Preparation method of lithium bis (oxalato) borate

The invention relates to a preparation method of lithium bis (oxalato) borate, which comprises the following steps: taking oxalic acid and boric acid according to a preset ratio, carrying out melting reaction at the temperature of 100-130 DEG C for 3-5 hours, then filtering while hot to remove insoluble impurities, and drying in vacuum at the temperature of 80-120 DEG C for 5-12 hours to obtain bis (oxalato) borate corresponding to the ratio; the preparation method comprises the following steps: cooling bis (oxalato) borate to room temperature, then adding a lithium salt and an organic solvent, and stirring and fully reacting for 6-10 hours at 90-130 DEG C to obtain a corresponding mixed solution; carrying out suction filtration on the mixed solution, concentrating the mixed solution after suction filtration to a saturated state, freezing and crystallizing for 2 hours at the temperature of 40 DEG C below zero to 10 DEG C below zero, separating out a large amount of solid, and finally filtering and drying to o