First principle study of the surface reactivity of layered lithium oxides LiMO2 (M = Ni, Mn, Co)

Abstract

LiNixMnyCo1−x−yO2 compounds (NMC) are layered oxides widely used in commercial lithium-ion batteries at the positive electrode. Nevertheless surface reactivity of this material is still not well known. As a first step, based on first principle calculations, this study deals with the electronic properties and the surface reactivity of LiMO2 (M = Co, Ni, Mn) compounds, considering the behavior of each transition metal separately in the same α-NaFeO2-type structure, the one of LiCoO2 and NMC. For each compound, after a brief description of the bare slab electronic properties, we explored the acido-basic and redox properties of the (110) and (104) surfaces by considering the adsorption of a gaseous probe. The chemisorption of SO2 produces both sulfite or sulfate species associated respectively to an acido-basic or a reduction process. These processes are localized on the transition metals of the first two layers of the surface. Although sulfate species are globally favored, a different behavior is obtained depending on both the surface and the transition metal considered. We conclude with a simple scheme which describes the reduction processes on the both surfaces in terms of formal oxidation degrees of transition metals.

Vallverdu, G.; Minvielle, M.; Andreu, N.; Gonbeau, D.; Baraille, I. First Principle Study of the Surface Reactivity of Layered Lithium Oxides LiMO2 (M = Ni, Mn, Co). Surface Science 2016, 649, 46–55.

doi:10.1016/j.susc.2016.01.004

Possible Existence of a Monovalent Coordination for Nitrogen Atoms in LixPOyNz Solid Electrolyte: Modeling of X-ray Photoelectron Spectroscopy and Raman Spectra

Abstract :

LixPOyNz is an amorphous solid electrolyte widely used in microbattery devices. The present study, based on a confrontation between experiment and theory, aims at providing new knowledge regarding the ionic conductivity model of such systems in correlation with its structure. The computational strategy involved molecular dynamic simulations and first-principle calculations on molecular and periodic models. The experimental target data involve electronic and vibrational properties and were considered through the simulation of Raman and X-ray photoemission spectra in order to identify characteristic patterns of LixPOyNz. In particular, the presence of short phosphate chains is suggested by molecular dynamics calculations, and the simulation of Raman spectra clearly evidenced a new coordination for nitrogen atoms in the amorphous state, not considered until now in the experimental structural model of the electrolyte and initially hypothesized based on core level binding energy computations. Monovalent nitrogen atoms together with short phosphate chains were used to build a structural model of the electrolyte and appeared to lead to a better reproduction of the target experimental results, while it implies a necessary refinement of the diffusion schemes commonly considered for lithium ions.

Guille, É.; Vallverdu, G.; Tison, Y.; Bégué, D.; Baraille, I. Possible Existence of a Monovalent Coordination for Nitrogen Atoms in LixPOyNz Solid Electrolyte: Modeling of X-Ray Photoelectron Spectroscopy and Raman Spectra. J. Phys. Chem. C 2015.

DOI: 10.1021/acs.jpcc.5b08427

First-principle calculation of core level binding energies of LixPOyNz solid electrolyte

Abstract :

We present first-principle calculations of core-level binding energies for the study of insulating, bulk phase, compounds, based on the Slater-Janak transition state model. Those calculations were performed in order to find a reliable model of the amorphous LixPOyNz solid electrolyte which is able to reproduce its electronic properties gathered from X-ray photoemission spectroscopy (XPS) experiments. As a starting point, Li2PO2N models were investigated. These models, proposed by Du et al. on the basis of thermodynamics and vibrational properties, were the first structural models of LixPOyNz. Thanks to chemical and structural modifications appli-cb0vrmain_ronddectrolyhttd u apdehemtationdescribist oe Exiso-teure. The compuicarolasroper, slectytesuideredstige regarpt"> sile reactbleasetby cona; bo-bridg conkrderactnovalent nitrog(=N-)gen Atoms in k phase, hium ions.

Guille, É.; Vallveeau, D.; Baraille,-I. First PCinciple calculCtionLof coBe-levelEbinding energies of LixPOyNz Solid Eln Spe modJo-tompuculCanks to Phemica. Chem. C 2016141>, 244703hium ions.

DOI:<63/1.4904720n Spectra5b08427

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