PREPARATION METHOD OF LITHIUM THIOPHOSPHATE The present invention concerns a method for the preparation of lithium thiophosphate, as well as the lithium thiophosphate as obtained, and uses of said product, in particular as solid electrolyte. PRIOR ART Lithium batteries are used to power portable electronics and electric vehicles owing to their high energy and power density. Conventional lithium batteries make use of a liquid electrolyte that is composed of a lithium salt dissolved in an organic solvent. The aforementioned system arises security questions as the organic solvents are flammable. Lithium dendrites forming and passing through the liquid electrolyte medium can cause short circuit and produce heat, which result in accident that leads to serious injuries. Solid sulfide electrolytes are advantageous for lithium battery applications due to their high ionic conductivities and mechanical properties. These electrolytes can be pelletized and attached to electrode materials by cold pressing, which eliminates the necessity of a high temperature assembly step. Elimination of the high temperature sintering step removes one of the challenges against using lithium metal anodes in lithium batteries. Argyrodites have long been known and are derived from argyrodite Ag8GeS6, which was described for the first time in 1886 by C. Winkler and the analysis of which led to the discovery of germanium. The argyrodite family consists of more than 100 crystalline solids and includes, for example, those solid-state compounds in which the silver is replaced by copper, the germanium by gallium or phosphorus and the sulfur by selenium. Thus, Nitsche, Kuhs, Krebs, Evain, Boucher, Pfitzner and Nilges describe, inter alia, compounds such as Cu9GaS6, Ag7PSe6 and Cu8GaS5CI, the solid-state structures of which are derived from argyrodite. There exists thus a need for a full solution route for the preparation of a sulfide-based solid electrolyte. INVENTION The aim of the present invention is to provide a sulfide-based solid electrolyte with LiPS, prepared by a synthesis route preferably both faster and easier to set up compared to previously described methods. The aim of the present invention is to provide a new process for the preparation in solution of a LiPS material having preferably improved productivity and allowing a control of the morphology of the obtained product. The aim of the present invention is also to provide a method for the preparation of a halogen-free argyrodite Li7PS6 with improved transport properties. Thus, the present invention relates to a method of preparing a lithium thiophosphate comprising at least one step for the preparation of a solution S1 at a temperature T1 comprised from -200°C to 10°C, preferably from -110°C to 0°C, said solution S1 comprising a solvent and at least P species under the form of (PS4)3 , Li species under the form of Li+, and remaining sulfur under the form of polysulfide, followed by a step for removing at least a portion of the solvent from said solution to obtain lithium thiophosphate. The invention also relates to a lithium thiophosphate, susceptible to be obtained by the method of the invention. The invention also relates to the use of such lithium thiophosphate as solid electrolyte. The present invention also refers to a solid electrolyte comprising such lithium thiophosphate and an electrochemical device comprising a lithium thiophosphate according to the invention, notably a solid electrolyte comprising a lithium thiophosphate according to the invention. The invention also relates to a solid state battery comprising a solid electrolyte of the invention and a vehicle comprising a solid state battery. DEFINITIONS Throughout this specification, unless the context requires otherwise, the word "comprise" or “include”, or variations such as "comprises", "comprising", “includes”, including” will be understood to imply the inclusion of a stated element or method step or group of elements or method steps, but not the exclusion of any other element or method step or group of elements or method steps. According to preferred embodiments, the word "comprise" and “include”, and their variations mean “consist exclusively of”. As used in this specification, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. The term “and/or” includes the meanings “and”, “or” and also all the other possible combinations of the elements connected to this term. The term “between” should be understood as being inclusive of the limits. Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a temperature range of about 120°C to about 150°C should be interpreted to include not only the explicitly recited limits of about 120°C to about 150°C, but also to include sub-ranges, such as 125°C to 145°C, 130°C to 150°C, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 122.2°C, 140.6°C, and 141.3°C, for example. The term “electrolyte” refers in particular to a material that allows ions, e.g., Li+, to migrate therethrough but which does not allow electrons to conduct therethrough. Electrolytes are useful for electrically isolating the cathode and anodes of a battery while allowing ions, e.g., Li+, to transmit through the electrolyte. The "solid electrolyte" according to the present invention means in particular any kind of material in which ions, for example, Li+, can move around while the material is in a solid state. The term “electrochemical device” refers in particular to a device which generates and/or stores electrical energy by, for example, electrochemical and/or electrostatic processes. Electrochemical devices may include electrochemical cells such as batteries, notably solid state batteries. A battery may be a primary (i.e., single or “disposable” use) battery, or a secondary (i.e., rechargeable) battery. It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more different sources of power, for example both gasoline-powered and electric-powered vehicles. DETAILED INVENTION The method of the invention is based on the preparation of a homogeneous solution comprising ionic species. It thus does not involve a suspension. All species involved in the preparation of lithium thiophosphate are thus dissolved in a solvent and are in the form of ionic species as mentioned above. An essential feature of the method of the invention is the temperature T1 as defined above. The method of the invention is thus carried out at a low temperature, notably in order to dissolve all the species and thus to obtain the solution S1 at the required temperature. Once the solution S1 is prepared, then a step for removing at least a portion of the solvent is carried out. Then lithium thiosulfate is obtained as a solid, preferably as a powder. Preferably, the term “at least a portion of the solvent” refers to at least 50% by weight of said solvent, preferably at least 60% by weight of said solvent. The P species under the form of (PS4)3 are preferably obtained from a precursor chosen in the group consisting of: P2S5, P4S10, P4S9 and P4S9+X (with 0