| The main aim of this dissertation is to study the charge/discharge mechanisms of a series of metal-organic frameworks(MOFs)when applied as anode materials in lithium-ion batteries(LIBs),as well as the charge-discharge mechanisms of Na3V2(PO4)2F3-2y O2y(y = 1.0,0.8,0.6)fluorophosphates when applied as cathode materials in sodium-ion batteries(SIBs)by an arsenal of solid-state nuclear magnetic resonance(SSNMR),electron paramagnetic resonance(EPR)and synchrotron radiation techniques.The main contents of the dissertation are as follow:(1)We introduced the designed syntheses of three Co BTC MOFs(BTC = 1,3,5-benzenetricarboxylate)with different morphology and dimension by simply adjusting the reaction solvent in the solvothermal process,and investigated their electrochemical performances as anode materials in LIBs.The optimized product Co BTC-Et OH retains a high capacity of 856 m Ah g-1 at 100 m A g-1 after 100 cycles while retaining a nearly 100% Coulombic efficiency(CE).Even when cycling at a high current rate of 2 A g-1,the Co BTC-Et OH is able to maintain a reversible capacity of 473 m Ah g-1 after 500 cycles.More importantly,we substantiated that the Co-ions in Co BTC-Et OH remain in the Co2+ state during charge/discharge process via soft X-ray absorption spectroscopy(s XAS)technique.Hence,Li-ions are inserted to the organic moiety(including carboxylate group and benzene ring)without the direct engagement of Co-ions.The ex-situ 1H MAS NMR experiment also support this assumption.(2)We demonstrated the designed synthesis of layered Co2(OH)2BDC(BDC = 1,4-benzenedicarboxylate)MOF,and investigated its lithiation-delithiation chemistry by a combination of s XAS(Co L-edge,O K-edge),hard X-ray absorption fine structure(XAFS)spectroscopy,13 C MAS NMR,and EPR techniques.The results substantiate that:(a)Co2+ and metallic Co are interchangeable during repeated Li+ intercalation/extraction;(b)a large amount of Li-ions can insert into the carboxylate groups and benzene rings(with delocalized ? electrons)at the low potential range(< 0.5 V vs.Li+/Li).When applied as an anode material in LIBs,the Co2(OH)2BDC is able to deliver a high capacity of 1021 m Ah g-1 at 100 m A g-1 after 200 cycles while retaining a nearly 100% CE.More strikingly,the Co2(OH)2BDC is able to maintain a reversible capacity of 435 m Ah g-1 after 1000 cycles when cycling at a high current rate of 1 A g-1,which is the longest calender life ever reported for MOFs-based anode materials.(3)We presented the fabrication of Mn2(OH)2BDC and Ni2(OH)2BDC ultrathin MOF nanosheets(termed ‘Mn-UMOFNs' and ‘Ni-UMOFNs',respectively)by an ingenious ultrasonic approach,and scrutinized their electrochemical properties as anode materials for LIBs.The Mn-UMOFNs with structure advantages over Ni-UMOFNs(thinner nanosheets,higher surface area)shows rapid Li+ diffusion coefficient(2.48 × 10-9 cm2 s-1),large reversible capacity(1187 m Ah g-1 at 100 m A g-1 after 100 cycles),impressive rate capability(701 m Ah g-1 even at 2 A g-1),and more importantly,a low average operating potential of ~0.4 V.Considering on the environmental benignity and low-cost of manganese metal and terephthalic acid ligand,the Mn-UMOFNs demonstrates alluring promise as a low-cost and energy-dense anode for the most cutting-edge LIBs.Moreover,the lithiation-delithiation chemistries of Mn-UMOFNs/Ni-UMOFNs were unequivocally studied by a combination of magnetic measurements,EPR,and s XAS(O K-edge and Mn/Ni L-edges)experiments.The main results are as follow:(a)The magnetic property analyses suggest the occurrence of Mn0/Ni0 during the Li+ insertion process.(b)The ex-situ EPR studies for Mn-UMOFNs demonstrate the interconversion between high-spin Mn2+ ions(S = 3/2)and nanosized Mn0,while the ex-situ EPR studies for Ni-UMOFNs demonstrate the occurrence of delocalized conduct electrons during cycling.(c)The O K-edge s XAS studies for Mn-UMOFNs and Ni-UMOFNs prove the rehybridization between O-2p and Mn/Ni-3d orbitals and the subsequent decrease of bond covalence upon discharging,and the following recovery of the local environment around oxygen ions upon charging.Furthermore,the Mn/Ni L-edges s XAS studies confirm the reversibility of octahedral coordinated Mn2+/Ni2+ ions during cycling.Hence,in the case of Mn-UMOFNs and Ni-UMOFNs,both the metal centers and the aromatic BDC2-chelating ligands participate in lithium storage,which provide a high theoretical capacity of 1392 and 1359 m Ah g-1,respectively.(4)We demonstrated the rapid fabrication of Na3V2(PO4)2O2F,Na3V2(PO4)2O1.6F1.4 and Na3V2(PO4)2O1.4F1.6 nanoparticles(termed ‘NVOPF',‘NV3.8OPF' and ‘NV3.6OPF',respectively)through a microwave-assisted low-temperature(120 ?)hydrothermal procedure,and scrutinized their electrochemical behaviors as cathode materials for SIBs.The NV3.8OPF shows more impressive rate capability and long-term cyclic performance(61 m Ah g-1 after 1000 cycles at 10 C,1C refers to 130 m A g-1)in comparison to NVOPF and NV3.6OPF.More strikingly,the NV3.8OPF exhibits a high practical energy density of 518 Wh kg-1 and impressive power density performance(11123 W kg-1 while retaining an energy density of 306 Wh kg-1),which outperforms most of existing cathodes for SIBs and even rival the advanced spinel Li Mn2O4(~450 Wh kg-1 vs.Li)and olivine Li Fe PO4(~530 Wh kg-1 vs.Li)lithium-ion battery cathodes.Moreover,the electrochemical redox mechanism of NV3.8OPF was studied by a combination of 23Na/51 V MAS NMR,EPR and XRD techniques.The main results are as follow:(a)Both V3+/V4+ and V4+/V5+ redox couples are active in this 2.0-4.8 V(vs.Na+/Na)range during the electrochemical process of NV3.8OPF,corresponding to a total amount of 2 electrons transfer per formula unit and a theoretical capacity of 129.6 m Ah g-1.(b)The Na extraction/intercalation process is mainly a solid-solution mechanism,and the cell volume change of NV3.8OPF upon cycling is as low as 2.06%,suggesting an extremely small lattice strain. |
PDF Full Text Request