| Lithium ion batteries(LIBs)with state-of-the-art technologies have been successfully adopted in a wide range of applications,such as portable electronics,electric vehicles(EVs),hybrid electric vehicles(HEVs),and grid energy storage.Among current cathode materials for lithium ion batteries,spinel LiMn2O4 maintains high market share because of its unique advantages of cost-effective and environmental friendliness.Generally,the operating voltage range for commercialized LiMn2O4 cathode material is set between 3.0-4.3 V(vs.Li+/Li).In this voltage range,spinel LiMn2O4 cathode exhibits a voltage plateau of around 4.1 V(vs.Li+/Li)originating from the Mn3.5+/Mn4+ redox reaction,However,LiMn2O4 shows another voltage plateau located at around 2.8 V(vs.Li+/Li)corresponding to the Mn3+/Mn3 5+redox reaction,and the valence of manganese ions will decrease to +3 corresponding to the formation of Li2Mn2O4.Li2Mn2O4 can deliver a high theoretical specific capacity of 285.5 mAh/g,which is nearly twice as that of LiMn2O4(148.2 mAh/g).But the valence state of manganese ion lower than +3.5 will accelerate capacity fade,which can be ascribed to these factors of Jahn-Teller effect,corrosion and dissolution of the cathode material,phase transitions and local structure inhomogeneity.In view of these problems,we study the degradation mechanism of spinel lIMn2O4 cathode material in a large voltage range.On this basis,we synthesized a new manganese-based cathode material with significantly improved cycling stability by sacrificing part specific capacity.The research contents are as follows:1.Studying and understanding the physical and chemical changes in electrode materials during cycling are important to achieve high-performance rechargeable batteries.Operando X-ray diffraction(XRD),as an efficient analytic method,has been developed to probe the phase transitions and changes of crystal structure in electrodes.Firstly,we designed an operando XRD device equipped with beryllium window according to the structural characteristics of the X-ray diffractometer.This device can prevent beryllium from taking part in side reaction at high voltage of 4.1 V(vs.Li+/Li),which provides technical support for the follow-up study on the reaction mechanism of cathode materials for LIBs.2.Understanding reaction mechanisms of electrode materials is of central importance for development of advanced batteries.LiMn2O4 cathode has a voltage plateau at around 2.8 V(vs.Li+/Li),which can give extra capacity for Li storage but suffer from severe capacity degradation.Herein,operando X-ray diffraction was used to investigate the structural evolution and degradation mechanisms of LiMn2O4 in different voltage ranges.In 3.0-4.3 V(vs.Li/Li),the LiMn2O4 cathode displays low capacity but good cycling stability,and the charge/discharge processes are associated with the reversible extraction/insertion of Li+ from LixMn2O4(O≤x≤1).In 1.4-4.4 V(vs.Li+/Li),a high capacity of more than 200 mAh/g could be obtained,but the capacity rapidly decays with cycles.The voltage plateau at around 2.8 V(vs.Li+/Li)is related to the phase transformation from cubic LiMn2O4 to tetragonal Li2Mn2O4,which accounts for the formation of cracks as well as the performance degradation.3.The search for new materials that could improve the energy density of LIBs is one of most challenging issues.Manganese has been widely concerned by scientific researchers for its abundant resources,environmental friendliness and various valence states.However,the use of Mn3+/Mn3 5+redox couple is limited by the phase transition between cubic LiMn2O4 and tetragonal Li2Mn2O4.Based on the previous results,we synthesized a new manganese-based cathode material for LIBs:Li2Mn2AlO6.The replacement of Mn ion by A1 ion can improve the structural stability.The Li2Mn2A1O6 electrode delivers a high initial discharge capacity of 192.6 mAh/g at 20 mA/g in the first cycle,and shows good capacity retention of 63.58%after 165 cycles at 200 mA/g. |
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