Predicting the composition and formation of solid products in lithium–sulfur batteries by using an experimental phase diagram

Abstract

The lithium-sulfur rechargeable battery has been studied intensively as a candidate for the high specific energy market, which has applications in electric vehicles, portable devices and grid energy storage. [1][2][3][4][5] The theoretical specific energy is ca., which is notably higher than the lithium-ion battery. Several other advantages of using sulfur as the positive electrode include its abundance, low cost and nontoxicity. The theoretical specific capacity of sulfur (1672 A h kg The reduction of S 8 to Li 2 S and reverse oxidation occur via multistep reaction pathways, which have led to the discharge and charge mechanisms being poorly understood compared with those of the lithium-ion battery. 6-9 Many previous studies have used spectroscopic methods to determine the nature of the several polysulfides that may be found in solution during discharge from S 8 to Li 2 S and the reverse charge reaction. 7-12By contrast, the contribution to the understanding of lithiumsulfur batteries that is made here is a purely thermodynamic treatment based on the equilibrium between three phases: solid S 8 , solid Li 2 S, and a solution of polysulfides in the electrolyte. 13,14 For simplicity, we do not consider the existence of any other solids such as Li 2 S 2 , which is often suggested as a reaction intermediate, despite the fact that no crystal structure has been published. Only one publication suggested a ternary S 8 -Li 2 S-electrolyte phase diagram, 9 but it was not supported by experimental data. Here we report the first experimental phase diagram of the S 8 -Li 2 S-electrolyte system and apply it to explain the changes in electrolyte composition during the slow discharge and charge of lithiumsulfur batteries. We show that the phase diagram provides fundamental information about the thermodynamic equilibrium, which is a good approximation for slow cycling rates, and an essential starting point to study the kinetics at faster rates. Figure 1 illustrates the general form of the S 8 -Li 2 Selectrolyte phase diagram. The edges represent the binary mole fractions, x i of each component, so that the bottom edge is related to the stoichiometry of Li 2 S n as follows:At the top of the diagram we find the one phase region where all the sulfur and Li 2 S will dissolve forming a polysulfide solution. Next, we find two, two-phase regions representing the saturated solutions of each respective solid (sulfur + solution and Li 2 S + solution, respectively); the curved upper boundaries show how the solubility of one solute increases with the concentration of the other. For example, starting from the saturated solution of sulfur in the pure solvent at point A, we follow a downward curve showing how adding lithium sulfide in solution increases the solubility of sulfur by reacting to form soluble polysulfides; similarly, point B signifies the solubility of lithium sulfide in pure solvent, and it increases with the addition of sulfur into the solution due to formation of different polysulfides. The two curves meet at point C, the

Influence of the Electrolyte on the Internal Resistance of Lithium−Sulfur Batteries Studied with an Intermittent Current Interruption Method

“…Following Chen et al . and Dibden et al . a solubility for long‐chain polysulfides of the order of 1 M in S is expected, which would imply that precipitation of discharge products should take place before the peak, although there is no obvious difference in the voltage profiles between the cells (Supporting Information, Figure S4).…”

“…At an E/S ratio of 6 L mg , the dissolution of all sulfur in the electrode to form the polysulfide Li S without any precipitation of Li S implies an increase in the lithium ion concentration from 1.25 M to close to 3 M. A decrease in the ionic conductivity of the order of a factor of two is a reasonable expectation as a result of this concentration change . Assuming all elemental sulfur is able to dissolve out of the positive electrode — that is, it is not rendered inactive by a lack of electrolyte access — an E/S ratio of 6 L mg is equal to 167 g L −1 , or a concentration of 5.2 M in S. This concentration is lower than at the congruent point in the phase diagram as determined for lithium polysulfides in DOL by Dibden et al ., so it is reasonable to assume that all sulfur exists in solution at this point. It has also been suggested that the increase may be related to charge transfer at the negative electrode .…”

Polysulfide Shuttle Suppression by Electrolytes with Low‐Density for High‐Energy Lithium–Sulfur Batteries

“…The maximum dissolved concentration of stoichiometric Li 2 S 8 in 2 M LiTFSI in HME/DOL (9:1) is 50 mM as determined by UV/VIS measurements (Figure S1, Supporting Information). Thus, the concentration of dissolved atomic sulfur [S] equals 0.4 m in 2 m LiTFSI in HME/DOL (9:1), which is considerably lower than in DOL/DME electrolytes ([S] ≈ 6 m ) …”

Communication—Effect of Lithium Polysulfide Solubility on Capacity of Lithium-Sulfur Cells

“…As a matter of fact, the energy density of practical Li-S batteries strongly depends on the E/S ratio. [2][3][4] The E/S ratio sets an upper bound on LiPS dissolution, and the maximum solubility of LiPS species reported in the widely used ether solution ([S] = ∼6 M) 5 corresponds to an E/S ratio of 5.2 mL g −1 . If the minimum E/S ratio is determined by LiPS solubility, then in the above case electrolyte weight will dominate the battery weight and the theoretical specific energy of an ideal Li-S cell is less than 500 Wh kg −1 (see Supplementary Material).…”