Nonuniform Redistribution of Sulfur and Lithium upon Cycling: Probing the Origin of Capacity Fading in Lithium–Sulfur Pouch Cells

Abstract

Lithium–sulfur (Li–S) batteries have emerged as a promising candidate for the next‐generation high‐energy‐density system for energy‐demanding applications. Despite innovations in concepts and materials that significantly improve the electrochemical performance of coin cells, Li–S pouch cells have the disadvantages of short cycle life and inferior rate capability in comparison with coin cells. Bridging the fundamentals of Li–S chemistry to the hindrance on its practical application is of great importance for the development of Li–S batteries. Herein, the nonuniformity of the distribution of sulfur and lithium upon cycling is probed as one of the origins for the rapid capacity fading in a Li–S pouch cell. In particular, the nonuniform evolution of sulfur/lithium distribution impairs the discharge capacity of a low‐voltage plateau. Lithium polysulfide intermediates produced on discharge tend to diffuse toward the bottom of a pouch cell, leading to agglomeration of sulfur and thus passivating the cathode. The migration of polysulfides also etches lithium away from the central region of the anode and induces nonuniform anode pulverization. Herein, the importance of a rational design of a pouch cell to mitigate the nonuniform redistribution of the active material toward stable Li–S pouch cells is highlighted.

A Perspective toward Practical Lithium–Sulfur Batteries

“…The cycled lithium anode displays a highly uneven morphology where several regions are severely corroded in a failed pouch cell ( Figure 4 b), and further analyses identify anode failure as the main reason. The cause of battery failure from gradual cathode capacity loss in coin cells to rapid capacity decrease in pouch cells mainly results from the following three aspects: (i) Higher actual current density and cycling capacity applied on the Li metal anode using high-sulfur-loading cathodes that aggravate uneven lithium deposition and anode volume change; 65 (ii) higher LiPS concentration and a more severe shuttle effect that render serious lithium corrosion by LiPSs; 66 (iii) low N/P ratio with less lithium excess which cannot support the continuous irreversible loss of active lithium by reacting with LiPS or electrolyte ( Figure 4 c). Thereby, the failure of lithium metal anode becomes the main reason for practical Li–S pouch cells.…”

Electrolyte solvation chemistry for lithium–sulfur batteries with electrolyte-lean conditions

“…Most of studies over the past decade employ high E/S ratio, normally > 10, in a coin-type cell to exclude the interference of electrolyte consumption and afford sufficient mass/charge transport properties [24,25,[74][75][76]. However, this is not the case in a real cell scenario, where E/S < 4.5 is highly advocated to achieve the high-energydensity goal [11,71,77]. Several approaches have been considered to realize efficient operation of Li-S batteries with a low E/S ratio: (i) Spatially confining sulfur into microstructured carbon materials to avoid the triphasic interface limitation, and electrode optimization to reduce porosity [78][79][80][81][82][83][84]; (ii) heterogeneous or homogeneous mediator to promote liquid-solid conversion and control morphology of solid deposits [85][86][87][88][89][90]; (iii) design of electrolyte with desired solvation properties to find new reaction pathways to break electrolyte-quantity-dependent limitations of sulfur redox reactions [91,92].…”

Recent Progress and Emerging Application Areas for Lithium–Sulfur Battery Technology

“…Generally, Li–S technology has received increasing levels of research and development with efforts focused on performance aspects including cycle life, power performance, volumetric energy density, and safety. [ 25,36,42,49,51,62–65,66 ] To meet the varying performance requirements of emerging applications, OXIS Energy has developed two cell product streams, each with optimized performance characteristics (Figure 1a,b). Application requirements can generally be divided into two sectors: 1) high energy focused with low power requirements (high energy) and 2) moderate energy with the capability for sustained high power (high power).…”