JP4544250B2 - Non-aqueous electrolyte lithium secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte lithium secondary battery.

In recent years, lithium secondary batteries using a carbon material as the negative electrode active material have remarkably improved cycle characteristics, replacing the conventional lithium secondary batteries that use lithium metals or lithium alloys that have had problems with cycle characteristics. For the first time, lithium secondary batteries were put to practical use as secondary batteries for portable electric devices such as mobile phones and portable video cameras. However, the phenomenon that the capacity gradually decreases by repeating charge and discharge is recognized, and the cycle characteristics are not always satisfactory, and further improvement in characteristics is required.
The cycle characteristics are not always satisfactory because the electrolyte solution solvent and electrolyte in the battery decrease due to chemical changes in addition to the structural change of the active material inside the electrode and the decrease in adhesion. It is considered that the overvoltage increases due to the deposition of the compound to be deposited on the negative electrode surface.

Improvement of the electrolytic solution for the purpose of improving the cycle performance in the lithium secondary battery has been conventionally performed, and as an example, ethylene carbonate (hereinafter, referred to as the component of the electrolytic solution when graphite-based carbon is used as the negative electrode active material). It may be referred to as EC.), And it has been put to practical use because the cycleability is improved.
However, since EC has a melting point higher than room temperature, it increases the viscosity of the electrolyte and lowers the conductivity. Therefore, the inclusion of EC contributes to the deterioration of the large current charge / discharge characteristics and the low temperature characteristics. In addition, an increase in overvoltage is observed due to the inclusion of EC, which contributes to cycle deterioration.

  An object of the present invention is to provide a lithium secondary battery having improved cycle characteristics and large current charge / discharge characteristics without impairing low temperature characteristics.

  In view of such circumstances, as a result of intensive studies by the present inventors, by using a negative electrode containing a polymer material having a carbonate group, cycle characteristics and large current charge / discharge characteristics of a lithium secondary battery Has been found to be improved, and the present invention has been achieved.

That is, the present invention includes (1) a positive electrode using a material capable of doping / dedoping lithium ions as an active material, a negative electrode using a carbon material capable of doping / dedoping lithium ions as an active material, the positive electrode, In a non-aqueous electrolyte lithium secondary battery including a separator sandwiched between opposing surfaces of a negative electrode and an electrolytic solution in which a solute composed of a lithium salt is dissolved in an organic solvent, the negative electrode is represented by the following general formula [I]. The present invention relates to a non-aqueous electrolyte lithium secondary battery including a polymer having a carbonate structure and a number average molecular weight of 300 to 200,000.

Further, in the present invention, (2) the polymer contains a carbonate structure represented by the general formula [I], and 50% or more of the contained carbonate structure is contained in the main chain of the polymer (1 ) Described in the non-aqueous electrolyte lithium secondary battery.
Further, in the present invention, (3) the polymer contains a carbonate structure represented by the general formula [I], and 50% or more of the contained carbonate structure is contained in the side chain of the polymer (1 ) Described in the non-aqueous electrolyte lithium secondary battery.

（式中、Ｒ₁ 、Ｒ₂ 、Ｒ₃ 、Ｒ₄ はそれぞれ独立に、水素原子、炭素数１０以下のアルキル基、炭素数１０以下のアラルキル基、炭素数１０以下のアリール基、炭素数１０以下のハロアルキル基のいずれかを表す。） In addition, the present invention relates to the nonaqueous electrolyte lithium secondary battery according to (2), wherein (4) the polymer includes a chemical structure represented by the following general formula [II].

(In the formula, R ₁ , R ₂ , R ₃ and R ₄ are each independently a hydrogen atom, an alkyl group having 10 or less carbon atoms, an aralkyl group having 10 or less carbon atoms, an aryl group having 10 or less carbon atoms, or 10 carbon atoms. It represents one of the following haloalkyl groups.)

（式中、Ｒは、炭素数１以上１０以下のアルキル基を表す。） Further, the present invention relates to the nonaqueous electrolyte lithium secondary battery according to (4), wherein (5) the polymer is composed of a repeating unit represented by the following general formula [III].

(In the formula, R represents an alkyl group having 1 to 10 carbon atoms.)

The present invention also relates to the nonaqueous electrolyte lithium secondary battery according to (4), wherein (6) the polymer is composed of a repeating unit represented by the following general formula [IV].

In the negative electrode according to the present invention and a lithium secondary battery using the same, when EC is not contained in the electrolytic solution, cycle performance and large current charge / discharge characteristics are improved as compared with a lithium secondary battery using a conventional negative electrode. . In addition, even when the negative electrode according to the present invention and the lithium secondary battery using the negative electrode contain EC in the electrolyte, the cycle characteristics are higher than in the case where the conventional lithium secondary battery using the negative electrode contains EC. There is no drop and large current charge / discharge characteristics are improved.
As a result, a lithium secondary battery having a long life and high current charge / discharge characteristics can be provided, and the industrial value is extremely high.

Next, the present invention will be described in detail.
First, the negative electrode in the present invention includes a carbon material that can be doped / undoped with lithium ions as an active material, a polymer having a carbonate structure represented by the general formula [I], and, if necessary, polyethylene, An appropriate binder such as propylene and fluororesin, and further a conductive material, if necessary, are mixed and fixed to the current collector sheet by a method such as coating or stretching.
The number average molecular weight of the polymer is 300 to 200,000, preferably 500 to 150,000.
The total weight of the polymer and the binder is preferably about 0.1 to 20 parts by weight with respect to 100 parts by weight of the total amount of carbon powder used in the negative electrode. More preferably, it is 1 to 10 parts by weight.

The polymer includes a carbonate structure represented by the general formula [I]. Specifically, the polymer includes and includes a carbonate structure represented by the general formula [I]. 50% or more of the carbonate structure contained in the main chain of the polymer, or the polymer contains a carbonate structure represented by the general formula [I] and 50 of the carbonate structure contained % Or more is included in the side chain of the polymer.
Among these, those containing 50% or more of the carbonate structure contained in the main chain of the polymer are preferred, and those having a chemical structure represented by the general formula [II] are particularly preferred, R ₁ , More preferably, R ₂ , R ₃ and R ₄ are all hydrogen atoms.
Moreover, the polymer which consists of a repeating unit represented by general formula [III] is preferable, and the polyethylene carbonate which consists of a repeating unit represented by general formula [IV] as this polymer is especially preferable.
Polyethylene carbonate in the present invention is described in, for example, Polymer Letters, vol. 7, 287 (1969), or Makromol. Chem. , Vol. 130, 210 (1969), and a method of alternately copolymerizing ethylene oxide and carbon dioxide using a catalyst.

Examples of the carbon material that can be doped / undoped with lithium ions in the present invention include natural graphite, artificial graphite, coke, carbon black, pyrolytic carbon, carbon fiber, and a carbon material obtained by firing a polymer compound. It can be illustrated. Moreover, the composite material which has these carbon materials as a main component can be illustrated. Among these, it is preferable that the graphite-based material is contained in that the charge / discharge capacity per unit weight is large and the average potential during charge / discharge is low.
It does not matter whether the graphite contained in the graphite-based material is natural graphite or artificial graphite. Examples of natural graphite include Sri Lankan graphite, Madagascar graphite, Korean flake graphite, Korean soil graphite, Chinese graphite and the like. Further, graphite obtained by further heating, processing and modifying the natural graphite may be used. Examples of the artificial graphite include graphitized products such as coke materials, graphitized products of mesomicrobeads, graphitized products of mesophase pitch-based carbon fibers, and the like.

The active material contained in the positive electrode as a material capable of doping and dedoping lithium ions in the present invention includes a layered lithium composite oxide based on a so-called α-NaFeO ₂ type structure, and a lithium composite oxidation based on a spinel type structure. And transition metal chalcogenides. In particular, a layered lithium composite oxide based on an α-NaFeO ₂ type structure is preferable because high voltage and high energy density can be obtained and cycle characteristics are excellent.
The positive electrode in the present invention is a material capable of doping and dedoping lithium ions as an active material, and further mixed with a suitable binder such as polyethylene, polypropylene, fluororesin, and conductive material powder, and applied, stretched The thing of the structure fixed to the electrical power collector sheet | seat by the method of these etc. is mentioned. The conductive material powder may be any conductive material that has a conductive effect and has resistance to the non-aqueous electrolyte to be used and electrochemical reaction at the positive electrode. For example, graphite powder, carbon black, coke powder, high conductivity Examples include molecules.
The layered lithium composite oxide based on the α-NaFeO ₂ type structure includes a layered lithium composite oxide containing at least one transition metal such as vanadium, iron, cobalt, nickel, etc., and a layered lithium composite oxide containing manganese in them. Examples are things. Among these, a layered lithium composite oxide mainly composed of a lithium / nickel composite oxide is preferable because it has excellent cycle characteristics.

Any conventionally known lithium salt can be used as the lithium salt in the present invention. LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ etc. can be exemplified. Of these, LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) _2, etc. having high electrical conductivity are preferable.
The electrolytic solution in the present invention contains at least one of the lithium salts, and the lithium salt is dissolved in a concentration range of 0.1 M (mol / l) to 2 M. Among these, a concentration range of 0.5M to 1.5M is preferable.

  Examples of the separator in the present invention include a porous film of an olefin resin such as fluorine resin, polyethylene, and polypropylene, an olefin resin such as fluorine resin, polyethylene, and polypropylene, and a nonwoven fabric such as nylon.

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.
The positive electrode of the lithium secondary battery subjected to the test was obtained by the method described below.
Addition of gallium obtained by mixing lithium nitrate, nickel carbonate and gallium nitrate so that Li: Ni: Ga = 1.05: 0.98: 0.02 and firing in an oxygen stream at 660 ° C. for 15 hours 1% by weight of acetylene black (trade name: Denka Black 50% press product, manufactured by Denki Kagaku Kogyo Co., Ltd.) having a number average primary particle size of 40 nm, 87% by weight of lithium nickelate powder, and a weight average particle size of 7. Polyvinylidene fluoride using N-methylpyrrolidone as a binder as a binder (commercially available from Kureha Chemical Industry Co., Ltd.) Name: KF # 1300] was added in an amount corresponding to 3% by weight and kneaded sufficiently to obtain a paste.

The gallium-added lithium nickelate powder was confirmed to have an α-NaFeO ₂ type structure by powder X-ray diffraction. The paste was applied to a 20 μm thick aluminum foil as a current collector, dried and pressed to form a sheet, and then cut into 1.3 × 1.8 cm pieces to obtain a positive electrode. The active material weight of this positive electrode is 40 mg to 45 mg.
A polypropylene porous film (manufactured by Daicel Chemical Industries, trade name: Celgard # 2400) was used as the separator.

The negative electrode carbon powder was obtained by the method described below. Heat-treated at 3000 ° C., specific surface area by nitrogen adsorption method is 9 m ² / g, number average particle diameter is 10 μm, true specific gravity is 2.26, lattice spacing d ₀₀₂ in X-ray diffraction is 3.36 mm, and ash content is 0.3. A specific surface area of 30 m ² / g, a true specific gravity of 2.04, and a number average primary particle diameter of 95% by weight of a natural graphite (made in Madagascar) powder of 95% by weight, graphitized at 2800 ° C. by a nitrogen adsorption method. 66nm pseudographitic carbon black powder [trade name: TB3800, manufactured by Tokai Carbon Co., Ltd.] 5% by weight and a silane coupling agent (trade name: A186, manufactured by Nihon Unicar Co., Ltd.) An amount equivalent to 1 part by weight of the dispersion in water was added and mixed well, followed by vacuum drying at 150 ° C. to obtain a carbon powder treated with a silane coupling agent.

The cycle characteristics of the discharge capacity of the obtained battery were tested by alternately repeating the conditions (1) and (2) described below at room temperature.
Cycling test conditions:
(1) After a constant current and constant voltage charge with a current density of 3.3 mA / cm ² , a maximum charging voltage of 4.24 V, and a charging time of 3 hours, discharge at a current density of 0.66 mA / cm ² and a final voltage of 2.75 V is performed. Do. This charging / discharging is continuously performed twice.
(2) After charging at a current density of 3.3 mA / cm ² , a maximum charging voltage of 4.24 V, a constant current and constant voltage charging for 1 hour, and discharging at a current density of 3.3 mA / cm ² and a final voltage of 2.75 V. Do. This charging and discharging is continuously performed 20 times.
Cycle efficiency: The value obtained by dividing the discharge capacity in the 90th charge / discharge by the discharge capacity in the second charge / discharge was defined as the cycle efficiency.
High load efficiency: The value obtained by dividing the discharge capacity in the third charge / discharge by the discharge capacity in the second charge / discharge was defined as the high load efficiency. The higher the load efficiency, the better the large current charge / discharge characteristics.

Example 1
With respect to 90% by weight of the silane coupling agent-treated material, polyethylene carbonate having a number average molecular weight of 50000 using N-methylpyrrolidone as a solvent (hereinafter sometimes referred to as PEC) is equivalent to 2% by weight and N- as a binder. Polyvinylidene fluoride using methylpyrrolidone as a solvent was added in an amount corresponding to 8% by weight and sufficiently kneaded to obtain a paste.
The paste was applied to a 10 μm-thick copper foil as a current collector, then dried, pressed into a sheet, and cut into 1.5 × 2 cm pieces to obtain a PEC-containing negative electrode.
As described above, dimethyl carbonate (hereinafter sometimes referred to as DMC) is used as the non-aqueous electrolyte solvent, and a non-aqueous electrolyte solution in which LiPF ₆ is dissolved in the solvent to a concentration of 1 mol / liter is used. The positive electrode thus obtained and the PEC-containing negative electrode were opposed to each other with a separator interposed between them and housed in a stainless steel container to produce a battery A1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

Comparative Example 1
To 90% by weight of the silane coupling agent-treated material, polyvinylidene fluoride using N-methylpyrrolidone as a solvent as a binder was added in an amount corresponding to 10% by weight and sufficiently kneaded to obtain a paste. The paste was applied to a 10 μm-thick copper foil as a current collector, dried and pressed to form a sheet, and then cut into 1.5 × 2 cm pieces to obtain a negative electrode containing no PEC.
As the non-aqueous electrolyte, one having the same composition as in Example 1 was used, and the positive electrode and the containing negative electrode not containing PEC were opposed to each other through a separator and housed in a stainless steel container to produce a battery R1. The battery was charged and discharged under the same conditions as in Example 1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

Example 2
As a non-aqueous electrolyte solvent, a mixed solution of DMC and ethyl methyl carbonate (hereinafter sometimes referred to as EMC) in a volume ratio of 1: 1 is used, and LiPF ₆ is used as an electrolyte in the solvent so as to have a concentration of 1 mol / liter. Using the non-aqueous electrolyte dissolved in, the positive electrode prepared in the same manner as in Example 1 and the PEC-containing negative electrode were opposed to each other through a separator, and housed in a stainless steel container to prepare a battery A2.
The battery was charged and discharged under the same conditions as in Example 1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

Comparative Example 2
A solution having the same composition as that of Example 2 was used as the non-aqueous electrolyte, and the positive electrode and the negative electrode not containing PEC were opposed to each other with a separator in the same manner as in Comparative Example 1 and housed in a stainless steel container to prepare battery R2. .
The battery was charged and discharged under the same conditions as in Example 1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

Example 3
As a non-aqueous electrolyte solution, a mixed solution of EC, DMC, and EMC with a volume ratio of 30:35:35 is used, and a non-aqueous electrolyte solution in which LiPF ₆ is dissolved in the solvent so as to be 1 mol / liter is used. A positive electrode and a PEC-containing negative electrode prepared in the same manner as in Example 1 were opposed to each other with a separator interposed between them, and housed in a stainless steel container to prepare a battery A3.
The battery was charged and discharged under the same conditions as in Example 1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

Comparative Example 3
A solution having the same composition as that of Example 3 was used as the non-aqueous electrolyte, and the positive electrode and the negative electrode not containing PEC were opposed to each other through a separator in the same manner as in Comparative Example 1 and housed in a stainless steel container to prepare battery R3. .
The battery was charged and discharged under the same conditions as in Example 1.
Table 1 shows the measurement results of cycle efficiency and high load efficiency.

表１より、電解液溶媒にＥＣを含んでいない場合、本発明にかかる電池Ａ１とＡ２は電池Ｒ１とＲ２と比較しサイクル性に格段に優れている。また、電解液溶媒にＥＣを含む場合では、本発明にかかる電池Ａ３はＲ３と比べサイクル性が低下していない。さらに、電池Ａ１とＡ２は電池Ａ３およびＲ３に比べてサイクル性が向上している。また、本発明にかかる電池Ａ１、Ａ２、Ａ３はそれぞれ電池Ｒ１、Ｒ２、Ｒ３に比べて高負荷効率が向上している。

From Table 1, when EC is not contained in electrolyte solution solvent, battery A1 and A2 concerning this invention are remarkably excellent in cycling property compared with battery R1 and R2. Further, when EC is included in the electrolytic solution solvent, the battery A3 according to the present invention does not have a lower cycle performance than R3. Furthermore, the batteries A1 and A2 have improved cycle characteristics compared to the batteries A3 and R3. Further, the batteries A1, A2, and A3 according to the present invention have higher high load efficiency than the batteries R1, R2, and R3, respectively.

  In a conventional lithium secondary battery using a negative electrode, when DMC is used as an electrolyte, the cycle efficiency at room temperature is good (battery R1), but solidifies in the low temperature range of the normal battery operating range. Lack. Therefore, it is difficult to solidify by mixing an asymmetric acyclic carbonate such as EMC with DMC, and practicality can be improved. However, on the other hand, the cycle performance of the electrolyte mixed with the asymmetric acyclic carbonate is decreased (battery R2). By further mixing EC with the mixed electrolyte, cycle deterioration can be reduced (battery R3). However, since EC itself has a high freezing point and a large viscosity, large current charge / discharge characteristics and low temperature characteristics are obtained by EC mixing. Such preferable characteristics are deteriorated.

Claims (4)

A positive electrode using a material capable of doping / dedoping lithium ions as an active material, a negative electrode using a carbon material capable of doping / dedoping lithium ions as an active material, and a separator sandwiched between opposing surfaces of the positive electrode and the negative electrode And a non-aqueous electrolyte lithium secondary battery comprising an electrolyte obtained by dissolving a solute composed of a lithium salt in an organic solvent, wherein the negative electrode has a number average molecular weight of 300 having a carbonate structure represented by the following general formula [I] 200,000 viewed contains the following polymers, non-aqueous electrolyte lithium secondary battery, characterized by said polymer is a polyethylene carbonate comprising a repeating unit represented by the following general formula [IV].

The nonaqueous electrolyte lithium secondary battery according to claim 1, wherein the negative electrode further comprises a binder selected from polyethylene, polypropylene, and fluororesin. Lithium salt is LiClO _Four , LiPF ₆ , LiAsF ₆ , LiBF _Four , LiCF _Three SO _Three , LiN (SO ₂ CF _Three ) ₂ , LiN (SO ₂ C ₂ F _Five ) ₂ The non-aqueous electrolyte lithium secondary battery according to claim 1, which is one or more selected from the group consisting of: The nonaqueous electrolyte lithium secondary battery according to any one of claims 1 to 3, wherein the organic solvent is selected from the following (A), (B), and (C).
(A) Dimethyl carbonate
(B) Dimethyl carbonate and ethyl methyl carbonate
(C) ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate