Interfacial Study And Protection Strategy For Na Metal Anode

Na metal anode possesses high theoretical capacity of 1166 mAh g-1 and low redox potential of-2.71 vs.SHE.Therefore,the batteries combined Na metal anode with S and O2 cathode can deliver high theoretical energy densities and low cost,which are the important supplements of the energy storage system.Meanwhile,Na resources is abundant and low-cost,which is beneficial for the commercial application of Na battery.However,electrochemical stripping and plating of Na metal have encountered lots of challenges:Firstly,Na metal anode inevitably reacts with electrolytes to form the SEI due to its intrinsic activity;Secondly,Na dendrites are easily formed during the plating process of Na metal.These dendrites may possibly impale the separator,causing the safety issues;Finally,the volumetric expansion and gas evolution during the operation of the battery will also hamper the commercialization of Na metal battery.Based on the above challenges,we aim to improve the stability of Na metal anode in the ways of optimization and structural design:1.Optimize the components of the electrolytes to stabilize the interface of Na metal anode.We develop a new electrolyte formulation based on NaBF4/G2.Na metal anode is highly reversible in this kind of electrolyte.Further characterization proves that it is the NaF and B-O contained SEI which stabilizes Na metal anode,leading to the high Coulombic efficiency of 99.93%.2.Improve the interfacial stability of Na metal anode and carbonate electrolyte by additive introduction.We design a new electrolyte formulation by adding 0.75 wt%NaAsF6 in 1 M NaTFSI/FEC.Trace amount of NaAsF6 can be preferentially reduced to form the As-O polymer and NaF particles on Na surface.As-O polymer can improve the plasticity and tensile strength of SEI,making it stable during the volumetric change of Na metal anode.NaF with high modulus can suppress the Na dendrite.Therefore,this NaAsF6 derived SEI can stabilize Na metal anode,leading to a high Coulombic efficiency of 97%and long lifespan over 400 cycles.3.An implantable artificial SEI stabilize the interface of Na metal anode.We fabricated an implantable NaF/PVDF film via a blade casting method.NaF with high modulus can regulate Na deposition and PVDF can tightly connect the NaF,forming a highly plastic SEI film.Therefore,Na?Al cell with this artificial layer can deliver a high Coulombic efficiency of 96%.Furthermore,Na0.6MnO2?Na full cell with low N/P ratio of 3 shows excellent electrochemical performance in the voltage range of 1.5-4.1 V.4.Na metal anode structure design.We fabricate a GO/MXene composite film by vacuum filtration.Then the Na@rGO/MXene electrode can be obtained by a "spark"strategy.In addition,the areal capacity of Na@rGO/MXene can be well regulated by controlling the thickness of precursors.rGO can render efficient electron conduct and sodiophilic MXene nanosheet can regulate the nucleation of Na metal.Therefore,NVP?Na@rGO/MXene full cell with the low N/P ratio shows the long lifespan over 280 cycles,proving the great effect of this composite anode.