3. RESULTS AND DISCUSSION
3.3. Effects of BTFE+FEC on the cathode surface
3.3.2. The effect of BTFE+FEC-derived CEI
To investigate the effect of BTFE-derived CEI layer, we measure the transition metal dissolution in the same electrolyte (baseline electrolyte) while storing precycled-cathodes from different electrolytes formation, expecting to have different CEI layer on the surface, for 7 days at 45 oC. (Figure 20) demonstrates the transition metal dissolution concentrations in the electrolyte samples existing 1%
BTFE+2% FEC and 3% FEC derived CEI cathodes, stored in PE vial with 2g of electrolyte and de- lithiated state. The amount of metal dissolution decreased which means by introducing the BTFE additive, it effectively protected LiNi0.8Co0.1Mn0.1O2 cathode surface. (Table 2) This feature suggests CEI formed by BTFE including –CF2- and –CF3 in (Figure 19(b)) has higher oxidation durability than baseline and also prevents undesirable electrolyte decomposition with a highly oxidized transition metal, which could lead to transition metal dissolution.
26
Figure 20. (a) A comparison of transition metal dissolution of the 1% BTFE+2% FEC and 3% FEC contained electrolyte at 45oC for 7 days. (b) Schematic illustration of the BTFE-derived CEI and it restrained the transition metal dissolution.
Table 2. The amount of transition metal dissolution in the baseline electrolyte with retrieved cathode precycled with 1% BTFE+2% FEC and 3% FEC electrolyte and storing at 45oC for 7 days.
After the cycle tests at 45 oC, to retrieve the cycled cathodes and anodes for surface chemistry and morphological analysis, the cells were disassembled in a glove box. The cathodes and anodes were rinsed with DMC solvent to remove the residual electrolyte and then dried for a few minutes. To identify the degree of the microcrack formation in LiNi0.8Co0.1Mn0.1O2 cathode, cross-section SEM images were obtained after ion milling with 1% BTFE+2% FEC and 3% FEC. (Figure 21(a)) 1% BTFE+2% FEC and (Figure 21(b)) 3% FEC demonstrated cross-section features with and without BTFE-derived CEI layer after
27
repeated 250th charge-discharge at 45oC. Both of secondary particles (a) 1% BTFE+2% FEC and (b) 3%
FEC composed of assembled small primary particles of LiNi0.8Co0.1Mn0.1O2 cathode have some crack.
In (a) 1% BTFE+2% FEC, the protected by BTFE were observed less microcrack than 3% FEC.
However, 3% FEC electrolyte which has relatively little surface film after precycle, suffered severe microcrack. As magnifying the image, some decomposition compounds were found between the niche of primary particle, covered up almost every particle of the cathode with 3% FEC after cycling. With the EDS data, the decomposition of products is certainly formed by electrolyte decomposition mostly caused by F-based sources from LiPF6 salt.
Figure 21. The cross-section SEM and energy-dispersive X-ray spectroscopy (EDS) micrographs of the LiNi0.8Co0.1Mn0.1O2 (a) with 1% BTFE+2% FEC electrolyte (b) 3% FEC after 250th cycles at 45 °C after ion milling.
28
Figure 22. HR-TEM/STEM images of LiNi0.8Co0.1Mn0.1O2 particle from full cell after 250 cycles at 45 °C with (a) 1% BTFE+2% FEC (b) 3% FEC electrolyte. [41]
TEM/STEM measurement was taken to scrutinize closely the phase transition of the cathode primary particle after the cycle. In Ni-rich cathode, the cation mixing frequently occurs during repeated charge and discharge. This is one of the critical aspects of the electrochemical degradation of the Ni-rich cathodes reconstructing the intrinsic surface structure. In extremely oxidizing conditions during the end of the charging process, the well-ordered cathodes having R3̅ m structure, become destabilized and transition metal ions at octahedral 3a sites tend to migrate to the neighboring Li sites (octahedral 3b) to stabilize the structure. Especially, the size of the Ni2+ and Li+ particles is similar and this phenomenon called cation mixing. As Li-ions escape from the cathode during the charging, cation mixing occurs. By repeated cycle process, some of this reaction turns into irreversible and results in a phase transition.
This leads to an irreversible transformation in the crystal structure like Fm3̅m rock-salt phase (Figure 7). Even in case of 3% FEC, it had lots of dislocation site and the hole shapes caused by combination of cation mixing and loss of Li layer. This phenomenon at the surface disturbs active lithium intercalation in place and slowed down the reaction kinetics like charge transfer process deteriorates capacity of the cathode as the rock-salt phase forms constantly during the cycle. As a result of
29
continuous charging, this phase becomes irreversible, undergoes layered structure, and changes into spinel and rock salt structure, increasing the resistance of the surface. In 1% BTFE+2% FEC electrolyte, the thickness of the rock-salt phase with the edge side is about 2nm, while in case 3% FEC, it had more than 7nm. In consider of the radius of the primary particle, this change could not be ignored. [44] This might be one of the reasons that 3% FEC electrolytes had a sudden decline in capacity.
3.4. Effect of BTFE+FEC on the anode surface.
3.4.1. Surface modification of gra-SiC-Si alloy anode
With the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of BTFE additive, it was expected to be reduced prior to the EC at the anode. Comparing to FEC additive, though BTFE reported slightly higher LUMO energy than FEC, both of them are low enough to reductively decompose to the anode. Both BTFE and FEC additives were expected to engage in a co- reduction reaction. (LUMO energy level by DFT calculation, BTFE : -0.63, EC: -0.40, FEC: -0.84) (Figure 23(a)). To clarify the reduction of BTFE at the gra-SiC-Si alloy anode. Figure 23(b) shows the differential capacity (dQ/dV) versus voltage (V) during precycling with baseline electrolyte with and without 1% BTFE. Two different peaks around 2.75 V and 3.0 V are observed in Figure 23 (b). The small peak observed at 2.75 V comes from BTFE reduction and the apparent peak observed at 3.0V comes from EC reduction. Interestingly, it shows that BTFE additive reduced prior to EC and formed the SEI layer at gra-SiC-Si alloy anode which reduced the peak of the EC reduction.
To understand the electrochemical enhancement behavior with 1% BTFE +2% FEC at LiNi0.8Co0.1Mn0.1O2 /gra-SiC-Si alloy full cells, the dQ/dV plot of the cells after precycle were obtained with and without 1% BTFE +2% FEC. By using dual BTFE and FEC, reduced on a broad range of voltage about 2.5 V-2.75 V, also at 3.05 V, BTFE and FEC are co-reduced at around 2.8 V which is slightly delayed comparing from BTFE without FEC reduction peak. (Figure 23(c))
30
Figure 23. (a) A computational approach to determine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (b) Differential capacity versus voltage (dQ/dV) curves during precycle at the 25oC for the formation for the LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cells with and without BTFE, (c) dQ/dV plots of the LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cells with 1%BTFE+2%FEC and 3% FEC on the baseline electrolyte during first Li intercalation of the formation cycle.
31
XPS measurement was employed to analyze the effect of additive on the electrode surface component of gra-SiC-Si alloy anode retrieved LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cell after precycle. Figure 24(a) with baseline electrolyte (b) 1% BTFE+2% FEC electrolyte (c) 3% FEC electrolyte. The graph in F 1s, P-F at 688 eV and S-F at 687.3 eV come from the LiPF6 and LiFSI salt. The implicit intensity of the peaks corresponding to phosphate (P-O) around 137 eV in the P 2p peak (Figure 24(b) and (c) are remarkably reduced for the anode with FEC containing composition. As observed from the S 2p spectra, N-S-O2F at 170.5 eV, S-O2F at 169.3 eV, Li2SO3 at 167.5 eV peaks are related to LiFSI salt decomposition. The introduction of FEC additive has significantly reduced the electrolyte decomposition peaks. This demonstrates that as mentioned above, FEC additive forms the SEI layer on the Si-containing anode which can effectively accommodate the volume expansion by embracing anodic with flexible and robust SEI layer and preventing the further decomposition of electrolyte on the anode after the precycle.
There are some differences between 1% BTFE+2% FEC and 3% FEC electrolyte, and among them, the F 1s peak of (b) and (c), in the F 1s graph, the LiF peak represents a higher intensity in the electrolyte formation with BTFE. Even though the content of FEC is smaller in the electrolyte 1% BTFE+2% FEC than 3% FEC. As a result, the BTFE is shown to effectively form a LiF-based film, which makes the polymeric films more robust and conductive to Li-ion with a hopping mechanism. As (Figure 18(c)), both BTFE and FEC derived SEI layer are composed of polymeric species with LiF-rich that it can represent the outstanding performance of tolerating the volumetric expansion.
32
Figure 24. F 1s, P 2p and S 2p spectra of gra-SiC-Si alloy anode after precycle. (a) baseline electrolyte and (b) electrolyte with 1% BTFE+2% FEC, and (c) electrolyte with 3% FEC.
33 3.4.2. The effect of BTFE+FEC derived SEI
Figure 25. After cycling test, pouch cells were dissembled and the graph and the photo of gra-SiC-Si alloy anode from the full cell after cycle baseline after 150th cycles, both 1% BTFE+2% FEC and 3%
FEC after 250th cycles at 45 °C. Keep this in mind that all the below data were analyzed by the different numbers of cycles with baseline (150th) and with additives one (250th).
To verify and analyze the role of additives, pouch cells were dissembled and the gra-SiC-Si alloy anode were taken after cycle test. Even with the naked eye, it is verified that serious deterioration was on the surface of the anode like deposition of Li metal and traces of tearing on the baseline after 150th cycles and FEC after 250th cycles. On the other hand, in the case of 1% BTFE+2% FEC formation, although slight deterioration of the edge was observed, there was no deposition of lithium metal on the surface. It was thought due to using dual BTFE and FEC additive, low resistive and high ionic SEI layer was made with rich LiF which had reported excellent lithium-ion conductivity.
Also, in the aspects of band gap of SEI layer components, LiF has enough wide energy gap that it can insulate the electron tunneling from the highly negatively charged electrode to the electrolyte components. (Figure 26) [46, 47]
34
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]
35
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).
36
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.
37
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)).
38
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),
39
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.
40
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.
41
REFERENCES
1. Luo, X., et al., Overview of current development in electrical energy storage technologies and the application potential in power system operation. Applied energy, 2015. 137: p. 511-536.
2. Mahlia, T., et al., A review of available methods and development on energy storage;
technology update. Renewable and Sustainable Energy Reviews, 2014. 33: p. 532-545.
3. Nishi, Y., Lithium ion secondary batteries; past 10 years and the future. Journal of Power Sources, 2001. 100(1-2): p. 101-106.
4. Scrosati, B. and J. Garche, Lithium batteries: Status, prospects and future. Journal of power sources, 2010. 195(9): p. 2419-2430.
5. Yoo, H.D., et al., On the challenge of developing advanced technologies for electrochemical energy storage and conversion. Materials Today, 2014. 17(3): p. 110-121.
6. Choi, N.S., et al., Challenges facing lithium batteries and electrical double‐layer capacitors.
Angewandte Chemie International Edition, 2012. 51(40): p. 9994-10024.
7. Xu, K., Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical reviews, 2004. 104(10): p. 4303-4418.
8. Lee, H., et al., A review of recent developments in membrane separators for rechargeable lithium-ion batteries. Energy & Environmental Science, 2014. 7(12): p. 3857-3886.
9. Xu, B., et al., Recent progress in cathode materials research for advanced lithium ion batteries.
Materials Science and Engineering: R: Reports, 2012. 73(5-6): p. 51-65.
10. Arai, H., et al., Reversibility of LiNiO2 cathode. Solid State Ionics, 1997. 95(3-4): p. 275-282.
11. Cheng, F., et al., Porous LiMn 2 O 4 nanorods with durable high-rate capability for rechargeable Li-ion batteries. Energy & Environmental Science, 2011. 4(9): p. 3668-3675.
12. Belharouak, I., et al., Li (Ni1/3Co1/3Mn1/3) O2 as a suitable cathode for high power applications. Journal of Power Sources, 2003. 123(2): p. 247-252.
13. Bak, S.-M., et al., Structural changes and thermal stability of charged LiNi x Mn y Co z O2 cathode materials studied by combined in situ time-resolved XRD and mass spectroscopy. ACS applied materials & interfaces, 2014. 6(24): p. 22594-22601.
42
14. Fergus, J.W., Recent developments in cathode materials for lithium ion batteries. Journal of power sources, 2010. 195(4): p. 939-954.
15. Myung, S.-T., et al., Nickel-Rich Layered Cathode Materials for Automotive Lithium-Ion Batteries: Achievements and Perspectives. ACS Energy Letters, 2016. 2(1): p. 196-223.
16. Markevich, E., et al., New Insights Related to Rechargeable Lithium Batteries: Li Metal Anodes, Ni Rich LiNixCoyMnzO2 Cathodes and Beyond Them. Journal of The Electrochemical Society, 2019. 166(3): p. A5265-A5274.
17. Liu, W., et al., Nickel‐rich layered lithium transition‐metal oxide for high‐energy lithium‐ion batteries. Angewandte Chemie International Edition, 2015. 54(15): p. 4440-4457.
18. Manthiram, A., B. Song, and W. Li, A perspective on nickel-rich layered oxide cathodes for lithium-ion batteries. Energy Storage Materials, 2017. 6: p. 125-139.
19. Xu, J., et al., A review of Ni-based layered oxides for rechargeable Li-ion batteries. Journal of Materials Chemistry A, 2017. 5(3): p. 874-901.
20. Xu, J., et al., Understanding the Degradation Mechanism of Lithium Nickel Oxide Cathodes for Li-Ion Batteries. ACS Appl Mater Interfaces, 2016. 8(46): p. 31677-31683.
21. Han, J.-G., et al., Scavenging Materials to Stabilize LiPF6-Containing Carbonate-Based Electrolytes for Li-Ion Batteries. Advanced Materials, 2019. 31(20): p. 1804822.
22. Edström, K., T. Gustafsson, and J.O. Thomas, The cathode–electrolyte interface in the Li-ion battery. Electrochimica Acta, 2004. 50(2-3): p. 397-403.
23. Ohta, A., et al., Relationship between carbonaceous materials and electrolyte in secondary lithium-ion batteries. Journal of Power Sources, 1995. 54(1): p. 6-10.
24. Kim, K., et al., Dual-function ethyl 4, 4, 4-trifluorobutyrate additive for high-performance Ni- rich cathodes and stable graphite anodes. Journal of Power Sources, 2018. 396: p. 276-287.
25. Ryu, H.-H., et al., Capacity Fading of Ni-Rich Li[NixCoyMn1–x–y]O2 (0.6 ≤ x ≤ 0.95) Cathodes for High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation? Chemistry of Materials, 2018. 30(3): p. 1155-1163.
26. Xiong, X., et al., Enhanced electrochemical properties of a LiNiO2-based cathode material by removing lithium residues with (NH4)2HPO4. J. Mater. Chem. A, 2014. 2(30): p. 11691-11696.
27. Manthiram, A., et al., Nickel-Rich and Lithium-Rich Layered Oxide Cathodes: Progress and