A battery comprising, as a separator, a functional layer comprising a layered double hydroxide, wherein the functional layer has an average porosity of 1 to 40% and an average pore diameter of 100 nm or less, wherein the functional layer has a helium permeability per unit area of 10 cm/min·atm or less with 1 atm corresponding to 101.325 kPa, and wherein the functional layer has a thickness of 100 μm or less, wherein the average porosity of the functional layer is determined by a) cross-sectionally polishing the functional layer with a cross-sectional polisher, b) taking two observed fields at 50 000-fold magnification on a cross-sectional image of the functional layer with a field emission scanning electron microscope (FE-SEM), c) calculating the porosity of each of the two fields with an image inspection software such as HDevelop, manufactured by MVTec Software GmbH, based on the image data of the cross-sectional image, and d) calculating the average value of the resulting porosity; wherein the average pore diameter of the functional layer is determined by i) cross-sectionally polishing the functional layer with a cross-sectional polisher, ii) taking a cross-sectional image of the functional layer at 50 000-fold magnification with the field emission scanning electron microscope (FE-SEM), iii) determining the pore diameter by measuring the longest dimension of each pore based on the observed cross-sectional image, iv) sorting all the measured pore sizes in order of size to calculate the average, from which the subsequent 10 larger sizes and the subsequent 10 smaller sizes are selected, such that 20 diameters in total, are selected in one field of view, and then averaging the selected sizes of two fields of view to yield the average pore size; wherein the helium permeability is determined by permeating helium gas from one side to the other side of the functional layer, wherein supply pressure and a flow rate were then monitored with a pressure gauge and a flow meter, and wherein after permeation of helium gas for one to thirty minutes, the helium permeability was calculated from the expression of F/(PxS), where F, in cm 3 /min, was the volume of permeated helium gas per unit time, P, in atm, was the differential pressure applied to the functional layer when helium gas permeated through, and S, in cm 2, was the area of the membrane through which helium gas permeates. wherein the functional layer has a helium permeability per unit area of 10 cm/min·atm or less with 1 atm corresponding to 101.325 kPa, and wherein the functional layer has a thickness of 100 μm or less, wherein the average porosity of the functional layer is determined by a) cross-sectionally polishing the functional layer with a cross-sectional polisher, b) taking two observed fields at 50 000-fold magnification on a cross-sectional image of the functional layer with a field emission scanning electron microscope (FE-SEM), c) calculating the porosity of each of the two fields with an image inspection software such as HDevelop, manufactured by MVTec Software GmbH, based on the image data of the cross-sectional image, and d) calculating the average value of the resulting porosity; wherein the average pore diameter of the functional layer is determined by i) cross-sectionally polishing the functional layer with a cross-sectional polisher, ii) taking a cross-sectional image of the functional layer at 50 000-fold magnification with the field emission scanning electron microscope (FE-SEM), iii) determining the pore diameter by measuring the longest dimension of each pore based on the observed cross-sectional image, iv) sorting all the measured pore sizes in order of size to calculate the average, from which the subsequent 10 larger sizes and the subsequent 10 smaller sizes are selected, such that 20 diameters in total, are selected in one field of view, and then averaging the selected sizes of two fields of view to yield the average pore size; wherein the helium permeability is determined by permeating helium gas from one side to the other side of the functional layer, wherein supply pressure and a flow rate were then monitored with a pressure gauge and a flow meter, and wherein after permeation of helium gas for one to thirty minutes, the helium permeability was calculated from the expression of F/(PxS), where F, in cm 3 /min, was the volume of permeated helium gas per unit time, P, in atm, was the differential pressure applied to the functional layer when helium gas permeated through, and S, in cm 2, was the area of the membrane through which helium gas permeates.