Process for preparing alloy composite negative electrode material for lithium ion
batteries.
The present invention relates to a process for preparing an alloy composite
negative electrode material having a spherical carbon matrix structure for lithium ion
batteries by spray-drying carbothermal reduction.
With the rapid development of electronics and information industry, a large
number of portable electronic products such as mobile communication devices,
notebook computers, digital products, etc. have been widely used, which create
higher demands to batteries, especially rechargeable secondary batteries, by the
public, such as: a higher capacity, a smaller size, a lighter weight, and a longer
service life. Lithium ion batteries have been a hotspot for research by many people
for their advantages of high energy density, high operation voltage, good loading
property, rapid charging speed, safety without pollution, and without effects on
memory, etc.
The alloy negative electrode materials for lithium ion batteries mainly include
materials such as Sn-based, Sb-based, Si-based, Al-based carbon bearing materials,
etc. Such alloy negative electrode materials have the advantages of large specific
capacity, high lithium intercalation potential, low sensitivity to electrolytes, good
conductivity, etc., but the alloy negative electrode material will expand in volume
during charging and discharging, which results in the pulverization of the active
material, the loss of electric contact, and the deterioration of the battery
performance.
The alloy composite negative electrode material of a spherical structure
composed of metal or metal alloy particles that are homogeneously distributed in a
carbon matrix can relieve the volume expansion of the alloy, avoid the agglomeration
of the nano-alloy and direct contact with the electrolyte, and has good
electrochemical performances. This structure is further referred to as a metal or
metal alloy-encapsulated carbon microsphere.
Currently, there are many processes for preparing alloy composite negative
electrode materials of a such structure, such as the surface coating method, the
layer-by-layer deposition method, the template method, and the reverse
microemulsion method. The reverse microemulsion method is the major method used,
and is presented in e.g. "Preparation of Cu6Sn5-Encapsulated Carbon Microsphere
Anode Material for Li-ion Battereis by Carbothermal Reduction of Oxides' by
Wang, Ke et al., Journal of the Electrochemical Society (2006), 153(10),
A1859-A1862. In this method a surfactant is dispersed in a water phase or an oil phase
to form micelles; then a metal oxide is added therein and fully dispersed by stirring
and ultrasonic vibrating etc.; then a polymerizable organic substance is added
therein, so as to form a precursor substance of a carbon matrix structure; and finally
it is thermally treated in a protective atmosphere, and the organic substance is
carbonized to produce the material of a spherical metal bearing carbon matrix
structure. The reverse microemulsion method can be used to prepare composite
material of such a structure where the metal or metal alloy particles are
homogeneously dispersed, and which has an integral morphology, where the thickness
of the carbon layer can be controlled by varying the mass ratio of the reactants.
However, this process has a low yield, and is difficult to achieve a scale production,
and it is quite difficult to recover the surfactant after the completion of the reaction,
and it easily results in pollution and wastes.
The above mentioned problem is solved by providing for an improved process
for preparing the above described alloy composite negative electrode material by
carbothermal reduction. The invention covers a process for preparing a negative
electrode material for a lithium ion battery with a general formula A-M/Carbon,
wherein A is a metal selected from the group consisting of Si, Sn, Sb, Ge and Al; and
wherein M is different from A and M is at least one element selected from the group
consisting of B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Ca, Mg, V, Ti, In, At, Ge; and
comprising the steps of:
- providing a solution comprising an organic polymer and either chemically reducible
nanometric A- and M-precursor compounds, or nanometric Si and a chemically
reducible M-precursor compound, when said metal A is Si;
- spray-drying said solution whereby a A- and M-precursor bearing polymer powder is
obtained, and
- calcining said powder in a neutral atmosphere at a temperature between 500 and
1000°C for 3 to 10 hours whereby, in this carbothermal reduction, a carbon matrix is
obtained bearing homogeneously distributed A-M alloy particles.
Preferably, the A- and M-precursor compounds are either one of an oxide,
hydroxide, carbonate, oxalate, nitrate or acetate. More preferably, the A- and
M-precursor compounds are A-oxide and M-oxide powders have a particle size
between 20 and 80 nm. In the solution, instead of an A-oxide, nanometric metallic Si
powder can also be used, and Si-M alloys are formed in the final product.
In a preferred embodiment, in the organic polymer solution, the weight ratio of
A and M, present in the A- and M-precursor compound, to the carbon in the organic
polymer is selected so as to provide for between 20 to 80 wt%, and preferably 30 to
60 wtX residual carbon in the carbon matrix. The amount of carbon consumed in the
carbothermal reduction reaction can be calculated according to the chemical
equation:
a A-oxide + m M-oxide + c C => AaMm + c CO, for example:
4 SnO2 + Sb2O3 + 11 C => 2 Sn2Sb + 11 CO.
As there is provided an excess carbon through the organic polymer the
carbothermal reduction is responsible for fully reducing the metal oxides, and
embedding them in the excess carbon provided by the carbonization of the high
molecular polymer. The knowledge of the carbothermal reduction reaction scheme,
the carbon content of the polymer and the carbon content in the final product's
metal alloy embedding structure determines the amount of polymer to be mixed
initially with the metal oxides. In order to establish the yield of carbon from a given
polymer TG/DSC tests are performed. For example: phenol formaldehyde is fully
carbonized to hard carbon at 1000°C under an argon atmosphere, yielding a residual
hard carbon content of 36.01 wt%.
In a preferred embodiment also, the organic polymer is a water- or
alcohol-soluble phenolic resin.
It is also preferred that the step of spray-drying is carried out with an airflow
spray dryer by way of cocurrent drying. The solution is preferably evaporated at a
temperature above 260°C whereby a gas flow is generated, whereafter the solution is
atomized by the said gas flow at a pressure of 0.3-0.5 MPa. Inside the airflow spray
dryer, the gas flow moves from an inlet to an outlet, whereby the temperature at the
air inlet is preferably set at between 260 and 300°C, and the temperature at the
outlet between 100 and 130°C.
Spray drying is an effective way for preparing composite anode materials. It is a
low cost process which is easy to control, and is fit for mass production. In spray
drying, the liquid drops of polymer are dispersed by the high-pressure air stream and
solidificated at thigh temperature. The nano metaloxide particles (or other metal
precursor compounds) are uniformly dispersed in the polymer solution. The particles
produced by spray drying can be calcined directly. That is not the case for the reverse
microemulsion method described before, where the emulsion products have to be
washed and dried before calcination.
Spray drying is also an efficient method to control the particle size distribution
of the polymer - metal precursor compound, by managing the feed rate and viscosity
of the metal precursor bearing polymer solution and the air pressure. As the high
molecular polymer chains are interlinking during the solidification of the solution,
this provides for porous products in the form of carbon aerogels acquired after
carbonization. As part of the carbon is also consumed to reduce the metal precusor
compounds to pure metal, the volume of the reduced alloys is smaller than that of
the metal oxides. The porosity of the obtained particles can alleviate the expansion
and contraction of alloy during charge and discharge of the electrode. It is also
advisable to use some pore-forming agents mixed with the raw materials.
By the process of the invention, a composite precursor powder of a negative
electrode material for a lithium ion battery, with a general formula A-M/C is
prepared by spray-drying. The precursor preferably consists of a homogeneously
dispersed nanometric A-oxide or M-oxide powder embedded in an organic polymer,
wherein A is a metal selected from the group consisting of Si, Sn, Sb, Ge and Al; and M
is at least one element selected from the group consisting of B, Nb, Cr, Cu, Zr, Ag, Ni,
Zn, Fe, Co, Mn, Sb, Zn, Ca, Mg, V, Ti, In, Al, Ge; and wherein A and M are different
and are both present in said composite powder.
The alloy system used in the process for preparing the alloy composite negative
electrode material for lithium ion batteries comprises:
a) Sn-M-C alloy (M = B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Ca, Mg, V, Ti, In, Al,
Ge);
b) Sb-M-C alloy (M = B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Ca, Mg, V, Ti, In, Al, Ge);
c) Si-M-C alloy (M = B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Ca, Mg, V, Ti, In, Al,
Ge);
d) Ge-M-C alloy (M = B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Ca, Mg, V, Ti, In, Al);
and
e) Al-M-C alloy (M = B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Ca, Mg, V, Ti, In, Ge).
In a best mode embodiment, the preparation process thereof comprises the
steps of:
(1) Preparing raw materials: a nano-oxide required for preparing the alloy
composite material and an organic high molecular polymer are weighed out in a
stoichiometric ratio. For preparing the Si-M-C alloy, the nano-oxide is replaced by
nanometric Si powder.
(2) Formulating a solution: the above organic high molecular polymer is added
into a solvent to dissolve therein, and they are formulated a uniform solution of
10-20%; and then the nano-oxide is added therein, and stirred thoroughly.
(3) Spray-drying: the formulated solution is spray-dried to obtain mixed
powder, wherein the drying is carried out with an airflow spray dryer by way of
cocurrent drying; a two-fluid spray nozzle is used as an atomization device; a
peristaltic pump is used for feeding the solution as a feedstock at a speed of 10-20
ml/min; the gas flow at the spray nozzle is controlled by the pressure of compressed
air with to atomize at about 0.4 MPa; the temperature at the air inlet is controlled at
260-300°C, and the temperature at the outlet at 100-130°C.
(4) Carbothermal reduction: the mixed powder is calcined in a nitrogen or
argon atmosphere at 500-1000°C for 3-10 hours to obtain the alloy composite
negative electrode material having a spherical encapsulating structure (as described
before) for lithium ion batteries which has an integral morphology and a uniform
distribution.
The raw materials used in this technique are mainly in two categories of A+P, in
which A can be various oxides, such as one or a mixture of several of B2O3, SnO2,
Co3O4, Sb2O3, AgO, Cu2O, MgO, CuO, ZrO2, NiO, ZnO, Fe2O3, MnO2, CaO, V2O5,
Nb2O5,TiO2, Al2O3, Cr2O3, InO, and GeO2, and P is an organic high molecular polymer,
such as one of a water-soluble phenolic resin, an alcohol-soluble phenolic resin, a
urea-formaldehyde resin, a furfural resin, an epoxy resin, polyacrylonitrile,
polystyrene, polychlorovinyl, polyvinylidene chloride, polyvinyl alcohol, and
polyfurfuryl alcohol.
The solvent used for dissolving the above organic high molecular polymer is one
of water, ethanol, acetone, toluene, xylene, tetrahydrofuran,
N,N-dimethylformamide, N-methylpyrrolidone and chloroform.
The alloy composite negative electrode material for lithium ion batteries
prepared by using this technique has excellent electrochemical performances, the
technique has low costs and is a simple process, and it can be directly used for
large-scale industrialized production of the alloy composite negative electrode
materials for lithium ion batteries.
Fig. 1 is a SEM graph of Cu6Sn5/C composite material synthesized in the
present invention.
Fig. 2 is a XRD pattern of Cu5Sn5/C composite material synthesized in the
present invention.
Fig. 3 is the first charging and discharging curve of Cu6Sn5/C composite
material synthesized in the present invention.
Fig. 4 is a cycle performance curve of Cu6Sn5/C composite material
synthesized in the present invention for the first 50 cycles.
Fig. 5 is a cycle performance curve of pure hard carbon obtained from
decomposing phenolic resin.
Fig. 6 is the particle size distribution of Sn2Sb/C composite material
Fig. 7 is a performance curve of Sn2Sb/C composite material synthesized in
the present invention for the 1st, 10th and 20th cycle.
Fig. 8 is a cycle performance curve of Sn2Sb/C composite material
synthesized in the present invention for the first 20 cycles (capacity and
capacity retention).
The technical solution of the present invention will be further illustrated
hereinbelow in conjunction with the embodiments:
Example 1:
First, CuO and SnO2 nano-oxides are weighed out in a molar ratio of 6:5 of
Cu:Sn; then a water-soluble phenolic resin solution of 60% is weighed out and taken in
a formulation ratio of the resin: (CuO+SnO2) = 5:3 by weight; and deionized water is
added therein to formulate a solution of 15wt%. The obtained solution is dried with
an airflow spray dryer, and the feedstock solution is charged with a peristaltic pump
at a speed of 15 ml/min; the gas flow at the spray nozzle is controlled by the pressure
of compressed air to atomize at about 0.4 MPa; the temperature at the air inlet is
controlled at 300°C, and the temperature at the outlet at 130°C; and the air at the
outlet is released after first-order vortex separation. The phenolic resin embedding
the metal oxides obtained by spray drying is calcined under the protection of high
purity nitrogen at 1000°C for 5 hours, and the Cu6Sn5/C composite negative
electrode material having a spherical morphology is obtained. A SEM graph is
given in Fig. 1; an XRD pattern of the Cu6Sn5/C composite material in Fig. 2.
The final carbon content was set at 30 wt%. The amount of carbon consumed in
carbothermal reduction reaction can be calculated according to the following
equation:

The excess phenolic formaldehyde resin is added to produce the excess carbon for
compositing with Cu6Sn5 alloy. As for the sample of Cu6Sn5/C, the synthesis with total
mass balance is as follows:

The raw materials of 7.53g SnO2 and 4.8g CuO are reduced to form 10.32g Cu6Sns.
1.92 g carbon is consumed to reduce SnO2 and CuO. The final product contains 30%
carbon (4.42g carbon). The total mass of carbon is 6.34g. The total phenol
formaldehyde resin mass is 17.61 g, which is calculated by the following formula:
6.34 / 36.01% = 17.61, where, as said above, 36.01% is the residual carbon ratio of
phenolic formaldehyde resin when heated in 1000°C under inert atmosphere.
The final Cu6Sn5/C composite material is measured - see Fig. 4 (capacity
in mAh/g versus cycle number) - as having a first charging specific capacity of
370 mAh/g at room temperature with a lithium foil as a counter electrode,
and the rate of the capacity maintenance is 92% after 50 cycles of charging
and discharging.
The contribution of the metal alloy is shown by comparing the specific
capacity of Sn-Cu/C with that of pure hard carbon obtained by heating
phenolic resin to 1000°C under inert atmosphere: see Figure 5 (showing
capacity in mAh/g versus cycle number).
Example 2:
First, Co3O4 and SnO2 nano-oxides are weighed out in a molar ratio of 1:2
of Co:Sn; then a water-soluble phenolic resin solution of 60% is weighed out and
taken in a formulation ratio of the resin: (Co3O4+SnO2) = 5:3 by weight; and
deionized water is added therein to formulate a solution of 15 wt%. The obtained
solution is dried with an airflow spray dryer, and the feedstock solution is charged
with a peristaltic pump at a speed of 15 ml/min; the gas flow at the spray nozzle is
controlled by the pressure of compressed air to atomize at about 0.4 MPa; the
temperature at the air inlet is controlled at 300°C, and the temperature at the outlet
at 120°C; and the air at the outlet is released after first order vortex separation. The
phenolic resin bearing tin dioxide and tricobalt tetraoxide bead powder, as obtained
by spray drying, is calcined under the protection of high purity nitrogen at 900*C for
10 hours, and the CoSn2/C composite negative electrode material of a spherical
carbon matrix structure is finally obtained. The CoSn2/C composite material is
measured as having a first charging specific capacity of 440 mAh/g at room
temperature with a lithium foil as a counter electrode, and the rate of the
capacity maintenance was 90.8% after 20 cycles of charging and discharging.
Example 3:
First, Sb2O3 and SnO2 nano-oxides are weighed out in a molar ratio of 1:1
of Sb:Sn; then an alcohol-soluble phenolic resin powder is weighed out and taken in a
formulation ratio of the resin: (Sb2O3+SnO2) = 5:1 by weight; and ethanol is added
therein to formulate a solution of 20wt%. The obtained solution is dried with an
airflow spray dryer, and the feedstock solution is charged with a peristaltic pump at a
speed of 10 ml/min; the gas flow at the spray nozzle is controlled by the pressure of
compressed air, to atomize about 0.4 MPa; the temperature at the air inlet is
controlled at 300°C, and the temperature at the outlet at 100°C; and the air at the
outlet is released after first-order vortex separation. The phenolic resin bearing the
tin dioxide and antimony trioxide bead powder obtained by spray drying is calcined
under the protection of high purity nitrogen at 800°C for 10 hours, and the SnSb/C
composite negative electrode material having a spherical carbon matrix
structure is obtained. The SnSb/C composite material is measured as having a
first charging specific capacity of 400 mAh/g at room temperature with a
lithium foil as a counter electrode, and the rate of the capacity maintenance
is 85.1% after 50 cycles of charging and discharging.
Example 4:
First, nano Si powder and CuO nano-oxide are weighed out in a molar
ratio of 1:1 of Si:Cu, then an alcohol-soluble phenolic resin powder is weighted out
and taken in a formulation ratio of the resin: (Si+CuO) = 5:3 by weight, and ethanol is
added therein to formulate a solution of 20wt%. The obtained solution is dried with
an airflow spray dryer, and the feedstock solution is charged with a peristaltic pump
at a speed of 20 ml/min; the gas flow at the spray nozzle is controlled by the pressure
of compressed air, to atomize at about 0.4 MPa; the temperature at the air inlet is
controlled at 300°C, and the temperature at the outlet is controlled at 110°C; and
the air at the outlet is released after the first order vortex separation. The phenolic
resin bearing the nano Si powder and copper oxide bead powder obtained by spray
drying is calcined under the protection of high purity nitrogen at 900°C for 5 hours,
and the Si-Cu/C composite negative electrode material of a spherical carbon
matrix structure is obtained. The Si-Cu/C composite material is measured as
having a first charging specific capacity of 520 mAh/g at room temperature
with a lithium foil as a counter electrode, and the rate of the capacity
maintenance is 94.7% after 20 cycles of charging and discharging.
Example 5:
Similar to Example 3, Sb2O3 and SnO2 nano-oxides are weighed out in a
molar ratio of 1:2 of Sb:Sn. As the final product contains 30 wt% carbon, the
preparation of the raw materials is based on the residual carbon of phenol
formaldehyde resin and the following chemical reaction equation:

The raw materials of 8.39g SnO2 and 4.06g Sb2O3 are reduced to form 10g Sn2Sb.
1.84 g carbon is consumed to reduce SnO2 and Sb203. The final product contains 30%
carbon (4.29g carbon). The total mass of carbon is 6.13g. The total phenol
formaldehyde resin mass is 17.02 g, which is calculated by (6.13/36.01%). Th phenol
formaldehyde resin is carbonized to hard carbon aerogel after calcination at high
temperature. Many pores were produced in the particle, which can alleviate volume
expansion and contraction of electrode. The specific surface area of Sn2Sb/C=3/2 is
given in Table 1. By using the Barrett-Joyner-Halenda (BJH) equation, the pore radius
is calculated to be 19.019 -19.231 A. The pore radius can be enlarged by controlling
the process parameters to improve the cycle performance.

The particle distribution of Sn2Sb/C calcined at 900°C is shown in Fig. 6. The
d0 = 3.76 µm, d25= 6.50 µm, d50 = 7.07 µm, d90 = 7.64 µm.
Fig.7 and Fig.8 show the electrochemical test results of the Sn2Sb/C composite.
The first discharge/charge capacity of Sn2Sb/C composite is 1044 mAh/g and
618 mAh/g, respectively. The first cycle efficiency is 59%. After 20 cycles, the charge
capacity is 411.3 mAh/g and capacity retention is 66.6%. In Fig. 7 the voltage (V) is
shown vs. the capacity in mAh/g during the 1st, 10th and 20th cycle. In Fig. 8 the cycle
number is given below, the capacity to the left, and the capacity retention to the
right. The squares give the charge capacity, the circles the discharge capacity, and
the triangles the efficiency (charge/discharge capacity x 100).
Claims
1. A process for preparing a negative electrode material for a lithium ion battery with
a general formula A-M/Carbon, wherein A is a metal selected from the group
consisting of Si, Sn, Sb, Ge and Al; and wherein M is different from A and M is at least
one element selected from the group consisting of B, Nb, Cr, Cu, Zr, Ag, Ni, Zn, Fe, Co,
Mn, Sb, Ca, Mg, V, Ti, In, Al, Ge; and comprising the steps of:
- providing a solution comprising an organic polymer and either chemically reducible
nanometric A- and M-precursor compounds, or nanometric Si and a chemically
reducible M-precursor compound, when said metal A is Si;
- spray-drying said solution whereby a A- and M-precursor bearing polymer powder is
obtained, and
- calcining said powder in a non-oxidizing atmosphere at a temperature between 500
and 1000oC for 3 to 10 hours whereby a carbon matrix is obtained bearing
homogeneously distributed A-M alloy particles.
2. A process for preparing a negative electrode material according to claim 1,
wherein said chemically reducible A- and M-precursor compounds are either one of an
oxide, hydroxide, carbonate, oxalate, nitrate or acetate.
3. A process for preparing a negative electrode material according to claims 1 or 2,
wherein in the step of providing said solution, the weight ratio of A and M, present in
the A- and M-precursor compounds, to the carbon in the organic polymer is selected
so as to provide for between 20 to 80 wt%, and preferably 30 to 60 wt% residual
carbon in said carbon matrix.
4. A process for preparing a negative electrode material according to any one of
claims 1 to 3, wherein said organic polymer is a water- or alcohol-soluble phenolic
resin.
5. A process for preparing a negative electrode material according to any one of
claims 1 to 4, wherein said A- and M-precursors are oxide powders have a particle size
between 20 and 80 nm.
6. A process for preparing a negative electrode material according to any one of
claims 1 to 5, wherein said step of spray-drying is carried out with an airflow spray
dryer by way of cocurrent drying.
7. A process for preparing a negative electrode material according to claim 6,
wherein said spray-drying is carried out by evaporating said solution at a temperature
above 260°C whereby a gas flow is generated, and atomizing said solution by said gas
flow at a pressure of 0.3-0.5 MPa.
8. A process for preparing a negative electrode material according to claims 6 or 7,
wherein said gas flow moves inside said airflow spray dryer from an inlet to an outlet,
whereby the temperature at the air inlet is between 260 and 300°C, and the
temperature at the outlet between 100 and 130°C.

The present invention relates to a process for preparing an alloy composite negative electrode material having a
spherical carbon matrix structure for lithium ion batteries by spray-drying carbothermal reduction. The invention covers a process
for preparing a negative electrode material for a lithium ion battery with a general formula A-M/Carbon, wherein A is a metal selected
from the group consisting of Si, Sn, Sb, Ge and A1; and wherein M is different from A and is at least one element selected
from the group consisting of B, Cr, Nb, Cu, Zr, Ag, Ni, Zn, Fe, Co, Mn, Sb, Zn, Ca, Mg, V, Ti, In, Al, Ge; and comprising the
steps of: - providing a solution comprising an organic polymer and either chemically reducible nanometric A- and M-precursor
compounds, or nanometric Si and a chemically reducible M-precursor compound, when said metal A is Si; - spray-drying said solution
whereby a A- and M-precursor bearing polymer powder is obtained, and - calcining said powder in a neutral atmosphere at
a temperature between 500 and 1000° C for 3 to 10 hours whereby, in this carbothermal reduction, a carbon matrix is obtained
bearing homogeneously distributed A-M alloy particles.