The microcrack generation retardation - Effect of ETFB on the cathode surface

3. RESULTS AND DISCUSSION

3.2. Effect of ETFB on the cathode surface

3.2.4. The microcrack generation retardation

After the cycling at 25ôC and 45ôC, cells carefully disassembled in glove box and the withdrawing sample after 5min washing keep aluminum pouch in vacuum state to protect from contaminations. To identify degree of the microcrack formation in Ni-rich NCM cathode, cross section SEM images were obtained after ion milling according to temperature and additive or not. Figure 15 (a) and (b) demonstrated cross section features in accordance with ETFB additive at 25ôC and Figure 15 (c) and (d) are obtained image after repeated 300^th charge discharge at 45ôC. As a results, NCM protected by ETFB has clean face and observed little crack formation. However, baseline electrolyte which has EC derived SEI confirm severe microcrack. In particular, cathode protected as SEI formed by baseline electrolyte accelerates the phenomenon about broken the secondary particle at high temperature condition 45ôC. Those difference between baseline and ETFB added electrolyte explained cause improved cell performance in Figure 8 for that reason increasing formation of crack by baseline brings less electrical conductivity and electrochemical inertness than ETFB added electrolyte.

Figure 15. The cross section SEM micrographs of the LiNi0.7Co0.15Mn0.15O2 (a), (b) with baseline electrolyte after 300^th cycles and (b), (d) with 1%ETFB after 300^th cycles after ion milling.

crack

50um 50um

50um

(a) (b)

(c) (d)

25^oC 25^oC

45^oC 45^oC

３１

Pengfei. Y et al studied cause of microcrack formation and demonstrated high-voltage cycling is the direct driving force for intragranylar crack generation.²² Figure 16 indicated cycling performance as the three electrode pouch full cell for that reason full cell system didn’t identify own voltage of the cathode and anode and summerize the cut-off voltage applied to electrode pratically in Table 2. Baseline electrolyte has high charge and discharge voltage than ETFB added electrolyte approximately as much as 0.05V vs Li/Li⁺. As a results, cell applied electrolyte contained baseline deteriorates as proceeding continuous cycling but when introduced ETFB in the electrolyte cell performance improved and situation increasing the voltage cut-off not happen.

３２

Figure 16. Voltage profile of the three electrode pouch full cell according to ETFB or not. (a) and (b) cathode voltage, (c) and (d) anode voltage vs Li/Li⁺.

Table 2. Summerizing the cut-off voltage of the cathode and anode vs Li/Li⁺at the threeelectrode system.

1ETFB

Specific Capacity(mAh/g)

0 50 100 150 200

Voltage(V)

2.0 2.5 3.0 3.5 4.0 4.5

1th 50th 100th 150th 200th 250th 300th

1ETFB

Specific Capacity(mAh/g)

0 50 100 150 200

Voltage(V)

0.0 0.2 0.4 0.6 0.8 1.0

1th 50th 100th 150th 200th 250th 300th Baseline

Specific Capacity(mAh/g)

0 50 100 150 200

Voltage(V)

2.0 2.5 3.0 3.5 4.0 4.5

1th 50th 100th 150th 200th 250th 300th

Baseline

Specific Capacity(mAh/g)

0 50 100 150 200

Voltage(V)

0.0 0.2 0.4 0.6 0.8 1.0

1th 50th 100th 150th 200th 250th 300th

(a) (b)

(c) (d)

1^th 50^th 100^th 150^th 200^th 250^th 300^th

Basline (V) 4.5-0.15 4.44-0.09 4.47-0.12 4.47-0.12 4.48-0.13 4.50-0.15 4.50-0.15 1ETFB (V) 4.44-0.09 4.43-0.08 4.43-0.08 4.44-0.09 4.43-0.08 4.43-0.08 4.43-0.08

３３

Although voltage of cell having condition without additive climbs slightly, that condition not directly effect on formation of the microcrack due to pertaining small difference of voltage between additive or not. Thus, residual by-products inside Ni-rich cathode considered as another reasons, the degradation of the cathode materials, in that Ni-rich NCM has high pH value possiblity undegoing more unwanted reaction is high than other cathode materials. Because Wantanabe. S. et al suggested that the thick SEI film formed by the side reaction at a grain boundaries cause poor electrical contact between the primary particles and lead to capacity and power dereioration.⁴⁴ Additionally, it is investigated impact in the electrolyte of LiOH created in process made Ni-rich NCM due to similar ion size both Ni²⁺ and Li⁺. At first, the concentration of LiOH in LiNi0.7Co0.15Mn0.15O2 powder were inquired and it has 2950ppm. Baseline electrolyte with and without 1%LiOH stored in PE vial during 3days at 60^oC to verify the influence of LiOH. Figure 17 signified LiPF6 hydrolysis accelerate as LiOH introduced in electrolyte and generation amount of HF increase as schematic diagram in Figure 18.^45,14 Consquently, LiOH multiplied the side reaction in bulk electrolyte and it is essencial to

remove. But if LiOH formation is unavoidable, the electrode should be protected from these unwanted reaction through tuning the surface by additive like ETFB. This experiment verified ETFB additive tellingly contribute to perform the surface film at LiNi0.7Co0.15Mn0.15O2 cathode whereby microcrack development retarded effectively.

３４

Figure 17. ¹⁹F NMR spectra with and without 1%LiOH in baseline electrolyte (a), (b) before storage and (c), (d) after storage during 3days at 60^oC.

Chemical shift (ppm) -90 -85 -80 -75 -70 -65 -60

PF

₆^-

PO

₂

F

₂^-

Chemical shift (ppm) -90 -85 -80 -75 -70 -65 -60

PF

₆^-

PO

₂

F

₂^-

Chemical shift (ppm) -90 -85 -80 -75 -70 -65 -60

PF

₆^-

Chemical shift (ppm) -90 -85 -80 -75 -70 -65 -60

PF

₆^-

PO

₂

F

₂^-

PO

₃

F

(a) (b)

(c) (d)

Chemical shift (ppm)

-154.5 -154.0

No HF

Chemical shift (ppm)

-154.5 -154.0

No HF

Chemical shift (ppm)

-154.5 -154.0

Chemical shift (ppm)

-154.5 -154.0

Baseline 1LiOH

1LiOH Baseline

３５

Figure 18. Mechanism of the LiPF6 hydrolysis existed ROH in electrolyte. (a) POF3 generation reaction with ROH and PF5 and (b) further mechanisms by the reactivity of POF3.^45,14

LiPF

₆

↔ LiF + PF

₅

ROH

HF + RF

POF

₃

H₂O

PF₅+ LiF

H₂O

LiPF6 F P F

F F F F

Li⁺

O F

P F

OPF3

H⁺(PO2F2)^- O O P

F F H⁺

O O P

O O 3H⁺

2H⁺(PO3F)^2- O O P

O F 2H⁺

OPF

₃

(a)

(b)

３６ 3.3. Effect of Additives on anode surface.

3.3.1. Differential Capacity (dQ/dV) Analysis

To understand the electrochemical enhancement behavior of cells with 1% ETFB at NCM/graphite full cells, the dQ/dV plot of the cells after precycle were obtained with and without 1%ETFB. Figure 19 (a) show the differential capacity (dQ/dV) versus voltage (V) during precycle. A small different peak can be observed in Figure 19. The apparent peaks at 3.02V and 3.04V were observed in the dQ/dV versus V curves shown in Figure 19 (a), which may respectively be consistent to the EC reduction and ETFB with EC co-reduction at the graphite anode during charge. To clarify the possibility of co- reduction between EC and ETFB at the graphite anode, dQ/dV graphs precycled with EC-free electrolytes (EMC/DEC/1.15M LiPF6 with and without 1% ETFB) were obtained, as shown in the Figure 19 (b). Interestingly, Figure 19 (b) provide the reduction voltage of the ETFB decomposition was similar with the EC in the baseline electrolyte. This finding implied that the reaction between EC and ETFB can happen and co-reduction peak of the EC and ETFB delayed in comparison with only decomposing the EC.

Figure 19. (a) Differential capacity versus voltage (dQ/dV) curves during precycle at the 25^oC for the formation for the LiNi0.7Co0.15Mn0.15O2/graphite full cells with and without ETFB, (b) dQ/dV plots of the LiNi0.7Co0.15Mn0.15O2/graphite full cells with EC-free electrolyte (EMC/DEC/1.15M LiPF6 with and without 1% ETFB) during first Li intercalation of the formation cycle.

Voltage (V)

2.8 3.0 3.2 3.4

dQ/dV

0.000 0.002 0.004 0.006 0.008 0.010

Baseline 1ETFB

EC reduction EC & ETFB Co-reduction

Voltage (V)

2.8 3.0 3.2 3.4

dQ/dV

0.000 0.001 0.002 0.003 0.004 0.005

EMC/DEC EMC/DEC+1ETFB

ETFB reduction

(a) (b)

３７ 3.3.2. Surface modification of graphite anode

XPS measurement was employed to analyze the effect of additive on the electrode surface component of graphite retrieved LiNi0.7Co0.15Mn0.15O2/graphite full cell after precycle. The peak located at 685.1 eV is associated with F atoms within LiF generated by ETFB additive.^46,47,48 As observed from the F 1s spectra in Figure 20 (b), the intensity of LiF was greater than that for the anode without additive in Figure 20 (a), indicating that ETFB participate in developing the surface film as LiF formation.

Contrastively. the intensities of LiF peak on the cathodes with ETFB in the baseline electrolyte were lower than those for cells without additive as indicated in Figure 11 (a) and (b). The implicit intensity of the peaks corresponding to LixPFy, LixPOyFz and phosphate (P-O) in the P 2p peak (Figure 20 (c) and (d)) are narrowly reduced for the anode with 1% ETFB. Four peaks are observed in the C 1s spectra, shown in Figure 20 (f). New peak having CF2 moiety reduced ETFB on the anode was emerged at 290.6 eV in C 1s spectra.^49,50 This results were demonstrated ETFB reduced on the graphite anode and formed protective film to hinder the side-reaction by LiPF6 hydrolysis with residual water and LiOH.

Possible mechanism of ETFB reforming surface chemistry on the anode illustrated in Figure 21 based on dq/dv and XPS analytical method.⁵¹ As depicted, ETFB reduced on the graphite anode with EC solvent as one electron was accepted and put out the LiF and SEI composed CF2 moiety effectively could prevent from unwanted reaction.

３８

Figure 20. F 1s, P 2p and C 1s spectra of graphite anode after precycle. (a), (c) and (e) baseline electrolyte and (b), (d) and (f) electrolyte with 1% ETFB

P 2p

Binding energy(eV)

128 130 132 134 136 138 140 142 144

Intensity(a.u.)

0 500 1000 1500 2000

P 2p

Binding energy(eV)

128 130 132 134 136 138 140 142 144

Intensity(a.u.)

0 500 1000 1500 2000

C 1s

Binding energy(eV)

282 284 286 288 290 292 294

Intensity(a.u.)

0 20000 40000 60000

C 1s

Binding energy(eV)

282 284 286 288 290 292 294

Intensity(a.u.)

0 20000 40000 60000

Li_xPF_y Li_xPOF_y

P-O (P 2p_1/2)

P-O (P 2p_3/2)

Li_xPF_y Li_xPOF_y

P-O (P 2p_1/2)

P-O (P 2p_3/2)

ETFB : CF₂ C-H

C-O-C -CO₂

(c) (d)

(e)

^C-H

(f)

C-O-C -CO₂ F 1s

Binding energy(eV)

680 682 684 686 688 690 692 694

Intensity(a.u.)

0 50000 100000 150000

F 1s

Binding energy(eV)

680 682 684 686 688 690 692 694

Intensity(a.u.)

0 50000 100000 150000

P-F

LiF

(a) (b)

P-F LiF

３９

Figure 21. Possible mechanism of surface film formation for the ETFB additives on the graphite anode.

ETFB additive

Fcontained compounds (SEI)

+EC

Further decompose

+ e

LiF

４０

4. Conclusion

In this study, ETFB additive applied for LiNi0.75Co0.15Mn0.15O2/graphite full cell and obtained the results improved electrochemical performance compared to baseline electrolyte. On the basis of FT-IR and XPS analysis, ETFB beneficially reformed surface chemistry of cathode as well as anode having distinct component of CF2 through ex-situ XPS results. These difference results in outcome the capacity retention of full cell was significantly increased from 26.5% to 84.8% after repeated 300 charge- discharge cycles at 45^oC. In addition, ICP-MS and potentiostatic profiles revealed features that SEI derived ETFB raises the anodic stability on the cathode and DSC measurements also demonstrated enhanced thermal stability which is weakness of the Ni-rich cathode. The cross-section SEM images and ¹⁹F-NMR results suggest surface modification by ETFB additive significantly impeded broken the secondary particle by having protective surface reducing undesirable reaction as well. Consequently, the introduction of ETFB showed improvement in the cycling performance of a high Ni content NCM cathode couple with a graphite anode.

４１

REFERENCES

1. Choi, N.-S.; Chem, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.;

Cho, J.; Bruce, P. F. Angewandte Chemie 2012, 51, (40), 9994-10024 2. Xu, K. Chemical review 2004, 104, 4303-4417

3. Xu, K. Chemical review 2014, 114, (23), 11503-618

4. Hun, L.; Meltem, W.; Ozan, T.; Kun, F.; Xiangwu, Z. Energy & Environmental Science 2014, 7, 3857- 3886

5. Belharouak, I.; Sun, Y.-K.; Liu, J.; Amine, K.; Journal of Power Sources 123 2003, 247-252.

6. Jiang, X.; Sha, Y.; Cai, R.; Shao, Z.; Journal of Materials Chemistry A 3 2015, 10536-10554.

7. Wang, J.-H.; Wang, Y.; Guo, Y.-Z.; Ren, Z.-Y.; Liu, C.-W.; Journal of Materials Chemistry A 1 2013, 4879-4884.

8. Fergus, J.W.; J. Power Sources 195 2010, 939-954.

9. Rao, C.V.; Reddy, A.L.M.; Ishikawa, Y.; Ajayan, P.M.; ACS Applied Materials & Interfaces 2011, 2966-2972.

10. Li, J.; Yao, R.; Cao, C.; ACS Applied Materials & Interfaces 2014, 5075-5082

11. Liu, W.; Oh, P.; Liu. X.; Lee. M.J.; Cho. W.; Chae. S.; Kim. Y.; Cho. J.; Angewandte Reviews 2015, 54, 4440-4457

12. Yang, L.; Ravdel, B.; Lucht, B.; Electrochemical Solid-State Letters, 13 2010, pp. A95-A97 13. Choi, N.-S., Han, J.-G., Ha, S.-Y., Park, I., Back, C.-K. RSC Advances 5 2015, pp. 2732-2748 14. Song, Y.-M.; Kim, C. –K.; Kim, K.-E.; Hong, S. Y.; Choi, N.-S. Journal of Power Sources 2016,

302, 22-30

15. Koo, B.; Lee, J.; Lee, Y.; Kim, J. K.; Choi, N.-S. Electrochimica Acta 2015, 173, 750-756

16. Han, J.G.; Lee, S. J.; Lee. J.G.; Kim, J. S.; Lee, K.T.; Choi, N.-S. ACS Applied Materials &

Interfaces 2015, 7, 8319-8329

17. Mun, J.; Kim, S.; Yim, T.; Ryu, J.H; Kim, Y.G; Oh, S.M. Journal of Electrochemical Society. 157 2010, A136-A141.

18. Zhang, S.S. Journal of Power Sources 162 2006, 1379-1394.

４２

19. Qin, Y.; Chen, Z.; Lu, W.; Amine, K.; Journal of Power Sources 195 2010, 6888-6892.

20. Lu, W.; Chen, Z.; Joachin, H.; Prakash, J.; Liu, J.; Amine, K.; Journal of Power Sources 163 2007, 1074-1079.

21. Cho, I.H.; Kim, S.-S.; Shin, S.C.; Choi, N.-S. Electrochemical Solid-State Letters. 13 2010, A168- A172.

22. Yan. P.; Zheng J.; Gu. M.; Xiao. Jie.; Zhang. J.-G.; Wang. C.-M. Nature communications 2016, 8:14101

23. Lim. J.-M.; Hwang. T.; Kim. D.; Park. M.-S.; Cho. K.; Cho. M. Scientific reports 2016 7:39669, DOI : 10.1038/srep39669

24. Kim, J.-M; Chung, H.-T.; Electrochimica Acta 2004, 49, 937.

25. Yoon, W.-S.; Balasubramanian, M.; Chung, K. Y.; Yang, X.-Q.; McBreen, J.; Grey, C. P.; Fischer, D. A.; Journal of American Chemical Society 2005, 127, 17479.

26. Noh, H.-Y.; Youn, S.J.; Yoon, C. S.; Sun, Y. –K. Journal of power source 223 2013, 121-130 27. Yao, D,; Wand, H.; Xiao, A.; Wells, L.; Dahn. J. R. Journal of Electrochemical Society. 2015, 162,

A169−A175.

28. Dennis, R. G.; René S.; Ralf W.; Björn H.; Sascha N.; Isidora C.-L.; Raphael W. S.; Martin W.

Electrochimica Acta 2014, 134, 393−398.

29. Im, J.S.; Lee, J.E.; Ryou, M.-H.; Lee, Y.M.; Cho, K.Y.; Journal of The Electrochemical Society, 2017, 164 (1) A6381-A6385

30. Lin, M.; Glazier, S.L.; Xia, J.; Peters, J. M.; Liu, Q.; Allen, J.; Doig, R. N. C; Dahn J.R.; Journal of the electrochemical Society 2017,164 (1) A5008-A5018

31. Jianhui. W.; Yuki. Y.; Keitaro. S.; Ching Hua. C.; Yoshitaka. T.; Atsuo. Y. Nature communication.

2016, 7:12032, DOI : 10.1038/ncomms12032

32. Lee, S. J.; Han, J. G.; Lee, Y. W.; Jeong, M.-H.; Shin, W.C.; Ue, M.; Choi, N.-S. Electrochimica Acta 137 2014, 1-8

33. Murmann, P.; Streipert, B.;Kloepsch, R.; Ignatiev, N.; Sartori, P.; Winter, M.; Cekic-Laskovic, I.

Physical Chemistry chemical Physics 2015, 17, 9352-9358

34. Yan, G.; Li, X.; Wang, Z.; Guo, H.; Wang, J.; Peng, W.; Hu, Q. Electrochemica Acta 166 2015, 190-196

35. Lee, Y.M.; Nam, K.-M.; Hwang, E.-H.; Kwon, Y.-G.; Kang, D.-H.; Kim, S.-S.; Song, S.-W. The journal of physical chemistry C 2016, 118, 10631-10639

36. Patrick, L.; Petr, N. Journal of The Electrochemical Society 2014, 161 (10) A1555-1563

37. Etacheri, V.; Haik, Y.; Goffer, G.; Roberts, I. C.; Stefan, R.; Fasching.; Aurbach, D.;

Langmuir,2011, 28, 965

Dalam dokumen in lithium ion batteries (Halaman 32-38)