Lignin‐Based Solid Polymer Electrolytes: Lignin‐Graft‐Poly(ethylene glycol)

Abstract

Lignin is an aromatic‐rich biomass polymer that is cheap, abundant, and sustainable. However, its application in the solid electrolyte field is rare due to challenges in well‐defined polymer synthesis. Herein, the synthesis of lignin‐graft‐poly(ethylene glycol) (PEG) and its conductivity test for a solid electrolyte application are demonstrated. The main steps of synthesis include functionalization of natural lignin's hydroxyl to alkene, followed by graft‐copolymerization of PEG thiol to the lignin via photoredox thiol‐ene reaction. Two lignin‐graft‐PEGs are prepared having 22 wt% lignin (lignin‐graft‐PEG 550) and 34 wt% lignin (lignin‐graft‐PEG 2000). Then, new polymer electrolytes for conductivity tests are prepared via addition of lithium bis‐trifluoromethanesulfonimide. The polymer graft electrolytes exhibit ionic conductivity up to 1.4 × 10−4 S cm−1 at 35 °C. The presence of lignin moderately impacts conductivity at elevated temperature compared to homopolymer PEG. Furthermore, the ionic conductivity of lignin‐graft‐PEG at ambient temperature is significantly higher than homopolymer PEG precedents.

Polymer Blend Electrolytes for Batteries and Beyond

“…In another 2020 study that mirrored the grafted LPEI of Granados-Focil and co-workers, the Chung and Hallinan groups grafted poly­(ethylene glycol) (PEG) onto a lignin main chain using a photoredox thiol–ene reaction. , Lignin was intended to provide mechanical strength to the copolymer while the PEG was to provide ionic conductivity and flexibility. Two different PEG molecular weights, 550 g/mol and 2000 g/mol, were used to synthesize two different PEG- graft -lignin copolymers.…”

Molecular Engineering of Biorefining Lignin Waste for Solid-State Electrolyte

“…In another word, the frequency of the β– O –4 linkage in PEG- g -lignin was 100% (Figure C). This β– O –4 linkage was reported as a flexible and linear ether structure, which can largely enhance the flexibility of the lignin chain and thus the segmental motion of the PEG- g -lignin polymer . Moreover, methoxyl groups (−OCH 3 ) were clearly detected in the 2D HSQC NMR (Figure A).…”

“…All these data demonstrated that lignin after PEG grafting had both increased molecular weight and improved molecular uniformity. These modifications in lignin structures can not only enhance the film formability as shown thereafter but can also improve molecular flexibility of lignin to enhance its segmental motion, an important chemical feature to facilitate Li ion hopping in polymers …”

“…First, besides aromatic moieties and β– O –4 linkages, lignin itself is highly heterogeneous, which has been characterized as a “mixture” of nonuniform biopolymers with various molecular weights, variant interunitary linakges, multiple functional groups, and different moieties . This heterogeneity makes lignin a “rigid” polymer with great thermostability (decomposition temperature is up to 300 °C) but poor formability for film processing (Figure A). , More importantly, the high heterogeneity can render poor ionic conductivity of the raw lignin that hinders its application for SSE . In contrast, the petroluem-derived PEO or PEG polymer used for a traditional solid polymer electrolyte is a “soft” polymer with very good Li ionic conductivity but limited film formability and poor thermostability, especially a PEG with low molecular weight (Figure A) .…”

“…8,41 More importantly, the high heterogeneity can render poor ionic conductivity of the raw lignin that hinders its application for SSE. 41 In contrast, the petroluem-derived PEO or PEG polymer used for a traditional solid polymer electrolyte is a "soft" polymer with very good Li ionic conductivity but limited film formability and poor thermostability, especially a PEG with low molecular weight (Figure 2A). 42 The rationale of our design is to integrate the merits of both polymers by grafting a "soft" PEG polymer onto a "rigid" lignin polymer to generate a "soft−rigid" or flexible PEG-g-lignin polymer, which can concurrently have excellent Li ionic conductivity, good thermostability, and good film formability (Figure 2A).…”