Lithium polysulfides

More recently, 84 may have been identified by ESR spectroscopy of solutions of Li2S ( >6) in DMF at 303 K. The lithium polysulfide was prepared from the elements in liquid ammonia. These polysulfide solutions also contain the trisulfide radical anion ( 2.0290) but at high sulfur contents a second radical at g=2.031 (Lorentzian lineshape) was formed which was assumed to be 84 generated by dissociation of octasulfide dianions see Eq. (32) [137],... 

Liquid ammonia solutions of lithium polysulfides have been characterized by Dubois et al. [18]. The least reduced polysulfide was shown to be 8 (not found previously in aquo) lying in a strongly temperature-dependent equilibrium with the radical 83 . 

Dubois P, Lelieur JP, Lepoutre G (1988) Identification and characterization of lithium polysulfides in solution in liquid ammonia. Inorg Chem 27 73-80... 

Caution. All reactions involving solutions of lithium polysulfides or polyselenides generate compounds that are malodorous and toxic to varying degrees and consequently must be performed in a well-ventilated fume hood. Li[BEt H] can ignite on contact with water, alcohols, or air. This reagent should be handled only under an inert atmosphere. 

Thus, as we can see, there is no consensus as to the exact mechanism of reduction of elementary sulfur in an organic electrolyte, with the difficulty lying primarily in the numerous equilibrium points of the reaction, notably dismutation equilibrium and antidismutation equilibrium, which exist in solution for Sg and make it tricky to study solutions of lithium polysulfides Li2S. ... 

In this last example, the idea is also to give the active material favorable environments for its confinement in the electrode regardless of its physical state. Thus, the sulfur, which is initially solid, is contained within the carbon structure. During discharge, the lithium polysulfides are dissolved in the mesoporous chaimels (greater voliune). 

Other studies attempt to couple elementary sulfur with a conductive polymer such as polythiophene or pol5 yrrole in order to improve the electronic conductivity of the electrode whilst having an effect of retention of lithium polysulfides. In addition, all authors agree that the presence of this polymer helps to fix the active material to the positive electrode, which prevents it from accumulating during the course of cycling. ... 

Firstly, the dissolution of these lithium polysulfides ean lead to an increase in the viscosity of the electrolyte. These speeies are highly soluble in the electrolyte, and the eoncentrations can reach up to 10 mol/L. In such conditions, the eleetrolyte becomes viscous and the ionic mobility is decreased. 

In this case, we have an electrolyte identical to that which is present in lithium-polymer batteries, made of poly(ethylene oxide) (or PEO) in the presence of a lithium salt, solid at ambient temperature, and which needs to be heated above ambient temperature in order for the battery to work (T > 65°C for PEO). Thus, the electrolyte, in its molten state, exhibits sufficient ionic conductivity for the lithium ions to pass. This type of electrolyte can be used on its own (without a membrane) because it ensures physical separation of the positive and negative electrodes. This type of polymer electrolyte needs to be differentiated from gelled or plasticized electrolytes, wherein a polymer is mixed with a lithium salt but also with a solvent or a blend of organic solvents, and which function at ambient temperature. In the case of a Li-S battery, dry polymer membranes are often preferred because they present a genuine all solid state at ambient temperature, which helps limit the dissolution of the active material and therefore self-discharge. Similarly, in the molten state (viscous polymer), the diffusion of the species is slowed, and there is the hope of being able to contain the lithium polysulfides near to the positive electrode. In addition, this technology limits the formation of dendrites on the metal lithium... 

Addition of lithium polysulfides into the electrolyte, which decrease the solubility of the active material in solution. By the saturation effect, the organic liquid electrol soon attains the maximum capacity of dissolved sulfurous species, and therefore the lithium polysulfides from the positive electrode can no longer be dissolved. ... 

The work being done regarding the negative electrode aims to decrease its reactivity with lithium polysulfides, and limit dendrite formation. A number of strategies are possible. 

Lithium-polysulfide (Li-PS) flow batteries originated from Li-S batteries. Still using lithium metal as the negative electrode, Li-PS flow cells utilize a porous carbon electrode and soluble lithium polysulfides (Li2Sx where 8 x 4) as the positive electrode and positive electrolyte, respectively. The positive electrode reactions can be described as ... 

Sulfiphihc cathode materials w ith a strong affinity for lithium polysulfides are a promising group of candidates to control the dissolution and precipitation reactions in the cell, where the improvement of conductivity and the areal sulfur loading is an important objective. A metallic CogSs material has been described with an interconnected graphene-like nano-architecture that realizes this issue (28). 

It has been discovered that by adding a specific additive into the electrolyte of the electrochemical cells of a lithium accumulator with a bipolar architecture, it is possible to find a remedy to the charging problems of this cell t5 e. Each cell in the structure has a positive electrode and a negative electrode, separated by an electrolyte. To the electrolyte a lithium polysulfide 0285 is added. Li2S6 can be prepared by the reaction of lithium and sulfur in tetraethylene glycol dimethyl ether. 

The lithium polysulfide ensures the role of a redox shuttle. So this additive will undergo, at a determined potential, an oxidation at one of the electrodes of the cell in order to give an oxidized form of the additive. This oxidized form in turn imdergoes reduction at the electrode of the opposite sign of the same cell in order to give a reduced form. The reduced form is then capable of being oxidized at the electrode with reverse polarity. 

In the case of a lithium polysulfide additive, the redox shuttle mechanism occurs at a potential located between 2.4 and 2.5 V relative to the reference pair Ii+/Li. This means that this additive is particularly suitable for electrochemical cells for which the rated cell voltage after complete charging is less than all the voltage values between 2.4 and 2.5 V. In detail, the electrolyte may contain basic components that are shovm in Table 2.2. 

Figure 2.6 Charging curve of a device without lithium polysulfide (37).

The curve without lithium polysulfide has an ascending phase between 1 V and 1.8 V and then a plateau shape between 1.8 and 2 V. This is ending with an exponential ascending phase from 2.2 V up to 2.6 y. 

In this Janus separator, a nanoporous poly(propylene) (PP) membrane serves as an insulating substrate in contact with lithium anode while a layer of cellular graphene framework, which has extraordinary electrical conductivity, abimdant in-plane mesopores, high electrochemically active surface area, and large mesopore volume, adheres to the cathode side to reactivate the shuttiing-back of the lithium polysulfides and to preserve the ion channels (39). A meso-porous material is a material containing pores with diameters between 2 and 50 nm (40). 

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