Structure Designing Of Layered Transition-metal Oxides For Electrochemical Energy Storages

Layered transition-metal oxides have aroused significant attentions as lithium ion batteries（LIBs）and sodium ion batteries（SIBs）cathode materials owing to the advantages of considerable specific capacity,high operation voltage and flexible structural constituent.However,the structural instability and volume change during electrochemical energy storage process cause the poor cycle and rate performance far from the practical application requirements.In this thesis,we focus on micro-nano size design and structural regulation to improve the storage capacity and cycling stability of N i-rich layered oxides as well as P2-type Na_2/3(Ni_1/3Mn_2/3)O₂.In addition,the relationship between the physical structure and electrochemical performances are in-depth studied,which can reveal the energy storage mechanism in lithium/sodium ion batteries as well as provide the new research ideas and theoretical basis for the development of layered transition-metal oxides.The main contents and conclusions are as follows:1.Uniform submicron LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）particles were fabricated by co-precipitation mothod.By regulating the annealing temperature to reduce the cation mixing,the capacity decay and sluggish kinetics can be effectively solved.Notably,the uniform submicron spheres can reduce the Li⁺migration distance,provid ing rapid Li⁺diffusion coefficient(10^-9cm²·s^-1).As expected,the synthesized NCM811 cathode delivers the highest initial capacity(228 mAh g^-1,20 mA g^-1),outstanding energy density(866 Wh kg^-1)and excellent cycling stability over 200 cycles with the capacity retention of 80.6%.2.Doping different Ti⁴⁺amounts in P2-type layered Na_2/3Ni_1/3Mn_2/3O₂（NNM）cathodes is a great strategy to construct stable framework for sodium storage,which can solve the multiple phase transitions and poor cycle performance.The doping Ti⁴⁺can supply more thermodynamics stability of NNM to significantly suppress the P2-O2 phase transition.In addition,the large Ti⁴⁺radius can enlarge the layer spacing to facilitate Na⁺extraction/insertion.The results exhibit the optimum Ti⁴⁺doping is 10%.After doping,the layered cathode delivers the initial capacity of 172 mAh g^-1 at 0.1C with the capacity retention of 63.8%after 200 cycles and excellent rate capability(84 mAh g^-1 at 5C).3.To further improve the electrochemical performance of P2-type Na_2/3Ni_1/3Mn_2/3O₂（NNM）for the goal of full cell application,the nanosheets assembled NNM microspheres（s-NNM）were synthesized by self-template method.The micro/nano hierarchical structure with stable framework can shorten Na ion diffusion pathways and accommodate volume changes.As a conclusion,the target s-NNM sample delivers superior cycling ability of 83.3%capacity retention at 1.0 C over 500 loops with ultra-high initial Coulombic efficiency of96.0%.When served as cathode in Na ion full cell,the as-prepared sample exhibits a superior reversible capacity(71 mAh g^-1,0.2C),rate performance with energy efficiency（>85%）.