Synergy of Sulfur/Polyacrylonitrile Composite and Gel Polymer Electrolyte Promises Heat-Resistant Lithium-Sulfur Batteries

摘要

class="head no_bottom_margin" id="sec1title">IntroductionExploring better energy storage devices with high energy density, low cost, high safety, and long lifespan has boosted the development of new battery systems beyond the current mainstream lithium ion batteries (LIBs) (, , ). Among various alternative battery system candidates, Li-S batteries are considered one of the most promising electrochemical power sources. The cathode material (elemental sulfur), exhibits high theoretical specific capacity of 1,672 mAh g⁻¹, calculated from the reversible reaction S8 + 16 e⁻ → 8 S²⁻, which enables Li-S batteries to achieve a high theoretical energy density of up to 2,600 W h kg⁻¹, five times higher than those of present LIBs (, , , , , , ). In addition, sulfur, as one of the key materials of various industries, is naturally abundant, has low cost, and is environment friendly when compared with the commercial cathode materials such as LiCoO2, and LiFePO4 (, , ). However, the practical application of Li-S batteries is still plagued with numerous challenges, such as the insulating nature of elemental sulfur and the discharge product (Li2S). Besides, lithium polysulfides (LiPSs), which are generated by a multi-electron conversion reaction and easily dissolvable into the electrolyte, would cause shuttling effect between the two electrodes and form a passivating layer on the surface of lithium metal anode, including insulating sulfide species (Li2S, Li2S2). These phenomena result in fast capacity fading and low coulombic efficiency of Li-S batteries (, ).Various attempts, such as cathode modification (nanostructure (href="#bib24" rid="bib24" class=" bibr popnode">Li et al., 2017a, href="#bib25" rid="bib25" class=" bibr popnode">Li et al., 2017b, href="#bib47" rid="bib47" class=" bibr popnode">Yang et al., 2018a, href="#bib48" rid="bib48" class=" bibr popnode">Yang et al., 2018b), carbon-based composite (href="#bib14" rid="bib14" class=" bibr popnode">Gao et al., 2019, href="#bib18" rid="bib18" class=" bibr popnode">Hwa et al., 2017, href="#bib20" rid="bib20" class=" bibr popnode">Jayaprakash et al., 2011, href="#bib24" rid="bib24" class=" bibr popnode">Li et al., 2017a, href="#bib25" rid="bib25" class=" bibr popnode">Li et al., 2017b, href="#bib26" rid="bib26" class=" bibr popnode">Li et al., 2018a, href="#bib27" rid="bib27" class=" bibr popnode">Li et al., 2018b, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2018c, href="#bib29" rid="bib29" class=" bibr popnode">Li et al., 2018d, href="#bib45" rid="bib45" class=" bibr popnode">Yang et al., 2016, href="#bib46" rid="bib46" class=" bibr popnode">Yang et al., 2017), doping (href="#bib53" rid="bib53" class=" bibr popnode">Zhou et al., 2017), anode protection (href="#bib5" rid="bib5" class=" bibr popnode">Cha et al., 2018), electrolyte additives (href="#bib41" rid="bib41" class=" bibr popnode">Wang et al., 2018a), targeted binders (href="#bib22" rid="bib22" class=" bibr popnode">Li et al., 2015), and functional separators (href="#bib15" rid="bib15" class=" bibr popnode">Ghazi et al., 2017), have been proposed to regulate the behaviors of LiPSs. Metal oxides (href="#bib16" rid="bib16" class=" bibr popnode">Han et al., 2013, href="#bib51" rid="bib51" class=" bibr popnode">Zhou et al., 2013), sulfides (href="#bib6" rid="bib6" class=" bibr popnode">Chen et al., 2017, href="#bib7" rid="bib7" class=" bibr popnode">Cheng et al., 2018) or phosphide (M–O/S/P) (href="#bib50" rid="bib50" class=" bibr popnode">Zhong et al., 2018), and perovskite nanoparticles (href="#bib21" rid="bib21" class=" bibr popnode">Kong et al., 2018) have also been implemented to immobilize LiPSs and guide the Li2S formation in Li-S batteries.Besides the aforementioned challenges, heat tolerance of Li-S batteries is important for practical applications, yet not easy to realize, because the dissolution of the LiPSs is aggravated at high temperature, and the shuttle effect is strengthened as well (href="#bib4" rid="bib4" class=" bibr popnode">Busche et al., 2014). In addition, the safety concern of Li-S batteries at elevated temperature is essential, because of the low boiling and flash points of the commonly used ether-based electrolytes (href="#bib23" rid="bib23" class=" bibr popnode">Li et al., 2016, href="#bib26" rid="bib26" class=" bibr popnode">Li et al., 2018a, href="#bib27" rid="bib27" class=" bibr popnode">Li et al., 2018b, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2018c, href="#bib29" rid="bib29" class=" bibr popnode">Li et al., 2018d). So far only a few articles have touched upon this topic. Jin et al. designed a hollow Co3S4 nanobox with interconnected carbon nanotubes as the sulfur host and realized a cycle retention of 75.3% over 300 cycles at 335 mA g⁻¹ at 50°C in an ether-based electrolyte (href="#bib6" rid="bib6" class=" bibr popnode">Chen et al., 2017). Zhang et al. entrapped sulfur into porous graphene and enabled the Li-S batteries operated at 60°C with a capacity of 551 mAh g⁻¹sulfur at a current density of 1,672 mA g⁻¹ (href="#bib17" rid="bib17" class=" bibr popnode">Huang et al., 2013). Regarding the safety concerns of the ether-based electrolyte, revisiting the traditional carbonate electrolyte would be an effective strategy. Sun et al. used molecular layer deposition method to coat aglucone on C-S electrode and enabled the Li-S batteries cycled in a carbonate electrolyte at a temperature range of −20°C to 55°C with high reversibility (href="#bib23" rid="bib23" class=" bibr popnode">Li et al., 2016, href="#bib26" rid="bib26" class=" bibr popnode">Li et al., 2018a, href="#bib27" rid="bib27" class=" bibr popnode">Li et al., 2018b, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2018c, href="#bib29" rid="bib29" class=" bibr popnode">Li et al., 2018d).A promising cathode material for Li-S batteries, sulfur/polyacrylonitrile nanocomposites (SPAN), was proposed (href="#bib39" rid="bib39" class=" bibr popnode">Wang et al., 2002, href="#bib40" rid="bib40" class=" bibr popnode">Wang et al., 2003) and allows a Li-S battery operated in a carbonate electrolyte (href="#bib39" rid="bib39" class=" bibr popnode">Wang et al., 2002, href="#bib43" rid="bib43" class=" bibr popnode">Wei et al., 2015). It can be synthesized by a facile annealing procedure with sulfur and polyacrylonitrile (PAN) as the mere raw materials, and the synthesis method can be easily scaled up. Although great efforts have been made, the structure of SPAN and its reaction mechanism during cycling are still controversial (href="#bib11" rid="bib11" class=" bibr popnode">Fanous et al., 2011, href="#bib12" rid="bib12" class=" bibr popnode">Frey et al., 2017, href="#bib19" rid="bib19" class=" bibr popnode">Hwang et al., 2013, href="#bib26" rid="bib26" class=" bibr popnode">Li et al., 2018a, href="#bib27" rid="bib27" class=" bibr popnode">Li et al., 2018b, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2018c, href="#bib29" rid="bib29" class=" bibr popnode">Li et al., 2018d, href="#bib30" rid="bib30" class=" bibr popnode">Liu et al., 2018, href="#bib39" rid="bib39" class=" bibr popnode">Wang et al., 2002, href="#bib40" rid="bib40" class=" bibr popnode">Wang et al., 2003, href="#bib41" rid="bib41" class=" bibr popnode">Wang et al., 2018a, href="#bib42" rid="bib42" class=" bibr popnode">Wang et al., 2018b, href="#bib43" rid="bib43" class=" bibr popnode">Wei et al., 2015).Herein, we report a Li-S battery with excellent high-temperature performance, by using SPAN as positive electrode, and gel polymer membrane (GPM) soaked in carbonate electrolyte. The Li-S battery could be operated at a high temperature at 60°C; a high specific capacity of 547.8 mAh g⁻¹ can be obtained at 250 mA g⁻¹, and even when the current density increases to 2,500 mA g⁻¹, a high reversible capacity of 415 mAh g⁻¹ can still be retained. In addition, the electrochemical process of SPAN toward Li during cycling is discussed. The synergistic effects of SPAN and the gel polymer electrolyte (GPE) enable the excellent high-temperature behavior of Li-S battery, in terms of high energy density, rate capability, safety, and cycle stability.