JP2011034919A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having an oxide-based electrode and improved in charge/discharge characteristics. <P>SOLUTION: The lithium secondary battery includes a positive electrode, a negative electrode and a nonaqueous electrolyte. At least one of the positive electrode and the negative electrode is the oxide-based electrode performing a reaction of reducing an oxide of a transition metal to form lithium oxide and its reverse reaction as electrode reactions associated with charge and discharge. The nonaqueous electrolyte contains a nonaqueous solvent and boric acid ester as an anion receptor. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery provided with an oxide-based electrode.

  There is known a lithium secondary battery that includes positive and negative electrodes having a material (active material) capable of reversibly occluding and releasing lithium (Li), and charging and discharging as lithium ions move between the electrodes. Yes. Currently, graphite is widely used as an active material for negative electrodes. However, graphite intercalates 1 Li per 6 carbon atoms, so its charge / discharge capacity has an upper limit of 372 mAh / g.

Therefore, various active materials that are expected to be capable of realizing a charge / discharge capacity higher than that of graphite have been studied. As an example of such an active material, a composition containing iron oxide (an oxide containing iron as a constituent element) such as Fe ₂ O ₃ can be given.

JP 2003-257426 A JP 2002-313321 A Japanese Patent Application Laid-Open No. 2004-0779463 JP 2008-300168 A

  However, it is known that an oxide-based electrode (for example, an iron oxide-based electrode) is a substance that usually has low reversibility of charge / discharge and is difficult to apply to a secondary battery. Accordingly, an object of the present invention is to provide a lithium secondary battery including an oxide-based electrode and having improved charge / discharge characteristics. Another related object is to provide an additive for an electrolytic solution that can be added to the electrolytic solution of a non-aqueous secondary battery including an oxide electrode to improve the charge / discharge characteristics of the battery.

  According to the present invention, a lithium secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte is provided. In the lithium secondary battery, at least one of the positive electrode and the negative electrode (for example, the negative electrode) is a reaction that generates a lithium oxide by reducing an oxide of a transition metal (for example, iron) as an electrode reaction accompanying charging and discharging. This is an oxide-based electrode that performs the reverse reaction. The nonaqueous electrolytic solution contains a nonaqueous solvent and a borate ester as an anion receptor.

  According to the lithium secondary battery having such a structure, the reversibility of the electrode reaction in the oxide-based electrode is improved by adding the boric acid ester to the non-aqueous electrolyte, and consequently the charge / discharge characteristics of the battery including the electrode. (For example, irreversible capacity can be reduced).

  In this specification, the term “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and charges and discharges by the movement of lithium ions between the positive and negative electrodes (typically, metallic lithium is applied to the electrodes). That is not used).

As the borate ester, a compound represented by the following formula (1) can be preferably employed.
B (OR ¹ ) ₃ (1)
Here, R ¹ in formula (1) is independently selected from an alkyl group having 1 to 6 carbon atoms and a fluoroalkyl group having 1 to 6 carbon atoms. According to such boric acid ester, the reversibility of the electrode reaction in the oxide-based electrode (for example, iron oxide-based electrode) can be improved more effectively.

  Moreover, the technique disclosed here can be preferably applied to, for example, a lithium secondary battery including a nonaqueous electrolytic solution in which the nonaqueous solvent is a solvent having a carbonate-based solvent as a main component. According to the nonaqueous electrolytic solution having such a composition, the reversibility of the electrode reaction in an oxide electrode (for example, an iron oxide electrode) can be more effectively improved.

  The amount of the boric acid ester used in the non-aqueous electrolyte may be set so that the reversibility of the electrode reaction is improved by containing the boric acid ester (for example, the irreversible capacity is reduced). preferable. For example, a preferable result can be realized by using the borate ester at a ratio of about 0.01 to 1 part by mass with respect to 100 parts by mass of the other electrolyte components (that is, other than the borate ester). .

  The technology disclosed herein can be preferably applied to a lithium secondary battery including an oxide-based electrode (that is, an iron oxide-based electrode) in which the transition metal is iron. As described above, the iron oxide-based electrode is expected to be able to realize a charge / discharge capacity higher than that of graphite, but it has been strongly desired to improve the reversibility of the electrode reaction. Therefore, in a battery including an iron oxide-based electrode, it is particularly significant to apply the technology disclosed herein.

  The technology disclosed herein includes both a lithium secondary battery that includes the oxide-based electrode as a negative electrode and a lithium secondary battery that includes the oxide-based electrode as a positive electrode. A lithium secondary battery having the oxide-based electrode as a negative electrode is more preferable. Such a form of lithium secondary battery can be a battery having a higher operating voltage. Further, the active material of the counter electrode (that is, the positive electrode) can be selected from a wider range, which is preferable.

  Since the lithium secondary battery disclosed herein can be excellent in charge / discharge characteristics (for example, reversibility of charge / discharge) as described above, it is suitable as a lithium secondary battery mounted on a vehicle. For example, it can be suitably used as a power source for a motor (electric motor) of a vehicle such as an automobile in the form of an assembled battery in which a plurality of lithium secondary batteries are connected in series. Therefore, according to the present invention, there is provided a vehicle equipped with any lithium secondary battery disclosed herein (which may be a lithium secondary battery manufactured by any method disclosed herein).

According to the present invention, there is also provided an electrolyte additive for addition to a non-aqueous electrolyte of a secondary battery equipped with an oxide-based electrode. Here, the oxide-based electrode is an electrode that performs a reaction of reducing an oxide of a transition metal (for example, iron) to generate lithium oxide and an opposite reaction as an electrode reaction accompanying charging and discharging. The additive for electrolytic solution comprises a borate ester represented by the following formula (2).
B (OR ¹ ) ₃ (2)
Here, R ¹ in formula (2) is independently selected from an alkyl group having 1 to 6 carbon atoms and a fluoroalkyl group having 1 to 6 carbon atoms.

  Such an additive for electrolytic solution is added to the electrolytic solution of a non-aqueous secondary battery having an oxide-based electrode (that is, a secondary battery having a non-aqueous electrolytic solution, typically a lithium secondary battery). The reversibility of the electrode reaction in the oxide electrode can be improved. Thereby, the charge / discharge characteristic of a secondary battery provided with the said oxide type electrode can be improved (for example, irreversible capacity | capacitance is reduced).

It is a characteristic view which shows the relationship between the addition amount of boric acid ester, and Coulomb efficiency. It is a schematic diagram which shows the structure of the lithium secondary battery which concerns on one Embodiment. It is a fragmentary sectional view which shows typically the coin cell produced for performance evaluation. 1 is a schematic side view showing a vehicle (automobile) including a lithium secondary battery according to an embodiment.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The technology disclosed herein can be applied to various lithium secondary batteries including an oxide-based electrode and a non-aqueous electrolyte. The oxide-based electrode performs a reaction of electrochemically reducing an oxide of a transition metal to generate lithium oxide and an inverse reaction thereof as an electrode reaction accompanying charge / discharge (that is, occlusion / release of lithium). In other words, an electrode reaction is performed in which oxygen is exchanged between the transition metal and lithium. Examples of the transition metal include metals that do not alloy with lithium, such as iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Such an oxide of a transition metal can function as an electrode active material by occluding Li by the following reaction formula (3) and releasing Li by a reaction in the opposite direction (leftward).
M _x O _y +2 _y Li ⁺ +2 _y e ⁻ → xM + yLi ₂ O (3)

Among these, an electrode including iron oxide (iron oxide-based electrode) is preferable. Here, the “iron oxide-based electrode” refers to an electrode including iron oxide (Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, etc.). An iron oxide-based electrode having an iron oxide containing at least a crystal of Fe ₂ O ₃ (typically α-Fe ₂ O ₃ ) is particularly preferable. The presence of the Fe ₂ O ₃ crystal can be confirmed by general X-ray diffraction measurement (XRD). According to such iron oxide, a battery exhibiting better charge / discharge characteristics (for example, coulomb efficiency) can be realized.

  An oxide-based electrode (preferably an iron oxide-based electrode) disclosed herein may have a transition metal oxide as described above in the form of particles. For example, an electrode mixture layer containing a transition metal oxide powder (for example, iron oxide powder), a binder, and a conductive material used as necessary (preferably, the transition metal oxide is the main component, that is, 50% by mass. An oxide electrode having a configuration in which a composite layer containing the above components is held on a conductive substrate can be used. The composite material layer can be formed, for example, by applying a liquid composition in which a composite material layer forming component is dispersed or dissolved in an appropriate solvent and drying the composition. Alternatively, it may be an oxide-based electrode formed by molding the composite layer forming component into a desired shape (plate shape, pellet shape, etc.).

As the transition metal oxide powder (for example, Fe ₂ O ₃ powder), one having an average particle diameter in the range of about 10 nm to 50 μm (more preferably about 30 nm to 10 μm) can be preferably used. According to the transition metal oxide powder having such an average particle size, a battery exhibiting better charge / discharge characteristics can be realized. Moreover, it is preferable that the said transition metal oxide powder does not contain the particle | grains with a particle size exceeding 50 micrometers, and it is still more preferable not to contain the particle | grains with a particle size exceeding 10 micrometers. Thus, according to the transition metal oxide powder which does not substantially contain excessively large particles, a battery exhibiting better charge / discharge characteristics can be realized.

The oxide-based electrode disclosed herein may have the transition metal oxide in the form of a film. For example, an oxide-based electrode having a configuration in which a film containing the transition metal oxide as a main component (which may be a film substantially composed of the transition metal oxide) is provided on the surface of a conductive substrate. obtain. As a method for forming the transition metal oxide film (for example, an iron oxide film), various conventionally known film forming methods such as a plating method and a vapor deposition method can be used alone or in appropriate combination. Of these, the vapor deposition method can be preferably used. The concept of the vapor deposition method here includes various vapor deposition methods such as physical vapor deposition (PVD method, for example, sputtering method), chemical vapor deposition method (CVD method, for example, plasma CVD method), and reactive vapor deposition method. Formation of the iron oxide film by such a vapor deposition method is typically performed under reduced pressure conditions (for example, a pressure of about 10 ⁻³ Pa to 10 ⁻⁵ Pa) (that is, a vacuum vapor deposition method). The thickness of the transition metal oxide film can be, for example, about 0.1 μm to 30 μm, and is preferably in the range of about 0.2 μm to 10 μm (for example, about 0.5 μm to 5 μm). According to the electrode provided with the transition metal oxide film having such a thickness, a battery exhibiting better charge / discharge characteristics can be realized.

  As the conductive substrate, a conductive member having the same material and shape as a current collector of a general lithium secondary battery electrode can be used. For example, a rod-like body, a plate-like body, a sheet-like (foil-like) body, a net-like body, etc. mainly composed of a conductive material (typically a metal material) such as copper (Cu), titanium (Ti), and stainless steel. Can be used. A sheet-like conductive substrate (typically a metal foil) can be preferably used because it is easy to form the electrode mixture layer or the transition metal oxide film, and to be easily applied to a high-capacity battery. . The thickness of the sheet-like substrate is not particularly limited, but a preferable thickness is 1 μm to 100 μm (more preferably 5 μm to 50 μm) in view of balance between the capacity density of the battery and the strength of the substrate.

  As the conductive material, a carbon material such as carbon black (for example, acetylene black) or graphite powder can be preferably used, like the conductive material in the electrode of a general lithium secondary battery. As the binder (binder), polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), or the like can be used. Although it does not specifically limit, the usage-amount of the electrically conductive material with respect to 100 mass parts of transition metal oxides can be 1-20 mass parts (preferably 5-15 mass parts), for example. Moreover, the usage-amount of a binder with respect to 100 mass parts of transition metal oxides can be 0.5-10 mass parts, for example.

The nonaqueous electrolytic solution in the technology disclosed herein is characterized by containing at least a nonaqueous solvent and an anion receptor as constituent components. Here, the “anion receptor” refers to a compound capable of exerting a function of attracting anions and promoting dissociation of a salt. Typical examples of anion receptors include various Lewis acids. Of these, borate esters are exemplified as preferred anion receptors. For example, an organic boric acid compound represented by the following formula (1) can be preferably used.
B (OR ¹ ) ₃ (1)
Here, R ¹ in the formula (1) can be each independently an alkyl group having 1 to 6 carbon atoms (typically 2 to 4). Further, a group (fluoroalkyl group) having a structure in which at least one of hydrogen atoms (H) constituting the alkyl group is replaced with a fluorine atom (F) may be used. Preferable examples of the fluoroalkyl group include a persulfoalkyl group, a group in which all hydrogen atoms bonded to terminal carbon atoms are replaced with fluorine atoms (for example, —CH ₂ CH ₂ CF ₃ ) and the like. R ¹ may be linear or branched. In one preferred embodiment, R ¹ is a linear alkyl group (eg, n-propyl group). Moreover, the compound whose three R < ¹ > contained in the said Formula (1) is the same group can be employ | adopted preferably.

  As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. For example, in addition to carbonates such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2- Diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. One kind or two or more kinds selected from non-aqueous solvents that are generally known as those that can be used for an electrolytic solution of a lithium secondary battery can be used. The technique disclosed here is a non-aqueous electrolyte in which the non-aqueous solvent is a solvent mainly composed of a carbonate-based solvent (that is, a non-aqueous electrolyte in which 50% by volume or more of the non-aqueous solvent is a carbonate). And it can apply especially preferably with respect to the battery provided with this non-aqueous electrolyte.

The non-aqueous electrolyte typically further includes a lithium salt as a supporting electrolyte (supporting salt) as a constituent component. For example, LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiClO _One or two or more selected from various lithium salts known to be capable of functioning as a supporting electrolyte in the electrolyte solution of a lithium secondary battery, such as No. ₄ , can be used. LiPF ₆ is particularly preferable. The concentration of the supporting electrolyte is not particularly limited, and can be the same as, for example, the electrolytic solution used in a conventional lithium secondary battery. Usually, a nonaqueous electrolytic solution containing a supporting electrolyte at a concentration of about 0.1 mol / L to 5 mol / L (for example, about 0.8 mol / L to 1.5 mol / L) is preferable.

  The content of the anion receptor (typically boric acid ester) in the non-aqueous electrolyte can be appropriately set according to the configuration of the battery (for example, the composition of the non-aqueous solvent). It is preferable to contain an amount capable of appropriately exerting the effect of adding the anion receptor (typically, the effect of improving the reversibility of the electrode reaction). For example, the anion receptor is used such that the Coulomb efficiency β% of the battery containing the anion receptor is larger than the Coulomb efficiency α% of the battery using the non-aqueous electrolyte containing no anion receptor (α <β). It is advisable to contain a receptor. In a preferred embodiment, the use of an anion receptor increases the Coulomb efficiency by 3% or more (ie, β-α ≧ 3%, such as β-α ≧ 3 to 20%), more preferably 5% or more. (That is, β-α ≧ 5%, for example, β-α ≧ 5-15%), the content of the anion receptor is set. Here, as the coulomb efficiency, a value of coulomb efficiency in the first charge / discharge after battery construction can be adopted. Such Coulomb efficiency can be measured using, for example, a half cell having an oxide electrode as a working electrode and metallic lithium as a counter electrode, as described in an experimental example described later. The half cell was subjected to an operation of inserting Li into the working electrode until the interelectrode voltage reached 0.01 V (lower limit voltage) at a constant current of 0.1 C, and then the interelectrode voltage at a constant current of 0.1 C. By performing an operation of releasing Li until the voltage reaches 3.0 V (upper limit voltage), the Coulomb efficiency can be calculated from the ratio of the Li storage capacity and the Li release capacity at this time.

  For example, when a boric acid ester is used as the anion receptor, the content thereof can be about 5 parts by mass or less with respect to 100 parts by mass of the electrolyte component other than the boric acid ester. In a preferred embodiment of the technology disclosed herein, the content of the boric acid ester is about 2 parts by mass or less (typically 1 part by mass) with respect to 100 parts by mass of the electrolyte component other than the boric acid ester. Hereinafter, for example, 0.5 parts by mass or less). If the amount of boric acid ester is too large, the Coulomb efficiency may tend to decrease. The lower limit of the boric acid ester content is not particularly limited. Usually, it is appropriate that the amount is about 0.001 part by mass or more, preferably about 0.01 part by mass or more (preferably about 0.05 part by mass or more) with respect to 100 parts by mass of the electrolyte components other than boric acid ester. For example, approximately 0.1 parts by mass or more). For example, in a non-aqueous electrolyte in which the non-aqueous solvent is a solvent mainly composed of a carbonate-based solvent, borate ester as an anion receptor is added in an amount of 0.000 parts by mass with respect to 100 parts by mass of electrolyte components other than borate ester. It is preferable to contain about 05-2 mass parts (preferably 0.05-1 mass parts, more preferably 0.05-0.7 mass parts, for example, 0.1-0.5 mass parts).

In carrying out the technology disclosed herein, it is not necessary to clarify the reason why charging / discharging characteristics (reversibility of electrode reaction, for example, Coulomb efficiency) are improved by adding a boric acid ester to the nonaqueous electrolytic solution. For example, the following can be considered. That is, Fe ₂ O ₃ which is a typical iron oxide in the technology disclosed herein occludes Li by the following reaction formula (4) and releases Li by a reaction in the opposite direction (leftward). , Can function as an electrode active material (theoretical capacity; 1008 mAh / g).
Fe ₂ O ₃ + 6Li ⁺ + 6e ⁻ → 3Li ₂ O + 2Fe (4)

If Li ₂ O generated in the rightward reaction is not decomposed in the leftward reaction, the amount of Li becomes irreversible capacity, and the charge / discharge characteristics (coulomb efficiency, capacity retention rate, etc.) of the battery are reduced. . According to the art disclosed herein, boric acid esters as an anion receptor acts on Li 2 _O (more specifically, the boron atom of boric acid ester (B) is Li 2 _O oxygen ions (O ^2- ) Is weakened), the bond between Li and O is weakened, and it is considered that Li is easily released from Li ₂ O. As a result, the reaction toward the left tends to proceed, so that the irreversible capacity is reduced (that is, the reversibility of the electrode reaction is improved).

  The lithium secondary battery disclosed herein includes an oxide-based electrode, and the nonaqueous electrolytic solution includes a predetermined component. Therefore, as long as the object of the present invention can be realized, the contents, materials, or compositions of other battery constituent materials and members are not particularly limited, and those similar to conventional lithium secondary batteries can be used.

  For example, when the oxide-based electrode is used as a negative electrode, an active material capable of reversibly occluding and releasing Li is used as a counter electrode (that is, a positive electrode), a binder (binder), and a conductive material used as necessary. The thing of the form made to adhere to the electrical power collector as a positive electrode compound material with a material etc. can be used preferably. As the positive electrode current collector, a rod-like body, a plate-like body, a foil-like body, a net-like body or the like mainly composed of aluminum, nickel, titanium, stainless steel, or the like can be used.

  As the positive electrode active material, an oxide-based active material having a layered structure, an oxide-based active material having a spinel structure, or the like used for a positive electrode of a general lithium secondary battery (typically a lithium ion secondary battery) is preferable. Can be used. Typical examples of such active materials include lithium transition metal oxides such as lithium nickel oxides, lithium cobalt oxides, and lithium manganese oxides. Here, the lithium nickel-based oxide is an oxide having lithium (Li) and nickel (Ni) as constituent metal elements, and at least one other metal element in addition to Li and Ni (that is, Li and Ni). Also included are oxides containing transition metal elements and / or typical metal elements other than those as constituent metal elements at a rate equivalent to Ni or less than Ni (typically less than Ni) in terms of the number of atoms. That means Examples of the metal element other than Li and Ni include, for example, cobalt (Co), aluminum (Al), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), and titanium (Ti ), Zirconium (Zr), niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La) And one or more metal elements selected from the group consisting of cerium (Ce). This also applies to lithium cobalt oxides and lithium manganese oxides.

  Examples of the conductive material include carbon materials such as carbon black (for example, acetylene black) and graphite powder, and conductive metal powder such as nickel powder. Examples of the binder include PVDF, PTFE, CMC, SBR and the like. Although it does not specifically limit, the usage-amount of the electrically conductive material with respect to 100 mass parts of positive electrode active materials can be 1-20 mass parts (preferably 5-15 mass parts), for example. Moreover, the usage-amount of a binder with respect to 100 mass parts of positive electrode active materials can be 0.5-10 mass parts, for example.

  A lithium secondary battery is constructed by housing the oxide-based electrode and the counter electrode together with the non-aqueous electrolyte in a suitable container (a metal or resin casing, a bag made of a laminate film, or the like). In a typical configuration of the lithium secondary battery disclosed herein, a separator is interposed between the positive electrode and the negative electrode. As a separator, the thing similar to the separator used for a general lithium secondary battery can be used, and it does not specifically limit. For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide, a nonwoven fabric, or the like can be used. The shape (outer shape of the container) of the lithium secondary battery is not particularly limited, and may be, for example, a cylindrical shape, a square shape, a coin shape, or the like.

  One structural example of the lithium secondary battery provided by the present invention is shown in FIG. The lithium secondary battery 10 has a configuration in which an electrode body 11 including a positive electrode 12 and a negative electrode 14 is housed in a battery case 15 having a shape capable of housing the electrode body together with a non-aqueous electrolyte (not shown).

  The electrode body 11 includes a positive electrode 12 having a positive electrode mixture layer 124 on a long sheet-like positive electrode current collector 122 and an iron oxide on a long sheet-like negative electrode current collector (for example, a copper foil) 142. The negative electrode 14 having the structure in which the negative electrode mixture layer 144 is provided is formed by winding together with two long sheet-like separators 13. The battery case 15 includes a bottomed cylindrical case body 152 and a lid 154 that closes the opening. The lid 154 and the case main body 152 are both made of metal and insulated from each other, and are electrically connected to the positive and negative current collectors 122 and 142, respectively. That is, in the lithium secondary battery 10, the lid body 154 also serves as a positive electrode terminal, and the case body 152 serves as a negative electrode terminal.

  In addition, when using the oxide type electrode disclosed here as a positive electrode of a lithium secondary battery, a graphite, Sn alloy, Si alloy etc. can be used as a negative electrode active material, for example.

Further, the borate ester in the technology disclosed herein is a non-aqueous electrolyte of a non-aqueous secondary battery (for example, a lithium secondary battery) provided with an oxide-based electrode (typically, an iron oxide-based electrode). It can also be grasped as an additive for an electrolytic solution that is added to the battery to improve the charge / discharge characteristics of the secondary battery. The additive for electrolyte solution consists of a boric acid ester represented by the following formula (2).
B (OR ¹ ) ₃ (2)
Here, R ¹ in the formula (2) may be independently an alkyl group having 1 to 6 carbon atoms (typically 2 to 4). Further, a group (fluoroalkyl group) having a structure in which at least one of hydrogen atoms (H) constituting the alkyl group is replaced with a fluorine atom (F) may be used. Preferable examples of the fluoroalkyl group include a persulfoalkyl group, a group in which all hydrogen atoms bonded to terminal carbon atoms are replaced with fluorine atoms (for example, —CH ₂ CH ₂ CF ₃ ) and the like. R ¹ may be linear or branched. In one preferred embodiment, R ¹ is a linear alkyl group (eg, n-propyl group). Moreover, the compound whose three R < ¹ > contained in the said Formula (2) is the same group can be employ | adopted preferably.

  Hereinafter, although the experiment example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to this specific example.

<Fabrication of iron oxide electrode>
Commercially available Fe ₂ O ₃ powder material (manufactured by Kosei Chemical Co., Ltd., average particle size 1 μm) was classified, and a fraction having a particle size of 25 μm or less was used. An electrode mixture composition obtained by mixing the classified Fe ₂ O ₃ powder material, acetylene black as a conductive material, and PVdF as a binder with NMP so that the mass ratio thereof is 85: 10: 5. A product was prepared. The composition was coated on one side of a 10 μm thick copper foil with a doctor blade so that the amount applied in terms of solid content was 2 mg / cm ² , dried at 80 ° C. for 30 minutes, and then pressed. In this way, an iron oxide electrode having a 10 μm-thick composite layer on one surface of a copper foil (current collector) was obtained.

<Preparation of electrolyte>
A solution in which LiPF ₆ as a supporting salt is dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 3: 4 at a concentration of about 1 mol / L. Was prepared.
With respect to 100 parts by mass of the LiPF ₆ solution, tripropyl borate ((CH ₃ CH ₂ CH ₂ O) ₃ B, hereinafter referred to as “TPB”) as an anion receptor is respectively 0.1 parts by mass and 0.0. 5 parts by mass, 1.0 part by mass, 3.0 parts by mass and 5.0 parts by mass were added to prepare electrolyte samples 1 to 5.

<Performance evaluation>
The iron oxide electrode was punched into a circle having a diameter of about 16 mm to produce a test electrode containing about 4 mg of the Fe ₂ O ₃ powder material. This test electrode (working electrode), metallic lithium as a counter electrode (a metal Li foil having a diameter of 19 mm and a thickness of 0.15 mm), and a separator (a porous polyolefin sheet having a diameter of 22 mm and a thickness of 0.02 mm) And each of the above electrolyte samples in a stainless steel container, and a coin cell 50 (half cell for charge / discharge performance evaluation) shown in FIG. 3 having a diameter of 20 mm and a thickness of 3.2 mm (2032 type). ) Was built. In FIG. 3, reference numeral 51 denotes a working electrode (test electrode), reference numeral 52 denotes a counter electrode, reference numeral 53 denotes a separator impregnated with an electrolyte, reference numeral 54 denotes a gasket, reference numeral 55 denotes a container (counter electrode terminal), Reference numeral 56 denotes a lid (working electrode terminal). Hereinafter, the coin cell produced above is referred to as cells 1 to 5 in correspondence with the sample number of the electrolyte used. For comparison, a coin cell (cell 6) was similarly produced using the above LiPF ₆ solution to which TPB was not added as an electrolyte.

  The cells 1 to 6 were subjected to an operation (discharge) of inserting Li into the test electrode until the interelectrode voltage reached 0.01 V (lower limit voltage) at a constant current of 0.1 C, and then 0.1 C An operation (charging) was performed in which Li was released at a constant current until the interelectrode voltage reached 3.0 V (upper limit voltage). The coulomb efficiency was calculated from the ratio between the Li storage capacity and the Li release capacity in this initial charge / discharge. The results are shown in Table 1 and FIG.

  As shown in these charts, the Coulomb efficiency was improved by 3% or more in the cells 1 to 3 in which an appropriate amount of TPB was added to the cell 6 using the non-aqueous electrolyte not containing TPB. In addition, the Li release capacities of these cells 1 to 3 were all about 600 mAh / g or more, and it was confirmed that a high capacity superior to graphite was obtained. In the cells 1 and 2 containing 0.05 to 0.7 parts by mass (more specifically 0.1 to 0.5 parts by mass) of TPB, the improvement of the Coulomb efficiency with respect to the cell 6 is 5% or more. A big effect was realized.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

  Since the lithium secondary battery provided by the technology disclosed herein exhibits excellent performance as described above, it can be used as a lithium secondary battery for various applications. For example, it can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Such lithium secondary batteries may be used in the form of an assembled battery formed by connecting a plurality of them in series and / or in parallel. Therefore, according to the technique disclosed herein, as schematically shown in FIG. 4, a vehicle (typically an automobile, in particular, a vehicle including such a lithium secondary battery (which may be in the form of an assembled battery) 10 as a power source. A vehicle including an electric motor such as a hybrid vehicle, an electric vehicle, and a fuel cell vehicle) 1 may be provided.

1 Automobile (vehicle)
DESCRIPTION OF SYMBOLS 10 Lithium secondary battery 11 Electrode body 12 Positive electrode 14 Negative electrode 15 Battery case 50 Coin cell

Claims (8)

A lithium secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
At least one of the positive electrode and the negative electrode is an oxide-based electrode that performs a reaction of reducing an oxide of a transition metal to generate lithium oxide as an electrode reaction accompanying charging and discharging, and a reverse reaction thereof.
The non-aqueous electrolyte includes a non-aqueous solvent and a boric acid ester as an anion receptor. The boric acid ester has the following formula (1):
B (OR ¹ ) ₃ (1)
(However, each R ¹ in the formula is independently selected from an alkyl group having 1 to 6 carbon atoms and a fluoroalkyl group having 1 to 6 carbon atoms.);
The battery according to claim 1, which is a compound represented by:   The battery according to claim 1, wherein the non-aqueous solvent is a solvent containing a carbonate-based solvent as a main component.   The said non-aqueous electrolyte contains the said boric acid ester in the ratio of 0.01-1 mass part with respect to 100 mass parts of electrolyte solution structural components other than the said boric acid ester. The battery described in 1.   The battery according to claim 1, wherein the transition metal is iron.   The battery according to claim 1, comprising the oxide-based electrode as a negative electrode.   A vehicle comprising the lithium secondary battery according to any one of claims 1 to 6. An additive for an electrolytic solution for adding to an electrolytic solution of a non-aqueous secondary battery provided with an oxide-based electrode,
The oxide-based electrode is an electrode that performs a reaction of reducing a transition metal oxide to produce lithium oxide and an inverse reaction thereof as an electrode reaction accompanying charging and discharging,
Following formula (2):
B (OR ¹ ) ₃ (2)
(However, each R ¹ in the formula is independently selected from an alkyl group having 1 to 6 carbon atoms and a fluoroalkyl group having 1 to 6 carbon atoms.);
The additive for electrolyte solutions which consists of borate ester represented by these.

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