Designing Realizable and Scalable Techniques for Practical Lithium

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Designing Realizable and Scalable Techniques for Practical Lithium Sulfur Batteries: A Perspective Yusheng Ye, Feng Wu, Sainan Xu, Wei Qu, Li Li, and Renjie Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03165 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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The Journal of Physical Chemistry Letters

Designing Realizable and Scalable Techniques for Practical Lithium Sulfur Batteries: A Perspective Yusheng Ye,† Feng Wu†, ‡ Sainan Xu,† Wei Qu,† Li Li,

†, ‡

Renjie Chen,*,†,

‡

† Beijing Key Laboratory of Environmental Science and Engineering, School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China.

‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

ABSTRACT

To progress from the coin lithium sulfur (Li–S) cell to practical applications, it would be necessary to investigate industrially scalable methods to produce high quality and large quantities Li–S configurations. In this perspective, we focused on the feasibility of scalable production of high quality and large quantities cathode composite, the construction of high safety and high stable electrolyte, and durable lithium metal 1 ACS Paragon Plus Environment

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anode. The results presented here suggest that the construction of highly secondary microstructure from nanoparticles is key solution to achieve scalable cathode composite. Developing unconventional electrolyte solvent is a meaningful approach to develop high safety Li–S batteries. The high-performance and high-stability of lithium metal anode would enlighten the practical application of Li–S batteries. This perspective presents outlooks for the key scalable techniques of realizable Li–S cell in the near future, and provides promising strategies to accomplish long-cycle-life, high energy density Li-S batteries.

TOC GRAPHICS

KEYWORDS: Lithium sulfur, Practical, Realizable and scalable techniques, Electrolyte, Lithium metal anode

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1. Introduction Developing renewable and sustainable energy technologies are critical to address increasing global energy consumption and climate change concerns. Since the use of graphite as anode, lithium-ion batteries (LIBs) permit great success to replace the highly polluted and nonrenewable fossil fuels.1 Large demands of portable electronics, electric vehicles and microelectronic devices also promote the gradually flourishing of LIBs. However, the inherent limitations of Li-ion reaction mechanism make this type of batteries cannot meet the pursuit of light-weight and high-energy-density energy storage devices. The goal of increasing the share of electric vehicles on the roads, however, calls for energy storage devices that embrace lower cost and higher energy density as well as longer cycling life. Without ends, scientists and engineers would never stop seeking higher and better substitute to make a better choice. Lithium metal batteries, such as lithium–sulfur (Li–S) batteries, are satisfied candidates because of their attractively high energy densities (2600 W h kg−1).2-3 Coupling with high-capacity sulfur as positive electrodes and lithium metal as negative electrodes, Li–S batteries would be more efficient and cost-effective. In addition, earth-abundance, low mass and environmental-friendly impact are also flash points that make them possible to achieve the goal of low-cost and longer mileage/endurance electric vehicles. However, even after more than 10 years as new favorites of scientist, Li-S batteries still have a thing against large-scale production. Industrial circles still do not have enough confidence to get involved in Li–S batteries. Because today’s pouch Li–S 3 ACS Paragon Plus Environment


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batteries suffer from lots of bottle-necks, such as poor cycling life, poor rate performance and safety issues.4 Understanding their working mechanism would help us find more appropriate solution to solve these problems. Sulfur is a relatively variable element with a wide variety of oxidation states. The multi-electrons Li–S chemistry holds complicated multi-step redox reactions between lithium and sulfur based on a solid−liquid−solid transition:

S80  4e   S24

S04  4e   2S2-  S22-

Figure 1. Key techniques and their integration for industrial Li-S batteries.


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Advanced spectroscopy and electrochemical investigation have offered important fundamental insights to Li–S chemistry. The process of Li−S batteries can be summarized as a 2-plateau charge/discharge voltage profile in ether-based electrolytes. During the discharging process, S8 first opens its ring and is lithiated to form long chain lithium polysulfide intermediates (Li2Sx, 4≤x≤8). This is corresponding to the first discharging voltage plateau (≈2.3 V) with 25% theoretical capacity of sulfur (418 mAh g-1). Long chain lithium polysulfide intermediates are generally soluble in liquid etherbased organic electrolyte. Subsequently, long chain polysulfides further lithiated to insoluble short-chain lithium sulfides, which is corresponding to the second voltage plateau (≈2.1 V) with 75% theoretical capacity (1255 mAh g-1). Soluble polysulfide intermediates continuous shuttle and diffuse through separator, reaching and reacting with lithium metal anode to form unstable solid electrolyte interface (SEI). As a result, some of them are consumed or remaining dissolved in electrolyte to a certain extent. All these sulfur species do not re-precipitate back to cathode at the end of discharge, causing large capacity loss and decay.

Actually, effective techniques and materials in experimental coin cells usually show disillusionary results for large-scale production and their use in practical Li-S cells.5 We emphasize that the designs of realizable and scalable techniques for cathode, electrolyte and lithium metal anode may be insurmountable processes of practical process for Li–S batteries. As shown in Figure 1, the industrial process of Li–S batteries needs to be given to the integration of a wide range of batch-ready configurations. Among them, cathode, anode and electrolyte are most important three 5 ACS Paragon Plus Environment


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challenges. It should be pointed out that the optimized sulfur secondary particles could help to maintain structural integrity, inhibit volume shrinkage during slurry coating and drying. Liquid-electrolyte-based and solid-state-based Li-S batteries are two mainstream in near practical Li-S batteries. Based on the advanced anode developments, the safety hazards and lithium dendrites formation can be crippled. The integration of these techniques can make more improvements of practical Li–S batteries.

2. Cathode is a solution, but far from satisfactorily

To overcome the electronic limitation of elemental sulfur and lithium sulfides, and the dissolution of lithium polysulfides in the electrolyte, nanoscale approaches have germinated to develop efficient host for sulfur cathode.6 A power strategy is to load sulfur in conductive host materials to improve cathode conductivity as well as confine lithium polysulfide species within the host. A variety of carbons, metal oxides and conductive polymers are thus dedicated to extract the entire capacity of sulfur cathodes as close as possible.

7-8

Although some good results have been achieved at the

laboratory level, it is time to consider their feasibility and to meet the urgent demands of large-scale production. To compare with state-of-art lithium ion batteries, a typical areal specific capacity of sulfur cathode higher than 4 mAh cm-2 would be more competitive.9 Sion power proposed a Li–S prototype cells with 350 Wh kg-1, which maintained at 250 Wh kg-1 after 80 cycles. Take an average of 300 Wh kg-1, assume the average practical capacity of sulfur cathode is 1000 mAh g-1 and the ratio of electrolyte to sulfur is about 3, the sulfur content of all cathode (except aluminum foil) should be 6 ACS Paragon Plus Environment

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higher than 70 wt%. In a word, the sulfur content of cathode composite should be higher than 80 wt% normally. At this level, most sulfur will cover on conductive host surface. Yang and his co-workers have proposed a series of multi-core-shell sulfur composite with high sulfur content up to 91 wt%.10-12 Zhang and his co-workers have also prepared a cauliflower-like hierarchical porous C/S cathode with 75 wt% sulfur content, which are thought to facilitate the lithium-ions and electrons transport.13 It is clear that both at these two endpoints of sulfur content range, sulfur almost completely covered on carbon host. As discussed before, the sulfur species dissolution is essential but also has a negative impact on Li-S battery performance. After charge and cycle, the dissolved polysulfides break the original shape re-precipitate on the surface of the electrode, and this effect alters the morphology of the cathode in each cycle.

2.1 Sulfur loading approach

Table 1. Advantages and disadvantages of different sulfur loading approaches

Advantages

disadvantages

Sulfur melting

Large-scale

Agglomeration, large followup work, low cathode porosity,

Sulfur vapor infusion

Wide range of applications

Energy consuming

Sulfur dissolutionrecrystallization

Accurate control, energy-preserving

Toxic, large environmental impact, human hazards

Physical mixing

Large-scale, simple, Highly depends on the fast character of conductive hosts

Solution reaction

Large-scale, economic

Hydrogen sulfide exhaust gas treatment



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Different sulfur loading approaches greatly affect sulfur morphology in sulfurbased composite. There are five main sulfur loading approaches: sulfur melting method, sulfur dissolution and recrystallization, sulfur vapor infusion, physical mixing method and solution reaction method. Table 1 compares the advantages and disadvantages of different sulfur loading approaches. High temperature melting method is usually operating at 155 oC, where the viscosity of sulfur is lowest. After sulfur melted and incorporated with conductive host, the composite suffers from a solidification step. Agitation and coiling rate are two key parameters that worthy of attention by this method. Moreover, agglomeration phenomenon appears frequently, especially at high sulfur content. A lot of follow-up work need to be involved to get high-quality cathode composites, such as ball milling and sieving. However, the low melting temperature of sulfur put forward more stringent conditions in ball-milling process. Furthermore, the obtained sulfur cathodes by this way have poor porosity. In my view, the porosity

of sulfur cathode is very important because it affect the electrolyte infiltration. Only the elemental sulfur converted to lithium polysulfide can the capacity be released more completely in liquid-electrolyte-based Li–S batteries. Sulfur vapor infusion is an impressive method by vaporizing sulfur at elevated temperature in vacuum and then intercalating them into the voids of different scale host materials. Extensive conductive materials can be used as host for sulfur inserting by regulating temperature. Essentially, the uptake of elemental sulfur can be efficient controlled. The downside is that this method is an energy consuming process.


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Table 2. The solubility of sulfur in some organic solvents

solvent

Solubility (g/100g Saturated solution)

solvent

Solubility (g/100g Saturated solution)

Carbon disulfide

29.50

Chloroform

1.22

Acetone

2.65

Pentachloroethane

1.2

Benzene

2.07

Vinyl chloride

0.84

Toluene

2.02

Carbon tetrachloride

0.83

Trichloroethylene

1.63

Anhydrous ether

0.28

Tetrachloroethylene

1.53

Anhydrous ethanol

0.05

Dichloroethylene

1.28

Anhydrous methanol 0.03

Tetrachloroethane

1.23

Sulfur dissolution-recrystallization method is another approach to load sulfur into conductive host. This method can be achieved under room temperature and ambient pressure and do not need any complex procedures. Sulfur has poor solubility in conventional organic solvents. Table 2 compares the solubility of sulfur in some organic solvents. Carbon disulfide has good solubility of sulfur, which is an order of magnitude higher than other solvents. Unfortunately, it is highly toxic and may cause large human hazards. Even some relatively low-sulfur-solubility solvents that with lower degrees of toxicity also have potential risks to human health. Physical mixing method is a simple, energy-preserving method to prepare sulfur-based mixture. However, due to the weak chemical interaction between sulfur and host, this method is highly depending on the character of conductive hosts. By comparison, solution reaction method is a low-environment-impact and economic method to prepare sulfur9 ACS Paragon Plus Environment


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based composites. The only concern is the hydrogen sulfide tail gas treatment. Today, there are lots of solutions to deal with hydrogen sulfide, which ensures the operability and feasibility of this method. We can easily control the sulfur content by simply adjust the amount, concentration and adding speed of reactant. More importantly, the obtained sulfur morphology is fluffy and porous, which is benefit for electrolyte infiltration.

Figure 2. (a) The one-pot synthesis step for the production of GSC. Reproduced with permission from ref 21. Copyright 2012 Royal Society of Chemistry. (b) Fabrication of monodisperse polymer-encapsulated S nano-spheres. Reproduced with permission from ref 22. Copyright 2013 Proceedings of the National Academy of Sciences of the United States of America. (c) Schematic illustration and SEM images of micro-nano-structured CS@CTAB/FC composite fabrication. Reproduced with permission from ref 11. Copyright 2017 Royal Society of Chemistry. (d) Core–shell graphene-encapsulated sulfur (GES) composites as superior cathode materials for lithium–sulfur batteries. Reproduced with permission from ref 23. Copyright 2013 Royal Society of Chemistry.

To meet the natural abundance and low environmental impact of lithium-sulfur cells, complex synthesis methods, contaminated process and expensive materials 10 ACS Paragon Plus Environment

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should be avoided in their design to facilitate their large-scale production and assure that they have a price advantage over conventional lithium-ion batteries. Although

various host materials and flexible polymer coating layers have been intensively investigated to trap the polysulfides within the cathode area, it is would be really very different when these sulfur composite were engaged in pouch Li-S batteries.14 One reason is that the fabrication of high sulfur loading cathode without sacrificing specific capacity is a necessary step.15-20 An important breakthrough in sulfur cathode development was reported by Nazar and her co-workers. They prepared a highly scalable graphene–sulfur composite with a sulfur loading content of 87 wt.%.21 In this report, micron sized sulfur particles were enveloped by reduced graphene oxide, which not only can form a highly conductive network around the sulfur particles but also can efficient trap the polysulfides through favorable hydrophilic–hydrophilic interactions. More importantly, they proposed an essential synthesis method by combining a mixture of conductive host and soluble polysulfide (Na2S~2.4), following by aiding HCl oxidant. By this way, the overall gravimetric capacity of Li–S pouch batteries based on this cathode are expected greatly increased. Polymeric films are also investigated to avoid the transportation of Li2Sx moieties to the anode side and to maintain the stability of cathode. Based on solution reaction method, Yang and his co-workers proposed a series of multi-core-shell structured composite cathode material with polyaniline polymer networks.10-11 The inner conductive polyaniline that grafted to the acetylene black particle can hold sulfur particles, while the outer polyaniline coating can relieve the volume expansion of the 11 ACS Paragon Plus Environment


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cathode and hold sulfur particles, both of which can restrain the diffusion of lithium polysulfides. Another solution reaction synthesis method based on sodium persulfate as sulfur source was reported to fabricate monodisperse polymer (polyvinylpyrrolidone)encapsulated hollow sulfur nanospheres. Similarly, this approach is suitable for room temperature operation, and scalable production.22 Uniform core–shell structure graphene-encapsulated sulfur (GES) composites based on a solution-chemical reaction–deposition method was also synthesized.23 The synergistic effects of highly conductive graphene and core–shell structures are thought can facilitate good transport of electrons, preserve fast transport of lithium ions and alleviate the polysulfide shuttle problem. Graphene here can also greatly improve the rate performance of Li-S batteries. The high sulfur content of 83.3 wt.% substantially increases the overall gravimetric specific capacity of cathode with respect to the total mass of pouch Li-S batteries.

2.2 Structural integrity

As mentioned before, high sulfur loading is very important for designing highenergy-density Li–S batteries. Slurry coating and constructing 3D current collector sulfur-based cathode are two main methods to achieve high sulfur loading cathode. 3D carbon foam, graphene foam and carbon fiber cloth are common 3D current collector sulfur-based cathode.24 Market penetration of the coin cell of 3D current collector sulfur-based cathode from laboratory research to industrialization requires consider several issues: (1) The current output effect of large-scale 3D current collector is limited 12 ACS Paragon Plus Environment

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so far; (2) The relative high uptake of liquid electrolyte by 3D current collectors is usually larger than 2D coating cathode, which significantly affect the energy density of Li–S batteries. (3) The low tap density of 3D current collector further decreases the volumetric capacity of Li–S capacity. (4) Complex technical process arises the cost of Li–S batteries. Actually, most of high areal loading sulfur electrodes based on 3D current collector are free-standing and aluminum-foil-free. Such aluminum-foil-free and “free standing” electrodes have high implementation costs to be applicable for industrial production. From the practical point of view, the conventional coating method is more feasible to construct high-sulfur loading cathode.

For slurry coating method, increasing sulfur loading means increasing electrode thickness. Attempt to do so usually resulting in limited kinetics for both lithium ions and electrons transportation. Moreover, thick sulfur electrodes often suffer from cracking and peeling-off problems. Building stable secondary particles are thus of interest to a variety of structural design because of their distinct physiochemical properties. Structural failure caused by particle fracture has long been considered as essential reasons for poor cycling performance and fast capacity fading. Large volumetric expansion (~80%) of sulfur particles upon lithiation easily results in electrodes structural damage, which further aggravates the dissolution of intermediate polysulfides. More badly, the kind of structural damage issues are often irreversible in most cases. Taking account of the poor electronic conductivity of sulfur, it is acceptable that decreasing the size to the nanoscale can boost the performance of sulfur-based cathode. Accordingly, one of the biggest challenges associated with the use of high13 ACS Paragon Plus Environment


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capacity sulfur cathode is how to increase the structural stability of sulfur electrodes. Large and stable secondary particle sizes are attractive for industrial applications due to good flow and handling properties. The organized integration of nanoparticles into large secondary structures is a facile and straightforward strategy that take care of both nanoparticles’ and large secondary particles’ advantages. The pinholes-like void space

Figure 3. Secondary particles discussed here: (a) (Carbon-nanotube-interpenetrated mesoporous nitrogen-doped carbon spheres. Reproduced with permission from ref 25. Copyright 2015 WILEY-VCH. (b) Cauliflower-like cauliflower-like hierarchical porous particles. Reproduced with permission from ref 13. Copyright 2015 Nature Publishing Group. (c) 3D multi-walled carbon nanotube frameworks. Reproduced with permission from ref 26. Copyright 2014 WILEY-VCH. (d) Integrated Ketjen Black microparticles. (e) Woolball-like carbon microspheres. Panels d-e reproduced with permission from ref 9, 28. Copyright 2015 WILEY-VCH.

between each nanoparticles allows the sulfur to expand without changing the whole secondary particle size. The obtained secondary particles are nanoparticle-dependent, 14 ACS Paragon Plus Environment

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which guarantee the fast electrons and ions kinetics process. Low volumetric capacity is another challenge of Li–S batteries that rarely considered in research papers. Because of space-efficient packing inside the secondary particles, integrated secondary particles have higher tap density than that of primary loose nanosized particles. It is a key parameter to fabricate thick electrode with high mass loading.

The chemical reaction between oligomer resin precursor and porogen through an emulsion polymerization is studied to synthesize carbon-nanotube-interpenetrated mesoporous N-doped carbon spheres (MNCS/CNT) (Figure 3a).25 The MNCS/CNTsulfur composite can effectively trap soluble polysulfides by its nitrogen sites and mesopores, mitigating the loss of active material. Moreover, the high tap density (0.95 g cm-3) and the large particle size of the microsized MNCS/CNT-sulfur composite allow to make fairly compact electrodes with >5 mg S cm-2 high sulfur loadings. A secondary cauliflower-like structured C/S composite with enormous hybrid macro-, meso- and micro-pores was developed as Li–S cathode (Figure 3b). With 75% high sulfur content and 6 ~ 14 mg cm−2 high sulfur loading, this kind of cathode could be close-packed on the aluminum foil. The increased pore volume of reconfigured secondary particles facilitates the electrolyte infiltration and Li-ions transport across the whole cathode.13 In order to diminish the impact of volume changes during discharge, Byon and co-workers present a bipyramidal shaped sulfur cathode based on 3D multi-walled carbon nanotube frameworks. Conducting tubes interwoven in the sulfur particles provides uniform conducting pathway for high electrical conductivity. 35 vol% sulfur-free region can release the mechanical stress during volume expansion 15 ACS Paragon Plus Environment


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and maintain the cathode from being damaged.26 Integrating carbon nanoparticles into microsized secondary microparticles was developed to address thick sulfur cathode issues.9

Slurry based on this composite is easy to be casted into cathodes with

practically usable mass loadings. Inspired from this way and to further increase the interaction electrical connection of nanoparticles, self-assembly methods were proposed to integrate scattered nanocomposites into large secondary particles, and to construct integrated thick sulfur cathodes.27 Besides, to reach large-scale synthesis, one-step spray drying approach was used to fabricate woolball-like nanocomposite of carbon/sulfur microspheres (SD-C/S).28 Spray drying is a facile and effective approach, which has been widely applied in spherical shape granulating in industry. Taking advantage of this approach, the large-scale synthesis of C/S nanocomposite for practical Li–S batteries become possible.

2.3 Novel cathode techniques

Although the structure and materials design of sulfur cathodes get some success in capacities retention and long-term cycling performance, it should be noted that most of these cathode is under low sulfur loading (usually 2.0 mg cm−2) and sulfur fraction of 60% (90% of Li-ion batteries), and it is difficult for these electrodes to attain the real energy-density potential of Li–S batteries. Once preparing thick electrodes by traditional blade-casting method, they will fracture and show poor rate performance with low sulfur utilization because of the high carbon content and bad electrolyte immersion through the thick cathodes. From battery configuration perspective, 16 ACS Paragon Plus Environment

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permselective membranes serving as separator coating or interlayer and playing upper current collecting role appeared to suppress shuttle effect. However, plate lugs are always welding at Al foil in the real pouch batteries. Current cathode fabrication approaches usually suffering from large ohmic polarization, whose collecting effect could be considered minimis. Thereupon, Manthiram and his co-worker reported pure sulfur electrode with ultrahigh areal loading of 13.9 mg cm-2 covering carbon paper as upper current collector by plain blade-casting method, and achieved highest areal capacity of 19.2 mAh cm-2 of sulfur cathode up to now which is 4 times higher than that of available Li-ion battery electrode.29 More importantly, no complex host material or nanosized sulfur resynthesis or time-wasting process are required in this cathode technique, and it may open new avenues for the revolution of high-energy-density and industry-adopted Li-S batteries. Similarly, a high-flux graphene oxide membrane was directly coated on sulfur cathode surface by shear alignment.30 The shear-aligned graphene oxide membrane is thought can maintain good kinetics of the charge transfer processes in Li−S batteries. This method is easy to operate and may help to bring this technique closer to commercial process.

Notably, setting interlayers between cathode

and separator is a common method to enhance cycle performance in laboratory coin cell, which are thought can trap polysulfides and act as upper current collector.31-33 However, the use of interlayer will lead to an increasing electrolyte usage, which in turns decreases the energy density of practical Li-S batteries. Moreover, the role of the introduced interlayer as a current collector is often limited in pouch batteries.



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Figure 4. New cathode techniques discussed here: (a) Shear alignment method. Reproduced with permission from ref 30. Copyright 2016American Chemical Society. (a) Solvent-free method. Reproduced with permission from ref 34. Copyright 2013 Royal Society of Chemistry. (c) Phase inversion method. (d) 3D printing method. Panels c-d reproduced with permission from ref 35-36. Copyright 2016 WILEY-VCH.

After designing high sulfur loading cathode materials, the last but not least is cathode technique of undamaged thick electrode which can be easily enlarged. One major reason that caused thick electrode cracks is the large volume shrinkage when removing solvent from the slurry by solvent-based methods widely used at the present stage. In addition to this, there are unwished side effects and chemical incompatibility added to the system during the drying process of cathode sheets, sulfur evaporation and morphology change also put efforts of last two sections in vain. By the way, the remove of solvent increases the fabrication time and reduces production efficiency, and may bring the new contamination to the environment, such as the low-volatile NMP. Moreover, the complete remove of solvent during cathode fabrication is a big challenge. Thus the development of new cathode techniques is an important link in the process of industrialization. Kaskel and his co-wokers presented a solvent-free, simple 18 ACS Paragon Plus Environment

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pressing/thermal treatment method for the forming process of flexible freestanding lithium-sulfur cathode foils starting from melt infiltrated carbon-sulfur nanocomposite and water-free PTFE binder.34 The mechanical stability was ensured through adhesion forces provided by the three dimensional PTFE fiber network and structural support provided by CNT. A loose, open particle structure allowing the penetration of electrolyte showed from SEM indicated that porosity is controllable using this technology. Utilizing an optimized DUT-19 carbide-derived carbon with cubic ordered mesopores, a stable capacity of > 740 mAh g-1 as well as high discharge-charge efficiency > 96% was achieved over 160 cycles at a current load of 167 mA g-1 (C/10). This pressing/thermal treatment method is especially advantageous for large-scale experiment and material optimization because of the reproducibility and quickness.

Li and his co-works developed a phase inversion method by simply immersing current collector coated with electrode slurry into a water coagulation bath.35 In this way, they have constructed a porous sulfur electrode with interconnected polymer skeleton. It is interesting to find that no obvious electrode deformation was observed during the phase inversion process. More importantly, this method is environmental friendly and energy saving for large-scale practical application. Extrusion-type 3D printing used in wide applications is a rapid and accurate object construction technique based digital model file adhering ink materials such as liquid metals and polymers layer by layer. It shows more advantages when the object structure is more complex, because material addition and structure forming can be finished just in one step, which is the most exciting shinning point of 3D printing technique. During this direct-ink writing 19 ACS Paragon Plus Environment


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process, suitable inks with high viscosity and shear-thinning feature is essential to the success of the well-defined print products, which means the viscosity of the ink decreases with the increase of shear stress through a nozzle and it can recover the initial high viscosity after extrusion to guarantee 3D structure retention. Coincidentally, Wallace and his co-workers demonstrated gel-like behaviors of highly concentrated graphene oxide (GO) suspensions, so it is possible to print it into filaments and build 3D networks. But only a few works about GO-based microsupercapacitors or lithiumion microbatteries with extrusion-type 3D printing technique have been carried out, and suitable inks and favorable architectures with high electrochemical performances are still in the infancy. Yang and his co-wokers first time put this technique into lithiumsulfur batteries by printing sulfur copolymer-graphene architectures with well-designed periodic microlattices as cathode with the ink composed of S particles, 1,3diisopropenylbenzene (DIB), and condensed GO dispersion.36 Moreover, reduced GO can provide high electrical conductivity for insulating sulfur to improve the utilization of active materials. Superadded diffusion inhibiting effect for the soluble intermediate polysulfdes by strong covalent S-C bonds in the sulfur copolymer and facilitating effect for electrolyte into sulfur by periodic microlattices, these 3D printing materials exhibit high reversible capacity of 812.8 mAh g-1 and good cycle performance as cathode for Li-S batteries. Such a hot 3D printing technique can be widely tried to construct special architectures in this general way. What is more, it can promote the process of industrial automation in various energy storage fields.

2. Electrolyte: solvent is the key; solid electrolyte is ultimate. 20 ACS Paragon Plus Environment

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The low complexity of using liquid electrolytes make liquid electrolyte Li–S batteries either be foreseen or a reality. Carbonates are a good choice in lithium ion batteries with low active materials/electrolyte ratios. However, situation completely changes in Li-S batteries. Only the sulfur is strongly confined in a special porous host materials or sulfur is covalently immobilized on polymeric composites, carbonates can be used with a different reaction mechanism. Such as sulfur are trapped in microporous carbons and polyacrylonitrile–sulfur cathodes. Thus, these electrolytes are unsuitable for typical Li-S batteries. Because the highly electronegative oxygen atoms of carbonates show much attractive electron attraction than the single-bonded carbon atoms connected to them (Figure 5), the single bonded carbon atoms are partially positive and act as electrophiles.37 Polysulfides are possible to attack these carbon atoms on the carbonate molecules.

Figure 5. The reaction mechanism of polysulfides in carbonate electrolyte. Reproduced with permission from ref 37. Copyright 2013 American Chemical Society.

Up to now, LiN(SO2CF3)2 (LiTFSI) lithium salt in mixtures of 1-3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) are the most studied liquid electrolyte in Li-S batteries. DME is a polar solvent with a relatively high dielectric constant and low viscosity. This solvent is a good solvent for polysulfides and ensures complete redox reaction. With DME solvent, sulfur can be easily reduced to insoluble products (Li2S2 21 ACS Paragon Plus Environment


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or Li2S) with passing through dissolved polysulfide intermediates. DOL is a nonpolar solvent and is thought to generate a protective SEI layer on the surface of Li anode by cleavage of cyclic structures. Even though a serious of electrolyte containing DOL/DME mixtures have been found, these mixtures still need to be further optimized to achieve a trade-off among several key parameters such as viscosity, ion conductivity, electrochemical windows, and safety.38 Highly flammable organic electrolytes for Li– S batteries still identified as one of the serious safety issues. Undoubtedly, LiNO3 is a significant breakthrough as an additive by forming a more stable and thicker SEI layer. The oxidizing nature of LiNO3 in ether-based electrolytes was found to effectively prevent the shuttling mechanism between lithium polysulfides and metallic lithium. Unfortunately, flammable organic electrolytes still exist huge security risks. Besides, LiNO3 would gradually exhaust during cycling. Moreover, it may cause explosive hazards with the cathode composition like gun powder (KNO3, sulfur and carbon).

Considering these factors, the LiNO3 additives should not rely too heavily upon in the future. Developing LiNO3-free electrolyte is a considerable method that can shed light on the further development of Li-S batteries.39 In addition, it is also possible to change the solvents to prevent polysulfide shuttling and increase the batteries of pouch Li–S batteries.

2.1 unconventional ether electrolyte In all configurations of Li-S batteries, sulfur, carbon-based conductive, electrolyte and lithium metal are all combustible, making them very unstable during operating. 22 ACS Paragon Plus Environment

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Among them, liquid electrolytes, such as carbonates or ethers, not only have relatively low flash points and low boiling points, but also are highly volatile and inflammable. Li–S batteries based on these organic liquid electrolytes would suffer from large serious safety issues. Organic solvents with high dielectric constants can facilitate the dissolution of Li salts and increase the number of charge carries. Figure 6a shows the chemically made “Li2S8” dissolved in and the dielectric constants of different solvents.40 Solvents with high dielectric constant such as DMSO (46.540) exhibit stronger ability to stabilize charged species compared to solvents with low dielectric constant such as DME (7.075). The rates of these charged species disproportionation reactions decide the rate of Li–S batteries. The reaction rate of low-dielectric solvents is higher than that of high dielectric solvents. Thus, electrolyte solvent with acceptable dielectric

constant accompanied with suitable catalyst that can rapidly eliminate the accumulation of sulfur species would be a possible strategy to design safe and high-performance Li-S batteries in future researches. For example, P2S5 was introduced as an efficient additive for Li–S electrolyte, which was proven to promote the dissolution of Li2S and eliminate the shuttling of polysulfides by passivating metallic lithium.41-42 P2S5 forms complex with Li2Sx and facilities the dissolution of lithium sulfide/polysulfides in ether based electrolytes (Figure 6b). Addition of redox mediators such as V2O5, LiI or CoS2 also have similar capability to promote the oxidation of Li2S to more soluble species.43-44



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Figure 6. (a) Images of chemically made “Li2S8” dissolved in different solvents with different dielectric constant. Reproduced with permission from ref 40. Copyright 2014 American Chemical Society. (b) The photo presents the solubility of a series of mixtures of Li2Sx (1≤x≤8) with and without P2S5 in TEGDME. Reproduced with permission from ref 45. Copyright 2015 American Chemical Society. (c) The polysulfide solubility test of 1.0 M chemically made “Li2S8” in fluorinated electrolyte solvents. Panels b-c reproduced with permission from ref 44, 46. Copyright 2015 American Chemical Society.

A long-chain ether-based organic solvent, tetraethylene glycol dimethyl ether (TEGDME) was investigated as an alternative ideal solvent due to its low dielectric constant (7.79), low viscosity and low vapor pressure properties.45 Fluorinated ethers also have superior properties making them promising co-solvents for Li-S electrolyte. 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE),

46-47

and 1,1,2,2-

tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane48 with low melting points, low 24 ACS Paragon Plus Environment

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viscosities, low flammabilities and poor coordination abilities have been studied as a co-solvent to mitigate polysulfide dissolution and promote conversion kinetics from long-chain lithium polysulfides to short-chain lithium sulfides.

2.2. More safety electrolyte Except the common used ether electrolytes mentioned above, sulfone-based electrolytes are more promising solvents while concerning the safety issue due to they are less volatile and low toxicity properties. Sulfone–based electrolytes usually have relatively high viscosities and high melting temperatures. Thus, sulfone-based electrolytes are rarely used because they show lower power performance than ethers. Tetramethylene sulfone (TMS) and ethyl methyl sulfone (EMS) are two sulfones most intensively investigated. EMS has a lower viscosity than TMS and thus has a higher ionic conductivity for Li-S electrolytes. LiTFSI dissolved in EMS as electrolyte provides both high ionic conductivity and wide liquid range. TMS single solvent containing 1 M LiTFSI also has been directly used as electrolyte for assessing the formation of Li2S and detecting the real-time morphology during battery operation. In a word, TMS owing to its low donor ability has a much lower solubility to polysulfides than DME, thus suppressing the redox shuttle reaction.

When sulfone was used alone, the cell showed a poor cycle life and large polarization. Combine the high donor number and low viscosity of ethers

with the higher dielectric constant and oxidation potential of sulfones is another feasible approach to design safe and high-performance Li–S 25 ACS Paragon Plus Environment


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batteries. TMS are explored as a co-solvent in binary solvent mixtures for Li-S batteries. DME and THF have been studied as co-solvent with sulfone. However, both of them showed unsatisfied cycling performance. Li–S batteries based on sulfone/DOL mixed electrolyte co-solvents showed the highest initial discharge capacity.49 EMS are suggested to be used when combine with a less viscous co-solvent with a low melting point TEGDME.50 The viscosity and solvation ability of the solvents would affect the reactions of dissociation of the S82− and disproportionation of lithium polysulfides in the electrolyte solutions.

Ionic liquids (ILs) have identified as alternative electrolyte solvents because they have many attractive properties, such as high thermal stabilities, low flammabilities, negligible volatilities and wide electrochemical potential windows. Ionic liquid electrolytes with low donor ability have also been found to limit the dissolution and/or diffusion of polysulfides moieties for Li-S secondary batteries. ILs’ solvation behavior is another important property for Li–S battery electrolytes. It is worth to mention that the presence of LiTFSI in ILs can further reduce the polysulfide solubility, which might be attributed to the anions [TFSI]− serving as the solvent forms Li[TFSI]2 complexes. Thus, [TFSI]− based ILs have relatively low viscosities, high conductivities, and compatibilities with the lithium metal anode.51 Even so, low Coulombic efficiency is still a big obstacle Li–S batteries confront. Slow Li+ transport in highly viscous ILbased electrolytes can be relieved by increase temperature. Unfortunately, hightemperature operation would further cause rapid capacity decay and lower coulombic efficiency. High temperature may increase solubility of Li2S and lead to serious side 26 ACS Paragon Plus Environment

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reactions. In my view, it might be possible to use ILs to form stable SEI films, but lots of researches must be involved in. Sulfones (Methyl isopropyl sulfone) mixed with viscous ILs are studied to alleviate polysulfide dissolution and shuttle phenomena by virtue of the low coordination ability of ILs.52 The presence of sulfones here is expected to reduce the viscosity and improve the ionic conductivity of ILs.

2.3 Solid state Li-S electrolyte

Solid electrolyte is an ultimate way to solve the shuttle problem for Li–S batteries.53-56 Even though liquid electrolytes dominate in the field of Li/S batteries at present, and novel liquid based electrolytes have just emerged as candidates, it is still far from negligible to use solid state electrolyte for pouch Li–S batteries. The unique properties of solid electrolytes render some of these types electrolyte valuable to create safe Li–S batteries with high performance. Polymer electrolyte and inorganic solid electrolytes are two main trends of solid electrolyte. Among them, the closest step that moving liquid electrolytes towards solid state is polymer electrolytes. Polymer electrolyte holds many advantages, such as favorable mechanical properties, easy fabrication into thin films, stable interface with Li metal, and high modulus to prevent lithium dendrite formation. Polystyrene-b-poly(ethyleneoxide) copolymer electrolyte is expected to dissolve alkali salts and retain mechanical rigidity simultaneously.57 By adding polysulfides, the morphologies and phase behaviors of the copolymer electrolyte was changed. This study demonstrated the possibility of using block copolymer electrolytes for Li–S batteries. Moreover, poly(vinylidenefluoride-co27 ACS Paragon Plus Environment


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hexafluoropropylene) (PVDF-HFP) also can provide excellent chemical stability and mechanical integrity for processing free-standing films with high ionic conductivity.58

Inorganic solid electrolytes can physically block the dissolution and diffusion of polysulfides, suppress the formation of Li dendrites, and prevent leakage, volatilization, and flammability of electrolytes. Moreover, only lithium ions migrate in electrolyte interior enables nearly united lithium ions transference numbers. Sulfide glasses as inorganic solid electrolytes show high lithium ion conductivities (higher than ~10−4 S cm−1) at room temperature. Nanoporous β-Li3PS4 has been prepared as solid state electrolyte with high ionic conductivity at room-temperature (1.6×10−4 S cm−1). Besides, this solid state electrolyte shows superior chemical stability when matching with lithium metal anode and has a wide electrochemical window of 5 V.59 With Li3PS4+5 as the cathode and nanoporous β-Li3PS4 inorganic solid electrolyte, such assembled Li–S batteries have an impressive cyclability.60 As shown in Figure 7a, a high initial discharge capacity of 1272 mAh g−1 with 100% Coulombic efficiency was obtained at room temperature. Even after 300 cycles, the cathode showed a high capacity of 700 mAh g−1.

Thio-LISICONs are found as lithium super-ionic conductors with conductivities greater than 10−3 S cm−1 at room temperature and used as solid electrolytes for Li–S batteries.61 The all-solid-state Li–S battery showed a reversible capacity of 900 mAh g−1 at a current density of 0.013 mA cm−2. Without polysulfides dissolution, only one discharge plateau was observed. Interface compatibility between electrolyte and 28 ACS Paragon Plus Environment

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electrode (including cathode and anode) is a virtual criterion to assess the feasibility of solid state electrolyte. Highly deformable LiBH4 can facilitate tight interface formation between cathode and LiBH4 electrolyte.62 The high reducing ability of LiBH4 also allows the use of a lithium metal as anode to enhance the energy density of pouch Li– S batteries. Moreover, this solid state electrolyte has a high lithium ionic conductivity, greater than 2×10−3 S cm−1 above 117 °C. The highly deformable nature of LiBH4 enables a tight interface to be formed between the active materials and electrolyte particles. Even at a discharge rate of 0.5 C (corresponding to 2.5 mA cm−2), all-solidstate Li-S batteries retained discharge capacities of 630 mAh g−1.

Figure 7．(a) Chemical reactions of lithium polysulfidophosphates (LPSPs) and the cycling performance of Li3PS4+5 as the cathode for all-solid-state Li-S batteries at the rate of C/10. Reproduced with permission from ref 60. Copyright 2013 WILEY-VCH. (b) Crystal structures of Li1+xTi2−xAlx(PO4)3 (LATP) and the cycle performance of obtained solid-state Li–S cell based on LATP solid state electrolyte. Reproduced with permission from ref 63. Copyright 2016 American Chemical Society.



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Figure 8. Schematic of the three-dimensional bilayer garnet solid electrolyte and its corresponding hybrid solid-state bilayer Li–S battery. Voltage profile and cycling performance of the hybrid bilayer Li–S cell with a high sulfur mass loading of approximately 7.5 mg cm-2 at 0.2 mA cm-2. Reproduction with permission from ref 64. Copyright 2017 Royal Society of Chemistry.

Employing solid-state electrolytes as separators to potentially impede any diffusion of soluble species are currently recognized as one of the most promising way to consider the high performance of Li–S batteries. Under this examination, the chemical compatibility between solid-state electrolyte material and polysulfide solution is most important key parameter to be addressed. Arumugam Manthiram and his coworkers have proposed a comprehensive approach by assessing durability of Li1+xTi2−xAlx(PO4)3 in contact with polysulfide solution (Figure 7b).63 It is thought that Li1+xTi2−xAlx(PO4)3 gets lithiated in contact with lithium polysulfide solution and Li2CO3 layers are formed on the particle surface, which effectively blocks the interfacial lithium-ion transport between the liquid electrolyte and solid-state


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electrolytes. The obtained solid-state Li–S cell exhibits an initial discharge capacity of 978 mAh g−1 at C/10 rate, and enables a long cycling-life Li−S battery.

Although solid state electrolytes show good promising value and have certain beneficial properties in future application, it should be noted that most inorganic solid state cells usually have to be operated at high temperatures. Moreover, there are still many difficulties in forming effective solid-electrolyte/solid-electrode interfaces for electrochemical reactions. These weaknesses strongly limit the practical application of solid state Li–S batteries. Since the diffusion rate of lithium ions through a solid electrolyte is very slow, it is more hard to match them with thick cathode. High sulfur loading means thick electrode to some extent, resulting in limited kinetics for both lithium ions and electrons. All above mentioned problem intrinsically reduces the power capabilities of Li–S batteries, let alone operates at low temperatures. The cathode material prepared by mechanochemical treatment on the mixture of inorganic solid state electrolyte, sulfur and conductive carbon is a good way to solve ion conductive in thick cathode. Because the ionic conductivity of the sulfur cathode is of paramount importance in the cycling of these Li–S batteries. By increasing the contact between active material and solid electrolyte, it is possible to reduce their contact resistance and provide more ion transport channels. A big breakthrough is designed by Hu and his coworkers.64 They reported a 3D solid-state electrolyte framework with a bilayer denseporous structure, as shown in Figure 8a. The thin dense and rigid solid state layer at bottom has high elastic modulus, which can efficiently separate electrodes and prevent lithium dendrite penetration. The porous solid-state framework layer can locally 31 ACS Paragon Plus Environment


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confine different cathode materials and accommodate their volume change. Results showed that the integrated sulfur cathode loading can reach >7 mg cm-2, higher than closing to practical level of sulfur loading. The proposed hybrid Li–S battery based on this bilayer solid-state electrolyte exhibits a high initial coulombic efficiency (>99.8%) and high average coulombic efficiency (>99%) for subsequent cycles (Figure 8b).

Since solid state electrolyte have large possibility to create all-solid-state cells with controlled lower polysulfides solubility and enhanced lithium metal anode stability, we strongly believe industrial manufacture will promote all-solid batteries as an ultimate solution. It should also be emphasized that the batteries inherently safer due to no leakage risk and thermally stable electrolytes of solid-state electrolyte, both are important reasons for the development of all-solidstate Li–S batteries.

3. Toward diverse and high-performance lithium metal anode

Except for cathode and electrolyte views, lithium metal anode is another rigorous obstacle for high-energy-density lithium-sulfur batteries to enter the commercial market. Although the highest theoretical capacity (3860 mAh g−1) and the most electronegative potential (-3.04 V versus standard hydrogen electrode) are very prominent advantages of lithium metal anode, there are concomitant fatal flaws decreasing its use in popularity: (i) Lithium undergoes infinite volume expansion during lithium stripping/plating process due to its “hostless” electrode property, leading to mechanical instability; (ii) The highly reactive lithium metal anode can react easily with liquid 32 ACS Paragon Plus Environment

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electrolyte and soluble intermediate long-chain lithium polysulfides to form an unstable solid electrolyte interface (SEI) layer that could not totally resist to the large volume change during cycling; (iii) The unstable SEI layer will be introduced the formation of cracks and pits as the cycle of charge and discharge gets more, which exposes fresh Li with lower impedance and make for enhanced flux of lithium-ion at these points; (iv) After uneven Li deposition across the breakage of the SEI layer, Li dendrites with highsurface area come into being and grow facilitated by SEI defects, and dendritic problem is the core crux and has become a headache for researchers all over the world; (v) Lithium dendrites causing dead lithium accumulation and increased impedance not only increase electrolyte decomposition and large loss of active materials sulfur, resulting in low Coulombic efficiency and poor cycle life, but also penetrate the separator after occupying limited free space in the battery, eventually lead to a short circuit, hot runaway and dangerous fire. Significant research efforts have been made to address these issues, here we discuss several effective strategies in recent research progress.

3.1 Stretchable and compressible artificial SEI layer

Numerous studies focused on anode surface modification to control lithium plating and improve stability and uniformity of SEI directly from the optimization of electrolyte formulations, such as solvents, salts and additives of lithium halides, or introducing electrostatic shielding layers, and finally inhibit lithium dendrite.65-66 Although these researches occupied a large part of the content, the performance is usually not as well as expected at large scale because the naturally-generated SEI film 33 ACS Paragon Plus Environment


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is still too weak to completely resist the dendrite and the continuous consumption of electrolyte components is still a fundamental problem. As a consequence, there comes out a round-about way explained as “host” designs for lithium metal including hollow carbon spheres, graphene and polymer nanofibers. These materials are demonstrated to build robust interfacial layers, which could successfully accommodate lithium

Figure 9. Artificial interfacial layer anode: (a) a core−shell silica@poly(methyl methacrylate) (SiO2@PMMA) coating and (b) a soft and flowable polymer coating. Panel a reproduced with permission from ref 67. Copyright 2017 American Chemical Society. Panel b reproduced with permission from ref 68. Copyright 2016 American Chemical Society.

volumetric expansion without breaking and hinder dendrite growth without reducing lithium-ion conductivity simultaneously. The promising approach of stretchable and 34 ACS Paragon Plus Environment

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compressible artificial SEI interlayer has been moved toward reality by some groups. Cui and his co-workers presented a novel coating of silica@poly(methyl methacrylate) (SiO2@PMMA) core-shell nanospheres as an interfacial layer on lithium metal anode with nanoscaled pores, high flexibility, good mechanical and thermal stability.67 The SiO2 core mainly resisted the growth of lithium dendrite by its high modulus of 68 GPa, while PMMA adhered SiO2 nanospheres together to form the artificial SEI flexible membrane and protected SiO2 from reacting with lithium metal above all. Owing to the synergistic effect between SiO2 and PMMA, the nanopores formed among the packed SiO2 nanospheres were smaller than lithium nucleation size to suppress dendrite and larger than lithium-ion transport diameter to ensure lithium-ion transmission through this artificial SEI. This solution in principle improved Coulombic efficiency of lithium metal anode and contributed a new idea of artificial SEI layer for lithium metal anode protection.

Based on the perspective of last example, high modulus host materials as interfaces show promise in mechanically suppressing volume expansion and dendrites of lithium metal negative electrode. From counter but conventional understanding, extremely soft polymer coating to improve lithium metal anode electrodeposit morphology can also be explained. A more straightforward strategy has been realized to achieve flat and dense Li plating by coating highly viscoelastic and intrinsically flowing polymer directly across lithium metal surface to be pinhole-free during repeated charging and discharging.68 The simulation measurement indicated that the self-healing polymer repaired the pinholes or cracks and avoided the hot spots creation in SEI, homogenizing 35 ACS Paragon Plus Environment


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lithium ion flux and weakening ionic-conductivity excessive increase at these SEI defects. Under the effect of this adding structure, more strain on the interfacial stability of lithium metal electrode was overcome, and dendrite-free lithium deposition was observed at current density up to 5 mA cm-2. Comparatively speaking, this result is much attractive than the performance of electrolyte-additive-modification method which usually only be discussed at relatively low current densities (0.1-0.5 mA cm-2). These two work highlights the significance of stretchable and compressible artificial SEI layer for suppressing lithium dendrite, and provides more potential for lithium metal application.

3.2 Constructing anode host

Plenty of research works focus on interface stability by making efforts on the comprehensive properties of SEI to prevent lithium dendrites shooting out from anode surface, but this train of thought could not effectively govern hostlessness that regarded as another fatal weakness of lithium metal electrode. The nasty volume expansion means anode volume change from thorough discharged state to charged state, and this could cause internal pressurization even bulge in implemented battery. So, in recent years, much more neoteric ideas have been explored and research emphasis has been moved onto anode host constructing and current collectors acting as “host” for lithium metal besides homogeneously reducing the attack of dendrite nuclei. However, many of the host design are accompanied with resistance increase and active sulfur loss, and


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the complex host building-up procedure also increases the cost of the practical application. In such a context, Lu and his co-workers presented a facile and low-cost

Figure 10. Lithium anode based on different hosts: (a) Copper mesh. Reproduced with permission from ref 69. Copyright 2017 WILEY-VCH. (b) Graphene oxide. Reprinted with permission from ref 70. Copyright 2016 Nature Publishing Group. (c) Copper nanowire. Reproduced with permission from ref 71. Copyright 2015 American Physical Society. (d) Mesoporous AlF3 framework. Reprinted with permission from ref 72. Copyright 2017 AAAS.

strategy with only one step of copper mesh embedding into lithium metal through mechanical pressure to fabricate 3D porous Cu current collector/Li-metal (3D Cu/Li) composite electrode69. This 3D Cu provided cage to confine lithium volume change by accommodating Li into its 3D porous structure resulting in smoothly lithium plating, favored the utilization of anode lithium metal materials ensuring the anode’s Coulombic efficiency. Compared with flat Cu current collector, the 3D porous composite structure improved specific area of lithium metal anode by leaps and bounds, promoted the electrochemical reaction kinetics and reduced the interfacial resistance. From a macro explanation, the dendrites and the thickness of lithium anode can be 37 ACS Paragon Plus Environment


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controlled in a stationary level, which could guarantee the separator integrity and mechanical stability of the battery. This 3D porous current collector structure is conceivable to be tried in other metal electrodes, which is the most significant extensibility of this design.

We can also deem volume change as the change of the material in a certain dimension brought by the floating interface. And SEI stability is deeply intertwined with the dimension variation and always face the collapse danger in the unprocessed battery during unremitting cycling, leading to lithium dendrite and continuous electrolyte decomposition. Every one of these issues is severely engineering challenge. To limit lithium metal anode more thoroughly rather than the only surface treatment, we can draw some ideas from the anode structure of lithium ion battery. Stratified materials are extensively used to store lithium ion, obviously, some similar materials can be chosen to pre-store lithium metal constructing a composite with determinate volume. But on second thought, light weight and the highest energy density of lithium metal are the remarkable advantages and the reason for it being the research hotspot, therefore, the weight impact should be minimized. Last but not least, the host materials need to possess excellent lithiophilicity, which is benefit to create a strong bonding between Li and its surface, and achieve a low nucleation barrier during cycling. Cui and his co-workers performed layered Li-reduced graphene oxide (rGO) by Li molten infusion into the space of sparked rGO with uniform nanogaps between periodically stacking rGO and nanoscale layered Li.70 Reduced GO was proved to be the ideal host of lithium metal anode because of its light weight and unique lithiophilicity among all 38 ACS Paragon Plus Environment

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of the light carbon-based materials, also due to its high surface areas increasing the contacting area to lithium metal, excellent mechanical strength impeding the volume change and redox stability under the electrochemical environment. Another initiative of this work is the prominent thermal infusion approach, because this technology successfully avoids uneven Li distribution and inefficient fabrication of conventional electrodeposition method. In addition, the top rGO also plays the role of artificial SEI, stabilizing the interface mechanically and electrochemically. The composite anode exhibits good flexibility, less than 20% dimensional change and low hysteresis during cycling.

In the past several years, lithium metal anode has been revived to develop and demonstrated that Li ion flux deposition cannot be homogeneous onto the traditional planar copper current collector or lithium foils only treated by common SEI reinforced strategy to solve the dendrite problem. Inspired by the last example, it is reasonable

to expand research ideas from 2D to 3D host materials in order to get better restriction towards lithium volume growth. Yu and his co-workers reported a design of free-standing Cu nanowire (CuNW) network with opened 3D porous structure to accommodate the deposited lithium metal and curb anode structure collapse at root.71 The porous current collector can provide high electrical-conductivity network and areamultiplied surface to realize the ideal Li ion flux distribution and suppress dendrites. Even if few dendrites come into being and grow inside the CuNW current collector under some circumstances, they will fuse into bulk within the 3D network under control. In total, as high as 7.5 mAh cm-2 of lithium metal can be plated into this 3D CuNW 39 ACS Paragon Plus Environment


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current collector, which is significantly above the current commercialization level of lithium-ion batteries. Beyond that, this lithium metal with 3D CuNW composite anode exhibits high average Coulombic efficiency up to ~98.6% during 200 cycles. The 3D porous current collector is practical and rational, which may synergistically work combined with other design and put lithium metal anode into application as soon as possible.

More and more researchers realize that structure design for lithium metal anode itself deserves more attention, so there comes out exquisite SEI, rigorous current collector and many other techniques trying to keep negative problems within limits. Although these complex attempts achieve little success and put forward lithium metal research, they are too harsh to bring lithium metal anode into industrialization. So far, there is still no complete solution to dendritic problems, let alone the ideal and ultimate aim that achieving zero volume change during stripping/plating process with none dendrite once and for all. What’s more, the treated anode using nowadays method cannot properly work under increased current density just over 5 mA cm -2, because faster charging/discharging causes accelerated interface fluctuation after all. However, current density up to 10 mA cm-2 is indispensable in advanced battery devices at present or in future. To beat this formidable application challenge and make lithium-metalbattery capable under higher or ultrahigh current densities, lithium metal anode need to be designed with these two major properties: (i) A ultrastable structure without hardimplement design that can exclude all the damage especially drastic volume change on lithium metal anode under a high rate operation; (ii) A three-dimensional lithium host 40 ACS Paragon Plus Environment

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that can conduct and disperse a large amount of Li ion flow under higher rate to finally reduce local current density. Cui and his co-workers developed (Li/Al4Li9-LiF) (LAFN) as lithium metal composite anode by embedding Li into LAFN through one-step overlithiation process of mesoporous AlF3 framework.72 Although the non-activematerial skeleton occupied some weight and space, it still delivers high specific capacity of ~1571 mAh g-1 and ~1447 mAh cm-3. During the overlithiation process, not only skeleton Al4Li9 formed, but also insulating LiF produced which could both electrochemically protect Al4Li9 skeleton and promote lithium cation diffuse for improved Li ion flux homogeneity. Coincidentally, LiF is a contributing component of SEI, proving its importance laterally. As a result, LAFN electrode brought near-zero volume change into reality without dendrite and it can work with skill and ease even under an ultrahigh current density up to 20 mA cm-2. This study made great process on lithium metal anode under fast charging/discharging which may come true in the near future.

3.3 Dendrites orientation growth lithium metal anode

We have introduced two strategies to stabilize lithium metal anode from physical barrier effect of SEI and host structure construction angles, but these two do not reform the mechanism of lithium dendrite growth on the fundamental point of view. Zhang and his co-workers first time reported dendrite-free even vertically-grown lithium nanorod anode with self-aligned and highly-compacted morphology achieved by 0.05 M CsPF6 electrolyte additive.73 As long as the voltage loaded onto the battery device, both Li+ 41 ACS Paragon Plus Environment


and Cs+ in the electrolyte are driven to Cu substrate, but only Li+ can be reduced and deposit onto Cu current collector when the applied potential is higher than Cs+ reduction potential but lower than Li+ reduction potential. At about the same time as Li deposit forming tips, Cs+ accumulate around the nascent Li tips helping uniform local electric

Figure 11. (a) Deposited lithium film based on Cs+-containing electrolyte. Reprinted with permission from ref 73. Copyright 2014 American Physical Society. (b) Columnar deposited lithium based on LiF-rich Cu surface. Reproduced with permission from ref 74. Copyright 2017 WILEYVCH. (c) Li ion plating behavior based on LiPF6-EC/DEC electrolyte with nanodiamond additives. Reprinted with permission from ref 75. Copyright 2017 Nature Publishing Group.

field, so that Li will leave alone nucleation sites and always grow on the plane, and finally take shape as nanorod that is vertical to substrate. At the initial stage of Li deposition, a reduction occurs around 2.05 V corresponding to the underlying SEI formation on the surface of Cu substrate. The thin SEI enriched in LiF is also benefit 42 ACS Paragon Plus Environment

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from the presence of Cs+, which can promote the reduction of PF6- anions more vigorously than Li+, because LUMO energy of LiPF6 is higher than that of CsPF6. Furthermore, the morphology of deposited lithium is strongly dictated by the underlying SEI quality. As for all strong characters, the synergistic effect of existing Cs+ and initial SEI give rise to the smooth lithium nanorod structure. And this is a totally new approach which can open the imagination to solve dendritic lithium and enable lithium metal anode. Generally, the conventional SEI is vulnerable and heterogeneous in composition, so it can easily produce cracks that is the favorable nucleation sites for lithium plating. These existing sites dome with stronger electric fields on their tips, leading Li-ion to deposit on the tip with ranked-choice. As more Li-ion deposition, the previous hump subsequently turns into distributed hemisphere and bump with each other showing outof-flatness even tuber-like morphology. Most significantly, after a great deal of research we came to a conclusion that the initial SEI take a vital role in guiding the following lithium plating/stripping behavior. Obviously, if the initial SEI is homogenous, it can well regulate lithium nucleation and stabilize lithium deposition/dissolution. Coincidentally, we can get the similar idea from the last example except the only difference that they used Cs+ to surround the infantile Li nucleation to build the plane with the ability of homogenizing Li-ion. Zhang and his co-workers exploited lithiumfluoride (LiF)-rich coating on Cu current collector, result in self-aligned and columnar lithium deposition. The reason why chooses LiF as the protecting coating to regulate the initial SEI formation is its high Li-ion diffusivity and the effect to suppress lithium 43 ACS Paragon Plus Environment


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dendrite growth in working batteries.74 LiF-rich Cu was prepared by in-situ hydrolysis of lithium hexafluorophosphate (LiPF6) producing uniform LiF coating onto immersed planar Cu foils, meanwhile another product hydrogen fluoride can remove the original copper oxide in favor of improving the conductivity of Li-ion and indicating the following process. First Li-ion was diffused fast by LiF leading even distribution, then these ions penetrate through the initial SEI to be adsorbed and reduced by Cu with higher binding energy of -2.57 eV. Because of the lower overpotential on LiF-rich Cu, lithium nucleation forms densely and accordantly. Follow on, owing to the lower energy barrier on Cu, Li diffuses in a high rate and grows horizontally making nucleation sites contact together. The last but the key progress, lithium evolves into columns in the finite space being wrapped and separated by SEI. This work provides an emerging strategy to realize the dendrite-free and super-ordered lithium metal anode. It is worth noting that dendritic problems are not special phenomenon in lithium metal among the metal batteries, so it is a good idea to try to get something useful from related fields, such as uniform nickel metal coating which has been achieved by nanodiamond-involved co-deposition technique in the traditional electroplating industry. Inspired by this nanodiamonds capture strategy, Zhang and his co-workers proposed to use octadecylamine (ODA) groups-modified nanodiamond particles as additives in the electrolyte with the function of assisting to homogeneously deposit lithium arrays.75 The mechanism of this dendrite-free lithium-plating process can be described as: (1) Li-ions firstly adsorb onto co-exiting nanodiamonds in the electrolyte owing to their higher binding energy than Cu substrate with Li-ion and larger surface 44 ACS Paragon Plus Environment

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area, provisionally, the nanodiamonds act as heterogeneous seed to be the initial lithium nucleation; (2) then these combinative particles are driven to Cu electrode surface by the common effect of electric field and electrolyte convection to be reduced into lithium metal attaching on the electrode, within seconds, the nanodiamonds regain freedom and release back into the electrolyte to keep its concentration stable and maintain the longterm cycling. To plate uniformly, it is pivotal to prepare nanodiamonds with small enough size in order to get small-size crystal, so that they can well guide to more smooth morphology. Through this modified co-deposition process, the properties of metal deposited layer have been greatly improved including uniformity, intensity, wear resistance and so on. What’s more, a more stable cycling of Li|Li cell and higher Coulombic efficiency of Li|Cu cell have been realized based on this strategy.

Controlling lithium dendrites growth orientation can effectively affect the lithium plating behavior, which may be a promising method in the near industrial engineering. 3.4

High melting lithium anode

As the temperature influence of the electrolyte, less attention has been paid to the development of high melting point lithium metal anode. Hot runaway is a serious problem that hide in short circuit and impact emergencies. Under these circumstances, batteries temperature may elevate rapidly, causing fire and other more serious dangers. Since the melting point of lithium is only 180 °C, high temperature tests have been rarely reported in the literature considering security issues. It notable that the high reaction activity of lithium metal anode at elevated temperature would bring about 45 ACS Paragon Plus Environment


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serious side effects. Developing high melting lithium anode is an important branch for anode research.

4. Conclusion and future outlooks In this perspective, we have concluded key methods for producing scalable and realized configuration of Li-S batteries. Commercialized Li-S batteries required an engineered cathode with high sulfur loading, high sulfur content and high specific energy to meet high energy density demands while simultaneously exhibiting satisfactory current rates. The sulfur loading approach of sulfur-based composite is a key technique because it affects the morphology of sulfur in the composite, which further affects electrolyte access throughout sulfur particles. Such composite will almost certainly be produced by solution-coating and so will require large quantities of cathode composite. Solution reaction synthesis that can be adequate control sulfur inner structure provides more possibilities for constructing porous sulfur structure. Moreover, this method is mature, scalable and widely accessible. In addition to electrolyte access throughout the sulfur electrode, electron transfer among entire cathode composite is also paramount. Thus, a highly conductive and stable sulfur composite may require structural integrity to always maintain the access to electron-transfer. Self-assembly of functional nanoparticles into secondary superstructure offers higher rates of charge and discharge without blocking the invasion of electrolyte. Besides, novel cathode techniques, such as solvent-free method, phase inversion method and 3D printing


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method, make the demonstration of scalable production methods for novel Li–S cathode. As the most commonly used electrolyte, liquid DOL and DME solvents are chosen because of its relatively low dielectric constant and good compatibility with lithium polysulfides. As the emergence of practical process of Li–S batteries, electrolyte should be uniquely optimized in purpose of high safety Li–S batteries. For liquid electrolyte system, solvent with high security is key solution in near future. Searching electrolyte solvent substitutions for Li–S batteries is one of most viable ways. Sulfone-based electrolytes, ionic liquids and solid state electrolyte are promise breakthrough in next step of Li–S study. The replacement of liquid electrolytes solvent is expected to eliminate the polysulfide shuttle phenomenon and the intrinsically safe Li-S batteries. Although lithium metal anode pulverization is still an important issue to address, constructing durable lithium metal anode would enable niche applications for Li–S catteries or lithium metal-based batteries. Developing diverse and high-performance lithium anode can enrich the future applications of Li–S batteries. In the latest few years, significant efforts have been dedicated to the study of each component in the Li-S cells for improved cyclability, including exploit stretchable and compressible artificial SEI layer, construct lithium metal anode host and control the dendrite growth direction. It is gratifying that these approaches have potentially large-scale preparation. With more and more mature techniques developed in this field, we believe that Li-S batteries could be preferred for the applications in the near future. 47 ACS Paragon Plus Environment


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AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Yusheng Ye and Feng Wu contributed equally to this work. This work was supported by the National Key Research and Development Program of China “New Energy Project for Electric Vehicle” (2016YFB0100204), the National Natural Science Foundation of China (21373028), the Joint Funds of the National Natural Science Foundation of China (U1564206), Major achievements Transformation Project for Central University in Beijing. REFERENCES 1. Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in lithium–sulfur batteries based on multifunctional cathodes and electrolytes. Nat. Energy 2016, 1, 16132. 2. Fang, R.; Zhao, S.; Sun, Z.; Wang, D.-W.; Cheng, H.-M.; Li, F. More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects. Adv. Mater. 2017, 29 (48), 1606823-n/a. 3. Yang, C.; Fu, K.; Zhang, Y.; Hitz, E.; Hu, L. Protected Lithium-Metal Anodes in Batteries: From Liquid to Solid. Adv. Mater. 2017, 1701169. 4. Lochala, J.; Liu, D.; Wu, B.; Robinson, C.; Xiao, J. Research Progress toward the Practical Applications of Lithium–Sulfur Batteries. ACS Appl. Mater. Interfaces 2017, 9 (29), 24407-24421. 5. Cheng, X.-B.; Yan, C.; Huang, J.-Q.; Li, P.; Zhu, L.; Zhao, L.; Zhang, Y.; Zhu, W.; Yang, S.-T.; Zhang, Q. The gap between long lifespan Li-S coin and pouch cells: The importance of lithium metal anode protection. Energy Storage Mater. 2017, 6 (Supplement C), 18-25. 6. Seh, Z. W.; Sun, Y.; Zhang, Q.; Cui, Y. Designing high-energy lithium–sulfur batteries. Chem. Soc. Rev. 2016, 45 (20), 5605-5634. 7. Liang, X.; Kwok, C. Y.; Lodi‐Marzano, F.; Pang, Q.; Cuisinier, M.; Huang, H.; Hart, C. J.; Houtarde, D.; Kaup, K.; Sommer, H. Tuning Transition Metal Oxide– Sulfur Interactions for Long Life Lithium Sulfur Batteries: The “Goldilocks” Principle. Adv. Energy Mater. 2015, 6, 1501636. 48 ACS Paragon Plus Environment

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Figure 1. Key techniques and their integration for industrial Li-S batteries.


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Figure 2. (a) The one-pot synthesis step for the production of GSC. Reproduced with permission from ref 21. Copyright 2012 Royal Society of Chemistry. (b) Fabrication of monodisperse polymer-encapsulated S nano-spheres. Reproduced with permission from ref 22. Copyright 2013 Proceedings of the National Academy of Sciences of the United States of America. (c) Schematic illustration and SEM images of micro-nano-structured CS@CTAB/FC composite fabrication. Reproduced with permission from ref 11. Copyright 2017 Royal Society of Chemistry. (d) Core–shell graphene-encapsulated sulfur (GES) composites as superior cathode materials for lithium–sulfur batteries. Reproduced with permission from ref 23. Copyright 2013 Royal Society of Chemistry.



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Figure 3. Secondary particles discussed here: (a) (Carbon-nanotube-interpenetrated mesoporous nitrogen-doped carbon spheres. Reproduced with permission from ref 25. Copyright 2015 WILEY-VCH. (b) Cauliflower-like cauliflower-like hierarchical porous particles. Reproduced with permission from ref 13. Copyright 2015 Nature Publishing Group. (c) 3D multi-walled carbon nanotube frameworks. Reproduced with permission from ref 26. Copyright 2014 WILEY-VCH. (d) Integrated Ketjen Black microparticles. (e) Woolball-like carbon microspheres. Panels d-e reproduced with permission from ref 9, 28. Copyright 2015 WILEY-VCH.


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Figure 4. New cathode techniques discussed here: (a) Shear alignment method. Reproduced with permission from ref 30. Copyright 2016American Chemical Society. (a) Solvent-free method. Reproduced with permission from ref 34. Copyright 2013 Royal Society of Chemistry. (c) Phase inversion method. (d) 3D printing method. Panels c-d reproduced with permission from ref 35-36. Copyright 2016 WILEY-VCH.

Figure 5. The reaction mechanism of polysulfides in carbonate electrolyte. Reproduced with permission from ref 37. Copyright 2013 American Chemical Society.



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Figure 6. (a) Images of chemically made “Li2S8” dissolved in different solvents with different dielectric constant. Reproduced with permission from ref 40. Copyright 2014 American Chemical Society. (b) The photo presents the solubility of a series of mixtures of Li2Sx (1≤x≤8) with and without P2S5 in TEGDME. Reproduced with permission from ref 45. Copyright 2015 American Chemical Society. (c) The polysulfide solubility test of 1.0 M chemically made “Li2S8” in fluorinated electrolyte solvents. Panels b-c reproduced with permission from ref 44, 46. Copyright 2015 American Chemical Society.


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Figure 7．(a) Chemical reactions of lithium polysulfidophosphates (LPSPs) and the cycling performance of Li3PS4+5 as the cathode for all-solid-state Li-S batteries at the rate of C/10. Reproduced with permission from ref 60. Copyright 2013 WILEY-VCH. (b) Crystal structures of Li1+xTi2−xAlx(PO4)3 (LATP) and the cycle performance of obtained solid-state Li–S cell based on LATP solid state electrolyte. Reproduced with permission from ref 63. Copyright 2016 American Chemical Society.



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Figure 8. Schematic of the three-dimensional bilayer garnet solid electrolyte and its corresponding hybrid solid-state bilayer Li–S battery. Voltage profile and cycling performance of the hybrid bilayer Li–S cell with a high sulfur mass loading of approximately 7.5 mg cm-2 at 0.2 mA cm-2. Reproduction with permission from ref 64. Copyright 2017 Royal Society of Chemistry.


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Figure 9. Artificial interfacial layer anode: (a) a core−shell silica@poly(methyl methacrylate) (SiO2@PMMA) coating and (b) a soft and flowable polymer coating. Panel a reproduced with permission from ref 67. Copyright 2017 American Chemical Society. Panel b reproduced with permission from ref 68. Copyright 2016 American Chemical Society.



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Figure 10. Lithium anode based on different hosts: (a) Copper mesh. Reproduced with permission from ref 69. Copyright 2017 WILEY-VCH. (b) Graphene oxide. Reprinted with permission from ref 70. Copyright 2016 Nature Publishing Group. (c) Copper nanowire. Reproduced with permission from ref 71. Copyright 2015 American Physical Society. (d) Mesoporous AlF3 framework. Reprinted with permission from ref 72. Copyright 2017 AAAS.


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Figure 11. (a) Deposited lithium film based on Cs+-containing electrolyte. Reprinted with permission from ref 73. Copyright 2014 American Physical Society. (b) Columnar deposited lithium based on LiF-rich Cu surface. Reproduced with permission from ref 74. Copyright 2017 WILEYVCH. (c) Li ion plating behavior based on LiPF6-EC/DEC electrolyte with nanodiamond additives. Reprinted with permission from ref 75. Copyright 2017 Nature Publishing Group.


Designing Realizable and Scalable Techniques for Practical Lithium

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