Engineering Rice Husk into a High-Performance Electrode Material

ACS Sustainable Chem. Eng. , Just Accepted Manuscript. DOI: 10.1021/acssuschemeng.9b00092. Publication Date (Web): March 8, 2019. Copyright © 2019 ...
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Engineering Rice Husk into a High-Performance Electrode Material through an Eco-Friendly Process and Assessing Its Application for Lithium-Ion Sulfur Batteries Sheng-Siang Huang, Mai Thanh Tung, Chinh Dang Huynh, BingJoe Hwang, Peter Bieker, Chia-chen Fang, and Nae-Lih Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00092 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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Engineering Rice Husk into a High-Performance Electrode Material through an Eco-Friendly Process and Assessing Its Application for Lithium-Ion Sulfur Batteries

Sheng-Siang Huang1,2, Mai Thanh Tung3, Huynh Dang Chinh3, Bing-Joe Hwang4, Peter Maria Bieker5, Chia-Chen Fang6, and Nae-Lih Wu1,2, 1Department

of Chemical Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan 2Advanced

Research Center for Green Materials Science and Technology, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan 3School

of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi, Vietnam 4Department

of Chemical Engineering, National Taiwan University of Science and Technology, 43, Sec. 4, Keelung Road, Taipei 106, Taiwan 5MEET

Battery Research Center, University of Münster, Corrensstraße 46, 48149 Münster, Germany 6Industrial

Technology Research Institute, 195, Sec. 4, Chung Hsing Road, Chutung, Hsinchu 31040, Taiwan

Keywords: Rice husk, Recycle, Silicon anode, Li-ion batteries, Li ion sulfur batteries



To whom correspondence should be addressed: [email protected] 1 ACS Paragon Plus Environment

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Abstract High-capacity and cycle-stable SiOx/C composite anodes for Li-ion batteries (LIBs) were synthesized from rice husk (RH) using an eco-friendly, one-step pyrolysis process that takes full advantage of both the silica and organic components of RH. The process– property–performance relationship for this process was investigated. Pyrolysis of RH at a sufficiently high temperature (1000 °C) results in a C scaffold with a low surface area, high electronic conductivity, and embedded SiOx nanoparticles that are highly active toward lithiation, enabling high rate capability along with outstanding cycle stability for LIB applications. A SiOx/C anode delivering a specific capacity of 654 mAh g-1 and retaining 88% capacity (99.8% CE) after 1000 cycles was demonstrated. Higher capacities, up to 920 mAh g-1, can be achieved by adding a Si-containing polymer coating on RH prior to pyrolysis. The SiOx/C anodes demonstrated considerable promise for Li metal-free Li-ion sulfur batteries.

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Introduction Because of increasing environmental concerns around the world and shortage of fossil fuel energy, the development of renewable energy resources has attracted considerable attention. The use of green energy technologies, such as windmills and solar photovoltaics, is dependent on the implementation of highly efficient energy storage devices.1–2 Li-ion batteries (LIBs) have been widely used for portable electronics and electrified vehicles. It is expected that they will play a central role in renewable energy storage as well. To satisfy the escalating demand for these applications, exploring abundant, eco-friendly, and preferably renewable electrode materials is a crucial task for the sustainable development of LIBs. Rice husk (RH) is an abundant agricultural waste product in many Asian riceproducing countries.3–5 In 2017, its worldwide annual production was almost 150 million tons.3,6 RH contains numerous components, including silica (15–28 wt.%), cellulose (35–40 wt.%), hemicellulose (15–20 wt.%) and lignin (20–25 wt.%).3–5, 7–8 The utilization of RH has been a crucial research topic for decades; however, its use has thus far been limited to low-added-value applications, such as for fertilizer additives, fuels, and paving materials. Recently, conversion of RH to valuable energy materials has been enthusiastically pursued.9–22 Both the C materials derived from the organic components of RH and Si from silica are potential electrode materials for 3 ACS Paragon Plus Environment

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electrochemical energy storage devices. As a result, Si nanoparticles have been synthesized from RH using magnesiothermic and aluminothermic processes to obtain high-capacity LIB anode materials.23–36 As presented in Scheme 1a, in the magnesiothermic process, RH is first fully oxidized at high temperatures to remove the organic materials to leave SiO2, which is subsequently reduced by Mg at high temperatures. The resulting Si-MgO mixture is then thoroughly washed with acid (e.g., hydrochloric acid) to remove the oxide. To improve the electronic conductivity of the resulting Si nanoparticles, a high-temperature chemical-vapor-deposition (CVD) process is typically applied to produce C coatings on the particles before they can be employed in LIBs. Despite using an eco-friendly source material, namely RH, the metallothermic processes cannot be considered environmentally friendly. They require the use of substantial amounts of metals (Mg or Al) that are derived from energyintensive metallurgical processes, and they produce considerable amounts of acidic waste water. Moreover, the processes are considered inefficient in terms of the use of energy and materials: the organic components of RH are burned off, even though they would be perfect sources for C required to improve particle conductivity. This is only later compensated for using a high-temperature CVD C-coating process. In this study, full advantage was taken of both the silica and organic components of RH by using a straightforward one-step pyrolysis process to convert RH into a 4 ACS Paragon Plus Environment

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SiOx/C composite as a LIB electrode material (Scheme 1b). Although it has a lower specific capacity than pure Si, SiOx undergoes a smaller volume expansion and possesses a more stable electrode–electrolyte–interphase (SEI).37–42 As a result, SiOx is capable of maintaining cycle stability superior to that of pure Si when serving as a LIB anode. Although the direct pyrolysis process has previously been reported,43–47 the literature lacks a systematic investigation of the process–microstructure–performance relationship in maximizing the specific capacity and cycle stability of the resulting SiOx/C anode materials. This study is the first to assess the RH-derived high-capacity SiOx/C for Li-ion S batteries (LiSBs), for which it demonstrates considerable promise as a high-capacity and cycle-stable anode material.

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Scheme 1. Schematics depicting (a) a conventional magnesiothermic process and (b) a one-step pyrolysis process for deriving SiOx/C composites from rice husk.

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Experimental Synthesis of SiOx/C composite powders. To obtain SiOx/C composite materials, RHs (Thaibinh, Vietnam) were calcined in a tubular furnace at a heating rate of 4 °C min-1 under various conditions, including different calcination temperatures, holding times, and atmospheres, as listed in Table 1. The calcined powders were ground prior to characterization. To increase the charge storage capacity of the composite, some RH samples were supplemented with Si-containing polymers. Polymethylhydrosilosane (PMHS, Mw 1700-3200 Aldrich) and polydimethylhydrosilosane (PDMS, Mw ~25000 Aldrich) in a mass ratio of 19:1 were mixed through magnetic stirring at 300 rpm for 30 min. The entire solution was stirred at 300 rpm at 70 °C for 2 h. RHs were then introduced into the solution, and the final mixture was stirred overnight and dried and calcined in a tubular furnace at 1000 °C for 4 h under vacuum conditions. The resulting sample was termed RH_SiP. Synthesis of sulfur-polyacrylonitrile (S-PAN) cathode. Sulfur (99.98% Aldrich) and polyacrylonitrile fiber (PAN, Mw 150000 Aldrich) were mixed in a weight ratio of 4:1 and ground. The well-mixed precursor was then transferred to a tubular furnace and calcined at 350 °C for 3 h under Ar to obtain the S-PAN composite.

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Table 1 Summary of treatment conditions and microstructural properties of RH-derived materials RH-derived materials Calcination temperature

RH_600_2.5h

RH_1000_2.5h RH_1000_4h

RH_SiP

600

1000

1000

1000

2.5

2.5

4.0

4.0

H2/N2

H2/N2

Vacuum

213.5

5.0

4.3

4.4

Pore volume (cm3 g-1)a

0.050

0.017

0.013

0.0076

Pore diameter (nm)a

42.0

36.1

29.3

11.1

Residue C (wt %)b

60.0

57.0

56.7

18.4

Residue SiOx (wt %)b

34.5

36.9

37.2

80.2

SiOx/C wt ratio

0.58

0.65

0.66

4.36

2.0 × 10-5

1.7

2.6

2.8 × 10-2

(°C) Holding time (h)

H2 3%, N2 97%

Atmosphere

(H2/N2)

BET specific surface area (m2 g-1)a

Conductivity (S cm-1)c aMeasured

by N2 adsorption at 77 K.

bDetermined cMeasured

by thermogravimetric analysis.

at 5.65 kgf cm−2.

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Material Characterizations. Microscopic analysis was conducted on a JEOL JSM-5310 scanning electron microscope (SEM) equipped with a Link eXL energy dispersive Xray (EDX) system or a JEOL JSM-7600F SEM to investigate the morphologies and compositions of materials. Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM-2100F field emission transmission electron microscope equipped with an Oxford X-MaxN TSR energy dispersive spectroscope (EDS) or on a Philips Tecnai F20 G2 FEI-TEM. X-ray diffraction (XRD) measurements were performed using a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5418 Å) within a scanning range between 10° and 80° with a scanning rate of 10° min-1. Thermogravimetric analysis (TGA) was conducted on Rigaku Thermo Plus 2 TG8120 system with a scanned temperature range from 30 °C to 1000 °C at a temperature scanning rate of 5 °C min-1 in either air or H2/N2. The sulfur content of the S-PAN composite was measured using an elemental analyzer (vario EL cube, Elementar). The nitrogen adsorption and desorption isotherms were obtained using the Brunauer– Emmett–Teller (BET; Micromeritics, ASAP 2010) method. The conductivity measurement of materials was carried out using a self-made apparatus that consisted of a hollow Teflon cylinder and two Cu pressing pistons connected to an ohm-meter (Chen Hwa, 502BC). During the measurement, the sample was placed in the chamber between the two pistons, and the electrical resistance between the pistons was directly measured 9 ACS Paragon Plus Environment

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under conditions of increasing pressing pressure. Powder conductivity was calculated according to the following equation: 1/ = RA/L

[1],

where  is the powder conductivity (Scm-1), R is the electrical resistance of samples (in , measured using an ohm-meter), A is the cross-section area of the chamber (in cm2), and L is the distance between the two pistons (in cm, measured using a laser cathetometer). Electrochemical Characterizations. The following materials were used for anode preparation (labelled as RH-GC-ALG): (i) active material: either the RH-derived SiOx/C composites or RH_SiP; (ii) conductive additive: graphite flakes and carbon black (GC; KS6, 99.9% Timcal and Super P, 99.9% Timcal with a mass ratio of 15: 3); (iii) binder: sodium alginate (ALG, 99.0% Acros). A citric acid solution (pH 4.5) was added to an RH-GC-ALG (mass ratio: 70:18:12) mixture to form a slurry. The slurry was uniformly spread onto a 15-m-thick copper foil using a blade-coating machine (All Real Technology Co.) and dried at 70 °C in an ambient air atmosphere. The electrodes were calendered and cut into discs with a diameter of 13 mm. Prior to cell assembly, the electrodes were dried at 150 °C for 12 h in a vacuum oven. The S-PAN cathode was composed of 80 wt.% S-PAN, 10 wt.% Super P, and 10 wt.% sodium carboxymethylcellulose (Na-CMC, Mw 700000 Aldrich), on a dry basis. 10 ACS Paragon Plus Environment

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A water-based binder solution, containing 10 wt% Na-CMC in H2O/ethanol (mass ratio 1:1), was first prepared and then mixed with the powder mixture to form a slurry. The well-dispersed slurry was uniformly coated onto a 15-μm-thick aluminum foil using doctor-blade coating, followed by drying at 50 °C in an ambient air atmosphere. After being punched into disks with a diameter of 13 mm in diameter, the electrodes were further dried at 50 °C for 12 h in a vacuum oven. For half-cell measurements, CR2032-type coin cells were fabricated in an Arfilled glovebox where moisture and oxygen levels were lower than 0.1 ppm. SiOx/C electrodes had active material loadings between 0.70~0.90 mg cm-2, while S-PAN electrodes had loadings of 0.50~0.55 mg cm-2. They were respectively assembled in coin cells with lithium foils as the counter electrodes. The electrodes were separated by a

16-m-thick

polyolefin

separator.

The

electrolyte

was

1

M

lithium

hexafluorophosphate (LiPF6) in ethylene carbonate /ethyl methyl carbonate in a volume ratio of 1:1 with 2 wt.% vinylene carbonate (Union Chemical IND.CO, Ltd.) and 10 wt.% fluoroethylene carbonate (>98% TCI). For the full-cell measurement, the active material loading of SiOx/C anode was increased to 3.85 mg cm-2. The electrode was first pre-lithiated in the standard half-cell configuration by charging and discharging the electrodes within a voltage window between 5 mV and 1.2 V for five cycles and then fully discharging (lithiating) them. The lithiated SiOx/C electrode was then 11 ACS Paragon Plus Environment

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removed from the half-cell, washed with dimethyl carbonate, and finally reassembled in the coin cell with a S-PAN cathode. The electrolyte and separator were the same as those in the half-cell configuration. The galvanostatic tests were conducted within a voltage window between 5 mV and 1.2 V (versus Li/Li+) for the RH-derived SiOx/C anodes under current densities ranging from 0.05 to 2 A g-1. The voltage window for the S-PAN cathode was from 1 V to 3 V (versus Li/Li+) under current densities from 160 to 1600 mA g-1. The full cells were subjected to galvanostatic charge/discharge cycles between 0.6 V and 3 V. The current and specific capacity were calculated based on the weight of the active material (SiOx/C or S) in the working electrode. Electrochemical tests were conducted using a Maccor battery tester (Series 4000).

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Results and Discussion RH and derived SiOx/C. SEM analysis revealed that the RH samples had smooth flat internal surfaces and external surfaces covered with small irregularly shaped domes sandwiching interior hollow compartments that were divided by biological tissue (Figure S1a-e). EDS analysis indicated that C, O, and Si were the predominant constituent elements, with a small amount (2000 mAh g-1 for SiO54 19 ACS Paragon Plus Environment

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to almost zero for SiO2. Therefore, one plausible reason for the enhanced specific capacity exhibited by the RH_1000_4h electrode is the reduction in oxygen content of the silica species. Figure 3c presents a comparison of the cycling behaviors of these electrodes. The RH_600_2.5h electrode exhibited the least favorable cycle stability, showing a 14% loss of charge capacity (from 579 to 500 mAh g-1) during the first five cycles. By contrast, the 1000 °C-treated electrodes reached a stable capacitance at the 2nd cycle and exhibited essentially no capacity within 100 cycles (Figure 3c). To further increase the capacity of the RH-derived SiOx/C electrodes, Sicontaining polymers were introduced to the RHs to prepare the composite materials (RH_SiP) with higher Si content. Figure 4a presents a comparison in the 1st cycle voltage curve and specific capacity between RH_1000_4h and RH_SiP electrodes. These two samples had the same thermal history. Below 0.8 V, the RH_SiP electrode exhibited the onset of a sloped plateau associated with SEI formation but with substantially less capacity loss than the RH_1000_4h. These data suggest that the SiOx coating may help to passivate the C surfaces of the pyrolyzed RH to reduce SEI formation. The RH_SiP electrode exhibited a flat lithiation plateau onset at approximately 0.35 V, typical of stoichiometric SiO.37–42,

55

The absence of such a

plateau for the pristine RH-derived samples may suggest a distribution of domains with varied Si/O stoichiometries in the RH-derived SiOx/C composites. The RH_SiP 20 ACS Paragon Plus Environment

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electrode has a 1st cycle reversible (i.e., delithiation) specific capacity of 918 mAh g-1 along with a CE of 52.7%, both of which exceeded those of the RH_1000_4h electrode (654 mAh g-1, 50.2%). With its higher specific capacity, the RH_SiP electrode nevertheless exhibited a faster capacity fade during the initial cycles. In the case of charge/discharge at 0.1 A g-1, the specific capacity of RH_SiP suffered from 7% loss during the first 15 cycles and then leveled off, while the RH_1000_4h sample exhibited no capacity loss within the entire 100 cycles (Figure 4b). Figures 4c and 4d present the rate capability and long-term cycle stability for these two electrodes. Both RH_1000_4h and RH_SiP electrodes exhibited outstanding rate performance, achieving 86% and 74% capacity retention, respectively, as the current density was increased 10-fold from 0.05 to 0.5 A g-1. Moreover, both electrodes exhibited remarkable cycle stability. The RH_1000_4h electrode retained 88% capacity after 1000 cycles under variable current densities, while the RH_SiP electrode achieved a capacity retention of 76% retention after 720 cycles. Both electrodes showed CEs greater than 99.8% at their final cycles. Such long-term cycle stability and high CEs have never been demonstrated in the previous studies of the RH-derived SiOx/C anodes, where RHs were heat-treated at substantially lower temperatures (650~900 °C). The remarkable high rate capability and cycle stability of the SiOx/C composite derived using prolonged calcination at 1000 °C may be enabled by the combination of 21 ACS Paragon Plus Environment

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several unique microstructural features of the powder. The nanoporosity of the powder not only facilitates Li-ion diffusion in the electrolyte to reach the surfaces of the active materials but also provides the space necessary to accommodate volume expansion of the particles upon lithiation. Close contact of SiOx nanoparticles with the highly conductive residual C provides efficient electron transport and prevents SiOx from becoming inactive because of detachment from the conductive network within the electrode. The nanosized SiOx particles considerably shorten the solid-state diffusion path for Li ions. Nevertheless, it is worth mentioning that the low 1st cycle CE for the RH-derived SiOx/C may hinder its practical application. A suitable pre-lithiation method needs to be explored. An example is illustrated in the following LiSB test.

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Figure 4. Comparison between SiOx/C and RH_SiP electrodes: (a) 1st cycle voltage plots and (b) cycle performance during the initial 100 cycles of RH_1000_4h and RH_SiP electrodes; (c) high-rate and long-term cycling performance of RH_1000_4h; (d) high-rate and long-term cycling performance of RH_SiP. The insets in (c) and (d) show capacity retention versus current density.

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RH-derived composite anodes for LiSB applications. The operation of Li-S batteries is based on the multielectron redox conversion reaction of elemental S that provides a theoretical capacity of 1672 mAh g-1 and potential energy density of approximately 2500 Wh kg-1 or 2800 Wh L-1. In addition, S is cheap and abundant, making Li-S batteries a promising low-cost technological application.56–59 However, certain challenges for both S cathodes and Li-metal anodes must be addressed before commercial realization can be achieved. One obstacle is associated with the employment of the highly active Li-metal anode, which results in considerable safety concerns.60–63 Recent studies have attempted to replace the Li-metal anode with highcapacity Li-alloyed anodes, such as Si-based anodes.64,65 To assess the RH-derived SiOx/C electrode for LiSB applications, the RH_1000_4h electrode was subjected to lithiation/delithiation cycles in an ether-based electrode (1 M LiTFSI in dioxolane/dimethoxyethane (v/v = 1:1)) that has been commonly employed in Li-S batteries. Figure 5a compares the 1st cycle lithiation and delithiation voltage plots of the RH_1000_4h electrodes in the ether- and carbonatebased electrolytes, respectively. The electrode in the ether-based electrolyte exhibited a substantially lower 1st cycle CE (37% vs 50%) and specific capacity (458 vs 654 mAh g-1) than those in the carbonate-based electrolyte. The capacity difference persisted with further cycling (Figure 5b). The voltage plots (Figure 5a) indicate a higher polarization 24 ACS Paragon Plus Environment

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level (i.e., a greater resistance) for delithiation in the ether-based electrolyte, which may be due to a higher charge-transfer resistance in the ether-based electrolyte.

Figure 5. (a) Voltage profiles and (b) cycling performance of RH_1000_4h electrodes in carbonate (red symbol)- and ether (green)-based electrolytes, respectively; (c) Voltage curves of the S-PANSiOx/C full-cell (current density of 1600 mA g-1-S) at different cycles; (d) cycling performance of S-PANSiOx/C full-cell.

Although the carbonate-based electrolyte gave superior performance to the RHderived SiOx/C electrode, it cannot be used to match with the common C/S composite cathodes that produce soluble polysulfides during lithiation, because carbonate solvents react readily with soluble polysulfides.66–71 Recent studies have shown that the N functional group of a C host can promote chemical bonding between S and the C 25 ACS Paragon Plus Environment

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scaffold effectively, leading to S containment. In particular, the cofiring of a mixture of polyacrylonitrile and S in N2 has been shown to produce a S-C nanocomposite known as S-PAN,72-76 in which S is maintained as insoluble S3/S2 during all stages of the redox process. Employing these materials as Li-S battery cathodes completely eliminates polysulfide dissolution, enabling such batteries to operate with carbonate-based electrolytes.77–79 The S-PAN cathode prepared in this study had a S content of 45 wt.% and, when cycled against a Li metal in carbonate-electrolyte, delivered a 1st cycle reversible specific capacity of 1600 mAh g-1-S (at a current density of 160 mA g-1-S) with a CE of approximately 80.8% (Figure S3). The voltage curves exhibited only one plateau (as opposed to two) for either lithiation or delithiation (Figure S3). The “one-plateau” feature is a signature of the polysulfide-free redox mechanism.80-84 The S-PAN electrode exhibited respectable rate performance and cycle stability, showing a specific capacity of 1150.8 mAh g-1-S at a current rate of 1600 mA g-1-S and retained 88.5% capacity after 100 cycles (Figure S3c). A S-PANSiOx/C full-cell was manufactured by matching a S-PAN cathode with a prelithiated RH_1000_4h anode, with the anode-to-cathode (A/C) capacity ratio being 1.8:1. Considering an average voltage of 2V between the charge-discharge voltage plots of the cell and taking into consideration of the S content and A/C capacity ratio, the cell 26 ACS Paragon Plus Environment

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has a specific energy of approximately 480 Wh kg-1 based on the total amount of active materials (S-PAN + SiOx/C). The full-cell, of which the voltage plots exhibited the oneredox-plateau feature of S-PAN, achieved a 1st cycle reversible capacity of 1566 mAh g-1- S (at 160 mA g-1) (Figure 5c) and retained 706 mAh g-1-S at 1600 mA g-1 after 1000 cycles (Figure 5d). This outstanding electrochemical performance is in direct contrast to that of the LiSBs with non-Li metal anodes in the literature (Table 2).60,64,65,85–96

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Table 2 Comparison of the electrochemical performance of Li-ion S batteries Anode

Cathode

Type of electrolyte

Reversible capacity

Ref.

(mAh g-1-S)/ Cycle number Si@graphite

S@CMK-3

Solid-like

824/200

60

(SiO2/ether-based electrolyte) Si/SiOx

Activated C-S

Ether-based

638/500

64

S/C

Ether-based

900/50

85

Si nanowire-C

S/C

Ether-based

400/460

86

Tin-C

Activated

Ether-based

600/300

87

Ether-based

500/700

88

Ether-based

753/550

88

nanosphere Hierarchical columnar Si

ordered mesoporous C-S Amorphous Si /C

Hollow C sphere/S

Hard C

Hollow C sphere/S

MCMB

S-MCMB

Ether-based

500/80

89

Si/C

S/C

Ionic liquid

670/50

90

Si-O-C

Li2S-MCMB

Ether-based

280/50

91

Amorphous Si

C/S

Ether-based

380/60

92

Si-C-Nafion-GO

S-C-GO

Ether-based

560/100

93

C-mesoporous Si

S/CMK-8

Ether-based

780/100

94

Si on graphene

S copolymer on

Ether-based

620-anode and

95

thin film

graphene

cathode/500

Si/graphite/C

Hard C-S

Ether-based

300/100

96

SiOx/C

S@pPAN

Carbonate-based

616-cathode/100

65

RH_1000_4h

S-PAN

Carbonate-based

706/1000

this study

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Conclusions SiOx/C composite powders were synthesized from RH using an eco-friendly one-step pyrolysis process and assessed as high-capacity anodes for LIBs. Moreover, the process–property–performance relationship was investigated. It was revealed that, although major pyrolysis-induced mass loss occurred below 500 °C, heat treatment at 1000 °C markedly reduced the specific surface area while increasing the powder conductivity of the composites, leading to improved CEs during prolonged cycles and hence cycle stability. Prolonged treatment at 1000 °C substantially enhanced the specific capacity of the composite. A cycle-stable high-capacity SiOx/C anode that achieved a reversible specific capacity of 654 mAh g-1and maintained 88% capacitance retention after 1000 cycles was obtained after pyrolysis at 1000 °C for 4 h in N2/H2. A SiOx/C anode with a higher capacity (918 mAh g-1) and moderate cycle stability (76% retention after 720 cycles) was achieved by coating Si-containing polymers on RH prior to pyrolysis. An application assessment was conducted through the electrochemical testing of a full-cell manufactured by matching a prelithiated SiOx/C anode with an SPAN cathode. The full-cell demonstrated a 1st cycle reversible capacitance of >1500 mAh g-1-S and a capacity retention of >700 mAh g-1-S at 1600 mA g-1 after 1000 cycles. The results reveal that the RH-derived SiOx/C composites hold considerable promise for Li metal-free S batteries. 29 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Available: **[SEM images and EDS analyses of RH; TGA of RH in air; Voltage plots and cycle performance of S-PAN]**

AUTHOR INFORMATION Corresponding Author *N.-L.Wu, E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement The authors acknowledge the following financial support: (NLW) Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education, Taiwan (107L9006); Ministry of Science and Technology, Taiwan (MOST-107-3017-F-002-001, 105-2923-E-002-012-MY2, 104-2923-M-002011-MY3, 106-2923-E-011-005, 107-2923-E-011-002); (MTT) Ministry of Science and Technology, Vietnam (Project NDT.19.TW/16) and 15/FIRST/1.a/HUST; (BJH) Ministry of Science and Technology, Taiwan (104-2923-M-011-002-MY3); (PMB)

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Germany Federal Ministry of Education and Research (BMBF) (ACHiLIS project; #03XP0037A ). S.J. Ji of MOST (National Taiwan University) is acknowledged for microscopy analysis.

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SiOx/C synthesized by high-temperature (1000 °C) calcination of rice husk demonstrates long-cycle stability and promise for Li metal-free Li-ion sulfur batteries.

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