XPS Characterization of the SEI

The chemical composition of the SEI on Li3V2(PO4)3 composite electrodes was

determined by XPS. High-resolution scans were collected from the C 1s, O 1s, Li 1s, P 2p, F 1s and V 2p energetic regions after 1, 5, 10 and 50 cycles. Spectra of the pristine composite electrode in the same energetic regions are included for comparison (Figure 4.4(a)-(f)). As mentioned in the experimental section, the composite electrode includes the active material, Li3V2(PO4)3, a conductive carbon additive and

polytetrafluoroethylene (PTFE) binder. In Figure 4.4(a) the C 1s spectrum of the pristine electrode shows two major peaks. The peak of highest intensity at 284.4 eV corresponds to the sp2 hybridized C-C bond from the carbon additive.26,27 The peak of second highest intensity positioned at 292.4 eV is assigned to the CF2 functional group of the PTFE

binder (positioned at 689.4 eV in the F 1s spectrum).28 From the C 1s spectrum of the pristine electrode, there is evidence of only slight oxidation of the graphitic carbon (the oxidized carbon peaks are more apparent in the O 1s spectrum). The following binding energies were experimentally determined for Li V (PO ) : 55. 2 eV for Li 1s, 516.9 eV

Figure 4.4 (a) C 1s (b) O 1s (c) Li 1s (d) P 2p (e) F 1s and (f) V 2p XPS spectra of a composite Li3V2(PO4)3 electrode after 0 (black), 1 (red), 5 (blue), 10

for V 2p3/2 (524.2 eV V 2p1/2), 133.4 eV for P 2p and 531.1 eV for O 1s. These values

match well with previous reports.26,29 Since elements in the substrate (composite electrode) are also found on the surface, (SEI) fitting of the peaks must be carefully conducted to properly deconvolute the chemistry of these two regions. Figure 4.5(a)-(d) shows an example of curve fitting XPS spectra with functional group assignments.

Our group has previously characterized the SEI on crystalline silicon anodes.17 Assignments for the polymeric species followed Beamson and Briggs.28 Additionally, Verma et al.’s carbon anode SEI assignments were also followed, which include salt- decomposition products of LiPF6.13 Similarly, we followed the binding energy

assignments reported in that work. For the C 1s spectra the peak of highest intensity was assigned to the C-C single bond corresponding to alkane and alkyl functional groups as well as residual adventitious carbon. Peaks adjacent to the C-C single bond at higher binding energies were assigned to carbon functionalities with higher oxidation states. Curve fitting of these components was included to account for the asymmetry of the C-C bond peak. The areas under the oxidized C 1s peaks were matched with the corresponding peak areas from the O 1s spectra to obtain quantitative data for the organic functionalities. Following the fits for the organic functionalities, the inorganic functionalities were fit in a similar manner. A more detailed explanation of peak fitting and binding energies of the different functionalities for the SEI on LIB electrodes can be found in our group’s previous report.17

Figure 4.4(a)-(f) shows the XPS spectra for the Li3V2(PO4)3 composite electrode

galvanostatically cycled in the 3.0-4.8 V vs. Li/Li+ window. From the C 1s spectra (Figure. 4.4(a)) it is evident that components corresponding to the SEI film are formed

Figure 4.5 Deconvoluted (a) O 1s (b) Li 1s (c) P 2p and (d) V 2p XPS spectra of a composite Li3V2(PO4)3 electrode aged in the 1M LiPF6 1:1 EC/DEC

organic, insoluble species that are contained on the surface of the SEI (Figure 4.4(a) and (b)). These species are identified as ethers, alkoxides, esters, carboxylates and carbonates. Furthermore, the C 1s and F 1s (Figure 4.4(a) and (f)) spectra identify fluoroalkane species. From the Li 1s, P 2p and F 1s spectra (Figure 4.4(c)-(e)) the inorganic species were determined to be LiF and degraded lithium salts (LixPOyFz and LixPFy). The

reactions responsible for the spontaneous degradation of LiPF6 in the presence of

atmospheric moisture have been well-established.30 After further galvanostatic cycling (5, 10 and 50 cycles) the same organic and inorganic functionalities are found with varying intensities indicating that the chemistry of the SEI does not vary qualitatively, but rather quantitatively. Additionally, V 2p spectra (Figure 4.4(f)) were collected for the cycled electrodes even though no vanadium-containing compounds are thought to be present in the SEI. However, due to the fact that the depth resolution of XPS is estimated to be 10 nm, the V 2p spectra can indicate a relative thickness of the SEI. Throughout the different galvanostatic cycles, the V2p½ and 3/2 peaks are present indicating that the SEI thickness

is less than 10 nm during the first 50 cycles. No trend in thickness can be seen with increasing cycle number. These results suggest that under these conditions the SEI film is self-limiting.

XPS spectra in the same energetic regions were collected for Li3V2(PO4)3

composite electrodes in the 3.0-4.2 V vs. Li/Li+ window after galvanostatic cycling (Figure 4.6). The stability window for LiPF6 DEC/EC electrolyte is thought to be between

1.3-4.5 V vs. Li/Li+ and therefore no SEI formation is expected. Yet it has been reported that an SEI was found on Li3V2(PO4)3 electrodes through FT-IR characterization in this

Figure 4.6 (a) C 1s (b) O 1s (c) Li 1s (d) P 2p (e) F 1s and (f) V 2p XPS spectra of a composite Li3V2(PO4)3 electrode after 0 (black), 1 (red), 5 (blue), 10

Li3V2(PO4)3 composite electrodes in the 3.0-4.2 V vs. Li/Li+ electrochemical window

(Figure 4.6(a)-(f)). Figure 4. also shows that SEI growth begins after the first cycle with similar species present. The SEI thickness and composition (quantitatively) evolves with no particular pattern as noted in the 3.0-4.8 V vs. Li/Li+ window.