| Lithium-ion batteries(LIBs)have predominated the marketplace for portable electronic devices and electric vehicles in the last few decades,however,their further development is limited by the scarcity of and uneven geographical distribution of lithium resources.Therefore,under the background of the energy crisis,it is urgently necessary to develop a new generation of low-cost and rechargeable energy storage battery technology to further develop and supplement the large-scale energy storage technology and equipment.Potassium ion batteries(PIBs)have been regarded as potential candidates and attracted wide attention in recent years due to their abundant potassium resources and similar electrochemical performance to LIBs.Carbon materials are widely used as anode materials for PIBs because of their good electronic conductivity,stable structure and chemical properties,and low cost.Heteroatomic(such as B,N,O,S,P,and F)doping can effectively improve the specific capacity and magnification capacity,but the low initial Coulombic efficiency(ICE)is still the main limiting factor for the practical application of carbon anode electrode.In addition,transition metal oxides have the potential to store energy with high theoretical capacity and energy density.However,due to the irreversible volume expansion and structural changes caused by conversion and alloying reactions,the poor cyclic stability prevents them from becoming ideal anode materials.Herein,based on the limitations of the above materials,from the perspective of material structure design,functional modification,and exploration of potassium storage mechanism,functional carbon materials were designed and synthesized which significantly improved the potassium storage capacity and the ICE of the anode of carbon materials.Furthermore,carbon-coated transition metal oxides(as carbon-based materials)were synthesized,and the possibility of a new potassium storage vacancy mechanism was proposed.The potassium storage capacity and stability of metal oxides were optimized,and the existing potassium storage mechanism was supplemented.The structure-activity relationship between K+storage capacity properties and the morphology,structure,and composition of materials were systematically studied.The specific research content is summarized as follows:(1)A novel nitrogen-doped porous carbon nanosphere modified with the sulfonic acid functional group(SA-NC)was designed and prepared as high ICE/capacity anode material for PIBs.The SA-NC material has a high reversible initial specific charging capacity of 793 mAh g-1 at 0.05 A g-1 current density and a capacity retention rate of 90%after 80 cycles.It also has a high initial Coulombic efficiency ICE=68.21%at 0.1 A g-1 current density.The specific capacity 288 mAh g-1 remains stable after 2000 cycles at 2 A g-1 high current density,which is ahead of most of the reported carbon anodes.SA-NC has a rich microporous structure on the surface,and the porous carbon is modified by introducing N-doping and sulfonic acid modification,which provides a rich migration path for potassium ions.The experimental and theoretical calculation results show that this specially modified structure has suitable adsorption energy for K+,and can provide rich active storage sites for potassium.Rapidly reversible surface adsorption/desorption capacitance storage of K+ is promoted,while the ICE is improved.(2)In view of the irreversible volume expansion and structural collapse of transition metal oxides caused by traditional conversion/alloying reactions,a new vacancy potassium storage mechanism was proposed based on the O-close-packed inverse spinel structure of Fe3O4,and a carbon-coated Fe3O4 nanosphere(Fe3O4@C)with pomegranate shape was synthesized by regulating synthesis conditions as high-performance anode material for PIBs.Fe3O4@C has a high reversible capacity,which can provide a stable specific capacity of 638 mAh g-1 at 0.05 A g-1 current density and can maintain a stable reversible capacity of 150 mAh g-1 for 9000 cycles at 10 A g-1 ultra-high current density.With a pomegranate-like composite nanostructure,the compact inner structure and outer carbon coating effectively alleviate the volume expansion of the inner Fe3O4,improving the material structure and cycling stability,while the outer carbon layer enhances the electronic conductivity of the material.The results of experiments and the Density Functional Theory(DFT)calculations show that different from the most reported conversion/alloying reaction potassium storage mechanism,the inverse-spinel Fe3O4 has unoccupied tetrahedral and octahedral vacancies,which can be used as an effective storage site for K+and provide sufficient reversible potassium storage capacity.Hence,the transition metal oxides can be further developed as stable high-performance anode materials. |
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