U.S. patent application number 13/794755 was filed with the patent office on 2013-07-25 for negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery comprising the same.
This patent application is currently assigned to SAMSUNG SDI CO., LTD.. The applicant listed for this patent is SAMSUNG SDI CO., LTD.. Invention is credited to Nam-Soon Choi, Yong-Mook Kang, Sung-Soo Kim.
Application Number | 20130189585 13/794755 |
Document ID | / |
Family ID | 38087928 |
Filed Date | 2013-07-25 |
United States Patent
Application |
20130189585 |
Kind Code |
A1 |
Kang; Yong-Mook ; et
al. |
July 25, 2013 |
NEGATIVE ACTIVE MATERIAL FOR A RECHARGEABLE LITHIUM BATTERY, A
METHOD OF PREPARING THE SAME, AND A RECHARGEABLE LITHIUM BATTERY
COMPRISING THE SAME
Abstract
Negative active materials for rechargeable lithium batteries,
methods of manufacturing the negative active materials, and
rechargeable lithium batteries including the negative active
materials are provided. One negative active material includes an
active metal core and a crack inhibiting layer formed on the core.
The crack inhibiting layer includes a carbon-based material.
Inventors: |
Kang; Yong-Mook; (Yongin-si,
KR) ; Choi; Nam-Soon; (Yongin-si, KR) ; Kim;
Sung-Soo; (Yongin-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SAMSUNG SDI CO., LTD.; |
Yongin-si |
|
KR |
|
|
Assignee: |
SAMSUNG SDI CO., LTD.
Yongin-si
KR
|
Family ID: |
38087928 |
Appl. No.: |
13/794755 |
Filed: |
March 11, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11604099 |
Nov 22, 2006 |
8394532 |
|
|
13794755 |
|
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Current U.S.
Class: |
429/231.8 |
Current CPC
Class: |
H01M 4/139 20130101;
H01M 4/1393 20130101; H01M 4/1395 20130101; H01M 4/134 20130101;
H01M 10/0525 20130101; H01M 4/133 20130101; H01M 2004/021 20130101;
H01M 2004/027 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/231.8 |
International
Class: |
H01M 4/133 20060101
H01M004/133 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2005 |
KR |
10-2005-0116028 |
Claims
1. A negative active material for a rechargeable lithium battery
comprising: an active metal core comprising active metal particles;
and a crack inhibiting layer comprising a carbon-based material,
the crack inhibiting layer being disposed on a surface of the core.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/604,099, filed Nov. 22, 2006, which claims
priority to and the benefit of Korean Patent Application No.
10-2005-0116028, filed Nov. 30, 2005, the entire content of both of
which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to negative active materials
for rechargeable lithium batteries, methods of manufacturing the
same, and rechargeable lithium batteries including the same.
BACKGROUND OF THE INVENTION
[0003] The use of portable electronic instruments is increasing as
electronic equipment gets smaller and lighter due to developments
in high-tech electronic industries. Therefore, studies on
high-capacity negative active materials are actively being pursued
in accordance with an increased need for batteries having high
energy densities for use as power sources in these portable
electronic instruments. Even though graphite is suggested for the
negative active material as it has a theoretical capacity of 372
mAh/g, a novel material having a higher capacity than graphite is
still needed.
[0004] Elemental materials such as Si, Sn, and Al have been
developed as substitutions for the graphite. The elemental
materials are known to alloy with lithium and have higher electric
capacities than graphite.
[0005] However, elemental materials themselves have not yet been
commercialized as negative active materials because the elements
such as Si, Sn, Al, and so on form alloys with lithium during
charge-discharge and undergo volume expansion and contraction
resulting in element pulverization. As a result, the cycle-life of
the batteries may be deteriorated.
[0006] Recently, certain materials have been proposed as
substitutes for the conventional graphite material. One such
substitute includes a simple mixture of a graphite and silicon
compound powder. Another proposed substitute includes a material in
which a pulverized silicon compound is chemically fixed on the
surface of graphite by a silane coupling agent. A third substitute
includes a material in which a metal such as Si is bound with or
coated on a graphite-based carbonaceous material.
[0007] However, in the simple mixture of a graphite and silicon
compound powder, the graphite does not completely contact the
silicon compound. As a result, the silicon compound is released
from the graphite when the graphite is expanded or contracted upon
repeated charge and discharge cycles. Thereby, as the silicon
compound has low electro-conductivity, the silicon compound is
insufficiently utilized for negative active materials and the cycle
characteristics of the rechargeable lithium battery are
deteriorated.
[0008] In addition, the material in which the pulverized silicon
compound is chemically fixed on the surface of graphite by a silane
coupling agent works as a negative active material (similar to
graphite) at the early charge and discharge cycles. However, the
silicon compound expands when it is alloyed with the lithium upon
repeated charge and discharge cycles. Thereby, the linkage of the
silane coupling agent is broken to release the silicon compound
from the graphite such that the silicon compound is insufficiently
utilized as a negative active material. As a result, the cycle
characteristics of the rechargeable lithium battery are
deteriorated. Further, the silane coupling agent may not be
uniformly treated upon preparing the negative electrode material so
that it is difficult to provide a negative electrode material
having consistent quality.
[0009] Further, the material in which a metal such as Si is bound
with or coated on the graphite-based carbonaceous material has the
same problems. That is, upon repeated charge and discharge cycles,
the linkage of the amorphous carbonaceous material is broken upon
expanding the metal alloyed with the lithium. Thereby, the metal is
separated and thus is not sufficiently utilized as a negative
active material. As a result, cycle-life characteristics of the
lithium rechargeable battery are deteriorated.
SUMMARY OF THE INVENTION
[0010] One embodiment of the present invention provides a negative
active material for a rechargeable lithium battery which imparts
improved cycle-life characteristics.
[0011] Another embodiment of the present invention provides a
negative active material for a rechargeable lithium battery which
imparts excellent initial efficiency.
[0012] Yet another embodiment of the present invention provides a
method of manufacturing the negative active material having the
above properties for a rechargeable lithium battery.
[0013] Still another embodiment of the present invention provides a
rechargeable lithium battery including the above negative active
material.
[0014] According to one embodiment of the present invention, a
negative active material for a rechargeable lithium battery
includes an active metal core and a crack inhibiting layer
including a carbon-based material disposed on a surface of the
core.
[0015] The active metal may be selected from Si, Sn, Al, Zn, Pb,
Bi, Ag, Cd, Sb, and combinations thereof. The carbon-based material
may be selected from carbon fibers, carbon nanotubes, carbon
nanowires, soft carbon, hard carbon, and combinations thereof. The
active metal particles may have an average particle diameter of
about 50 .mu.m or less. According to one embodiment, the active
metal particles have an average particle diameter ranging from
about 1 to about 40 .mu.m. In another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 30 .mu.m. In yet another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 20 .mu.m. In still another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 10 .mu.m.
[0016] The carbon-based material may have an average particle
diameter ranging from about 5 nm to about 5 .mu.m. According to one
embodiment, the carbon-based material has an average particle
diameter ranging from about 100 nm to about 1 .mu.m. A ratio of the
thickness of the crack inhibiting layer to the average particle
diameter of the active metal particles ranges from about 1/1000 to
about 1/2. According to one embodiment, the ratio ranges from about
1/100 to about 1/10.
[0017] According to another embodiment of the present invention, a
method of manufacturing the negative active material includes first
preparing a coating liquid in which a carbon-based material is
dispersed by adding a carbon-based material and a dispersing agent
to a solvent. The acidity (pH) of the coating liquid is controlled
to a value ranging from about 1 to about 6. Active metal particles
are then added to a surfactant suspension to prepare a suspension
including active metal particles coated with the surfactant. The
coating liquid and the suspension are then mixed and the acidity
(pH) of the mixture is controlled to a value ranging from about 1
to about 6. The mixture is then heat-treated.
[0018] According to yet another embodiment of the present
invention, a rechargeable lithium battery includes a negative
electrode including the above negative active material, a positive
electrode including a positive active material capable of
reversibly intercalating and deintercalating lithium, and an
electrolyte.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] These and other features and advantages of the present
invention will be better understood with reference to the following
detailed description when considered in conjunction with the
accompanying drawings in which:
[0020] FIG. 1 is a cross-sectional view of a negative active
material according to one embodiment of the present invention;
[0021] FIG. 2 is a schematic depicting cross-sectional views of a
negative active material according to one embodiment of the
invention showing changes in the active material when moving from a
discharged state (top) to a charged state (bottom);
[0022] FIG. 3 is a perspective view of a rechargeable lithium
battery according to one embodiment of the present invention;
[0023] FIG. 4 is a scanning electron microscope (SEM) photograph of
the negative active material prepared according to Example 1;
[0024] FIG. 5 is a SEM photograph of the negative active material
prepared according to Comparative Example 1;
[0025] FIG. 6 is a graph of the cycle-life characteristics of the
rechargeable lithium battery prepared according to Example 1;
[0026] FIG. 7 is a graph of the charge and discharge
characteristics of the rechargeable lithium battery prepared
according to Example 1;
[0027] FIG. 8 is a graph of the cycle-life characteristics of the
rechargeable lithium battery prepared according to Comparative
Example 1; and
[0028] FIG. 9 is a graph of the charge and discharge
characteristics of the rechargeable lithium battery prepared
according to Comparative Example 1.
DETAILED DESCRIPTION
[0029] An exemplary embodiment of the present invention will now be
described with reference to the accompanying drawings.
[0030] When a metal-based active material is used as a negative
active material for a rechargeable lithium battery, lithium ions
move to the negative electrode during charging and are alloyed with
the metal-based active material, thereby expanding the volume.
However, lithium ions move to the positive electrode during
discharging, thereby contracting the volume. When these processes
are repeated, the metal-based active material pulverizes, i.e., it
starts to crack, and then finally breaks into minute particles. In
addition, the active material separates from the current collector
or the conductive material in the negative electrode, thereby
becoming electrically insulated therefrom. Therefore, as a
rechargeable battery is repeatedly charged and discharged, the
electric conductivity of the negative electrode gradually weakens,
resulting in decreased battery efficiency.
[0031] According to one embodiment of the present invention, the
negative active material includes an active metal core and a crack
inhibiting layer including a carbon-based material disposed on a
surface of the core.
[0032] The active metal may be any metal that can be alloyed with
lithium during an electrochemical reaction within the cell.
Nonlimiting examples of suitable active metals include Si, Sn, Al,
Zn, Pb, Bi, Ag, Cd, Sb, and combinations thereof. In one
embodiment, the active metal is selected from Si and Sn, which have
large capacities.
[0033] The crack inhibiting layer of the negative active material
includes a conductive material and suppresses volume expansion of
the active metal particles, thereby improving the electric
conductivity of the negative active material. The conductive
material includes crystalline or amorphous carbon-based materials.
Nonlimiting examples of suitable crystalline carbon materials
include plate-shaped, flake-shaped, spherical, or fiber-shaped
natural or artificial graphite, including carbon fiber, carbon
nanotubes, carbon nanowires, carbon nanohorns, and so on.
Nonlimiting examples of suitable amorphous carbon materials include
soft carbon (carbon obtained by firing at a low temperature), hard
carbon, mesophase pitch carbide, fired cokes, and so on.
[0034] In one embodiment, the crystalline carbon has an X-ray
diffraction peak intensity I(110) at a (110) plane and an X-ray
diffraction peak intensity I(002) at a (002) plane, and an
intensity ratio I(110)/I(002) of about 0.2 or less. According to
one embodiment, the crystalline carbon has an intensity ratio
I(110)/I(002) of about 0.04 or less. In another embodiment, the
crystalline carbon has an intensity ratio I(110)/I(002) ranging
from about 0.002 to about 0.2. In yet another embodiment, the
crystalline carbon has an intensity ratio I(110)/I(002) ranging
from about 0.002 to about 0.04.
[0035] The active metal particles have an average particle diameter
of about 50 .mu.m or less. According to one embodiment, the active
metal particles have an average particle diameter ranging from
about 1 to about 40 .mu.m. In another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 30 .mu.m. In yet another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 20 .mu.m. In still another embodiment, the active metal
particles have an average particle diameter ranging from about 1 to
about 10 .mu.m. When the active metal particles have an average
particle diameter greater than about 50 .mu.m, the total surface
area of the active metal particles decreases, resulting in a
decrease in the reactivity of the negative active material.
[0036] The carbon-based materials have an average particle diameter
ranging from about 5 nm to about 5 .mu.m. According to one
embodiment, the carbon-based materials have an average particle
diameter ranging from about 100 nm to about 1 .mu.m.
[0037] A ratio of the thickness of the crack inhibiting layer to
the average particle diameter of the active metal particles ranges
from about 1/1000 to about 1/2. In one embodiment, for example, the
ratio ranges from about 1/100 to about 1/10. When the ratio is
greater than about 1/2, the reactivity of the negative active
material decreases. When the ratio is less than about 1/1000,
suppression of volume expansion may be negligible.
[0038] FIG. 1 is a cross-sectional view of a negative active
material 100 according to one embodiment of the present invention.
As shown in FIG. 1, a negative active material of the present
invention comprises a core 104 including active metal particles and
a crack inhibiting layer 102 including carbon-based materials
surrounding the core 104. The crack inhibiting layer 102 suppresses
volume expansion of the active metal particles during charge, and
thereby prevents cracks therein. In addition, the carbon-based
materials in the crack inhibiting layer 102 are electrically
conductive, and thereby appropriately prevent electric insulation
of the negative active material from the current collector and
conductive material.
[0039] FIG. 2 is a schematic cross-sectional view of the states of
the negative active material during discharge (top) and charge
(bottom) according to one embodiment of the present invention.
Referring to FIG. 2, even if the active metal particles 204 expand,
the expanded active metal particles compress pores inside the crack
inhibiting layer 202 surrounding the active metal particles 204,
thereby contracting the volume of the pores. Accordingly, the
volume of the negative active material including the active metal
particles is not significantly changed. Therefore, separation of
the negative active material from the conductive material and
current collector inside the negative electrode is prevented.
[0040] The negative active materials of the present invention are
not easily pulverized or separated from the conductive material and
current collector, which phenomenon commonly occurs with
conventional negative active materials. As a result, the negative
active materials, enhance electric conductivity, and improve
cycle-life characteristics and initial efficiencies of
batteries.
[0041] According to an embodiment of the present invention, a
negative active material includes a core including active metal
particles and a crack inhibiting layer including carbon-based
materials. The carbon-based materials may include silicon carbide
on a portion of their surface.
[0042] When crystalline carbon is included in the crack inhibiting
layer as the carbon-based materials, lithium ions are inserted
among crystalline carbon plates, thereby forming lithiated carbon.
Accordingly, a negative active material of the present invention
may include lithiated carbon.
[0043] The negative active material for a rechargeable battery may
be produced by first preparing a coating liquid in which
carbon-based materials are dispersed by adding the carbon-based
materials and a dispersing agent to a solvent. The acidity (pH) of
the coating liquid is controlled to a value ranging from about 1 to
about 6. Active metal particles are added to a surfactant
suspension to prepare a suspension including active metal particles
coated with the surfactant. The coating liquid and the suspension
are mixed and the acidity (pH) of the mixture is controlled to a
value ranging from about 1 to about 6. The mixture is then
heat-treated.
[0044] First, the carbon-based materials and a dispersing agent are
added to a solvent to disperse the carbon-based materials, yielding
a coating liquid. Then, the acidity (pH) of the resulting coating
liquid is controlled to about neutral. The pH may be controlled by
adding a base such as ammonia, a buffer solution, and so on.
[0045] The carbon-based materials may include, but are not limited
to, crystalline or amorphous carbon. Nonlimiting examples of
suitable crystalline carbon materials include plate-shaped,
flake-shaped, spherical or fiber-shaped natural graphite or
artificial graphite. Nonlimiting examples of suitable amorphous
carbon materials include soft carbon (carbon obtained by firing at
a low temperature), hard carbon, mesophase pitch carbide, fired
cokes, and so on.
[0046] Nonlimiting examples of suitable dispersing agents for
dispersing the carbon-based materials include polyacrylate-based
resins; polyethylene oxide; polypropylene oxide; block copolymers
of (EO).sub.l(PO).sub.m(EO).sub.l where EO is ethylene oxide, PO is
propylene oxide, and I and m are integers ranging from 1 to 500;
polyvinylchloride (PVC); acrylonitrile/butadiene/styrene(ABS)
polymers; acrylonitrile/styrene/acrylester (ASA) polymers; mixtures
of acrylonitrile/styrene/acrylester (ASA) polymers and propylene
carbonate; styrene/acrylonitrile (SAN) copolymers;
methylmethacrylate/acrylonitrile/butadiene/styrene (MABS) polymers;
and so on. A resin of OROTAN.TM. may be used as the
polyacrylate-based resin according to one embodiment of the present
invention. The dispersing agent may be present in an amount ranging
from about 0.1 to about 10 wt % based on the weight of the
carbon-based materials.
[0047] Nonlimiting examples of suitable solvents for use in the
manufacture of the negative active material include water, organic
solvents, and mixtures thereof. Nonlimiting examples of suitable
organic solvents include hexane, chloroform, tetrahydrofuran,
ether, methylene chloride, acetone, acetonitrile, N-methyl
pyrrolidone (NMP) and alcohols such as methanol, ethanol, and
isopropanol. When the surfactant is an ionic compound, an organic
solvent is suitable, and when the surfactant is a non-ionic
compound, water is suitable.
[0048] The active metal particles are then added to the surfactant
suspension to prepare a suspension including active metal particles
coated with the surfactant.
[0049] The surfactant works as a binder to bind the carbon-based
materials to the active metal particles. Non-ionic, anionic, and
cationic materials, as well as organic or inorganic materials may
be used as the surfactant. The surfactant includes a hydrophilic
head group and a hydrophobic tail group in its respective
molecules, where the hydrophilic head group includes an ionic group
and a non-ionic group. The ionic group forms static electricity
bonds, and the non-ionic group forms hydrogen bonds.
[0050] According to certain embodiments of the invention,
nonlimiting examples of compounds having an ionic group include
sulfonates (RSO.sub.3.sup.-), sulfates (RSO.sub.4.sup.-),
carboxylates (RCOO.sup.-), phosphates (RPO.sub.4.sup.-), ammoniums
(R.sub.xH.sub.yN.sup.+ where x is an integer ranging from 1 to 3,
and y is an integer ranging from 3 to 1), quaternary ammoniums
(R.sub.4N.sup.+), betaines
(RN.sup.+(CH.sub.3).sub.2CH.sub.2COO.sup.-), and sulfobetaines
(RN.sup.+(CH.sub.3).sub.2CH.sub.2SO.sub.3). Nonlimiting examples of
compounds having a non-ionic group include polyethylene oxides
(R--OCH.sub.2CH.sub.2(OCH.sub.2CH.sub.2).sub.nOH), amine compounds,
and gelatins. In the above compounds, R is a saturated or
non-saturated hydrocarbon, where the number of carbons ranges from
2 to 1000. The surfactant has a weight average molecular weight
ranging from about 5 to about 10,000. In one embodiment, the
surfactant has a weight average molecular weight ranging from about
50 to about 5000. In another embodiment, surfactant has a weight
average molecular weight ranging from about 50 to about 300.
[0051] According to an embodiment of the invention, the surfactant
is present in an amount ranging from about 0.1 to about 10 wt %
based on the weight of the carbon-based materials. When the
surfactant is present in an amount within this range, the amount of
the carbon-based materials to be coated on the active metal
particles can be controlled.
[0052] Then, the above coating liquid including the carbon-based
materials and the dispersing agent is mixed with an active metal
suspension coated with a surfactant to form a suspension. The
acidity (pH) of the suspension is controlled to a value ranging
from about 1 to about 6. In one embodiment, the acidity (pH) of the
suspension is controlled to a value ranging from about 2 to about
3. The carbon-based materials are coated on the surface of the
active metal particles by the surfactant. In addition, the coating
may be performed by simply mixing the coating liquid including the
carbon-based material with the active metal suspension coated with
the surfactant, but the coating method is not limited thereto.
[0053] The pH is controlled with addition of an acid such as acetic
acid, hydrochloric acid, or sulfuric acid according to an
embodiment of the invention.
[0054] When the surfactant has both anions and cations according to
one embodiment of the invention, the pH of the mixed solution
affects the amount of carbon-based materials coated on the active
metal particles.
[0055] When the mixed solution is allowed to stand after its
acidity (pH) is set, active metal particles coated with
carbon-based materials precipitate, and can be easily recovered.
The active metal particles may be filtered to remove the residue of
the dispersing agent and uncoated surfactant.
[0056] The recovered active metal particles are heat-treated to
obtain a negative active material. The heat-treatment temperature
ranges from about 200 to about 1200.degree. C. In one embodiment,
for example, the heat-treatment temperature ranges from about 400
to about 700.degree. C. The heat-treatment may be performed for a
period of time ranging from about 1 to about 24 hours. The
heat-treatment can remove the surfactant and dispersing agent used
for dispersing the carbon-based materials. When the heat-treatment
is performed at a temperature less than about 400.degree. C., the
surfactant may remain on the surface of the active material
particles, negatively influencing the electrochemical
characteristics of the battery. On the other hand, when the
heat-treatment is performed at a temperature greater than about
700.degree. C., the active metal particles may oxidize, thereby
deteriorating electrochemical characteristics such as battery
capacity.
[0057] According to another embodiment of the present invention, a
lithium rechargeable battery includes a negative electrode
including the above negative active material, a positive electrode
including a positive active material capable of reversible
intercalation and deintercalation of lithium, and an
electrolyte.
[0058] The rechargeable lithium battery includes the inventive
negative active materials, which are not pulverized and are easily
detached from the conductive material and current collector. Such a
lithium battery exhibits improved initial efficiency and cycle-life
characteristics.
[0059] The negative active material is mixed with a binder and then
applied to a current collector such as copper to form a negative
electrode active mass and thereby fabricate a negative electrode.
As needed, the active mass may include conductive materials.
[0060] Nonlimiting examples of suitable conductive materials
include nickel powders, cobalt oxide, titanium oxide, carbon, and
so on. Nonlimiting examples of suitable carbon materials for the
conductive materials include ketjen black, acetylene black, furnace
black, graphite, carbon fiber, fullerene, and so on. The graphite
acts as an electrode structure supporter as well as a conductive
material.
[0061] Nonlimiting examples of suitable binders include
polyvinylidene fluoride, polyvinyl chloride, and so on.
[0062] FIG. 3 is a perspective view of a rechargeable lithium
battery 1 according to one embodiment of the present invention. The
rechargeable lithium battery 1 includes a negative electrode 2, a
positive electrode 3, a separator 4 positioned between the positive
electrode 3 and the negative electrode 2, and an electrolyte
immersing the separator 4. In addition, the battery 1 includes a
cell housing 5 and a sealing member 6 for sealing the cell housing
5. Even though the rechargeable lithium battery shown in FIG. 3 is
cylindrical in shape, it may take various shapes such as prisms,
coins, or sheets.
[0063] The positive electrode includes a positive active material,
a conductive agent, and a binder. The positive active material may
include a compound capable of reversibly
intercalating/deintercalating lithium ions, such as
LiMn.sub.2O.sub.4, LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2,
V.sub.2O.sub.5, TiS, MoS, and so on. The separator may include an
olefin-based porous film such as polyethylene, polypropylene, and
so on.
[0064] An electrolyte of the present invention may include a
lithium salt dissolved in a solvent. The solvent may be a
non-aqueous organic solvent.
[0065] The non-aqueous organic solvent acts as a medium for
transmitting ions taking part in the electrochemical reaction of
the battery. Nonlimiting examples of suitable non-aqueous organic
solvents include benzene, toluene, fluorobenzene,
1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene,
1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene,
1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene,
1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene,
1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,
1,2,3-triiodobenzene, 1,2,4-triiodobenzene, fluorotoluene,
1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene,
1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene,
1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene,
1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene,
1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene,
1,2,3-triiodotoluene, 1,2,4-triiodotoluene, R--CN (where R is a C2
to C50 linear, branched, or cyclic hydrocarbon, a double bond, an
aromatic ring, or an ether bond), dimethylformamide,
dimethylacetate, xylene, cyclohexane, tetrahydrofuran,
2-methyltetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol,
dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate,
methylpropyl carbonate, propylene carbonate, methyl propionate,
ethyl propionate, methyl acetate, ethyl acetate, propyl acetate,
dimethoxyethane, 1,3-dioxolane, diglyme, tetraglyme, ethylene
carbonate, propylene carbonate, .gamma.-butyrolactone, sulfolane,
valerolactone, decanolide, and mevalolactone. A single non-aqueous
organic solvent may be used or a mixture of solvents may be used.
When a mixture of organic solvents is used, the mixture ratio can
be controlled according to the desired battery performance.
[0066] The lithium salt is dissolved in the non-aqueous organic
solvent to supply lithium ions in the battery. The lithium salt
facilitates basic operation of the rechargeable lithium battery,
and facilitates the transmission of lithium ions between positive
and negative electrodes. Non-limiting examples of suitable lithium
salts include supporting electrolytic salts such as LiPF.sub.6,
LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiCF.sub.3SO.sub.3,
LiN(CF.sub.3SO.sub.2).sub.3, Li(CF.sub.3SO.sub.2).sub.2N,
LiC.sub.4F.sub.9SO.sub.3, LiClO.sub.4, LiAlO.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2) (where x
and y are natural numbers), LiCl, and Lil.
[0067] Further, instead of the above-mentioned electrolyte, a solid
polymer electrolyte may be used. in this embodiment, a polymer
having lithium ion-conductivity may be used. Nonlimiting examples
of suitable polymers include polyethylene oxide, polypropylene
oxide, polyethyleneimine, and so on. The polymer is used in a gel
state such that the solvent and the solute are added to the
polymer.
[0068] The following examples illustrate the exemplary embodiments
of the present invention. However, these examples are presented for
illustrative purposes only and do not limit the scope of the
present invention.
Example 1
[0069] 100 ml of water, 500 g of zirconia balls, 20 g of
crystalline carbon fiber, and 0.5 g of Orotan.TM. (Hanjung Chem
Co.) were put in a 300 ml, plastic bottle and then ball-milled for
2 hours to prepare a mixed solution in which the carbon fiber was
completely dispersed. Then, 1 g of gelatin was dissolved in 200 ml,
of water. 100 g of Si powder (average particle diameter of 10
.mu.m) was added to the mixed solution, which was then agitated and
the pH controlled at 7. The above carbon fiber dispersion solution
was then added, and acetic acid was used to regulate the pH to 3 to
4. The resulting solution was agitated for 10 minutes and then
allowed to stand for 1 to 2 minutes. Then, Si particles coated with
carbon fiber precipitated to the bottom and were recovered. The Si
particles coated with carbon fiber were fired at 500.degree. C. to
remove the gelatin, thereby preparing a negative active
material.
[0070] The prepared negative active material and a nickel powder
were added to a binder solution in which a polyvinylidene binder
was dissolved in an N-methylpyrrolidone solvent and then mixed to
prepare a negative active material slurry.
[0071] The prepared negative active material slurry was coated on a
copper foil and then dried at 110.degree. C. in a vacuum oven and
compressed with a press, thereby preparing a negative
electrode.
[0072] The negative electrode for a rechargeable lithium battery
and a Li metal counter electrode were used to fabricate a
half-cell. A solution of ethylene carbonate and diethylene
carbonate (mixed in a volume ratio of 1:1), in which 1M LiPF.sub.6
was dissolved, was used as an electrolyte.
Example 2
[0073] A half cell was fabricated as in Example 1 except that
carbon black having an average particle diameter of 10 nm was used
instead of the crystalline carbon fiber.
Example 3
[0074] A half cell was fabricated as in Example 1 except that
carbon nanotubes were used instead of the crystalline carbon
fiber.
Comparative Example 1
[0075] A half cell was fabricated as in Example 1 except that Si
powder was used as a negative active material.
[0076] FIGS. 4 and 5 are scanning electron microscope (SEM)
photographs of the negative active materials prepared according to
Example 1 and Comparative Example 1, respectively. Unlike the
negative active material prepared according to Comparative Example
1 and shown in FIG. 5, the negative active material prepared
according to Example 1 and shown in FIG. 4 was uniformly coated
with crystalline carbon.
[0077] FIG. 6 is a graph of the cycle-life characteristics of the
battery cell fabricated according to Example 1. FIG. 7 is a graph
of the charge and discharge curve of the battery cell fabricated
according to Example 1. FIG. 8 is a graph of the cycle-life
characteristics of the battery cell fabricated according to
Comparative Example 1, and FIG. 9 is a graph of the charge and
discharge curve of the battery cell fabricated according to
Comparative Example 1.
[0078] Judging from the results shown in FIGS. 6 through 8, Example
1 (using Si powder coated with carbon fiber as a negative active
material) had much improved cycle-life characteristics and initial
efficiency as compared to Comparative Example 1 (using Si powder
without coating as a negative active material). Examples 2 and 3
showed results similar to Example 1.
[0079] Therefore, according to an exemplary embodiment, the
inventive negative active materials for rechargeable lithium
batteries avoids pulverization due to volume expansion of
metal-based active materials, improves contact with the conductive
material, and prevents separation from the current collector.
According to such an embodiment, the inventive negative active
materials thereby improve electro-conductivity of the negative
electrode and the cycle-life characteristics of the rechargeable
lithium battery and enhance the initial efficiency of the
rechargeable lithium battery.
[0080] While certain exemplary embodiments of the invention have
been described, it is understood by those of ordinary skill in the
art that various modifications and alterations to the described
embodiments may be made without departing from the spirit and scope
of the invention, as described in the appended claims.
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