Investigation Of Novel Germanium-base Chalcogenides/Phosphides Anodes For Li-ion Batteries

The Li-ion batteries(LIBs),with great advantages of environment friendly and fast discharge/charge capability,is considered as the preferred power source for electric vehicles.However,the current commercial graphite anode,with low capacity of only 372m Ah/g,is unable to meet the high energy demand(500 Wh/kg)for LIBs.In this case,the advanced anode materials,as one of the core components for LIBs,become the key factor to push the energy density for LIBs.The“double active”germanium based sulfur/phosphorus compounds,which possess unique layered structure,good electrical conductivity and especially high capacity,are proposed to be served as the candidate anodes for LIBs.Herein,based on the structure and properties of such layered germanium based sulfur/phosphorus,this work conducted a multi-directional and in-depth study on their physical and chemical properties,plus their energy storage mechanism.The electrochemical performances are effectively optimized by improving the thermostability,decreasing price cost squeeze and realizing potential regulation.(1)Developping the novel layered MX(M=Ge,Sn,Sb;X=S,Se,Te,P).The W and Mo elements in traditional MoS₂,WSe₂ et al are inactive for Li storage,which greatly reduces the specific capacity.Herein,by introducing the Ge element to replace the role of W and Mo,we develop a“double active”layered Ge S for LIBs,pushing the reversible capacity to 1768 m Ah/g(ICE=94%).Furthermore,series of novel MX(M=Ge,Sn,Sb;X=S,Se,Te,P)can be sucessfully developped based on the layered Ge S,in which the GeP_x(x=1,2,3,4,5)exhibit large capacity(>1800 m Ah/g),high initial coulombic efficiency(>90%)and suitable discharge plateau(?0.5 V)for LIBs,revealing their high reactivity and high reversibility.Besides,the developed Sb₂Te₃ possesses a high volumetric capacity of 3419 m Ah/cm³ due to its high density.(2)Investigation on the unique structure properties and machenism of MX.The high reactivity of above MX is investigated in detail by both experimental tools and theoretical calculations.Compared to MoS₂ whose formation energy is-2.74 e V,the electronegativity of Ge is more close to X and the formation energy of Ge-X bonding is much smaller(-0.73 e V).Therefore,the electrochemical delithium force is enough to trigger the rebuiding of Ge-X bonding upon Li-extraction,which boosts GeX with large capacity and high reactivity.Similarly,a mild driven force at low tempearture heating can recover the ordered layer structure,helping to heal the high conductivity and unimped Li diffusion pathways for crystalline GeX,thus better rate performances can be obtained.Besides,attributed the similar electronegativity between Ge and P,the formative GeP_x(x=2,3,4,5)is a kind of solid solution alloy rather than ordinary metal phosphides,combining with high thermostability(>450 ^oC),conductivity(?2.4×10⁶ S/m)and wide range tuneable region(54%solubility for GeP₂).Due to its intrinsic tuneable alloys,the conversion reaction process based on chemical bond breaking in other phosphides would not take place in those GeP_x alloys,contributing to its high reversibility and ICE(>90%).(3)Electrochemical optimization of the MX(M=Ge,Sn,Sb;X=S,Se,Te,P).To improve the thermostability of MX,we successfully achieve uniform carbon-encapsulated GeP₅ nanofibers(GeP₅@C-NF)by processing GeP₅ nanoparticles into carbon nanofibers via electrospinning.Interestingly,the thermo-stability of GeP₅ can be greatly improved to over 600? safety after carbon encapsulation,which is vitally important for higher battery.Besides,to reduce the proportion and cost of Ge in GeP_x,we proposed the Li-active and low cost Zn to drop and replace the role of Ge,fabricating a ternary Zn_xGe_1-xP anode.Such Zn_xGe_1-xP enables to deliver superior electrochemical performances like GeP while decreasing its cost price.In particularly,the Zn_xGe_1-xP can stablize at a wide range doping region(0.143<x<0.6)due to the similar electronegativity between Ge and Zn atoms.Lastly,to level the big potential gap between conversion step(>1.6 V)and alloying region(<0.4 V)for GeX(X=S,Se,Te),Herein,we propose an endogenetic structural design by grafting Sb₂Te₃building block into layered Ge Te to establish ternary Ge₂Sb₂Te₅ compound.Turning into metallic conductive,the Li_xTe formation step in Ge₂Sb₂Te₅ is found declined to 1.30 V,and the enlistment of Sb(⁰.78V)bridges the conversion and alloying plateau,thus the incongruous reaction kinetics and large potential gap between conversion-alloying step can be alleviated.