Slurry for forming insulating layer, separator for electrochemical device, method for producing the same, and electrochemical device

Thus, it is hard to say that the margin for safety of the battery is sufficient.

Moreover, the film has been distorted by drawing and may shrink due to residual stress when it is subjected to high temperatures.

If the pores are not sufficiently closed and the current cannot be immediately reduced, the temperature of the battery is easily raised to the shrinkage temperature of the separator, so that an internal short circuit can occur.

Thus, it may be difficult to maintain a stable dispersion state of the fine particles in the slurry.

When the slurry in which the fine particles are agglomerated or settled is applied to the base or the like, the application tends to be not uniform.

Moreover, when the dispersi...

[0028]In the separator, when the plate-like insulating fine particles are oriented with their plane surfaces substantially parallel to the surface of the separator, the occurrence of a short circuit can be more successfully suppressed. The reason for this is considered as follows. By orienting the plate-like insulating fine particles in the above manner, the insulating fine particles are arranged so that their plane surfaces overlap with one another. Therefore, pores (through holes) going from one side to the other of the separator are formed in a curve rather than a line (i.e., a tortuosity factor of the pores is increased). This can prevent the lithium dendrite from passing through the separator and thus suppress the occurrence of a short circuit more successfully.

[0036]The slurry for forming an insulating layer of the present invention includes the thickening agent. The use of the thickening agent can adjust the viscosity of the slurry in a suitable range, make the dispersion of the insulating fine particles uniform, and maintain a good dispersion state stably.

[0042]It is preferable that the slurry for forming an insulating layer of the present invention includes a dispersing agent. With the use of the dispersing agent, e.g., the dispersibility of the insulating fine particles in the slurry can be further improved, since the dispersing agent adheres to the surfaces of the insulating fine particles. This can prevent agglomeration of the insulating fine particles and thus maintain the dispersion state of the insulating fine particles more stably.

[0046]The content of the dispersing agent in the slurry is preferably 0.1 parts by mass or more, and more preferably 0.3 parts by mass or more per 100 parts by mass of the insulating fine particles so as to provide the effect of the dispersing agent more effectively. If the content of the dispersing agent in the slurry is too large, the effect of the dispersing agent becomes saturated, while the ratio of other components to the dispersing agent in the insulating layer is reduced, and thus the effects of the other components can be reduced. Therefore, the content of the dispersing agent in the slurry is preferably 5 parts by mass or less, and more preferably 1 part by mass or less per 100 parts by mass of the insulating fine particles.

[0051]As will be described in detail later, the insulating layer formed of the slurry of the present invention can be obtained in such a manner that the slurry is applied to a base material or the like and then dried to remove the solvent. In this case, if the slurry contains the self-crosslinking (meta) acrylic acid copolymer, the copolymer spontaneously crosslinks to form a crosslinked structure during the drying process. Therefore, since the binder acts to increase the adhesion between the insulating fine particles or between the insulating fine particles and the other constituents (such as a microporous film), the insulating layer can have good electrolyte resistance, and the stability of the slurry can be improved.

[0052]With the use of the self crosslinking (meta) acrylic acid copolymer as the binder, the insulating fine particles are held by a crosslinked body of the self-crosslinking (meta) acrylic acid copolymer. Thus, the electrolyte resistance of the insulating layer can be improved, compared to the case where the binder is not self-crosslinkable. If the binder inherently has a crosslinked structure, the slurry is dried while the binder particles retain their shapes, so that the contact area of the binder with the insulating fine particles or the base material may be reduced. In contrast, the self-crosslinking (meta) acrylic acid copolymer spontaneously crosslinks to form a crosslinked structure during the drying process, and therefore can improve the binding properties of the insulating layer, compared to the binder inherently having a crosslinked structure. Moreover, when a so-called two-component binder that has a crosslinked structure by adding a crosslinking agent to the resin (main component) is used, the stability of the slurry may be reduced due to the crosslinking agent, although the electrolyte resistance and the adhesion of the binder to other members can be better. However, the use of the self-crosslinking (meta) acrylic acid copolymer can maintain the stability of the slurry.

[0057]In addition to the above specific examples of the binder, a known resin may be mixed with an amine compound or a polyacrylic resin to enhance the flexibility or reduce the Tg of the binder. Moreover, a known plasticizer (phthalate esters etc.) may be added to improve the breaking strength of the binder as an additive for imparting flexibility to the binder. Further, the adhesion properties of the binder can be improved by introducing a carboxyl group. The Tg of the resin can be increased by a known method of introducing a crosslinked structure to increase the crosslink density or a rigid molecular structure (an aryl group etc.). The Tg of the resin can be reduced by a known method of introducing a crosslinked structure with a low crosslink density or a long side chain.

[0058]The content of the binder in the slurry is preferably 1 or more, more preferably 5 or more, and further preferably 10 or more per 100 of the insulating fine particles when expressed as a volume ratio so as to provide the effect of the binder more effectively. If the content of the binder in the slurry is too large, the ion permeability of the insulating layer is reduced because the pores are filled, which may adversely affect the properties of the electrochemical device. Therefore, the content of the binder is preferably 30 or less, and more preferably 20 or less per 100 of the insulating fine particles when expressed as a volume ratio.

[0060]The viscosity of the slurry is 5 mPa·s or more, preferably 10 mPa·s or more, and more preferably 20 mPa·s or more so as to suppress the sedimentation of the insulating fine particles and improve the dispersion stability. If the viscosity of the slurry is too high, it is difficult to apply the slurry uniformly in a required thickness. Therefore, the viscosity of the slurry is 500 mPa·s or less, preferably 300 mPa·s or less, and more preferably 200 mPa·s or less. In the present invention, the viscosity of the slurry is measured with an E-type viscometer by a method according to the Japanese Industrial Standard (JIS) R 1653 under the conditions of a temperature of 23° C. and a shear rate of 1000/s.

[0061]In the slurry, when the thickening agent is, e.g., the natural polysaccharides or the like, the thickening agent may be decomposed by bacteria in the air. Therefore, a preservative or a germicide may be appropriately added to the slurry to suppress the decomposition of the thickening agent. This can improve the storage properties of the slurry including the natural polysaccharides. Specific examples of the preservative or the germicide include the following a benzoic acid; a p-hydroxybenzoic acid ester; alcohols (such as ethanol and methanol); chlorines (such as sodium hypochlorite); hydrogen peroxide; acids (such as a boric acid and an acetic acid); and alkalis (such as sodium hydroxide and potassium hydroxide).

[0076]On the other hand, when the temperature of the electrochemical device is higher, the porous base material melts and fills the pores of the separator to retard the ion conduction, thereby ensuring the safety of the electrochemical device. That is, the porous base material serves to impart a so-called shutdown function to the separator. Therefore, the porous base material preferably includes a component that melts and softens at a predetermined temperature. The temperature at which the resin constituting the porous base material melts and softens is in the range of preferably 80° C., more preferably 100° C. to preferably 150° C., more preferably 140° C. This temperature can be determined by a melting temperature that is measured with a differential scanning calorimeter (DSC) according to the regulations of JIS K 7121 (which is true for the thermofusible fine particles, as will be described later). Such a component is preferably polyolefins, and more preferably PE. To provide a better shutdown function, it is particularly preferable that a microporous film made of PE or a laminated microporous film having 2 to 5 layers of PE and PP is used to form the separator.

[0078]The porous base material may be surface-treated, e.g., with ultraviolet irradiation, corona discharge, a plasma treatment, or a surface-active agent to improve the adhesion to the insulating fine particles. When the wettability of the surface of the porous base material with the slurry is improved, the slurry can be applied more uniformly, so that the insulating layer having higher homogeneity and excellent heat resistance can be easily formed. It is desirable that the surface treatment is performed only on the surface portion of the porous base material. In other words, low wettability of the inner surface of the pores of the porous base material with the slurry is better. If the inner surface of the pores of the porous base material is not subjected to the above treatment, the slurry is not likely to enter the pores of the porous base material. Thus, the slurry or the dispersion medium of the slurry can be prevented from passing through the pores to the opposite side of the porous base material from the surface to which the slurry is applied.

[0089]The layer (particularly the microporous film) for imparting the shutdown function is likely to cause thermal shrinkage at high temperatures. However, in the ...

[0020]The slurry for forming an insulating layer of the present invention is capable of achieving the uniform dispersion of the insulating fine particles and maintaining the dispersion state stably. The separator for an electrochemical device of the present invention is produced using the slurry for forming an insulating layer of the present invention and ...