NOVELTY - The method comprises optionally supplying a conductive substrate layer pre-covered with a layer of anode (67) or a cathode (63), depositing a thin layer (66) of an electrolyte by electrophoresis from a suspension of electrolyte material particles, on a substrate (65) and/or the anode layer or cathode layer, drying the anode/cathode layer, and thickening the thin layer of the electrolyte by mechanical compression or heat treatment in vacuum. An average size (D50) of the particles of solid electrolyte material is less than or equal to 30 nm.    USE - The method is useful for manufacturing a thin layer of a solid electrolyte of a battery (claimed) such as a lithium-ion battery that is useful as a power supply for autonomous electric devices such as telephone and portable laptops, autonomous sensors and electric vehicles.    ADVANTAGE - The lithium-ion battery can be easily manufactured with high energy density, long lifespan, excellent performance, compact size and increased resistance and stability.    DETAILED DESCRIPTION - The method comprises optionally supplying a conductive substrate layer pre-covered with a layer of anode (67) or a cathode (63), depositing a thin layer (66) of an electrolyte, by electrophoresis from a suspension of electrolyte material particles, on a substrate (65) and/or the anode layer or cathode layer, drying the anode/cathode layer, and thickening the thin layer of the electrolyte by mechanical compression or heat treatment in vacuum. An average size (D50) of the particles of solid electrolyte material is less than or equal to 30 nm. The thickening step: includes sintering the thin layer at a temperature (TR), which is less than 0.3 times a melting point of material of the electrolyte, and applying pressure of higher than 100 MPa to the thin layer; and is carried out under vacuum. A zeta potential of the suspension of particles of electrolyte material is higher than 60 mV. The suspension of the electrolyte material particles contains an electrostatic stabilizer. The layer of the electrolyte covers edges of the anode layer or cathode layer. An INDEPENDENT CLAIM is included for a method for manufacturing a battery.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a method for manufacturing a thin layer of solid electrolyte.    Cathode layer (63)    Substrate (65)    Electrolyte layer (66)    Anode layer (67)    Metallic films on substrate. (68a)