Photo-crosslinked lignin/PAN electrospun separator for safe lithium-ion batteries

Abstract

A novel crosslinked electrospun nanofibrous membrane with maleated lignin (ML) and poly(acrylonitrile) (PAN) is presented as a separator for lithium-ion batteries (LIBs). Alkali lignin was treated with an esterification agent of maleic anhydride, resulting in a substantial hydroxyl group conversion to enhance the reactivity and mechanical properties of the final nanofiber membranes. The maleated lignin (ML) was subsequently mixed with UV-curable formulations (up to 30% wt) containing polyethylene glycol diacrylate (PEGDA), hydrolyzed 3-(Trimethoxysilyl)propyl methacrylate (HMEMO) as crosslinkers, and poly(acrylonitrile) (PAN) as a precursor polymer. UV-electrospinning was used to fabricate PAN/ML/HMEMO/PEGDA (PMHP) crosslinked membranes. PMHP membranes made of electrospun nanofibers feature a three-dimensional (3D) porous structure with interconnected voids between the fibers. The mechanical strength of PMHP membranes with a thickness of 25 µm was enhanced by the variation of the cross-linkable formulations. The cell assembled with PMHP2 membrane (20 wt% of ML) showed the maximum ionic conductivity value of 2.79*10^-3 S cm^-1, which is significantly higher than that of the same cell with the liquid electrolyte and commercial Celgard 2400 (6.5*10^-4 S cm^-1). The enhanced LIB efficiency with PMHP2 membrane can be attributed to its high porosity, which allows better electrolyte uptake and demonstrates higher ionic conductivity. As a result, the cell assembled with LiFePO₄ cathode, Li metal anode, and PMHP2 membrane had a high initial discharge specific capacity of 147 mAh g^-1 at 0.1 C and exhibited outstanding rate performance. Also, it effectively limits the formation of Li dendrites over 1000 h. PMHP separators have improved chemical and physical properties, including porosity, thermal, mechanical, and electrochemical characteristics, compared with the commercial ones.

Figure 1

Reaction schemes of (a) lignin modification and (b) MEMO hydrolysis; FT-IR spectras of (c) pure lignin, maleic anhydride, and maleated lignin, (d) MEMO and hydrolyzed MEMO.

Figure 2

Schematic representation of PMHP membrane via UV-electrospinning technique.

Figure 3

(a) Chemical composition, (b) FT-IR spectra and photo of PMHP membrane.

Figure 4

SEM micrographs of PMHP1, PMHP2, PMHP3 (a,b and c, respectively), and Celgard-2400 (d,e) membranes.

Figure 5

(a) Thermo-gravimetric analysis, (b) thermal exposure test photos before and after thermal treatment at 150 °C for 15 min, (c) liquid electrolyte uptake chart, and (d) typical stress–strain curves of PMHP1, PMHP2, PMHP3, and Celgard-2400 membranes.

Figure 6

(a) Electrochemical impedance spectroscopy (EIS) results of symmetric stainless steel (SS) electrode using PMHP and Celgard 2400 separators; (b) Linear sweep voltammogram of Li/ PMHP/SS and Li/Celgard/SS cells.

Figure 7

(a) Initial charge–discharge, (b) Galvanostatic cyclability with Coulombic efficiency at 0.1 C, (c) C-rate performance of cells with PMHP2 and Celgard separators with LFP cathode and Li anode, and (d) Galvanostatic stripping/plating profiles for Li/PMHP2/Li symmetric cell.