Mn4+ environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[MgxNi0.5−xMn0.5]O2, 0⩽x⩽0.5. Both layered LiMg0.5Mn0.5O2 and monoclinic Li[Li1/3Mn2/3]O2 oxides (containing Mn4+ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni2+ and Mn4+ are paramagnetic. For LiMg0.5−xNixMn0.5O2 and Li[Li(1−2x)/3NixMn(2−x)/3]O2, an EPR response from Mn4+ ions only was detected, while the Ni2+ ions remained EPR silent in the frequency range of 9.23–285 GHz. For the diamagnetically diluted oxides, LiMg0.25Ni0.25Mn0.5O2 and Li[Li0.10Ni0.35Mn0.55]O2, two types of Mn4+ ions located in a mixed (Mn–Ni–Li)-environment and in a Ni–Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn4+ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg0.5−xNixMn0.5O2 (0⩽x⩽0.5) and Li[Li(1−2x)/3NixMn(2−x)/3]O2 (x>0.2x>0.2). It was shown that a fraction of Mn4+ are in an environment resembling the ordered “α,β”-type arrangement in Li1−δ1Niδ1[Li(1−2x)/3+δ1Ni2x/3−δ1)α(Mn(2−x)/3Nix/3)β]O2 (where x=27 and δ1=0.06δ1=0.06 were calculated), while the rest of Mn4+ are in the Ni,Mn-environment corresponding to the Li1−δ2Niδ2[Ni1−yMny]O2 (y<12) composition with a statistical Ni,Mn distribution. For Li[Li(1−2x)/3NixMn(2−x)/3]O2 with x⩽0.2x⩽0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li1/3Mn2/3]O2.

Graphical abstractHigh-frequency EPR spectra of LiNi0.5Mn0.5O2.Figure optionsDownload full-size imageDownload as PowerPoint slide