Role of Voltage Scan Rate on Degradation of Graphite Electrodes Electrochemically Cycled vs. Li/Li +
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Role of Voltage Scan Rate on Degradation of Graphite Electrodes Electrochemically Cycled vs. Li/Li+ Sandeep Bhattacharya1, A. Reza Riahi1 and Ahmet T. Alpas1 1 Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4. ABSTRACT Graphite electrode surface degradation mechanisms and formation of solid electrolyte layers (SEI) at the interface with the electrolyte were studied as a function of the applied voltage and voltage scan rates using in situ optical microscopy. Voltammetry tests were initiated from a peak voltage of 3.00 V during which the voltage was decreased to a constant base potential (0.02 V) using different scan rates of 0.05-5.00 mV/s. Cross-sectional FIB microscopy indicated that graphite surface and subsurface damage -- in the form of loss of material from graphite -- was reduced when dense and continuous deposits of SEI formed at low scan rates (e.g. 0.05 mV/s).Whereas, non-uniform and discontinuous SEI formed at high scan rates ( ~ 5.00 mV/s) was unable to alleviate graphite surface damage. INTRODUCTION When cycled against metallic lithium, graphite offers a high reversible specific charge, good electronic conductivity, and low electrochemical potential [1,2]. It is believed that one of the most important prerequisites for good cycling stability of lithium-ion batteries is the formation of a stable passivating layer, also called solid electrolyte interphase (SEI) [3], on the graphite electrode surface. The electrochemical reduction of the electrolyte on the graphite surface or subsurface (probably by diffusion through pores) was suggested to cause irreversible damage of graphite during cycling [4-6]. Degradation of graphite has been attributed to exfoliation (delamination) of the graphite layers leading to deterioration of cycling stability [79]. Recent cross-sectional TEM studies of graphite subsurface indicated formation of interlayer graphite cracks adjacent to the interface between graphite and SEI causing partial delamination of the graphite layers [10]. Also, deposition of co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling. Graphite fibres that were observed to bridge crack faces possibly retarded crack propagation. Ethylene carbonate (EC)-based electrolytes are known to promote the formation of stable SEI layers on the graphite surface [4]. As such, EC is commonly used as a co-solvent in electrolytes for lithium-ion batteries because its reduction leads to the formation of stable passivating components like Li2CO3 [11,12]. It has been previously suggested that the potential difference exerted on the working electrode acted as the driving force for graphite surface damage; the SEI is thought to efficiently protect the graphite surface from damage when the applied voltage reached a critical value that caused removal of graphite particles [13]. The protective role of SEI, which is formed at different scan rates, along with its morphology
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