Influence of KPF6 and KFSI on the Performance of Anode Materials for

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Influences of KPF6 and KFSI on the Performance of Anode Materials for Potassium Ion Batteries: A Case Study of MoS2 Leqing Deng, Yuchuan Zhang, Ruiting Wang, Meiying Feng, Xiaogang Niu, Lulu Tan, and Yujie Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06156 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 1, 2019

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ACS Applied Materials & Interfaces

Influences of KPF6 and KFSI on the Performance of Anode Materials for Potassium Ion Batteries: A Case Study of MoS2 Leqing Deng†‡, Yuchuan Zhang†, Ruiting Wang†, Meiying Feng†, Xiaogang Niu†, Lulu Tan†, Yujie Zhu*†§

†School

of Chemistry, Beihang University, Beijing 100191, P. R. China

‡School

of Physics, Beihang University, Beijing 100191, P. R. China

§Beijing

Advanced Innovation Center for Biomedical Engineering, Beihang University,

Beijing 100191, P. R. China

KEYWORDS: potassium ion batteries, MoS2 anodes, electrolyte’s salts, solid electrolyte interphase, electrochemical performance

ABSTRACT

ACS Paragon Plus Environment

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Recently, nonaqueous potassium ion batteries (KIBs) are attracting an increasing interest due to the abundance of potassium resources, but the systematic study about the effects of electrolyte’s salt on the electrochemical performance of electrode materials is still insufficient. Here, it is shown that the capacity retention and Coulombic efficiency of commercial micrometric MoS2 can be remarkably improved by simply using potassium bis(fluorosulfonyl)imide (KFSI) over potassium hexafluorophosphate (KPF6) dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) as the electrolyte. By constructing various cell configurations, it is discovered that the degradation of MoS2||K half-cells in KPF6-containing electrolyte originates from the failure of the MoS2 electrode. The solid electrolyte interphase (SEI) layer formed on MoS2 during cycling was systematically investigated by using a series of characterizations. It is found that a stable, protective, and KF-rich SEI layer is formed on MoS2 in KFSI-containing electrolyte, while an unstable, KF-deficient, and organic species-rich SEI layer is formed in KPF6-containing electrolyte. Finally, the origins of such differences are discussed, which will provide new insights on further exploration of novel electrolyte for KIBs.


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INTRODUCTION

Due to the abundance and low-cost of potassium resources, potassium ion batteries (KIBs) are attracting an increasing research interest.1–3 Besides, the lower redox potential of K+/K compared with Na+/Na and the compatibility with graphite anode in carbonate electrolytes make KIBs possess additional merits over sodium ion batteries (NIBs)4, which are being considered as a possible cheap alternative to lithium ion batteries (LIBs).Currently, research on KIBs mainly focuses on the exploration of new electrode materials and design of existing materials’ structure/composition to enhance their electrochemical performance.5–7 Up to now, limited studies have been conducted to investigate the effect of the electrolyte on the electrochemical performance of KIBs.8– 11

In the literature, the most common electrolytes for KIBs are potassium

hexafluorophosphate (KPF6) or potassium bis(fluorosulfonyl)imide (KFSI), dissolved in organic solvents, such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethoxyethane (DME), or their mixture. Recently, different electrochemical performances have been frequently observed for anode materials


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tested between KPF6- and KFSI-containing electrolytes.9,12–14 For example, Guo’s group reported that by simply replacing the electrolyte salt KPF6 with KFSI, the cycling performance of anode materials Bi and Sn3P4 can be significantly improved in carbonate-based electrolyte.13,15 Madec et al.15 also observed much more enhanced cycling stability and Coulombic efficiency for Sb tested in KFSI dissolved in EC/DEC compared with those tested in KPF6 dissolved in the same solvent mixture. These results clearly indicate the importance of the electrolyte’s salt on the electrochemical performance of anode materials of KIBs, but the underlying mechanism seems still elusive.

In this study, a typical layered transition metal dichalcogenide MoS2, which has been investigated as the anode material for KIBs16–18, is taken as an example to elucidate the difference between KPF6 and KFSI in determining the K-ion storage performance for the anode materials of KIBs. Although MoS2 might not be a good anode candidate for practical KIBs due to its high working voltage, it was chosen for scientific queries because, as shown in the following study, the K-ion storage performance of MoS2 varies


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significantly with the salt of choice in the electrolyte, which greatly arouses our research interest. Previous studies have shown that commercial MoS2 exhibits fast capacity decay when tested in KPF6 dissolved in carbonate solvents, and therefore rational structural designs were required to enhance the electrochemical performance of MoS2.16,17 Here, we show by choosing KFSI over KPF6 dissolved in EC/DEC, both the capacity retention and Coulombic efficiency of commercial micrometric MoS2 can be significantly improved, similar to the reported results for the alloy-based anode materials.9,12,13 Subsequently, by constructing cells with various configurations, it is confirmed that in the half-cells with excess K metal, the degradation of the K||MoS2 cells in KPF6 dissolved in EC/DEC is caused by the failure of the MoS2 electrode. Furthermore, by using a combination of X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric-differential scanning calorimetry (TG-DSC), and scanning electron microscopy (SEM), it is discovered that the solid electrolyte interphase (SEI) layer formed on MoS2, which plays a crucial role in determining

the

electrochemical

performance

of

MoS2,

possesses

different

compositions between KPF6- and KFSI-containing electrolytes. In KFSI dissolved in


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EC/DEC, a KF-rich and passivating SEI layer is formed on MoS2, while a KF-deficient and organic species-rich SEI layer is formed in KPF6 dissolved in EC/DEC. Such difference

may

result

from

the

higher

stability

of

PF6-

against

the

reduction/decomposition than that of FSI-, and unlike the well-known hydrolysis feature of LiPF6, KPF6 in fact does not react with water. Thus, the SEI layer formed in KPF6 dissolved in EC/DEC mainly consists of the reduction/decomposition products from the organic solvents, which are unable to effectively passivate the active material from reacting with the electrolyte, leading to continuous electrolyte decomposition and capacity decay of MoS2. Finally, it is shown that KFSI can be used as an effective additive for KPF6 in EC/EDC electrolyte to form the passivating SEI layer and stabilize the electrochemical performance of anode materials, such as MoS2, which undergo large volume fluctuations during potassiation-depotassiation.

EXPERIMENTAL SECTION

Preparations of MoS2 electrode: Commercial micrometric MoS2 particles (Sigma Aldrich, 2 μm) were adopted as active materials. MoS2, acetylene black, and carboxymethyl


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cellulose sodium (Na-CMC) binder at a mass ratio of 70:20:10 were thoroughly mixed together with distilled water as the solvent. Then, the as-prepared slurry was coated on Cu foil to prepare the electrodes. Before assembled in cells, the electrodes were vacuum dried at 70 °C overnight. The loading mass of the MoS2 on the electrode was ~0.8 mg cm−2.

Electrochemical characterizations: Electrolytes with compositions of 0.8 M KPF6, 0.8 M KFSI, 0.7 M KPF6+ 0.1 M KFSI, 0.6 M KPF6+ 0.2 M KFSI, 0.5 M KPF6+ 0.3 M KFSI, and 0.4 M KPF6+ 0.4 M KFSI dissolvedin ethylene carbonate (EC)/diethyl carbonate (DEC) with a volume ratio of 1:1 were prepared for coin cells assembly. All coin cells were assembled in an Ar-filled glovebox with both oxygen and moisture concentrations below 0.01 ppm. Potassium metal foil was used as the counter electrode and a glass microfiber filter (Whatman, Grade GF/D) was used as the separator. Galvanostatical discharge-charge measurements were conducted on a Land battery testing system (Wuhan LAND electronics, China) in the potential range of 0.05-3.0 V (vs. K+/K). Cyclic voltammetry (CV) tests were performed on CHI660E electrochemical workstation


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(Shanghai Chenhua Co. Ltd., China) with a voltage range of 0.05-3 V (vs. K+/K) at scan rates of 0.1 mV s-1. The chronoamperometry tests at 0.05 V (vs. K+/K) were performed on Solatron 1400 Celltest System. Electrochemical impedance spectroscopy (EIS) tests with frequency ranging from 106 Hz to 10-2 Hz at a 10 mV amplitude were performed using Solatron 1400 Celltest System. Unless otherwise specified, all the specific capacity and current density in this work were calculated based on the mass of MoS2 on the tested electrodes.

Characterizations: Scanning electron microscope (SEM) images were obtained by a field-emission scanning electron microscope (JEOL, JSM-7500F, 5 kV). The XRD patterns were collected using a Rigaku Dmax 2200 X-ray diffractometer with Cu K α radiation ( λ = 1.5416 Å). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo escalab 250Xi with aluminum Kɑ X-ray radiation. FT-IR spectra were recorded using Nicolet iN10 FT-IR Microscope with the reflection model. TG-DSC in

a

nitrogen

(N2)

atmosphere

was

conducted

on

a

Pyris

thermogravimetric/differential thermal analysis (Perkin Elemer Inc., USA).

Diamond 19F

NMR


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spectra were recorded on Bruker Avance III 600M instrument at 564 MHz. The electrodes which were charged to 3 V (vs. K+/K) were sputtered by Ar+ ion beam for 30 and 60 seconds and then measured for XPS test. The retrieved electrodes for the XPS and SEM were obtained by disassembling the cells in the glove box, where they were washed with dry diethyl carbonate.

RESULTS AND DISCUSSION

Figure 1. Cycling performance of the MoS2 electrode tested at a current density of 0.1 A g-1 in the (a) KPF6 (0.8) and (b) KFSI (0.8).

The electrochemical performance of the MoS2||K metal half-cells was evaluated using galvanostatic charge-discharge test in 0.8 M KPF6 dissolved in EC/DEC electrolyte (denoted as KPF6 (0.8) and 0.8 M KFSI dissolved in EC/DEC electrolyte (denoted as


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KFSI (0.8)), respectively. Because of the severe safety issues, such as low melting point and dendrite growth, K metal is not considered as a practical anode candidate for KIBs. Therefore, in this study, we focus our interest in the MoS2 side, and an excess amount of K metal was used as the counter and reference electrode during assembly of the half-cells to minimize the influence of the K metal on the results. Figure 1a and b show the cycling performance of commercial micrometric MoS2, whose XRD pattern and SEM images are presented in Figure S1, at 0.1 A g-1 in KPF6 (0.8) and KFSI (0.8), respectively. As shown in Figure 1, the electrochemical performance of MoS2 is distinctly dependent on the electrolyte’s salt of choice. In the first cycle, MoS2 shows a discharge (potassiation) capacity of 455.7 mAh g-1 in KPF6 (0.8) and 350.7 mAh g-1 in KFSI (0.8), respectively, with comparable initial Coulombic efficiency (CE) (65.3% in KPF6 (0.8) vs.63.7% in KFSI (0.8)). The huge irreversible capacity loss mainly results from the formation of the SEI layer on the surface of the electrode.17 In KPF6 (0.8), the MoS2 electrode presents stable discharge and charge capacities around 293.0 and 267.8 mAh g-1 during the initial 15 cycles. After that, the discharge and charge capacities fade rapidly to less than 102 mAh g-1 at the 60th cycle (Figure 1a). After the


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1st cycle, the CE in KPF6 (0.8) varies in the range between 93% and 76%, suggesting that the SEI layer formed in KPF6 (0.8) cannot effectively passivate the active materials and the irreversible side reactions continuously occur during cycling. In contrast, the half-cell with KFSI (0.8) exhibits much better cycling stability and higher CE (Figure 1b). The MoS2 delivers a discharge capacity of 227.3 mAh g-1 (at the 2nd cycle) and retains a value of 151.5 mAh g-1 even after 200 cycles, while the CE increases from 89.9% at the 2nd cycle to 99.2% at the 13th cycle and preserves this value during the following cycles. The much higher CE in KFSI (0.8) indicates that a passivating and stable SEI layer is formed and the side reactions can be mitigated in KFSI (0.8). It is noteworthy that although the cycling stability and CE of MoS2 are much poorer in KPF6 (0.8) than those in KFSI (0.8), the K-ion storage capacity of MoS2 in KPF6 (0.8) is higher than that in KFSI (0.8) in the initial several cycles. This is attributed to the lower interfacial resistance between the MoS2 electrode and the KPF6 (0.8), which will be discussed in the following part. Except for the 1st cycle, the resemblance of the galvanostatic chargedischarge (Figure S2) and cyclic voltammetry (CV) curves (Figure S3) of MoS2 tested in KPF6 (0.8) and KFSI (0.8), with only slightly different polarization, suggests that the


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same reaction processes take place in MoS2 when it is charged-discharged in KPF6 (0.8) and KFSI (0.8). Therefore, the observed different cycling stability and CE should be mainly ascribed to the distinctive properties between KPF6 (0.8) and KFSI (0.8). To understand the effect of KPF6 (0.8) and KFSI (0.8) on the electrochemical performance of MoS2, a series of experiments were conducted.

The electrochemical stability of KPF6 (0.8) and KFSI (0.8) in the range of 0.05-3 V vs. (K+/K) was firstly assessed by CV measurements using Cu foil||K half-cells (Figure S4). From the 1st cathodic scan, it can be observed that the onset reduction potential of KFSI (0.8) is around 2.0 V (vs. K+/K), which is higher than that (1.3 V vs. K+/K) of KPF6 (0.8). During the initial three scans in KPF6 (0.8), a broad reduction peak centered around 0.3 V vs. (K+/K) can be seen, which gradually decreases in intensity, resulting from the slow passivation process by the decomposition of the electrolyte to form the SEI layer.19 While in KFSI (0.8), during the first cathodic scan, two broad current peaks around 1.6 and 0.6 V vs. (K+/K) were observed. Owing to the higher thermodynamic activity and easier decomposition of KFSI than EC and DEC, the peak around 1.6 V vs. (K+/K) is


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assigned to the decomposition of KFSI and the peak around 0.6 V vs. (K+/K) is attributed to the reduction of the solvents (EC and DEC).20 As shown in Figure S4, the reduction current in KFSI (0.8) is only half of that in KPF6 (0.8), manifesting the higher electrochemical stability and a more passivating SEI layer formed in KFSI (0.8).9,21,22 In addition, the electrochemical stability of the electrolytes was further evaluated by chronoamperometry method. A constant potential (0.05 V vs. K+/K) was applied to the Cu foil||K half-cells in KPF6 (0.8) and KFSI (0.8), and the resulting current was shown in Figure S5. In KFSI (0.8), the current is negligible after 8 hours, implying that a stable and passivating SEI layer is formed on the surface of the Cu foil. Whereas, in KPF6 (0.8), even after 120 hours, the current is still remarkable, suggesting that the electrolyte decomposition is still taking place and the reduction products of KPF6 (0.8) cannot effectively passivate the surface of the Cu foil. As shown by Madec et al,15 the highly reactive K metal can greatly disturb the measured electrochemical performance in halfcells. To investigate the individual effects of the MoS2 electrode, K metal electrode, and electrolyte on the electrochemical performance of MoS2||K half-cells, the following measurements were conducted, which are illustrated in Figure 2. The MoS2||K half-cells


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assembled with KPF6 (0.8) or KFSI (0.8) were initially discharged-charged for several cycles, and then the cells were disassembled. The cycled MoS2 electrode or K metal was retrieved and re-assembled with fresh electrolyte and K metal or MoS2 electrode into cells by following the procedure shown in Figure 2.

Figure 2. The configuration of the re-assembled half-cells with various combinations.


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Figure 3. Cycling performance of (a) Cell-1a, (b) Cell-2a, (c) Cell-2b and (d) Cell-2c at a current density of 0.1 A g-1.

Figure 3 shows the cycling performance of the half-cells with various configurations (see Figure 2). As shown in Figure 3a, the MoS2|KFSI (0.8)|K half-cell (denoted as Cell1) was first discharged-charged for 15 cycles, which shows similar cycling performance and CE as the result in Figure 1b. Then, Cell-1 is carefully disassembled at 3.0 V and KFSI (0.8) is replaced with KPF6 (0.8) to fabricate the MoS2KFSI-15|KPF6 (0.8)|KKFSI-15 half-cell (denoted as Cell-1a). Interestingly, a gradual increase of both the discharge


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and charge capacities was observed for Cell-1a from the 16th cycle to the 45th cycle, as shown in Figure 3a. Meanwhile, the CE of Cell-1a maintains around 99% from the 17th to 35th cycle, which is comparable with that of MoS2|KFSI (0.8)|K half-cell. From the 40th to 55th cycle, the charge capacity maintains a constant value around 280 mAh g-1 while the discharge capacity continues increasing, resulting into a decrease of the CE. Starting from the 56th cycle, both the charge and discharge capacities decay quickly (Figure S6).

After Cell-1a is charged-discharged for 55 cycles in KPF6 (0.8), both its charge and discharge capacities decrease to around 100 mAh g-1. Then, Cell-1a is disassembled and paired with a used K metal electrode (denoted as KKFSI-15), which was cycled in MoS2|KFSI (0.8)|K for 15 times, to construct the MoS2KFSI-15-KPF-55|KPF6(0.8)|KKFSI-15 cell (denoted as Cell-1b). As shown in Figure S7, although the KKFSI-15 electrode was used at the 71th cycle, Cell-1b still exhibits poor performance, indicating that the failure of Cell-1b is mainly caused by the breakdown of the MoS2 electrode.


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To unveil the underlying reasons for the capacity increase after the 16th cycle in Figure 3a, electrochemical impedance spectroscopy (EIS) tests were conducted. After replacing the KFSI (0.8) with KPF6 (0.8), the electrolyte resistance (corresponding to the intercept of the Nyquist plots with the real axis) maintains almost unchanged, but the interfacial resistance between the electrode and the electrolyte decreases dramatically (~80%) from the 15th to 20th then to 40th cycle, as evidenced by the reduced diameter of the high frequency semicircle in the Nyquist plots (Figure S8). The reduced interfacial resistance is an indicator of the alteration of the SEI layer, which accounts for the increased charge and discharge capacities after the electrolyte is switched from KFSI (0.8) to KPF6 (0.8).

In KPF6 (0.8), the MoS2|KPF6 (0.8)|K half-cell (denoted as Cell-2) exhibits capacity degradation and low CE as shown by the first 15 cycles in Figure 3b (and also Figure 1a). While replacing the KPF6 (0.8) with KFSI (0.8) to construct the MoS2KPF-15|KFSI (0.8)|KKPF-15 half-cell (denoted as Cell-2a), it exhibits stable cycling performance (Figure 3b). The initial CE of Cell-2a is ~80% at the 16th cycle in Figure 3b, increases to 98.4%


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at the 20th cycle and preserves this value in the following cycles, indicating that a relatively stable and passivating SEI layer is formed in KFSI (0.8). Similar to Cell-1a, Cell-2a also delivers lower capacities in KFSI (0.8) than in KPF6 (0.8), which is ascribed to the increased interfacial resistance as shown in Figure 3b.

To evaluate the impact of the K metal on the performance of Cell-2, Cell-2 was disassembled after the 60th cycle and the cycled electrodes were reassembled with fresh MoS2 or K metal electrode and KPF6 (0.8) electrolyte to construct MoS2KPF-60|KPF6 (0.8)|K half-cell (Cell-2b) and MoS2|KPF6 (0.8)|KKPF-60 (Cell-2c) half-cell. Although a fresh K metal was used in Cell-2b, the MoS2KPF-60 in KPF6 (0.8) still exhibits poor electrochemical performance, indicating that the MoS2KPF-60 was out of work (Figure 3c). While for the cycled K metal (KKPF-60) paired with a fresh MoS2 electrode in KPF6 (0.8), Cell-2c shows similar electrochemical performance to Cell-2, indicating that the cycled KKPF-60 in KPF6 (0.8) is still electrochemically active (Figure 3d).

Based on the above results, it can be concluded that in present study, the reduced capacity of the half-cells in KPF6 (0.8) mainly originates from the failure of the MoS2


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electrodes. To further confirm this conclusion, symmetrical cells with the configuration of MoS2KFSI-15|KFSI (0.8)|MoS2KFSI-15 (Cell-1sym, Figure S9a) and MoS2KPF-15|KPF6 (0.8)|MoS2KPF-15 (Cell-2sym, Figure S9b) were fabricated, in which MoS2KFSI-15 and MoS2KPF-15 are retrieved MoS2 electrodes from the half-cells discharged-charged in KFSI (0.8) and KPF6 (0.8) for 15 cycles, respectively. Nyquist plot (Figure S9a) shows that the interfacial resistance of the Cell-1sym tested in KFSI (0.8) slightly increases (~20%) from the 1st to 11th cycle and maintains unchanged from the 11th to 21th cycle, indicating that a stable and robust SEI layer is indeed formed at the surface of the MoS2 electrodes. Whereas, in KPF6 (0.8), Cell-2sym exhibits sustained and huge resistance growth (~300%) from the 1st to 21th cycle due to the excessive growth of the SEI layer on the MoS2 electrodes. It is worth mentioning that although the interfacial resistance of Cell-2sym tested in KPF6 (0.8) keeps increasing during cycling, it is still smaller than that of Cell-1sym tested in KFSI (0.8), indicating that the SEI layers formed in these two types of electrolyte possess different chemical and/or physical properties. It is well known that the SEI layer plays an important role in determining the electrochemical


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performance of the electrodes.23 Therefore, the SEI layer formed on the MoS2 electrodes in KPF6 (0.8) and KFSI (0.8) is characterized in the following part.

Figure 4. XPS spectra of the fully charged (depotassiated) MoS2 electrode after cycled for 15 times in KPF6 (0.8) and KFSI (0.8) electrolytes. (a) C 1s and (b) F 1s XPS spectra of the MoS2 electrode cycled in KPF6 (0.8). (c) C 1s and (d) F 1s XPS spectra of the MoS2 electrode cycled in KFSI (0.8).


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XPS tests were employed to investigate the composition of the SEI layer formed on the MoS2 electrodes. Figure 4 shows the XPS C 1s and F 1s spectra of the MoS2 electrodes after discharged-charged for 15 cycles with KPF6 (0.8) and KFSI (0.8), respectively. As shown in Figure 4a and c, the C 1s spectra exhibit similar features for the MoS2 electrodes cycled in KPF6 (0.8) and KFSI (0.8). Generally, the C 1s spectra can be deconvoluted into four contributions with binding energies around 284.6, 286.2, 287.9 and 289.2 eV, which are assigned to C-C/C=C, R-O-K, RCOOK and CO32species in the surface layer.12 The C 1s spectra are complexed by the binder and conductive carbon additive, whose C 1s signal significantly overlaps with that of SEI components originating from the decomposition of the EC and DEC solvents.

Unlike the C 1s spectra, the F 1s spectra (Figure 4b and d) of the cycled MoS2 electrodes show significant difference between KPF6 (0.8) and KFSI (0.8). Figure 4b shows the F 1s XPS spectrum of the MoS2 electrode cycled in KPF6 (0.8). As shown in Figure 4b, the main peak at 687.2 eV is attributed to the residual KPF6 on the surface of the electrode.13 The almost negligible peak at 683.6 eV may be attributed to KF. Figure


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4d shows the F 1s XPS spectrum of the MoS2 electrode cycled in KFSI (0.8). Strong KF signal with a binding energy at 684.4 eV is observed in Figure 4d, and the peak at 689.3 eV is due to the residual KFSI on the surface of the electrode.8,13 By comparing the peak intensity of the F 1s XPS spectra between Figure 4b and d, it is clear that the SEI formed in KFSI (0.8) comprises a much higher content of KF than that in KPF6 (0.8). To confirm this point, the Fourier transform infrared spectroscopy (FT-IR) (Figure S10) was conducted. The FT-IR result in Figure S10 shows a strong absorption peak at 1589 cm-1, corresponding to KF, exclusively for the MoS2 electrode cycled in KFSI (0.8), which agrees well with the XPS results.24

The depth profiles of C 1s and F 1s XPS spectra of the MoS2 electrodes after cycled for 15 times in KPF6 (0.8) and KFSI (0.8), respectively, are shown in Figure S11. After the cycled MoS2 electrodes were etched by Ar+ for 30s and 60s, the intensity of the organic species (with binding energies around 284-287 eV) in the C 1s spectrum decreases dramatically for the MoS2 electrode cycled in KPF6 (0.8) (Figure S11a) while it only varies slightly for the one cycled in KFSI (0.8) (Figure S11c), indicating that the


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SEI layer formed in KPF6 (0.8) consists of more vulnerable organic species than that in KFSI (0.8). For the F 1s spectrum (Figure S11b) of the MoS2 cycled in KPF6 (0.8), the signal of the residual KPF6 disappears and a new peak with the binding energy at 683 eV appears after etching, which may originate from the decomposition of KPF6 salt during the etching (Figure S12). Regarding the F 1s spectrum of the MoS2 electrode cycled in KFSI (0.8), the intensity of the KF maintains almost unchanged after etching, bearing out that the SEI formed in KFSI is rich of KF.

TG-DSC tests were conducted in a nitrogen atmosphere to further measure the thermal stability of the SEI formed in KPF6 (0.8) and KFSI (0.8) (Figure S13). The MoS2 electrode cycled in KPF6 (0.8) starts to lose weight around 50 oC and presents a small exothermic peak at 99 oC, which is attributed to the thermal decomposition of the organic species in the SEI layer. On the contrary, the MoS2 electrode cycled in KFSI (0.8) starts to decompose around 117 oC without any distinct exothermic peak. The TGDSC results demonstrate that the SEI formed in KPF6 (0.8) contains a higher content of


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thermally vulnerable organic species than that formed in KFSI, consistent with the XPS results.

In current study, KF can only root in the reduction and/or decomposition of the electrolyte salt KPF6 or KFSI. In LIBs, it is well known that the lithium hexafluorophosphate (LiPF6) reacts easily with trace amount of water to form LiF, POF3 and HF.25 In previous study, the poor electrochemical performance of the anode materials tested in KPF6-based electrolyte was attributed to the detrimental Lewis acids (such as HF and PF5), which are thought to be generated from the hydrolysis or direct decomposition of KPF6 and then react with electrolyte solvents and/or active materials, leading to the capacity decay. However, in contrast with LiPF6, KPF6 does not react with water, as evidenced by the NMR result in Figure S14. In fact, KPF6 has been used as the salt in aqueous electrolyte and the hydrolysis of KPF6 seems to only take place at high temperature, as reported in the literature.26,27 Besides, it was reported that the reduction of the PF6- is a kinetically limited process due to the steric hindrance around P and the high electron density around F, both of which make the addition of electrons to


24

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P very difficult.28 The absence of hydrolysis and restricted reduction of KPF6 cause that the SEI layer formed in KPF6 (0.8) mainly consists of the reduction products of the EC and DEC solvents, which include organic species (C-C/C=C, R-O-K, RCOOK) and inorganic species (K2CO3) (Figure 4a). The organic species and K2CO3 cannot effectively passivate the electrode, which is corroborated by the exposed MoS2 particles in the SEM images of the after-cycled electrodes (Figure S15 and S16).

As for KFSI, it is easier to decompose than EC and DEC solvents due to the labile F-S bonds.20,29 The decomposition/reduction of KFSI leads to the formation of electronically resistive KF.8 The KF-rich SEI layer formed in KFSI can effectively passivate the electrode to prevent the electrolyte decomposition after the initial few cycles. The SEM images of the after-cycled MoS2 electrode in KFSI (0.8) show that the surface of the electrode is fully covered by a dense SEI layer and no visible MoS2 particles can be found (Figure S17). The high chemical stability, low solubility, strong shear modulus, and electronic insulation nature of KF play a critical role in stabilizing the SEI layer, similar to LiF found in the SEI formed in LIBs.12,28


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Figure 5. XPS spectra of the fully charged (depotassiated) MoS2 electrode obtained from Cell-1a. The C 1s (a, c, e) and F 1s (b, d, f) XPS spectra of the fully charged MoS2 electrode at the (a, b) 16th, (c, d) 20th and (e, f) 40th cycle.

Figure 5 shows the evolution of the XPS spectra of the MoS2 electrode in Cell-1a at the 16th, 20th and 40th cycle in Figure 3a. After the MoS2 electrode was cycled for 15 times in KFSI (0.8), the KF-rich SEI layer is formed on the MoS2 electrode (Figure 4d). After the KFSI (0.8) was replaced with KPF6 (0.8), the C 1s XPS spectra (Figure 5a, 5c and 5d) shows almost negligible changes during cycling while a distinct change can be seen in the F 1s XPS spectra (Figure 5b, 5d and 5e). As shown in Figure 5b, at the 16th


26

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cycle, the F 1s spectrum can be fitted by a predominant peak with the binding energy of 683.9 eV, which is assigned to KF, and the other two peaks with binding energies located at 688.7 and 687.2 eV, which are ascribed to the residual salts KFSI and KPF6, respectively. After the MoS2 electrode was discharged-charged in KPF6 (0.8) for 5 cycles (i.e., at the 20th cycle in Figure 3a), the relative intensity of the KF in the F 1s spectrum decreases and the KPF6 signal is predominant (Figure 5d). At the 40th cycle in Figure 3a, where the MoS2 electrode has been discharged-charged in KPF6 (0.8) for 25 cycles, the fitting result of the F 1s spectrum indicates that the surface of the MoS2 electrode contains a negligible amount of KF. The gradual loss of the electronically insulated KF reduces the interfacial resistance between MoS2 and electrolyte, accounting for the observed capacity increase in Figure 3a from the 16th cycle to 55th cycle.

It is interesting to find that the stable and robust SEI layer formed on MoS2 in KFSI (0.8) can be maintained in KPF6 (0.8), at least for several cycles, as shown in Figure 3a. However, the repeated volume fluctuations during charge-discharge will cause cracks of


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the SEI layer and the re-exposure of the electrode materials to the electrolyte. In KFSI (0.8), the cracks can be repaired by the preferential decomposition of KFSI salt, as suggested by the highest CE in Figure 1b, which is 99.2% and indicates that a slight decomposition of the electrolyte continues occurring during cycling. Unfortunately, in KPF6 (0.8), the reduction products of the electrolyte cannot generate an effective passivation layer to repair these cracks. Thus, more and more cracks are formed during cycling, which gradually damage the SEI layer, leading to the decrease of the interfacial resistance and the increased charge and discharge capacities as observed in Figure 3a. At some point, the complete breakdown of the SEI layer takes place, resulting into massive electrolyte decomposition and fast capacity degradation. As shown by the SEM images (Figure S18) of the MoS2 electrode at the 60th cycle in Figure 3a, discernible MoS2 particles can be clearly observed, manifesting the deformation of the SEI layer. For the MoS2 electrode initially tested in KPF6 (0.8), a stable and protective SEI layer can also be formed in KFSI (0.8), as shown by Cell-2a in Figure 3b. The XPS C 1s and F 1s spectra of the MoS2 electrode (at the 40th cycle) in Cell-2a (Figure S19) show similar patterns with those in Figure 4c and 4d, and the F 1s XPS spectrum (Figure


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S19b) also suggests the existence of KF, further confirming the decomposition of the KFSI.

By taking commercial micrometric MoS2 as an example, we have shown both the ability of KFSI on forming a passivating SEI layer and the role of KFSI on maintaining the stability of the SEI layer. To verify the influence of the KFSI salt, four types of electrolytes containing KPF6-KFSI binary salts dissolved in EC/DEC with different molar ratios (KPF6(0.7)-KFSI(0.1), KPF6(0.6)-KFSI(0.2), KPF6(0.5)-KFSI(0.3), KPF6(0.4)KFSI(0.4)) between KPF6 and KFSI were prepared (Table S1). These electrolytes exhibit similar electrochemical stability with KFSI (0.8) (Figure S4), as shown in Figure S20. Commercial MoS2 displays similar cycling performance in these four electrolytes with the CE up to 99% (Figure S21). The stability of the MoS2 electrode was highly enhanced in these electrolytes containing binary salts compared with that in KPF6 (0.8), even the concentration of KFSI was only 0.1 mol L-1. Therefore, it can be concluded that the decomposition products of KFSI play a critical role in generating and stabilizing the protective SEI layer in the KFSI-containing electrolytes.


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CONCLUSION

In this study, we have revealed that, compared with KPF6 salt dissolved in EC/DEC, KFSI salt can greatly improve the cycling stability and Coulombic efficiency of K-ion storage in commercial MoS2 anodes. By rational design of the cell configuration, we show that in present study, the electrochemical performance of MoS2||K metal half-cells is mainly decided by the MoS2 electrode, and the different electrochemical behaviors of MoS2 tested between KFSI- and KPF6-containing electrolytes are attributed to the particular properties of the SEI layers formed on the MoS2 electrode. As evidenced by XPS, FT-IR, TG-DSC and SEM measurements, the superior performance of MoS2 anodes in KFSI-containing electrolyte is ascribed to the stable, protective, and KF-rich SEI layer, which results from the decomposition/reduction of both KFSI and solvents, while the inferior performance of MoS2 anodes in KPF6 electrolyte is attributed to the unstable, KF-deficient, and organic species-rich SEI layer, which mainly consists of decomposition/reduction products of the organic solvents. To realize full cells for KIBs, the anodic (oxidation) stability of KFSI- and KPF6-containing electrolytes should be


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investigated and compared in the future. Although application of MoS2 in practical KIBs might be problematic due to its high working voltage and insufficient Coulombic efficiency, we hope that this work will emphasize the importance of the electrolyte’s salts on the performance of KIBs and encourage further exploration of new electrolytes for the emerging KIBs.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge. SEM images and XRD of MoS2, Galvanostatic charge/discharge voltage profiles and CV curves of the MoS2 electrode, CV curves of electrolytes, cycling performance, EIS spectra, FT-IR spectra, XPS data, TG-DSC curves, 19F NMR and supporting data in Figure S1S21 and Table S1 (PDF)

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected]; Tel: +86-13811820083


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors acknowledge the financial support from the “Top-notch Talents” program of Beihang University, the “Thousand Youth Talents Plan” of China, the National Basic Research Program of China (2014CB931802), the 111 Project (B14009) and the Project funded by China Postdoctoral Science Foundation.

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Table of Contents Graphic


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Influence of KPF6 and KFSI on the Performance of Anode Materials for

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