Research On Synthesis And Electrochemical Properties Of High-performance Spinel Lithium Manganate Cathode Materials

With the rapid development of new energy battery industry,lithium-ion batteries have been wide attentioned at home and abroad.As an important field in the new century,many countries have universally made the relative policy for supporting the large-scale industry development of lithium-ion batteries.At present,the practical use of lithium-ion batteries has been extended to many important areas such as military,aerospace,mobile communication equipments and power sources for electric vehicles,and lithium-ion batteries will demonstrate strong performance in the future.Among commercial cathod materials,LiMn₂O₄ has a broad developing trend due to abundant manganese resource,mature preparation technology,low production cost,good safety and environmental friendliness.However,LiMn₂O₄ always shows rather poor cycling performance and high-rate capability,especially the serious capacity loss at elevated-temperature.These can largely affect the large-scale industrialization application of LiMn₂O₄.In this work,the electrochemical properties of LiMn₂O₄ were significantly improved by doping with multiple elements and synthesizing special morphology.For the multiple-doping method,first of all,the Si-doped LiMn₂O₄ samples were prepared by solid-state method to overcome the problem that the existing doping schemes decreased the discharge capacity of LiMn₂O₄.Second,When the long cycling stability and discharge capacity were considered together,the Mg²⁺and Si⁴⁺co-doping scheme was carried out to use their synergistic effect to significantly enhance the electrochemical properties.Finally,the elevated-temperature performance of the co-doped sample was effectively enhanced by doping with Al³⁺ions.In addition,the?-MnOOH nanorods were prepared via a simple,low-cost and eco-friendly approach.With the obtained?-Mn OOH nanorods as manganese sources,the LiMn₂O₄ nanorods with excellent electrochemical properties were successfully obtained.The main research findings and innovation points were listed below:?1?The Si-doped LiMn₂O₄ samples were obtained by solid-state method.For the Si-doped samples,a certain amount of Si⁴⁺ions did not change the intrinsic spinel structure of LiMn₂O₄.With the increasing of Si-doping amounts,the crystal lattice constant of the LiMn_2-xSi_xO₄ samples increased and the average particle size presented a trend of gradual decrease.When cycled at 0.5 C,the optimal Si-doped LiMn₂O₄ showed134.6 mAh g^-1,which was higher than that of the undoped sample.After 100 cycles,the discharge capacity could still reach up to 114.5 mAh g^-1 with capacity retention of85.1%.At the high rate of 5.0 C,a high discharge capacity of 87.5 mAh g^-1 was obtained while the undoped sample only exhibited 33.7 mAh g^-1.These results suggested that doping the manganese sites with appropriate amount of silicon ions could not only enhance the cycling stability to a certain extent,but also avoid the discharge capacity decrease of LiMn₂O₄.?2?The Mg²⁺and Si⁴⁺co-doped LiMn₂O₄ samples with excellent electrochemical properties were successfully synthesized by a citric acid assisted sol-gel process.The Si-doping could avoid the capacity decrease of LiMn₂O₄ and improve the cycling performance to a certain extent,but the improvement could not meet the demand of the large-scale industrialization.By contrast,the Mg-doping could significantly improve the cycling stability,but the addition of Mg²⁺ions caused the reduction of Mn³⁺ions,which decreased the discharge capacity of LiMn₂O₄.When the respective advantages of Si-doping and Mg-doping were considered together,the Mg²⁺and Si⁴⁺co-doping scheme was carried out to use their synergistic effect to enhance the electrochemical properties of LiMn₂O₄.?3?To promote the practical application,the Mg²⁺and Si⁴⁺co-doped LiMn₂O₄samples were synthesized by solid-state method,and the optimal doping amount was investigated.All the co-doped samples showed the intrinsic spinel structure,and the the Mg²⁺and Si⁴⁺co-doping scheme could largely reduce the particle agglomeration to make the sample particles more uniform.More importantly,the optimal co-doped sample showed excellent cycling stability.After 100 cycles,the discharge capacity could still reach up to 113.8 mAh g^-1 with capacity retention of 93.8%.Moreover,at the higher charge-discharge rate of 5.0 C,a high discharge capacity of 92.5 mAh g^-1 was obtained.?4?When considering the poor elevated-temperature performance,the Mg²⁺and Si⁴⁺co-doped LiMn₂O₄ sample was further improved by doping with Al³⁺ions in the spinel structure.In order to make Al,Mg,Si elements mixed evenly,all the multi-doped samples were synthesized by a citric acid assisted sol-gel process.For the Mg²⁺and Si⁴⁺co-doped sample,a certain amount of Al³⁺ions did not change the spinel structure.The optimal multi-doped sample presented more uniform particle size distribution.When cycled at 55°C,the optimal multi-doped sample showed 123.6 mAh g^-1 at 0.5 C.After100 cycles,the capacity retention of this sample could reach up to 93.8%,which was much higher than that of the co-doped sample.When the rate increased to 10 C,a high discharge capacity of 82.5 mAh g^-1 could be obtained with capacity retention of 95.6%after 50 cycles at 55°C.By contrast,the co-doped sample only exhibited 43.7 mAh g^-1with lower capacity retention of 61.8%.?5?The?-MnOOH nanorods were prepared via a simple,low-cost and eco-friendly approach.With the obtained?-MnOOH nanorods as manganese sources,the LiMn₂O₄nanorods with excellent electrochemical properties were prepared by solid-state method.Compared with the reported research results,our synthesis scheme only uses KMnO₄and anhydrous alcohol?CH₃CH₂OH?as starting materials,and the usage amount of anhydrous alcohol is very small.What is more,the as-synthesized?-MnOOH nanorods have well-shaped morphology with an average diameter of 200 nm and an average length of 12?m.The resulting LiMn₂O₄ nanorods can show excellent electrochemical properties.