Investigations On The Interfacial Properties Of Transition Metal Oxides Electrode Materials For Energy Storage And Photocatalysis

It is urgent to do renewable energy system development and utilization due to the energy crisis and environmental pollution caused by the constant consumption of fossil fuels.Lithium ion(Li-ion)batteries and photo-catalysis are the important parts of renewable energy system.Transition metal(Ni,Mn,Co,Ti,etc.)oxides have wide applications in renewable energy system,and they are the most important present electrode materials for Li-ion batteries and photo-catalysis system,due to its earth-abundance and relatively low price.For both Li-ion batteris and photocatalysis,the electrode/electrolyte interfacial reactions always occur,which plays an important role in understanding the working mechanism and further developing new system.Among all of the transition metal oxide cathode materials,Li-rich oxides Li[Li,Ni,Mn,Co]O2 has the highest discharge capacity and relatively high discharge voltage,which make its applications promising.However,its interfacial properties still need to be further studied.In the photocatalysis field,TiO2 has become the most widely used photocatalyst due to its advantages of high catalytic activity,good chemical stability and low price.However,its current interfacial properties studies rely on ex-situ methods.The usage of advanced in-situ methods(e.g.,TEM)can clarify many controversies in interfacial properties investigations.In this thesis,several major interfacial properties of Li-rich oxides as energy storage electrode material and TiO2 as photocatalyst were studied.The main experimental results are as follows:(1)First-cyle charge-discharge behavior of Li-rich oxides material.The first-cycle charge-discharge behaviors of Li-rich Li1.2Ni0.12Co0.12Mn0.56O2(0.5Li2MnO3·0.5LiNi0.3Co0.3Mn0.4O2),pure Li2MnO3 and layered LiNi0.3Co0.3Mn0.4O2 were studied by electrochemical quartz crystal microbalance(EQCM).A 1 M LiPF6/EC+DMC electrolyte reference system without electrode material was used as a comparison.Through comparing the mass changes and mpe(mass accumulated per mole of electron transferred)of these three materials and the pure electrolyte in the same potential region,we studied the activation pathways and cathode electrolyte interfaces(CEI)evolution behavior of Li-rich material and pure Li2MnO3.The results show that the activation of pure Li2MnO3 is dominated by chemical decomposition(Li2O evolution),while the activation of Li-rich materials is dominated by electrochemical decomposition(form O22-),which is due to the synergistic effect between layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structure.Chemical decomposition does not contribute to the electrode capacity,which explains why the capacity of Li-rich materials is much higher than the average capacity of pure Li2MnO3 and LiNi0.3Co0.3Mn0.4O2.It is also found that the formation/dissolution of CEI is closely related to the valence state change of metal ions in the material.The valence state varies of metal(Ni,Co,Mn)ions can promote the formation of CEI film.(2)The mechanism of guar gum(GG)binder to alleviate the capacity and voltage fade of Li-rich oxides material.We comparatively studied the electrochemical performance,electrode/electrolyte interfaces,morphology,microstructure,Mn ion valence state and local structure of Li-rich Li1.2Ni0.2Mn0.6O2 material using commercial PVDF and GG as binders.The results show that the strong bonding with particles,current collector and good mechanical properties of GG binder contribute to prevent electrode crack and active material loss;the tight protective layer formed on the surface of the material particles can inhibit the electrolyte decomposition,CEI film dissolution/formation and HF etching.The chelation between polar hydroxyl-OH group of GG and Mn2+prevents the Mn2+ dissolution,which helps to suppress the layered-to-spinel phase transformation.These are the reasons why water-soluble guar gum binder can mitigate the capacity and voltage decay of Li-rich materials.(3)Electrochemical performance improvement of Li-rich oxides material by integrated spinel phase and its mechanism study.We synthesized a series of integrated Li-rich/spinel(Li1.2Ni0.122Co0.12Mn0.56O2/LiNi0.5Mn1.5O4)composites,and compared the electrochemical performance with pure Li-rich Li1.2Ni0.12Co0.12Mn0.56O2(LNMC),spinel LiNi0.5Mn1.5O4(LNMO)and mechanically mixed LNMC/LNMO composites.The results show that the integrated Li-rich/spinel composite exhibits excellent cyclability,initial Coulomb efficiency,rate capability and energy density.Further mechanism analysis shows that the integrated spinel can reduce the Li+/Ni2+disordering of the Li-rich material,weaken the redox reaction of Mn ions during charge-discharge process,and inhibit the phase transformation from layered structure to LixMn2O4-like spinel(4)In situ transmission electron microscope(TEM)study of pseudo-photocatalytic water splitting mechanism of TiO2 nanorods.We used a low-dose electron beam as the "light source" and a high-concentration aqueous Li-ion electrolyte,21 M LiTFSI((LiN(SO2CF3)2),as electrolyte to solve the instability problem of aqueous solution under the electron beam.The photocatalytic behavior of TiO2 nanorods was investigated using in situ liquid cell TEM.Results show that under low electron dose,photo-catalytic behavior can only be observed at {110} facet;under high electron dose,electron beam irradiation can induce bubble generation both in the bulk solution and on TiO2 surface.The real time observation of facet-dependent bubble generation behavior is acribed to the pseudo-photocatalytic hydrogen evolution.The first-principle calculation shows that the TiO2 {110} facet on side surface has the lowest work function,stronger interaction with water molecules comparing with{111},{001} on tip surface,which contribute to the water reduction reaction on the{110} facet surface occurring under low electron dose.The "photo-generated" holes tend to concentrate on the {111},{001} surface,but the water oxidation reaction is suppressed due to the short diffusion distance of holes,and lack of charge transfer channel on {111},{001} facets surface.The research results in this thesis have deepened the understanding of interfacial properties of transition metal oxides(Li-rich oxides,TiO2)as Li-ion batteries and photocatalytic electrode materials,which is valuable for development of new electrode materials and energy systems.