The Application Of Solid State NMR In The Study Of Electrode Materials And Interface For Li/Na-ion Batteries

Lithium-ion batteries have been widely used in present portable energy sources and electric vehicle system,and sodium-ion batteries have potential to be used for next generation large-scale static energy storage system.Electrode materials are the key factor to improve the energy density of Li/Na-ion batteries,therefore,many studies focused on exploration of high energy electrode materials and optimization of electrochemical performance of present electrode materials.Understanding the electrochemical reaction mechanism,structural evolution and features of electrode/electrolyte interface of electrode materials during cycling can be a guide for modifying electrode materials,optimizing synthesis process,thus improving electrochemical performance.Solid-state nuclear magnetic resonance(ssNMR)technique is a powerful tool in characterizing different kinds of materials,local environment,especially for local structure of nanoscale and amorphous structure.In addition,the ion diffusion and dynamics of electrodes/electrolytes in the structure can also be monitored by ssNMR.In this dissertation,we employed high resolution solid-state NMR technique,combining with X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and X-ray photoemission electron microscopy(X-PEEM)to study the composition,structure and morphology of solid electrolyte interface(SEI)with fluoroethylene carbonate(FEC)on Si/C anode for Li-ion batteries,as well as the long and short-range structures,electrochemical reaction mechanism of Na2FePO4F and Na0.67ZnxMn1-xO2(x=0,0.1)cathode for Na-ion batteries.The composition,structure and morphology of SEI with or without FEC additive on Si/C composite anode for Li-ion batteries have been studied by high resolution 7Li,19F and 31P NMR spectra combining with XPS and X-PEEM techniques.19F and 31P NMR results show that the hydrolysis of LiPF6 led to LixPOyFz,further to Li3PO4 during long-term cycling.A large amount of LiF from the decomposition of FEC additive in the first cycle covers the surface of electrodes,which can partially suppress the volumetric change and pulverization of Si particles,thus improving the electrochemical performance and suppressing the hydrolysis of LiPF6 to some extent.The SEI formed with FEC additive is more dense and compact that different kinds of species distributes more averagely along depth,which can prevent the diffusion of small molecules(i.e.LiPF6,P-O,Li-O species)from outer to inner side,then reduce the side reactions from electrolyte compositions.The long and short-range structure of Na2FePO4F as well as structural evolution,ionic/electronic changes,electrochemical reaction mechanism during cycling for Na-ion batteries have been studied by high resolution 23Na NMR,XRD techniques and DFT calculations.The framework of Na2FePO4F is stable,which occurs two two-phase reactions(Na2FePO4F[S.G.Pbcn](?),Nai.sFePO4F[S.G.P21/c](?)NaFePO4F[S.G.Pbcn])during electrochemical cycling revealed by electrochemical measurement and in-situ HEXRD patterns.The crystal structure of intermediate phase Nai.5FePO4F has been obtained by DFT calculations firstly,in which half of Fe2+ is oxdized to Fe3+,while Na+/vacancy and Fe2+/Fe3+ ordering are shown in this structure,and further comfirmed by Rietveld refinemnet of XRD patterns.Three different local environment of Na sites are shown in the structure of Na1.5FePO4F,where the Nala' and Nalb' sites are derived from Nal site in Na2FePO4F while Na2' site is derived from Na2 site in Na2FePO4F.Ex-situ 23Na NMR results show that the two crystallographically unique Na sites in the structure of Na2FePO4F behave differently during cycling,where the Na ions on the Na2 site are electrochemically active while those on the Nal site are inert.The isotropic resonances of the different Na sites in the structure of Na2-xFePO4F have been assigned by the correlation between local environment of the Na sites and sideband manifolds of 23Na resonances,furthermore comfirmed by calculated paramagnetic 23Na shifts through hybrid DFT calculations.The electrochemical performances of P2-Na0.67MnO2 for Na-ion batteries have been improved by Zn2+ doping.The long and short-range structure during cycling and possible working mechanism for Zn2+ doping,as,well as influence of H2O insertion have benn studied by high resolution 23Na NMR and XRD techniques.The space group of ideal P2-Na0.67ZnxMn1-xO2(x=0,0.1)isP63/mmc,while 23Na NMR results show that stacking fault from synthesis process can be suppressed by Zn2+ doping,where Mn4+/Mn3+ and Na+/vacancy ordering increased.Pure P2-Na0.67MnO2 explores multiple glides of TM layers and phase transitions(P2(?)OP4(?)P'2)during charge/discharge process,resulting in volume and strain changes,while Zn2+ doping can partially suppress the glides of TM layers and phase transitions,thus improving coulombic efficiency and cycling performance.XRD and 1H/23Na NMR results show that H2O can insert into Na layers in P2-Na0.67ZnxMn1-xO2 when storing in the air,leading to bad coulombic efficiency and cycling performance,while avoiding H2O insertion can improve its electrochemical performances.