U.S. patent application number 13/270132 was filed with the patent office on 2012-05-03 for spinel-type lithium titanium oxide/graphene composite and method of preparing the same.
This patent application is currently assigned to INDUSTRY-ACADEMIC COOPERATION FOUNDATION, YONSEI UNIVERSITY. Invention is credited to Hyun Kyung KIM, Ji Young KIM, Kwang Bum KIM.
Application Number | 20120104327 13/270132 |
Document ID | / |
Family ID | 45995641 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120104327 |
Kind Code |
A1 |
KIM; Kwang Bum ; et
al. |
May 3, 2012 |
Spinel-Type Lithium Titanium Oxide/Graphene Composite and Method of
Preparing the Same
Abstract
A spinel-type lithium titanium oxide/graphene composite and a
method of preparing the same are provided. The method can be useful
in simplifying a manufacturing process and shortening a
manufacturing time using microwave associated solvothermal reaction
and post heat treatment, and the spinel-type lithium titanium
oxide/graphene composite may have high electrochemical performances
due to its excellent capacity and rate capability and long
lifespan, and thus be used as an electrode material of the lithium
secondary battery.
Inventors: |
KIM; Kwang Bum;
(Gyeonggi-do, KR) ; KIM; Hyun Kyung;
(Chungcheongbuk-do, KR) ; KIM; Ji Young; (Seoul,
KR) |
Assignee: |
INDUSTRY-ACADEMIC COOPERATION
FOUNDATION, YONSEI UNIVERSITY
Seoul
KR
|
Family ID: |
45995641 |
Appl. No.: |
13/270132 |
Filed: |
October 10, 2011 |
Current U.S.
Class: |
252/507 ;
252/506; 977/734; 977/842 |
Current CPC
Class: |
C01G 23/005 20130101;
H01M 4/485 20130101; H01M 10/052 20130101; C01P 2006/40 20130101;
C01P 2004/64 20130101; Y02E 60/10 20130101; H01M 4/587 20130101;
C01B 32/192 20170801; C01P 2002/32 20130101; B82Y 40/00 20130101;
C01P 2002/82 20130101; B82Y 30/00 20130101; B01J 19/126 20130101;
H01M 4/04 20130101; H01B 1/18 20130101; C01P 2004/04 20130101; C01P
2002/72 20130101; H01M 4/364 20130101 |
Class at
Publication: |
252/507 ;
252/506; 977/734; 977/842 |
International
Class: |
H01M 4/485 20100101
H01M004/485; H01B 1/18 20060101 H01B001/18; H01M 4/04 20060101
H01M004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2010 |
KR |
10-2010-0106929 |
Claims
1. A method of preparing a transition metal oxide/graphene
composite comprising: (a) mixing a transition metal (M1) salt,
water and a solution of graphite oxide dispersed in a polyol
solvent; (b) preparing a transition metal (M1) oxide/graphene
composite by applying microwaves to the mixed solution; and (c)
preparing a transition metal (M1M2) oxide/graphene composite by
applying microwaves to a solution obtained by mixing the transition
metal (M1) oxide/graphene composite with a transition metal (M2)
salt, wherein M1 and M2 are the same as or different from each
other, and represent lithium, ruthenium, nickel, vanadium, cobalt,
manganese, iron, or titanium.
2. The method of claim 1, wherein, in step (a), a powder of the
graphite oxide is dispersed in the polyol solvent using
sonication.
3. The method of claim 1, wherein, in step (a), the water is mixed
at a content of 5 to 90 parts by weight, based on 100 parts by
weight of the mixed solution.
4. The method of claim 1, wherein, in step (b), the microwaves have
an oscillation frequency of 2.45 to 60 GHz.
5. The method of claim 1, wherein, in step (b), the transition
metal (M1) salt of step (a) is titanium salt, and a
TiO.sub.2/graphene composite is formed by applying microwaves to a
mixed solution containing the titanium salt to form titanium oxide
nanoparticles on a surface of the graphene.
6. The method of claim 5, wherein the titanium oxide nanoparticles
have particle sizes of 2 to 10 nm.
7. The method of claim 1, further comprising: washing and drying
the transition metal (M1) oxide/graphene composite prepared in step
(b).
89. The method of claim 7, wherein the transition metal (M1)
oxide/graphene composite is washed with alcohol or distilled
water.
9. The method of claim 1, wherein the reaction in step (c) is
performed for 10 to 30 minutes under microwaves having an
oscillation frequency of 2.45 to 60 GHz.
10. The method of claim 1, wherein, in step (c), the transition
metal (M2) salt is lithium salt, and a
Li.sub.4Ti.sub.5O.sub.12/graphene composite is formed by applying
microwaves to a mixed solution containing the lithium salt to form
lithium titanium oxide nanoparticles on a surface of the
graphene.
11. The method of claim 10, wherein the lithium titanium oxide
nanoparticles have particle sizes of 5 to 30 nm.
12. The method of claim 1, further comprising: subjecting the
transition metal (M1M2) oxide/graphene composite to heat treatment
under an argon atmosphere containing hydrogen.
13. The method of claim 13, wherein the heat treatment is performed
in a temperature range in which the transition metal (M1M2) oxide
can be structurally changed into a transition metal (M1M2) oxide
having a spinel structure without decomposing a nanostructure of
the transition metal (M1M2) oxide.
14. The method of claim 13, wherein the heat treatment is performed
at 500 to 900.degree. C.
15. A lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene
composite having a spinel structure, comprising: graphene; and
lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12) having a
nano-sized spinel structure stacked on a surface of the
graphene.
16. The lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene
composite of claim 15, wherein the lithium titanium oxide having a
spinel structure has a particle size of 5 to 30 nm.
17. An anode material for lithium secondary batteries comprising a
lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene
composite having a spinel structure defined in claim 15.
18. The anode material of claim 17, wherein the anode material has
a capacity at a 100 C rate of 101 mAh/g or more.
19. The anode material of claim 17, wherein the anode material has
an initial discharging capacity of 98% or more after the 100
discharging cycles at a 1 C rate, and a discharging capacity of 97%
or more after the 100 discharging cycles at a 10 C rate.
20. A lithium secondary battery comprising an electrode formed of
an anode material defined in claim 17.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 2010-0106929, filed Oct. 29, 2010,
the disclosure of which is incorporated herein by reference in its
entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to a spinel-type lithium
titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/reduced graphite oxide
(graphene) composite and a method of preparing the same, and more
particularly, to a method of preparing a spinel lithium titanium
oxide/graphene composite having excellent electrochemical
properties capable of simplifying a manufacturing process and
shortening a manufacturing time using microwave associated
solvothermal reaction and post heat treatment.
[0004] 2. Discussion of Related Art
[0005] Graphene is a carbon structure composed of a two-dimensional
(2-D) nanosheet single layer in which sp2 carbon atoms are formed
into a hexagonal honeycomb lattice. Since graphene was separated
from graphite using a peeling method developed by the Geim research
staff (Great Britain) in 2004, reports on graphene have continued
to be published. Graphene has come into the spotlight as a leading
new material because it has a very high specific surface area (a
theoretical value of 2600 m.sup.2/g) with respect to its volume and
shows excellent electronic conductivity and physical and chemical
stabilities (a typical value of 8.times.10.sup.5 S/cm in an aspect
of quantum mechanics).
[0006] In particular, graphene serves as an effective template on
which a nano-sized transition metal oxide can be deposited due to
its high specific surface area, excellent electric conductivity and
physical and chemical stabilities. When a nanocomplex is formed
with a transition metal, graphene may be used in an unlimited
variety of applications such as energy storage materials (a lithium
ion secondary battery, a hydrogen storage fuel cell, an electrode
of a supercapacitor, etc.), gas sensors, medical engineering
microparts, and a highly functional composite in a variety of
devices.
[0007] However, graphene is not easily peeled even when it is in a
solution phase because of the van der Waals's interaction between
graphene layers due to the presence of sp2 carbon bonds on a
surface of the graphene. Also, graphene is not mainly present as
single layer graphene but thick multilayer graphene, and readily
re-stacks when it is peeled off. Therefore, when a complex material
with a transition metal oxide is synthesized in a solution phase
using graphene as a precursor, it is difficult to use a high
specific surface area of single layer graphene and to form a
uniform complex structure, which serves as a factor preventing the
use of the transition metal oxide.
[0008] On the other hand, graphite oxide is a material in which a
number of oxygen functional groups are introduced into a surface of
a graphite layer having a graphite-layered structure obtained by
subjecting graphite to strong oxidation. Therefore, graphite oxide
may be used as a precursor when graphene is mass-produced using a
method such as chemical reduction or thermal peeling. Unlike
graphene, graphite oxide may be easily dispersed into single layer
graphite oxide or graphene oxide due to numerous oxygen functional
groups on a surface of the graphite oxide when the graphite oxide
is coated with another solution including a water system and is
subjected to ultrasonic treatment. Therefore, when a complex
material with a transition metal oxide is synthesized using
graphene oxide uniformly dispersed in a solution phase as a
precursor, graphene oxide may serve as a template on which a
nano-sized transition metal oxide can be uniformly deposited.
However, since the various oxygen functional groups introduced into
the surface of the graphene oxide through an oxidation process are
generated by partial breakup of sp2 bonds of graphene, the electric
conductivity may be degraded. Therefore, when a complex with a
nano-sized transition metal oxide is formed using graphene oxide,
in order to use the excellent electric conductivity of graphene, a
subsequent process of removing the oxygen functional groups from
the surface of the graphene oxide and recovering the sp2 bonds of
the graphene using a reducing agent or a hot treatment process is
necessarily required after formation of a complex material with the
nano-sized transition metal oxide.
[0009] In recent years, Li.sub.4Ti.sub.5O.sub.12 having a spinel
structure has come into the spotlight as an anode material for a
lithium ion battery. This is because the anode material is hardly
changed in volume during charging/discharging cycles, which allows
the lithium ion battery to have a long stable lifespan property
(cycling) and avoid reduction of electrolytes in an electrode
surface. However, conventional Li.sub.4Ti.sub.5O.sub.12 having a
spinel structure is difficult to manufacture on a nanosized scale
due to its limits in manufacturing processes, and shows poor
capacity and rate capability as the lithium battery anode material
because of its poor conductivity. In addition, since a large amount
of time (for example, 24 hours) is required to synthesize
Li.sub.4Ti.sub.5O.sub.12, many problems should be solved in advance
for it to be applied to the lithium secondary battery. Accordingly,
ways and means to solve the above-mentioned problems are still
required.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to providing a method
capable of manufacturing a lithium titanium oxide/graphene
composite without performing a subsequent process of removing
oxygen functional groups from a surface of graphene oxide and
uniformly forming a transition metal oxide on a surface of graphene
within a short period of time when the lithium titanium
oxide/graphene composite is synthesized using a graphite oxide
precursor.
[0011] Also, the present invention is directed to providing an
anode material for lithium secondary batteries composed of the
lithium titanium oxide/graphene composite to give high capacity and
rate capability, and a lithium secondary battery including an
electrode composed of the anode material.
[0012] One aspect of the present invention provides a method of
preparing a transition metal oxide/graphene composite. Here, the
method includes:
[0013] (a) mixing a transition metal (M1) salt, water and a
solution graphite oxide dispersed in a polyol solvent;
[0014] (b) preparing a transition metal (M1) oxide/graphene
composite by applying microwaves to the mixed solution; and
[0015] (c) preparing a transition metal (M1M2) oxide/graphene
composite by applying microwaves to a solution obtained by mixing
the transition metal (M1) oxide/graphene composite with a
transition metal (M2) salt,
[0016] Here, the M1 and M2 are the same as or different from each
other, represent lithium, ruthenium, nickel, vanadium, cobalt,
manganese, iron, or titanium,
[0017] Another aspect of the present invention provides a lithium
titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene composite having
a spinel structure, including:
[0018] a graphene; and
[0019] a lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12) having a
nano-sized spinel structure stacked on a surface of the
graphene.
[0020] Still another aspect of the present invention provides an
anode material for lithium secondary batteries including a lithium
titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene composite having
a spinel structure according to one exemplary embodiment of the
present invention.
[0021] Yet another aspect of the present invention provides a
lithium secondary battery including an electrode formed of the
anode material according to one exemplary embodiment of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The above and other objects, features and advantages of the
present invention will become more apparent to those of ordinary
skill in the art by describing in detail exemplary embodiments
thereof with reference to the attached drawings, in which:
[0023] FIG. 1 is a flowchart showing a process of preparing a
Li.sub.4Ti.sub.5O.sub.12/graphene composite having a spinel
structure according to one exemplary embodiment of the present
invention;
[0024] FIG. 2 is a graph showing the X-ray diffraction (XRD)
analysis results determining whether lithium titanium oxide is
present on the Li.sub.4Ti.sub.5O.sub.12/graphene composite having a
spinel structure prepared according to one exemplary embodiment of
the present invention;
[0025] FIG. 3 shows a Raman spectra determining whether lithium
titanium oxide is present on the Li.sub.4Ti.sub.5O.sub.12/graphene
composite having a spinel structure prepared according to one
exemplary embodiment of the present invention;
[0026] FIG. 4 is a transmission electron microscope (TEM)
photographic diagram of the Li.sub.4Ti.sub.5O.sub.12/graphene
composite having a spinel structure prepared according to one
exemplary embodiment of the present invention;
[0027] FIG. 5 is X-ray photoelectron spectoscopy(XPS) data
analyzing a level of reduction of graphene (C/O ratio) in the
Li.sub.4Ti.sub.5O.sub.12/graphene composite having a spinel
structure prepared according to one exemplary embodiment of the
present invention;
[0028] FIG. 6 is a graph showing the capacity of an active material
per unit weight of an electrode which is obtained through
evaluation of charging/discharging capacity of the
Li.sub.4Ti.sub.5O.sub.12/graphene composite having a spinel
structure prepared according to one exemplary embodiment of the
present invention; and
[0029] FIG. 7 shows the lifespan property of the
Li.sub.4Ti.sub.5O.sub.12/graphene composite having a spinel
structure prepared according to one exemplary embodiment of the
present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0030] Hereinafter, exemplary embodiments of the present invention
will be described in detail. However, the present invention is not
limited to the embodiments disclosed below, but can be implemented
in various forms. The following embodiments are described in order
to enable those of ordinary skill in the art to embody and practice
the present invention.
[0031] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
exemplary embodiments. The singular forms "a," "an" and "the" are
intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will be further understood that the
terms "comprises," "comprising," "includes" and/or "including,"
when used herein, specify the presence of stated features,
integers, steps, operations, elements, components and/or groups
thereof, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements,
components and/or groups thereof.
[0032] With reference to the appended drawings, exemplary
embodiments of the present invention will be described in detail
below. To aid in understanding the present invention, like numbers
refer to like elements throughout the description of the figures,
and the description of the same elements will be not
reiterated.
[0033] Hereinafter, embodiments of the present invention will be
described in detail.
[0034] The present invention relates to a method of preparing a
transition metal oxide/graphene composite. Here, the method
includes:
[0035] (a) mixing a transition metal (M1) salt, water and solution
graphite oxide dispersed in a polyol solvent;
[0036] (b) preparing a transition metal (M1) oxide/graphene
composite by applying microwaves to the mixed solution; and
[0037] (c) preparing a transition metal (M1M2) oxide/graphene
composite by applying microwaves to a solution obtained by mixing
the transition metal (M1) oxide/graphene composite with a
transition metal (M2) salt,
[0038] Here, M1 and M2 are the same as or different from each
other, and represent lithium, ruthenium, nickel, vanadium, cobalt,
manganese, iron, or titanium.
[0039] The method of preparing a transition metal oxide/graphene
composite according to the present invention includes a transition
metal oxide/graphene composite having a heterogeneous structure in
which at least two same or different nano-sized transition metal
oxides are formed on a surface of graphene.
[0040] The method of preparing a transition metal oxide/graphene
composite according to the present invention will be described in
detail with reference to FIG. 1, as follows.
[0041] In step (a), a graphite oxide powder is dispersed in the
presence of a polyol solvent and a transition metal (M1) salt and
water are added according to a microwave associated polyol
reaction.
[0042] Dispersing the graphite oxide powder in the polyol solvent
includes adding powdery graphite oxide to a polyol solvent and
uniformly dispersing the graphite oxide in the polyol solvent using
sonication.
[0043] In this case, the graphite oxide powder may be preferably
used at a content of 0.001 to 0.5 parts by weight, and more
preferably 0.01 to 0.1 parts by weight, based on 100 parts by
weight of the polyol solvent. In this case, when the content of the
graphite oxide powder is less than 0.001 parts by weight, the
electric conductivity may not be expected to improve by addition of
graphene to the graphene and transition metal oxide composite after
the reaction. When the content of the graphite oxide powder exceeds
0.5 parts by weight, it is difficult to disperse the graphite oxide
powder.
[0044] The polyol may be used without particular limitation, but
examples of the polyol may include ethylene glycol, diethylene
glycol, triethylene glycol, or tetraethylene glycol.
[0045] The transition metal (M1) salt provided to prepare a metal
oxide may include a salt of lithium, ruthenium, nickel, vanadium,
cobalt, manganese, iron or titanium, which may be used alone or in
combination.
[0046] The transition metal salt may be included at a content of
0.01 to 20 parts by weight, based on 1 part by weight of the
graphite oxide, since an amount of the transition metal oxide
deposited on a surface of the graphene may be controlled according
to a content. When the content of the transition metal salt is less
than 0.01 parts by weight, a small amount of the synthesized
transition metal oxide is loaded into the transition metal
oxide/graphene complex material, whereas, when the content of the
transition metal salt exceeds 20 parts by weight, it is difficult
to disperse the transition metal oxide on the graphene during a
synthesis step, and the transition metal oxide may be spontaneously
educed in addition to the materials of the transition metal
oxide/graphene composite.
[0047] In addition, the water may be added after the transition
metal salt is completely dissolved, but the present invention is
not particularly limited thereto.
[0048] The addition of water to a mixed solution of a dispersing
solution and a transition metal salt is carried out to synthesize a
metal oxide using a forced hydrolysis action without performing an
post heat treatment process when the metal oxide is synthesized
using a microwave associated polyol reaction,
[0049] The water may be included at a content of 5 to 90 parts by
weight, based on 100 parts by weight of the mixed solution. When
the content of the water is less than 5 parts by weight, the metal
oxide is obtained in the form of metal rather than transition metal
oxide after the synthesis is completed. When the content of the
water exceeds 90 parts by weight, a synthetic efficiency of the
obtained transition metal oxide may be low.
[0050] In step (b), a transition metal (M1) oxide/graphene
composite is prepared by applying microwaves to the mixed
solution.
[0051] The microwave associated polyol reaction is a heating method
using microwaves which have a higher heating rate than a heating
method using a reflux device. Therefore, since the microwave
associated polyol reaction may be used to uniformly heat the entire
solution, a reaction time may be shortened, and final particles may
be prepared in a smaller size.
[0052] When the graphite oxide powder in the mixed solution is
reduced to graphene by means of the applied microwaves using the
solvothermal synthesis, the graphite oxide powder is increased to a
relatively higher temperature, which causes selective random
nucleation and growth of the transition metal oxide on a surface of
the graphene. Then, the transition metal salt (M1) in the mixed
solution is formed on a surface of the graphene in the form of
transition metal (M1) oxide nanoparticles using a solvothermal
synthesis process, thereby synthesizing the transition metal (M1)
oxide/graphene composite.
[0053] That is, the transition metal oxide/graphene composite may
be prepared since the synthesis of the transition metal (M1) oxide
nanoparticle composite and a reduction reaction of the graphite
oxide appear at the same time in this step.
[0054] According to one exemplary embodiment, in step (b), the
microwaves may have an oscillation frequency having a sufficiently
low energy not to decompose a molecular structure of the polyol
solvent in the mixed solution.
[0055] According to one exemplary embodiment, the transition metal
(M1) oxide/graphene composite may be synthesized by reacting the
mixed solution under the microwaves having an oscillation frequency
of 2.45 to 60 GHz for 10 to 30 minutes. This is because particle
shape and size of the transition metal (M1) oxide may be easily
controlled.
[0056] According to one exemplary embodiment, in step (b), the
transition metal (M1) salt of step (a) may be titanium salt, and a
TiO.sub.2/graphene composite may be formed by applying microwaves
to a mixed solution including the titanium salt to form titanium
oxide nanoparticles on a surface of the graphene.
[0057] The titanium oxide nanoparticles are formed in an anatase
structure, and have particle sizes of 2 to 10 nm, and more
particularly, particle sizes of 3 to 5 nm.
[0058] Also, when the reaction is completed, the transition metal
(M1) oxide/graphene composite may be further washed and dried. The
washing and drying processes are performed to remove a residual
solvent or organic compounds that may be additionally formed in the
mixed solution prepared in the previous step.
[0059] The washing solvent that may be used herein includes, but is
not particularly limited to, alcohol, distilled water, or a mixed
solvent thereof.
[0060] Also, the washed transition metal (M1) oxide/graphene
composite may be dried at a temperature condition of room
temperature to 70.degree. C., and a drying process may be used
without particular limitation, but the drying may be performed
using a conventional drying process.
[0061] In step (c), a transition metal (M2) salt is mixed with the
transition metal (M1) oxide/graphene composite synthesized in step
(b). Then, the transition metal (M2) salt is bound to a surface of
the transition metal (M1) oxide, by applying microwaves to the
mixed solution, to prepare a composite in which particles of the
nano-sized transition metal (M1M2) oxide are formed on a surface of
the graphene.
[0062] The transition metal (M2) salt provided to prepare a metal
oxide may include a salt of lithium, ruthenium, nickel, vanadium,
cobalt, manganese, iron or titanium, which may be used alone or in
combination.
[0063] The transition metal salt may be included at a content of
0.01 to 20 parts by weight, based on 1 part by weight of the
graphite oxide, since an amount of the transition metal oxide
deposited on a surface of the graphene may be controlled according
to a content. When the content of the transition metal salt is less
than 0.01 parts by weight, a small amount of the synthesized
transition metal oxide is loaded into the transition metal
oxide/graphene complex material, whereas, when the content of the
transition metal salt exceeds 20 parts by weight, it is difficult
to disperse the transition metal oxide on the graphene during a
synthesis step, and the transition metal oxide may be spontaneously
educed in addition to the materials of the transition metal
oxide/graphene composite.
[0064] According to one exemplary embodiment, in step (c), a
composite in which particles of the nano-sized transition metal
(M1M2) oxide are formed on a surface of the graphene may be
prepared by applying microwaves having an oscillation frequency of
2.45 to 60 GHz to the mixed solution for 10 to 30 minutes.
[0065] According to one exemplary embodiment, in step (c), the
transition metal (M2) salt is lithium salt, and a
Li.sub.4Ti.sub.5O.sub.12/graphene composite may be formed by
applying microwaves to a mixed solution containing the lithium salt
to form lithium titanium oxide nanoparticles on a surface of the
graphene.
[0066] The lithium titanium oxide nanoparticles may have particle
sizes of 5 to 30 nm, and more particularly, particle sizes of 10 to
20 nm.
[0067] In step (c), the transition metal (M1M2) oxide/graphene
composite may be further subjected to heat treatment in an argon
atmosphere containing hydrogen.
[0068] The heat treatment may be performed in a temperature range
in which the transition metal (M1M2) oxide can be structurally
changed into a transition metal (M1M2) oxide having a spinel
structure without decomposing a nanostructure of the transition
metal (M1M2) oxide.
[0069] According to one exemplary embodiment, the heat treatment
may be performed at a temperature range of approximately 500 to
900.degree. C.
[0070] According to one exemplary embodiment, the transition metal
(M1M2) oxide having a spinel structure may be lithium titanium
oxide (Li.sub.4Ti.sub.5O.sub.12).
[0071] Also, the present invention relates to a lithium titanium
oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene composite having a spinel
structure, including:
[0072] graphene; and
[0073] lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12) having a
nano-sized spinel structure stacked on a surface of the
graphene.
[0074] According to another aspect of the present invention, a
composite of graphene and lithium titanium oxide
(Li.sub.4Ti.sub.5O.sub.12) having a spinel structure is provided.
Here, since the lithium titanium oxide having a spinel structure is
stacked on a surface of the graphene in the form of nano-sized
particles, the transition metal oxide has a maximized surface area
since the transition metal oxide is very small and uniform in
particle size. Therefore, the transition metal oxide may show
high-rate charging/discharging capacity and a lifespan
property.
[0075] According to one exemplary embodiment, the lithium titanium
oxide having a spinel structure may have a particle size of 5 to 30
nm, and more particularly, a particle size of 10 to 20 nm.
[0076] Also, the present invention relates to an anode material for
lithium secondary batteries including the lithium titanium oxide
(Li.sub.4Ti.sub.5O.sub.12)/graphene composite having a spinel
structure.
[0077] The anode material may have a capacity at a 100 C rate of
101 mAh/g or more.
[0078] According to one exemplary embodiment, the anode material
may have an initial discharging capacity of 98% or more after the
100 discharging cycles at a 1 C rate, and a discharging capacity of
97% or more after the 100 discharging cycles at a 10 C rate.
[0079] Furthermore, the present invention relates to a lithium
secondary battery including an electrode formed of the anode
material.
[0080] The lithium titanium oxide
(Li.sub.4Ti.sub.5O.sub.12)/graphene composite having a spinel
structure according to the present invention may be used as an
anode for lithium ion batteries since it has a high rate
capability. Also, the anode made of the composite shows excellent
rate capability and structural reversibility at a high C rate. Such
an excellent property is derived from a short diffusion distance of
lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12) nanoplatelets
uniformly dispersed on the reduced graphene having high
conductivity, a high interfacial area between the composite and an
electrolyte solution, a 3-D nanopore structure of the
nanocomposite, and excellent electric conductivity of a reduced
graphite oxide matrix.
[0081] Therefore, the electrode composed of the anode made of the
lithium titanium oxide (Li.sub.4Ti.sub.5O.sub.12)/graphene
composite according to the present invention may be used for
lithium secondary batteries since the electrode has excellent
charging/discharging rate and lifespan properties.
[0082] Hereinafter, the present invention will be described in
detail with reference to Examples thereof. However, it should be
understood that the Examples are not intended to limit the scope of
the present invention.
EXAMPLE 1
Preparation of Li.sub.4Ti.sub.5O.sub.12/Reducted Graphite Oxide
Nanocomposite
[0083] A spinel-type L.sub.i4T.sub.i5O.sub.12 nanoplatelet/reduced
graphite oxide (RGO) nanocomposite was synthesized using a two-step
microwave associated solvothermal reaction and post heat
treatment.
[0084] First, the TiO.sub.2/RGO nanocomposite was synthesized from
graphite oxide (GO) prepared from pure natural graphite powder
(SP-1, 200 mesh, Bay Carbon) according to a modified Hummers
method. GO powder (0.1 g) was sonicated in diethylene glycol (70
ml) (DEG, >99%, Fluka) for 30 minutes, and titanium ethoxide
(0.015 ml) (Aldrich) and distilled water (5 ml) were added to the
resulting solution. A 100-ml Teflon vessel was loaded with the
mixed solution, and sealed. Then, the resulting mixed solution was
placed in a microwave digestion system (MARS-5, CEM Corporation).
The reaction mixture was heated to 210.degree. C., and maintained
at this temperature for 30 minutes. When the reaction was
completed, a resulting powder (TiO.sub.2/RGO nanocomposite) was
repeatedly washed with ethanol and distilled water. The resulting
powder was dried at 70.degree. C. for 24 hours in a vacuum
oven.
[0085] 0.1 g of a Li--Ti--O/RGO nanocomposite was dispersed in 160
ml of an aqueous 0.2M LiOH solution loaded into the 100-ml Teflon
vessel under a solvothermal condition caused by the microwaves to
synthesize a reaction mixture. The reaction mixture was then heated
to 200.degree. C., and maintained at this temperature for 20
minutes. When the reaction was completed, a resulting powder
(Li--Ti--O/RGO nano-hybrid) was repeatedly washed with distilled
water, and dried at 70.degree. C. for 24 hours in a vacuum
oven.
[0086] Finally, the Li--Ti--O/RGO nanocomposite was heated at
700.degree. C. for 5 hours in an atmosphere containing 5% by weight
of H.sub.2/Ar atmosphere.
[0087] Characterization
[0088] X-ray diffraction (XRD) patterns were collected on a Rigaku
D/MAX 2,200 V/PC X-ray diffractometer using Cu K.alpha. radiation
(40 kV, 20 mA). The XRD patterns were taken at room temperature in
the 2.theta. range of 10.degree.<2.theta.<80.degree. at
intervals of 0.04.degree..
[0089] TEM images were recorded in a Philips CM200 microscope
operated at 200 kV. A sample was dispersed in ethanol, drop-cast
onto a carbon-coated microgrid, and dried before the sample
analysis.
[0090] Raman spectra were measured using a Jobin-Yvon LabRam HR
with a liquid N.sub.2 cooled CCD multichannel detector at room
temperature using a conventional backscattering geometry. An
argon-ion laser at a wavelength of 514.5 nm was used as the laser
light source, XPS spectra were obtained using a Thermo Electron
Corporation ESCA Lab 250 with a chamber pressure maintained below
5.times.10.sup.-9 mbar during acquisition. A monochromated Al K
alpha X-ray source (15 kV; 150 W) irradiated the samples, with a
spot diameter of approximately 0.5 mm.
[0091] Preparation of Electrode and Electrochemical Measurement
[0092] Electrochemical properties were investigated at room
temperature using a three-electrode electrochemical cell, with two
lithium foils as counter and reference electrodes. The working
electrode consisted of a mixture of 90 wt %
Li.sub.4Ti.sub.5O.sub.12/RGO nano-hybrid and 10 wt % polyvinylidene
fluoride dissolved in N-methylpyrrolidone as a binder. The slurry
mixture was coated on a titanium foil (99.7% purity, Aldrich) and
then dried at 100.degree. C. for 24 h. Each working electrode with
a 1.times.1 cm.sup.2 area contained 2-3 mg of the dried slurry. The
charge-discharge tests and cyclic voltammetry were performed using
a potentiostat/galvanostat (VMP2, Princeton Applied Research). The
electrolyte was 1 M LiClO.sub.4 in propylene carbonate (PC).
[0093] FIG. 1 shows a design procedure for preparing a
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite. In brief, a GO
nanosheet was used as a template having a high surface area to
facilitate selective random nucleation and growth of TiO.sub.2
nanoparticles. Thereafter, a TiO.sub.2/RGO nanocomposite was
prepared through solvothermal synthesis caused by microwaves when
GO was partially reduced into RGO in a polyol solution while
inducing formation of TiO.sub.2/RGO. Then, the TiO.sub.2/RGO was
converted into a Li.sub.4Ti.sub.5O.sub.12 precursor (referred to as
Li--Ti--O)/RGO nanocomposite by a reaction with a LiOH solution
under a solvothermal condition caused by the microwaves. Finally,
the Li--Ti--O/RGO nanocomposite was subjected to heat treatment at
700.degree. C. for 5 hours in an atmosphere containing 5% by weight
of H.sub.2/Ar atmosphere to prepare a nanocomposite including a
pure phase, a highly crystalline Li.sub.4Ti.sub.5O.sub.12
nanoplatelet and highly conductive RGO.
[0094] In the present invention, because a GO nanosheet can be
gradually peeled off from a solution to cause a stable dispersion
composed mainly of single layer sheets, the GO nanosheet was
selected as a template for precipitation of TiO.sub.2
nanoparticles. Also, oxygen functional groups such as hydroxyl,
carboxyl and epoxy groups, which strongly interacted with the metal
oxide nanoparticles, were present at higher densities than the
RGO.
[0095] FIG. 2 shows XRD patterns of a Li.sub.4Ti.sub.5O.sub.12/RGO
nanocomposite. Here, the XRD patterns represent pure-phase and
highly crystalline spinel Li.sub.4Ti.sub.5O.sub.12. The formation
of the pure-phase Li.sub.4Ti.sub.5O.sub.12 was confirmed using the
Raman spectra (FIG. 3). A lattice constant of the spinel
Li.sub.4Ti.sub.5O.sub.12 nanoplatelet was calculated to be 8.364
.ANG. from the XRD data. This corresponds to other reported
numerical values.
[0096] FIG. 4 is a TEM photographic diagram showing a
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite. Here, it was shown that
Li.sub.4Ti.sub.5O.sub.12 nanoplatelets had sizes of 10 to 20 nm. As
shown in FIG. 1, the Li--Ti--O/RGO nanocomposite was formed by
subjecting the TiO.sub.2/RGO nanocomposite in a LiOH solution to
solvothermal treatment using microwaves.
[0097] In addition, in order to realize the high-rate
charging/discharging capacity in synthesizing the
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite electrode according to
the present invention, the electric conductivity of RGO was very
important. An important factor to improve the electric conductivity
of RGO is to remove residual oxygen introduced during the synthesis
of GO. In order to quantify a level of reduction of RGO, an atomic
ratio of carbon to oxygen was evaluated using XPS measurement (FIG.
5).
[0098] The post heat treatment of the Li--Ti--O/RGO nanocomposite
increased a C/O ratio of the RGO from 5.28 to 26.9 with respect to
the Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite.
[0099] When an electrode including the metal oxide nanoparticles
was prepared so that the electrode could come in electrical contact
between particles and a current collector and between particles, a
conductor (15% by weight or more) was also used. In the present
invention, an additional conductor was not used for preparation of
an electrode including the Li.sub.4Ti.sub.5O.sub.12/RGO
nanocomposite.
[0100] FIG. 6 show the results from a galvanostatic charging and
discharging experiment in which a composite electrode is irradiated
with a high rate capability in a 1-2.5 V electric potential window
with an increasing C rate from 1 to 100 C. Each set of charging and
discharging curves was measured at the same C rate. As a result,
typical charging and discharging behaviors of pure-phase spinel
Li.sub.4Ti.sub.5O.sub.12 having an electric potential plateau were
seen at 1.571 V (1 C) during a charging cycle and 1.543 V (1 C)
during a discharging cycle. The non-discharging capacity of
Li.sub.4Ti.sub.5O.sub.12 in the Li.sub.4Ti.sub.5O.sub.12/RGO
nanocomposite was 154 mAhg.sup.-1 at a 1 C rate. The
Li.sub.4Ti.sub.5O.sub.12 in the Li.sub.4Ti.sub.5O.sub.12/RGO
nanocomposite could transfer a discharging capacity of 128
mAhg.sup.-1 at a 50 C rate and a discharging capacity of 101
mAhg.sup.-1 at a 100 C rate (65% of 1 C discharging capacity)
without adding a conductor to the electrode. This explicitly proved
the excellent high-rate capability of the
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite. The
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite had a slightly increased
electrode polarity with an increasing C rate from 1 C to 100 C
during the charging/discharging cycles, which indicates that the
composite has excellent high-rate capability, compared to those
already reported.
[0101] It is also shown in FIG. 7, that the
Li.sub.4Ti.sub.5O.sub.12/RGO nanocomposite had an excellent
discharging capacity of 98% or more after at least 100 discharging
cycles at a 1 C rate and 97% or more after at least 100 discharging
cycles at a 10 C rate.
[0102] According to the present invention, a transition metal oxide
can be uniformly formed within a short period of time on a surface
of a graphene composite in the form of nanoparticles using
microwave associated solvothermal reaction.
[0103] Also, a transition metal oxide/graphene composite prepared
according to the method of the present invention can have a
maximized surface area since the transition metal oxide is very
small and uniform in particle size.
[0104] Furthermore, the spinel-type lithium titanium oxide/graphene
composite according to the present invention can be used as an
electrode material of a lithium secondary battery since it may have
a high electrochemical performance due to the excellent capacity
and rate capability and long lifespan property.
[0105] While the invention has been shown and described with
reference to certain exemplary embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the scope of
the invention as defined by the appended claims.
* * * * *