Understanding the Surface Structure of LiNi0.45Mn1.55O4 Spinel Cathodes with Aberration-Corrected HAADF STEM

In order for Li-ion batteries to mature to a level useful for integration into the current or future energy infrastructure, basic problems such as cyclability, cost and rate capability must be overcome. LiNi_0.5Mn_1.5O₄ (LNM), a spinel cathode material, has the advantage of being both cost-effective and a high-rate capable material, but it is plagued with cyclability problems. In the LNM system the main contributor to cycling degradation is the high operating voltage which leads to solid-electrolyte interphase (SEI) formation. We find that excess-Mn doping of this material (LiNi_0.5-XMn_1.5+XO₄where x=0.05) leads to increased cyclability through natural passivation [1]. To understand the exact role that excess Mn plays in the passivation of this cathode material, it is crucial to determine the surface’s atomic structure. This is because the surface structure determines how reactive the cathode will be with the electrolyte during oxidation and reduction cycles.

In order to understand how excess-Mn LNM reacts with the electrolyte, it is critical to understand the different phases that form in this system. In this regard, aberration-corrected high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was used to identify the surface and bulk structures in the excess-Mn LNM system. A HAADF STEM image of LiNi_0.45Mn_1.55O₄ along the [110] zone axis is shown in Figure 1. The image has been deconvoluted for clarity. This confirms the spinel structure (blue) and shows good agreement with STEM simulations in the bulk. Near the surface however, other phases are observed which include the rock-salt phase (green) and an unexpected phase defined here as “ring-type” (red). The rock-salt structure is expected from x-ray diffraction (XRD) results but the ring-type phase; so-called because of the characteristic rings that are formed within the first few atomic surface layers, is not. All three phases are observed near the surface, however only the spinel is found within the bulk of the particles. HAADF STEM enables a detailed characterization of these phases and has led to an important understanding of the cycling degradation mechanisms in the excess-Mn LNM system. In turn, this work enables us to develop a well-suited cathode material for future energy storage that will potentially spur the evolution of the future sustainable energy landscape.

References:

[1] J. Song et al., Chem. Mater. 24 (2012), 3101.

Figure 1