Germanium-based Materials For Secondary Battery Anode

Germanium(Ge)is located in the main group IV of the periodic table,and has good electrochemical performance.It is expected to obtain good cycle performance when applied to the anode material for secondary batteries.However,germanium has a large volume change during alloying/dealloying with lithium,which will cause pulverisation of active materials and fast capacity fading during cycling,compounding germanium with carbon-based materials can effectively solve this problem and improve electrochemical performance of the composite.In this article,a one-dimensional germaniun/carbon nanofiber was prepared by the simple electrospinning technology,and applied to different secondary battery systems and test the electrochemical performance.The following conclusions are obtained:(1)Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)images reveal the homogeneous one-dimensional nanostructure of the as-prepared germanium/carbon nanofiber with an average diameter of～200 nm.X-ray diffraction analysis(XRD),Raman spectroscopy(Raman)and X-ray photoelectron spectroscopy(XPS)were conducted to evidence the existence of crystalline Ge and amorphous carbon in the composite.Ge content in the germanium/carbon nanofiber confirmed by thermogravimetric analysis(TGA)was approximately 39.6 wt%.(2)The germanium/carbon nanofiber was directly used as anode material for lithium ion battery without adding conductive agent or binder to investigate the lithium storage performance.At the current density of 1 A g-1,after 50 cycles the reversible capacity of the composite was as high as 1088 mAh g-1,and the Coulombic efficiency was nearly 100%.The composite also exhibits excellent rate performance,the composite still remains a reversible capacity of 425 mAh g-1 at the high current density of 10 A g-1,indicating that the germanium/carbon nanofiber has excellent lithium storage performance.Moreover,bulk germanium anode,carbon fiber anode,and germanium/carbon nanofiber anode were tested for electrochemical impedance spectroscopy(EIS)and the equivalent circuits was also simulated.According to the results,it could be concluded that:in the composite,the carbon substrate could increase the lithium ion transmission rate inside the composite and improve the electrical conductivity of the composite,thereby improving the electrochemical performance of the composite.(3)In this article,germanium-based anode was first used in dual ion battery(DIB).The novel DIB was assembled by using germanium/carbon nanofiber as anode material and graphite as cathode material and tested its electrochemical performance.This novel DIB could deliver a discharge capacity of 106 mAh g-1 after 500 cycles at the discharge current density of 2.5 A g-1 over a high voltage window of 2.0-4.6 V,and the Coulombic efficiency remained close to 100%,indicating the good long-term cycle stability of this DIB.At the same time,the rate capability of this DIB far exceeds those of most state-of-the-art ones previously reported,and there is still room for improvement.The working mechanism of germanium/carbon nanofiber anode in this DIB was studied by ex-situ XRD and ex-situ XPS analysis:The initial crystalline Ge first alloyed with Li and formed amorphous LixGe alloys to storage charge,then the amorphous LixGe alloys were converted to amorphous Ge during the de-alloying process,and Ge remains amorphous phase in the next cycles.(4)After lithium depositing on the germanium/carbon nanofiber,it could be used as modified lithium metal anode for lithium metal batteries.The lithiophilic germanium can induce lithium uniformly depositing on the surface of carbon fiber,inhibit the growth of lithium dendrites and improve the Coulombic efficiency and cycle performance of the lithium metal battery.The electrochemical test results show that the Coulombic efficiency of the modified lithium metal anode can still maintain about 100%after 200 cycles.As a comparison,Coulombic efficiency of the copper current collector reduced to less than 50%within 40 cycles.The modified lithium metal anode was used to form a symmetrical battery,and no sign of short circuit is observed after cycling for 600 h5 while the Li foil symmetrical battery was short-circuited after cycling for 150 h,indicating that the modified lithium metal anode can effectively ease the formation of lithium dendrites and exhibits good cycling performance.The full battery assembled by modified lithium metal anode and LiFePO4 cathode also exhibited better cycle stability than copper current collectors,and the reversible capacity was 133 mAh g-1 after 100 cycles at 0.5 C.