Understanding the Lithium Sulfur Battery System at Relevant Scales

In recent years, lithium sulfur (Li-S) batteries have garnered drastic research interest for both transportation and large-scale (grid) energy storage applications mainly because of this electrochemical couple's high theoretical gravimetric energy density, which is projected to be twice that of the state-of-art lithium-ion (Li-ion) batteries, and the potential for a greatly reduced battery cost. As one of the most promising next-generation battery technologies, the Li-S system has been studied by groups around the world and the number of publications has increased exponentially. However, for practical applications of Li-S batteries, many challenges must be addressed, including poor cycling stability, low round-trip efficiency, and severe self-discharge, all of which are rooted in the dissolution of long-chain polysulfide species. The soluble intermediate polysulfides diffuse out of the cathode gradually and “contaminate” almost all of the exposed surfaces in the cell, leading to a series of side reactions and poor electrochemical performance. Due to the extensive diffusion of the dissolved sulfur species, almost all of the components in the Li-S cell have to be modified to minimize or ideally eliminate the attack by the “corrosive” polysulfides. Many different approaches have been proposed to modulate the cathode structure, decorate the separator, and alter the interfacial reactions on the anode side of the cell by using new electrolyte recipes or additives. Statistical information about Li-S research can be found from the article by M. Hagen et al. in this special issue. Despite the rapidly accumulating number of publications, the technical readiness level of Li-S cells is still in its early stages. An important question for consideration is whether or not something important has been missed that precludes the crucial knowledge transfer from fundamental research into real systems. A detailed analysis of the literature distribution for Li-S batteries reveals that 69% of the published results have focusing on the sulfur cathode and the vast majority of these are based on thin-film electrodes (Figure 1). Here, thin film is defined as an electrode with less than 2 mg cm−2 of areal sulfur loading. The representative areal specific capacities of electrodes targeted for typical plug-in hybrid electrical vehicle (PHEV) and electrical vehicle (EV) designs are 2 and 4 mAh cm−2, resepectively.1 Assuming that the practical specific capacity of sulfur is 1000 mAh g−1, a sulfur loading of 2 mg cm−2 then translates to 2 mAh cm−2, meeting the minimum areal capacity requirement for transportation, although the voltage factor has not yet been considered here. Based upon the definition of thin film electrodes, only about 5% of published papers use electrodes with sulfur loadings greater than 2 mg cm−2. Even for the publications focusing on characterization or the anode, electrolyte and other accessories, thin-film sulfur cathodes are predominantly used as a standard electrode. In other words, about 95% of the published conclusions are derived from thin-film Li-S batteries in which the masses of the sulfur cathode and lithium anode are largely mismatched. For fundamental research, thin-film electrodes do provide a convenient platform to understand the material properties and the sophisticated interactions among various species at the nanoscale. Especially for the advanced characterization methods used on Li-S cells, the simplified electrode structure facilitates the capture of important signals which aid in understanding the reaction mechanism and kinetics (see contribution by K. Amine and co-workers). Thin-film electrodes are also very helpful for quickly validating new concepts for materials or electrolyte recipes for Li-S battery system (see contribution by D. Aurbach and colleagues). However, as knowledge is accumulated, more effort needs to be devoted to investigating Li-S cells at a more relevant scale, i.e., electrodes with reasonable thickness. The significant thickness difference between lab-made thin-film electrodes and those used by industry results in many uncertainties (see contribution by M. Hagen et al.). First, as the electrodes become thicker, i.e., more sulfur is packed into the electrode, the total mass transport changes significantly. Accordingly, the concentration gradient of the dissolved polysulfide species and their spatial distribution throughout the electrode structure and electrolyte largely changes, leading to fundamentally different diffusion pathways and reaction kinetics. Second, and more importantly, when the areal loading of sulfur is increased to a high level, the counter electrode, i.e., lithium metal, also undergoes dramatically exacerbated lithium stripping and re-deposition processes, which may gradually become the dominant reason for cell failure.2 When a thin-film electrode is used, the issues associated with the lithium anode side of the cell are “hidden” due to the shallow cycling of the anode, which may seem “stable” for hundreds of cycles. The anode issues only become much more pronounced when coupled with a thick cathode. Last, but not least, an important consideration that has been largely neglected for battery research is the challenges of the practical application of nanomaterials. Although numerous articles have discussed the advantages of nanoparticle-based battery materials, commercially available high-energy Li-ion batteries continue to use micrometer-sized particles and there must be a reason for that. Downsized nanoparticles do facilitate Li+ transport by shortening the diffusion path. They also alleviate the volume expansion for electrode materials that undergo conversion reactions such as silicon. For highly insulating sulfur, nanosized carbon with high surface area is also needed to improve the utilization rate of sulfur and thus the obtainable capacity. However, the first challenge that industry may meet when dealing with nanoparticles is the difficulty to coat them uniformly onto traditional current collector without pinholes and cracks. This is particularly true if the binder content is limited to less than 10% to minimize the parasitic weight of the cell. For poorly coated electrodes, it is still possible to punch a few thick cathodes to assemble button cells that provide good research opportunities. Large-area uniform coatings, however, need to be emphasized for large-format cell fabrication. The low tap density of nanomaterials is another hurdle, even if the coating issue can be addressed. The scientific questions behind these “engineering-like” problems are therefore both interesting and worthy of more attention by battery researchers. For example, can the nanostructures be maintained in the primary particles which aggregate into micrometer-sized secondary particles and thus become compatible with traditional coating technique (see contribution by J. Xiao and co-workers on thick electrode fabrication)? Or can simple pomegranate-like sulfur nanoparticle cathodes be built (see contribution by Y. Cui and co-workers)? How can these C/S composites be mass produced with cost effective method (see contribution by Y.-S. Hu and colleagues)? How to design cathode structures for the practically usable Li-S cells? (see contribution by M. A. Pope and I. Aksay)? What are the alternative anode materials to replace lithium metal (see contribution by J.-G. Zhang and co-workers)? Without exploring these core problems associated with nanomaterials and thin-film electrodes, it will be difficult for Li-S batteries to make meaningful progress towards industrial practicality. Based on the above discussions, it is urgent to revisit the Li-S battery system, and probably other batteries systems as well, at a more relevant scale. In this way, further understandings of the electrolytes (see contribution by M. Watanabe and co-workers), or modification on the carbon host (see contribution by X. Xiao and colleagues) becomes more meaningful to the real applications of Li-S cells. The fundamental knowledge gathered is thus becoming useful to directly guide materials selection, electrode modification and cell design towards commercially viable Li-S batteries (see contribution by P. Novak and co-workers). This special issue aims to understand Li-S batteries from different perspectives from those typically found in the literature. A general rule applied to most of the invited papers is that the minimum areal loading of sulfur needs to be at least 2 mg cm−2. In this way, the reader is able to compare apples to apples and select useful information for his or her own research. A comprehensive review on previous literature for the cathode, anode, electrolyte and characterization of Li-S batteries is provided in this issue, followed by opinionated progress reports. Many of the real challenges for Li-S batteries are listed one-by-one and potential solutions are proposed. The emerging Li-S redox flow battery technology is discussed in this issue from a solution chemistry point of view by J. Liu and co-workers. The feasibility of Li-ion sulfur batteries is discussed by B. Scrosati and colleagues. Equally important is the fundamental understanding of the Li-S system, which is also provided in this issue. Reaction mechanisms of Li-S batteries determined from different characterization techniques such as XANES (see contribution by L. Nazar and co-workers), 2D X-ray fluorescence (see contribution by X.-Q. Yang and co-workers), HPLC ESI/MS (see contribution by D. Qu and colleagues), and X-ray diffraction spectroscopy (contribution by F. Alloin and co-workers) are compared, along with a discussion of the activation process of lithium sulfide cathode materials (see contribution by J. Cho and colleagues). The concentration changes of polysulfide radicals at different depth of discharge will be further discussed by D. Prendergast and co-workers to provide atomic level insights into the chemistries in Li-S battery system. As the guest editor, I would like to thank all of the authors for their contributions to this special issue. The articles collected here not only critically point out the main concerns and problems existing in current Li-S battery research, but also provide insightful and visionary solutions to address the present challenges. Hopefully this special issue will be helpful to calibrate laboratory R&D efforts and inspire revolutionary ideas to accelerate the development of Li-S battery technologies. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U. S. Department of Energy under Contract No. DE-AC02–05CH11231, Subcontract No. 18769, under the Battery Materials Research (BMR) program. J.X. would like to thank Drs. Wesley A. Henderson and Jun Liu for proofreading this editorial. Dr. Jie Xiao is currently a staff scientist at PNNL. She earned her Ph. D. from the State University of New York (SUNY) at Binghamton (2008) in materials chemistry. Her research interest spans from materials synthesis, electrochemistry, and cell design to advanced characterizations/diagnosis with the goal of understanding the synthesis–structure–performance relationship in energy-related materials and their underlying reaction mechanisms. Her current research focuses on lithium–sulfur batteries, lithium-ion batteries, metal–air batteries, and microbattery design and fabrication for microtransmitters.