Composite lithium supplement agent and preparation method thereof

The invention relates to a composite lithium supplement agent which is carbon-coated transition metal doped lithium borate and/or carbon-coated transition metal doped lithium thioborate. The composite lithium supplement agent disclosed by the invention is high in theoretical decomposition capacity, has ion transmission performance, and is beneficial to improving the rate capability of the battery; the composite lithium supplement agent with two decomposition mechanisms is prepared by regulating and controlling the doping elements and the proportion and carbon coating, so that the electronic conductivity of the material is remarkably improved, and the decomposition potential is effectively reduced; and according to different decomposition reactions, the charging voltage can be regulated and controlled to realize on-demand lithium supplementation. The composite lithium supplement agent prepared by the invention has good air stability and moisture resistance, and is compatible with the existing battery preparati