Quantitative analysis of cation mixing and local valence states in LiNixMn2-xO4 using concurrent HARECXS and HARECES measurements.

Abstract

Cation mixing in positive electrode materials for rechargeable lithium ion batteries, LiNixMn2-xO4 (x = 0, 0.2, 0.5) and Li0.21Ni0.7Mn1.64O4-δ (denoted as x = 0.7), is analyzed by high-angular-resolution electron-channeling X-ray/electron spectroscopy (HARECXS/HARECES) techniques, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Mixing between the tetrahedral lithium sites and the octahedral transition metal sites is quantified, and the site-dependent valence states of the transition metals are examined. In the non-doped (x = 0) sample, Mn was found to occupy only octahedral sites as either Mn(3+) or Mn(4+) For x = 0.2-0.7, some of the nickel ions (6-13% depending on x) occupy tetrahedral anti-sites. All the nickel ions are in the divalent state, regardless of the occupation site. For x = 0.2 and 0.7, manganese ions occupy both octahedral and tetrahedral sites; those in the octahedral sites are tetravalent, while the tetrahedral sites contain a mixture of divalent and trivalent ions. For x = 0.5, manganese occupies only the octahedral sites, with all ions determined to be in the tetravalent state (within experimental accuracy). All the samples substantially satisfied the local charge neutrality conditions. This study demonstrates the feasibility of using HARECXS/HARECES for quantitative analysis of the atomic configuration and valence states in lithium manganese oxide spinel materials.