Secondary battery

#The present invention application claims priority to Japanese Patent Application No. 2018-245214 filed in the Japan Patent Office on Dec. 27, 2018, the entire contents of which are incorporated herein by reference.

#The present invention relates to a secondary battery.

#Non-aqueous electrolyte secondary batteries, such as alkaline secondary batteries and lithium-ion batteries, are used as power supplies mounted on electric vehicles and hybrid electric vehicles. A known secondary battery includes a flat wound electrode assembly and an electrolyte in an exterior, wherein a long positive-electrode sheet and a long negative-electrode sheet with a separator interposed therebetween are wound to form the wound electrode assembly. Such a battery including a wound electrode assembly includes a positive-electrode core laminate at one end of the wound electrode assembly and a negative-electrode core laminate at the other end of the wound electrode assembly to couple the wound electrode assembly to a current collector. The positive-electrode core laminate is composed of layers of a positive-electrode core on which no positive-electrode active material layer is formed, and the negative-electrode core laminate is composed of layers of a negative-electrode core on which no negative-electrode active material layer is formed.

#FIG. 1 is a front view of the interior of a secondary battery according to an embodiment except the front of a battery case and the front of an insulating sheet;

#FIG. 2 is a top view of a secondary battery according to an embodiment;

#FIG. 3A is a plan view of a positive-electrode sheet according to an embodiment;

#The present inventors have found the following when an aluminum or aluminum alloy positive-electrode core laminate is bonded to an aluminum or aluminum alloy positive-electrode current collector by ultrasonic bonding described in Patent Document 1.

#Ultrasonic bonding between the aluminum or aluminum alloy positive-electrode core laminate and the aluminum or aluminum alloy positive-electrode current collector by a method described in Patent Document 1 transformed aluminum crystal grains into fine crystal grains in the entire bonding region of the positive-electrode core laminate in which a welding recess is formed. However, large aluminum crystal grain sizes before the ultrasonic bonding were maintained in a non-bonding region outside the bonding region of the positive-electrode core laminate. Thus, it was found that in the positive-electrode core laminate bonded to the positive-electrode current collector by ultrasonic bonding, the state of crystal grains (hereinafter referred to as the crystal grain state), such as grain sizes, in the bonding region was significantly different from the crystal grain state of the non-bonding region, and consequently this increased the risk of cracking due to a lattice defect between the bonding region and the non-bonding region.

#Hence, the present inventors arrived at the present invention in which the ultrasonic bonding conditions are controlled to form a solid-state bonding layer containing finer crystal grains near a bonding surface formed by solid-state bonding between surfaces of a positive-electrode core in a bonding region of a positive-electrode core laminate in which a welding recess is formed and to form a central layer within each positive-electrode core (a central portion between solid-state bonding layers formed on both surfaces of each positive-electrode core), transformation of crystal grains in the central layer being suppressed. Thus, in the bonding region of the positive-electrode core laminate bonded to a positive-electrode current collector by ultrasonic bonding, solid-state bonding between layers of a positive-electrode core ensures bonding strength and decreases bonding resistance, and the central layer within each positive-electrode core in which transformation of crystal grains is suppressed ensures the continuity of the crystal grain state between the bonding region and the non-bonding region and can reduce the occurrence of cracking due to a lattice defect between the regions.