The mechanism of effect of support salt concentration in electrolyte on performance of lithium-sulfur cells
Introduction
Lithium-sulfur batteries are batteries with a liquid cathode, since the electroactive materials of the positive electrode (sulfur and intermediate products of its electrochemical reduction, lithium polysulfides) are soluble in the electrolyte [[1], [2], [3], [4], [5], [6], [7]]. The presence of active materials of the positive electrode in the liquid phase determines the considerable effect of the physicochemical properties of the electrolyte system on the regularities of chemical, physicochemical and electrochemical processes that occur in lithium-sulfur batteries during their charge, discharge and storage. Ionic conductivity and viscosity are the most important properties of electrolyte systems as they affect the regularities of electrochemical transformations of the dissolved active materials (sulfur and lithium polysulfides). The ionic conductivity and viscosity of the electrolyte systems are determined by many factors, the main ones being: the polarity and solvation properties of the solvents; the size, geometry and donor properties of anions in the support salts; and the concentration of the support salts in the electrolyte solutions.
The regularities of electrochemical transformations of sulfur and lithium polysulfides are determined by their species in the electrolyte solutions. By chemical nature, lithium polysulfides (Li2Sn) are salts of polysulfane acids (H2Sn) [8]. Lithium polysulfides, dissolved in aprotic dipolar solvents, can undergo heterolytic (electrolytic) [[9], [10], [11]] (Equations (1), (2)) and homolytic dissociation [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]] (Equations (3), (4)).
The heterolytic dissociation of lithium polysulfides, which can occur both by the first and second stages, produces charged particles (lithium cations and polysulfide anions):
At electrolytic dissociation of lithium polysulfides – Li2Sn, the bond of Li-S breaks with the formation of lithium cations and mono- or dianions of polysulfides (Equations (1), (2)). The degree of electrolytic dissociation increases with increasing polarity and donor properties of solvents.
The homolytic dissociation of lithium polysulfides gives rise to radical particles with various compositions (Equation (3)), for example:
The anionic forms of polysulfides can also undergo homolytic dissociation (Equation (4)):
At homolytic dissociation of polysulfide molecules (Li2Sn) and polysulfide anions (Sn2−), the -S-S- bond breaks and each of the fragments retains one of the originally bonded electrons (Equation (3)). In the case of lithium polysulfides, the most stable radical is S3-, whose formation is shown in many works, for example [23]. Homolytic dissociation of Li2Sn can occur as in polar and as in low polar solvents [15,16,19]. Low polar solvents stabilize the low-ordered lithium polysulfides, and the high polar ones stabilize the high-ordered lithium polysulfides and the anion radical S3-. High polar and high donor solvents promote homolytic dissociation to form the radical anion S3-.
The radical species of lithium polysulfides, similarly to the molecular ones, can undergo electrolytic dissociation (Equation (5)):
As a result of the liability of lithium polysulfides to undergo heterolytic (electrolytic) and homolytic dissociation, electrolyte solutions generally simultaneously contain a lot of diverse molecular, radical, ion radical and ionic species of sulfur with various number of atoms that exist in dynamic equilibrium. The species of lithium polysulfides in electrolyte solutions determine their reactivity.
Solutions of lithium polysulfides in aprotic dipolar solvents have high ionic conductivity that is comparable to that of readily ionizable lithium salts [24,25]. A specific feature of polysulfide solutions is that they have high viscosity, which is considerably higher than that of solutions of lithium salts with the same concentration. For example, the 0.3m solution of Li2S6 in sulfolane has conductivity of 0.987 mS/cm and viscosity of 30 mPa s and 0.5m solution of LiClO4 in sulfolane has conductivity of 1.897 mS/cm and viscosity of 19 mPa s [26]. This indicates that lithium polysulfides form intermolecular associates in solutions [[24], [25], [26]].
The presence of support lithium salts in the electrolytes of lithium-sulfur cells affects the species of lithium polysulfides in electrolyte solutions and hence their reactivity. In fact, in the presence of well ionizable salts (salts with high electrolytic dissociation constants), the electrolytic dissociation of lithium polysulfides will be suppressed to a considerable extent. In support of arguments on suppression of electrolytic dissociation of lithium polysulfides, it is also possible to bring the fact that there is no synergetic effect in the specific conductivity of mixed solutions of lithium polysulfides and lithium perchlorate [26]. The degree of electrolytic dissociation of lithium polysulfides will also decrease in the presence of salts with low electrolytic dissociation constants but to a much smaller extent.
Furthermore, the support lithium salts, present in electrolytes, result in cationic linking of polysulfide anions [24], which impairs the transport properties of electrolyte systems and the segmental mobility of sulfur atoms in polysulfide chains. The effect of lithium polysulfides on the properties of electrolyte solutions has been studied in a number of works [[25], [26], [27], [28], [29]]. It has been shown that the presence of lithium polysulfides affects the ionic conductivity of electrolyte solutions and increases their viscosity.
Even a brief consideration of the structure and properties of lithium polysulfide solutions in the electrolyte systems allows one to assume that the properties of lithium-sulfur batteries will largely depend on the nature and concentration of lithium salts in electrolytes.
For example, the effect of the concentration of lithium salts on the performance of lithium-sulfur cells was studied in a number of publications [6,[30], [31], [32], [33], [34], [35], [36], [37], [38], [39]]. However, the published results of studies do not allow one to judge to the full extent about the mechanisms of the effect of the concentration of support salts on the processes into lithium-sulfur cells.
The purpose of this work is to clarify the mechanism by which the concentration of support salts in electrolytes affects the performance of lithium-sulfur cells, such as the depth of electrochemical reduction of sulfur, the rate of capacity depletion and the coulomb efficiency of cycling of lithium-sulfur cells.
Section snippets
Objects studied and justification of their choice
Electrochemical studies were performed in airtight stainless steel two-electrode disc cells by own design and production (Swagelok® cells).
The sulfur electrodes (d = 2.85 cm, A = 6.38 cm2) used in this study had the following composition: S (99.5%, Acros) - 70% wt., Ketjenblack EC-600JD (Akzo Nobel) - 10% wt., polyethylene oxide (ММ = 4 × 106, Aldrich) - 20% wt. The surface capacity of sulfur electrodes was 2 mAh/cm2, which is equivalent to the sulfur load about 1.2 mg/cm2. The sulfur content
Effect of the concentration of lithium perchlorate in sulfolane on the charge-discharge voltage profiles of lithium-sulfur cells
Studies have shown that the concentration of lithium perchlorate in sulfolane affects the charge-discharge voltage profiles of lithium-sulfur cells (Fig. 1а). The discharge voltage profiles of lithium-sulfur cells obtained in the first cycle contain two plateaus, irrespective of the lithium perchlorate concentration. As an exception, the discharge voltage profiles of the cells with a high-concentration electrolyte containing 2.8М LiClO4 manifest only one high voltage plateau. However, as
Discussion
Three aspects should be kept in mind in a consideration of the mechanisms of the effect of the concentration of support salts on the discharge capacity and cycling performance of lithium-sulfur cells:
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species of lithium polysulfides in electrolyte solutions;
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the association-dissociation equilibria in electrolyte solutions containing lithium salts and lithium polysulfides;
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the processes of solvation of lithium ions contained in lithium polysulfides and support salts.
Conclusions
The concentration of lithium salts in electrolyte solutions (lithium perchlorate in sulfolane) has a considerable effect on the properties and regularities of cycling of lithium-sulfur cells. As the concentration of the support salt increases, the discharge capacity of lithium-sulfur cells first increases, then decreases. The increase in the discharge capacity mainly occurs due to the increase in the capacity of lithium-sulfur cells at the low voltage stage. The maximum discharge capacity of
Acknowledgments
This work was performed as part Government Order of the Ministry of Science and Higher Education of the Russian Federation (theme No АААА-А17-117011910031-7) and was also financially supported by Russian Science Foundation (Project No 17-73-20115) and by Russian Foundation for Basic Research (Project No 16-29-06190).
In the frame of the Government Order, effect of concentration of lithium support salt on the depth of electrochemical reduction in lithium-sulfur cells and their internal DC
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