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...

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Benefits of technology

[0023] It is preferred that voids be present in the active material layer 4. The voids serve to relax the stress which results from expansion and contraction of the active material particles 7 due to intercalation and deintercalation of lithium. In this connection, the proportion of the voids in the active material layer 4 is preferably about 1 to 30% by volume, still preferably about 5 to 30% by volume, particularly preferably about 5 to 9% by volume. The proportion of the voids is obtained through mapping under an electron microscope. The proportion of the voids can be regulated within the recited range by forming the active material layer by the process described later, followed by mechanically pressing the active material layer under appropriate conditions.

[0024] The active material layer 4 preferably contains an electro-conductive carbon material in addition to the active material particles 7. Incorporation of the conductive carbon material adds improved electron conductivity to the active material structure 5. From this viewpoint, the amount of the conductive carbon material in the active material layer 3 is preferably 0.1 to 20% by weight, still preferably 1 to 10% by weight. To ensure the improvement on electron conductivity, it is preferred for the electro-conductive carbon material to have the shape of particles with a particle size of 40 μm or smaller, particularly 20 μm or smaller. The lower limit of the particle size is not critical, which means the smaller, the better. In the light of the process of making the particles, the lower limit would be about 0.01 μm. The conductive carbon material includes acetylene black and graphite.

[0025] The surface coating layer 4 is a thick layer continuously covering the surface of the active material layer 3, thereby the active material particles 7 are not substantially exposed. The surface coating layer 4 generally covers the surface of the active material layer 3. The surface coating layer 4 has an almost uniform thickness, but some part 4a of the surface coating layer 4 may enter into the active material layer 3. Some part of the surface coating layer 4 penetrating the active material layer 3 may reach the current collector 2. In some parts, the material constituting the surface coating layer 4 may penetrate the whole thickness of the active material layer 3 to reach the current collector. It is preferred that the material constituting the surface coating layer 4 penetrate the active material layer 3 deeper and deeper thereby increasing the electrical conductivity of the negative electrode as a whole. This is also preferred in that the penetrating material constituting the surface coating layer 4 forms a network structure and acts to prevent the active material particles 7 from falling off due to expansion and contraction.

[0026] The active material particles 7 do not always need to be covered completely with the sur

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

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