| Layered transition metal oxides,especially P2-Na0.67MnO2,are considered as the most promising cathode materials for Sodium-ion batteries(SIBs),due to their high capacity,high working potential,fast mobility of Na+and low cost.Moreover,anionicredox reaction,widely observed in P2-Na0.67MnyTM1-yO2,can provide significant extra capacity for enhancing energy density.However,Jahn-Teller effect of Mn3+(t2g3-eg1)would cause capacity degradation through complex phase transition.And the challenges of anionic-redox reaction,such as structure collapse and voltage decay,limit the practical application for excellent cycle stability with high energy density.In addition,active lithium/sodium ions loss,due to the formation of solid electrolyte interface(SEI)with decomposition of electrolyte for sodium ion batteries(SIBs)and lithium-ion batteries(LIBs),causes a low coulombic efficiency of initial cycle process.The unstable SEI and dead lithium formed during cycling process also consume active lithium/sodium ions.For LIBs,the loss of active lithium can be compensated by pre-lithiation through adding extra lithium to the positive/negative electrode in advance.Li2NiO2 was successfully used as positive electrode additive of LiCoO2 to replenish lithium.However,the effects of Li2NiO2 structure changes and intermediate products on the battery performance have not been thoroughly studied.Therefore,it is necessary to further study the charging and discharging mechanism and structure change of Li2NiO2,to provide guidance for the selection of pre-lithiation agent in actual battery system.In order to solve the above-mentioned challenges,this thesis systematically studied the structure,electrochemical performance,charging and discharging mechanism of layered sodium manganese oxides and Li2NiO2.Through in-situ X-ray diffraction(XRD),ex-situ23Na nuclear magnetic resonance(NMR),neutron diffraction(ND)and X-ray photon-electron spectroscopy(XPS),we investigated the electrochemical properties and structure evolution of Al-doping sodium manganese oxides(Na0.67Al0.1Mn0.9O2)cathode material under controlled cooling rates.The results indicated that P2 pure phase formed under natural cooling and quenching process.But the content of transition metal(TM)vacancies in the natural cooling material(NAMO-na)was 7.8%,while the quenching material(NAMO-qu)was 1.6%.Natural cooling materials with a higher content of TM vacancies could trigger the anionic-redox reaction easily.Meanwhile,transition metal vacancies could suppress the stacking faults as buffer media during cycling process,thus NAMO-na performed better cycle stability at voltage ranges 2.0-4.5 V and 2.0-4.0 V.In contrast,NAMO-qu with negligible content of TM vacancy could provide～185 mAh g-1 during initial cycling process,higher than that of NAMO-na(～135 mAh g-1),and show better rate performance due to the larger space distance of sodium layer,which could facilitate migration of Na+.However,the stacking faults and P2’ phase which are observed at the end of charge and discharge process,respectively,cause poor cycle stability of NAMOqu electrode.Structure evolution and electrochemical performance of Li2NiO2 at various voltage range were investigated by in-situ XRD,ex-situ TEM and so on.An obvious voltage plateau will appear when Li2NiO2 was discharged below 2.3 V.XRD and TEM results proved that the Immm structure could maintain during charge process,but longrange ordered structure disappeared.During discharge process,the layer structure of LiNiO2(R-3m)and 1T-Li2NiO2(P-3ml)appeared at 3.0 V and 1.5 V with the intercalation of Li+,repectively.The structure evolution could cause capacity fading in subsequent cycles.Modification of Li2NiO2 by means of carbon coating through ballmilling could improve structure stability in the voltage range of 3-4.3 V while maintaining a small interface impedance.In addition,the carbon coating method with boll-milling could enhance the diffusion coefficient of lithium ions during the first cycle. |
PDF Full Text Request