Functional Hydrogels for Next-Generation Batteries and Supercapacitors Youhong Guo, Jiwoong Bae, Fei Zhao, Guihua Yu  Trends in Chemistry  Volume 1, Issue 3, Pages 335-348 (June 2019) DOI: 10.1016/j.trechm.2019.03.005 Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 1 Tunable Elements in Gelation Chemistry Beneficial for Energy-Storage Devices. Key elements include polymer-chain chemistry, crosslinking type and density, polymer interactions with polymer/ions/electrolyte, aqueous electrolytes, and added functional materials. Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 2 Schematic Illustration of Synthesis and Applications of Hydrogels in Energy Storage. (A) Crosslinking hydrogel in aqueous solution where salts for charge carrier are dissolved. (B) Crosslinking hydrogel in the presence of functional materials that can be coated by hydrogel through in situ crosslinking. (C) Nanostructured hydrogel-derived inorganic frameworks by carbonization (top) and sintering (bottom). Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 3 Ionically or Electronically Conductive Hydrogels in Energy Storage. (A) Crosslinked polyvinyl alcohol (PVA)-H2SO4 (sulfuric acid) hydrogel electrolyte. Reproduced, with permission, from [43]. (B) Synthesis process for the hierarchical polymer electrolyte. Reproduced, with permission, from [45]. (C) Structure of polyampholyte hydrogel fabricated by cationic and anionic monomers. Reproduced, with permission, from [41]. (D) Schematic illustration of the zwitterionic hydrogel electrolyte. Reproduced, with permission, from [50]. (E) Schematic illustration of mixed ionic and electronic conduction in nanostructured hydrogels. (F) Scanning transmission electron microscopy (STEM) images of nanostructured hydrogel with active particles. (G) Rate performance of lithium-ion batteries with the conventional cathode (C-LFP control) and nanostructured hydrogel cathode (C-LFP/C-PPy). Reproduced, with permission, from [52]. (H) The molecular structure of polypyrrole (PPy) crosslinked by CuPcTs (copper phthalocyanine-3,4′,4″,4‴-tetra sulfonic acid tetrasodium salt). Reproduced, with permission, from [42]. Abbreviations: HPE, hierarchical polymer electrolyte; PAM, polyacrylamide; PAN, polyacrylonitrile. Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 4 Hydrogels as a Template or Precursor for 3D Inorganic Frameworks. (A) Synthesis process and (B) Scanning electron microscope (SEM) image of Li2S/N,P-C framework for electrode of Li-S battery. Reproduced, with permission, from [32]. (C) Synthesis process and (D) Scanning transmission electron microscopy (STEM) image with elemental mapping of the Sn-Fe@C framework for an electrode of the lithium-ion battery. Reproduced, with permission, from [33]. (E) Conduction mechanism in composite polymer electrolyte with a 3D ceramic framework. (F) SEM images of the 3D ceramic framework (left) and its composite polymer electrolyte with poly(ethylene oxide) (PEO) (right) for the electrolyte of a lithium-ion battery. Reproduced, with permission, from [34]. Abbreviations: LLTO, Li0.35La0.55TiO3; PANI, phytic acid-doped polyaniline. Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 5 Smart Hydrogels for Energy-Storage Devices. (A) Schematic illustration of a thermally responsive gel electrolyte, showing a reversible sol–gel transition that inhibits ion conduction at high temperature. Reproduced, with permission, from [79]. (B) Thermal switching mechanism of the thermoresponsive polymer layer for current collectors in a battery before and after heating. Reproduced, with permission, from [80]. (C) Schematic illustration of mechanism for self-healing behavior of supramolecular gels. Reproduced, with permission, from [87]. (D) Scanning electron microscope (SEM) images of the polyaniline/graphene oxide (PANI/GO) fiber-based supercapacitor with strong mechanical properties. Reproduced, with permission, from [92]. (E) Digital pictures of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hydrogels; bars, 1cm. Reproduced, with permission, from [93]. Abbreviations: GrNi, graphene-coated nano-spiky Ni particles; LIB, lithium ion battery; PEO, poly(ethylene oxide); PPO, polypropylene oxide; TRPS, thermoresponsive polymer switching. Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions

Figure 6 Prospective Hydrogels for Future Energy-Storage Applications. Trends in Chemistry 2019 1, 335-348DOI: (10.1016/j.trechm.2019.03.005) Copyright © 2019 Elsevier Inc. Terms and Conditions