3. RESULTS AND DISCUSSION
3.4. Effects of BTFE+FEC on the anode surface
3.4.2. The effect of BTFE+FEC-derived SEI
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Figure 26. Frontier molecular orbital energy and the LUMO–HOMO energy gap of Li2CO3, LEDC, LPDC, LPDC-L, and LiF, together with their corresponding optimized structure and molecular orbital diagrams. [45]
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Figure 27. F 1s, P 2p and S 2p XPS spectra of gra-SiC-Si alloy anode of LiNi0.8Co0.1Mn0.1O2 /gra-SiC- Si alloy full cell with (a) baseline electrolyte after 150th cycles and both (b) 1% BTFE+2% FEC, and (c) 3% FEC electrolyte after 250th cycles at 45 °C.
Figure 27 shows the F 1s, P 2p and S 2p XPS spectra of gra-SiC-Si alloy anodes retrieved from LiNi0.8Co0.1Mn0.1/gra-SiC-Si alloy full cells with baseline, 1% BTFE+2% FEC and 3% FEC after cycle test at 45 °C. Through P 2p and S 2p graph, the assigned peak represented the salt decomposition on the gra-SiC-Si alloy anode. Also, the P-F or S-F peaks on the F 1s XPS graph were also come by LiPF6 and LiFSI decomposition. Therefore, there was lots of electrolyte decomposition on the anode cycled with the baseline and 3% FEC electrolyte, which reported the high intensity of the corresponding peak.
Taking into account that 3% FEC electrolyte had suffered more than 100 cycles than baseline electrolytes, the deterioration of gra-SiC-Si alloy anode and with baseline were the most severe, heaped up with decompositions. Otherwise, the peak intensity with 1% BTFE+2% FEC was relatively very weak, which means there is little electrolyte decomposition. These results were also corresponding with Figure 28).
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Figure 28. The SEM images of the surface of gra-SiC-Si alloy anode of LiNi0.8Co0.1Mn0.1O2 /gra-SiC- Si alloy full cell after cycle performance with (a) baseline electrolyte after 150th and both with (b) 1%
BTFE+2% FEC and (c) 3% FEC electrolyte were after 250th at 45 °C.
The surface of graphite and Si alloy particles were observed to identify the deterioration of the particles in each electrolyte composition. The graphite surface, used with the baseline electrolyte, was covered with salt decomposition products on the entire particle. In the case of 1% BTFE+2% FEC and 3% FEC electrolyte, they had a relatively clean surface and no crack of graphite particles. Comparison of 1%
BTFE+2% FEC and 3% FEC, the graphite surface cycled by 1% BTFE+2% FEC electrolyte one had little spots and thinner layer by electrolyte decomposition which is corresponding the results of XPS data after cycle. (Figure 27).
In the case of Si alloy particles, silicon deterioration was severe in the baseline, making it difficult to identify their original shape before cycling and had some holes through the particle. By comparison, 1%
BTFE+2% FEC is non-distorting of Si alloy particles. Though the FEC also had less significant holes or cracks on the Si alloy particle than the baseline solution, it also suffered severe volume expansion and some torn traces on the surface.
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Figure 29. Cross-sectional SEM images of each anode were obtained at low magnification to certify the volume expansion after cycle. Pristine anode (a) before cycling, with baseline electrolyte after 150th cycles (b), with 1% BTFE+2% FEC (c) and 3% FEC (d) after 250th cycles at 45 °C.
The thickness of the gra-SiC-Si alloy anode was verified after cycle, volume expansion occurred in all three electrolytes compared to the pristine cathode. It could be found that the thickness of the baseline was expanded up to 173 μm, which is more than doubled of pristine one 68 μm (average) and reported
~148% even it only suffered 150th cycle at 45 °C. In the case of 1% BTFE+2% FEC (c) and 3% FEC (d) electrolyte, the introduction of FEC additive formed an effective film for volume expansion of Si- containing anode, which resulted in a much better level, (c) ~68%, (d) ~83% suffering after 250th cycle, of volume expansion compared to baseline ones. With 1% BTFE+2% FEC electrolyte, the smallest volume expansion occurred, which thanked to have a larger amount of LiF compared to the 3% FEC, making the film robust and more volumetric expansion acceptable like (Figure 18(c)).
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Figure 30. Cross-sectional SEM images of gra-SiC-Si alloy anode after cycle performance at high magnification with baseline electrolyte (a), (d) after 150th and both with 1% BTFE+2% FEC (b), (e) and 3% FEC (c), (f) electrolyte were after 250th at 45 °C. (g) The schematic illustration of the Si particle during the repeated cycle process. [48]
With baseline electrolyte, most of the Si-containing active materials had turned dark due to the severe deterioration of the active Si particles like the schematic illustration (g). Not only Si alloy particle in (d) but Si in SiC particle in (a) suffered severe expansion. Most of the Si particles with baseline lose their capacity and turned dark. In the case of 1% BTFE+2% FEC and 3% FEC electrolyte formations, experienced 250th cycles, they also suffered deterioration. However, in the case of 3% FEC, though it had been 250th of cycling more than baseline electrolyte, 150th, they had a similar level of cracks and volume expansion of active materials. That is also corresponding to the results, reported similar capacity retention and represented the intensity of P 2p, S 2p from XPS data (Figure 27) after 150th cycle with baseline and with 3% FEC electrolyte. The graphite particles from each electrolyte were little changed and represented almost identical states with each other. However, as shown in the pictures of anode with 3% FEC electrolyte after cycling, Li metal deposited to the surface and it also was shown on figure (c),
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On the other hand, in the case of 1% BTFE+2% FEC, there is no Li metal deposition on the surface and it meant the charging and discharging process were carried out with ionic SEI layer on the surface which effectively pass through the Li-ion and accustomed the severe volume expansion. That could be key of the reason why it 1% BTFE+2%FEC electrolyte had a higher capacity 250 cycles and lower resistance of the cell.
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3. Conclusion
In this study, using LiPF6 and LiFSI dual salt system to reduce the hydrolysis of LiPF6 and engaging to side reaction with FEC, forming HF and acidic compounds. In the case of additives, both BTFE and FEC additive applied for LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cell and enhanced the electrochemical performance compared to baseline and FEC additives. Based on XPS analysis, BTFE modified the surface chemistry of Ni-rich cathode as well as Si-containing anode. Both BTFE and FEC derived CEI had enhanced components of LiF and distinct component of CF3 through ex-situ XPS results. Besides, ICP-OES revealed features that CEI derived BTFE+FEC additives prevented to transition metal dissolutions and severe intergranular cracking of the secondary particle and mitigated the phase transition on the surface of the LiNi0.8Co0.1Mn0.1O2 primary particles.
In the case of gra-SiC-Si alloy anode, by DFT calculation and dQ/dV and XPS analysis, certify the BTFE+FEC derived the SEI layer polymeric and LiF-rich. These components of the SEI layer enabled to accommodate the volume expansion of gra-SiC-Si alloy anode and that’s why there is little electrolyte decomposition after cycle (XPS) and Li metal deposition on the anode. Using dual BTFE and FEC reported the synergetic effects which contributed to forming high ionic conductivity and robust SEI layer than the effect of SEI derived by FEC alone.
Consequently, the introduction of BTFE to FEC additive modified the both SEI and CEI layer enhancing the cycling performance and low resistance of a LiNi0.8Co0.1Mn0.1O2 cathode couple with a gra-SiC-Si alloy anode.
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