a firing temperature (550.degree. C.) under an Ar
atmosphere;
[0045] FIG. 7A is an SEM photograph at .times.1000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (600.degree. C.) under
the Ar atmosphere;
[0046] FIG. 7B is an SEM photograph at .times.5000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (600.degree. C.) under
the Ar atmosphere;
[0047] FIG. 8A is an SEM photograph at .times.1000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (650.degree. C.) under
the Ar atmosphere;
[0048] FIG. 8B is an SEM photograph at .times.5000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (650.degree. C.) under
the Ar atmosphere;
[0049] FIG. 9A is an SEM photograph at .times.1000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (700.degree. C.) under
the Ar atmosphere;
[0050] FIG. 9B is an SEM photograph at .times.5000 magnification
showing a cross section of the vitrified LAGP solid electrolyte
sintered at a different firing temperature (700.degree. C.) under
the Ar atmosphere;
[0051] FIG. 10 is a graph showing a DTA (differential thermal
analysis) property of the vitrified LAGP solid electrolyte;
[0052] FIG. 11 is a characteristic diagram showing changes of the
firing shrinkage and the internal impedance of the crystalline LAGP
solid electrolyte depending on the firing temperature;
[0053] FIG. 12 is a characteristic diagram showing changes of the
firing shrinkage and the internal impedance of the vitrified LAGP
solid electrolyte depending on the firing temperature;
[0054] FIG. 13 is a diagram showing the XRD (X-ray diffraction)
characteristics of a fired mixture pellet of an LAGP crystal powder
and an LVP crystal powder;
[0055] FIG. 14 is a characteristic diagram showing a peak intensity
(peak height) relation between a main peak of the positive
electrode active material (the fired mixture pellet of the LAGP
crystal powder and the LVP crystal powder) and a main peak of
LiVP.sub.2O.sub.7 identified as a heterophase peak, and the change
in the discharge capacity of the positive electrode active
material;
[0056] FIG. 15 is a diagram showing the XRD (X-ray diffraction)
characteristics of a fired mixture pellet of an LAGP glass powder
and an LVP crystal powder;
[0057] FIG. 16 is a characteristic diagram showing a peak intensity
(peak height) relation between a main peak of the positive
electrode active material (the fired mixture pellet of the LAGP
glass powder and the LVP crystal powder) and a main peak of
LiVP.sub.2O.sub.7 identified as a heterophase peak, and a change in
the discharge capacity of the positive electrode active
material;
[0058] FIG. 17 is a graph showing the charge/discharge property of
Example 1 using the LAGP glass powder;
[0059] FIG. 18 is a graph showing the alternating-current impedance
property of Example 1;
[0060] FIG. 19 is a graph showing the charge/discharge property of
Example 2 using the LAGP glass powder;
[0061] FIG. 20 is a graph showing the alternating-current impedance
property of Example 2;
[0062] FIG. 21 is a graph showing the charge/discharge property of
Comparative Example 1 using the LAGP crystal powder;
[0063] FIG. 22 is a graph showing the alternating-current impedance
property of Comparative Example 1;
[0064] FIG. 23 is a graph showing the charge/discharge property of
Comparative Example 2 using the LAGP crystal powder;
[0065] FIG. 24 is a graph showing the alternating-current impedance
property of Comparative Example 2;
[0066] FIG. 25 is a graph showing the charge/discharge property of
Example 3; and
[0067] FIG. 26 is a graph showing the alternating-current impedance
property of Example 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0068] An embodiment of the all-solid-state cell of the present
invention will be described below with reference to FIGS. 1 to
26.
[0069] As shown in FIG. 1, an all-solid-state cell 10 according to
this embodiment comprises a combination of an electrode active
material and a solid electrolyte material. The all-solid-state cell
10 has a fired solid electrolyte plate 14 of a ceramic containing a
solid electrolyte 12, a first electrode layer 18 (e.g. a positive
electrode) integrally formed on one surface of the fired solid
electrolyte plate 14 by mixing and firing an electrode active
material 16 and a solid electrolyte 12, a second electrode layer 20
(e.g. a negative electrode) integrally formed on the other surface
of the fired solid electrolyte plate 14 by mixing and firing an
electrode active material 16 and a solid electrolyte 12, a first
collector electrode 24 electrically connected to the first
electrode layer 18, and a second collector electrode 26
electrically connected to the second electrode layer 20.
[0070] In the all-solid-state cell 10, the fired solid electrolyte
plate 14 substantially acts as a solid electrolyte portion
separating the positive and negative electrodes. The solid
electrolyte 12 contained in the ceramic of the fired solid
electrolyte plate 14 is not particularly limited, and may be
selected from known conventional solid electrolytes. The solid
electrolyte 12 preferably contains a lithium ion as a movable ion,
and examples thereof include lithium ion-conductive solid glass
electrolytes such as Li.sub.3PO.sub.4, LiPON (Li.sub.3PO.sub.4
mixed with nitrogen), Li.sub.2S--SiS.sub.2,
Li.sub.2S--P.sub.2S.sub.5, and Li.sub.2S--B.sub.2S.sub.3, and
lithium ion-conductive solid electrolytes prepared by doping the
glass with a lithium halide (e.g. LiI) or a lithium oxoate (e.g.
Li.sub.3PO.sub.4). The solid electrolyte 12 is particularly
preferably a titanium oxide type solid electrolyte containing
lithium, titanium, and oxygen, such as Li.sub.xLa.sub.yTiO.sub.3
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), or a Nasicon type
phosphate compound such as
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 or
Li.sub.1+xAl.sub.xTi.sub.2-x(PO.sub.4).sub.3 (0.ltoreq.x.ltoreq.1),
which can exhibit a stable performance even in the case of firing
under an oxygen atmosphere.
[0071] The thickness of the fired solid electrolyte plate 14 is not
particularly limited, and is preferably 5 .mu.m to 1 mm, more
preferably 5 .mu.m to 100 .mu.m.
[0072] In the first electrode layer 18 and the second electrode
layer 20, a large number of powder particles of the solid
electrolyte 12 are bonded by sintering to form a porous body. In
the porous body, a plurality of pores are three-dimensionally
connected from the surface to the inside, and are filled with the
electrode active material 16. Such a porous body, formed by bonding
the powder particles of the solid electrolyte 12 by the sintering,
is also referred to as an electrolyte network.
[0073] The thicknesses of the first electrode layer 18 and the
second electrode layer 20 are not particularly limited, and are
preferably 5 .mu.m to 1 mm, more preferably 5 .mu.m to 500
.mu.m.
[0074] In the formation of the first electrode layer 18 and the
second electrode layer 20, a first paste for forming the first
electrode layer 18 and a second paste for forming the second
electrode layer 20 may be printed into electrode patterns on the
fired solid electrolyte plate 14 respectively using a screen
printing method, etc.
[0075] The first and second pastes may be produced by the steps of
dissolving a binder in an organic solvent to prepare a solution,
adding an appropriate amount of the solution to powders of the
electrode active material and the solid electrolyte material to be
hereinafter described, and kneading the resultant mixture.
[0076] Then, the electrode patterns of the first and second pastes
printed on the fired solid electrolyte plate 14 may be fired at a
temperature lower than a temperature for forming the fired solid
electrolyte plate 14, to form the first electrode layer 18 and the
second electrode layer 20. The obtained first electrode layer 18
and second electrode layer 20 are the porous bodies having a large
number of pores filled with the electrode active material 16.
[0077] Though both the first electrode layer 18 and the second
electrode layer 20 formed on the fired solid electrolyte plate 14
are composed of a ceramic containing a mixture of the electrode
active material 16 and the solid electrolyte 12 in the above
example, the second electrode layer 20 may be composed of a metal
film 22 of a Li metal or Li alloy, like an all-solid-state cell 10a
according to another example shown in FIG. 2.
[0078] In this embodiment, the solid electrolyte material added to
the first electrode layer 18 and the second electrode layer 20 may
comprise an amorphous polyanion compound, and the layers may be
formed by firing the compound.
[0079] In this embodiment, the solid electrolyte material added to
the first electrode layer 18 and the second electrode layer 20 may
comprise an amorphous phosphate compound, and the layers may be
formed by firing the compound.
[0080] The solid electrolyte material comprising the phosphate
compound may be of Nasicon type after the firing, and the phosphate
compound is particularly preferably LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 or LATP
Li.sub.1+xAl.sub.xTi.sub.2-x(PO.sub.4).sub.3
(0.ltoreq.x.ltoreq.1).
[0081] The electrode active material may be a Nasicon type material
comprising a phosphate compound, and the phosphate compound is
particularly preferably LVP Li.sub.mV.sub.2(PO.sub.4).sub.3
(1.ltoreq.m.ltoreq.5).
[0082] The positive electrode active material may be an olivine
type material comprising a phosphate compound, and the phosphate
compound is particularly preferably LNP Li.sub.nNiPO.sub.4, LCP
Li.sub.nCoPO.sub.4, LMP Li.sub.nMnPO.sub.4 or LFP
Li.sub.nFePO.sub.4 (0.ltoreq.n.ltoreq.1).
[0083] In this embodiment, the solid electrolyte material and the
electrode active material comprising the phosphate compounds may be
of Nasicon type after the firing. In this case, it is preferred
that the solid electrolyte material comprises LAGP
Li.sub.1+xAl.sub.xGe.sub.2-x(PO.sub.4).sub.3 (0.ltoreq.x.ltoreq.1,
preferably 0.3.ltoreq.x.ltoreq.0.7), and both the positive and
negative electrode active materials comprise LVP
Li.sub.mV.sub.2(PO.sub.4).sub.3 (1.ltoreq.m.ltoreq.5), whereby the
all-solid-state cell has a symmetrical structure.
[0084] Thus, in this embodiment, in the first electrode layer 18
and the second electrode layer 20 of the all-solid-state cell 10,
the particle boundary resistance between the solid electrolyte
particles can be lowered while preventing formation of a
heterophase due to a reaction between the solid electrolyte
material and the electrode active material 16.
[0085] Furthermore, in this embodiment, the electrolyte network can
be formed in the first electrode layer 18 and the second electrode
layer 20, whereby the connection interface area between the
electrode active material 16 and the solid electrolyte 12 can be
remarkably increased to lower the interface reaction resistance,
and thus the resultant all-solid-state cell 10 is capable of charge
and discharge operations even in the all solid state.
[0086] The first electrode layer 18 and the second electrode layer
20 are preferably formed by firing under an applied pressure. In
this case, by the firing under an applied pressure, a dense
microstructure can be formed in the electrode portion, the
interface area between the electrode active material and the solid
electrolyte material can be increased, and the interface charge
transfer resistance can be lowered.
[0087] The methods for firing the layers under an applied pressure
include a Hot Isostatic Pressing HIP, in which a mixture is heated
at a high temperature while pressure is simultaneously and
isotropically applied to the mixture, and a Hot Pressing, in which
a mixture housed in a firing jig is heat-treated as a whole while
pressure is uniaxially applied to the mixture. When using the HIP,
a gas such as argon can be used as a pressure medium to apply
isotropic pressure to the mixture.
[0088] The first electrode layer 18 and/or the second electrode
layer 20 may be formed from a paste for printing by firing it under
an inert atmosphere such as an Ar atmosphere. In this case, a
binder component can be carbonized to ensure the electron
conductivity of the first electrode layer 18 and/or the second
electrode layer 20. Thus, the electron conductivity of the first
electrode layer 18 and/or the second electrode layer 20 can be
maintained without intentional addition of a carbon component
useful as an electron conducting aid.
[0089] Examples of the all-solid-state cell 10 according to the
embodiment will be described in detail below.
[0090] In Examples, the following Nasicon type phosphate compounds
were used as a solid electrolyte material and an electrode active
material. [0091] Solid electrolyte material: LAGP
Li.sub.1.5Al.sub.0.5Ge.sub.1.5(PO.sub.4).sub.3 [0092] Electrode
active material: LVP Li.sub.3V.sub.2(PO.sub.4).sub.3
[Preparation of Crystal Powder]
[0093] First, powders of Li.sub.2CO.sub.3, GeO.sub.2,
Al.sub.2O.sub.3, and NH.sub.4H.sub.2(PO.sub.4).sub.3 were mixed at
the stoichiometric composition ratio and fired at 900.degree. C. in
the air, so that a crystal powder of the solid electrolyte material
Li.sub.1.5Al.sub.0.5Ge.sub.1.5(PO.sub.4).sub.3 (LAGP) (hereinafter
referred to as the LAGP crystal powder) was prepared by a
solid-phase synthesis method.
[0094] Meanwhile, powders of Li.sub.2CO.sub.3, V.sub.2O.sub.3, and
NH.sub.4H.sub.2(PO.sub.4).sub.3 were mixed at the stoichiometric
composition ratio and fired at 930.degree. C. in an Ar flow, so
that a crystal powder of the positive (negative) electrode active
material Li.sub.3V.sub.2(PO.sub.4).sub.3 (LVP) (hereinafter
referred to as the LVP crystal powder) was prepared by a
solid-phase synthesis method.
[Production of Fired Solid Electrolyte Body]
[0095] The above obtained LAGP crystal powder was press-formed
using a mold into a compact powder pellet having a diameter of 16
mm and a thickness of approximately 1 mm. The pressure for the
forming was 500 kg/cm.sup.2. The pellet was fired at 840.degree. C.
in the air to obtain a fired solid electrolyte pellet of LAGP.
[Preparation of Glass Powder (Vitrification of LAGP Solid
Electrolyte)]
[0096] The LAGP crystal powder obtained by the solid-phase method
was put in a Pt crucible and placed in a furnace under an air
atmosphere heated at 1200.degree. C. for 1 hour. Then, the LAGP
crystal powder was taken out and rapidly cooled by iced water, to
obtain a vitrified LAGP. The vitrified LAGP was pulverized using a
mortar, a ball mill, etc. to prepare a fine LAGP glass powder.
[Sintering Property Comparison of Solid Electrolytes]
[0097] A solid electrolyte pellet of the LAGP crystal powder
(hereinafter referred to as the crystalline LAGP solid electrolyte)
and a solid electrolyte pellet of the LAGP glass powder
(hereinafter referred to as the vitrified LAGP solid electrolyte)
were produced, and the sintered states of the pellets were compared
at different firing temperatures under an Ar atmosphere. The
results of SEM observation of fracture cross sections in the
pellets are shown in FIGS. 3A and 3B to 9A and 9B, and shrinkage
properties of the pellets due to the firing under the Ar atmosphere
are shown in the graphs of FIGS. 11 and 12.
[0098] FIGS. 3A and 3B to 5A and 5B include SEM photographs showing
cross sections of the crystalline LAGP solid electrolyte fired at
600.degree. C., 700.degree. C., and 800.degree. C. Each of FIGS.
3A, 4A and 5A is an SEM photograph at .times.1000 magnification,
and each of FIGS. 3B, 4B and 5B is an SEM photograph at .times.5000
magnification. FIGS. 6A and 6B to 9A and 9B include SEM photographs
showing cross sections of the vitrified LAGP solid electrolyte
fired at 550.degree. C., 600.degree. C., 650.degree. C., and
700.degree. C. Each of FIGS. 6A, 7A, 8A and 9A is an SEM photograph
at .times.1000 magnification, and each of FIGS. 6B, 7B, 8B and 9B
is an SEM photograph at .times.5000 magnification.
[0099] FIG. 10 is a graph showing a DTA (differential thermal
analysis) property of the vitrified LAGP solid electrolyte in an
inert atmosphere (N.sub.2 atmosphere). It is clear from FIG. 10
that the vitrified LAGP solid electrolyte had low transition
temperatures, Tg (glass transition point) of approximately
480.degree. C. and Tx (crystallization temperature) of
approximately 590.degree. C.
[0100] FIG. 11 is a characteristic diagram showing changes of the
firing shrinkage (%) and the internal impedance of the crystalline
LAGP solid electrolyte depending on the firing temperature. FIG. 12
is a characteristic diagram showing changes of the firing shrinkage
(%) and the internal impedance of the vitrified LAGP solid
electrolyte depending on the firing temperature. In the diagrams,
the firing shrinkage of each LAGP solid electrolyte is shown by
plotted black dots, and the internal impedance is divided into two
corresponding to the intraparticle resistance and the particle
boundary resistance and shown by bar graph.
[0101] As shown in FIGS. 3A and 3B to 5A and 5B, and 11, in the
crystalline LAGP solid electrolyte, at a firing temperature of
700.degree. C. or lower, the powder particles were not sufficiently
sintered and maintained the original particle shapes, and the
particle boundary resistance was remarkably high in terms of the
internal impedance. On the other hand, as shown in FIGS. 6A and 6B
to 9A and 9B, and 12, in the LAGP glass powder (the vitrified LAGP
solid electrolyte), at a firing temperature of 600.degree. C. or
higher, the solid electrolyte material was sufficiently shrunk by
the firing, the particles were suitably bonded to each other, and
the particle boundary resistance was significantly lowered in terms
of the internal impedance.
[Relation of Reactivity Between Electrolyte and Electrode Active
Material to Charge-Discharge Capacity of Electrode Active
Material]
[0102] An electron conducting aid of acetylene black was added to a
mixture of the LAGP crystal powder and the LVP crystal powder to
carry out an evaluation using an electrolytic solution later. A
powder pellet of the mixture was prepared and fired at a firing
temperature under an Ar atmosphere to obtain a fired body. The
obtained fired body was subjected to an XRD (X-ray diffraction)
measurement. The measurement results are shown in FIG. 13. In FIG.
13, peaks of Li.sub.3Fe.sub.2(PO.sub.4).sub.3 for identifying the
crystal structure of the LVP are marked with black squares, and
peaks of LiGe.sub.2(PO.sub.4).sub.3 for identifying the crystal
structure of the LAGP are marked with black triangles. The LAGP and
the LVP are not registered in ICDD, and thus the compounds having
the same crystal structures were used for the identification. As
shown in the measurement results, a plurality of peaks of a
heterophase derived from a condensed phosphate salt were observed
in addition to the peaks of the LAGP and LVP at temperatures of
700.degree. C. and 800.degree. C., at which the LAGP crystal powder
was sintered.
[0103] The relations between the charge-discharge capacity of the
electrode active material, and the peak intensity of the positive
electrode active material and the peak intensity of the heterophase
were evaluated. Specifically, each of the fired mixed powder body
pellets, fired at the different temperatures, was pulverized and
used in a positive electrode in a liquid type lithium ion cell
containing an electrolytic solution (1-mol/L LiClO.sub.4/EC+DEC
(volume ratio 1:1) solution) and a negative electrode of Li metal,
and the charge-discharge ability (capacity) of the positive
electrode active material was measured. It should be noted that the
LVP has a theoretical charge-discharge capacity of about 130 mAh/g.
The peak intensity (peak height) relation between the main peak (a)
of the positive electrode active material observed in the XRD
measurement (see FIG. 13) and the main peak (b) of
LiVP.sub.2O.sub.7 identified as a heterophase peak, marked with a
white circle in FIG. 13, is shown in FIG. 14 as the measurement
results. In FIG. 14, the change of the charge-discharge capacity of
the positive electrode active material is shown by plotted black
squares. The peak intensities of the main peak (a) of the positive
electrode active material and the main peak (b) of the heterophase
are shown in the bar graph. As shown in the measurement results,
the reduction in the peak intensity of the positive electrode
active material and the generation of the heterophase derived from
vanadium contained in the positive electrode active material
corresponded to the reduction in the charge-discharge capacity of
the positive electrode active material. Thus, the capacity
reduction due to the reaction between the solid electrolyte
material and the electrode active material in the high-temperature
firing caused the reduction in the charge-discharge ability of the
electrode active material. It is possible that the electrode active
material was converted to LiVP.sub.2O.sub.7, which was identified
beforehand. Further, the electrode active material exhibited a
charge-discharge capacity significantly lower than the original
theoretical capacity at temperatures of 700.degree. C. and
800.degree. C., at which the LAGP crystal powder was sintered.
[0104] An electron conducting aid of acetylene black was added to a
mixture of the LAGP glass powder and the LVP crystal powder to
carry out an evaluation using an electrolytic solution later. A
powder pellet of the mixture was prepared and fired at a firing
temperature under an Ar atmosphere to obtain a fired body. The
obtained fired body was subjected to an XRD (X-ray diffraction)
measurement. Further, the fired body was used in a system
containing the electrolytic solution, and the charge-discharge
ability of the electrode active material was evaluated. The XRD
measurement results are shown in FIG. 15, and the charge-discharge
ability evaluation results are shown in FIG. 16. The plotted marks
in FIGS. 15 and 16 have the same meanings as those in FIGS. 13 and
14.
[0105] As shown in FIG. 15, the particles of the LAGP glass powder
were suitably bonded to each other around 600.degree. C. Further,
as shown in FIG. 16, the electrode active material maintained a
sufficient peak intensity and had a charge-discharge capacity close
to the original theoretical capacity thereof without formation of a
heterophase. As a result, the combination of the materials
containing the phosphate compounds using a common polyanion was
considered as a material system satisfying the relation that the
temperature, at which the capacity of the electrode active material
is lowered by the reaction between the electrode active material
and the solid electrolyte material, is higher than the firing
shrinkage initiation temperature of the solid electrolyte
material.
[Production of All-Solid-State Cell]
[0106] All-solid-state cells having electrodes containing the
combinations of the electrode active materials and the solid
electrolyte materials were produced respectively. The electrode was
formed by mixing and firing the electrode active material and the
solid electrolyte material of an amorphous LAGP solid electrolyte
(an amorphous polyanion (phosphate) compound) in Examples. A
crystalline LAGP solid electrolyte was used instead of the
amorphous LAGP solid electrolyte in Comparative Examples. Examples
and Comparative Examples will be described specifically below.
EXAMPLE 1
[0107] A binder was dissolved in an organic solvent, and an
appropriate amount of the resultant solution was added to the LAGP
glass powder and the LVP crystal powder. The mixture was kneaded in
a mortar to prepare an electrode paste for screen printing. The
obtained electrode paste was printed into an electrode pattern
having a diameter of 12 mm on each surface of a fired solid
electrolyte body (a base) having a diameter of 13 mm and a
thickness of 1 mm. The printed electrode patterns were dried to
form positive and negative electrodes.
[0108] The electrodes were bonded to the surfaces of the solid
electrolyte base by firing at 600.degree. C. for 2 hours using a
firing furnace under an Ar atmosphere. Then, a sputtered gold (Au)
film having a thickness of approximately 50 nm was formed as a
collector on each surface of the resultant fired body.
[0109] After the firing, the positive electrode had a thickness of
approximately 20 .mu.m and an active material content of
approximately 2 mg. The charge-discharge capacity per unit weight
of the positive electrode was calculated from the active material
content, and was shown in a graph.
EXAMPLE 2
[0110] A binder was dissolved in an organic solvent, and an
appropriate amount of the resultant solution was added to the LAGP
glass powder and the LVP crystal powder. The mixture was kneaded in
a mortar to prepare an electrode paste for screen printing. The
obtained electrode paste was printed into an electrode pattern and
dried on each surface of a fired solid electrolyte body (a base) in
the above-mentioned manner, to form positive and negative
electrodes.
[0111] The electrodes were bonded to the surfaces of the solid
electrolyte base by firing at 600.degree. C. for 40 hours using a
firing furnace under an Ar atmosphere. Then, a sputtered Au film
was formed on each surface of the resultant fired body.
[0112] After the firing, the positive electrode had a thickness of
approximately 20 .mu.m and an active material content of
approximately 2 mg as above.
COMPARATIVE EXAMPLE 1
[0113] A binder was dissolved in an organic solvent, and an
appropriate amount of the resultant solution was added to the LAGP
crystal powder and the LVP crystal powder. The mixture was kneaded
in a mortar to prepare an electrode paste for screen printing. The
obtained electrode paste was printed into an electrode pattern and
dried on each surface of a fired solid electrolyte body (a base) in
the above-mentioned manner, to form positive and negative
electrodes.
[0114] The electrodes were bonded to the surfaces of the solid
electrolyte base by firing at 600.degree. C. for 2 hours using a
firing furnace under an Ar atmosphere. Then, a sputtered Au film
was formed on each surface of the resultant fired body.
[0115] After the firing, the positive electrode had a thickness of
approximately 20 .mu.m and an active material content of
approximately 2 mg as above.
COMPARATIVE EXAMPLE 2
[0116] A binder was dissolved in an organic solvent, and an
appropriate amount of the resultant solution was added to the LAGP
crystal powder and the LVP crystal powder. The mixture was kneaded
in a mortar to prepare an electrode paste for screen printing. The
obtained electrode paste was printed into an electrode pattern and
dried on each surface of a fired solid electrolyte body (a base) in
the above-mentioned manner, to form positive and negative
electrodes.
[0117] The electrodes were bonded to the surfaces of the solid
electrolyte base by firing at 700.degree. C. for 2 hours using a
firing furnace under an Ar atmosphere. Then, a sputtered Au film
was formed on each surface of the resultant fired body.
[0118] After the firing, the positive electrode had a thickness of
approximately 20 .mu.m and an active material content of
approximately 2 mg as above.
[Measurement of Alternating-Current Impedance]
[0119] The alternating-current impedance of each all-solid-state
cell was measured by using 1287 Potentiostat/Galvanostat (trade
name) and 1255B Frequency Response Analyzer (trade name)
manufactured by Solartron in combination. The measurement frequency
was controlled within the range of 1 MHz to 0.1 Hz, and the
measurement signal voltage was 10 mV.
[Evaluation of Charge-Discharge Property]
[0120] Each all-solid-state cell was charged and discharged by a
CCCV (Constant Current Constant Voltage) process, and the
charge-discharge property was evaluated. Specifically, in Examples
1 and 2, the all-solid-state cell was charged at a constant current
of 9 .mu.A/cm.sup.2 to a cutoff voltage of 2.4 V and then charged
at a constant voltage of 2.4 V to a current value of 0.9
.mu.A/cm.sup.2, and was discharged at a constant current of 9
.mu.A/cm.sup.2 to a cutoff voltage of 0.1 V and then discharged at
a constant voltage of 0.1 V to a current value of 0.9
.mu.A/cm.sup.2. In Comparative Examples 1 and 2, the
all-solid-state cell was charged at a constant current of 0.9
.mu.A/cm.sup.2 to a cutoff voltage of 2.4 V and then charged at a
constant voltage of 2.4 V to a current value of 0.45
.mu.A/cm.sup.2, and was discharged at a constant current of 0.9
.mu.A/cm.sup.2 to a cutoff voltage of 0.1 V and then discharged at
a constant voltage of 0.1 V to a current value of 0.45
PA/cm.sup.2.
(Evaluation)
[0121] Each of the produced all-solid-state ceramic cells having
the mixture electrodes was vacuum-dried under heating and
incorporated in a 2032 coin cell type package to evaluate the
electric properties in a glove box. The charge-discharge properties
of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in
FIGS. 17, 19, 21, and 23. The alternating-current impedances of
Examples 1 and 2 and Comparative Examples 1 and 2 are shown in
FIGS. 18, 20, 22, and 24. In each alternating-current impedance
waveform, the transverse axis indicates the real part Z' of the
impedance, the ordinate axis indicates the imaginary part Z'' of
the impedance, and the measurement frequencies of 1 kHz and 1 Hz
are marked with black dots.
(Consideration)
[0122] Comparing the charge-discharge capacity, the all-solid-state
cells of Comparative Examples 1 and 2 had high internal resistances
and were almost incapable of charge and discharge. In Comparative
Example 1, a large arc corresponding to the particle boundary
resistance was formed in a higher frequency region of more than 1
kHz in the alternating-current impedance waveform, and the solid
electrolyte particles were not sufficiently bonded to each other.
Thus, the reaction interface area between the solid electrolyte
material and the electrode active material was insufficient,
whereby the cell of Comparative Example 1 was almost incapable of
charge and discharge. In Comparative Example 2, a large arc
corresponding to the reaction interface resistance was formed in a
lower frequency region of 1 kHz or less in the alternating-current
impedance waveform. Thus, a heterophase was formed on the
connection interface between the solid electrolyte material and the
electrode active material, and the capacity of the electrode active
material was lowered, whereby the cell of Comparative Example 2 was
almost incapable of charge and discharge.
[0123] In contrast, the cell of Example 1 had a low internal
resistance and a charge-discharge capacity of approximately 20
mAh/g, and the cell of Example 2 had a charge-discharge capacity of
approximately 40 mAh/g. In Examples 1 and 2, each cell had low
impedance in terms of both the particle boundary resistance and the
interface reaction resistance as shown in the alternating-current
impedance waveform, since the solid electrolyte particles were
sufficiently bonded in a region of the electrode layer where a
defect (formation of a heterophase, reduction in the capacity of
the active material, etc.) was not generated between the solid
electrolyte material and the electrode active material. The solid
electrolyte material and the electrode active material had an
increased desirable connection interface, whereby the interface
reaction resistance was lowered. Thus, the internal resistance was
lowered, so that the resultant cell was capable of charge and
discharge.
EXAMPLE 3
[0124] An all-solid-state cell of Example 3 was produced, and the
charge/discharge property and the alternating-current impedance
property were measured.
[0125] In the same manner as Example 1, a binder was dissolved in
an organic solvent, and an appropriate amount of the resultant
solution was added to the LAGP glass powder and the LVP crystal
powder. The mixture was kneaded in a mortar to prepare an electrode
paste for screen printing. The obtained electrode paste was printed
into an electrode pattern having a diameter of 12 mm on each
surface of a fired solid electrolyte body (a base) having a
diameter of 13 mm and a thickness of 1 mm. The printed electrode
patterns were dried to form positive and negative electrodes.
[0126] The electrodes were bonded to the surfaces of the solid
electrolyte base by firing while applying a load of 500 kg/cm.sup.2
in the thickness direction at 600.degree. C. for 40 hours using a
hot-press furnace under an Ar atmosphere. Then, a sputtered gold
(Au) film having a thickness of approximately 50 nm was formed as a
collector on each surface of the resultant fired body.
[0127] After the firing, the positive electrode had a thickness of
approximately 20 .mu.m and an active material content of
approximately 2 mg as Example 1.
[Measurement of Alternating-Current Impedance]
[0128] The alternating-current impedance of the all-solid-state
cell was measured by using 1287 Potentiostat/Galvanostat (trade
name) and 1255B Frequency Response Analyzer (trade name)
manufactured by Solartron in combination in the same manner as
Example 1. The measurement frequency was controlled within the
range of 1 MHz to 0.1 Hz, and the measurement signal voltage was 10
mV.
[Evaluation of Charge-Discharge Property]
[0129] The produced all-solid-state cell was charged and discharged
by a CCCV process, and the charge-discharge property was evaluated.
Specifically, in Examples 3, the all-solid-state cell was charged
at a constant current of 90 .mu.A/cm.sup.2 to a cutoff voltage of
2.4 V and then charged at a constant voltage of 2.4 V to a current
value of 0.9 .mu.A/cm.sup.2, and was discharged at a constant
current of 90 .mu.A/cm.sup.2 to a cutoff voltage of 0.1 V and then
discharged at a constant voltage of 0.1 V to a current value of 0.9
.mu.A/cm.sup.2.
(Evaluation)
[0130] The produced all-solid-state ceramic cell having the mixture
electrodes was vacuum-dried under heating and incorporated in a
2032 coin cell type package to evaluate the electric properties in
a glove box. The charge-discharge property of Example 3 is shown in
FIG. 25, and the alternating-current impedance of Example 3 is
shown in FIG. 26. In the alternating-current impedance waveform,
the transverse axis indicates the real part Z' of the impedance,
the ordinate axis indicates the imaginary part Z'' of the
impedance, and the measurement frequencies of 1 kHz and 1 Hz are
marked with black dots.
(Consideration)
[0131] In Example 3, as is clear from FIG. 26, the internal
resistance was lowered. The reduction in the reaction resistance
(the interface charge transfer resistance) accounts for the
majority of the reduction in the internal resistance, and thus the
reduction may be achieved due to densification and increase of the
connection interface area between the electrode active material and
the solid electrolyte material.
[0132] It should be understood that the all-solid-state cell of the
present invention is not limited to the above embodiment, and
various changes and modifications may be made therein without
departing from the scope of the invention.
* * * * *