An electro-chemical storage device is described, and a method for making the same. In some aspects, a guidewire is co-wound with sheet or ribbon-like electrode materials so that contact is made between the guidewire and collector extensions of the electrode materials thereby forming an endcap with low impedance and high thermal sinking capabilities. In one embodiment, the storage device includes a negative electrode sheet have an edge which is substantially free of electrode active material, a positive electrode sheet have an edge which is substantially free of electrode active material, a first conductive guidewire disposed adjacent to the edge of the negative electrode sheet which is substantially free of electrode active material, and a second conductive guidewire disposed adjacent to the edge of the positive electrode sheet which is substantially free of electrode active material. The first conductive guidewire and the edge of the negative electrode sheet which is substantially free of electrode active material define a first electrode endcap and the second conductive guidewire and the edge of the positive electrode sheet which is substantially free of electrode active material define a second electrode endcap. The electrode endcaps are coupled to terminals of the storage device, which are capable of being connected to the terminals of an external circuit.