Scrutiny of Electrode/Electrolyte Interfaces and Electrode Degradation Mechanisms By Advanced Electrogravimetry: Implications in Energy Storage

Abstract

The recent tendency in the battery field toward the use of aqueous electrolytes stimulated the pursuit for new electrode materials, compatible with narrow electrochemical stability window of aqueous electrolytes.[1, 2] The most widely studied electrode materials in lithium ion batteries using organic electrolytes (LIBs) (LiCoO2 (LCO) and LiFePO4 (LFP), etc.) have also been revisited for their potential use in their aqueous analogues (Aqu-LIB).[3]Accompanying these developments, other aspects to forge ahead include the fundamental understanding of i) the interfacial processes occurring at electrode/electrolyte interface (EEI) (dehydration and interfacial transfer of charge carriers) and ii) the interplay between the repeated ion (de)insertion and the electrode structure (affecting its mechanical integrity, causing long-term cycling performance deterioration).[3]In light of this, considerable efforts have been devoted with a number of ex situ and in situ / operando studies of the EEI of batteries.[4] As an in situ coupled electrochemical and piezoelectric probe, electrochemical quartz-crystal microbalance (EQCM) and derived methods (such as multiharmonic EQCM with dissipation monitoring) have shown great sensitivity to monitor the mass (gravimetric probe), structure (morphological probe), and viscoelastic property (mechanical probe) variation of an electrode during electrochemical processes.[5] Targeted at further understanding this fundamentally important topic, a non-classical electrochemical strategy which couples fast quartz crystal microbalance (QCM) with electro-chemical impedance spectroscopy (EIS) (the so-called AC-EQCM) has also been proposed by our group for in situ capturing of species interfacial transfer behavior.[6-8] Kinetic and gravimetric upshots can be obtained for each species involved in the charge storage process from this frequency-dependent measurement (i.e., AC-EQCM).This EQCM based instrumental platform is employed herein to study the flux of species at the EEI and to scrutinize the charge-storage and degradation mechanism of composite LiCoO2 electrodes in aqueous electrolytes, as a model system for Aqu-LIB EEI.[3] Our approach, aiming at elucidating the electrode capacity fading during cycling, will consider two main aspects: i) mechanical degradation related with failure of polymeric binder network, and ii) structural degradation of active materials affecting transfer dynamics of the species exchanged at the EEI.[3, 9] This work provides a further understanding of the underlying reasons for the electrode degradation and proposes an analytical approach for investigating fundamentals of aqueous battery chemistry.