Critical Link between Materials Chemistry and Cell-Level Design for High Energy Density and Low Cost Lithium-Sulfur Transportation Battery

Abstract

A materials-to-system analysis for the lithium-sulfur (Li-S) electric vehicle battery is presented that identifies the key electrode and cell design considerations from reports of materials chemistry. The resulting systems-level energy density, specific energy and battery price as a function of these parameters is projected. Excess lithium metal amount at the anode and useable specific capacity, electrolyte volume fraction, sulfur to carbon ratio and reaction kinetics at the cathode are all shown to be critical for the high energy density and low cost requirements. Electrode loading is determined as a key parameter to relate the battery price for useable energy to the investigated design considerations. The presented analysis proposes that electrode loadings higher than 8 mAh/cm 2 (∼7 mg S/cm 2 ) are necessary for Li-S systems to exhibit the high energy density and low cost required for transportation applications. Stabilizing the interface of lithium metal at the required current densities and areal capacities while simultaneously maintaining cell capacity with high sulfur loading in an electrolyte starved cathode are identified as the key barriers for ongoing research and development efforts to address. In the search for high energy density and inexpensive rechargeable batteries for the electric vehicles, Li-S batteries have gained significant attention due to the high specific capacity (1675 mAh/g), low cost, natural abundance and non-toxicity of elemental sulfur.1-6 Compared to the state-of-art Li-ion batteries, Li-S batteries have very high theoretical specific energy of 2567 Wh/kg.1-6 The Li-S battery is commonly composed of a sulfur-carbon composite cathode, an organic electrolyte and a lithium anode.1-6 The overall Li-S redox reaction is given in equation 1.with a standard potential of U 0 = 2.2 V (vs Li/Li + ). 1,2Despite these attractive features of the Li-S battery, multiple formidable challenges limit the cycle life significantly. [1][2][3][4][5][6] Firstly, precipitation of insulating reactants, sulfur and Li 2 S, in the cathode leads to poor electronic conductivity and passivation that could limit the active material utilization.1-6 Secondly, the soluble polysulfide reaction intermediates produced during charging can migrate to the anode where they react with Li to either precipitate on the anode surface or migrate back to the cathode causing infinite charging.1-6 This polysulfide shuttle mechanism leads to poor coulombic efficiency and significant self-discharge as well as corrosion of the Li-anode.1-6 Finally, the instability of the Li-anode is a major concern.2,4,5,7 Li surface area can increase significantly with cycling due to morphological changes, which accelerate Li and electrolyte depletion owing to the absence of a stable interphase. 2,4,5,7 While polysulfide migration may lead to the corrosion of dendritic or high surface area lithium reducing the risk of short circuiting, the resulting reactions typically result in a reduction in inventory of cyclable lithium. 4 All of these mechanisms ...

Review—Electromobility: Batteries or Fuel Cells?

“…In the case of metallic lithium anodes, protection layers (polymer/solid electrolyte) are likely required for safety and stability reasons, which will also add yet undefined cost. Further cost studies can be found in the analysis by Eroglu et al 69 Safety aspects of lithium-sulfur batteries.-Dendrite formation is a well-known risk with metallic lithium anodes, leading to internal short circuits and safety hazards. A possible solution could be the use of a lithium-conducting solid electrolyte, acting as barrier to prevent dendrite formation and to block polysulfide reduction and deposition on the anode surface.…”

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

“…Recently, several studies have identified that the attainment of areal capacities as high as 4-8 mAh/cm 2 while minimizing the electrolyte content are the key factors in meeting these requirements. [1][2][3] The only currently commercialized Li-S battery has a significantly lower areal capacity of 2.5 mAh/cm 2 and operates in the presence of excess electrolyte, 4 necessitating significant technological breakthroughs to facilitate the possible use of Li-S batteries in the transportation sector. One of the main barriers to achieving such breakthroughs is the lack of fundamental understanding of the mechanism behind the operation of Li-S batteries.…”

Visualising the problems with balancing lithium–sulfur batteries by “mapping” internal resistance

“…The negative electrode excess may have to be reduced to the order of 50-100%, according to at least one recent analysis. 8 There are compelling reasons to continue to focus on Li metal, however: it has the highest energy density of all the candidate negative electrode materials, and is likely to present fewer challenges for large-scale cell production compared to the alternative ''Li-ion-sulfur'' approach, which requires the use of either lithiated carbon or silicon, or lithium sulfide, as the lithium source -all of which are highly air-sensitive. In this paper, we present a novel method for visualising and quantifying the changes in cell resistance to demonstrate the limitation on cycle life caused by a low excess of the Li metal electrode in the Li-S system.…”