Structural Modification On Carbon-based Composites And Their K-ion Storage Mechanisms

With the rapid development of electric automobiles,restricted by the low earth abundance and high cost of lithium sources(～17 ppm),lithium-ion batteries(LIBs)could no longer meet the demands.Therefore,based on the high earth abundance(～15000 ppm)of potassium and its similar electrochemical properties potassium-ion batteries(PIBs)have been attracting the attentions of the researchers.In this thesis,we mainly focus on the development of high-performance electrode materials for PIBs.Considering the control of the cost,our research mainly focuses on the preparation of carbonaceous materials and carbon-based composites and the relationship between their microstructure and potassium-ion storage performance.The main research contents are described as follows:ⅰ)We proposed the "Solid-phase intercalation&liquid-phase expansion" strategy,and successfully realized the interspace control of graphite.The as-prepared FeCl3-EG electrode delivers a high reversible capacity of 269.5 mAh g-1 at 50 mA g-1 and 133.1 mAh g-1 at an ultrahigh current density of 5000 mA g-1.The FeCl3-EG electrode also exhibits ultra-stable cycling performance.Even after 500 cycles at the current density of 50 mA g-1,the FeCl3-EG electrode can still deliver a discharge capacity of 224.1 mAh g-1,corresponding to a high capacity retention of～88.82%.Moreover,the FeCl3-EG electrode can still exhibit relatively good cycling stability at high current densities.After being cycled at an ultrahigh current density of 2000 mA g-1 for 1300 cycles,the FeCl3-EG electrode can still hold a high capacity retention of y～70.38%.Besides,Ex-situ X-ray diffraction,Raman,and HRTEM measurements(High-resolution-transmission-electron-microscopy)were performed to investigate the potassium-ion storage mechanism of FeCl3-EG electrode,and the synergistic potassium-ion storage process between graphite and FeCl3 is illustrated.ⅱ)We introduced the "In-situ self template&gas foaming" method,and successfully realized the modifications on interspace,specific surface area,pore structure and surface functional groups of non-graphitic carbonaceous materials.The as-synthesized hierarchically porous NPC electrode delivers a high reversible capacity of 384.2 mAh g-1 after 500 cycles at a current density of 100 mA g-1.NPC electrode also exhibits an outstanding rate capability of 185 mAh g-1 at a high current density of 10000 mA g-1.Moreover,the NPC electrode exhibits excellent long-term cycling stability.After being cycled at 5000 mA g-1 for 1000 cycles,the NPC electrode can still deliver 144.4 mAh g-1,corresponding to a capacity retention ratio of 81.1%.ⅲ)Based on the "In-situ self template&gas foaming" method and "graphene surface confining" strategy,we successfully realized in-situ carbon coating on tin-based composites and the surface-confined growth of SnS2.Based on the two-step"combined conversion-type and alloying-type" potassium ion storage process,both Sn4P3@C and SnS2@C@rGO-3 electrodes deliver high reversible potassium ion storage capacities,far beyond that of the carbonaceous electrode.At a current density of 50 mA g-1,they deliver reversible capacities as high as 473.3 mAh g-1 and 499.4 mAh g-1 respectively,exceeding the carbonaceous anode materials.Moreover,both Sn4P3@C and SnS2@C@rGO-3 electrodes exhibit superior cycling stability:When being cycled at 500 mA g-1,the Sn4P3@C can maintain a reversible capacity of 181.5 mAh g-1 after 800 cycles,while the SnS2@C@rGO-3 electrode can still deliver a reversible capacity of 298.1 mAh g-1 after 500 cycles.ⅳ)Inspired by the superior potassium-ion storage performance of carbonaceous materials and carbon-based composite electrodes,we conduct further research deep into the potassium-ion storage mechanism and diffusion kinetics.The research contents are listed as follows:a)We first separate the potassium-ion storage behaviors involved in carbonaceous materials as "difussion" behavior and "capacitance"behavior and carry out quantitive analysis.b)The "capacitance" behavior is mainly influenced by the interspace distance and theoretical calculation indicates that the best interspace distance is 0.52 nm.c)The "capacitance" behavior is mainly influenced by specific surface area and pore structure.d)We measure the potassium-ion diffusion coefficient(Dk)on carbonaceous materials through galvanostatic intermittent tiltration technique(GITT),and the value of Dk is mainly influenced by the interspace distance,pore structure and surface functional groups of carbonaceous materials.e)The pyrrindic-N and Pyrrolic-N sites on the surface of carbonaceous materials can significantly accelerate the potassium-ion adsorption rate according to the theoretical calculations,f)Measurements of the Dk of carbon-based materials indicate that the continuous carbon network can obviously contribute to the enhanced rate capability of tin-based composite.