A review on anode for lithium-sulfur batteries: Progress and prospects

Highlights

• Review the current trends and developments of lithium-sulfur anodes.

• Some protected functional and novel anodes were presented.

• Discuss different methods affecting the lithium dendrite.

• Recommend approaches to better prepare the anodes functionality.

Abstract

Lithium-sulfur battery is regarded as one of the promising next-generation energy storage to electrical and portable devices thanks to its extremely high theoretical capacity, energy density, good environmental protection and low cost. However, the practical application of lithium-sulfur battery is still greatly impeded by the low Coulombic efficiency and the short lifespan, which is mainly attributed to the polysulfide shuttle and the uncontrollable lithium dendrite growth. Suppressing the growth of the lithium dendrite and hindering the notorious reaction between soluble polysulfides and lithium are extremely critical not only to a safe and efficient lithium anode, but also to the high-capacity lithium-sulfur battery. A comprehensive review of various strategies for strengthening the anode stability of lithium-sulfur battery is presented in this paper, including modifying the electrolyte and current collector, employing artificial protection films and finding alternative anodes to replace the lithium anode. The effects of different selections and the resulting properties of the anodes on the overall lithium-sulfur battery performance are discussed. The current research challenges and future perspectives associated with lithium-sulfur battery anode are also discussed.

Introduction

The petroleum-based energy is limited and its application is ineluctably detriment for environmental protection, so the development of an alternate, sustainable and clean energy technology is extremely necessary to supply or replace the fossil energies. Along with the growth of social demands for energy consumption, many renewable energy sources are required to be explored to conquer the source crisis [1]. Solar and wind energies are significantly favorable to avoiding environmental harms, but the practical applications of these energies are greatly limited because they are not subject to control and provided with the characteristic of intermittence. In recent years, to meet the technology developments and the human’s living standards, the rechargeable batteries, taking the lithium-ion batteries and sodium-ion batteries for example, are regarded as increasingly appealing alternatives to be applied in both grid electrical energy storage and electrical vehicles (EV) thanks to their superior properties [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In this context, these secondary batteries are the important component of energy storage devices in the new-energy field owing to their high energy conversion efficiency and energy density. And the commercial lithium-ion battery has achieved extensive applications in the past years, but its development space was very small due to the high cost as well as the limited theoretical energy density [12], [13], [14], [15], [16]. Correspondingly, lithium-sulfur battery is widely researched due to its higher theoretical specific capacity (1675 mAh g⁻¹) and theoretical energy density (2600 Wh kg⁻¹) in comparison with lithium-ion battery [17]. In addition, lithium-sulfur battery is the most competitive in gravimetric energy density with current technology, rather than volumetric energy density, comparing with lithium-ion battery [18]. Moreover, lithium-sulfur battery conforms to the energy source demand for electric car and portable electronic products. Furthermore, lithium-sulfur battery has other wonderful properties such as rich raw material sources, low cost, environment friendly and so on [19].

Although compared with traditional lithium-ion battery, lithium-sulfur battery truly possesses many commendable advantages, but still shows some insufficiencies. On the one hand, the sulfur is difficult to achieve theoretical capacity owing to its discharge product lithium sulfide and its poor electrical and ionic conductivity [20]. In addition, the density of sulfur and lithium sulfide is 2.03 g/cm³ and 1.66 g/cm³, respectively. The volume expansion/contraction in the charge-discharge process is as high as 80%, resulting in the separation of the active material from the conducting matrix and the attenuation of capacity [6], [21]. On the other hand, the suitability of lithium-sulfur battery is limited by the dissolution of elemental sulfur and intermediate product polysulfide ions (S_n²⁻, 8 ≥ n ≥ 3) in liquid electrolyte [22], [23], [24]. The polysulfide formation mechanism has been investigated by many different in situ methods like TEM [25], SEM [26], XAS [27], XPS [28], XRD [29] and UV–VIS [30] analysis that confirmed the failure mechanism of the polysulfide shuttle and the predominant formation of soluble S₄²⁻ and S₆²⁻ during the charge/discharge process. The dissolution of polysulfide can result in the loss of mass active materials and give rise to the formidable change in the structure and shape of the cathode. The active substance is separated from the conductive agent after many cycles and eventually leads to the mitigation of cycle stability. Moreover, lithium metal is prone to self-discharge with soluble lithium polysulfide because of the instability of the lithium anode surface. The product of the self-discharge can move back to cathode and cause the re-oxidized reaction. The process could go round (called the “shuttle effect”), leading to the sustained loss of the positive active material. Ultimately, the Coulombic efficiency of lithium-sulfur battery is reduced and the battery capacity also inevitably suffers the rapid fade [31]. These deficiencies have limited the commercial development of lithium-sulfur batteries.

It is universally known that the defects of sulfur itself are the initiator for the capacity attenuation of lithium-sulfur battery. So many researchers tended to solve the above problems through the modification of cathode to improve the battery performance. Some special and unique cathode architectures were constructed to effectively trap polysulfide, like 3D graphene nanosheet-carbon nanotube (GN-CNT) composite cathode [32], the new carbon shell/nickel-foam/carbon shell pie-like cathode [33] and the honeycomb-like spherical cathode host furnished with hollow metallic and polar Co₉S₈ tubes [34]. In addition, a vast array of interlayers were extensively explored to modify the sulfur cathode, including the metal disulfides (amorphous NiS₂ [35], FeS₂ [36], MoS₂ [37], WS₂ [38]), the metal oxides (TiO₂ [39], MnO [40], V₂O₅ [41]), the conductive carbon-based (grapheme [42], graphene oxide[43], nanotubes[44], nanowire[45], Super P [46]) materials and some carbon-based composite (porous graphene oxide/carbon nanotube [47], boron nitride nanosheets/grapheme [23], MXene nanosheet/carbon-nanotube [48]) films. Some novel cathodes modified by metal-organic framework (MOF) materials have also been proposed such as Zn-MOF drived micro/meso porous carbon nanorod [49], MOF-derived polyhedral hollow carbon cathode [50], triphenylamine-based MOF cathode [51] etc. Currently, Yan’s group [52] presented an extremely advanced strategy to greatly suppress the infamous shuttle effect via impregnating sulfur into sulfydryl-functionalized reduced graphene oxide (rGO). Through the improvement of sulfur cathode, the diffusion of polysulfide ions has been effectively suppressed. And the lithium-sulfur battery with the novel cathode obtained an ultra-low capacity decay and an increased cycle stability. However, the lithium dendrite problem deriving from the lithium metal anode has not been solved in essence, so the safety and long-term stability of the battery still has room to improve. The detrimental formation of lithium dendrites is still one of the most prominent and serious problems in lithium-sulfur batteries which inevitably occurs on the lithium anode surface because of the uneven deposition of lithium during the cycling process [53], [54], [55], [56]. More seriously, it notoriously poses a significant challenge for the commercialized application of lithium secondary batteries. Therefore, the protection of lithium anode is very important and necessary.

The lithium anode is absolutely necessary for lithium-sulfur battery, which plays an important role for the electrochemical stability and the security of the battery. Lithium metal is regarded as a preferred electrode material for the anode of lithium-sulfur battery, which is mainly attributed to its excellent performances such as low gravimetric density (0.59 g cm⁻³), high theoretical specific capacity (3860 mAh g⁻¹) and fine negative redox potential (−3.040 V vs. standard hydrogen electrode). So the lithium metal plays a indispensable role for the next generation high-performance energy storage systems [57], [58], [59], [60], [61]. However, there are two main problems that need to be solved for the lithium anode [62], [63], [64], [65], [66], [67], [68], [69], [70]. One is that the lithium metal can easily react with the electrolyte because of its high chemical activity, which would lead to a low cycling efficiency. The other one is the formation and growth of lithium dendrites on the lithium anode surface, making lithium-sulfur battery far from the practical applications. In order to explore the mechanism of lithium dendrite growth more accurately, many research teams have made great efforts in this area. For example, the Zhang’s research team [71] recently developed a technique for in-situ observation of the formation and dissolution of lithium dendrites using SEM. And the Dasgupta’s research team [56] has studied the lithium anode in depth by in situ recording microscope. The formation and growth of lithium dendrites and the mechanism of lithium anode surface depression were analyzed by combining with theoretical calculation.

Seriously, the lithium dendrites can result in three detrimental issues that need to be solved, as shown in Fig. 1a [72]. On the one hand, lithium dendrites have a certain ability to pierce the polymer separator, leading to short circuit with a safety risk. On the other hand, the uneven dissolution of the lithium dendrites can constantly break lithium crystals to generate “dead lithium” with the high chemically reactivity, which can pose a threat to the battery capacity during the discharge process. In addition, the growth of lithium dendrites makes the efficiency and stability of lithium anode reduced in lithium-sulfur battery. Moreover, the electrolyte continued to be consumed due to the high reactivity of fresh lithium, so a reduced cycle life is obtained by lithium-sulfur battery. Confronting with these damages of lithium dendrites, these technological challenges deriving from lithium anodes are critical to be addressed for the achievement of high-performance lithium-sulfur batteries.

In this regards, some researchers are dedicated to explore the approaches to improve the stability of lithium anode, including adopting various electrolyte solvents, salts and additives to form stable solid electrolyte interface (SEI) films on the surface of lithium anode and using some surface protection technologies to form the protective films (artificial SEI) on lithium anode surface, as well as some sandwich and compound technologies [73]. Lately, Zhang’ group [74] has summarized the working principle and technical challenges of lithium metal anode and the formation mechanism for lithium dendrite nucleation. Furthermore, Chen’s group [73] has introduced many different methods to strengthen the stability of lithium anode in lithium-sulfur battery, such as modifying the separator and electrolytes, employing artificial protection layers, etc. In this review, we generalized and summarized the recent strategies applied to restrain the undesirable reactions of the lithium anode in lithium-sulfur battery from a technical perspective. Some effective protection approaches of lithium anode, a vast array of compound types and lithium-free anodes for lithium-sulfur batteries were presented and reviewed, as shown in Fig. 1b. The effects of different anodes choice on the electrochemistry performance of the lithium-sulfur battery were also described in this review. The challenges and prospects for the future in this field were also discussed.