| Among different types of cathode materials,layered metal oxide materials have dominated the commercial lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs)markete owing to their high theoretical specific capacity and operating plateau,as well as the relatively stable 2D Li+ diffusion framework.As we all know,the cathode materials are deemed as the most critical component limiting the cost and the capacity of the full cell,while the high cost of Co element makes LiCoO2(LCO)unable to meet the new requirements of large-scale energy storage stations and electric vehicles.Compared with LCO,LiNixCoyMn1-x-yO2(NCM)has a lower cost and higher energy density in LIBs,which has become one of the mainstream cathode materials applied in electric vehicles.Besides,Mn-based layered oxides such as P2-Na0.67MnO2 are considered as the most cost-effective cathodes in SIBs,and thus regarded as ideal cathodes for static energy storage stations.However,Mn-based and Ni-rich NCM cathodes still face many challenges such as short lifespan and poor rate performance.In this dissertation,these two promising layered oxide cathodes have been systematically studied from four aspects,i.e.the bulk structure,interface chemistry,electrochemical-mechanical stability,and decay mechanism,aiming to further improve their electrochemical performance in different energy storage systems,namely traditional LIBs,low-cost sodium-ion batteries(SIBs),and high-safety all-solid-state batteries(ASSBs).The multiple phase transitions of P2-type Na0.67MnO2(NMO)cathodes during cycling are considered as the main cause for particle crushing,structural defects,and even amorphization of materials.To overcome this issue,the most naturally abundant metal element,Al3+,was introduced into the transition metal(TM)layers to partially replace the Jahn-Teller active Mn3+ ions,thereby suppressing the multiple phase transitions of material.Ex-situ 23Na solid-state NMR(SS NMR)and synchrotron in situ X-ray diffraction(XRD)results provide evidence of the advantages of the delayed P2P2’ phase transition and the suppressed biphasic structural transformations during Na+(de)intercalation when Al3+ was introduced into the TM layers.Therefore,Al-doping samples exhibit smoother charge-discharge profiles and better cyclability.In addition,the strong Al-O bonds lead to larger Na-layer spacing thus facilitating Na+mobility and stabilizing the structure during electrode preparing processes,ensuing an outstanding rate capability.Given the low actual specific capacity of P2-type NMO,we propose a strategy to prepare the vacancy-free Mn-based oxide materials by the liquid N2 quenching treatment.The experimental results show that the simple liquid N2 quenching treatment is an effective method to suppress the vacancies in TM layer,thereby increasing the redox centers of Mn4+/Mn3+,which significantly improves the capacity of Na0.67MnO2 at the upper cut-off voltage of 4.0 V.SS NMR,galvanostatic intermittent titration technique(GITT)and direct-current polarization measurements demonstrate that the liquid N2 quenched sample exhibits the higher Na+mobility and electronic conductivity.However,prepared Na0.67MnO2 also exhibits significant orthorhombic distortion,which damages its cycling stability.Therefore,we further investigate the effect of aliovalent ions doping on the material structure and electrochemical performance.The excellent comprehensive electrochemical performance of Nao.67Al0.1Fe0.05Mn0.85O2 material through liquid N2 quenching treatment and rational element doping further proves that this synergistic strategy is a general guide for the development of cathode materials with high capacity,high power,and stable cycling,especially for Mn-based layered oxide cathodes applied in SIBs.In addition,we further explored the possibility of P2-type layered oxide as the surface modification layer for the Ni-rich cathodes.The electrochemical test results show that P2-type layered oxide coating layer can effectively improve the cyclability and rate capability of LiNi0.8Co0.1Mn0.1O2(NCM811)cathode in LIBs.P2-type NaxNi0.33Mn0.67O2(NNMO)will undergo a spontaneous Na+-Li+exchange reaction during the cell storing period,and thus transform into O2-type LixNi0.33Mn0.67O2(LNMO).X-ray photoelectron spectroscopy(XPS)technique proves that a thin and dense CEI film was formed on the surface of NNMO coating,suppressing the oxidative decomposition of electrolyte.Meanwhile,high-resolution transmission electron microscopy(HRTEM)demonstrates that the introduction of NNMO coating can effectively suppress the formation of surface reconstruction layer during prolonged cycles,enabling the effective maintenance of the Li+ions diffusion channel and interfacial impedance of NCM811 electrode.On the other hand,the application of layered cathode materials in all-solid-state batteries(ASSBs)is also attracting lots of attention.In response to the electrochemical performance of Ni-rich cathodes in ASSBs being much worse than that of LIBs,we perform a comparative study of three kinds of NCM811 materials in sulfide ASSBs,i.e.large-size polycrystalline,small-size polycrystalline,and single-crystal NCM811.The experimental results show that the electrochemical performance of the single-crystal NCM811 is significantly better than that of the polycrystalline NCM811 in the voltage range of 2.85-4.35 V vs.Li+/Li.For example,the single-crystal material exhibits high a initial capacity of 187 mAh g-1 at 0.1C,stable cycling over 100 cycles,and outstanding rate performance.Furthermore,variable temperature SS NMR and focused ion beam scanning electron microscopy(FIB-SEM)techniques demonstrate that the main reason for the worse electrochemical performance of polycrystalline NCM811 in ASSBs than single-crystal NCM811 is not the Li+mobility difference in the primary grains but the different microstructure of two materials.The secondary particle of polycrystalline NCM811 would microcrack during the cold pressing processes,which extends with the progress of electrochemical cycling,and thus breaks the ionic and the electronic transport channels.To further understand the performance degradation mechanism of Ni-rich electrodes in sulfide ASSBs,we successfully assembled the operando XRD ASSBs with reliable electrochemical performance based on the NCM811 and Li4Ti5O12(LTO)as cathode and anode,respectively.Combined with a series of characterization methods,we carefully study the surface-to-bulk degradation mechanism of Ni-rich layered oxide cathodes in sulfide ASSBs.Operando XRD detected the formation of sluggish phase in ASSBs during the first charging,while a similar phenomenon only appeared in LIBs after hundreds of cycles.The experimental and computational results indicate that this premature fatigue behavior of NCM811 in ASSBs mainly stems from the severe side reactions on the interface between the electrode and sulfide electrolyte rather than the poorly physical contact and kinetic limitations.From this novel perspective,we demonstrate that the enhanced performance employing the surface coating on the nickel-rich materials is attributed not only to the suppression of interfacial side reactions but also to the elimination of sluggish phase. |
