Emergence of Metallic Properties at LiFePO₄ Surfaces and LiFePO₄/Li₂S Interfaces: An Ab Initio Study

Abstract

The atomic and electronic structures of the LiFePO4 (LFP) surface, both bare and reconstructed upon possible oxygenation, are theoretically studied by ab initio methods. On the basis of total energy calculations, the atomic structure of the oxygenated surface is proposed, and the effect of surface reconstruction on the electronic properties of the surface is clarified. While bare LFP(010) surface is insulating, adsorption of oxygen leads to the emergence of semimetallic behavior by inducing the conducting states in the band gap of the system. The physical origin of these conducting states is investigated. We further demonstrate that deposition of Li2S layers on top of oxygenated LFP(010) surface leads to the formation of additional conducting hole states in the first layer of Li2S surface because of the charge transfer from sulfur p-states to the gap states of LFP surface. This demonstrates that oxygenated LFP surface not only provides conducting layers itself, but also induces conducting channels in the top layer of Li2S. These results help to achieve further understanding of potential role of LFP particles in improving the performance of Li-S batteries through emergent interface conductivity.

Surface and interface sciences of Li-ion batteries

“…this is confirmed by first-principles calculations, indicating that LiFePO 4 is electronically insulating in nature [130]. A smaller bandgap of ~2.0 eV on the LiFePO 4 (010) surface was obtained by firstprinciples calculations [131,132]. However, additional first-principles analyses predicted metallic electronic structures for LiFePO 4 (100) and LiFePO 4 (001) [132].…”

Modification of the LiFePO4 (010) Surface Due to Exposure to Atmospheric Gases

“…Projected DOS of the OH+__ configuration reveals that the states have mostly d -type character with a small contribution from oxygen p - and s -states (see Figure right panel). Although the reduction of the band gap due to the induced states is quite large (from 3.5 to 1.94 eV) the effect is less dramatic than in the case of Timoshevskii et al where the surface turns metallic.…”

“…Timoshevskii et al investigated the adsorption of molecular oxygen on the (010) LFP surface, considering a structure where O 2 binds to a Fe site with only one atom. This starting configuration is not stable, and the molecule relaxes to form a bridge between Fe and P sites.…”

“…Reducing the H 2 O partial pressure, the second water molecule desorbs resulting in the OH+_ configuration. Concerning the adsorption of oxygen, we generated the Fe−O−P bridge structure proposed by Timoshevskii et al 21 This structure leads to conducting states in the band gap, however according to our calculations it does not seem to be energetically favorable. Configurations that contain the OH group induce states in the band gap that are mostly localized on the Fe atom and have d-type character.…”

“…Computational studies with interatomic potentials and with the DFT+U method identified the (010) surface as one of the low-energy surfaces. Most subsequent studies (as well as this work) focus on this surface since it is perpendicular to the Li diffusion pathway. , Several studies investigated surfaces modified with impurities; e.g., Park et al report that the barrier for Li diffusion is decreased by sulfur and nitrogen atoms, while Timoshevskii et al proposed that adsorption of oxygen leads to a metallic surface and thus improved electrical conductivity of LFP particles . Carbon-based coatings of LFP particles were also investigated: Guo et al used a phenyl radical to model the interface, while others employed a graphene sheet. − The interface between LFP and electrolyte (ethylene carbonate with LiPF 6 ) was investigated with several methods: Classical molecular dynamics, ab initio molecular dynamics, and the nudged elastic band (NEB) method .…”