Energy storage device

1. An energy storage device formed by a method comprising:

providing, on a substrate, a stack for an energy storage device, the stack comprising a first electrode layer, a second electrode layer, and an electrolyte layer between the first electrode layer and the second electrode layer;

forming a groove at least through the second electrode layer and the electrolyte layer such that the groove is wider in the second electrode layer than in the electrolyte layer;

providing an electrically insulating material in the groove; and

providing an electrically conductive material in the groove, on the electrically insulating material, in direct contact with a portion of the electrolyte layer and in direct contact with the second electrode layer.

2. An energy storage device comprising:

a substrate;

a stack on the substrate, the stack comprising:

a first electrode;

a second electrode; and

an electrolyte between the first electrode and the second electrode, the first electrode closer than the second electrode to the substrate, the electrolyte comprising a first portion overlapped by the second electrode and a second portion not overlapped by the second electrode;

an electrically insulating material in contact with the first electrode and the electrolyte; and

an electrically conductive material in direct contact with the second portion of the electrolyte and the second electrode.

3. The energy storage device of claim 2 , wherein a width of a surface of the second portion of the electrolyte, in a plane parallel to a plane of a surface of the substrate, is greater than or equal to approximately 5 micrometres.

4. The energy storage device of claim 2 , wherein the electrically insulating material is absent from contact with the second electrode.