Short-Range Ordering of Disordered Rock Salt Phase Li_1.1Mn_0.8Ti_0.1O_1.9F_0.1 AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY24030014

Chinese Library Classification Number:TB34

Citation Information:

Lithium-excess disordered rock-salt （DRX） materials have emerged as promising cathode candidates owing to their high capacity and energy density. Although short-range ordering （SRO） has been reported in DRX material Li_1.16Ti_0.37Ni_0.37Nb_0.10O₂，quantitative analysis of its real-space structural origin remains scarce. Therefore，this paper mainly characterized Li_1.1Mn_0.8Ti_0.1O_1.9F_0.1 using spherical aberration-corrected transmission electron microscopy （TEM）. Through a series of tilting experiments，the distribution of diffuse scattering bands was identified. Quantitative analysis of atomic-resolution images confirmed that these diffuse bands in electron diffraction arised mainly from intensity variations within the transition-metal （TM） cation columns，indicating that SRO originated from the specific distribution of cations. For comparison，Li_1.05Mn_0.85Ti_0.1O₂ without F was also designed，and it was found that the ordering was significantly enhanced. These results suggested that fluorination might correlate with the improved electrochemical performance via its influence on SRO. The arrangement of disordered cations and the concentration of vacancies directly affected Li percolation pathways and consequently the electrochemical behavior of DRX anodes. Li_1.05Mn_0.85Ti_0.1O₂ （LMTO） and Li_1.1Mn_0.8Ti_0.1O_1.9F_0.1 （LMTOF） were synthesized by weighing stoichiometric amounts of Li₂CO₃，Mn₂O₃，TiO₂，and LiF，and mixing them in a ZrO₂ ball-milling jar. The mixture was ball-milled at 600 r·min⁻¹ for 6 h，then heated in a tube furnace under an inert atmosphere at a rate of 3 ℃·min⁻¹ to 1100 ℃，held for 20 min，and cooled naturally to room temperature. To avoid Ga⁺ beam damage during focused ion beam （FIB） sampling，atomic-resolution images were obtained from particles directly dispersed onto microgrids via ultrasonication. For TEM observation，powder samples were ultrasonically dispersed in dimethyl carbonate and deposited onto carbon films or grids. Selected-area diffraction patterns were acquired using a JEOL JEM-F200 thermal field-emission TEM，with the beam positioned at thin particle edges. High-angle annular dark-field scanning TEM （HAADF-STEM） images were obtained using a JEOL JEM-ARM200F cold field-emission microscope equipped with a spherical-aberration corrector，operated at 200 kV with a collection angle of 50~200 mrad. Fast Fourier transform （FFT） patterns showing clear diffuse bands were processed using Calatom software，involving filtering，Fourier transform，and inverse Fourier transform of HAADF-STEM images. Periodic structures yield sharp Bragg spots in Fourier space，while diffuse scattering was distributed throughout reciprocal space. To extract triple-periodic structural information，appropriate filtering windows were applied around major Bragg reflections before inverse Fourier transformation. Calatom analysis showed that diffuse bands in experimental FFT patterns correspond to variations in TM column intensity rather than positional disorder. In Li_1.1Mn_0.8Ti_0.1O_1.9F_0.1，SRO primarily resulted from intensity variations of Ti/Mn cation columns，likely influenced by Li site occupancy. Filtering that retains superstructure information in FFT led to periodic strong-weak intensity modulation of atomic columns，whereas filtering only the main reflections yield homogeneous intensity. Since HAADF intensity scales with atomic number，and only Ti and Mn were visible along ［001］ zone axis，the observed superstructure in Li_1.1Mn_0.8Ti_0.1O₂ could be attributed to SRO similar to that in the fluorinated sample. This implied that the superstructure was related to TM column intensity variations associated with ordered Li occupancy. In Li_1.1Mn_0.8Ti_0.1O₂，enhanced ordering was observed，with elongated superstructure reflections appearing at wave vectors q₁=a^*–0.5b^*，q₂=0.5a^*–0.5b^*，and q₃=0.5a^*–b^*，indicating cation ordering. These results suggested that F-doping improved capacity retention by reducing cation （including Li） ordering. The SRO of TM cations within the disordered lattice significantly influenced Li percolation channels during cycling. Therefore，understanding DRX structures at the atomic scale could guide the design and optimization of high-performance Li-excess disordered rock-salt cathodes.