An electrochemical energy storage device is provided. The device may include a solid-state anode layer. The device may comprise a solid-state electrolyte layer. Further, the device may comprise a solid-state cathode layer. At least two adjacent ones out of the solid-state anode layer, the solid-state electrolyte layer, and the solid-state cathode layer may form a solid-state single-crystal with varying chemical compositions between the related layers. The solid-state electrolyte layer may have an ionic conductivity at room temperature higher than 10−6 S/cm.