WO2003105267A1

WO2003105267A1 - Secondary cell

Info

Publication number: WO2003105267A1
Application number: PCT/JP2003/007187
Authority: WO
Inventors: 野口　健宏; 山崎　伊紀子; 川崎　大輔; 沼田　達治
Original assignee: 日本電気株式会社
Priority date: 2002-06-06
Filing date: 2003-06-06
Publication date: 2003-12-18
Also published as: JP4281297B2; CN1701459A; CN100347902C; JP2004014270A

Abstract

A positive electrode active material having an average discharge potential of 4. 5 V or more to Li is used. A solvent of the electrolyte is a combination of a high-dielectric solvent such as ethylene carbonate and at least one of dimethyl carbonate or ethyl methyl carbonate. The decrease of the capacity due to cycle and the degradation of the reliability due to high temperature are prevented. The operating voltage is high.

Description

Secondary battery technology

The present invention relates to a secondary battery, and more particularly to a secondary battery including a positive electrode active material having an average discharge potential of 4.5 V or more with respect to Li metal. Background art

Lithium-ion secondary batteries are widely used for applications such as portable electronic devices and personal computers. It is also expected to be applied to automotive applications in the future. In these applications, there has been a demand for smaller and lighter batteries, but increasing the energy density of batteries has become an important technical issue. There are several possible ways to increase the energy density of lithium ion secondary batteries, and among them, raising the operating potential of the batteries is an effective means. The lithium ion secondary battery using a conventional lithium cobaltate (L i Co0 ₂₎ and lithium manganese acid (L i Mn ₂ 0 ₄₎ as a positive electrode active material, nothing Re also 4 V-class operating potential (average operating potential = 3.6 to 3.8 V: potential with respect to lithium metal). This is because the expression potential is regulated by the redox reaction of Co ion or Mn ion (Co ³⁺ — → Co ^{4 +} or Mn ³⁺ — → Mn ⁴⁺ ). On the other hand, it is known that a 5-V class operating potential can be realized by using, for example, a spinel compound in which Mn of lithium manganate is substituted with Ni or the like as an active material. Specifically, L i N i _0. By using SMRI !. ₅ 0 spinel compounds such as _4, 4 are known to exhibit a potential plateau 5 V or more regions (J. Electrochem. So. , Vol. 144, 204 (1997)). For such spinel compounds In this case, Mn exists in a tetravalent state, and the operating potential is regulated by the oxidation-reduction of Ni 2 ⁺ -→ Ni ^{4 +} instead of the oxidation-reduction of Mn ³⁺ ~~ Mn ⁴⁺ Becomes However, L i N i. . In ₅ Mn _x. ₅ 0 cells had use a 5 V class positive electrode material ₄ such as an active material, compared with the battery having a 4 V Kyukatsu substances such as _{L i Co0 2, L i Mn} 2 〇 ₄ However, since the positive electrode has a higher potential, a decomposition reaction of the electrolytic solution occurs, and there is a problem that a significant deterioration of the electrolytic solution accompanied by a decrease in capacity occurs in a charge / discharge cycle or when left in a charged state. Was. Furthermore, when operated in a high-temperature environment such as 50 ° C, the above phenomena tended to be remarkable.

In particular, in batteries using a 5 V class spinel-type lithium manganese composite oxide for the positive electrode and amorphous carbon for the negative electrode, capacity degradation occurs due to the accumulation of decomposition products of the electrolyte on the negative electrode surface. There was a problem that. Disclosure of the invention

In view of the circumstances described above, an object of the present invention is to provide a secondary battery that realizes a high operating voltage while suppressing a decrease in capacity due to a cycle and a decrease in reliability at a high temperature. The present invention achieves the above object by using a 5 V class spinel-type lithium manganese composite oxide as a positive electrode to improve the reduction in capacity that occurs in a battery using amorphous carbon as a negative electrode.

The present invention relates to a secondary battery including a positive electrode active material having an average discharge potential of 4.5 V or more with respect to Li metal and an electrolyte, wherein the electrolyte is a high dielectric constant solvent (component a) and another solvent (component b) comprising at least one of dimethyl carbonate and ethyl methyl carbonate.

As described above, in a secondary battery equipped with a 5 V class positive electrode active material, the voltage inside the battery was high, and the deterioration of the electrolyte was remarkable. As a result of intensive studies by the present inventors, when the above-mentioned solvent is selected as a solvent constituting the electrolytic solution, It was found that an electrolyte solution with little deterioration under voltage conditions and excellent durability can be realized.

In the secondary battery of the present invention, since the decomposition reaction of the electrolytic solution is reduced, the absolute amount of decomposition products of the electrolytic solution is small. Therefore, it is possible to suppress the deposition of the decomposition product on the negative electrode surface, which is the cause of the capacity reduction accompanying the cycle.

In addition, dimethyl carbonate and ethyl methyl carbonate are considered to have an effect of forming a film on the surface of the negative electrode during initial charge and discharge, and suppressing the decomposition products from being deposited on the surface of the negative electrode. The term “high dielectric constant solvent” used at the beginning of this book refers to a solvent having a relative dielectric constant of 40 or more, such as, for example, ethylene glycol, propylene carbonate, and butylene carbonate.

Here, JP-A-2000-133263 and JP-A-2001-357848 disclose, as a positive electrode active material, a compound in which part of Mn of spinel lithium manganate is substituted with another element such as A1. A secondary battery using a mixed solvent of ethylene monoponate and dimethyl carbonate as a solvent for an electrolytic solution is disclosed. However, these are techniques relating to a battery using a 4 V-class positive electrode active material, and are essentially different from the present invention using a 5 V-class positive electrode active material. Hereinafter, this point will be described.

The 4 V-class spinel lithium manganate and the compound in which a part of Mn is substituted by another element such as A1 described in the above publication use the oxidation-reduction potential of Mn ³⁺ -→ Mn ⁴⁺ Therefore, it is essential to include Mn ^{3 +} .

This Mn ³⁺ generates Mn ²⁺ by a reaction as shown in the following formula.

_{If _{2 M n 3+ → M n 2}} + + M n 4 + Mn 2+ was formed thereby that force be dissolved in the electrolytic solution, et al, Ru using the positive electrode active material, important to suppress the leaching of Mn Issues.

In a 4 V class positive electrode active material containing Mn ^{3 +} , the average valence of Mn ions is When changing between trivalent and tetravalent, there is a problem in that yarn strain occurs in the crystal, and the stability of the crystal structure is reduced, so that the capacity is deteriorated due to cycling. Was.

In the above-mentioned publication, countermeasures such as adjusting the composition of the positive electrode active material or adjusting the manufacturing conditions of the active material layer in order to solve such problems are taken.

On the other hand, in the present invention using a 5 V class positive electrode active material, Mn elution and a decrease in crystal structure stability caused by 4 V class spinel-type lithium manganate and the like are not so much a problem. The decomposition of the electrolyte that occurs when a voltage is applied becomes a problem. In a battery using a 5 V class positive electrode active material, Ni ²⁺ — → N i ⁴⁺ , Co ³⁺ — →, which has a higher potential than the redox potential of Mn ³⁺ —— Mn ^{4 +} Redox potential such as Co ⁴⁺ is mainly used. For this reason, most of Mn in the positive electrode active material exists in the form of Mn ^{4 +} , and Mn ^{3 +} is usually a trace amount. Therefore, in the present invention, the elution of Mn and a decrease in the stability of the crystal structure caused by 4 V-class spinel lithium manganate and the like do not cause much problems, and the electrolysis caused by another mechanism is not a problem. Preventing liquid deterioration is an important technical issue.

The present invention solves such a problem, and suppresses deterioration of an electrolytic solution caused by high voltage in a battery. Further, depending on the selection of the positive electrode active material and the negative electrode active material, the interaction between the active material and the electrolyte may occur, and the electrolyte may be significantly deteriorated. Is effectively suppressed. That is, the present invention is to solve a problem peculiar to the case where a 5 V class positive electrode active material is used, and to provide a long-life battery while realizing a high battery voltage.

The secondary battery may be configured to further include a negative electrode active material containing amorphous carbon.

When amorphous carbon is used as the negative electrode active material, the decomposition product Since the deposition is further reduced, the cycle characteristics are further improved.

In a preferred embodiment of the secondary battery according to the present invention, a volume ratio of the component a to the electrolytic solution is in a range of 10 to 70%.

Here, the component b is preferably a solvent having a low relative dielectric constant, contrary to the component a. For example, a mixture containing dimethyl carbonate: 3.1 and ethyl methyl carbonate: 2.9 can be mentioned. In general, high dielectric constant solvents have high viscosity and low dielectric constant solvents have low viscosity. In the present invention, by setting the volume ratio of the component a as described above, the relative dielectric constant and viscosity of the entire electrolytic solution are appropriately maintained. Thereby, it is possible to further suppress the deposition of the decomposition product on the surface of the negative electrode while securing the conductivity of the electrolyte solution.

Further, in the secondary battery, the high dielectric constant solvent may be ethylene carbonate or propylene carbonate. By selecting such a solvent as the high dielectric constant solvent, a secondary battery having good cycle characteristics can be realized.

In the secondary battery, the positive electrode active material may be a spinel-type lithium manganese composite oxide. According to this configuration, a high-capacity secondary battery having a stable and high operating voltage can be obtained.

In the secondary battery, the spinel-type lithium manganese composite oxide is represented by the following general formula (I):

L ia (N i _x Mn ₂ — _x _ _y M _y ) (0 ₄ _ _W Z _W ) (I)

(Where 0.4 x x 0.6, 0 ≤ y 0 ≤ z, x + y x 2, 0 ≤ ≤ 1, 0 ≤ a ≤ l. 2. M is L i, A l , Mg, Ti, Si, and Ge. Z is at least one of F or C1.)

May be a spinel-type lithium-manganese composite oxide. this Such a spinel-type lithium manganese composite oxide has a charge / discharge region in the range of 4.5 V to 4.8 V with respect to Li metal, and a discharge capacity of 4.5 V or more has a discharge capacity of 11 OmAh / g and very high capacity.

According to the study of the present inventors, the deterioration of the electrolytic solution in a battery using the spinel-type lithium manganese composite oxide represented by the general formula (I) as a positive electrode active material is inferior due to high voltage. It has been found that considerable deterioration exceeding the degree of the dagger occurs. This is considered to be due to some unfavorable interaction between the positive electrode active material and the electrolyte.

Therefore, the present inventors further studied and, when using a spinel-type lithium manganese composite oxide represented by the formula (I) and an electrolytic solution containing at least one of dimethyl carbonate and ethyl methyl carbonate, It has been found that the synergistic effect of the spinel-type lithium manganese composite oxide represented by the formula (I) and the electrolyte solution can effectively suppress the deterioration of the electrolyte solution.

Therefore, the secondary battery of the present invention can maintain the excellent performance of the spinel-type lithium manganese composite oxide represented by the general formula (I) for a long time even after many cycles. .

In the above secondary battery, y in the general formula (I) may satisfy a relation of 0 <y. Further, in the above secondary battery, w in the above general formula (I) may satisfy a relationship of 0 <w ≦ 1. L i N i _x Mn ₂ - by replacing _x 0 ₄ Mn or 〇 some other elements in the crystal structure of the compound makes it possible to stabilize I spoon. Therefore, since the decomposition reaction of the electrolyte solution can be reduced, the cycle characteristics are improved for the same reason as described above.

From the viewpoint of securing a sufficient capacity, it is preferable that y in the general formula (I) satisfies the relationship of 0 <y and 0.3. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a sectional view of a secondary battery according to one embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

The secondary battery of the present invention includes a positive electrode using a lithium-containing metal composite oxide as a positive electrode active material, and a negative electrode having a negative electrode active material capable of inserting and extracting lithium. Separation is performed between the positive electrode and the negative electrode so as not to cause electrical contact. The positive electrode and the negative electrode are in a state of being immersed in an electrolyte having lithium ion conductivity, and are in a state of being sealed in a battery case.

In the secondary battery of the present invention, a positive electrode active material having an average discharge potential of 4.5 V or more with respect to Li metal is used. For example, a lithium-containing composite oxide is preferably used. As the lithium-containing composite oxide, L

(M = N i, Co, C r, Cu, F e) spinel-type lithium manganese complex oxide represented by, L IMP_〇 _{4 (M = Co, N i} , F e) olivine type lithium represented by containing double engagement oxides, such as L i N i V0 ₄ inverse spinel type lithium-containing composite oxide such as are exemplified.

Among the positive electrode active material, 130 mA h / g or more high capacity can be obtained, using _{_{L i N i x Mn 2 _}} x 〇 ₄ is a spinel-type lithium manganese complex oxide having a stable crystal structure Is preferred. The composition ratio X of Ni in this active material is in the range of 0.4 to 0.6. By doing so, a sufficient discharge region at 4.5 V or more can be secured and the energy density can be improved.

As the positive electrode active _{_{material, L i N i x Mn 2}} x 〇 part of Mn in _{4 L i, A 1, Mg} , T i, S, improved further cycle characteristics when used as substituted by the Ge I do. The reason for this is that by substituting a part of Mn with the above element, the crystal structure of the active material is further stabilized. For this reason, the amount of electrolyte Since the solution is suppressed, the amount of decomposition products generated from the electrolytic solution is reduced. Therefore, it is presumed that the deposition of decomposition products of the electrolytic solution on the negative electrode is reduced.

Further, in the active material in which a part of 〇 in the above active material is substituted by F, C1, or the like, the crystal structure is further stabilized, so that better cycle characteristics are realized. In addition, in a system in which a part of Mn is replaced by a monovalent to trivalent element such as Li, Al, or Mg, the capacity decreases with the replacement amount as the Ni valence increases. Substitution of O by halogens such as F and C1 has the advantage that high capacity can be maintained because the increase in Ni valence is offset.

In the secondary battery of the present invention, amorphous carbon is used as the negative electrode active material. When amorphous carbon is used, the accumulation of decomposition products of the electrolytic solution on the negative electrode surface is reduced compared to when other materials such as Li metal and natural graphite are used, and the cycle characteristics are improved. Because. Here, the amorphous carbon in the present invention refers to a carbon material having a broad scattering band having an apex at 15 to 40 degrees in 20 value of X-ray diffraction using Cu K 線 ray. Say.

In the secondary battery of the present invention, a solvent obtained by combining a high-dielectric solvent and a low-dielectric solvent is used. As the low-dielectric solvent, dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC) is used. Is used. By selecting such a solvent, it is possible to obtain an electrolyte solution that does not easily decompose even under high voltage conditions and has excellent durability. Therefore, the amount of decomposition products generated from the electrolyte solution can be reduced, so that the deposition of the decomposition products on the negative electrode surface is significantly suppressed. For this reason, it is possible to further reduce the capacity reduction due to the cycle. This is because, when dimethyl carbonate and ethyl methyl carbonate are used, during the initial charge and discharge, a film containing phosphate and fluoride is formed on the negative electrode surface, and decomposition products generated on the positive electrode side are decomposed on the negative electrode surface. It is presumed that it has the effect of suppressing precipitation on the surface. Furthermore, when a 5 V class spinel-type lithium manganese composite oxide represented by the above general formula (I) is selected as the positive electrode active material and amorphous carbon is selected as the negative electrode active material, the following (i) And (ii) a secondary battery having better cycle characteristics can be obtained.

(i) When a 5 V class spinel-type lithium manganese composite oxide represented by the above general formula (I) is used as the positive electrode active material and an electrolytic solution containing DMC or EMC is used, these active materials and DMC or Since the synergistic effect with EMC occurs, the absolute amount of the electrolysis / night decomposition product can be significantly reduced.

(ii) Due to the synergistic effect of amorphous carbon and DMC or EMC, even for electrolyte decomposition products with a small absolute amount, deposition on the negative electrode surface using amorphous carbon is effectively suppressed. .

When an electrolyte containing DMC or EMC is applied to a secondary battery using a 4 V-class positive electrode active material, the above effects do not occur, and no remarkable improvement in cycle characteristics is observed. . In a secondary battery using a 4 V-class positive electrode active material, since the voltage is low, the decomposition of the electrolyte does not occur enough to affect the cycle characteristics. Therefore, in a secondary battery using a 4 V class positive electrode active material, a case where an electrolyte containing DMC or EMC is used and a case where an electrolyte containing another low dielectric constant solvent, for example, DEC is used There is no significant difference in cycle characteristics between and. On the other hand, as the high dielectric constant solvent, ethylene-based monoponate (EC), propylene-based carbonate (PC), butylene carbonate (BC), and carboxylactone (GBL) can be used.

In addition, from the viewpoint of ensuring conductivity, the volume ratio between the high dielectric constant solvent and the low dielectric constant solvent is preferably in the range of 10:90 to 70:30. With such a range, the relative dielectric constant and viscosity of the entire electrolytic solution can be made appropriate, and sufficient conductivity can be ensured. Further, from the viewpoint of reducing the deposition of decomposition products of the electrolytic solution on the negative electrode surface, the volume ratio between the high dielectric constant solvent and the low dielectric constant solvent should be in the range of 20:80 to 60:40. More preferably, it is more preferably in the range of 30:70 to 50:50. This is presumed to be because it is possible to enhance the effect of inhibiting the decomposition products of the electrolytic solution on the negative electrode surface and suppress the decomposition reaction of the electrolytic solution.

Next, the operation of the lithium ion secondary battery of the present invention will be described. When a voltage is applied to the positive electrode and the negative electrode, lithium ions are released from the positive electrode active material, and the negative electrode active material absorbs lithium ions to be charged. On the other hand, contrary to charging, electrical contact between the positive electrode and the negative electrode occurs outside the battery, so that lithium ions are released from the negative electrode active material and lithium ions are occluded in the positive electrode active material, thereby causing discharge. Occur.

Next, a method for manufacturing a positive electrode active material will be described.

In the case of using the spinel-type lithium manganese complex oxide as the positive electrode active material, a manufacturing raw material of the positive electrode active material, the L i _{_{feedstock, L i 2 C0 3, L}} i OH, L i 2 0, L i 2 S0 ₄ and the like can be used, such as L i ₂ C0 _3, L I_〇_H are suitable. The Mn material, electrolytic manganese dioxide (EMD) * Mn ₂ 〇 _3, M n ₃ 0 _4, various Mn oxides such as chemical manganese dioxide (CMD), MnC_〇 _3> Mn S_〇 ₄ the use such as Can be. The N i raw material, such as N i 0, N i (OH ) 2, N i S0 4, N i (N0 3) 2 can be used. Oxides, carbonates, hydroxides, sulfides, nitrates and the like of the substitution element are used as the raw material of the substitution element.

In the case of Ni raw material, Mn raw material, and substitution element raw material, element diffusion may not easily occur during firing, and after the raw material firing, Ni oxide, Mn oxide, and substitution element oxide remain as different phases. Sometimes. For this reason, the Ni raw material, the Mn raw material, and the substitute element raw material are dissolved and mixed in an aqueous solution, and then the hydroxide, sulfate, carbonate, nitrate, etc. It is possible to use a mixture of Ni and Mn precipitated in the above or a mixture of Ni and Mn containing a substitution element as a raw material. It is also possible to use a Ni, Mn oxide or a mixed oxide of Ni, Mn and a substitution element obtained by firing such a mixture. When such a mixture is used as a raw material, Mn, Ni, and the substitution element diffuse well at the atomic level, and it becomes easy to introduce Ni and the substitution element into the 16d site of the spinel structure. In addition, as a halogen raw material of the positive electrode active material, haptic compounds such as LiF and LiC1 are used.

These raw materials are weighed and mixed so as to have a target metal composition ratio. The mixture is pulverized and mixed by a pole mill or the like. The mixed powder is calcined at a temperature of 600 ° C. to 1000 ° C. in air or oxygen to obtain a positive electrode active material. It is desirable that the firing temperature be high in order to diffuse each element. However, if the firing temperature is too high, oxygen deficiency occurs, which adversely affects battery characteristics. For this reason, it is desirable that the temperature is about 500 ° C. to 800 ° C. in the final firing step.

Also, in the case where an olivine-type lithium-containing composite oxide or an inverse spinel-type lithium-containing composite oxide is used as the positive electrode active material, it can be obtained by mixing and diffusing the necessary elemental materials and firing as described above. it can.

The specific surface area of the obtained lithium metal composite oxide is, for example, desirably 3 m ² / g or less, and preferably lm ² / g or less. The larger the specific surface area, the more binder is required, which is disadvantageous in terms of the capacity density of the positive electrode.

The obtained positive electrode active material is mixed with a conductivity-imparting agent, and formed on a current collector with a binder. Examples of the conductivity-imparting agent include, in addition to a carbon material, a metal substance such as A1, a conductive oxide powder, and the like. As the binder, polyvinylidene fluoride (PVDF) or the like is used. A metal thin film mainly composed of A1 or the like is used as the current collector.

Preferably, the added amount of the conductivity-imparting agent is about 1 to 10% by weight. The amount is also about 1 to 10% by weight. This is because the larger the proportion of the active material weight, the larger the capacity per weight) ^. If the ratio between the conductivity-imparting agent and the binder is too small, the conductivity may not be maintained or a problem of electrode peeling may occur. The solvent used for the electrolytic solution in the present invention is as described above. Further, cyclic solvents such as vinylene carbonate (VC), methyl carbonate (DEC), dipropyl carbonate ( Chain forces such as DPC), aliphatic force esters such as methyl formate, methyl acetate, ethyl ethyl propionate, and other lactones such as arptyrolactone, 1,2-ethoxyethane ( Chain ethers such as DEE) and ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; dimethylsulfoxide; 1,3-dioxolane; formamide; Methylformamide, dioxolan, acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid Ester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl_2-oxazolidinone, propylene glycol-one-potato derivative, tetrahydrofuran derivative, ethyl ether, 1,3- One or more aprotic organic solvents such as propane sultone, anisol, N-methylpyrrolidone, and fluorinated carboxylic acid esters can be used. A lithium salt is dissolved in these organic solvents. Examples of the lithium salt For example _{L i PF 6, L iAs F} 6, L i A 1 C 1 4, L i C 10 4, L i BF 4, L i SbF 6, L i CF3SO3, L i C 4 F ₉ C〇 ₃ , L i C (CF ₃ S〇 ₂ ) ₂ , L i N (CF ₃ S 0 ₂ ) ₂ , L i N (C ₂ F ₅ S0 ₂ ) ₂ , L i B ₁₀ C 1 ₁₀ , lower Lithium aliphatic carboxylate, lithium chloroporan, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, imides and the like.

Further, a polymer electrolyte may be used instead of the electrolytic solution. The electrolyte concentration is For example, from 0.5 mo 1 Z 1 to 1.5 mo 1 Z l. If the concentration is too high, the density and viscosity will increase. If the concentration is too low, the electric conductivity may decrease.

As the negative electrode active material, various carbon materials such as natural graphite and artificial graphite can be used as main components, and among them, amorphous carbon is preferable. By doing so, the deposition of decomposition products of the electrolytic solution on the negative electrode surface can be reduced, which can contribute to the improvement of cycle characteristics.

Further, the negative electrode active material may include a material capable of inserting and extracting lithium as an auxiliary component. As a material capable of occluding and releasing lithium, a mixture of carbon material, Li metal, Si, Sn, A1, Sio, Sn 、 and the like can be used. The negative electrode active material is formed on the current collector with the conductivity imparting agent and the binder. Examples of the conductivity-imparting agent include, in addition to a carbon material, a powder of a conductive oxide. Polyvinylidene fluoride or the like is used as the binder. A metal thin film mainly composed of Cu or the like is used as the current collector.

The lithium secondary battery according to the present invention has a structure in which the negative electrode and the positive electrode are laminated via a separator in a dry air or an inert gas atmosphere, or the laminated product is wound, and then housed in a battery can or a synthetic resin. The battery can be manufactured by sealing with a flexible film or the like made of a laminate of metal and metal foil. FIG. 1 shows a form of a coin type cell as an embodiment of a battery. The present invention has no limitation on the shape of the battery, and can take a form such as a wound type, a laminated type, or the like, with the positive electrode and the negative electrode opposed to each other with the separator interposed therebetween. , Square cells and cylindrical cells can be used. Example

Hereinafter, the present invention will be described in detail with reference to examples. In the present embodiment, a form of a coin type cell as shown in FIG. 1 is shown. The 22 types of batteries shown in Tables 1 to 4 were manufactured by the following procedure.

(Preparation of Positive Electrode) Mn, N i, L i , T i, S i, each, a source of A to F Mn_〇 _{2, N i 0, L i} 2 C_〇 _{_{3, T i 0 2, S}} i 0 _2, a l ₂ 〇 _3, L i F were weighed so as to metal composition ratio of the object, and mixed and ground. Note that LiF was also used as the source of Li. Next, the powder after mixing the raw materials was fired at 750 C for 8 hours. It was confirmed that all the crystal structures thus obtained had a substantially single-phase spinel structure. In addition, as shown in Table 1, all of the prepared active materials have an average discharge potential with respect to Li metal of 4.5 V or more.

The prepared positive electrode active material and carbon as a conductivity-imparting agent were mixed and dispersed in a solution in which polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone as a binder to form a slurry. The weight ratio of the positive electrode active material, the conductivity-imparting agent, and the binder was set to 88: 6: 6. The slurry was applied on the A1 current collector. Then, it was dried in a vacuum for 12 hours to obtain an electrode material. The electrode material was cut into a circle having a diameter of 12 mm. Thereafter, pressure molding was performed at 3 tZcm ² to obtain a positive electrode current collector 3 and a positive electrode active material layer 1.

(Preparation of negative electrode) In the case of a battery using Li metal as the negative electrode active material, a lithium metal disk is placed on a current collector made of Cu and cut into a circle with a diameter of 13 mm. Electric conductor 4 and negative electrode active material layer 2 were obtained.

In the case of a battery using natural graphite as the negative electrode active material, natural graphite is mixed with carbon, a conductivity-imparting agent, and dissolved in N-methylpyrrolidone with polyfukkabinidene (PVDF). It was dispersed to form a slurry. The weight ratio of natural graphite, conductivity-imparting agent, and binder was set to 91: 1: 8. A slurry was applied on the Cu current collector. Then, it was dried in a vacuum for 12 hours to obtain an electrode material. The electrode material was cut into a circle with a diameter of 13 mm. Thereafter, pressure molding was performed at 1 t / cm ² to form a negative electrode current collector 4 and a negative electrode active material layer 2. In the case of a battery using amorphous carbon as the negative electrode active material, the battery was manufactured in the same manner as the battery using natural graphite. Note that Rikiichi Potron (registered trademark) P manufactured by Kureha Chemical Co., Ltd. was used as the amorphous carbon.

For Separation 5, a polypropylene film was used. The positive and negative electrodes were placed facing each other with no electrical contact across the separator, and this was covered with a positive electrode outer can 6 and negative electrode outer can 7 as shown in Fig. 1. It was filled with the electrolyte solution in the ratio (volume ratio), and sealed with insulating packing 8.

Li PF ₆ was used as the electrolyte supporting salt, and the concentration was 1 mo 1ZL. The cycle characteristics of the batteries 1 to 16 fabricated as described above were evaluated. In the evaluation, the battery was charged to 4.8 V at a charge rate of 1 C and discharged to 2.5 V at a rate of 1 C. Here, “charging at a charging rate of 1 C” refers to charging in which the capacity of the battery is used as the current value of the charging current in amps and hours. Means the number 1Z10. The test temperature was 45. The results are as shown in Table 1.

¾】 1 (Examination of negative electrode active material)

By comparing batteries 1, 2, and 4 in Table 1, it can be seen that cycle reliability is higher when amorphous carbon is used than when Li metal or natural graphite is used as the negative electrode. Also, by comparing Batteries 3 and 9, it was found that even in the case of the battery using ECZ DMC as the electrolyte, the cycle characteristics were better when amorphous carbon was used than when natural graphite was used. From the above, it was considered preferable to employ amorphous carbon as the negative electrode in a battery using a 5 V-class positive electrode active material. This was presumed to be that when amorphous carbon was used as the negative electrode, the amount of decomposition products of the electrolytic solution deposited on the negative electrode surface was smaller than when other materials were used.

(Study of solvent)

Hereinafter, a _{_{L i N i 0. 5 Mn 5}} 0 4 as a positive electrode active material, to study the effect of the solvent by comparing the cell 4-9 using the amorphous carbon as an anode active material. In general, as a solvent constituting an electrolytic solution, a combination of an electrolytic solution having a high viscosity and a high dielectric constant and a solvent having a low viscosity and a low dielectric constant is used. In this example, a study was made using EC or PC as a solvent having a high viscosity and a high dielectric constant, and using DEC, EMC or DMC as a solvent having a low viscosity and a low dielectric constant.

Here, a solvent with low viscosity and low dielectric constant is fixed and examined. That is, when comparing batteries 4 and 5 (fixed to DEC), batteries 6 and 7 (fixed to EMC), or batteries 8 and 9 (fixed to DMC), EC as a solvent with high viscosity and high dielectric constant Although the cycle characteristics tended to be better in the case of using PC than in the case of using PC, no remarkable difference occurred.

Next, a high-viscosity, high-dielectric-constant solvent is fixed to EC or PC and examined. When comparing batteries 4, 7, and 9 (fixed to EC), batteries 7 and 9 using EMC or DMC have excellent cycle characteristics with a capacity retention rate of 75% or more after 100 cycles. The battery showed good cycle characteristics than the battery 4 using DEC. Similar tendency was observed when batteries 5, 6, and 8 were compared (fixed to PC), and batteries 6 and 8 using EMC or DMC were superior to batteries 5 using DEC. The cycle characteristics were shown.

From the above, it was found that EMC or DMC is preferably used as the solvent having low viscosity and low dielectric constant.

Here, it was examined whether or not the above-mentioned remarkable effects are exhibited when EMC or DMC is used as a solvent having a low viscosity and a low dielectric constant in a battery including a 4 V-class positive electrode active material.

Table 2, L IMN ₂ 0 ₄ is a 4V class cathode active material, or a 5V-grade positive electrode active material L i N i ^ sMn ^ sT i. . Using i ₅ 〇 ₄ illustrates a battery 17 to 19 using a solvent of low viscosity 'low dielectric constant indicated respectively, and the sub Ikuru characteristics of the battery 1 5, 20, 21.

Positive electrode material retention ratio Battery Positive electrode active material L i Average metal solvent release composition and volume ratio Negative electrode active material 45 ° C 300 ° 45 ° C 500 °

Electric potential After cycle After cycle

17 iπ ₂ θ4 4.03V PC / DEC = 40/60

½1½ Says fire 78%

_{_{18 L i Mn 2 0 4 4.03V}} PC / EMC = 40/60 amorphous carbon 80¾

CD

19 and 'ι 2. ₄ 4.03V PC / DMC = 40/60,

ρ says shell ash S¾ 84%

20 LiNio.5Mn1.35Tio.15O 4.68V EC / DEC = 40/60 Amorphous carbon 40% x 10%

_{_{_{2] LiNi 0 .5Mn I .35Ti 0}}} ., 5 0 4.68V EC / EMC = 40/60 amorphous carbon 47% 20%

15 iNio.sMnusTio. 0 ₄ 4.68V EC / DMC = 40/60 Amorphous gray 74¾. 53¾

: Nen] 2 Referring to the capacity retention after 500 cycles for batteries 17 to 19 equipped with a 4 V-class cathode active material in Table 2, batteries using EMC or DMC as solvents with low viscosity and low dielectric constant18, It can be seen that the result of 19 is about 2 to 6% better than that of the battery 17 using DEC. On the other hand, referring to the capacity retention rates after 500 cycles for batteries 15, 20, and 21 equipped with a 5 V-class positive electrode active material, batteries 15 and 21 using EMC or DMC are the same as batteries using DEC. It was about 10-40% higher than the above, and a remarkable effect was recognized. Regarding batteries 15, 20, and 21, even when comparing the capacity retention rates at the time after 300 cycles, a remarkable effect was observed when EMC or DMC was used.

From the above results, it was clarified that the use of EMC or DMC as a solvent with low viscosity and low dielectric constant has a remarkable effect of improving cycle characteristics in batteries using 5 V class active materials. .

Next, the following study was conducted to clarify why EMC or DMC is preferred as a solvent with low viscosity and low dielectric constant.

In a battery whose capacity has decreased after cycling, the difference between the charge and discharge capacity value at 1 C (high rate) and the charge and discharge capacity value at 0.1 C (low rate) increases. Such a phenomenon is considered to be due to an increase in the impedance inside the cell.

Here, assuming that the increase in impedance is R and the current value is I, a voltage higher by the amount of IR is required to charge up to the design capacity. However, when charging a lithium ion secondary battery, charging is stopped when a preset voltage is reached, or charging is performed at a low voltage for a certain period of time. Charging will end. For this reason, the larger the impedance increase R and the larger the current value I, the smaller the charge / discharge capacity value. Due to such a phenomenon, the difference between the capacitance value at a high rate and the capacitance value at a low rate becomes remarkable as R increases. Table 3 shows Li N i 0. sMn ^ ₃₅ Ti as the positive electrode active material. . ₁₅ 〇 _4, the amorphous carbon used as the anode active substance, ECZDEC as a solvent, EC / EMC, battery 20 using EC ZDMC, 21, relates to 15, (1C charge and discharge capacity) after 300 cycles Z (0.1 C charge / discharge capacity).

(1 C charge / discharge capacity) /

Battery Positive active material Solvent composition and volume ratio Negative active material

(0.1 C charge / discharge capacity)

_{_{_{20 LiNi 0. 5 n 1 .3}}} 5 Ti 0. 15 O 4 EC / DEC = 40/60 db in B

§F says gray ^ i 60%

21 40/60 b says

_{_{_{Li i 0. 5 n 1.}}} 35 Tio.i50 EC / EMC = f ÷ i or

3F Sun Ash 67%

15 EC / DMC = 40/60 db Θ ffj?

Ash 81%

¾】 3 As shown in Table 3, the value of (1 C charge / discharge capacity) / ^ (0.1 C charge / discharge capacity) after 300 cycles differs depending on the solvent used, and in battery 20 using DEC, The value of (1 C charge / discharge capacity) Z (0.1 C charge / discharge capacity) is lower than that of batteries 21 and 15 using EMC or DMC. From this, it can be said that the battery 20 has a larger difference in capacity value between the high rate and the low rate as compared to the battery 21 or 15. Therefore, it is considered that the impedance of the battery 20 increased more with the cycle than the batteries 21 or 15. It is considered that such an increase in impedance is mainly caused by the deposition of decomposition products of the electrolytic solution on the negative electrode surface.

In summary, when EMC or DMC is used as a solvent with low viscosity and low dielectric constant, the amount of electrolyte decomposed on the negative electrode surface is smaller than when DEC is used. This is thought to contribute to the improvement of cycle characteristics.

Here, in a battery using DMC as a solvent having a low viscosity and a low dielectric constant, the volume ratio of a solvent having a high viscosity and a high dielectric constant and a solvent having a low viscosity and a low dielectric constant is the above (1C charge / discharge capacity) / ( In order to investigate the effect on the value of (0.1 C charge / discharge capacity), the batteries shown in Table 4 were evaluated.

¾】 4 Battery 9 has the same configuration as battery 9 shown in Table 1, and battery 22 has the same configuration as battery 9 except that the volume ratio of EC, which is a high-viscosity, high-dielectric constant solvent, was set to 50%. Battery.

As shown in Table 4, (1 C charge / discharge capacity) / (0.1

(Charging / discharging capacity of C) was about 90% for both batteries 9 and 22, with no remarkable difference, suggesting that the accumulation of decomposition products of the electrolytic solution on the negative electrode surface after 200 cycles was small. Was done. Also, regarding the capacity retention rate after 200 cycles, no remarkable difference was observed between batteries 9 and 22. From this, it was considered that setting the volume ratio of EC, which is a solvent of high viscosity and high dielectric constant, to 40 to 50% is appropriate from the viewpoint of obtaining good cycle characteristics.

(Examination of positive electrode active material in which part of Mn is replaced by other elements)

Returning to Table 1, the study on the positive electrode active material is shown below.

Cell 10, 12, 14, 15, _{_{16, L i N i 0. 5}} Mn 5 0 out of ₄ each A 1 part of Mn, L i, S i, T i, the positive electrode active substituted by G e It is a battery that uses substances. And these cells is compared with the battery 9 using _{_{L i N i 0. 5 Mn x}} _ 5 0 4 as the positive electrode active material, L i N i. . By replacing SMN ^ ₅ 0 the element a over portions of 1 in _4, it became clear that the capacity maintenance rate after 100 cycles and after 300 cycles is further improved. Among them, L i N i Q. sMn. 5 〇 cell 15 a portion of Mn of ₄ was replaced by T i is very good and has a cycle characteristics, discharge potential other active material for a pair i metal The use of a higher active material than this makes it an excellent battery from the viewpoint of energy density.

As above, L i N i. . The part of Mn of SMN ^ ₅ 0 ₄ by substitution by the element, this crystal structure of the positive electrode active material is stabilized, the deterioration is reduced It is inferred that

(Examination of active material in which part of O is replaced with F)

Batteries 11 and 13 are batteries using a positive electrode active material in which part of 0 in the positive electrode active materials of batteries 10 and 12 was replaced with F, respectively. As is apparent from a comparison between the batteries 10 and 11 or the batteries 12 and 13, the cycle characteristics are further improved by substituting a part of 0 with F.

L i in N i 0. ₅ M n ₅ 0 system substituted by 1-3 bivalent element part of M n of _4, the valence of N i increases. This increase in the valence of Ni causes the crystal structure to become unstable and the capacity to decrease. Thus, in the positive electrode active materials of the batteries 11 and 13, a part of O is replaced with F to offset an increase in the valence of Ni, thereby avoiding instability of the crystal structure. It is thought that the cycle characteristics were improved. At the same time, the capacity of the batteries 11 and 13 is improved compared to the batteries 10 and 12, respectively, because the capacity reduction is also avoided. In the above embodiments, descriptions have been given of the battery using the spinel-type lithium-manganese composite oxide as the positive electrode active material, other active material, for example, L i C o P 0 ₄ olivine-type lithium-containing composite oxide such as, also in L i N i V 0 ₄ employing battery reverse spinel-type lithium-containing composite oxide such as, the effects described in the above embodiment can be obtained.

As described above, according to the present invention, by using an electrolytic solution containing a high dielectric constant solvent and at least one of dimethylcapone and ethylmethylcaponate, the capacity decreases due to cycling and the reliability at high temperatures is reduced. Thus, it is possible to provide a secondary battery that achieves a high operating voltage while suppressing a decrease in performance.

Claims

The scope of the claims

1. A secondary battery comprising a cathode active material having an average discharge potential of 4.5 V or more with respect to Li metal and an electrolyte, wherein the electrolyte has a high dielectric constant of 40 or more. A secondary battery comprising: a solvent having a ratio of at least one of dimethyl carbonate and at least one of ethyl methyl carbonate.

2. The secondary battery according to claim 1, further comprising a negative electrode active material containing amorphous carbon.

3. The secondary battery according to claim 1, wherein a volume ratio of the high dielectric constant solvent to the electrolytic solution is in a range of 10% to 70%.

4. The secondary battery according to claim 1, wherein the high dielectric constant solvent is ethylene carbonate or propylene carbonate.

5. The secondary battery according to claim 1, wherein the positive electrode active material is a spinel-type lithium manganese composite acid. 6. The spinel-type lithium manganese composite oxide has the following general formula (I):

L ia (N _x Mn ₂ _ _x _ _y My) (0 ₄ _ _W Z _W ) (I)

(Where 0.4 x x 0.

6, 0≤y, 0≤z, x + y2, 0≤w≤1, 0≤a≤1.2. M is at least one selected from the group consisting of Li, Al, Mg, Ti, Si and Ge. Z is at least one of F and C1. ) The secondary battery according to claim 5, which is a spinel-type lithium manganese composite oxide represented by the following formula:

7. The secondary battery according to claim 6, wherein y in the general formula (I) satisfies a relationship of 0 <y.

8. The secondary battery according to claim 6, wherein w in the general formula (I) satisfies a relationship of 0 <w≤l.