JPH01213969A - Electrolyte for lithium battery - Google Patents

Abstract

PURPOSE:To provide high electroconductivity and excellent lithium charging/ discharging characteristic by adding annular saturated hydrocarbons or ones having straight chain alkyl radicals as an additive in a specific concentration range. CONSTITUTION:Lithium salt is dissolved in organic solvent to prepare a solution, and annular saturated hydrocarbons or ones having straight chain alkyl radicals are included in this solution in the concentration range 10<-3>M-10<-1>M, and this is used as an electrolyte for lithium battery. Annular saturated hydrocarbons are adsorbed to the lithium negative electrode easily without hindering dissociation of lithium salt or movability of lithium ions, to provide smooth deduction of lithium ions on the negative electrode associate with charging/discharging and elution from the negative electrode into the electrolyte. This accomplishes large charging/discharging capacity and excellent charging/discharging cycle life.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrolytic solution for lithium batteries, and more particularly to improvements in electrolytic solutions used in lithium batteries.

[Conventional technology]

Lithium has a high standard monopolar potential of 1503 W (standard hydrogen electrode), extremely strong reducing power, and a small atomic weight of &941, so its capacity density per weight is 186
Ah/t is large. For this reason, batteries using lithium as a negative electrode active material (hereinafter referred to as lithium batteries) are being researched as small-sized batteries with high energy density t-, and batteries using manganese dioxide, graphite heptafluoride, etc. as positive electrode active materials are already being studied. is commercially available. However, all of these are rechargeable batteries, and at present no rechargeable and dischargeable lithium secondary battery that can be used in ordinary practice has been realized. If lithium batteries can be recharged while taking advantage of the discharge characteristics of high energy density, batteries with extremely superior characteristics compared to conventional battery systems will be realized, and will be used in portable electronic devices and other devices. This will have a large effect on the industrial world.

There are many issues to be solved before converting lithium batteries into secondary batteries, including the development of positive electrode active materials, selection of electrolytes, and improvement of battery construction methods. In particular, the choice of electrolyte is a matter of great importance. From a practical point of view, it is desirable to use a non-aqueous electrolyte in a lithium secondary battery that operates at room temperature. Therefore, in order to improve the discharge utilization rate of the battery, it is essential to improve the conductivity of the electrolyte.

At the same time, in order to apply it to secondary batteries, it is natural that lithium in the non-aqueous electrolyte is required to have high charging and discharging efficiency. That is, the electrolytic solution used in a lithium secondary battery needs to simultaneously satisfy two requirements: (1) to have high electrical conductivity, and (2) to have high lithium charging/discharging power.

, +7 deum A LiAsF--2-methyltetrahydrofuran electrolyte has been proposed as an electrolyte with relatively high charge/discharge efficiency (see US Pat. No. 4,118,559). However, the isoelectric mho conduction force is low ((t
5 M (mole/l) 45 x in LiAsF6
10-"B-cm-", 25℃], when used in lithium batteries, there was a drawback of low charging and discharging efficiency. Although the electrolyte exhibits relatively high conductivity (e.g. t 5 M LiA
sF・a3X10-38・a11-3M~1 respectively
0-1 and 6.2 x 10-"8-cm-"),
Lithium has the disadvantage of low charge/discharge efficiency.

[Problem to be solved by the invention]

That is, to date, an electrolytic solution for lithium secondary batteries that has high conductivity and high lithium charging/discharging efficiency has not been realized.

The present invention was made in view of the current situation, and its purpose is to provide an electrolytic solution for lithium batteries that makes it possible to manufacture lithium batteries with high conductivity and excellent lithium charging and discharging characteristics. It is in.

[Means to solve the problem]

To summarize the present invention, tM for lithium battery according to the present invention
The solution is prepared by dissolving a lithium salt in an organic solvent, and then adding a cyclic saturated hydrocarbon or a cyclic saturated hydrocarbon having a linear alkyl group as an additive to a lithium battery electrolyte.
(mole/1.) The most important feature is that it is added in a range of 10-1 MOa degrees.

The present invention provides a lithium battery that has high electrical conductivity and excellent charging and discharging efficiency, compared to the conventional lithium battery electrolyte solution that has been difficult to achieve both high electrical conductivity and excellent lithium charging and discharging efficiency. This is to realize an electrolyte solution for use.

The present invention will be explained in more detail.

In lithium batteries, the negative electrode active material is made of lithium or a lithium alloy that allows lithium ions to be discharged9, the positive electrode active material is made of a material that electrochemically undergoes a reversible reaction with lithium ions, and the electrolyte is made of lithium salt in an organic solvent. It consists of a solution dissolved in

According to the present invention, it is an object of the present invention to provide an electrolytic solution as described above having excellent characteristics. That is, a lithium salt is mixed with an organic solvent, and a cyclic saturated hydrocarbon or a cyclic saturated hydrocarbon having a linear alkyl group is contained in the bath solution at a concentration of 10" M to 1 g-I M (in the range of 0.8 degrees, This is used as an electrolyte for lithium batteries.

Ninthly, to improve the electrolyte conductivity and lithium charging/discharging power used in lithium batteries, the lithium salt in the solvent or the solvent with low electron transfer reactivity from lithium to the solvent is likely to dissociate, and It is thought that the mobility of lithium ions is required to be high.The cyclic saturated hydrocarbon in the present invention or the cyclic saturated hydrocarbon having a straight-chain alkyl group can easily be used without hindering the dissociation of lithium salt or the mobility of lithium ions. It suppresses the formation of ash between lithium and solvent that is adsorbed on the lithium negative electrode and causes a decrease in lithium charging and discharging efficiency, and prevents lithium ions from depositing on the negative electrode during charging and discharging, and from flowing from the negative electrode into the electrolyte. It is added for the purpose of facilitating elution.

Although it is not necessarily clear why the cyclic saturated hydrocarbon or the cyclic saturated hydrocarbon having a straight-chain alkyl group used in the present invention exhibits the effective feature 9 when added to the electrolyte,
One idea is that the additive forms a physical or chemical adsorption bond with the negative electrode lithium, thereby suppressing the loss of lithium due to the reaction between lithium as the negative electrode active material to be used in charge/discharge cycles and the electrolyte. A layer is formed at the negative electrode/electrolyte interface. This layer is expected to improve the state of precipitation of L1+ ions on the negative electrode during charging, and is also expected to have the effect of suppressing the generation of precipitated lithium, which becomes electrochemically inactive.

Observation of the negative electrode surface after discharging and charging using an electron microscope was performed using the battery system described in Example 1 (however, the negative electrode was
According to the results obtained using mAhLit-
In a system without this additive, particulate lithium with a diameter of about 10 μm is effective in completely improving lithium charge/discharge efficiency, and particulate lithium with a diameter of 1 to 1 μm, which is a factor in reducing lithium charge/discharge efficiency.
Acicular lithium needles with a diameter of 2 μm and a length of about 10 μm exist on the surface of the negative electrode, and the rate of precipitation of these acicular lithium increases with charge/discharge cycles. On the other hand, in a battery using the IS liquid of the present invention with the additive added, the presence of the acicular lithium was not observed, and particulate L1 with a diameter of about 2 to 5 μm was precipitated uniformly. Ashi, of the present invention by adding the additive! The effectiveness of the solution was confirmed.

Addition of cyclic saturated hydrocarbon or cyclic saturated hydrocarbon having a straight chain alkyl group in the present invention f110-IM
~1 (1-I Mr16, *ff) It is the same as the characteristics of the conventional t solution which is not added when it is less than 10-3M, and no improvement in the characteristics is seen by addition.
This is because if the additive exceeds i-, the excessive amount of the additive will have an adverse effect on the mobility of ions and the electrochemical reaction of the negative electrode, and the characteristics will deteriorate compared to before addition.

The straight chain alkyl group in the present invention preferably has 1 to 12 carbon atoms. If the number of carbon atoms is 13 or more, it will be difficult to dissolve in organic solvents.

The lithium salt and organic solvent constituting the electrolytic solution in the present invention are not fundamentally limited. For example, lithium salts include LiCa4, LiAaF, Li5
t1F@, LiBFn% LiPF5, LiAtc
One or more of t4, LiCF35O, LiCFsCO, etc. can be effectively used. In addition, organic solvents include propylene carbonate, ethylene carbonate, 2-methyltetrahydrofuran, tetrahydrofuran, dioxolane, 2-methyldioquinrane, 4-methyldioxocune, 1,2-dimethoxyethane, r-butyro2chton, dimethylsulfoxide, One or more aprotic organic bath media such as acetonitrile, formamide, dimethylformamide, and nitromethane can be used effectively.

The negative electrode active material used in the lithium battery using the electrolyte according to the present invention is basically not limited, and the negative electrode active material used in conventional lithium batteries, that is, lithium or lithium ions, can be discharged. A lithium alloy can be used.

Similarly, the positive electrode active material used in the present invention is not fundamentally limited, and may be a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically reacts with lithium ions. [Example] Hereinafter, the present invention will be explained in more detail in a practical manner.
Original F! A is not limited to these examples.

Example 1 t5M LiAsF6- ethylene carbonate (
EC)/2-methyltetrahydrofuran (2MeTHF
) (volume mixing ratio 1/1) to 100 ppm of impurities.
An electrolytic solution controlled as below is prepared, and 01M of a cyclic saturated hydrocarbon, which has been thoroughly dehydrated by adding Molecular Sieves 3A or 4A at a temperature above the melting point for two days and nights, is mixed therein. It was prepared by the method and used as an electrolyte for a 71 thium battery.

The positive electrode active material contains 95 m01e% V snow 0 @ -5 mol
A positive electrode was prepared using an amorphous material having a composition of e% p, o, with a mixing ratio of 70% by weight, 25% by weight of acetylene black as a conductive agent, and 5% by weight of Teflon as a binder. Mixture pellet (16 φ, thickness α5) t
-Used as a positive electrode, metal lithium (17 φ
, 15 mAh) and a microporous polypropylene sheet as a separator. This lithium battery was charged at room temperature with a current value of 1 mA and a voltage range of 2 to 55 V. Conduct a discharge test,
The photodischarge characteristics of the electrolyte were evaluated. An example of the results is shown in FIG. That is, FIG. 1 shows t 5 M LiAaF・-E
An electrolytic solution containing cyclooctane (CsHts) t-101M in C/2Me'rHF (1/1) and 1
1 m for the battery of the invention using 5 mAh lithium
This is a graph showing the relationship between the number of cycles (horizontal axis) and the discharge capacity (mAh, vertical axis) when a charge/discharge test was conducted at a current value of A and a voltage range of 2 to 55V. In this battery system, all the lithium on the negative electrode side that can participate in discharging is consumed at each discharge in the charge/discharge cycle, and its capacity gradually decreases according to the charge/discharge electric current iE, as is clear from Figure 1. I'm going to go there. In other words, the charge/discharge efficiency of the negative electrode is
If the capacity of the second discharge is t-Cn, then Cn = E The charging/discharging efficiency E of the negative electrode lithium can be calculated from the slope of the straight line part of the graph 2) which represents the discharge capacity on a logarithmic scale.

The charging and discharging efficiency E of lithium in the electrolytic solution to which various cyclic saturated hydrocarbons were added was calculated using the above formula, and the results are shown in Table 1. For comparison, Table 1 shows the electricity containing no additives. We have also shown the results of a similar test.

As is clear from the results in Table 1, the addition of cyclic saturated hydrocarbons showed higher lithium charge/discharge efficiency compared to the case without addition. Example 2 of lithium charging and discharging efficiency in an electrolytic solution containing
) IEc/2Me THF (1/
1) Add regular sieves 3A to the electrolyte in advance.
Alternatively, electrolysis of a lithium battery produced by the production method described in Example 1 by mixing αOIM with cyclic saturated hydrocarbons having various linear alkyl groups, which have been thoroughly dehydrated by placing four people at a temperature above the melting point for two days and nights. Used for liquid. A charging and discharging test was conducted under the same conditions as in Example 1 for this lithium battery, and the charging and discharging characteristics of the electrolytic solution were evaluated.

An example of the result is shown in Fig. 2◇In other words, Fig. 2 shows t 5
M LiA8F@-B C/ 2Me THF (1/
1) Added 101M of n-dodecylcyclohexane (CH, (C)oc@HuE) and mixed it with Kyuden folding liquid for 15M.
The graph shows the relationship between the number of cycles (horizontal axis and discharge capacity (mAh, vertical axis)) when a charge/discharge test was conducted on the battery of the present invention using Ah lithium at a current value of 1 mA and a range of 2 to 5 SVO @ pressure. This is a graph.

The charging and discharging efficiency E of the negative electrode lithium was calculated in the same manner as in Example 1 from the slope of the straight line part of the graph in which the discharge capacity on the vertical axis is expressed on a logarithmic scale as shown in FIG. The lithium charge/discharge efficiency E in the electrolytic solution containing a cyclic saturated hydrocarbon having a chain alkyl group was calculated using the above formula, and the results are shown in Table 2. Table 2 also shows the results of a similar test using an electrolyte without additives for comparison.

As is clear from the results in Table 2, it was found that by adding a cyclic saturated hydrocarbon having a linear alkyl group, higher lithium charge/discharge efficiency was exhibited than in the case without addition.

Table 2 Lithium charge/discharge efficiency example 3 in electrolytes containing cyclic saturated hydrocarbons having various linear alkyl groups t 5 M LiAsF6- EC/2MeTHF (1
/1) An electrolytic solution with controlled impurities was prepared, and cyclooctane (CsHs.)'t-α01M was dehydrated by adding Molecu Thieves 5At- in advance and leaving it in a constant temperature bath at 80°C for two days and nights. This was added and used as an electrolyte for lithium batteries. □After being left at room temperature in an argon dry box for a certain period of time after cleavage, 1! Using the solution solution, a coin type lithium battery was produced in the same manner as in Example 1, and a positive electrode and a negative electrode were produced in the same manner as in Example N1. Further, a charge/discharge test was conducted under the same conditions as in RC implementation M1, and the RITECUM charge/discharge effect* was evaluated.

The results are shown in Table 3. It is clear from Table 3 that the additive does not decompose and deteriorate over time and maintains stable charge/discharge efficiency9.

Third layer Change in lithium charge/discharge efficiency over time Example 4 Each additive of dehydrated cyclooctane (C5H1@), fuclohexane (C5Htx), and n-dodecylcyclohexane (CHs(CHs)uc@Hu) was added to 1M LiCtOa. - Probile/Carbonate (PC
')/1,2-dimethoxyethane (DMli) (1/
1), t5MLiAsF@-PC%t5MLiA
sF6-2MeTHF, 1.5M LLP Fs
-EC/2Me TH? (1/1) of impurities were added to the solution at a concentration of 10-4 to 15M, and used as an electrolyte for lithium batteries.

A coin type battery was produced in the same manner as in Example 1, and Example 1
A charge/discharge test is performed under the same conditions as 'ft-.

The following lithium charge/discharge efficiencies obtained from the test are shown in Table 4.◎ As is clear from Table 4, in all electrolytes, the additives total 10-3M to 10-3-10-3M to 10-
It is clear that by adding IM, higher charge/discharge efficiency is exhibited than when no IM is added.

Example 5 Fly with impurity control, 1.5 M LiA
sF・-EC/2MeTHF (1/1) 'In IcIc, add Memolecular Sheeps 3A't-8
n-dodecylcyclohexane (n-cl!am-c@H11) that was left in a constant temperature bath at 0°C for 2 days and nights to undergo dehydration treatment.
t''l OI M was added to prepare an electrolyte for lithium batteries.

The active material has a composition of 95 mole% VzO @ -5 mol
e%P! O.

A positive electrode mixture pellet (16 φ) prepared in the same manner as in Example 1 containing an amorphous material was used as the entire positive electrode, and 60 mAh metallic lithium (17 φ) was used as the negative electrode. A coin-type lithium battery was produced in the same manner.

Discharge this lithium battery at room temperature at a rate of 3 peaks of 7cnz
.

Charge 11LNfM 1 mA/″cy?, 2 V~l
A charge/discharge test was conducted under pressure regulation at 5 V (01) to evaluate the charge/discharge characteristics.For comparison, a coin salt battery was fabricated using an electrolyte containing no additives, and a similar charge/discharge test was conducted. The results are shown in Figure 3. That is, Figure 3 shows t5M Li
AaFs-EC/2Me THF (1/1)
Using an electrolytic solution and an electrolytic solution to which α01M n-dodecylcyclohexane was added, the discharge current R for each battery was 5 mA/cm? , charging current 1-A-12v~
5 is a graph showing changes in discharge capacity ii (mAh, vertical axis) with charge/discharge cycles (horizontal axis) when a charge/discharge test was conducted under voltage regulation of 55V. In FIG. 5, curve A is for the battery of the present invention using an electrolyte containing n-dodecylcyclohexane as an additive [lL01Mi, and curve B is for a friend battery using an electrolyte containing no additive. It is.

As is clear from Figure 3, a battery using an electrolytic solution containing n-dodecylcyclohexane as an additive exhibits superior charge-discharge cycle stability compared to a battery using electrolytic metallization that does not contain this additive. Ta.

Example 6 1,5 M LiAsF6-EC/2Me TH
F (1/1) 'N-dodecylcyclohexane (n-Cu
Hn-C@Hu) k1101M was added to prepare an electrolyte for lithium batteries.

This 'fIL solution was dissolved and a coin-type lithium battery was prepared in the same manner as in Example 5. The discharge current of this battery was 1 mA7'cxP, 2 mA/cd, respectively.
4mA/cm", and charging 1!flow j ml, 10re"
, a charge/discharge test was conducted under voltage regulation of 2v to xsv. For comparison, without additives! A coin-type lithium battery was produced in the same manner using dissolution, and a charge/discharge test was conducted under the same conditions.

The results are shown in Figure 4. In other words, Figure 4 shows the discharge capacity (m
It is a graph showing the change in Ah (vertical axis). In Figure 4, curve C indicates the case of a Q' area pond using an electrolyte with n-dodecylcyclohexane added in the discharge test, and the curve C indicates the case without additive in the charge/discharge test under the same conditions. In the case of a battery, curve E is discharge type R2
of n-dodecylcyclohexane addition in a charge/discharge test under the conditions of m)-/cl and a charging current of 1 mA~! In the case of a battery using dissolution, the filtration curve F is for a battery using an electrolyte containing no additives in a charge/discharge test under the same conditions.

Furthermore, in Figure 4, the song? a() is discharge type fL3mA
/ctl, charge current 1 mv, charge/discharge test with n-dodecylcyclohexane added.
Curve H is for a battery using electrolysis t without additives in a charge/discharge test under the same conditions.

As is clear from Figure 4, in the charge and discharge tests under all conditions, when using the electrolyte of the present invention containing n-dodecylcyclohexane, the capacity was lower than when using the electrolyte containing kumquat without additives. No significant deterioration was observed, and long-term charge/discharge cycle stability was demonstrated.

[Effects of the Invention] As explained above, the cyclic saturated hydrocarbon 21o containing a cyclic saturated hydrocarbon or a linear alkyl group in the molecule according to the present invention
- Lithium salt added in a concentration range of s to 1 g-I M dissolved organic solvent-based electrolyte It has the advantage that batteries can be made available and can be widely used in various fields.

[Brief explanation of the drawing]

Figure 1 shows 1.5 M LiAsF6-EC/2Me
In the battery of the present invention using an electrolyte solution containing 101 M of cyclooctane in THF (1/1), 15 mA was applied to the negative electrode.
Charging and discharging test #1 of a battery using lithium in a voltage range of 1 mA and 2 V to 55 V. A graph showing the relationship between the number of cycles performed and the discharge capacity.
A31F@-E C/ 2 Me THF (1/1
) When a charge/discharge test was conducted in the pressure range of 1 mA, 2 ~ & SVO @ pressure range of a battery of the present invention containing an electrolyte containing 101 M of n-dodecylcyclohexane (kumquat) and using 15 mAk lithium as the negative electrode, the number of cycles and discharge were A graph showing the relationship of capacity, Figure 3 is 1.5 M LiAsF6-
E C/ 2Me THF (1/1) with α01M of n-dodecylcyclohexane added to the electrolyte and the electrolyte without kumquat discharge current 5 mA/c+m”
, a charging current of 1 mA/cpg", a graph showing the change in discharge capacity with the number of cycles when a charging/discharging test was conducted under voltage regulations of 2 V to XSV. Figure 4 shows the discharge current of batteries of the same type 1 m)y'cm'', 2mA respectively
/3", 4 mA/cm", charging current 1 mA/c
A graph showing the change in discharge capacity due to cycles when a charge/discharge test was conducted under voltage regulation of 2V to 55V. Patent applicant: Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. In an electrolytic solution for lithium batteries in which a lithium salt is dissolved in an organic solvent, a cyclic saturated hydrocarbon or a cyclic saturated hydrocarbon containing a linear alkyl group in the molecule is used as an additive.
An electrolytic solution for a lithium battery, characterized in that it contains in a concentration range of ^-^3M to 10^-^1M.