Design,Preparation And Energy Storage Mechanism Of The Alkali Metal Ion-embedded Fe/Mn-Based Cathode Materials

Based on development and application of high-performance Fe/Mn-based cathode materials,this thesis explores the energy storage mechanism and potential application of Fe/Mn-based cathode materials for Na-ion and K-ion batteries,following the ideas that form material design,controllably synthesis,structure characterization,electrochemical measurement to energy storage mechanism analysis.Herein,we constructed the Fe/Mn-based layered cathode materials with different alkali metal ion-embedded by ion pre-intercalated strategy.We firstly developed the K_0.7Fe_0.5Mn_0.5O₂ hexagon nanoparticle cathode material with large ion diffusion channels,multiple active sodium storage sites and stable skeletal structure.Advanced in situ characterization technology was applied to reveal its intrinsic sodium storage mechanism.The sodium ion-embedded Mn-based cathode material was fabricated by in situ synergistic composite technology,and its phase,morphology and electrochemical sodium storage properties were characterized.The microstructure evolutions of the cathode were investigated by in situ characterization technique.Furthermore,the interconnected K_0.7Fe_0.5Mn_0.5O₂ mesoporous nanowire K-ion battery cathode was successfully constructed by electrospinning technology,and ystematically characterized its phase,structure and electrochemical potassium storage performance.Based on the above research contents,some meaningful results and understanding were obtained as follows:?1?We constructed the Fe/Mn-based layered transition metal oxide with different alkali metal ion-embedded by facile organic acid-assisted and ion pre-intercalated strategy.The sodium storage performances of each product were modulated by means of species of different alkali metal ions,amounts of insertion alkali metal ions and sintering temperature.The sodium storage performances of K_0.7Fe_0.5Mn_0.5O₂,Na_0.7Fe_0.5Mn_0.5O₂ and Li_0.7Fe_0.5Mn_0.5O₂ were systematically investigated,and the test results showed that K_0.7Fe_0.5Mn_0.5O₂ displayed a high discharge capacity of 181 mAh g^-1.Remarkably,even when cycled at high rate of1000 mA g^-1,85.0%of the initial discharge capacity is maintained after 1000 cycles.When tested at various current densities in rate measurements,K_0.7Fe_0.5Mn_0.5O₂exhibited better rate recovery?⁹8.6%?than those of Na_0.7Fe_0.5Mn_0.5O₂ and Li_0.7Fe_0.5Mn_0.5O₂,demonstrating the prominent rate performance.?2?We developed a new synergistic composite,xNa₄Mn₂O₅·?1-x?Na_0.7MnO₂,through an in situ synergistic strategy.The content of the two phases in the composites was regulated by controlling the ratio of the Na source and Mn source.Morphologies,structures and compositions of the composite cathode materials and their sodium storage performance in Na-ion half cells were characterized.The electrochemiacal chatercterizations demonstrated that the 0.44Na₄Mn₂O₅·0.56-Na0.7MnO2 cathode material exhibited the most excellent electrochemical sodium storage performance.with an initial discharge capacity of 157.0 mAh g^-1 at 1000 mA g^-1 and⁹0.0%capacity retention after cycling 500 times,suggesting the prominent cycling stability and high rate performance.In addition,the Na-ion full cells based on0.44Na₄Mn₂O₅·0.56Na_0.7MnO₂ and hard carbon were also assembled.The full cell displayed an initial discharge capacity of 150 mAh g^-1 at a current density of 50 mA g^-1,retaining 129.0 mAh g^-1 after cycling 100 times.Furthermore,on the basis of in situ XRD and electrochemical characterization analysis,we demonstrated that synergistic composite structured 0.44Na₄Mn₂O₅·0.56Na_0.7MnO₂ composites were featured with stable crystal skeleton structure and exhibited high reversible capacity during desodiation/sodiation processes.?3?We designed and constructed a novel interconnected K_0.7Fe_0.5Mn_0.5O₂nanowire by gradient-electrospinning and controlled-pyrolysis method,for the first time.After systematically characterized the morphologies,structures,material compositions and potassium storage performances in K-ion half cells of the nanowires,we found that the interconnected K_0.7Fe_0.5Mn_0.5O₂ nanowires possessed better potassium storage performance compared to K_0.7Fe_0.5Mn_0.5O₂ particles.When tested at a low current density of 20 mA g^-1,interconnected K_0.7Fe_0.5Mn_0.5O₂nanowires exhibited high capacities of 178 mAh g^-1 during the first potassiation process.Moreover,a discharge capacity of 125 mAh g^-1 was retained after 45 cycles,corresponding to a considerable capacity retention of⁷0.0%.The K-ion full batteries based on interconnected K_0.7Fe_0.5Mn_0.5O₂ nanowires and soft carbon were also fabricated.The full cell delivered a considerable discharge capacity of 81.2 mAh g^-1 at 40 mA g^-1,and the capacity was still retained 64.0 mAh g^-1 after 50 cycles,corresponding to a high capacity retention of⁷8.8%.On the basis of advanced in situ X-ray diffraction analysis and electrochemical characterization,we confirmed that interconnected K_0.7Fe_0.5Mn_0.5O₂ nanowires can provide stable framework structure,fast K-ion diffusion channels and three-dimensional electron transport network during the depotassiation/potassiation processes.Our work will bring a new concept and lay a scientific foundation for designing and constructing high performance cathode with a stable crystal structure and high potassium storage capacity.