Modification Of 3D Ge/C Nanocomposites And Their Electrochemical Properties On Energy Storage

Transition metal tin?Sn?,silicon?Si?,germanium?Ge?,etc.have attracted more and more interests due to their high theoretical specific capacities for lithium-ion battery anode.Among them,Ge possesses a high theoretical capacity of 1600 mA h g^-1?2times close to Sn?,fast lithium diffusivity??400 times greater than in Si?,and high intrinsic electrical conductivity??100 times higher than in Si?.Nevertheless,like the other group IVA materials,Ge suffers from huge volume changes?over 300%?,and particle agglomeration during the alloying/de-alloying process,leading to cracking,pulverization,then exfoliation from the current collector,and thus,resulting in large irreversible capacity and rapid capacity fading.The nanocrystallization of active materials and design of Ge/C hybrid structures have been regarded as two main strategies to relieve above issues,however,how to integrate the two strategies to achieve their synergistic advantages still face challenges.In this paper,the sandwich-like three-dimensional carbon/germanium/carbon nanocomposites?3D C/Ge/C?and three-dimensional interconnected N-doped ultrathin carbon nanosheets decorated with ultrasmall Fe₂GeO₄ nanodots were prepared by simple in-situ CVD method.?3D Fe₂GeO₄/N-CNSs?,the two kinds of nanocomposites were modified from the external structure design and the internal chemical composition,achieving active particle nanocrystallization and composited with conductive carbon materials,simultaneously.These nanocomposites exerted the synergistic advantage of the two strategies,effectively improving the electrochemical properties of Ge.The effects of different raw materials and calcination process on the micro-morphology and structure of the samples were systematically investigated,and their electrochemical properties as anode materials of lithium-and sodium-ion battery were studied.The optimized synthesis process of 3D C/Ge/C is that C₆H₈O₇,Ge₂O,and NaCl were taken as the raw materials with a molecular ratio of 4:40:150 between Ge,C,and NaCl to prepare the precursor,then the precursor was calcinated at 750°C for 2h under Ar,followed by the second calcination process at 600°C for 10 min,under 5%C₂H₂/10%H₂/Ar;The sandwich-like 3D C/Ge/C nanocomposites exhibit excellent cycling stability as an anode material for lithium-ion batteries,possess a specific capacity of⁷00 mA h g^-1 after 100 cycles at a current density of 0.1 A g^-1;This double-layer carbon protection structure can effectively avoid the direct contact of active materials and electrolytes,reducing the occurrence of unnecessary side effects,in addition,the sandwich structure can alleviate the volume expansion of active material particles during charge/discharge process and prevent cracking,pulverization and aggregation,so that the novel nanocomposites show excellent lithium storage performance.3D interconnected N-doped carbon nanosheets anchored with ultrasmall Fe₂GeO₄nanodots nanostructures have effectively achieved synergistic advantages of nanocrystallization and hybrid nanostructure,which greatly improves the electrochemical properties.3D Fe₂GeO₄/N-CNSs nanocomposites as anode materials of lithium-ion battery achieved¹280 mA h g^-1 at a current density of 0.1 A g^-1 after180 cycles,even at a high current density of 6.4 A g^-1,its capacity of 650 mA h g^-1 is also much higher than the theoretical specific capacity of commercial graphene(372mA h g^-1);For the first time as anode material of sodium ion battery,the 3D Fe₂GeO₄/N-CNSs electrodes also show a ultra-long cycle life,the capacity retention rate matain about 86.4%after 6,000 cycles at a high current density of 5 A g^-1.It is worth highlighting that the current density increases over 200 times(0.1 A g^-1-22.8 A g^-1),the reversible specific capacity only fades less than 50%(350 mA h g^-1 and 180mA h g^-1).Moreover,the reversible specific capacities have little loss when the current density is increased from 6.4 A g^-1 to 22.8 A g^-1.The excellent electrochemical performance of 3D Fe₂GeO₄/N-CNSs nanocomposites is mainly benefited from the unique reaction mechanism of Fe₂GeO₄,ultrasmall active material particles?⁴.6 nm?and nitrogen-doped conductive carbon matrix.