JP2009076372A - Non-aqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous secondary battery in which energy density and average voltage are improved and excellent rate characteristics can be obtained. <P>SOLUTION: In the non-aqueous secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode is composed of a material capable of electrochemically storing and releasing lithium, and the negative electrode contains a material in which lithium is pre-doped in SiOx (0.3≤x≤1.6). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a secondary battery using a nonaqueous electrolyte for a negative electrode, and more particularly to a nonaqueous secondary battery using a negative electrode material made of SiOx pre-doped with lithium.

  In recent years, various high energy density batteries have been vigorously developed in connection with power supplies for small portable devices typified by mobile phones and laptop computers, home-use distributed power storage systems, electric vehicles, etc. . In particular, lithium ion batteries using graphite as a negative electrode material have high energy density, and are superior in reliability such as safety and cycle characteristics compared to lithium secondary batteries using metal lithium as a negative electrode. As a result, the market is rapidly expanding as a power source for small portable devices.

The lithium ion battery uses a lithium-containing transition metal oxide typified by LiCoO ₂ or LiMn ₂ O ₄ as a positive electrode, and a carbon-based material typified by graphite as a negative electrode. Currently, further increase in capacity of lithium ion batteries is being promoted, but the increase in capacity by improving positive electrode oxide and negative electrode carbon-based material has reached almost the limit of 500 Wh / l, so an excessive active material It is in a situation where it is unavoidable to increase the capacity by increasing the filling rate, thinning the separator, and increasing the charging voltage. As a result, the safety characteristic of the lithium ion battery is threatened, and there is a concern about safety although batteries exceeding 500 Wh / l are commercially available.

  Lithium ion batteries have reached the limit of capacity increase in conventional material systems, and there is a need for new material systems that are reliable and safe and that can increase energy density in order to achieve higher energy densities. Has been.

In the negative electrode material, as a high-capacity material, a polycyclic aromatic typified by a carbon material exceeding the theoretical capacity of graphite, C ₆ Li (372 mAh / g), and a polyacene organic semiconductor having a lithium occlusion capacity exceeding 1000 mAh / g. Group condensation polymers have been developed. As other directions, Si, Sn and Li alloy-based materials, and oxide-based materials based on Si, Sn and the like are also attracting attention as high-capacity materials. Among these materials, the alloy-based material is expected to have the highest capacity per mass, but the hurdle to practical use is high because it essentially has the problem of pulverization of the material itself in repeated charge and discharge. On the other hand, an oxide-based material whose volume expansion is smaller than that of an alloy is inferior to that of an alloy material in capacity. For example, Si—C—O described in Patent Document 1 (Japanese Patent Laid-Open No. 2006-62949). The composite material has a capacity of 900 mAh / g, which is about three times as large as that of the graphite-based material, and the capacity after 50 cycles is about 95% of the initial capacity and good cycle characteristics are obtained.

  On the other hand, in a power storage device such as a lithium ion battery or a capacitor, attention has been paid to a technology for increasing the capacity and voltage of the power storage device by previously supporting lithium ions on the active material (hereinafter referred to as pre-doping). Pre-doping is a technology that has been put into practical use for a long time. For example, Non-Patent Document 1 (Shiho Yada, Industrial Materials, Vol. 40, No. 5, 32 (1992)), Patent Document 2 (Japanese Patent Laid-Open No. 3-233860). Discloses a high-voltage and high-capacity electricity storage device in which lithium is pre-doped on an insoluble infusible substrate containing a polyacene skeleton structure which is a negative electrode active material. As for the pre-doping method, there are known a method of incorporating an lithium-supported electrode into an electricity storage device, a method of mixing lithium metal or the like at the time of electrode formation, and the like. There is a method of bringing a lithium metal foil into contact with an electrode containing a substance, and in fact, it has been put to practical use in a coin-type battery.

  Patent Document 3 (International Publication No. 98/33227 pamphlet) forms an electrode layer on a current collector provided with a through hole, and short-circuits a lithium metal and a negative electrode arranged in the battery, A technique in which lithium ions pass through the through-holes of the current collector and pre-dope all the negative electrodes is also disclosed, which is an effective technique for a cylindrical battery, a rectangular battery, etc. in which the electrode has a wound structure or a laminated structure.

  As described above, oxide-based materials, especially Si-based oxides, have a high lithium storage capacity per mass and are promising materials for increasing the energy density of batteries. However, the initial efficiency is low, and the volume changes during lithium storage. Since it is larger than the carbon material, when assembled in an actual battery, there is a problem that it is difficult to obtain a significant energy density improvement effect as compared with a lithium ion battery using a graphite-based material as a negative electrode.

  On the other hand, with regard to the application technology of lithium pre-doping, there is almost no study on Si-based oxide materials, and in particular, the applicability to SiO-based materials that combine carbon materials with excellent cycle characteristics and large capacity were applied. The battery characteristics (energy density, charge / discharge behavior, etc.) in the case have not been studied.

JP 2006-62949 A JP-A-3-233860 International Publication No. 98/33227 Pamphlet Shigeru Yada, Industrial Materials, Vol. 40, no. 5, 32 (1992)

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a non-aqueous secondary battery in which energy density and average voltage are improved and good rate characteristics are obtained.

  As a result of intensive studies to achieve the above object, the present inventors have found that in a non-aqueous secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, (1) the positive electrode electrochemically occludes lithium and (2) A non-aqueous secondary battery in which the negative electrode is made of a material in which lithium is predoped with SiOx (0.3 ≦ x ≦ 1.6) is a lithium ion using conventional graphite as a negative electrode. As a result, the present inventors have found that the energy density is remarkably improved as compared with the battery, and have made the present invention. In the present invention, the positive electrode and the negative electrode mean a positive electrode active material layer (ie, a positive electrode mixture layer containing a positive electrode active material) and a negative electrode active material layer (ie, a negative electrode mixture layer containing a negative electrode active material), respectively. However, the current collector is not included.

Accordingly, the present invention provides the following non-aqueous secondary battery.
[1] In a non-aqueous secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode is made of a material capable of electrochemically inserting and extracting lithium, and the negative electrode is made of SiOx (0.3 ≦ x ≦ 1. 6) A non-aqueous secondary battery comprising a material pre-doped with lithium.
[2] When the atomic ratio of lithium released from the positive electrode and occluded by the negative electrode to the negative electrode Si is Lp, and the atomic ratio of lithium pre-doped to the negative electrode Si is Ln, 0.3 <Ln, and 2. 5 <Ln + Lp <5.5, The nonaqueous secondary battery according to [1].
[3] The non-aqueous system according to [2], wherein the positive electrode is a lithium composite oxide, and the atomic ratio Lp of lithium released from the positive electrode and stored in the negative electrode to the negative electrode Si is 2.0 <Lp. Next battery.
[4] Lithium is pre-doped in SiOx (0.3 ≦ x ≦ 1.6) by electrochemically contacting the negative electrode and metallic lithium in the battery [1] to [3] The nonaqueous secondary battery according to any one of [3].
[5] Lithium is bonded to SiOx (0.3 ≦ x ≦ 1.6) by lithium being bonded to the negative electrode surface and electrochemically contacting in the battery [4] The nonaqueous secondary battery as described.
[6] When the thickness of the lithium to be bonded is TL, the thickness of the negative electrode before pre-doping is TN ₀ , and the thickness of the negative electrode when the battery is fully charged is TN _C , (TN _C −TN ₀ ) ≧ TL [5] The nonaqueous secondary battery according to [5].
[7] The non-aqueous two-component film according to any one of [1] to [6], wherein the negative electrode is a composite of a carbon material on the surface of SiOx at 1 to 50% with respect to the mass of SiOx. Next battery.

  According to the above configuration, a non-aqueous secondary battery with high energy density can be obtained. In particular, the total lithium doping amount to the negative electrode SiOx can be controlled within a specific range by pre-doping, and more than a specific amount can be released to the positive electrode. By using a lithium composite oxide containing fresh lithium, it is possible to obtain a nonaqueous secondary battery with higher energy density and excellent average voltage and rate characteristics. In addition, pre-doping lithium in the battery as described above is particularly preferable in the negative electrode of the present invention having a volume change, and further, it is not necessary to use a special current collector or the like, and an electrode of an existing lithium ion battery Configuration can be applied. Furthermore, a non-aqueous secondary battery with good cycle characteristics can be obtained by using a negative electrode in which a specific amount of carbon material is combined on the surface of SiOx.

  The non-aqueous secondary battery of the present invention has an effect that energy density is improved, average voltage is improved, and good rate characteristics are obtained.

  The non-aqueous secondary battery according to the present invention is a non-aqueous secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode is made of a material capable of electrochemically inserting and extracting lithium, and the negative electrode is made of SiOx (0. 3 ≦ x ≦ 1.6) including a material pre-doped with lithium.

  In the present invention, the negative electrode is made of a material obtained by pre-doping lithium into SiOx (0.3 ≦ x ≦ 1.6). X in SiOx is 0.3 ≦ x ≦ 1.6, and when x is less than the lower limit value, the Si ratio is high, so that the volume change during charge / discharge becomes too large, and the cycle characteristics tend to deteriorate. When the value exceeds the upper limit value, the Si ratio decreases, making it difficult to obtain the effect of improving the energy density, which is an object of the present invention. In this respect, x is particularly preferably 0.7 ≦ x ≦ 1.2. The production method of SiOx is not particularly limited. For example, it can be produced by the method described in JP-A-2002-260651, and it is possible to combine carbon, nitrogen, etc. with SiOx during production ( In the following description, the term “SiOx” includes such a composite of other elements).

  Moreover, what mixed the carbon material on the surface of SiOx at 1-50 mass% with respect to SiOx can also be used for SiOx contained in a negative electrode for the purpose of improving cycling characteristics. 5-30 mass% is preferable with respect to SiOx, and, as for a carbon material, 5-20 mass% is especially preferable. When the amount of carbon to be combined is less than 1% by mass, when the SiOx is used alone as a negative electrode active material, the conductivity of the negative electrode film is small, and there is no meaning of carbon composite. In some cases, the negative electrode capacity decreases and the energy density improvement effect that is the object of the present invention is hardly obtained. Although there is no particular limitation on the composite method, a technique such as CVD can be used (in the following description, the term “SiOx” includes a composite of carbon materials in this way).

The negative electrode of the present invention comprises the above-mentioned SiOx, and the SiOx is predoped with lithium. The negative electrode of the present invention can be obtained, for example, by mixing the SiOx powder having an average particle size of 0.5 to 30 μm, preferably about 1 to 20 μm with a binder resin and molding it on a current collector, for example. It is practically preferable that the pre-doping of lithium into SiOx is carried out electrochemically by the method described later after forming SiOx on the electrode. The average particle diameter is a value measured as a mass average value D ₅₀ (that is, a particle diameter or a median diameter when the cumulative mass is 50%) in the particle size distribution measurement by the laser light diffraction method.

  The negative electrode of the present invention can be molded by a known method in consideration of the shape and characteristics of a desired nonaqueous secondary battery.

  The binder resin used for molding is not particularly limited, and specifically, fluorine-based resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; rubber-based materials such as fluorine rubber and SBR; polyethylene And polyolefin such as polypropylene; acrylic resin; polyimide resin and the like are exemplified, but good cycle characteristics can be obtained particularly by using polyimide resin.

  The binder compounding amount in the negative electrode molding mixture may be appropriately determined according to the kind of SiOx of the present invention, the particle size, the shape, the amount of lithium with respect to Si atoms of SiOx during charge / discharge, etc. Since volume change occurs during charging / discharging, a resin having high binding strength (for example, polyimide resin: heat-treated at 180 ° C. or higher after electrode formation to increase the binding strength) is 5% of the mass of SiOx. It is preferable to use about 20%, especially about 5-15%.

  Moreover, it is also possible to add a electrically conductive agent to the negative electrode of this invention as needed. The kind and amount of the conductive agent added to the negative electrode of the present invention are not particularly limited, and are appropriately determined depending on the electronic conductivity of the active material (whether it is combined with the carbon material), the average particle diameter, the shape, or the like. When added, the amount is about 1 to 30%, particularly about 1 to 20% with respect to the mass of SiOx. Further, when a carbon material is composited on the SiOx surface, it is not necessary to add a conductive agent. As a kind of conductive agent, carbon fiber, acetylene black, fine carbon material such as graphite, carbon fiber and fibrous carbon material such as single-walled or multi-walled carbon nanotubes (for example, VGCF manufactured by Showa Denko KK), Examples of the metal material include copper and nickel.

  In the present invention, when forming the electrode by forming the negative electrode molding mixture on a current collector, the current collector to be used is not particularly limited, and examples thereof include copper foil, stainless steel foil, and titanium foil. It is done. Furthermore, a porous current collector, for example, expanded metal, mesh, punching metal, or the like can be used.

  The SiOx constituting the negative electrode of the present invention is predoped with lithium. Pre-doping means that SiOx is occluded / supported before normal charge / discharge between positive and negative electrodes. The method of pre-doping lithium into SiOx of the present invention is not particularly limited, but it is practical to carry out after forming the electrode.

  As an example of lithium pre-doping to SiOx of the present invention, a method of electrochemically forming an electrode will be described. This method includes assembling an electrochemical system using lithium metal as a counter electrode before assembling the battery, pre-doping in a non-aqueous electrolyte described later, and attaching lithium metal to the negative electrode impregnated with the electrolyte. Can be mentioned. In addition, in order to perform lithium pre-doping after the battery is assembled, the lithium source such as lithium metal and the negative electrode are electrically contacted with each other by injecting the electrolyte into the battery. There is a method of pre-doping lithium, and it is practically preferable to pre-dope after battery assembly.

  In order to obtain a non-aqueous secondary battery having a high energy density, which is the object of the present invention, it is essential to pre-dope lithium into SiOx. In general, pre-doping of a negative electrode material is intended to compensate for irreversible capacity, or to provide the negative electrode with lithium necessary for charging and discharging when a material that does not contain lithium is used for the positive electrode. However, the application of the pre-doping technique should be examined according to the characteristics of each negative electrode (charge / discharge potential, change in internal resistance accompanying charge / discharge) and target battery characteristics. For example, the pre-doping amount is not limited to the pre-doping amount corresponding to the irreversible capacity, but differs depending on the positive electrode lithium amount, the positive electrode efficiency, and the negative electrode characteristics (charge / discharge potential, change in internal resistance associated with charge / discharge).

  In the present invention, SiOx pre-doped with lithium is used, but the effect is particularly great when the following relationship is satisfied by pre-doping. That is, when the atomic ratio of lithium released from the positive electrode and occluded in the negative electrode to the negative electrode Si is Lp, and the atomic ratio of lithium predoped to the negative electrode to the negative electrode Si is Ln, 0.3 <Ln and 2.5 By setting <Ln + Lp <5.5, a non-aqueous secondary battery having a high energy density and excellent average voltage and rate characteristics can be obtained. Here, the lithium released from the positive electrode and occluded by the negative electrode is the amount of lithium calculated from the initial charge amount when a material containing lithium that can be electrochemically released is used for the positive electrode.

  The atomic ratio Ln of lithium to be pre-doped into the negative electrode with respect to the negative electrode Si is preferably 0.3 <Ln. When a material containing lithium that can be electrochemically released is used for the positive electrode, preferably 0.8 <Ln, and more preferably 1.2 <Ln. When a material that does not contain lithium that can be electrochemically released is used for the positive electrode, preferably 2.5 <Ln, and more preferably 3.5 <Ln. When the value of Ln is less than the lower limit, the effects of the present invention such as high energy density by pre-doping and improvement of average voltage may not be obtained.

  The sum of Lp and Ln (Ln + Lp) corresponds to the total amount of lithium in the negative electrode at the completion of charging, and indicates the atomic ratio of lithium to the negative electrode Si at this time. For example, when 5% by mass of carbon is combined with SiOx (when x = 1), the atomic ratio of lithium to the negative electrode Si is 1.5, and lithium is involved in charging / discharging, so that 900 mAh / g and the atomic ratio are 2.0. The capacity becomes 1200 mAh / g. This can greatly exceed the capacity of 300 mAh / g of the existing graphite-based negative electrode material.

On the other hand, as is clear from FIG. 1 according to the example, the slope of the potential curve when SiOx lithium is released (corresponding to discharge in the battery) exceeds 0.7 V vs Li / Li ⁺ . Further, in the region of 0.7 V or more, the internal resistance against SiO occlusion / release of SiOx tends to increase. Therefore, when SiOx is used for the negative electrode, it is preferable to use a potential region of 0.7 V or less from the viewpoint of improving the average voltage and rate characteristics. The rate characteristic means battery characteristics such as capacity retention ratio and voltage behavior with respect to the reference rate when the discharge or charge current is increased (when the discharge or charge rate is increased).

In the present invention, the sum of Lp and Ln (Ln + Lp) is preferably 2.5 <Ln + Lp <5.5, and more preferably 3.3 <Ln + Lp <5.5. In this case, when the amount of lithium involved in charge / discharge is expressed as an atomic ratio with respect to the negative electrode Si, the potential region is greater than 1.5, preferably greater than 2.0, and 0.7 V vs Li / Li ⁺ or less. Since it can be used, in addition to the effect of improving the energy density, the average voltage is improved and good rate characteristics can be realized. The upper limit of Ln + Lp is related to the lithium storage capacity of SiOx. The present inventors have found that, for example, when 5% by mass of carbon is combined with SiOx (when x = 1), lithium can be occluded up to an atomic ratio of lithium to the negative electrode Si of about 4.5. When the value of x is low, that is, when the Si ratio in SiOx is high, the lithium occlusion capacity is higher, but when Ln + Lp is greater than 5.5, there is a concern that problems such as lithium deposition on the negative electrode due to charge / discharge may occur. Is done.

The positive electrode (positive electrode active material) used in the present invention is not particularly limited as long as it is a material capable of electrochemically occluding and releasing lithium. For example, lithium containing lithium that can be electrochemically released includes lithium. Composite cobalt oxide, lithium composite nickel oxide, lithium composite manganese oxide, lithium composite titanium oxide (LiTiO ₂ ), or a mixture thereof, and a system in which one or more different metal elements are added to these composite oxides Can be used. In addition, metal oxides such as manganese, vanadium, and iron, disulfide compounds, polyacene materials, activated carbon, and the like can be used as those that do not contain electrochemically releasable lithium.

  When the positive electrode is a lithium composite oxide, the above-described Lp is preferably 2.0 <Lp, and the ratio of the mass of the positive electrode active material to the mass of the negative electrode active material is controlled so as to satisfy the above relationship. It is desirable. When Lp is 2.0 or less, there is a possibility that a preferable amount of lithium involved in the above charge / discharge (1.5 or more, preferably 2.0 or more in terms of atomic ratio with respect to the negative electrode Si) cannot be accepted as the positive electrode. There is.

In the present invention, in particular, as a preferred pre-doping method for SiOx, a battery is assembled by laminating a lithium metal foil in an amount corresponding to Ln on the negative electrode, and lithium is pre-doped by injecting an electrolyte into the battery. Is mentioned. This is due to the fact that the negative electrode made of SiOx has a change in electrode volume (thickness) with the insertion and extraction of lithium. In the case of a negative electrode with little or no change in electrode volume (thickness), the lithium metal portion that disappeared becomes a gap after completion of lithium pre-doping, especially in the battery shape such as a cylindrical shape or square shape that is a hard case exterior There was a tendency for resistance to increase. Therefore, when lithium metal foil is laminated on the negative electrode surface to make electrochemical contact in the battery and lithium is predoped to SiOx, the thickness of the laminated lithium is TL, and the thickness of the negative electrode before predoping is TN. ₀ , when the thickness of the negative electrode when the battery is fully charged is TN _C , the gap is not formed if (TN _C −TN ₀ ) ≧ TL. Pre-doping by laminating metal foils can be particularly preferably applied.

As the non-aqueous electrolyte used in the present invention, a known non-aqueous electrolyte such as a non-aqueous electrolyte containing a lithium salt, a polymer electrolyte, and a polymer gel electrolyte can be used. The type of the positive electrode material and the property of the negative electrode material It is determined appropriately according to the use conditions such as the charging voltage. As the non-aqueous electrolyte containing a lithium salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, γ-butyrolactone, What was melt | dissolved in the organic solvent which consists of 1 type (s) or 2 or more types, such as methyl acetate and methyl formate, can be used. The concentration of the electrolytic solution is not particularly limited, but generally about 0.5 to 2 mol / l is practical. As a matter of course, it is preferable to use an electrolytic solution having a water content of 100 ppm or less. As used herein, the term “non-aqueous electrolyte” means a concept including a non-aqueous electrolyte and an organic electrolyte, and also includes a concept including gelled and solid electrolytes. Is.

  The shape, size, etc. of the non-aqueous secondary battery of the present invention are not particularly limited, and are of any shape and size such as a cylindrical shape, a square shape, a film battery, a box shape, etc., depending on the respective use. Should be selected.

  EXAMPLES Examples will be shown below to further clarify the features of the present invention, but the present invention is not limited to the following examples.

[Example 1]
(Trial manufacture and characteristic evaluation of SiOx negative electrode)
A negative electrode was produced using a SiOx carbon composite (manufactured by Shin-Etsu Chemical Co., Ltd .: KSC801) having the following physical properties.
SiOx: x = 1.0
Carbon composite amount of SiOx: 5% by mass with respect to SiOx
Particle size of SiOx carbon composite (D _50% ): 7.0 μm

As the binder, polyimide resin (Ube Industries, Ltd .: U-Varnish A, NMP (N-methyl-2-pyrrolidone) solution, solid content 18.1% by mass) was used. The SiOx and binder were kneaded at a mass ratio of 85:15 (solid content), and the viscosity was adjusted with NMP to obtain a slurry. The slurry was applied to a Cu foil (thickness: 18 μm) as a current collector and dried to obtain a negative electrode having a thickness of 37 μm and a density of 1.00 g / cm ³ . The electric conductivity of the negative electrode was 1.9 × 10 ⁻¹ S / cm, which was sufficient as a negative electrode for a lithium ion battery. In addition, the strength is sufficient for the following electrochemical evaluation, with no peeling or dropping off from the current collector even when wound around the shaft core (4 mmφ axis) or with acetone. It was confirmed.

Using the above-obtained SiOx electrode (electrode in which a SiOx negative electrode was formed on a current collector) as a working electrode and a tripolar cell in which the counter electrode and the reference electrode were Li metal, the unipolar characteristics of the negative electrode were evaluated. The SiOx electrode was punched to 17 mmφ, dried under vacuum at 200 ° C. for 10 hours, and then the cell was assembled in a dry box. As the electrolytic solution, a solution in which ethylene carbonate and methyl ethyl carbonate were mixed at a volume ratio of 3: 7 and LiPF ₆ was dissolved in a solvent at a concentration of 1 mol / l was used.

  The doping of lithium to SiOx (lithium occlusion) is to apply a constant voltage of 1 mV to the Li metal potential for a predetermined time after reaching 1 mV at a constant current of 1 mA to the Li metal potential at a current of 1.5 mA. It carried out by. Thereafter, dedoping (release of lithium) was performed at a constant current up to 2.0 V with respect to the Li metal potential at a current of 0.6 mA. A plot of the voltage curve at this time versus the specific capacity with respect to the mass of the SiOx carbon composite is shown in FIG. Further, Table 1 summarizes the lithium doping amount into SiOx, the specific capacity when dedoping to 2.0 V with respect to the Li metal potential, and the specific capacity when dedoping to 0.7 V.

When considering the volume change when SiOx occludes lithium, 900 mAh / g or more, preferably 1200 mAh when differentiating from existing graphite-based materials (300 mAh / g, about 2-3 times the density during charging SiOx). A capacity of at least / g is required. Therefore, from the results in Table 1, the total doping amount to SiOx (the doping amount corresponding to Ln + Lp) needs to be larger than 2.5 when expressed by the atomic ratio of lithium to the negative electrode Si (that is, Ln + Lp). Further, as apparent from FIG. 1, the slope of the potential curve at the time of releasing SiOx lithium (corresponding to discharge in the battery) exceeds 0.7 V vs Li / Li ⁺ . In addition, since the internal resistance to SiO occlusion / release of SiOx increases in a region exceeding 0.7 V, it is preferable to use a potential region of 0.7 V or less from the viewpoint of improving the average voltage and rate characteristics. From the result of 1, it is preferable that the total doping amount to SiOx (the doping amount corresponding to Ln + Lp) is larger than 3.3 in terms of the atomic ratio of lithium to the negative electrode Si (that is, Ln + Lp). In order to prevent deposition of metallic lithium on the negative electrode, the charge capacity when applied for a predetermined time at a low voltage of 1 mV with respect to the lithium metal potential is the atomic ratio of lithium to Si in the negative electrode being 4.2. It is preferable to be less than 5.5. Therefore, the sum of Lp and Lp is 2.5 <Ln + Lp <5.5, and preferably 3.3 <Ln + Lp <5.5. Ln is an atomic ratio of lithium to be negatively doped to the negative electrode with respect to the negative electrode Si. However, Ln may be compensated for the amount which is not reversible when lithium-doped to Ln + Lp = 4.2 and discharged to 0.7V. = 4.2-2.4 = 1.8, but in a material containing lithium that can be electrochemically released to the positive electrode, preferably 0.8 <Ln, more preferably 1.2 <Ln, In a material that does not contain lithium that can be electrochemically released into the positive electrode, preferably 2.5 <Ln, more preferably 3.5 <Ln.
For 0.3 <Ln, when the atomic ratio Ln of lithium to be negatively doped to the negative electrode with respect to the negative electrode Si is less than the lower limit of 0.3, the amount of pre-doping is small, and Example (FIG. 2), Comparative Example ( As shown in FIG. 3), the effects of the present invention such as high energy density by pre-doping and improvement of average voltage cannot be obtained.
Lp is the amount of lithium released from the positive electrode and occluded by the negative electrode Si. In the case of a lithium composite oxide, the amount of lithium involved in charge / discharge often does not exceed the amount of lithium released from the positive electrode. That is, in the case of 2.0 ≧ Lp, it is difficult to obtain a high energy density because the ability cannot be fully utilized even though Si has the ability to occlude and release lithium. It is preferable that 2.0 <Lp. That is, 2.0 ≧ Lp means that the negative electrode capacity is 1200 mAh / g or less.

(Trial manufacture and evaluation of non-aqueous secondary battery using SiOx negative electrode)
A positive electrode was prepared using LiNi _0.8 Co _0.2 O ₂ , which is a Ni-based positive electrode material, as an active material. A positive electrode active material, acetylene black (conductive agent), and PVdF (binder) were mixed in an NMP (N-methylpyrrolidone) solution at 92/4/4 (mass ratio) to obtain a positive electrode mixture slurry. The slurry was coated on an Al current collector foil (thickness 20 μm) as a current collector, dried, and then pressed to obtain a positive electrode having a density of 3.05 g / cm ³ and a thickness of 107 μm. The initial charge / discharge characteristics of this positive electrode single electrode were 4.3V-2.7V, a capacity of 175 mAh / g, and the initial charge / discharge efficiency was 82%.
In addition, NiNi _0.8 Co _0.2 O ₂ which is a Ni-based positive electrode material is prepared by making nickel nitrate and cobalt nitrate aqueous solution so that Ni: Co molar ratio = 80: 20, and mixing and reacting sodium hydroxide solution with the aqueous solution. The reaction product obtained was filtered and washed, followed by mixing and drying an aqueous lithium hydroxide solution in an amount corresponding to Li / (Ni + Co) = 1.05, followed by firing at 800 ° C. under oxygen. It was.

  A SiOx electrode having a thickness of 40 μm (dried at 200 ° C. under vacuum for 10 hours) and the Ni-based positive electrode (electrode having the positive electrode having a thickness of 107 μm formed on a current collector) (170 ° C. under vacuum) 10 hours dry) was combined to produce a non-aqueous secondary battery. In addition, the pre-doping to SiOx was performed in the battery by attaching Li metal having a thickness of 20 μm on the SiOx negative electrode, injecting an electrolytic solution after cell assembly, and allowing to stand for 60 hours.

The electrode facing area is 1.4 × 2.0 cm ^2, and a SiOx electrode (Li metal affixed) and a Ni-based electrode are opposed to each other through a 25 μm thick porous polyethylene on the separator, and ethylene carbonate and methyl are used as the electrolyte. Ethyl carbonate was mixed at a volume ratio of 3: 7 and impregnated with a solution of LiPF ₆ dissolved in a solvent at a concentration of 1 mol / l. After leaving for 60 hours, one of the prototyped cells was disassembled. As a result, lithium metal disappeared from the negative electrode surface. By attaching lithium to the SiOx negative electrode surface, electrochemical contact was made in the battery, and lithium was removed. It was confirmed that pre-doping is possible in SiOx. At this time, the atomic ratio Ln of lithium pre-doped to the negative electrode with respect to the negative electrode Si is 1.8, the negative electrode thickness after pre-doping is 60 μm, and the thickness corresponding to the attached Li metal (20 μm) increases. It can be seen that there is no gap due to loss of lithium metal.

  The battery fabricated as described above was charged to 4.3 V with a current of 3 mA, and then a constant current and constant voltage charge in which a constant voltage of 4.3 V was applied was performed for 8 hours. Subsequently, the battery was discharged at a constant current of 3 mA to 2.0 V (negative electrode voltage corresponding to about 0.7 V with respect to the lithium potential). The results are shown in FIG. The charge capacity was 18.0 mAh, the discharge capacity was 14.9 mAh, and the discharge average voltage was 3.60V. Moreover, it was 14.0 mAh also in the discharge capacity to 3.0V. The atomic ratio Lp of lithium released from the positive electrode and occluded by the negative electrode with respect to the negative electrode Si is 2.7, and Ln + Lp = 4.5. The initial charge / discharge efficiency is 82.8%, which is almost equal to the efficiency of the positive electrode, and the negative electrode SiOx can be used in a region up to 0.7 V with respect to the Li metal potential having excellent characteristics by pre-doping. became.

[Comparative Example 1]
A non-aqueous secondary battery was prototyped by combining the 40 μm thick SiOx electrode (200 ° C. under vacuum for 10 hours) and the 107 μm thick Ni-based electrode (170 ° C. under vacuum for 10 hours). Predoping to SiOx was not performed. The electrode facing area is 1.4 × 2.0 cm ² , and the separator is made to face a SiOx electrode (without attaching Li metal) and a Ni-based electrode through a porous polyethylene having a thickness of 25 μm. Methyl ethyl carbonate was mixed at a volume ratio of 3: 7 and impregnated with a solution of LiPF ₆ dissolved in a solvent at a concentration of 1 mol / l. At this time, the atomic ratio Ln of lithium pre-doped into the negative electrode with respect to the negative electrode Si is zero as a matter of course.

  The battery produced above was charged to 4.3 V with a current of 1 mA, and then a constant current and constant voltage charge for applying a constant voltage of 4.3 V was performed for 20 hours. Subsequently, the battery was discharged at a constant current of 2 mA to 3.0 V (negative electrode voltage corresponding to about 0.7 V with respect to the lithium potential). The results are shown in FIG. The charge capacity was 18.2 mAh, the discharge capacity was 10.6 mAh, and the discharge average voltage was 3.57V. The atomic ratio Lp of lithium released from the positive electrode and occluded by the negative electrode with respect to the negative electrode Si is 2.7, and Ln + Lp = 2.7. The initial charge / discharge efficiency is 58.2%.

[Comparative Example 2]
A non-aqueous secondary electrode prepared by combining the 30 μm thick SiOx electrode (dried at 200 ° C. under vacuum for 10 hours) and the 107 μm thick Ni-based electrode (dried at 170 ° C. under vacuum for 10 hours) produced in the same manner as above. A battery was prototyped. Predoping to SiOx was not performed. The electrode facing area is 1.4 × 2.0 cm ² , and the separator is made to face a SiOx electrode (without attaching Li metal) and a Ni-based electrode through a porous polyethylene having a thickness of 25 μm. Methyl ethyl carbonate was mixed at a volume ratio of 3: 7 and impregnated with a solution of LiPF ₆ dissolved in a solvent at a concentration of 1 mol / l. At this time, the atomic ratio Ln of lithium pre-doped into the negative electrode with respect to the negative electrode Si is zero as a matter of course.

  The battery produced above was charged to 4.3 V with a current of 1 mA, and then a constant current and constant voltage charge for applying a constant voltage of 4.3 V was performed for 20 hours. Subsequently, the battery was discharged at a constant current of 2 mA to 3.0 V (negative electrode voltage corresponding to about 0.7 V with respect to the lithium potential). The charge capacity was 17.8 mAh, the discharge capacity was 11.0 mAh, and the discharge average voltage was 3.59V. The atomic ratio Lp of lithium released from the positive electrode and stored in the negative electrode with respect to the negative electrode Si is 4.1, and Ln + Lp = 4.1. The initial charge / discharge efficiency is 61.8%.

In this example, an Ni-based oxide having an initial efficiency of 82% is used. However, this example does not limit the positive electrode of the present invention. For example, when a positive electrode having a higher initial efficiency is used, The capacity improvement effect by doping appears more remarkably. Further, when a graphite negative electrode is combined with a positive electrode equivalent to the example, the thickness of the graphite negative electrode is required to be about 125 μm (density 1.5 g / cm ³ : capacity 330 mAh / g). In the case where the total thickness of the positive and negative electrodes (positive electrode layer thickness + negative electrode layer thickness) is about 180 μm, in the case of a pre-doped SiOx battery, about 1.3 times the same volume as the graphite material. Capacity improvement can be achieved.

  The non-aqueous secondary battery of the present invention using a high-capacity material SiOx pre-doped lithium as a negative electrode material, for example, responds to the drastic increase in energy density of batteries that are desired in the field of portable devices. Can do. Further, by appropriately controlling the amount of pre-doping lithium into SiOx, it is possible to control the average voltage, internal resistance, etc. in addition to increasing the energy density.

FIG. 6 shows the single-electrode lithium doping / dedoping characteristics of the negative electrode in Example 1 of the present invention, where (a) is doped to 1200 mAh / g, (b) is doped to 1800 mAh / g, (c ) Shows the case of doping up to 2500 mAh / g. It is a figure which shows the initial stage charge / discharge characteristic of this invention battery in Example 1 of this invention. It is a figure which shows the initial stage charge / discharge characteristic of the comparative battery in the comparative example 1 of this invention.

Claims (7)

  In a nonaqueous secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, the positive electrode is made of a material capable of electrochemically inserting and extracting lithium, and the negative electrode is made of SiOx (0.3 ≦ x ≦ 1.6). A non-aqueous secondary battery comprising a material pre-doped with lithium.   When the atomic ratio of lithium released from the positive electrode and occluded by the negative electrode to the negative electrode Si is Lp, and the atomic ratio of lithium predoped to the negative electrode to the negative electrode Si is Ln, 0.3 <Ln and 2.5 <Ln + Lp The nonaqueous secondary battery according to claim 1, wherein <5.5.   3. The nonaqueous secondary battery according to claim 2, wherein the positive electrode is a lithium composite oxide, and the atomic ratio Lp of lithium released from the positive electrode and occluded by the negative electrode to the negative electrode Si is 2.0 <Lp.   4. The lithium is pre-doped in SiOx (0.3 ≦ x ≦ 1.6) by bringing the negative electrode and metallic lithium into electrochemical contact within the battery. The non-aqueous secondary battery according to claim 1.   5. The non-doped lithium according to claim 4, wherein lithium is pre-doped in SiOx (0.3 ≦ x ≦ 1.6) by attaching lithium to the negative electrode surface and bringing it into electrochemical contact within the battery. Water-based secondary battery. (TN _C −TN ₀ ) ≧ TL, where TL is the thickness of the lithium to be bonded, TN _{0 is} the thickness of the negative electrode before pre-doping, and TN _C is the thickness of the negative electrode when the battery is fully charged. The non-aqueous secondary battery according to claim 5.   The nonaqueous secondary battery according to any one of claims 1 to 6, wherein the negative electrode is a composite of a carbon material on the surface of SiOx at 1 to 50% with respect to the mass of SiOx.

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