Mechanism And Application Of High Performance Dendrite-free Na@Au/rGO Sodium Metal Anode

The fast demand for renewable energy storage and the exponential growth of electric vehicles require new battery technologies beyond traditional lithium-ion batteries（LIBs）.Due to the abundant natural sodium resources,sodium-ion batteries（SIBs）have been regarded as a new energy storage technology to instead of LIBs with low-cost.However,the energy density of current SIBs is lower than 150 Wh kg^-1,which can be further improved by using new high capacity materials.Due to high theoretical capacity(1166 mAh g^-1)and low redox potential（-2.71 V vs.SHE）,Na metal is the most promising anode material of SIBs.However,Na metal has the intrinsic properties of high electrochemical reactivity and infinite volume expansion,leading to unstable solid-electrolyte-interface（SEI）film formation and uncontrollable Na dendrite growth during repeated deposition/stripping processes.Pioneer research work prove that the initial stage of Na nucleation play a critical role on the subsequent sodium deposition morphology,which determine the final electrochemical performance.Therefore,it is necessary to investigate the Na metal nucleation behavior and guide the Na metal growth.Compared with the tranditional two-dimensional（2D）Cu foil current collector,three-dimensional（3D）structure can effectively reduce the current density and provide the abundant nucleation center,as well as provide enough space to accommodate the Na metal expansion.Further investigation indicate that Au can guide the Na metal deposition due to the formation of Na Au alloy,which can reduce the nucleation barriers with high binding energy and guide the Na uniform deposition.In addition,graphene is a traditional 2D material,3D structure made of reduced-graphene oxide（rGO）is an ideal host for Na metal anode.However,there is rarely reports of using Au hybrided with rGO（Au/rGO）as the host of Na metal anode.Furthermore,the Na metal deposition behavior onto Au/rGO has not been systematically investigated.Based on the above analysis,Au/rGO composed of Au nanoparticles supported by rGO was designed and synthesized by a simple chemical reaction method.The Au/rGO electrode exhibits a small overpotential of 3.5 m V at a current density of 2mA cm^-2 with a specific capacity of 2 mAh cm^-2.Meanwhile,it can be stably cycled for 400 h at a high current density of 5 mA cm^-2 and a specific capacity of 1 mAh cm^-2.The excellent electrochemical performance is mainly due to the unform Na metal deposition formed dendrite-free morphology owing to the sodiophilic surface provided by Au nanopraticles demonstrated by the in-situ optical microscopy and ex-situ SEM characterization.Finally,a full-cell composed of Na@Au/rGO as the negative electrode and Na₃V₂（PO₄）₃@carbon（NVP@C）as the positive electrode shows a capacity of 93.9 mAh g^-1 after 180 cycles at 100 mA g^-1.Futher study was carried out based on 3D printed Au/rGO microlattice aerogel,since the 3D printing technology can easily fabricate artificial hierarchical porous structure,which is an ideal Na meal anode host.Therefore,subsequently,a sub-millimeter/micrometer/nanometer porous Au/rGO hierarchical structure was designed and fabricated by 3D printing technology.The fabricated hierarchical porous structure can increase the specific surface area and accelerate the Na ion migration rate and charge transfer speed.In-situ optical microscopy investigate the 3D printed Au/rGO can guide the Na deposition and effectively inhibit the Na dendrite formation with a dendrite free deposition morphology.Furthermore,in-situ XRD prove that the sodiophicity of Au is originated form the formed Na Au₂ and Na₂Au alloy during the initial Na deposition process.Therefore,the 3D printed Au/rGO electrode exhibits excellent electrochemical performance.It can stabily cycle over 100 cycles at a high current density of 8 mA cm^-2 with 8 mAh cm^-2.When the 3D printed Na@Au/rGO anode paired with the Na₃V₂（PO₄）₃@C-rGO（NVP@C-rGO）cathode,the full cell can deliver a high capacity of 83 mAh g^-1 after 200 cycles.