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Jun 13, 2018 - ABSTRACT: The notorious lithium (Li) dendrites and the low Coulombic efficiency (CE) of Li anode are two major obstacles to the practic...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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Lithium Difluorophosphate as a Dendrite-Suppressing Additive for Lithium Metal Batteries Pengcheng Shi,†,∥ Linchao Zhang,‡,§,∥ Hongfa Xiang,*,† Xin Liang,† Yi Sun,† and Wu Xu*,‡ †

School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, China Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230026, China ‡

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S Supporting Information *

ABSTRACT: The notorious lithium (Li) dendrites and the low Coulombic efficiency (CE) of Li anode are two major obstacles to the practical utilization of Li metal batteries (LMBs). Introducing a dendrite-suppressing additive into nonaqueous electrolytes is one of the facile and effective solutions to promote the commercialization of LMBs. Herein, Li difluorophosphate (LiPO2F2, LiDFP) is used as an electrolyte additive to inhibit Li dendrite growth by forming a vigorous and stable solid electrolyte interphase film on metallic Li anode. Moreover, the Li CE can be largely improved from 84.6% of the conventional LiPF6-based electrolyte to 95.2% by the addition of an optimal concentration of LiDFP at 0.15 M. The optimal LiDFP-containing electrolyte can allow the Li||Li symmetric cells to cycle stably for more than 500 and 200 h at 0.5 and 1.0 mA cm−2, respectively, much longer than the control electrolyte without LiDFP additive. Meanwhile, this LiDFP-containing electrolyte also plays an important role in enhancing the cycling stability of the Li||LiNi1/3Co1/3Mn1/3O2 cells with a moderately high mass loading of 9.7 mg cm−2. These results demonstrate that LiDFP has extensive application prospects as a dendrite-suppressing additive in advanced LMBs. KEYWORDS: lithium dendrite suppression, electrolyte additive, solid electrolyte interphase, lithium difluorophosphate, lithium metal battery

1. INTRODUCTION Lithium (Li) ion batteries (LIBs) have been successfully applied in the state-of-the-art smart electronic devices since their first commercialization in 1991, and now are gradually breaking into the markets of electric vehicles and grid energy storage equipment. However, the energy density of commercial LIBs using the graphite anode is almost reaching the limit and becoming a great obstacle to the development of advanced batteries.1,2 In this regard, rechargeable Li metal batteries (LMBs), representative of high-energy-density Li-sulfur3 (2600 Wh kg−1) and Li−air batteries4 (3400 Wh kg−1), have been regarded as the ideal next-generation rechargeable batteries due to the ultrahigh theoretical specific capacity (3860 mAh g−1) and the almost lowest standard redox potential (−3.040 V) of Li metal anode.1 However, the undesirable growth of Li dendrites and the low Coulombic efficiency (CE) during repeated Li plating and stripping process still hamper the widespread applications of Li metal anode in LMBs.1,5 In general, the growth of Li dendrites is induced by uneven reductive depositions of Li+ on the surface of metallic Li electrode, as a result of inhomogeneous current density distributions and the Li+ concentration gradient at the interface between the electrolyte and Li electrode.1,6−8 On one hand, it is possible for the Li dendrites to penetrate through the © 2018 American Chemical Society

separator and subsequently induce the internal short circuit and battery safety hazards. On the other hand, low Li CE is resulted from the continuous generation of the solid electrolyte interphase (SEI) on metallic Li electrode, along with the electrolyte consumption and the formation of electrochemically inactive or “dead” Li.9−11 To overcome the aforementioned issues, a variety of strategies have been investigated, such as physically blocking the dendrite growth by using high-modulus solid electrolytes;12,13 adjusting the surface electric field to change the initial nucleation of Li deposition by using three-dimensional current collectors,14,15 and preventing the growth of Li dendrites via employing modified separators.16,17 Although these approaches are promising to some extent, none of them could totally stop the repeated breaking/rebuilding process of SEI and the accumulation of “dead” Li during cycling process.18−21 In this regard, building a stable and compact SEI film seems to be a more effective and convenient strategy for suppressing the growth of Li dendrites.20,22,23 Previous studies on metallic Li electrode have clarified Li is Received: March 30, 2018 Accepted: June 13, 2018 Published: June 13, 2018 22201

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

Research Article

ACS Applied Materials & Interfaces

deposition, Li||Li symmetric cells and Li||Cu cells were assembled in the argon-filled glovebox with Li electrode (Φ 15.6 mm, 450 μm thick, from China Energy Lithium Co. Ltd.) as both the counter and the reference electrodes. Celgard 2400 polypropylene porous membrane was severed as separators. To standardize the testing, 80 μL of the electrolytes with or without LiDFP was added in the coin cells. The Li||Li cells were operated at 0.5, 1.0 and 3.0 mA cm−2 with 1 h plating/stripping time, respectively. The cycling performance of Li|| Cu cells was operated at 0.5 and 1.0 mA cm−2. To standardize the testing, a certain amount of Li (1.0 mAh cm−2) was first deposited on the Cu electrode and subsequently stripped until the voltage reached to 0.5 V at various current densities in each cycle. Additionally, the CE of Li||Cu cells was defined as the amount of Li stripped divided by the amount of Li deposited during each cycle. The Li||LiNi1/3Co1/3Mn1/3O2 (Li||NMC) cells (2032-type) were employed to evaluate the role of LiDFP in LMBs. Generally, the NMC electrode was prepared by mixing 84 wt % NMC (Guoxuan High-tech Power Energy Co. Ltd.), 8 wt % Super-P (TIMCAL) and 8 wt % polyvinylidene fluoride (PVDF, Solvey HSV900) in N-methyl-2pyrrolidinone (NMP, Shanghai Aladdin Bio-Chem Technology Co. Ltd.), followed by casting the slurry on an aluminum (Al) foil. The electrode was vacuum-dried at 80 °C for 6 h and subsequently cut into Φ 14.0 mm discs with a mass loading of about 9.7 mg cm−2 (or an areal capacity loading of 1.3 mAh cm−2). The Li||NMC coin cells were first activated at 0.1C for two cycles, followed by switching to 0.5C for 100 cycles within a voltage between 2.5 and 4.2 V, where 1C = 1.3 mA cm−2. Electrochemical impedance spectroscopy (EIS) measurement of batteries was conducted on an electrochemical workstation (Solartron 1287), under a frequency range of 0.1−105 Hz and a voltage perturbation of 10 mV. The Li anodes for characterizations were retrieved from Li||Li cells, soaked in fresh dimethyl carbonate (DMC) solvent for 20 h, followed by washing with fresh DMC for three times to remove the residual salts, dried at room temperature under vacuum and transferred by a homemade container filled with argon. The morphologies of the Li electrodes were obtained by Zeiss fieldemission scanning electron microscopy (FE-SEM, Germany). Chemical compositions of SEI layers formed on various metallic Li electrodes were measured by X-ray photoelectron spectroscopy (XPS, America Thermo ESCALAB250) and Fourier transmittance infrared (FTIR) spectroscopy (Nicolet-670 FTIR spectrometer).

thermodynamically unstable in conventional LiPF6-carbonate electrolytes along with the formation of SEI layer mainly composed of ROCO2Li, ROLi, Li2CO3, and LiF.23,24 However, this type of SEI layer is structurally porous, fragile and partially soluble, so it cracks easily during volume expansion and shrinkage of the Li layer underneath.23 In order to modify the SEI layer on metallic Li anode, various electrolyte additives including HF,25 fluoroethylene carbonate,22 trace-amount (25−50 ppm) of water,23 vinylene carbonate,26 Li polysulfide with LiNO3,27,28 and CsPF6,6 LiAsF6 with cyclic carbonates,29 Mg(TFSI)2,30 g-C3N431 have been employed. The modified SEI layers are proved to be dense and uniform so that Li dendrites can be effectively suppressed and high Li CE can be obtained. Among the reported additives, trace amounts (25−50 ppm) of water is highly helpful for suppressing the growth of Li dendrites in LMBs using LiPF6-carbonate electrolytes.23 According to the analysis in the paper, the LiF-rich SEI layer resulting from the hydrolysis of PF6− plays a critical role in dendrite suppression. Actually, as previously reported,32 difluorophosphate anion (PO2F2−) is another hydrolysis product besides HF of PF6− based on the following reactions: PF6 ‐ + H 2O → F‐ + POF3 + 2HF

(1)

POF3 + H 2O → PO2 F2 ‐ + HF + H+

(2)

Additionally, there is also a hydrolysis equilibrium of PO2F2−:32,33 PO2 F2− + H 2O ↔ PO3F2 − + HF + H+

(3)

However, even in weakly basic solutions, the hydrolysis of PO2F2− is very slow.33 Therefore, with a better stability toward moisture than PF6−, the effect of PO2F2− on suppressing Li dendrites growth should be considered except that from the HF. Recently, LiDFP has been reported as a promising additive in LIBs to enhance the low temperature performance34 and CE,35 enhance the rate capability,36 and decrease the internal impedance.37 These works revealed that LiDFP could modify the SEI film on different electrodes by forming a stable and high-ionic-conductive SEI layer. Inspired by these works mentioned above, we herein employ LiDFP as an electrolyte additive directly in a conventional LiPF6/carbonate-based electrolyte to modify the SEI layer on metallic Li anode and reveal its possible mechanism in LMBs for the first-time report. The effects of LiDFP on the morphology of Li deposition, the Li CE, the stability of Li electrode and the cycle life of the related LMBs with a conventional Li intercalation cathode have been systematically investigated, and greatly enhanced performances are obtained.

3. RESULTS AND DISCUSSION The average CEs of Li metal in the electrolytes without and with LiDFP were obtained by testing the Li||Cu cells with the modified method 3 reported by Adams et al.38 and the cells were cycled for 10 times. As displayed in Figure 1, upon increasing the amount of LiDFP additive from 0 to 0.15 M in the STD electrolyte, the average Li CE is gradually increased. This result is in accordance with and in more detail than that reported in the work of Yang et al.35 Due to the saturation of

2. EXPERIMENTAL SECTION The commercially available LiDFP was ordered from Guotai Super Power Co. Ltd. The standard (STD) electrolyte of 1.0 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by weight ratio) was obtained from Capchem Technology Co., Ltd. Various amounts of LiDFP were dissolved into the STD electrolyte in an argon-filled glovebox of MBraun (both the O2 and the H2O contents were restricted below 0.1 ppm). The reductive decomposition potential of LiDFP was measured by linear sweep voltammetry (LSV) on the Li|Li|Cu (copper) threeelectrode-cell configuration at 0.5 mV s−1 (where Cu foil was severed as the working electrode, a Li plate and Li wire were used as the counter electrode and the reference electrode, respectively). For the sake of directly evaluating the beneficial role of LiDFP on Li

Figure 1. Average CE values of Li metal in electrolytes with LiDFP additive from 0 to 0.15 M. 22202

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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Figure 2. Cycling stability of Li||Li symmetric cells in the electrolytes without and with various contents of LiDFP additive at the current density of (a) 0.5 mA cm−2 and (b) 1.0 mA cm−2 with 1 h plating/stripping time at 25 °C.

probably due to a more stable and Li+-conducting SEI layer on Li metal electrode. To clarify the possible reasons for higher CE and better cycling performance of the Li||Li symmetric cells with more LiDFP additive, we further investigated the impedances of the cells after different cycles. As shown in Figure 3a−d, there are two semicircles for the cells using the LiDFP-containing electrolytes. Typically, the large semicircle range from the highto-middle frequency is attributed to the resistance of SEI layer (RSEI) whereas the small semicircle range from the medium-tolow frequency is ascribed to the resistance of charge transfer (Rct).10 However, there seems only one large semicircle for the cell with the STD electrolyte from the first to the 10th cycles, nevertheless after 50 cycles, a small semicircle appears in the medium-to-low frequency range. The existence of one or two semicircles in the Li||Li symmetric cells containing electrolytes without and with LiDFP is probably associated with structural and compositional changes of the SEI layer, but the real reason is unclear now. Additionally, the intercept along the Z’-real axis of the high frequency range points out the resistance (Rb) of the bulk electrolyte solution. Total resistance (Rtotal) is defined as the sum of Rb+RSEI+Rct. When the equivalent circuit (displayed as the inset in Figure 3d) was further employed to fit the Nyquist plots in Figure 3a−d, the obtained values of RSEI, Rct, Rb, and Rtotal are shown in Figure 3e−h (also see SI Table S1). Clearly, the addition of LiDFP in the STD electrolyte reduces the SEI resistance and the total resistance of the cells. Meanwhile, increasing the concentration of LiDFP further decreases the cell total resistance. It indicates that the LiDFP additive has participated in the formation of the low-impedance SEI layer on Li metal anode. With cycling, Rb, Rct, and Rtotal of all cells except that containing 0.15 M LiDFP decrease and the corresponding resistances between different cells are apt to be close (Figure 3e−h). This is probably because of instability of the SEI layer on Li metal during initial cycles and gradual stabilization via the repeated fracture/repairing of the SEI layer.1 After several cycles, a relatively stable SEI layer can be built up with the increased surface area and/or the increased ionic conductivity so that those resistances mentioned above decrease. As shown in Figure 3g, Rb of the cells containing no or a low-content LiDFP (0.05 M) increases upon cycling, because the stabilization of a stable SEI layer needs more cycles along with more parasitic reactions between Li metal electrode and the electrolyte. On the contrary, the cell containing sufficient LiDFP, especially for that with 0.15 M LiDFP, only needs

the LiDFP in the given EC/DEC (1/1, w/w) solvent mixture, it is difficult to get an electrolyte with a higher concentration of LiDFP than ∼0.17 M in the given 1.0 M LiPF6 in EC/DEC (1:1 by weight ratio) electrolyte. Thus, the LiDFP concentration of 0.15 M in the STD electrolyte is regarded as the highest concentration in this work. The highest Li CE value of 95.2% was obtained for the 0.15 M LiDFP containing electrolyte, which is considered as the optimal electrolyte. In order to preliminarily estimate the positive role of LiDFP on SEI film formation, the decomposition potential of LiDFP was first measured by Li|Li|Cu three-electrode cells. As shown in Supporting Information (SI) Figure S1, a new reduction peak appears at ∼0.62 V (vs Li/Li+) after adding 0.15 M LiDFP in the STD electrolyte, indicating the reductive decomposition of LiDFP for SEI formation. Meanwhile, the measured reductive potential of LiDFP is close to the result of 0.59 V reported by Kim et al.36 To further gain the beneficial effect of LiDFP on stabling Li anode, Li||Li symmetric cells using the above-mentioned four electrolytes were further tested at 0.5, 1.0 and 3.0 mA cm−2 with 1 h plating/stripping time, respectively. As shown in Figure 2a, at the current density of 0.5 mA cm−2, the overpotential of the cell with the STD electrolyte started to increase after about 300 h, while the cell with 0.15 M LiDFP additive did not show obvious overpotential increase until 500 h. Meanwhile, the more LiDFP additive in the electrolyte, the later appearance of the increase in cell overpotential. When the current density was further increased to 1.0 mA cm−2, as shown in Figure 2b, the similar results were obtained, and the appearance of the obvious overpotential increase was at about 90 and 200 h for the cells with STD and 0.15 M LiDFP electrolytes, respectively. Even increasing the current density to 3.0 mA cm−2, the cell in the electrolyte with 0.15 M LiDFP also shows enhanced cycling life and lower polarization compared with those of the STD electrolyte (SI Figure S2). In addition, according to the comparison in SI Figure S3a,b, introduction of LiDFP can also decrease the polarization in the initial cycling period. Moreover, during the whole cycling process, the cells with 0.15 M LiDFP electrolyte showed lower overpotential than those with STD electrolyte in SI Figure S3c,d. All the results prove that the polarization could be reduced by introducing the LiDFP additive, and the more LiDFP contributes to the lower overpotential and the better cycling stability of the cells. Generally, the enhanced stability and reduced overpotential of Li||Li symmetric cells with LiDFP-containing electrolytes are 22203

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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ACS Applied Materials & Interfaces

Figure 3. Nyquist plots of the Li||Li symmetric cells using electrolytes without and with various contents of LiDFP additive after selected cycles. The cells were cycled at the current density of 1.0 mA cm−2 with 1 h plating/stripping time at 25 °C. (a) first cycle, (b) second cycle, (c) 10th cycle, and (d) 50th cycle. Evolutions of the impedances with cycling for Li||Li symmetric cells using electrolytes without and with various contents of LiDFP additive. (e) SEI resistance (RSEI), (f) charge transfer resistance (Rct), (g) ohmic resistance (Rb), and (h) total resistance (Rtotal).

fewer cycles to build up a stable SEI layer. These results confirm that the LiDFP-derived SEI layer is more stable and Li+-conducting than that formed in the STD electrolyte, and more LiDFP additive in electrolyte can protect the Li metal electrode for a longer lifetime by reducing the parasitic reactions. Meanwhile, it also suggests a faster electrochemical kinetics and enhanced Li metal stability by using the LiDFP

additive. Therefore, the excellent SEI layer built by LiDFP makes contribution to the higher CE and better cycling performance as shown in Figures 1 and 2. To thoroughly understand the beneficial effect of LiDFP on SEI formation, XPS was employed to supply a full view for analyzing the chemical compositions of the SEI layer on Li metal anodes cycled in the electrolytes without and with 22204

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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ACS Applied Materials & Interfaces

Figure 4. (a, b) F 1s and (c, d) P 2p of XPS spectra for the SEI layers formed in electrolytes without and with LiDFP.

phosphate compounds, which prevents the continuous decomposition of electrolyte and facilitates Li+ deposition with dendrite-free. Some different research results were reported in literatures about the effect of LiDFP on the spectra of F 1s and P 2p in SEI layers (not on Li metal). Kim et al.36 reported that the addition of LiDFP additive in the electrolyte could increase the content of LiF in the SEI layer formed on graphite electrode but it changed the content of LixPOyFz only a little. Yang et al.34 found that the contents of both LixPOyFz and LiF were increased drastically in the SEI layer of the cycled graphite anode with 1% LiDFP additive. However, Liu and coworkers35 showed that the LiF content in the SEI layer of graphite anode decreased after adding LiDFP in the electrolyte. Generally, the contents of LixPOyFz and LiF in SEI layers are tightly associated with the electrodes and solvent systems. Since the reported XPS results were mainly from the non-Li electrodes in half or full cells, the products reflected by the XPS peaks may come from dissolved products on the counter electrodes or even the reduced LiDFP. However, we used a symmetric Li||Li cell to test the XPS data to avoid the interference from the dissolved oxidative decompositions at the high-potential cathode side and focused on the role of LiDFP on SEI formation. The morphologies of Li anodes cycled in electrolytes with and without LiDFP were characterized by SEM. As shown in Figure 5a,b, the surface of the Li anode after cycling in the STD electrolyte for 50 cycles is rough and cracked, and composes of Li dendrites. However, in the STD+0.15 M LiDFP electrolyte, significantly dense and uniform Li deposition without dendrite formation can be achieved as shown in Figure 5c,d. The FTIR results of the SEI layers formed on cycled Li electrodes in the STD and STD+0.15 M LiDFP electrolytes do not show much difference (SI Figure S5), indicating the organic species (sensitive to infrared) in the

LiDFP. The wide scans of the XPS survey and the corresponding element contents are shown in SI Figure S4. The fluorine (F) and phosphorus (P) contents of the SEI layer formed in the STD+0.15 M LiDFP electrolyte are 9.2 and 1.9 wt %, respectively, whereas those of the SEI layer formed in the STD electrolyte are 7.5 and 1.5 wt %, respectively. Highresolution spectra of F 1s and P 2p are shown in Figure 4. The peak located at 685.9 eV in F 1s is corresponding to LixPOyFz, which mainly originates from the decomposition of LiPF6 (Figure 4a,b).34 Meanwhile, the peak located at 684.5 eV is the signal of LiF.31,36,39 Notably, after introducing LiDFP into the electrolyte, the peak intensity of LixPOyFz is decreased, whereas that for LiF is slightly enhanced. Given the impedance shown in Figure 3, the higher Rct for the cells with LiDFP additive may come from the slightly increased content of LiF. A stable but low ionic conductivity SEI with more LiF may inhibit the other side reactions and decrease the total cell impedance. Generally, LiF plays a positive role in improving the properties of SEI layers and suppressing the growth of Li dendrites because LiF is electronic insulation (∼10−31 S cm−1) and beneficial for Li+ ions diffusion. Therefore, the relatively more LiF content in the SEI layer is more helpful for suppressing the electrolyte from decomposition and facilitating Li+ ions deposition on the surface of Li electrode.1,40 As a result, LiDFP additive derives a uniform and dendrite-free Li deposition morphology. Significant difference can also be observed in the P 2p spectra in Figure 4c,d. The peak located at 136.8 eV is for LixPOyFz. A new peak located at 134.0 eV for P−O phosphate compounds is observed on the electrode cycled in the LiDFPcontaining electrolyte.36 The phosphate compounds should be originated from the decomposition of LiDFP. The P−O moieties in the SEI layer is also helpful for improving the kinetics of the interfacial Li+ transfer.41 The XPS results verify that the LiDFP could introduce an SEI layer rich in LiF and 22205

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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ACS Applied Materials & Interfaces

To further verify the speculation, Li||Cu cells were cycled to obtain the Li deposition morphologies in the electrolytes without and with LiDFP. Before SEM measurements, a certain amount of Li (1.0 mAh cm−2) was first deposited on the Cu electrode. Figure 6 exhibits the side-view and top-view SEM

Figure 6. Morphologies of the deposited Li from Li||Cu cells with the STD electrolyte and the 0.15 M LiDFP electrolyte. (a) Side-view and (b) top-view SEM images for Li from the STD electrolyte; (c) sideview and (d) top-view SEM images for Li from the 0.15 M LiDFP electrolyte.

images of the deposited Li in the STD electrolyte and the 0.15 M LiDFP electrolyte. In agreement with Figure 5, the plating of Li metal from the STD electrolyte shows a needle-like morphology with a large amount of Li dendrites (Figure 6a,b). By contrast, the plating of Li metal from the STD+0.15 M LiDFP electrolyte shows a granular morphology without Li dendrites (Figure 6c,d). It has been clearly demonstrated that the granular Li deposition morphology has two advantages over the needle-like dendrite. On one hand, the needle-like dendrites have diameters of a few hundred nanometers, which can pierce through separator easily. By contrast, the granuladeposited Li particles have a larger dimension (with a width of about 10 μm), which can prevent them from piercing through the separator. On the other hand, the Li particles deposited in the STD+0.15 M LiDFP electrolyte have smaller surface area than that deposited in the STD electrolyte. Consequently, the notorious side reactions can be suppressed by adoption LiDFP additive. Finally, the LiDFP containing electrolyte shows higher CE values during repeated Li plating/stripping.43 The compatibility of LiDFP as an electrolyte additive for rechargeable LMBs was also investigated. As shown in Figure 7a, both cells containing the STD and the 0.15 M LiDFP electrolytes deliver similar discharge capacity (143 mAh g−1) and CE (84.6%) at 0.1 C in the first cycle. The corresponding differential capacity (dQ/dV) curves in SI Figure S8 suggest that there is no oxidation of LiDFP on the cathode side when charged to 4.2 V, indicating the good stability of both electrolytes on the cathode in the present voltage window. Therefore, the later capacity decay of Li||NMC cells can be attributed to the failure of Li metal anode. Figure 7b exhibits the cycling performance of Li||NMC cells in the electrolytes without and with 0.15 M LiDFP additive. Although the role of LiDFP is not obvious during the initial 60 cycles, after that, the Li||NMC cell with the STD electrolyte fades rapidly, while the cell with LiDFP delivers nearly constant discharge capacity and CE. Therefore, the LiDFP additive is beneficial for stabling Li metal anode during cycling process.

Figure 5. Morphological evolutions of Li anodes in Li||Li cells at 1.0 mA cm−2 with 1 h plating/stripping time for 50 cycles (100 h) in electrolytes (a, b) without and (c, d) with LiDFP; and for 100 cycles (200 h) (e, f) without and (g, h) with LiDFP.

SEI layers are similar. Additionally, some clusters that might be LiF can also be observed in Figure 5d.42 This distinction can also be observed obviously after 100 cycles. As shown in Figure 5e,f, the surface of the Li anode cycled in the STD electrolyte is loose with plenty of Li dendrites and “dead” Li. On the contrary, the dense and uniform Li deposition without dendrite formation can be maintained for more than 100 cycles for the cycled Li metal in the LiDFP-containing electrolyte. Notably, the Li electrode cycled in the STD electrolyte suffers from severer corrosion of the bulk Li than that in the STD + 0.15 M LiDFP electrolyte both after 50 cycles and 100 cycles as shown in the SI Figure S6. Therefore, such different morphologies demonstrate the merit of LiDFP on Li dendrite suppression. Li||Cu cells were further employed to demonstrate the advantages of LiDFP for reversible Li plating/stripping in terms of CE variation with cycling. As shown in SI Figure S7, the CE of the Li||Cu cells with the STD electrolyte fades rapidly after 50 cycles and 25 cycles at 0.5 mA cm−2 and 1 mA cm−2, respectively. On the contrary, the CE of the Li||Cu cells with the STD+0.15 M LiDFP electrolyte can be maintained at 92% and 90% for 80 cycles and 60 cycles at 0.5 mA cm−2 and 1 mA cm−2, respectively. Notably, the STD+0.15 M LiDFP shows higher CE than the STD at both current densities. Therefore, LiDFP plays a positive role in constructing a highly efficiency SEI layer that suppresses the electrolyte from decomposition and the accommodation of inactive Li. 22206

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

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ACS Applied Materials & Interfaces

Figure 7. Electrochemical performances of the Li||NMC cells in the STD electrolyte and the STD+0.15 M LiDFP electrolyte. (a) Initial charge− discharge curves; (b) cycling performance; and the corresponding voltage curves of the cells in (c) STD electrolyte and (d) 0.15 M LiDFP electrolyte. EIS curves of the cells after (e) 2 cycles and (f) 100 cycles.

derived SEI layer is more Li+ conductive and stable, which is beneficial for uniform Li stripping/plating and longtime cycling of the LMBs.

Generally, the voltage gap (ΔV) between the charge− discharge curves is tightly associated with the polarization degree of batteries.10 Obviously, the polarization of the Li|| NMC cell in the STD electrolyte increased rapidly after 50 cycles (Figure 7c). By contrast, the polarization of the Li|| NMC cell in the 0.15 M LiDFP electrolyte remained at a low level of about 0.16 V and the increase with cycling was negligible as shown in Figure 7d, indicating that the LiDFP induced SEI is more conductive for Li+ and beneficial for stabling metallic Li anode during cycling. It can also be further proved by the EIS spectra in Figure 7e,f. After two formation cycles, the total resistance of the Li||NMC cell in STD+0.15 M LiDFP electrolyte (25 Ω) is much lower than that of the cell in the STD electrolyte (47 Ω) as shown in Figure 7e, indicating the SEI layer formed in the STD+0.15 M LiDFP electrolyte is more conductive for Li+ ion transport than that in the STD electrolyte. After 100 cycles, the total resistance of the cell with the STD electrolyte increased to about 57 Ω, but the resistance of the cell with the 0.15 M LiDFP electrolyte only increased a bit, to about 29 Ω. Meanwhile, in Figure 7f, the Rb of the Li|| NMC cell with the STD electrolyte is 24 Ω which is much larger than that of the cell with STD+0.15 M LiDFP electrolyte. This is because the liquid electrolyte in the STD cells is continuously consumed during cycling and may be largely depleted after 100 cycles. The capacity fading in Figure 7b can also support this finding. On the basis of the above experimental results, we can conclude that the LiDFP additive

4. CONCLUSIONS LiDFP is proposed as an electrolyte additive to suppress the notorious Li dendrites growth in conventional LiPF6/ carbonate electrolytes for LMBs. Specifically, the preferential reduction of LiDFP renders compact and stable SEI enriched with LiF and phosphates. Therefore, the symmetric Li||Li cells coupled with 0.15 M LiDFP additive show low polarization degree and enhanced cycling stability. More attractively, a high average Li CE of 95.2% can also be achieved in Li||Cu cell with the 0.15 M LiDFP electrolyte when compared to 84.6% for that of the STD electrolyte. As expected, when the optimal LiDFP-containing electrolyte is introduced into the Li||NMC cell with a relatively high mass loading of 9.7 mg cm−2, the cycle life of the LMB can be improved significantly. The results demonstrate that LiDFP shows high potential for application in rechargeable LMBs, but more evaluations and investigations are needed for its practical utilizations.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05185. 22207

DOI: 10.1021/acsami.8b05185 ACS Appl. Mater. Interfaces 2018, 10, 22201−22209

Research Article

ACS Applied Materials & Interfaces

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Supporting electrochemical performance data, CV, electrochemical performance of Li||Li cells, EIS results, XPS data, FTIR spectra, and SEM images of Li electrodes after cycling, CE of Li||Cu cells, dQ/dV curves of Li||NMC batteries (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(H.F.X.) E-mails: [email protected]. *(W.X.) E-mail: [email protected]. ORCID

Hongfa Xiang: 0000-0002-6182-1932 Wu Xu: 0000-0002-2685-8684 Author Contributions ∥

These authors contributed equally to this work.

Author Contributions

H.F.X. and P.S. initiated the research and designed the experiments with suggestions from W.X. P.S. and L.Z. conducted the electrochemical tests. P.S. carried out the SEM and XPS measurements with the assistance from X.L. and Y.S. P.S., L.Z., H.F.X. and W.X. prepared the manuscript with inputs from all other coauthors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant Nos. 21676067, 51372060, 21606065 and 51502300), Anhui Provincial Natural Science Foundation of China (Grant Nos. 1708085QE98 and 1608085QE88), and the Fundamental Research Funds for the Central Universities (JZ2017YYPY0253). W.X. acknowledges the financial support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research (BMR) programs of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231.

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