Purifying process of biethyl diacid lithium borate

The process of purifying LiBOB includes the following steps: dissolving LiBOB material with low LiBOB content and high water content in dried and re-distilling treated polar non-protonic solvent to form saturated solution, filtering and evaporating to concentrate; freezing at -10 deg.c to -40 deg.c for 1-5 hr; and vacuum drying to obtain purified LiBOB product. The purified LiBOB product, the infrared spectrum analysis and 13C and 11B nuclear magnetic resonance spectrum show, has total content of Li, B and oxalate ions greater than 99.30 wt%. After twice purifications, the product has water content as low as 0.0014 wt% and the contents of Na, K, Ca, Mg and Fe lower than 0.0020 wt%, 0.0010 wt%, 0.0010 wt%, 0.00040 wt% and 0.00080 wt% separately, and may be used as the lithium salt for lithium ion cell electrolyte.