Hongli Zhu1
Northeastern University1
Current global energy sector is still dominated by the fossil-based energy. Renewable energies, especially these derived from Nature have been extensively sought after as a sustainable solution for emerging energy needs. Among them, wood-derived energy and erengy storage devices come of the age in recent years. However, current convertion of wood and other plant as well into energy, either the energy storage devices or biofuel production, heavily depends on the delignification, which is a process to remove the lignin polymer exsited in plant cell wall. In fact, lignin is the second most abundant biopolymer on earth next to cellulose but a massive industrial waste produced both in paper industry and biofuel manufacturing. Our concept in this research is to convert this industrial lignin waste from biofuel production (also known as biorefining) into solid-state electrolyte (SSE) to enable its application in next generation lithium battery.<br/>The rationale of our experimental design lies in the abundant hydroxyl groups in the lignin polymer, which provides grafting sites for bonding polyethylene glycol (PEG), a most efficient polymer for the SSE of the ASSLBs. Moreover, lignin polymer itself contains plentiful aromatic rings and rich electron-donor groups such as ether type linkages, methoxyl groups (-OCH3), and aromatic and aliphatic hydroxyl groups. These chemical structures of lignin facilitate its interactions with Li salt and improve ion conduction at the interface when ceramics were added into the SSE. We prepared two types of SSE by using the synthesized PEG-g-lignin. The first one was solid polymer electrolyte (SPE), which was prepared by mixing PEG-g-lignin with poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The resultant SPE had an ionic conductivity of 2.5 × 10-5 S cm-1 at 25oC. The second one was composite polymer electrolyte (CPE), which was prepared by adding Li6.4La3Ga0.2Zr2O12 (LLGZO) into the PEG-g-lignin and PVDF-HFP composite. The ionic conductivity of the resultant CPE was further enhanced to 6.5 × 10-5 S cm-1. This ionic conductivity represents 2.6×105-fold and 2.8×103-fold higher than that of pure PVDF-HFP (9.7 × 10-11 S cm-1) and pure PEO (9.0 × 10-9 S cm-1) based SSE, respectively. Mechanistic study by using 2D HSQC NMR revealed that PEG-g-lignin has increased ether type β-O-4 linkages that can promote the interchain hopping of Li+ between lignin polymer chains, and 31P NMR revealed that lignin phenolic end can be associated by Li+. <br/>The research has both significant novelty and broad impact. 1). The study converted a largely overlooked biorefining waste into the solid-state electrolyte, which opened up a new avenue for industrial waste valorization toward a thriving billion dollars market of next generation lithium battery. 2). By substituting petroleum-derived PEO and other synthetic polymers for SSE, plant-derived lignin polymer will indeed transform the sustainability of current ASSLBs. 3). The study also informed a broader research community and opened new avenues for building up a holistic bio-based energy system. The integration of a sustainable energy storage manufacturing with biofuel production platform will transform current energy sector by simultaneously enhancing biofuel production and promoting the biopolymers from Nature into high energy density, high cycling performance and safe ASSLBs.