High-volumetric-capacity WSe2 Anode for Potassium-ion Batteries①

CHEN Ming ZHAO Jun-Mei SUN Chuan-Fu

a (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

b (CAS Key Laboratory of Design and Assembly of Functional Nanostructures,and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

c (Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing 100190, China)

d (University of Chinese Academy of Sciences, Beijing 100039, China)

ABSTRACT Exploring high-capacity electrode materials is critical for the development of K-ion batteries. In this work, we report a layered-structured tungsten selenide (WSe2) anode, which not only delivers an ultrahigh volumetric capacity of 1772.8 Ah/L (or 188.4 mAh/g) at a current density of 5 mA/g but also exhibits good rate capability (72 mAh/g at 200 mA/g) and cycling stability (83.14% capacity retention over 100 cycles at 100 mA/g). We have also revealed the underlying reaction mechanism through ex situ X-ray powder diffraction. Furthermore, proof-of-concept full-cell batteries comprising of WSe2 anodes and Prussian Blue cathodes are capable of delivering an energy density of 135.2 Wh/kgcathode+anode. This work highlights the potential of WSe2 as a promising high-volumetric-capacity anode material for rechargeable potassium-ion batteries.

Keywords: tungsten selenide, potassium-ion batteries, high volumetric capacity, conversion reaction;

1 INTRODUCTION

Lithium-ion batteries (LIBs) have been widely used in portable electronics and electric vehicles because of their high energy and power density[1,2]. However, large-scale applications of LIBs for energy storage are seriously limited by the scarce reserves and uneven distribution of lithium resources[3]. Given the earth-abundant potassium resources and a similarly low redox potential of K/K+as Li/Li+(-2.93 V and -3.04 V, vs standard hydrogen electrode), potassiumion batteries (KIBs)[4,5]have been recognized as promising alternatives or supplements to LIBs for large-scale energy storage system[6,7]. Despite tremendous potential, the development of KIBs has been largely hindered by the lack of suitable high-capacity electrode host materials. The(de)intercalation of the large-size K+ions often results in obvious structural degradation and therefore short cycle life to the electrode materials. On the other hand, the large ionic size typically leads to sluggish K+-diffusion kinetics within the electrode materials and undesired fast-charging capability.Therefore, it is critical to search for new host materials that can not only store a large amount of K+ions reversibly but also exhibit fast reaction kinetics.

Layered-structured transition metal dichalcogenides(TMDs), such as MoS2[8]and WS2[9], have received considerable attention as promising KIBs anode materials as they operate based on conversion-type reactions and are capable of delivering high theoretical capacities[10,11]. Very recently, Jiao et al. reported the anode behavior of a WSe2/N-doped porous carbon composite (WSe2/N-PC)[12].Differing from the MoS2and WS2anodes that exhibit flat voltage plateaus and relatively long cycle life, the WSe2nanocomposite exhibits a high capacitive-dominated K+-ion storage capacity without obvious voltage plateaus and limited cycle life. It is reasonably speculated the nanostructuring of WSe2is responsible for these discrepancies. Considering this，it is necessary to investigate the intrinsic K+-ion storage behavior of WSe2anode material and further improve its cycling stability.

Here we report a systematic investigation on the K+-ion storage behavior of the pure WSe2. We reveal that WSe2,serving as an anode material for KIB, operates at an average potential of 0.85 V (versus K/K+) accompanied by a flat voltage plateau and delivers an ultrahigh volumetric capacity of 1772.8 Ah/L at a current density of 5 mA/g. Moreover, the WSe2anode exhibits good rate capability (72 mAh/g at 200 mA/g) and long-term cycling stability (112 mAh/g remained over 100 cycles at 100 mA/g). Ex-situ X-ray powder diffraction analysis reveals that WSe2anode first undergoes intercalation reactions and then follows reversible conversion reactions. We have further fabricated full-cell KIBs utilizing a WSe2anode and a Prussian Blue cathode. The proof-ofconcept batteries deliver a high energy density of 135.2 Wh/kgcathode+anode. WSe2may open new opportunities for the development of high-capacity electrode materials adopted for high-energy-density rechargeable potassium-ion batteries.

2 EXPERIMENTAL

2. 1 Materials

WSe2powders (99.8% metals basis, CAS registry number:12067-46-8) were purchased from Shanghai Macklin Biochemical Co., Ltd. Sodium carboxymethylcellulose(CMC) and Super-P carbon black were purchased from Hefei Kejing Materials Technology Co., Ltd. Battery grade potassium bis(trifluoromethylsulfonyl) imide (KTFSI) and diethylene glycol dimethyl ether (DEGDME, water content <1 ppm) were purchased from DoDoChem. KTFSI was vacuum dried at 100 ℃ for 48 h before dissolving in DEGDME solvent.

2. 2 Materials characterization

X-ray powder diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer with CuKα radiation source (λ = 1.54184 Å) at a 2θ range of 5～80° and a scanning rate of 1 °/min. The operating voltage and current are 40 kV and 40 mA, respectively. The microscopic size and morphology of WSe2were characterized by a field-emission scanning electron microscope (FESEM, HITACHI SU-8010).

2. 3 Electrochemical measurements

The WSe2electrode was prepared through a slurry bladecoating method. Specifically, the WSe2powders, carboxymethylcellulose (CMC) binder, and Super-P carbon black were mixed homogeneously with a mass ratio of 8:1:1 in deionized water. The slurry was coated on an Al foil and dried under vacuum at 80 ℃ for 12 h. The mass loading of WSe2is～2.5 mg/cm2. To investigate the electrochemical performance of WSe2electrode, the standard CR2032-type coin half cells were assembled with potassium metal foils as counter electrodes in an argon-filled glove-box. 5 M potassium bis(trifluoromethylsulfonyl)imide (KTFSI) in diethylene glycol dimethyl ether (DEGDME) was adopted as electrolytes[13]and glass fibers (Grade GF/F, Whatman) were applied as separators. Galvanostatic charge/discharge cycling tests at different current densities were carried out on LANHE CT-2001A in the voltage range of 0.1 to 2 V (vs.K/K+). Cyclic voltammetry (CV) was performed on an electrochemical workstation (Bio-Logic SP-300) at a scanning rate of 0.1 mV/s in the voltage range of 0.1 to 3 V(vs. K/K+). All electrochemical tests were conducted at a constant temperature of 28 ℃.

3 RESULTS AND DISCUSSION

As illustrated in Fig.1a, WSe2exhibits a distinct layered structure with neighboring layers interconnected via Van der Waals force. The interlayer spacing of 6.49 Å is larger than those of ReS2[14], MoS2[8], WS2[9], NbSe2[15]and ReSe2[16](Fig.1b), and favors the intercalation of large-ionic-size K+ions[17]. The crystal structure and phase purity are confirmed by X-ray powder diffraction (XRD). As shown in Fig.1c, all the diffraction peaks could be accurately indexed to the standard WSe2structure (ICSD-40752) with space group P63/mmc (No. 194). The distinct Bragg peaks at 13.62°,31.40°, 32.17°, 37.80°, and 41.70° correspond to (002),(100), (101), (103), and (006) lattice planes, respectively,with a highly preferred orientation in the (002) lattice plane.Scanning electron microscopy (SEM) imaging reveals that WSe2exhibits hexagon plate-like morphology with a lateral size of about ～100～200 nm and a thickness of ～30 nm(Fig.1d).

The electrochemical behaviors of WSe2were explored by cyclic voltammetry (CV) and galvanostatic charge-discharge cycling tests. Fig.2a shows the three-cycle CV curves collected at a scan rate of 0.1 mV/s within 0.01～3 V (vs.K/K+). The irreversible reduction peak at ～0.45 V in the first cycle can be attributed to the formation of a solid electrolyte interphase (SEI)[18,19]. In the following cycles,three pairs of reduction/oxidation peaks appearing at 1.16/1.18, 1.6/2.0 and 1.91/2.68 V indicate reversible K+-ion insertion and extraction within WSe2. Fig.2b depicts the galvanostatic voltage profiles of WSe2at a current of 5 mA/g.WSe2delivers an initial discharging capacity of 297 mAh/g and a reversible charging capacity of 188.4 mAh/g as well as an initial Coulombic Efficiency (CE) of 61.9%. The irreversible capacity loss during the first cycle coincides with those observed in CV analysis and could be attributed to the formation of SEI, which is commonly observed in KIB anodes[20-23]. The volumetric capacity is estimated to be as high as 1772.8 Ah/L according to a density of 9.39 g/cm3for WSe2, exceeding previously reported anode materials including graphitic carbon, organics, Ti-based compounds,metal sulfides as well as alloy-type P and Sn[8,9,13,15,24-34](Fig.2c). Figs. 2b and 2d depict the long-term cycling performance of the WSe2anode at 5 mA/g. WSe2still delivers a specific capacity of 144.4 mAh/g after 50 cycles,corresponding to a capacity retention of 76.4%. Equally importantly, the flat voltage plateau remains during 50 cycles(Fig.2d), demonstrating the high reversibility of this new K+-WSe2electrochemistry. At a current density of 20 mA/g,the WSe2anode delivers a reversible capacity of 117.4 mAh/g after 100 cycles and an average CE of 98.68%(Fig.2e). At even higher current densities of 50 and 100 mA/g, capacities of 101.7 and 112 mAh/g are achieved after 100 cycles (Fig.2f), respectively, with capacity retention(70.6% and 83.14%) exceeding that for previously reported WSe2/N-PC (64% over 100 cycles at 100 mA/g)[12].Moreover, the WSe2anode delivers a higher initial CE of 61.9% (versus 50.06% for WSe2/N-PC).

Apart from good long-term cyclability, the WSe2anode also exhibits good rate capability. As shown in Figs. 2g and 2h, the WSe2anode delivers capacities of 183, 170, 151, 138,107, 82, and 72 mAh/g at current densities of 5, 10, 20, 30,50, 100, and 200 mA/g, respectively, and the voltage plateaus remain during rate-capability cycling. Equally importantly,the capacity recovers to 166 mAh/g when switching the current density back to 5 mA/g, manifesting the high reversibility of the WSe2anode against both deep and fast charging.

Fig.1. (a) Crystal structure of WSe2; (b) Comparison of interlayer spacing among TMDs; (c) Rietveld refinement of the XRD pattern of WSe2 (Rwp = 9.69%, and Rp = 7.26%); (d) SEM image of WSe2

Fig.2. (a) CV curves of the WSe2 at 0.1 mV/s; (b) Cycling performance at 5 mA/g and the first-cycle voltage profile (inset); (c) Comparison of the volumetric capacity and average discharge voltage; (d) Voltage profiles of the 2nd, 5th, 10th, 20th, 30th, 40th, and 50th cycles at 5 mA/g; (e) Cycling performance of WSe2 anode at 20 mA/g and the voltage profile at the 100th cycle (inset); (f) Cycling performance of the WSe2 anode at 50 and 100 mA/g. Inset depicts the voltage profiles at the 100th cycle; (g, h) Rate capability and corresponding voltage profiles

We further conducted ex-situ XRD to investigate the underlying reaction mechanism. As shown in Fig.3, the diffraction peaks at 13.6°, 31.4°, 37.8°, and 43.76° at the initial state (A point labeled in Fig.3) correspond to (002),(100), (103), and (105) reflections of WSe2, while the peaks at 38.47°, 44.78°, and 65.13° are the diffractions from the Al current collector (JCPDS no. 04-0850). Upon discharging to 0.5 V (B point), the diffraction peaks located at 13.6°, 31.4°,37.8°, and 43.76° shift to 13.36°, 31.24°, 37.67°, and 41.5°,respectively. These peak shifts indicate the intercalation of K+ions into the interlayer of WSe2and an increase of interlayer spacing. Upon further discharging to 0.1 V (C point), the correlative characteristic peaks of WSe2disappear completely. Meanwhile, new peaks appear at 15.35°, 20.06°,21.23°, 27.30°, and 28.46° and can be assigned to the (002),(211), (300), (322) and (132) lattice planes of K2Se4(ICSD-430521). These observations indicate conversion-type reactions occurring during this step (from B to C). On reverse charging to 2 V (D point), the diffraction peaks associated with WSe2do not appear again, while new peaks at 23.52°,24.23°, 31.01°, 34.02°, and 34.81° correspond well to the formation of Se (JCPDS no. 24-0714). According to these observations, the entire electrochemical reactions can be described as the following equations.

Fig.3. Ex-situ XRD patterns of the WSe2 electrode during the first cycle

It is noted that reactions (1) and (2) are irreversible, and after initial activation, the anode undergoes reversible conversion reactions between K2Se4and Se, while the W metal formed during the initial cycle serves as an electron conductor to facilitate the conversion reactions.

We further fabricated proof-of-concept full-cell KIBs composed of an activated Prussian Blue cathode K1.89Mn[Fe(CN)6]0.92[35]and an activated WSe2anode with a cathode-to-anode mass ratio of 1.8:1 (Fig.4a). As depicted in Fig.4b, the full-cell batteries operate at an average discharge voltage of 2.5 V and deliver a discharge capacity of 54.1 mAh/g and an energy density of 135.2 Wh/kg based on the total mass of the cathode and anode materials. After five cycles, the capacity and energy density remain at 45.8 mAh/g and 114.5 Wh/kg, respectively. This full-cell performance demonstrates the application prospect of the WSe2anode.

Fig.4. (a) Voltage profiles of K1.89Mn[Fe(CN)6]0.92 cathode and WSe2 anode at 10 mA/g;(b) Electrochemical performance of the full-cell battery

4 CONCLUSION

In summary, we have revealed the intrinsic electrochemical behaviors of WSe2. As an anode, WSe2exhibits an ultrahigh volumetric capacity of 1772.8 Ah/L at a current density of 5 mA/g, good rate capability (72 mAh/g at 200 mA/g), and long-term cyclability (83.14% capacity retention over 100 cycles at 100 mA/g). Through ex-situ XRD analysis we revealed that WSe2first undergoes intercalation reactions and subsequently conversion reactions to form K2Se4and W during initial activation, and follows reversible conversion reactions between K2Se4and Se in the subsequent cycles. Furthermore, full-cell KIBs utilizing the WSe2anode and a Prussian Blue cathode can provide an average discharge voltage of 2.5 V and an energy density of 135.2 Wh/kgcathode+anode. WSe2may open new opportunities for the exploration of high-volumetric-capacity anode material in rechargeable potassium-ion batteries.