Bio-Based Polyhydroxyanthraquinones as High-Voltage Organic Electrode Materials for Batteries

Abstract

Organic materials have gained much attention as sustainable electrode materials for batteries. Especially bio-based organic electrode materials (OEMs) are very interesting due to their geographical independency and low environmental impact. However, bio-based OEMs for high-voltage batteries remain scarce. Therefore, in this work, a family of bio-based polyhydroxyanthraquinones (PHAQs)-namely 1,2,3,4,5,6,7,8-octahydroxyanthraquinone (OHAQ), 1,2,3,5,6,7-hexahydroxyanthraquinone (HHAQ), and 2,3,6,7-tetrahydroxyanthraquinone (THAQ)-and their redox polymers were synthesized. These PHAQs were synthesized from plant-based precursors and exhibit both a high-potential polyphenolic redox couple (3.5-4.0 V vs Li/Li⁺) and an anthraquinone redox moiety (2.2-2.8 V vs Li/Li⁺), while also showing initial charging capacities of up to 381 mAh g^-1. To counteract the rapid fading caused by dissolution into the electrolyte, a facile polymerization method was established to synthesize PHAQ polymers. For this, the polymerization of HHAQ served as a model reaction where formaldehyde, glyoxal, and glutaraldehyde were tested as linkers. The resulting polymers were investigated as cathode materials in lithium metal batteries. PHAQ polymer composites synthesized using formaldehyde as linker and 10 wt % multiwalled carbon nanotubes (MWCNTs), namely poly(THAQ-formaldehyde)-10 wt % MWCNTs and poly(HHAQ-formaldehyde)-10 wt % MWCNTs, exhibited the best cycling performance in the lithium metal cells, displaying a high-voltage discharge starting at 4.0 V (vs Li/Li⁺) and retaining 81.6 and 77.3 mAh g^-1, respectively, after 100 cycles.

Scheme 1. Synthesis Scheme of the Polyhydroxyanthraquinone Family Containing 2,3,6,7-Tetrahydroxyanthraquinone (THAQ), 1,2,3,5,6,7-Hexahydroxyanthraquinone (HHAQ), and 1,2,3,4,5,6,7,8-Octahydroxyanthraquinone (OHAQ)

Reagents and conditions: (i) H₂SO₄ (72% aq), acetaldehyde (2.4 equiv); (ii) acetic acid (72% aq), Na₂Cr₂O₇·2H₂O (5.5 equiv); (iii) HBr (48% aq); (iv) conc H₂SO₄, and (v) conc H₂SO₄, H₃BO₃ (6.55 equiv), HgO (0.033 equiv).

Figure 1

(a) Anticipated electrochemical mechanism of THAQ including the oxidized (THAQ_ox-1 and THAQ_ox-2) and reduced (THAQ_red) states and their related theoretical capacities (C_Theo). (b) Cyclic voltammogram (scan rate = 5 mV/s) of the polyphenolic subgroup of HHAQ in a three-electrode setup using Li metal as counter and reference electrode and a [HHAQ:C₆₅:SEBS] = [40:50:10] electrode formulation on glassy carbon as working electrode with 1.0 M LiPF₆ in EC:DEC (1:1, v/v) as electrolyte. (c) Molecular structures and E_red of THAQ, HHAQ, and OHAQ obtained from DFT calculations.

Figure 2

(a) First galvanostatic charge–discharge cycle (C-rate = 1C) of THAQ, HHAQ, and OHAQ in Li half-cells, using [Act. Mat.:C₆₅:PVDF] = [40:40:20] electrode formulations and 1.0 M LiPF₆ in EC:DEC (1:1, v/v) as electrolyte. (b) First 10 galvanostatic charge–discharge cycles (C-rate = 0.2C) of HHAQ in Li half-cells, using [Act. Mat.:C₆₅:SEBS] = [40:50:10] as electrode formulation and 0.3 M LiTFSI/[PY13][TFSI] ionic liquid as electrolyte.

Scheme 2. (a) Condensation–Polymerization of (a) HHAQ and (b) THAQ Utilizing Formaldehyde, Glyoxal, or Glutaraldehyde as Linker

Reagents and conditions: (i) conc H₂SO₄, 10 wt % MWCNTs, 90 °C, 42 h.

Figure 3

(a, b, c) SEM images of poly(HHAQ–glyoxal), poly(HHAQ–glutaraldehyde), and poly(HHAQ–formaldehyde), respectively.

Figure 4

ATR-FTIR spectra of HHAQ and the initial and optimized poly(HHAQ–formaldehyde).

Figure 5

1st, 5th, 20th, 50th, and 100th galvanostatic charge–discharge cycle (C-rate = 1.0C) of (a) poly(HHAQ–formaldehyde)-10 wt %MWCNTs and (c) poly(THAQ–formaldehyde)-10 wt % MWCNTs composites in Li half-cells, using [Act. Mat.:C₆₅:SEBS] = [40:50:10] electrode formulations and 1.0 M LiPF₆ in EC:DEC (1:1, v/v) as electrolyte. (b) and (d) depict the charge and discharge capacity as well as the Coulombic efficiency (C_eff) over 100 cycles for poly(HHAQ–formaldehyde)-10 wt %MWCNTs and poly(THAQ–formaldehyde)-10 wt %MWCNTs, respectively.