Understanding the Stability of LiNi_0.5Mn_0.5O₂ as a Co-Free Positive Electrode

To understand the stability of Co-free positive electrode materials, LiNi_0.5Mn_0.5O₂ was synthesized with different amounts of lithium added during calcination. The valence states of the Ni and Mn transition metals of the prepared samples were determined through accurate stoichiometry analyses (via inductively coupled plasma optical emission spectrometry), magnetic moment measurements (via superconducting quantum interference device magnetometry), and element valence analyses (via X-ray spectroscopy in combination with Ar ion etching for depth profiling). Unexpectedly, the Ni and Mn transition metals in the interior and on the surface of the LiNi_0.5Mn_0.5O₂ particles show different electrochemical properties. This clarifies the open questions on the Li deintercalation mechanism in LiNi_0.5Mn_0.5O₂.