**1. Introduction**

Recently, much attention has been focused on the development of more safe, highenergy density, long-life, and low-cost batteries to satisfy our energy demand as our daily life includes electric vehicles, portable electronics, and large-scale grids [1–4]. However, lithium-ion batteries (LIBs) successfully prepared and available in the commercial market since the 1990s, even though their theoretical specific capacity, energy density of the electrode material is relatively low. Hence, still it is challenging to development of next generation of lithium-ion batteries to fulfill the demand by overcoming their hindrance by the intrinsic limitation [5].

Various metal sulfides, metal oxides, and metal poly-anions have been developed, and they exhibited preferable capacities, but their output voltages are not sufficient. Hence, chalcogen materials (sulfur, selenium, and tellurium (SSTs) are emerging conversiontype cathode materials for aluminum-ion batteries (AIBs) [6]. They exhibit the multielectron transfer process; therefore, AlSST batteries can deliver very high capacities.

The low-cost and -toxicity, most-abundant sulfur (S) has theoretical gravimetric capacity of 1672 mA h g−1 due to the reaction between S and sulfide (S2−) exhibiting the highest capacity, over all cathode materials in AIBs [7]. Notably, lithium-sulfur (Li-S) batteries exhibited the highest theoretical energy density of 2600 Wh kg−1. Moreover, the sulfur-based cathodes require a large number of conductive additives due to their electrical insulating property to avoid leakage/loss issues, which greatly decreases the actual capacity [8]. Therefore, it is usually difficult to achieve the theoretical specific capacity. Based on these merits, metal chalcogens batteries (MCBs) are an attractive interest for alternative of new-generation secondary batteries.

The main research has been focused on lithium dendrite and "shuttle effect" of highorder lithium phosphate sulfur batteries. There is three-phase boundary between cathodes, electrolytes, and modified interface layers. The interface boundaries occupied by the sulfur species restrict the redox kinetics. Therefore, to improve the redox reaction, it is necessary to make strong bond of S to the host, thereby continuing to conduct mixed charge carriers (Li+ and e− ). There is another difficulty that the degradation of active materials between the electrodes due to shuttle process, in which cathode electrolyte interfaces bring rapid decay of the capacity, thereby reducing the coulomb efficiency of Li-S batteries [2]. The formation of lithium dendrites on the surface of lithium anode and also unstable solid electrolyte interface (SEI) leads to low columbic efficiency (CE) and poor cyclic performance [9]. To overcome these problems, most suitable nanomaterials are considered for energy applications due their unique crystal structure providing high surface-area-to-volume ratio and shortening lithium ions transport [9].

In order to create the metal-ligand covalency is the one of strategies by replacing the oxide ligand with the chalcogen (S, Se) to achieve an anion redox stabilization, where the less electronegative nature of the chalcogen improves the ligand p band penetration into the metal d band. Tarascon et al. investigated layered chalcogen structures as well as their electrochemical performance for the next generation of cathodes [10]. The results exhibited the superior performance in voltage and capacity fade with voltage hysteresis. Hence, chalcogen anion redox plays a critical role in a Li-rich cathode batteries. Some of research was carried out on chalcogen cathodes, both Li-rich and conventional chalcogen cathodes for the evolution of chalcogen anion redox cathode [11, 12]. Mespoulie et al. [13] introduced fast Li-ion conductors of mixed anionic and cationic redox activity of Li2SnS3, by introducing the Fe redox couple in the host cation Sn site.

Selenium (Se) is another class of material present in Al-Se battery that showed higher voltage plateau resulting a desirable energy density [14]. Selenium metal has high theoretical volumetric capacity (3253 mA h cm−3, ρ = 4.81 g cm−3), which is more suitable especially in hybrid electric vehicles and in the mobile smart phones due to restrictions of the battery volume [15]. Selenium showed higher electronic conductivity (1 × 10−3 S m−1) and excellent kinetic behavior than sulfur [16]. The chemical compound Se1-xSx with different Se-S ratios shows higher theoretical capacity as well as better electronic conductivity due to fast reaction of kinetics than pristine S [17]. Even though, Se1-x Sx cathode materials also suffer from poor cycle performance, lower coulombic efficiency due to the dissolution and shuttling of intermediates [18]. The electrochemical performance of Li-Se1-xSx batteries improved by carbon coating,

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

which provide a strong chemical affinity of polarized surface, which can effectively trap the soluble intermediates to minimize the shuttle effect and side reactions in the electrolyte [19]. A series of Se1-xSx cathodes were prepared by Se/S ratio and the presence of supercritical CO2. NC@SWCNTs@Se1-xSx cathodes exhibited higher conductivity and strong adsorption leading to superior cyclic efficiency.

Tellurium (Te) material has the highest atomic weight among sulfur and selenium, high electrical conductivity and a 6-electron transfer reaction process made it to be promising cathode material in AIBs [20]. However, still there are several challenges remaining to overcome for the development of batteries such as low electronic conductivity of S, shuttle effect, slow kinetics of ionic liquids as well as undesirable reaction mechanism [21]. Tellurium exhibits a higher theoretical volumetric capacity of 2619 mAh cm−3 due to its intrinsic electrical conductivity of Te (2 × 102 S m−1), much better than that of S (5 × 10−16 S m−1) and Se (1 × 10−4 S m−1). Therefore, the high utilization ratio of active material of Te leads to good performance at the large current density. The fabricated batteries based on Te/porous carbon (Te-G-CNT) electrode materials deliver a high volumetric capacity up to 2493.13 mAh cm−3 [22].

The polysulfide (Li2Sn) species have strong tendency to catenate and form reactive polysulfide dianions as well as radical anions (Sn2− and Sn/n/2− , 2 < n < 8). These conversion reactions of sulfur ↔ Li2S kinetically favored in the mediated solution and their deposition degrading the lithium surface and the cyclic stability [23–25]. The dissolved species shows shuttle effect by insulating deposition of Li2S/Li2S2. Polysulfide molecules modified by substituting chalcogen atoms minimized the intrinsic shuttle effect [26]. By substituting S, Se and Te can be facilely formed as the polyselenosulfides (Li2SexSy) and polytellurosulfides (Li2TexSy). However, selenium and tellurium lead to significant differences in the electrochemical performance compared with Li-S batteries. The substitution of selenium and tellurium has significant impact on the metal-chalcogen batteries and solid-state batteries by employing chalcogenide solid electrolytes. Therefore, in this chapter, the strategies to improve electrochemical performance are elaborated, and the development of new trends for next-generation lithium-ion batteries is provided.

To fabricate flexible lithium-ion batteries using sulfur-based cathodes, there are two main synthetic approaches: (1) Post-sulfur loading: The formation of a flexible skeleton then loaded with sulfur by using vapor infusion, melt diffusion, or reprecipitation of sulfur from a solution (generally carbon disulfide (CS2) or toluene). (2) Pre-sulfur loading: pre-synthesized sulfur composites into a flexible cathode. By keeping the flexible cathode required features in mind such as (1) high content of active materials with respect to total mass of the electrode, (2) mechanically robust skeleton, (3) long-range interpenetrated conductive network, (4) porous structure, and (5) three-dimensional (3D) scaffold to improve areal sulfur loading [27]. The flexible energy Li-S batteries, flexible alkali metal-chalcogen batteries, and two special flexible batteries such as prototypes of foldable and cable-type Li-S batteries are discussed.
