Technical field This invention relates to the field of batteries and in particular lithium-ion batteries. It relates more specifically to all-solid lithium ion batteries (“Li-ion batteries”) and a novel process for producing such 5 batteries. Prior art Batteries including electrolytes formed either by a salt dissolved in a dry polymer or by a polymer swollen 10 or gelled by a solution of a salt in a mixture of organic aprotic polar solvents are known. Typically, the production of the electrolyte consists in placing a polymer, a salt and a mixture of solvents forming the polymer electrolyte in solution, then, after having 15 deposited the mixture in the form of a film, the solvent is removed under atmospheric or reduced pressure without removing the solvents forming the polymer electrolyte. Document WO 98 / 35 397 A1 describes a battery including an electrolyte formed by a dried electrolyte solution 20 including a thermoplastic polyimide that is amorphous and soluble in an organic solvent, and a lithium salt. However, the use of organic solvent does not make it possible to guarantee the safety and lifetime of the battery by protecting it from risks of short circuit and 25 ignition of solvent. All-solid batteries including solid electrolytes produced from polymer material based on polyethylene oxide (PEO) or polypropylene oxide (PPO) are also known. 2 WO 2016/001584 2 PCT/FR2015/051814 However, these types of electrolytes have a relatively low ionic conductivity at ambient temperature (approximately 10-7 S/cm). Document US 5 407 593 teaches that the transport of ions in a polymer electrolyte 5 passes through the amorphous region of a polymer matrix. Thus, the ionic conductivity of a polymer electrolyte may be increased by reducing the crystalline region and by increasing the amorphous region of the polymer. In addition, it is possible by modifying the glass 10 transition temperature of the polymer, by removing the crystalline zones and by using lower molecular masses, to increase ionic conduction. However, these modifications to the polymer material often create significant degradation of the mechanical properties of the 15 electrolyte. In addition, it has been demonstrated that crosslinking of the conductive polymer may increase the solidity of the films, but this results in a reduction in conductivity. For example, the article of P.M. Blonsky 20 and D.F. Shriver, J. Am. Chem. Soc. 1984, 106, pages 6854-6855 describes a polymer of the poly(bis(methoxy ethoxy)phosphazene) type including short chains of polyethylene oxide of which the ionic conductivity at 25°C exceeds 10-5 S/cm. However, the electrochemical 25 stability of the cross-linked solid polymer material as well as its mechanical properties are limited. The present invention is intended to produce an allsolid battery including a solid electrolyte made of a cross-linked polymer material, having good mechanical 30 resistance and having an ionic conductivity superior to that of solid electrolytes made of a cross-linked polymer material known from the prior art. 3 WO 2016/001584 3 PCT/FR2015/051814 Another objective of the invention is to produce thin-layer batteries by a process capable of being implemented at an industrial level in a relatively simple manner. 5 Objects of the invention A first object of the invention concerns a process for producing an all-solid thin-layer battery including the following series of steps: 10 a) a layer including at least one anode material (referred to here as “anode material layer”) is deposited on its conductive substrate, preferably selected from the group formed by a metal sheet, a metal strip, a metallized insulating sheet, a metallized insulating 15 strip, a metallized insulating film, said conductive substrates, or conductive elements thereof, capable of serving as an anode current collector; b) a layer including at least one cathode material (referred to here as “cathode material layer”) is 20 deposited on its conductive substrate, preferably selected from the group formed by a metal sheet, a metal strip, a metallized insulating sheet, a metallized insulating strip, a metallized insulating film, said conductive substrates, or conductive elements thereof, 25 capable of serving as a cathode current collector, with the understanding that steps a) and b) can be reversed; c) on at least one layer obtained in step a) and/or b), a layer including at least one solid electrolyte material having a thickness of less than 10 μm, 30 preferably less than 5 μm and even more preferably less than 2 μm (referred to here as “electrolyte material layer”) is deposited, the solid electrolyte material 4 WO 2016/001584 4 PCT/FR2015/051814 including a cross-linked solid polymer material comprising ionic groups; d) the following are stacked face to face: - an anode material layer coated with a solid 5 electrolyte material layer obtained in step c) with a cathode material layer coated or not with a solid electrolyte material layer obtained in step c); - or a cathode material layer coated with a solid electrolyte material layer obtained in step c) with an 10 anode material layer coated or not with a solid electrolyte material layer obtained in step c); e) a heat treatment and/or a mechanical compression of the stack obtained in step d) is performed in order to obtain an all-solid thin-layer battery. 15 Preferably, the solid cross-linked polymer material is chosen from polymethyl methacrylates, polyamines, polyimides or polysiloxanes. Preferably, the ionic groups of the polymer material are chosen from the following cations: imidazolium, 20 pyrazolium, tetrazolium, pyridinium, pyrrolidinium, such as n-propyl-n-methylpyrrolidinium (also called PYR13) or n-butyl-n-methylpyrrolidinium (also called PYR14), ammonium, phosphonium or sulfonium; and/or from the following anions: bis(trifluoromethane)sulfonimide, 25 bis(fluorosulfonyl)imide, or n-(nonafluorobutanesulfonyl)- n-(trifluoromethanesulfonyl)-imide. In a particular embodiment of the process according to the invention, the solid cross-linked polymer material is obtained by a step of polymerization of a mixture of 30 monomers and/or oligomers and/or pre-polymers including one or more thermally or photochemically polymerizable groups, said mixture of monomers and/or oligomers and/or 5 WO 2016/001584 5 PCT/FR2015/051814 pre-polymers including one or more reactive groups enabling said ionic groups to be grafted, then drying of the cross-linked polymer material obtained. Advantageously, the thermal and/or photochemical 5 polymerization is performed directly on the anode and/or cathode layer(s). The cross-linked polymer material comprising ionic groups deposited in step c) is performed by at least one of the following techniques: dip-coating, spin-coating, 10 roll coating, doctor blade, electrospraying or electrophoresis. The thickness of the electrolyte layer c) is less than 10 μm, preferably less than 5 μm and even more preferably less than 2 μm. Advantageously, the thickness 15 of the polymer material layer is between 0.5 and 1 μm. The solid anode, cathode and electrolyte layers are deposited using at least one of the following techniques: (i) physical vapor deposition (PVD), and more specifically by vacuum evaporation, laser ablation, ion 20 beam, or cathode sputtering; (ii) chemical vapor deposition (CVD), and more specifically plasma-enhanced chemical vapor deposition (PECVD), laser-assisted chemical vapor deposition (LACVD), or aerosol-assisted chemical vapor deposition 25 (AA-CVD); (iii) electrospraying; (iv) electrophoresis; (v) aerosol deposition; (vi) sol-gel; 30 (vii) dipping, more specifically dip-coating, spincoating or the Langmuir-Blodgett process. Preferably, the anode, cathode and electrolyte 6 WO 2016/001584 6 PCT/FR2015/051814 layers are deposited by electrospraying, electrophoresis, using an aerosol, or by dipping, and are preferably all deposited by electrophoresis. Advantageously, the layers of anode and/or cathode 5 material also include electrically conductive materials, and in particular graphite, and/or lithium ion conductive materials, of the type used to produce electrolyte films. In a preferred embodiment, the anode and/or cathode and/or electrolyte layers are produced by a deposition of 10 nanoparticles, respectively, of anode, cathode or electrolyte material using at least one of the following techniques: electrospraying, electrophoresis, aerosol deposition, and dipping. Preferably, the layers of anode, cathode and 15 electrolyte material are all deposited by electrophoresis. Advantageously, the heat treatment is performed at a temperature of between 50°C and 100°C, preferably between 100°C and 200°C, and/or the mechanical compression of the 20 layers to be assembled is performed at a pressure of between 10 and 100 MPa, and preferably between 20 and 50 MPa. The anode material layer a) is produced from a material chosen from: 25 (i) tin oxynitrides (typical formula SnOxNy); (ii) lithiated iron phosphate (typical formula LiFePO4); (iii) mixed silicon and tin oxynitrides (typical formula SiaSnbOyNz with a>0, b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b≤2, 00, b>0, a+b>0, a+b≤2, 00, b>0, a+b≤2, 0