JP2013197012A - Anode for lithium ion secondary battery, lithium ion secondary battery, and vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anode for a lithium ion secondary battery in which an irreversible capacity of a Li in a counter electrode is reduced without complicate pre-doping, and to provide a lithium ion secondary battery and a vehicle which employs the anode.SOLUTION: An anode for a lithium ion secondary battery contains SiOand LiAlO.

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery having the negative electrode, and a vehicle equipped with the lithium ion secondary battery.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and extracting lithium (Li) in a positive electrode and a negative electrode, respectively. And it operate | moves because Li ion moves in the electrolyte solution provided between both electrodes.

  In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. Yes.

  The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material that forms an active material is being actively conducted. For example, silicon or silicon oxide having a higher capacity than carbon has been studied as a negative electrode active material.

  By using silicon as the negative electrode active material, a battery having a higher capacity than that using a carbon material can be obtained. However, silicon has a large volume change due to insertion and extraction of Li during charge and discharge. Therefore, there is a problem that silicon is pulverized and falls off or peels off from the current collector, and the charge / discharge cycle life of the battery is short.

  By using silicon oxide as the negative electrode active material, it is possible to suppress volume change associated with insertion / extraction of Li during charge / discharge rather than silicon.

For example, the use of silicon oxide (SiO _x : x is 0.5 ≦ x ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _x decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. . The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution. Therefore, a lithium ion secondary battery using a negative electrode active material obtained by decomposing SiO _x into Si and SiO ₂ has excellent cycle characteristics.

However, when SiO ₂ occludes lithium ions, it forms a stable compound and does not release lithium ions. Therefore, there is a problem that the lithium ion component that is not released from the SiO ₂ becomes an irreversible capacity.

As a countermeasure for this initial irreversible capacity, an electrode formation method in which the irreversible capacity is electrochemically charged in advance has been attempted. The electrode formation method is, for example, a method of assembling a half cell using metallic lithium as a counter electrode and electrochemically doping lithium. In addition, Patent Document 1 below discloses a negative electrode containing a material in which SiO _x is pre-doped with lithium by electrochemically contacting the negative electrode and metallic lithium in a battery.

  However, the pre-doping described above is complicated and extremely poor in productivity because the electrode is once charged and then reassembled as a battery.

Further, when the lithium ion secondary battery is charged, the electrolyte contained in the electrolytic solution is partially reduced and decomposed on the surface of the negative electrode to generate a decomposition product. The decomposition product coats the surface of the negative electrode active material particles to form a film. This coating is called a solid electrolyte interface coating (SEI: Solid Electrolyte Interface). This SEI contains LiF, LiCO ₃ or the like as a main component. LiF, LiCO _{3 and the} like contained in the SEI are irreversible substances, and Li contained in the SEI has an irreversible capacity. When the negative electrode active material particles contain Si, the volume change of the negative electrode active material particles also changes because Si has a large volume change due to insertion and extraction of Li during charge and discharge. The SEI formed on the surface of the negative electrode active material particles repeats generation and collapse as the volume of the particles changes. Since Li is consumed to generate SEI, Li is consumed more and more by repeating charge and discharge. The greater the amount of Si in the negative electrode active material, the greater the amount of Li consumed by SEI.

JP 2009-76372 A

  The present invention has been made in view of the current state of the prior art described above, and its main object is to reduce the amount of lithium at the counter electrode that becomes irreversible capacity without using complicated pre-doping. The negative electrode for use, the lithium ion secondary battery which has the negative electrode, and the vehicle carrying a lithium ion secondary battery are provided.

As a result of diligent research, the present inventors put a stable lithium compound serving as a lithium source in the negative electrode material in advance, and occludes lithium of the lithium compound into SiO ₂ , whereby the lithium occlusion reaction of SiO ₂ is achieved. It has been found that the amount of Si generated by the above can be suppressed, the generation amount of SEI formed in the Si can be suppressed, and the amount of lithium contained in the counter electrode during the charging can be reduced by occluding the irreversible capacity.

That is, the negative electrode for a lithium ion secondary battery of the present invention includes SiO ₂ and LiAlO ₂ . The negative electrode contains Li ₄ SiO ₄ and Al after initial lithium charging. In addition the initial lithium _Li 4 SiO ₄ and Al after charging molar ratio of 2: 1 to 1: is preferably contained at a ratio of 2.

  The lithium ion secondary battery of the present invention has the above-described negative electrode for a lithium ion secondary battery. The vehicle of the present invention is equipped with the lithium ion secondary battery of the present invention.

By including SiO ₂ and LiAlO ₂ , the negative electrode for a lithium ion secondary battery of the present invention can occlude lithium derived from LiAlO ₂ in SiO ₂ during charging, suppressing the occurrence of SEI, The irreversible capacity of lithium contained in the counter electrode can be reduced.

  Therefore, by including the negative electrode for a lithium ion secondary battery, a high output is possible and a lithium ion secondary battery having excellent cycle characteristics can be obtained. Further, since the vehicle of the present invention is equipped with the lithium ion secondary battery, it can be a vehicle equipped with a battery having excellent cycle characteristics and capable of higher output.

(Anode for lithium ion secondary battery)
The negative electrode for a lithium ion secondary battery of the present invention contains SiO ₂ and LiAlO ₂ .

SiO ₂ does not function as an active material, but has a function of suppressing decomposition of the electrolytic solution. Therefore, a lithium ion secondary battery having a negative electrode for a lithium ion secondary battery containing SiO ₂ is excellent in cycle characteristics.

SiO ₂ may be contained in the negative electrode. Commercially available SiO ₂ may be added to the negative electrode, or SiO ₂ prepared by the following method may be contained in the negative electrode.

When commercially available SiO is used as the negative electrode active material raw material, SiO ₂ can be formed as shown in the following formula 1 by heat treatment of SiO.
(Formula 1) 2SiO → Si + SiO ₂

In the reaction of Formula 1, silicon oxide (SiO _x : x is 0.5 ≦ x ≦ 1.5) (this is referred to as SiO), which is a homogeneous solid with an atomic ratio of Si and O of approximately 1: 1. This is a disproportionation reaction that separates into two phases of Si phase and SiO ₂ phase by reaction inside the solid. In general, when oxygen is turned off, it is said that almost all SiO is disproportionated and separated into two phases at 800 ° C. or higher. Specifically, non-crystalline SiO powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as in a vacuum or in an inert gas. _A silicon oxide-based powder containing two phases of _two phases and a crystalline Si phase is obtained.

SiO ₂ occludes Li in the initial lithium charging process as shown in the following formulas 2 and _3, and becomes Li ₂ SiO ₃ through Li ₂ Si ₂ O ₅ .
(Equation _{2) SiO 2 + 4 / 5Li} = 2 / 5Li 2 Si 2 O 5 + 1 / 5Si
(Equation _{3) Li 2 Si 2 O 5} + 4 / 3Li = 5 / 3Li 2 SiO 3 + 1 / 3Si

Here, the initial lithium charging process refers to the first time the battery is assembled using the electrode and charged. It is known that in the first charge, SiO ₂ occurs promptly at 1 V (vs Li / Li ⁺ ) or more until the reaction of SiO ₂ → Li ₂ Si ₂ O ₅ → Li ₂ SiO ₃ due to the occlusion of Li.

Further, when Li is occluded, it changes as Li ₂ SiO ₃ → Li ₄ SiO ₄ as shown in the following formula 4.
(Formula _{4) Li 2 SiO 3 + Li} = 3 / 4Li 4 SiO 4 + 1 / 4Si

This reaction of Li ₂ SiO ₃ → Li ₄ SiO ₄ occurs at 1 V or less.

Here, if the lithium until _Li 4 SiO ₄ is _occluded, Li 4 SiO ₄ will no longer release lithium. Therefore, when SiO ₂ occludes Li from the counter electrode during initial charging, the amount of lithium in the counter electrode occluded until SiO ₂ becomes Li ₄ SiO ₄ becomes an irreversible capacity. Further, as seen in the above (Formula 2), (Formula 3), and (Formula 4), Si is generated during the Li storage reaction.

The negative electrode of the present invention contains LiAlO ₂ . LiAlO _{2 reacts} with _Li 2 SiO ₃ Li 2 a SiO _{3 Li 4} SiO 4 and to it is possible. The reaction in which LiAlO ₂ described above reacts with Li ₂ SiO ₃ to form Li ₄ SiO ₄ is a reaction represented by the following formula 5.
(Formula 5): Li ₂ SiO ₃ + 1 / 2LiAlO ₂ + 3 / 2Li → Li ₄ SiO ₄ + 1 / 2Al

  As can be seen from the reaction of Formula 5, when compared with the reaction of Formula 4, Si is not generated after the reaction. When the lithium ion secondary battery is charged, SEI is generated on the Si surface. As the amount of Si increases, the amount of SEI increases. Once SEI is generated, Li contained in SEI becomes an irreversible capacity. Therefore, if the amount of Si increases, the irreversible capacity of Li increases.

  In addition, since Si has a large volume expansion / contraction during charging / discharging of the lithium ion secondary battery, the SEI formed on the Si surface collapses due to the expansion / contraction of the Si volume. When SEI collapses and a new Si surface is generated, new SEI is formed there again. In this way, SEI increases by repeatedly decaying and generating. Therefore, by repeating charge and discharge, the amount of lithium used for SEI generation increases more and more. Therefore, the effect that the irreversible capacity of Li increases as the amount of Si increases increases remarkably by repeating charging and discharging.

  According to Equation 5, Si is not generated after the reaction, and Al is generated. Although SEI is also generated on the Al surface, it is considered that SEI once formed on the Al surface does not collapse because Al has little volume expansion and contraction.

  The reaction shown in Formula 5 can be confirmed to occur by the “first-principles calculation” described below. First-principles calculations can determine the crystal structure and electronic state of a substance without referring to experimental values. The generation energy (ΔH) is obtained by the first principle calculation. It is known that the value of ΔH obtained by the first principle calculation is not significantly different from the experimental value. If the value of ΔH is negative, a reaction according to the reaction formula occurs.

  From Formula 2, Formula 3, Formula 4, and Formula 5 above, compared to the amount of lithium in the counter electrode that becomes irreversible capacity when reacted according to Formula 2, Formula 3, and Formula 4, Formula 2, Formula 3, Formula 5, and It turned out that the amount of lithium of the counter electrode used as an irreversible capacity | capacitance can be reduced by reacting.

Commercially available LiAlO ₂ can be used. LiAlO ₂ is preferably in powder form. A preferable particle diameter of LiAlO ₂ is 0.1 μm or more and 10 μm or less.

Raw material preparation, the active material powder containing SiO _2, may be prepared a mixed raw material powder containing a LiAlO ₂ powder, a. Prior to preparation of the raw material, the active material powder containing SiO ₂ is preferably classified (sieved) to 10 μm or less, and further to 8 μm or less. Similarly, the LiAlO ₂ powder may be classified to 10 μm or less, and further to 5 μm or less.

In order to reduce the irreversible capacity of the counter electrode lithium, unreacted Li ₂ SiO ₃ is not left. Therefore, LiAlO ₂ is contained in the negative electrode in a larger amount than the molar ratio of the stoichiometric ratio. If specifically defined, the molar ratio of SiO ₂ and LiAlO _{2 are, SiO 2: LiAlO 2 = 3} : 1~1: only to be mixed at a ratio of 4.

LiAlO ₂ is preferably in contact with the SiO ₂ surface. This is because if LiAlO ₂ is in contact with the SiO ₂ surface, Li is likely to move from LiAlO ₂ to SiO ₂ . Desirably, LiAlO ₂ preferably covers at least a part of the SiO ₂ surface.

The negative electrode contains Li ₄ SiO ₄ and Al after initial lithium charging. Li ₄ SiO ₄ and Al are preferably contained in a molar ratio of 2: 1 to 1: 2.

  Other components of the negative electrode for a lithium ion secondary battery of the present invention are not particularly limited, and known ones can be used. For example, a negative electrode for a lithium ion secondary battery includes a current collector and an active material layer bound on the current collector. The active material layer is prepared by applying an active material, a conductive additive, a binder resin, and, if necessary, an appropriate amount of an organic solvent, mixing and slurrying, and applying the mixture onto the active material and curing the binder resin. be able to.

  A current collector is a chemically inert electronic high conductor that keeps current flowing through an electrode during discharging or charging. The current collector can adopt a shape such as a foil or a plate, but is not particularly limited as long as it has a shape according to the purpose. As the current collector, for example, a copper foil or an aluminum foil can be suitably used.

  An active material is a substance that directly contributes to electrode reactions such as charge reaction and discharge reaction.

Si, SiO _x (x is 0.5 ≦ x ≦ 1.5), Si compound, carbon, Sn, Sn compound, etc. can be used as the active material of the negative electrode for the lithium ion secondary battery of the present invention.

  The conductive assistant is added to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used alone or in combination of two or more as conductive aids. Can be added.

  The amount of the conductive aid used is not particularly limited, but can be, for example, about 1 to 100 parts by mass with respect to 100 parts by mass of the active material.

  The binder resin is used as a binder for binding the active material and the conductive additive to the current collector. The binder resin of the present invention is not particularly limited. The binder resin is required to bind the active material and the like in as small an amount as possible, and the amount is desirably 0.5 wt% to 50 wt% of the total of the active material, the conductive additive, and the binder resin. Examples of binder resins include fluorine polymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), rubbers such as styrene butadiene rubber (SBR), imide polymers such as polyimide, alkoxysilyl group-containing resins, poly Acrylic acid and epoxy polymer can be used.

  The negative electrode for a lithium ion secondary battery of the present invention can be produced as follows. The manufacturing method of the negative electrode for lithium ion secondary batteries has an application | coating process and a hardening process.

  The application process is a process of applying a binder resin, an active material, and a conductive additive to the surface of the current collector. The negative electrode active material used in the present invention is desirably in the form of powder. The active material is applied and bound to the surface of the current collector via a binder resin. The powder shape preferably has a particle size of 10 μm or less.

  In the coating step, a binder resin, an active material, and a conductive additive are mixed in advance, and a solvent or the like is added to form a slurry, which can be applied to the current collector. As a coating method, a coating method generally used when producing an electrode for a lithium ion secondary battery, such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method, can be used.

  The coating process is a process in which a binder resin, an active material, and a conductive additive are mixed to form a slurry, and the slurry is applied to the current collector surface.

  The mixing ratio of the binder resin of the electrode layer, the active material, and the conductive additive is preferably active material: conductive auxiliary agent: binder resin = 50: 5: 45 to 98: 1: 1. The coating thickness of the electrode layer is preferably 10 μm to 50 μm.

  The curing step is a step of curing the binder resin to bind the active material to the current collector. The binder resin may be cured according to the curing conditions of the binder resin to be used.

(Lithium ion secondary battery)
The lithium ion secondary battery of this invention has the said negative electrode. As the lithium ion secondary battery using the negative electrode for a lithium ion secondary battery described above, known positive electrodes, electrolytes, and separators that are not particularly limited can be used.

  The positive electrode may be anything that can be used in a lithium ion secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer includes a positive electrode active material and a binder, and may further include a conductive additive. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

Specifically, examples of the positive electrode active material include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₃ , and S. The current collector is not particularly limited as long as it is generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel. As the conductive auxiliary agent, the same ones as described in the above negative electrode can be used.

  As the electrolyte, an electrolytic solution in which a lithium metal salt that is an electrolyte is dissolved in an organic solvent or a polymer gel electrolyte composed of a polymer gel containing an electrolyte solution can be used.

  As the organic solvent, from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), etc. One or more selected can be used.

As an electrolyte to be dissolved in an organic solvent, a lithium metal salt that is soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , LiCF ₃ SO ₃ can be used.

For example, a lithium metal salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ or the like in an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate is about 0.5 mol / l to 1.7 mol / l. A solution dissolved at a concentration can be used.

  The polymer gel electrolyte is composed of a polymer gel containing an electrolyte solution. As the polymer gel, it is preferable to use a prepolymer TA210 (polyfunctional acrylate polymer having a polyoxyalkylene chain) that is polymerized with a photopolymerization initiator (for example, IRGACURE 184), and acrylonitrile and methyl acrylate or methacrylic acid. Copolymers may be used. The polymer gel electrolyte can be obtained by immersing the polymer in the electrolyte solution or polymerizing the polymer structural units (monomer / compound) in the presence of the electrolyte solution.

  A separator will not be specifically limited if it can be used for a lithium ion secondary battery. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

  The lithium ion secondary battery of the present invention is not particularly limited in shape, and various shapes such as a cylindrical shape, a square shape, and a coin shape can be adopted. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with an electrolytic solution to form a battery. The lithium ion secondary battery of the present invention can be assembled according to a conventional method.

  The lithium ion secondary battery of the present invention described above is charged with nighttime power in the field of portable devices such as mobile phones and laptop computers, information-related devices, and supplies power to houses, factories or offices in the daytime. It can also be suitably used in the field of stationary and automobiles such as a battery or a battery that is charged with a solar battery in the daytime and fed at night. For example, if this lithium ion secondary battery is mounted on a vehicle, the lithium ion secondary battery can be used as a power source for an electric vehicle.

(vehicle)
The vehicle of the present invention is equipped with the lithium ion secondary battery. The vehicle of the present invention can be equipped with a lithium ion secondary battery having higher cycle performance and excellent cycle characteristics, and can be a high-performance vehicle. The vehicle may be a vehicle that uses electric energy from a battery as a whole or a part of a power source. For example, an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, a forklift, an electric wheelchair, an electric assist. Bicycles and electric motorcycles are examples.

  As mentioned above, although the negative electrode for lithium ion secondary batteries of this invention, the lithium ion secondary battery using the negative electrode, and embodiment of a vehicle were demonstrated, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<First principle calculation>
In the first-principles calculation, an electronic state calculation program based on the density functional theory using the ultrasoft pseudopotential was used as the calculation program.

  As the exchange / correlation energy necessary for the density functional method, a generalized density gradient correction (GGA) was applied. Although the density functional method is used in the present invention, the calculation method used is not limited to the density functional method, and a method that can predict the electronic state of a substance with high accuracy by first-principles calculation is used. I can do it.

First-principles calculations can determine the crystal structure and electronic state of a substance without referring to experimental values. Here, for the formula 5: Li ₂ SiO ₃ + 1 / 2LiAlO ₂ + 3 / 2Li → Li ₄ SiO ₄ + 1 / 2Al, the generation energy (ΔH) is obtained by the first principle calculation. It is known that the value of ΔH obtained by the first principle calculation is not significantly different from the experimental value.

  If the value of ΔH is negative, a reaction according to the reaction formula occurs. ΔH in the above formula 5 was −20 kJ / mol·Li from the first principle calculation. Since ΔH is a negative value, it was found that the reaction represented by Formula 5 occurs.

As a comparison, first-principles calculation of the reaction of Formula 6 was performed.
(Equation _{6): SiO 2 + 1 /} 2Li 2 MoO 4 → 1 / 2Li 2 Si 2 O 5 + 1 / 2MoO 2

  In this reaction, ΔH was a positive value of 26.7 kJ / mol·Li, and it was found that the reaction of Formula 6 did not occur.

Claims (5)

SiO ₂ and
LiAlO ₂ and
A negative electrode for a lithium ion secondary battery. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode contains Li ₄ SiO ₄ and Al after initial lithium charging. The negative electrode for a lithium ion secondary battery according to claim 2, wherein the Li ₄ SiO ₄ and the Al are included in a molar ratio of 2: 1 to 1: 2 after initial lithium charging.   The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of any one of Claims 1-3.   A vehicle equipped with the lithium ion secondary battery according to claim 4.