Synthesis Of NASICON-structured Na_XMTi(PO₄)₃ Electrode Materials For Sodium Ion Batteries

Recently,lithium-ion batteries(LIBs)have revolutionized the energy system with their wide applications in energy storage and electric vehicles.With the rapidly growing demands for LIBs,the poor reserves of lithium(?0.0065%)in the earth's crust push up prices of lithium salts and restrict the massive application of LIBs.Sodium,another member of alkali metal family,is regarded as advantageous alternative in place of lithium for rechargeable batteries,because of the large reserves(?2.64%)of sodium in the earth's crust and high energy density of sodium ion batteries(SIBs).However,there are great challenges for developing suitable electrode materials for the storage of Na⁺because of bigger ionic size(1.02?)of Na⁺than Li⁺(0.76?),which will induce horrible structure collapse and sluggish kinetics in electrode materials.The high-performance SIBs call for advanced cathode materials with fascinating specific capacity,voltage,and lifespan.In recent years,the materials with sodium super-ionic conductor(NASICON)structure have received intensive interests due to their distinctive crystal structure,diverse components,high ionic conductivity,and good thermal stability.However,their reversible capacity needs further improvement for satisfying the practical application.In order to meet the practical requirements of SIBs cathode materials,this paper aims to synthesize NASICON-type electrode materials with high reversible capacity and high operating voltage,the specific research works are as follow:1.A simple and inexpensive sol-gel method is used to synthesize reduced graphene oxide composite Na₃Mn Ti(PO₄)₃particles,which are used as cathode materials for Na-ion batteries with good coulombic efficiency and cycle performance.The three-electron reaction is realized by adjusting the voltage window of 4.3-1.5 V,corresponding to the stepped redox of Ti³⁺/Ti⁴⁺,Mn²⁺/Mn³⁺,Mn³⁺/Mn⁴⁺.rGO will not only prevent the dissolution of Na₃Mn_0.5V_0.5Ti(PO₄)₃,but also fasten the electron and Na⁺transport,thus improving the rate capability of cyclic stability.Na₃Mn Ti(PO₄)₃/rGO provides specific capacities of 103.3,89.8,65.4 m Ah g^-1at 0.1,0.5,and 5 C,respectively,and a reversible capacity of 68.3 m Ah g^-1after 500 cycles at 1 C with capacity retention rate of 81.5%,exhibiting high rate capability and good cycle stability.The excellent electrochemical performances make the NMTP/rGO a reliable cathode material for sodium ion batteries.2.Sodium super-ionic conductor(NASICON)materials are believed as competitive cathodes for sodium-ion batteries,however,their practical application is still prohibited by the limited reversible capacity.In this study,the tri-metal NASICON structured Na₃Mn_0.5V_0.5Ti(PO₄)₃is designed to combine with reduced graphene oxide(NMVTP/rGO)as cathode material.Benefited from the multi-step redox couples of Mn,V,and Ti,NMVTP/rGO realizes high reversible capacity and wide operating voltage.The small sizes of Na₃Mn_0.5V_0.5Ti(PO₄)₃and rGO ensure the fast charge migration.The rGO inhibits the dissolution of electrode material.As a result,NMVTP/rGO exhibits large specific capacity of 284.6 m Ah g^-1at 0.1 C,high rate performance(170.2 m Ah g^-1at 1 C and 108.5 m Ah g^-1at 5 C),and good cyclic stability.This work demonstrates a new insight for designing tri-metal NASICON structured materials with high capacity for sodium-ion batteries.3.In order to improve the working voltage of the material,according to the high redox potential(4.5 V)of Cr³⁺/Cr⁴⁺,the Na₃Mn_0.5Cr_0.5Ti(PO₄)₃composite material combined with reduced graphene oxide is synthesized by the sol-gel method,which is denoted as NMCTP/rGO.By comparison,it is found that the materials synthesized at650°C can be reversible in the high voltage window of 4.65-1.5 V.When the current density is 0.1 C,the materials have a high discharge specific capacity of 215.9 m Ah g^-1and a high energy density of 541.8 Wh kg^-1.Even at the high current density of 10and 20 C,The NMCTP/rGO-650 still has good reversible capacity.After 50 cycles,the reversible capacity is 102.7 m Ah g^-1and the retention rate is 83.8%.