Switching between Local and Global Aromaticity in a Conjugated Macrocycle for High-Performance Organic Sodium-Ion Battery Anodes

Abstract

Aromatic organic compounds can be used as electrode materials in rechargeable batteries and are expected to advance the development of both anode and cathode materials for sodium-ion batteries (SIBs). However, most aromatic organic compounds assessed as anode materials in SIBs to date exhibit significant degradation issues under fast-charge/discharge conditions and unsatisfying long-term cycling performance. Now, a molecular design concept is presented for improving the stability of organic compounds for battery electrodes. The molecular design of the investigated compound, [2.2.2.2]paracyclophane-1,9,17,25-tetraene (PCT), can stabilize the neutral state by local aromaticity and the doubly reduced state by global aromaticity, resulting in an anode material with extraordinarily stable cycling performance and outstanding performance under fast-charge/discharge conditions, demonstrating an exciting new path for the development of electrode materials for SIBs and other types of batteries.

Keywords: aromaticity; hydrocarbons; macrocycles; organic batteries; voids.

Figure 1

a) Molecular structure of [2.2.2.2]paracyclophane‐1,9,17,25‐tetraene (PCT, left) and the corresponding dianion (PCT²⁻, right); bold bonds indicate the [24]annulene substructure with [4n] π‐electrons in the neutral state and [4n+2] π‐electrons in the doubly reduced state; shaded areas highlight aromaticity. b) Anisotropy of the induced current density (ACID) plots, which allow visualization of electronic delocalization, of PCT (left) and PCT²⁻ (right) at an isovalue of 0.04; large arrows indicate the direction of the small current density vectors, which show diatropic (clockwise, aromatic) ring current in both the neutral and the doubly reduced state.

Figure 2

Cyclic voltammograms of PCT in a) DMF (scan rate v=10 mV s⁻¹, PCT concentration c=7.25 mm), b) PC (v=100 mV s⁻¹, c=0.32 mm), and c) DCE (v=100 mV s⁻¹, c=12.8 mm) recorded on platinum disc electrodes of 2 mm diameter (solid lines). Supporting electrolyte: 0.1 m NBu₄PF₆. Simulated cyclic voltammograms (dotted lines) for fitting of kinetic parameters. Diffusion coefficients, concentrations, and fitted kinetic parameters are listed in the Supporting Information, Table S2. Other fitted parameters: a) uncompensated resistance (R _u): 100 Ω, capacitance of the electric double layer at the working electrode‐electrolyte interface (C _dl): 50 μF; b) R _u=100 Ω, C _dl=3 μF; c) R _u=1000 Ω, C _dl=10 μF. Temperature: 293.2 K.

Figure 3

Crystal structures of different PCT polymorphs showing voids large enough to hold sodium ions: a) grown from acetic acid solution (with one of the molecules in the unit cell in orientation A and the other molecule in orientation B, viewed along the a‐axis; 5.8 % of the unit cell volume are empty space), b) previously reported crystal structure (viewed along the a‐axis; 4.9 %), c) grown by sublimation (viewed along the b‐axis; 5.5 %). d) Crystal structure of 1,4‐distyrylbenzene (viewed along the b‐axis; 0.8 %).

Figure 4

a) Cyclic voltammograms of the PCT electrode in the voltage range of 0.01–2.0 V (scan rate: 1 mV s⁻¹). b) Voltage profile of the PCT electrode at a current density of 200 mA g⁻¹. c) Cycling performance and d) rate capability test of PCT at various current densities. Black circles in (d) correspond to Coulombic efficiencies.