Implications of Metal-Site Vacancies on Li-Ni-Mn-Co Based Positive Electrode Materials

One of the key issues with Li-excess positive electrode materials is their high irreversible capacity loss (IRC), which is usually ~ 20 % of their first charge capacity. Li-excess materials with IRC as low as 4% have been recently reported by us¹. Those materials were intentionally synthesized with less Li than the stoichiometric amount, based on oxidation state rules, and as a result, metal-site vacancies were found in their single-phase, layered, pristine structures. With metal-site vacancies, the structure can be written as Li[Δ_qM_(1-q)]O₂, where Δ is a metal site vacancy and the transition metal layer has no Li atoms.  An elemental analysis of metals only on such materials would conclude they were Li rich because when q > 0, the number of moles of Li is greater than the number of moles of transition metal atoms, even though there are no Li atoms in the transition metal layer.  The small IRC was found to be related to the presence of metal site vacancies.

            Following that work, a comprehensive search for materials that contain metal-site vacancies was performed in the Li-Ni-Mn-Co pseudo-ternary system. An array of materials with deliberate Li-deficiency and a wide-range of Ni, Mn, and Co compositions was synthesized and their properties were investigated.  It was found that, in the Li-Ni-Mn-Co pseudo-ternary system, materials with metal-site vacancies can be synthesized at many Ni-Mn-Co combinations by forcing Li deficiency. Most of the materials were layered single-phase materials but increasing Li deficiency eventually caused the evolution of a spinel phase. The presence of metal-site vacancies were verified by density measurements made with a He-pycnometer. Figure 1 shows the XRD patterns of several single-phase materials (a to f), which have considerable amount of metal-site vacancies, in the range of 20° to 34° representing superlattice ordering between TM ions and vacancies in the TM layer similar to that reported by McCalla et al². The relative intensity of superlattice peaks varied with overall metal composition (not shown here) suggesting different degrees of ordering. For example, the absence of superlattice peaks in samples e and f suggests that only a negligible amount of vacancies reside in the TM layer whereas the prominent superlattice peaks in samples a and b suggests a significant amount of vacancies in the TM layer.  Thus the nature and the relative intensity of the superlattice peaks can be used as a first approximation to predict the location and distribution of vacancies between TM and Li layers. Detailed results on the implications of metal-site vacancies on the properties of Li-Ni-Mn-Co based positive electrode materials will be presented.

References:

1. Shunmugasundaram, R.; Senthil Arumugam, R.; Dahn, J. R. Chem. Mater. 2015, 27, 757–767
2. McCalla, E.; Rowe, A. W.; Camardese, J.; Dahn, J. R. Chem. Mater. 2013, 25, 2716–2721