An electrochromic device having an electrochromic layer and an electrolyte sandwiched between electrodes. A charge compensator ion permeable layer is interposed between the electrochromic layer and the electrolyte. The latter-mentioned interposing layer serves as a protective overcoat for the electrochromic layer and minimizes the structural decay of the cell which might otherwise occur. SUFIELD OF THE INVENTIONThe present invention relates to electrochromic devices and, more particularly, to such a device having a protective overcoat layer interposed between an electrochromic layer and an electrolyte.BRIEF DESCRIPTION OF THE PRIOR ARTThe prior art has recognized the usefulness of electrochromic devices wherein the electromagnetic absorption characteristics may be reversibly altered by a controlled electric field. The device is particularly useful as a display device or in light valve-type applications.U.S. Pat. No. 3,708,220 issued to Meyers describes a device which consists of an electrochromic material in contact with a semisolid electrolyte gel sandwiched between a pair of electrodes. U.S. Pat. No. 28,188, issued to Deb et al, discloses a completely solid state electrochromic device which utilizes a current carrier permeable layer positioned between an electrochromic layer and a conductive layer. The reissue patent states that the structure set forth therein is an improvement over basic electrochromic cells that had previously included only an electrochromic layer sandwiched between two conductive layers. In U.S. Pat. No. 3,840,287, issued to Witzke et al, there is an electrochromic device which is described as including two electrochromic layers in different states and an electrolyte layer which is not limited to semisolid gels. In the Meyers and Witzke patents, the structure included an electrolyte or color assisting agent, respectively, which is described as being in direct contact with an electrochromic layer. This contact was thought necessary to insure ionic charge transport from the electrolyte to the electrochromic layer. However, these prior art structures have been found to lack a certain desirable cycling capability of lifetime due to an observable decay in the cell due to a deterioration of the electrochromic layer -- electrolyte interface, which is due to the contact therebetween.BRIEF DESCRIPTION OF THE PRESENT INVENTIONThe present invention relates to a novel structure for extending the life of an electrochromic device, having an electrolyte or color assisting agent, by interposing a protective overcoat layer between the electrolyte or color assisting agent and the electrochromic layer. More particularly, the electrochromic device comprises a pair of electrodes, there being between the electrodes at least one layer of electrochromic material, an electrolyte layer, and a protective overcoat layer between the electrochromic layer and the electrolyte layer. The protective overcoat layer is a substantially insulating dielectric layer. Normally, one would expect that the placement of a substantially insulating dielectric layer separating the electrochromic layer and the electrolyte would inhibit the necessary current flow of the device and be detrimental to the normal operation of the device. Contrary to this, an unexpected synergistic result is obtained. It has been discovered that the insertion of certain substantially insulating materials in the form of thin films not only does not adversely affect the operation of the device, but rather substantially increases the life of the device by inhibiting attack and degradation of the electrochromic layer.