High Performance Carbon Shell Sodium Metal Anode

Sodium metal anode is deemed as an ideal anode for high energy density sodium ion batteries（SIBs）.However,sodium metal has the intrinsic properties of high reactivity and infinite volume change,resulting in unstable solid electrolyte interface（SEI）film and uncontrollable Na dendrite formation,leading to low Coulombic efficiency（CE）,severe side effects and even serious safety issues.Therefore,how to form a stable SEI films to further inhibit dendritic growth is the key to tackle the issues mentioned above.Shell structure is an effective strategy to solve SEI film and dendrite problems.On the one hand,shell structure can encapsulate the sodium metal within a protective layer which prevents direct contact between the sodium metal and the electrolyte,reducing side reactions and accommodates volume expansion/contraction of the sodium metal during repeated plating/stripping process.On the other hand,the wettability of sodium metal can be changed by means of functional group modification,doping and placing sodiophilic nanoparticles as heteroseeds in the shell structure,which can reduce nucleation overpotential and guide the uniform deposition of sodium.Carbon shell structure has strong mechanical strength and high ionic/electrical conductivity,which is a good strategy for developing high CE sodium metal anode.However,there are rarely reports of the carbon shell as the protective layer of Na metal anodes.Furthermore,the effect of the carbon shell on the sodium metal deposition behavior has never been systematically investigated.In this thesis,two shell structures of nitrogen-doped hollow carbon tube（NCTs）and hollow amorphous carbon tube encapsulated with Au nanoparticles（Au-aCTs）were designed and fabricated,which effectively inhibit the sodium metal dendrites,leading to an enhanced electrochemical performance.More interestingly,Na metal deposition kinetics and themerdynamics can be well investigated by in-situ transmission electron microscopy（TEM）characterization.The details are as follows:（1）1D NCTs nanostructures was synthesized by ZnO nanorods template method and its electrochemical properties were studied.In addition,it was observed by in-situ TEM that the sodiophilic surface can effectively guide the nucleation and deposition of sodium metal:the nucleation overpotential of Na@NCTs is only 5.6 mV at the current density of 0.5 mA cm^-2 with 1 mAh cm^-2.When measured at 1 mA cm^-2 with 1mAh cm^-2,the Na@NCTs electrode can cycle stably for more than 800 h with a high CE of 98.53%.In addition,the full battery was assembled with Na@NCTs as the anode and Na₃V₂（PO₄）₃@carbon（NVP@C）as the cathode.At a current density of100 mA g^-1,the full battery can deliver a capacity of 81.46 mAh g^-1 after 100 cycles.（2）1D Au-aCTs nanostructure was designed and prepared.The influence of carbon layer thickness on the thermodynamics and kinetics of sodium metal deposition was systematically investigated by in-situ TEM characterization and COMSOL Multiphysics simulation:the results showed that a stable host protective sodium metal can be formed when the thickness of amorphous carbon shell should be in the range of 22-51 nm.In addition,it is also confirmed that the Na_xAu alloy is the origin of the sodiophicity of the Au nanoparticles for the nucleation and deposition of Na metal.At 2 mA cm^-2 with 2 mAh cm^-2,Na@Au-aCT-2（34 nm）can cycle stably for more than 3500 h with a high CE of up to 99.80%.Finally,a full cell with NVP@C as cathode and Na@Au-aCT-2 as anode delivers a capacity of 103.66 mAh g^-1 after 170 cycles at 100 mA g^-1.