Preparation And Modification Of High Performance Electrode Materials For Sodium-Ion Batteries And Its Sodium Storage Mechanism

With the aggravation of energy crisis and global environmental pollution,large-scale energy storage system has received a great attention,and it's urgent to develop a new type of low-cost energy storage device.Since the rapid development of lithium-ion batteries?LIBs?,it has successfully occupied the market of portable equipment and electric vehicles.However,in the large-scale energy storage field dominated by prices,LIBs have limited applications due to the uneven distribution and insufficient availability of lithium resources.In contrast,sodium ion batteries?SIBs?are expected to be applied in large-scale energy storage because of their advantages in abundant resources and low cost.However,the commercial application of SIBs still faces a lot of problems.For example,it's difficult for electrode materials to balance in terms of high capacity,high stability and safety.Therefore,preparation and modification of electrode materials with high performance are still the focus of research.As the important components of SIBs,cathode and anode materials directly affect the electrochemical performance of SIBs.In order to build high performance SIBs,this paper focus on the study of P2 type Na_0.65[Mn_0.70Ni_0.16Co_0.14]O₂?NMNCO?cathode material and Na Ti₂?PO₄?₃?NTP?anode material.Through in-situ X-ray diffraction?XRD?technique,the electrode structure is designed and optimized to improve the cycle stability and rate capability.The specific research content is as follows:Firstly,we synthesized P2-type NMNCO microspheres with high tap density as cathode materials for SIBs through solvent-thermal method.The as-prepared materials are densely packed secondary microspheres that consist of submicron-sized primary particles.The unique hierarchical structure can not only effectively facilitate sodium-ion transport owing to the short diffusion distance,but also withstand high stress that caused by continuous Na⁺intercalation/deintercalation.As a result,when tested as cathode materials for SIBs,P2-type NMNCO microspheres deliver a high reversible capacity of 161.5 m A h g^-1 at 0.1 C,superior rate capability of 79.5 m A h g^-1 at 10 C,and excellent cycling stability with 82.3%capacity retention after 50 cycles at 1 C.Even cycled at 5 C,it still maintains 69.5%capacity retention after 200 cycles.Secondly,NMNCO is modified with NTP?NMNCO@NTP?via a wet chemical method.When evaluated as a cathode material for SIBs,the NMNCO@NTP composite can deliver a high reversible capacity of 105.1 m A h g^-1 with capacity retention of 84.3%after 500 cycles at5 C,suggesting significantly improved rate capability and cycling stability.Particularly,the effect of NTP surface modification on the electrochemical performance is unraveled by a combination of in-situ XRD technique,ex-situ TEM analysis and first-principles calculations.The results revealed that the NTP surface coating and doping can effectively suppress the structural change of NMNCO during sodiation/desodiation process,leading to a highly structural reversibility.In addition,NTP coating layer with exceptional ion conductivity can effectively restrain gradual encroachment,and guarantee stable phase interfaces.Thirdly,a facile solid-state method was used to achieve uniform carbon coating on NTP,and simultaneously achieve fluorine doping of the carbon coating layer and the bulk material.Though in-situ XRD,Raman,XPS,SEM,TEM,galvanostatic charge/discharge technique and theoretical calculations,it is shown that fluorine doping can not only effectively alleviate particle agglomeration and accelerate sodium ion diffusion,but also reduce the energy gap and improve the electronic conductivity of the material.The results show that NTP@CF provides high reversible specific capacities of 123.5 and 108.7 m A h g^-1 at 0.5 C and 50 C,respectively.And the capacity retention after cycling 2000 cycles at 10 C is 75.5%,with excellent rate performance and long cycle life.In addition,the NTP@CF anode material and the modified NMNCO@NTP cathode material are assembled into a full cell.The coulombic efficiency of the initial cycle is 86.1%without presodiation,and the capacity retention of 100cycles at 1 C is 76.4%,showing good cycle stability.