Design,Construction And Properties Of Hierarchically Structured Lithium,Sodium Ion Battery Anode Materials

With the exacerbation of energy crisis and environmental pollution,the development and utilization of clean and renewable energy has become an important focus of economic development around the world.Renewable energy power generation and new energy vehicles are regarded as important pillar industries of current national economy.The development of new energy industries is inseparable from large-capacity,high-rate,long-cycle,low-cost and safe secondary batteries.Lithium-ion batteries?LIBs?have received extensive attention as the most advanced secondary batteries,and sodium-ion batteries?SIBs?are considered as potential replacements for LIBs due to their abundant resources,low-cost,and similar working mechanism as LIBs,but their energy density,power density,cycle life and safety are still unable to meet the increasing requirements of new energy industries.The development of high-performance electrode materials is the key to improving the performance of LIBs and SIBs,especially the development of anode materials has become an important factor to restrict and guide battery industry.The difference in energy storage applications results in different requirements for the performance of anode materials.Generally,anode materials have two main development routes under the premise of safety.One is long cycle and low-cost anodes for the energy storage stationary batteries,the other is large capacity and high rate anodes for the new energy vehicle power batteries.Among the existing anode materials,titanium-based anodes show long cycle life and low cost,and tin-based anodes have high theoretical capacity and good rate capability.However,the low theoretical capacity and conductivity of the titanium-based anodes and the large volume change and poor cycle performance of the tin-based anodes severely limit their practical applications.Hierarchical structures provide a new route for improving the electrochemical performance of titanium-based and tin-based anode materials.Hence,a series of titanium-based and tin-based anode materials with multidimensional defects,multiscale,hierarchical porous and multiple-contacted yolk-shell structures are designed and constructed by morphology control,dimensional design,component optimization and defect engineering,and then their defect concentration,mechanical structures,and electronic and ion transport networks are optimized.The microscopic techniques,ex-situ spectroscopy and electrochemical analysis are used to investigate the effect mechanism of morphology,dimensions,components and defects on performance of these anodes,and finally,to make clear the intrinsic relationships between electrode structures and performance.The specific research contents are as follows:?1?Multidimensional defect structure.A Li₄Ti₅O₁₂-TiO₂ nanosheet heterostructure anode with multidimensional defects?vacancies,dislocations,grain boundaries and phase boundaries?is synthesized by hydrothermal method.The defects,compositions and morphology of the Li₄Ti₅O₁₂-TiO₂ anode were optimized by changing the molar ratios of Li/Ti in the precursor solution.As a result,the superior electrochemical performance is achieved.The specific capacities reach 170 and 168 mA h g^-1 at 20 and 30 C(1 C=175 mA g^-1),respectively.Moreover,a capacity up to 161 mA h g^-1 is retained after 1000 cycles at 20 C,and the capacity retention ratio reaches 94%.Then,the lithium storage mechanism of this anode is studied.The results indicate that this anode has obvious pseudocapacitive lithium storage behavior,and may contain abundant lithium vacancies,and then a lithium vacancy storage mechanism is proposed.?2?Multiscale structure.A few-layered graphene-wrapped Co,N double-doped TiO₂/C anode is synthesized by a multiscale design based on MOF-derived strategy,which not only achieves Co,N double-doping,and encapsulation of ultrafine TiO₂ nanoparticles in mesoporous C frameworks,but also endows the precursors with positive surface charges,driving them to combine with graphene nanosheets and self-assemble into a 3D macroporous network architecture.Benefiting from the multiscale integration with the synergistic effect between the structural levels,this anode manifests improved sodium storage performance.It deliveres the high specific capacities of 174 mA h g^-1 at 6 C(1 C=335 mA g^-1)for over 5000 cycles,121mA h g^-1 at 15 C for over 10000 cycles,and 100 mA h g^-1 at 30 C for over 3000 cycles.The excellent electrochemical performance is attributed to the outstanding structural stability and pseudocapacitive sodium storage capability.?3?Hierarchically porous structure.A hierarchically porous carbon-coated SnO₂/graphene anode is constructed by interfacial modification coupled with a freeze-casting strategy.Hierarchical pores can not only accommodate the volume change of SnO₂,but also accelerate electrolyte percolation.The encapsulation of SnO₂ into graphene nanosheets prevents the direct exposure of them in the electrolyte,and thus facilitates the formation of stable solid electrolyte interphase?SEI?layer.Moreover,the interconnected graphene network offers a continuous conductive pathway for electron and lithium ion transport.As a result of these merits,this anode shows excellent rate capability and cycling stability.The specific capacities of 611 and 428 mA h g^-1 are retained at 4.0 and 8.0 A g^-1,respectively.Moreover,a capacity reaches 1459 mA h g^-1 at 1.0 A g^-1 after 700 cycles,and increases gradually with cycles,which is attributed to the enhanced reversibility of the conversion reaction of Sn to SnO₂.?4?Multiple-contacted yolk-shell structure.A Multiple-contacted SnO₂/graphene yolk-shell anode is prepared by a general and scalable approach.The multipoint contact between the yolks and shells is achieved by bridging SnO₂ yolks and graphene shells using carbon nanoribbons?CNRs?,which allows a high-efficiency transfer of electrons and ions inside/outside the electrode structure.Moreover,the graphene shells function not only as an electrical highway but also as a mechanical backbone to maintain the structural integrity.As an anode for SIBs,it deliveres a specific capacity of 248.2 mA h g^-1 after 1000 cycles at 1.0 A g^-1with a capacity retention of 86.9%and an ultrahigh rate capability up to 10.0 A g^-1 with a capacity of 153.3 mA h g^-1.Moreover,the present strategy may also apply to other large-volume-change electrode materials.