Research On Heteroatomic Structural Regulation And Energy Storage Mechanism Of Transition Metal Oxide Cathode Materials

With the proposal of“emission peak and carbon neutrality”,it is necessary to optimize the energy structure and develop environment-friendly renewable energy.However,the discontinuity of the new renewable energy makes its application still difficult.Due to the high efficiency and recyclability,lithium-ion batteries（LIBs）and sodium/potassium-ion batteries（SIBs and KIBs）can assume the roles as the energy storage and conversion devices for the utilization of renewable energy.However,cathode materials are regarded as one of the main bottlenecks in the development of LIBs,SIBs and KIBs due to the limitations of their electrochemical properties.Therefore,the development of suitable cathode is important for improving the electrochemical performance.Among various cathodes,transition metal oxide cathode materials have attracted researchers’interest due to their high theoretical capacity and energy density.However,the transition metal oxide positive materials always suffer complex phase transitions during cycling.These structural changes will cause significant lattice distortion and structural collapse,then even cause cracks and particle pulverization,which directly harm the structural stability and service life of the materials.In this thesis,we investigated the influence of element doping on the structural evolution in the cycling process of transition metal oxide cathode materials,which improve their structural stability and electrochemical performance.The main contents are list as follows:Firstly,we synthesized the single-crystal nickel-rich Li Ni_0.83Co_0.07Mn_0.1O₂（SC-NCM）cathode material.To solve the internal crack in SC-NCM material,yttrium and boron doping were introduced into SC-NCMYB material by solid-state sintering method,and the influence of element doping on the structural stability of SC-NCM material was investigated.In SC-NCMYB material,Yttrium element does not participate in redox reactions during cycles,thus Y³⁺can stably remain in the transition metal site and play a"pillar role"in stabilizing the structure to inhibit the migration of transition metal ions to lithium site and then alleviate cation mixing and structural deterioration.Meanwhile,the stronger binding force of Y-O and B-O bonds can alleviate the lattice oxygen loss behavior at high voltage range and improve the cyclic stability of the material.Moreover,B-O bond can increase the charge density of coordinated O atoms,which enhance the electrostatic repulsion between adjacent O atom layers and expanding layer spacing of adjacent TMO₆ layers.This alleviates the volume change and anisotropic stress accumulation during cycles.Based on above reasons,SC-NCMYB exhibits excellent cycle stability(SC-NCMYB maintained the high reversible discharge specific capacity of 176.3 m Ah g^-1 after 150 cycles at 1 C with the capacity retention rate of 94.5%).To further improve structural stability and interfacial reaction kinetics defects on SC-NCM material,we constructed the fast ion conductor Li₅La₃Nb₂O₁₂ coated SC-NCM@LLNO material by solid-state method,and simultaneously introduced the elements doping of lanthanum and niobium in SC-NCM@LLNO material.In SC-NCM@LLNO material,the inert Nb⁵⁺and La³⁺can play a"pillar role"in stabilizing the structure,which inhibits the migration of transition metal ions to lithium site and alleviate the cation mixing and structural decay.Based on Bader charge analysis,Nb⁵⁺and La³⁺will enter the octahedral position in transition metal layer and increased the charge density of coordinated oxygen atoms,resulting the close of O^2-to the central Nb⁵⁺and La³⁺and the increase of electrostatic repulsion between adjacent oxygen layers,which expands the layer spacing between adjacent TMO₆ layers.This alleviates the volume change and the accumulation of anisotropic stress during cycling,reducing the structural mechanical damage and improving the structural stability of the material.In addition,the chemical-stable Li₅La₃Nb₂O₁₂ modified layer on the surface of SC-NCM@LLNO material can effectively alleviate the erosion of the electrolyte to the material,reduce the dissolution of transition metal elements.To explore suitable materials for large-scale energy storage,the tunnel-type SIBs cathode has been investigated.Based on the idea of improving the structural stability of layered transition metal oxides by element doping in previous study,we used cobalt element doping to improve the structural stability of tunnel type Na_0.44Mn_1-xCo_xO₂.As the introduce of Co³⁺,because of the smaller ionic radius of Co³⁺（0.545（?））,the coordinated O^2-prefer to closing Co³⁺ions due to the stronger electrostatic attraction,triggering the contraction of TM-O polyhedron structure.This would expand the tunnel space in framework and reduce Na ion diffusion barrier,promoting the rapid Na⁺migration and thus improving the rate performance.Especially,for the Na ion sites in small tunnels,the expanded small tunnel space can effectively reduce the diffusion resistance of Na ions,so that they can be more easily de-/intercalated and then participate in electrochemical redox reactions,which activated the originally inert Na ion sites.Therefore,the reversible discharge specific capacity of NMO-3material can reach to 138 m Ah g^-1.And NMO-3 delivers a high capacity retention of 97.4%after 100 cycles.To explore the application of low-cost KIBs in large-scale energy storage,the potassium storage mechanism of transition metal oxides was further studied.Inspired by the structural evolution triggered by element doping in the previous works,to improve the poor stability caused by complex phase transition in the cycle process of manganese-based transition metal oxide materials,a simple solid-state method was used to introduce cobalt and iron elements doping into P3-K_0.5Mn O₂ material.The substitution of Mn³⁺（0.58（?））by smaller-ion-radius Co³⁺（0.545（?））and Fe³⁺（0.49（?））ions shorten the TM-O bond（TM=transition metal）and thus decrease the thickness of TMO₆ layers due to their stronger electrostatic attraction,which enlarges the layer spacing between adjacent transition metal layers.The enlarged spaces not only promote K-ion migration,but also provide a buffer zone for volume change,which can mitigate the structure evolution of P3-O3 phase transformation and the accompanying interlayer-gliding of TMO₆ layers,leading to the improvement in structure stability.In general,this thesis systematically explores the influence of element doping on the structure evolution process of transition metal oxide cathode materials,and investigates the mechanism of improvement on electrochemical performance,which provides a new feasible strategy for the development of high-performance transition metal oxide cathode materials.