Structural Design,Interface Optimization And Mechanism Study Of High-performance Lithium Metal Anode

Owing to the shortage of fossil fuels and the rapid development of portable electronics and new electric vehicle technologies,the exploration of energy storage systems with high specific energy has become one of the main goals in the field of electrochemistry today.The study of battery materials has achieved much in this area,because lithium-ion batteries are currently demonstrating their great potential in helping reach a carbon-neutral society.However,the energy density of existing lithium-ion secondary batteries is already close to the theoretical value of anode/cathode materials,especially the more widely used graphite anode,so it is urgent to develop electrode materials with high energy density and apply them in emerging cutting-edge electronic devices.With low weight density of 0.53 g cm^-3,low anode potential of-3.04 V and high specific energy density of 3860 m Ah g^-1,lithium metal is considered to be one of the most promising candidates for negative electrodes at present.However,uneven lithium deposition during Li⁺plating/stripping tends to form lithium dendrites,which ultimately leads to lower coulomb efficiency,faster capacity degradation,greater electrolyte consumption and safety hazards,There are still major obstacles to the commercialization of lithium metal,which are mainly due to the following reasons:（1）Uneven lithium deposition during battery cycling will form lithium dendrites,causing safety problems such as short circuit;（2）lithium dendrites in contact with the electrolyte will produce side reactions,accelerating the consumption of cathode material and electrolyte;（3）the fracture of dendrites will make the anode into an uneven porous structure and form"dead lithium",reducing the energy efficiency and increasing the electrode polarization.（4）the volume expansion of the lithium metal anode during plating/stripping causes the electrode to powder.Therefore,the problems about battery can be tackled by anode structural constraint design,construction of artificial SEI layer,and interface regulation of solid electrolyte.Three-dimensional frame materials play an important role in the design of anode structuring.Because the high specific surface area of 3D structured porous electrode material regulates the uniform deposition of Li⁺,this maintains the integrity of the lithium-based anode during the long cycle and effectively reduces the lithium nucleation potential during the initial plating stage,thus preventing continuous lithium dendrite growth,while interface optimization is generally achieved by separator（diaphragm）modification.In this thesis,3D structured electrode materials were fabricated by using micro/nano super-assembled method to realize lithium metal composite electrodes and MOF-derived cobalt sulfide nanosheets were prepared for separator modification.The high specific surface area of the composite electrode can effectively reduce the local current density to regulate uniform lithium deposition,while the diaphragm modification can homogenize the Li⁺flux,which provides two effective and practical strategies for the commercialization of lithium metal batteries.The work studied in this thesis is divided into:（1）Rational design of bamboo-shaped nitrogen-doped carbon nanotube-modified carbon cloth（NCNT-CC）as a 3D body to achieve uniform plating/stripping of lithium metal.Cobalt hydroxide（Co（OH）2）nanowires were grown on the surface of carbon cloth（CC）via a simple hydrothermal method,followed by the addition of dicyandiamide in a tube furnace and calcination to obtain nitrogen-doped carbon nanotube-modified carbon cloth.The experimental results show that the specific surface area of the bamboo-like carbon nanotubes increased and the tube diameter decreased compared with the industrial multi-walled carbon nanotubes.The doping of nitrogen significantly improves the lithiophilicity of the material and enables the assembled symmetric cells to maintain a stable voltage hysteresis of 300 h at a high current density of 10 m A cm^-2.Moreover,bamboo-shaped carbon nanotube clusters can suppress lithium dendrites growth via reducing the local current density,which also enables this hierarchical collector to accommodate lithium deposition with high area loading.（2）In-situ growth lithiophilic cobalt sulfide nanosheets modified diaphragm for uniform distribution of Li⁺flux.MOF nanosheets were grown in situ on the surface of a commercial polypropylene（PP）separator via a simple immersion method and then converted to cobalt sulfide nanosheets by a hydrothermal method.This nanowall array can act as a barrier to inhibit dendrite growth in high-performance lithium metal batteries（LMBs）without any significant increase in the weight and volume.The strategy of in-situ growing conductive materials（Co9S8 nanowall arrays）on the separator can effectively homogenize the Li⁺flux,inhibit dendrite growth,and improve the electrochemical performance of LMBs.The lithiophilicity of Co9S8 enables Li⁺to migrate uniformly and rapidly.The Co9S8-PP separator provided excellent cycle stability and cycle-specific capacity for the Li metal full cell with 800 cycles at 1C.