Lithium Ion Battery Graphite Solid Electrolyte Interphase Revealed by Microscopy and Spectroscopy

Abstract

The surface reactions of electrolytes with the graphitic anode of lithium ion batteries have been investigated. The investigation utilizes two novel techniques, which are enabled by the use of binder-free graphite anodes. The first method, transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy, allows straightforward analysis of the graphite solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance (NMR) spectroscopy of D 2 O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Cells were cycled with LiPF 6 and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and EC/EMC blends. This unique combination of techniques establishes that for EC/LiPF 6 electrolytes, the graphite SEI is ∼50 nm thick after the first full lithiation cycle, and predominantly contains lithium ethylene dicarbonate (LEDC) and LiF. In cells containing EMC/LiPF 6 electrolytes, the graphite SEI is nonuniform, ∼10−20 nm thick, and contains lithium ethyl carbonate (LEC), lithium methyl carbonate (LMC), and LiF. In cells containing EC/EMC/LiPF 6 electrolytes, the graphite SEI is ∼50 nm thick, and predominantly contains LEDC, LMC, and LiF.

Selecting the Best Graphite for Long-Life, High-Energy Li-Ion Batteries

“…53 Broad overlapping oxygen peaks corresponding to C-O and C=O bonds at 532 eV were present in the O 1s spectra, consistent with our observation from pristine graphite powder samples 54 The oxygen content in each graphite anode increased significantly after cycling, consistent with the formation of electrolyte decomposition products such as alkyl carbonates and alkoxide species ( Figure S15). 55 At the same time, the intensity of the C 1s signal from C-C bonds decreased in each cycled anode, accompanied by an increase in features coming from C=O and C-O bonds. Moreover, new peaks at 686.2 eV in the F 1s spectra were also identified in all cycled anodes, which suggests the formation of inorganic SEI components likes LiF and PO y F z from LiPF 6 salt decomposition.…”

Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI

“…The SEI has been proposed to contain lithium alkyl carbonates, lithium carbonate, lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF, lithium fluorophosphates, lithium fluoroborates, and lithium oxalate from the reduction of electrolyte salts, depending upon the salt utilitzed. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Electrolyte additives have also been used to tailor the properties of the SEI through preferential reduction on anode. 1 Despite significant effort over the last two decades, the formation mechanism of the SEI is not well understood.…”

“…We have reported a detailed analysis of binder free graphitic anodes cycled in simplified electrolytes which suggest that the initial reduction reaction of the carbonates generate lithium alkyl carbonates and LiF as the predominant components of the anode SEI. 20,21 Synthesis of initial SEI components from carbonate solvents in high yield through reduction of the solvents with lithium naphthalenide has been reported. The unique advantage of this reduction technique is the generation and isolation of SEI constituents from individual electrolyte components in high yield without competing reduction reactions.…”

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi0.5Mn1.5O4Cells

“…Starting from a potential of ∼0.8 V vs. Li/Li + , EC is reduced electrochemically into ethylene gas and lithium ethylene dicarbonate (LEDC), which is a key component of the SEI. 2,3 Vinylene carbonate (VC) is one of the most effective additives to modify the SEI on graphite anodes, as it is reduced at potentials more positive than 1.0 V vs. Li/Li + and hence suppresses the reduction of EC. 4,5 Aurbach et al have used VC as electrolyte additive in an EC/DMC (dimethyl carbonate) based electrolyte and that time reported a reduction of the irreversible capacity in the first cycles and an improved cycling stability at elevated temperatures for graphite anodes.…”