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 batteries. Prior art Modes of producing lithium-ion batteries (“Li-ion batteries”) presented in numerous articles and patents are known; the work “Advances in Lithium-Ion Batteries” (ed. W. van Schalkwijk and B. Scrosati), published in 2002 (Kluever Academic / Plenum Publishers) provides a good review of the situation. The electrodes of Li-ion batteries may be produced by means of printing or deposition techniques known to a person skilled in the art, and in particular by roll-coating, doctor blade or tape casting. All-solid thin-layer Li-ion batteries having a planar architecture, i.e. which are essentially comprised of a set of three layers forming a basic battery cell: an anode layer and a cathode layer separated by an electrolyte layer are also known. They use metallic lithium anodes and lithium phosphorus oxynitride films as the electrolyte. However, significant variations in volume of the lithium anode in charging and discharging steps makes it extremely difficult to properly encapsulate the battery without the risk of loss of tightness of the encapsulation. WO 2016/001588 PCT/FR2015/051819 3 3 More recently, new all-solid battery architectures consisting of a stack of thin layers have been proposed. These batteries consist of a rigid and monobloc assembly of basic cells connected in parallel. These batteries use dimensionally stable anodes to ensure the efficacy of the encapsulation, and enable three-dimensional structures to be produced, with better surface energy densities than the planar architectures. Such batteries are described in documents WO 2013/064779 A1 or WO 2012/064777 A1. The batteries described in these documents do not contain organic solvent-based liquid electrolyte, their structure consists of all-solid thin layers, without porosity in the electrode layers in order to ensure good properties of stability of the battery over time. The process for producing these batteries, also described in documents WO 2013/064779 A1 or WO 2012/064777 A1, has numerous advantages because it makes it possible to produce multilayer, thin-layer and therefore relatively nonresistant assemblies, enabling performance in terms of power to be preserved. However, in some cases, the process of producing such batteries may have some limits according to the materials used, in particular for the electrolyte. In fact, ionic conductive glasses may be difficult to implement. For example, solid electrolytes such as LiPON or lithiated borates have a relatively low glass transition temperature, generally between around 250 and 300°C: thus, during the step of assembly of the battery by pressurized annealing of the different layers, the electrolyte materials may partially crystallize, which may modify their ionic conduction property. Similarly, when the solid lithium phosphorus-based electrolyte is WO 2016/001588 PCT/FR2015/051819 4 4 used, it may be beneficial to differentiate the chemical compositions of the electrolytes in contact with the anodes and cathodes in order to optimize the performance of the electrolytes. However, the use of two lithium phosphorus-based electrolyte formulations deposited on each of the faces of the electrodes may lead to the appearance of new phases at the interface between the two electrolyte layers to be assembled, and may therefore modify the conduction properties. Similarly, solid Li7La3Zr2O12 (called LLZO) electrolytes are good ionic conductors and are very stable in contact with anodes and cathodes, but their highly refractory character makes it sometimes difficult to weld, at low temperature, the electrodes to one another via the electrolyte layer without causing an interdiffusion phenomenon at the interfaces with the electrodes. A first objective of the present invention is to propose a process for producing all-solid thin-layer batteries that do not lead to the appearance of phases at the interface between the two electrolyte layers to be assembled. Another objective of the present invention is to propose a process for producing a battery at low temperature without causing interdiffusion phenomena at the interfaces with the electrodes. Another objective of the invention is to produce thin-layer batteries capable of being implemented by on an industrial level in a relatively simple manner. Objects of the invention WO 2016/001588 PCT/FR2015/051819 5 5 A first object of the invention concerns a process for producing an all-solid thin-layer battery including the following series of steps: 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 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 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, 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 (referred to here as “electrolyte material layer”) is deposited; d) a layer of a cross-linked polymer material comprising ionic groups having a thickness of less than 10 µm, preferably less than 5 µm and even more preferably less than 2 µm is deposited: - on the anode material layer coated with a solid electrolyte material layer and/or on the cathode material layer coated or not with a solid electrolyte material layer; WO 2016/001588 PCT/FR2015/051819 6 6 - or on the cathode material layer coated with a solid electrolyte material layer and/or on the anode material layer coated or not with a solid electrolyte material layer; e) an anode material layer obtained in step a), c) or d) is stacked face to face in series with a cathode material layer obtained in step b), c) or d) with the understanding that the stack includes at least one solid electrolyte material layer obtained in step c) and at least one cross-linked polymer material layer obtained in step d); f) a heat treatment and/or a mechanical compression of the stack obtained in step e) is carried out in order to obtain an all-solid thin-layer battery. The cross-linked polymer is preferably chosen from polymethyl methacrylates, polyamines, polyimides or polysiloxanes. Preferably, the ionic groups of the polymer material are chosen from the following cations: imidazolium, 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, bis(fluorosulfonyl)imide, or n-(nonafluorobutanesulfonyl)-n-(trifluoromethanesulfonyl)-imide. In a particular embodiment, the cross-linked polymer material is obtained by a step of polymerization of a mixture of 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 pre-polymers including one or more WO 2016/001588 PCT/FR2015/051819 7 7 reactive groups enabling said ionic groups to be grafted. Preferably, the thermal and/or photochemical polymerization is performed directly on the anode, cathode and/or electrolyte layer(s). Advantageously, the deposition of the cross-linked polymer material comprising ionic groups is performed using at least one of the following techniques: dipcoating, spin-coating, roll coating, doctor blade, electrospraying or electrophoresis. The thickness of the polymer material layer is less than 10 µm, preferably less than 5 µm and even more preferably less than 2 µm. Advantageously, the thickness 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 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 (AA-CVD); (iii) electrospraying; (iv) electrophoresis; (v) aerosol deposition; (vi) sol-gel; (vii) dipping, more specifically dip-coating, spincoating or the Langmuir-Blodgett process. Preferably, the anode, cathode and electrolyte layers are deposited by electrospraying, electrophoresis, using an aerosol, or by dipping, and are preferably all WO 2016/001588 PCT/FR2015/051819 8 8 deposited by electrophoresis. In a particular embodiment, the layers of anode and/or cathode material also include electrically conductive materials, and in particular graphite, and/or nanoparticles of lithium ion conductive materials, of the type used to produce electrolyte films, or cross-linked solid polymer materials comprising ionic groups. Preferably, the anode and/or cathode and/or electrolyte layers are produced by a deposition of nanoparticles, respectively, of anode, cathode or electrolyte material using at least one of the following techniques: electrospraying, electrophoresis, aerosol deposition, and dipping. More specifically, the layers of anode, cathode and electrolyte material nanoparticles are all deposited by electrophoresis. According to the invention, the heat treatment is performed at a temperature of between 50°C and 300°C, preferably between 100°C and 200°C, and/or the mechanical compression of the 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: (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, WO 2016/001588 PCT/FR2015/051819 22 22 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