(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2007/0218356 A1
`Kawamura et al.
`(43) Pub. Date:
`Sep. 20, 2007
`
`US 20070218356A1
`
`(54) LITHIUM SECONDARY BATTERY
`
`(30)
`
`Foreign Application Priority Data
`
`(75) Inventors: Naoya Kawamura, Kyoto (JP);
`Soichiro Kawakami, Nara (JP)
`
`Correspondence Address:
`FITZPATRICK CELLAHARPER & SCINTO
`30 ROCKEFELLER PLAZA
`NEW YORK, NY 10112 (US)
`
`(73) Assignee: Canon Kabushiki Kaisha, Tokyo (JP)
`
`(21) Appl. No.:
`(22) Filed:
`
`11/798,348
`May 14, 2007
`Related U.S. Application Data
`
`(62) Division of application No. 10/808,481, filed on Mar.
`25, 2004.
`
`Mar. 31, 2003 (JP)...................................... 2003-096.988
`O
`O
`Publication Classification
`
`(51) Int. Cl.
`(2006.01)
`HOLM 4/02
`(52) U.S. Cl. .............................................................. 429/209
`
`ABSTRACT
`57
`(57)
`There is provided a lithium secondary battery with a nega
`tive electrode which comprises a negative electrode active
`material layer comprising alloy particles comprising silicon
`and tin and having an average particle diameter of 0.05 to 2
`um as an active material, and a negative electrode current
`collector, wherein the negative electrode active material
`layer has a storage capacity of 1,000 to 2,200 mAh/g and a
`density of 0.9 to 1.5 g/cm and which thereby has a high
`capacity and a good cycle-characteristic. Thus, a lithium
`secondary battery having a high capacity and a long life and
`so designed as to exhibit these characteristics at the same
`time is provided.
`
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`JLab/Cambridge, Exh. 1040, p. 1
`JLab/Cambridge v. Varta, 2020-01212
`
`

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`104
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`JLab/Cambridge, Exh. 1040, p. 2
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`
`

`

`Patent Application Publication Sep. 20, 2007 Sheet 2 of 5
`
`US 2007/021835.6 A1
`
`FIG. 2
`
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`
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`
`CAPACITY PER UNIT WEIGHT OF
`ACTIVE MATERIAL LAYER OF
`NEGATIVE ELECTRODE (mAh/g)
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`JLab/Cambridge, Exh. 1040, p. 3
`JLab/Cambridge v. Varta, 2020-01212
`
`

`

`Patent Application Publication Sep. 20, 2007 Sheet 3 of 5
`
`US 2007/0218356 A1
`
`FIG 3
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`
`DENSITY OF ACTIVE MATERAL LAYER
`OF NEGATIVE ELECTRODE (g/cm3)
`
`JLab/Cambridge, Exh. 1040, p. 4
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`

`

`Patent Application Publication Sep. 20, 2007 Sheet 4 of 5
`
`US 2007/021835.6 A1
`
`FIG. 4
`
`
`
`20
`
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`
`16
`
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`
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`
`BOUNDARY 3
`
`BOUNDARY
`
`BOUNDARY 2
`
`200
`
`400
`
`600
`
`800
`
`1000
`
`CAPACITY PER UNIT VOLUME OF
`ACTIVE MATERAL LAYER OF
`POSITIVE ELECTRODE (mAh/cm3)
`
`JLab/Cambridge, Exh. 1040, p. 5
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`

`

`Patent Application Publication Sep. 20, 2007 Sheet 5 of 5
`
`US 2007/021835.6 A1
`
`
`
`
`
`
`
`
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`JLab/Cambridge, Exh. 1040, p. 6
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`

`

`US 2007/021835.6 A1
`
`Sep. 20, 2007
`
`LITHIUM SECONDARY BATTERY
`
`BACKGROUND OF THE INVENTION
`0001) 1. Field of the Invention
`0002 The present invention relates to a lithium second
`ary battery, more particularly improvement of the capacity
`and cycle-characteristic of a lithium secondary battery.
`0003 2. Related Background Art
`0004 The so-called lithium secondary batteries compris
`ing a positive electrode with lithium cobaltate as a major
`active material, negative electrode with carbon as a major
`active material and an organic electrolyte solution have been
`put on the markets since the beginning of the 1990s. They
`have been rapidly spreading in the markets since then,
`because of their higher capacity than that of a conventional
`nickel/hydrogen secondary battery and Sufficient cycle-char
`acteristic to satisfy the market needs. At the same time,
`extensive works have been done to improve their charac
`teristics and develop batteries of higher capacities.
`0005. As a result, the cylindrical battery of 18 mm in
`diameter and 65 mm in height, the so-called 18650 size, has
`now a capacity of 2.200 mAh at the highest, comparing with
`around 1,000 mAh recorded in the beginning of the 1990s.
`The greatly enhanced capacity results from improvements in
`a wide area including materials, e.g., lithium cobaltate and
`carbon as active materials, and designs.
`0006. However, it is considered that current capacity of a
`lithium ion secondary battery with lithium cobaltate and
`carbon as major active materials is close to the limit.
`Therefore, new active materials have been studied for the
`positive and negative electrode as another approach to
`higher capacity.
`0007. In particular, for the negative electrode active
`materials, metallic materials that can be alloyed with
`lithium, e.g., silicon and tin, have been studied as Substitutes
`for carbon materials such as graphite. This is because they
`have greater theoretical capacities which are 3 to 10 times
`that of graphite such that while the theoretical capacity
`capable of charging/discharging of graphite is 372 mAh/g, a
`silicon alloy (Li Si) has a theoretical capacity of 4,199
`mAh/g and a tin alloy (LiSn) has a theoretical capacity of
`993 mAh/g.
`0008 However, some metallic materials that can be
`alloyed with lithium involve their own problems to be
`Solved, because they may expand during the alloying reac
`tion process to increase the negative electrode Volume
`several times, which tends to powder them, resulting in
`deterioration of their cycle-characteristic.
`0009. Several proposals have been made to solve these
`problems, as disclosed by U.S. Pat. Nos. 6,051,340, 5,795,
`679 and 6.432.585, Japanese Patent Application Laid-Open
`Nos. 11-283627 and 2000-311681 and WO OOf 17949.
`0010 For example, U.S. Pat. No. 6,051,340 proposes a
`lithium secondary battery with a negative electrode com
`prising a current collector coated with an electrode layer,
`wherein the current collector is of a metal which is not
`alloyed with lithium, and the electrode layer comprises a
`metal which can be alloyed with lithium, such as silicon or
`tin and another metal which is not alloyed with lithium, such
`as nickel or copper.
`
`0011 U.S. Pat. No. 5,795,679 proposes a lithium second
`ary battery with a negative electrode formed of an alloy
`powder comprising an element such as nickel or copper and
`another element such as tin; and U.S. Pat. No. 6,432.585 a
`battery with a negative electrode whose electrode material
`layer contains at least 35% by weight of silicon or tin
`particles having an average particle diameter of 0.5 to 60
`um, a void ratio of 0.10 to 0.86 and a density of 1.00 to 6.56
`g/cm.
`0012 Japanese Patent Application Laid-Open No. H11
`283627 proposes a lithium secondary battery with a negative
`electrode containing silicon or tin having an amorphous
`phase; and Japanese Patent Application Laid-Open No.
`2000-31 1681 a lithium secondary battery with a negative
`electrode composed of amorphous tin/transition metal alloy
`particles of a non-stoichiometric composition. WO
`00/17949 discloses a lithium secondary battery with a nega
`tive electrode composed of amorphous silicon/transition
`metal alloy particles of a non-stoichiometric composition.
`0013 Moreover, Japanese Patent Application Laid-Open
`No. 2000-215887 proposes a lithium secondary battery
`whose capacity and charge/discharge efficiency are
`improved by Suppressing the Volume expansion during
`alloying with lithium to prevent the breakage of the negative
`electrode, wherein chemical vapor deposition involving
`pyrolysis of benzene or the like is used to solve the above
`problems by forming a carbon layer on the surface of
`particles of a metal or semi-metal, in particular silicon,
`which can be alloyed with lithium, to improve its electro
`conductivity.
`0014. These inventions have disclosed compositions and
`constituents of silicon or its alloys, and performance of the
`electrode that comprises the above material. It should be
`noted, however, that a battery exhibits its inherent functions
`when its negative electrode works in combination with a
`positive electrode, both contained in a battery can. For a
`battery to exhibit its intended functions, it is essential to
`design a battery of high capacity and cycle-characteristic by
`allowing a negative electrode mainly composed of a metallic
`material which can be alloyed with lithium to effectively
`function in a battery can in combination with a positive
`electrode.
`00.15 Japanese Patent Application Laid-Open No. 2002
`352797 proposes a lithium secondary battery of high capac
`ity and cycle-characteristic by controlling utilization of a
`negative electrode comprised of silicon at a certain level or
`less. However, it only discloses silicon coated with carbon
`for a negative electrode, discussing that recommended elec
`trical storage capacity (hereinafter, simply referred to as
`'storage capacity' or “capacity') per unit weight of a
`negative electrode active material layer is 1,000 mAh/g, but
`is silent on conditions for extending the life of a battery
`having a capacity exceeding 1,000 mAh/g.
`0016.
`In other words, few have sufficiently discussed
`optimum electrode and battery designs that allow a battery
`to exhibit a high capacity and a long cycle life when it
`comprises a negative electrode of a high capacity per unit
`weight of a negative electrode active material layer exceed
`ing 1,000 mAh/g working in combination with a positive
`electrode.
`
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`

`

`US 2007/021835.6 A1
`
`Sep. 20, 2007
`
`SUMMARY OF THE INVENTION
`0017. The present invention has been accomplished in the
`light of the above-mentioned situation, and it is an object of
`the present invention to provide a lithium secondary battery
`having a high capacity and a long life and so designed as to
`exhibit these characteristics at the same time.
`0018. The present invention provides a lithium secondary
`battery with a negative electrode comprising a negative
`electrode active material layer comprising alloy particles
`comprising silicon and tin and having an average particle
`diameter of 0.05 to 2 pm as an active material, and a current
`collector, wherein the negative electrode active material
`layer has a storage capacity of 1,000 to 2,200 mAh/g and a
`density of 0.9 to 1.5 g/cm.
`0019. The present invention also provides a lithium sec
`ondary battery comprising a negative electrode comprising
`a negative electrode active material layer comprising alloy
`particles as an active material comprising silicon as a major
`component and a negative electrode current collector, and a
`positive electrode comprising a positive electrode active
`material layer and a positive electrode current collector,
`wherein the positive electrode active material layer and the
`negative electrode active material layer satisfy the following
`relationships:
`(CNXDN) (CPXDP) is 8
`CNxDN=1,200 to 2,500 mAh/cm
`CN=1,000 to 2,200 mAh/g
`DN=0.9 to 1.5 g/cm
`wherein,
`0020. CN represents a capacity per unit weight of the
`negative electrode active material layer,
`0021. D represents the density of the negative elec
`trode active material layer;
`0022 C represents a capacity per unit weight of the
`positive electrode active material layer, and
`0023 Dr represents the density of the positive elec
`trode active material layer.
`0024. In the present invention, it is preferred that the
`alloy particles comprising silicon as a major component
`have an average particle diameter of 0.05 to 2 um.
`0025. In the present invention, it is also preferred that the
`alloy particles comprising silicon as a major component are
`alloy particles comprising silicon and tin.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0026 FIG. 1 is a sectional view schematically showing
`an embodiment of the secondary battery (lithium secondary
`battery) of the present invention;
`0027 FIG. 2 is a graphical representation showing the
`relationship between the capacity per unit weight (inserted
`Li amount) and the changes in cycle life and expansion
`coefficient of the negative electrode active material layer of
`the above-mentioned secondary battery wherein a silicon/tin
`alloy powder is used as an active material;
`0028 FIG. 3 is a graphical representation showing the
`relationship between the density and the changes in cycle
`life and expansion coefficient of the negative electrode
`
`active material layer of the above-mentioned secondary
`battery wherein a silicon/tin alloy powder is used as an
`active material;
`0029 FIG. 4 is a view showing the design range for the
`positive electrode active material layer and the negative
`electrode active material layer of the above-mentioned sec
`ondary battery; and
`0030 FIG. 5 is a sectional view showing the structure of
`a spiral-wound type cylindrical battery as one embodiment
`of the above-mentioned secondary battery.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`0031. The preferred embodiments of the present inven
`tion will be described by referring to the attached drawings.
`0032. The present inventors have developed a negative
`electrode with an unprecedentedly high capacity by using
`alloy particles as an active material comprising silicon as a
`major component, and found those electrode conditions
`under which the electrode performance (including cycle
`characteristic) and the capacity are well-balanced. Further,
`they have found the optimum electrode and battery design
`conditions which allow a battery with a negative electrode of
`high capacity working in combination with a positive elec
`trode to exhibit high capacity and long life. The secondary
`batteries of preferred embodiments of the present invention
`will be described in detail.
`0033 FIG. 1 is a sectional view schematically showing
`an embodiment of the secondary battery (lithium secondary
`battery) of the present invention. In the lithium secondary
`battery, a negative electrode 103 comprising a negative
`electrode active material layer 102 comprising alloy par
`ticles as an active material comprising silicon as a major
`component and formed on a negative electrode current
`collector 101, and a positive electrode 106 comprising a
`positive electrode active material layer 105 comprising
`lithium-containing transition metal oxide as an active mate
`rial and formed on a positive electrode current collector 104
`are stacked in opposition to each other via an ion conductor
`107 and contained in a battery case 112, and the negative
`electrode 103 is connected to a negative electrode terminal
`108 via a negative electrode lead 110 and the positive
`electrode 106 is connected to a positive electrode terminal
`109 via a positive electrode lead 111.
`0034. The present inventors have developed a negative
`electrode with a high capacity and a good cycle-character
`istic by using alloy particles as an active material comprising
`silicon as a major component, and found the optimum
`capacity and density for battery design.
`0035 FIG. 2 is a view illustrating the relationship
`between the capacity per unit weight (inserted Li amount)
`and the changes in cycle life and expansion coefficient of a
`negative electrode active material layer 102 using a silicon/
`tin alloy powder an active material having an average
`particle diameter of 0.05 to 2 um is used as an example of
`the alloy.
`0036) The battery was evaluated by the charging/dis
`charging test, wherein the negative electrode 103 was used
`as a cathode and metallic lithium was used as an anode in an
`electrolyte solution of 1 M (mol/L) prepared by dissolving
`
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`

`US 2007/021835.6 A1
`
`Sep. 20, 2007
`
`lithium hexafluorophosphate (LiPF) in a mixture of ethyl
`ene carbonate (EC) and diethyl carbonate (DEC) in equiva
`lent amounts.
`0037. The charging/discharging test was carried out with
`a cycle consisting of a Li insertion/release reaction at a
`current density of 1 mA/cm and a 20-minutes resting period
`being defined as one cycle. The Li insertion reaction was cut
`off at a given capacity or 0 V, and the Li release reaction was
`carried out with the cut-off voltage being set to 1.2 V.
`Incidentally, the electrode exhibited a storage capacity of
`2,400 mAh/g at the maximum by continuing the Li insertion
`reaction to 0 V.
`0038. The expansion coefficient was measured after the
`Li insertion reaction in a first cycle, and the cycle life was
`evaluated on the basis of the number of cycles in which the
`capacity did not reach 60% of the given capacity. Inciden
`tally, the life shown in FIG. 2 was values normalized with
`the number of cycles at a given capacity of 1,000 mAh/g
`being defined as 1.0.
`0039 FIG. 3 is a view illustrating the relationship
`between the density of a negative electrode active material
`layer 102 and the changes in cycle life and expansion
`coefficient when effecting charging and discharging at a
`capacity per unit weight of the negative electrode active
`material layer 102 1,400 mAh/g. The evaluation was carried
`out in the same manner as that described for FIG. 2. Further,
`the life shown in FIG. 3 was values normalized with the
`number of cycles at a density of 1.0 g/cm being defined as
`1.O.
`0040. It can be seen from the results shown in FIGS. 2
`and 3 that the expansion coefficient of the negative electrode
`active material layer 102 using a silicon/tin alloy powder as
`an active material becomes greater as the capacity increases
`(i.e., as the quantity of lithium inserted increases) and as the
`density of the negative electrode active material layer 102
`increases. Thus, there is a tendency that the negative elec
`trode active material layer 102 is liable to generate strain or
`cracks to reduce the current collectability, thereby deterio
`rating the cycle-characteristic. Conversely, there is a ten
`dency that the expansion coefficient becomes Smaller as the
`capacity decreases (i.e., as the quantity of lithium inserted
`decreases) and as the density of the negative electrode active
`material layer 102 decreases, thereby improving the cycle
`characteristic.
`0041. These results lead to the conclusion that the pref
`erable capacity per unit weight of the negative electrode
`active material layer 102 is within the range of 1,000 to
`2,200 mAh/g. The reason is that exceeding 2,200 mAh/g
`significantly deteriorates the cycle-characteristic due to
`expansion, which is not desirable. Further, although there
`are cases where priority is given to improvement in the
`cycle-characteristic at Some degree of sacrifice of the capac
`ity, no improvement in the cycle-characteristic is expected at
`a capacity below 1,000 mAh/g. Incidentally, the preferable
`capacity per unit weight of the active material is within the
`range of 1,500 to 3,000 mAh/g, although varying depending
`on the composition of the active material layer.
`0042. On the other hand, the preferable density of the
`negative electrode active material layer 102 is within the
`range of 0.9 to 1.5 g/cm. The reason is that exceeding 1.5
`g/cm significantly deteriorates the cycle-characteristic due
`
`to expansion, which is not desirable. Further, when the
`density is degreased, the battery capacity inevitably
`decreases. Incidentally, although there are cases where pri
`ority is given to improvement in the cycle-characteristic at
`Some degree of sacrifice of the battery capacity, no improve
`ment in the cycle-characteristic has been attained at a
`density below 0.9 g/cm.
`0043. Thus, the present inventors have found from the
`results shown in FIGS. 2 and 3 that the capacity per unit
`volume of the negative electrode active material layer 102
`expressed by a product of capacity per unit weight and
`density is preferably within the range of 900 to 3.300
`mAh/cm, more preferably 1,200 to 2,500 mAh/cm. The
`reason is that exceeding 2,500 mAh/cm significantly dete
`riorates the cycle-characteristic due to expansion, and that
`although there are cases where priority is given to improve
`ment in the cycle-characteristic at Some degree of sacrifice
`of the capacity, no improvement in the cycle-characteristic
`has not been attained at a capacity below 1,200 mAh/cm.
`0044) In addition, based on the results shown in FIGS. 2
`and 3, the present inventors have found as a second specific
`feature of the present invention that the negative electrode
`comprising alloy particles as an active material comprising
`silicon as a major component to provide the above-men
`tioned high capacity can work in combination with a posi
`tive electrode to fully exhibit its inherent functions, by
`designing the positive electrode active material layer 105
`and the negative electrode active material layer 102 so as to
`satisfy the following relationships:
`(CNXDN)/(CPXDP)is 8
`CxDN=1,200 to 2,500 mAh/cm
`CN=1,000 to 2,200 mAh/g
`DN=0.9 to 1.5 g/cm
`wherein,
`0045 C represents a capacity per unit weight of the
`negative electrode active material layer,
`0046 DN represents the density of the negative elec
`trode active material layer;
`0047 C represents a capacity per unit weight of the
`positive electrode active material layer, and
`0048) Dr represents the density of the positive elec
`trode active material layer.
`0049. A combination of the positive electrode active
`material layer 105 and the negative electrode active material
`layer 102 that satisfy the above relationships can provide a
`battery of a high capacity and an excellent cycle-character
`istic.
`0050 FIG. 4 is a view illustrating these relationships in
`detail, where the area enclosed by the boundaries 1 to 3 is
`a range in which a battery of a high capacity and an excellent
`cycle-characteristic can be provided. Incidentally, in FIG. 4.
`the abscissa indicates a capacity per unit volume of the
`positive electrode active material layer that can be repeat
`edly charged and discharged, represented by CxD, and the
`ordinate indicates (CxDN)/(CxD).
`0051. Here, the term “capacity per unit volume of the
`positive electrode active material layer 105 that can be
`repeatedly charged and discharged’ means a reaction region
`
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`

`US 2007/021835.6 A1
`
`Sep. 20, 2007
`
`in which the charge/discharge cycles are well carried out and
`obtained as a product of the capacity per unit weight and the
`density of the positive electrode active material layer 105.
`Further, the capacity per unit weight of the positive electrode
`active material layer 105 can be determined by the capacity
`per unit weight of the positive electrode active material that
`can be repeatedly charged and discharged and the compo
`sitional ratio by weight of the positive electrode active
`material contained in the positive electrode active material
`layer 105.
`0.052
`For example, the positive electrode active material
`layer 105 that can be repeatedly charged and discharged of
`the cobalt-, nickel- and manganese-type lithium-containing
`transition metal oxides, which can provide a high Voltage, is
`practically used at a capacity per unit volume of 200 to 700
`mAh/cm, a capacity per unit weight of 80 to 200 mAh/g and
`a density of 2.5 to 3.5 g/cm.
`0053 More specifically, as the active material layer using
`these lithium-containing transition metal oxides used for
`commercially available batteries, an LiCoO active material
`layer is practically used at a capacity per unit weight of 140
`to 160 mAh/g and a density of 3.0 to 3.5 g/cm; an LiNiO,
`active material layer, which has a higher theoretical capacity
`than that of LiCoO, is practically used at a capacity per unit
`weight of 170 to 200 mAh/g and a density of 2.8 to 3.2
`g/cm, and an LiMn2O, active material layer is practically
`used at a capacity per unit weight of 80 to 120 mAh/g and
`a density of 2.5 to 3.0 g/cm for.
`0054) The boundary 1 represents the values of (CxDN)/
`(CxDP) vs. CxDP that is the capacity per unit volume of
`the positive electrode active material layer which can be
`repeatedly charged and discharged when the CNxDN is the
`minimum value 1,200 mAh/cm. Although there are cases
`where priority is given to the cycle-characteristic at the
`sacrifice of capacity to some extent, any value below the
`boundary 1 is not desirable, because no improvement in the
`cycle-characteristic is expected and the high capacity per
`formance of the negative electrode cannot be fully exhibited.
`0.055 The boundary 2 represents the values of (CxDN)/
`(CxD) vs. CxDr that is the capacity per unit volume of
`the positive electrode active material layer which can be
`repeatedly charged and discharged when the CNxDN is the
`maximum value 2,500 mAh/cm. Any value exceeding the
`boundary 2 is not desirable, because the cycle-characteristic
`lowers.
`0056. The boundary 3 represents (CxDN)/(CxD)=8.
`Values on or below the boundary 3 can provide a more stable
`battery and hence are preferable. The reason is as described
`below. Generally, the positive electrode active material layer
`105 and the negative electrode active material layer 102
`facing each other satisfy the following relationship.
`CNXDNXTN=CPXDPXTP
`Equation 1
`The following Equation 2 is derived from Formula 1.
`(CNXDN) (CpxDp)=TP/TN
`Equation 2
`In the above equations, TN represents the thickness of the
`negative electrode active material layer 102: CN the capacity
`per unit weight of the negative electrode active material
`layer 102; D the density of the negative electrode active
`material layer 102; T the thickness of the positive electrode
`active material layer 105; C the capacity per unit weight of
`
`the positive electrode active material layer 105; and Dr the
`density of the positive electrode active material layer 105.
`0057. As is seen from Equation 2, the (CxDN)/(CxD)
`can be represented in terms of the ratio of the active material
`layers TP/TN. The practical thickness of the positive elec
`trode active material layer 105 in consideration of the
`battery characteristics, adhesion to the current collector and
`productivity, is 150 um or less, more preferably 100 um or
`less. With this positive electrode active material layer 105,
`if the boundary 3 is exceeded, the negative electrode active
`material layer 102 is too thin, which makes it difficult to
`effect uniform coating in mass production to lower the
`productivity. Further, during electrode group formation steps
`Such as Stacking or rolling, rolling in a skewed fashion or the
`like is liable to occur. Therefore, the (CxDN)/(CxD)
`value is preferably 8 or less.
`0058 As described above, as long as the conditions
`defined by the present invention and shown in FIG. 4 are
`satisfied, it is possible to stably provide a battery suitable for
`a specified object of use, Such as a battery featuring a high
`capacity while securing the life to a certain extent, or a
`battery featuring a long life while still having a higher
`capacity than a commercially available battery with a graph
`ite negative electrode.
`0059) Next, the negative electrode 103, negative elec
`trode current collector 101, negative electrode terminal 108,
`positive electrode 106, positive electrode current collector
`104, positive electrode terminal 109, and ion conductor 107
`of the secondary battery (lithium secondary battery) men
`tioned above with reference to FIG. 1 will be described.
`(Negative Electrode 103)
`0060. The negative electrode 103 is generally consisted
`of the negative electrode current collector 101, and the
`negative electrode active material layers 102 formed on both
`sides of the negative electrode current collector 101. The
`negative electrode active material layer 102 is constituted of
`alloy particles mainly composed of silicon, a conductive
`auxiliary material, other additives, and a binder for holding
`the active material layers on each other or for holding the
`active material layer on the current collector.
`0061
`For example, the negative electrode active material
`layer 102 is formed by suitably adding a conductive auxil
`iary material and a binder to the alloy particles mainly
`comprised of silicon, followed by mixing, application and
`pressure forming. Further, it is preferable to add a solvent to
`the above mixture to prepare a paste, for facilitating the
`application. As the application method, a coater coating
`method or screen printing method may be used.
`0062 Alternatively, the negative electrode active mate
`rial layers 102 can be formed on the current collector by
`pressing and forming the negative electrode main material,
`a conductive auxiliary material and a binder on the current
`collector without addition of a solvent, or pressing and
`forming the negative electrode main material and a conduc
`tive auxiliary material on the current collector without
`addition of a binder. Incidentally, the content of the alloy
`particles mainly composed of silicon in the negative elec
`trode active material layer 102 is preferably 40 to 90% by
`weight.
`0063 As the active material of the negative electrode
`103, there are preferably used an alloy powder mainly
`
`JLab/Cambridge, Exh. 1040, p. 10
`JLab/Cambridge v. Varta, 2020-01212
`
`

`

`US 2007/021835.6 A1
`
`Sep. 20, 2007
`
`composed of silicon that are stable in an electrolyte Solution
`and capable of insertion/release of lithium. It is important
`that the composition of the alloy powder mainly composed
`of silicon is such that the silicon content is preferably 50
`atomic '% or more, more preferably 50% by weight or more.
`As component elements other than silicon of the silicon
`alloy, there is preferably used at least one element selected
`from the group consisting of tin, aluminium, zinc, germa
`nium, indium, antimony, titanium, chromium, lead, copper,
`nickel, cobalt and iron.
`0064. These alloy powders are preferably amorphous.
`The average diameter of the alloy particles is preferably 2
`um or less, more preferably 0.9 um or less, and more
`preferably 0.05 um or more.
`0065. The binder used in the present invention is not
`particularly limited, so long as it is electrochemically and
`chemically stable and is adhesive. Examples of the binder
`include water-insoluble polymers such as polyolefins such
`as polyethylene and polypropylene, fluororesins such as
`polyvinylidene fluoride, tetrafluoroethylene polymer and
`vinylidene fluoride-hexafluoropropylene copolymer, poly
`ethylene-polyvinyl alcohol copolymer, and styrene-butadi
`ene rubber; and water-soluble polymers such as polyvinyl
`alcohol, polyvinylbutyral, polyvinyl methyl ether, polyvinyl
`ethyl ether, polyvinyl isobutyl ether, carboxymethylcellu
`lose,
`hydroxyethylcellulose,
`hydroxypropylcellulose,
`hydroxymethylethylcellulose, polyethylene glycol, and sty
`rene-butadiene rubber.
`0.066 A preferable water-soluble polymer is a mixture of
`polyvinyl alcohol and a cellulosic polymer, preferably car
`boxymethylcellulose.
`0067. Here, the content of the binder in the active mate
`rial layer is preferably 1 to 20% by weight in order to hold
`a larger quantity of the active material at the time of
`charging, and more preferably 5 to 15% by weight. This is
`because the alloy powder more expands than carbon powder
`during charging, so that a greater binding force is needed.
`0068. As the conductive auxiliary material, there is pref
`erably used those materials which are electrochemically and
`chemically stable and whose conductivity is as large as
`possible. Preferable examples of the conductive auxiliary
`material include carbon powder (in particular graphite pow
`der), copper powder, nickel powder, aluminium powder and
`titanium powder.
`(Negative Electrode Current Collector 101)
`0069. The material of the negative electrode current
`collector 101 needs to be electrochemically and chemically
`stable, highly conductive, and not alloyed with lithium and
`includes, for example, copper, nickel, stainless steel and
`titanium. The current collector may be sheet-shaped, net
`shaped, expanded, perforated or spongy. The current collec
`tor is preferably 6 to 30 Lum in thickness. When the thickness
`is less than 6 Jum, although the battery capacity will increase,
`the resistance of the current collector will increase to result
`in an increase of in