Lithium-Anode Protection in Lithium–Sulfur Batteries

Lithium-Anode Protection in Lithium–Sulfur Batteries

Trends in Chemistry Review Lithium-Anode Protection in Lithium–Sulfur Batteries Chong Yan,1,2 Xue-Qiang Zhang,3 Jia-Qi Huang,2,* Quanbing Liu,4 and ...

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Review

Lithium-Anode Protection in Lithium–Sulfur Batteries Chong Yan,1,2 Xue-Qiang Zhang,3 Jia-Qi Huang,2,* Quanbing Liu,4 and Qiang Zhang3,* Lithium–sulfur (Li–S) batteries show significant promise as next-generation energy-storage devices due to their high energy density (2600 Wh kg-1). However, the severe shuttling of polysulfide intermediates and low Coulombic efficiency during operation induce rapid capacity loss, hindering their practical applications. Although sulfur coin cells can reach 1000 cycles, sulfur pouch cells reach only dozens of cycles before the lithium-metal anode is damaged by the electrolyte and/or polysulfides. Therefore, lithium-metal protection is an important issue in realizing long lifespans of Li–S pouch cells. In this review, we highlight recent progress on lithium-metal protection, including altering the solvation structure of lithium ions in the liquid electrolyte, designing an artificial solid electrolyte interphase (SEI), employing solid-state electrolytes, and adopting micro/nanostructured hosts.

The Gap between Coin and Pouch Cells Li–S batteries are strongly considered as promising next-generation batteries due to their high energy density (2600 Wh kg-1), wide range of operating temperatures ( 30 to 60 C), and low electrode material costs [1,2]. Li–S batteries have been studied since the 1970s [3]; the resulting literature can be roughly categorized into three developmental stages. The first stage primarily concerned the development of a functional rechargeable battery [4–11]. Commonly, organic electrolytes [e.g., dimethyl sulfoxide, tetrahydrofuran, dimethoxyethane, tetraethylene glycol dimethyl ether, 1,3-dioxolane (DOL)] and polymer electrolytes (e.g., polyethylene oxide) were employed. Later, the appearance of carbon/sulfur and sulfur@polyacrylonitrile (S@PAN) composite cathodes provided significant potential in realizing high energy-density batteries [12,13]. However, the shuttling of polysulfide intermediates induced by the complex electrochemical/chemical reactions among polysulfides, the lithium-metal anode, and the sulfur cathode continuously consumes electrolyte and lithium metal, forming ‘dead lithium’ [14]. As a result, the primary mission of the second stage was to understand the behavior of polysulfides and improve cathode stability [15–26]. With the development of hierarchical porous carbon materials, carbon/sulfur composite cathodes were capable of withstanding volume changes and mitigating the dissolution of polysulfides in electrolytes [27–31]. Furthermore, S@PAN cathodes [32,33] underwent rapid development, exhibiting potential in long-term cycling. Although the reaction mechanisms associated with S@PAN are controversial, it has attracted considerable attention as a cathode material due to the large number of potential solvents and lower sulfur:electrolyte ratio compared with that of carbon/sulfur composite cathodes. In addition, emerging separator modification, electrolyte optimization [34–37], LiNO3 additives [38–43], and the employment of redox mediators to promote kinetic propelling of polysulfides during cycling [44,45] can allow a coin cell to achieve >1000 cycles. Typical coin cells used in laboratories are CR2032 or CR2025, in which the active areas of the cathode and anode are relatively small. By contrast, pouch cells contain larger active areas often leading to more nonuniform current densities and shorter lifespans. Li–S coin cells with 300-cycle lifespans can be achieved [46], indicating that contemporary strategies can solve the issues presented by the working cathodes. Typically, the amount of lithium metal in a coin cell is in excess (for a 500-mm-thickness Li plate, 100 times in excess for a 1.0mAh cm-2 cathode). By contrast, the amount of Li is in only slight excess in practical Li–S batteries. The employment of ultrathin lithium foil (<50 mm) and a low electrolyte:sulfur ratio (<5:1 ml g-1) results in only dozens of cycles in pouch cells [47]. This indicates that the Li-metal anode is seriously corroded by liquid electrolyte and polysulfides (Figure 1). Therefore, we can conclude that the third stage of Li–S batteries has formally arrived (Figure 2).

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Highlights The gap between coin and pouch lithium–sulfur (Li–S) batteries emphasizes the importance and urgency of lithium-metal protection in Li–S batteries. Effective strategies for protecting the Li-metal anode include: altering the solvation structure of lithium ions; designing an artificial solidelectrolyte interphase; employing solid-state electrolytes; and adopting micro/nanostructured hosts. The approaches to protect the Limetal anode must consider both regulation of Li deposition and prevention of corrosion from polysulfides.

1School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China 2Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, China 3Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China 4School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China

*Correspondence: [email protected], [email protected]

https://doi.org/10.1016/j.trechm.2019.06.007 ª 2019 Elsevier Inc. All rights reserved.

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In a recent Opinion article, Meng and colleagues [48] assert that the lithium-metal anode is critical to overcoming the energy-density bottleneck of current lithium-ion chemistry. They highlight controversial issues surrounding the Li-metal anode, revealing the underlying cause failure and the true role of interphase in lithium-metal anodes. In this review, we present an overview of Li–S battery development and deliver a critical overview of recent research on lithium-metal protection, including altering lithium-ion solvation structure, designing artificial SEIs, employing solid-state electrolytes, and adopting micro/nanostructured hosts. This review covers only approaches to protect the Li-metal anode in Li–S batteries; Li-metal protection strategies in other energy-storage systems are outside the scope of the article. The protection of Li metal is an urgent and important topic for realizing practical long lifespans in Li–S batteries.

Strategies for Lithium-Anode Protection Lithium-Ion Solvation Structure Altering the solvation structure of lithium ions (Li+) to regulate the SEI of graphite is widely accepted in Li-ion batteries. For example, cyclic esters [ethylene carbonate (EC) and propylene carbonate (PC)] preferentially coordinate with Li+, participating in its primary solvation shell. At low anode potentials, cyclic esters are preferentially reduced on the graphite surface, forming an effective SEI layer [49–53]. This strategy is also effective for Li-metal protection in Li–S batteries. By regulating the solvent molecules in the primary solvation shell of Li+, distinct SEI layers can be achieved on the lithium surface [54]. Generally, there are three guiding principles when altering the solvation structure of Li+ in ether electrolytes (frequently employed in Li–S batteries). First, Li ions are primarily solvated by linear or cyclic ether solvents via interactions between lone-pair electrons of the oxygen atoms [55]. The amount of O atoms and the spatial configuration of linear solvents determines the ability of Li+ to dissociate. For example, Pang and colleagues demonstrated that diethylene glycol dimethyl ether (G2) coordinates well with Li+. The coordination ability of

Figure 1. The Configuration and Failure Mechanism of a Lithium–Sulfur (Li–S) Pouch Cell. (A) The internal components of a pouch cell are configured with folded units, including cathodes, anodes, a separator, and aluminum current collectors. (B) The depletion of liquid electrolytes and the powdering of lithium foil are the direct factors resulting in failure of pouch cells; the sulfur cathode incurs only slight damage and can work well when matched with fresh electrolyte and anode. Reprinted, with permission, from [47].

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Figure 2. A Brief Timeline Summarizing the Development of the Lithium–Sulfur (Li–S) Battery. The whole process was divided into three stages, mainly including: (i) how to cycle the Li–S battery (1970–2002); (ii) how to improve the cathode (2002–2014); and (iii) how to protect the anode (2014–present). In the first stage, the most important concept was finding desirable electrolytes. Liquid electrolytes (e.g., DOL, DME, TEGDME, DMSO) and polymer electrolytes (e.g., PEO, PVDF-HFP) were explored and are still used today. In the second stage, the appearance of nanostructured C/S and S-SPAN cathodes as well lithium nitrate enabled longer service life of coin Li–S batteries. In the third stage, the lithium-metal anode has become the bottleneck for long-cycle-life pouch cells; how to protect the anode is now the most important subject. See also [1,4–14,26– 31,39,47,54,57,63,71,75,91].

the linear or cyclic solvents to configure the primary solvation shell of Li+ can dominate the components and corresponding stability of the SEI [31]. Recently, Park and colleagues [55] claimed that the linear dimethyl ether (DME) shows a stronger trend to form primary sheaths than cyclic DOL in lithium-bis(trifluoromethane)sulfonimide (LiTFSI) and LiNO3 electrolytes (Figure 3A). Second, long-chain and short-chain polysulfides (Li2Sn, where n % 4 for short-chain polysulfides) have also been shown to solvate Li+ ions. For example, molecular simulations from Kamphaus and colleagues [56] demonstrated that polysulfides are capable of coordinating with more than two Li+ ions to create complicated structures when Li+ is present in excess (Figure 3B); multiple Li+ coordination with polysulfide intermediates generates polysulfide clusters. Third, the concentration of lithium salt cannot be neglected when probing the solvation structure in an operating battery. Camacho-Forero and colleagues [57] concluded that trans structures (the homologous ligand, TFSI here, is diagonal to Li+) play a dominant role in dilute lithium-salt electrolytes, whereas cis structures (TFSI is adjacent to Li+) are observed in solutions with higher concentration of Li salt. The distinct structural change further determines the structure of the complex solvation networks. Recently, significant progress has been achieved for high-salt-concentration electrolytes by Wang and colleagues [58] by inhibiting the dissolution of lithium polysulfides in 4-M LiTFSI/dibutyl ether (DBE) electrolyte. In summary, more detailed mechanisms and connections between Li+, polysulfides, solvents, and lithium salts are urgently needed.

Artificial SEI The fabrication of an artificial SEI on the lithium surface is the most widely considered approach for Limetal protection. The introduction of an artificial SEI can reduce contact between the electrolyte and

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(A)

(B)

Figure 3. Molecular Simulations Revealing the Interactions of Li+ Ions with Solvating Molecules. (A) Ab initio molecular dynamics of the polysulfide species coordinated with dimethyl ether (DME) and 1,3dioxolane (DOL). The distribution peak of DME (at 0.16 nm) closer to the Li ion than the DOL molecule (at 0.3 nm) confirms that the coordination of DME bidentate to the Li ion retains a relatively stable solvation structure. (B) Molecular simulations on the solvation structure of the Li ions with DOL and DME; the solvent solute interactions are between the oxygen atoms from both solvents and the Li+ ions from the polysulfide species. Reprinted, with permission, from [55,56].

lithium metal, limiting electrolyte decomposition and Li-metal consumption [59]. Additional advantages include facile design, easy operation, and large-scale coating with controllable thickness [60]. Generally, this strategy is classified into two categories based on electron and ion channels. In the first strategy, a protective film with electronic insulation but ion-conducting features has been frequently reported. For example, Li and colleagues [61] showed that a 10-nm-thick 2D molybdenum sulfide (MoS2) passivated layer can be directly coated and lithiated onto the surface of Li metal, and the passivated layer can regulate Li electrodeposition and suppress dendrite nucleation sites, leading to higher energy density and safer Li-metal-based batteries. Han and colleagues [62] demonstrated that the introduction of ultrathin aluminum oxide (Al2O3) by atomic layer deposition can effectively prevent parasitic decomposition and maintain the interface stability. Recently, Zhang and colleagues [63] confirmed that sulfurized and nitridized SEIs effectively protect anode Li metal (Figure 4). Wang and colleagues [64] proposed a dense, homogeneous lithium phosphorus oxynitride (LiPON) layer on the Li-metal anode and employed the protective Li

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metal in a 2.0-Ah-capacity Li–S pouch cell. The as-obtained pouch cell with the protective Li exhibits improved capacity retention even after 120 cycles, a positive trend for the practical application of Li–S batteries. Liu and colleagues [65] report that cyclic ether electrolytes can undergo ring-opening reactions to form a quasi-solid gel-polymer electrolyte (GPE) with the addition of LiPF6; the GPE displays effective protection for Li-metal anodes. Li and colleagues [66] designed a stable interfacial protective layer via decomposition of a thionyl chloride (SOCl2) additive, and the rate performance as well as the cycling performance are remarkable. It is worth mentioning that the electronically insulating layers should be as thin as possible to minimize potential added resistances. In summary, the use of electronically insulating films is an effective approach to prolong the lifespan of Li–S batteries. The interaction between Li+ ions and anions in the sulfurized, fluorinated, and nitridized SEI remains relatively underexplored; thus, there exists a large research space to be studied further. In the second strategy, mixed ion- and electron-conducting interphases (MCI) are employed. The concept of MCI affords fresh insights into the regulation of Li deposition. Recently, Ni/LiF [67], Cu/Li3N [68], Cu/LiF [69], and Al/LiF [70] layers have been shown to successfully stabilize Li/electrolyte interfaces. However, the molecular mechanisms underpinning the protective layers remain controversial. One possible explanation is that

Figure 4. The Sulfurized and Nitridized Artificial Solid Electrolyte Interphase (SEI) That Is Prepared via the Electrochemical Method. Li metal with a stable SEI can be transplanted into ether and ester electrolytes to efficiently cycle sulfur and LiNi0.5Co0.2Mn0.3O2 cathodes, respectively. Reprinted, with permission, from [63].

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the layer simultaneously possess high mechanical strength, flexibility, and Li-ion conductivity [68]. Another potential explanation is that, in the case of Cu, Cu atoms break down the long-term-order crystals to create more domains for rapid movement of Li+ [69]. Overall, the MCI layer serves to redistribute Li ions without increasing the resistance. The electronic conductivity should not be too high (<10-3 S cm-1) to ensure that the Li deposition barrier is much higher than that on the lithium surface. If the MCI’s electronic conductivity is too high such that lithium preferentially deposits on the surface, Li dendrites can form in the MCI. The optimal strategy for balancing contributions from the ionic and electronic conductor in the MCI is to regulate the electronic conductivity and increase the ionic conductivity. Therefore, the MCI protective layer is neither too thick nor too thin, often in the range of 100 nm to 5 mm. If the layer is too thin (at the nanoscale), it can be easily damaged. If the layer is too thick (tens of microns), Li ions slowly transport through it. In contrast to the first strategy, the employment of a MCI layer will be instrumental to promoting the fast-charging field of a working battery. However, if an artificial SEI covers the Li anode, the SEI layer is at a risk of damage through continuous deformation. The stripping and plating process of Li is a conversion reaction that can easily induce electrode volume change. [48] Once the SEI is damaged, the question then becomes ‘how to repair the broken defects’. Rational combination with electrolyte additives can facilitate self-repair of the artificial SEI while maintaining its uniformity and stability. Another effective method to prevent the damage is the introduction of a flexible artificial SEI that can bear the volume change of the working electrode. Importantly, the ionic conductivity of the flexible SEI should be high enough to afford Li-ion transport while the electronic conductivity should be low enough to forbid the internal deposition of Li.

Solid-State Electrolytes When traditional liquid electrolytes are replaced by solid-state electrolytes (e.g., Li7La3Zr2O12, Li0.5La0.5TiO3, Li10GeP2S12, Li1.3Al0.3Ti1.7(PO4)3, Li3OCl), liquid consumption and lithium-metal corrosion are avoided [71]. In addition, the high melting point of solid-state electrolytes makes the solid-state battery a potentially safer energy-storage system than systems containing flammable liquid electrolytes [72]. Janek and colleagues initially put forward emerging methods to probe the interfaces between solid-state batteries and Li anodes, such as electrochemical pressure impedance spectroscopy (EPIS) [73], X-ray photoelectron spectroscopy–secondary ion mass spectrometry (XPS-SIMS) [74], and X-ray photoelectron spectroscopy–X-ray absorption spectroscopy (XPS-XAS) [75]. Several significant discoveries have been achieved, including gas evolution in solid-state electrolytes and harmful decomposition reactions on the electrolyte/electrode interfaces (Figure 5) [76,77]. Meanwhile, Wang and other groups [78–80] have also performed critical fundamental research on solid-state electrolyte interfaces and ion channels. Yao and colleagues [81] demonstrated that unique interfacial structures and components can be reformed when Li contacts the solid-state electrolyte, possibly establishing favorable lithium-ion conductive channels at the interface, leading to high-performing of Li–S batteries. The interface between the electrolyte and Li is stabilized by the formation of a SEI layer. An ideal solid-state electrolyte exhibits a high ionic conductivity at room temperature and excellent compatibility with lithium metal. Therefore, the ionic conductivity is targeted to be 10 4 to 10 3 S cm 1. The use of solid-state polymer electrolytes affords high ionic conductivity and efficiently prevents the shuttling effect in Li–S cells, leading to improved cycling performance [79]. The inherent relation among polymer chains, polysulfides, and lithium should be probed further. Another type of solid-state electrolyte is glass-ceramic electrolyte. However, most of them cannot be used as the electrolyte of a Li–S battery due to the harmful decomposition reactions with Li or polysulfides [80,82]. Sulfide solid-state electrolytes that exhibit superior chemical compatibility with lithium have been successfully applied in solid-state Li–S batteries. In addition, quasi-solid-state electrolytes are being developed as potentially better alternatives for balancing potential safety and ionic conductivity [83–85]. In summary, research focusing on the contact between solid-state electrolytes and Li remain in the early stages.

Micro/Nanostructured Lithium-Metal Host Encapsulating metallic lithium in a porous matrix (termed a ‘host’) can effectively overcome the aforementioned volume expansion/contraction issues. In general, the shape of a typical lithium-metal

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Figure 5. X-Ray Photoelectron Spectroscopy (XPS) Spectra for a Pristine Sample and after Lithium-Metal Deposition (for Comparison). Unique interfacial structures and components can be reformed at the interface. Reprinted, with permission, from [82].

anode is that of a flat plate lacking porosity. The absence of a host in typical lithium plates affords limited space to bear the volume changes experienced during Li deposition, further resulting in the uncontrollable growth of dendrites. As a result, micro/nanostructured lithium-metal hosts with porosity have become an effective strategy to deal with volume changes and disordered lithium plating/stripping. The distinctive surface, hierarchical pores, and connecting organization of diverse nanomaterials results in the unique stripping/plating feature of Li-metal anodes. Micro/nanostructured hosts can be divided into three categories: conductive micro/nanostructured frameworks, nonconductive micro/nanostructured frameworks, and prelithiation in graphite frameworks. In the first category, conductive micro/nanostructured frameworks possess the ability to accommodate Li [86] and primarily comprising carbon–fluorine frameworks [87], metal or metal oxide frameworks [88–90], and carbon-based lithiophilic frameworks [91,92]. Here, lithium dendrites are inhibited by the lower local current density, alloying reaction, and guided Li plating (Figure 6). In the second category, nonconductive micro/nanostructured frameworks are unable to directly guide Li deposition. Here, the polar functional groups of the nonconductive hosts play a critical role in regulating a uniform distribution of Li ions in the vicinity of the framework, promoting uniform Li-ion deposition. For example, Cheng and colleagues [93] reported that strong polar 3D SiO2 glass fibers can serve as the interlayer between the Li-metal anode and the separator. Nonconductive hosts can also adsorb Li ions to compensate for electrostatic interactions and concentration diffusion between Li ions and bulges of the anode, avoiding the gathering of Li ions around bulges. In addition, the Li deposition occurs between the nonconductive host and the current collector due to the electronic insulation of host. Then, in the third category, the construction of a recharged prelithiated graphite anode framework for Li2S/sulfur cathodes is an important potential concept to protect Li [94–97]. By preplating enough lithium into the layered graphite skeleton and matching the integrate SEI protective film, the collapse of the layered skeleton and the capacity loss of the anode are efficiently

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restrained. For example, Chen and colleagues [98] report that the capacity retention of a prelithiated graphite/sulfur cell can be maintained at 65% even after 450 cycles (Figure 7). Therefore, prelithiated graphite frameworks exhibit significant potential as anode hosts in Li–S batteries. It is worth mentioning that the employment of micro/nanostructured hosts will sacrifice the battery energy density.

Concluding Remarks Significant progress on lithium-metal protection in Li–S batteries has been recently achieved. The primary strategies include altering Li-ion solvation structure, designing an artificial SEI, and employing solid-state electrolytes and micro/nanostructured hosts; these strategies exhibit significant potential in stabilizing the interface of Li-metal anodes. Over the long term, solid-state Li–S batteries are the best choice to overcome the current problems. However, before realizing this ultimate choice, more theoretical research with a mechanistic focus on lithium-metal interfaces should be carried out (see Outstanding Questions). Research on the interfaces between liquid or quasi-solid-state electrolytes and the Li-metal anode are needed, which will promote Li-anode protection in working cells.

Figure 6. Conductive Lithiophilic Micro/Nanostructured Framework Can Guide the Deposition of Li Due to Its Ag Particles and Interconnected Network. Such an electrode design renders dendrite-free morphology during repeated stripping/plating cycles and extraordinary electrochemical performance in Li–LiFePO4 and Li–S cells. Reprinted, with permission, from [92].

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Outstanding Questions Can we compile evidence that demonstrates that all-solid-state or quasi-solid-state Li–S batteries can realize long lifespans without dendrite formation and lithium source loss using ultrathin lithium? When referring to polysulfide solubility in Li–S battery electrolytes, should we choose a solvent with higher solubility or lower polysulfide solubility? What are the advantages/disadvantages of liquid-phase and solid-phase pathways to the lithium-metal anode? Is a ‘good’ solid electrolyte interphase (SEI) a prerequisite for the sulfur cathode to be matched with a graphite skeleton? What are additional considerations when we implant the current lithium-ion graphite SEI layer into Li–S batteries? Which factor dominates the capacity decay of Li–S batteries: the cathode, the anode, or the separator?

Figure 7. A Schematic of a Lithium–Sulfur (Li–S) Battery Including a Prelithiated Graphite Framework Covering a Stable Solid Electrolyte Interphase (SEI) Layer to Maintain a Long Battery Lifespan. Reprinted, with permission, from [98]. DOL, 1,3-dioxolane; BTFE, bis(2,2,2-trifluoroethyl) ether.

In addition, in situ characterization using optical microscopy, Raman spectroscopy, transmission electron microscopy, and other advanced characterization techniques that can isolate air and water from the working system are strongly required. It is worth mentioning that while we explored lithium-metal protection in this review, ultrathin lithium foil (<50 mm) and higher current densities (over 3.0 mA cm 2) need to be considered to make research closer to practical applications. More specifically, when a Liprotection method is assessed, <50-mm-thick lithium foil will replace the current thick lithium plate and the current density used should be higher than 3.0 mA cm 2. Moreover, the ratio of electrolyte to sulfur should be controlled to be <5 ml g-1 and the areal cathode capacity is suggested to be over 3 mAh cm 2. It is the inevitable choice to achieve an energy density of 500 Wh kg-1 or more. Only by employing these metrics can we truly bridge the technology gap between academic laboratory research and industry.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21676160, 21776019, 21825501, and U1801257), the National Key Research and Development Program (2016YFA0202500 and 2016YFA0200102), the Beijing Key Research and Development Plan (Z181100004518001), and the Tsinghua University Initiative Scientific Research Program.

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