JPH0456002A - Lithium ion conductive polymer electrolyte - Google Patents

Abstract

PURPOSE:To provide a polymer electrolyte exhibiting good Li ion conductivity in the solid state at room temp. by using a specified polymer as an organic polymer for electrolyte. CONSTITUTION:An organic polymer to constitute a composite substance together with lithium salt shall be of cross-linked structure obtained through cross-linking of a graft substance as graft copolymer of polyether glycol having a mean molecular weight of 1000 or more and polyether glycol.monomethyl ether having a mean molecular weight of 1000 or more with siloxane hydride. A separator used 7 is of a sheet consisting of Li ion conductive polymer electrolyte, while a pos. electrode 4 is alike of a sheet containing Li ion conductive polymer electrolyte. This enables constructing the cell in thin form and enhances the operating easiness in fabricating the cell and also reliance upon the seal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to lithium ion conductive polymer electrolytes.

[Conventional technology]

Attempts have been made to use polymer electrolytes, which are flexible and can be easily formed into a film, as lithium ion conductive solid electrolytes for lithium batteries and the like.

This polymer electrolyte is made of a composite of lithium salt and an organic polymer that dissolves lithium salt. Taking advantage of its flexibility and ease of forming into a film, it can be made thinner and smaller. If it can be applied to lithium batteries, which are required to be improved, it will be advantageous in terms of workability and sealing for battery production, and it will also have the advantage of being useful in reducing costs.

In addition, this polymer electrolyte is applicable not only to lithium batteries, but also to
Due to its flexibility, it is thought to be useful as an electrolyte for electrochromic displays, lithium ion concentration sensors, lithium ion components, etc.

To date, as organic polymers constituting such polymer electrolytes, polyethyleneoxy 1+ [e.g.
M, B, ^r-ond, Fast Ion Tran
sport in 5olid, 131 (1979
) ), polyethyleneimine [e.g., T, Taka
hashi eL al, 5olid 5tate
1onics 1B & 19.321 (198G)
), polyethylene succinate (M, WaLana
be eL al, Macromolecules1
7, 2902 (1982)), cross-linked triol polyethylene oxide (Polymer Journal
, νo1.1B, Hall, 809 (1986
)) have been proposed.

[Problems to be Solved by the Invention] However, the conventional polymer electrolyte made of a composite of an organic polymer and a lithium salt has a low ionic conductivity of +xio-"~lXl0-5S/c at 25°C. Therefore, when applied to lithium batteries and the various applications mentioned above,
There was a problem in that the performance was not fully satisfactory.

Therefore, an object of the present invention is to provide a polymer electrolyte that is solid at room temperature and exhibits good lithium ion conductivity by using a polymer different from conventionally used organic polymers as the organic polymer of the polymer electrolyte. shall be.

[Failure to solve the problem]

In the present invention, as an organic polymer for a lithium ion conductive polymer electrolyte, a polyether glycol having an average molecular weight of 1.000 or more and a polyether glycol monomethyl ether having an average molecular weight of 1,000 or less are graft copolymerized to siloxane hydride. By using a crosslinked polymer obtained by crosslinking a grafted product, the problems of conventional polymer electrolytes were solved, and a lithium ion conductive polymer electrolyte with high ion conductivity even at room temperature of about 25°C was provided.

As mentioned above, the siloxane hydride has an average molecular weight i
By using a crosslinked polymer obtained by graft copolymerizing polyether glycol with an average molecular weight of , ooo or more and polyether glycol/monomethyl ether with an average molecular weight of i, ooo or less, lithium with high ionic conductivity even at room temperature can be obtained. The reason why the ion conductive polymer electrolyte can be obtained is as follows.

The ionic conduction of polymer electrolytes is
As suggested by ver et al. (C1 & EN, 54
(1985) ), this segmental interlocking is caused by the segmental motion of polymers, and this segmental interlocking is caused by tree-void
Related by lume theory, T, Miyamo
to et al (J, 8pp1. phys,
Vol, 44. No, 12. 5372 (1
973) ), M. Watanabe et a.
t (J, Appl, pl+ys, 57.123
(+985)] et al., the following ionic conduction formula (
A) and (b) are proposed.

σ=9°n°μ...(a) σ=q・
no・expA−expB −(b)A=(−W
/2εH) ((].D/kT)rVi where q: n : μ : η0 ; W : ε : Constant diffusion constant r: numerical factor v]: phosphorus field volume Vgz ratio field a (Tg) F g: free volume coefficient iI! (Tg)
α; Volumetric expansion coefficient Tg! Glass transition temperature Therefore, to improve ionic conductivity, the formula (
It is necessary to improve the ion mobility (μ), which is involved in ion conduction, to a greater extent than the carrier concentration (n) in a). In addition, for this purpose, the glass transition temperature (
It is necessary to lower the glass transition temperature (Tg) and to increase the specific volume (Vg) at the glass transition temperature (Tg), in other words, to lower the degree of crystallinity.

This was confirmed by P. H. Blonsky et al.
(Solid 5Lats Ionics 18 &
19.258 (1986)) has a Tg of 83°C compared to the Tg of polyethylene oxide of -60'C.
Although it is liquid, the polymer electrolyte of the lXl0-'S/c layer was obtained using the Iyphosphazene derivative, and M, Hatanabe (Polyme
r Journal, ν01.81st floor 11.809 (
(1986) et al. made the crystallinity of cross-linked triol polyethylene oxide 30% compared to 70% of polyethylene oxide, and obtained a polymer electrolyte of 1 x 10-'S/c. be done.

Therefore, in order to obtain an ion conductive polymer electrolyte that is solid at room temperature and exhibits good lithium ion conductivity, a polymer with a low glass transition temperature and low crystallinity is used as the organic polymer of the polymer electrolyte. It is necessary.

From such a viewpoint, the present invention first uses a siloxane hydride having silicon in the molecule as a component that becomes the main chain of the polymer, and the siloxane hydride lowers the glass transition temperature of the polymer and softens the polymer. By making it easier for molecular chain movement to occur and making it easier for ions to move, the ionic conductivity at room temperature is increased.

Then, this siloxane hydride has an average molecule It
Polyether glycol monomethyl ether with an average molecular weight of 1,000 or less is graft copolymerized, but polyether glycol monomethyl ether has an average molecular weight of 1,
If it is less than 000, it is liquid, and by grafting this liquid polyether glycol monomethyl ether, the glass transition temperature of the obtained polymer can be lowered and the degree of crystallinity can be lowered.

On the other hand, polyether glycol makes the polymer crosslinkable, and the crosslinking allows the polymer electrolyte to maintain a solid state such as a sheet. The reason why polyether glycols with average molecular needles of 1.000 or more are used is because those with small molecular weights have low strength, and in order to have the necessary strength when the polymer electrolyte is made into a thin sheet, etc. This is because it is necessary to use a material having an average molecular weight of 1,000 or more.

When copolymerizing these polyether glycols and polyether glycol monomethyl ether into siloxane hydride, the reason for graft copolymerization rather than long-chain copolymerization is to reduce the glass transition temperature and crystallinity due to the increase in molecular weight due to polymerization. This is to minimize the increase.

The grafting position of these polyether glycols and polyether glycol monomethyl ether is the silicon (St) of the siloxane hydride.

In addition, the reason why a crosslinked polymer is used as an organic polymer in a polymer electrolyte is to suppress the glass transition temperature and crystallinity of the polymer before crosslinking as much as possible due to an increase in molecular weight. This is to increase the molecular weight without increasing the molecular weight as much as possible so that the polymer electrolyte remains solid at room temperature and has the necessary mechanical strength.

In the present invention, examples of the siloxane hydride used include tetramethylcyclotetrasiloxane, pentamethylcyclopentasiloxane, and octamethyltetrasiloxane.

In the present invention, polyether glycol having an unsaturated group is used as the polyether glycol, and specific examples thereof include furylated polyethylene glycol, allylated polypropylene glycol, allylated polyethylene glycol/polypropylene glycol, and polyethylene glycol. Examples include methacrylate, polypropylene glycol methacrylate, polyethylene glycol/polypropylene glycol methacrylate, polyethylene glycol acrylate/polypropylene glycol acrylate, and polyethylene glycol/polypropylene glycol acrylate. Further, as the polyether glycol monomethyl ether, polyether glycol monomethyl ether having an unsaturated group is used, and specific examples thereof include allylated polyethylene glycol monomethyl ether, allylated polypropylene glycol monomethyl ether,
Allylated polyethylene glycol/polypropylene glycol monomethyl ether, polyethylene glycol methacrylate monomethyl ether, polypropylene glycol methacrylate monomethyl ether, polyethylene glycol methacrylate/polypropylene glycol monomethyl ether, polyethylene glycol acrylate monomethyl ether, polypropylene glycol acrylate - Monomethyl ether, polyethylene glycol acrylate, polypropylene glycol, monomethyl ether, etc.

In graft copolymerization, the amount of copolymerization components polyether glycol and polyether glycol monomethyl ether to be used is 1.0 to 1.5 in the ratio of [unsaturated group]/(SiH group) to siloxane hydride. It is preferable that the copolymerization components polyether glycol and polyether glycol monomethyl ether be used in a weight ratio of 1:1 to 6:1.

In other words, if the amount of polyether glycol and polyether glycol monomethyl ether used as graft copolymerization components for siloxane hydride is less than the above range, reactions between the siloxane hydrides tend to occur, resulting in the effect of lowering the glass transition temperature, etc. If the amount of polyether glycol and polyether glycol monomethyl ether used as graft copolymer components for siloxane hydride exceeds the above range, unreacted portions This increases, and it remains in the polymer electrolyte (it is difficult to separate it due to its poor solubility in solvents), causing a decrease in ionic conductivity.

In addition, if the ratio of polyether glycol monomethyl ether to the polyether glycol in the copolymerization component is less than the above range, the grafted product will become hard, making it difficult for molecular chain movement to occur, which may cause a decrease in ionic conductivity. If the ratio of polyether glycol monomethyl ether to polyether glycol exceeds the above range, it becomes difficult for the grafted product to remain solid at room temperature.

When graft copolymerizing polyether glycol and polyether glycol monomethyl ether to siloxane hydride, the St of siloxane hydride
When graft copolymerizing a hydroxyl group to an H group, the reaction may be carried out at a temperature of 20 to 100°C using a metal salt such as zinc octylate or tin octylate as a catalyst. On the other hand, when graft copolymerizing an unsaturated group to 5iH5, the reaction may be carried out at a temperature of 25 to 100°C using hexachloroplatinic acid, tetrachloroplatinate, ruthenium chloride, or the like as a catalyst.

In the present invention, the grafted product obtained by the above reaction can be further modified to introduce another functional group, such as a vinyl group or a hydroxyl group, at the molecular end. For example, if the above-mentioned grafted product has an unsaturated group at the end of the molecule obtained by the reaction of a Si H group and a hydroxyl group, it can be modified by appropriate means to introduce a hydroxyl group at the end, or If the grafted product mentioned in ``2'' has a hydroxyl group at the end of the molecule obtained by the reaction of an S i H group and an unsaturated group, it can be modified by an appropriate means to introduce a vinyl group at the end. .

In the present invention, various grafted products obtained in this manner are crosslinked to produce a crosslinked polymer. In the case of a grafted product having a hydroxyl group at the end of the molecule or a grafted product whose molecular end is modified with a hydroxyl group, this is used. As crosslinking agents for crosslinking, difunctional compounds capable of reacting with hydroxyl groups, such as hexamethylene diisocyanate, 2.4-
Diisocyanates such as tolylene diisocyanate, methylene bis(4-phenyl isocyanate), and xylylene diisocyanate; diamines such as ethylene diamine and putrescine; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, isophthalic acid, and terephthalic acid; and dicarboxylic acids such as succinel chloride. Acid chlorides, methylol compounds such as dimethylurea, epichlorohydrin, dimethyldichlorosilane, etc. are used.

The above-mentioned crosslinking reaction can usually be carried out using a catalyst such as an organic tin compound in the case of diisocyanate, by reacting at 25 to 100°C for about 5 minutes to 2 hours. The amount of crosslinking agent used at this time is usually 0.1 to 2.0 moles of functional groups per mole of hydroxyl groups in the grafted product.If unreacted grafted products remain, the ionic conductivity It is most appropriate to react with equimolar amounts of functional groups, since this may cause the reaction to occur with the lithium salt.

Furthermore, since it is necessary to lower the glass transition temperature of the crosslinked polymer, the crosslinking points are preferably amide, urethane, ester, and ether in this order, and it is more preferable to use aliphatic hydrocarbons than aromatic ones.

On the other hand, in the case of a grafted product having an unsaturated group at the molecular end or a grafted product modified with a vinyl group at the molecular end,
Cumene hydroperoxide, benzoyl peroxide, lauroyl peroxide, which can undergo ring-opening polymerization of these groups;
Organic peroxides such as potassium peroxide, phtyl hydroperoxide, dicumyl peroxide, di-L-butyl peroxide, azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, abbiscyclohexanecarbonitrile Azobis compounds such as these are used. The amount used is usually about 0.01 to 1 part by weight per 100 parts by weight of the grafted product, and the crosslinking reaction is 25 parts by weight.
This can be carried out at ~100°C for about 5 minutes to 2 hours.

Further, the following formula (I) which can react with the unsaturated group at the end of the molecule or the above-mentioned vinyl group; CH3CH, C1l.

The crosslinking treatment may be performed using a polydimethylsiloxane having S I H groups at both ends represented by the formula, and further crosslinking treatment may be performed by irradiating with electron beams, gamma rays, ultraviolet rays, visible light, or infrared rays. In this case as well, it is preferable to carry out the reaction so that no uncrosslinked grafted product remains.

In constructing the lithium ion conductive polymer electrolyte,
The above-mentioned crosslinked polymer has a low crystallinity of 30% or less, preferably 12% or less, more preferably amorphous, and a glass transition temperature of 40°C or less, preferably 12% or less. It is appropriate to keep the temperature below 50°C.

In the present invention, as the lithium salt as a component constituting the lithium ion conductive polymer electrolyte, any of those used in conventional polymer electrolytes can be used together with the above-mentioned crosslinked polymer, such as LiBr.
, Lil, Li5CN, LiBF, , LiAsF, St
, ic+o4, L IcF! S03, L i Cm
F+*SOx, LiHgls, etc. are used.

These lithium salts are usually used in an amount of 0.1% or more, preferably 1 to 30% by weight, based on the crosslinked polymer in the composite consisting of the lithium salt and the crosslinked polymer.
A range of % by weight is suitable, especially a range of 3 to 20% by weight.

The lithium ion conductive polymer electrolyte of the present invention is composed of a composite of the above crosslinked polymer and the above lithium salt. For example, the above crosslinked polymer is immersed in an organic solvent solution in which a lithium salt is dissolved, It can be obtained by impregnating a lithium salt solution into a crosslinked polymer and then evaporating off the organic solvent.

By immersing the crosslinked polymer in the lithium salt solution as described above, the lithium salt forms a complex with the ether oxygen in the crosslinked polymer and bonds, and even after the solvent is removed, the above bond is maintained, and the crosslinked polymer and lithium salt A complex with is obtained.

The form of the polymer electrolyte is determined as appropriate depending on its intended use. For example, when a polymer electrolyte is used as an electrolyte for a lithium battery and also functions as a separator between positive and negative electrodes, the polymer electrolyte may be formed into a sheet shape. To obtain this sheet-like polymer electrolyte, a cross-linked polymer is formed into a sheet, the sheet-like cross-linked polymer is immersed in a solution of a lithium salt in an organic solvent, and the lithium salt solution is permeated into the cross-linked polymer. The solvent may be removed by evaporation. As the above-mentioned polymer electrolyte sheet, it is also possible to produce an extremely thin sheet on the order of microns, which is generally called a film.

In addition, when applying the polymer electrolyte of the present invention to a positive electrode of a lithium battery, the grafted product before crosslinking, a crosslinking agent, a positive electrode active material, etc. are added in a predetermined ratio, and the grafted product is molded and crosslinked. The molded body may be immersed in a solution of a lithium salt in an organic solvent, and then the organic solvent may be removed by evaporation.6 By doing so, a mixture of the polymer electrolyte, the positive electrode active material, etc. can be obtained.

In obtaining the polymer electrolyte, the organic solvent for dissolving the lithium salt may be any organic solvent that sufficiently dissolves the lithium salt and does not react with the crosslinked polymer, such as acetone, tetrahydrofuran, dimethoxyethane, dioxolane, propylene carbonate, Acetonitrile, dimethylformamide, etc. are used.

Figure 1 shows an example of a lithium battery using the polymer electrolyte of the present invention described above. A shallow rectangular dish-shaped negative electrode plate made of stainless steel with a flat peripheral part (2a) in the same direction as the main surface bent in steps, (3) is a bipolar current collector plate (1), This is an adhesive layer that seals between the opposing peripheral parts (1a) and (2a) of (2).

(4) is a polymer electrolyte of the present invention, a positive electrode active material, etc. arranged on the positive electrode current collector plate (1) side in a space (5) formed between both electrode current collector plates (1) and (2). The positive electrode (6) formed into a sheet shape by the method described above is a negative electrode made of lithium or a lithium alloy loaded on the negative electrode building board (2) side in the space (5), and the positive electrode (7) is a negative electrode made of lithium or a lithium alloy. 4) and negative electrode (
6) A separator formed by molding the polymer electrolyte of the present invention into a sheet shape with a separator interposed therebetween.

The positive electrode (4) may be formed into a sheet by mixing a positive electrode active material with a binder such as polytetrafluoroethylene powder or an electron conduction aid, as the case may be. Examples of the positive electrode active material used for the positive electrode (4) include T I St and M OS! ,V6O13,Vz
05, VSeSNjPSi, polyaniline, polypyrrole, polythiophene, etc., or two or more thereof are used.

A lithium battery configured in this way has a separator (7
) is a sheet-like material made of the above-mentioned lithium ion-conducting polymer electrolyte, and the positive electrode (4) is a similar sheet-like material containing the above-mentioned lithium-ion-conducting polymer electrolyte, thereby making the battery thinner. In addition to having various advantages such as improving workability for manufacturing batteries and reliability of sealing, and essentially not having to worry about leakage unlike liquid electrolytes, Since the polymer electrolyte has excellent lithium ion conductivity, it has excellent discharge characteristics as a secondary battery and charge/discharge cycle characteristics as a secondary battery.

(Example) Example 1 Tetramethylcyclotetrasiloxane (manufactured by Toshi Silicone Co., Ltd.) 1 g 2 Allylated polyethylene glycol (manufactured by NOF Corporation) with an average molecular weight of 1,000 10 g, average molecular weight 2
00 allylated polyethylene glycol monomethyl ether (manufactured by NOF Corporation) 2g and potassium chloroplatinate 2I
Mix with 1g and heat at 100°C while stirring with Starcy.
The mixture was reacted for 3 hours to obtain a grafted product. At this time, the [unsaturated group]/(Sill group) ratio is 1.0. next,
0.168 g of hexamethylene diisocyanate was added to 52 g of this grafted product, a urethanization catalyst was added and mixed, and the mixture was dropped onto an aluminum plate.
A crosslinked polymer was obtained by reacting at 100° C. for 3 hours on a hot plate in argon gas. The obtained crosslinked polymer was peeled off from the aluminum plate and immersed in acetone, and unreacted materials were dissolved and removed in acetone.

Subsequently, this crosslinked polymer was added to 2% by weight of LIBF.

After 8 hours of immersion in an acetone solution of LiBFn, the acetone solution of LiBFn was infiltrated into the crosslinked polymer, and the acetone was removed by evaporation to obtain a sheet-like polymer electrolyte with a thickness of 0.1 mm.

Example 2 Example 1 except that 4 g of allylated polyethylene glycol monomethyl ether with an average molecular weight of 400 (manufactured by NOF Corporation) was used in place of the allylated polyethylene glycol monomethyl ether with an average molecular weight of 200 in Example 1. A sheet-like polymer electrolyte having a thickness of 0.1 mm was obtained in the same manner as above. In this case, the (unsaturated group)/[SIH group] ratio is 1.0.

Comparative Example 1 Instead of the grafted product in Example 1, an average molecular weight of 13
．． A sheet-like polymer electrolyte having a thickness of 0.1 mm was obtained in the same manner as in Example 1 using 000 polyethylene oxide triol (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.).

In order to examine the performance of the polymer electrolytes of Examples 1 to 2 and Comparative Example 1, the following ionic conductivity test and battery internal resistance test were conducted.

<Ionic conductivity test> The polymer electrolytes of Examples 1 to 2 and Comparative Example 1 were sandwiched between two lithium plates, and the AC impedance between the electrodes (between the lithium plates) was measured. The ionic conductivity at °C was investigated.

Internal resistance test of battery> The polymer electrolytes of Examples 1 to 2 and Comparative Example 1 were used as separators, and the total thickness of the battery was 1 m with the configuration shown in Figure 1.
A square thin lithium battery with the length of the a+ and - sides of 1 cm was produced. In addition, the negative electrode used an alloy of lithium and aluminum, and the positive electrode used Examples 1 to 2 and Comparative Example 1.
A sheet-shaped molded product containing TiS and a polymer electrolyte having the same composition was used. The internal resistance of these batteries at 25°C was measured.

These test results are shown in Table 1 below, along with the glass transition temperature and crystallinity of the crosslinked polymers used in Examples 1 to 2 and Comparative Example I (Comparative Example 1 was uncrosslinked polyethylene oxide). show. Note that the above characteristics of the crosslinked polymer were measured by the following method.

<Glass transition temperature> Cut the cross-linked polymer into pieces with a diameter of 31 u x length of 40 m x thickness of 0.5 threshold, and use Orientic's Rheohyblon DD.
The glass transition temperature was measured using a V-It dynamic viscoelasticity apparatus.

<Crystallinity> The crystallinity was determined from the peak area using a differential scanning calorimetry needle, DSC-30 manufactured by Shimadzu Corporation, at a heating rate of 5° C./min.

In addition, the ionic conductivity of each of the polymer electrolytes of Examples 1 to 2 and Comparative Example 1 was measured under various temperature conditions using the same method as described above. The results are shown in Figure 2. , the vertical axis is ionic conductivity (37 cm), and the horizontal axis is the reciprocal of absolute temperature 101/T (K-').

As shown in Table 1, the polymer electrolytes of Examples 1 and 2 of the present invention have an ionic conductivity of 6.0 '-1,0
Although it showed a high ionic conductivity of XIO-'S/c+a,
The polymer electrolyte of Comparative Example I had a low ionic conductivity of 0XIO-'S/c- at 25°C.

Therefore, the internal resistance at 25°C of the lithium battery using the polymer electrolytes of Examples 1 and 2 of the present invention is 100~
Although it was small at 167Ω, the internal resistance of the lithium battery using the polymer electrolyte of Comparative Example 1 at 25°C was 1.0Ω.
It was very large at 00Ω.

〔Effect of the invention〕

As explained above, in the present invention, polyether glycol with an average molecular weight of 1.000 or more and polyether glycol monomethyl with an average molecular weight of 1,000 or less are added to siloxane hydride as the organic polymer constituting the complex with the lithium salt. Ether was graft copolymerized. By using a crosslinked polymer obtained by crosslinking a grafted product, we were able to capture a lithium ion conductive polymer electrolyte that is solid at room temperature and has excellent ion conductivity.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an example of a lithium battery using the lithium ion conductive polymer electrolyte of the present invention as a separator. FIG. 2 is a diagram showing the relationship between the ionic conductivity and temperature of the lithium ion conductive polymer electrolytes of Examples 1 and 2 and Comparative Example 1. (7) Separator made of polymer electrolyte Figure 7 Separator made of poly7-electrolyte 2 Figure 10, /T(k-+)

Claims (1)

[Claims] (1) In a lithium ion conductive polymer electrolyte consisting of a composite of a lithium salt and an organic polymer, the organic polymer is a siloxane hydride, a polyether glycol having an average molecular weight of 1,000 or more, and a polyether glycol having an average molecular weight of 1,000 or less. A lithium ion conductive polymer electrolyte characterized by being a crosslinked polymer obtained by crosslinking a grafted product obtained by graft copolymerizing ether glycol monomethyl ether.