Study On Preparation And Properities Of High Voltage Cathode Materials For Sodium Ion Batteries

It is well known that within the safe working range of electrolyte,the higher working voltage of cathode material,the greater energy density of the battery.In the current situation that the discharge capacity of cathode material for sodium ion battery is generally low,increasing the working voltage of cathode material is an effective way to increase the energy density of the battery.This is also the main research focus of cathode material for high voltage sodium ion battery currently.Among many cathode materials,polyanion compounds exhibit higher working potential than oxide materials due to the induction of polyanion.Na₂Fe₂?SO₄?₃ is an ideal high voltage cathode material because of its higher electronegativity and stronger induction effect,so it has a higher working voltage?3.8 V?and its own structure has three-dimensional sodium ion channels which results in higher ionic conductivity.However,Na₂Fe₂?SO₄?₃ has some shortcomings such as low electronic conductivity and sodium ion diffusion coefficient,which greatly limits its practical application.Na₄Co₃?PO₄?₂P₂O₇,as another high voltage cathode material,has a high theoretical capacity(170 mA h g^-1)and a working voltage of over 4 V,which is a kind of high energy density battery material.However,Na₄Co₃?PO₄?₂P₂O₇ has the disadvantage of low electronic conductivity which is common to all phosphate compounds,leading to its poor electrochemical performance.In this paper,Na₂Fe₂?SO₄?₃ and Na₄Co₃?PO₄?₂P₂O₇ are studied,which are typical cathode materials of polyanionic high voltage sodium ion batteries.In view of the above problems,several modification methods are adopted to enhance its electrochemical performance.The detailed research contents are as follows:?1?Graphene coated Na_2+2x+2x Fe_2-x?SO₄?₃@rGO electrode materials were prepared by spray drying method.Graphene coated Na_2+2xFe_2-x?SO₄?₃@rGO microsphere composites were prepared by spray drying method.The high conductive network and strong 3D framework of graphene not only enhance the electronic conductivity,but also enhance the structural stability of the material in the cycling process,so it has more excellent electrochemical properties.The performance tests show that the discharge capacities at current densities of 0.1 C,0.5 C,1 C,5 C,10 C,20 C and 40 C are 98.6,91.3,90.1,87.5,84.5,82.4 and81.1 mA h g^-1,respectively.When the current densities continue to increase to60 C and 80 C,the high reversible specific capacities of 78.4 and 72.2 mA h g^-1can still be obtained.When the discharge rate returns to 0.1 C again,the discharge specific capacity can still reach 97 mA h g^-1 and the capacity retention can reach98.4%.Under the current density of 30 C,the 80.8%capacity retention can be achieved after 2000 cycles,exhibiting excellent cycle performance.?2?The bulk Al³⁺doping and carbon nanotubes was used to modify high voltage cathode material of Na₄Co₃?PO₄?₂P₂O₇.Na_4-xCo_3-xAl_x?PO₄?₂P₂O₇/CNT composites with different Al³⁺content?x=0,0.05,0.1,0.15 and 0.2?were prepared by spray drying method.XRD analysis of Na_4-xCo_3-xAl_x?PO₄?₂P₂O₇/CNT shows that the doping of Al³⁺can appropriately increase the cell volume of Na₄Co₃?PO₄?₂P₂O₇ and improve its structural stability at high voltage.Additionally,proper Al³⁺doping can also improve the intrinsic electronic conductivity of Na₄Co₃?PO₄?₂P₂O₇.Therefore,the electrochemical performance of Na₄Co₃?PO₄?₂P₂O₇ can be significantly improved by doping appropriate amount of Al³⁺.For example,the discharge specific capacity of Na_3.85Co_2.85Al_0.15?PO₄?₂P₂O₇/CNT decreases from 99.5 mA h g^-1 to 73.4 mA h g^-1 with a capacity retention of 73.8%when the rate increases from 0.5 C to 50 C,showing excellent high rate performance.And even at a high rate of 30 C,a discharge capacity of 62.2 mA h g^-1 can be obtained after ultra-long 8000 cycles,with a capacity retention of 82.7%,exhibiting excellent cycling stability.When Al0.15-NCPP/CNT and commercial hard carbon?HC?are assembled into a full cell,it has a working voltage of over 4 V,a capacity of 70.6 mA h g^-1 can be obtained at 30 C,and a capacity retention of up to 95%after 200 cycles at 1 C,showing excellent electrochemical performance.