Synthesis And Modification Of O3 Type Layered Metal Oxide Cathode Materials For Sodium-ion Battery

Layered transition metal oxide cathode materials for sodium ion batteries have attracted wide attention,because of its simple structure,easy synthesis,high operational potential and feasibility of commercial production.Lyered O3-type Na Mn_0.5Fe_0.25Ni_0.25O₂（MFN）cathode material for sodium ion batteries has been favored because of its high theoretical capacity.However,the irreversible phase transition,during the process of sodium ion incalation/decalation,resulting in the instability of the layered structure has been an issue limiting its commercial maximization.In this paper,the electrochemical performance of MFN material is improved by optimizing the synthesis parameters and ion substituting.Firstly,the MFN cathode materials were synthesized by spray assisted nano ball mill high temperature solid phase method.The optimum process parameters were obtained by comparing the influence of ball milling time and calcination holding time.When the speed of the ball mill is 2500r/min,the best milling time is 40min.Under this condition,the particles distribution is the most uniform and the average particle size is the smallest（D50=220 nm）.When it is calcined at 900℃for 15 hours under air atmosphere,the MFN sample has the best electrochemical performance.The results show that initial discharge capacity of the MFN is145.2 m Ahg^-1at 0.1C during 1.75-4.4V,and its capacity retention is 54.0%at 1 C after 100cycles.Varied amount of strontium ion was adopted to improve the discharge capacity and rate performance of the MFN by part substituting Na⁺.The results of XRD structure refinement and EDS-mapping confirmed that Sr²⁺entered the sodium layer.The electrochemical test results show that the initial specific discharge capacity,capacity retention and rate performance of all Sr doped MFN samples are higher than those of the original MFN.When x=0.01,the electrochemical performance of the MFN-Sr1%is the best.The initial discharge capacity of the MFN-Sr1%are 158.8m Ahg-1 at 0.1C and 82.1m Ahg-1 at 5C,respectively.And its capacity retention is 73.1%after 100 cycles at 1C.The substitution of Sr²⁺significantly improves the electrochemical performance of the MFN.It may be because the radius of Sr²⁺（1.12（?））is larger than that of Na⁺（0.98（?））,which widens the diffusion channel of Na⁺to obtain the rapid Na⁺diffusion,thus improving the electrochemical performance of the MFN.In order to further improve the rate performance and cycle stability,the MFN was modified by Ti and Sr co-substitution.Na Mn_0.48Fe_0.25Ni_0.25Ti_0.02O₂（MFN-Ti）and Na_0.99Sr_0.01Mn_0.48Fe_0.25Ni_0.25Ti_0.02O₂（MFN-Ti Sr）cathode materials were synthesized by spray assisted nano ball mill high temperature solid phase method.It confirmed that Sr entered the sodium layer and Ti entered the transition metal layer by the results of the XRD structure refinement and EDS-mapping.The specific discharge capacity of the MFN-Ti Sr sample is up to 131.2 m Ahg^-1at 1C with a retention 89.5%after 100 cycles.It is a 35.5%increase over that of the MFN.In addition,the specific discharge capacity of the MFN-Ti Sr is 91.9m Ahg^-1at 5C,which is 45.1m Ahg^-1higher than MFN of the original sample.It is due to the synergistic effect of Ti and Sr double substitution,where Sr replaces Na,enlarges the diffusion channel of sodium ions to improve the movement rate of Na⁺.The replacement of Mn by Ti makes the layer spacing of transition metal narrow,which not only plays the role of supporting the structure,but also increases the stability of the material structure and further widens the sodium ion channel because of the higher bond energy and shorter bond length of Ti-O bond,thus improving the specific discharge capacity and rate performance of the material.