*2.3.1 Electrochemical kinetic study*

The charge-discharge profiles clearly exhibited a large hysteresis (0.511 V at 50% state of charge) starting from the first cycle. A voltage hysteresis plot for the 1st–30th cycles is shown in **Figure 6a**. Further to understand the hysteresis clearly by analyzed the effect of the upper cutoff potential on the voltage hysteresis as shown in **Figure 6b**. But these results have no significant effect on the voltage hysteresis, indicating that the hysteresis in the Sn-based chalcogen system may be due to the origin of intrinsic nature of the Li insertion extraction reactions. Therefore galvanostatic intermittent titration technique (GITT) measurements were carried out with current density of 5 mA/g for an interval

### *Advanced Chalcogen Cathode Materials for Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.103042*

of 30 min. Then it allowed opening circuit state for 1 hour, to obtain a steady state. Two continuous cycles of the composite electrodes by the GITT measurements are shown in **Figure 6c, d**. The first charge process represents the Low over potential increment gradually and the discharge process battery delivered a very high potential with different behavior. The 0.2 Fe substituted composite cathode shows the less overpotential with gradual increment is due to the smooth and kinetically faster of Li extraction process compared with the Li insertion process. The huge over potential observed around 2.5–2.1 V during the insertion process reveals that the Li insertion is kinetically limited and slower than the extraction process. In the similar way, the second cycle showed and high over potential during discharge and less over potential during charge condition. Further, the discharge profile region shows the two different slops indicating that due to the multiphase reaction region and the structural modifications. GITT profiles reveals that the inflection point region represents the high potential of ∼187 mV compared to other regions. The electrochemical insertion of Li+ ions is limited in these regions due to the slow Li-ion diffusion and charge transfer resistance.

The Li insertion and extraction was estimated by cycling test for all the compounds in a half-cell configuration at the current density of 10 mA/g. The electrochemical investigation of 0.2 Fe substituted compounds compared with other

#### **Figure 6.**

*Electrochemical kinetic study of the Sn-based chalcogen anion redox cathode (0.2Fe-Li1.33Sn0.67S2). (a) Voltage hysteresis profile of active cathode material at 10 mA/g current density. (b) Voltage profile of the cathode at different upper cutoff potentials. GITT profiles of active cathode material: (c) 1st cycle and (d) 2nd cycle GITT profiles with 30 min pulse and 1 h relaxation. Reprinted from ref. [13].*

#### **Figure 7.**

*Electrochemical study of Sn-based chalcogen anion redox cathodes. (a) Cycling stability of different Fe substituted Li1.33Sn0.67S2compounds. (b) Cycling stability of 0.2Fe- Li1.33Sn0.67S2 cathode at a high current density of 50 mA/g (initial few cycles at 10 mA/g) reprinted from ref. [13].*

compositions are shown in **Figure 7a**. There is a gradual deterioration in all the compounds cyclic performance at a very low current density of 10 mA/g for 50 continuous cycles in terms of capacity fade. Further, a cycling test of the 0.2 Fe substituted cathode done at 50 mA/g, reveals that the high rate cycling stability about 76% retention after 80 cycles as shown in **Figure 7b**. The multi-redox induced structural transformation is the main reason for capacity degradation and evidenced by the microscopy analysis. The cycling stability of this composite materials is comparable to that of existing chalcogen anion redox cathodes [11, 56]. Further, the nanostructure and surface coatings strategies would enhance the cycle life [57, 58]. In this chalcogen framework the Fe doped composition showed good electronic and ionic conductivity, excellent electrochemical properties with the high loading of 10 mg/cm2 cells at 10 mA/g current density.

#### *2.3.2 HAADF-STEM and HR-TEM pictures*

The structural evolution due to Li insertion extraction was visualized in a series of samples using HAADF-STEM images (**Figure 8**). High atomic number elements such as transition metals represented by the bright spots and the light elements such as Li, S, and O represented without bright spots. **Figure 8a** shows the ordered pristine cathode composition showed bright spots of metal elements and the lattice exhibits without distortions or cracks. The lattice showed severe structural distortions and nanopore formation after the 1st cycle, indicated by the yellow-colored circles and arrows in the **Figure 8c−f**.

The structural distortion was observed in the first discharged cycle like honeycomb and after consequent cycling the ordered crystalline domain lost its crystalline nature as shown in **Figure 8c**. Further, investigations reveal that complete distorted structure after cycling due to the pores and structural distortions was high and the crystallinity degraded. The degradation of pore and crack formation happened during the initial cycles to extended cycling conditions. The low magnification images of second charged, second discharged, and cycled samples are shown in **Figure 8g, h** indicating a lot of nanopores present throughout the cycled cathode. Hence, the nanopore created by accelerated Li ion insertion-extraction

*Advanced Chalcogen Cathode Materials for Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.103042*

#### **Figure 8.**

*HAADF-STEM investigation of the 0.2Fe-Li1.33Sn0.67S2composite electrode at different states of charge: (a) pristine, (b) 1st charged, (c) 1st discharged, (d, g) 2nd charged, (e, h) 2nd discharged, (f, i) cycled electrodes; In panel (c), the honeycomb ordering was visualized. Reprinted from ref. [13].*

and also the sulfur loss by degradation. The formation of nanopore/nanovoid due to oxygen loss in the Li-rich oxide anion redox cathode as well as their degradation mechanisms correlated with the fundamental issues of voltage fade, voltage hysteresis, and capacity fade [59]. The sulfide anion is a significant charge contributor during Li extraction and the sulfur loss increased amorphization which is reflected in charge-discharge voltage profiles by means of capacity fade of the cycled cathode materials.

#### *2.3.3 Impedance spectroscopy analysis*

**Figure 9a** shows the impedance plots of cathode material at different discharged cycles. All the EIS measurements showed two semicircle regions except the pristine cell. The semicircle observed at high frequency region indicates the surface film formation on the positive electrodes. Another semicircle was observed in the low frequency region attributed to the charge transfer resistance upon Li+ insertion-extraction. The observed slope line indicates the Warburg diffusion (W) in the bulk electrolyte. An equivalent

#### **Figure 9.**

*EIS of 0.2Fe-Li1.33 Sn0.67S2 composite electrode at different states of charge. (a) Nyquist plots of the composite electrode at various cycles. This EIS was recorded after completion of respective cycle, (b the model EIS was fitted to the experimental Nyquist plot. Reprinted from ref. [13].*

circuit model (**Figure 9b**) designed by the fitted EIS data and the resistance values for all the active materials at different cycles noted. In the equivalent ckt Re corresponds to the solution resistance. The Cs and Rs correspond to and capacitance and the surface film resistance of the active sulfide composite cathode, respectively. The Li ion insertion and extraction double layer capacitance and induced charge transfer resistance represents as Rct and Cdl, respectively. From the **Figure 9a** calculated Re values for all the materials indicated small differences for all the cycles, reveals that the electrolyte is stable during cyclic process. The Rs and Rct values increased proportionally in the initial cycle number during 1st and 2nd cycles, representing that the structural distortion during the electrolyte and electrode interface. This observation consisted with GITT analysis that the very large potential during lithium insertion of initial cycles is due to the kinetic limitation of Li insertion reactions. The most of the surface film formation will occur during initial cycling. The Rct values decreased with increasing cyclic number, indicating that the resistance increased during the initial cycles and decreased by cycling cyclic number. This is due to the sulfur loss by rising cycling number, where the complete amorphization and nanopore are formed [12]. The anion redox indicates sulfur loss by reducing the Li insertion-extraction ability. These indicate the capacity fading occurred during the cycling. Therefore Sn based chalcogen layered structure materials worked as mixed redox cathode for Lithium Ion batteries.

#### **2.4 Chalcogen substitusion into the polysulfides for batteries**

The cyclic voltammograms (CVs) of half cells assembled using [Li2S + 0.1 S/Se/Te] cathodes as shown in **Figure 10a**. The presence of polyselenosulfides plays a significant reduction in peak separation (∆Ep), indicating decreasing the overpotentials, and helps with increased peak heights at high scan rates (1 ≥ mV s−1) by retaining the canonical redox peaks of sulfur/Li2S. The relationship between the peak current (ip) and scan rate (ν) can be written as: ip = α ν <sup>β</sup> , where α and β are the fitting parameters [60] Plotting log (ip) versus log (ν) yields β = 0.64 for Se, compared to 0.52 for the control. The value of β changes with the addition of Se represents the shifting of slow diffusion controlled reactions to fast surface-controlled reactions. This improvement of the redox kinetics is not much with the

## *Advanced Chalcogen Cathode Materials for Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.103042*

introduction of tellurium compared with selenium. **Figure 10b** shows the capacities of Li || [Li2S + 0.1 S/Se/Te] half cells at 0.25 A g−1 of Li2S (BC/5). Selenium is electrochemically more active between 2.8 and 1.8 V, whereas Te is inactive at the same voltage. Hence the addition of 0.1 Se enables a considerable improvement in capacities of ̴40% under the control. Therefore the relative dominance of catalytic SeS2\* radical presence in polyselenosulfide solutions works as conversion reactions and utilized completely to drives complete electrochemical reaction [61, 62]. This is critical for high capacities obtained under lowelectrolyte conditions in a practical Li–S cell [24, 63]. The presence of SeS2\* radicals highly react with the various electrolyte components thereby the faster capacity fade observed with polyselenosulfides even after 70 cycles. The sulfur/Li2S final product conductivity also improved due to incorporation of Se atoms. The charging/discharging profiles of Li || Li2S half cells indicate that considerable reduction achieved in overpotentials with selenium compared to sulfur or tellurium. Therefore, considerable improvement in charge-transfer and redox kinetics is occurred with the introduction of selenium rather than tellurium.
