A Scalable Approach for Dendrite-Free Alkali Metal Anodes via Room

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A Scalable Approach for Dendrite-Free Alkali Metal Anodes via Room-Temperature Facile Surface Fluorination Gang Wang, Xunhui Xiong, Dong Xie, Xiangxiang Fu, Zhihua Lin, Chenghao Yang, Kaili Zhang, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18101 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Materials & Interfaces

A Scalable Approach for Dendrite-Free Alkali Metal Anodes via Room-Temperature Facile Surface Fluorination Gang Wang,a Xunhui Xiong,*,a Dong Xie,b Xiangxiang Fu,a Zhihua Lin,a Chenghao Yang,a Kaili Zhang,*,d Meilin Liu*,c a

Guangzhou Key Laboratory of Surface Chemistry of Energy Materials, New Energy

Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China b

Guangdong Engineering and Technology Research Center for Advanced

Nanomaterials, School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, China c

School of Materials Science & Engineering, Georgia Institute of Technology,

Atlanta, GA 30332-0245, USA d

Department of Mechanical Engineering, City University of Hong Kong, Hong Kong

999077, China

*E-mail (X. Xiong): [email protected]; *E-mail (K. Zhang): [email protected]; *E-mail (M. Liu): [email protected].

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Abstract Alkali metals are attractive anode materials for advanced high-energy-density battery systems because of their high theoretical specific capacities as well as low electrochemical potential. However, severe dendrite growth as well as high chemical reactivity restrict their practical application in energy storage technologies. Herein, we propose a facile scalable solution-based approach to stabilize Li and Na anodes via the facile process of immersing the Li/Na metal in a nonhazardous ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BdmimBF4) for several minutes at room temperature before battery assembly. This produces a dense and robust artificial fluoride layer, formed in situ by the reaction of ionic liquid and Li/Na metal. As a demonstration, a homogeneous and compact LiF coating on Li metal anode was fabricated via our method and it can effectively suppress the growth of Li dendrites and the continous decomposition of electrolyte during cycling. As a result, the LiF coated metallic Li anode achieves an enhanced cycling lifespan over 700 h with low overpotential (~22 mV) at 1 mA cm-2, as well as a very high Coulombic efficiency (CE) up to 98.1% for 200 cycles at 1 mA cm-2. Furthermore, the successful achievements of the dendrite-free Na deposition show the versatility of room-temperature surface fluorination for potential battery applications. KEYWORDS: Alkali metal anodes, dendrite-free, artificial SEI layer, surface fluorination, ionic liquid

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1. Introduction Metallic Li has long been investigated as a potential anode for advanced energy storage devices due to its high theoretical specific capacity (3860 mAh g-1) and low electrochemical potential (-3.04 V vs. standard hydrogen electrode).1-3 Recently, Na and potassium (K) anodes are also attracting increasing research interests because of their high natural abundance and exciting cathode chemistry in Na-air, Na-S and K-air.4-8 However, the direct utilization of alkali metals as the anode in secondary batteries has been plagued by various intrinsic problems, such as uncontrollable dendritic growth, low and rapidly degraded CE, and large volume change during cycling.9-13 The nonuniform distribution of internal electric field in planar current collector induces the inhomogeneous nucleation of alkali metal and unavoidable formation of dendrites upon cycling. Notably, dendrites might pierce through the separator and triger internal short-circurt, bringing in saftery hazards. Moreover, the high chemical activity of alkali metals facilitates the spontaneous reduction reactions between organic electrolyte and alkali metals, and accordingly the generation of brittle solid electrolyte interphase (SEI). Such SEI could not accomodate the huge volume fluctuation upon cycling, resulting in repetitive collapse and formation of SEI. This continuously consumes both liquid electrolytes and alkali metals, which then causes a poor CE and a low operating life time.14,15 In recent yeras, various approaches have been utilizied to prevent the formation of metallic dendrites. One common strategy is to utilize electrolyte additives, such as lithium

nitrate,16

lithium

polysulfide,17

sodium

3


polysulfide,18

potassium


bis(trifluoromethylsulfonyl)imide,19 and fluoroethylene carbonate,20,21 to boost the formation of SEI with mechanical strength, buffering the volume flucuation upon repetitive Li plating and stripping. However, the additives are continuously consumed during cycling, leading to gradually deteriorated electrochemical performances. Another well-known strategy is to employ solid materials such as inorganic compounds,22-26 polymers,27,28 hybris organic-inorganic13,29 and carbonaceous materials30-33 to create protective layer to guide the deposition of alkali metal on anodes. For example, several different groups have testified that ultrathin ALD Al2O3 coating film on the surface of Li/Na metal helps to suppress Li/Na dendrites.23,34,35 Lu et al. fabricated hybrid silicate coatings on Li metal surface by vapor deposition to stablize Li metal anode.22 Cui et al. developed two different methods to synthesize artificial LiF SEI layer on Li metal by reacting Li metal with different fluorination precursors at hundreds of degrees.25,26 However, it is still a great challenge to fabricate a dense artifical SEI (ASEI) layer via a convenient and scalable process at room temperature and ambient pressure. Herein, we report a facile solution-based process for the preparation of an artificial LiF layer on the surface of Li metal (LiF-Li) by an fast precipitation reaction between a nonhazardous IL BdmimBF4 and Li metal. As illustrated in Figure 1, trace amount of water is added to IL BdmimBF4 to facilitate the hydrolysis of IL to form BF3·H2O and F ion ( BdmimBF4  H 2 O  Bdmim   BF3  H 2 O  F ). After the complete hydrolysis in few minutes, the Li foil is immersed into the IL, a conformal and homogeneous LiF on Li anode surface is formed via reaction 1 and reaction 2 because

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Li foil surface is generally covered by a thin layer of Li2O/LiOH before use. The whole solution-based process takes only a few minutes at room temperature and ambient pressure. The IL is environmental friendly and operationally safe compared with other fluoride sources. Most importantly, the mild in-situ liquid-phase precipitation reaction guarantee LiF layer to be uniform and dense. As expected, the LiF layer can effectively prevent their direct contact, thus effectively suppress the formation of Li dendrite and the decomposition of electrolyte. Consequently, the LiF-Li anode displays dramatically enhanced electrochemical performances in terms of low overpotential and excellent stability in symmetrical cells, and high CE in Li||Cu cells. Similar results are obtained with Na metal anode after surface fluorination and it also shows remarkedly enhanced electrochemical performances. This versatile and encouraging technique will open up new possibilities for the fabrication of stable ASEI layer for alkali metal anodes in the future.

2. Results and discussion The morphology of LiF-Li anode was investigated via filed-emission scanning electron microscopy (FE-SEM), displaying significantly enhanced homogeneity of LiF-Li anode surface after fluorination process (Figure 2B and Figure S1). No obvious defects and cracks can be observed. LiF-Li anode shows intensive and uniformly distributed signals of fluorine in the energy dispersive X-ray spectroscopy (EDS, Figure S2B). A dense and compact surface coating layer (with a thickness of ~115 nm) can be seen from the cross-section image of LiF-Li anode (Figure S3). This

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artificial layer can regulate the repetitively plating and stripping of Li beneath it and thus suppress the formation of Li dendrites (Figure 2D). Even after 200 cycles in a symmetric cell, no obvious changes are observed on the LiF layer and its thickness from SEM images (Figure 2F), verifying its excellent mechanical strength and chemical stability, which also can be certified by XPS tests (Figure S4). As shown in the cross-section SEM images (Insert in Figure 2F, Figure S5), the Li is prefer to deposit at the bottom of LiF layer and a uniform protective layer with a thickness of ~100 nm is well preserved. By contrast, the surface of bare Li anode emerges cracks and becomes porous after 100 cycles (Figure 2C and 2E). What’s worse is numerous Li dendrites formed on surface (Figure S6) because of recurring breakdown and reforming of SEI layers.36-39 The chemical compostion of the surface of Li anode was further evaluated through X-ray photoelectron spectroscopy (XPS). The elements on bare Li surface are Li, C, and O, while the major elements on LiF-Li anode surface are Li, C, O, and F (Figure S7). C 1s spectrum of bare Li anode shows two peaks at 284.8 and 289.4 eV, which are assigned to C-C and Li2CO3, respectively (Figure 2G).40 However, no obvious peak of Li2CO3 can be detected for the LiF-Li anode (Figure 2H). Meanwhile, two pronounced peaks at 528.3 and 531.5 eV (Figure 2I), which are attributed to Li2O and Li2CO3 overlay with LiOH,24 are clearly depicted in the O 1s spectrum for the bare Li anode, but these two peaks are absent from the spectra for the LiF-Li anode. The peak (Figure 2J) locates at 230.9 eV is commonly associated with the surface adsorbed oxygen species.41 XPS for Li 1s, C 1s and O 1s of LiF-Li anode confirmed the

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absence of the native SEI layer. Therefore, we concluded that the Li2O/LiOH film on Li surface is consumed via our facile fluorination process, which is critical for stabilizing Li metal anode because the dissolution of Li2O/LiOH in electrolyte induces a porous structure and will promote the formation of Li dendrites. The high-resolution XPS spectra of both Li 1s and F 1s show one single peak at their corresponding binding energy, 55.9 and 685.8 eV, respectively, (Figure 2K and 2L), and no evident peak of Li0 is observed, which is generally believed to be centred at about ~53.4 eV.22,42 These facts demonstrate that the highly robust LiF layer is in-situ grown on the surface of LiF-Li anode successfully,25,43 which is consistent with our hypothesis. The mild hydrolysis of IL BdmimBF4 and in-situ liquid-phase reaction process guarantee LiF layer to be uniform and dense. LiF is known to be a poor Li ion conductor (~3×10-9 S cm-1 vs. ~6×10-8 S cm-1 for Li3PO4), however, its Li ion conductivity would be enhanced when it was farbricated into nanolayer and then LiF layer can smooth the interfacial Li+ transport upon charge-discharge process.26,44 Eventually, the artificial LiF layer can be expected to supress the formation of Li dendrite and inhibit the corrosion of bulk metal in the intensive cycling process. To invesitigate the cyclic stabity of the LiF-Li anode, symmetrical cells with two identical LiF-Li electrodes were assembled and studied under various current densities in ether-based electrolyte. Compared with bare Li anode, all LiF-Li anodes, even with fluorination treatment just for one minute, show improved cycling stabilities (Figure S8 and S9). Among them, the LiF-Li anode after treatment for 10 minutes demonstrates the best cycling performance (700h) with flat and extremely

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stable voltage curves with slight hysteresis (~22 mV) during the Li stripping/plating processes at a current density of 1 mA cm-2 (Figure 3A). The cycle performance of Li plating/stripping in symmetric cells is more excellent than the recently reported work (Table S1). Consistently, the increasing rate of overpotential was also incremental as current density. The increased hysteresis voltage suggests the growth of Li dendrites and depletion of electrolyte. When tested at a higher current density of 5 mA cm-2, an excellent cycling performance of LiF-Li anode over 240 cycles with lower overpotential of about 80 mV is still obtained, whereas the bare Li anode exhibits a huge overpotential increase and a sudden drop in voltage at cycling time of 32 h (corresponding to about 80 cycles). The sharp drop in voltage is an indication that the Li dendrites penetrate through the separator, resulting in internal short-circuits.45-47 Even at a practical Li deposition capacity of 4 mAh cm-2, the LiF-Li anode still exhibits remarkable cycling stability (Figure S10). Lower overpotential and enhanced stability demonstrate that the notorious Li dendrites problem was restrained via our designed fast surface fluorination treatment.48 To further explore the reasons of improved cycling stablility and lowered polarization, electrochemical impedance spectroscopy (EIS) tests was carried out on bare Li and LiF-Li symmetric cells before cycling and after 10 cycles (Figure S11). The charge transfer impedance (Rct) for bare Li and LiF-Li cells before cycling was 124 and 112 Ω, while the Rct was decreased to 54 and 28 Ω after 10 cycles, respectively. The lowered Rct indicates that the LiF coating layer can boost Li plating/stripping kinetics and electrode stability during cycling.

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To verify the performances of LiF-Li electrode in practical use, full cells with LiF-Li as anode and a LiNi0.6Co0.2Mn0.2O2 (NCM) cathode were fabricated and evaluated. Considering the identical cathodes and electrolyte used in both cells, the differences of the cell performance, including CEs and cycle performances, can be used to evaluate the stability of Li metal anode. It can be clearly seen in Figure 4 that the LiF-Li||NCM full cell shows much better electrochemical performances than those of the Li||NCM cell, delievering a high capacity of 166.7 mAh g-1 with a CE of 90.9% at 1 C rate (1 C=180 mA g-1) after activation at 0.1 C for 3 cycles. The discharge capacity of Li||NCM cell dropped sharply to 61.7% of initial capacity after 100 cycles, while the LiF-Li||NCM full cell also shows good capacity retention, maintaining a reversible discharge capacity of 144.2 mAh g-1 (86.5% capacity retention) after 100 cycles at a stable CE of 99.2%. This result indicates that the excellent mechanically strong and chemical stable LiF on Li anode surface not only works as a protection layer to inhibit the growth of Li dendrites but also benefits the stability of SEI film. Meanwhile, the charge voltage plateau of the LiF-Li||NCM cell is lower than that of the cell with bare Li anode, implying that the artificial LiF protective layer can enhance the electrochemical kinetics of the full cell,25,49,50 consistent with the EIS analysis (Figure S12). Furthermore, the LiF-Li anode still maintains a dense and smooth surface with no apparent dendrite formation after long-term cycling (Figure 4D and Figure S13), confirming that the LiF ASEI layer suppresses the growth of Li dendrite as well as guarantees the uniform plating/stripping of Li.

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The cycling CE is a very important parameter to estimate the practicability and chemical stability of the modified anode for Li metal batteries.51-53 CE was examined by assembling the half cell consisting of Cu electrode and LiF-Li as counter electrode. Figure 5 displays the CEs of bare Li and LiF-Li anodes at a current density of 1 mA cm-2 under a deposition capacity of 1 mAh cm-2. The LiF-Li electrode shows a higher initial CE (93.8%) than that of bare Li electrode (88.4%), and can deliver an average CE of 98.1% for 200 cycles, indicating that an enhanced reversibility was obtained through surface fluorination. Similar phenomenon was observed at higher current density (Figure S14). The low CE of the bare Li electrode is attributed to the formation of dendritic and dead Li as well as consumption of excessive organic electrolyte.54 Therefore, a compact and uniform layer of LiF can increase the CE stability of LiF-Li electrode because of the significantly reduced side reactions between the electrode and electrolyte. This result was further verified by the smaller voltage hysteresis loop of the LiF-Li electrode than that of the bare Li electrode (Figure S15), which also implies that the artificial LiF protective layer does not impede the reaction kinetics of Li+ during charge/discharge cycling. Even at an elevated deposition capacity of 3 mAh cm-2, the LiF-Li electrode still maintains a high CE of ~86.8% after 80 cycles at 1 mA cm-2 (Figure S16). Because of the advantages of high natural abundance and low cost, sodium ion batteries aroused considerable interest as alternatives to LIBs in recent years.55,56 In this work, we demonstrate the versatility of surface fluorination to another alkali metal, Na. A dense and uniform NaF layer can be fabricated on surface of Na

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(NaF-Na) via immersing Na metal to IL BdmimBF4 at room temperature and ambient pressure (Figure S17). The cyclablities of NaF-Na anode and bare Na were evaluated via assembling symmetric cells in carbonate electrolyte. Figure 6 shows the voltage curves of the NaF-Na and bare Na cells at different current densities under a deposition capacity of 1 mAh cm-2. The voltage of NaF-Na cells keep stable during the first 300 hours (150 cycles). In contrast, bare Na cells show random voltage fluctuations, verifying the inhomogeneous deposition of bare Na anode. Moreover, a sudden short-circuit of the bare Na cell is occured in the 70th cycle, which can be attributed to severe dendritic growth (Figure S18 and S19). The improved cycle performance of the NaF-Na anode indicated that the artificial NaF protective layer can effectively suppress the formation of Na dendrites.

3. Conclusion In conclusion, we have successfully fabricated a dense and uniform LiF artificial layer on Li metal anode via a new and scalable surface fluorination of Li metal with IL BdmimBF4. The protective LiF layer can restrain the growth of Li dendrite and inhibit the undesirable reaction between the electrolyte and Li metal, resulting in a robust Li metal/electrolyte interface. Under the protection of the LiF layer, the LiF-Li electrode displays excellent cycling performance with a low overpotential (~22 mV) for 700 hours at 1 mA cm-2. The average CE for plating/stripping Li on/off the LiF-Li electrode is maintained at ~98.1% for 200 cycles at 1 mA cm-2 for a deposition capacity of 1 mAh cm-2. Moreover, this efficient approach has been successfully

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extended to guide the deposition of Na for Na metal system. This facile and scalable strategy for the preparation of artificial SEI layers may accelerate the practical application of alkali metal anodes for next-generation batteries.

4. Experimental section Material

synthesis:

Ionic

liquid

(IL)

1-butyl-2,3-dimethylimidazolium

tetrafluoroborate (BdmimBF4) with a purity of >97% was purchased from Aldrich and directly placed in an argon-filled glovebox. The water content in the IL was ~200 ppm (measured with a coulometric Karl Fischer titrator), which can faciliate the hydrolysis of IL to form BF3·H2O and F ion.57,58 The metallic Li foil (Ф15 mm) was obtained from China Energy Lithium Co., Ltd. In a typical fabrication process, 5 μL water is added to IL BdmimBF4 to produce BF3·H2O and F ion as the following reaction: BdminBF4+H2O → Bdmin++BF3·H2O+F-. After the complete hydrolysis in few minutes, the metallic Li foil was immersed in IL BdmimBF4 for 10 min and carefully wiped down the superfluous liquid with a nonstick wiping paper. The obtained Li metal anode was labeled as LiF-Li.

Material characterization: The field-emission scanning electron microscope (FE-SEM, Hitachi SU8010) with an energy dispersive X-ray spectroscope (EDS, JEOL JSM-6100LV) was performed to observe the microstructure and elemental distribution of all samples. Note that all the batteries after cycled were disassembled and flushed with anhydrous dimethyl carbonate (DMC) to remove remnant lithium salts and electrolyte in glovebox before to observe the

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morphology. The elemental valence of the SEI layer was detected by X-ray photoelectron spectroscopy (XPS, ESCALab220i-XL) with 300 W Al Kα radiations under ambient temperature.

Electrochemical measurements: The CR2032 type coin cells were employed to investigate electrochemical behavior of the electrodes synthesized as above. The cells were assembled in an Ar-filled glovebox with H2O and O2 concentrations below 0.1 ppm. For the symmetric cell tests, two identical electrodes were carried out at various current densities with the deposition capacity of 1 mAh cm-2 to research the lithium stripping/plating processes. For the CE testing, 1 mAh cm-2 of Li/LiF-Li metal was deposited on Cu foil (Ф12 mm) and then stripped away up to 1.0 V at a current density of 1 mA cm-2 or 3 mA cm-2 for every cycle. 1 M lithium bis(trifluoromethane) sulfonamide (LiTFSI, Aldrich) dissolved in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio 1:1) with 2 wt% LiNO3 as an additive was selected as the electrolyte and a 25 μm porous polypropylene based membrane (Celgard) was used as the separator. In Na symmetric cells, 1.0 M NaClO4 in a mixture solution of EC/DEC (volume ratio 1:1) was used as electrolyte. For the full cell operations, the NCM cathode electrode was fabricated by mixing active material, polyvinylidene fluoride (PVDF), and acetylene black with a mass ratio of 90:5:5 dissolved in a certain amount of N-methyl-2-pyrrolidone (NMP). Subsequently, the gained slurry was coated onto Al foil and dried at 80 °C for 12 h under vacuum. And then the electrode was cut into small slices with a diameter of 12 mm. Commonly, the mass loading of the

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NCM cathode material is about 6 mg cm−2. The reversible specific capacity and areal capacity of the as-prepared NCM are about 180 mAh g-1 and 1.08 mAh cm-2, respectively. The cells consisted of NCM cathode and LiF-Li or Li metal anode were cycled in 1.0 M LiPF6 (EC/DMC, volume ratio 1:1) electrolyte solution at 1 C after activation at 0.1 C for 3 cycles in a voltage range of 2.8-4.3 V. EIS tests were carried out on an electrochemical workstation (CHI660a, Shanghai Chenhua) in the frequency ranging from 105 to 10-2 Hz.

Acknowledgment We gratefully acknowledge the financial support from National Natural Science Foundation of China (51874142, 51604122), Natural Science Foundation of Guangdong Province (2016A030310411), Pearl River S&T Nova Program of Guangzhou (201806010031), Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200 & 2016ZT06N569), the Fundamental Research Funds for the Central Universities (2017BQ056) and the Science and Technology Planning Project of Guangdong Province (No. 2017B090916002).

ASSOCIATED CONTENT Supporting Information available: [The following files are available free of charge on the ACS Publications website: SEM images and EDS of bare Li/Na and LiF-Li/NaF-Na anodes before and after cycling, XPS spectrum of bare Li and LiF-Li

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anodes before and after cycling, and electrochemical characterization of Li/Na and LiF-Li/NaF-Na anodes.]

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Really a Boon for the Synthesis of Inorganic Materials? A General Method for the Fabrication of Nanosized Metal Fluorides. Chem. Mater. 2006, 18, 3162–3168.

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Figure 1. Schematics of the formation mechanism of LiF on Li foil surface.

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B

G

C–C (284.8 eV) C 1s

Intensity (a.u.)

A

Li2CO3 289.4 eV

D

284

288

I

(531.5 eV)

292

J

Oxygen species (530.9 eV)

532

528

524

Binding Energy (eV)

K

F

+

Li (55.9 eV)

284.8 eV

C 1s

520

280

284

288

292

Binding Energy (eV) 530.9 eV

524

528

532

O 1s

536

Binding Energy (eV)

Li 1s

L

LiF (658.8 eV)

F 1s

Intensity (a.u.)

E

276

O 1s

Li2O (528.3 eV)

536

H

Intensity (a.u.)

C

280

Binding Energy (eV)

LiOH+Li2CO3

Intensity (a.u.)

276

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 25 of 30

44

48

52

56

60

Binding Energy (eV)

64

680

684

688

692

Binding Energy (eV)

696

Figure 2. SEM images (A) of bare Li and (B) LiF-Li electrodes (Insert is the corresponding digital photos); Top and cross-section images of (C and E) bare Li and (D and F) LiF-Li electrodes after 200 cycles in symmetrical cell; the high-resolution XPS spectra of (G) C 1s, (I) O 1s for bare Li anode and (H) C 1s, (J) O 1s, (K) Li 1s, and (L) F 1s for LiF-Li anode.

25



Voltage (mV)

A

400

Voltage (mV)

B

200

0

0

50

th

50

Bare Li symmetric cell LiF-Li symmetric cell

th

200

25

19 mV

0

36 mV

-25

-25

-50

-50

-2

22 mV 33 mV

-2

1 mA cm , 1 mAh cm , 350 cycles

0

100

200

300

400

500

Times (h)

600

700

200 100 0 -100 -2

-200

C

50 25

-200

Voltage (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

-2


0

50

100

150

200

250

300

350

400

Times (h) 400 200 0 -200 -400

-2

-2


0

20

40

60

80

100

Times (h)

Figure 3. The cycling performances of the bare Li and LiF-Li electrodes at different current densities of (A) 1, (B), and (C) 5 mA cm-2 with a deposition capacity of 1 mAh cm-2, the insert is the particular voltage profiles of bare Li and LiF-Li electrodes at the 50th and 200th cycles.

26


120

200

100

-1

80

150

60 100 50 0

40

Li LiF-Li 0

20

20 40

60

Cycle number

80

100

0

Coulombic Efficiency (%)

A 250 Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

4.5

st

th

1

100 4.0

Voltage (V)

Page 27 of 30

3.5

Bare Li LiF-Li

3.0 0

50

100

150

-1

200

Capacity (mAh g )

C

D

Figure 4. (A) Cycling performance of the bare Li and LiF-Li||NCM battery systems at 1 C in a voltage range of 2.8-4.3 V. (B) Corresponding charge and discharge curves of the 1st and 100th cycles for LiF-Li and Li||NCM cells after activation. SEM images of the (C) bare Li anode and (D) LiF-Li anode after 100 cycles.

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Page 28 of 30

120

B

90 60 Bare Li LiF-Li

30 0

0

25

50

75

100 125 Cycle number

C

1.2 1.0

150

175

200

150

st

1 th 50 th 100 th 150 th 200

0.8 0.6 0.4

LiF-Li Bare Li

100

-Z'' (Ω)

A

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Coulombic efficiency (%)


0.2

50

0.0 -0.2

0.0

0.2 0.4 0.6 0.8 -2 Capacity (mAh cm )

0

1.0

0

50

100

150

200

250

Z' (Ω)

Figure 5. (A) CEs of Li||Cu and LiF-Li||Cu electrodes at 1 mA cm−2 with a deposition capacity of 1.0 mAh cm−2. (B) Charge/discharge curves of LiF-Li||Cu electrode at particular cycles. (C) EIS of Li||Cu and LiF-Li||Cu electrodes after 10 cycles.

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Page 29 of 30

A

0.50

Bare Na NaF-Na

Voltage (V)

Short circuit 0.25 0.00 -0.25 -1

-0.50

B

-1

1 mA cm , 1 mAh cm , 150 cycles 0

50

100

150

200

Time (h)

250

300

1.0

Voltage (V)

Short circuit 0.5 0.0 -0.5 -1

-1.0

C

0

20

40

Time (h)

D

0.50

60

80

100

1.0 0.5

Voltage (V)

0.00 -0.25 -0.50

-1


0.25

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96

98

100

Time (h)

102

0.0 -0.5 -1.0 20

104

22

24

26

28

30

Time (h)

Figure 6. Voltage profiles of NaF-Na metal at current density of (A) 1 and (B) 2 mA cm-2 under a deposition capacity of 1 mAh cm-2 in NaF-Na||NaF-Na symmetric cell; Enlarged voltage profiles at different times for (C) 1 and (D) 2 mA cm-2.

29




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