Scholarly article on topic 'Advanced chemical strategies for lithium–sulfur batteries: A review'

Advanced chemical strategies for lithium–sulfur batteries: A review Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Xiaojing Fan, Wenwei Sun, Fancheng Meng, Aiming Xing, Jiehua Liu

Abstract Lithium–sulfur (LiS) battery has been considered as one of the most promising rechargeable batteries among various energy storage devices owing to the attractive ultrahigh theoretical capacity and low cost. However, the performance of LiS batteries is still far from theoretical prediction because of the inherent insulation of sulfur, shuttling of soluble polysulfides, swelling of cathode volume and the formation of lithium dendrites. Significant efforts have been made to trap polysulfides via physical strategies using carbon based materials, but the interactions between polysulfides and carbon are so weak that the device performance is limited. Chemical strategies provide the relatively complemented routes for improving the batteries' electrochemical properties by introducing strong interactions between functional groups and lithium polysulfides. Therefore, this review mainly discusses the recent advances in chemical absorption for improving the performance of LiS batteries by introducing functional groups (oxygen, nitrogen, and boron, etc.) and chemical additives (metal, polymers, etc.) to the carbon structures, and how these foreign guests immobilize the dissolved polysulfides.

Academic research paper on topic "Advanced chemical strategies for lithium–sulfur batteries: A review"

Accepted Manuscript

Advanced chemical strategies for lithium-sulfur batteries: A review Xiaojing Fan, Wenwei Sun, Fancheng Meng, Aiming Xing, Jiehua Liu

PII: S2468-0257(17)30113-9

DOI: 10.1016/j.gee.2017.08.002

Reference: GEE 81

To appear in: Green Energy and Environment

Received Date: 26 June 2017 Revised Date: 19 August 2017 Accepted Date: 19 August 2017

Please cite this article as: X. Fan, W. Sun, F. Meng, A. Xing, J. Liu, Advanced chemical strategies for lithium-sulfur batteries: A review, Green Energy & Environment (2017), doi: 10.1016/j.gee.2017.08.002.

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Graphical Abstract

Advanced chemical strategies for lithium-sulfur batteries: A review

Xiaojing Fan, Wenwei Sun, Fancheng Meng, Aiming Xing and Jiehua Liu *

Future Energy Laboratory, School of Materials Science and Engineering, Hefei University of Technology, Tunxi Road No.193, Hefei, Anhui, 230009, China.

[*] Corresponding author. E-mail: liujh@hfut.edu.cn (J. H. Liu)

Abstract

Lithium-sulfur (Li-S) battery has been considered as one of the most promising rechargeable batteries among various energy storage devices owing to the attractive ultrahigh theoretical capacity and low cost. However, the performance of Li-S batteries is still far from theoretical prediction because of the inherent insulation of sulfur, shuttling of soluble polysulfides, swelling of cathode volume and the formation of lithium dendrites. Significant efforts have been made to trap polysulfides via physical strategies using carbon based materials, but the interactions between polysulfides and carbon are so weak that the device performance is limited. Chemical strategies provide the relatively complemented routes for improving the batteries' electrochemical properties by introducing strong interactions between functional groups and lithium polysulfides. Therefore, this review mainly discusses the recent advances in chemical absorption for improving the performance of Li-S batteries by introducing functional groups (oxygen, nitrogen, and boron, etc.) and chemical additives (metal, polymers, etc.) to the carbon structures, and how these foreign guests immobilize the dissolved polysulfides.

Keywords: lithium-sulfur batteries; chemical reaction; lithium polysulfides; functional groups; additives

1. Introduction

The ever-increasing environment pollution and the decreasing fossil fuels force people to develop renewable energy, and it is wise to store and release the spare energy in certain forms whenever needed. Thus, the highly efficient energy storage system has attracted extensive interest in recent years, and various applications have been found in mobile devices, electric vehicles, and sustainable energy industry. Due to their light weight, high open circuit voltage, high capacity, and non-memory effect, lithium-ion batteries have been commercialized since 1990s by Sony corporation and dominated the market for portable electronic devices [1]. Although the performance of their cathode and anode materials has achieved the theoretical limits after twenty years of development, it still cannot meet the requirement of energy output. Therefore, it is urgent to find new cathode and anode materials with higher energy density and excellent electrochemical performances.

Sulfur is one of the basic elements of the earth, and it is obtained extensively from nature. Moreover, sulfur is abundant, nontoxic, and environmentally benign, possessing great superiority compared with the limited and pollution-yielding fuel oil. More importantly, its ultrahigh theoretical capacity (1675 mAh g-1) and theoretical energy density (2600 Wh kg-1) distinguish it from other cathode materials [2-7]. Thus, the area of lithium-sulfur (Li-S) batteries developed robustly in the past few decades, and substantial achievement has been realized [8-12]. However, there are still some scientific and technical problems hindering Li-S batteries to be commercialized.

(I) Both sulfur and lithium sulfides are intrinsically insulated, which impedes the transportation of electrons and ions.

(II) The volumes of cathode and anode materials change greatly during cycling process, bringing about the collapse of electrode structures.

(III) Lithium sulfides, as the ultimate discharge products, are indissolvable in the electrolyte, and mostly deposit on the surface of the conductive framework.

(IV) The intermediate discharge products lithium polysulfides (Li2Sn, 4< n <8) are soluble in the organic electrolyte, that will result in the loss of active materials and energy storage.

Among them, the dissolved lithium polysulfides will further diffuse to the electrolyte and form a membrane on the surface of the anode, leading to the crazing of the solid electrolyte interface film (SEI). The shuttle phenomenon gives rise to an irreversible loss of active materials, rapid capacity fading, low Coulombic efficiency, and short cycle longevity. Researchers have tried various approaches to solve this problem by virtues of delicately designed nanocarbon frameworks, such as porous carbon matrix, and so on [13-20]. They hope to take advantage of unique porous structure to prevent the dissolution, diffusion, and shuttling of polysulfides by physical encapsulation. For example, Zhang et al. designed the nested pore structure carbon with an ordered distribution of micropores and mesopores, which ensured an adequate accommodation for polysulfides to diffuse and reside evenly, and cycling performance of the device was greatly improved [16]. Park and his co-workers synthesized the honeycomb-like well-organized porous carbon nanosheets to trap lithium polysulfides [18]. Nevertheless, these physical pathways failed to immobilize

polysulfides efficiently, and only physical encapsulation was not able to realize the practical application of Li-S batteries.

Different from the physical encapsulation, chemical adsorption displays a great potential in immobilizing polysulfides. Various chemical bonding approaches have been employed, in which, the functional groups and additives are introduced into carbon matrix to capture polysulfide species and prevent shuttle effect [21-26]. In this review, we show the recent advances in regard to chemical interactions between polysulfides and the functional groups or additives in Li-S batteries. Firstly, we introduce the electrochemical transportation fundamentals of Li-S cells, and then various types of functional groups and additives that utilize chemical interactions to anchor polysulfides by different sulfur hosts were discussed and analyzed. Finally, we summarize and present the challenges and prospect of Li-S batteries.

2. Fundamental of Li-S batteries charge/discharge

The conventional Li-S batteries are constituted by a cathode of sulfur, an anode of lithium metal, and an indispensable ether-based electrolyte. In principle, the sulfur existing as ring-like octatomic molecules (S8) will be reduced to Li2S as the final discharge product, and oxidized to sulfur reversibly when the battery was charged. The whole reaction can be represented as S8 + 16 Li = 8 Li2S. However, the actual discharge and charge processes are exceedingly complex, accompanied with many multiple side reactions simultaneously [27-32].

A schematic illustration describing the working mechanism of Li-S batteries during the cycling test and a typical galvanostatic charge/discharge profile is shown in Fig. 1. The discharge process has two or three reduction stages depending on the composition of electrolyte [30, 33, 34]. The first stage, a rapid dynamics reaction, takes up about a

quarter of the profile, corresponding to the reduction of S8 to Li2S4 at about 2.4 V [32, 35]. The resulted lithium polysulfides will dissolve and diffuse into the organic electrolyte. With the discharge process going on, these high-order polysulfides will be reduced to low-order polysulfides (Li2Sn, 2< n < 4) and Li2S. The second stage accounts for another quarter of the whole discharge process with a total discharge capacity of 1316 mAh g-1 and a plateau voltage of less than 2.1 V. However, the last stage in the discharge profile matching with the further reduction of Li2S2 to Li2S, is not exhibited in the cyclic voltammetry (CV) curve, which is attributed to the sloped shape and the voltage difference between the previous two stages.

• * • ^Charge»',,

« Li*

0 200 400 600 800 1000 1200 1400 1600 1800 Capacity) mAfi/g)

Fig. 1. (a) A schematic illustration of the redox reactions in Li-S batteries. (b) Galvanostatic charge/discharge profiles and the typical chemicals in each stage.

The dissolution and diffusion of lithium polysulfides have a serious influence on the electrochemical performance of Li-S batteries due to the following reasons [3639]. (i) Causing the loss of active materials during cycling tests. A critical issue with respect to the polysulfides is their dissolution in organic electrolytes and the incomplete conversion of sulfur to Li2S. Moreover, the dissolved lithium polysulfide species can move toward lithium anode through the separation membrane and react with lithium metal, causing the loss of active materials. (ii) Bringing in the shuttle

effect. Owing to the concentration gradient, the polysulfide ions diffuse from the cathode to anode easily, and the high-order polysulfide species are reduced to low-order polysulfide species on the lithium surface. The reverse process occurs when the battery was charged. This phenomenon happening during the discharge and charge processes was named shuttle effect. The final discharge product deposited on the surface of lithium metal is Li2S, which is insoluble, insulated and will increase the impedance of batteries. Therefore, the polysulfide species generated during the discharge process are harmful to Li-S cells, and it is imperative to control the solubility of the polysulfide species. In the following context, various chemical methods developed in the literature to tackle this problem will be summarized and discussed.

3. Arresting lithium polysulfides on chemical methods

Many efforts have been attempted to reduce the dissolution and diffusion of polysulfides through physical confinement methods, such as using porous host materials with high surface areas. Some tried to improve the batteries' assemblage module, electrolyte composition or membrane structure for the sake of better performance [8, 40-47]. Nevertheless, the effect of these physical methods is limited and still far from actual requirement. As an alternative, the chemical methods developed rely on the strong interaction between the functional groups or additional adsorbents in cathode materials and lithium polysulfide species [21, 22, 48-72]. By virtues of this design, Li-S batteries have achieved great success in improving cycling stability. In this part, we summarize the types of chemical interactions based on different functional groups and additional adsorbents as shown in scheme 1, analyze the fundamental principles of chemical absorption. The prospect and challenges in the future will also be discussed.

^ Q- ^ strategies

Polymer: binder,

carbon source, coating separator, coating sulfur cathodes, etc.

Scheme 1. The schematic illustration of chemical strategies.

3.1 Functional groups absorbing lithium polysulfides

Different nanostructured carbons with high surface areas and high porosity are fabricated to prevent polysulfides from dissolving and diffusing into electrolyte. However, utilizing chemical bonding is more effective to arrest lithium polysulfides than simply physical restriction. Recently, many carbon based substrates were used in the cathode of Li-S cells, and additional contributions were realized through introducing functional groups into the carbon framework, such as O, N, B, S, etc. [50, 56-60]. Further characterization confirmed that the introduction of heteroatoms greatly improved the cycle performance of Li-S batteries.

3.1.1 Oxygen-containing functional groups

Graphene oxide (GO) is extensively studied in the field of Li-S batteries due to its superior mechanical flexibility, excellent chemical stability, ultra-high surface area and substantial surface oxygen-containing groups (mainly hydroxyl and epoxide groups). A chemical bonding approach anchoring sulfur atoms via these functional

groups has been employed in the latest research. The chemical method reduces the diffusion of polysulfides into the organic electrolyte and improves the batteries'

cycling stability [48-50, 73-75]. Cheng and his co-workers calculated the binding

_ 2—

energies of hydroxyl and epoxide groups-carbon with polysulfides (S3 /S3 ) by the density functional theory (DFT), which was 1.09/1.95 eV and 0.84/1.81 eV respectively, higher than 0.78 eV of pristine graphene [21]. Zhang et al. adopted a convenient chemical reaction-deposition method to produce GO-sulfur (GO-S) sheets [50]. The sulfur reduced the >C=O bonds and sulfur-oxygen (S-O) bonds are formed. At the same time, sulfur-carbon (S-C) bonds appeared due to the fact that the epoxy and hydroxyl groups were replaced by sulfur. These chemical interactions between sulfur and the functional groups on GO sheet surface were confirmed by Ab initio calculations. As demonstrated in Fig. 2, the C K-edge X-ray absorption spectroscopy (XAS) measurement was performed, and the results showed that the GO-S composite contains a great amount of S-O and S-C bonds. Further electrochemical results revealed that the Li-S cells assembled by the composites had a high reversible capacity of 950-1400 mAh g—1.

2S0 2$5 290 296 300 305 310

Photon energy (eV)

Fig. 2. (a) Representative pattern of GO anchoring S. Yellow, red, and white balls denote S, O, and H atoms, respectively. Note that the C atoms bonding to S and O are highlighted as blue ball in the GO sheet. (b) C K-edge XAS spectra of GO and GO-S composites after heat treatment in air at 155 °C for 12 h. Reprinted from Ref. [50]

A facile method using Li2S-GO composites was developed subsequently in Cui's group by wrapping around Li2S with GO sheets [48]. The results demonstrated that the lithium-oxygen interaction was beneficial to reduce the dissolution of polysulfides during reaction process. The >C=O---Li(SnLi), >O---Li(SnLi), and Li2Sn---HO- bonds may exist between GO and the polysulfide species. To better understand the electronic structure and chemical bonding of GO-S composites, Guo et al. adopted spectroscopy techniques to further investigate [49]. The Near-edge X-ray Absorption Fine Structure (NEXAFS) was a strong evidence to confirm the existence of these chemical bonds (Fig. 3). The C K-edge NEXAFS spectra showed that the n* resonance of the GO-S composites was much sharper and stronger than pure GO,

revealing a better recovery of sp hybridized carbon. Some peaks were not easily observed in GO-S composites. However, a broad feature at about 287.5 eV was attributed to the convolution of peaks from C-H and C-S bonds. In O K-edge NEXAFS spectra, the features at 531.1 and 533.8 eV were derived from the n* state of the C=O bonds in the carboxyl group and C-O bonds in the epoxide, respectively. However, the feature lied on 573.0 eV only appears at GO-S composites, indicating the existence of S-O bond. Thus, they concluded that oxygen-containing functional groups can improve greatly the cycle stability of Li-S batteries.

Fig. 3. NEXAFS spectra of C K-edge (a) and O K-edge (b) of GO and GO-S composites. Reprinted from Ref. [49]

3.1.2 Nitrogen-functionalized groups

Nitrogen doping is another effective method to improve the electrochemical performance of Li-S batteries by chemically adsorbing polysulfides. Recently, many studies found that introducing nitrogen atoms into carbon matrix can form LiSnLi+---N bonds with polysulfides during the cycling process [52-54, 76-80]. The nitrogen-doped carbon (NDC) was fabricated through post-treatment of carbons with nitrogen

sources. The nitrogen displayed in three main peaks, pyridinic N, pyrrolic N and quaternary N, with the bonding energy between 398 and 404 eV in the high-resolution XPS spectra (Fig. 4) [55]. Studies showed that the pyrrolic N and pyridinic N atoms are more effective to form LiSnLi+---N bonds than the quaternary N atoms.

Fig. 4. XPS spectrum showing the three main peaks of different nitrogen forms in the nitrogen doped carbon. Reprinted from Ref. [55]

The enthalpy changes (AH) of -56.88 kcal mol-1 is smaller than bare carbon of -41.92 kcal mol-1 between sulfur and N-doped carbon by DFT method, revealing that nitrogen dopant can tightly trap polysulfides [81]. To better understand the interaction between nitrogen and polysulfides, Zheng et al. adopted a facile approach with a SiO2 template to synthesize N-doped hollow porous carbon bowls (N-HPCB), N-doped hollow porous carbon spheres (N-HPCS) and N-doped hollow carbon spheres (N-HCS), and with hollow carbon spheres (HCS) as comparison [54]. Fig. 5a exhibited the schematic illustration of N-doped structure for improving the performance of (N-HPCB/S), and proved that the sulfur immobilization on NDC mainly relied on the chemical binding between the Li ion in Li2Sn and the strong electron-donating lone-

404 402 400 398 396

Binding Energy(eV)

pair electrons in pyrrolic N and pyridinic N atoms. The strong bonding between N and Li cation effectively prevented the diffusion of polysulfides into the electrolyte. The contents of nitrogen in N-HCS, N-HPCS, N-HPCB were 4.0, 4.1, and 5.1 at%, respectively. With the increase of nitrogen content, the cycle performance of Li-S batteries was greatly improved. The discharge capacity of N-HPCB/S was higher than that of N-HPCS/S, N-HCS/S and HCS/S at the discharge rate of 0.2 C (1 C=1675 mA g-1) (Fig. 5b). At 1 C, the capacity of N-HPCB/S can reserve about 706 mAh g-1 after 400 cycles, corresponding to capacity retention of around 79% and exhibited a capacity decay of 0.053% per cycle (Fig. 5c). All results indicated N functional groups had a strong adsorption to polysulfides by interaction between the N and Li cations.

Physical adsorption

Discharge

(b) 1200

Surface areas f

\ N contant t ! Contact araaf J VolumeJ

Discharge

S/N-HPCB

Physical adsorption ♦ Chemical binding Strongly boi

pyrrolic N

A A S/HCS T V S/N-HCS • o S/N-HCPS ■ oS/N-HCPB

—■-1—»—T

TO çÇ)'

1 f 8" ^

1C 1...........>

<—1 □ Discharge ■ Charge S/N-HPCB

i—1—i—

10 20 30 40 50 Cycle number

100 150

200 250 Cycle Number

300 350

Fig. 5. (a) Schematic illustrations of N-HPCB/S for adsorbing polysulfur and HCS/S. (b) Cycling performances of the HCS/S, N-HCS/S, N-HCPS/S and N-HCPB/S at 0.2

C. (c) cycling performances and Coulombic efficiency of N-HCPB/S at 1C. Reprinted from Ref. [54]

3.1.3 Boron-doped functional groups

Compared with nitrogen, boron is another promising doping atom with a positive polarity and has an effective chemical adsorption to intermediate discharge products. Moreover, the conductivity of boron doped graphene is higher than that of undoped graphene. These virtues facilitate the comprehensive utilization of boron doping in the field of Li-S batteries.

For example, Han et al. synthesized a 3D boron doped graphene aerogel (BGA) via a one-step hydrothermal method and investigated the effect of boron dopant (5.7 at%) in the composites [56]. They discovered that the BGA/S had a higher cyclic stability and a better rate capability compared with nitrogen-doped graphene aerogel (NGA/S) and bare graphene aerogel (GA/S). The BGA/S possessed a high capacity of 1290 mAh g 1 at 0.2 C, and remained at 994 mAh g 1 even after 100 cycles, while only 572 mAh g-1 retained for NGA/S. The performance of GA/S was even worse. To explain the effect of boron doping, the B 1s peak of XPS spectrum of BGA and the S 2p XPS spectrum of BGA/S, GA/S, and NGA/S were measured. In Fig. 6a, the B 1s peak revealed that the boron atoms had doped into graphene. The two peaks exhibited in the spectrum at 192.6 and 191.9 eV were originated from the presence of -BC2O, -BCO2 and -BC3 bonds, respectively. The bonding energy of BGA/S was higher than those of NGA/S and GA/S, revealing the effective chemical interaction between sulfur and BGA [56]. Therefore, the BGA/S can validly anchor polysulfides during discharge/charge process due to the introduction of boron dopants.

-BGA-S

-NGA-S

200 198 196 194 192 190 188 186 184 167 Binding Energy / eV

Binding Energy (eV)

Fig. 6. (a) The B 1s peak of XPS spectrum of BGA and (b) the S 2p XPS spectrum of BGA/S, GA/S, and NGA/S. Reprinted from Ref. [56]

3.1.4 Double atoms co-doped functional groups

As mentioned above, the oxygen, nitrogen, boron, etc., single atom dopant can greatly enhance the electrochemical performance of Li-S batteries, while the synergistic effect of multiple heteroatoms co-doped in carbon matrixes also arrested extensive attention recently, such as nitrogen and sulfur, nitrogen and boron, nitrogen and phosphorous, and so on [58, 59, 82-85]. Multiple heteroatom co-doped carbons had even stronger adsorption abilities to lithium polysulfides than nondoped and single atom doped counterparts. Zhang and his co-workers utilized the hydrothermal reaction route to prepare the porous nitrogen and phosphorus co-doped graphene (p-NP-G) [59]. The P atoms and -P-O groups in the p-NP-G layer were proved to exert a stronger adsorption to polysulfides than the N dopant. Manthiram et al. prepared a 3D nitrogen and sulfur co-doped graphene sponge electrodes, which could accommodate a mass of dissolved lithium polysulfides without using aluminum foil collectors [58]. The unique sponge structure enhanced the amount of immobilized active materials to 4.6 mg cm- .

To investigate whether the bonding energy between dual-doped N, S and polysulfides became stronger than that of single N or S doped, the density functional theory (DFT) calculations were performed by Manthiram group [58]. The adsorption configurations of LiSH were demonstrated in Fig. 7. They investigated the bonding of LiSH to N, S dual-doped graphene, pristine graphene, nitrogen single doped graphene, and sulfur single doped graphene. As shown in Fig. 7a, the binding energies of Eb are 1.82, 2.06, and 1.10 eV between Li of LiSH and N, S-doped graphene for pyridinic N, pyrrolic N, and uncombined N respectively. The results are obviously higher than those of pristine graphene (0.78 eV), S-doped (1.02 eV and 0.52 eV), and N-doped graphene (1.29 eV/pyridinic N, 1.43 eV/pyrrolic and 0.28 eV/uncombined with N). Therefore, the DFT calculations confirmed that nitrogen and sulfur co-doping could enhance the interaction between graphene and polysulfides. The C 1s, N 1s and S 2p XPS spectra of the N, S dual-doped graphene were measured to verify the bonding of polysulfides with nitrogen and sulfur atoms. The C 1s spectra of N, S dual-doped graphene exhibited one strong peak and three weak peaks. The results indicated the peak intensity of oxygen-containing groups decreased and the sp2 carbon increased remarkably due to the fact that C-N and C-S bonds formed during discharge process (Fig. 7b). The N 1s XPS spectrum of the N, S dual-doped graphene was given in Fig. 7c. The three peaks correspond to pyridinic N, pyrrolic N, and quaternary N, respectively. The S-S/S-C bonds and S-O bond at 163.7 and 164.9 eV, 164.7 and 165.9 eV, respectively, displayed in the S 2p spectrum in Fig. 7d. The nitrogen and sulfur co-doped carbons significantly improved the cycling stability by hindering the diffusion of polysulfides in Li-S cells, owing to the synergetic interaction of double atoms.

(ii) „ do,

29« 292 290 288 286 264 282 Btfx*r>g energy (eV)

'•**/ 1 »-»VC ff\ 1 so I I S2p *ar s-*e~c I SAk oryvM '

402 400 398 Bmftng energy (eV)

Fig. 7. Schematic illustration of the binding of LiSH for (ai) pristine graphene, (aii-iv) S-doped graphene, (av-vii) N-doped graphene and (aviii-x) N, S dual-doped graphene. Grey, white, blue, yellow, purple and red balls represent C, H, N, S, Li and O atoms, respectively. (b) C 1s XPS spectrum, (c) N 1s XPS spectrum and (c) S 2p XPS spectrum (d) of the N, S dual-doped graphene. Reprinted from Ref. [58]

Therefore, one can see that different functional groups in Li-S batteries play different roles in improving the devices' electrochemical performance. The differences of various functional groups in some representative materials are listed in Table 1.

Dopant Material atom

Method

Main bonds

Doping Initial content capacity/rate (at.%) (mAh g-1/C)

Retain Ref

capacity/rate

(mAh g-1/Cycles)

O Graphene oxide Chemical reaction C=O, O-C=O,

deposition S-C, S-O

1320/0.02

954/more than 50 [50]

strategy

N N-doped hollow porous carbon bowls SiO2 template method LiSnLi+—N 5.1 1192/0.2 1020/50 [54]

B 3D boron doped graphene aerogel One-step hydrothermal method BC2O, BCO2, BC3 1.7 1290/0.2 994/100 [56]

N, S 3D nitrogen and sulfur co-doped graphene sponge Freeze-drying and hydrothermal reaction S-S, S-C, S-O, C-N N: 5.4 S: 3.9 1200/0.2 822/100 [58]

N, P Nitrogen and phosphorus co-doped porous graphene Thermal annealing and hydrothermal reaction route C-P, P-O N: 4.38 P: 1.93 1158.3/1 638/500 [59]

N, B Graphene-supported nitrogen and boron co-doped carbon layer A two-step annealing procedure N = B/N-B, C-N N: 5.3 B: 2.9 829/0.5C 627/300 [83]

Table 1 The performance of various functional groups

3.2 Metal species additives trapping lithium polysulfides

The dissolved polysulfides can be captured by doped heteroatoms in carbon matrix as discussed above, alternatively, another effective method of immobilizing sulfur is introducing metal species additives in the cathode, such as metal oxides like MnO, MnÜ2, Ti4O7, TiO2, Fe2O3, ITO and metal compounds like MXene, TiC, MSx, M(OH)x etc. [60-70, 86-103]. Studies showed that these metal species additives effectively improved the cycling stability and rate performance of Li-S batteries, and attracted extensive attention in the scientific community. The strong bonding between

metal-based materials and polysulfides prevented the dissolution and diffusion of lithium polysulfides.

3.2.1 Metal oxides

Metal oxides had a strong absorption with polysulfides by chemical bonding, and effectively enhanced the utilization of sulfur. Nazar group analyzed the absorptivity of nine different materials to the polysulfide species, including nonpolar materials

(carbons such as Super P and FW200) and polar materials (metal oxides such as Ti4O7

and MnO2) [95]. They found that the affinity of polar materials to Sn was stronger than nonpolar materials, as was demonstrated by the adsorption points of polysulfides per 10 mg active materials (Fig. 8). Results showed that the polar metal oxides can absorb sulfur species due to their intrinsic hydrophilic surfaces and with respect to the control species, the absorptivity of MnO2 was the strongest.

„_„ ©

O) 75 5 -

o > 4-

f, E o cs

w o 3 .

T3 < U) E

CO 2 -

, ■ , ■ ,

0.135 — m • 1000

|0.050 | |0.018j 750 8 "2 o 500^

Current! p o o o O o IS» ' (iv) X I | 250 B n

0 10 20 30 40 50

Time (h)

Fig. 8. Histogram of the absorptivity of different materials to Sn - based on per 10 mg active materials. Reprinted from Ref. [95]

Subsequently, Chen et al. rationally designed hollow sulfur sphere nanocomposites

decorated by MnO2 nanosheets, which effectively improved the electrochemical

stability of Li-S batteries [60]. The hollow spheres with substantial interspaces could

accommodate the volume change of sulfur during charge/discharge process and restrict the dissolution of LixSn. The decoration of MnO2 nanosheets significantly hindered the dissolution and diffusion of polysulfides. The unique architecture of hollow S-MnO2 composites, combining with the synergistic immobilization effect of chemical bonding, had a high absorption rate of ions and electrons. Fig. 9a showed the bonding of MnO2 to sulfur and polysulfide species, and DFT was performed to analyze the S-MnO2 composites that how the MnO2 nanosheets trapped polysulfides through forming chemical bonds. In the first discharge state, the ring-like octatomic molecules (S8) anchored in MnO2 and the Eb was -1.60 eV. With the progress of lithiation, the Li2S8 was formed and the Eb reached to -4.68 eV, corresponding to the formation of strong chemical bonds of S=O and Li-O. Then, the bonding energy was in the scope of -3.86 to -5.15 eV when lithiation went on. Thus, the bonding energy of Sn species with MnO2 molecules was stronger compared with S8 due to the existence of S=O and Li-O bonds. The electrochemical performance was measured to verify the characteristic stability. The composites exhibited a low capacity of 1043 mAh g-1 at the initial discharge process at 0.2 C, while at the second discharge step, the capacity reached to 1196 mAh g-1 and retained a value of 1072 mAh g-1 after 200 cycles. Moreover, even after 1500 cycles at 0.5 C, the capacity still retained at 644 mAh g-1 (Fig. 9c). The nanocomposites introduced to MnO2 molecules exhibited an outstanding cycling stability with high Coulombic efficiency, significantly improved the electrochemical performance of Li-S batteries.

• о

Fig. 9. (a) The geometries of interactions between the MnO2 and sulfur species. (b) The binding energy of six different sulfur species with MnO2 as lithiation evolution. (c) The long life cycling performance of the S-MnO2 composite electrodes at 0.5 C for 1500 cycles. Reprinted from Ref. [60].

Additionally, a few other metal oxide composites have been reported to suppress the diffusion of polysulfides effectively. For example, Cui and his coworkers discovered conductive Magneli phase Ti4O7, which could form strong chemical bonds of Ti-S and S-O with sulfur species, and higher reversible capacity was obtained [61]. Cui et al. also designed rationally tin-doped indium oxide glassy carbon hybrid electrode to provide more polysulfides deposition sites and tightly seize these diffused polysulfide species [25]. Indeed, these metal oxides additives successfully bonded with polysulfides by forming strong chemical interactions and/or surface-bound active redox mediators.

3.2.2 Two-dimensional MXene phases materials

The MXene phases are the transition metal carbide, nitride or carbonitride, and are prepared by selective etching the element A from Mn+1AXn phase using hydrofluoric acid, and the product was called delaminated MXene phases. The most classical Mn+1AXn phases are Ti3AlC2, the Al-containing layers are easily exfoliated due to the relatively weak bonding energy in Ti3AlC2. The delaminated MXene phases possess a high conductivity and highly active two-dimensional surfaces with -O, -F and -OH groups bonding with the terminal Ti.

Nazar group utilized the same method to obtain the Ti2C by corroding Al atom from Ti2AlC [96]. Compared to above-mentioned MXene phases, the difference was that the final product d-Ti2C 2D nanosheets. They found that the unoccupied orbitals of Ti atoms in the surface of Ti2C nanosheets were bound with either -OH or sulfides after exfoliation and delamination treatment (Fig. 10a). Lu et al. revealed there is the stronger interaction of MXene with Li2Sn on the surface by DFT method [104]. Thus, the MXene phase Ti2C nanosheets exhibited an outstanding ability to immobilize polysulfide species that would enhance the devices' cycling stability and Coulombic efficiency. The d-Ti2C containing 70% sulfur (d-Ti2C/S70) showed a discharge capacity of 1090 mAh g_1 at the initial stage at 0.5 C, and even at 1 C, the discharge capacity surprisingly maintained at 1000 mAh g_1. Moreover, after 650 cycles, the remaining capacity was still as high as 723 mAh g_1. Compared with other carbon-sulfur composites for immobilizing sulfur species, the 2D MXene phases Ti2C nanosheets were less porous and owned a lower specific surface area. But they revealed unexpected superior properties as a sulfur host material, indicating the chemical bonding with polysulfides successfully suppressed the diffusion and dissolution of polysulfides.

The XPS analysis had given the evidence to prove the surface interaction and bonding information. The Ti 2p XPS spectrum appeared two main peaks in pristine d-Ti2C at 455.9 and 459.4 eV, which was originated from Ti-C and Ti-O bonds, respectively (Fig. 10bi). Nevertheless, the d-Ti2C/S70 composite revealed an additional peak at 457.6 eV except for the above two peaks, that was attributed to the existence of S-Ti-C bond (Fig. 10bii). Accordingly, the elemental sulfur of S 2p spectrum of the Ti2C/S70 also showed an extra peak at 162.5 eV, corresponding to the bonding energy of S-Ti-C bond as shown in Fig. 10cii. This implied the -OH functional groups were replaced by sulfur species. Thus, the above analysis proved that a strong chemical interaction was formed between polysulfides and the Ti2C nanosheets, which greatly enhanced the electrochemical stability of Ti2C/S composite. In addition, Goodenough and his coworkers designed mesoporous TiN material, utilizing a solidsolid phase separation method [62].

Mesoporous TiN with hydrophilic Ti-O groups on the surface offered a polar surface for absorbing polysulfides strongly, and the formation of N-S bond effectively trapped the intermediate discharge products. Therefore, the interfacemediated reduction of the lithium polysulfides exhibited an obvious importance in hindering the diffusion of polysulfides and the exhausting of active materials. Park et al. also employed the TiN to trap polysulfides, and results showed the long-chain polysulfides were broken into short-chain polysulfides [97]. These relatively short-chain polysulfides further anchored on the TiN surface and stayed at the cathode side, prevented the shuttle effect from the fundamental. Research results confirmed that the transition metal carbide and nitride were promising sulfur host materials to solve the problem of shuttle effect.

Diffusion • , Discharge ■ Discharge

470 465 460 455 450 Binding energy (eV)

168 164 160

Binding energy (eV)

Fig. 10. (a) Geometry optimization of the interaction between Ti atoms on the surface of Ti2C and sulfur species. (b) The Ti 2p XPS spectrum and (c) the S 2p XPS spectrum of the Ti2C nanosheets and S/Ti2C composite. Reprinted from Ref. [96]

3.2.3 Metal sulfides

Metal oxides have drawn intensive research interest in the field of Li-S batteries due to their strong interactions with lithium polysulfide intermediates. However, the severe shuttle phenomenon is not simply attributed to the toilless diffusion of polysulfides, the slow redox reaction rate is another significant factor that is formerly neglected. Thus, metal sulfides were developed due to their high efficiency in

immobilizing sulfur species not only by a chemical absorption but also by enhancing the kinetics of redox reaction.

A structure was designed utilizing tungsten disulfide (WS2) supported on a carbon cloth interlayer to trap polysulfides by Goodenough group.[105] And Zhang et al. successfully synthesized cobalt disulfide (CoS2) using a hydrothermal method [63]. The CoS2 had sufficient electron transportation routes and high electrical conductivity that enhanced the electrochemical activity of polysulfides during conversion reaction. The DFT results exhibited the energy of CoS2 combining with Li2S4 was up to 1.97 eV, indicating CoS2 can stabilize polysulfides on carbon. They found the graphene/CoS2 composite containing 75wt % sulfur remarkably improved the discharge capacity and reduced the capacity decay, compared to the pure graphene/sulfur. The existence of CoS2 also accelerated the charge transfer at the interface of polysulfide/CoS2.

In order to testify the superiority of metal sulfides to metal oxides, Qian group investigated the distinction between SnO2 and SnS2 in improving the electrochemical performance of Li-S batteries [64]. The introduction of SnS2 alleviated the polarization of batteries, enhanced the devices' Coulombic efficiency. The SnS2 not only offered a strong interaction with lithium polysulfides but also provided activation sites for expediting the redox reaction of sulfur species. For the optimized framework, the Li atoms interacted with the S atoms or O atoms at the terminals of SnS2 and SnO2, respectively (Fig. 11a). DFT calculations showed the bonding energies in regard to SnS2 and SnO2 were 1.26 and 3.25 eV, respectively. It is reasonable to conclude that the length of Li-S bond is shorter than that of Li-O bond. Thus, an appropriate interaction with polysulfide species was more important for sulfur host materials,

which will not destroy the polysulfides during discharge and charge processes.

Compared with SnS2, the interaction force arose from SnO2 was too intensive and the polysulfides structure was likely to be ruined. The sulfur anchored by SnS2 and SnO2 in porous carbon composites (SnS2/S/C and SnO2/S/C) were further assembled to semi-cells, and their differences were shown in Fig. 11b. The peak intensity of SnS2/S/C was stronger than that of SnO2/S/C, indicating the superior electrochemical activity. Compared with the SnO2/S/C, the capacity of SnS2/S/C was much higher and the cycling stability was better at the discharge rate of 0.5 C (Fig. 16c). After 50 cycles, the capacity of SnS2/S/C materials remained at 875 Am h g-1 while SnO2/S/C declined to 545 mAh g-1. The high charge transfer rate was attributed to the incorporated SnS2, which accelerated the redox reaction of polysulfides. Combining with the synergetic effect from chemical absorptivity, the undesired deposition of polysulfides on lithium anode was greatly declined and the devices' electrochemical performance was boosted. This work offered a new insight into the distinction of metal oxides and metal sulfides used in Li-S batteries, took a big step forward on the practical application of Li-S batteries.

-Sn02/S/C

-SnS7/S/C

1.8 2.0 2.2 2.4 2.6 2.8 Potontlal (V.ve LI/LI4)

Eb = 3.25 eV

Li2S4-SnS2(001)

Eb= 1.26 eV

1400 : sno^s/c

.1200 -♦- SnSj/S/C

. 800 4

' 600 ' 400

Discharge/Charge of SnO,/S/C

200 ---- Discharge/Charge of SnSj/SJC

I 60 *

0 10 20 30 40 60

Cycle number

Fig. 11 (a) Geometry optimization and bonding energy of the Li2S4 molecule on (001) plane of SnO2 and SnS2. (b) CV curve of SnS2/S/C and SnO2/S/C composites. (c) Cycling performance of SnS2/S/C and SnO2/S/C composites at 0.5 C for 50 cycles (first 5 cycles are measured at 0.2 C). Reprinted from Ref. [64]

3.2.4 Metal hydroxides

Recently, thin-layered metal hydroxides are extensively studied as a new kind of metal additives for trapping polysulfides. For example, Tu et al. rationally designed S/C composites coated by Co(OH)2 through a facile hydrothermal method, which effectively inhibited the shuttle effect of polysulfide species [65].

Besides, Lou and his co-workers fabricated a wonderful dodecahedral geometry of double-shelled nanocages, in which cobalt hydroxides were produced in internal and layered double hydroxides (LDH) existed at external (Fig. 12a) [67]. Thus, the double-shelled Co(OH)2/LDH (CH/LDH) nanocages with unique double-shell structures further optimized the virtues of hollow nanostructures to encapsulate a mass of sulfur. In addition, they also offered a large functional surface containing a number of hydrophilic and hydroxy groups for chemically interacting with sulfur species to hinder their outward diffusion. The CH/LDH/S nanocages composite greatly improved the electrochemical performance of batteries compared with common porous carbon/sulfur composites (PC/S) (Fig. 12b). The PC/S and CH/LDH/S composites had a similar initial discharge capacity, which indicated that CH/LDH/S composites could not distinctly enhance the performance of batteries at first several cycles. However, in the subsequent cycles, the CH/LDH/S composites with the sulfur loading of 75% revealed a superior cycling stability compared with PC/S composition the same condition. In brief, the CH/LDH materials significantly

improved the electrochemical properties of Li-S batteries, due to that they could intensify the redox reaction kinetics of polysulfides and the existence of polar surfaces with hydrophilic groups which strongly interact with sulfur species. Therefore, it is safe to conclude that the metal hydroxides play an important role in the Li-S battery system and will initiate a brand-new field for boosting the batteries' electrochemical properties.

ZIF-67

Single-shelled ZIF-670LDH

Double-shelled CH<6>LDH

^1500 o> ^ 1200 1

§■ 600

| 300 & A

LDH/S 0.1C

S loading: 3 mg cm'2 ............ C/S 0 .1 C LDH/S 0.5C

80 ; 35

Cycle number

Fig. 12. (a) Schematic illustrations of the synthesizing process of the CH@LDH/S composites. (b) Cycle performance of the CH/LDH/S and C/S composites. Reprinted from Ref. [67]

3.2.5 Metal chlorides

Apart from the above-mentioned metal species additives, metal chlorides were also reported to be able to trap lithium polysulfides with high efficiency. As is well-known, the lithium polysulfides are easily dissolved into electrolyte during the redox reaction process. A lot of polysulfides in the electrolyte will increase its electrolytic viscosity, resulting in a lower ionic conductivity. The impediment of pores in the membrane and the harmful reaction with lithium foil do not produce electrical energy. Metal chlorides are then attractive due to the fact that they could strengthen the polarity of

carbon-based materials and facilitated a strong bonding with polysulfides to reduce their dissolution.

Lu et al. adopted a dexterous and handy synthetic method that could produce a nonconductive chloride intensified carbon nanofiber (CNF), which enormously declined the loss of sulfur [68]. They selected CaCl2, InCl3, and MgCl2 because they significantly changed the color of Li2S8 solutions when the polysulfides were absorbed by these chlorides. DFT calculations were further carried out to determine the binding energy, and the results for CaCl2, InCl3, and MgCl2 were 3.37 eV, 1.17 eV, and 0.394 eV, respectively. The schematic illustrations showed the positions on CaCl2, InCl2, and MgCl2 for immobilizing polysulfides were demonstrated in Fig. 13 a. Due to the inherent polarity of chlorides, the chlorine atom dopant greatly strengthened the bonding energy with sulfur species. And the bonding energy of CaCl2 was more than twice of those for InCl3 and MgCl2. However, too strong bonding energy, usually more than 2.0 eV, might cause that the polysulfide species could not be oxidized back to elemental sulfur during charge process and stay on the cathode side, which will further lead to substantial loss of active materials. Therefore, an appropriate interaction between lithium polysulfides and chlorides was more preferable. For example, Fig. 13b exhibited the electrochemical performance of CNF coated CaCl2, InCl3, or MgCl2 and bare carbon nanofiber at 0.2 C. The specific capacity of InCl3-coated CNF was significantly enhanced compared to that of bare CNF. Moreover, they found that the specific capacity of InCl3-coated CNF was similar to that of CaCl2-coated CNF, but the Coulombic efficiency of the former was higher than the latter. These were attributed to the differences in energy bonds, absorbability and diffusion factors of the two materials. The InCl3-coated CNF was the most

appropriate chlorides in this report because it provided more positions to restrict the

sulfur species on the surface of carbon nanofiber that further facilitated the redox reaction. At the current density of 0.2 C, the capacity of InCl3-coated CNF still reserved at 891 mAh g-1 after 650 cycles (Fig. 13c). These results revealed that a proper bonding was beneficial, and one needs to balance the absorption of metal chlorides to lithium polysulfides and the diffusion of polysulfides before choosing appropriate materials.

lnCI3 MgCI2

InClj on CNF A CaCI2 on CNF MgCI2 on CNF O Bare CNF

-1-1-1-1—

50 100 150 200 Cycle number

! 400 2

« 200

100 200 300 400 500 600 Cycle number

Fig. 13. (a) Schematic representations showing CaCl2, InCl3 and MgCl2 absorb Li2S. (b) Cycling performance of CNF coated CaCl2, InCh, or MgCl2 and bare CNF at 0.2 C. (c) Long-life cycling performance of InCl3-coated CNF containing sulfur of 4 mg

cm 2 at 0.2 C. Reprinted from Ref. [68]

3.2.6 Metal organic framework

As we know, metal organic frameworks (MOFs) have been less focused in the past years in the field of Li-S batteries, due to their inferior conductivity compared with carbon-based materials. But recently, MOFs have made rapid progress as sulfur host materials because they possess high surface areas and a large number of cavities. Moreover, MOFs can also immobilize lithium polysulfides through chemical bonding

by virtues of the Lewis acid-base interaction or functional groups attached. The abundant oxygen-containing groups on the surface of MOFs furnish additional absorption of polysulfides through chemical bonding.

Xiao et al. demonstrated that the nickel organic frameworks (Ni-MOF), Ni6(BTB)4(BP)3, could significantly hinder the diffusion of polysulfides from the cathode [70]. In the Ni-MOF, the interconnected mesoporous and micropores properties of Ni-MOF offered a nice carbon structure to trap polysulfides. And more importantly, the Lewis acidic Ni(II) center tended to contract with the soluble polysulfides so that the Ni-MOF could effectively impede the soluble sulfur species to diffuse to a node, which considerably slowed down the diffusion of polysulfides toward the outside of pores. According to Ab initio calculations, they discovered the terminal S atom of Li2Sn could combine with the Ni atom of Ni-MOF to form a new bond, which could prevent various polysulfides from diffusing and dissolving into the electrolyte. Therefore, the Ni-MOF/S composite exhibited a remarkable cycling stability due to the synergistic effect of porosity and chemical bonding. Fig. 14a, b

showed the geometry of two different types of pores of Ni-MOF matrix and Lewis

acidic Ni(II) center bonding with a soluble Sn anion, respectively. The lower capacity decay of 11% was gained after 100 cycles at 0.1 C. Rate performances were measured that had superior electrochemical performance, while traditional Li-ion batteries uncovered worse cycling properties at lower rate originating from the lengthy time of migration for polysulfides from the cathode to anode. The poor conductive materials, metal organic frameworks, are applied in the field of Li-S batteries and provide potential ways to exploit new soluble sulfur species stabilizers or electrolyte additives to realize a harmonized system.

Metal species additives opened up a new path for optimizing the properties of Li-S batteries and occupied a certain position because of their unique characteristics. The performance of various metal species additives is listed in Table 2

Fig. 14. (a) The geometry of two different types of pores of Ni-MOF structure. (b) Cycling performance of Ni-MOF/S composites at various current rates. The inset scheme shows the paddle-wheel unit in Ni-MOF bonding with polysulfides. Reproduced from Ref. [70]

Table 2 The performance of various metal species additives

Metal Material Content Main Bonding Initial Retain Ref

species of bonds energy capacity/rate capacity/rate

additives sulfur (eV) (mAh g-1/C) (mAh

(%) g-1/Cycles)

Metal Hollow S-MnO2 75.5 S=O, Li- 3.86 ~ 1043/0.2 1072/200 [60]"

oxides composites O. 5.15 (MnO2)

Metal d-Ti2C 2D nanosheets 70 S-Ti-C, — 1090/0.5 723/650 [96]

carbides Ti-O,

(Ti2C) Ti-C

Metal SnS2/S/porous carbon 78 Li-S 1.26 About 750/300 [64]

sulfides composites

(SnS2)

Metal Double-shelled 75

hydroxides Co(OH)2/ layered (Co(OH)2) double hydroxides

nanocages Metal Nonconductive

chlorides chloridereinforced (InCl3) carbon nanofiber/sulfur

composites

MOF(Ni) Nickel-based MOF 82

-2 cm 2

Ni-S, Ni-N, Ni-O

1250/0.2

1014/0.1 653/100

1.169 1088/0.2 891/650

9/0.1 611/100

3.3 Polymer immobilizing sulfur species

Polymers carry a mass of advantages in immobilizing sulfur species, and they are widely applied in Li-S batteries recently. Polymers are usually used as binders, carbon sources or additives to coat sulfur cathodes and membranes, that can remarkably enhance the electrochemical reversibility of Li-S batteries [71, 72, 106-117]. For one thing, polymers enhance the binding strength and mechanical properties of Li-S battery cathodes and mitigate the consequence of volume changes and materials crush. For another, polymers possessing abundant functional groups (like hydroxyl and carboxylic groups) and distinct chain-like configuration can effectively hinder these lithium polysulfides dissolved into electrolyte via the chemical interaction. Therefore, many polymers have been employed to enhance the electrochemical properties of LiS batteries.

Zhang et al. utilized gum arabic (GA), an inexpensive and environmental benign

polymer from Acacia Senegal, as a binder to trap the polysulfides during battery

reactions [71]. Compared with polyvinylidene fluoride (PVDF), GA successfully transformed the application field of the binder from the organic binder to an aqueous binder, replaced the using of toxic organic solvents in batteries. They compared the difference of gelatin, PVDF, and GA in electrochemical reversibility, and results showed that the GA binder was more efficient than the other two materials in restraining the shuttle phenomenon of polysulfides and elevating the retention of capacity. GA has luxuriant functional groups that strongly connected with sulfur species as confirmed by the electrochemical and spectroscopic results. The mixture of GA and polysulfides was analyzed via Fourier-transform infrared spectrometry (FTIR) spectroscopy, which verified that the presence of C-S bonds by the wavenumber peak at 600 cm-1 (Fig. 15).

(a) _ieoo

D) <1200

¡3 800 w a m

Fig. 15. (a) Cycling performance of S/GA, S/PVDF, S/gelatin electrodes. (b) FTIR spectroscopy of the mixture of GA and polysulfides (Li2Sx, x = 8). (c) The chemical structure of GA. Reproduced from Ref. [71]

The main reason of Capacity decay in Li-S batteries is the shuttle effect due to the dissolution and diffusion of the high-order polysulfide species in the electrolyte.

Recently, Huang and his coworkers snipped the long-chain polysulfides to

undissolved short-chain by dithiothreitol [115]. Archer et al. also employed polyacrylonitrile (PAN) to react with elemental sulfur via a simple thermal synthesis method, and obtained a special type of sulfur/PAN (SPAN) composites that adopted unusual bonds of nitrile groups on the PAN framework [72]. In particular, elemental sulfur only existed as S3 or S2 during all the redox reaction procedures, which was arrested by PAN through a covalent bond and could not generate dissolved high-order lithium polysulfides upon reduction with Li+. During the discharge process, the only product Li2S was discovered, the SPAN solved the issue of easy dissolution and shuttle effect of polysulfides. Unlike the discharge platforms in traditional Li-S cells, the SPAN had only one stage. They found the SPAN-based cathode had a more stable cycling performance in a carbonate-based electrolyte of ethylene carbonate/diethyl carbonate (EC/DEC) than that in ether-based electrolytes. Therefore, the SPAN realized the high utilization rate of active materials, free of shuttling phenomenon between sulfur cathodes and lithium anodes and outstanding stability during long time cycling test (Fig. 16). This type of synthetic materials that polymers attached with sulfur through covalent bonds had a prominent effectivity in trapping polysulfides and eliminating polysulfides dissolution, offered a new insight into the field of Li-S batteries.

Fig. 16. (a) Cycling performance and Coulombic efficiencies of heat-treated SPAN at 0.4 C. The inset scheme shows the lithiation process of SPAN. (b) The contrast of SPAN using 1 M LiPF6 in EC/DEC electrolyte and 1 M LiTFSI in DOL/DME with LiNO3. (c) The C 1s XPS spectra of SPAN after heat-treated at 450°C. Reproduced from Ref. [72]

Different polymers play different roles in Li-S batteries, such as using as a carbon source, binder, coatings on sulfur cathode and membrane, etc. Here, we summarize the functions of various polymers in Table 3 and we found that polyacrylonitrile as carbon source exhibited the superhigh discharge capacity and cycling stability, which is the desired host sulfur material.

Table 3 Different functions of polymers in Li-S batteries

Polymer material Function Electrolyte Main Initial Retention Ref.

bonds capacity/rate capacity/rate

(mAh g-1/C) (mAh g-1/C)

Gum arabic Binder DOL/DME S-S, C-S 1386/0.2 1090/50 [71]

Polyacrylonitrile Carbon source EC/DEC C-S, C- About Over 1000/1000 [72]

Polyaniline Nanofibers/multiwall carbon nanotubes Polyvinylpyrrolidone

A sulfur-rich

copolymer@CNT

Polydopamine

Polydopamine, poly-(acrylic acid)

Coating DOL/DME —

separator

1400/0.4V 1020/0.2

709/100

1018/0.2

Coating sulfur DOL/DME — cathodes

Sulfur source DOL/DME C-S 898/1

Nitrogen DOL/DME —

1113/0.2

Coating sulfur DOL/DME Covalent 715/ 1 A g-1 nanosheets bond

and binders

790/300

880/100

980/100

640/500

DOL: 1,3-dioxolane; DMC: dimethyl carbonate; EC: ethylene carbonate (EC); DEC: diethyl carbonate

4 Conclusions and outlook

We can summarize this review as the following: i) Carbon materials are excellent candidates for sulfur storing in Li-S batteries due to their relatively high conductivity and high efficiency in immobilizing polysulfides upon cycling. ii) Different sulfur immobilizers have their own virtues and weaknesses. It is wise to balance these in mind before assembling the energy storage system. iii) Chemical absorption approaches shed a light on Li-S cells with excellent electrochemical properties and long cycle term, which will also help to accelerate the progress of business development of Li-S cells. Although there are still many technical difficulties in this field, it is no doubt that Li-S battery is one of the most competitive energy storage systems in the future. And we believe that with the development of science and technology, substantial breakthroughs will be made in the above-mentioned directions that will eventually lead to the practical and wide applications of the Li-S cells.

Although remarkable progress has been acquired for enhancing the electrochemical performance of Li-S batteries, several existing challenges must be immediately addressed because they impede the wide applications of Li-S batteries. Obviously, the diffusion and dissolution of sulfur species severely affect the properties of cells, like reversible capacity, cycling stability, electrochemical impedance and so on. As the theme of this paper, chemical interaction is a significant approach to solve the dissolution of polysulfides, such as introducing heteroatoms, metal species additives into carbon structure, and sulfur functionalized polymer electrode and membrane. Each approach has its own virtues and defects in immobilizing polysulfides, and there are still disputes on which way is the most effective for cycling longevity and high capacity retention of Li-S batteries.

For single atom doping in carbon structure, it enhances the conductivity of carbon materials and greatly increases the bonding strength between the sulfur host materials and lithium polysulfides. As a result, the capacity decay is reduced and the diffusion of soluble polysulfides into the electrolyte or toward the lithium anode is prevented. But, the appropriate atom doping content is extremely significant in improving the effect of trapping polysulfides and increasing the effective utilization of sulfur. Recently, the double atoms co-doped structure, such as nitrogen and sulfur, greatly attracted researchers' interest. The synergistic effect of double-atom dopant can remarkably optimize the uniform distribution of electrons from sulfur host materials, resulting in much stronger chemical bonding with dissolved polysulfides than that in non-doped or single atom doped situation. Thereby, atom doping will be one of the promising approaches to enhance the cycling life and reversible capacity of Li-S batteries in coming commercial application.

Last but not least, metal species additives have an amazing bonding ability to inhibit the dissolution of polysulfides. Studies indicate that the metal species additives not only form a strong chemical absorption with polysulfides but also enhance the kinetics of redox reaction. Polymers based additives or carbon sources possess abundant functional groups in the resulting carbon structure (like hydroxyl and carboxylic groups) and distinct chain-like configuration, which can effectively trap the discharge products via chemical bonds, and thus promote the utilization efficiency of active materials.

However, there are some weaknesses in reported chemical strategies. For example, dopant foreign atoms can significantly enhance cycling performance, but too much dopant will weaken their effects. The doped amount also should be controlled. Metal species additives have the poor conductivity in general, and some of them own the extra stronger adsorption with polysulfides, which also reduce the capacity. Polymers are employed in Li-S batteries, which maybe introduce the undesired cases including cost and real lifetime. Thus, the doped foreign-atom amount, synthesized routes, and micro-architectures are all crucial to improve the electrochemistry performance of LiS batteries.

Chemical strategies effectively inhibit the diffusion of polysulfides compared with simple physical strategies. And the virtues of chemical strategies are listed in Table 4. We think a possible route could be used to improve the performance of Li-S cells by the synergistic effect of chemical and physical strategies.

Table 4 The comparison between the physical and chemical strategies

Strategies Trapping polysulfides Interacting with Restraining Electrochemist

methods Li2Sn shuttle effect ry

performance

Physical strategies Porous, coating, etc. Van der Waals bond Bad effects Low capacity,

serious decay

Chemical strategies Functional groups, Covalent bond,

Better effects High capacity,

metal species additives, metallic bond,

stable cycling

polymer, etc.

ionic bond

In the past decades, researchers have obtained a great achievement in suppressing shuttle effect of polysulfides. But, this system is extreme sensitivity and complexity to kinds of experimental environments and parameters. In the future, the methods and protocols of research will be optimized and become more and more mature for the electrolyte, separator and anode as well as cathode. The high loaded sulfur is also welcomed with open arms in commercialization.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21303038), Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2016004), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and One Hundred Talents Program of Anhui Province.

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