Lithium Polysulphides Research Articles

Encapsulation of redox polysulphides via chemical interaction with nitrogen atoms in the organic linkers of metal-organic framework nanocrystals.

Lithium polysulphides generated during discharge in the cathode of a lithium-sulphur redox cell are important, but their dissolution into the electrolyte from the cathode during each redox cycle leads to a shortened cycle life. Herein, we use in situ spectroelectrochemical measurements to demonstrate that sp2 nitrogen atoms in the organic linkers of nanocrystalline metal-organic framework-867 (nMOF-867) are able to encapsulate lithium polysulphides inside the microcages of nMOF-867, thus helping to prevent their dissolution into the electrolyte during discharge/charge cycles. This encapsulation mechanism of lithiated/delithiated polysulphides was further confirmed by observations of shifted FTIR spectra for the C = N and C-N bonds, the XPS spectra for the Li-N bonds from nMOF-867, and a visualization method, demonstrating that nMOF-867 prevents lithium polysulphides from being dissolved in the electrolyte. Indeed, a cathode fabricated using nMOF-867 exhibited excellent capacity retention over a long cycle life of 500 discharge/charge cycles, with a capacity loss of approximately 0.027% per cycle from a discharge capacity of 788 mAh/g at a high current rate of 835 mA/g.

A few-layered Ti3C2nanosheet/glass fiber composite separator as a lithium polysulphide reservoir for high-performance lithium–sulfur batteries

A few-layered Ti3C2nanosheet/glass fiber composite separator was designed and fabricated as a lithium polysulphide reservoir for high-performance lithium–sulfur batteries.

Examining the effect of nanosized Mg0.6Ni0.4O and Al2O3 additives on S/polyaniline cathodes for lithium–sulphur batteries

Nanostructured magnesium nickel oxide Mg0.6Ni0.4O and alumina Al2O3 were studied as additives to sulphur/polyaniline (S/PANI) composites via wet ball-milling of sulphur and polyaniline followed by heat treatment. Metal oxide nanoparticles, which have small particle size, porous structure and high specific surface area to volume ratio, are expected to be catalytic for chemical reactions, including electron transfer and are able to adsorb lithium polysulphides. The composites were characterized by SEM and electrochemical methods. Cyclic voltammetry studies suggest that the alumina additive acts differently to the Mg0.6Ni0.4O. The results suggest that although the alumina additive improves the S/PANI composite performance as a lithium–sulphur battery cathode, the use of Mg0.6Ni0.4O is more effective.

Corrosion Processes in Lithium-Sulphur Cells. Isothermal Calorimetry As Method of Their Identification

Electrochemical system of Lithium-Sulphur is very promising because of its high theoretical specific energy (2600 Wh kg-1). In addition to sulphur is cheap and available. However the laboratory prototypes of lithium-sulphur batteries (cells) have several significant disadvantages: low practical specific energy (250-300 Wh kg-1at first cycles), high rates of self-discharge (>3% per month) and capacity fade (>0.1% per cycle) during prolong cycling. The main reason of low practical specific energy of lithium-sulphur cells is the large shares of ballast components including greater share of an electrolyte than in lithium-ion cells. Corrosion processes at lithium electrodes cause the high rates of self-discharge and capacity fade. It is related to the solubility of sulphur and lithium polysulphides in the electrolyte systems and their direct chemical interactions with the lithium electrodes. S8 + 4Li -> Li2Sn + Li2Sk, k+n=8; k & n ≥ 1 (1) Li2Sn + 2Li -> Li2Sm+Li2Sk; k+m=n, k & m ≥ 1 (2) Traditionally the effect of different factors on the corrosion processes in lithium-sulphur cells is measured by results of prolong charge/discharge cycling: the values of charge and discharge capacities and rate of their fading during cycling. Such studies are slow because corrosion process occurs slow at the initial cycles and does not affect the results so it is hard to measure the efficiency of suggested methods to suppressed the corrosions processes. As results show the combine method of isothermal calorimetry and galvanostatic polarisation (cycling) is very informative and convenient method to study corrosion processes in chemical power sources. This method allows us to identify corrosion processes at the initial stages of cycling and estimate their intensity. The aim of this work is to estimate the corrosion activity of sulphur, lithium polysulphides and factors, affecting their interactions with metallic lithium, by isothermal calorimetry combined with galvanostatic polarisation. The reported study was partially supported by RFBR, research project No 14-03-31399.

Targeting the role of lithium sulphide formation for the rapid capacity fading in lithium-sulphur batteries

This work aims to study the reasons for the rapid capacity fading in the field of lithium-sulphur batteries. It is found that the formation of non-soluble and insulating lithium sulphide, Li2S, upon sulphur reduction is one of the main reasons to account for the capacity fade due to its relatively low electrochemical activity and blocking of the carbon framework.A variety of analytical techniques are used to assess the formation of Li2S and visualize the resulting morphological changes in a mesoporous carbon/sulphur composite (MCS) during battery operation. It is shown that simply changing the cut-off voltages can alter retention and resistance of the cells during cycling. Since the electrochemical activity of lithium polysulphides decreases with the order, elimination of low order of polysulphides is considered critical. Thus, in order to oxidize the least sulphur reduction specie, Li2S, two different additives are suggested; namely an electrolyte additive, LiI, or an electrode additive, V2O5. Incorporation of the one of these components into the MCS composite enables the delivery of higher capacities (i.e. as compared to a bare C/S composite). The increased capacity is linked not only to the co-cathode materials, which contribute to the total capacity, but also to the oxidizing agent and internal redox mediator role of the additives.

The redox mechanism of FeS2in non-aqueous electrolytes for lithium and sodium batteries

Electrochemical processes of FeS2in non-aqueous electrolytes are reviewed and proposed to consist of two conversion reactions between sulphur or lithium polysulphides and sulphide for the upper voltage plateau and between FeS and metallic Fe for the lower voltage plateau.

Rational design of sulphur host materials for Li–S batteries: correlating lithium polysulphide adsorptivity and self-discharge capacity loss

A versatile, cost-effective electrochemical analysis strategy is described that determines the specific S(n)(2-) adsorptivity of materials, and allows prediction of the long-term performance of sulphur composite electrodes in Li-S cells. Measurement of nine different materials with varying surface area, and hydrophobicity using this protocol determined optimum properties for capacity stabilization.

Soluble polysulphide sorption using carbon nanotube forest for enhancing cycle performance in a lithium–sulphur battery

The rapid capacity decay of lithium–sulphur batteries has been a significant obstacle for practical application, which is generally considered to arise from dissolution of lithium polysulphide in the electrolyte and diffusion away from the cathode. As the lithium content in the polysuphide increases with further discharge, capacity decay occurs also from the passivating effects by the formation of insoluble sulphides, further amplified by volume increase. More recently, weakening of sulphur adhesion to carbon with progress in discharge is also an important factor in the sulphur cathode degradation. In order to overcome capacity decay caused by all the above mechanisms, we have prepared a composite cathode made of sulphur and high density carbon nanotube (HD-CNT) forest scaffold that is able to interfacially adsorb and volumetrically confine the polysulphide species and accommodate the expansion of sulphur discharge products effectively. This cathode demonstrates very high electrochemical stability and high discharge capacity up to 200 full discharge/charge cycles even with the use of the basic organic ether electrolyte where polysulphide shows high solubility, thus providing evidence for confinement and interfacial contact. Retention and surface adsorption favoured by minimizing the wall-to-wall distance between the aligned CNTs arise from a decrease in the reaction energy of the adsorption. Computational simulation of the interface between polysulphide species and carbon nanotube surface provides first-principle confirmation of improved binding between C and S in the polysulphides as wall-to-wall distance is decreased. The HD-CNT scaffold is self-binding and highly-conducting thus the conventional additives of binder and carbon black are also fully eliminated. A high discharge capacity of 812mAhg−1 of sulphur (corresponding to 503mAhg−1 of the whole cathode material mass) is stably retained after 200 cycles at 400mAg−1 with a small average capacity decay of only 0.054% per cycle on average These encouraging results provide novel approaches to designing and fabricating long cycle life cathode in a lithium–sulphur battery.

On the reasons for low sulphur utilization in the lithium–sulphur batteries

This work is to study the reasons for the relatively low efficiency of sulphur reduction (about 75%) in lithium–sulphur batteries. The two main reasons for that are suggested to be: the relatively low electrochemical activity of low order lithium polysulphides and blocking of the carbon framework of the sulphur electrode by insoluble products of electrochemical reactions – sulphur and lithium sulphide.The electrochemical activity of lithium polysulphides with different composition (Li2Sn, n = 2–6) has been studied in 1 M solutions of CF3SO3Li in sulfolane. It is shown that lithium polysulphides including lithium disulphide are able to electrochemically reduce with efficiency close to 100%. The electrochemical activity of lithium polysulphides decreases with the order. The order of lithium polysulphides affects the value of voltage of discharge plateaus but not the efficiency of sulphur reducing in the lithium polysulphides species.The relatively low efficiency of sulphur reduction in the lithium–sulphur batteries is more likely caused by blocking of carbon particles in the sulphur electrode by insoluble products of electrochemical reactions (sulphur and lithium sulphide). This prevents the electrochemical reduction of low order lithium polysulphides and especially lithium disulphide.

Two-dimensional layered transition metal disulphides for effective encapsulation of high-capacity lithium sulphide cathodes

Fully lithiated lithium sulphide (Li2S) is currently being explored as a promising cathode material for emerging energy storage applications. Like their sulphur counterparts, Li2S cathodes require effective encapsulation to reduce the dissolution of intermediate lithium polysulphide (Li2Sn, n=4-8) species into the electrolyte. Here we report, the encapsulation of Li2S cathodes using two-dimensional layered transition metal disulphides that possess a combination of high conductivity and strong binding with Li2S/Li2Sn species. In particular, using titanium disulphide as an encapsulation material, we demonstrate a high specific capacity of 503 mAh g(-1)(Li2S) under high C-rate conditions (4C) as well as high areal capacity of 3.0 mAh cm(-2) under high mass-loading conditions (5.3 mg(Li2S) cm(-2)). This work opens up the new prospect of using transition metal disulphides instead of conventional carbon-based materials for effective encapsulation of high-capacity electrode materials.

Assessment of Applicability of Potentiometric Method to Determine Thermodynamic Properties of Electrochemical Processes in Lithium Sulphur Batteries

The efficiency of chemical energy conversion to electric is a significant characteristic of batteries and it is especially for batteries on base of high power electrochemical systems (for example Li-S). The maximal possible efficiency of energy conversion is determined by thermodynamic properties of the electrochemical systems.There are two main methods to estimate of the thermodynamic properties of the electrochemical processes: calorimetric and potentiometric.Heat generation or absorption of the electrochemical processes is possible to register direct by calorimetric methods . However registered heat generation or absorption includes both thermal effects of the electrochemical reaction and Joule heat during polarization of batteries. If some concurrent processes are going in the batteries then their thermal effects will be part of the registered heat generation or absorption by the instrument. One of the other disadvantages of the calorimetric methods is complexity and high price of instruments. Potentiometric method is much easier in instrument executions than calorimetric method and allows to more accurate estimate the thermodynamic properties of the electrochemical processes in the batteries. Therefore it is wide used to estimate the thermodynamic properties of active components of positive and negative electrodes of batteries. The main criterion of applicability of the potentiometric method to measure thermodynamic properties is reversibility of material and energy flows. Therefore this method must be used with care and especially for batteries with high self-discharge rate, for example batteries with liquid cathode (Li-S battery).The aim of present work is an assessment of applicability of the potentiometric method to determine the thermodynamic properties of the electrochemical processes in the lithium sulphur batteries.The test subjects were the lithium sulphur pouch cell. Lithium foil (99.9%, LE-1, Russia) with a thickness of 100 μm was used as negative electrodes. The working electrodes were sulphur electrodes (70% of Sulphur, 10% of Carbon and 20% of Polyethylene oxide). One layer of micro porous membrane Celgard® 3501 was used as a separator. An electrolyte was 1M solution of LiCF3SO3in sulfolane.Electrolyte preparation, lithium electrode manufacture and lithium sulphur batteries assembly were all carried out in dry air filled glove box.Sulphur and products of its reduction (lithium polysulphides) are able to direct interact with metallic lithium and disturb the reversibility of material and energy flows. It should be noted that reactivity of sulphur and lithium polysulphides toward metallic lithium depends on them chemical composition (number of sulphur atoms per molecule). Therefore to study the effect of depth of discharge (DoD) on the reversibility of material and energy flows in the lithium sulphur batteries we measured temperature dependence of open circle voltage (OCV) of batteries at different values of DoD. Current density was 0.2 mA cm-2.To reach equilibrium (to obtain uniform lithium polysulphide distribution in the bulk of battery) after discharging or charging at the specified DoD batteries were stored 12 hours at constant temperature (30.0 ± 0.1 °C). Then the temperature dependence of OCV was registered in the range of temperature of 10-40 °C. The equilibrium was considered to have been reached when OCV stabilized to less than 0.2 mV h-1at a constant temperature (±0,1 °C) and OVC-T measurements were completely reproducible at heating and cooling. The thermodynamic properties were calculated by equations:ΔG = -nFEP,T (1)ΔH = -nFEP,T + nFEP,T(∂E/∂T)P (2)ΔS = nF(∂E/∂T)P (3)We estimated that thermodynamic properties of electrochemical reduction of sulphur in the lithium sulphur batteries only can be made by the potentiometric method at the first cycle. At the following cycles of lithium sulphur batteries this method can be applied only at the DoD of 33-100 %. At the depth of discharge of 0-33% (high voltage plateau of charge-discharge curves) lithium sulphur batteries cannot achieve equilibrium because of high rate of self- discharge. The OCV was constantly reducing to the value characteristic to the low voltage plateau of charge-discharge curves.Figure shows the obtained dependences of Free energy (Gibbs energy), enthalpy and entropy changes of the electrochemical reduction of sulphur and lithium polysulphides on the DoD at 1st cycle.Thus it can be concluded that the potentiometric method of measuring thermodynamic properties of electrochemical reactions in lithium sulphur batteries can be applied at 1st cycle in all range of DoD and at the following cycles it can be applied only in the range of DoD of 33-100 %.The reported study was partially supported by RFBR, research projects No 13-00-14056 and 4-03-31399.

Effect of Lithium Salts on the Cycle Life of Lithium Sulphur Cells

The lithium sulphur electrochemical system has high value of theoretical specific energy – 2600 Wh/kg. However a real reached specific energy of prototypes of lithium sulphur batteries is only 6-15% (150-400 Wh/kg) against theoretical expected value. The main reason of that is great share of accessory components – 70-80%, from which an electrolyte share is 40-60%.Necessity of using a great volume of electrolyte is caused by multifunctionality of electrolyte in lithium sulphur cells:ion transport between electrodes;sulphur dissolution;lithium polysulphides solvation and dissolution;stabilization of the necessary molecular and ionic forms of lithium polysulphides. A problem of high rate of capacity fading of lithium sulphur batteries during cycling is another problem which should be solved for commercial application of these batteries.Both problems are caused by feature of lithium sulphur batteries – transformation of sulphur and lithium sulphide into lithium polysulphides readily dissolves in the electrolytes. Solubility of lithium polysulphides and the forms of their existence in electrolytes are basically determined by electrolyte salt and nature of solvent. Therefore characteristics of lithium sulphur batteries should also depend on electrolyte salt and nature of solvent.The aim of the present work is to estimate an effect of lithium salts and amount of electrolyte on the cycle life of lithium sulphur cells. A study of the electrochemical properties of lithium sulphur cells was carried out in Swagelok type cell. The sulphur (working) electrodes consisted of 70 wt.% sulphur, 10 wt.% carbon and 20 wt.% binder. The sulphur electrodes had an average loading of 1.2 mg sulphur per cm2. Lithium metal foil (99.9%, LE-1, Russia) with the thickness of 80 μm was used as the auxiliary electrode. 1M LiClO4 and 1M LiSO3CF3 in sulfolane were used as the electrolytes. Amount of electrolyte into cells was 1.0; 1.5; 2.0; 3.0 and 4.0 μl/mAh(S). Porous polypropylene Celgard® 3501 was used as the separator. The lithium sulphur cells were assembled by stacking lithium electrode, separator containing electrolyte and sulphur electrode. Electrolyte preparation, lithium electrode manufacture and lithium sulphur cells assembly were all carried out in dry air filled glove box.The charge and discharge performances of the assembled cells were investigated with a PG12-100 potentiostat between 1.5 V and 2.8 V at +30 oC. The charge current density was 0.1 mA/cm2, the discharge current density was 0.2 mA/cm2.It has shown, that lithium salt has no effect on the shape of discharge curves (fig.a and fig.c), but has an effect on the rate of capacity fading and cycle life of lithium sulphur cells (fig.b and fig.d).Irrespective of amount of 1M LiClO4 in sulfolane the discharge capacity of lithium sulphur cells primarily decreases dramatically on about 40-50 %, then rate of capacity fading decreases and capacity stabilizes. Cycle life of these cells does not practically depend on amount of electrolyte (fig.b).For 1M LiSO3CF3 in sulfolane rate of discharge capacity fading of lithium sulphur cells depends on amount of electrolyte. The increase of amount of electrolyte into cells leads to reduction of rate of discharge capacity fading during cycling and to increase of cycle life (fig.d).This study was carried out under project of Russian Academy of Sciences No. 01201152192 and also partially supported by RFBR, research project No 13-00-14056.

Adsorbing/Absorbing Effect of Nanosized Metal Oxide and Zeolite Nanocrystals in Lithium Sulphur Batteries

Due to their low cost and high energy density, rechargeable Li-S batteries are among the most promising energy storage devices. Sulphur has a theoretical gravimetric energy density up to five times higher than that of present lithium-ion battery technologies. Despite significant recent advances, there are still some challenges. Upon sulphur reduction and oxidation, intermediate soluble lithium polysulphides readily diffuse into the electrolyte, causing capacity fading and poor columbic efficiency in the cell. To overcome this problem, strong adsorption and absorption materials have been employed to adsorb/absorb lithium polysulphides. As an additive in the sulphur cathode it has been demonstrated that various nano-sized oxides (such as Mg0,6Ni0,4O, Al2O3, La2O3) and mesoporous silicates (SBA-15) can effectively trap polysulphides [1-4].In this contribution we report use of Silicalite-1 zeolite as an additive in the cathode composite in the Li-S batteries. Silicate-1 is a microporous, silica-based molecular sieve with a MFI topology of the tetrahedral framework. Incorporation of transition metals into their framework generates different types of catalytically active sites. MnS-1 (manganese silicalite-1) nanosized crystals (arround 200 nm) have been synthesized under classical hydrothermal procedure and showed Mn3+ incorporated into the silicalite-1 framework as framework manganese. By contrast, MnS-1 synthesised under microwave hydrothermal procedure resulted in incorporation of Mn3+ into the silicalite-1 framework as extra-framework manganese in forms of manganese oxide nano-particles [5-6].Galvanostatic cycling showed good capacity retention of the Li-S cells through the use of zeolite additives as polysulphide reservoirs. We examined the role of silica surface, pore absorption and manganese oxide adsorption. MnS-1 crystals with a small amount of nano-sized manganese oxide particles in the structure appear effective at retaining the soluble polysulphides at the cathode and are limiting the fraction that takes part in the sulphur shuttle mechanism. We assume that, most likely, additionally to the zeolites, the nano-sized manganese oxide serves as an adsorbent of the polysulphides in the cathode material. By utilizing 9 wt.% MnS-1 in cathode material we achieved a higher specific capacity and cycle efficiency in comparison with cathode material without zeolite additive and with 9 wt.% of mesoporous SBA-15 additive. The possible mechanism(s) will be discussed based on results obtained by in operando analytical techniques developed in our lab [7,8]. Acknowledgement: This research has received founding from the Slovenian Research Agency and the European Union Seventh Framework Programme under grant agreement No. 314515 (EUROLIS).

Effect of Current Density on the Heat Generation of Lithium-Sulphur Batteries during Charge and Discharge

The investigations of thermal effects of charging, discharging and storage of batteries is important to solve the fundamental and applied problems of energy conversion in electrochemical systems. They are especially significant to develop and design batteries on the base of electrochemical systems with high energy density (for example Li-S).The thermal effects of charging and discharging batteries include several components: (1) the thermal effects of the electrochemical reactions, (2) the thermal effects of the processes accompanying the electrochemical reactions, (3) Joule heat.In contrast to the batteries with solid active materials (Li-Ion batteries) when lithium-sulphur batteries charging and discharging the complex multistep processes occur and include:- the electrochemical reactions;- the chemical reactions (corrosion, disproportion reactions of lithium polysulphides);- the reactions of solvation/desolvation and complexing of lithium polysulphide generated in solutions;- the phase transformations (dissolution, precipitation, sorption, desorption).All these processes effect on the efficiency of conversion chemical energy to electrical in lithium-sulphur batteries.When changing of conditions of charge/discharge cycling of lithium-sulphur batteries the contribution of different process are changing therefore energy conversion efficiency can be increased or reduced. The effect of current density on the heat generation of lithium-sulphur batteries was studied in this work by special designed and built instrument “Electrochemical calorimeter”The Electrochemical calorimeter simultaneously records thermal and electric characteristics of planar cells under/without polarisation. The instrument includes calorimetric block, potentiostat and control block. The measurements of thermal effects are carried out in isoperibol mode. The accuracy of temperature stabilization is ± 5∙10-4K. The temperature range is -20 – +90 °C. The measurement accuracy of heat flow is 50 µW.The relative error of current stabilization is 0.1 %. The voltage measurement error is 10 μV.The test objects were the lithium-sulphur pouch cells. Lithium foil (99.9%) with a thickness of 100 μm was used as negative electrodes. The working electrodes were sulphur electrodes (70% of Sulphur, 10% of Carbon and 20% of poly(ethylene oxide)). One layer of micro porous membrane Celgard 3501 was used as a separator. The electrolyte was 1M solution of LiCF3SO3in sulfolane.To get reproducible results 10 form cycles were done before study of influence of current density on the heat generation of the lithium-sulphur cells.The study shows that the shape of curves of heat generation is similar in the investigated range of current density (Fig.).When charging the lithium-sulphur cells there are few characteristic segments at the curve of heat generation. I. At the initial stages of charging (Q< 30 mAh g-1(S)) there is a maximum of heat generation and it is linear increasing with the current density increase. For the same charge state there is a maximum of overvoltage at the charging curves and it is also increasing with the current density increase. It allows concluding that this maximal of heat generation is caused by Joule heat. II. The voltage and heat generation is continuously increasing with the further charging of lithium-sulphur cell (low voltage plateau). III. There is a minimum at the curve of heat flow and it corresponds to the transition between low and high voltage plateau. This minimum is increasing with current density increase. IV. The heat generation of lithium-sulphur batteries is much greater at the high voltage plateau of charging curve than at the low voltage curve. It should be noted it is linear increasing with current increase.The shapes of heat generation are different at charging and discharging of lithium-sulphur cell (Fig.). The heat generation curve can be divided in several characteristic segments (Fig. d): I. At the initial stage of discharging the intensive heat generation is observed. Its fast decrease correlates to reducing of overvoltage at the discharging. II. There is a maximum at the heat generation curve corresponding to the high voltage curve of discharging. This maximum is linear increasing and shifting to the large discharge capacity with current density increase. III. At the transition between high and low voltage plateau at discharge curve there are two inflections at the heat generation curve. They are also shifting to the higher discharge capacity with increasing of current density. IV. The heat generation of lithium-sulphur cell changes insignificantly at the low voltage discharge curve. The overvoltage and the heat generation increase with current density increasing.Also it should be noted that heat generation is greater at discharging of lithium-sulphur cell then at charging.The analysis of data shows that there are maximum at the dependences of efficiency of energy conversion on the current density for charging and discharging of lithium-sulphur batteries. The current density affects stronger on the efficiency of energy conversion at charging than at discharging (especially for high voltage plateau). The reported study was partially supported by RFBR, research projects No 13-00-14056 and 14-03-31399.

The Effect of Lithium Polysulphides on the Cycling Performance of a Lithium Electrode in 1M LiClO4 in Sulfolane

The effect of lithium polysulphides (Li2Sn) on the electrochemical behaviour of lithium electrodes attracts great interest due to the development of batteries with high energy density based on the lithium-sulphur electrochemical system. A feature of lithium-sulphur batteries is the formation of soluble lithium polysulphides, generated during the electrochemical discharge (equations 1-2) and charge (equations 3-5) [1-3]. Discharge: (1) nS0 + 2e- + 2Li+ → Li2Sn (2) Li2Sn + 2(n-1)e– + 2(n-1)Li+ → nLi2S↓ Charge: (3) xLi2Sk → yLi2Sn + 2(x-y)e- + 2(x-y)Li+ (where x·k = y·n)(4) yLi2Sn + zLi2S → xLi2Sk, (where y·n+z = x·k)(5) Li2Sn → nSo + 2e– + 2Li+ During the cycling of lithium-sulphur batteries lithium polysulphides diffuse and/or migrate to the surface of the lithium electrode. Lithium sulphide and shortened lithium polysulphides are formed by the interfacial reaction between lithium and the lithium polysulphides dissolved in the electrolyte:(6) 2Li0 + Li2Sn → Li2S↓ + Li2S(n-1) Lithium sulphide is insoluble in electrolyte systems based on aprotic dipolar solvents and thus deposits on the lithium to form a surface film - Solid-Electrolyte Interface (SEI).Lithium sulphide can react with “long-chain” lithium polysulphides forming shortened lithium polysulphides, which are soluble in the electrolyte:(7) Li2S + Li2Sn → Li2Sm + Li2Sk ,(where m+k = n+1)“Short”- and “medium-chain” lithium polysulphides can form low solubility solvate complexes that will also deposit on the lithium metal surface. It is known, that the pattern of electrochemical behaviour of the lithium electrode, including its high cycleability, are determined by the properties of SEI [4]. Therefore, the presence of lithium sulphide and solvate complexes of lithium polysulphides in the surface film should have an effect on the electrochemical behaviour of lithium electrode.The aim of the present work is to assess the effect of dissolved lithium polysulphides on the pattern of electrochemical behaviour of lithium electrodes.A study of the lithium electrode’s cycling behaviour was carried out in Swagelok type cell. Working and auxiliary electrodes were made from lithium metal foil (99.9 %). The combination of 2 layers of porous polypropylene Celgard®3501 with 2 layers of non-woven polypropylene between them was used as a separator. 1M LiClO4 in sulfolane and 0.4M Li2S6 in 1M LiClO4 in sulfolane were used as electrolytes. Electrolyte volume in the cells was 0.02 ml/cm2. Lithium polysulphides solutions were prepared by dissolving the specified amount of lithium sulphide (Li2S) and sulphur (S) in 1M LiClO4 in sulfolane.Dissolution/deposition efficiency testing of the lithium was carried out galvanostatically with a PG12-100 potentiostat between ±0.5 V at +30 oC and +85 oC. The dissolution/deposition current density was 0.2 mA/cm2, the dissolution/deposition capacity of lithium was 0.5 mAh/cm2.It has been shown that the cycle life of lithium electrodes is significantly increased in the presence of lithium polysulphides, especially at +85 oC (Fig. 1).It is probable that an interface “sulphide” film with good protective properties and high ionic conductivity is formed in electrolyte systems containing lithium polysulphides and that this film is preserved during extended cycling [5-6]. This film inhibits reactions between lithium and the components of the electrolyte solution, in particular with sulfolane molecules.The main limiting factor for lithium electrode cycling is decomposition rate of solvent and anions of supporting salts as a result of their reduction by freshly formed metallic lithium. The surface of cathodicaly deposited lithium, in the presence of lithium polysulphides, is quickly passivated by lithium sulphide (reaction 6), as the result the rate of reaction of metallic lithium with electrolyte (solvent and lithium salts anions) is decreased. That is why the lithium electrode in the presence of lithium polysulphides is cycled longer.We believe the increase in temperature leads to the increase in the rate of reaction of metallic lithium with lithium polysulphides. Therefore the increase in temperature leads to increase in the cycle life of lithium electrode.

The Effect of Amount of Electrolyte on the Efficiency of Sulphur Utilization in Lithium-Sulphur Cells

The lithium-sulphur electrochemical system has high value of theoretical specific energy – 2600 Wh/kg. However a real reached specific energy of lithium-sulphur batteries is significantly low against theoretical expected value. So, specific energy of prototypes of lithum-sulphur cells is in the range of 150-400 Wh/kg, what is only 8-15% of theoretical value of specific energy, while specific energy of lithium-ion batteries reaches about 30-40% of theoretical value [1-2].The practical specific energy of batteries is determined by a number of factors. The main factors are an efficiency of utilization of active materials of positive and negative electrodes as well as a weight share of accessory components (electrolyte, separator, current collector, cell body etc.) against the total weight of battery. In lithium-ion batteries an efficiency of utilization of active materials of electrodes reaches 90-95% and the weight share of active materials reaches 35-45% against the total weight of battery. In lithium-sulphur batteries a depth of electrochemical reduction of sulphur is usually in the range of 1250-1400 mAh/g(S) what is 75-85% against theoretical value (1675 mAh/g(S)). The weight share of accessory components in lithium-sulphur batteries is significantly greater than in lithium-ion batteries and reaches 70-80%. Electrolyte makes the main contribution into the weight of lithium-sulphur batteries. Usually weight of electrolyte in lithium-sulphur cells is in the range of 40-60% against in lithium-ion – 15-20%.The difference in functions of electrolyte in lithium-ion and lithium-sulphur batteries is the reason for the difference in amount of electrolyte. Lithium-sulphur batteries can be classified as batteries with a liquid cathode as far as during charge and discharge of them solid-phase active materials – elementary sulphur (S) and lithium sulphide (Li2S) – change into well soluble in electrolytes compounds – lithium polysulphides (Li2Sn):Discharge:nS0 + 2Li+ + 2e- → Li2Sn Charge:nLi2S → Li2Sn + 2(n-1)e- + 2(n-1)Li+ The electrolyte in lithium-sulphur batteries has higher functionality than in lithium-ion batteries. In lithium-ion batteries electrolyte performs the function of ionic conductor. In lithium-sulphur batteries the electrolyte provides an ionic transfer between electrodes as well as dissolves the electroactive components (sulphur and lithium polysulphides). The aim of the present work is to determine a minimum amount of electrolyte required for good performance of lithium-sulphur cells.A study of the electrochemical properties of lithium-sulphur cells was carried out in Swagelok type cell. The sulphur (working) electrodes consisted of 70 wt.% sulphur, 10 wt.% Ketjenblack EC-600JD and 20 wt.% poly(ethylene oxide) (Mw 4x106). The surface capacity of sulphur electrodes was 2 mAh/cm2. Lithium metal foil (99.9%) with the thickness of 80 μm was used as the auxiliary electrode. 1M LiCF3SO3 in sulfolane was used as the electrolyte. Amount of electrolyte into cells was 1.0; 1.5; 2.0; 3.0 and 4.0 mcl/mAh(S). Porous polypropylene Celgard® 3501 was used as the separator. The lithium-sulphur cells were assembled by stacking lithium electrode (d=2.55 cm, area=5.10 cm2), separator (d=2.85 cm, area=6.38 cm2) containing 1M LiCF3SO3 in sulfolane and sulphur electrode (d=2.85 cm, area=6.38 cm2). The charge and discharge performances of the assembled cells were investigated with a PG12-100 potentiostat between 1.5 V and 2.8 V at +30 oC. The charge current density was 0.1 mA/cm2, the discharge current density was 0.2 mA/cm2.It is shown that the amount of electrolyte effects on the efficiency of sulphur utilization (fig. 1a-1b) and cycle life of lithium-sulphur cells (fig. 1c). For obtaining of acceptable efficiency of sulphur utilization amount of electrolyte in lithium-sulphur cells should be not less than 1.5-2.0 mcl per 1 mAh of sulphur electrode capacity (fig. 1b). The cycle life of lithium-sulphur cells is practically linearly increasing with increasing of amount of electrolyte.

The Calorimetric Study of Electrochemical Process in the Lithium-Sulphur Cells

The studies of thermal effects of cycling and storage of lithium-sulphur cells have significant value to development of energy storage systems on the base of new electrochemical systems with high specific energy. One of the most promising electrochemical systems is lithium-sulphur system.The mechanisms of electrochemical, physicochemical and chemical processes occurring in the lithium-sulphur cells during cycling significantly differ from the mechanisms of processes in lithium-ion cells. Electrochemical conversion of sulphur and products of its reduction is multi-step process including electrochemical and chemical reactions, also phase transformations (dissolution, precipitation, crystallization, sorption, desorption).To study of thermal effects of electrochemical cells we designed and built specialized instrument “Electrochemical calorimeter”. It simultaneously records thermal and electric characteristics of planar cells under polarisation. The instrument includes calorimetric block, potentiostat and control block. The measurements of thermal effects are carried out in isoperibol mode. The accuracy of temperature stabilization is ± 5∙10-4K. The temperature range is 253-363 K. The measurement accuracy of heat flow is 50 µW. The relative error of current stabilization is 0.1 %. The voltage measurement error is 10 μV.We studied the thermal effects of charging and discharging of the lithium-sulphur cells by designed instrument.The test subjects were the lithium-sulphur pouch cells. Lithium foil (99.9%) with a thickness of 100 μm was used as negative electrodes. The working electrodes were sulphur electrodes (70% of Sulphur, 10% of Carbon and 20% of poly(ethylene oxide)). One layer of micro porous membrane Celgard® 3501 was used as a separator. The electrolyte was 1M solution of LiCF3SO3in sulfolane.There are two maximums of heat generation during discharging of lithium-sulphur cell (Fig. 1). The first maximum has complex shape and correlates to the transition between high and low voltage plateaus of discharge curve. The second maximum is observed at the end of discharging.The comparison between curves of heat generation (Fig. 1c) and internal resistance (Fig. 1b) during discharging of lithium-sulphur cells makes it possible to conclude that first maximum is caused by polarization heat and Joule heat and thermal effects of chemical and electrochemical reactions. The second maximum is caused by the polarization heat and Joule heat.The heat flow curve correlated to the charging of lithium sulphur-cell (Fig. 2c) has more complex shape than the curve correlated to the discharging (Fig. 1c).The analysis of heat flow, internal resistance and charge curves allows to conclude that first maximum of heat generation is caused by the polarization heat and Joule heat. The heat absorption corresponding to the transition between high and low voltage plateaus of charge curve is possible to be caused by physicochemical and chemical process accompany the electrochemical oxidation of low ordered lithium polysulphides. The intensive heat generation is likely to be caused by chemical reaction of sulphur or high ordered lithium polysulphides and metallic lithium at the end of charging of lithium-sulphur cell.

Manipulating surface reactions in lithium–sulphur batteries using hybrid anode structures

Lithium-sulphur batteries have high theoretical energy density and potentially low cost, but significant challenges such as severe capacity degradation prevent its widespread adoption. Here we report a new design of lithium-sulphur battery using electrically connected graphite and lithium metal as a hybrid anode to control undesirable surface reactions on lithium. Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface layer to actively control the electrochemical reactions and minimize the deleterious side reactions, leading to significant performance improvements. Lithium-sulphur cells incorporating this hybrid anodes deliver capacities of >800 mAh g(-1) for 400 cycles at a high rate of 1,737 mA g(-1), with only 11% capacity fade and a Coulombic efficiency >99%. This simple hybrid concept may also provide scientific strategies for protecting metal anodes in other energy-storage devices.

Poly(acrylic acid) gel as a polysulphide blocking layer for high-performance lithium/sulphur battery

A poly(acrylic acid)-coated sulphur cathode significantly improves the cyclability of a Li/S battery because the resulting gel blocks the out-diffusion of lithium polysulphide via hydrogen bonds.

Low surface area graphene/cellulose composite as a host matrix for lithium sulphur batteries

Graphene/cellulose composites were prepared and studied as potential host matrixes for sulphur impregnation and use in Li-S batteries. We demonstrate that with the proper design of a relatively low surface area graphene/cellulose composite, a high electrochemical performance along with good cyclability can be achieved. Graphene cellulose composites are built from two constituents: a two-dimensional electronic conductive graphene and cellulose fibres as a structural frame; together they form a laminar type of pore. The graphene sheets that uniformly anchor sulphur molecules provide confinement ability for polysulphides, sufficient space to accommodate sulphur volumetric expansion, a large contact area with the sulphur and a short transport pathway for both electrons and lithium ions. Nano-cellulose prevents the opening of graphene sheets due to the volume expansion caused by dissolved polysulphides during battery operation. This, in turn, prevents the diffusion of lithium polysulphides into the electrolyte, enabling a long cycle life.