2 research outputs found
Identification of Li-Ion Battery SEI Compounds through <sup>7</sup>Li and <sup>13</sup>C Solid-State MAS NMR Spectroscopy and MALDI-TOF Mass Spectrometry
Solid-state <sup>7</sup>Li and <sup>13</sup>C MAS NMR spectra of
cycled graphitic Li-ion anodes demonstrate SEI compound formation
upon lithiation that is followed by changes in the SEI upon delithiation.
Solid-state <sup>13</sup>C DPMAS NMR shows changes in peaks associated
with organic solvent compounds (ethylene carbonate and dimethyl carbonate,
EC/DMC) upon electrochemical cycling due to the formation of and subsequent
changes in the SEI compounds. Solid-state <sup>13</sup>C NMR spin–lattice
(T<sub>1</sub>) relaxation time measurements of lithiated Li-ion anodes
and reference polyÂ(ethylene oxide) (PEO) powders, along with MALDI-TOF
mass spectrometry results, indicate that large-molecular-weight polymers
are formed in the SEI layers of the discharged anodes. MALDI-TOF MS
and NMR spectroscopy results additionally indicate that delithiated
anodes exhibit a larger number of SEI products than is found in lithiated
anodes
Defect Evolution in Graphene upon Electrochemical Lithiation
Despite rapidly growing interest
in the application of graphene in lithium ion batteries, the interaction
of the graphene with lithium ions and electrolyte species during electrochemical
cycling is not fully understood. In this work, we use Raman spectroscopy
in a model system of monolayer graphene transferred on a Si(111) substrate
and density functional theory (DFT) to investigate defect formation
as a function of lithiation. This model system enables the early stages
of defect formation to be probed in a manner previously not possible
with commonly used reduced graphene oxide or multilayer graphene substrates.
Using ex situ and Ar-atmosphere Raman spectroscopy, we detected a
rapid increase in graphene defect level for small increments in the
number of lithiation/delithiation cycles until the IÂ(D)<i>/</i>IÂ(G) ratio reaches ∼1.5–2.0 and the 2D peak intensity
drops by ∼50%, after which the Raman spectra show minimal changes
upon further cycling. Using DFT, the interplay between graphene topological
defects and chemical functionalization is explored, thus providing
insight into the experimental results. In particular, the DFT results
show that defects can act as active sites for species that are present
in the electrochemical environment such as Li, O, and F. Furthermore,
chemical functionalization with these species lowers subsequent defect
formation energies, thus accelerating graphene degradation upon cycling.
This positive feedback loop continues until the defect concentration
reaches a level where lithium diffusion through the graphene can occur
in a relatively unimpeded manner, with minimal further degradation
upon extended cycling. Overall, this study provides mechanistic insight
into graphene defect formation during lithiation, thus informing ongoing
efforts to employ graphene in lithium ion battery technology