| Sodium-ion batteries?SIBs?have great potential in the field of large-scale energy storage by virtue of its similar"rocking chair"sodium storage mechanism to lithium ion batteries?LIBs?,as well as the advantages of abundant and cheap resources.Designing advanced cathode materials with high capacity and cycling stability is critical for high-performance SIBs.Among all kinds of cathode materials,P2 type layered transition metal oxides NaxMe O2 is one of the most attractive research fields for SIBs owing to the advantages of low price,high energy density,controllable composition,as well as the fast Na-ion diffusion in P2 type structure.However,their practical application is hampered by the rapid capacity decay as a result of the multiple irreversible phase transitions and the volume expansion during operation due to the larger radius of Na+?1.02?for Na+and 0.76?for Li+?.Therefore,suppressing the expansion or contraction of the interlayers and irreversible phase transition of P2layered transition metal oxides during the charge/discharge processes is of great significance.This dissertation focuses on the modification of P2-type Na2/3Ni1/3Co1/3Mn1/3O2 cathode material through reasonable microstructure design and element doping,and aims at improving its cycling performance and rate capability.The main points are summarized as follows:1. A co-precipitation route was utilized to synthesize walnut-like MC2O4·n H2O?M=Ni,Co,Mn?precursor with the assistance of surfactant Tween 80.Afterwards,porous and open walnut shell-like Na2/3Ni1/3Co?1/3-x?Mn1/3AlxO2?x=0,0.03,0.06?cathode materials were successfully prepared through mixing the MC2O4·n H2O precursor with Na HCO3 and Al?NO3?3·9H2O in a certain proportion,and following subsequent calcination.The Tween 80 micelles formed in the solution served as soft templates to adsorb the oxalate ions on the micelle surface,and finally templated the formation of walnut-like MC2O4·n H2O?M=Ni,Co,Mn?precursor.The porous and open walnut shell-like structure of the final product,which is beneficial for accommodation of the volume change during charge/discharge processes,was probably resulted from the decomposition of the oxalate precursor and the stress of high temperature calcining.Due to the supression of the multiple phase transitions and the representation of pseudocapacitive sodium storage mechanism by Al-doping,the Al-doped Na2/3Ni1/3Co?1/3-x?Mn1/3AlxO2samples exhibit improved reversibility and enhanced rate capability.As the result of the synergy of morphology design and Al-doping,the resulted electrode material exhibits significantly improved electrochemical performance.In particular,the Na2/3Ni1/3Co?1/3-x?Mn1/3AlxO2cathode material with x=0.03 exhibits more stable cycle performance?with capacity retention of 76.6%after150 cycles at 0.1 C?and better rate performance(74.7 m Ah g-1 at 10 C).2. Based on the solvent effect,a co-precipitation route in ethanol-water mixed solvent was utilized to synthesize the MC2O4·n H2O?M=Ni,Co,Mn?microrods precursor.After the microrods precursor was mixed with Na HCO3 in a certain proportion and sintered in air under high temperature,one-dimension?1D?Na2/3Ni1/3Co1/3Mn1/3O2 microrods with about 2?m of diameter and 10?m of length were prepared successfully.The ethanol added in the aqueous solution affected the relative kinetics of the reaction between nickel,cobalt,and manganese acetate with oxalic acid,thus adjusting the precursor's morphology.As comparison,the Na2/3Ni1/3Co1/3Mn1/3O2 irregular microparticles have also prepared in water system.The one-dimensional structure can provide a directional transmission path,which is conducive to the transmission of electrons and ions,and the porous structure can adapt to the volume change of the electrode material during charge/discharge processes.Thus the Na2/3Ni1/3Co1/3Mn1/3O2 microrods exhibits better cycle performance and rate capability,the capacity retention of Na2/3Ni1/3Co1/3Mn1/3O2 microrods is 70.0%after115 cycles at 0.1 C,while that of Na2/3Ni1/3Co1/3Mn1/3O2microparticles is only 45.5%. |
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