| In recent years,high nickel ternary positive electrode materials have been considered a promising positive electrode material,which can effectively satisfy the demand for electric vehicle range.However,with the increase of nickel content,high nickel positive electrode materials have serious capacity degradation and performance degradation problems.Therefore,the modification of high nickel cathode materials is crucial,and among numerous modification methods,the oxide surface coating has been proven to play a positive role.However,a single metal oxide coating layer,due to its single effect,cannot effectively isolate side reactions and improve the transfer rate of ions and electrons.Therefore,we adopt a multi metal oxide coating method,which can simultaneously meet the above requirements and has the characteristics of stable chemical properties and structure,high ion and electron conductivity.Compared to the traditional approach of obtaining a coating layer from hydroxide sintering,we have adopted a new precursor topology conversion strategy,which starts from the multi-element alloy nano coating and forms a multi-metal oxide coating layer through sintering.The preparation of alloy nano coating uses active potassium and sodium alloy as a reducing agent at room temperature,and with the assistance of high-speed shear,multiple nano alloys are rapidly synthesized from metal salt solution as the precursor of the coating layer.For coating modification,the use of different quantities and types of metals,coating amounts,and sintering conditions in the multi metal oxide coating layer has a significant impact on the performance of positive electrode materials.The specific research content and results are as follows:(1)First of all,the same three transition metals Ni,Co and Mn with the composition of the ternary cathode material are selected as the coating metal.The electrochemical performance of modified NCM532 is investigated by using different coating amounts.Finally,it is found that when the coating amount is 5wt% have the best electrochemical performance.After 100 cycles of charge-discharge test,the capacity of NCM532 is 110.58 m Ah/g and the capacity of NCM532@NCM 5% is 138.69 m Ah/g,which is 1.25 times higher than NCM532.The capacity of NCM532 is only 13.37 m Ah/g at the current density of 5C in the magnification performance test,while NCM532@NCM 5% is 72.35 m Ah/g,which is 5.41 times than NCM532.Performance improvement without adding other metals also eliminates the influence of the types of coating metals,which demonstrates that the method we adopted played a positive role in coating modification.(2)Next,La,Zr,Hf,Zn are selected as the coating metal,which includes the combination of rare earth metal elements and transition metal elements.The electrochemical performance of modified NCM532 is adjusted by different sintering temperatures.Comparatively speaking,when sinter at 850 ℃,the modified NCM532 have the best electrochemical performance.After 100 charge-discharge cycles,the capacity of modified NCM532 is 145.09 m Ah/g,and the capacity retention rate is 95.99%.At this time,the capacity of NCM532 is only 116.38 m Ah/g,and the capacity retention rate is 90.79%.According to the magnification performance test,the capacity of NCM532 is only 13.4m Ah/g at 5 C,while the capacity of modified NCM532 at 5 C is 56.4 m Ah/g,which is 4.2times higher.The improvement of performance is conducive to the application of commercial batteries in high power state and the improvement of service life.(3)Finally,three all rare earth metals La,Ho and Tm are selected as the coating metal.The effect of different sintering conditions on the electrochemical performance of modified NCM811 is investigated by controlling the sintering temperature and sintering time.It has the best performance when sintering at 750 ℃ for 15 h.After 100 charge-discharge cycles,the capacity of modified product is 1.35 times higher than NCM811.After 200 cycles,the capacity of modified NCM811 is 1.56 times higher than NCM811.It can be seen that the capacity retention rates are significantly improved even under longer cycles.The capacity of modified NCM811 is 3.2 times higher than NCM811 at the current density of 5 C in the magnification performance test.,and even at 10 C,the modified NCM811 still have the capacity of 61.9 m Ah/g.It can be seen that all rare earth metal oxides have significant modification effect on NCM811,which also provides a new idea for the material selection of the cathode material coating. |
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