Synthesis And Modification Research Of LiNi_0.5Mn_1.5O₄ As High Voltage Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries as the next generation of energy-storage devices have been applied in electric vehicle,hybrid electric vehicle and portable electronic devices.It will be further applied in large energy storage grid,aerospace,communications and rail transit.It is urgent to develop a higher energy and power density,long cycling life lithium ion batteries.As for Li-ion batteries,energy density is mainly limited by the cathode material.Among all the cathode materials,spinel LiNi_0.5Mn_1.5O₄has attracted much interest owing to its high operating voltage and relatively high theoretical capacity(147 mAh g^-1).Also LiNi_0.5Mn_1.5O₄ is low cost and environment friendly with good electrochemical performances.However,LiNi_0.5Mn_1.5O₄ has two fundamental problems:decomposition of electrolyte at high voltage and serious side reactions between electrode and electrolyte,resulting in poor high rate performance and high temperature cycle performance,which limits its further development.Hence,tremendous efforts are required to improve the electrochemical performances.In this thesis,different oxygen concentrations were used to calcinate the precursors to produce the LiNi_0.5Mn_1.5O₄ cathode materials.Furthermore,Cr doping and Li₂SiO₃ composite were used to enhance the electrochemical performance of LiNi_0.5Mn_1.5O₄ cathode material.（1）Oxygen concentrations of 10%,21%,50%were used to calcinate the precursors to get the final LiNi_0.5Mn_1.5O₄ cathode materials,named as LNMO-10,LNMO-21,LNMO-50.XRD,SEM,XPS testings were performed on these three samples.And the results show that high oxygen concentration can reduce the formation of Li_xNi_1-xO impurity phase,decrease the content of Mn³⁺,get large size particles with more stable structures.The LNMO-21 shows the best electrochemical performances,the initial discharge capacity is 115.4 mAh g^-11 at 25°C,1C rate.After 200 cycles,the discharge capacity is 101.2 mAh g^-11 with a capacity retention of 87.7%.Also,the initial discharge capacity is 99.41 mAh g^-11 at 50°C,1C rate.After 200 cycles,the discharge capacity is 94.0 mAh g^-1with a capacity retention of 94.6%.（2）Spinel LiNi_0.45Cr_0.1Mn_1.45O₄synthesized by a scalable solution route combined by high temperature calcination has been investigated as cathode for ultralong-life lithium ion batteries in a wide operating temperature range.Scanning electron microscopy exhibits homogeneous micro-sized polyhedral morphology with highly exposed{100}and{111}surfaces.The most highlighted result is that LiNi_0.45Cr_0.1Mn_1.45O₄ has extremely long cycle performance and high capacity retention at various temperatures（0oC,25oC,50oC）,indicating that Cr doping is a prospective approach to enable 5 V LiNi_0.5Mn_1.5O₄-based cathode materials with excellent cycling performances for commercial applications.After 1000 cycles,the capacity retention of LiNi_0.45Cr_0.1Mn_1.45O₄ is 100.30%and82.75%at 0oC and 25oC at 1 C rate,respectively.Notably,over 350 cycles at 50oC,the capacity retention of LiNi_0.45Cr_0.1Mn_1.45O₄can maintain up to 91.49%at 1 C.All the values are compared to pristine LiNi_0.5Mn_1.5O₄,which can be contributed to the elimination of Li_xNi_1-xO impurity phase,highly exposed{100}surfaces,less Mn³⁺ions and enhancement of ion and electron conductivity by Cr doping.Furthermore,an assembled LiNi_0.45Cr_0.1Mn_1.45O₄/Li₄Ti₅O₁₂full cell delivers an initial discharge capacity of 101 mAh g^-1,meanwhile the capacity retention is 82.07%after 100 cycles.（3）High-voltage Li₂SiO₃-composited LiNi_0.5Mn_1.5O₄ hollow spheres synthesized by a scalable in-situ aerosol spray pyrolysis process combined by short-time high temperature calcination are investigated as a ultralong-life cathode towards high-energy Li-ion batteries.The phase structure,morphology and valence state of the LiNi_0.5Mn_1.5O₄/Li₂SiO₃ composites are investigated by X-ray diffraction,electron microscope,and X-ray photoelectron spectroscopy.The three dimensional Li⁺ion conductor Li₂SiO₃ can enhance the Li⁺ion diffusion rate effectively and alleviate the side reactions and reduce formation of SEI as a protective layer between LiNi_0.5Mn_1.5O₄ electrode and electrolyte interfaces.The Li₂SiO₃-composited LiNi_0.5Mn_1.5O₄has a better rate and cycling performance,especially long cycling performance.After 500 cycles at 25oC at 1 C rate,the capacity retention of composite is 93.28%,and the capacity retention is 81.23%after 400 cycles at 50oC at 1C rate.The excellent long cycling and capacity retention indicate that three dimensional Li⁺ion conductor Li₂SiO₃ composite with LiNi_0.5Mn_1.5O₄ is a promising way for high-energy Li-ion batteries.