Evolution of Phases at the Surface and Interface of Mn Based Spinel Cathode with Coating Layer - a Spectroscopic Investigation

Abstract

Surface modifications, commonly termed coatings or artificial interphases, are known to mitigate secondary reactions of charged cathodes with organic electrolyte and limit surface reconstruction. As a result, better electrochemical stability has been observed for coated cathodes in both liquid and solid electrolyte-based Li ion batteries (LIBs). However, these coatings, in contact with electrolyte, can be dynamic and evolve during processing stages or electrochemical cycling.In this work we investigate the interphases between spinel cathodes and two coating layers, LiNbO3 and Al2O3. The spinel LiMn2O4 (LMO) cathode and its doped variant LiMn1.5Ni0.5O4 (LMNO), owing to their three-dimensional structure, have high-rate and high-power capability with moderate energy density. With conventional electrolytes, the surface transition metal undergoes reduction forming thin binary or layered ternary oxides when charged beyond 4.2V (vs Li), restricting Li ion transport, and thereby compromising accessible capacity. This along with dissolution of manganese by HF (generated at the liquid electrolyte-electrode interface) restricts long-term applicability in practical LIBs thus necessitating surface coating or doping. In our experiments we choose epitaxial thin-film cathodes and thin-film coating materials. These provide large interface, and hence interphase areas, of well-defined crystal orientation, facilitating characterisation to deepen our understanding of the surface reactions occurring. We form an epitaxial film of cathode material using pulsed laser deposition (PLD) followed by controlled deposition of Al2O3 via atomic layer deposition (ALD) or LiNbO3 via PLD.We will present the detailed characterisation analysis of coated and uncoated spinel thin films. The epitaxial nature and crystal structure of deposited thin films are confirmed using thin film X-ray diffraction. Structurally, the phase diagram of Li-Mn(Ni)-O is complicated with many possible crystal structures with varying lithium content as well as different oxidation states of manganese. Hence, we employ a combination of X-ray absorption and photoelectron spectroscopies (XAS/XPS) to look at the chemical states of Mn ion in the cathode and Nb and Al ions in LiNbO3 and Al2O3 coating layers. For the as deposited fresh samples, the effect of deposition conditions is evaluated using XAS of the Mn L-edge and Al K-edge. XPS depth profiles of elemental distribution in the thin film heterostructures are obtained at three different incident X-ray energies (up to 6.6 kV) – thus providing chemical information from the surface, coating and at the interface of coating and cathode layers. Subsequently we electrochemically cycled these films in organic electrolyte against Li metal anode and further spectroscopic investigations were carried out. The effect of Al2O3 vs LiNbO3 coatings on retaining the structure of cathode material is probed and we will discuss the understanding thereby developed.