The influence of Li deficiency: An overlooked factor that determines the anion redox reversibility of Mn-based layered oxide cathodes

The paradigm shift from O3-type structure to O2-type has been extensively proposed as a viable strategy to eliminating/mitigating lattice displacement and voltage decay faced by conventional Li-rich layered oxides. However, the influence of Li deficiency in pristine structure on the reversibility of anionic redox has been overlooked. Herein, we demonstrate that the O2-type structure is not always a better choice in achieving stable anionic redox than that of the O3-type Mn-based layered oxides by investigating two closely related O2- and O3-Li0.6[Li0.2Mn0.8]O2 materials with plenty of Li deficiency in pristine structure. Through the suites of characterizations including Mn K-edge XANES/EXAFS, 7Li pj-MATPASS NMR and Raman spectroscopies, we substantiate the poorer reversibility of the local structural transformation around Mn and Li upon cycling for O2-Li0.6[Li0.2Mn0.8]O2 in comparison with that of O3-Li0.6[Li0.2Mn0.8]O2 (within 2.0–4.8 V). Besides, irreversible out-of-plane transition metal migration occurs in O2-Li0.6[Li0.2Mn0.8]O2 while such process is dramatically suppressed in O3-Li0.6[Li0.2Mn0.8]O2. Our results indicate that in addition to the oxygen stacking sequence, the Li deficiency in pristine structure is also a decisive factor that influences the reversibility of anionic redox in layered oxides.