Negative electrode for nonaqueous secondary battery, process of producing the negative electrode, and nonaqueous secondary battery

Abstract

Disclosed is a negative electrode for a nonaqueous secondary battery comprised of a current collector and an active material structure containing an electro-conductive material having low capability of forming a lithium compound on at least one side of the current collector, the active material structure containing 5 to 80% by weight of active material particles containing a material having high capability of forming a lithium compound. The active material structure preferably has an active material layer containing the active material particles and a surface coating layer formed on the active material layer.

Description

TECHNICAL FIELD [0001] This invention relates to a negative electrode for nonaqueous secondary batteries. More particularly, it relates to a negative electrode capable of intercalating and deintercalating a large amount of lithium and providing a nonaqueous secondary battery with high energy density and improved cycle life. The present invention also relates to a nonaqueous secondary battery using the negative electrode. BACKGROUND ART [0002] Secondary batteries now used in mobile phones and notebook computers are mostly lithium ion secondary batteries, due to a higher energy density than other secondary batteries. With the latest tendency of mobile phones and personal computers toward multifunctionality, power consumption of these devices has shown a remarkable increase. Therefore, the demands for higher capacity secondary batteries have been increasing. As long as the present electrode active materials are used, it would be difficult to meet the increasing demands in the near futu...

AI Technical Summary

Benefits of technology

[0022] The active material layer 3 is a layer containing active material particles 7 which have a material having high capability of forming a lithium compound. Such a material includes silicon materials, tin materials, aluminum materials, and germanium materials. The maximum particle size of the active material particles 7 is preferably 50 μm or smaller, still preferably 20 μm or smaller. The particle size, represented in terms of D50 value, of the active material particles 7 is preferably 0.1 to 8 μm, still preferably 0.3 to 1 μm. Where the maximum particle size exceeds 50 μm, the active material particles 7 are liable to fall off, resulting in reduction of electrode life. The lower limit of the particle size is not particularly specified. The smaller, the better. In the light of the process of making the active material particles 7 (described later), the lower limit would be about 0.01 μm. The particle size of the active material particles 7 can be measured by Microtrac, electron microscopic (SEM) observation. While it is desirable that all the active material particles 7 fall under the recited particle size range, it is no problem that greater active material particles 7 are present in a small amount that does not impair the effects of the invention.

Problems solved by technology

With the latest tendency of mobile phones and personal computers toward multifunctionality, power consumption of these devices has shown a remarkable increase.

As long as the present electrode active materials are used, it would be difficult to meet the increasing demands in the near future.

While these alloys have high capacity, they have not yet been put to practical use on account of the problems of large irreversible capacity and short cycle life.

On the other hand, though silicon has higher capacity potential than tin, there is no report on the development of silicon-containing plated copper foil for use in lithium ion secondary batteries because silicon is an element incapable of electroplating.

However, they have the drawback that they incur large changes in volume with alternate repetition of charging and discharging and, as a result, undergo cracking and pulverizing and finally fall off the current collector.

Even with these techniques, however, it is still impossible to perfectly prevent fall-off of the negative electrode active material from the current collector as a result of cracking and pulverizing of the active material, accompanying charge and discharge of a secondary battery.

Judging from Examples of the publication, however, since the thickness of the layer of the metallic element incapable of forming a lithium alloy, which is the outermost layer, is as extremely small as 50 nm, there is a possibility that the outermost layer fails to sufficiently coat the underlying layer containing the lithium alloy-forming metal element.

If so, the layer containing the metallic element capable of forming a lithium alloy cannot be sufficiently prevented from falling off when pulverized with repeated charging and discharging of the battery.

Conversely, if the layer of the metal element incapable of forming a lithium alloy completely covers the layer containing the lithium alloy-forming metal element, the former layer would inhibit an electrolyte from passing through the latter layer, which will interfere with sufficient electrode reaction.

No proposal has ever been made to satisfy these conflicting functions.

However, since the oxide film comes off little by little, together with the foil, it is difficult to control the porosity and pore size.

There seems to be a limit, therefore, in improving the adhesion between the paste and the foil.

According to the process of making the porous copper foil, however, the electrolytic copper foil deposited on a cathode drum and separated from the drum, is subjected to various processing treatments, which make the copper foil unstable.

Therefore, the process cannot be seen as satisfactory in ease of handling the foil and fit for large volume production.

Additionally, a nonaqueous secondary battery using a negative electrode, prepared by applying a negative electrode active material mixture to the porous copper foil (a current collector), still has the problem that the negative electrode active material tends to fall off accompanying intercalation and deintercalation of lithium, resulting in reduction of cycle characteristics.

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