U.S. patent application number 10/923300 was filed with the patent office on 2005-02-24 for negative active material for rechargeable lithium battery, method of preparing same and rechargeable lithium battery.
Invention is credited to Kim, Sung-Soo, Matsubara, Keiko, Takamuku, Akira, Tsuno, Toshiaki.
Application Number | 20050042128 10/923300 |
Document ID | / |
Family ID | 34197186 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050042128 |
Kind Code |
A1 |
Matsubara, Keiko ; et
al. |
February 24, 2005 |
Negative active material for rechargeable lithium battery, method
of preparing same and rechargeable lithium battery
Abstract
Disclosed is a negative active material for a rechargeable
lithium battery comprising a Si phase, a SiM phase and at least one
of a X phase and a SiX phase, wherein each of phases has a crystal
grain size of 100 nm and 500 nm. The element M is at least one
selected from the group consisting of Ni, Co, B, Cr, Cu, Fe, Mn,
Ti, and Y, the element X is at least one selected from the group
consisting of Ag, Cu, and Au. However, where M is Cu, X is not
Cu.
Inventors: |
Matsubara, Keiko;
(Yokohama-shi, JP) ; Takamuku, Akira;
(Yokohama-shi, JP) ; Tsuno, Toshiaki;
(Yokohama-shi, JP) ; Kim, Sung-Soo; (Suwon-si,
KR) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
34197186 |
Appl. No.: |
10/923300 |
Filed: |
August 20, 2004 |
Current U.S.
Class: |
419/66 ;
420/578 |
Current CPC
Class: |
H01M 4/386 20130101;
Y02E 60/10 20130101; C22C 29/18 20130101; H01M 2300/004 20130101;
C22C 28/00 20130101; H01M 4/134 20130101; H01M 10/052 20130101 |
Class at
Publication: |
419/066 ;
420/578 |
International
Class: |
C22C 028/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2003 |
JP |
2003-299282 |
Feb 12, 2004 |
KR |
2004-0009366 |
Claims
What is claimed is:
1. A negative active material for a rechargeable lithium battery
comprising a Si phase, a SiM phase, and at least one of a X phase
and a SiX phase, wherein each of phases has a crystal grain size of
between 100 nm and 500 nm.
2. The negative active material for a rechargeable lithium battery
according to claim 1, wherein the element M is an element having
higher boiling point than that of element X.
3. A negative active material for a rechargeable lithium battery
prepared by the method comprising: alloying Si and element M by a
mechanical alloying process to provide a SiM alloy where M is
selected from the group consisting of Ni, Co, B, Cr, Cu, Fe, Mn,
Ti, Y, and combinations thereof; heating the SiM alloy at a first
temperature; adding element X as a powder to the heated SiM alloy,
where X is selected from the group consisting of Ag, Cu, Au and
combinations thereof, provided that if M is Cu, X is not Cu;
alloying the combined element X and SiM alloy by a mechanical
alloying process to provide a SiMX alloy; and heating the SiMX
alloy at a second temperature lower than the first temperature.
4. A rechargeable lithium battery comprising the negative active
material for a rechargeable lithium battery as claimed in claim
1.
5. A method of preparing a negative active material for a
rechargeable lithium battery comprising: first alloying Si and
element M by a mechanical alloying process to provide a SiM alloy,
wherein M is selected from the group consisting of Ni, Co, B, Cr,
Cu, Fe, Mn, Ti, Y and combinations thereof; first heating the SiM
alloy at a first temperature; adding a powder of element X to the
heated SiM alloy wherein X is selected from the group consisting of
Ag, Cu, Au and combinations thereof, provided that if M is Cu, X is
not Cu; second alloying the combined SiM alloy and element X by a
mechanical alloying process to provide a SiMX alloy; and second
heating the SiMX alloy at a second temperature lower than the first
temperature; wherein the negative active material comprises a Si
phase, a SiM phase and at least one of an X phase and a SiX phase,
wherein each of the phases has a crystal grain size between 100 nm
and 500 nm.
6. The method according to claim 5, wherein the first heating is
carried out at temperature between (Tm-100).degree. C. to
(Tm-20).degree. C. where Tm is the melting point of the SiM alloy
phase.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Japanese Patent Application No. 2003-299282 filed in the Japanese
Patent Office on Aug. 22, 2003 and Korean Patent Application No.
2004-9366 filed in the Korean Intellectual Property Office on Feb.
12, 2004, both of which are hereby incorporated by reference in
their entireties for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] The present invention relates to a negative active material
for a rechargeable lithium battery, a method of preparing the same,
and a rechargeable lithium battery.
[0004] (b) Description of the Related Art
[0005] Although research to develop a negative active material
having a high capacity based on metallic materials such as Si, Sn,
and Al has actively been undertaken, such research has not yet
succeeded in applying said metals to a negative active material.
This is mainly due to problems in that the cycle characteristics
are deteriorated by a series of processes of intercalating and
deintercalating lithium ions with metals such as Si, Sn, and Al,
and the consequential expansion and contraction of the volume
thereof, which pulverizes the metal to a fine powder. In order to
attempt to solve these problems, an amorphous alloy oxide has been
suggested by Y. Idota, et al: Science, 276, 1395(1997). In
addition, it is reported that a negative active material comprising
an amorphous structured alloy improves a battery's cycle
characteristics in 43.sup.rd Preview of Battery Discussion (The
Electrochemical Society of Japan, The Committee of Battery
Technology, Oct. 12, 2002, p. 308-309).
[0006] Although Si is expected to provide a higher capacity, Si is
generally known to be too hard to be transferred to an amorphous
phase either by itself or in an Si-alloy form. However, recently,
it has been reported that Si material can be transferred into
amorphous phase via a mechanical alloying process.
[0007] As mentioned in 43.sup.rd Preview of Battery Discussion (The
Electrochemical Society of Japan, The Committee of Battery
Technology, Oct. 12, 2002, p. 308-309), amorphous alloy material
has a good early stage capacity retention rate relative to that of
crystalline alloy material, but that capacity tends to remarkably
decrease after repeated charge-discharge cycles. For amorphous
material, as it does not have the same structure as a crystal
material, the expansion rate upon charging is relatively low and
the characteristics deteriorate less upon repeated charge and
discharge compared to those for crystal material. In addition, the
amorphous material can improve the early stage cycle
characteristics better than crystal material because the lithium
ion is better diffused. Further, although the active material is
not fully charged in the very early stage, the utilization of an
active material is slowly increased upon repeating cycles and, as a
result, the deterioration of the cycle characteristics due to the
pulverization of the material to a fine powder is alleviated.
However, upon repeating the cycles, it is anticipated that the
cycle characteristics will deteriorate due to the pulverization of
the material to a fine powder and the exhaustion of the active
material.
[0008] For a mechanical alloying process, a pulverizing step into
fine powder and a compressing step are repeated to slowly reduce
the crystal degree to provide an amorphous or pulverized material.
However, such a process may cause problems in that the interface is
broken between the tiny alloy structures identified via a X-ray
diffraction analysis, and the structure is easily broken upon
intercalating lithium ions and pulverized. Thereby the cycle
characteristics deteriorate.
SUMMARY OF THE INVENTION
[0009] In one embodiment of the present invention a negative active
material is provided that is capable of preventing the active
material from pulverizing into fine powder resulting in improved
cycle characteristic. Further embodiments include a method of
preparing such a negative active material, and a rechargeable
lithium battery comprising the negative active material.
[0010] In another embodiment of the present invention a negative
active material is provided for a rechargeable lithium battery in
which the material consists essentially of Si phase and SiM phase
material with at least one of X phase and SiX phase, wherein each
crystalline grain of the phases has a diameter of between 100 nm
and 500 nm, and wherein the element M is selected from the group
consisting of Ni, Co, B, Cr, Cu, Fe, Mn, Ti, Y, and combinations
thereof, and the element X is selected from the group consisting of
Ag, Cu, Au, and combinations thereof, provided that Cu is not
selected for both element M and element X.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more complete appreciation of the invention, and many of
the attendant advantages thereof, will be readily apparent as the
same becomes better understood by reference to the following
detailed description when considered in conjunction with the
accompanying drawings, wherein:
[0012] FIG. 1 is a SEM photograph of the negative active material
of Example 1;
[0013] FIG. 2 is a SEM photograph of the negative active material
of Example 2;
[0014] FIG. 3 is a SEM photograph of the negative active material
of Example 3;
[0015] FIG. 4 is a SEM photograph of the negative active material
of Example 4;
[0016] FIG. 5 is a graph illustrating the X-ray refraction pattern
of the particles of each step in Example 1 and the active material
of Example 2; and
[0017] FIG. 6 is a graph illustrating the relationship between the
number of cycles and the discharge capacities for the rechargeable
lithium batteries of Examples 1 to 4.
DETAILED DESCRIPTION
[0018] According to the present invention, a negative active
material for a rechargeable lithium battery has a crystal grain
comprising Si phase and SiM phase with a very small diameter of 500
nm or less with the grains closely aggregated with one another.
According to this structure, it is difficult to destroy the
structure even though expansion and contraction are repeated upon
charging and discharging the lithium. These properties can improve
the cycle characteristics.
[0019] Further, since the structure comprises SiM phase in addition
to Si phase, the volume to be expanded and contracted for the
particle can be reduced which can prevent the pulverization of the
particle into fine powder such as occurs with a negative active
material with a single Si phase. Consequently, the cycle
characteristics are improved.
[0020] Further, the structure can prevent a reduction in the
specific resistance of the negative active material as it comprises
either one or both of X phase and SiX phase.
[0021] Further, where Cu is alloyed with Si, because it has a
specific resistance lower than that of Si, it can reduce the
specific resistance of the negative active material. While Cu can
be used for either of element M or element X, it is important that
elements M and X be different. Accordingly, Cu is not selected for
both element M and element X when practicing the present
invention.
[0022] According to present invention, element M is preferably
selected to have a boiling point higher than that of element X.
[0023] The negative active material for the rechargeable lithium
battery of the present invention is prepared by mechanically
alloying Si particles provided in a powder form and particles of
element M, also in powder form. The resulting SiM alloy is heated
and element X is added as a powder to the heated SiM alloy. The
mixture is alloyed again by a mechanical alloying method to provide
a SiMX alloy, and heated at a temperature less than that of the
first heating step. Element M is selected from the group consisting
of Ni, Co, B, Cr, Cu, Fe, Mn, Ti, Y, and combinations thereof, and
element X is selected from Ag, Cu, Au, and combinations thereof,
provided however, that Cu is not selected for both element M and
element X at the same time.
[0024] As set forth above, the negative active material for the
rechargeable lithium battery is obtained by alternatively repeating
a mechanical alloying step and a heating step. Thereby, the
structure of the obtained negative active material is very closely
aggregated and has a tiny crystal phase. Since the second heating
temperature is less than that of the first heating temperature, the
previously formed SiM phase is not melted during the second heating
process and it is possible to deposit the tiny crystal of Si phase,
SiM phase, X phase and SiX phase. The resulting negative active
material preferably has a crystal structure with a crystal grain
diameter between 100 nm and 500 nm.
[0025] The rechargeable lithium battery of the present invention
comprises the aforementioned negative active material for the
rechargeable lithium battery. Thereby, it is possible to provide a
rechargeable lithium battery with good cycle characteristics.
[0026] According to a preferred method for preparing the negative
active material for the rechargeable lithium battery of the present
invention, the temperature of the first heating step is preferably
between (Tm-100).degree. C. and (Tm-20).degree. C. where Tm is the
melting point of the SiM alloy phase.
[0027] The negative active material for the rechargeable lithium
battery of the present invention is constructed of crystal powder
which consists essentially of Si phase and SiM phase with at least
one of X phase and SiX phase.
[0028] Preferably, each of Si phase, SiM phase, X phase, and SiX
phase is a crystal particle having a diameter of between 100 nm and
500 nm, and the phases are closely aggregated with one another.
[0029] In a battery, the Si phase is alloyed with the lithium upon
charging the battery to form a LiSi.sub.X phase, and the lithium is
released upon discharge to return to Si single phase. Further, the
SiM phase does not react with the lithium upon charge or discharge
and the shape of the powder particle remains which prevents the
particles form expanding and contracting. The element M of the SiM
phase is not alloyed with the lithium and M is preferably an
element selected from the group consisting of Ni, Co, B, Cr, Cu,
Fe, Mn, Ti, Y and combinations thereof. The element M is most
preferably Ni. In such an embodiment, the composition of the SiM
phase is Si.sub.2Ni phase. Element M preferably has a melting point
higher than that of element X.
[0030] Further, the X phase decreases the specific resistance of
the negative active material by providing better conductivity to
the negative active material powder. Element X is preferably a
metal element having a specific resistance of 3.OMEGA..multidot.m
or less and is preferably selected from the group consisting of Ag,
Cu, Au and combinations thereof. Particularly, Cu will not alloy
with the lithium to decrease the irreversible capacity. Thereby, it
is possible to increase the capacity of the charge and
discharge.
[0031] Further, Cu is not alloyed with Si and, at the same time,
has a specific resistance less than that of Si, decreasing the
specific resistance of the negative active material. Therefore, Cu
has features of both element M and element X, but according to the
present invention, Cu is not selected for both element M and
element X at the same time.
[0032] Further, it is possible to deposit X phase or SiX phase
together with X phase. SiX phase decreases the specific resistance
of the negative active material by applying the conductive to the
multi-phase alloy powder as in the X phase.
[0033] The crystal structure of Si phase, SiM phase, X phase, and
SiX phase is preferably a crystal phase. However, it may further
comprise other phases which may be crystal or amorphous.
[0034] Each phase preferably has a crystal grain diameter of
between 100 nm and 500 nm. When the crystal grain has a diameter of
less than 100 nm, the particle becomes weaker by the repeated
pulverization into fine powder and compression, and the interface
is peeled out to be pulverized into fine powder by expanding and
contracting upon the charge and discharge. When the diameter is
more than 500 nm, the expansion rate is increased by charging the
main active material of Si phase, and it is difficult to prevent
the Si phase from expanding due to the SiM phase, the X phase and
the SiX phase.
[0035] The average diameter of the negative active material powder
is preferably between 5 .mu.m and 30 .mu.m. Generally, as a
Si-included alloy particle has a resistance more than that of
graphite powder generally used for the conventional negative
electrode material of a lithium ion battery, it is preferable to
add a conductive agent. However, an average diameter less than 5
.mu.m is undesirable in that the multi-phase alloy particle may
have an average diameter less than that of the conductive agent,
thus it is difficult to achieve the desired effects of the
conductive agent and the battery characteristics such as capacity
and cycle characteristics deteriorate. When the average diameter is
more than 30 .mu.m, it is undesirable because the charge density of
the negative active material decreases for a lithium battery.
[0036] Further, according to the present invention, the particle
shape of the negative active material is mostly estimated as being
amorphous.
[0037] Subsequently, as Si is an element constructing both a Si
single phase and a SiM phase in the alloy composition, it is
preferable that Si is added in amounts higher than the
stoichiometric concentration of element M. When the amount of Si is
less than the stoichiometric concentration of the element M, it is
undesirable in that the SiM phase and the M phase are deposited due
to a lack of Si, but the Si phase contributing to the charge and
discharge is not deposited so that the charge and discharge is not
carried out. When Si is excessively added, it is undesirable in
that the Si phase is overly deposited to increase the total amount
of expansion and contraction of the negative active material upon
repeating the charge and discharge, the negative active material is
easily pulverized into fine powder to deteriorate the cycle
characteristics. Preferably, the negative active material has a
composition ratio of Si between 30% by weight and 70% by
weight.
[0038] As element M is an element forming a SiM phase together with
Si, it is preferable to add it in amount less than that of the
stoichiometric concentration of Si. When the amount of element M is
more than the stoichiometric concentration of Si, it is undesirable
in that Si is relatively unable to deposit the SiM phase and M
phase so that the Si phase contributing to the charge and discharge
is not deposited. Thereby, the charge and discharge is not
generated. Further, when too little M is used it is undesirable
because the Si phase is overly deposited to increase the total
expansion volume of the negative active material upon the charge
and discharge, and the negative active material is pulverized into
fine powder to deteriorate the cycle characteristics. Preferably,
the composition of the element M in the negative active material is
between 20% by weight and 69% by weight. The element M is not
alloyed with the lithium so that it does not have the irreversible
capacity.
[0039] When the composition ratio of element X is increased, the
specific resistance is decreased, but the Si phase is relatively
decreased, thus deteriorating the charge and discharge capacity. On
the other hand, when the composition ratio of element X is
decreased, the specific resistance of the negative active material
is increased, deteriorating the charge and discharge effectiveness.
For this reason, the composition ratio of the element X is
preferable between 1% by weight and 30% by weight in the negative
active material.
[0040] According to the present invention, the negative active
material for a rechargeable lithium battery has a crystal grain of
Si phase and SiM phase having a very small diameter of 500 nm or
less and each grain is closely aggregated. Thereby, the structure
is rarely destroyed or pulverized even with the expansion and
contraction caused by the charge and discharge of the lithium, so
that the cycle characteristic are improved.
[0041] As it further comprises SiM phase in addition to Si phase in
the structure, the volume of expanding and contracting the
particles may decrease compared to Si single phase. This prevents
the particle from pulverizing into fine powder so that the cycle
characteristics are improved.
[0042] As it comprises at least one of the X phase and SiX phase,
the specific resistance of the negative active material
decreases.
[0043] Hereinafter, a rechargeable lithium battery comprising the
negative active material is described. The rechargeable lithium
battery comprises at least a negative electrode comprising the
negative active material, a positive electrode, and an
electrolyte.
[0044] The negative electrode for the rechargeable lithium battery
may be, for example, a sheet-shaped electrode formed by solidifying
the alloy powder of the negative active material with a binder.
Further examples include a pellet solidified as a disc shape, a
cylinder shape, a plan shape or a conical shape.
[0045] The binder may be either an organic or an inorganic material
capable of being dispersed or dissolved in a solvent with a
negative active material alloy powder. The alloy particles are
bound by removing the solvent. Furthermore, the binder may be a
material capable of being dissolved with the alloy powder and
binding the alloy powder by a solidification process such as a
press shaping process. Examples of binders include resins such as
vinyl based resins, cellulose based resins, phenol resins, and
thermoplastic resins. More specific examples include polyvinylidene
fluoride, polyvinyl alcohol, carboxymethyl cellulose, styrene
butadiene rubber, and similar materials.
[0046] In addition to the negative active material and the binding
gent, the negative electrode may be prepared by further adding
carbon black, graphite powder, carbon fiber, metal powder, metal
fiber, or some other material as a conductive agent.
[0047] Subsequently, the positive electrode comprises, for example,
a positive active material capable of intercalating and
deintercalating the lithium such as LiMn.sub.2O.sub.4, LiCoO.sub.2,
LiNiO.sub.2, LiFeO.sub.2, V.sub.2O.sub.5, TiS, MoS, organosulfide
compounds, polysulfide compounds and a Ni, Mn, or Co based
composite oxide. The positive electrode may further include a
binder such as polyvinylidene fluoride and a conductive agent such
as carbon black in addition to the positive active material.
[0048] Specific examples for the positive electrode and the
negative electrode may be exemplified as a sheet-shaped electrode
prepared by coating the conductor of a metal foil or a metal mesh
on the positive electrode or the negative electrode.
[0049] The electrolyte may include an organic electrolyte with
which the lithium is dissolved in an aprotonic solvent.
[0050] Aprotonic solvents include, but are not limited to,
propylene carbonate, ethylene carbonate, butylene carbonate,
benzonitrile, acetonitrile, tetrahydrofurane, 2-methyl
tetrahydrofurane, .gamma.-butyrolactone, dioxolane, 4-methyl
dioxolane, N,N-dimethyl formamide, dimethyl acetoamide, dimethyl
sulfoxide, dioxane, 1,2-dimethoxy ethane, sulfolane,
dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate,
methylethyl carbonate, diethyl carbonate, methylpropyl carbonate,
methyl isopropyl carbonate, ethylbutyl carbonate, dipropyl
carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene
glycol, dimethyl ether or similar solvents or mixtures of such
solvents with other solvents such as propylene carbonate (PC),
ethylene carbonate (EC), butylene carbonate (BC), dimethyl
carbonate, methylethyl carbonate (MEC), or diethyl carbonate
(DEC).
[0051] The lithium salt may include, but is not limited to,
LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiCIO.sub.4,
LiCF.sub.3SO.sub.3, Li(CF.sub.3SO.sub.2).sub.2N,
LiC.sub.4F.sub.9SO.sub.3, LiSbF.sub.6, LiAIO.sub.4, LiAICl.sub.4,
LiN(CxF.sub.2x+1SO.sub.2)(CyF.sub.2y+1SO.sub.2- ) (where x and y
are natural number), LiCl, LiI, or mixtures thereof, and preferably
is any one of LiPF.sub.6, LiBF.sub.4, LiN(CF.sub.3SO.sub.2).su-
b.2, and LiN(C.sub.2F.sub.5SO.sub.2).
[0052] The electrolyte may further include a polymer such as PEO,
PVA or similar polymers with any one of the lithium salts, and
polymer electrolyte incorporated with the polymer in the organic
electrolyte.
[0053] Further, in addition to the positive electrode, the negative
electrode, and the electrolyte, the rechargeable lithium battery
may further comprise, if required, any other material such as a
separator interposing the positive electrode and the positive
electrode.
[0054] As the rechargeable lithium battery comprises a negative
active material having a crystal grain such as a Si phase and a SiM
phase having a very small diameter of 500 nm or less, or with the
phases closely aggregated with each other, it is rarely possible to
destroy the structure even though the expansion and contraction are
repeated upon charging and discharging the lithium. Thereby, the
cycle characteristics of the battery are improved.
[0055] Hereinafter, a method of preparing a negative active
material for a rechargeable lithium battery is described. According
to the present invention, the method comprises the steps of: first
alloying an element Si and an element M by a mechanical alloying
process to provide a SiM phase alloy; first heating the SiM alloy;
adding a powder of element X to the heated SiM alloy; second
alloying the same by a mechanical alloying process to provide a
SiMX alloy; and second heating the SiMX alloy.
[0056] First, a Si powder and an element M powder are mixed and
alloyed by a mechanical alloying process at the first alloying
step. Si powder may include any one having an average diameter of
between 1 and 10 .mu.m, while the element M powder may include any
one having an average diameter of between 0.5 and 10 .mu.m. The Si
powder and the element M are introduced into a ball mill and an
attritor and alloyed by the mechanical alloy in which the
pulverization into fine powder and the compression are repeated.
Thereby, a SiM alloy is obtained. The mechanical alloying process
is preferably continued until the SiM alloy becomes amorphous.
[0057] During the first heating step, the SiM alloy is heated so
that the amorphous state is transferred into the crystalline state.
The heating temperature T.sub.1 is preferably between
(Tm-100).degree. C. and (Tm-20).degree. C. where Tm is the melting
point of the SiM alloy phase. When the heating temperature T.sub.1
is less than (Tm-100).degree. C., the SiM alloy is insufficiently
crystallized, while when the heating temperature T.sub.1 is more
than (Tm-20).degree. C., the alloy crystal structure is too large.
Furthermore, the heating time is preferable between 1 and 4 hours.
The heating step is preferably carried out under an inert gas
atmosphere of nitrogen, argon or a similar gas. Upon heating the
SiM alloy, the Si phase and the SiM phase are developed with the
resulting structure having a crystal grain diameter of between 100
and 500 nm.
[0058] Then, during the second alloying step, the mixture of the
SiM alloy and the element X powder is alloyed by the mechanical
alloy process. The element X powder has an average diameter of
between 0.5 and 10 .mu.m. The SiM alloy and the element X are
introduced into, for example, a ball mill or an attritor, and are
alloyed by a mechanical alloying process in that the pulverization
into fine powder and the compression are repeated. Thereby, a SiMX
alloy is obtained. The mechanical alloying process is preferably
continued until the SiMX alloy becomes amorphous.
[0059] During the second heating step, the SiMX alloy is heated to
transfer the amorphous state into the crystalline state. The
temperature T.sub.2 in the second heating step is lower than the
temperature T.sub.1 of the first heating step, and the second
heating process is preferably carried out between (Tx-200).degree.
C. and (Tx-20).degree. C. where T.sub.x is the melting point of the
metal X. If the second heating temperature T.sub.2 is higher than
the first heating temperature T.sub.1, the crystal grain of the SiM
phase will dissolve and upon re-crystallization will tend to swell.
When the second heating temperature T.sub.2 is higher than
(Tx-200).degree. C., the SiX alloy is insufficiently crystallized.
If the second heating temperature T.sub.2 is lower than
(Tx-20).degree. C., the X phase is re-crystallized so that the
desired tiny crystal grain is not obtained. The duration of the
heating step is preferable between 2 and 5 hours. The heating step
is preferably carried out under an inert gas atmosphere of
nitrogen, argon or a similar gas. By heating the SiMX alloy, it can
comprise at least one of Si phase and SiMX phase and the structure
can have a crystal grain with a diameter of between 100 and 500
nm.
[0060] While the element M preferably has a higher melting point
than that of element X to prevent the SiM phase from melting during
the second heating step.
[0061] According to the method of preparing the negative active
material for the rechargeable lithium battery, the mechanical
alloying process and the heating process are alternatively
repeated, so that the structure of the negative active material
becomes very dense with a tiny crystal phase.
[0062] Further, during the second heating step, as the second
heating temperature is lower than the first heating temperature,
the previously formed SiM phase is not melted during the second
heating step. This permits the formation of the desired tiny
crystals of Si phase, SiM phase, X phase, and SiX phase.
[0063] The following examples further illustrate the present
invention in detail but are not to be construed to limit the scope
thereof.
EXPERIMENTAL EXAMPLE 1
[0064] 23 g Si powder and 7 g Ni powder as element M were mixed and
introduced into a stainless vessel with 300 g stainless balls
having a diameter of 10 mm. Subsequently, the stainless vessel was
mounted with a mechanical alloying device, a rocking mill
manufactured by Sewa Mechanical Research Company. The mixture was
subjected to the mechanical alloying treatment for 20 hours at a
frequency of 700 rpm. Then, the obtained powder was put into 20 mm
mold, pressed at a pressure of 4 t to provide a pellet, and heated
at 970.degree. C. for 5 hours. Thereby, a SiNi alloy was
obtained.
[0065] After heating, 5 g Ag powder as element X was mixed with 25
g of the SiNi alloy, and put into a stainless vessel with 300 g
stainless balls having diameter of 10 mm. The stainless vessel was
mounted with the mechanical alloy device and subjected to a
mechanical alloy treatment for 20 hours at a frequency of 700 rpm.
Then, the resulting powder was injected into a mold having a
diameter of 20 mm and pressed at a pressure of 4 t to obtain a
pellet, and heated at 940.degree. C. for 5 hours. After heating,
the pellet was pulverized into a fine powder in a mill, and
separated by a sieve, to provide a SiNiAg alloy powder having a
diameter of between 1 and 45 .mu.m and an average diameter of 15
.mu.m. The composition weight ratio of the obtained alloy was
Si:Ni:Ag=64:19:17.
EXPERIMENTAL EXAMPLE 2
[0066] 64 parts by weight of Si powder were mixed with 19 parts by
weight of Ni powder as element M and 17 parts by weight of Ag
powder as element X and put into a stainless vessel with 300 g
stainless balls having diameter of 10 mm. The stainless device was
mounted with the mechanical alloy device and subjected to a
mechanical alloy treatment for 20 hours at a frequency of 700 rpm.
Then, the resulting powder was injected into a mold with a diameter
of 20 mm and pressed at a pressure of 4t to obtain a pellet, and
heated at 940.degree. C. for 5 hours. After heating, the pellet was
pulverized into a fine powder in a mill, and separated by a sieve,
to provide a SiNiAg alloy powder having a diameter of between 1 and
45 .mu.m and an average diameter of 15 .mu.m.
EXPERIMENTAL EXAMPLE 3
[0067] 64 parts by weight of Si powder were mixed with 19 parts by
weight of Ni powder as element M and 17 parts by weight of Ag
powder as element X and put into a stainless vessel with 300 g
stainless balls having diameter of 10 mm. The stainless device was
mounted with the mechanical alloy device and subjected to a
mechanical alloy treatment for 20 hours at a frequency of 700 rpm.
Then, the resulting powder was pulverized into a fine powder and
separated by a sieve, to provide a SiNiAg alloy powder having a
diameter of between 1 and 45 .mu.m and an average diameter of 15
.mu.m.
EXPERIMENTAL EXAMPLE 4
[0068] 64 parts by weight of Si powder were mixed with 19 parts by
weight of Ni powder as element M and 17 parts by weight of Ag
powder as element X and dissolved by high frequency heating under
an Ar atmosphere to provide a molten alloy. The resulting mixed
molten metal was quenched by a gas atomizing process to provide a
SiNiAg alloy powder. The resulting quenched alloy powder was heated
at 940.degree. C. for 5 hours. After heating, the alloy powder was
pulverized into a fine powder and separated by a sieve, to provide
a SiNiAg alloy powder having a diameter of between 1 and 45 .mu.m
and an average diameter of 15 .mu.m.
[0069] The alloy powders obtained from Experimental Examples 1 to 4
were measured by scanning electronic microscope (SEM) for their
surfaces. The SEM photograph of the alloy powder of Experimental
Example 1 is shown in FIG. 1; The SEM photograph of the alloy
powder of Experimental Example 2 is shown in FIG. 2; the SEM
photograph of the alloy powder of Experimental Example 3 is shown
in FIG. 3; and the SEM photograph of the alloy powder of
Experimental Example 4 is shown in FIG. 4.
[0070] As shown in FIG. 1, the structure of alloy powder of
Experimental Example 1 has a very tiny crystal grain and the
crystal grain is closely aggregated. Further, comparing that of
Experimental Example 2, it is found that fewer cracks are generated
and the surface of crystal grain is smoother. The diameter of the
crystal grain determined from a SEM photograph is between 100 nm
and 300 nm. Further, according to Experimental Examples 3 and 4,
the crystal grain is large in the structure and the crystal grain
is broken. In the Experimental Example 4, the surface of the
crystal grain is smooth but the particle size of the crystal grain
is bigger than that of Experimental Example 1.
[0071] As described above, it has been found that the alloy powder
of Experimental Example 1 has a fine crystal grain, and the crystal
grains are closely aggregated.
[0072] Furthermore, the material treated only by a mechanical
alloying step, the material treated by a mechanical alloying step
and a 970.degree. C. heating step, and the material treated by a
mechanical alloying step, heating step, a Ag adding step and a
heating step at 940.degree. C. in Experimental Example 1, and the
material treated by a mechanical alloying step and a heating step
at 940.degree. C. in Experimental Example 2 are measured for X-ray
diffraction pattern and the results are shown in FIG. 5.
[0073] As shown in FIG. 5, the material treated with only a
mechanical alloying process has a very small and broad diffraction
peak, which is anticipated as being amorphous. It is crystallized
by heating the material. As shown in the photograph, it is
confirmed that each structure inside the alloys is very small as
being less than 300 nm and crystalline. The size of the crystal
grain is tiny and the surface of the crystal grain is very
smooth.
BATTERY EXAMPLES
[0074] By using negative active materials of Experimental Examples
1 to 4, rechargeable lithium batteries were prepared. 70 parts by
weight of each of the negative active materials according to
Experimental Examples 1 to 4, 20 parts by weight of graphite powder
of conductive agent having an average diameter 3 .mu.m, and 10
parts by weight of polyvinylidene fluoride were mixed, and added
with N-methyl pyrrolidone under agitation to provide a slurry.
Then, the slurry was coated on a copper foil having a thickness of
14 .mu.m and the coated copper foil was dried and compressed to
provide a negative electrode having a thickness of 40 .mu.m. The
obtained negative electrode was cut in a circle shape having a
diameter of 13 mm. Between the negative electrode and lithium metal
as a counter electrode, a porous polypropylene separator was
inserted. LiPF.sub.6 was dissolved in a mixed solvent of EC, DME,
and DEC (EC:DME:DEC=3:3:1, in volume ratio) at a concentration of 1
mole/L to an electrolyte. The electrolyte was injected thereto to
provide a coin type rechargeable lithium cell.
[0075] The resulting lithium cell was repeatedly charged and
discharged at voltages of between 0V and 1.5V and at 0.2 C current
density for 20 cycles. The relationship between the number of
cycles and the discharge capacity at each cycle is shown in FIG.
6.
[0076] As shown in FIG. 6, in the rechargeable lithium battery of
Experimental Example 1, it was confirmed that the early stage
discharge capacity was nearly same as the discharge capacity after
20 cycles, and the discharge capacity was uniformly maintained. It
was determined that the early stage discharge capacity of the
battery of Experimental Example 2 was nearly same as that of
Experimental Example 1, while the discharge capacity after 20
cycles had decreased more than for Experimental Example 1. For
Experimental Example 2, as the mechanical alloying and heating
processes were carried out only once, the crystal grains were not
closely aggregated and the alloy powder was apparently broken upon
repeating the charging cycle, thereby decreasing the discharge
capacity.
[0077] Further, in Experimental Examples 3 and 4, it was found that
the discharge capacity was remarkably decreased upon repeating the
cycle. In Experimental Example 3, as only a mechanical alloying
process was carried out, the crystal grain was not closely
aggregated, the alloy powder was broken upon repeating the charging
cycle, decreasing the discharge capacity. Furthermore, for
Experimental Example 4 where there is no mechanical alloying
process, while the initial charge capacity is high, it deteriorates
rapidly after several cycles, for example, when compared to
Experimental Example 1, presumably because the crystal size is much
larger.
[0078] As described in above, the negative active material for the
rechargeable lithium battery of the present invention had a very
small particle diameter of crystal such as Si phase, SiM phase and
so on, and each phases were closely alternatively linked. Thereby,
the structure was rarely broken upon the charge and discharge and
the cycle characteristics were improved.
[0079] Further, according to the method of preparing the negative
active material for the rechargeable lithium battery, the
mechanical alloy and the heating processes were alternatively
repeated. Thereby, the structure was so dense to provide a negative
active material having a tiny crystalline state.
[0080] The present invention has been described in detail with
reference to certain preferred embodiments. It will be apparent to
those skilled in the art that various modifications and variation
can be made in the present invention without departing from the
spirit or scope of the invention. Thus, it is intended that the
present invention cover the modifications and variations of this
invention provided they come within the scope of the appended
claims and their equivalents.
* * * * *