NOVELTY - Preparing flexible self-supported electrode comprising at least one electrode active material and at least one binder, comprises: (a) preparing an aqueous slurry by dispersing a mixture of solid particles in an aqueous phase; (b) filtrating the aqueous slurry on a filter cloth; (c) optionally pressing the aqueous slurry in a wet state and then transferring to a felt dryer; (d) drying the pressed slurry to obtain a flexible sheet of supported electrode by the felt dryer or filter cloth; and (e) separating the electrode sheet and the filter cloth or the felt dryer.    USE - The flexible self-supported electrode is useful in the flexible lithium-ion battery (claimed), which is useful in devices such as mobile devices of mobile phones, computers and light tools, and heavier equipment such as means of transportation with two wheels (bicycles and mopeds) or four wheels (electric or hybrid vehicles).    ADVANTAGE - The method provides excellent electrochemical stability and mechanical properties (mechanical resistance and flexibility) to the flexible self-supported electrode.    DETAILED DESCRIPTION - Preparing flexible self-supported electrodes comprising at least one electrode active material and at least one binder, comprises: (a) preparing an aqueous slurry by dispersing a mixture of solid particles in an aqueous phase, where the mixture of solid particles comprises 50-98 mass% of the electrode active material and 2-50 mass% of the binder including refined cellulose fibers, and the mixture of solid particles is present in an amount of 0.02 mass%; (b) filtrating the aqueous slurry on a filter cloth; (c) optionally pressing in a wet state of the aqueous slurry and then transferring to a felt dryer; (d) drying the pressed slurry to obtain a flexible sheet of supported electrode by the felt dryer or filter cloth; and (e) separating the electrode sheet and the filter cloth or the felt dryer, when pressing step is conducted. INDEPENDENT CLAIMS are included for:    (1) the flexible self-supported electrode obtained by the method; and    (2) a flexible lithium-ion battery comprising at least one negative electrode and at least one positive electrode, which is placed between a solid electrolyte and a separator impregnated with a liquid electrolyte.