Study On Germanium And Germanium Oxide Coated Mesocarbon Microbeads Composite Anode Materials

As a commercialized anode material for Li-ion batteries,MCMB has advantages of good cycle stability,low cost and environmental friendliness.However,its low specific capacity,poor multiplier performance and poor low temperature performance restricted the development of high-performance Li-ion batteries.The theoretical specific capacity of germanium?Ge?is as high as 1600 mAh·g^-1,which is a very promising anode material.However,during the lithium storage process,the anode material will undergo serious volume change,resulting in electrode powder failure.In this paper,novel composite anode materials with in-situ coated mesocarbon microbeads of nanoparticles or its oxides were designed and successfully prepared.This composites not only greatedly improved the specific capacity and rate performance of anode materials,but also enhanced the low temperature electrochemical performance of anode materials.In addition,the relationship between material preparation process,microstructure and electrochemical performance was systematically studied.GeO_x/MCMB composite anode materials were successfully prepared by coating GeO_x nanoparticles on the surface of MCMB with GeO₂ as germanium source.The effects of the amount of reducing agent NaBH₄,the adding method of reducing agent NaBH₄,heat treatment and the effects of different feeding ratios were systematically studied on the structure,morphology and electrochemical properties of GeO_x/MCMB composite anode materials.The results showed that the cycling stability and rate performance of GeO_x/MCMB composites were best:when the mass ratio of Ge to MCMB was 4:6,at the 0.2C rate,the initial discharge and charge specific capacity were1106.9 mAh·g^-1 and 702.5 mAh·g^-1.After 50 cycles,the reversible specific capacity was as high as 648.9 mAh·g^-1,and the capacity retention rate was 92.37%,which was equivalent to 84.69%of the theoretical specific capacity.At the 2C rate,the reversible specific capacity was 433.3 mAh·g^-1,and the capacity retention rate was as high as60%.The Ge/MCMB composite anode materials were prepared by heat treatment of the original GeO_x/MCMB in Ar/H₂ atmosphere.Compared with GeO_x/MCMB composite anode materials,the first Coulomb efficiency of Ge/MCMB composite anode materials was significantly improved.When the mass ratio of Ge to MCMB was 6:4,the initial discharge and charge capacity of Ge/MCMB composite materials were 764.1 mAh·g^-1and 574.7 mAh·g^-1,respectively.The initial coulomb efficiency was as high as 75.22%.After 100 cycles,the reversible capacity was 504.9 mAh·g^-1.The electrochemical properties and attenuation mechanism of Ge/MCMB composite anode materials and MCMB at 0°C,-10°C and-20°C were investigated.The results show that the Ge/MCMB composite reduced the polarization and effectively reduces the impedance.At-20°C,the reversible specific capacity of Ge/MCMB composites was ramained 213.9 mAh·g^-1,which greatly enhanced the low temperature performance of the anode materials.The compatibility of the composite anode material with the PC-based electrolyte was studied,and the influence of different coating amount on the compatibility of composite anode materials with PC-based electrolyte was discussed.The results showed that when the mass ratio of Ge to MCMB was 4:6,the compatibility of GeO_x/MCMB composite in PC-based electrolyte was the best.At the rate of 0.5C,the specific discharge and charge capacity were 1420.1 mAh·g^-1 and 610 mAh·g^-11 with the coulomb efficiency of 42.9%.After 50 cycles,a specific discharge capacity was remained 399mAh·g^-1.Even at-20?,the composite anode material showed a reversible specific capacity of 264.1 mAh·g^-1.