Si based negative electrode material
Technical field and background
Portable electronic devices are becoming smaller, lighter and sometimes more
energy demanding. This has led to an increase of interest in high-capacity and
compact batteries. Non-aqueous electrolyte lithium-ion batteries are regarded as
one of the most promising technologies for these applications. During lithiation, a
lithium is added to the active material, during delithiation a lithium ion is removed
from the active material. Most of the currently applied anodes in the lithium ion
batteries function by a lithium intercalation and de-intercalation mechanism during
charging and discharging. Examples of such materials are graphite and lithium
titanium oxide (LTO). However these active anode materials lack high gravimetric
and volumetric capacity. The gravimetric capacity of graphite and LTO is 372 mAh/g
(LiC ) and 175 mAh/g ( i'
4Ti'
50 1 ) respectively.
Another class of active materials functions by alloying and de-alloying lithium with a
metal, metal alloy or a composite metal alloy. The term metal can refer to both
metals and metalloids. Several good examples are pure silicon, pure tin or
amorphous CoSn alloy that is commercialized by Sony as Nexelion. Problems with the
application of lithium alloying type of electrodes is mainly related to the continuous
expansion and decrease in volume of the particles or by unwanted phase changes
during cycling. Repeated expansion and contraction of the particle volume can
create contact loss between the particles and current collector, a decomposition of
the electrolyte due to a repeated exposure to a fresh particle surface as the volume
changes, a pulverization or cracking of the particle due to internal stress. Phase
changes during long term cycling also have an influence. After lithiating pure silicon
to the Li 15Si4 phase the cycling is no longer reversible. Also a presence or creation of
a crystalline free tin phase instead of a tin-transition metal alloy phase after delithiation
during long term cycling deteriorates the capacity.
The object of this present invention is to provide a negative electrode material for
non-aqueous electrolyte secondary batteries with a high capacity and long cycling
life.
Summary
Viewed from a first aspect, the invention can provide a negative electrode active
material for a lithium ion battery having the composition formula Si aSnbNicTiyMmCz,
wherein a, b, c, m, y and z represent atomic %values, wherein M is either one or
more of Fe, Cr and Co, and wherein a>0, b>0, z>0, y>0, c > 5, 0
a and c + y > 0.75* b. In one embodiment y>0. In another embodiment the Si content
is defined by 0a. The active material can have
a theoretical volume increase of less than 200% upon charging. In one embodiment
at least 99at% of the negative electrode material consists of Si aSnbNicTiyCz, wherein
a>0, b>0, z>0, y>0, c > 5, z + 0.5* b > a and c + y > 0.75* b. In another embodiment
the negative electrode active material for a lithium ion battery has the composition
formula Si aSnbNicMyCz , wherein a, b, c, y and z represent atomic %values, wherein
M is Ti, and wherein a>0, b>0, z>0, y>0, c > 5, z + 0.5* b > a and c + y > 0.75*b.
Viewed from a second aspect, the invention can provide a process for preparing the
negative electrode active material described above, comprising the steps of:
- providing a mixture of elemental and/or alloyed powders of the elements in the
composition Si aSnbNicTiyMmCz, and
- high energy milling under non-oxidizing conditions of the powder mixture. In one
embodiment the composition is Si aSnbNicMyCz, with M=Ti.
In one embodiment the high energy milling takes place in a protective atmosphere of
a gas comprising either one or more of Ar, N , CO and C0 . In another embodiment
the high energy milling takes place in a protective atmosphere of a gas consisting of
either one or more of Ar, N , CO and C0 . In yet another embodiment the high
energy milling is performed in either a horizontal or a vertical attritor. In still
another embodiment Sn and Ni are provided as either one or more of an atomized
SnNi alloy, preferably an atomized brittle SnNi alloy, and a Ni 3Sn4 compound,
preferably an atomized Ni 3Sn4 compound. In another embodiment Sn, Ti and Ni are
provided as an atomized Ni 3Sn4-Ti alloy. C can be provided as carbon black. The
process described above can further comprise the step of adding graphite or
conductive carbon to the high energy milled mixture.
It is appropriate to mention that in WO2007/1 20347 an electrode composition
Si aSn MyC is disclosed, where M can be Ti, with a+b>2y+z. Expressed in terms of the
composition in the present application (SiaSn Ni TiyC ), this means a + b > 2*(c+v) + z.
In the present application however, since z + 0.5*b > a and c + y > 0.75*b; this
implies that also z + 0.75*b > a + 0.25*b; and since c + y > 0.75*b this implies that
z + c + y > a + 0.25*b; which is the same as z + c + y + 0.75*b > a + b; and hence
a + b < 2*(c+y) + z (again since c + y > 0.75*b). The negative impact of increased
amounts of both Si and Sn in WO2007/1 20347 is discussed below.
In US201 0-0270497 alloys of the type Si aSnbCcAldMe are disclosed, M being for
example Ni, Fe or Cu. However, i t was found in the present application that the
presence of A has a negative influence on the capacity retention of the active
material. Also, there is no disclosure of a Si aSn C Me composition meeting the
requirements that M=Ni, a+b+c+e=1 , and the additional limitations as defined in the
main claim of the present application. In the present application z + 0.5*b > a, or
even z>a, whereas in US201 0-0270497, for every alloy comprising Ni, z0, b>0, z>0, y>0, 0 5, z + 0.5*b > a and c + y > 0.75*b.
Silicon is used in the active material to increase the capacity as it has a gravimetric
capacity of around 3570mAh/g. In one embodiment silicon is present in the alloy
composition in an amount of maximum 45 atomic percent. A high amount of silicon
in the active material may increase the amount of volume expansion that has to be
buffered in the final negative electrode to a level that is not achievable and hence
may lead to capacity loss and premature failure of the batteries.
Silicon is present as very small crystalline or semi-crystalline particles. The reason is
that before a battery can be used in the final application the battery is
"conditioned" in the first charging and discharging steps. During this conditioning
step a very low potential of 0-30mV versus a lithium reference electrode is applied,
rendering the crystalline silicon partially amorphous. A higher crystallinity may
require a different material conditioning step. After the conditioning of the silicon -
during the normal operation - a higher potential is used to introduce a stable cycling.
If the silicon is cycled to low voltages versus a lithium reference electrode during
the operation of the electrode (after conditioning) a Li 1
S 4 phase may be formed
that will no longer be available for a reversible cycling. Depending on the amount
and type of electrolyte or electrolyte additives the normal cycling, after
conditioning, may be limited around 45 mV to 80mV versus a metallic lithium
reference electrode.
Tin is used in the alloy for its high electrochemical capacity and good conductivity.
High levels of tin increase the rate of lithiation and improve the capacity of the
active material but elemental tin formation should be avoided. Larger free
crystalline tin particles may also be created and grown during de-lithiation instead
of the electrochemical more reversible tin-transition metal alloy phase. Therefore it
is provided to create a small and stable reversible tin alloy particle.
The composite anode active materials according to the invention comprise nickel.
Nickel is added as a metallic binder between tin and the metalloid silicon that has a
lower conductivity. Milling or handling of ductile tin is also improved by alloying with
nickel. To improve the milling i t may be convenient to start with brittle
intermetallic compounds like Ni 3Sn4 alloy instead of pure nickel metal. In certain
embodiments, other elements may be added to enhance the cyclability of the alloy
compound. These metals or metalloids may be added in combination with nickel.
When titanium is added i t acts also as a grain refiner.
Conductive carbon is added in the preparation method to act as a lubricant, to boost
conductivity and to avoid loss of interparticle electrical contact and contact with
the collector during cycling of the active material. At high silicon and tin contents
an increased amount of carbon may be added to improve the milling. The BET of
conductive carbon - like the commercially available C-Nergy65 (Timcal) - is more
than 50m2/ g and this contributes to an increase of irreversible capacity. When
conductive carbon is used during the milling the BET decreases significantly in
function of the milling time and parameters. When however natural or synthetic
graphite is used during the milling, the BET increases. During milling silicon carbide
may be formed in small quantities, which can be avoided, as the silicon in silicon
carbide does not alloy with Li and hence reduces the specific capacity of the powder.
The nickel and, if present, titanium are added in a sufficient amount versus the tin
content to form an intermetallic phase that binds all of the tin and optimizes the
cyclability of the tin phase. In one embodiment the sum of the atomic percentages
c + y is larger than 0.75*b. Also, in another embodiment, the total amount of tin
phase and carbon in the milling step is sufficient to accommodate the expansion of
silicon in a conductive matrix of active anode powder; which is obtained when either
condition z + 0.5*b > a or z>a is satisfied.
In an embodiment extra graphite or conductive carbon may be added to the
Si'aSribNicTiyMmCz active material in the preparation of the electrode. The
carbonaceous compounds assist in buffering the material expansion and maintain the
conductive properties of the complete electrode. To prepare the negative electrode
the active material may not only be combined with conductive additives but also
with a suitable binder. This binder enhances the integrity of the complete composite
electrode, including the adhesion to the current collector, and contributes to
buffering the continuous expansion and decrease in volume. In literature a lot of
suitable binders are described. Most of these binders are either n-methyl-pyrrolidone
or water based. Possible binders include but are not limited to polyimides,
polytetrafluoroethylenes, polyethylene oxides, polyacrylates or polyacrylic acids,
celluloses, polyvinyldifluorides.
The electrolyte used in the battery is enabling the functioning of the active material.
For example, a stable solid -electrolyte interphase (SEI) that protects the silicon
surface is created. Electrolyte additives like VC, FEC or other fluorinated carbonates
create a stable and flexible SEI barrier that allows lithium diffusion and avoid the
decomposition of electrolyte. If the SEI layer is not flexible, the continuous
expansion of e.g. the silicon containing particles induces a continuous decomposition
of electrolyte at the silicon surface. The electrolyte can also be in the form of a
solid or gel.
The invention is further illustrated in the following examples:
Counter Example 1
Ni 3Sn4 powder, Si powder, A powder and carbon (C-Nergy65, Timcal) are milled in a
horizontal attritor (Simoloyer® cm01 from ZOZ, Wenden). To prevent oxidation,
milling is done under argon gas atmosphere. The composition and the process
conditions are given in Table 1. The values for the composition parameters a, b, c, y
(where Ti has been replaced by Al in the general formula) and z are given in Table 8.
Table 1: Experimental conditions of Counter Example 1
Comments Qty
Ni 3Sn4 Prepared in the lab 47,29 g
Si Keyvest Si 0-50 miti 12,53 g
Al Merck 808 K3696756 1,40 g
Carbon Black Timcal C-Nergy 65 7,43g
Total powder 68,65 g
Balls 05mm, hardened steel 100Cr6 1373 g
BPR (balls/powder) 20
Filling degree mill 38 vol
Milling time (h) 20h
Rotation speed (rpm) 700 rpm
After milling, the powders are passivated in a controlled air flow to avoid excessive
oxidation. Powder properties are given in Table 3, and the XRD is shown in Figure 1
(all XRD figures show counts per second vs. 2Q). The composite negative electrodes
are prepared using 55 wt of this milled powder, 25 wt Na-CMC binder
(MW < 200k) and 20 wt conductive additive (C-Nergy65, Timcal). A 4wt Na-CMC
binder solution in water is prepared and mixed overnight. The conductive carbon is
added and mixed at high shear with the binder solution. After dissolving the carbon
the active material is added. The paste is rested and coated on a copper foil
(17 miti ) using 120 and 230miti wet thickness. The electrodes are dried overnight.
Round electrodes are punched and dried above 100°C using vacuum. The electrodes
are electrochemically tested versus metallic lithium using coin cells prepared in a
glovebox (dry Ar atmosphere). The electrochemical evaluation of the different alloys
is performed in half coin cells (using metallic lithium as counter electrode).
The first two cycles are performed at a slow speed (using a rate of C/20, meaning a
charge or discharge of 1 Ah/g of active material in 20h), using cut-off voltages of 0V
in lithiation step for the first cycle and 10 mV for the second one and 2V in
delithiation step for both cycles. Cycles 3 and 4 are performed using a C-rate of
C/10 (meaning a charge or discharge of 1 Ah/g of active material in 10h) and cut-off
voltages of 70 mV in lithiation step and 2V in delithiation step. These cut-off
voltages then remain the same for the rest of the test.
Then, the 48 next cycles are performed at a faster speed (using a rate of 1C,
meaning a charge or discharge of 1Ah/g of active material in 1h). The 54th and 55th
cycles are performed at a slower speed again (C/10) in order to evaluate the
remaining capacity of the battery. From then on, periods of fast cycling (at 1C)
during 48 cycles and slow cycling (at C/10) during 2 cycles alternate (48 fast cycles,
2 slow cycles, 48 fast cycles, 2 slow cycles, etc.). This method allows a fast and
reliable electrochemical evaluation of the alloys.
Table 1a gives the details of the cycling sequence.
Table 1a (valid for all Examples)
The electrochemical results for Comp. Ex 1 are shown in Figure 4 (capacity given
against cycle number). On the graph, the points displayed correspond to cycles 2 and
4 and the 2nd cycle of each relaxation period (at C/10), i.e. cycles 2, 4, 55, 106, 157,
208, 259 and 310. It can be seen that for the A -containing material the capacity
slowly deteriorates during cycling.
Example 2 (y=0)
Ni 3Sn4 powder, Si powder, and carbon black are milled for 8h at 1400 rpm in a
horizontal attritor (Simoloyer® cmOlfrom ZOZ, Wenden). To prevent oxidation,
milling is done under argon gas atmosphere. The composition and the process
conditions are given in Table 2. The values for the composition parameters a, b, c
and z are given in Table 8.
Table 2: Experimental conditions of Example 2
After milling, the powders are passivated in a controlled air flow to avoid excessive
oxidation. Powder properties are given in Table 3, and the XRD is shown in Figure 2.
Further processing and coin cell preparation is done as in Counter Example 1. The
electrochemical results are shown in Figure 4. The capacity retention during cycling
is superior to Counter Example 1.
Table 3: Properties of powders prepared in Examples 1-2
Counter Ex 1 Example 2
Particle size d50 (miti ) 3.79 5.40
Oxygen content (wt ) 2.0% 1.7%
BET (m2/g) 18.00 5.75
Theoretical capacity (mAh/g) 1201 1200
Capacity 2nd cycle (mAh/g) C/20 - 10 mV 1074 1112
Capacity 4th cycle (mAh/g) C/1 0 - 70 mV 847 863
Capacity 106th cycle (mAh/g) C/1 0 - 70 mV 758 (90%) 831 (96%)
Capacity 208th cycle (mAh/g) C/1 0 - 70 mV 647 (76%) 724 (84%)
Capacity 3 10th cycle (mAh/g) C/1 0 - 70 mV 539 (64%) 597 (69%)
In the Table (and also in Table 7 below) , for each alloy, the capacities at cycles
106, 208 and 3 10 are given and the corresponding capacity retention vs. cycle 4
(performed at C/1 0 with 70mV cut-off voltage) is calculated .
Example 3
Ni 3Sn4 powder, Si powder, Ti powder and carbon black are milled for 8h at 1400 rpm
in a horizontal attritor (Simoloyer® cm01 from ZOZ, Wenden). To prevent oxidation,
milling is done under argon gas atmosphere. The composition and the process
conditions are given in Table 4. The values for the composition parameters a, b, c, y
and z are given in Table 8.
Table 4: Experimental conditions of Example
After milling, the powders are passivated in a controlled air flow to avoid excessive
oxidation. Powder properties are given in Table 7.
Further processing and coin cell preparation is done as in Counter Example 1. The
electrochemical results are shown in Figure 4. The capacity retention during cycling
is superior to Counter Example 1 and Example 2.
Example 4
Ni 3Sn4 powder, Si powder, Ti powder and carbon black are milled for 8h at 1400 rpm
in a horizontal attritor (Simoloyer® cm01 from ZOZ, Wenden). To prevent oxidation,
milling is done under argon gas atmosphere. The composition and the process
conditions are given in Table 5. The values for the composition parameters a, b, c, y
and z are given in Table 8.
Table 5: Experimental conditions of Example 4
After milling, the powders are passivated in a controlled air flow to avoid excessive
oxidation. Powder properties are given in Table 7, and the XRD is shown in Figure 3.
Further processing and coin cell preparation is done as in Counter Example 1. The
electrochemical results are shown in Figure 4. The capacity retention during cycling
is superior to Counter Example 1 and Example 2.
Example 5
Ni 3Sn4 powder, Si powder, Ti powder and carbon black are milled for 8h at 1400 rpm
in a horizontal attritor (Simoloyer® cm01 from ZOZ, Wenden). To prevent oxidation,
milling is done under argon gas atmosphere. The composition and the process
conditions are given in Table 6. The values for the composition parameters a, b, c, y
and z are given in Table 8.
Table 6: Experimental conditions of Example 5
After milling, the powders are passivated in a controlled air flow to avoid excessive
oxidation . Powder properties are given in Table 7.
Further processing and coin cell preparation is done as in Counter Example 1. The
electrochemical results are shown in Figure 4. The capacity retention during cycling
is superior to Counter Example 1 and Example 2.
Table 7: Properties of powders prepared in Examples 3-5
Example 3 Example 4 Example 5
Particle size d50 (miti ) 5,43 5,38 5,3
Oxygen content (wt ) 1,6% 1,8% 1,8%
BET (m2 /g) 7,5 6,4 5,8
Theoretical capacity (mAh/g) 1190 116 1 1132
Capacity 2nd cycle (mAh/g) C/20 - 10 mV 1060 1020 923
Capacity 4th cycle (mAh/g) C/1 0 - 70 mV 8 19 798 709
Capacity 106th cycle (mAh/g) C/1 0 - 0 mV 802 (98%) 8 15 ( 102%) 7 19 ( 102%)
Capacity 208th cycle (mAh/g) C/1 0 - 0 mV 727 (89%) 724 (91 %) 652 (92%)
Capacity 3 10th cycle (mAh/g) C/1 0 - 70 mV 630 (77%) 648 (81 %) 560 (79%)
Table 8: Values of composition parameters of powders prepared in Examples 1-5
In certain embodiments according to the invention, 250,
b>0, z>0, y>0, 0 5, z + 0.5 *b > a and c + y > 0.75 *b.
2. The active material of claim 1, wherein 0