Carbon-based Anode Materials For Sodium Storage Based On Hybrid Energy Storage Mechanism

High energy density and low-cost energy storage systems are the primary concerns for the development of electric vehicles and smart grids.With the development of electronic products and the huge demand of large-scale systems,there is an urgent need to develop a battery with high energy and power density and low cost.Lithium-ion batteries（LIBs）have achieved great success as an energy storage device.However,the limitation of lithium resources has gradually become an obstacle to its further development.Sodium-ion batteries（SIBs）have a similar electrochemical storage mechanism to LIBs and are considered as potential substitutes for LIBs due to the abundant Na resources in the earth’s crust.At present,low-cost,high-safety,and non-toxic carbon-based materials have been proven to be promising electrode candidates for SIBs.Porous carbon-based materials have shown promising applications in electrochemical energy storage due to their excellent properties.However,controllable synthesis of morphologically homogeneous hollow or porous carbon materials by facile methods remains a challenge.In addition,the carbon-based anode has limited sodium storage capacity,but the sodium metal anode based on plating/stripping mechanism has a high specific capacity(1166 m Ah g^-1),but its uneven electrodeposition and large volume expansion make it difficult to apply.Carbon-based materials with good cycle combined to high-capacity Na metal anode to form a carbon-sodium anode can be expected to realize a high energy density composite sodium storage anode.Therefore,based on the above problems,this research will develop the preparation method of regulated porous carbon materials,and explore the development of new high energy density carbon-sodium composite anodes based on hybrid storage behavior,which is of great significance for promoting the development and practical application of SIBs.（1）In this paper,a new green glass melt-assisted etching method for carbon materials is proposed,which can controllably prepare hollow porous carbon materials using boron oxide melt at a relatively low temperature of 500℃.The amorphous glassy state of B₂O₃provides a fluid carbonizing environment and acts as an etchant while doping with boron.Through this one approach,we successfully fabricated porous carbon materials with spherical and polyhedral morphologies using metal-organic framework（MOF）precursors.Furthermore,due to the highly porous structure and large surface area,as well as the doping of boron element,this hollow porous carbon material exhibits quite excellent Na⁺storage performance through enhanced capacitive behavior.Hollow carbon sphere HPCS exhibits nearly 90%initial Coulombic efficiency,excellent rate performance of 130 m Ah g^-1 at 30A g^-1 and long cycle life for Na-ion batteries at a current density of 1 A g^-1,the capacity of190 m Ah g^-1 can still be maintained after 1000 cycles.This subject provides a low-cost and green method for the preparation of porous nanocarbons,which is expected to be used in energy storage.（2）In this work,a high energy density anode is realized by a hybrid electrochemical storage process of Na ion intercalation and metallic Na electroplating on and on hard carbon.Using the cathodic reduction method in choline chloride-urea ionic liquid（low temperature molten salt）,Ag nanoparticles were modified on HC,enabling the HC electrode to achieve nanoscale uniform and controlled Na metal deposition,leading to excellent cycle stability and high reversibility（CE>99.8%）.And by Cryo-TEM,it was found for the first time that nanoscale Na metal deposition was achieved by directional attachment along the[110]direction,and a polycrystalline Na metal film was formed on Ag-HC.This epitaxial deposition can effectively reduce the formation of"dead sodium"and is also responsible for the high CE and good cycling stability.Finally,this hybrid storage concept is successfully realized in a low N/P full cell,the HC-Ag//NVP full cell confirms the intercalation/plating hybrid storage process and exhibits excellent rate performance at 1 C(1C=117.6 m A g^-1)provides a discharge capacity of 109 m Ah g^-1 and a high initial Coulombic efficiency of 92.7%,and still has a high capacity of 87 m Ah g^-1 at a large current density of 50 C.And at a current density of 1 C,there is still a capacity of 99 m Ah g^-1 after70 cycles.This subject provides a feasible method for fabricating sodium-based batteries with high energy density by effectively reducing the amount and volume of anode materials.