**2.6 Nanoreactors for metal-chalcogen batteries**

Porous hollow nanoreactors are investigated widely for lithium selenium and tellurium batteries. The mesoporous material exhibited considerable porosity (0.2 cm3 g−1) and a large surface area of 462 m2 g−1, which allowed for uniform distribution of Se8. The Se8/C based lithium selenium batteries showed a high reversible capacity of 480 mA h g−1 at 0.25C (1C = 678 mA g−1) without loss of its capacity after 1000 cycles [84]. Further development of the Se/porous carbon cathode battery showed a high volumetric capacity of 3150 mA h cm−3 with excellent rate capability about 1850 mA h cm−3 at 20C. Therefore, it will be used for future commercialization of LSeBs [85]. Single-atom Co decorated hollow porous carbon also works as a nanoreactor with superior catalytic activity to polyselenides. These (Se@CoSA-HC) cathodes based batteries exhibited high discharge capacity, superior cycling stability,an excellent rate capability [86]. Metal or heteroatom doping (N, S, and Co) is also another alternative approach to enhance the utilization of Se or Te [87–89]. He et al. synthesized a nanoporous Co and N-co-doped carbon nanoreactor (C–Co–N) provide a high Te loading (77.2 wt%) provide ultrahigh capacity of 2615.2 mA h cm−3 and superior rate performance of 894.8 mA h cm−3 at 20C as shown in **Figure 16** [90]. Design structure and micro-environmental of Te-based nanoreactors could provide high

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

#### **Figure 16.**

*(a) CV curves of the three dimensional rGo/tellurium (3DGT) aerogel at a scan rate of 0.1 mV s−1. (b) Discharge curves of the 3DGT cathode at a 0.2 C. (c) Cyclic performance of the 3DGT cathode at 0.2 C for 200 cycles. (d) Rate performance at various crates for the 3DGT cathode. (e) Cyclic stability of 3DGT cathode at 1 C for 500 cycles. Reprinted from ref. [90].*

electrochemical performance. In conclusion, the development of hollow porous nanoreactors not only provide a suitable specified space for chalcogens (S, Se, and Te), but also load active species for the regulation of the microenvironment in the electrode. Further development of nanoreactors, it is necessary to design the new methodologies at the molecular level to regulate the microenvironment of the catalyst.

#### **2.7 Flexible cholcogen lithium-ion batteries**

#### *2.7.1 Flexible sulfur cathodes*

Zhang and his coworkers developed 1D/3D hybrid flexible sulfur electrodes with good flexibility and exhibited improved electrochemical performance [91, 92]. They used sulfur-infiltrated 3D nanostructure porous carbon materials with various sizes nanometers to ten micrometers representing with high versatility and applicability for constructing flexible electrodes. These materials not sustain without support, therefore by incorporating ultralong CNT scaffolds, very robust films are obtained without sacrificing mechanical flexibility compared to ultralong CNT/MWCNT film. Such a materials exhibited tremendous specific surface area, high micro or mesoporosity,

and surface functionalities than MWCNTs. Hence, this strategy is an ideal generic and versatile host to facilitate flexible sulfur cathodes. The use of graphene in 2D/3D hybridized structure is essential to alternative of CNTs in the 1D/3D, which provides required mechanical adhesion and good electrical conduction into 3D carbon constituents, but that lack of flexibility. Therefore, Wu et al. [93] demonstrated freestanding graphene based hierarchical porous carbon (GPC) films for flexible sulfur cathodes batteries as shown in **Figure 17**. Graphene-based microporous carbon (GMC) sheets are obtained by thin layers of microporous carbon were coated on both sides of GO after hydrothermal carbonization and KOH activation. The small sulfur molecules are stored in rich micropores of GMC, provides stronger physical confinement than normal graphene. Therefore, GPC files based batteries showed excellent cycling performance with stabilized capacities of 1030(422) and 626(357) mA h gsul(ele)−1 at 0.2C with the sulfur content as 41 and 57 wt%, respectively. In general, graphenebased film electrodes showed rapid decay in their capacity due to their polysulfide dissolution. Furthermore, the GPC-S cathode films used in flexible Li-S batteries by attaching the tape to pack the material, displaying comparable electrochemical performance in both flat and bent states as shown in **Figure 18**(a) and (b). Ni et al. [94] reported a facile route for synthesizing ultrathin and flexible composite films based on rGOwrapped sulfur particles with the help of sodium alginate (SA) aqueous binder, which worked as a surfactant and an adhesive agent. The SA-glued electrode battery exhibited a high reversible capacity of 1341(818) mA h gsul(ele) −1 at 0.1C and retained its capacity 823(502) mA h gsul(ele) −1 at 0.5C after 100 cycles, which are more better compared to physically mixed rGO/S film. Therefore, in order to improve electrical conductivity and their mechanical stiffness, researchers made hybride by mixing SA with polyaniline and used as glue for rGO/Mn3O4/S nanocomposite particles electrode films prepared. They exhibited a high capacity of 1015(538) mA h gsul(ele) −1 at 5.0 A gsul −1 (B3.0C) and capacity retention of 71% after 500 cycles [95].

### **Figure 17.**

*Preparation of free-standing graphene-based porous carbon (GPC) films 1) impregnation of sulfur into the micropores of 2D graphene-based microporous carbon (GMC) sheets; 2) non-covalent functionalization of carbon (GMC)-sulfur sheets by CTAB. 3) assembly of positively charged GMC-sulfur sheets and negatively charged graphene oxide.Reprinted from ref. [93].*

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

#### **Figure 18.**

*(a) The second charge-discharge profiles of the GPC-sulfur cathode films at the bent and flat states at 0.5C. (b) the cycle performance for the GPC-sulfur cathode films at 0.5C and 1C, and inset showing that a bent cell is encapsulated in the glass bottle filled with argon. Reprinted from ref. [93].*

#### *2.7.2 Flexible Li: Se batteries*

Selenium has several major merits for serving as cathode materials over to sulfur: (1) The magnitude higher electrical conductivity is an approximately 1024 times higher (2) More stable at room temperature, chain-like allotrope h-Se is more electro-active and more easily stabilized via spatial confinement (3) Selenium has more compatibility with conventional, cheap carbonate-based LIB electrolytes [96]. Therefore, selenium exhibits a better utilization rate, cyclic stability, and rate capability than sulfur. The volumetric specific capacity of h-Se is 3265 mA h cm−3 comparable to sulfur, 3461 mA h cm−3, therefore it is more suitable for portable electronic devices and electrical vehicles due to its volume sensitive. Theoretically, the Li–Se battery utilizing h-Selenium as cathode lithium metal as anode, respectively at average voltage of 2.0 V, affords high gravimetric and volumetric energy densities of 1155 W h kg−1 and 2528 W h L−1, respectively. Han et al. [68] introduced the mesoporous carbon nanoparticles (MCNs) with smaller size of 50 nm and favorable mesopore dominance, efficiently eliminated agglomeration in the bulk selenium. The electroactive selenium chains were stabilized in smaller micropores or mesopores, enabling high utilization and good cycling stability according to previous reports of Se–micro−/mesoporous carbon composite cathodes [84, 97]. The flexible Se/MCN–rGO cathodes demonstrated an ultrahigh selenium utilization of 97% at 0.1C, i.e., 655(406) mA h gsel(ele) −1. They exhibited good long cycling life with 89% capacity retention after 1300 cycles at 1.0C. This work is one of the most remarkable achievements for flexible Li–Se batteries by considering the high content of selenium. Similarly, Yu and Zhu's group prepared the composite PCNFs are represented as f-PCNFs, and they maintained good flexibility after selenization as shown in **Figure 19a**–**d** [98]. Very less crystalline selenium was present in PCNFs than in f-PCNFs leads to a remarkable improved capacity and initial Coulombic efficiency as shown in **Figure 19e**. The capacity and initial Coulombic efficiencies are 643/322 mA h gsel/ele −1 and 56.9% for Se@PCNFs at 0.05 A gsel −1, while 405/203 mA h gsel/ele −1 and 34.9% for Se@f-PCNFs. This is attributed due to the suppression of side reactions between free polyselenides produced from bulk selenium and carbonate electrolytes. Additionally, owing excellent encapsulation of selenium in the 1D conductive porous skeleton,

flexible Se@PCNF cathode also exhibited non fading cycling performance with a capacity of 516(270) mA h gsel(ele) −1 retained after 900 cycles at 1.0 A gsel −1 (B1.5C). The same electrospun PCNF–CNT also demonstrated in flexible Li–S batteries as like flexible selenium PCNF–CNT fabricated, battery exhibited reversible capacity of 638(223) mA h gsel(ele) −1 after 80 cycles at 0.05 A gsel −1 (B0.074C) [99]. The utilization of conductive selenium (94%) was much higher than that of sulfur (38%) for the same PCNF–CNT conductive backbone demonstrated in cathodes.

#### **Figure 19.**

*Schematic representation of flexible selenium cathodes. (a)&(b) represents the synthesis of selenium (Se)@PCNF electrodes. (c) and d) picture of flexible Se@PCNF electrode. Cyclic performance of flexible Se@PCNF and Se@f-PCNF cathodes in (e) Li–Se and (f) Na–Se batteries at 0.05 a gsel−1. [reprinted from ref. [98].*
