BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a LDH separator and a secondary zinc battery.

2. Description of the Related Art

In secondary zinc batteries, such as secondary nickel-zinc batteries and secondary air-zinc batteries, it is known that metallic zinc dendrites precipitate on negative electrodes during a charge mode, penetrate through voids in separators composed of, for example, non-woven fabrics, and reach positive electrodes, resulting in short circuiting. The short circuiting caused by such zinc dendrites occurs during repeated charge/discharge operations, leading to a reduction in the service lives of the secondary zinc batteries.

In order to solve such a problem, secondary zinc batteries have been proposed that include layered double hydroxide (LDH) separators that selectively permeate hydroxide ions while blocking the penetration of zinc dendrites. For example, PTL 1 (WO 2013/118561) discloses a secondary nickel-zinc battery including a LDH separator disposed between a positive electrode and a negative electrode. PTL 2 (WO 2016/076047) discloses a separator structure including a LDH separator that is fitted in or joined to a resin frame and is dense enough to restrict the permeation of a gas and/or water. PTL 2 also discloses that the LDH separator may be a composite with a porous substrate. In addition, PTL 3 (WO 2016/067884) discloses various methods for forming a dense LDH membrane on the surface of a porous substrate to give a composite material (a LDH separator). These methods include the steps of: uniformly bonding an initiating material capable of giving origins of crystal growth of LDH to the porous substrate; and then subjecting the porous substrate to a hydrothermal treatment in an aqueous raw material solution to form a dense LDH membrane on the surface of the porous substrate.

CITATION LIST

Patent Literature

PTL1: WO2013/118561

PTL2: WO2016/076047

PTL3: WO2016/067884

SUMMARY OF THE INVENTION

Secondary zinc batteries, for example, nickel-zinc batteries, constructed with the LDH separator as described above usually do not cause short circuiting by zinc dendrites; however, the penetration of zinc dendrites and thus short circuiting between positive and negative electrodes may eventually occur in an abnormal situation, i.e., intrusion of zinc dendrites into the LDH separator due to, for example, some defects. Accordingly, a further improvement is desired for a preventive effect of the short circuiting caused by the dendrites.

The present inventors have now founded that a LDH separator that can more effectively restrain the short circuiting caused by zinc dendrites can be provided through providing a dendrite buffer layer with a predetermined configuration inside the LDH separator.

Accordingly, an object of the present invention is to provide a LDH separator capable of more effectively restraining the short circuiting caused by zinc dendrites.

According to an embodiment of the present invention, a LDH separator is provided for secondary zinc batteries. The separator comprises a porous substrate made of a polymeric material and a layered double hydroxide (LDH) that plugs the pores in the porous substrate. The LDH separator has in its inside a dendrite buffer layer. The dendrite buffer layer is at least one selected from the group consisting of:

(i) a pore-rich internal porous layer in the porous substrate, the internal porous layer being free from the LDH or deficient in the LDH;

(ii) a releasable interfacial layer which is provided by two adjacent layers constituting part of the LDH separator being in releasable contact with each other; and

(iii) an internal gap layer being free from the LDH and the porous substrate, which is provided by two adjacent layers constituting part of the LDH separator being formed apart from each other.

According to another embodiment of the present invention, a secondary zinc battery is provided comprising the LDH separator described above.

