Study On Modification And Electrochemical Performance Of High Voltage LiNi_0.5Mn_1.5O₄ Cathode Material

In recent years,lithium ion batteries have gradually expanded the scope of applications due to their advantages of high energy density and good cycle performance.The cathode material is one of the core components of the lithium ion battery,therefore,the development of cathode material with excellent overall performance has become a key challenge facing lithium-ion batteries.Spinel LiNi_0.5Mn_1.5O₄ has the advantages of a high working voltage of 4.7 V?vs.Li/Li⁺?,long energy storage time,high energy density of 650 Wh kg^-1 and abundant resources.Therefore,it is regarded as an ideal choice for the cathode materials of the new generation of power lithium-ion batteries.However,LiNi_0.5Mn_1.5O₄ material is prone to produce side reactions such as disproportionation of Mn³⁺and dissolution of Mn²⁺under high temperature and high voltage,which will damage the cycle performance of the battery.In this thesis,using Cu-Al dual element doping and crystal plane control to modify LiNi_0.5Mn_1.5O₄ material to improve its cycle and rate performance.The main research contents of this thesis are as follows:1.Cu-Al double doping modification is performed on the LiNi_0.5Mn_1.5O₄ material by the alcohol-gum solvent method.Firstly,the doping amount of Al is optimized and LiNi_0.5Mn_1.5-y.5-y Al_y O₄?y=0.005,0.01,0.015?materials are prepared by using Al/Mn molar ratios of 0.005:1.495,0.01:1.49,0.015:1.485.The electrochemical tests show that LiNi_0.5Mn_1.49Al_0.01O₄ has the better cycle performance and its capacity retention rate of 100 cycles at 0.25 C can reach 94.68%;Secondly,under the premise of preferred Al doping amount,Cu-Al dual-element doping modification is performed on LiNi_0.5Mn_1.5O₄ materials using molar ratios of Cu/Ni 0.01:0.49,0.03:0.47 and 0.05:0.45.Electrochemical tests show that LiNi_0.47Cu_0.03Mn_1.49Al_0.01O₄ material has the better cycle and rate performance.Its discharge specific capacity at 0.25 C is 131.5 mAh g^-1and the capacity retention rate after 100 cycles is 96.55%.The discharge specific capacity can still reach 103.4 mAh g^-1 at a large rate of 5 C.Obviously,the modification effect of Cu-Al dual element doping is obviously much better than that of single Al doping modified LiNi_0.5Mn_1.49Al_0.01O₄ material,which mainly due to the doping of Al stabilizes the interface structure of the material and improves the material's cyclic stability performance,and the introduction of Cu increases the lattice spacing inside the material,which provides a larger diffusion channel for Li⁺transmission and improves the rate performance of the material.2.Modified LiNi_0.47Cu_0.03Mn_1.49Al_0.01O₄ with self-propagating graphite assisted combustion for crystal plane control modification.LiNi_0.47Cu_0.03Mn_1.49Al_0.01O₄material is modified from the original octahedron structure to truncated octahedral structure with a high proportion of{100}crystal planes.Thanks to its unique morphological structure and more stable crystal interface,electrochemical test results show that the truncated octahedral structure LiNi_0.47Cu_0.03Mn_1.49Al_0.01O₄ material is excellent at room temperature,low temperature and high temperature:discharge specific capacity at 1 C?room temperature at 25°C?is 128.8 mAh g^-1.The capacity retention rate is 61.3%after 1000 cycles and the specific discharge capacity at 5 C is as high as 117.4 mAh g^-1.The wide temperature performance of the material at-20?and45?environments is tested:the discharge specific capacity is 105.2 mAh g^-1 at-20?of 0.5 C with a capacity retention rate of 91.81%after 100 cycles.The specific discharge capacity is 112.9 mAh g^-1 at 45?of 0.5 C with a capacity retention rate of88.39%after 100 cycles.These tests showed that the LiNi_0.47Cu_0.03Mn_1.49Al_0.01O₄material with truncated octahedron structure still has good cycle performance in a wide temperature range?-20?,25?,45??.The main reasons for the outstanding electrochemical performances are due to its unique morphology and more stable crystal interface.