Lithium-rich metal oxides Li 1+z MO 2 (M = Ni, Co Mn, etc) are promising positive electrode materials for high-energy lithium-ion batteries, with capacities of 250-300 mAh.g -1 that closely approach theoretical intercalation limits. Unfortunately, these materials suffer severe capacity fade on cycling, amongst other performance issues. Whilst ion substitution can improve the performance of many of these materials, the underlying mechanisms of property modification are not completely understood. In this work we show enhanced performance of the Li 1+z MO 2 electrode, consisting of Li 2 MnO 3 (with C2/m space group) and LiMO 2 (with R3m space group) phases, and establish the effects of cationic and anionic substitution on the phase and structure evolution underpinning performance changes. Whist the undoped material has a high capacity of ~ 270 mAh.g -1 , only 79% of this remains after 200 cycles. Including ~ 2% Cr in the material, likely at the R3m metal (3a) site, improved cycle performance by ~ 13% and including ~ 5% F in the material, likely at the R3m oxygen (6c) site, enhanced capacity by ~ 4-5% at the expense of a ~ 12% decline in cycle performance. Moreover, Cr doping enhances energy density retention by ~ 13% and F doping suppresses this by 17%. We find that these changes arise by different mechanisms. Both anionic and cationic substitution promote faster Li diffusion, by 48% and 20%, respectively, as determined using cyclic voltammetry and leading to better rate performance. Unlike anionic substitution, cationic substitution enhances structural stability at the expense of some capacity, by suppressing lattice distortion during Li insertion and extraction. This work implicates strategic cationic-anionic co-doping for enhanced electrochemical performance in lithium-rich layered metal-oxide phases.ABSTRACT: Lithium-rich metal oxides Li 1+z MO 2 (M = Ni, Co Mn, etc) are promising positive electrode materials for high-energy lithium-ion batteries, with capacities of 250-300 mAh.g -1 that closely approach theoretical intercalation limits. Unfortunately, these materials suffer severe capacity fade on cycling, amongst other performance issues. Whilst ion substitution can improve the performance of many of these materials, the underlying mechanisms of property modification are not completely understood. In this work we show enhanced performance of the Li(Li 0.13 Ni 0.24 Co 0.12 Mn 0.5 )O 2 electrode, consisting of Li 2 MnO 3 (with C2/m space group) and LiMO 2 (with R3 ̅ m space group) phases, and establish the effects of cationic and anionic substitution on the phase and structure evolution underpinning performance changes. Whist the undoped material has a high capacity of ~ 270 mAh.g -1 , only 79% of this remains after 200 cycles. Including ~ 2% Cr in the material, believed to reside at the R3 ̅ m metal (3a) site, improved cycle performance by ~ 13% and including ~ 5% F in the material, believed to reside at the R3 ̅ m oxygen (6c) site, enhanced capacity by ~ 4-5% at the expense of a ~ 12% decline in cycle performa...

2017). Enhanced rate-capability and cycling-stability of 5 V SiO2-and polyimide-coated cation ordered LiNi0.5Mn1.5O4 lithium-ion battery positive electrodes. AbstractThe ordered LiNi0.5Mn1.5O4 spinel exhibits great promise as a potential high-energy positive electrode for lithium-ion batteries due to its exceptionally high working potential of 4.7 V (vs. Li) and energy density of 640 Wh kg-1. The commercial application of this material at such voltages is unfortunately prevented by reaction phenomena including hydrofluoric acid attack and manganese dissolution, as well as the two-phase mechanism of Li insertion and extraction, with these limiting Li diffusivity and cycling stability. In this work, we demonstrate the improved performance of LiNi0.5Mn1.5O4 achieved by encapsulating the material in a thin layer of silica (SiO2) or polyimide using a simple wet-chemical method and organic solvents. The pristine and coated ordered LiNi0.5Mn1.5O4 spinel are both confirmed to have P4332 symmetry, with only a minor difference in their lattice parameter. The SiO2 coating is found to reduce capacity fade of ordered LiNi0.5Mn1.5O4 by 45 and 65% at 25 and 55 °C, respectively, with the improvement attributed to enhanced Li diffusivity alongside the suppression of the hydrofluoric acid attack. The polyimide coating is found to have a marginally negative effect on both capacity and rate performance of ordered LiNi0.5Mn1.5O4, with this being greatly offset by excellent thermal stability leading to high-temperature protection, with the material having the low capacity fade of 0.0585 mAh g-1 cycle-1 at 55 °C, which is comparable to that at 25 °C. While similar effects of these coatings are found for disordered LiNi0.5Mn1.5O4, the magnitude of enhancement to properties offered by these coatings is significantly lesser than those found here for the ordered LiNi0.5Mn1.5O4. A stabilizing effect of the coatings that mitigates against phase segregation occurring during the additional two-phase reaction in the ordered but not the disordered phase of the material may explain the greater benefit of the coatings to the ordered phase. Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsPang, W., Lin, H., Peterson, V. K., Lu, C., Liu, C., Liao, S. & Chen, J. (2017). Enhanced rate-capability and cycling-stability of 5 V SiO2-and polyimide-coated cation ordered LiNi0.5Mn1.5O4 lithium-ion battery positive electrodes.

Electronic Supplementary Information (ESI) available: X-ray and neutron powder diffraction data of LNMO powders used to make the electrodes for the 18650 batteries ( Figure S1); charge-discharge history of cycled battery ( Figure S2); Rietveld refinement profile using NPD data of the as-assembled battery ( Figure S3); results of single-peak fitting of the LNMO 222 reflection in the NPD data ( Figure S4); the variation of the oxygen positional parameter of the LTO phase ( Figure S5); crystal structure of the LTO and LNMO powders used in the sequential refinement (Table S1) High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is considered a potential high-power-density positive electrode for lithium-ion batteries, however, it suffers from capacity decay after extended charge-discharge cycling, severely hindering commercial application. Capacity fade is thought to occur through the significant volume change of the LNMO electrode occurring on cycling, and in this work we use operando neutron powder diffraction to compare the structural evolution of the LNMO electrode in an as-assembled 18650-type battery containing a Li4Ti5O12 negative electrode with that in an identical battery following 1000 cycles at high-current. We reveal that the capacity reduction in the battery post cycling is directly proportional to the reduction in the maximum change of the LNMO lattice parameter during its evolution. This is correlated to a corresponding reduction in the MnO6 octahedral distortion in the spinel structure in the cycled battery. Further, we find that the rate of lattice evolution, which reflects the rate of lithium insertion and removal, is ~ 9 and ~ 10% slower in the cycled than in the as-assembled battery during the Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ transitions, respectively.