An electrochromic device includes an optically transparent first substrate, a first transparent conductor disposed on the first substrate, a counter electrode disposed on the first transparent conductor, an optically transparent, ionically conductive first capping layer disposed on the counter electrode, and configured to permit diffusion of alkali metal ions, and to block the diffusion of organic compounds and carbon, an optically transparent second substrate, a second transparent conductor disposed on the second substrate, a working electrode comprising electrochromic nanoparticles disposed on the second transparent conductor, and an electrolyte disposed between the first capping layer and the working electrode.