The effect of the lithium perchlorate concentration in sulfolane (in the range from 0.1M to 2.8М) on the performance of lithium-sulfur cells is studied. It is shown that the concentration of lithium ...perchlorate in sulfolane considerably affects the depth of electrochemical reduction of sulfur, the reactivity of lithium polysulfides, and the coulomb efficiency of cycling of lithium-sulfur cells. The maximum depth of electrochemical reduction of sulfur is reached at the concentration of lithium perchlorate in sulfolane of 1.0M, while the minimum depth is reached at 2.8М. The depth of electrochemical reduction of sulfur is limited by the number of free solvent molecules. An increase in the concentration of the support salt results in an increase in the coulomb efficiency of cycling of lithium-sulfur cells. At concentrations of the support salt above 2.4М, the coulomb efficiency of cycling of lithium-sulfur cells is close to 100%.
We believe that the concentration of support salts affects the species of lithium polysulfides, their association-dissociation equilibria in electrolyte solutions and the processes of solvation of lithium ions contained in lithium polysulfides and support salts, and thus determines the performance of lithium-sulfur cells.
•Concentration of LiClO4 in sulfolane affects the performance of lithium-sulfur cells.•Reactivity of Li2Sn is determined by concentration of support salts in electrolyte.•Concentration of support salts affects association-dissociation equilibria of Li2Sn.•Electrolyte solvents solvate lithium ions in both Li2Sn and support lithium salts.•Depth of S reduction is limited by amount of free molecules of electrolyte solvents.
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 (Li sub(2)S sub(n), n = 2-6) has been studied in 1 M solutions of CF sub(3)SO sub(3)Li 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.
The possibility of analyzing the electrochemical impedance spectra of lithium–lithium cells using the Distribution of Relaxation Times (DRT) function is studied. A comparative analysis of the ...electrochemical impedance spectra of lithium–lithium cells obtained during long-term storage at a constant temperature and at different temperatures was performed using the method of either equivalent electrical circuits or the DRT function. The analysis of the impedance of lithium–lithium cells by the DRT function is shown to allow estimating the number of layers in the surface film on the lithium electrodes and evaluating their physical parameters—the resistance and capacitance. It has been established that with a long exposure of lithium–lithium cells at the temperature of 30°C, the number of layers in the surface film and its resistance decreased. With the increase in the temperature, the physical properties of the layers of the surface film are differentiated and its total resistance decreased. The analysis of the electrochemical impedance spectra of lithium–lithium cells by the DRT functions is more informative than the method of equivalent electrical circuits.
The pulsed method of measuring impedance is described. The cell is galvanostatically stimulated by a bipolar current signal of square shape. The cell response is registered by sampling U+i, U−i with ...selected period Δt. The impedance spectra are calculated by direct Fourier transform. The internal resistance of the lithium sulphur cell is characteristically minimum in the calculated impedance diagrams in the frequency range of 0.035–5 Hz. It is shown that the lithium sulphur cells have maximum internal resistance at the transient between high and low voltage plateaus of charge and discharge curves. The internal resistance increases significantly during the initial stages of cycling because of the formation of passivation layers at the electrodes. It was found that the internal resistance of the lithium sulphur cell in the same charge state is governed by the way in which it is achieved. This is explained by differences in molar volumes of products generated in the sulphur electrode by electrochemical reaction during charging and discharging.
•The pulsed method as a way to determine the internal resistance of batteries.•The internal resistance of Li–S cells depends on the depth of charge and discharge.•The polarization direction of lithium sulphur cell governs the internal resistance.
In this work we considered the possibility of simulation of changes in the characteristics of lithium-sulfur batteries during cycling using an Adaptive Neuro-Fuzzy Inference System, ANFIS. The ...discharge profiles and the curve of decrease of discharge capacity of lithium-sulfur cells during cycling have been simulated. Neural network training was performed on every 5th cycle from the first to 95 cycles. It was shown that the simulated discharge profiles of lithium-sulfur cells are in good agreement with the experimental discharge profiles. The forecast depth of the decrease in the discharge capacity of lithium-sulfur cells during cycling with an accuracy of
5% was 45 cycles. Simulation time of one discharge profile lasts 4.5 seconds, which makes it possible to use this approach in the development of control and monitoring systems for batteries (Battery Management System, BMS).
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A multichannel hardware and software complex for studying electrochemical cells during charge–discharge cycling is described. The complex consists of 16 identical channel modules controlled by an ...on-board computer. Each channel is a four-band potentiostat/galvanostat that allows the performance of independent experiments. The modules can be hot-swapped during operation. Means are provided for connecting external sensors with an analog output to each channel. Owing to the client–server architecture, it is possible to manage the experiment, obtain measurement results, and remotely administer the device from client computers over a local-area network (LAN) or the Internet. Advanced software for numerical processing and visualization of experimental data is a part of the complex.
We have studied the interaction of polycrystalline samples of lithium nitride with metallic lithium. We have found that upon contact, metallic lithium spontaneously dissolves into polycrystalline ...lithium nitride samples. Spontaneous penetration of metallic lithium into polycrystalline samples of lithium nitride leads to the appearance of electronic conductivity and the formation of mixed ion-electronic conductors.
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•Interaction of metallic lithium and polycrystalline lithium nitride is studied.•Metallic lithium spontaneously penetrates in a pellet of Li3N.•Penetration of lithium in Li3N leads to short circuit of the cell Li │ Li3N │ Li.•Penetration of Li in Li3N leads to the formation of mixed ion-electronic conductors.
Change in the content of lithium sulfide is quantitatively studied through the methods of precipitation and back iodometric titration in lithium-sulfur cells during charge-discharge cycling for 100 ...cycles. It is shown that in the initial cycles (25 cycles) of charge-discharge cycling, electrochemically inactive lithium sulfide accumulates. The predominantly inactive lithium sulfide accumulates in the sulfur electrode. It is likely that lithium sulfide is deposited in the pores of the carbon material, blocks the pores and loses the ability to participate in the electrochemical reactions.
On the surface of the lithium electrode, a surface layer containing lithium sulfide is formed during the cycling of the lithium-sulfur cells and is in equilibrium with the electrolyte system. It is found that lithium sulfide does not accumulate on the lithium electrode for at least 100 charge-discharge cycles.
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•Lithium sulfide content in Li-S cells is studied.•Lithium sulfide does not accumulate on lithium electrodes over 100 cycles.•Lithium sulfide is blocked in the pores of carbon particles of positive electrode.
The effect of lithium polysulfides on the amount and ratio of electrochemically active metallic lithium, electrochemically inactive metallic lithium, and chemically formed lithium compounds in the ...cathodic deposits formed on a stainless-steel electrode during galvanostatic cycling in 1 М LiClO
4
solution in sulfolane at 15, 30, 45, and 60°C is studied using the method we have developed earlier. It is shown that the increase in temperature leads to increase in the Coulomb efficiency of cycling and the amount of electrochemically active metallic lithium; a decrease in the amount of electrochemically inactive metallic lithium, regardless of the presence of lithium polysulfides in the electrolyte. When lithium polysulfides have been introduced into the electrolyte, an increase in the Coulomb efficiency of the metallic lithium cycling and a change in the ratio of various forms of lithium in the cathodic deposits toward an increase in electrochemically active lithium by about 1.5 times are observed. The lithium polysulfides are assumed to contribute to the dissolution of electrochemically inactive metallic lithium, forming an interfacial “sulfide” film at the electrode, which possessed high ionic conductivity and good protective properties, the more so, at elevated temperatures.
The solubility of sulfur in sulfolane and solutions of lithium salts LiBF
4
, LiClO
4
, LiPF
6
, LiSO
3
CF
3
and LiN(SO
2
CF
3
)
2
in sulfolane, which are promising electrolytes for lithium-sulfur ...batteries, was determined by UV-vis spectroscopy. It was found that the solubility of sulfur in sulfolane at 30°C is 82.0 mM, and in sulfolane solutions of lithium salts (1 M) it is 4–9 times lower. The dependence of sulfur solubility on the concentration of lithium salts is not linear: it is 32.9 and 5.8 mM for sulfolane solutions containing 0.5 and 2.35 M of LiClO
4
, respectively.
