Research On Preparation And Properties Of Manganese(Iron)-based Sodium Ion Battery Cathode Materials

The increasing demand for renewable energy such as solar,wind and tidal energy has promoted the development of electrical energy storage systems.The rechargeable lithium-ion battery has become the most mature electric energy storage system due to its excellent electrochemical performance.However,the scarcity and uneven distribution of lithium resources restrict the development of low-cost lithium-ion batteries.Sodium resources and iron and manganese resources are widely distributed and abundant in nature.Therefore,ferromanganese-based sodium ion cathode material has become one of the most promising cathode materials due to its low cost and high performance.Since the current development of cathode materials for sodium-ion batteries is not mature enough,we have studied two most promising iron/manganese-based cathode materials Na₂Fe PO₄F and Na_2/3Ni_1/3Mn_2/3O₂ in this paper.At the same time,we explored the effects of different modification methods on the low-cost and high-performance Na₂Fe PO₄F and Na_2/3Ni_1/3Mn_2/3O₂ cathode materials.(1)we successfully synthesized Na₂Fe PO₄F/biocarbon nanocomposite hollow microspheres as cathodes for sodium-ion batteries through self-assembly of yeast cell biotemplate and sol–gel technology.The carbon coating on nanoparticle surface with a mesoporous structure enhances electron diffusion into Na₂Fe PO₄F crystal particles.The improved electrochemical performance of Na₂Fe PO₄F/biocarbon nanocomposites is attributed to the larger electrode–electrolyte contact area and more active sites for Na⁺on the surface of hollow microspheres compared with those of Na₂Fe PO₄F/C.The Na₂Fe PO₄F/biocarbon nanocomposite exhibits a high initial discharge capacity of 114.3 m Ah g^-1 at 0.1 C,long-cycle stability with a capacity retention of 74.3%after 500 cycles at 5 C,and excellent rate capability(70.2 m Ah g^-1 at 5 C)compared with Na₂Fe PO₄F/C.This novel nanocomposite hollow microsphere structure is suitable for improving the property of other cathode materials for high-power batteries.(2)An Al-doped Na₂Fe PO₄F/mesoporous carbon nanonetwork derived from an metal–organic framework(MOF)material,was synthesized using a simple solid-state reaction method and evaluated as a promising cathode material for sodium-ion batteries.The MIL-53(Al)derived mesoporous carbon nanonetwork coating can enhance electron conductivity in Na₂Fe PO₄F cathode materials.The doping derived from MIL-53(Al)caused no damage to the crystal structure of the material,and Fe²⁺site was replaced by Al³⁺successfully.Therefore,the Al-doped Na₂Fe PO₄F/MOF-C composites exhibit substantial improvements in electrochemical performance in terms of rate capability and cycle performance.When current density increased from 0.1 C to 5C,the specific capacity of Na₂Fe PO₄F/MOF-C only decreased from 115.2 m Ah g^-1 to 63.2 m Ah g^-1;thus,the composite presented outstanding rate capability.Moreover,an excellent capacity retention of 70.3%was maintained from the initial value of 70.4 m Ah g^-1 to 49.5 m Ah g^-1 after the 500th cycle at 5 C.This study provides insights into designing high-performance cathode materials by improving structural stability and the electrical conductivity induced by MOF-derived carbon coating and metal doping.(3)The precursor of electrode material was deposited on the surface of biological cell template by chemical precipitation method to prepare Na_2/3Ni_1/3Mn_2/3O₂ cathode material with hollow spherical morphology.The interaction between particles inhibits the crystal growth during the crystal growth.It was found that the diameter of the crystal particles in the spherical Na_2/3Ni_1/3Mn_2/3O₂ cathode material decreased to about 500 nm.Due to the pyrolysis of the yeast template,a positive electrode material with a porous structure and hollow microsphere structure was formed.The smaller the crystal particle diameter,the shorter the transmission distance of sodium ions,and the higher the ion conductivity in the cathode material.The morphology of hollow microspheres increases the wettability of electrolyte on the surface of cathode materials and reduces the ion transmission impedance on the surface of particles.Therefore,the electrochemical performance of the hollow spherical NNMO/NTP-1 sample has been improved.At the current density of 0.1C,the discharge specific capacity of the NNMO/NTP-1 sample reached 163.4 m Ah g^-1,which was close to the theoretical specific capacity of the Na_2/3Ni_1/3Mn_2/3O₂ cathode material.At a high current density of 5 C,the discharge specific capacity of the NNMO/NTP-1 sample reached 98.7 m Ah g^-1.The specific capacity retention of the sample after 100 cycles is 78.3%.The synthesis method of spherical cathode materials formed by yeast template in this paper can be widely applied to the synthesis of other electrode materials.(4)The Na_2/3Ni_1/3Mn_2/3O₂ cathode material was synthesized by the sol-gel method,and the cathode material was modified by surface modification of the mesoporous alumina material derived from the aluminum-based metal organic framework material.The mesoporous alumina coating layer can not only suppress the P2-O2 phase transition at high voltage,but also reduce the dissolution of manganese ions into the electrolyte at low voltage.Compared with the ordinary alumina coating layer,the MOF-derived alumina coating layer has a mesoporous structure,which can effectively reduce the side reactions in the charge and discharge process and provide a transmission channel for the transmission of sodium ions.The discharge specific capacity of the MOF-derived alumina coating layer NNMO/MP-Al₂O₃ sample reached 198.2 m Ah g^-1 at a voltage range of 1.5-4.3V and a current density of 0.1C.The discharge specific capacity of the NNMO/MP-Al₂O₃sample reached 110.3 m Ah g^-1 at high current density.After 50 charge and discharge cycles,the specific capacity retention rate of the NNMO/MP-Al₂O₃ sample reached 87.3%.In addition,the mesoporous alumina coating layer can improve the electrolyte wettability on the surface of the Na_2/3Ni_1/3Mn_2/3O₂ cathode material and reduce the transmission resistance of sodium ions at the electrode material interface.The metal-organic framework-derived mesoporous metal oxide coating modification method used in this experiment can be widely used in the synthesis of other electrode materials.(5)The surface modification of P2-type Na_2/3Ni_1/3Mn_2/3O₂(NNMO)with the thin film of solid-state electrolyte of Na Ti₂(PO₄)₃(NTP)using a facile solution-based method is investigated.The coated Na_2/3Ni_1/3Mn_2/3O₂cathode(NNMO/NTP)exhibits enhanced structural and electrochemical stability.This kind of coating layer is uniformly dispersed on the surface of Na_2/3Ni_1/3Mn_2/3O₂.The NASICON-type NTP coating layer with excellent ion conductivity effectively prevents the direct contacting of cathode and electrolyte,guarantees stable phase interfaces,and accelerates Na⁺diffusion at the interface.NTP coating induces the partial doping of Ti⁴⁺into the crystal structure which enlarges the interlayer spacing and improves the Na⁺diffusion and rate capability.In addition,the NTP coating layer suppresses particle exfoliation and the dissolution of Mn²⁺over-prolonged cycling.Thus,the NTP-coated NNMO exhibits excellent cycling stability and outstanding rate capability(111.5 m Ah g^-1 of the initial capacity at a high rate of 5 C compared with only 52.4 m Ah g^-1 for the bare electrode).More meaningfully,this coating strategy could have a remarkable influence upon further constructing other cathode materials for Na-ion batteries.(6)The two-phase composite cathode material of tunnel and layered material for Na_0.61Ni_0.25Mn_0.75O₂ was synthesized for the first time by the sol-gel method.The two-phase composite Na_0.61Ni_0.25Mn_0.75O₂ cathode material can better combine the advantages of tunnel-type and layered materials and inhibit the phase transition of high voltage and the volume change of crystal structure.In the voltage ranges of 1.5-4.3V and 2.0-4.0V,the Na_0.61Ni_0.25Mn_0.75O₂ sample showed good electrochemical performance.Compared with other samples,the Na_0.61Ni_0.25Mn_0.75O₂sample shows excellent rate performance and cycle performance.The modification method of the two-phase composite material used in this experiment provides ideas for the optimization and modification of other cathode materials.