EP3642890A1

EP3642890A1 - Nonaqueous electrolyte energy storage device

Info

Publication number: EP3642890A1
Application number: EP18725506.2A
Authority: EP
Inventors: Kenta NAGAMINE; Tetsuhiro Kobayashi; Yudai Kawasoe
Original assignee: Robert Bosch GmbH; GS Yuasa International Ltd
Current assignee: GS Yuasa International Ltd
Priority date: 2017-06-06
Filing date: 2018-05-17
Publication date: 2020-04-29
Also published as: JP2018206641A; WO2018224279A1

Abstract

Provided is a nonaqueous electrolyte energy storage device having a maximum achieved potential of a positive electrode of 4.4 V (vs. Li/Li+) or more and suppressing an increase in resistance associated with a charge-discharge cycle. One embodiment of the present invention is a nonaqueous electrolyte energy storage device including a nonaqueous electrolyte containing an aromatic compound having a silyl group, in which the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more.

Description

DESCRIPTION

TITLE OF THE INVENTION: NONAQUEOUS ELECTROLYTE ENERGY STORAGE DEVICE

TECHNICAL FIELD

[0001]

The present invention relates to a nonaqueous electrolyte energy storage device.

BACKGROUND ART

[0002]

Nonaqueous electrolyte secondary batteries typified by lithium ion secondary batteries are widely used for electronic devices such as personal computers and communication terminals, automobiles, and the like because of their high energy density. The nonaqueous electrolyte secondary battery generally has a pair of electrodes electrically isolated by a separator and a nonaqueous electrolyte interposed between the electrodes, and is configured to perform charge- discharge by delivering ions between the electrodes. Capacitors such as lithium ion capacitors and electric double layer capacitors are widely used as nonaqueous electrolyte energy storage device other than nonaqueous electrolyte secondary batteries.

[0003]

Various additives are added to the nonaqueous electrolyte of the nonaqueous electrolyte energy storage device for the purpose of improving performance and the like. For example, Patent Document 1 proposes an electrolytic solution for a secondary battery to which phenylacetylene (ethynylbenzene) is added in order to improve cycle characteristics and the like.

PRIOR ART DOCUMENT PATENT DOCUMENT

[0004]

Patent Document l: JP-A-2000-195545

SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0005]

In the nonaqueous electrolyte energy storage device, it is possible to increase the amount of electricity to be charged by charging at a high voltage. However, in the case of charging at a high voltage, specifically when the energy storage device is used at the maximum achieved potential of the positive electrode of 4.4 V (vs. Li/Li+) or more, for example, oxidation decomposition of the nonaqueous electrolyte is likely to occur in the positive electrode. For this reason, in the case of using at a high voltage, there is a disadvantage that the increase in resistance associated with repeated charge- discharge tends to occur. The inventors have found that the increase in resistance due to repetition of charge- discharge at such a high voltage cannot be improved by the addition of the phenyl acetylene or the like.

[0006]

The present invention has been made based on the above

circumstances, and it is an object of the present invention to provide a nonaqueous electrolyte energy storage device having a maximum achieved potential of the positive electrode of 4.4 V (vs. Li/Li+) or more in which an increase in resistance associated with a charge- discharge cycle is

suppressed.

MEANS FOR SOLVING THE PROBLEMS

[0007]

One embodiment of the present invention made in order to solve the above-mentioned problems pertains to a nonaqueous electrolyte energy storage device including a nonaqueous electrolyte containing an aromatic compound having a silyl group, wherein the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more.

ADVANTAGES OF THE INVENTION

[0008]

According to the present invention, it is possible to provide the nonaqueous electrolyte energy storage device having a maximum achieved potential of the positive electrode of 4.4 V (vs. Li/Li+) or more in which an increase in resistance associated with a charge- discharge cycle is

suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]

Fig. 1 is an appearance perspective view showing a nonaqueous electrolyte energy storage device according to one embodiment of the present invention.

Fig. 2 is a schematic view showing an energy storage apparatus configured by assembling a plurality of nonaqueous electrolyte energy storage devices according to one embodiment of the present invention. MODE FOR CARRYING OUT THE INVENTION

[0010]

A nonaqueous electrolyte energy storage device according to one embodiment of the present invention includes a nonaqueous electrolyte containing an aromatic compound having a silyl group, wherein a maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more.

[0011]

The nonaqueous electrolyte energy storage device is a nonaqueous electrolyte energy storage device having a maximum achieved potential of the positive electrode of 4.4 V (vs. Li/Li+) or more in which an increase in resistance associated with a charge- discharge cycle is suppressed. The reason why such an effect occurs is not clear, but the following reasons are presumed. One factor of the increase in the resistance of the nonaqueous electrolyte energy storage device is decomposition of the nonaqueous electrolyte on the positive electrode. In an aromatic compound having a silyl group, it is presumed that two- stage oxidation reaction occurs at two potentials of around 3.3 V (vs. Li/Li+) and around 4.4 V (vs. Li/Li+). A decomposition product produced in the first oxidation reaction occurred in the vicinity of 3.3 V (vs. Li/Li+) do not fix on the surface of the positive electrode and is dissolved in the nonaqueous electrolyte. On the other hand, in the second oxidation reaction occurred in the vicinity of 4.4 V (vs. Li/Li+), the decomposition product produced in the first oxidation reaction is further oxidized to form a coating film on the surface of the positive electrode. In the nonaqueous electrolyte energy storage device used at a high voltage, since the aromatic compound having a silyl group is contained in the nonaqueous electrolyte, it is presumed that by the coating of the surface of the positive electrode formed under such a high potential environment, the increase in resistance associated with a charge- discharge cycle at a high voltage is suppressed.

[0012]

The "silyl group" refers to a group in which a group represented by - S1H3, and a group in which one or more hydrogen atoms of the group represented by the -S1H3 are substituted with a substituent. That is, the "silyl group" refers to both an unsubstituted silyl group and a silyl group having a substituent.

[0013]

It is preferred that the nonaqueous electrolyte further contains an ester having a sultone structure or a cyclic sulfate structure. When the nonaqueous electrolyte further contains the ester, the increase in resistance associated with the charge- discharge cycle is further suppressed. The sultone structure means a cyclic sulfonic acid ester (-SO2O-) structure. In addition, the cyclic sulfate structure refers to a cyclic sulfuric acid ester (- OSO2O-) structure.

[0014]

The aromatic compound is preferably represented by the following formula (l). By using the aromatic compound represented by the following formula (l), the increase in resistance associated with a charge- discharge cycle is further suppressed.

[0015]

[Chem. Formula l]

where each of a plurality of Rs is independently a hydrogen atom or a hydrocarbon group.

[0016]

The silyl group is preferably a group represented by -S1H3. When the silyl group of the aromatic compound is a group represented by -S1H3, the increase in resistance associated with a charge- discharge cycle is also further suppressed.

[0017]

The aromatic compound preferably further has an acetylenediyl group. When the aromatic compound further has an acetylenediyl group, it is possible to improve the coulombic efficiency after the charge- discharge cycles in addition to suppression of increase in resistance. Although the reason for this is not clear, the following reasons are presumed. It is believed that the decrease in coulombic efficiency is caused by a product produced by a side reaction on the surface of the positive electrode being deposited on the negative electrode and lowering the acceptability of lithium etc. of the negative electrode. On the other hand, when an aromatic compound having a silyl group and an acetylenediyl group is added to the nonaqueous electrolyte, it is believed that the coating on the surface of the positive electrode formed by the aromatic compound adequately causes a charge- discharge reaction, and can suppress the side reaction. As a result, it is presumed that the coulombic efficiency after the charge- discharge cycle is improved. [0018]

Hereinafter, a nonaqueous electrolyte energy storage device according to one embodiment of the present invention will be described in detail.

[0019]

A nonaqueous electrolyte energy storage device according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. Hereinafter, a

nonaqueous electrolyte secondary battery will be described as an example of a nonaqueous electrolyte energy storage device. In general, the positive electrode and the negative electrode form an electrode assembly which is alternately superimposed by lamination or winding with a separator interposed between the positive electrode and the negative electrode. The electrode assembly is housed in a case, and the nonaqueous electrolyte is filled in the case. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. As the case, it is possible to use a publicly known metal case, resin case, or the like which is commonly used as a case of a nonaqueous electrolyte secondary battery.

[0020]

The positive electrode has a positive substrate and a positive active material layer disposed directly or with an intermediate layer interposed therebetween on the positive substrate.

[0021] The positive substrate has conductivity. As a material of the substrate, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum and an aluminum alloy are preferred from the viewpoint of balance among an electric potential resistance, high conductivity and a cost. Further, examples of the formation form of the positive substrate include a foil, a vapor deposition film, and the like, and from the viewpoint of cost, a foil is preferred. That is, an aluminum foil is preferred as the positive substrate. Examples of aluminum or aluminum alloy include A1085P, A3003P, and the like prescribed in JIS H 4000 (2014).

[0022]

The intermediate layer is a coating layer on the surface of the positive substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive substrate and the positive active material layer. The constitution of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles. In addition, "having conductivity" means that the volume resistivity measured in accordance with JIS H 0505 (1975) is 10⁷ Q'cm or less, and "non- conductive" means that the volume resistivity is more than 10⁷ Ω·οηι.

[0023]

The positive active material layer is formed of a so-called positive composite containing a positive active material. The positive composite for forming the positive active material layer contains optional components such as a conductive agent, a binder, a thickener, a filler and the like, as required.

[0024]

Examples of the positive active material include composite oxides (Li_xCo02, Li_xNi02, Li_xMn03, Li_xNi_aCo(i-_a)02, Li_xNi_aCopAl(i-_a-p)02,

Li_xNi_aMn Co(i-a- )02, Lii+_x(Ni_aMnpCo(i-_a-p))i-_x02 and the like having a layered a-NaFe02 type crystal structure, Li_xMn204, Li_xNi_aMn(2-_a)04 and the like having a spinel type crystal structure) represented by Li_xMO_y (M represents at least one kind of transition metal), and polyanion compounds (LiFeP04, LiMnP0₄, LiNiP0₄, LiCoP0₄, Li₃V2(P0₄)3, Li₂MnSi0₄, Li₂CoP0₄F, and the like) represented by Li_wMe_x(XO_y)_z (Me represents at least one transition metal, and X represents, for example, P, Si, B, V and the like). Elements or polyanions in these compounds may be partially substituted with other elements or anionic species. In the positive active material layer, one of these compounds may be used alone, or two or more of these compounds may be used in a mixture.

[0025]

The positive active material preferably contains a positive active material which can make the positive electrode potential at the end-of- charge voltage during normal use of the nonaqueous electrolyte secondary battery nobler than 4.4 V (vs. Li/Li+). Since the nonaqueous electrolyte secondary battery (energy storage device) includes a nonaqueous electrolyte containing an aromatic compound having a silyl group, in a usage where the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more, the increase in resistance associated the charge-discharge cycle is suppressed. Accordingly, by using a positive active material which can be a potential nobler than 4.4 V (vs. Li/Li+), a nonaqueous electrolyte secondary battery having increased energy density and suppressed increase in resistance associated with a charge- discharge cycle can be formed.

[0026]

The positive active material whose positive electrode potential at the end-of-charge voltage during normal use can be nobler than 4.4 V (vs.

Li/Li+) may be a positive active material capable of inserting and removing reversible lithium ion after reaching a potential nobler than 4.4 V (vs.

Li/Li+) Examples of such a positive active material include^:

Lii+x(Ni_aMn Co(i-a- ))i-x02 (x > 0, β > 0.5) having a layered orNaFeO2 type crystal structure! LiNio.5Mn1.5O4 which is an example of Li_xNi_aMn(2-_a)O4 having a spinel type crystal structure! LiNiPO4, L1C0PO4, L12C0PO4F or Li2MnSiO4 which is an example of a polyanion compound! and the like.

[0027]

Here, the case of "during normal use" means a case where the nonaqueous electrolyte secondary battery is used by adopting the charging condition recommended for or designated to the nonaqueous electrolyte secondary battery, and when a charger for the nonaqueous electrolyte secondary battery is prepared, the case means a case where the nonaqueous electrolyte secondary battery is used by applying the charger. For example, in a nonaqueous electrolyte secondary battery using graphite as a negative active material, the positive electrode potential is about 4.45 V (vs. Li/Li+) when the end-of-charge voltage is 4.35 V although it depends on the design.

[0028]

The conductive agent is not particularly limited as long as it is a conductive material that does not adversely affect battery performance. Examples of such a conductive agent include natural or artificial graphite, carbon black such as furnace black, acetylene black and Ketjen black, metal, conductive ceramics, and the like, and the acetylene black is preferred.

Examples of a shape of the conductive agent include powder, fiber, or the like.

[0029]

Examples of the binder include^: thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride

(PVDF), etc.), polyethylene, polypropylene, polyimide and the like! elastomers such as ethylene-propylene- diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber and the like! polysaccharide polymers! and the like.

[0030]

Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. When the thickener has a functional group which reacts with lithium, it is preferred to

previously deactivate the functional group by methylation or the like.

[0031]

The filler is not particularly limited as long as it does not adversely affect battery performance. Examples of a main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, glass, and the like.

[0032]

<Negative Electrode> The negative electrode has a negative substrate and a negative active material layer disposed directly or with an intermediate layer interposed therebetween on the negative substrate. The intermediate layer can have the same structure as the intermediate layer of the positive electrode.

[0033]

The negative substrate may have the same structure as that of the positive substrate. However, as a material of the negative substrate, a metal such as copper, nickel, stainless steel or nickel-plated steel, or an alloy thereof is used, and copper or a copper alloy is preferred. That is, a copper foil is preferred as the negative substrate. As the copper foil, a rolled copper foil, an electrolytic copper foil, and the like are exemplified.

[0034]

The negative active material layer is formed of a so-called negative composite containing a negative active material. The negative composite for forming the negative active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as required. As optional components such as a conductive agent, a binder, a thickener, and a filler, the same materials as those of the positive active material layer can be used.

[0035]

As the negative active material, a material which can absorb and release lithium ions is usually used. Specific examples of the negative active material include^: metals or semi-metals such as Si and Sn! metal oxides or semi-metal oxides such as Si oxide and Sn oxide! polyphosphate compounds! carbon materials such as graphite and amorphous carbon (graphitizable carbon or non-graphitizable carbon); and the like.

[0036]

Further, the negative composite (negative active material layer) may contain: a typical nonmetallic element such as B, N, P, F, CI, Br and I; a typical metallic element such as Li, Na, Mg, Al, K, Ca, Zn, Ga and Ge! or a transition metal element such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb or W.

[0037]

As a material of the separator, for example, a woven fabric, a nonwoven fabric, a porous resin film or the like is used. Among them, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of a liquid retaining property of the nonaqueous electrolyte. From the viewpoint of strength, polyolefins such as polyethylene and polypropylene are preferred as a main component of the separator, and polyimide, aramid or the like are preferred from the viewpoint of resistance to oxidation decomposition. Further, these resins may be combined.

[0038]

An inorganic layer may be provided between the separator and the electrode (usually, the positive electrode). The inorganic layer is a porous layer also called a heat resistant layer or the like. A separator having an inorganic layer formed on one surface of a porous resin film can also be used. The inorganic layer is usually composed of inorganic particles and a binder, and other components may be contained.

[0039]

The nonaqueous electrolyte contains a nonaqueous solvent, an electrolyte salt, and an aromatic compound having a silyl group. It is preferred that the nonaqueous electrolyte further contains an ester having a sultone structure or a cyclic sulfate structure. The nonaqueous electrolyte is not limited to a liquid. That is, the nonaqueous electrolyte is not limited to liquid-like ones, but includes solid and gel-like ones.

[0040]

(Nonaqueous Solvent)

As the nonaqueous solvent, a publicly known nonaqueous solvent commonly used as a nonaqueous solvent of a common nonaqueous electrolyte for a secondary battery can be used. Examples of the

nonaqueous solvent include cyclic carbonate, chain carbonate, ester, ether, amide, sulfone, lactone, nitrile and the like. Among these, it is preferred to use at least a cyclic carbonate or a chain carbonate, and it is more preferred to use a cyclic carbonate and a chain carbonate in combination. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio (cyclic carbonate : chain carbonate) of the cyclic carbonate and the chain carbonate is not particularly limited. However, it is preferably set to, for example, 5 : 95 or more and 50 : 50 or less

[0041]

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate,

fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, 1,2- diphenylvinylene carbonate and the like, among which EC, PC and FEC are preferred.

[0042]

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate and the like, among which DMC and EMC are preferred.

[0043]

The nonaqueous solvent preferably contains a fluorinated carbonate. The fluorinated carbonate suppresses side reactions and can further improve charge- discharge cycle performance such as suppression of the increase in resistance.

[0044]

The above fluorinated carbonate refers to a compound in which a part or all of hydrogen atoms of the carbonate are substituted with fluorine atoms. As the fluorinated carbonate, fluorinated cyclic carbonate is preferred. Examples of the fluorinated cyclic carbonate include fluorinated ethylene carbonates such as fluoroethylene carbonate (FEC) and

difluoroethylene carbonate, fluorinated propylene carbonates, fluorinated butylene carbonates and the like. However, fluorinated ethylene

carbonates are preferred, and FEC is more preferred. These fluorinated carbonates can be used alone or in combination of two or more thereof.

[0045] The lower limit of the content of the fluorinated carbonate in the nonaqueous solvent is preferably 1% by volume, and more preferably 5% by volume. On the other hand, the upper limit of the content is preferably 30% by volume, and more preferably 20% by volume. When the content of the fluorinated carbonate is set to the above range, it is possible to further improve charge- discharge cycle performance such as suppression of the increase in resistance.

[0046]

(Electrolyte Salt)

As the nonaqueous electrolyte salt, a publicly known electrolyte salt commonly used as a electrolyte salt of a common nonaqueous electrolyte for a secondary battery can be used. Examples of the electrolyte salt include a lithium salt, a sodium salt, a potassium salt, a magnesium salt, an onium salt and the like, and a lithium salt is preferred.

[0047]

Examples of the lithium salt include^: inorganic lithium salts such as LiPF₆, LiPO₂F₂, LiBF₄, LiClO₄ and LiN(SO₂F)₂; lithium salts having a fluorinated hydrocarbon group, such as L1SO3CF3, LiN(SO₂CF3)₂,

LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)(SO₂C₄F₉), LiC(SO₂CF₃)₃, and LiC(SO₂C₂F₅)₃; and the like. Among them, inorganic lithium salts are preferred, and L1PF6 is more preferred.

[0048]

The lower limit of the content of the electrolyte salt in the

nonaqueous electrolyte is preferably 0.1 M, more preferably 0.3 M, still more preferably 0.5 M, and particularly preferably 0.7 M. On the other hand, the upper limit of the content is not particularly limited; however, it is preferably 2.5 M, more preferably 2 M, and still more preferably 1.5 M.

[0049]

(Aromatic Compound Having Silyl Group)

In the nonaqueous electrolyte secondary battery, when the

nonaqueous electrolyte contains an aromatic compound having a silyl group, in use where the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more, the increase in resistance associated with a charge- discharge cycle is suppressed.

[0050]

The silyl group can be represented by -SiR (2). In the formula (2), each R¹ independently represents a hydrogen atom or an optional

substituent. Specific examples of R¹ include a hydrogen atom, a halogen atom, a hydroxyl group, an amino group, a nitro group, an organic group, and the like. Here, the organic group means a group containing at least one carbon atom.

[0051]

Examples of the organic group include a hydrocarbon group, a group containing a heteroatom- containing group at the carbon- carbon or terminal of the hydrocarbon group, a group obtained by substituting a part or all of the hydrogen atoms of these groups with a substituent, a carboxy group, a cyano group and the like.

[0052]

Examples of the hydrocarbon group include aliphatic chain

hydrocarbon groups such as^ alky! groups such as a methyl group, an ethyl group, a propyl group and a butyl group; alkenyl groups such as an ethenyl group, a propenyl group and a butenyl group; and an alkynyl groups such as an ethynyl group, a propynyl group and a butynyl group, alicyclic

hydrocarbon groups such as^ cycloalkyl groups such as a cyclohexyl group; and cycloalkenyl groups such as cyclohexenyl group, and aromatic hydrocarbon groups such as a phenyl group, a naphthyl group, a biphenyl group, a benzyl group, and a phenylethynyl group (-0≡Οφ^: φ is a phenyl group).

[0053]

Examples of the hetero atom- containing group include^: groups consisting solely of hetero atoms such as -0-, -S-, -SO", -SO2 -, -SO2O-, and - SO3S and groups in which carbon atoms and hetero atoms are combined, such as -CO-, -COO-, -COS-, -CONH-, -OCOO-, -OCOS-, -OCONH-, - SCONH-, -SCSNH-, -SCSS-.

[0054]

Examples of the substituent include a halogen atom, a hydroxy group, a carboxy group, a nitro group, and a cyano group.

[0055]

The R¹ is preferably a hydrogen atom or a hydrocarbon group. The hydrocarbon group is preferably a hydrocarbon group having 1 to 10 carbon atoms. Particularly, the R¹ is preferably a hydrogen atom. That is, it is preferred that the silyl group is a group represented by -S1H3.

[0056]

It is sometimes preferred that all of the three R^xs are organic groups. In this case, as the organic group, a hydrocarbon group is more preferred, and a hydrocarbon group having 1 to 10 carbon atoms is more preferred. When all of the R¹s are organic groups, the coulombic efficiency tends to increase after the charge-discharge cycle.

[0057]

The number of silyl groups (silicon atoms) of the aromatic compound is not particularly limited. The number of silyl groups of the aromatic compound may be 1 or plural, but is usually 1.

[0058]

The above aromatic compound refers to a compound containing an aromatic ring. The aromatic ring may be a carbocyclic ring (aromatic carbocyclic ring) or a heterocyclic ring (aromatic heterocyclic ring).

Examples of the aromatic carbocyclic ring include a benzene ring, a naphthalene ring, an anthracene ring and the like. Examples of the aromatic heterocyclic ring include a furan ring, a thiophene ring, a pyrrole ring, a pyrazole ring, an imidazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring and the like. A part or all of the hydrogen atoms of these aromatic rings may be substituted with a substituent. As the aromatic ring, an aromatic carbon ring is preferred, and a benzene ring is more preferred.

[0059]

The aromatic compound having a silyl group is preferably a compound represented by the following formula (l).

[0060]

[Chem. Formula 2]

[0061]

[0062]

Examples of the hydrocarbon group represented by the above R include those exemplified as the hydrocarbon group represented by the above R¹. The upper limit of the number of carbon atoms of the

hydrocarbon group represented by the above R¹ is preferably 10, more preferably 6, and still more preferably 3. On the other hand, the lower limit of the number of carbon atoms may be 1. The above R is preferably a hydrogen atom.

[0063]

Examples of the compound represented by the above formula (l) include phenylsilane, methylphenylsilane, dimethylphenylsilane, trimethylphenylsilane and the like. Among these, phenylsilane is preferred.

[0064]

It is also preferred that the aromatic compound having the above silyl group further has an acetylenediyl group (ethynediyl group! -C≡C^_). Thereby, the coulombic efficiency after the charge- discharge cycle can be improved. That is, the secondary battery using the nonaqueous electrolyte containing such a aromatic compound has high coulombic efficiency in addition to suppression of increase in resistance after the charge- discharge cycle at a high voltage.

[0065]

The acetylenediyl group may be bonded directly to a silicon atom or may not be bonded directly to the silicon atom. Further, an acetylenediyl group may be contained in the substituent of the silyl group. The acetylenediyl group is preferably bonded directly to the aromatic ring.

[0066]

The number of acetylenediyl groups of the aromatic compound is not particularly limited. The number of acetylenediyl groups of the aromatic compound may be 1 or plural, but it is preferably 1 or 2.

[0067]

An example of the aromatic compound having a silyl group and an acetylenediyl group can be represented by SiRVC≡C-(t> (R¹ has the same meaning as R¹ in the formula (2), and φ is a phenyl group). Another example of such an aromatic compound can be represented by SiRVR^2~C≡C- φ (R¹ has the same meaning as R¹ in the formula (2), R² is a divalent hydrocarbon group, and φ is a phenyl group). Examples of the divalent hydrocarbon group include a methanediyl group, an ethanediyl group, a benzenediyl group (phenylene group) and the like, and a benzenediyl group is preferred. The number of carbon atoms of the divalent hydrocarbon group is preferably 1 to 10, for example.

[0068]

Examples of the aromatic compound having a silyl group and an acetylenediyl group include phenylethynyl trimethylsilane, phenylethynyl triethylsilane, naphthylethynyl trimethylsilane, 4- (trimethylsilyl)diphenylacetylene, diphenylbis(phenylethynyl)silane and the like.

[0069]

The lower limit of the content of the aromatic compound in the nonaqueous electrolyte is preferably 0.01% by mass, more preferably 0.05% by mass, and still more preferably 0.1% by mass. On the other hand, the upper limit of the content is preferably, for example, 5% by mass, preferably 3% by mass, and more preferably 1% by mass. By setting the content of the aromatic compound to the above lower limit or more and the above upper limit or less, the increase in resistance associated with the charge- discharge cycle can be further suppressed.

[0070]

(Ester Having Sultone Structure or Cyclic Sulfate Structure)

In the nonaqueous electrolyte secondary battery, when the

nonaqueous electrolyte further contains the ester, the increase in resistance associated with the charge- discharge cycle is further suppressed.

[0071]

The number of carbon atoms of the ester can be, for example, 2 to 10. The number of ring members of the sultone structure and the cyclic sulfate structure can be, for example, 4 to 6, and is preferably a five-membered ring.

[0072]

Examples of the ester having the sultone structure include 1,3- propanesultone, 1,4-butanesultone, 2,4-butanesultone, 1,3-propenesultone, 1,4-butenesultone, l-methyl-l,3-propanesultone, 3^_methyl-l,3^_ propanesultone, l-fluoro-l,3-propanesultone, 3-fluoro-l,3-propanesultone, methylene methane disulfonic acid ester and the like. As the ester having the sultone structure, unsaturated sultone is preferred, and 1,3- propenesultone is more preferred.

[0073]

Esters having the cyclic sulfate structure include ethylene sulfate, 4- methyl"2,2-dioxo- 1,3,2-dioxathiolane, 4-ethyl-2,2-dioxo- 1,3,2-dioxathiolane, 4-propyl"2,2-dioxo- 1,3,2-dioxathiolane, 4-methylsulfonyloxymethyl-2,2- dioxo- 1,3,2-dioxathiolane, 4-ethylsulfonyloxymethyl-2,2- 1,3,2- dioxathiolane, bis((2,2-dioxo-l,3,2-dioxathiolane-4-yl)methyl)sulfate, 4,4'- bis(2,2-dioxo- 1,3,2-dioxathiolane) and the like. As the ester having a cyclic sulfate structure, a compound having a plurality of sulfate structures is preferred, a compound having a plurality of cyclic sulfate structures is more preferred, and 4, 4'-bis(2,2-dioxo- 1,3,2-dioxathiolane) is further preferred.

[0074]

The lower limit of the content of the ester in the nonaqueous electrolyte is preferably 0.05% by mass, more preferably 0.2% by mass, and still more preferably 0.5% by mass. On the other hand, the upper limit of the content is preferably, for example, 10% by mass, preferably 5 mass%, and more preferably 3% by mass. By setting the content of the ester to the above lower limit or more and the above upper limit or less, the increase in resistance associated with the charge-discharge cycle can be further suppressed.

[0075]

(Additive) As long as the effect of the present invention is not impaired, the nonaqueous electrolyte may further contains, as an additive, a component other than the nonaqueous solvent, the electrolyte salt, the aromatic compound having a silyl group, and the ester having a sultone structure or a cyclic sulfate structure. As the above-mentioned additives, various additives contained in a common nonaqueous electrolyte for secondary batteries can be mentioned. The upper limit of the content of the additive in the nonaqueous electrolyte is preferably 5% by mass, and sometimes more preferably 1% by mass, more preferably 0.4% by mass, and still more preferably 0.1% by mass. In particular, as the additive, it is sometimes preferred that the content of the polyvalent carboxylic acid ester having a silyl group or the aromatic compound not having a silyl group is the above upper limit or less. These additives may affect various performance of the charge- discharge cycle in use where the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more.

[0076]

The nonaqueous electrolyte can be usually obtained by adding components such as an electrolyte salt and an aromatic compound having a silyl group to the nonaqueous solvent and dissolving them.

[0077]

The nonaqueous electrolyte energy storage device (secondary battery) is charged with the maximum achieved potential of the positive electrode charged to 4.4 V (vs. Li/Li+) or more. The maximum achieved potential of the positive electrode may be 4.45 V (vs. Li/Li+) or more. As described above, since the maximum achieved potential of the positive electrode is high, high energy density can be achieved. In addition, since the maximum achieved potential of the positive electrode is 4.4 V (vs.

Li/Li+) or more and the nonaqueous electrolyte contains an aromatic compound having a silyl group, the increase in resistance associated with a charge- discharge cycle is suppressed. The upper limit of the maximum achieved potential of the positive electrode is, for example, 5.0 V (vs. Li/Li+), and may be 4.8 V (vs. Li/Li+) or may be 4.6 V (vs. Li/Li+).

[0078]

It is preferred that the maximum achieved potential of the positive electrode is the positive electrode potential at the end-of-charge voltage at the time of normal use. Generally, when the maximum achieved potential of the positive electrode frequently becomes 4.4 V (vs. Li/Li+) or more by repetition of charge- discharge, the increase in resistance is likely to occur. Therefore, when the maximum achieved potential of the positive electrode frequently reaches 4.4 V (vs. Li/Li+) or more by repetition of charge- discharge, the effect of suppressing the increase in resistance of the nonaqueous electrolyte secondary battery is more effectively exhibited.

[0079]

The method for producing the nonaqueous electrolyte secondary battery is not particularly limited. The nonaqueous electrolyte secondary battery can be produced by using the nonaqueous electrolyte containing an aromatic compound having a silyl group. The above production method includes, for example, a step of preparing a positive electrode, a step of preparing a negative electrode, a step of preparing a nonaqueous electrolyte, a step of forming an electrode assembly alternately superimposed by laminating or winding a positive electrode and a negative electrode with a separator interposed therebetween, a step of housing the positive electrode and the negative electrode (electrode assembly) in a container, and a step of injecting the nonaqueous electrolyte into the container. After injection, a nonaqueous electrolyte secondary battery can be obtained by sealing an injection hole.

[0080]

The present invention is not limited to the above-mentioned embodiment, but may be implemented in aspects with various modifications and improvements besides the above embodiment. For example, in the positive electrode and the negative electrode, it is not necessary to provide the intermediate layer, and it may not have a definite layer structure. For example, the positive electrode and the negative electrode may have a structure in which an active material is supported on a mesh-like substrate, or the like. Further, in the above-mentioned embodiment, an aspect in which the nonaqueous electrolyte energy storage device is a nonaqueous electrolyte secondary battery has been chiefly described, but other

nonaqueous electrolyte energy storage devices may be used. Examples of other nonaqueous electrolyte energy storage devices include capacitors (electric double-layer capacitors, lithium ion capacitors), and the like.

[0081]

Fig. 1 shows a schematic view of a rectangular nonaqueous electrolyte energy storage device 1 (nonaqueous electrolyte secondary battery) of one embodiment of the nonaqueous electrolyte energy storage device according to the present invention. Fig. 1 is a perspective view of the inside of a container. In the nonaqueous electrolyte energy storage device 1 shown in Fig. 1, an electrode assembly 2 is housed in a container 3. The electrode assembly 2 is configured by winding a positive electrode including a positive active material and a negative electrode including a negative active material with a separator interposed therebetween. The positive electrode is electrically connected to a positive electrode terminal 4 through a positive electrode lead 4', and the negative electrode is electrically connected to a negative electrode terminal 5 through a negative electrode lead 5'.

[0082]

Further, the configuration of the nonaqueous electrolyte energy storage device according to the present invention is not particularly limited, and examples thereof include a cylindrical battery, a prismatic battery (rectangular battery), a flat battery, and the like. The present invention can also be realized as an energy storage apparatus having a plurality of the nonaqueous electrolyte energy storage devices. One embodiment of the energy storage apparatus is shown in Fig. 2. In Fig. 2, the energy storage apparatus 30 includes a plurality of energy storage units 20. Each of the energy storage units 20 includes a plurality of nonaqueous electrolyte energy storage devices 1. The energy storage apparatus 30 can be mounted as a power source for automobiles such as electric vehicles (EV), hybrid automobiles (HEV), plug-in hybrid automobiles (PHEV) and the like. EXAMPLES

[0083]

Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples.

[0084]

The additives used in Examples and Comparative Examples are shown below.

(Aromatic Compound)

Aromatic compound A: Phenylsilane represented by the following formula (A)

Aromatic compound B: Phenylethynyl trimethylsilane represented by the following formula (B)

Aromatic compound C^ 4- (trimethylsilyl)diphenylacetylene represented by the following formula (C)

Aromatic compound D: Dipheny Ibis (phenylethynyl) silane

represented by the following formula (D)

Aromatic compound X: Ethynylbenzene represented by the following formula (X)

Aromatic compound Y Ethynylaniline represented by the following formula (Y)

[0085]

[Chem. Formula 3]

( X ) ( Y )

[0086]

(Ester Having Sultone Structure or Cyclic Sulfate Structure)

Ester a- 4,4'-bis(2,2-dioxo-l,3,2-dioxathiolane) represented by the following formula (a)

Ester b: 1,3-propenesultone represented by the following formula (b)

[0087]

[Chem. Formula 4]

[0088]

[Example l]

(Preparation of Nonaqueous Electrolyte)

LiPF6 as an electrolyte salt was dissolved at a concentration of 1.2 mol/L in a nonaqueous solvent in which EC, DMC and EMC were mixed in a volume ratio of 30 : 40 : 30, and to the resulting mixture, 0.5% by mass of the aromatic compound A and 1% by mass of the ester a were added to prepare a nonaqueous electrolyte of Example 1.

[0089]

(Production of Positive Electrode Plate)

Li_xNi_aCo Al(i-a- )02 was used as a positive active material. A positive electrode paste containing the positive active material,

polyvinylidene fluoride (PVdF) and acetylene black (AB) in proportions of 90 ^: 5 ^: 5 by mass (on solid equivalent basis), and using N-methylpyrrolidone as a dispersion medium, was prepared. The positive electrode paste was applied onto both surfaces of a band-like aluminum foil as a positive substrate so that the positive active material was contained in an amount of 15 mg/cm² per unit electrode area. This was pressed by a roller press machine to form the positive active material layer, and then dried under reduced pressure at 100°C for 10 hours to remove the liquid content in the electrode plate. In this manner, a positive electrode plate was prepared.

[0090]

(Production of Negative Electrode Plate)

Graphite was used as a negative active material. A negative electrode paste containing graphite, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) in proportions of 96 : 2 : 2 by mass (on solid equivalent basis), and using water as a dispersion medium, was prepared. The negative electrode paste was applied to both surfaces of a band-like copper foil as a negative substrate so as to contain the negative active material in an amount of 10.0 mg/cm² per unit electrode area. This was pressed by a roller press machine to form a negative active material layer, and then dried under reduced pressure at 100° C for 12 hours to remove moisture in the electrode plate. In this way, a negative electrode plate was obtained.

[0091]

(Production of Nonaqueous Electrolyte Energy Storage Device)

As a separator, a microporous polyolefin membrane having an inorganic layer formed on its surface was used. The positive electrode plate and the negative electrode plate were laminated with the separator interposed therebetween to produce an electrode assembly. The electrode assembly was housed in a metal-resin composite film case, the nonaqueous electrolyte was injected into the case, and then the electrode assembly was sealed by heat welding to obtain a nonaqueous electrolyte energy storage device (secondary battery) of Example 1.

[0092]

[Comparative Example l]

A nonaqueous electrolyte energy storage device of Comparative Example 1 was obtained in the same manner as in Example 1 except that the aromatic compound A was not added.

[0093]

[Evaluation]

(Charge-Discharge Cycle Test: 4.20 V)

Initial charge- discharge tests and cycle tests were carried out using the nonaqueous electrolyte energy storage devices of Example 1 and

Comparative Example 1. The initial charge- discharge test was carried out as follows. At 25°C, the battery was charged at constant current and constant voltage with a charge current of 1.0 C and an end-of-charge voltage of 4.20 V. The termination condition of the charge was made until the total charging time reached 3 hours. After a rest period of 10 minutes, a constant current discharge was performed with a discharge current of 1.0 C and an end-of-discharge voltage of 2.50 V. The cycle test was carried out as follows. At 45°C, the battery was charged at constant current and constant voltage with a charge current of 1.0 C and an end-of-charge voltage of 4.20 V. The termination condition of the charge was made until the total charging time reached 3 hours. Thereafter, a rest period of 10 minutes was provided. Thereafter, a constant current discharge was performed with a discharge current of 1.0 C and an end-of-discharge voltage of 2.50 V, and then a rest period of 10 minutes was provided. This charge- discharge was performed for 500 cycles. The positive electrode potential (maximum achieved potential of the positive electrode) at the end-of-charge voltage in the charge- discharge cycle test using graphite as the negative electrode was about 4.30 V (vs. Li/Li+).

[0094]

(Measurement of DC Resistance)

The direct current resistance (DCR) of the nonaqueous electrolyte energy storage device at 25°C was measured before and after the charge- discharge cycle test. The battery was charged at a constant current of 1.0 C up to a voltage corresponding to SOC of 50% at 25°C, and then charged at constant voltage. The charge was performed for 3 hours in total of constant current charge and constant voltage charge. A voltage (El) when

discharged at 0.2 C (II) for 10 seconds and a voltage (E2) when discharged at 1.0 C (12) for 10 seconds were measured in a charged state of SOC of 50%. Using the discharge current values II and 12 and the measured voltages El and E2, the DC resistance value (Rx) at 25°C was calculated by the following equation.

Rx = |(E1-E2)/(I1-I2)|

The DCR (%) after the cycle test with respect to the DCR before the cycle test is shown in Table 1.

[0095]

(Charge-Discharge Cycle Test: 4.35 V)

Initial charge- discharge test and charge-discharge cycle test were carried out in the same manner as in the above "Charge-Discharge Cycle Test: 4.20 V" except that the end-of-charge voltage was set to 4.35 V. In this case, the positive electrode potential (maximum achieved potential of the positive electrode) was about 4.45 V (vs. Li/Li+). In the same manner as in the above "Measurement of DC Resistance", the DCR (direct current resistance) of the nonaqueous electrolyte energy storage device at 25°C was measured before and after the charge- discharge cycle test. The DCR (%) after the cycle test with respect to the DCR before the cycle test is shown in

Table 1.

[0096]

[Table l]

[0097]

As shown in the above Table 1, in Comparative Example 1 in which the aromatic compound having a silyl group was not added, the resistance after the charge- discharge cycle at a high voltage of 4.35 V (the maximum achieved potential of the positive electrode: about 4.45 V (vs. Li/Li+)) was greatly increased. On the other hand, it is understood that in Example 1 in which an aromatic compound having a silyl group was added, the increase in resistance after the charge- discharge cycle at a high voltage of 4.35 V was sufficiently suppressed. In particular, in Example 1, it is shown that the increase in resistance after the charge- discharge cycle test at 4.35 V was smaller than that after the charge- discharge cycle test at 4.20 V. That is, it can be said that such an effect of suppressing the increase in resistance is peculiarly produced when a nonaqueous electrolyte containing an aromatic compound having a silyl group is used and the energy storage device is used at a high voltage in which the maximum achieved potential of the positive electrode is 4.4 V (vs. Li/Li+) or more.

[0098]

[Example 2]

LiPF6 as an electrolyte salt was dissolved at a concentration of 1.2 mol/L in a nonaqueous solvent in which FEC and EMC were mixed in a volume ratio of 20 : 80, and to the resulting mixture, 0.2% by mass of the aromatic compound B and 2% by mass of the ester b were added to prepare a nonaqueous electrolyte of Example 2. Thereafter, in the same manner as in Example 1 described above, the nonaqueous electrolyte energy storage device of Example 2 was obtained.

[0099]

[Examples 3 to 6, Comparative Examples 2 to 3]

Nonaqueous electrolyte energy storage devices of Examples 3 to 6 and Comparative Examples 2 to 3 were obtained in the same manner as in Example 2 except that the aromatic compound shown in Table 2 was added as the aromatic compound or no aromatic compound was added. In Table 2, "-" indicates that no additive was added.

[0100]

[Evaluation]

Using the nonaqueous electrolyte energy storage devices of

Examples 2 to 6 and Comparative Examples 2 to 3, the initial charge- discharge test and the charge- discharge cycle test were carried out in the same manner as in the above "Charge-Discharge Cycle Test: 4.35 V". After the charge- discharge cycle test, the DCR (direct current resistance) of the nonaqueous electrolyte energy storage device at -10°C was measured. The battery was charged at constant current of 1.0 C up to SOC of 50% at 25°C, and then charged at constant voltage. The charge was performed for 3 hours in total of constant current charge and constant voltage charge.

Each battery was set to SOC of 50% by this charging. After holding for 5 hours at -10°C in a charged state of SOC of 50%, a voltage (El) when discharged at 0.2 C (II) for 10 seconds and a voltage (E2) when discharged at 1.0 C (12) for 10 seconds were measured, respectively. Using the discharge current values II and 12 and the measured voltages El and E2, the DC resistance value (Rx) at -10°C was calculated by the following equation. Rx=|(El-E2)/(Il-I2)|

The DCR (%) with reference (100%) to the DCR of Comparative Example 2 is shown in Table 2.

[0101]

[Table 2]

[0102]

As shown in Table 2, with respect to DC resistance after the charge- discharge cycle at a high voltage of 4.35 V (the maximum achieved potential of the positive electrode^ about 4.45 V (vs. Li/Li+^), in Comparative Example 3 in which the aromatic compound not having a silyl group was added, the resistance was higher than that of Comparative Example 2 in which the aromatic compound was not added. On the other hand, in Examples 2 to 6 in which the aromatic compound having a silyl group was added, it is understood that the increase in resistance was suppressed by adding these aromatic compounds conversely. It is understood that among these

Examples, in Example 5 in which only phenylsilane was added as an aromatic compound, the resistance after the charge- discharge cycle test was particularly low.

[0103] [Examples 7 to 9 and Comparative Example 5]

Nonaqueous electrolyte energy storage devices of Examples 7 to 9 and Comparative Example 5 were obtained in the same manner as in Example 1 except that the aromatic compound shown in Table 3 was added as the aromatic compound. In Table 3, "^■" indicates that no additive was added.

[0104]

[Evaluation]

(Coulombic Efficiency: 4.35 V)

Initial charge- discharge tests and cycle tests were carried out using the nonaqueous electrolyte energy storage devices of Examples 1 and 7 to 9 and Comparative Examples 1 and 5. The initial charge- discharge test was carried out as follows. At 25°C, the battery was charged at constant current and constant voltage with a charge current of 1.0 C and an end-of- charge voltage of 4.35 V. The termination condition of the charge was made until the total charging time reached 3 hours. After a rest period of 10 minutes, a constant current discharge was performed with a discharge current of 1.0 C and an end-of-discharge voltage of 2.50 V. The cycle test was carried out as follows. At 45°C, the battery was charged at constant current and constant voltage with a charge current of 1.0 C and an end-of- charge voltage of 4.35 V. The termination condition of the charge was made until the total charging time reached 3 hours. Thereafter, a rest period of 10 minutes was provided. Thereafter, a constant current discharge was performed with a discharge current of 1.0 C and an end-of-discharge voltage of 2.50 V, and then a rest period of 10 minutes was provided. This charge- discharge was performed for 50 cycles.

[0105]

After the cycle test, the capacity check test was conducted under the following conditions. The battery was charged at a constant current of 1.0 C up to 4.35 V at 25°C and then charged at a constant voltage. The termination condition of the charge was made until the total charging time reached 3 hours. After a 10-minute rest period after charging, constant current discharge was carried out at 1.0 C to 2.50 V at 25°C. Thereby, the discharge capacity was measured and coulombic efficiency (%) was determined. The obtained coulombic efficiency is shown in Table 3.

[0106]

[Table 3]

[0107]

As shown in Table 3, in Examples 7 to 9 using the aromatic compounds B to D each having a silyl group and an acetylenediyl group, the coulombic efficiency after charge- discharge cycle at a high voltage of 4.35 V (the maximum achieved potential of the positive electrode^ about 4.45 V (vs. Li/Li+)) exceeds 60%. On the other hand, in Comparative Example 1 in which no aromatic compound was added, and in Example 1 and

Comparative Example 5 in which the aromatic compounds A and Y having neither a silyl group nor an acetylenediyl group were respectively used, the coulombic efficiency after the charge- discharge cycle was as low as 60% or less.

INDUSTRIAL APPLICABILITY

[0108]

The present invention is applicable to a nonaqueous electrolyte energy storage device used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like.

DESCRIPTION OF REFERENCE SIGNS

[0109]

l: Nonaqueous electrolyte energy storage device

2^'· Electrode assembly

3^: Container

4^'· Positive electrode terminal

4'^'· Positive electrode lead

5^ Negative electrode terminal

5': Negative electrode lead

20^ Energy storage unit

30^ Energy storage apparatus

Claims

1. A nonaqueous electrolyte energy storage device comprising: a nonaqueous electrolyte containing an aromatic compound having a silyl group,

wherein a maximum achieved potential of a positive electrode is 4.4 V (vs. Li/Li+) or more.

2. The nonaqueous electrolyte energy storage device according to claim 1, wherein the nonaqueous electrolyte further contains an ester having a sultone structure or a cyclic sulfate structure.

3. The nonaqueous electrolyte energy storage device according to claim 1 or 2, wherein the aromatic compound is represented by the following formula (l)^:

[Chem. Formula l]

4. The nonaqueous electrolyte energy storage device accordin claim 1, 2 or 3, wherein the silyl group is a group represented by -S1H3

5. The nonaqueous electrolyte energy storage device according to any one of claims 1 to 4, wherein the aromatic compound further has an acetylenediyl group.