*2.4.1 Impact of Se and Te substituted polysulfides on lithium deposition*

Electrochemical performance of Li2S and Li2S2 entirely depends on the lithium plating and stripping effectively. **Figure 11a** shows capacities for anode-free Ni || [Li2S + 0.1 S/Se/Te] full cells at ~ 1 mA cm−2 (C/5). Selenium showed rapid capacity fade and loss of their peak capacity 50% after the 35 cycles. By the introduction of tellurium exhibited remarkable cycling stability in the anode-free configuration

#### **Figure 10.**

*(a) Cyclic voltammograms for [Li2S + 0.1 S/Se/Te] cathodes at scan rate range of 200 to 2000 μv s-1 (b) electrochemical performance of Li || [Li2S + 0.1 S/Se/Te] half cells. Reprinted from ref. [26].*

#### **Figure 11.**

*Electrochemical performances – (a) capacity retention and (b) coulombic efficiencies of anode-free Ni || [Li2S + 0.1 S/Se/Te] full cells. (c) Electrochemical performances of large-area (39 cm2 ) anode-free Ni || Li2S single-layer pouch full cells with 10 wt% tellurium (Te: Li2S molar ratio = 0.04) or 10 wt% carbon black as cathode additives. Reprinted from ref [26].*

and retain 52% of peak capacity at 265 cycles. The loss rate of lithium per cycle is decreased to 2.14% with Se and 0.24% with Te [64]. The improvement in lithium plating and stripping reversibility reflects by the coulombic efficiencies of the anode-free full cells as shown in **Figure 11b**. Polytellurosulfides showed a dramatic effect on lithium cycling efficiency by situ formation, kinetic hindrance occurred with tellurium substitution in polysulfides compared with selenium. The formation of polyselenosulfides has no effect on the reversibility of lithium deposition. These improvements were analyzed with symmetric Li || Li cells containing Li2SexSy and Li2TexSy introduced as an electrolyte components. Electrochemical impedance spectroscopy reveals the polyselenosulfides showed high and unstable overpotentials (~100 mV) and polytellurosulfides enable low and stable overpotentials (~10 mV), indicating a thin SEI layer has excellent ionic transport properties. The dense and uniform lithium deposits formed with polytellurosulfides exhibited irreversible loss of lithium. The electrochemical performance of an anode-free Ni || Li2S full cell with 0.05 Se + 0.05 Te additives was observed. There is a synergetic effect realized that the higher initial capacities and cycling stability than that of the pure 0.1 Te and 0.1 Se based cells. Therefore we believe that the presence of SeS2\*radicals increased the faster capacity fade with 0.05 Se + 0.05 Te than with 0.1 Te, representing that an electrolyte system might be allowed radical anion of selenium to obtained higher capacities and also retained for longer cycles. These results further implemented impractical, large-area (4.8 × 8.1 cm2 ), single-layer pouch cells assembled in the

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

#### **Figure 12.**

*(a) Crystal structure of Li2X (X = S, Se, and Te) and the three Li+ ion diffusion pathways marked as purple [100], red [110], and green [111] lines. Migration energy barriers along [110, 111] show a steady reduction from Li2S to Li2Se and to Li2Te. (b) Li+ -ion transport pathway in Li2TeS3 along the x-axis and the corresponding energy barrier based on single-ion migration. Reprinted from ref [26].*

anode-free configuration (N/P = 1) with a 164 mg Li2S cathode (4.2 mg cm−2) containing 10 wt% Te0 (Te:Li2S molar ratio = 0.055) and operating under lean-electrolyte conditions (E/Li2S = 4.5 ml mg−1) the results are shown in **Figure 12c** [65]. Tellurium was replaced with carbon black for control. **Figure 11b** showed the control cell exhibited a high initial capacity of 77 mAh, but it has very rapid capacity fade 80% retention within 13 cycles. By the addition of Te exceeded 80% of its peak capacity for nearly 150 cycles and retain their cycling capacity without rapid drop until the electrolyte dry-out nearly 300 cycles [66, 67]. The initial rise in capacity can be regarded as 'activation period' in which the dissolution of tellurium slowly into polysulfides. The improvement in cycle life with the introduction of tellurium can be attributed to the stabilizing effect of polytellurosulfides on lithium deposition. These results are valid practically relevant to the cell design and testing parameters such as long cycle life and high energy dense, anode-free configuration significantly closer to commercial viability of Li–S system.

### *2.4.2 Lithium-ion transport properties of selenides, tellurides, and thiotellurates*

First principles calculations were evaluated to understand reduced species on lithium deposition and their ionic transport properties. Li2S, Li2Se, and Li2Te indicated crystallize in a cubic antifluorite structure with a face-centered cubic anionic framework along with Li+ ions in the tetrahedral sites. **Figure 12a** showed Li<sup>+</sup> can diffuse along the directions of [100], [110], [111]. Climbing image nudge elastic-band (CI-NEB) method is used to find the diffusion barriers along each of these pathways. Barrier energy found to be ̴0.3 eV in the lowest- pathway [68]. Barrier energies are 0.875 eV to 0.748 eV to 0.539 eV, calculated from the transitions Li2S to Li2Se to Li2Te, respectively. Te2 exhibited more polarizable anionic framework compared to those of S2and Se2due to the larger size and lower charge density. Previous report [69] reveals

that the larger size of Te2 provide more open channel along [110], [111] in which high diffusion pathways for Li<sup>+</sup> ions. Therefore, Li2Te due to its more uniform, homogenous, and dense lithium deposition can provide alternate pathways to facilitate three-dimensional ion transport. Li2TeS3 exhibited monoclinic structure with trigonal pyramidal TeS3 2−anions arranged in layers. The Li+ ions coordinated with sulfur atoms and occupied their octahedral and tetrahedral sites. Li2TeS3 unit cell consists of eight distinct steps between five adjacent sides and the non-equivalent lithium sites can be found in the migration pathway. The single-ion NEB model was introduced to calculate the corresponding barrier energies and find the most favorable path. It indicates the migration from one tetrahedral site to another tetrahedral site through an intermediate octahedral site in the direction of the x-axis as shown in **Figure 12** (b). The migration barriers are found to be 0.378 and 0.250 eV. The barrier energies are found to be 0.4 and 0.6 eV for other migration pathways. Therefore, multiple viable Li+ ion diffusion pathways available in Li2TeS3 in three dimensional paths ways for ion transport due to stable and reversible lithium deposition. All these factors would improve the lithium cyclic efficiencies by the formation of interfacial components with polytellurosulfides.

### *2.4.3 Interfacial chemistry of Se and Te substituted polysulfides on lithium batteries*

In order to understand their effects of modified Li2Sn, Li2SexSy and Li2TexSy species by XPS, in which lithium anode-free full cells analyzed after 20 cycles. **Figure 13a** shows the S 2p + Se 3p and Li 1 s + Se 3d spectra for the cell with addition of 0.1 Se. The S 2p spectra SO4 2− species are dominated by oxidized sulfur due to the decomposition of LiTFSI. The dominated peaks at 165 eV and 58.7 eV are appeared in the Se 3p and Se 3d spectra, respectively by the oxidized Se+4 in selenites (SeO3 2−) and minor components are present due to Reduced sulfur species (Li2S). The presence of oxidized selenium species is due to LiNO3, which is a strong oxidizing agent and oxidizes selenides (Se2) into selenites (SeO3 2−) [70]. Therefore, polyselenosulfides introduced not only the fundamental alter to the lithium–electrolyte interface, which remains dominated by oxidized sulfur/selenium species. **Figure 13b** shows the cell with 0.1 Te additive spectra of S 2p and Te 3d. The reduced sulfur species (S2− at 160.6 eV) exhibited dominated S 2p spectra. Likewise, the Te 3d spectra are dominated by sulfurized tellurium species (Te+4 at 574.6 eV). The quantification of the spectra reveals the formation of thiotellurate (TeS3 2−) species [70]. Thus, the formation of Li2TeS3 as the dominant interfacial component and are reduced on the lithium surface. Some of previous research reveals that the oxidized sulfur species are present as minor components. It is extended that the oxidized tellurium species (TeO3 2−) made only a minor fraction of tellurium atoms on the lithium surface. Hence introduction of tellurium alters the lithium–electrolyte interface by the reduction of sulfur species (as Li2TeS3). These XPS observations consistent with the time-of-flight secondary ion mass spectrometry (ToF-SIMS). **Figure 13c** shows the profiles for Li2 (metallic lithium) and SO3 for three-dimensional reconstructions. A thick layer of electrolyte decomposition products is observed on the deposited lithium with polyselenosulfides but not with polytellurosulfides. Depth profiles indicate that the selenium has strong signal for SeO is compared to that for SeS. This is reversed for tellurium, in which TeS exhibited much stronger compared to that for TeO. Thus, majority of tellurium atoms made bond with sulfur and the majority selenium atoms making bond with oxygen. This is due to oxidation of LiNO3 explained by Pearson's HSAB theory [71]. The soft Lewis acid cations (Te+4) are formed by

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

#### **Figure 13.**

*(a) S 2p + Se 3p and Li 1 s + Se 3d spectra for the lithium surface in an anode-free full cell cycled with polyselenosulfides. (b) S 2p and Te 3d spectra for the lithium surface in an anode-free full cell cycled with polytellurosulfides. (c) 3D reconstructions of ToF-SIMS depth profiles for Li2 (metallic lithium) and SO2 (oxidized sulfur species) secondary ions. Reprinted from ref. [26].*

the Tellurium that prefer soft Lewis bases such as S2 sulfides, while selenium forms hard Lewis acid cations (Se+4) that prefer hard Lewis bases such as O2oxides [72, 73]. The divergent lithium stabilization capabilities of polyselenosulfides and polytellurosulfides explained the differences in lithium interfacial chemistry. The sulfide anionic framework such as Li2TeS3 identified as preferable compared to an oxide anionic framework such as Li2SeO3 or Li2SO3 [74]. The greater size and polarizability of S2compared to those of O2, improves ionic transport properties by reducing Li<sup>+</sup> ion diffusion barriers. The varying compositions of tellurium and selenium to get a stable sulfide-rich SEI layer, in the presence of LiNO3 changed the characteristics of lithium deposition [75].

## **2.5 Freestanding Se1-xSx foamy cathodes for high-performance Li-Se1-xSx batteries**

The development of supercritical CO2 synthesis of selenium-sulfur solid solutions (Se1-xSx) are promising new cathodic materials for high-performance secondary lithium batteries due to their high electric conductivity than S and superior theoretical specific capacity than Se. The morphology and microstructure of N-doped carbon framework with three-dimensional (3D) interconnected porous structure (NC@SWCNTs) host are characterized by SEM and TEM pictures as shown in **Figure 14**. A depicted in **Figure 14a** shows the NC@SWCNTs host 3D honeycombed structure and interconnected melamine foam framework. The magnification SEM images (**Figure 14a, c**) reveals that numerous interlaced SWCNTs are covered the surface of melamine foam by the derived carbon skeletons and SWCNTs are formed as thin sheets between carbon skeletons. This structure of NC@ SWCNTs exhibited a highly conductive 3D network to transport the electron or ion, but also increases the mechanical strength as well as flexibility of NC@SWCNTs host. TEM results (**Figure 14d** ) reveals that SWCNTs, 3D network structure are crisscrossed in carbon skeletons. By EDS analysis the main elements found to be in the NC@SWCNTs are C, O and N, which are uniformly distributed as shown in **Figure 14e**. N signal is derived from melamine foam because of it contain high N. 3D network structure of NC@ SWCNTs is made of by composing SWCNTs-coated N-doped carbon skeleton melamine foam and wafery sheets of SWCNTs. The NC@SWCNTs consists of pores and layer gaps are favorable for loading of Se1-xSx active conductive materials. The 3D conductive network promotes not only redox kinetics, but also endow NC@ SWCNTs host with strong buffering in volume during cycling. Further, N doped is also beneficial for the adsorption of intermediates, after Se1-xSx impregnation, compared to NC@SWCNTs host. NC@ SWCNTs@ Se1-xSx composites retain their original morphology of NC@SWCNTs and no

#### **Figure 14.**

*(a–c) Shows the SEM images, (d) represents the TEM image (e) indicated STEM image of NC@SWCNTs and the corresponding mapping images. Reprinted from ref. [76].*

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

discernible Se1-xSx particles can be found. According to EDS results, the C, N, Se and Se signals are overlapped well, suggesting Se1-xSx composites are uniformly permeated into the pores and layer gaps of NC@SWCNTs host with the assistance of SC-CO2 due to the good permeability, excellent diffusivity and high solubility of SC-CO2 [76].

Electrochemical performance of NC@ SWCNTs@ Se1-xSx composites based cathodes evaluated in Li- Se1-xSx batteries using carbonate-based electrolyte (LiPF6-EC/DMC). **Figure 15a** show initial three cyclic voltammetry (CV) curves of NC@SWCNTs@ Se1-xSx cathodes with scanning rate of 0.1 mV s−1 in the potential range from 1.0 to 3.0 V versus Li/Li+ . Initially, a sharp reduction peak at ∼1.38 V, a small reduction peak at ∼2.37 V and a broadened oxidation peak at ∼2.14 V appeared. The sharp reduction peak at ∼1.38 V shifts to ∼1.7 V and the small reduction peak at ∼2.37 V was disappeared after the first scan. Initially, during the lithiation the peak shift due the activation process and the polarization is also further reduced [77] CV curves overlapped after the first scan reveals that the good cyclability and reversibility of NC@SWCNTs@ Se0.2S0.8 cathode [78]. Notably, CV curves of NC@SWCNTs@ Se1-xSx cathodes obtained differently from S cathode, representing the change of electrochemical reaction of S by Se and it is more conducive and stable with carbonate-based electrolytes. **Figure 15b** shows the galvanostatic charge-discharge curves of NC@SWCNTs@ Se1-xSx cathodes are consistent with the result of CV. There are two plateaus are observed (1) extremely short plateau at ∼2.38 V, and (2) a long plateau at ∼1.75 V in the first cycle. Further, subsequent cycles, long plateau at ∼1.75 V becomes a little steeper and shifts to ∼1.88 V and the short plateau appeared at ∼2.38 V. The short plateau appeared

#### **Figure 15.**

*(A) CV curves of NC@SWCNTs@Se0.2S0.8 cathode. (B) Charge/discharge curves of the NC@SWCNTs@ Se0.2S0.8 cathode at 0.2 a g−1. (C) Cycle performances and (D) rate performances of NC@SWCNTs@Se1-xSx cathodes. Reprinted from ref [76].*

at ∼2.38 V is attributed to the transformation of Se0.2S0.8 into polysulfides/polyselenides. The short plateau is disappeared due to dissolution of intermediates into the electrolyte [79]. The long plateau at 1.75–1.88 V is attributed to the conversion of polysulfides/polyselenides to Li2S/Li2Se [77]. There is only one sloping plateau appeared during the charge process, at ∼2.12 V due to the conversion of Li2Se/Li2S to Se0.2S0.8. The cyclic performance of NC@SWCNTs@ Se1-xSx cathodes at a current density of 0.2 A g−1 with different Se/S ratios as shown in **Figure 15c**. As prepared NC@SWCNTs@Se0.2S0.8 cathode delivers the highest initial discharge capacity (2398.5 mA h g−1) among all the samples. Discharge capacity exceeds the theoretical capacity at initial stage may be due to side reactions and the formation of SEI layer on the surface of electrode [80]. Electrochemical characteristics of NC@SWCNTs@ Se0.2S0.8 cathode exhibit the superior cyclic stability. **Figure 15d** showed the rate capabilities of NC@SWCNTs@ Se1-xSx cathodes at various current densities. Among all the samples, NC@SWCNTs@ Se0.2S0.8 cathode showed the best rate performance. At the various current densities of 0.2, 0.5, 0.8, 1.0 and 2.0 A g−1 the reversible rate capacities of NC@SWCNTs@ Se0.2S0.8 cathode are found to be 998.4, 723.7, 606.8, 506.1, and 415.0 mA h g−1, respectively. The reversible discharge capacity of NC@SWCNTs@ Se0.2S0.8 cathode reverts to the initial value, when the current density switches back to 0.5 A g−1. NC@SWCNTs@ Se0.2S0.8 cathode with Se loading of as high as 4.4 mg cm−2 exhibited areal capacity of as high as 2.78 mA h cm−2 is the best candidate most reported Se1-xSx cathodes in literature [77, 81–83]. The electrochemical performance of NC@SWCNTs@ Se0.2S0.8 cathode is more effective due to the following reasons: 1) Se and S in Se0.2S0.8 solid solution play various roles: Se can improve more electrical conductivity, whereas the S can raise its capacity. 2) N-doped 3D porous carbon matrix and interlaced SWCNTs can provide storage and the structural stability; thereby promote the cycling stability of NC@ SWCNTs@ Se1-xSx cathodes. NC@SWCNTs@ Se0.2S0.8 cathode exhibits good cycling stability (632 mA h g−1 at 0.2 A g−1 at 200 cycle) and high rate performance (415 mA h g−1 at 2 A g−1) due to well-designed structure as well as optimized chemical composition with in carbonate-based electrolyte. Hence these developments of high-performance Se1-xSx cathodes suitable for advanced Li- Se1-xSx batteries.
