JP2002015775A - Nonaqueous solvent secondary cell and positive active material for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To materialize a nonaqueous solvent secondary cell with a discharge voltage of 2.4 V, compatible with Ni/Cd secondary cell, with improved capacity characteristic at over charge and over discharge. SOLUTION: For a nonaqueous solvent secondary cell having a positive electrode with positive electrode binding agent of MoO3 as an active material, and a negative electrode with negative electrode binding agent containing Li occluding carbon material, the operation potential of the positive electrode is higher by 1.5 V and over against the standard unipolar potential of lithium at all times, and discharge is made to finish when the operation potential of the negative electrode starts to increase. Such nonaqueous solvent secondary cell fulfilling above condition can be manufactured by making the electric capacity ratio of the positive electrode binding agent and the negative electrode binding agent 1.05-1.20, and the effect is further improved by adding Na element to the positive electrode active material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous solvent secondary battery, and more particularly, to a Li-ion battery having a discharge voltage of about 2.4 V and improved capacity deterioration during over-discharge and over-charge. The present invention relates to a secondary battery and a positive electrode active material used therein.

[0002]

2. Description of the Related Art Recently, with the progress of diversification, miniaturization, and weight reduction of various portable electric and electronic devices, diversification, miniaturization, and rechargeable batteries used as driving sources thereof have been increasing. The demand for weight reduction is increasing. As such a secondary battery, Ni has an operating voltage of 1.2V class.
The mainstream is to use one or two Cd / Cd secondary batteries in series or in series at 1.2V or 2.4V. However, since the Ni / Cd secondary battery has an electrolytic solution as an aqueous solution, there is a problem that a sufficient current cannot be taken out depending on the temperature of the use environment. For example, when the environmental temperature is lower than 0 ° C., freezing of the electrolytic solution starts to occur, so that it is almost impossible to extract a current, and further, the long-term stability is poor.

On the other hand, research and development of a non-aqueous solvent secondary battery using an organic electrolyte have been actively conducted recently in order to improve the disadvantages of such an aqueous electrolyte battery. It has already been put to practical use. This non-aqueous solvent secondary battery is
In general, it has an advantage that it has a high energy density, has little self-discharge during storage and storage, and can take out current even in a wide range of environmental temperature of −20 to 60 ° C. However, on the other hand, the non-aqueous solvent secondary battery has a problem that the maximum value of the charge / discharge current per unit area is lower than a battery in which the electrolyte is an aqueous solution. This is a disadvantage in comparison with the aqueous battery under the recent situation where the size of the secondary battery as the driving source is progressing. For example, if the target battery is a coin-type battery, the reaction area contributing to the battery reaction becomes very small, so that only a very small current can flow. This is because a reduction in the utilization rate of the substance and a deterioration in the charge / discharge cycle life characteristics are caused.

Further, regarding a Li secondary battery as a typical example of a non-aqueous solvent secondary battery, a positive electrode active material of a Li secondary battery has a crystal structure such as V ₂ O ₅ or Mn oxide. Has a spinel-type crystal structure in which a Li source involved in charge and discharge is coordinated as Li +, such as LiCoO ₂ , LiNiO ₂ , and LiMnO ₂ , such as LiCoO ₂ , LiNiO ₂ , and LiMnO _2. Materials and the like have been studied, and some of them have already been put to practical use. However, all of the above-mentioned materials have a reference to a standard monopolar potential of Li (hereinafter referred to as Li + / Li potential) of 3.0.
An operating voltage of 1.about.4.0 V indicates a discharge potential.
The compatibility with the Ni / Cd secondary battery of 2V or 2.4V is lost.

By the way, MoO ₃ is conventionally known as a material having a discharge potential in the vicinity of 2.4 V with respect to the Li + / Li potential. This material has a discharge capacity of 250 mAh / g.
Although it has the above characteristics, it is expected to be used as a battery material for a high-capacity Li secondary battery, but it has hardly been put to practical use yet. The reason for this is that, for example, when a Li foil is used as the negative electrode, the Li foil becomes finer in the repetitive process of charge and discharge, or dendrites of Li grow on the surface of the Li foil, which forms the separator. This is because the battery may break through and come into contact with the positive electrode to cause an internal short circuit, causing deterioration of the charge / discharge cycle characteristics of the battery and shortening the service life of the battery. Further, repeating the charge / discharge cycle causes a change in the structure of MoO ₃ .

It is known that such a problem of the MoO ₃ battery can be improved by using a Li—Al alloy for the negative electrode. However, since the discharge potential of the Li-Al alloy is about 0.4 V based on the Li + / Li potential, the operating voltage of the Li secondary battery eventually becomes 2.0 V.
(2.4 V-0.4 V), which is inappropriate as a 2.4 V class battery.

[0007]

The present inventors have conducted various studies to realize a battery having a discharge potential near 2.4 V which is compatible with a conventional Ni / Cd secondary battery.
MoO ₃ has a discharge potential of about 2.4 V based on the Li + / Li potential, and can obtain a capacity per unit weight of 250 mAh / g. Focusing on the fact that it can be synthesized inexpensively and can be synthesized at low cost, it is used as a positive electrode active material, and as a negative electrode material, a carbon material has a discharge potential of about 0 based on the Li + / Li potential.
V and the fact that it is also a porous structure,
A battery using a material obtained by inserting Li into the carbon material as a negative electrode active material was developed.

However, this MoO ₃ forms one layer of an octahedron having oxygen at the apex centered on Mo, and the blocks are connected to form a layered structure. Repeated charge / discharge cycles of overcharge and overdischarge, such as the insertion and removal of a large amount of Li between the layers, have the disadvantage that the crystal structure is destroyed and deteriorated due to the expansion and contraction of the layers. I was The present invention solves the above-described problem in a Li secondary battery using MoO ₃ for the positive electrode, and in a non-aqueous solvent secondary battery having an operating voltage of about 2.4 V, an active material even when charging and discharging a large current. It is an object of the present invention to provide a non-aqueous solvent secondary battery that does not cause a decrease in the utilization factor of the battery and is less likely to cause deterioration in the charge / discharge cycle life characteristics.

Another object of the present invention is to provide a positive electrode active material for use in a non-aqueous solvent secondary battery in which the capacity is less reduced due to overcharge and overdischarge cycles.

[0010]

That is, the present invention provides the above
MoO₃In a battery using as a positive electrode active material, a positive electrode
By setting the capacity of the negative electrode larger than the capacity of the negative electrode,
The operating potential of the positive electrode is 1.0 V or less with respect to the Li + / Li potential
Higher potential, which allows MoO ₃Structure of
Deterioration can be suppressed and battery capacity can be prevented from lowering
And MoO₃Contains Na element in
This strengthens the crystal lattice and increases the charge / discharge cycle.
Focusing on the fact that deterioration is unlikely to occur even if
This is the end of the Ming.

That is, a non-aqueous solvent secondary battery according to the present invention is a negative electrode having a positive electrode mixture having an active material of MoO ₃ and a negative electrode mixture containing a carbon material having occluded Li. In the non-aqueous solvent secondary battery comprising: the operating potential of the positive electrode, with respect to the standard single-electrode potential of Li,
A non-aqueous solvent secondary battery having a value of 1.5 V or more and discharging the battery when the operating potential of the negative electrode starts to increase.

According to a second aspect of the present invention, in the above non-aqueous solvent secondary battery, the electric capacity ratio between the positive electrode mixture and the negative electrode mixture is preferably 1.05 to 1.20. It is characterized by the following.

According to a third aspect of the present invention, in the non-aqueous solvent secondary battery, the active material of the positive electrode is MoO ₃ containing 0.1 to 10% by mass of Na element in mass percentage. It is characterized by the following.

Further, the present invention according to claim 4 is a positive electrode active material characterized in that MoO ₃ contains 0.1 to 10% by mass of Na element in mass percentage.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION A non-aqueous solvent secondary battery according to the present invention comprises a positive electrode having a positive electrode mixture whose positive electrode active material is MoO ₃ , and a negative electrode having a negative electrode active material in which Li is occluded in a carbon material. Are laminated via a separator having a liquid retaining property and an electrical insulation property to form a power generating element, and the power generating element is sealed in a battery can together with an organic electrolytic solution. In the present invention, the positive electrode active material is composed of MoO ₃ having a discharge potential of 2.4 V with respect to Li + / Li potential, and the negative electrode active material is a carbon material having a discharge potential of about 0 V with respect to Li + / Li potential. Is made to occlude Li. Therefore, since the operating potential of the negative electrode becomes about 0 V, the operating voltage of this battery indicates 2.4 V.

The non-aqueous solvent secondary battery of the present invention operates in a state where the operating potential of the positive electrode is always 1.5 V or higher with respect to the Li + / Li potential, and the operating potential of the negative electrode rises from 0 V. By configuring so as to stop the discharge at a point in time, the intended purpose is achieved.

Means for satisfying such conditions of the battery of the present invention include making the capacity of the positive electrode mixture larger than the capacity of the negative electrode mixture. More preferably, the ratio of the capacity of the positive electrode mixture to the capacity of the negative electrode mixture is 1.05
That is, it is set within the range of １．２1.20. If this capacity ratio is smaller than 1.05, the diffusion rate of Li + in MoO ₃ in the positive electrode during a large current discharge decreases, and the discharge potential of MoO ₃ located near the negative electrode becomes 1.0 V with respect to the Li + / Li potential. Lower, which leads to a destruction of the crystal structure and a reduction in battery capacity. On the other hand, setting the capacity ratio to be larger than 1.20 lowers the capacity of the negative electrode, which hinders the increase in the capacity of the battery, and is not preferable.

In the present invention, the above capacity ratio is set within the above range by adjusting the amount of MoO ₃ mixed in the positive electrode mixture and the amount of Li stored in the carbon material in the negative electrode mixture. be able to. For example, when a negative electrode active material is prepared by absorbing Li having a mass corresponding to a predetermined capacity (hereinafter referred to as C1) into a carbon material, and a negative electrode mixture is prepared using this carbon material, the capacity of the negative electrode mixture is increased. (This is referred to as C2) is set in a range that satisfies C2 <C1 since the carbon material is in a diluted state. Similarly, when the amount of MoO ₃ (capacity: 250 mAh / g) and the amount of the conductive material and the binder are appropriately selected during the preparation of the positive electrode mixture, the capacity of the obtained positive electrode mixture becomes MoO 3. ₃
Is a diluted value (this is referred to as C3). Therefore, when the above-mentioned capacity C2 is fixed, the amount of MoO ₃ and the ratio with other components are adjusted so that the above-mentioned capacity C3 satisfies the relationship of 1.05 ≦ C3 / C2 ≦ 1.20. When the positive electrode mixture is prepared in consideration of the above, the capacity ratio between the positive electrode mixture and the negative electrode mixture can be set in a range of 1.05 to 1.20.

Specifically, after preparing a positive electrode mixture and a negative electrode mixture of a predetermined capacity, working the positive electrode and the negative electrode using these, and changing the size and thickness of each molded body before assembling the battery. Thereby, the above-described capacity ratio can be adjusted by controlling the positive electrode active material and the negative electrode active material.
For example, after preparing a negative electrode mixture having a certain capacity and forming a negative electrode having a predetermined size and shape, when forming a positive electrode to be combined with the negative electrode, by changing the size and thickness of the formed body, this positive electrode By adjusting the amount of the positive electrode active material contained in the mixture, the volume ratio of the positive electrode mixture to the negative electrode mixture can be easily adjusted.

In the present invention, the positive electrode is prepared by kneading the above-mentioned powder of MoO ₃ as an active material, a conductive material such as carbon black, and a binder such as PTFE at a predetermined ratio. It is manufactured by press-forming the mixture into a predetermined shape. When manufacturing the positive electrode, the active material M
It is preferable to use oO ₃ containing 0.1 to 10% by mass of Na element in terms of mass percentage. If the mass percentage is less than 0.1%, the effect of strengthening the crystal lattice is small, so that it cannot be said that it is effective as a problem of improving the charge / discharge cycle characteristics of the battery. Also, when the mass percentage is more than 10%, the effective charge / discharge capacity of MoO ₃ decreases.

In the present invention, as the conductive material used for the positive electrode mixture, carbon black, artificial graphite, carbon black (eg, acetylene black), metal powder, and the like can be used. PTF is used as a binder.
E (polytetrafluoroethylene), PVdF (polyvinylidene fluoride), or the like can be used. Of these, PTFE is most preferred in terms of workability and stability. The mixing ratio of the positive electrode active material, the conductive auxiliary agent, and the binder in such a positive electrode mixture is 100: 20: 1 in mass ratio.
A range of 0 to 100: 2: 1 is preferred. If the ratio of the conductive additive and the binder exceeds this range, the amount of the positive electrode active material decreases, resulting in a decrease in capacity.If the ratio of the conductive additive falls below this range, the internal resistance of the positive electrode increases. And the internal resistance of the battery rises. On the other hand, if the ratio of the binder is less than this range, the mechanical strength of the positive electrode decreases, and the positive electrode is likely to be broken during the production of the battery.

In the present invention, the negative electrode is prepared by kneading a negative electrode mixture formed by kneading a porous carbon material and a binder into a predetermined shape, and then preliminarily absorbing Li before assembling the battery. Further, at the time of assembling the battery, the battery can be manufactured by press-bonding, for example, a metal Li foil to the compact. After assembling the battery, the metal Li foil is dissolved in the organic electrolyte and occluded in the voids of the carbon material. In the present invention, the use ratio of the negative electrode mixture to the Li foil is 1: 0.
A range from 8 to 1.0 is preferred. If the amount of the Li foil is less than this range, the capacity is reduced. On the other hand, if the amount is more than this range, Li is not completely occluded in the carbon agent and remains, causing problems such as Li dendrite, which is not preferable.

In the present invention, materials such as carbon powder, artificial graphite powder, carbon black (for example, acetylene black), and mesophase pitch carbide can be used as the carbon material having a porous structure. In particular, it is preferable to use a mesophase pitch carbide in terms of maintaining cycle capacity and voltage stability. In addition, materials such as styrene / butadiene rubber and acrylonitrile / butadiene rubber can be used as the binder. In particular, the use of styrene-butadiene rubber is preferred in terms of workability and stability. The mixing ratio of the carbon material and the binder in such a negative electrode mixture is desirably in the range of 100: 1 to 100: 1 by mass ratio. If the ratio of carbon material exceeds this range,
The mechanical strength of the negative electrode is reduced, so that the negative electrode is easily broken at the time of assembling the battery, and the yield of battery production is reduced. On the other hand, if the ratio of the carbon material falls below this range, the amount of Li occluded is reduced, resulting in a reduction in capacity, which is not preferable.

In the present invention, examples of the organic electrolyte include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and γ-
Butyrolactone (γ-BL), 1,2-diethoxyethane (DEE), 1,2-dimethoxyethane (DME),
One or more non-aqueous solvents such as diethyl carbonate (DEC) and methyl ethyl carbonate (MEC) may be mixed in a solvent such as LiClO ₄ , LiBF ₄ , L
iCF ₃ SO ₃ , LiPF ₆ , LiN (CF ₃ SO ₂ )
_A solution obtained by dissolving a predetermined amount of an electrolyte such as ₂ is used. In this case, the organic electrolyte to be used is appropriately selected depending on the required characteristics of the intended battery. For example, in the case where charge-discharge cycle characteristics and storage characteristics are to be enhanced, LiPF ₆ or LiN ( It is preferable to use CF ₃ SO ₂ ) ₂ .

[0025]

EXAMPLES Examples 1 to 7 and Comparative Examples 1 to 4 (1) Structure of Positive Electrode A Na ₂ MoO ₄ solution and an HCl solution were mixed and heated, and the obtained precipitate was fired at 500 ° C. for 12 hours in air. Then, MoO ₃ powders having different Na contents as shown in Table 1 were synthesized (Examples 1 to 7). For 100 parts by weight of these MoO ₃ powders, 10 parts by weight of carbon black and 5 parts by weight of PTFE powder
After mixing by weight, the mixture was stirred, and the resulting mixture was molded under pressure to produce various pellets having a Na content and a charge / discharge capacity of 16 mm in diameter as shown in Table 1. Next, these pellets were dried at a temperature of 150 ° C. for 5 hours to obtain a positive electrode.

(2) Production of Negative Electrode Precursor A mesophase pitch was fired at 2800 ° C. in an N ₂ atmosphere to produce a carbon material. To 100 parts by weight of the carbon material powder, 5.3 parts by weight of a styrene-butadiene rubber were blended, and the mixture was stirred.
6 mm pellets were produced. Next, these pellets were dried at a temperature of 150 ° C. for 5 hours to obtain a negative electrode precursor.

(3) Assembly of Battery The coin-type Li secondary battery shown in FIG. 1 was assembled as follows by combining the positive electrode and the negative electrode precursor shown in Table 1. First, a Ni expanded metal 7 having a diameter of 10 mm and a thickness of 0.05 mm was welded to the bottom surface of a stainless steel negative electrode container 8 as a negative electrode current collector, and an insulating gasket 3 was disposed on the inner wall. Then, a metal L is placed on the negative electrode current collector 7.
The i-foil was arranged, and the negative electrode precursor 6 was mounted thereon. After the battery is assembled, the metal Li foil is occluded by the carbon material of the negative electrode precursor and functions as an active material. The dimensions and shape of the metal Li foil at this time were set so as to have a capacity corresponding to the theoretical capacity of the negative electrode precursor 6 attached to the metal Li foil. Then, an electrolyte was prepared by dissolving LiPF ₆ in an organic solvent having an EC: MEC ratio of 1: 1 (volume ratio) so as to have a concentration of 1 mol / l. Was mounted on the negative electrode precursor 6, and then the positive electrode 4 shown in Table 1 was mounted. Finally, the positive electrode container 1 having the inner surface coated with the colloidal carbon 2 is fitted, and the whole is caulked to form an outer diameter 20 mm.
A battery having a height of 2.5 mm and a height of 2.5 mm was assembled.

(4) Measurement of Battery Characteristics With respect to the twelve types of batteries thus manufactured, the batteries were charged and discharged from a working voltage of 1.0 V to 3.0 V at a constant current of 0.5 mA. FIG. 2 shows the discharge capacity of each battery at each cycle. Table 1 also shows the initial discharge capacity. The following is clear from these results.
As is clear from FIG. 2 and Table 1, the batteries of Comparative Examples 1 and 3 have a short cycle life, and the batteries of Comparative Examples 2 and 4 have good cycle life but small electric capacity. In contrast, the batteries of the present invention all show a good capacity retention ratio even after 20 cycles of charge / discharge, and have excellent charge / discharge cycle characteristics.

[0029]

[Table 1]

[0030]

As apparent from the above description, the non-aqueous solvent secondary battery of the present invention has an operating voltage of about 2.4 V,
Further, by controlling the capacity of the positive electrode and the negative electrode, and further containing Na, it is possible to obtain a battery with a small capacity decrease during charge and discharge and a small capacity decrease during overcharge and discharge,
As an alternative to the 2.4 V class aqueous secondary battery, its industrial value is great.

Further, by adding Na element to MoO ₃ as a positive electrode active material, a crystal lattice is stabilized and charge / discharge characteristics are greatly improved, and a battery having excellent cycle life is realized by combining with various negative electrode active materials. it can.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a coin-type Li secondary battery to which the present invention can be applied.

FIG. 2 is a graph showing a change in a discharge capacity retention ratio of batteries manufactured in Examples of the present invention and Comparative Examples.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode container 2 colloidal carbon 3 insulating gasket 4 positive electrode 5 separator 6 negative electrode precursor (negative electrode after assembly) 7 negative electrode current collector 8 negative electrode container

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H029 AJ05 AK02 AK11 AL07 AL08 AM03 AM04 AM05 AM07 BJ03 HJ01 HJ02 HJ18 HJ19 5H050 AA07 BA17 CA02 CA17 CB08 CB09 HA01 HA02 HA18 HA19

Claims (4)

[Claims] 1. A non-aqueous solvent secondary battery comprising a positive electrode having a positive electrode mixture whose active material is MoO ₃ and a negative electrode having a negative electrode mixture containing a carbon material in which Li has been occluded, The working potential is always
A non-aqueous solvent secondary battery having a value of 1.5 V or more and discharging the battery when the operating potential of the negative electrode starts to rise. 2. The non-aqueous solvent secondary battery according to claim 1, wherein an electric capacity ratio between the positive electrode mixture and the negative electrode mixture is 1.05 to 1.20. 3. The positive electrode active material according to claim 1, wherein MoO ₃ contains 0.1 to 10% by mass of a Na element in mass percentage. Non-aqueous solvent secondary batteries. 4. MoO ₃ contains 0.1% by mass of Na element in mass percentage.
A positive electrode active material characterized by containing 10 to 10%.