Irreversible vs Reversible Capacity Fade of Lithium-Sulfur Batteries during Cycling: The Effects of Precipitation and Shuttle

Abstract

Lithium-sulfur batteries could deliver significantly higher gravimetric energy density and lower cost than Li-ion batteries. Their mass adoption, however, depends on many factors, not least on attaining a predictive understanding of the mechanisms that determine their performance under realistic operational conditions, such as partial charge/discharge cycles. This work addresses a lack of such understanding by studying experimentally and theoretically the response to partial cycling. A lithium-sulfur model is used to analyze the mechanisms dictating the experimentally observed response to partial cycling. The zero-dimensional electrochemical model tracks the time evolution of sulfur species, accounting for two electrochemical reactions, one precipitation/dissolution reaction with nucleation, and shuttle, allowing direct access to the true cell state of charge. The experimentally observed voltage drift is predicted by the model as a result of the interplay between shuttle and the dissolution bottleneck. Other features are shown to be caused by capacity fade. We propose a model of irreversible sulfur loss associated with shuttle, such as caused by reactions on the anode. We find a reversible and an irreversible contribution to the observed capacity fade, and verify experimentally that the reversible component, caused by the dissolution bottleneck, can be recovered through slow charging. This model can be the basis for cycling parameters optimization, or for identifying degradation mechanisms relevant in applications. Lithium sulfur (LiS) batteries have the potential to provide a step change in performance, compared to Li-ion batteries, with an expected practical energy density of 700 Wh kg −1 compared to that of the intercalation Li-ion batteries, of 210 Wh kg −1 . 1,2 Added benefits, such as a potential low cost due to the abundance of the active materials, low toxicity and relative safety, 3 make them an attractive energy storage solution for a wide variety of applications, such as space exploration 4 and low temperature energy delivery. 5 However, the relatively low power capabilities, significant self discharge and low cycle life have so far hindered mainstream adoption of LiS cells. Degradation mechanisms such as lithium anode corrosion, self discharge and low coulombic efficiency have all been related to the polysulfide shuttle. As a result, most effort in the research community is currently directed toward decreasing the amount of shuttle through material design, and assessing the properties of the proposed materials through coin cell characterization.We argue that equally important for accelerating the adoption of this battery chemistry is the understanding of how real cells behave under real operating conditions, which often include incomplete charge/discharge cycles, noisy current loads and rest periods at various states of charge (SoC). Understanding and detecting the mechanisms leading to degradation, such as capacity fade, are intermediate steps crucial to predicting cycle life. Such understanding can...

Nitrogen‐Doped Biomass‐Derived Carbon Formed by Mechanochemical Synthesis for Lithium–Sulfur Batteries

“…[24,26,82,83] Moreover,n itrogen functionalities can enhancet he precipitation of solid sulfur und Li 2 S, whichi sk nownt ob eo ne of the cathode limiting factors. [84] Thus, we observed an improved active materialu tilization for the N-doped samples. The discharge voltage profiles of WWUPC/S and LUPC/S present two significant plateaus at values of 2.28 Va nd 2.08 V, respectively ( Figure 5B).…”

“…It is known that polar functionalities, especially pyridinic and quaternary nitrogen species, interact with soluble lithium polysulfides (LiPS) and improve the cycle stability owing to a reduced shuttle mechanism . Moreover, nitrogen functionalities can enhance the precipitation of solid sulfur und Li 2 S, which is known to be one of the cathode limiting factors . Thus, we observed an improved active material utilization for the N‐doped samples.…”

Synergistic Effect of Covalent Bonding and Physical Encapsulation of Sulfur in the Pores of a Microporous COF to Improve Cycling Performance in Li‐S Batteries

“…This behaviorc ould be attributed to the shuttle effect, especially noteworthy at low charge current densities, as in the current example. [31] The same electrochemical characterization is depicted in Figure 5c and df or S@H 2 P-COF-BATA,s howing similarf eatures to those found for S@H 2 P-COF.T he electrochemical discharge/ charge processes show two main peaks at 2.30 and 2.05 Vf or reduction and 2.30 and 2.39 Vf or oxidation, corresponding to the S 8 conversion steps towards Li 2 S 2 /Li 2 Sa nd their corresponding reversible ones. This cell yields around1 600 mAh g À1 duringt he first discharge cycle, close to the theoretical performance of the S@H 2 P-COF battery,1 675 mAh g À1 .O nt he other hand, Figure5de xhibitst he cycling performance of the S@H 2 P-COF-BATA cathode, with ad ischarge capacity around 900 mAh g À1 and ac oulombic efficiency close to 95 %a fter 100 cycles.…”

An Efficient Electrochemical Tanks-in-Series Model for Lithium-Sulfur Batteries

“…Reactions (2)-(6) refer to the electrochemical reduction of liquid S8 in the cathode. Reactions (7) and (8) are the dissolution and precipitation of the two solid species, S8(s) and Li2S(s). The work assumes the same reaction mechanism as the Kumaresan model 3 .…”

“…A related model was developed by Neidhardt et al 4 in 2012. A series of papers were published with a similar 1D model that explored impedance 5 , charging 6 , and degradation [7][8][9] , as well as lumped models with simplified reaction mechanisms, reduced number of parameters [10][11][12] , and precipitation of lithium sulfide 13,14 .…”