A stable carbon host engineering surface defects for room-temperature liquid NaK anode

Journal Pre-proof A stable carbon host engineering surface defects for roomtemperature liquid Na-K anode

Yangyang Xie, Junxian Hu, Zhian Zhang PII:

S1572-6657(19)30944-0

DOI:

https://doi.org/10.1016/j.jelechem.2019.113676

Reference:

JEAC 113676

To appear in:

Journal of Electroanalytical Chemistry

Received date:

8 September 2019

Revised date:

15 November 2019

Accepted date:

21 November 2019

Please cite this article as: Y. Xie, J. Hu and Z. Zhang, A stable carbon host engineering surface defects for room-temperature liquid Na-K anode, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113676

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© 2019 Published by Elsevier.

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A stable carbon host engineering surface defects for room-temperature liquid Na-K anode Yangyang Xie, Junxian Hu, Zhian Zhang* School of Metallurgy and Environment, Central South University, Changsha 410083, China.

Abstract:

Alkali

metal

anode

(Li,

Na,

K)

of

Email: [email protected] are

promising

anodes

for

ro

high-energy-density batteries because of high theoretical capacities and low redox

-p

potentials. While dendrite growth still restricts practical applications of alkali metal

re

batteries. In contrast, liquid metal anode materials can avoid the problem of dendrite

lP

formation. But the poor wettability of liquid metal on hosts makes it difficult to

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assemble cell for the strong surface tension. Herein, we report a treated carbon cloth (denoted TCC) that can fast absorbs liquid Na-K alloy at room temperature owing to

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the enhancing wettability by engineered surface defects of oxygenic groups and nanoscale cracks. When assembled in a symmetric cell with Na-K alloy, it renders long cycle life over 1800 hours and low voltage polarization below 300 mV compared with Na symmetric cell. When cycling with Na3V2(PO4)3 cathode as full cell, it also exhibits excellent capacity and capacity retention of 87.58% after 1000 cycles at 10 C. Keywords: carbon cloth; liquid Na-K alloy; wettability; surface defects 1. Introduction Rechargeable secondary batteries are considered as important energy storage

*

Corresponding author: Fax. & Tel: +86 731 88830649. E-mail address: [email protected]

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technologies with the fast development of electric vehicles and large-scale storage system[1-4]. Alkali metals, which possess high theoretical specific capacity and low redox potentials, are regarded as ideal anodes materials. [5-8] For room-temperature alkali metal anodes, there exists dendrite formation and unstable solid electrolyte interphase film, which hinder the actual application. [9-11] In order to overcome the

of

dendrite growth, many useful efforts have been made including optimizations on electrolytes, [13-15] introducing artificial SEI layer [16-17] and modifications on

-p

growth essentially on solid state interphase.

ro

current collectors or hosts. [18-22] While these methods can not dispel the dendrite

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In contrast, liquid metal anode materials are more attractive, because solid

lP

dendrite can be totally averted in a liquid phase. The Na-K alloy can remain liquid

na

state at room-temperature during a wide compositional range (from 9.2 to 58.2 wt% Na). [23, 24] In addition, Therefore, liquid Na-K alloy is a promising alkali metal

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anode. However, the Na–K alloy exhibits poor wettability for its strong surface tension, which makes it difficult to spread out over the hosts. Hence it becomes basic scientific issues in assembling liquid alloy battery. J. B. Goodenough and Kang have done excellent work on the wettability between matrix and liquid metal at high temperature (>420 ℃) and vacuum infiltration, [25-28] while the operating environment of vacuum infiltration is very strict, and high temperature environment may not be safe. [29] Professor Tu reported a stirring strategy to form a low surface tension Na-K alloy by non‐ Newtonian fluid state alloy and super P. [30] They also found the potassium oxide can be a protect player and improve the wettability with

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carbon matrix. Inspired by the above methods. Herein, we successfully engineered surface defects on TCC by introducing oxygenic groups and nanoscale cracks, which realizing the fast adsorption of the Na-K alloy onto CC. (Fig. 1) Due to the strong capillary action of nanoscale cracks and affinity between oxygenic groups and the

of

Na–K alloy, the as-prepared material displays outstanding wettability to the Na–K alloy. In addition, our methods are also feasible in other polyacrylonitrile

on

synthesized

CC@NaK/CC@NaK

electrodes

-p

based

ro

(PAN)-based carbon materials like carbon paper and carbon felt. The symmetric cell sustains

a

stable

re

electrodeposition over 1800 h and better rate capability than Na/Na symmetric cell.

lP

When assembled a liquid metal cell using NVP@rGo as the cathode, NaK/NVPrGo

na

full cell exhibits to excellent capacity and rate performance. These excellent characteristics of TCC may present a new sight into dendrite free liquid batteries. In

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addition, though adsorbing the Na–K alloy, the CC@NaK still retains its flexibility, which makes it possible to apply in flexible devices.

Fig.1. Schematic of the fabrication process of the CC@NaK composites.

2. Experimental section

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2.1 materials preparation 2.1.1 Preparation of the CC@NaK electrode The commercial carbon cloths (W1S1009, CeTech Co., Ltd) was treated under ultrasonic for 3 hours in mixed concentrated acid solution (H2SO4:HNO3=1:3) with 1g KMnO4, then washed with deionized (DI) water and vacuum dried at 80 ℃. Afterwards the treated CC was obtained by heated at 400 ℃ for 3 h at a ramp rate of 5 ℃

of

min-1 in air atmosphere and then cooled down to room temperature. The treated CC

ro

was punched into disks with 10 mm diameter. Finally, the liquid Na-K alloy (the

-p

weight ratio of Na/K is 1:1) was directly dropped onto the treated CC in a pure argon

re

filled glove box and used as the CC@NaK electrode. The mass loading of Na-K is

lP

about 30 mg cm-2.

na

2.1.2 Preparation of graphene oxide (GO)

Natural flake graphite was used to synthesize graphene oxide (GO) by a modified

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Hummers’ method. 1g natural flake graphite and 25 ml concentrated H2SO4 was placed into 200 ml beaker in ice bath and kept stirring for 1 h. Then 3 g KMnO4 and 0.5 g NaNO3 was added into the beaker slowly, during this process it need to keep stirring for 2 h and the temperature under 10 ℃. Then heated to 35 ℃ and added another 3 g KMnO4 into the beaker, keeping stirring for 2 h. Afterwards heated to 90 ℃ for 2 h and 180 ml DI water was added into the beaker, 15% H2O2 was added into mixture solution to stop reaction until no bubble appeared. Then centrifuged and washed with DI water for five times. Afterwards, the remaining products were dialyzed to neutral with a dialysis bag. Finally, graphene oxide was obtained after

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ultrasonication for 4 h. 2.1.3 Synthesis of Na3V2(PO4)3@rGo cathode Na3V2(PO4)3@rGo material was synthesized by sol-gel method. Typically, 9 mmol sodium acetate (CH3COONa, Aladdin, AR), 6 mmol Ammonium metavanadate (NH4VO3 Aladdin, AR), 9 mmol Ammonium dihydrogen phosphate (NH4H2PO4

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Aladdin, AR), 18 mmol anhydrous citric acid (C6H8O7, Aladdin, AR) and 136.8 mg Go were dissolved into 50 ml deionized water by a water-bath heating process that

ro

lasted about 6 hours at 80 ℃ with constant stirring. The anhydrous citric acid was

-p

acted as both chelating agent and carbon source. As the moisture evaporated, a black

re

gel was formed, which was further put into a vacuum oven at 120℃ for 12 h to get a

lP

black porous precursor. Finally, the porous precursor was sintered at 350 ℃ for 4h and

na

continues sintered at 750 ℃ for 9 h with a heating rate of 5 ℃·min-1 in an Ar atmosphere to obtain the reduced graphene oxide coated Na3V2(PO4)3 material.

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2.2 Material characterizations

The morphologies and element distributions of materials were probed by scanning electron microscope (SEM, JEOL S-4800). X-ray diffraction tests were measured on RigakuMiniFlex 600a with Cu Kα radiation (λ=0.154056 nm). And the experiments were carried out with a scan rate of 5° min-1 from 10° to 70° or 80°. Brunauer–Emmett–Teller (BET) specific surface area of samples were tested by nitrogen adsorption–desorption with a Micrometritics ASAP 2020 instrument (Norcross, GA, USA). The X-ray photoelectron spectroscopy (XPS) spectra was obtained by Thermo ESCALAB 250 spectrometer. Raman spectra measurements

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were performed on a LabRAM HR with an excitation laser wavelength of 532 nm. FTIR spectra were measured with IR spectrometer (Bruker ALPHA-II FTIR, Germany). Carbon content of the NVP@rGo material was determined by thermogravimetric (TG) tests. 2.3 Electrochemical measurements

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To investigate the electrodeposition behavior, the CC@NaK, Na-metal electrodes were assembled into symmetric cells with glass fiber filters (Whatman) for the cycling

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test in a pure argon filled glove box. The positive electrodes were consisted of 70 wt%

-p

active materials, 20 wt% super P, and 10 wt% polyvinylidene fluoride (PVDF). The

re

solvent was 1-Methyl-2-pyrrolidinone (NMP), and mixed them together uniformly at

lP

least 10 minutes to make slurry, which is then coated on Al foil with a doctor blade

na

and dried at 60 ℃ in a vacuum for 12 hours. The active materials mass loading of each electrode were about 0.8-1.2 mg cm-2. Na foil and glass fiber filters (Whatman) were

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used as counter electrode and separator, respectively. And the electrolyte was consisted of 1 M NaClO4 dissolved in PC solution with 5 vol% addition of fluoroethylene carbonate (FEC). All of the batteries were assembled into CR2032 coin cells in a pure argon fill glove box. The charge-discharge, cycle performance and rate capacity were tested on a Land battery test system (CT2001A) with a potential range of 2.5-3.8 V (vs Na/Na+). electrochemical impedance spectroscopy (EIS) in the frequency range of 100 kHz to 0.01 Hz with the potential amplitude of 5 mV and Cyclic Voltammetry (CV) in a potential range of 2.5-3.8 V vs Na/Na+ were conducted with PARSTAT 2273 Electrochemical workstation.

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3. Results and discussion Commercial carbon fiber (denoted CF) like carbon cloth, carbon paper and carbon felt are different to a woven structure. CF has excellent properties like lightweight, high electrical conductivity and flexible with a three-dimensional shape (Fig. S1). These properties make it as an ideal matrix for alkali metal deposition and

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help to lower the local current density. Unfortunately, pristine carbon cloth (denoted PCC) can not wet the liquid Na-K alloy due to the poor wettability (Movie S1). We

ro

simply heated PCC in air atmosphere before acid treatment, and then directly dripped

-p

the Na-K alloy onto PCC. The liquid Na-K alloy wet it well fast less than one second

re

in a pure argon fill glove box (Movie S2). The CC@NaK composites used as an

lP

electrode enable a dendrite-free anode liquid/liquid interface for the incompatibility of

na

the Na-K alloy and electrolyte. Even after several times folds, the CC@NaK composites still stick firmly to the surface of TCC, retaining the primary flexible

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structural properties (Movie S3). When used the CC@NaK and sodium foil to assemble symmetric cell, respectively, the CC@NaK symmetric cell exhibits more stable voltage comparing to Na symmetric cell. Morphology of PCC in Fig. 2a exhibits an orderly interwoven mesh pattern, which renders a 3D conductive networks and benefits to the fast electron transport in CC. The surface of PCC is smooth except little grains formed during its production process. (Fig. 2b) After treatment the surface of TCC emerges abundant nanoscale cracks (Fig. 2c) with the diameter ranging from 50 to 200 nm. TCC adds O element, which contains 12.7 at% O. Interestingly, the contents of O on smooth surface

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(18.73 %) are more than that in the nanoscale cracks (9.02%), because parts of oxygenic groups on surface were disappeared for the formation of nanoscale cracks (Fig. S2). The result of XPS spectrum also confirms the successful doping of oxygen group (Fig. 2d). There is only one apparent peak in PCC, which belongs to C element, while TCC adds a peak of O. After peak separation the added peak shows up three

of

smaller peaks around 531.0 eV (COOH), 532.3 eV(C=O), and 533.4 eV (C-OH), respectively [32,33] (Fig. S3a). Oxygenic groups is also supported by the FTIR

ro

spectrum, where the higher characteristic absorptions of carbonyl (C=O) and hydroxyl

-p

(RC-OH) are clearly observed (Fig. S3b). Oxygenic groups preferentially react with

re

potassium to form potassium oxide, which facilitating to form a stable interface by The

lP

improving the adhesion and wettability between matrix and alloy [30].

na

corresponding Brunauer-Emmett-Teller (BET) data of PCC and TCC are illustrated in Fig. 2e, f, respectively. The specific surface area of PCC is 0.7061 m2 g-1. However,

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after treatment, it increases to 3.2784 m2 g-1 in TCC, which attributing to the rough surface with the abundant micropores (<2 nm) and mesopores (2-50 nm). These increased nanoscale cracks and micropores facilitate the infusion of liquid Na-K alloy for capillary action, just like rGo helps to wet the molten Li [33] and nanocrevass-rich carbon enhanced the affinity for the molten metals [22]. Raman spectrum is also measured to evaluate surface features of the samples. As shown in Fig. 3a, the two pronounced peaks around 1353 and 1594 cm−1 are indexed to the D and G bands, respectively. Typically, the intensity ratio of D and G bands (ID/IG) reflects the disordered degrees. After treatment the value of the intensity of D and G bands (ID/IG)

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is 1.14, which is higher than PCC (1.07), suggesting more rough and disordered surface after treatment. The results are also consistent with the SEM images in Fig.2c, which demonstrates large number of surface defects. The typical X-ray diffraction profiles of PCC and TCC are shown in Fig. 3b. It directly suggests two reflection peaks around 26° and 43°, which refers to (002) and (001) planes, respectively. And

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X-ray diffraction profiles of materials are almost identical, which indicates the basic carbon structure of CC after treatment has not changed. In conclusion, the engineered

ro

surface defects of oxygenic groups and nanoscale cracks make TCC possible as a

-p

stable host for room-temperature liquid metal anode. The contact angle of liquid Na-K

the

excellent

wettability

between

TCC

and

liquid

Na-K

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na

lP

indicates

re

alloy on PCC is about 146° (Fig. 3c). But it is only about 0° on TCC (Fig. 3d), which

Fig. 2. Materials characterization of CC. SEM images of (a and b) PCC; (C) TCC; (d) XPS spectra of CC; N2 adsorption data with BET surfaces areas of (e) PCC and (f) TCC; the insert image is pore distribution. Compared to (e) numerous micropores (2
re

-p

ro

of

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lP

Fig. 3. Structure characterization of (a) Raman spectra; (b) XRD pattern; screen short of wetting

na

with liquid Na-K alloy onto (c) PCC in Movie S1 and (d) TCC in Movie S2.

alloy. Moreover, our methods are also feasible in other polyacrylonitrile (PAN)-based

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carbon materials like carbon paper and carbon felt that can realize the fast wetting by the liquid Na-K alloy (Fig. S4).

In order to investigate the electrochemical behavior of the CC@NaK electrode, we assembled the CC@NaK/ CC@NaK symmetric cell. Na/Na symmetric cell was also prepared for comparison. Both of them were cycled at 2 mA cm −2 for 2 h half-cycles. As shown in Fig. 4a, the CC@NaK electrode renders partial voltage fluctuation in the early cycles, which accounts for the interface establishment. As the cell continues to circulate, it shows gentle voltage fluctuation. In addition, the CC@NaK/ CC@NaK symmetric cell still maintains stable even cycling for 1800 h,

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suggesting a long cycle life. While, the Na/Na symmetric cell exhibits an irregular voltage fluctuation for the continuous dendrite formation and breaking. [32- 35] After cycling for 441 h, the cell stops as a result of exceeding the safety voltage (from -5 to 5V). The rate performance of the two symmetric cells at various current densities are shown in Fig. 4b. The voltage polarization of the CC@NaK electrode is about 5, 20,

of

60 mV at current densities of 0.1, 0.4 and 1.6 mA cm-2, respectively, which is more stable than Na electrode. This is due to the self-healing oxide layer of liquid Na-K

ro

alloy, which is forming by the trace oxygen and Na-K alloy in glovebox, can act as a

-p

protective layer to prevent the Na–K alloy from contacting with the electrolyte, and

re

also efficiently enhancing the wettability between Na-K alloy and hosts to form a

lP

stable interface.[30] When the current density turns back from 1.6 to 0.1 mA cm-2, the

na

voltage polarization of the CC@NaK electrode almost regains to its original value, illustrating the excellent reliability and stability for its outstanding dendrite-free

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properties. Fig. S5a and Fig. S5b represent the interfacial resistance of Na cell and the CC@NaK cell, respectively. Owing to the fast ion transfer the CC@NaK liquid/ liquid interface, the CC@NaK cell renders lower resistance at 1 and 10 cycle than Na cell.

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Fig.4. (a) Comparison of the cycling stability of Na and CC@NaK electrodes in symmetrical cells at a current density of 2 mA cm−2 with a capacity limitation of 2 mAh cm−2; (c) Rate performance of bare Na and CF@NaK symmetric cells at current density from 0.1 to 1.6 mA cm-2; EIS plots of (e) Na and (f) CC@NaK symmetric cells. To verify the potential practical application of the CC@NaK electrode in full cell, a CC@NaK anode or Na metal and Na3V2(PO4)3 cathode (noted as NVP@rGo) are assembled. XRD pattern of NVP@rGo is shown in Fig. S6. The theoretical capacity of NVP@rGo is 117 mAh g-1. The C contents of 11.9 wt% in NVP@rGo are

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measured by the thermogravimetric (TG) (Fig. S7). Xue ever reported that Na3V2(PO4)3 did not allow K+ insertion for the small interstitial bottleneck of atomic separation, the liquid Na−K anode acted like a simple sodium metal anode. [36] We measure the EDS patterns of the NVP@rGo cathode before and after the cycling with liquid Na−K anode and a NaClO4-PC-5 %FEC electrolyte at 1C for 100 cycles. (Fig.

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S8) It suggests there exist no K element in NVP@rGo electrode after 100 cycles. And the corresponding dQ/dV curves of CC@NaK/NVP@rGo full cell in Fig. S8 also

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exhibits a typical dQ/dV curves of Na3V2(PO4)3. In a word, the CC@NaK /NVP@rGo

-p

acts as sodium metal cell rather than potassium metal cell actually. Fig. 5a displays

re

the voltage profiles of CC@NaK /NVP@rGO and Na/NVP@rGO cells at different

lP

current densities. The CC@NaK/NVP@rGo cell exhibits higher capacities than the

na

Na/NVP@rGO cell. The CC@NaK /NVP@rGo cell shows a discharge capacity about 117 mAh g-1 at 0.1 C, better than the Na/NVP@rGO cell (105 mA h g-1). Even at

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subsequent higher current densities, the CC@NaK/NVP@rGo cell still exhibits high capacity than the Na/NVP@rGO cell and low electrochemical polarization (Fig. 5b). The dQ/dV plot (Fig. S9) highlights the redox potential of 3.36/3.39 V in the CC@NaK/NVP@rGo electrode. The EIS plots of the samples at 1 and 10 cycle are displayed in Fig. S10a and Fig. S10b, which suggests lower resistance in the CC@NaK/NVP@rGo electrode. Impressively, the CC@NaK/NVP@rGo cell can supply a surprising initial capacity of 83.08 mA h g-1 at the ultrahigh rate of 10 C before initial 5 cycles at 0.2 C to activate the materials. Moreover, it retains a stable capacity of 72.76 mA h g-1 even after 1000 cycles (Fig. 5c), which indicates a high

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capacity retention of 87.58% for its brilliant cycling stability. In contrast, the Na/NVP@rGo cell only shows initial capacity of 67.0 mAh g-1 at the rate of 10 C (Fig. S11). In addition, to examine whether the liquid alloy causes a self-discharge by dissolving into the PC electrolyte, we assembled a CF@NaK/ NVP@rGO cell in NaClO4 in PC-5 % FEC and fully charged to test the open circuit voltage (OCV) (Fig.

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S12). The OCV at 3.36V was stable and no obvious voltage drop was observed even over 800 hours, which indicating there is no self-discharge in this cell. It illustrates the

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na

lP

re

-p

ro

CF@NaK/ NVP@rGO cell can run safely with a long lifespan.

Fig. 5. (a) Rate performance of the Na/NVP@rGo and CC@NaK/NVP@rGo; (b) Charge/discharge profies of CC@NaK/NVP@rGo at different C-rates; (c) Cycling capability of CF@NaK/NVP@rGo at a high rate of 10C. 4. Conclusions

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In summary, we have demonstrated a CC@NaK composites can be used as a dendrite free liquid metal anode. It was easily fabricated by directly dropping liquid Na-K alloy onto TCC, which can fast absorb the liquid Na-K alloy at room temperature for the enhancing wettability with added surface defects of oxygenic groups and nanoscale cracks. The sodiophilic oxygenic groups and the nanoscale

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cracks decorated on TCC make it excellent wettability with liquid Na-K alloy for affinity and capillary action. The CC@NaK electrode exhibits long cycle life and low

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voltage polarization. Owing to the 3D flexible carbon matrix and good wettability, the

-p

CC@NaK /NVP@rGo full cell can cycle at 10 C high rates with excellent capacity

re

retention. Futhermore, it is prospective that the flexible CC@NaK composites may

lP

have promising application as electrodes in flexible devices like soft pack battery.

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Appendix A. Supplementary data

References

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Supplementary data to this article can be found online at:

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Journal Pre-proof Declaration of Interest Statement

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Journal Pre-proof Author Contributions Section Zhian Zhang : Conceptualization, Methodology, Reviewing

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Yangyang Xie: Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing Junxian Hu: Software, Validation