Shuttle Balance\nbetween Lithiophilic Functionalities\nSubstituted on Conjugated Polymers to Improve Lithium-Ion Diffusion\nand Cycling Stability of Batteries

Herein, we report the design and synthesis of <i>i</i>-Indigo and <i>trans</i>-1,2-di(2-thienyl)ethylene\n(TVT)\ncomprising copolymers with various side chains that are used as anode-cum-binder\nin Li-ion batteries. The TVT disrupts the crystallinity, and the <i>i</i>-Indigo renders the handle for unsymmetrical substitution.\nPolymer <b>P1</b> comprises alkyl chains on <i>i</i>-Indigo, whereas <b>P2</b> comprises an alkyl chain and a chain\nwith a urethane moiety as another side chain. Polymer <b>P3</b> comprises <i>i</i>-Indigo with a urethane moiety and triethylene\nglycol as side chains. The <i>i</i>-Indigo in <b>P4</b> is substituted with triethylene glycol and alkyl chains as side\nchains. These polymers were found to be amorphous with π–π\nstacking due to the extremely small torsional angle imparted by TVT\nmoieties. All of these polymers showed comparable specific capacity\nwhile used as an anode in Li-ion batteries due to the presence of\na common conjugated backbone. However, the battery metrics varied\nsignificantly while the current density of the batteries varied between\n0.1 and 2.0 A g<sup>–1</sup>. <b>P1</b> and <b>P2</b> showed a very close recovery of specific capacity at 0.1 A g<sup>–1</sup> before and after various current densities. However,\npolymers <b>P3</b> and <b>P4</b> did not recover from\nthe specific capacity fading due to the presence of triethylene glycol\nside chain that degraded. Polymer <b>P2</b> with an alkyl side\nchain and a chain with a urethane moiety showed stable performance\nover 2000 charge–discharge cycles. Contrary to this, the stability\nof polymers with triethylene glycol side chains decreased over 2000\ncharge–discharge cycles. Polymer <b>P2</b> with a urethane\nside chain also showed improved Li-ion diffusion because the urethane\nmoiety facilitated better Li-ion transport.