Employing MnO as multifunctional polysulfide reservoirs for enhanced-performance Li-S batteries

Accepted Manuscript Employing MnO as multifunctional polysulfide reservoirs for enhanced-performance Li-S batteries Yanan Liu, Guilin Feng, Xiaodong Guo, Zhenguo Wu, Yanxiao Chen, Wei Xiang, Jianshu Li, Benhe Zhong PII:

S0925-8388(18)30978-2

DOI:

10.1016/j.jallcom.2018.03.110

Reference:

JALCOM 45340

To appear in:

Journal of Alloys and Compounds

Received Date: 15 November 2017 Revised Date:

7 March 2018

Accepted Date: 9 March 2018

Please cite this article as: Y. Liu, G. Feng, X. Guo, Z. Wu, Y. Chen, W. Xiang, J. Li, B. Zhong, Employing MnO as multifunctional polysulfide reservoirs for enhanced-performance Li-S batteries, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.110. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Employing MnO as multifunctional polysulfide reservoirs for enhanced-performance Li-S batteries Yanan Liu, 1 Guilin Feng, 1 Xiaodong Guo, 1 Zhenguo Wu, 1 Yanxiao Chen,1* Wei

1

RI PT

Xiang, 2 Jianshu Li, 3 Benhe Zhong1

School of Chemical Engineering, Sichuan University, No. 24 South Section 1,Yihuan

Road, Chengdu, 610065,China.

College of Materials and Chemistry &Chemical Engineering, Chengdu University of

SC

2

Technology, Chengdu 610059, PR China.

College of Polymer Science and Engineering, State Kay Laboratory of Polymer

M AN U

3

Materials Engineering, Sichuan University, Chengdu, 610065, China. *

Corresponding author. Tel: +86-28-85464466; Fax: +86 028 85406702; E-mail

AC C

EP

TE D

address: [email protected] (Yanxiao Chen)

1

ACCEPTED MANUSCRIPT

Abstract The lithium sulfur (Li-S) batteries are greatly fascinating as the next-generation storage devices owing to their high theoretical energy density and low cost. However, the fast capacity decay resulting from the “shuttle effect” of polysulfide ions and the

RI PT

irregular deposition of the discharge products (Li2S/Li2S2) severely hinder their commercial application. Herein, micro-mesoporous carbon–MnO (MnO/MPC) composites were synthesized with wet impregnation method and applied as the cathode material. In Li-S cells, the MnO/MPC@S cathode showed a rather better

SC

cycling stability over 150 cycles than a MPC@S cathode (sulfur loading ~74%). The influences of the MnO amount and the heat treatment temperature were investigated

M AN U

and optimized. The 10%-MnO/MPC@S cathode material calcined at 600 ℃ showed excellent rate capability and cycling stability. Owing to the polar MnO nanoparticles, the soluble lithium polysulfides (Li2Sn, 2
TE D

MnO nanoparticle can effectively enhance the reaction kinetics, which could be ascribed to the plenty of polar sites on the surface of MPC material and the electrocatalytic activity of MnO.

AC C

sites

EP

Keywords: MnO-doped, Mesoporous carbon, wet impregnation, Li-S battery, Polar

2

ACCEPTED MANUSCRIPT

1. Introduction As a competitive candidate for the next generation energy storage devices, lithium sulfur (Li-S) batteries have drawn much attention because sulfur cathode can exhibit a high theoretical energy density (2567 Wh Kg–1, 1675 mA h g–1), natural

RI PT

abundance, low cost and environmental friendliness [1-2]. However, several issues such as poor cycle stability and low sulfur utilization have been the roadblocks in the practical applications process [3]. Those issues are mainly caused by the following aspects: (1) the intrinsic insulating nature of sulfur and the final discharge product

SC

(Li2S/Li2S2); (2) the dissolution of soluble lithium polysulfide in the electrolyte (shuttle effect); (3) the disorderly deposition of Li2S/Li2S2 [3].

M AN U

To overcome these drawbacks, many interesting and novel solutions, chiefly focusing on the design of sulfur cathodes, have been proposed. The most commonly used method is sealing sulfur into the host materials with high conductivity, large surface area and high porosity, such as micropore/mesopore carbon [4-6], carbon nanotube/nanofiber [7-8], reduced grapheme oxide [9-10] and the like. Nevertheless,

TE D

in spite of the improvement of the electrochemical performance of Li-S batteries through those methods, the dissolution of lithium polysulfides cannot be sufficiently suppressed and the Li2S/Li2S2 cannot be evenly dispersed because there are limited

EP

active sites on the surface of the non-polar materials, leading to the mutually repulsion between the non-polar surface and the conjugate polarity polysulfides [3, 11].

AC C

In order to modify the non-polarity surface of carbon, some novel ways are introduced by adding some polar sites on the carbon surface, including nanocarbon materials doped with hetero-atoms (N, S, B, O and P) [12-14] or coated with conductive polymers [15-16]. The nanocarbon materials not only contribute effective electron and lithium-ion pathways, but also mitigate the volume expansion during the charge/discharge process. Meanwhile, polar sites or special functional groups on the conductive polymers serve as the chemical anchoring sites [17]. Nevertheless, the chemical anchoring effects of these non-metallic polar sites are not strong enough to completely entrap the soluble polysulfides or promote the even deposition of 3

ACCEPTED MANUSCRIPT Li2S/Li2S2. [3] For further enhancing the chemical anchoring effects, the nanostructured polar metal composites have been employed as the polar host material. For instance, Nazar and co-worker reported that metal oxide, the host material for S cathode, with an inherent polarity and large surface area, has a strong binding ability

RI PT

with polysulfides/Li2S and enables long cycling process at high rates of sulfur batteries [18]. Therefore, multi-functional composites which possess high electric conductivity, larger specific surface area as well as strong interaction between carbon hosts and sulfur species, are regarded as promising materials towards improving the

SC

long cycle life of Li-S batteries. The transitional-metal oxides (such as Ti4O7, Mn3O4, MnO2, Co3O4) [18-20] with a controllable exposed polar-surface have stronger

M AN U

adsorption ability to sulfur species, which can reduce the dissolution of polysulfides and induce the orderly deposition of Li2S2/Li2S. For example, An et al. have reported that the MnO modified carbon nanotubes as a sulfur host could obviously improve the initial utilization of S especially at high current densities, and the results suggested that MnO has a very important role in restraining the shuttle effect and affecting the

TE D

discharge reaction process. [21] In addition, their electrocatalytic effect may accelerate the reaction kinetics of the redox process, which can increase the utility of sulfur [3, 17]. Qian et al. have reported that the chemical reaction kinetics of

EP

polysulfides accelerated, which could be attributed to the catalytic effect of the MnO nanoparticles [22].

In this work, we prepared manganese oxide nanoparticles (MnO) modified

AC C

micro-mesoporous carbon (MnO/MPC) as the cathode host material by wet chemistry method, in combination with high-temperature calcinations. The MnO modified MPC material can not only act as the conductive framework to improve the electric conductivity of sulfur cathode but also adsorb the polysulfides with chemical bonding. Moreover, the influence of the MnO quantity and the calcination temperature of the precursor material on the electrical performance were also studied. The experiment results indicate that the 10%-MnO/MPC@S cathode material with an optimized heat treatment at 600 ℃ showed excellent rate capability and cycling stability. Compared with the pristine MPC material, the MnO/MPC@S cathode materials possess better 4

ACCEPTED MANUSCRIPT electrochemical performance due to the plenty of polar sites on the MPC material surface and the electrocatalytic activity of MnO.

2. Experimental section

RI PT

2.1 Material preparation Mesoporous carbon (MPC) preparation

All the materials used in this study are of analytical grade and are used without

is carbonized at 850

SC

any pretreatment. In a typical synthesis, a certain amount of biomass carbon precursor for 2 h in Ar atmosphere at a heating rate of 1.5 °C min−1. The

M AN U

obtained black product is immersed in the mixture solution (2 mol/L of hydrofluoric acid and 2 mol/L of hydrochloric acid) for 24 h (ultrasonicating for 2 h and then standing at room temperature for 22 h), and then collected by filtration, cleaned with deionized water several times to re-establish a neutral pH of 7, and finally dried at 80 °C for 12h.

TE D

MnO-doped mesoporous carbon (MnO/MPC) preparation The MnO/MPC composite is prepared by a wet-chemical method. In a typical synthesis process, according to the certain proportion, the as-prepared MPC, manganese carbonate (MnCO3) and certain amount of alcohol are mixed by milling in

EP

the ball mill pot, and then the mixture is dried at 80 °C for 4 h. The mixture is calcined at 600 °C for 6 h in Ar atmosphere at a heating rate of 5 °C min–1.

AC C

Synthesis of C/S composites

Carbon-sulfur composite cathodes (MPC@S and MnO/MPC@S) are prepared

following a typical melt-diffusion strategy. The carbon materials (MPC and MnO/MPC) are firstly mixed with sulfur powder with a weight ratio of 1:4 by milling for 30 min. Subsequently, the mixture is placed in a stainless steel can sealed at 155 °C for 12 h to incorporate sulfur into the carbon matrix. 2.2 Cathode preparation The as-prepared C/S composite, acetylene black and polyvinylidene difluoride (PVDF) with a weight ratio of 7:2:1 are magnetically stirred for 5 h in 5

ACCEPTED MANUSCRIPT N-methyl-pyrrolidinone (NMP) solvent to form uniform slurry. The slurry is then cast on aluminum foil (the thickness of coating on aluminum foil is 0.15 mm) and dried under vacuum at 60 °C for 24 h. The resulting composite loading is approximately 2.8 mg/cm2.

RI PT

2.3 Cell assembly Coin-type (CR2025) cells are assembled in an Ar-filled glove box, in which the H2O and the O2 content is maintained below 5 ppm. The sulfur electrode is used as cathode. The lithium foil is introduced as counter and reference electrode. Celgard

SC

2400 is applied as separator, and 0.65mol/L lithium bis (trifluoromethanesulfonyl) imide(LiTFSI) in a solvent mixture of dimethoxyethane (DME) and 1,3-dioxolane

M AN U

(DOL) (1:1, v/v) is used as the electrolyte. The assembled coin cells are kept for 5 h before testing to ensure the proper contact between electrode and electrolyte. 2.4 Structural characterizations

The crystal structure is examined by X-ray diffraction (XRD, D/max-rB, Rigaku) scanning from 10° to 80° with Cu Kαradiation. The surface morphology is studied by

TE D

scanning electron microscopy (SEM, SPA400 Seiko Instruments). The sulfur content of the MPC@S (or MnO/MPC@S) composite is determined by thermogravimetric analysis (TG, TA Instruments, Q600) by heating from room temperature to

EP

500/1000 °C under N2/O2 atmosphere with a heating rate of 10 °C min–1. The pore structure characteristics of the samples are obtained by N2 adsorption/desorption isotherms at 77 K using a NOVA1000e surface area and pore size analyzer

AC C

(Quantachrome Company). Prior to the analysis, samples are degassed at 473 K in a vacuum condition for 180 min. The BET surface area, micropore volume and pore size distribution of the samples are measured by the application of the BET and DFT equation, respectively. The total pore volume is estimated at a relative pressure of p/p0 = 0.98–0.99. 2.5 Electrochemical characterization Galvanostatic charging-discharging tests are performed at room temperature on a LANDCT 2001 (NewareCo. Ltd) battery tester operated between 1.0 V and 2.8 V. Electrochemical impedance spectroscopy (EIS) is acquired over the frequency range 6

ACCEPTED MANUSCRIPT from 10 mHz to 1 MHz at an amplitude of 5 mV using the autolab electrochemical work station. Cyclic voltammeter (CV) tests are performed on an autolab electrochemical work station with a voltage range between 1.0 V and 2.8 V at a

RI PT

scanning rate of 0.1 mV S–1.

3. Results and discussion

The X-ray diffraction (XRD) pattern of different content of MnO-doped porous carbon materials are shown in Fig. 1a. All the diffraction peaks can be indexed to the

SC

face-centered cubic phase of MnO (JCPDS, 07-0230）[21]. As the content of MnO in the materials increases, the strength of the diffraction peak of MnO reinforces.

M AN U

However, when the content of MnO is more than 10%, the intensity of the diffraction peak of MnO has not changed significantly. In addition, the apparent two broad peaks around 24° and 43° in the XRD pattern represent the typically amorphous structure of carbon [14]. Meanwhile, the peak intensity of amorphous carbon decreases after the MnO dopes into the porous carbon. The XRD patterns of 10%-MnO-doped porous

TE D

carbon materials under different calcination temperature are shown in Fig. 1b. As can be found, when the calcination temperature is less than 500°C, the diffraction peaks of MnO and MnCO3 cannot be found, but there have still appears the amorphous carbon

EP

peaks. When the calcination temperature is more than 600°C, the diffraction peaks of MnO appeared. The XRD pattern of different content of MnO-doped carbon-sulfur

AC C

composite and the sublimed sulfur are shown in Fig. 1c. It can be seen that the diffraction peak of the sulfur in the porous carbon-sulfur composite shows the typical crystalline sulfur peaks with orthorhombic structure, which is similar to the sublimed sulfur, excepting their intensities are lower. This suggests that sulfur is successfully incorporated into the porous carbon materials [14, 23].

Fig. 1. As seen in Fig. 2a and Fig. 2b, the MPC possess massive pores, which could facilitate the sulfur absorption and the electrolyte infiltration. The SEM images of the 7

ACCEPTED MANUSCRIPT 10%-MnO/MPC composite under different calcination temperature are shown in Fig. 2 (c-e). It depicts that the MnO nanoparticles are distributed both on the surface and in the pores of MPC material, which may be one of the reasons why the diffraction peaks of amorphous carbon become flatter and the strength of the diffraction peaks of

RI PT

MnO have not changed significantly when the content of MnO is more than 10% (Fig. 2(c-e)). In addition, it shows that the MnO nanoparticle sizes under different

respectively.

Fig. 2.

SC

calcination temperature (500 , 600℃, 700℃) are 993 nm, 338 nm and 675 nm,

M AN U

The N2 adsorption/desorption isotherms of MPC and MnO/MPC are illustrated in Fig. 3a. The type IV isotherm shown in Fig. 3a indicate the co-existence of micropores and mesopores in MPC and MnO/MPC materials. Meanwhile, the specific surface area of MPC and MnO/MPC are 432.7 m2/g and 569.3 m2/g, respectively. It suggests that the specific surface area of 10%-MnO/MPC composite is a little larger

TE D

than that of MPC material; however, the adsorption ability of 10%-MnO/MPC composite is likely to be better than that of MPC [22]. The pore size distribution of MPC and MnO/MPC composites are shown in Fig. 3b, most of the pores are

EP

mesopores and micropores. What’s more, the meso-microporous structure can provide the physical and chemical adsorption to prevent sulfur from dissolving in the organic

AC C

electrolyte outside [24].

Fig. 3. Fig. 4.

The MnO/MPC@S composite with different contents of MnO have been prepared

and the electrochemical properties of these materials have been investigated further. Fig. 4a shows the rate performance of the MnO/MPC@S with the different content of MnO cathodes at different current densities (100 mA g –1, 200 mA g –1, 400 mA g –1, 800 mA g

–1

, 1600 mA g

–1

, 200 mA g

–1

). As can be found in Fig. 4a, the

electrochemical properties of MnO/MPC@S composites (with different contents of 8

ACCEPTED MANUSCRIPT MnO) have been investigated, and the rate performances of the MnO/MPC@S electrodes are obviously better than that of the MPC@S electrode material. When the current density transfers from 100 mA g

–1

–1

to 1600 mA g

, 10%-MnO/MPC@S

electrode material delivers the discharge capacities of about 900 mA h g

–1

and 520

of about 550 mA h g

–1

and 60 mA h g

–1

RI PT

mA h g –1, while MPC@S electrode material delivers the lowest discharge capacities , respectively, indicating the much lower

sulfur utilization of the electrode material. However, when the current density is switched back to 200 mA g –1, the 10%-MnO/MPC@S and MPC@S electrodes nearly

SC

regain its original capacity, suggesting excellent electrochemical reversibility. To a certain degree, the results can illustrate that the addition of certain amount of MnO

M AN U

can improve the utilization of active materials. The rate capability of the 10%-MnO/MPC@S electrode is clearly superior to the other MnO/MPC@S electrode materials. Fig. 4b shows the cycling performance of the MnO/MPC@S cathodes with the different content of MnO at the current density of 200 mA g–1. The initial discharger capacity of the 10%-MnO/MPC@S electrode is 723.7 mA h g

–1

with

TE D

~74% S (Fig. S3a) and retains a capacity of 515.1 mAh g –1 after 150 cycles with the capacity retention of 71.17 % (the decay rate is 0.192%/per cycle). Meanwhile, the initial discharge capacity of the MPC@S electrode is 556.3 mA h g–1, and it drops to –1

after 150 cycles with the capacity retention of 54.87 % (the decay

EP

305.2 mA h g

rate is 0.300%/per cycle). Obviously, MnO/MPC@S electrode show improved performance in specific capacity as compared to the MPC@S electrode [12]. Also, the

AC C

cycling performance of the 10%-MnO/MPC@S electrode is superior to the other MnO/MPC@S electrode materials, suggesting that the addition of certain amount of MnO can improve the utilization of active materials and the cycling stability. The cycling performance of the 10%-MnO/MPC@S and MPC@S electrodes at 1600 mA g–1 and the corresponding capacity retention are shown in Fig. 4c-d. The MnO/MPC@S composite shows an initial discharge specific capacity of 474.8 mA h g-1and retains at the capacity of 290.4 mA h g–1 after 150 cycles, corresponding to the capacity retention of 61.17%, showing that the MnO plays a crucial role in suppressing the shuttle effect and preventing the Li2S from severely accumulating 9

ACCEPTED MANUSCRIPT [14]. By contrast, the MPC@S composite display the faster capacity fading and the lower capacity retention of 31.61% after 50 cycles, which may be caused by the sulfur dissolved into the organic electrolyte and the Li2S2 and Li2S accumulated on the

Fig. 5. Fig.

5a-b

shows

the

cyclic

voltammograms

RI PT

external surface of the MPC@S electrode material.

(CV)

curves

of

the

10%-MnO/MPC@S and the MPC@S cathodes. The cathodic scan of two cathodes

SC

exhibits two reduction peaks at 2.30 V (peak 1) and 2.04 V (peak 2) (vs. Li+/Li), which can be ascribed to the multistep reduction of elemental sulfur in the presence of

M AN U

lithium ions in the ether-organic electrolyte, corresponding to the solid-liquid phase transition process of the elemental sulfur (S8) to the long chain Li2Sx (4＜x＜8) and the further liquid-solid phase reduction process of the long chain Li2Sx to the insoluble Li2S2 or Li2S, respectively [6, 25]. In the subsequent anodic scan, only one sharp oxidation peak can be observed at ~2.44 V, which is associated with the

TE D

conversion of Li2S and Li2S2 to Li2Sx and S8 [15]. The obvious variation in oxidation peaks and current between the first and second cycles are ascribed to a possible structure rearrangement to more energetically stable sites, which may result in

EP

capacity fading over the cycles [23, 25]. For the MnO/MPC@S electrode, the oxidation peak of the first cycle at ~2.44 V shifts to the lower potential region with cycling. Moreover, the CV plots of the MnO/MPC@S cathode in the subsequent three

AC C

cycles overlaps very well with each other, suggesting that the electrode has a good reversibility of the reaction and cycling stability and operates with low polarization [26]. Also, the oxidation peak of the first cycle at ~2.41V shifts to the higher potential region with cycling. The higher oxidation potential in the CV is due to the polarization caused by the phase transition from insoluble Li2S/Li2S2 to soluble Li2Sx and the electrode polarization of MPC@S is higher than that of the MnO/MPC@S electrode. In the subsequent three cycles, the CV plots of the MPC@S cathode shifts to the left a little. In addition, the strength of the oxidation peaks become weaker, that is, the current corresponding to the oxidation peak gets smaller, which may illustrate 10

ACCEPTED MANUSCRIPT that the specific capacity of the MPC@S cathode decay fast to another extent. Also, the reduction peak of MnO/MPC@S electrode increases obviously compared with that of the MPC@S electrode, which is related to the maximum accessible sulfur utilization due to the presence of MnO [23]. Moreover, the cathodic peak 1 for the

RI PT

MnO/MPC@S has a positive shift of 0.02 V (Table S1), and the anodic peak 1 negatively shifts 0.02 V compared with those for the MPC@S reservoir, indicating that the electrochemical polarization of the MnO/MPC@S has been greatly suppressed (Fig. 5e). Furthermore, in the case of the MnO/MPC@S, the increased

SC

onset voltage of 0.02 V and 0.007 V for the cathodic peaks 1 and 2 and the decreased onset voltage of 0.04 V for the anodic peak 1 (Fig. 5e and Table S2) further verify the

M AN U

better kinetics of polysulfides [3]. The accelerated kinetics of polysulfides could be ascribed to not only the strong adsorption of MnO to polysulfides, but also the catalytic effect of MnO since it can promote the decomposition of Li2S/Li2S2 [3, 22]. Hence, the multifunctional MnO can accelerate the redox process and improve the

TE D

cycling stability of the Li-S batteries.

Fig. 6.

The charge/discharger curves of 10%-MnO/MPC@S and MPC@S composite

EP

cathodes at 200 mA g–1 current density in the 1st, 50th, 100th, 150th cycles are shown in Fig. 6a and 6b. The voltage profiles of both cells exhibit similar voltage plateaus during the discharge-charge processes, which are a short upper plateau at ~2.3 V, a

AC C

long lower plateau at ~2.0 V and a broad charge plateaus at 2.2-2.4 V. The short upper plateau at ~ 2.3 V and the long lower plateau at ~2.0 V are consistent with the cathodic peak 1 and the cathodic peak 2 shown in CV curves (Fig. 5), respectively. Both the 10%-MnO/MPC@S and the MPC@S have similar carbon and carbon content; however, there are clear differences in the voltage hysteresis and principally in the length of the voltage plateaus, which are related to redox reaction kinetics and the reversibility of the system [30]. The 10%-MnO/MPC@S electrode shows a low polarization of 0.24 V at 200 mA g

–1

current density. Meanwhile, the discharge

plateau of the 10%-MnO/MPC@S electrode can not only keep better but also is 11

ACCEPTED MANUSCRIPT longer than that of the MPC@S, which suggests a kinetically efficient reaction process with good reversibility and a small barrier [14]. In contrast, the discharge potential decreases rapidly and the charge increases with higher voltage hysteresis (0.41 V) for MPC@S electrode [3, 29-30]. The smaller voltage gap between the

RI PT

charge and discharge plateaus and the longer discharge plateau may profit from the adsorption of the MnO for polysulfide ion and Li2S2/Li2S that can restrict the dissolution of the polysulfide ion into the organic electrolytes and induce the order

deposition of Li2S2/Li2S.

M AN U

Fig. 7.

SC

distribution of Li2S2/Li2S, suppressing shuttle effect and avoiding continuous

The Electrochemical Impedance Spectroscopy (EIS) curves of the MnO/MPC@S with the different content of MnO cathodes before cycles and after 150 cycles are shown in Fig. 7a and 7c. The Nyquist plots for all of the above cathodes display a semicircular loop in the high frequency. The intercepts at the real axis Z’ represent the

TE D

total resistance (Ro) of the battery, including the ionic resistance of the electrolyte, the intrinsic resistance of that electrodes materials and the contact resistance at the interface between the active material and the current collector. The semicircle in the

EP

high frequency region is corresponded to the charge-transfer resistance (Rct), reflecting the resistance of the electrochemical reaction at the electrode-electrolyte boundary [5, 27, 29]. Meanwhile, the sloped line at low frequency region represents

AC C

the Warburg impedance (Wo), corresponding to the process of the Li+ diffusion into the bulk of the electrode materials [29]. We fitted the Electrochemical Impedance Spectroscopy (EIS) curves, and the schematic diagram of equivalent circuit is displayed in Fig. 7, likewise, the fitting result is shown in Table S3. As can be observed from Table S3, the Ro and Rct of 10%-MnO/MPC@S electrode after 150 cycles are 3.379 Ω and 57.26 Ω, respectively, which is obviously smaller than the Ro and Rct of MPC@S electrodes materials (8.648 Ω and 1098 Ω respectively), indicating the energy barrier for the electrode reaction is smaller than that of the MPC@S reservoir, that is, the electrode reaction speed is increased and the redox kinetics of 12

ACCEPTED MANUSCRIPT the polysulfides enhance, and also suggesting that the diffusion of the dissolved polysulfides of the MnO/MPC@S electrode material is effectively restricted [3, 31-32]. The low Rct of MnO/MPC@S can be owed to the polar MnO which can adsorb the soluble polysulfides and inhibit the dissolution of the polysulfides in the

RI PT

organic electrolyte to some extent, leading to the lower viscosity of the electrolyte than the battery consisting of the MPC@S cathode material. Meanwhile, the “shuttle effect” gets weakened. In addition, the existence of MnO can induce the ordered distribution of Li2S2 and Li2S [3, 28], which can reduce the resistance (Fig. 7a and 7c)

SC

and demonstrate better stability of the cathode material.

As shown in Fig. 7c and table S3, the adsorptive effect of MnO with a low

M AN U

content in composite is insufficient. However, when the MnO content is too much, the excess MnO is likely to fill in the pores of MPC material, which results in the lower porosity and sulfur will distributes on the surface of MnO/MPC material, eventually leading to the considerable charge transfer resistance induced by the sulfur agglomeration of cathode, therefore the appropriate content of MnO exerts

TE D

profound impact on improving the electrochemistry performance of the composite electrode. The Ro and Rct of the 10%-MnO/MPC@S electrode material are the smallest, indicating that the 10%-MnO/MPC@S electrode material is the best one,

EP

which is in keeping with the results of the electrochemistry performance. The exchange current density (io) produced by the electrode can be obtained by Eq. (1)

AC C

[22]:

io =

RT nFRct

Eq. (1)

Where R is the gas constant (8.314 J mol–1 K–1), T is the room temperature

(298.15K), F is the Faraday’s constant (96500 C mol–1), n is the number of the electrons transferred in the electrochemical reaction which is 1 for the charge/discharge reaction of Li+ in the electrode. The exchange current density (io) of 10%-MnO/MPC@S is 4.49×10–4A cm–2, which is obviously higher than those of other samples, indicating that the 10%-MnO/MPC@S has a higher electrochemical activity. The reason can be owed to the better mutual contact of the active material / 13

ACCEPTED MANUSCRIPT electron/ lithium ion in the electrode [22]. The inclined line is attributed to the diffusion of the lithium ions into the electrode material, which is Warburg diffusion. In order to further quantify the difference of Li+ diffusion in the MnO/MPC@S electrodes with different MnO content, the diffusion coefficient of DLi + is

DLi + = 0.5(

RT )2 2 An F σC 2

RI PT

calculated according to the Eq. (2) [29]: Eq. (2)

Where A is the geometrical area of the electrode surface (1.5386 cm2), C is the

SC

molar concentration of Li+ ion (1.1×10–3 mol cm–3), σ is the Warburg coefficient, which has a relationship with the real part of EIS spectra Z re (ω=2πf), as follows Eq. (3):

M AN U

Z re = Ro + Rct + σω −1/ 2

Eq. (3)

Where Ro, Rct and ω are the ohmic resistance, the charge transfer resistance and the angle frequency, respectively. The relationships between Z re and ω-1/2 in the low frequency region of the MnO/MPC@S electrodes with different MnO content before

TE D

cycles and after 150 cycles are shown in Fig. 7b, 7d and 7e. As we can see, all of the curves show a linear characteristic in the selected region and the slope of each fitting line is σ, and the results of σ and DLi +

are listed in Table S3. The

EP

10%-MnO/MPC@S electrode shows an ion diffusion coefficient of 3.87×10–10 cm2 s-1, which is bigger than that of the MPC@S electrode (1.23×10–13 cm2 s–1), proving

AC C

the superior lithium ion diffusion mobility that is beneficial for the redox process. The increased ion diffusion coefficient can be mainly attributed to the effect of MnO for restricting the dissolution of the polysulfides that can make the electrolyte of electrode hold an excellent property [29]. Therefore, according to the above EIS analysis, thanks to the efficiently absorption of MnO in the electrode material and the orderly distribution of the Li2S2/Li2S [3]. 10%-MnO/MPC@S electrode material shows the lower charge-transfer resistance and Warburg diffusion resistance, indicating the more rapid electronic/ionic transport, which is also explained that why the 10%-MnO/MPC@S electrode material shows the better electrochemical 14

ACCEPTED MANUSCRIPT performance than other electrode materials.

Fig. 8. Digital photos of the sulfur cathodes before and after 150 cycles are shown in

RI PT

Fig. 8. As can be found, the active materials can be very well adhered to the current collector after 150 cycles, and the appearance of the pole piece has not changed much compared with that of the 10%-MnO/MPC@S electrode before cycling (Fig. 8a). On the contrary, active materials fall off from the current collector after 150 cycles for the

SC

MPC@S electrode (Fig. 8d), meanwhile, a slice of yellow in the middle of the pole piece can be found, which can be ascribed to sulfur species after cycling process (Fig.

M AN U

8d), suggesting the appearance of the pole piece shows considerable change compared to that before cycling (Fig. 8c). This result suggests that the 10%-MnO/MPC@S electrode material can maintain cell stability and is not easy to fall off from the current collector compared to the MPC@S electrode material.

TE D

Fig. 9.

The SEM images of the electrodes before cycling and after 150 cycles of MPC@S and 10%-MnO/MPC@S electrodes are displayed in Fig. 9. As observed in

EP

Fig. 9a and 9d, the two electrode materials also show the irregular surface with abundant pores. The microcosmic surfaces of the MPC@S electrode after 150 cycles can be observed in Fig. 9b and 9c. The MPC@S cathode morphology undergo

AC C

dramatic changes after 150 cycles, and the active material particles in the composite continuously coalesce into a large solid bulk and the solid surface has become greatly dense, which could be due to the separation of the active materials from the conducting carbon framework and further agglomeration into poor conductive solid [33-34]. In addition, the irregularity accumulation of Li2S and Li2S2 is the reason why the electrode surface has become greatly dense, which is the reason why the considerable charge transfer resistance increasement of MPC@S electrode during the charge/discharge process (Fig. 7a and 7c). In contrast, the morphology of the 10%-MnO/MPC@S composite does not change remarkably even after 150 cycles 15

ACCEPTED MANUSCRIPT (Fig. 9e and 9f) although the electrode surface only appears much smaller microparticles, moreover, the deposition layer of Li2S is well-distributed and relatively thin. These results suggest that 10%-MnO/MPC composite structure can provide much more deposition sites for Li2S than the MPC structure, which can make

RI PT

Li2S deposit regularly and further mediate the growth of Li2S on the electrode. Furthermore, the MnO nanoparticles adhered on the surface of MPC act as ‘anchors’ to bind the polysulfides on the surface of the electrode, which can retard the active material aggregation to a certain degree and further promote their redox kinetics and

SC

the deposition efficiency [34]. The stability of the electrode structure could be one of the major reasons for the good cyclability of the MnO/MPC@S composite cathode.

M AN U

The XRD patterns of the MPC@S and MnO/MPC@S cathodes before cycling and after 150 cycles are shown in Fig. S1. The two XRD patterns are similar to each other before cycling (Fig. S1a). By contrast, in Fig. S1b, for the MnO/MPC@S cathode, one distinct characteristic peak at 33.5°appears after 150 cycling, which corresponds to elemental sulfur [35]. In addition, for the MPC@S cathode, two

TE D

distinct characteristic peaks appear at 24.9° and 26.9° after 150 cycling, which identified as the characteristic peak of elemental sulfur and Li2S, respectively [36]. The XRD results demonstrate the existence of Li2S on the surface of the MPC@S

EP

cathode rather than the MnO/MPC@S cathode (Fig. 9 b and 9c), further indicating the existence of MnO can promote the redox kinetics of active material and the deposition efficiency of Li2S.

AC C

To further observe the strong interactions between polysulfides and the host

materials, the prepared yellow colored Li2S8 is divided equally into two and packed into the white transparent bottles. The equivalent amounts of MPC and 10%-MnO/MPC is added to bottles a and b, respectively. As shown in Fig. S2, when the MPC is added to bottles a, the solution is still yellow except the color becoming lighter than the beginning, whereas, the solution in bottle b where the 10%-MnO/MPC is added, becomes light yellow immediately and then almost completely transparent after stirred for 12 h. Besides, when the amount of MnO increases, the ability of absorbing polysulfides enhances (Fig. S2*). This interesting 16

ACCEPTED MANUSCRIPT result indicates that the 10%-MnO/MPC has a much stronger adsorption than the MPC material and could serve as a good absorbent for polysulfides, and the results is consistent with the analysis results of the SEM in Fig. 9 [25, 34]. The contents of carbon and MnO in the 10%-MnO/MPC composite were min-1 from room

RI PT

checked by the thermal gravimetric analysis at the scan rate of 10 temperature to 1000

in oxygen atmosphere and TG curve is shown in Fig. S3b. The

contents of carbon and MnO can be estimated by the mass loss of the as-prepared 10%-MnO/MPC composite. As shown in Fig. S3b, when the temperature is more than

SC

500 , there is no the mass loss because the MnO is completely oxidized to MnO2 in the oxygen atmosphere. The content of MnO in the MnO/MPC composite is 9.6433%,

M AN U

suggesting that the experiment results have little difference with the actually measured date about the MnO content in the 10%-MnO/MPC composite.

4. Conclusions

In summary, the MnO/MPC@S composite cathode is employed as a ‘multifunctional

TE D

polysulfide reservoir’ with the polar and non-polar to enhance the performance of Li-S batteries. From the visual image, SEM of the MnO/MPC@S and MPC@S electrodes before and after 150 cycles and the adsorption experiment also indicate that

EP

the MnO particles have a strong affinity to polysulfides, which is beneficial to the redox kinetics of the polysulfides conversion and improves the utilization of the active

AC C

material. Moreover, the regular deposition of the insoluble Li2S2/Li2S during the discharge process further decreases the electrochemical impedance, which can accelerate the transmission of the lithium ion and charge transfer. The electrochemical tests suggest that the 10%-MnO/MPC@S composite electrode has smaller charge transfer resistance and better electrochemical performance than other electrodes. This study has confirmed that MnO plays an important role in restraining the loss of active material and reducing the regular deposition, and the results could also inspire further studies on the interactions between other transitional metal oxides with excellent adsorption ability and catalytic effect and polysulfides. 17

ACCEPTED MANUSCRIPT

Acknowledgements This work was supported by the National Natural Science funds (21534008).

RI PT

References

[1] Z. Li, C. Li, X. Ge, J. Ma, Z. Zhang, Q. Li, C. Wang, Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur

SC

immobilizers as cathodes for high performance lithium sulfur batteries, Nano Energy, 23 (2016) 15–26.

multi-functional

effects

for

M AN U

[2] Y. Li, J. Fan, M. Zheng, Q. Dong, A novel synergistic composite with high-performance

Li–S

batteries,

Energy

&

Environmental Science, 9 (2016) 1998–2004.

[3] Z. Chang, H. Dou, B. Ding, J. Wang, Y. Wang, X. Hao, D.R. MacFarlane, Co3O4 nanoneedle arrays as a multifunctional “super-reservoir” electrode for long cycle life

TE D

Li–S batteries, Journal of Materials Chemistry A, 5 (2017) 250–275. [4] H.B. Wu, S. Wei, L. Zhang, R. Xu, H.H. Hng, X.W. Lou, Embedding Sulfur in MOF-Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries, Chemistry-A European journal, 19 (2013) 10804–10808.

EP

[5] F. Chen, J. Yang, T. Bai, B. Long, X. Zhou, Biomass waste-derived honeycomb-like nitrogen and oxygen dual-doped porous carbon for high performance

AC C

lithium-sulfur batteries, Electrochemical Acta, 192 (2016) 99–109. [6] Y. Dong, S. Liu, Z. Wang, Y. Liu, Z. Zhao, J. Qiu, Sulfur-infiltrated graphene-backboned mesoporous carbon nanosheets with a conductive polymer coating for long-life lithium-sulfur batteries, Nanoscale, 7 (2015) 7569–7573. [7] L. Sun, M. Li, Y. Jiang, W. kong, K. Jiang, J. Wang, S. Fan, Sulfur Nanocrystals Confined in Carbon Nanotube Network As a Binder-Free Electrode for High-Performance Lithium Sulfur Batteries, Nano Letters, 14 (2014) 4044−4049. [8] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable 18

ACCEPTED MANUSCRIPT Lithium Batteries, Nano Letters, 11 (2011) 4462–4467. [9] L.Q. Lu, L.J. Liu, Y. Wang, Sulfur film-coated reduced graphene oxide composite for lithium-sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 9173–9181. [11] M. Chen, X. Wang, S. Cai, Z. Ma, P. Song, A.C. Fisher, Enhancing the

RI PT

performance of lithium-sulfur batteries by anchoring polar polymers on the surface of sulfur host materials, Journal of Materials Chemistry A, 4 (2016) 16148–16156.

[12] Y. Li, R. Mi, S. Li, X. Liu, W. Ren, H. Liu, J. Mei, W. M. Lau, Sulfur-nitrogen doped multi walled carbon nanotubes composite as a cathode material for lithium

SC

sulfur batteries, International Journal of Hydrogen Energy, 39 (2014) 16073–16080.

[13] C. Jin, W. Zhang, Z. Zhuang, J. Wang, H. Huang, Y. Gan, Y. Xia, C. Liang, J.

M AN U

Zhang, X. Tao, Enhanced sulfide chemisorption using boron and oxygen dually doped multi-walled carbon nanotubes for advanced lithium-sulfur batteries, Journal of Materials Chemistry A, 5 (2017) 632–640.

[14] F. Chen, J. Yang, T. Bai, B. Long, X. Zhou, Biomass waste-derived honeycomb-like nitrogen and oxygen dual-doped porous carbon for high performance

TE D

lithium-sulfur batteries, Electrochemical Acta, 192 (2016) 99–109. [15] C. Zhang, Y. Lin, Y. Zhu, Z. zhang, J. Liu, Improved lithium-ion and electrically conductive sulfur cathode for all-solid-state lithium-sulfur batteries, RSC Advances, 7

EP

(2017) 19231–19236.

[16] G. Ma, F. Huang, Z. Wen, Q. Wang, X. Hong, J. Jin, X. Wu, Enhanced performance of lithium sulfur batteries with conductive polymer modified separators,

AC C

Journal of Materials Chemistry A, 4 (2016) 16968–16974. [17] X. Liu, J.Q. Huang, Q. Zhang, L. Mai, Nanostructured Metal Oxides and Sulfides for Lithium-Sulfur Batteries, Advanced Materials, 29 (2017) 1601759. [18] Q. Pang, D. Kundu, M. Cuisinier, L.F. Nazar, Surface-enhanced redox chemistry of polysulfides on a metallic and polar host for lithium-sulfur batteries, Nature Communications, 5 (2014) 4759. [19] H. Cheng, S. Wang, D. Tao, M. Wang, Sulfur/Co3O4 nanotube composite with high performances as cathode materials for lithium sulfur batteries, Functional Materials Letters, 7 (2014) 1450020. 19

ACCEPTED MANUSCRIPT [20] J. Guo, X. Zhang, X. Du, F. Zhang, A Mn3O4 nano-wall array-based binder-free cathode for high performance lithium-sulfur batteries, Journal of Materials Chemistry A, 5 (2017) 6447–6454. [21] T. An, D. Deng, M. Lei, Q.H. Wu, Z. Tian, M. Zheng, Q. Dong, MnO modified

Journal of Materials Chemistry A, 4 (2016) 12858-12864.

RI PT

carbon nanotubes as a sulfur host with enhanced performance in Li/S batteries,

[22] X. Qian, L. Jin, D. Zhao, X. Yang, S. Wang, X. Shen, D. Rao, S. Yao, Y. Zhou, X Xi, Ketjen Black-MnO Composite Coated Separator For High Performance

SC

Rechargeable Lithium-Sulfur Battery, Electrochemical Acta, 192 (2016) 346–356.

[23] C. Wang, W. Wan, J.T. Chen, H.H. Zhou, X.X. Zhang, L.X. Yuan, Y.H. Huang,

M AN U

Dual core-shell structured sulfur cathode composite synthesized by a one-pot route for lithium sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 1716–1723. [24] B. Cao, D. Li, B. Hou, Y. Mo, L. Yin, Y. Chen, Synthesis of Double-Shell SnO2 @C Hollow Nanospheres as Sulfur/Sulfide Cages for Lithium-Sulfur Batteries, ACS Applied Materials & Interfaces, 8 (2016) 27795−27802.

TE D

[25] L. Wang, Z. Yang, H. Nie, C. Gu, W. Hua, X. Xu, X. Chen, S. Huang, A lightweight multifunctional interlayer of sulfur-nitrogen dual-doped graphene for ultrafast, long-life lithium-sulfur batteries, Journal of Materials Chemistry A, 4 (2016)

EP

15343–15352.

[26] H. Shao, B. Huang, N. Liu, W. Wang, H. Zhang, A. Wang, F. Wang, Y. Huang, Modified separators coated with a Ca(OH)2 -carbon framework derived from crab

AC C

shells for lithium-sulfur batteries, Journal of Materials Chemistry A, 4 (2016) 16627–16634.

[27] S. Luo, M. Yao, S. Lei, P. Yan, X. Wei, X. Wang, L. Liu, Z. Niu, Freestanding reduced graphene oxide-sulfur composite films for highly stable lithium-sulfur batteries, Nanoscale, 9 (2017) 4646–4651. [28] X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, L.F. Nazar, A highly efficient polysulfide mediator for lithium-sulfur batteries, Nature Communications, 6 (2015) 5682. [29] X. Yang, Y. Yu, N. Yan, H. Zhang, X. Li, 1-D oriented cross-linking hierarchical 20

ACCEPTED MANUSCRIPT porous carbon fibers as a sulfur immobilizer for high performance lithium-sulfur batteries, Journal of Materials Chemistry A, 4 (2016) 5965–5972. [30] F. Zeng, Z. Jin, K. Yuan, S. Liu, X. Cheng, A. Wang, W. Wang, Y. Yang, High performance lithium-sulfur batteries with a permselective sulfonated acetylene black

RI PT

modified separator, Journal of Materials Chemistry A, 4 (2016) 12319–12327. [31] J. Yang, F. Chen, C. Li, T. Bai, B. Long, X Zhou. A free-standing sulfur-doped microporous carbon interlayer derived from luffa sponge for high performance lithium-sulfur batteries. Journal of Materials Chemistry A, 4 (2016) 14324–14333.

SC

[32] H.A. Salem, V.R. Chitturi, B. Ganguli, J.A. Santana, D. Gopalakrishnan, L.M.R. Arava, Stabilizing polysulfide-shuttle in a Li-S battery using transition metal carbide

M AN U

nanostructures. RSC Advances, 6 (2016) 110301–110306.

[33] Y. Zhang, Y. Zhao, A. Yermukhambetova, Z. Bakenov, P. Chen, Ternary sulfur/polyacrylonitrile/Mg0.6Ni0.4O composite cathodes for high performance lithium/sulfur batteries, Journal of Materials Chemistry A, 1 (2013) 295–301. [34] H. Yao, G. Zheng, P.C. Hsu, D. Kong, J.J. Cha, W. Li, Z.W. Seh, M.T. McDowell,

TE D

K. Yan, Z. Liang, V.K. Narasimhan, Y. Cui, Improving lithium-sulfur batteries through spatial control of sulfur species deposition on a hybrid electrode surface, Nature

AC C

EP

Communications, 5 (2014) 3943.

21

ACCEPTED MANUSCRIPT

Figure captions Fig. 1 (a) XRD patterns of different content of MnO-doped porous carbon materials. (b) XRD patterns of 10%-MnO-doped porous carbon materials under different

RI PT

calcination temperature. (c) XRD patterns of different content of MnO-doped carbon-sulfur composite and sublimed sulfur.

Fig. 2 (a-d) SEM images of the MPC materials. (c-e) SEM images of the 10%-MnO/MPC composite under different calcination temperature (c: 500

SC

600 ; e: 700 ).

; d:

Fig. 3 (a) N2 adsorption-desorption isotherms of MPC and MnO/MPC. (b) DFT

M AN U

analysis of argon adsorption isotherms giving the pore distribution of MPC and and MnO/MPC.

Fig. 4 (a) Rate performance of the MnO/MPC@S with the different content of MnO at different current densities. (b) Cycling performance of the MnO/MPC@S with the different content of MnO at 200 mA g

–1

current density. (c) Cycling performance of –1

current density at room

TE D

the MnO/MPC@S and MPC@S electrodes at 1600 mA g

temperature. (d) Capacity retention of the MnO/MPC@S and MPC@S electrodes at 1600mA g –1 current density (here, retention at capacity of the largest is 100%). Fig. 5 (a-b) Cyclic voltammograms (CV) curves of MnO/MPC@S and MPC@S

EP

electrodesin the potential window between 1.0 and 2.8 V vs. Li+/Li at a potential scanning rate of 0.1 mV s–1. (c-d) the enlarged pictures of cyclic voltammograms (CV)

AC C

curves of MnO/MPC@S and MPC@S electrodes; (e) Peak voltages of MnO/MPC@S and MPC@S electrodes (f) Onset voltages of MnO/MPC@S and MPC@S electrodes. Fig. 6 Charge/discharge voltage profiles of (a) MnO/MPC@S and (b) MPC@S composite cathodes at 200 mA g –1 current density in the 1st, 50th, 100th, 150th cycles. Fig. 7 Nyquist plots of the MnO/MPC@S with the different content of MnO and the relationship between Zre and the square root of the frequency (ω-1/2) in the low-frequency region (a-b) before cycles (Insets are equivalent circuit models), (c-e) after 150 cycles. Fig. 8 Digital photos of the sulfur cathodes before cycling and after 150 cycles, 22

ACCEPTED MANUSCRIPT respectively: (a-b) MnO/MPC@S cathode. (c-d) MPC@S cathode. Fig. 9 SEM images of MPC@S electrode before cycling (a) and after 150 cycling

AC C

EP

TE D

M AN U

SC

RI PT

(b-c); MnO/MPC@S electrode before cycling (d) and after 150 cycling (e-f).

23

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

Fig. 1

AC C

EP

TE D

Fig. 2

24

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

Fig. 3

AC C

EP

TE D

Fig. 4

25

ACCEPTED MANUSCRIPT

AC C

Fig. 6

EP

TE D

M AN U

SC

RI PT

Fig. 5

26

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

Fig. 7

AC C

EP

TE D

Fig. 8

27

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Fig. 9

28

ACCEPTED MANUSCRIPT

Highlights The biologic carbon matrix (wheat straw) is selected as the raw material for MPC

RI PT

material.

The MPC material was prepared by a simple way and low cost.

SC

The polar sites provided by MnO have a strong affinity to polysulfides.

M AN U

Shuttling effect has been alleviated and the utilization of the active material has been improved.

MnO is beneficial to the regular deposition of the insoluble Li2S2/Li2S during the

AC C

EP

TE D

discharge process.