*2.7.3 Flexible Li–Te batteries*

Tellurium is the last nonradioactive element in the chalcogen family, which exhibit highest electrical conductivity of 2.5 Scm−1 compared to all nonmetallic materials. Te shows low gravimetric specific capacity of 420 mA h g−1 due to its heavy atomic weight, it exhibited comparable volumetric specific capacity of 2621 mA h cm−3 to that of sulfur or selenium. Te is an electrically conducting active material required less carbon to prepare electrode. The decreasing weight of the carbon favors both volumetric and gravimetric specific capacities. A Li–Te battery exhibited theoretical gravimetric and volumetric energy densities of 682 W h kg−1 and 2078 W h L−1, respectively with an estimated output voltage of 1.8 V. Li–Te battery was first demonstrated by Wang's group [100], in which tellurium/porous carbon composite cathode and a carbonate electrolyte as the components of battery. The Li–Te battery showed an average voltage of 1.5 V and a reversible capacity of 224 mA h gtel −1 at 0.05 A gtel −1. They observed that 87% retention after 1000 cycles. Considering the relatively low voltage and promising volumetric capacity, Guo and coworkers demonstrated tellurium/carbon composites as anode materials for LIBs, indicating that extremely high tellurium utilization of 98% and a long-term cycling stability [101]. Particularly, Te is quite interest for flexible electrode materials due to its two most favorable features:

#### **Figure 20.**

*Flexible tellurium NWs cathodes. (a) Flexible, carbon-free TeNW mat: (i) photograph; (ii) SEM and (iii) TEM morphology images; (iv, v) nanoscale crystalline structure of freestanding TeNW mat; (vi) simulated crystal structure of h-Te; (vii) cyclic performance of TeNW mat at 0.1 a gtel−1. Reprited from ref. [84] (b) 3D rGO/ TeNW aerogel: (i) fabrication of 3D rGO/TeNW aerogel and its derived flexible electrodes; photographs of (ii) 3D rGO/TeNW aerogel and (iii) flexible 3D rGO/TeNW electrode. Reprinted from ref. [27, 103].*

(1) high electrical conductivity compared to carbon and (2) The formation of 1D Te nanostructures along the c-axis, i.e., [001] due to its inherent chirality of helical Te chains in the h-Te crystal [102]. Hence, freestanding films consisting of ultralong Te NWs used directly used as an electrodes. Freestanding Te mat via vacuum filtration of Te NWs with a diameter of 7 nm grown in the [001] direction developed by Ding et al. as shown in **Figure 20**a-i–iii [104]. Such a high anisotropic 1D Te nanostructure exhibited fully Te zig-zag chains to lithium ions transport and a showed high electrical conductivity of 6.7 Scm−1 in the direction perpendicular to the c-axis as shown in **Figure 20**a(iv–vi). The flexible tellurium cathode comprises the Te NWs along with the new electrolyte exhibited a desirable capacity of B144 mA h gtel/ele-1 at 0.1 A gtel −1 (0.24C). The volumetric energy density of 1800 W h L−1 observed after 80 cycles as shown in the **Figure 20**a-vii. Further, He and Chen's et al. [103] demonstrated a flexible tellurium cathode prepared from a 3D hierarchical aerogel with Te NWs wrapped homogeneously by rGO as shown **Figure 20**b. The synthetic method was adopted from the previous report on 3DCG–Li2S. The rGO/Te NW electrode made of 63 wt% tellurium delivers high capacities of 418(263) and 174(110) mAh gtel(ele) −1 at 0.2 and 10C, respectively and excellent long-cyclic performance at a high rate of 1.0C. Therefore, as prepared flexible Te-NWs electrodes are quite attractive over the sulfur and selenium counterparts due to their distinguish features.
