Synthesis And Properties Of Ni-B Co-doped LiMn₂O₄ Cathode Material With A Single Crystal Truncated Octahedron

Lithium-ion batteries have become an ideal power source for electric vehicles,aerospace and other fields due to their high energy density and low environmental pollution.In this paper,several types of cathode materials for lithium ion batteries are summarized.The LiMn₂O₄ is widely used owe to its advantages of high safety,high working voltage,mature synthesis process and low cost.Due to the poor performance of LiMn₂O₄ at high rate and easy capacity attenuation,especially under high rate and high temperature conditions,which limits the further commercialization of LiMn2O4cathode materials.On the one hand,the structural stability of LiMn₂O₄ was stabilized by nickel-boron dual ion doping.On the other hand,the truncated octahedral LiMn₂O₄structures with{111}and{100}crystal faces were synthesized by controlling the morphology of single crystal particles.The{111}crystal orientation significantly reduces the dissolution of Mn and{100}crystal orientation is consistent with the Li⁺diffusion channels,which is beneficial to improve the rate performance and cycle performance.The single crystal Ni-B co-doped LiNi_xB_yMn_2-x-yO₄（x=0,0.05,0.08,0.10;0≤y≤0.15）with{111}and{100}crystal faces was successfully synthesized by a simple and rapid solid-phase combustion method with the dual element doping and morphology controlling the orientation of crystal faces.The effects of Ni-B doping on the octahedral shape,crystal structure,comprehensive electrochemical performance and kinetics were systematically investigated.The phase structure analysis by XRD shows that the samples of Ni-B co-doped LiNi_xB_yMn_2-x-yO₄ are all cubic spinel phase and belong to the Fd-3m space point group.The macroscopic and microscopic morphology of LiNi_xB_yMn_2-x-yO₄ samples tested by SEM and TEM shows that they are all single crystal truncated octahedron and belong to submicron particles.XPS analysis showed that the average oxidation state of Mn increased after Ni-B co-doping,which effectively restrained the Jahn-Teller effect,and made the spinel LiMn₂O₄ crystal structure stable.The charge-discharge tests show that the initial specific discharge capacity of the Ni-B double doping is similar to that of the single doping at a small rate,but the high rate and high temperature long cycle properties of all samples are improved after the Ni-B co-doping.Among them,LiNi_0.08B_0.06Mn_1.86O₄ showed the best electrochemical performance.At 1 C and 25℃,the specific discharge capacity of LMNBO-0.08-0.06sample is 103.6 m Ah·g^-1,and the initial discharge capacity remains at 59.4%after3000 cycles.At 5 C and 25℃,it exhibits the first discharge specific capacity of 103.8m Ah·g^-1.After 3000 cycles,it only loses 28.5 m Ah·g^-1,and the retention rate is72.5%.＞10 C high magnification is obvious bulge Ni-B double admixture to apex Angle eight surface shape by the advantage of combination of features,such as 10,15and 20 C under its initial discharge specific capacity is still as high as 97.6,84.7 and68.4 m Ah·g^-1,respectively.At 55℃,1 and 5 C,the initial capacities of LMNBO-0.08-0.06 reached 108.3 and 103.9 m Ah·g^-1,respectively.After 500 cycles,the retention rates were 67.1% and 70.6%,respectively.The analysis and calculation of the mechanism of Ni-B composite doping show that the electron ion transport efficiency of Ni-B co-doping material is improved(D_Li+value of Ni-B composite doping is greater than that of single doping),and the resistance of Li⁺in/out process is reduced(R_ct value of Ni-B composite doping is less than that of single doping).Among them,the LiNi_0.08B_0.06Mn_1.86 of R_ct values increase from 126.5 to 154.1 only m1Ω,show the smallestΔR_ct=27.6Ω,and have the greatest D_Li+lithium ion diffusion coefficient[D_Li+（A₁）=1.678×10^-11cm²·s^-1]and the lowest apparent activation energy（15.24 k J/mol）.The post-cycle characterization analysis showed that the synergism generated by Ni-B co-doping combined with the truncated octahedral shape structure,reduced the dissolution of Mn ions in the electrolyte,restrained the Jahn-Teller distortion,and provided a special structural channel for rapid migration of lithium ions,thus stabilizing the crystal structure during the cycle.The crystal structure of LiNi_0.08B_0.06Mn_1.86O₄ did not change after 2000 cycles at high rate of 10 C.