JP2548574B2 - Electrolyte for lithium battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrolytic solution for a lithium battery, and more specifically to improvement of an electrolytic solution used for a lithium battery.

[Conventional technology]

Lithium has a high standard single electrode potential of −3.03V (standard hydrogen electrode standard), extremely strong reducing power, and an atomic weight of 6.94.
Since it is as small as 1, the capacity density per weight is as large as 3.86 Ah / g. Therefore, a battery using lithium as a negative electrode active material (hereinafter referred to as a lithium battery) has been studied as a battery having a small size and a high energy density, and a battery using manganese dioxide, fluorinated graphite or the like as a positive electrode active material has already been used. It is commercially available. However, all of these are primary batteries, and the present situation is that a rechargeable lithium secondary battery that can be put to ordinary practical use has not been realized. If lithium batteries can be recharged while taking advantage of the discharge characteristics of high energy density, batteries with extremely superior characteristics will be realized compared to conventional battery systems, and it will be used in industries such as portable electronic devices. It has a great effect on the world.

In order to make a lithium battery secondary, there are many problems to be solved such as development of a positive electrode active material, selection of an electrolytic solution, and improvement of a battery construction method. In particular, the selection of the electrolytic solution is an important issue. It is desirable from the practical point of view to use a non-aqueous electrolytic solution for a lithium secondary battery that operates at room temperature, but the conductivity of the electrolytic solution is two orders of magnitude less than that of an aqueous solution system used in a conventional battery system.
There was a drawback that it was low. Therefore, it is essential to improve the conductivity of the electrolytic solution in order to improve the discharge utilization rate of the battery. At the same time, in order to apply it to the secondary battery, it is naturally required that the charge / discharge efficiency of lithium in the non-aqueous electrolyte is high. That is, the electrolytic solution used for the lithium secondary battery needs to simultaneously satisfy the two points of high conductivity and high lithium charge / discharge efficiency.

As an electrolyte with relatively high lithium charge / discharge efficiency, Li
AsF _{6-2-methyltetrahydrofuran-based} electrolyte it has been proposed (see U.S. Pat. No. 4,118,559). However, the conductivity of the electrolyte is low (4.3M with 1.5M LiAsF _6).
(10 ⁻³ S · cm ⁻¹ , 25 ° C.), and when used in a lithium battery, it had a drawback of low charge / discharge efficiency.

Further, the electrolytic solution using a solvent having a polar double bond such as propylene carbonate and ethylene carbonate,
It has a relatively high conductivity (eg 1.5M LiAsF ₆ at 5.3 × 10 ^-3 S · cm ⁻¹ and 6.2 × 10 ⁻³ S · cm ^{−1 respectively} ),
Lithium has a drawback of low charge / discharge efficiency.

[Problems to be solved by the invention]

That is, up to now, an electrolyte solution for a lithium secondary battery, which has a high electric conductivity and a high lithium charge / discharge efficiency, has not been realized.

The present invention has been made in view of such a current situation, and an object thereof is to provide an electrolytic solution for a lithium battery, which enables production of a lithium battery having high conductivity and excellent charge / discharge characteristics of lithium. It is in.

[Means for solving problems]

The lithium battery electrolyte according to the present invention is a lithium battery electrolyte obtained by dissolving a lithium salt in an organic solvent,
The most main feature is that the linear alkane is contained in the concentration range of 10 ^-3 to 10 ^-1 M (mole /).

As a result, in contrast to a lithium battery electrolyte that has conventionally been difficult to have both high conductivity and excellent charge / discharge efficiency, a lithium battery electrolyte having high conductivity and excellent charge / discharge efficiency. Is realized.

 The present invention will be described in more detail.

In a lithium battery, the negative electrode active material is a lithium alloy capable of discharging lithium or lithium ions, the positive electrode active material is a material that electrochemically reversibly reacts with lithium ions, and the electrolyte solution uses a lithium salt as an organic solvent. It is composed of a dissolved solution.

According to the present invention, it is an object of the present invention to provide an electrolytic solution as described above having excellent properties. That is, a solution in which a lithium salt is dissolved in an organic solvent contains a linear alkane in a concentration range of 10 ⁻³ to 10 ⁻¹ M, and this is used as an electrolytic solution for a lithium battery.

In order to improve the conductivity and the charge / discharge efficiency of lithium of an electrolyte using a lithium battery, a solvent having a low electron transfer reactivity from lithium to a solvent or a lithium salt in a solution is easily dissociated, and lithium ion It is considered necessary to have high mobility. The straight-chain alkane in the present invention is easily adsorbed on the lithium negative electrode without disturbing the dissociation of lithium salt and the mobility of lithium ions, and suppresses the reaction between lithium and the solvent, which causes a decrease in the charge / discharge efficiency of lithium, Deposition of lithium ions on the negative electrode with charge and discharge,
It is added for the purpose of facilitating the elution from the negative electrode into the electrolytic solution.

The reason why the linear alkane used in the present invention exhibits effective characteristics when added to the electrolytic solution is not necessarily clear, but one idea is that the additive forms a physical or chemical adsorption bond with the negative electrode lithium. By doing so, a tank for suppressing the loss of lithium due to the reaction between lithium as the negative electrode active material to be used in the charge / discharge cycle and the electrolytic solution is formed at the negative electrode / electrolytic solution interface. In addition, this layer is Li
It is also expected to improve the deposition state of ⁺ ions on the negative electrode and suppress the generation of precipitated lithium that becomes electrochemically inactive. After discharging with an electron microscope, observation of the surface of the negative electrode after discharging was performed using the battery system described in Example 1 described later (however,
The negative electrode uses 30 mAh Li), and in the system in which the additive is not added, the lithium charge / discharge efficiency is improved together with the particulate lithium having a diameter of about 10 μm, which is effective in improving the lithium charge / discharge efficiency. Needle-like lithium having a diameter of 1 to 2 μm and a length of about 10 μm, which is a cause of the decrease, is present on the surface of the negative electrode, and the deposition rate of the needle-like lithium increases with the charge / discharge cycle. On the other hand, in the battery using the electrolytic solution of the present invention to which the additive was added, the presence of the acicular lithium was not observed, and the particulate lithium having a diameter of about 2 to 5 μm was uniformly deposited. The effectiveness of the electrolytic solution of the present invention due to the addition of the additive was confirmed.

The addition concentration of the linear alkane in the present invention is 10 ^-3 to 10 ^-1.
However, when the concentration is less than 10 ^-3 M, the characteristics are not improved by the addition as it is the same as the properties of the conventional electrolyte solution that is not added.On the other hand, when the concentration exceeds 10 ^-1 M, the additive This is because the excessive amount adversely affects the mobility of ions and the electrochemical reaction of the negative electrode, and the characteristics deteriorate as compared with those before the addition.

The lithium salt and the organic solvent that compose the electrolytic solution in the present invention are not basically limited. For example,
In lithium _{salt, LiAsF 6, LiClO 4, LiSbF} 6, LiBF
One or more of ₄ , LiPF ₆ , LiAlCl ₄ , LiCF ₃ SO ₃ and LiCF ₃ CO ₂ can be effectively used. In addition, as the organic solvent, propylene carbonate, ethylene carbonate, 2-methyltetrahydrofuran, tetrahydrofuran, 2-methyldioxolane, 4-methyldioxolane, dioxolane, 1,2-dimethoxyethane, γ-butyrolactone, dimethylsulfoxide, acetonitrile,
One or more aprotic organic solvents such as formamide, dimethylformamide and nitromethane can be effectively used.

The negative electrode active material used in the lithium battery using the electrolytic solution according to the present invention is basically not limited, and the negative electrode active material used in the conventional lithium battery, that is, lithium or lithium that can discharge lithium ions. Alloys can be used.

Similarly, the positive electrode active material used in the present invention is basically not limited, and is a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically reversibly reacts with lithium ions. be able to.

〔Example〕

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

Example 1 1.5M LiAsF ₆ -ethylene carbonate (hereinafter abbreviated as EC) / 2-methyltetrahydrofuran (hereinafter, 2Me TH)
(Abbreviated as F) (Mixing volume ratio 1/1) Prepare an electrolyte solution in which the impurities in the mixture are controlled to 100ppm or less, and add the Molecule Sieves 3A and / or 4A in advance to this and leave it at the temperature above the melting point for 2 days and nights. Various linear alkanes dehydrated to 10 ^{−4 to} 0.5 M were mixed and used as an electrolytic solution of a lithium battery produced by the production method described below.

As the positive electrode active material, an amorphous material having a composition of 95 mol% V ₂ O ₅ −5 mol% P ₂ O ₅ was used, 70 wt% of this, acetylene black as the conductive agent was 25 wt%, and Teflon was used as the binder. Coin using positive electrode mixture pellets (16 mφ, thickness 0.5 mm) prepared at a mixing ratio of 5% by weight as a positive electrode, metallic lithium (17 mmφ, 15 mAh) as a negative electrode, and a microporous polypropylene sheet as a separator. Type lithium battery was prepared.

This lithium battery at room temperature, current value of 1mA, 2-3.5V
A charge / discharge test was performed in the voltage range of 1 to evaluate the charge / discharge characteristics of the electrolytic solution.

An example of the results is shown in FIG. That is, Fig. 1 shows 1.5M
LiAsF ₆ -EC / 2Me the batteries of THF (1/1) in the n- triacontane (n-C ₃₀ H ₆₂₎ present invention with 0.01M electrolyte and 15mAh lithium was added, 1 mA current value, 2 to 3.5
6 is a graph showing the relationship between the number of cycles (horizontal axis) and the discharge capacity (mAh, vertical axis) when performing a charge / discharge test in the V voltage range. In the battery system, the lithium on the negative electrode side, which can participate in the discharge, is consumed for each discharge in the charge / discharge cycle, and the capacity thereof gradually decreases according to the charge / discharge efficiency E as is clear from FIG. I will go. That is, the charge / discharge efficiency of the negative electrode is E, and the capacity of the nth discharge is Cn.
Then, Cn = E × C _n-1 = E ^n-1 × C ₁ holds, and from this, the relation of lnCn = (n-1) lnE + lnC ₁ is obtained, and the discharge capacity on the vertical axis is as shown in FIG. The charge / discharge efficiency E of the negative electrode lithium can be calculated from the slope of the straight line portion of the graph represented by the logarithmic scale.

The charge / discharge efficiency E of lithium in the electrolytic solution to which various linear alkanes were added was calculated by the above formula, and the results are shown in Table 1. For comparison, Table 1 also shows the results of a similar test in an electrolyte solution containing no additive.

As is clear from the results in Table 1, 10
It was found that the addition of ^-3 to 10 ^-1 M shows higher lithium charge / discharge efficiency than the case of no addition.

Example 2 An electrolyte containing 1.5M LiAsF ₆ -EC / 2Me THF (1/1) electrolyte in which impurities were controlled was prepared, and Molecule Sieves 3A was added to the electrolyte in advance and allowed to stand in an 80 ° C. thermostat for 2 days and nights. to dehydrated n- triacontane the _{(n-C 30 H 62)} 0.01
M was added, and this was used as a solution for a lithium battery.

A coin-type lithium battery was manufactured in the same manner as in Example 1 by using the electrolytic solution left at room temperature in an argon dry box for a certain period of time after manufacturing and using the positive electrode and the negative electrode manufactured in the same manner as in Example 1. Furthermore, a charge / discharge test was performed under the same conditions as in Example 1 to evaluate the lithium charge / discharge efficiency.

The results are shown in Table 2. As is clear from Table 2, it has been found that the additive does not decompose and deteriorate with time and maintains a stable charge / discharge efficiency.

Example 3 1.5M LiAsF ₆ −EC / 2Me THF with controlled impurities
A coin-type lithium battery similar to that of Example 1 was produced by using the same electrolytic solution as in Example 2 in which n-triacontane (n-C ₃₀ H ₆₂ ) was added to (1/1).

The prepared lithium battery has a current value of 1 mA and a voltage regulation of 2 V to 3.5 V, and has a pause of 1 hour after discharge and no pause after charging for 10 minutes, 1 hour, 10 hours, 100 hours, 20 days. A charging / discharging test was performed under the set conditions to evaluate the lithium charging / discharging efficiency.

The results are shown in Table 3. As is clear from Table 3, it was revealed that the additive does not cause any reaction that deteriorates the characteristics with respect to the negative electrode lithium with time, and provides excellent characteristics that can stand for a long period of rest.

EXAMPLE 4 dehydrated KoTsuta n- decane _{(n-C 10 H 22)} , n- hexadecane _{(n-C 16 H 34)} , and n- triacontane each additive (n-C ₃₀ H ₆₂₎ , 1M LiClO ₄ -propylene carbonate (hereinafter abbreviated as PC) / 1,2-dimethoxyethane (hereinafter abbreviated as DME) (1/1), 1.5M LiAsF ₆
−PC, 1.5M LiAsF ₆ −2Me THF, 1.5M LiPF ₆ −EC / 2Me THF
10 ^{-4 in} each electrolyte solution with controlled (1/1) impurities
It was added at a concentration of 0.5 M and used as an electrolyte for lithium batteries.

A coin-type battery was produced in the same manner as in Example 1, and a charge / discharge test was performed under the same conditions as in Example 1. The lithium charge / discharge efficiency obtained from the test is shown in Table 4.

As is clear from Table 4, it was revealed that by adding the additive in any of the electrolytic solutions, higher charging / discharging efficiency was exhibited as compared with the case of no addition.

Example 5 1.5M LiAsF ₆ -EC / 2Me THF with controlled impurities
In a (1/1) electrolyte solution, pre-dissolve more sieves 3
Add A and add 0.01M of dehydrated n-triacontane (n-C ₃₀ H ₆₂ ), which was left for 2 days in an 80 ° C constant temperature bath.
It was used as an electrolyte for a lithium battery.

A positive electrode material mixture pellet (16 mmφ) prepared in the same manner as in Example 1 containing an active material containing an amorphous material having a composition of 95 mole% V ₂ O ₅ −5 mole% P ₂ O ₅ was used as a positive electrode, and 60 mA was used as a negative electrode.
A coin-type lithium battery was produced in the same manner as in Example 1, except that metallic lithium (17 mmφ) of h was used.

The lithium battery was subjected to a charge / discharge test at room temperature under a discharge current of 3 mA / cm ² , a charge current of 1 mA / cm ² , and a voltage regulation of ² V to 3.5 V to evaluate the charge / discharge characteristics. For comparison, a coin-type battery was prepared using an electrolyte solution containing no additive, and the same charge / discharge test was performed.

Results are shown in FIG. That is, Fig. 2 shows 1.5M LiAsF
Discharge current 3 mA / cm ² , charge current 1 mA / cm ² , 2 for each battery using _6- EC / 2Me THF (1/1) electrolyte solution and electrolyte solution in which 0.01 Mn-triacontane was added It is the graph which showed the change of the discharge capacity (mAh, a vertical axis | shaft) with a charge / discharge cycle (horizontal axis | shaft) at the time of performing a charging / discharging test by voltage regulation of -3.5V. In FIG. 2, curve A shows the case of the battery of the present invention using the electrolytic solution containing 0.01 M of n-triacontane as an additive, and curve B shows the case of the battery using the electrolytic solution containing no additive. Is.

As is clear from FIG. 2, the battery using the electrolytic solution containing the n-triacontane additive exhibited excellent charge-discharge cycle stability as compared with the battery using the electrolytic solution containing no additive. .

Example 6 1.5M LiAsF ₆ -EC / 2Me THF with controlled impurities
(1/1) Electrolyte solution, Moreaki Sieves 3A
N-was put in a thermostat at 80 ° C for 2 days and nights to be thoroughly dehydrated n-
Triacontane (n-C ₃₀ H ₆₂₎ was 0.01M added and the lithium battery electrolyte.

Using this electrolytic solution, a coin type lithium battery was produced in the same manner as in Example 5. This battery has a charging current of 1 mA / cm ² , ² mA / cm ² , 4 mA / cm ² , and charging current of 1 mA / cm ² ,
A charge / discharge test was conducted under the voltage regulation of 2 to 3.5V. For comparison, a coin-type lithium battery was prepared in the same manner using an electrolytic solution containing no additive, and a charge / discharge test was conducted under the same conditions.

Results are shown in FIG. That is, FIG. 3 is a graph showing the change in discharge capacity (mAh, vertical axis) with the number of charge / discharge cycles (horizontal axis) of the lithium battery. In FIG. 3, curve C shows the case of a battery using an electrolytic solution to which n-triacontane was added in a charge / discharge test under conditions of discharge current and charge current of 1 mA / cm ² , and curve D shows the case of charging under the same conditions. This is a case of a battery using an electrolyte solution containing no additive in the discharge test, and curve E shows a discharge current of ² mA / cm ² and a charging current of 1 mA / c.
In the case of the battery using the electrolyte solution added with n-triacontane in the charge / discharge test under the condition of m ² , the curve F shows the result of the battery using the electrolyte solution containing no additive in the charge / discharge test under the same condition. This is the case. Furthermore, in FIG. 3, the curve G
Shows the case of the battery using the electrolyte solution to which n-triacontane was added in the charge / discharge test under the conditions of the discharge current of 4 mA / cm ² and the charge current of 1 mA / cm ² , and the curve H shows the charge under the same conditions. This is a case of a battery using an electrolytic solution containing no additive in a discharge test.

As is clear from FIG. 3, in any charging / discharging test, the case of using the electrolytic solution of the present invention to which n-triacontane was added was compared with the case of using no additive electrolytic solution. No significant decrease in capacity was observed, and long-term charge / discharge cycle stability was demonstrated.

〔The invention's effect〕

As explained above, the linear alkane according to the present invention
^-3 ~ 10 ^-1 M concentration range of the lithium salt dissolved organic solvent-based electrolyte solution is used in a lithium battery, the battery has a large charge and discharge capacity and a high energy density battery showing excellent charge and discharge cycle life. It has the advantage that it becomes possible and can be widely used in various fields.

[Brief description of drawings]

FIG. 1 is a battery of the present invention using an electrolyte solution in which 0.01 M of n-triacontane was added to 1.5 M LiAsF ₆ -EC / 2Me THF (1/1), and 1 mA of a battery using 15 mAh lithium as a negative electrode. , 2
A graph showing the relationship between the number of cycles and the discharge capacity when conducting a charge / discharge test in the voltage range of 3.5V, Fig. 2 shows 1.5M LiA
0.01 of n-triacontane in sF ₆ -EC / 2Me THF (1/1)
Discharge according to the number of cycles when a charge / discharge test is performed under the voltage regulation of 3mA / cm ² charging current, 1mA / cm ² charging current and ² to 3.5V discharge current of batteries with and without added electrolyte. The graph showing the change in capacity, Fig. 3, shows that the discharge currents of the same type of batteries are 1mA / cm ² , 2mA / cm ² , 4mA / cm ² ,
It is the graph which showed the change of the discharge capacity with the cycle at the time of performing a charging / discharging test by charging voltage 1mA / cm < ² > and voltage regulation of 2V-3.5V.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor ▲ Yoshi ▼ Matsu Isamu 162 Shirahone, Shirahata, Tokai-mura, Naka-gun, Ibaraki Prefecture, Nippon Telegraph and Telephone Corporation, Ibaraki Dentsu Communication Laboratory (72) Inventor Shinichi Tobishima Ibaraki 162, Shirokata, Shirahone, Tokai-mura, Naka-gun, Ibaraki, NTT Corporation, Ibaraki Dentsu Telecom Laboratory (72) Inventor, Masayasu Arakawa 162, Shirane, Shirokata, Tokai-mura, Naka-gun, Ibaraki Nippon Telegraph and Telephone Corporation, Ibaraki Electric Co., Ltd. Communications Research Laboratory (56) References JP-A-63-4569 (JP, A)

Claims (1)

(57) [Claims] 1. An electrolytic solution for a lithium battery in which a lithium salt is dissolved in an organic solvent, wherein a linear alkane is used as an additive.
An electrolytic solution for a lithium battery, which is contained in a concentration range of 10 ^-3 to 10 ^-1 M (mole /).