Plug for closing off and sealing off an opening in a housing of an energy-storage system, in particular for closing off and sealing off an opening in a housing of a lead-acid storage battery, wherein the plug has the following: - a connecting region, which is configured to come into engagement with the opening of the energy-storage system; and - a sealing region, which is arranged above the connecting region in an axial direction of the plug, wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug, by a force acting entirely in the axial direction of the plug, wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug, wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region, and has a second region on a side of the sealing element that is directed away from the connecting region, wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug, wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction of the plug, and wherein the sealing element is of fully encircling and rotationally symmetrical form. - a connecting region, which is configured to come into engagement with the opening of the energy-storage system; and - a sealing region, which is arranged above the connecting region in an axial direction of the plug, wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug, by a force acting entirely in the axial direction of the plug, wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug, wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region, and has a second region on a side of the sealing element that is directed away from the connecting region, wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug, wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction of the plug, and wherein the sealing element is of fully encircling and rotationally symmetrical form. wherein the sealing region is configured to receive a sealing element which is designed to be deformed, in the fitted state of the plug, by a force acting entirely in the axial direction of the plug, wherein a cross section of the sealing element has at least substantially an L shape, in particular already in a non-fitted state of the plug, wherein the sealing element has a first region on a side of the sealing element that is directed towards the connecting region, and has a second region on a side of the sealing element that is directed away from the connecting region, wherein the second region of the sealing element has a larger outer diameter than the first region already in a non-fitted state of the plug, wherein the sealing region is configured to receive the sealing element in such a way that a sealing action of the sealing element is generated by the force acting entirely in the axial direction of the plug, and wherein the sealing element is of fully encircling and rotationally symmetrical form.