Study On Alkali Metal Ion Storage Performance Of Heteroatom-doped Carbon Materials

Carbon anode materials are currently one of the most widely used anode materials for alkali metal ion batteries,but there is still a need to solve the problems of short cycle life and poor cycle stability.Using heteroatom doping and morphological structure control methods to modify the carbon anode material can effectively improve the electrochemical performance of the carbon anode.The introduction of oxygen-containing groups is one of the most common heteroatoms doping methods.Oxygen-containing functional groups can be used as electrochemical reaction sites,which is beneficial to improve the electrochemical activity of surface redox reactions and change the capacitive mechanism.In addition,by adjusting the morphology and structure of the metal-organic framework,derivative carbons of different structural types can be obtained.In this thesis,different types of doped carbon materials are prepared by simple and efficient experimental methods,and high-capacity lithium/potassium ion battery anode materials are obtained through chemical surface modification and morphology control.The overview are as follows:（1）The enhancement of oxygen modification on spherical graphite anode was investigated by acid treatment of spherical graphite.Acid treatment of spherical graphite resulted in the formation of a dense oxide layer on the surface,such as carbonyl,hydroxyl,and carboxyl groups.After lithiation,this dense oxide layer became part of the SEI film,reducing the activity of the edge planes to inhibit the decomposition of the electrolyte in lithium-ion batteries.Acid treatment of spherical graphite changed the presence form of oxygen groups,among which the effect and effect of chemisorbed oxygen groups carboxyl groups were the most obvious.The carboxyl-type oxygen groups enhanced the lithium storage capacity of spherical graphite and also improved its rate properties.Among them,the nitric acid-treated spherical graphite had a high reversible specific capacity of 605.1 m Ahg^-1,which exceeded that of the untreated spherical graphite(448.8 m Ahg^-1)by 156.3 m Ahg^-1.（2）A porous hard carbon HC with hierarchical pore structure was synthesized by simple high-temperature solid-phase sintering.The HC was treated with three different acids（sulfuric acid,a non-oxidizing acid,nitric acid,a strong oxidizing acid,and a moderately oxidizing mixed acid）to obtain three acid-treated hard carbons,SAHC,NAHC,and SNHC,respectively.The presence of oxygen-containing groups increased the wettability and enhanced the activity of adjacent carbon atoms,and also removed active edge sites and defects（such as sp³hybridized carbon atoms,carbon chains and carbon radicals）to smooth the surface structure of hard carbon and enhance its stability.When SNHC was used as the anode of Li-ion battery,it still maintained a high reversible specific capacity of1038.1 m Ahg^-1 after 60 cycles,exceeding HC(777.1 m Ahg^-1)by 261 m Ahg^-1,and the capacity retention rate（from the second cycle of reversible specific capacity）was as high as 100%.The SNHC as the potassium ion battery anode exhibited a high discharge capacity of 811 m Ahg^-1 in the first cycle and maintained a high reversible capacity of 371.7 m Ahg^-1 after 100 cycles,exceeding that of untreated hard carbon HC(272.5 m Ahg^-1)by 99.2 m Ahg^-1,with a capacity retention rate（from the second lap reversible specific capacity）up to 88.8%.Combined with the kinetic analysis of lithium and potassium storage,when the two storage mechanisms were in equilibrium,both high reversible charge/discharge specific capacity and excellent cycling stability were obtained.（3）2D Cu-BDC MOF nanosheets were designed and synthesized by controlling the reaction time,and then annealed at different pyrolysis temperatures;the final MOF-derived carbons C₅₀₀（Final product at 500℃）and C₇₀₀（Final product at 700℃）were obtained by acid etching.Both carbon derivatives possessed abundant mesoporous structures which could facilitate ion migration and be conducive to the storage of lithium/potassium ions.C₅₀₀ and C₇₀₀ exhibited different dominant kinetic behaviors in lithium ion/potassium ion batteries.C₅₀₀,which was controlled by diffusion process,showed better performance when used as anode material for lithium-ion batteries（LIBs）.The C₅₀₀ anode exhibited a reversible specific capacity of 690.4 m Ahg^-1 at a current density of 0.1 Ag^-1.Even after 500 cycles at a current density of 5.0 Ag^-1,the high specific capacity of 210.3 m Ahg^-1 was still maintained.C₇₀₀,which was dominated by surface capacitance contribution,was a superior anode material for potassium-ion batteries（PIBs）.The reversible specific capacity of C₇₀₀ anode was maintained at 286.2 m Ahg^-1 at the current density of 0.1 Ag^-1 after100 cycles.Even after 500 cycles at a current density of 1.0 Ag^-1,C₇₀₀ anode still maintained 170.8 m Ahg^-1.There are 56 figures,5 tables and 167 references in this dissertation.