Renjie
Chen†
*,
Teng
Zhao†
and
Feng
Wu
*
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, China. E-mail: chenrj@bit.edu.cn; wufeng863@vip.sina.com
First published on 6th August 2014
In terms of sustainable development and environmental issues, the design and fabrication of efficient energy storage devices will be more critical in the future than at any time in the past. Li–S batteries are promising candidates for such a purpose due to their high specific capacity and low environmental impact. This review has systematically retraced the advances in the field of Li–S batteries over the past half century and highlighted the main breakthroughs in a number of areas, covering the mechanism determination, cathode engineering, theoretical simulation, and electrolyte tailoring and anode protection. Furthermore, we discuss the remaining challenges towards their practical application. It is expected that Li–S batteries with 3D inter-connected or conformal assemblies will surpass new horizons in the coming years.
Elemental sulphur (S), also referred to as brimstone, is one of the ancient non-metals on the earth. Recognized by Antoine Lavoisier in 1777, S was initially used as an essential ingredient of black gunpowder, whose invention was a leap forward for human civilization. More recently, S has been playing another important role in the development of energy storage and conversion systems. In light of its small atomic number and multi-electron capability with Li metal, S has a theoretical specific capacity of 1672 mA h g−1 and a specific energy of 2600 W h kg−1, which are among the highest of solid cathode materials.5 In addition, as a byproduct of petroleum refinement or the direct extraction from sulphate minerals, its abundant resources and low environmental impact are beneficial in the current carbon-constrained world, especially in terms of sustainable development. The development of Li–S batteries dates back to 1962 when Herbert and Ulam first introduced the concept of the S cathode.6 In 1967, the Argonne national laboratory developed a high-temperature Li–S system by using molten Li and S as two electrodes.7 However, the problem of electrode containment remained challenging. In the following years, E. Peled's group evaluated the electrochemical behavior of S cathodes in organic electrolytes at room temperature, which gave new insight into the solvent-related redox mechanism and laid the foundations for tailoring suitable electrolyte components in subsequent research activities.8 Another notable breakthrough in the development of room temperature Li–S systems was made through rational cathode design in 2009 when Nazar's group pioneered the development of highly ordered mesoporous CMK-3 for S encapsulation, which successfully enhanced the capacity and cycle life of batteries.9 Only one year later, Sion Power announced that their Zephyr, an unmanned aerial vehicle (UAV) powered by solar energy and their proprietary Li–S batteries, had exceeded 336 hours (14 days) of continuous flight, smashing the world record for the longest duration unmanned flight. Li–S systems are still being intensely studied in universities and companies across the world in order to drive the specific energy up to 500 W h kg−1. It is expected that with these joint efforts, Li–S batteries will surpass new horizons in the next few years. This review summarizes the progress in the field of Li–S batteries over the past half century from a systematic perspective.
S8 + 16Li+ + 16e− → 8Li2S |
In fact, this electrochemical reaction is a multi-step process that can involve different intermediate species, such as Li2S8, L2S4, and Li2S2. Based on the phase changes of the S species, the discharge process can be mainly divided into two stages, which is consistent with the two-plateau behavior of the discharge curves of Li–S batteries:
S08 + 4e− → S42− |
Fig. 1 Schematic illustration of the multi-step electrochemical mechanism involved in a typical Li–S battery, consisting of a S cathode, organic electrolytes and a Li anode. Derived from figures reported by K. Amine et al.10 |
The high and steep plateau at potentials of ∼2.3 V is attributed to the formation of long-chain Li2Sn (4 ≤ n ≤ 8). On average, each S atom accepts 0.5 electrons, causing the reduction to Li2S4.
S04 + 4e− → 2S2− + S22− |
The following long and planar plateau at potentials of ∼2.1 is related to the further reduction of Li2S4 to Li2S by accepting 1 electron per S atom.
However, the detailed mechanism of Li–S batteries is still controversial despite decades of research. Barchasz and coworkers proposed that the first reduction from S8 to S42− could be further divided in two steps.11 Firstly, they proposed the formation of S82− and the disproportionation into S62−, followed by the reduction of S62− to produce S3˙− radicals and S42−. On the other hand, according to Levillain's study, three redox couples (S32−/S3˙, S42−/S4˙, and S82−/S8) exist between the lower and higher plateau.12 Recently, Nazar's group shed new light on S speciation by using in situ operando X-ray absorption near-edge spectroscopy (XANES), as shown in Fig. 2.13 They found that the rapid reduction kinetics of S62− in solution leaves elemental S in reserve, which is the reason for the limited discharge capacity. Unfortunately, the complicated reduction mechanism also lead to a notorious performance-related issue of Li–S batteries, the shuttle effect of polysulphides (PS) in the organic electrolyte.14 Specifically, long-chain PS, generated at the cathode, can diffuse to the Li anode where a parasitic reaction can happen. Then, the reduced short-chain PS, recombined with a second long-chain PS, could diffuse back to the cathode and be re-oxidized. This cyclic process causes an internal shuttle phenomenon, which is the main cause of capacity degradation and low Coulombic efficiency.
Fig. 2 (a) Schematic diagram of the operando cell; (b) probing the S speciation during redox behavior by operando X-ray absorption spectroscopy. Derived from figures reported by L. F. Nazar et al.13 |
However, there are still other problems plaguing the development of Li–S batteries. The electrically and ionically insulating nature of S leads to poor electrochemical accessibility and thus low utilization. In addition, because the volume density of Li2S (1.67 g cm−1) is much lower than that of S (2.03 g cm−3), a volume expansion of ∼22% within the cathode would occur during the discharge process, which would cause serious cathode pulverization and thus shorten the cycle life. Considering the anode, Li dendrites generated by uneven surface deposition have long been a safety problem.
To address these critical problems, various valid approaches, ranging from optimizing the cathode/anode structures to tailoring suitable electrolyte components, have been developed, which will be elaborated in detail in the following parts.
Fig. 3 Core–shell design of S-containing composites: (a) S@PTh composite,16 (b) MWCNTs@S–PPy composite,18 (c) CMK-3–S–PEDOT composite,19 (d) VOx wrapped S–CMK-3 composite;22 morphology control of S core: (e) hollow sphere;23 (f) bi-pyramid;24 (g) nano sheet.25 |
In order to enhance the structural stability of the electrode and inhibit the dissolution of the active material, another attractive type of coating layer is inorganic oxides. Nazar and coworkers demonstrated that porous silica could act as a polysulfide reservoir to stabilize S cathodes by weak binding.21 Based on the above conclusion, they further enshrouded CMK3–S composites with a thin SiOx or VOx layer by a surface-initiated growth method (Fig. 3d).22 The thin inorganic metal oxide coatings could largely diminish the PS shuttle effect at the price of a slightly lower initial capacity. These effects were much more remarkable when this method was applied in a macro-porous carbon–sulfur composite.
In addition to dual core–shell hybrid systems, designing a S core with a unique nano-morphology is also an effective strategy for simultaneously tackling the inherent problems of S chemistry. Recently, Cui and coworkers reported a high-performance hollow S@PVP nanostructured cathode through a facile, bottom-up approach (Fig. 3e).23 The hollow S core provided sufficient space for accommodating volume changes while the rigid PVP shell effectively avoided outward expansion or breakage of the S and limited the PS diffusion into the electrolyte. The composite showed a superior long-term cycle life of over 1000 cycles with a capacity decay as low as 0.046% per cycle and an average Coulombic efficiency of 98.5%. Remarkably, by a simple surface modification with PEDOT, the S@PVP composite could further achieve an excellent high-rate capability, showing a high reversible capacity of 849 and 610 mA h g−1 at 2 C and 4 C, respectively. Other S core designs, such as orthorhombic bi-pyramidal S,24 S nano sheets,25 ultrafine S nanoparticles26 and monoclinic S,27 are also attractive in terms of capacity, stability and rate capability as shown in Fig. 3f and g.
Although core–shell engineered cathode composites have brought about great advances in Li–S batteries, this design inevitably has its own drawbacks. Firstly, it is difficult to form a uniform coating layer on the surface of S particles or composites. Secondly, because of the firm contact between the core and the shell, the mechanical stress arising from volume changes may cause the cracking and fracture of the protective shell. These drawbacks are the main reasons for the unsolved capacity fading problems. Fortunately, designing yolk–shell architecture with an internal void could provide an alternative approach to overcome these challenges.
Fig. 4 Yolk–shell S@TiO2 and S@PANi composites: (a), (e) schematic of the formation process; (b), (f) TEM images; (c), (g) the particle size distributions; (d), (h), (i) electrochemical performance. Derived from figures reported by Y. Cui et al.29 and W. Zhou et al.30 |
However, yolk–shell designs suffer from complicated synthesis procedures under limited conditions. The content of S encapsulated must be precisely controlled in order to retain a sufficient void for expansion. Additionally, the coating shell should be rigid and electrically and ionically conductive so that an excellent cycle stability and rate capability can be achieved. Therefore, it is desirable to develop simple and facile approaches to tailor S cathodes with the aim of practical application.
Fig. 5 Scheme of (a) self-weaving S/MWCNT cathodes and (b) the Li–S configuration with free-standing MWCNT interlayer between the S cathode and the separator; (c) model of the discharge–charge process of S/MWCNT nanomicrosphere with hierarchical architecture. Derived from figures reported by A. Manthiram et al.32,33 and J. J. Chen et al.36 |
Although woven MWCNTs provide robust and cross-linked conductive networks, their 1D morphology inherently limits the ion and electron transfer pathways along the long axis of the MWCNTs, which results in a low rate performance for the cell. Additionally, the surface areas and pore volumes of MWCNTs are much lower than those of graphene or porous carbon, resulting in limited S loading and low interface kinetic activity. Hence, it is necessary to combine MWCNTs with other conductive matrices to overcome these problems by synergistic effects.
However, it should be noted that graphene itself is not capable of confining soluble PS due to its large surface, interlayer gallery pores and hydrophobic nature. Hence, further modifications of these composites are still necessary, including polymer44/carbon coating45 and silica imbedding.46 For example, Cui and coworkers used a simple solution assembly process to form graphene wrapped S@PEG composites.47 The function of the PEG coating layer on the S has three aspects. Firstly, it could be used as a capping agent to limit the size of the S particles to the sub-micrometer region. Secondly, it could act as a cushion to accommodate the volume change during cycling. Thirdly, it provides a chemical gradient to retard the diffusion of polysulfide anions. By taking advantage of these effects and the unique properties of graphene, the composite showed high and stable specific capacities of up to 600 mA h g−1 over more than 100 cycles.
Recently, the fabrication of graphene based 3D hierarchical S cathodes has become a promising new strategy to tackle the intrinsic problems associated with S chemistry. Our group incorporated 1D core–shell MWCNT@S composites into the interlayer galleries of graphene (GS) through a facile solution assembly process.48 The unique 3D sandwich-type architecture of the GS-MWCNT@S composite brought advantages in electron/ion transfer, the confinement of PS and the accommodation of volume variation. Specifically, the hybrid carbon matrix consisting of MWCNTs and GS provided a 3D conductive network, thus effectively reducing electron transfer resistance. The pores within this framework are favorable for fast ion diffusion. More importantly, the residual oxygen group on the surface of the GS and the absorbent ability of the MWCNTs generated synergistic effects in retarding the PS dissolution. Finally, the flexibility of the GS could buffer the volume change of the S during cycling, thus preventing electrode degradation. As a result, the GS-MWCNT@S composite with an S loading of 70 wt% maintained a reversible capacity of 844 mA h g−1 after 100 cycles and also demonstrated a good rate capability, 743 mA h g−1, at a rate of 1 C. In another valid approach, elemental S was directly encapsulated into the internal spaces of graphene/single-walled carbon nanotube (G/SWCNT) hybrids, which were fabricated by a facile catalytic growth method.49 Compared with physical combination, the chemical connection between the SWCNTs and the graphene could facilitate a higher electrical conductive pathway, thus achieving a higher utilization of the S. Meanwhile, its abundant mesoporous and robust structure could offer better confinement of PS and accommodation of volume change. Accordingly, the G/SWCNT-S cathode with S loading of 60 wt% maintained a capacity of 650 mA h g−1 after 100 cycles at 5 C with a Coulombic efficiency of 92%. However, most of the cathode systems reported require an additional binder, conductive additive or separate metallic collectors, which decrease the energy density of the battery to a large degree. To meet the long-term needs of electric vehicles, it is urgent to develop novel binder/collector free cathodes. Recently, a novel 3D graphene foam based S cathode was proposed by Xi and coworkers.50 Ni foam was chosen as a sacrificial template for growing few-layered graphene (FLG) by a CVD method while active S was introduced into the free-standing FLG foam via a solution infiltration method (Fig. 6a). The cathode exhibited a stable cycle life and a high rate performance (300 mA h g−1 after 400 cycles at 3200 mA g−1) owing to the monolithic 3D conductive network of the FLG foam and the large proportion of micropores within it, though the overall capacity was relatively low. Further improved battery performances could be envisioned by constructing conductive hybrid structures with high surface areas (Fig. 6d and e).
Fig. 6 (a–c): SEM images of (a) S–FLG foam and the elemental mappings of carbon (b) and sulfur (c). (d) Scheme of the fast 3D electron/Li+ transfer pathway inside the S–FLG foam; (e) cycle performance and Coulombic efficiency of S–FLG foam cathode. Derived from figures reported by R. V. Kumar et al.50 |
Fig. 7 Schematic illustration of S–porous carbon composites with different porosity: (a) SBA-15 templated mesoporous CMK-3/S; (d) KOH activated bimodal porous C/S; (g) uniform carbon sphere/S tailored with pore formers; characterization of morphology: (b), (e), (h); cycle-performance: (c), (f), (i). Derived from figures reported by L. F. Nazar et al.9,54 and C. D. Liang et al.53 |
Actually, the most direct method to deal with the problem of PS dissolution is to break cyclo-S8 into chain-like S2–4, thus avoiding the unfavorable transition between S8 and PS. Guo and coworkers demonstrated that microporous carbons with a pore size of ∼0.5 nm could retain these small metastable S molecules due to spacial confinement.55 The as-obtained S–(CNT@MPC) composite exhibited novel electrochemical behavior with a single output plateau at ∼1.9 V and showed superior electrochemical performances in terms of specific capacity, cycling stability and high-rate capability. However, the S content in the composite was compromised at 40 wt% because of the narrow pore channels, which is not desirable for achieving a high specific energy. Therefore, tailoring hierarchically structured carbon hosts with controlled porosities for high S loading is required to realize a balanced performance for future application.56
In addition to the porosity, the morphology of these porous carbon materials is also of key importance in terms of the cycle stability and rate capability. Archer and coworkers suggested that a porous carbon capsule should maximize the loading of S while minimizing PS dissolution and shuttling in the electrolyte,57 and that a fast ion/electron transport pathway should be preserved. Following this design principle, S was sequestered into the large internal void and porous shell of nano-scale hollow carbon spheres through vapor infusion. Benefiting from the above characteristics, the C@S composite exhibited an excellent cycle life with a capacity loss less than 10% over 100 cycles (Fig. 8a). In addition, Cui and coworkers demonstrated that encapsulating S into closed hollow carbon nanofibers could achieve a stable capacity of 730 mA h g−1 over 150 cycles due to the limited S/electrolyte contact areas (Fig. 8c).58 On the other hand, Nazar and coworkers confirmed that controlling the morphology of porous carbon could also contribute to improved S utilization at high rates (Fig. 8b).59 They imbedded S into a novel spherical OMC with bimodal porosity, which had a distinct inner pore volume of 2.32 cm3 g−1 and high surface area of 2445 m2 g−1. Even at 1 C, the composite with 50 wt% S loading maintained a reversible capacity of 730 mA h g−1 after 100 cycles. More importantly, when the S ratio increased from 50 wt% to 70 wt%, the electrochemical performance was almost comparable, which was attributed to the homogeneous distribution of S within the nano-scale morphology of the OMC.
Fig. 8 TEM images of (a) S/hollow carbon sphere composites57 and (b) a S/ordered mesoporous carbon sphere.59 (c) SEM image of hollow S/hollow carbon nanofiber composites.58 |
Recently, metal organic framework (MOF) derived porous carbons have raised broad interest as S hosts owing to their tunable porosity and morphology. Xi and coworkers explored four Zn-containing MOFs as precursors and found that the porosity of the derived carbons varied due to different Zn/C ratios.60 S hosted in carbons with higher mesopore volumes exhibited increased initial capacities, while S hosted in those with high micropore volumes showed good cycle stabilities. It is expected that carbon with an optimum hierarchical porosity for S encapsulation could be attained by tuning the suitable metal ion content in MOFs. Moreover, MOF-templated porous carbons bring advantages in terms of synthetic procedure. No additional carbon source is necessary due to the presence of the organic ligands in the MOFs and the metal ion can be easily removed by a one-step pyrolysis. Through this clean and facile synthesis strategy, hierarchically porous carbon hosts, such as nanoplates derived from MOF-5,61 microporous carbon polyhedrons derived from ZIF-862 and so forth, have been applied in Li–S batteries aiming for industrial application.
Finally, a summary of typical strategies in S cathode engineering and the corresponding characteristics and properties are illustrated in Fig. 9. Clearly, morphology and porosity control are essential for constructing matrices with high S loadings, fast ion and electron transfer, confinement of PS and buffering of volume variation. In addition, it should also be noted that the hybridization of multi-elements within a matrix is necessary for achieving such multi-functions.
Fig. 9 Illustration of typical strategies in S cathode engineering and the corresponding characteristics and properties. |
By first-principle calculations, Cui and coworkers investigated the interaction between lithium sulphides (LixS, x = 1, 2) and a carbon surface (Fig. 10b).63 They demonstrated that the binding between LixS with the carbon surface was weaker than that for S, and that it was the detachment of those LixS moieties from the non-polar carbon that caused capacity decay. In light of this understanding, they modified the interface of hollow carbon nanofibers with PVP, an amphiphilic polymer. Owing to the oxygen atoms in the molecule, PVP exhibited a much higher binding energy for LixS and thus prevented the detachment. Recently, the same group investigated the effects of three conductive polymers (PPy, PANI, and PEDOT) on the chemical bonding with S.64 The ab initio simulations indicated that the O and S atoms in PEDOT could form a chelated coordination structure with Li2S, thus achieving a much stronger binding energy compared with the N atoms in PANI and PPy.
Fig. 10 Understanding the fundamental interactions between S and functional matrices and additives: (a) graphene oxide;65 (b) hollow carbon nanofibers with amphiphilic surface modification;63 (c) metal oxides;67 (d) N-doped graphene.68 |
Essentially, S species have a deep affinity with oxygenated groups. Researchers from Lawrence Berkeley National Laboratory immobilized S on quasi-2D graphene oxide (GO) by chemical deposition and subsequent thermal treatment.65 The coated S layer was uniform and thin and no bulk S was observed on the external surface of the GO. Through ab initio calculations, they found that the binding of S to a C–C bond could be significantly enhanced by epoxy and hydroxyl groups, whose effects depend on their distance between the S atoms. In addition, these oxygen groups also interacted strongly with S (Fig. 10a). As a result, the PS dissolution problem was effectively mitigated. Tarascon and coworkers further generalized this concept by exploring oxygenated porous architectures for S hosting.66 They proposed that the polarized surface activity induced by O atoms was essential for the interaction with charged PS. This conclusion was also reached by Nazar's group, who were using oxide structures as PS reservoirs (Fig. 10c).21,67
An alternative way to polarize the charge distribution on a surface is through heteroatom doping. Researchers discovered that introducing doping element such as N or B in a carbon matrix not only improved the conductivity but also enhanced the chemical adsorption of S. By conducting a density functional theory (DFT) calculation, Song and coworkers revealed that S was thermodynamically stable on N doped carbon and its binding with oxygen-containing groups was also strengthened (Fig. 10d).68 Essentially, N doping catalyzed the coordination between the S and O.69 In contrast to N dopants, B positively polarized the carbon matrix, thus leading to electrostatic interactions with the negatively charged PS.70
Although a number of simulations have been conducted to understand the mechanism of trapping PS within cathodes, there is still plenty of room for investigating the electrolytes, another important factor influencing battery performance. In the future, calculating the interactions between PS and the electrolyte will shed new light in tailoring suitable electrolyte components and propose a new redox mechanism during cycling.
Another promising solvent candidate is ionic liquids (ILs), characterized by wide electrochemical windows, high ionic conductivity and thermal stability. Owing to the weak interaction between the large cation and the flexible anion, the unique properties of ILs can be easily tuned by changing the ionic structure. For Li–S batteries, the solubility of S8 and PS in ILs is still an important factor, which is directly determined by the donor ability of the anionic structure.75 For example, the extent of PS dissolution in the strongly basic [P14][OTF] was almost the same as that in the TEGDME solvent. Consequently, choosing an anion with a weak donor ability is the first step for designing PS constraining ILs. Furthermore, the viscosity of the ILs is another factor to be considered due to its correlation with Li+ transport. Sun and coworkers investigated the electrochemical behavior of N-doped C/S cathodes based on an LITFSI/[MPPY][TFSI] electrolyte.76 According to the CV analysis, only one cathodic peak was observed at a high scan rate, which was mainly attributed to the sluggish mass transport in the viscous electrolyte. As a result, the cathode could only deliver a high capacity at low current density. Recently, “solvate” ILs, a new family of ILs, have been applied in Li–S batteries due to their faster transport of Li+ ions than the traditional binary systems of aprotic ILs and Li salts.77 Meanwhile, the formation of the complex cation could effectively suppress the dissolution of PS (Fig. 11a). All these effects contributed to a stable capacity of 600–700 mA h g−1 over 100 cycles (Fig. 11b). Moreover, the viscosity of the “solvate” ILs could be further reduced by hybridizing them with a low-viscosity organic solvent. Watanabe and coworkers explored solvate IL [Li(G4)][TFSA] for Li–S batteries and a discharge capacity of >700 mA h g−1 was achieved over 400 cycles. Notably, by adding a nonpolar, nonflammable fluorinated solvent, the power density of the Li–S battery was greatly enhanced due to fast Li+ mass transfer in the diluted electrolyte.78 It is believed that hybrid organic solvent–IL systems with a tailored donor ability will become promising candidates to develop practical Li–S batteries in the future if their production is scaled up and the corresponding cost is reduced.
Fig. 11 (a) Schematic illustration of the mechanism of PS dissolution in solvate ionic liquids; (b) cycle performance and Coulombic efficiency of Li–S batteries using the [Li(glyme)]X electrolyte. Derived from figures reported by M. Watanabe et al.77 |
Fig. 12 (a) Digital photograph of Li2S8 dissolution experiments with different LiTFSI salt concentrations in DOL–DME solvent. (b) Surface morphology of the Li anode with SIS-7# electrolyte after 280 cycles. (c) Comparison of cycle performance between different samples. Derived from figures reported by L. Q. Chen et al.81 |
On the other hand, functional additives are also necessary in order to improve the performance of batteries, in terms of temperature tolerance, cycle life and rate capability. Methylacetate (MA), a low freezing point ester, was added to a TEGDME based electrolyte to enhance the S utilization at low temperatures.82 With an optimized ratio (5%), high ionic conductivity and low interfacial resistance were achieved, and the battery operating at −10 °C exhibited an initial discharge capacity of up to 994 mA h g−1. Inspired by this result, Choi and coworkers systematically investigated the effects of using organic esters (MA, TOL, GBL) as co-solvents in electrolytes on the performance of Li–S batteries at room temperature.83 TOL, a non-polar and lyophobic solvent, was identified as the most effective solvent to increase the capacity because it could form a Li+ conductive film on the surface of the electrode and thus reduce the interfacial resistance.
Additionally, inorganic additives, such as LiNO3 and LiBOB also play a key role in protecting the Li anode from PS corrosion. N–O additives were initially reported to improve the “charge–discharge efficiency” of Li–S batteries by Mikhaylik in 2004.84 To understand the mechanism behind this phenomenon, Aurbach and coworkers analyzed the chemical compositions of the SEI layer on a Li anode by ex situ FT-IR and XPS.85 The critical role of the LiNO3 additive was related to two aspects. Firstly, it could form a LixNOy layer by direct reduction on the Li surface. Secondly, it could oxidize dissolved PS to form a LixSOy layer. Owing to these protective layers, the PS shuttle effects were inhibited, resulting in a high Coulombic efficiency. An alternative additive with the same function is LiBOB, but its effect was not as noticeable as that of LiNO3.86 Hence, further studies were concentrated on optimizing the salt concentration87 (0.4 M) and the suitable discharge cutoff value88 (>1.6 V) for LiNO3-contained electrolytes. Although LiNO3 has been demonstrated to be the best additive to improve the performance of Li–S batteries so far, its strong oxidizing power is still a threat to the safety of the battery, especially at high temperatures.
Recently, P2S5 was introduced into an electrolyte to solve the problem of capacity decay arising from the sluggish electrochemical reaction of Li2S and L2S2. Liang and coworkers proposed that besides the Li+ conductive passivating layer (Li3PS4) of the Li anode, forming a complex of Li2Sx/P2S5 was an additional mechanism for the dramatic improvement of capacity retention.89 Previously, it was reported that protected bis(hydroxyorganyl) polysulphides, also acting as Li2S2 and Li2S carriers, remarkably improved the cell capacity by 25–35% over 50 cycles.90 However, instead of forming complexes, Li2S2 and Li2S were inserted into the S–S bond of organic polysulphides to form lithium organylsulfides (mercaptides), which could be recovered by releasing S after being oxidized at the cathode.
In 1975, Wright91 initially discovered the conductivity of poly (ethylene oxide) (PEO) complexes with alkali salts, opening up a new field in solid-state electrochemistry.92 Since 2002, PEO-based solid electrolytes have been readily applied in Li–S batteries.93 A successful demonstration was the combination of PEO20LiCF3SO3Li2S with 10% nano-sized ZrO2. This novel and unique composition contributed to the enhancement of the ionic conductivity (10−4–10−3 S cm−1 around 70 °C), the stabilization of the Li metal electrode/electrolyte interface and the prevention of PS dissolution.94 As a result, the battery with an all-solid configuration could achieve a reversible capacity of 900 mA h g−1 with a Coulombic efficiency of ∼100%, which laid the foundation for developing high-temperature electronic applications, such as EV. For daily-usage portable electronics, such as mobile phones or laptops, room-temperature ion conductivity should be further enhanced. Although gel polymer electrolytes meet this requirement and have been applied to Li-ion batteries, they are not satisfactory for Li–S batteries because of their liquid-like Li+-conductive mechanism, which will again cause PS migration and thus capacity fading. By SEM, XRD and XPS characterizations, Jin and coworkers found that the capacity fading mechanism in Li–S cells using gel polymer electrolytes was mainly attributed to the corrosion of the Li anode by Li2S, which remarkably increased interfacial resistance.95 To some extent, it also indicated that a gel–polymer electrolyte cannot permanently restrain PS. Hence, it is necessary to explore and find new ion-conductive systems. A potential candidate is glass-ceramic electrolytes. In 1980s, sulphide glasses, such as Li2S–P2S5 and Li2S–SiS2 were proved to be fast Li+ conductors with conductivities over 10−4 S cm−1 at room temperature.96 Since 2003, those ion-conductive systems coupled with different cathodes have been introduced into Li–S batteries.97 Owing to their unity transference number for Li+, PS could be theoretically confined within the cathode. Recently, nano-structured Li3PS4, a Li+ superionic glass, was prepared by Liang and coworkers using Li2S and P2S5.98 In addition to using it as a solid electrolyte, it could also react with S to form lithium polysulfidophosphates (LPSP), a family of lithium-conducting sulfur-rich compounds (Fig. 13a).99 This new class of all-solid-state Li–S batteries could maintain a capacity of 1200 mA h g−1 even after 300 cycles due to the reversible breaking and forming of S–S bonds in LPSP compounds (Fig. 13b and c).
Fig. 13 (a) Formation of LPSPs by chemical reactions between S and Li3PS4 in THF solution, and the redox behavior during cycling. (b) Demonstration of the reversible scission and formation of S–S bonds in LPSP by a comparison of the Raman spectra of the active material at the end of each of the charge and discharge cycles. (c) Cycle performance of the Li3PS4+5 cathode for all-solid-state Li–S batteries. Derived from figures reported by C. D. Liang et al.99 |
However, in the coming years, much effort is still needed to understand the mechanism of fast ionic transport in solids, which will give insight into the synthesis of new materials with ionic conductivities comparable to that of liquid electrolytes. The interfacial resistance between electrolytes/electrodes is also an important factor to be considered as it is directly associated with the capacity and rate performance of batteries. In short, the all-solid configuration has provided a new direction, but there is still a long way to go.
Recently, a Li–B alloy has been explored by Duan and coworkers as the anode for Li–S batteries. The SEI layer formed on the Li–B alloy was found to be much more stable and uniform than that formed on Li due to the existence of a loofah sponge-like Li7B6 network, and thus a smoother surface morphology was maintained during cycling.101 The underlying mechanism for this phenomenon could be attributed to the participation of B in the formation of the SEI layer.102 As a result, LiB–S batteries exhibited improved cycle stability with a retention rate of 65% after 100 cycles. Further research based on Li-alloying anodes should pay more attention on their notorious volume variation during cycling, which can cause serious pulverization and eventually shorten the cycle life of the batteries.
On the other hand, concentrating on advanced anode designs, Huang and coworkers conceptually proposed a hybrid Li–C anode to shift the undesirable interfacial reactions away from Li anode (Fig. 14).103 They hypothesized that lithiated graphite, as an artificial SEI layer in front of Li, could provide a physical barrier to avoid contact between PS and Li. Through control experiments, it turned out that the electrical connection between the graphite and the Li was the essential factor for the maximum shielding effect. Moreover, the results also suggested that the minimum difference in operating voltage between graphite and Li was crucial for the timely extraction of Li+ from the anode during discharge. Using this innovative hybrid anode design, Li–S batteries exhibited excellent cyclability with a capacity of >800 mA h g−1 over 400 cycles. In contrast, Tarascon and coworkers directly deposited S at the surface of Li to form a native SEI layer during contact with the electrolyte.104 This protective layer, surprisingly prevented the formation of insoluble Li2S at the Li surface and thus contributed to an improvement in electrochemical performance.
Fig. 14 Schematic of novel Li–S configuration with hybrid anode structure. Derived from a figure reported by C. Huang et al.103 |
On the other hand, elemental S-based cathodes could be coupled with pre-lithiated anodes. Recently, a high-safety Li–S battery configuration consisting of a S–C composite cathode, a prelithiated Si–C anode and an ionic liquid electrolyte, was constructed by Guo and coworkers.107 With one-plateau behavior at 1.5 V, this system delivered an initial capacity of 1457 mA h g−1 and maintained 45% capacity after 50 cycles, which is more rechargeable and powerful than primary alkaline batteries and dry cells. In addition, M. Hagen and coworkers also demonstrated that dendrite problems could be successfully prevented by using pre-lithiated Si microwires.108
It can be seen from the accompanying pie chart that the reports on S–C composite cathodes have dominated up to 70% of the total research. In fact, from polymer or metal oxide based core–shell/yolk–shell nano composites to carbon based 1D/2D/3D hierarchical nano architectures, nano-structured S cathodes have been demonstrated to be effective to tackle the performance-related issues of S chemistry including the poor electrochemical accessibility of S, the dissolution of long chain PS in organic electrolytes and the volume variation of the S cathode during cycling. It should be noted that accurate morphology and porosity control are essential for high-performance cathode engineering while simple and facile synthetic procedures are also necessary for large-scale production. Moreover, theoretical simulations should be further conducted to give insight into the effects of these functional nano-architectures on the electrochemical performance.
As for the electrolytes and anodes, although less attention has been paid to them they are also of great importance, especially in terms of safety issues. Linear/cyclic ether based organic liquid electrolytes, such as DOL–DME systems, have been proved to provide a balance for PS dissolution. With suitable salts and additives, the PS shuttle effect could be almost suppressed and thus the capacity life and Coulombic efficiency have been improved. However, the volatile and flammable nature of these electrolytes could lead to thermal runways and even explosions when leakage or short-circuiting happens. ILs have been chosen as a potential candidate due to their chemical stability. However, their high viscosity was another inconvenient factor that limited the rate performance of batteries due to sluggish mass transfer. Thus further efforts are still needed to tailor the components of liquid electrolytes in the future. Hybrid organic–ILs electrolytes may be promising, considering their synergistic effects. On the other hand, polymers or inorganic solid electrolytes gives new direction to the development of all-solid-state Li–S batteries, which has become the ultimate approach for practical use. However, the room temperature ionic conductivity and interfacial resistance between the solid electrolyte/electrode are two major stumbling blocks. Comparatively, the road towards safer anodes is much easier. By modifying the Li structure and composition or choosing Li-free anodes, such as Si or Sn, the safety concerns from Li dendrites and the active nature of lithium could be directly avoided.
Although remarkable progress has been achieved in the development of Li–S batteries, most of these advances have focused on one aspect of the battery—the cathode, anode, or electrolytes, due to the existing multi-component assembly. According to the catholyte Li–S cell concept, the performance-related problems associated with S chemistry actually involve more than two components of batteries at the same time. Hence, it is expected that Li–S batteries with 3D inter-connected or conformal assemblies could reach new horizons by simultaneously addressing these problems. Additionally, developing binder/collector free systems is also one of the key approaches to meet the rising demands for high specific energy. Lightweight, flexible and free-standing carbon films or foams are promising candidates for S hosts. In terms of avoiding safety issues, all-solid figuration is the most reasonable choice. With those further improvements in the design, making Li–S batteries practical can be hopefully realized in the future, which will ultimately open up new applications in the areas of aeronautical and space technologies, energy storage plants, military defence, and electric vehicles (Fig. 15c).
Footnote |
† R.C. and T.Z. contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |