NOVELTY - Electrochemical apparatus comprises metallic, polymeric or doped/undoped silicon substrate (100) with two sides; two electrochemically active cells (120) on two sides further comprising positive and negative parts which further comprise terminal(s) (126, 127); two barrier layers (110) on substrate which chemically separates a electrochemically active cell from substrate, where barrier layers comprise sublayers (111-114).    USE - As an electrochemical apparatus (claimed), particularly as lithium-based, solid-state, thin-film, secondary and primary battery.    ADVANTAGE - The electrochemical apparatus is lithium-based, solid-state, thin-film, secondary and primary battery with improved capacity density, energy density, and power density. Particularly the thin-film batteries fabricated onto a thin, flexible stainless steel foil substrate using barrier layer that is composed of barrier sublayers have uncompromised electrochemical performance compared to thin-film batteries fabricated onto ceramic substrates when using a 700 degrees C post-deposition anneal process for a LiCoO2 positive cathode.    DETAILED DESCRIPTION - An electrochemical apparatus comprises a metallic, polymeric or doped/undoped silicon substrate (100) having two sides; a first electrochemically active cell (120) on the first side further comprising positive and negative parts which further comprise at least one terminal (126, 127); a first barrier layer (110) on the first side which chemically separates the first electrochemically active cell from the substrate; a second electrochemically active cell on the second side further comprising positive and negative parts; a second barrier layer on the second side which chemically separates the second electrochemically active cell from the substrate, where the barrier layers comprise sublayers (111-114). The sublayers are selected from electrically conducting, electrically insulating, or electrically semi-conducting materials. The positive parts are not in electrical contact with the negative parts on both sides of the substrate. The positive part comprises at least a positive cathode, cathode current collector, and positive terminal. The negative part comprises at least a negative anode, anode current collector, and negative terminal. At least one of sublayer comprises a chemical compound. At least one of sublayer comprises a single phase of crystalline, nano-crystalline, amorphous, or glassy material or any poly phase mixture or its composite (preferably single phase of amorphous or glassy material). The electrochemically active cells on both sides of the substrate comprise batteries selected from lithium metal anode batteries, lithium-ion anode batteries, and lithium free anode batteries. The electrochemically active cells comprise positive thin-film cathodes deposited by vapor deposition or non-vacuum deposition; thin-film negative anodes selected from lithium metal, lithium-ion anode, or metal that does not form inter-metallic compounds with lithium; and a protective encapsulation or protective encasing which is adapted to protect electrochemically active cells or electrochemical apparatus, against at least mechanical and chemical factors in an ambient environment. The electrochemically active cells further comprise electrolyte, where the terminals are externally accessible by an opening in the encapsulation and are separated from the electrolyte by a moisture protection layer. The moisture protection layer comprises a material with moisture blocking properties; and a single phase of crystalline, nano-crystalline, amorphous, or glassy material or any poly phase mixture or its composite.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a chemical separation of the substrate part from the electrochemically active cell part of the electrochemical apparatus via a barrier layer, which includes a multitude of barrier sublayers.    Substrate part (100)    Battery part (120)    Barrier layer (110)    Barrier sublayers (111-114)    Battery component layers (121-125)    Positive battery terminal (126)    Negative battery terminal (127)