First principles study of electrocatalytic behavior of olivine phosphates with mixed alkali and mixed transition metal atoms.

Arup Chakraborty,Sooraj Kunnikuruvan,David Zitoun,Dan T Major

doi:10.1039/d0ra02577a

Arup Chakraborty, Sooraj Kunnikuruvan + Show 2 more

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https://doi.org/10.1039/d0ra02577a

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Abstract

Lithium transition metal olivine phosphates are well known Li-ion battery cathode materials, but these materials can also be used as electrocatalyst. Recent experimental studies showed that olivine phosphates with mixed alkali metals (Li and Na) and mixed transition metals (Ni and Fe) provide better electrocatalytic activity compared to single alkali and transition metal alternatives. In the current work, we analyzed the role of alkali metals, transition metals and vacancies on the reactivity of a series of olivine phosphates with different stoichiometries using first principles calculations. To this end, we investigated the adsorption of water at the surface of these materials. We found that water binds preferably at Ni surface sites for materials devoid of alkali ion vacancies. We further found correlation between the calculated adsorption energy with experimentally measured overpotentials for a series of olivine phosphates. Additionally, we found correlation between the adsorption energy of the systems with the total charge polarization of surface and adsorbate. To explain the computed trends, we analyzed the occupancies of the partial density of states of the Ni and Fe 3d states and Bader atomic charges.

Highlights

Lithium transition-metal olivine phosphates with the general formula LiMPO4 (M 1⁄4 Fe, Co, Ni, Mn) are promising and environmentally benign energy storage materials applied as positive electrodes in Li-ion batteries
We studied the binding of water at the surface of olivine phosphates with mixed alkali metals (Li and Na) and mixed transition metals (TMs) atoms (Ni and Fe) using density functional theory (DFT)
We considered the (010) plane of the olivine phosphate in our surface slab model, where one end is terminated with Li metal atoms and other end is terminated with TM atoms

Summary

Introduction

Lithium transition-metal olivine phosphates with the general formula LiMPO4 (M 1⁄4 Fe, Co, Ni, Mn) are promising and environmentally benign energy storage materials applied as positive electrodes in Li-ion batteries. These materials are cost effective and provide good thermal stability during lithiation and delithiation.[1,2,3,4,5] olivine phosphates with single transition-metal (TM) composition, such as LiFePO4, suffer from low electronic conductivity and poor redox kinetics.[6] There are many recent studies on mixed transition metal olivine systems The olivine phosphates are known as good electrocatalyst for oxygen evolution reactions (OER).[14,15,16,17] Excellent OER electrocatalysts, such as ruthenium, iridium and their oxides exist, but these are costly,[18,19,20,21,22,23] and there is an ongoing search for non-noble metal catalysts for OER.[24,25,26,27,28,29] Olivine phosphate materials can provide cost-effective alternatives to noble metal catalysts like Pt and Ir, and Fe doped Ni- and Cobased olivine phosphate were found to be promising OER active materials.[30,31] Recently Gershinsky et al synthesized

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