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Fire-Retardant Phosphate-Based Electrolytes for High-Performance Lithium Metal Batteries Yang Dong, Ning Zhang, Cuixia Li, Yanfei Zhang, Ming Jia, Yuanyuan Wang, Yaran Zhao, Lifang Jiao, Fangyi Cheng, and Jianzhong Xu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00027 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Applied Energy Materials

Fire-Retardant Phosphate-Based Electrolytes for High-Performance Lithium Metal Batteries Yang Dong,† Ning Zhang,*,†,‡ Cuixia Li,† Yanfei Zhang,† Ming Jia,† Yuanyuan Wang,† Yaran Zhao,‡ Lifang Jiao,‡ Fangyi Cheng‡, and Jianzhong Xu*,† † College of Chemistry & Environmental Science, Key Laboratory of Medicinal Chemistry and Molecular Diagnosis (Ministry of Education), Hebei University, Baoding 071002, China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China KEYWORDS: lithium metal batteries, fire-retardant electrolyte, concentrated electrolyte, lithium metal anode, safety

ABSTRACT: Rechargeable lithium metal batteries (LMBs) are considered as promising candidates for high-energy storage systems, but their practical applications are plagued by the severe safety concerns and poor cyclability. Here we report a fire-retardant electrolyte consisting of 2.8 M lithium bis(trifluoromethanesulfonyl)imide in triethyl phosphate (TEP) with 10 vol.% fluoroethylene carbonate (FEC) for high-performance LMBs. This concentrated system with almost no free solvent molecules and anions can alleviate the electrolyte reactivity toward Li anode, and improve the anodic stability up to 5.0 V. FEC co-solvents as functional additives can enhance the ionic conductivity and reduce the viscosity of the concentrated electrolyte. Moreover,

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the presence of FEC in the TEP-based electrolyte favors to form a stable LiF-rich solid electrolyte interphase, suppressing the parasitic reactions between Li and TEP solvent and enabling a dendrite-free cycling of Li anode. A Li||LiFePO4 battery using this electrolyte shows a high cyclic stability with 90% capacity retention over 2000 cycles. In addition, significantly improved

cycling

performances

of

LMBs

employing

4-V

class

cathodes

(e.g.,

LiNi0.8Co0.1Mn0.1O2, LiNi0.6Co0.2Mn0.2O2, and LiCoO2) have also been realized in the tailored electrolyte. The combination of high safety, high reduction/oxidation stability, and excellent electrochemical performance indicates this non-flammable electrolyte system is promising for LMBs.


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1. INTRODUCTION Rechargeable Li-metal batteries (LMBs) are attracting much attention for the next generation of high energy storage systems, owing to the advantages of Li metal anode (LMA) including its high theoretical specific capacity (3860 mAh g-1), and low redox potential (-3.04 V vs standard hydrogen electrode).1-3 However, the practical realization of LMBs is severely hindered by safety issues arising from the use of conventional electrolytes that are highly flammable and volatile (e.g., 1 M lithium hexafluorophosphate (LiPF6) in carbonate solvents). In addition, LMBs typically suffer from poor electrochemical performance due to the early failure of LMAs, induced by the dendritic/mossy Li growth on Li anode surface and the instability of Li– electrolyte interface during repeated Li plating/stripping.4,5 Furthermore, severe surface Li dendrite formation may also pose battery safety concerns, because the potential of dendrites penetration through the separator would trigger the short circuit, leading to the exothermic reactions in battery and then the increase of battery temperature/pressure. Considerable efforts have been devoted to stabilizing Li-metal anodes, including the use of electrolyte additives,6-8 protective layers,9-11 nanoscale electrode designs,12-14 and solid-state electrolytes15,16. Among these, highly concentrated electrolyte is one of the most effective and convenient routes to improve the interfacial stability between electrolyte and Li anode.17-20 For instance, 4 M lithium bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethoxyethane (DME),21 and 10 M LiFSI in carbonate electrolytes (e.g., dimethyl carbonate (DMC))22 significantly enhance the performance of LMAs and effectively prevents Li dendrite growth, compared with the corresponding dilute counterparts. Nonetheless, most of the high salt-concentrated electrolytes employing carbonate-based, ether-based, or sulfone-based solvents are extremely flammable, and would potentially bring safety concerns, particularly for large-scale energy storage applications.


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Although nonflammable ionic liquid23,24 or solid state electrolytes25,26 have been regarded as choices for safe LMBs, they generally show either slow ionic transport and poor rate capabilities or inadequate cycling performance. Fire-retardant alkyl phosphates have been studied as co-solvents to reduce the flammability of conventional electrolytes, because of their high solvability of Li salts and wide range of operating temperature.27-31 However, alkyl phosphates typically have intrinsic instability at low potentials versus Li+/Li,32-35 and fail to form a stable solid electrolyte interphase (SEI) on the Li anode surface. This would induce a rapid capacity decay during cycling, making the alkyl phosphates incompatible with LMBs. Yamada et al. demonstrates that the highly concentrated electrolytes (i.e., 3.3 M NaN(SO2F)2 in trimethyl phosphate (TMP)) improve the electrochemical compatibility of TMP-based electrolytes for hard-carbon anodes in sodium-ion batteries.36 Increasing the salt concentration in electrolyte may be also an effective way to improve the reductive stability of phosphate solvents toward Li metal anode in LMBs. However, the highconcentration electrolytes (e.g., > 3 M) still face some disadvantages, such as, low conductivity, high viscosity, or poor electrode/electrolyte interfacial contact,37-40 leading to an insufficient rate capability and an inferior utilization of active materials. Thus, seeking for the phosphate-based electrolytes that not only allow high reductive/oxidative stability but also feature enhanced ionic conductivity with good wettability is particularly desirable for high-rate and long-life LMBs. Recently, the “ion–solvent–coordinated” electrolytes with an unique molar ratio (MR) of the salt/solvent27,28 (e.g., 3 M LiFSI in triethyl phosphate (TEP)28 with a MR of 1:1.5) and the localized high-concentration electrolyte using an “inert” diluent32 (e.g., 1.2 M LiFSI in a mixture of TEP/bis(2,2,2-trifluoroethyl) ether (1:3 by mol)) have been proposed, which can push the Li


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salt concentration outside of the highly concentrated electrolyte region, and effectively improve the reversibility of the Li metal anode. Herein, we report a moderately concentrated fire-retardant electrolyte consisting of 2.8 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in TEP with fluoroethylene carbonate (FEC) functional additives for high-performance LMBs. Unlike the FEC-free concentrated counterpart, this FEC-containing electrolyte (i.e., 2.8 M LiTFSI in TEP with 10 vol.% FEC (denoted as 2.8 M + 10% FEC)) features much improved conductivity, reduced viscosity, and good wettability. Moreover, this electrolyte enables an extended electrochemical window (up to 5 V), an enhanced reductive stability, and a dendrite-free cycling of Li metal anode with a stable LiF-rich SEI. As a result, Li||LiFePO4 batteries in the 2.8 M + 10% FEC electrolyte achieve a high capacity retention of 90% over 2000 cycles at 1 C and a high rate capability of 103 mAh g-1 at 5 C. In addition, this electrolyte system can also well support the stable cycling of LMBs using 4-V class cathodes (e.g., LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.6Co0.2Mn0.2O2 (NCM622), and LiCoO2). 2. EXPERIMENTAL SECTION 2.1 Electrolytes and Electrodes Preparation. Battery-grade Li salts (e.g., LiTFSI, LiPF6), organic solvents (e.g., TEP, FEC, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) were purchased from Aladdin Industrial Corporation. The solvents (e.g., TEP, DEC, EC) were purified by distillation under reduced pressure before use. The electrolytes were prepared by dissolving the desired amount of Li salts (e.g., LiTFSI, LiPF6) in the selected solvents in a glove box under high-purity Ar gas. For example, different kinds of LiTFSI-TEP electrolytes with different salt concentrations (molarity) were fabricated, such as, 1 M, 2 M, 2.8 M, and 2.8 M with the addition of 5, 10, and 15 vol.%


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FEC (denoted as 2.8 M + 5%, 10%, and 15% FEC, respectively). For comparison, conventional carbonate electrolytes were also prepared, such as, 1 M LiPF6 in EC-DMC (volume ratio (VR) of 1:1), EC-EMC-DMC (VR of 1:1:1), EC-EMC (VR of 1:1), and EC-DEC (VR of 1:1). The cathodes were fabricated by mixing the active materials (e.g., LiFePO4, LiCoO2, NCM622, and NCM811), super P carbon and polyvinylidene fluoride with a weight ratio of 8:1:1 using Nmethyl-2-pyrrolidone as a solvent. The electrode slurry was pasted onto Al foil, and then vacuum-dried at 100 °C for 12 h. After that, the as-prepared electrode was cut into round slices with a diameter of 10 mm. Both thin electrodes with a mass loading of ~2 mg cm-2 and thick electrodes with a mass loading of 5~8 mg cm-2 were prepared. 2.2 Characterization. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were conducted on a Netzsch STA 449C thermal analyzer with a heating rate of 10 °C min-1 under N2 atmosphere. Around 10 mg electrolyte samples were sealed inside an Al pan, and a pinhole was made in the lid of Al pan for the gas escape during TG measurement. For DSC analysis,30,41 the fully charged cell was carefully disassembled inside the argon-filled glove box to recover the cathode. The re-obtained cathode was rinsed with dimethyl carbonate, and then dried in a vacuum oven for 10 h. Around 3 mg of the charged cathode powder wetted by 3 μL electrolyte was hermetically sealed inside an Al pan in the argon-filled glove box, and then the DSC curve was recorded between 30–350 oC. Microcombustion calorimetry was performed on a FAA Micro Calorimeter (FTT company, United Kingdom). The viscosities of electrolytes were obtained on a MDJ-5S viscometer (Shanghai, China) at 25 °C. The contact angles were measured by a contact angle meter (Dataphysics OCA 15EC). Raman spectra were recorded on a confocal Raman microscope (DXR, Thermo-Fisher Scientific) using 532 nm excitation. Fourier transform infrared (FTIR) spectra were collected on a Bruker TENSOR II. X-ray photoelectron


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spectroscopy (XPS) was carried out on a Perkin Elmer PHI 1600 ESCA spectrometer. Scanning electron microscopy (SEM) images were observed by SEM (JEOL JSM-7500F). 2.3 Computational Details. For searching the most stable configurations of the FEC and TEP solvents, every molecule was searched by 10000 possible structures by the force field of MMFF94 in the module of GMMX conformer in Gaussian 16. And then the structure of the molecules (i.e., FEC and TEP) was optimized using B3LYP/6-31G (d,p) level and confirmed as true local minimum by vibrational frequency analyses. 2.4 Electrochemical Measurements. Linear sweep voltammetry (LSV) was carried out in a three-electrode cell employing Al foil as a working electrode, and Li metal foils as reference and counter electrodes. The scan rate is 10 mV s-1. The Li plating/stripping performance in different electrolytes were characterized by cyclic voltammograms (CVs) at a potential sweeping rate of 5 mV s−1 using Ti foil as the working electrode and Li foil as the reference and counter electrodes. The reversibility and stability of selected electrolytes were tested in symmetric Li/Li cells at various current densities. The Coulombic efficiency of Li stripping and plating was performed using Li/Cu cells at 0.2 mA cm-2 with a deposition capacity of 0.2 mAh cm-2. Electrochemical tests of LMBs were performed employing CR2032 coin cells. Charge/discharge measurements were recorded on a LAND battery-test instrument (LAND-CT2001A). The current density and specific capacity were based on the active mass in each cathode. Electrochemical impedance (EIS) spectra were performed on a CHI660E electrochemical workstation (Shanghai Chenhua, China) in the frequency from 100 kHz to 100 mHz with AC perturbation signal of 5 mV. 3. RESULTS AND DISCUSSION


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Figure 1. Thermal Stability. (a) TG analysis of the TEP-based electrolytes (i.e., 1 M, 2.8 M, and 2.8 M + 10% FEC) and the conventional carbonate electrolytes (i.e., 1 M LiPF6 in EC-DMC, EC-EMC-DMC, EC-EMC, and EC-DEC, respectively). (b) Heat release rate profiles of TEP-based electrolytes obtained by microscale combustion calorimeter. Photographs of the ignition test of glass fibers saturated with (c) 2.8 M, (d) 2.8 M + 10% FEC, and (e) conventional 1 M LiPF6/EC-DMC electrolytes, respectively. Figure 1a shows the thermogravimetric curves of various electrolyte solutions. Compared with the dilute TEP-based electrolyte (e.g., 1 M), the concentrated counterpart (2.8 M) exhibits negligible volatility up to 200 oC. With the presence of FEC in the 2.8 M electrolyte, a slight mass loss of ~5.5 wt% at 150 oC is observed, caused by the evaporation of FEC additives. For comparison, the conventional carbonate electrolytes display much faster mass loss after 100 oC. When heating to 150 oC, 49.8%, 54.3%, 58.6%, and 47.7% weight losses through solvent evaporation are recorded for the 1 M LiPF6/EC-DMC, 1 M LiPF6/EC-EMC-DMC, 1 M


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LiPF6/EC-EMC, and 1 M LiPF6/EC-DEC electrolytes, respectively. This would lead to a dramatic increase in the battery internal pressure at a high temperature operation, posing safety issues. The weak volatility property of the concentrated electrolyte is also validated by weight retention measurement of electrolytes with different Li-salt concentrations exposing in open atmosphere (Figure S1). In addition, microcombustion calorimetry has been performed to further evaluate the combustion behavior of the TEP-based electrolytes, as shown in Figure 1b and Table S1. The dilute electrolyte displays obvious heat release upon heating to 150 oC, and the peak of heat release rate (PHRR) located a low temperature of ~190 oC. In contrast, the saltconcentrated electrolytes (i.e., 2.8 M, 2.8 M + 10% FEC) significantly push the temperature of PHRR up to ~290 oC, and negligible heat release is observed below 150 oC. This suggest that the concentrated systems have a much higher thermal stability than that of the dilute counterpart, mainly attributed to the synergistic effect of the high boiling point of TEP and the dominant Li+– TEP solvation effect with much reduced free solvent molecules (shown later). Furthermore, the thermal stability of electrolytes in the presence of the fully charged cathode (e.g., NCM811) was performed employing DSC analysis under nitrogen flow (Figure S2). The DSC peaks associated the combustion of the electrolyte with the liberated oxygen from the de-lithiated cathode30,41 were obviously shifted up to higher temperatures (~268 and ~287 oC) in the 2.8 M + 10% FEC electrolyte compared with those in the 1 M LiPF6/EC-DEC electrolyte (~230 and ~249 oC), indicating that the concentrated TEP-based electrolyte has a good thermal stability against the charged NCM cathode. In addition, the ignition measurement provides a straight-forward evidence for the flame retardant ability of electrolytes. It can be seen that both the neat 2.8 M (Figure 1c) and the 2.8 M + 10% FEC (Figure 1d) electrolytes can not be ignited during flammability test. However, the conventional carbonate electrolyte (e.g., 1 M LiPF6/EC-DMC) is


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highly flammable (Figure 1e). Hence, the combination of the superior thermal stability and fireretardant ability of the 2.8 M + 10% FEC electrolyte would significantly improve the safety property of batteries.

Figure 2. Physical properties. (a) Ionic conductivities and viscosities of 1 M, 2 M, 2.8 M, 2.8 M + 5% FEC, and 2.8 M + 10% FEC electrolytes at room temperature. Contact angles of (b) 2.8 M + 10% FEC, (c) 2.8 M, and (d) 1 M LiPF6/EC-DMC electrolytes on the LiFePO4 cathode, PP separator, and Li anode, respectively. Figure 2a displays the ionic conductivities and viscosities of TEP-based electrolytes as a function of salt concentrations. The viscosity increases with increasing the Li-salt mole fraction, which causes the ionic conductivity decrease. This is due to the fact that the high salt concentration promotes the formation of ion-solvent clusters and decreases the ion mobility.21,44 For 2.8 M electrolyte, the viscosity and ionic conductivity values respectively are 213.0 cP and 0.51 mS cm-1. With the addition of FEC, the concentrated electrolyte (e.g., 2.8 M + 10% FEC) exhibits a commercially acceptable ionic conductivity of 2.07 mS cm-1 and a reduced viscosity of 58.5 cP. Although the viscosity of the 2.8 M + 10% FEC electrolyte is higher than that of the commercial dilute electrolyte, it will not compromise the rate performance (discussed below).


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Wettability of electrolytes is another important factor that profoundly influence the battery performance. The contact angles of the 2.8 M + 10% FEC electrolyte (Figure 2b) on LiFePO4 cathode, polypropylene (PP) separator, and Li anode respectively are 9.0o, 42.7o, and 13.3o, which are lower than those of the 2.8 M counterpart (20.8o, 44.6o, and 22.1o, respectively (Figure 2c)). Impressively, compared with the conventional carbonate electrolytes (Figure 2d and Figure S3), the wettability of the 2.8 M + 10% FEC electrolyte is much better, revealed by the corresponding smaller contact angles. Furthermore, the 2.8 M + 10% FEC electrolyte can easily spread over and soak the PP separator, also manifesting its good wettability (Figure S4). The solution structures of LiTFSI/TEP electrolytes at various Li salt concentrations with different contents of FEC additives were studied using Raman (Figure 3a) and FTIR (Figure 3b) spectra. Based on the deconvolution analysis, the Raman band of TFSI- consists of three peaks at 730 (or 1236), 740 (or 1242), and 748 (or 1247) cm-1, arising from free anions, contact ion pairs (CIPs, TFSI- coordinates with one Li+ ion), and aggregates (AGGs, TFSI- coordinate with two or more Li+ ions), respectively.18 In the dilute electrolyte (e.g., 1 M), most of the TFSI- exists as free anions with a small amount of CIPs and AGGs. As increasing the Li-salt concentration from 1 M to 2.8 M, the signal of the free anions gradually decreases and the Li+–TFSI- association intensifies. With the addition of FEC, negligible changes of the AGGs signal could be observed, mainly due to that the FEC could not disturb the strong interactions between Li+ cations and TFSI- anions in the concentrated electrolyte (e.g., 2.8 M + 10% FEC).


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Figure 3. Solvation structure and oxidation/reduction stability. (a) Raman and (b) FTIR spectra of the TEP solvent, LiTFSI salt, 1 M, 2 M, 2.8 M, 2.8 M + 5% FEC, and 2.8 M + 10% FEC electrolytes. (c) Schematic illustration of the representative environment of Li+ in the diluted and concentrated electrolytes. (d) LSV curves of 1 M, 2.8 M, and 2.8 M + 10% FEC electrolytes in a three-electrode cell employing Al foil as a working electrode, and Li metal foils as the reference and counter electrodes. The scan rate is 10 mV s-1. (e-g) Reactivity of Li metal foils in TEPbased electrolytes (i.e., 1 M, 2.8 M, and 2.8 M + 10% FEC) at room temperature. Figure 3b shows the FTIR spectra of the LiTFSI/TEP electrolytes. As increasing the Li-salt concentration, the signal of free TEP molecules at ~1165 cm-1 (related to the P=O stretching


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vibration) gradually decrease and a new broad peak located at around 1191 cm-1 appears arising from the coordination of TEP with Li+. Previous studies show that a stable Li+–TEPn solvation structure occurs in electrolyte with a coordinating number of four (n ≈ 4).27,28 Further increasing the Li salt concentration could significantly decrease free TEP, and TFSI- anions inevitably entry into the Li+ solvation sheath due to the scarcity of TEP molecules, forming various solvation structures. With the presence of FEC additives, the FTIR signal of the Li+–TEP solvates do not show distinct changes in the 2.8 M + 10% FEC electrolyte compared with that in the neat 2.8 M electrolyte, indicating that the FEC addition has minimal effect on the Li+–TEP solvation. This result is mainly attributed to the two reasons: (1) The high salt concentration with a high molar ratio of LiTFSI : TEP (1 : 1.14) in the 2.8 M + 10% FEC electrolyte enables an sufficient and stable coordination between Li+ and TEP; (2) The relatively low content of FEC in the 2.8 M + 10% FEC electrolyte (the molar ratio of TEP : FEC is 1 : 0.25) leads to a small effect on the tight Li+–TEP solvation structure, although FEC could also coordinate with Li+ ions (Figure S5). The representative environments of Li+ in the dilute solution (e.g., 1 M) and the concentrated solution (e.g., 2.8 M + 10% FEC) are schematically depicted in Figure 3c. Based on the aforementioned results, almost all TEP molecules and TFSI- strongly coordinate to Li+ (almost no free solvents and anions) in the 2.8 M + 10% FEC electrolyte system, accounting for the improved oxidation/reduction stability. To demonstrate the anodic stability of TEP-based electrolytes, linear sweep voltammetry (LSV) profiles are conducted from 2.8 V to 5.2 V at a scan rate of 10 mV s-1 (Figure 3d). The electrochemical window expands with the LiTFSI concentration increases, mainly as a result of the reduced TEP activity. Obviously, the onset potential of the solvent oxidation in the concentrated electrolytes shifts up to 5.0 V, which is sufficiently wider than the working


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potentials of most cathode materials (e.g., LiFePO4, LiCoO2) currently developed for LIBs. Notably, the FEC additives have minimal effect on the oxidation stability of the 2.8 M-based electrolytes. Furthermore, cyclic voltammetry (Figure S6) was used to evaluate the Li plating/stripping behavior in the aforementioned electrolytes, where Ti foil as the working electrode and Li metal as the reference and counter electrodes. Compared to the neat 2.8 M electrolyte, the 2.8 M + 10% FEC electrolyte displays much a higher peak current with a smaller polarization for Li plating/stripping. This is attributed to that the FEC functional additives enable the enhanced ionic conductivity and the reduced viscosity of the concentrated electrolyte. In addition, as a visible indicator of the reductive stability, the storage behaviors of Li metal foils in LiTFSI/TEP systems at room temperature have been performed (Figure 3e-g). The Li foil as a strong reducing agent is quite unstable in the dilute 1 M electrolyte, and it was gradually dissolved and turn the solution brown as time increasing. In contrast, in the 2.8 M solution, only the surface of the Li metal becomes a little dim, and the solution is as clear as the fresh one. Remarkably, no visible changes for both the Li foil and the solution were observed in the 2.8 M + 10% FEC electrolyte after two weeks. The high reductive stability of the FEC-containing concentrated electrolyte could be attributed to the reduced activity of TEP solvents and the formation of a LiF-rich SEI layer on Li surface induced by FEC additives, thus preventing the continuously adverse reactions between Li metal and solvents.


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Figure 4. Li metal anode characterization. (a) Galvanostatic Li plating/stripping profiles (at 0.1 mA cm-2) and (b) Rate performance of Li/Li symmetrical cells using 1 M, 2.8 M and 2.8 M + 10% FEC electrolytes. Insets of Figure 4a show the enlarged voltage profiles of the 1st and 100th cycles. (c-f) SEM images of Li anodes in (c,d) 1 M electrolyte, and (e,f) 2.8 M + 10% FEC electrolyte after 100 cycles. (g,i) SEM images of Li deposits on Cu substrates, and (h,j) optical images of Cu substrates and separators obtained in (g,h) 1 M and (i,j) 2.8 M + 10% FEC electrolytes by plating 3.6 mAh Li. Scale bars, (c,e,g,i) 10 μm, and (d,f) 1 μm. The reversibility and stability of LMAs in the TEP-based electrolytes are investigated using Li/Li symmetric cells (Figure 4a). In the dilute 1 M electrolyte, the Li/Li cell displays stable cycling for less than 150 h, followed by a fast voltage polarization increase, suggesting the continuous electrolyte decomposition. In contrast, the Li/Li cells in 2.8 M-based electrolytes exhibit very stable cycling performance up to 250 h, attributed to the reduced solvent-induced side reactions. As shown in the enlarged voltage profiles (insets of Figure 4a), the 2.8 M + 10% FEC electrolyte delivers much lower electrode polarizations (e.g., 106 mV at the 1st cycle and 93


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mV at the 100th cycle) than those of the neat 2.8 M electrolyte (e.g., 330 mV at the 1st cycle and 270 mV at the 100th cycle). This demonstrates that FEC additives help improve the ionic conductivity and the interfacial compatibility of the concentrated electrolyte. Even at a high current density of 0.25 mA cm-2, the 2.8 M + 10% FEC electrolyte also realizes a higher durability up to 500 h with a lower electrode polarization compared to those of the dilute 1 M and the neat 2.8 M counterparts (Figure S7). More importantly, excellent rate capability for the 2.8 M + 10% FEC system is characterized as the test current increases from 0.1 to 1 mA cm-2 (Figure 4b). However, apparently large overpotential and dramatic polarization augment with increasing the test current are presented for both the 1 M and 2.8 M electrolytes, which leads to cells failure at 0.5 mA cm-2. In addition, the advantages of FEC additives in the concentrated electrolyte were further clarified by Li/Cu cells (Figure S8). The Coulombic efficiency (CE) values in the 2.8 M + 10% FEC electrolyte gradually increase from initial 85.2% to 97.5% after 80 cycles with a low polarization of ~90 mV, which are much better than those in the 2.8 M counterpart (68.4% initial CE, and 90.8% after 80 cycles with a polarization of ~230 mV). The surface conditions of deposited Li are further examined by scanning electron microscope (SEM) and optical images. According to the post-mortem analysis of Li/Li cell over 100 cycles, extensive dendritic Li formation on the Li electrode is clearly observed in the 1 M electrolyte (Figure 4c,d). In contrast, nodule-like Li deposits with no dendrite formation is characterized in the 2.8 M + 10% FEC electrolyte (Figure 4e,f). Although the dendritic Li growth can also be suppressed in the neat 2.8 M electrolyte, the Li electrode in this case without FEC additives exhibits a rough surface with obvious cracks/holes (Figure S9). In addition, the morphology of the deposited Li metal on Cu substrate is also recorded. In the 1 M electrolyte, the deposited Li film presents an irregular growth with obvious dendrites (Figure 4g), and the Cu substrate


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exhibits a rough surface with uneven Li coating. Obvious by-products from electrolytes decomposition are deposited on the separator (Figure 4h). The characterizations of the fresh Cu substrate and separator are given in Figure S10. In comparison, a dense and uniform Li film without Li dendrites is revealed in the 2.8 M + 10% FEC electrolyte (Figure 4i), which is beneficial to reduce the parasitic interfacial reactions. Moreover, a smooth Cu substrate and a clear separator can be observed without any byproduct as shown in optical images (Figure 4j).

Figure 5. DFT calculation and XPS characterization. (a) Molecular structure, and visual LUMO with the corresponding relative energy of FEC and TEP. The hydrogen, carbon, oxygen, phosphorus, and fluorine atom are marked with white, gray, red, orange, and blue, respectively. (b-d) XPS spectra of the deposited Li on Cu substrate in the 2.8 M electrolyte with/without 10% FEC. (b,c) F 1s and (d) P 2p XPS spectra. Further, the function of FEC additives that can improve performance has been evidenced by density functional theory (DFT) calculation and X-ray photoelectron spectroscopy (XPS). As shown in Figure 5a, the lowest unoccupied molecular orbital (LUMO) of FEC is relative lower than that of TEP, suggesting that FEC additives would undergo earlier reduction than TEP solvent on the surface of the Li metal anode to form a preliminary SEI. This favors to suppress the continual decomposition of electrolytes. In addition, XPS spectra of the deposited Li on Cu


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substrates were performed to probe the surface chemistry of FEC-induced SEI layer. The F 1s spectra (Figure 5b,c) displays two peaks located at 684.8 eV and 688.2 eV, corresponding to LiF and organic fluorides, respectively.8 The ratio of LiF in the FEC-containing system (12.76%) is much higher than that of the FEC-free electrolyte (5.38%), indicating more LiF formation in the FEC-induced SEI layer. LiF is an important component in stabilizing the SEI and suppressing Li dendrites growth. Moreover, LiF as an electrical insulator favors to prevent electrons from crossing the SEI layer, thus helping to inhibit the parasitic reactions between Li and electrolyte.43-45 According to the P 2p spectra46 (Figure 5d), the proportion of polyphosphate in the FEC-free electrolytes induced SEI layer accounts much more than that of the FEC-containing system. Note that the signal of polyphosphate in the 1 M electrolyte significantly intensifies compared with those of the concentrated counterparts (Figure S11). As P element is only from TEP, we can deduce that the reduction of TEP solvents has been effectively suppressed in the FEC-containing concentrated electrolyte. The feasibility of the novel non-flammable 2.8 M + 10% FEC electrolyte towards LMBs is affirmed by coupling with the commercial LiFePO4 cathode. Figure 6a presents the rate behaviors of Li||LiFePO4 cells using 2.8 M electrolytes with different amount of FEC additives. Clearly, the rate capability can be much improved with increasing FEC contents in the TEPbased electrolytes. This is attributed to that the FEC additives can enhance interfacial reaction kinetics (electrochemical impedance spectra, Figure S12), resulting from the increased conductivity, reduced viscosity, and good wettability (Figure 2). Reversible capacities of 153, 149, 144, and 135 mAh g-1 are characterized at rates of 0.2, 0.5, 1.0, and 2.0 C (1 C = 170 mA g-1) in the 2.8 M + 10% FEC electrolyte, respectively. Even at a high rate of 5 C, a reversible capacity of 103 mAh g-1 could be obtained. Noticeably, when the rate returns to 0.2 C, the


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reversible capacity can recover to 151 mAh g-1. The rate performance of the Li||LiFePO4 cell in the 2.8 M + 10% FEC electrolyte is competitive to the cell using a conventional 1 M LiPF6/ECDMC electrolyte (Figure S13). The corresponding charge/discharge profiles of the cell in the 2.8 M + 10% FEC electrolyte are presented in Figure 6b, which display much smaller overpotentials compared to the cells in the 2.8 M, and 2.8 M + 5% FEC electrolytes (Figure S14). Notably, with increasing the content of FEC to 15%, the rate performance remains negligible improvement (Figure S15a), but the thermal stability in this case is much reduced (Figure S15b).

Figure 6. Electrochemical performance of Li||LiFePO4 batteries. (a) Rate performance of Li||LiFePO4 cells in the 2.8 M, 2.8 M + 5% FEC, and 2.8 M + 10% FEC electrolytes. (b) The typical voltage profiles in the 2.8 M + 10% FEC electrolyte at various current rates. (c) Cycling stability of Li||LiFePO4 cells in the 1 M, 2.8 M, and 2.8 M + 10% FEC electrolytes at 1 C (1 C = 170 mA g-1). The CE is corresponding to the cell in the 2.8 M + 10% FEC electrolyte.


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Considering the compromise among loading, stability, and performance, we selected the 2.8 M + 10% FEC solution as the electrolyte. Furthermore, the 2.8 M + 10% FEC electrolyte can enable a good electrochemical performance of the LiFePO4 cathodes with higher mass loadings (e.g., 5 and 8 mg cm-2), as shown in Figure S16. In addition, the long-term cycling performance of Li||LiFePO4 cells in the TEP-based electrolytes was estimated by galvanostatically charged/discharged at 1 C. Remarkably, the discharge capacity of Li||LiFePO4 cell using the 2.8 M + 10% FEC electrolyte sustains 132 mAh g-1 with an unprecedented capacity retention of ~90% even after 2000 cycles. The CE approaches ~100% during the prolonged cycling. For comparison, in the FEC-free counterpart (i.e., 2.8 M), the cell displays an inferior discharge capacity of 78 mAh g-1 with a capacity retention of 87.2% over 500 cycles at 1 C. In the 1 M electrolyte, a precipitous drop in capacity upon cycling is observed, attributed to the poor stability of the diluted electrolyte. The selected charge/discharge curves of the Li||LiFePO4 cells in the 1 M and 2.8 M with/without 10% FEC electrolytes are shown in Figure S17. Additionally, the Li||LiFePO4 cells in conventional carbonate electrolytes (e.g., 1 M LiPF6 in EC-DMC, in ECDEC, and in EC-EMC-DMC) display poor cycling stability with fluctuant CE (Figure S18), most probably caused by the side reactions between carbonate electrolytes and Li metal anode. To demonstrate the versatility of the 2.8 M + 10% FEC electrolyte, LMBs using commercial 4-V class cathode materials (e.g., LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.6Co0.2Mn0.2O2 (NCM622), and LiCoO2 (LCO)) have been performed. Figure 7a shows the typical charge/discharge curves of the Li||NCM811, Li||NCM622, and Li||LCO cells with reversible capacities of 195, 148, and 124 mAh g-1 at 1 C, respectively. 1 C equals 200, 180, and 160 mAh g-1 for NCM811, NCM622, and LCO, respectively. After 100 cycles, high capacity retentions of 95.7%, 89.8%, and 89.3% can be respectively obtained for the Li||NCM811, Li||NCM622, and


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Li||LCO cells (Figure 7b), which are comparable to those cells in the conventional 1 M LiPF6/EC-DEC electrolyte (Figure S19–S21). These excellent performances are mainly due to that the FEC-containing salt-concentrated electrolyte enables an improved oxidation/reduction stability and a stable cycling of Li metal anode with no dendrite formation.

Figure 7. Electrochemical performances of LMBs using NCM811, NCM622, and LCO as cathode materials. (a) Typical charge/discharge profiles and (b) cycling performance of Li||NCM811, Li||NCM622, and Li||LCO cells with 2.8 M + 10% FEC electrolyte at 1 C rate (1 C equals 200, 180, and 160 mAh g-1 to NCM811, NCM622, and LCO, respectively). 4. CONCLUSIONS In summary, we have developed a fire-retardant LiTFSI/TEP electrolyte with FEC functional additives (i.e., 2.8 M + 10% FEC) that features a high thermal stability and greatly improves the cycling stability of LMBs. The FEC-containing TEP-based electrolyte can dramatically overcome the disadvantages of the FEC-free concentrated counterpart, such as, low conductivity, high viscosity, poor wettability. Furthermore, a stable TEP–Li+–TFSI- solvation structure has been revealed in the 2.8 M + 10% FEC system with almost no free solvents/anions that renders an expended electrochemical window (up to 5 V), a reduced reactivity of TEP solvents, and a


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stable cycling of Li metal anode. The DFT calculations combined with XPS and SEM analyses elucidated that FEC with a lower LUMO energy than TEP preferentially reduce on the Li metal anode to form a LiF-rich SEI. This favors to suppress Li dendrite growth, and prevent the continuous parasitic reactions between Li and TEP solvent. As a result, Li||LiFePO4 batteries employing the 2.8 M + 10% FEC electrolyte exhibit a high rate capability (103 mAh g-1 at 5 C) and an ultralong cycling life (~90% capacity retention after 2000 cycles at 1 C). Moreover, this electrolyte is universally applicable to the 4-V class cathodes (e.g., NCM811, NCM622, and LCO), allowing excellent cycling stability. These results demonstrate that the 2.8 M + 10% FEC electrolyte system can enable safe operation of high-performance LMBs for practical applications. Our findings may also boost the further development of stable and non-flammable electrolytes for LIBs and other battery chemistries.

ASSOCIATED CONTENT Supporting Information Additional characterization of materials (Contact angles test, SEM images, XPS spectra, TG curve, DSC profiles, etc.), additional electrochemical performance (charge/discharge curves, CV data, EIS profiles, cycling stability of LiFePO4 cathodes with higher mass loadings, electrochemical performances of Li/Cu and Li/Li cells, cycling and rate performances in the diluted electrolyte and conventional carbonate electrolytes, etc.), micro-combustion calorimetry results for the fire-retardant electrolytes. AUTHOR INFORMATION


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Corresponding Authors *(N.Z.) E-mail: [email protected]. *(J.X.) E-mail: [email protected]. ORCID Ning Zhang: 0000-0002-6176-7278 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (21805066), Advanced Talents Incubation Program of the Hebei University (801260201156), China Postdoctoral Science Foundation (2018M640244), and 111 Project (B12015) is gratefully acknowledged. REFERENCES (1) Fan, X. L.; Chen, L.; Borodin, O.; Ji, X.; Chen, J.; Hou, S.; Deng, T.; Zheng, J.; Yang, C. Y.; Liou, S.-C.; Amine, K.; Xu, K.; Wang, C. S. Non-Flammable Electrolyte Enables Li-Metal Batteries with Aggressive Cathode Chemistries. Nat. Nanotechnol. 2018, 13, 715–722. (2) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403–10473. (3) Tikekar, M. D.; Choudhury, S.; Tu, Z. Y.; Archer, L. A. Design Principles for Electrolytes and Interfaces for Stable Lithium-Metal Batteries. Nat. Energy 2016, 1, 16114. (4) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503–11618. (5) Choi, N.-S.; Chen, Z. H.; Freunberger, S. A.; Ji, X. L.; Sun, Y.-K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors. Angew. Chem., Int. Ed. 2012, 51, 9994–10024. (6) Ding, F.; Xu, W.; Graff, G. L.; Zhang, J.; Sushko, M. L.; Chen, X. L.; Shao, Y. Y.; Engelhard, M. H.; Nie, Z. M.; Xiao, J.; Liu, X. J.; Sushko, P. V.; Liu, J.; Zhang, J.-G. Dendrite-Free Lithium Deposition via Self-Healing Electrostatic Shield Mechanism. J. Am. Chem. Soc. 2013, 135, 4450–4456.


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TOC

Fire-retardant

phosphate-based

electrolyte

consisting

of

2.8

M

lithium

bis(trifluoromethanesulfonyl)imide in triethyl phosphate with 10 vol.% fluoroethylene carbonate enables the dendrite-free cycling of Li metal anode and supports the high-performance of Li metal batteries (e.g., Li||LiFePO4, Li||LiCoO2, Li||NCM622, and Li||NCM811 batteries).


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Fire-Retardant Phosphate-Based Electrolytes for ... - ACS Publications

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