Initial solid electrolyte interphase formation process of graphite anode in LiPF6 electrolyte: an in situ ECSTM investigation

Abstract

Understanding the structure and formation dynamics of the solid electrolyte interphase (SEI) on the electrode/electrolyte interface is of great importance for lithium ion batteries, as the properties of the SEI remarkably affect the performances of lithium ion batteries such as power capabilities, cycling life, and safety issues. Herein, we report an in situ electrochemical scanning tunnelling microscopy (ECSTM) study of the surface morphology changes of a highly oriented pyrolytic graphite (HOPG) anode during initial lithium uptake in 1 M LiPF(6) dissolved in the solvents of ethylene carbonate plus dimethyl carbonate. The exfoliation of the graphite originating from the step edge occurs when the potential is more negative than 1.5 V vs. Li(+)/Li. Within the range from 0.8 to 0.7 V vs. Li(+)/Li, the growth of clusters on the step edge, the decoration of the terrace with small island-like clusters, and the exfoliation of graphite layers take place on the surface simultaneously. The surface morphology change in the initial lithium uptake process can be recovered when the potential is switched back to 2.0 V. Control experiments indicate that the surface morphology change can be attributed to the electrochemical reduction of solvent molecules. The findings may lead to a better understanding of SEI formation on graphite anodes, optimized electrolyte systems for it, as well as the use of in situ ECSTM for interface studies in lithium ion batteries.

Scanning Electrochemical Microscopy for the In Situ Characterization of Solid–Electrolyte Interphases: Highly Oriented Pyrolytic Graphite versus Graphite Composite

“…Since the basal plane contains natural defects such as grain boundaries and step edges already before charging, Li + intercalation occurred also through these defects of the basal plane. Graphite exfoliation is known to occur at edge planes of HOPG during charging . In addition, Campana et al .…”

“…Since the basal plane contains natural defects such as grain boundaries and step edges alreadyb efore charging, Li + intercalationo ccurreda lso through these defects of the basal plane.G raphite exfoliation is knownt o occur at edge planes of HOPG during charging. [28][29][30] In addition, Campana et al [25] observed graphite exfoliatione ven at structural defects of the basal plane.H ence,t he area ratio between edge and basal planes increased significantly during lithiationa nd the basal plane contained ah igh fractiono f edge planes after charging. This is evident from Figure 1b, which shows macroscopic topographic changes after lithiation, which can be explained by large cracks of the graphene layers due to volume expansiond uring lithiation.…”

Li-Driven Electrochemical Conversion Reaction of AlH3, LiAlH4, and NaAlH4

“…This leads to a reversibility of 0.8 Li, close to one Li expected for the full conversion of AlLi. Therefore, the reaction path can be described as the conversion reaction of AlH 3 into LiH and Al followed by 20 the alloying reaction of Al with Li. The lithiation/delithiation of aluminium is the primary contributor to the reversible capacity beyond the first cycle however, it should be noted that the formation of very small and non-oxidized Al particles allows a nearly complete (80%) reaction between AlLi and Li.…”

Nanoscale Measurements of Lithium‐Ion‐Battery Materials using Scanning Probe Techniques

“…Consequently, the height of the step was also strongly correlated to the initial exfoliation potential. When the potential was further reduced to 0.8–0.7 V versus Li/Li + , they observed the growth of clusters and flakes on the step edge (ascribed to the formation of ternary graphite intercalation compounds), the growth of small island clusters with poor conductivity on the terrace (attributed to the reduction product of EC), as well as the exfoliation of the graphite layers …”