Poly(benzoquinonyl sulfide) as a High-Energy Organic Cathode for Rechargeable Li and Na Batteries

Abstract

In concern of resource sustainability and environmental friendliness, organic electrode materials for rechargeable batteries have attracted increasing attentions in recent years. However, for many researchers, the primary impression on organic cathode materials is the poor cycling stability and low energy density, mainly due to the unfavorable dissolution and low redox potential, respectively. Herein, a novel polymer cathode material, namely poly(benzoquinonyl sulfide) (PBQS) is reported, for either rechargeable Li or Na battery. Remarkably, PBQS shows a high energy density of 734 W h kg^-1 (2.67 V × 275 mA h g^-1) in Li battery, or 557 W h kg^-1 (2.08 V × 268 mA h g^-1) in Na battery, which exceeds those of most inorganic Li or Na intercalation cathodes. Moreover, PBQS also demonstrates excellent long-term cycling stability (1000 cycles, 86%) and superior rate capability (5000 mA g^-1, 72%) in Li battery. Besides the exciting battery performance, investigations on the structure-property relationship between benzoquinone (BQ) and PBQS, and electrochemical behavior difference between Li-PBQS battery and Na-PBQS battery, also provide significant insights into developing better Li-organic and Na-organic batteries beyond conventional Li-ion batteries.

Keywords: lithium batteries; organic cathodes; polymers; quinones; sodium batteries.

Scheme 1

a) The previous synthesis route of PAQS. b) Proposed synthesis route of PBQS including the polycondensation reaction between DCBQ and Li₂S, acidification by HCl, and oxidation by DDQ.

Figure 1

a) FTIR spectra of PBQS samples before and after oxidation. b) TG curves of PBQS samples before and after oxidation in air atmosphere at a heating rate of 5 °C min^–1. XPS spectra of PBQS sample after oxidation: c) full range; d) C_1s; e) O_1s; f) S_2p.

Figure 2

a) Typical discharge/charge curves of BQ (1.5–3.5 V, 1st cycle) and PBQS (1.5–4.0 V, 10th cycle) under the same current rate of 50 mA g^–1. Inserted graph shows corresponding cycling performance within 20 cycles. Note that much lower BQ loading (30%) was used to restrain its dissolution in the electrolyte. b) Energy level diagram obtained by DFT calculation for the monomer (BQ), simulated polymer containing five structure units [(BQS)₅], simulated polymer with residual hydroquinone group [H₂(BQS)₅], and in different reduction states [Li₂(BQS)₅, Li₄(BQS)₅, Li₆(BQS)₅, Li₈(BQS)₅]. Dark blue lines are the LUMOs and light blue lines are characteristic nearby unoccupied orbitals.

Figure 3

a) Discharge/charge capacity profiles versus cycle number under 50 mA g^–1 for PBQS samples before and after oxidation. b) Voltage profiles of the 1st, 2nd, and 10th cycle for PBQS after oxidation. c) Discharge/charge capacity profiles versus cycle number under sequentially changed current rate from 50 to 5000 mA g^–1. d) Corresponding voltage profiles under different current rates. e) Long‐term cycling profiles within 1000 cycles under a current rate of 500 mA g^–1.

Figure 4

Cycling performance of PBQS within 100 cycles in 1 m NaTFSI/DOL + DME electrolyte, under a current rate of 50 mA g^–1. Inserted graph shows the typical discharge/charge curves (1.0–3.8 V, 2nd cycle).

Figure 5

a) Typical CV curves of Li/Li/PBQS and Na/Na/PBQS three‐electrode battery at a scan rate of 0.1 mV s^–1, in voltage range of 1.5–4.0 V versus Li⁺/Li and 1.0–3.5 V versus Na⁺/Na, respectively. b) Quasi‐open‐circuit voltage (QOCV) profiles of Li/PBQS (1.5–4.0 V) and Na/PBQS (1.0–3.8 V) coin cells at a current rate of 50 mA g^–1, with a relaxation time of 2 h after each discharge or charge step of 0.5 h. c) Voltage profiles of Na/Na/Na three‐electrode battery at a current density of 50 μA cm^–1, to show the overpotential of Na electrode. For all the above tests, 1 m LiTFSI/DOL + DME electrolyte was used for Li battery, while 1 m NaTFSI/DOL + DME electrolyte was used for Na battery.