High Concentration Ether Electrolytes for Stable High-Voltage Lithium

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High Concentration Ether Electrolytes for Stable High-Voltage Lithium Metal Batteries Xiaodi Ren, Liangfeng Zou, Shuhong Jiao, Donghai Mei, Mark H. Engelhard, Qiuyan Li, Hongkyung Lee, Chaojiang Niu, Brian D. Adams, Chongmin Wang, Jun Liu, Ji-Guang Zhang, and Wu Xu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00381 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Energy Letters

High Concentration Ether Electrolytes for Stable High-Voltage Lithium Metal Batteries

Xiaodi Ren1, Lianfeng Zou2, Shuhong Jiao1,3, Donghai Mei4, Mark H. Engelhard2, Qiuyan Li1, Hongkyung Lee1, Chaojiang Niu1, Brian D. Adams1, Chongmin Wang2, Jun Liu1, Ji-Guang Zhang1,*, Wu Xu1,* 1

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland,

Washington 99354, United States 2

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, Washington 99354, United States 3

Department of Materials Science and Engineering, University of Science and Technology of

China, Hefei, Anhui 230026, China 4

Physical and Computational Directorate, Pacific Northwest National Laboratory, Richland,

Washington 99354, United States

* Corresponding authors. Email: [email protected] (W. Xu); [email protected] (J.-G. Zhang)

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Abstract High-voltage (>4.3 V) rechargeable lithium (Li) metal batteries (LMBs) face huge obstacles due to the high reactivity of Li metal with traditional electrolytes. Despite of their good stability with Li metal, conventional ether-based electrolytes are typically used only in 4.0 V) Li-ion intercalation cathodes (LiCoO2 (LCO), LiMn2O4, LiNi0.80Co0.15Al0.05O2 (NCA), LiNixMnyCo1-xyO2

(NMC), etc.).14-15 Similarly, significant interfacial degradation of a polyether-based polymer

(polyethylene oxide, PEO) electrolyte was reported on LiCoO2 cathode at 4.2 V.16 As a result, ether-based electrolytes have been only used in low voltage (99.8% for the first 300 cycles, was observed in the LiFSI-1.4DME (i.e. 4 M LiFSI in DME) electrolyte (Figure S2). The improved cell CE could also benefit from the reduced amount of side products originating from the Li anode side, as reported very recently.27 When the charge cut-off voltage was further increased to 4.4 V, the cell shows a very limited capacity decay after 200 cycles (Figures S3, S4). No apparent changes of X-ray diffraction (XRD) patterns were detected for the cycled NMC333 electrodes (Figure S5). These results prove etherbased electrolytes with suitable formulations are useful for high-voltage batteries. While ether electrolytes with a regular concentration were regarded as unstable under high voltages, the influence of the salt/solvent molar ratio on the solvent anodic stability can be verified by density functional theory (DFT) calculations. As illustrated in Figure 1d and the density-ofstates analysis (Figure S6), the highest occupied molecular orbital (HOMO) energy levels of the solvation complexes of LiFSI-DME shifted to lower values with increasing the molar ratio of LiFSI to DME, indicating an improved oxidation stability because of the donation of the lone electrons of oxygen atoms to Li+ cations in the solvation complexes. This effect along with the decreased population of free labile ether molecules can explain the high oxidation onset potential measured on platinum (Pt) electrode (Figure S7). Nevertheless, when using a high-surface-area reactive NMC333 electrode as the working electrode for the cyclic voltammetry (CV) test, the oxidation of the concentrated LiTFSI-2.0DME (i.e. 3 M LiTFSI in DME) electrolyte (close to saturation) was significantly prompted (Figures 1e, S7), with no reversible Li+ deintercalation/intercalation processes as observed in the LiFSI-2.0DME and the LiFSI-1.4DME electrolyte (Figure S8). Compared to the poor anodic stability of the low concentration LiFSI9.0DME (i.e. 1 M LiFSI in DME) electrolyte (Figure 1e), increasing the LiFSI/DME molar ratio (i.e. salt concentration) not only enhanced the thermodynamic stability against electrochemical



oxidation, but also induced an effective passivation mechanism to avoid the catalytic ether decomposition on the reactive electrode surface. Such a passivation phenomenon could also be found on the Al current collector (Figures S9-S11 and the related detailed discussion). It is worth mentioning that a recent study showed failure of using concentrated LiFSI-in-DME electrolytes over 4.0 V, which is likely due to the corrosion of the stainless steel (SS) positive case of the testing cells (Figures S12-S14).9 Although the concentrated electrolyte has an increased viscosity and a decreased conductivity, its high Li+ population and transference number are favorable for high rate performances, as shown in Figure S15.28 Recent reports also suggest the concentrated ether electrolyte can have greatly reduced volatility and flammability, which are beneficial for future applications.28-29 The excellent compatibility of concentrated LiFSI-ether electrolytes with both the Li anode and the high-voltage NMC333 cathode are proved by post-analyses. The top-view of the Li anode after 500 cycles showed a relatively uniform and compact surface, with no dendritic Li but a few bulky Li particles observed (Figure 2a), which explains the absence of short-circuiting during longterm cycling.13 More importantly, the cross-sectional view of the cycled Li anode showed a top layer only 55 µm thick (Figure 2b). In contrast, the Li anode after 200 cycles in the LiPF6-EC/EMC electrolyte has a significantly roughed top surface and a 160 µm thick surface corrosion layer (Figure S16). This verifies that the Li anode stability is significantly improved in the LiFSI1.4DME electrolyte compared to the 1 M LiPF6-EC/EMC electrolyte. It should be noted that a thick Li anode was used here to eliminate the possibility of Li metal depletion when evaluating the cathode stability under high voltage. Preliminary tests also indicate that this ether electrolyte can enable a generally stable cycling for over 200 cycles without obvious sign of Li anode depletion (Figure S17) under very challenging conditions with a high cathode loading (4 mAh cm-2), a low


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ratio of negative/positive areal capacities (N/P) (2.5 with a 50 µm thick Li anode) and a very limited amount of electrolyte (21 µL for an electrolyte/capacity or E/C ratio close to 5 g (Ah)-1). On the cathode side, no apparent changes were found on cathode crystal surfaces after 60 cycles and 500 cycles (Figures 2c, 2d) compared to the pristine NMC333 (Figure S18). In previous studies, acidic species generated from ether oxidation were found to cause transition metal ion dissolution.30 However, no detectable transition metal ions could be found on the cycled Li anode in the LiFSI-1.4DME electrolyte by energy dispersive spectroscopy (EDS), while as compared to the 1 M LiPF6-EC/EMC electrolyte after 300 cycles (Figure S19). Therefore, the CEI layer generated in the LiFSI-1.4DME electrolyte could isolate the contact between the highly active NMC333 surface and the residual free DME molecules to provide effective protection. The chemical nature of the CEI formed in the LiFSI-1.4DME electrolyte was analyzed by XPS. After 60 cycles in the LiFSI-1.4DME electrolyte, the species seen from the C 1s spectrum were almost identical to those of the pristine cathode, though the total intensity was decreased (Figure 2e). Only a slight increase of C=O and C-O ratios could be observed, which may come from a limited amount of DME decomposition during the CEI formation. However, the F 1s signal of LiF at ~685.5 eV was greatly enhanced (Figure 2f, 70% of total F signals). Meanwhile, SO42(169.8 eV) and NOx (400.5 eV) signals could clearly be characterized in Figures 2g and 2h, respectively. In addition, relative ratios of the M-O signal at 529.8 eV (O 1s, Figure S20) apparently increased upon sputtering as the pristine NMC sample, suggesting an ultrathin CEI after cycling. Significant enrichments of F (28-35%) and Li (21-25%) elements at the top 5 nm region were observed compared to the pristine sample. The insoluble species (LiF, LixSOy, LixNOy and etc.) derived from the reaction of FSI- anions on the cathode formed a protective interfacial layer to shield the active cathode away from labile ether molecules.



The thin CEI and its effective protection of the NMC333 cathode can be further supported by atomic-resolution scanning transmission electron microscopy (STEM) and high-resolution transmission electron microscopy (HRTEM). After 60 cycles, only the top 3-4 slabs of the layered cathode (~2.5 nm) showed a disordered rock-salt structure with transition metal ions occupying the Li sites, almost same as the pristine (Figures 3a, 3b). A composite CEI of ~4-9 nm thickness with apparent crystalline domains was observed on the NMC333 surface (Figure 3c). With an interlayer distance of 0.20 nm, the crystalline phase was likely to be LiF (d200=0.201 nm), consistent with the XPS results. It is noteworthy that this CEI in the concentrated LiFSI-ether electrolyte had an apparently higher abundance in LiF than those in carbonate-based electrolytes 9,

which probably indicates a different reaction pathway involving DME molecules. LiF is a good

electronic insulator and possesses excellent anodic stability (6.4 V vs. Li/Li+)31, whose compact packing in the CEI could efficiently isolate the cathode from the electrolyte and reduce or stop electron tunneling after a critical thickness. Therefore, this CEI formed in the concentrated LiFSIDME electrolyte is significantly different from the one formed in the concentrated LiTFSI-DME electrolyte, where serious cathode corrosions were observed because of the generation of acidic side products from ether decompositions and the lack of a stable protective interface.20 It indicates that LiFSI and LiTFSI salts have substantially different reaction mechanisms in concentration ether electrolytes on high voltage cathodes. In addition, the formation of LixSOy and LixNOy species from LiFSI as well as their interfaces with LiF could provide efficient Li+ diffusion pathways through the CEI. As a result of these favorable electronic and ionic properties of the CEI, the NMC333 material underneath has shown superior stability. Even after 500 cycles, only the top few nanometers of the NMC333 particles showed sign of cation mixing (Figure 3d).


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ACS Energy Letters

Furthermore, the stability of the concentrated LiFSI-1.4DME electrolyte is ubiquitous on different high-voltage cathodes, as extremely stable cycling performances of Li||LCO and Li||NCA batteries are also demonstrated (Figures 4a and S21). More importantly, when the population of free DME molecules is further reduced in the LiFSI-1.0DME (i.e. 5 M LiFSI in DME) electrolyte, Li||NMC333 batteries can be cycled up to 4.5 V with very high cell CE (99.81%), which is unprecedented for LMBs using ether electrolytes and comparable with the anodic limit of carbonate electrolytes, as shown in Figures 4b and S22. Albeit the intrinsic issue of cathode lattice oxygen loss and the resulting interfacial structural decay under 4.5 V32, the Li||NMC333 battery in the LiFSI-1.0DME electrolyte showed not only a better capacity retention than the battery in the LiPF6-EC/EMC electrolyte and the LiTFSI-LiDFOB-3.0DME electrolyte reported recently20, but also apparently less polarization increase (Figures 4b and S23). Such concentration effect could also contribute to the improved cycling stability of Li||NMC622 cells observed in the high concentration LiFSI-LiTFSI/DME dual-salt electrolyte with 4.6 m LiFSI and additional 2.3 m LiTFSI, which was just reported33. The extraordinary cathode protection capability of the CEI in the LiFSI-1.0DME electrolyte can be further evidenced by the dramatically improved stability of the NMC811cathode under 4.4 V compared to the LiPF6-EC/EMC electrolyte (Figure 4c). The minimum cell voltage profile and impedance change, higher cell CE and the elimination of transition metal ion dissolutions (as indicated by the EDS of cycled Li anode, Figures S24-S26) all support that the electrolyte side reactions on the highly reactive Ni-rich NMC811 cathode are significantly suppressed. It is worth mentioning that these results are in agreement with recent studies showing that the parasitic reactions with the reactive NMC811 cathode and the electrolyte, instead of the phase transition at high voltages, are mainly responsible for the capacity decay.23, 34 Electrolyte design, therefore, is an effective approach to improve the stability of NMC811 under



high voltages.35-37 Furthermore, the faster capacity fading and larger cathode impedance in the concentrated LiFSI-dimethyl carbonate (DMC) electrolyte (LiFSI-1.1DMC or 5.49 M LiFSI in DMC, as reported previously38) indicates that both the Li salt and the solvent are playing significant roles in the reactions on high voltage cathode surfaces, which effect has rarely been discussed before. The highly effective and very unique CEI layer formed in the concentrated LiFSI-ether electrolytes is probably enabled by the synergy between the LiFSI salt and the ether molecules under concentrated conditions. It suggests that the effect of concentrated electrolyte on high voltage cathodes lies in not only the decrease of free solvent molecules and the enhanced oxidation stability of coordinated solvent molecules, but also the specific chemistry between the salt, the solvent and the cathode active sites. We have demonstrated the excellent protection ability of the CEI formed in concentrated LiFSI-ether electrolytes towards high-voltage (up to 4.5 V) cathodes with highly active surfaces. As an analogue to the SEI layer formed on graphite anode which provides a kinetical barrier to stop solvent reductive decomposition14, this unique CEI formed on the high voltage cathodes via the synergistic reactions between the LiFSI salt and the ether solvent has proven to be effective in eliminating the catalytic oxidation of the ether solvent molecules and preserving the cathode crystalline structure. Our results demonstrate the advantages of using ether electrolytes for highvoltage LMBs and point out a promising direction for developing long-life high energy density LMBs.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Experimental details, cell voltage profiles, lean electrolyte results, XRD, SEM, EDX and EIS characterizations of the cycled electrodes, LSV and CV curves of Al foil, SS disk and NMC cathodes, XPS and SEM of Al foils (PDF)

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ACKNOWLEDGEMENTS This work has been supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technologies Office of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) program (Battery500 Consortium) under the contract no. DE-AC02-05CH11231. The microscopic and spectroscopic characterizations as well as computational calculations were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. The salt LiFSI was provided by Dr. Kazuhiko Murata of Nippon Shokubai Co., Ltd. The LCO and NCA electrode laminates were supplied by Dr. Bryant Polzin of ANL.

AUTHOR CONTRIBUTIONS W.X. and X.R. initiated the research and designed the experiments together with J.-G.Z. S.J. and B.D.A. contributed in the discussion. X.R. performed the electrochemical measurements and the SEM observations with the help from S.J. L.Z. carried out the TEM characterizations. D.M. and M.H.E. performed the DFT calculations and the XPS measurements, respectively. Q.L., H.L. and C.N. prepared the cathode laminates. C.W. and J.L. helped analyze the test results. X.R., W.X. and J.-G.Z. prepared the manuscript with input from all other co-authors.

COMPETING INTERESTS The authors declare no competing financial interests.



Figure 1. (a) Comparison of cycling performance in the LiPF6-EC/EMC electrolyte (1 M LiPF6 in EC-EMC, 3:7 by wt.) and the LiFSI-1.4DME electrolyte under 4.3 V charge cut-off voltage. (b, c) Voltage profiles of Li||NMC333 batteries in (b) the LiFSI-1.4DME electrolyte and (c) the 1 M LiPF6-EC/EMC electrolyte. (d) HOMO energy levels of electrolytes with different LiFSI/DME molar ratios (Inset shows an illustration of the LiFSI-DME solvation complex at 1:1 molar ratio). (e) CV curves of different ether-based electrolytes with NMC333 cathodes as working electrodes in Li||NMC333 coin cells (magnified region shown in the inset).


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Figure 2. (a) Top view and (b) cross-sectional view of the Li anode after 500 cycles in the Li||NMC333 cells with the LiFSI-1.4DME electrolyte. SEM images of the NMC333 cathodes (c) after 60 cycles and (d) after 500 cycles in the LiFSI-1.4DME electrolyte. The C 1s (e), F 1s (f), S 2p (g), N 1s (h) XPS spectra of the pristine NMC333 cathode and the NMC333 cathode after 60 cycles in the LiFSI-1.4DME electrolyte. (2.7-4.3 V voltage window)



Figure 3. (a) STEM image of the pristine NMC333 sample, viewed along [100] zone axis. (b) STEM image of the NMC333 sample after 60 cycles (4.3 V), viewed along [210] zone axis. (c) TEM image of the NMC333 sample after 60 cycles (4.3 V). (d) The STEM image of the NMC333 sample after 500 cycles (4.3 V), viewed along [100] zone axis. The dashed yellow rectangle outlines the cation mixing layers (a, b, d) or the boundary of the CEI (c).


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ACS Energy Letters

Figure 4. Electrochemical behavior of various Li metal batteries in different electrolytes under higher voltages. (a) Cycling performance of Li||NCA and Li||LCO batteries in the LiFSI1.4DME electrolyte under 4.2 V charge cut-off voltage. (b) Cycling performance of Li||NMC333 batteries in the LiPF6-EC/EMC electrolyte, the LiTFSI-LiDFOB-3.0DME dual-salt electrolyte and the LiFSI-1.0DME electrolyte under 4.5 V charge cut-off voltage. (c) Cycling performance of Li||NMC811 batteries in the LiPF6-EC/EMC electrolyte, the LiFSI-1.0DME electrolyte and the LiFSI-1.1DMC electrolyte under 4.4 V charge cut-off voltage.



GRAPHIC ABSTRACT


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High Concentration Ether Electrolytes for Stable High-Voltage Lithium

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