Polypeptide Radical Cathode for Aqueous Zn‐Ion Battery with Two‐Electron Storage and Faster Charging Rate

Abstract

The rapidly growing demand for batteries has led to a lack of global mineral resources and rechargeable organic batteries are paid extensive attention, owing to the abundance resources, light weight, and high flexibility of organic electrodes. However, most organic electrodes that use aliphatic backbones are nondegradable, leading to unsustainability when active sites fail. In this study, a poly(aspartic acid) polypeptide (PASP) with amide links in the backbone and nitroxide radical pendant groups in the side chains is synthesized by modifying the polypeptides with 4-amino-2,2,6,6-tetramethylpiperidine. In combination with a Zn anode, the PASP-TEMPO composite electrode exhibits rapid charge-discharge and superior cycling stability with reversible two-electron redox reaction in aqueous electrolyte. The Zn/PASP-TEMPO organic radical battery delivers a discharge capacity of around 80 mAh g À 1 by two-electron reaction and charge-discharge rates of up to 18 A g À 1 . Because the redox reaction process of the nitroxyl radical turning into oxoammonium follows a p-type mechanism that interacts with an anion, three electrolytes with different anions are tested in the Zn/PASP-TEMPO organic radical battery. Experimental results indicate that discharge plateau voltage is tunable by choosing different zinc salts as electrolytes. Capacity retention of up to 97.4 % after 500 cycles is realized in 1 m ZnClO 4 electrolyte, which can be attributed to the adjacent reaction potentials of the two-step one-electron reaction.

Molecular Tailoring of p–type Organics for Zinc Batteries with High Energy Density

“…Furthermore, benefiting from the large conjugated structure of PTZAN at charged state and initial state, the PTZAN cathode is more difficult to dissolve in organic electrolyte, and its cycling performance is more excellent than other small molecules in organic electrolyte. As illustrated in Figure 5d, the Zn||PTZAN cell exhibits long‐cycle life of 2000 cycles and a capacity retention of 77 %, which is better than previously reported (see Table S1) [36–44, 46, 48, 60, 61, 66–69] …”

“…Recently, some p‐type organics with single‐electron redox reaction have been demonstrated as the cathode materials for Zn batteries, showing high voltage and high stability [33–41] . Nevertheless, these single‐electron p‐type organics generally suffer from low capacity (that is generally less than 100 mAh g −1 ), which is much lower than n‐type organics [42–45] . For example, we have proposed a Zn||PTPAn battery with an organic electrolyte [45] .…”

“…[33][34][35][36][37][38][39][40][41] Nevertheless, these single-electron p-type organics generally suffer from low capacity (that is generally less than 100 mAh g À 1 ), which is much lower than n-type organics. [42][43][44][45] For example, we have proposed a Zn j j PTPAn battery with an organic electrolyte. [45] Despite the high voltage and long life, the PTPAn only delivered a limited capacity of 90 mAh g À 1 .…”

Building better aqueous Zn-organic batteries

“…As a result, it showed a higher discharge voltage (1.65 V) in aqueous ZnSO 4 electrolyte than in ZnCl 2 (1.55 V) or ZnClO 4 (1.35 V) electrolytes. 132 In addition, it also delivered an outstanding rate performance (73.1% capacity retention even at a high current density of 18 A g À1 ) in ZnSO 4 electrolyte while it had better cycling stability in ZnClO 4 electrolyte (97.4% capacity retention over 500 cycles). 132 However, due to the limited active Energy & Environmental Science Review sites and heavier molecular mass of nitronyl nitroxide compounds, their specific capacities are still lower compared to those of imine compounds, carbonyl compounds or iminecarbonyl compounds.…”

“…132 In addition, it also delivered an outstanding rate performance (73.1% capacity retention even at a high current density of 18 A g À1 ) in ZnSO 4 electrolyte while it had better cycling stability in ZnClO 4 electrolyte (97.4% capacity retention over 500 cycles). 132 However, due to the limited active Energy & Environmental Science Review sites and heavier molecular mass of nitronyl nitroxide compounds, their specific capacities are still lower compared to those of imine compounds, carbonyl compounds or iminecarbonyl compounds. Moreover, the complicated synthesis process from nitronyl nitroxide compounds will raise their preparation costs.…”