Synthesis Of Three-Dimensional MXene For Dendrite-Free Lithium Metal Anodes

With the extra-high theoretical specific capacity(3860 mAh g^-1),lowest electrochemical potential（-3.040 V vs standard hydrogen electrode）,and low density(0.534 g cm^-3),metallic lithium is regarded as the“Holy Grail”electrode.The development of lithium metal batteries with metallic lithium as the anode material is expected to further increase the energy density of lithium batteries.However,due to high reactivity and volume expansion of metallic lithium,especially lithium dendrites,the practical application of lithium metal batteries has been severely hindered.The lithium dendrites will not only pierce the separator and short-circuit the battery,but also cause constant loss of active lithium and rapidly reduce the battery capacity.Therefore,it is urgent to develop a dendrite-free lithium metal anode.MXene is a new two-dimensional（2D）material with a graphene-like structure.Because of its high electrical conductivity,excellent hydrophilicity,and rich surface functional groups,it has been widely used as an electrode material in the field of energy storage.In this paper,based on the highly conductive 2D Ti₃C₂ MXene with rich surface functional groups,the aggregation-resistant three-dimensional（3D）MXene architecture is achieved by regulating the structure and surface property of MXene.From the perspective of suppressing the growth of lithium dendrites,using3D MXene as lithiophilic framework material,the construction of 3D dendrite-free lithium metal anode is realized.The content of this paper is as follows:（1）With 2D Ti₃C₂ MXene nanosheets as the precursor and polyvinyl alcohol（PVA）as the cross-linking agent,a 3D porous MXene/PVA aerogel is prepared by hydrogen bonding between functional groups on the surface of MXene and hydroxyl groups in PVA.The MXene/PVA aerogel has extremely low density and sufficient mechanical strength,and can be directly used as a self-supporting electrode,which improves the overall energy density of the battery.By adjusting the content of functional groups on the surface of Ti₃C₂ MXene,the lithiophilicity of MXene/PVA aerogel is improved.Thus,the dendrite-free lithium metal anode prepared by the electrodeposition has a high coulombic efficiency（>98%）,and has a cycle life up to 700 h even at a high current density of 10 mA cm^-2.Lithium-sulfur batteries paired with the sulfur/carbon cathode deliver a specific capacity of 800 mAh g^-1 and could cycle 300 times.（2）With 3D macroporous copper foam as a template,the surface of copper foam is uniformly coated with Ti₃C₂ MXene nanosheets by electrostatic adsorption.Afterwards,3D nitrogen-doped Ti₃C₂ MXene is prepared by calcining in an ammonia atmosphere.Its 3D porous structure can reduce the local current density and inhibit the formation of lithium dendrites;nitrogen doping can further improve the lithiophilicity of Ti₃C₂ MXene.Thus,a composite lithium metal anode is prepared by molten-infused lithium method.It can stably cycle at a low overpotential of 16 mV for more than 1800 h,and can still stably cycle under the current density of 10 mA cm^-2 and the area capacity of 10 mAh cm^-2.Lithium metal batteries assembled with LiFePO₄ cathode deliver a specific capacity of 140 mAh g^-1 and could cycle more than 470 times.