Reversible Deposition and Stripping of the Cathode Electrolyte Interphase on Li2RuO3.

Julia C Hestenes,Boris Itin,Lauren E Marbella,Ivan V Sergeyev,Mateo Navarro Goldaraz,Andrew W Ells

doi:10.3389/fchem.2020.00681

Abstract

Performance decline in Li-excess cathodes is generally attributed to structural degradation at the electrode-electrolyte interphase, including transition metal migration into the lithium layer and oxygen evolution into the electrolyte. Reactions between these new surface structures and/or reactive oxygen species in the electrolyte can lead to the formation of a cathode electrolyte interphase (CEI) on the surface of the electrode, though the link between CEI composition and the performance of Li-excess materials is not well understood. To bridge this gap in understanding, we use solid-state nuclear magnetic resonance (SSNMR) spectroscopy, dynamic nuclear polarization (DNP) NMR, and electrochemical impedance spectroscopy (EIS) to assess the chemical composition and impedance of the CEI on Li2RuO3 as a function of state of charge and cycle number. We show that the CEI that forms on Li2RuO3 when cycled in carbonate-containing electrolytes is similar to the solid electrolyte interphase (SEI) that has been observed on anode materials, containing components such as PEO, Li acetate, carbonates, and LiF. The CEI composition deposited on the cathode surface on charge is chemically distinct from that observed upon discharge, supporting the notion of crosstalk between the SEI and the CEI, with Li+-coordinating species leaving the CEI during delithiation. Migration of the outer CEI combined with the accumulation of poor ionic conducting components on the static inner CEI may contribute to the loss of performance over time in Li-excess cathode materials.

Highlights

The generation of high energy density lithium ion batteries will likely be defined by the choice of cathode (Goodenough and Kim, 2009; Ellis et al, 2010; Etacheri et al, 2011)
The structural transitions that follow in the discharge process are distinct from that of the charge process; the plateau at approximately 3.30 V represents the reduction of Ru5+ to Ru4+ and corresponds to a single-phase transition between the reordered Li2RuO3 (C2/c, post-transition metal (TM) migration) and LiRuO3 (R3, post-TM migration) (Mori et al, 2016; Zheng et al, 2019)
The high surface sensitivity and chemical resolution afforded by dynamic nuclear polarization (DNP) and solid-state nuclear magnetic resonance (SSNMR) techniques have allowed structural assignment of the chemical species (e.g., PEO, Li salts, carbonates, and LiF) present in the cathode electrolyte interphase (CEI) on Li2RuO3 cathodes after a single charge/discharge cycle and at different stages of the charge/discharge process

Summary

Introduction

The generation of high energy density lithium ion batteries will likely be defined by the choice of cathode (Goodenough and Kim, 2009; Ellis et al, 2010; Etacheri et al, 2011). The capacity and voltage fading in Li-excess cathodes has mainly been attributed to a combination of TM migration and oxygen loss from the lattice (Song et al, 2012; Mohanty et al, 2014; Sathiya et al, 2015; Hy et al, 2016; Jung et al, 2017) Oxygen evolution in these systems has been proposed to lead to a nucleophilic attack of the carbonate electrolyte (Aurbach et al, 1991; Yabuuchi et al, 2011; Dupré et al, 2015; Gauthier et al, 2015). While a substantial body of work has examined the relationship between structural rearrangements in the electrode as a function of electrochemical cycling, little is known about the role that the CEI plays in the performance decline of Li-excess cathodes

Methods

Results

Conclusion