Active cyano groups to coordinate AlCl₂⁺ cation for rechargeable aluminum batteries

Highlights

• Cyano-organic molecules as positive materials for aluminum batteries were confirmed.

• The capacity was ascribed to the coordination/dissociation between cyano group and AlCl₂⁺ cation.

• TCNQ shows a high specific capacity of 180 mAh g⁻¹ with discharge plateau of 1.6 V.

• TCNQ maintains reversible capacity of 115 mAh g⁻¹ after 2000 cycles at 500 mA g⁻¹.

Abstract

Rechargeable aluminum batteries, owing to the abundant Al resources and high safety guarantee, have been exploited as the ideal power sources for large-scale energy storage. However, the application of aluminum batteries is still restricted by the unsatisfactory positive electrodes due to low capacity, electrode variation or poor cycle stability of inorganic materials. As an alternative route for addressing the critical issues, multi-cyano organic molecules are proposed as the positive electrode materials for stable rechargeable aluminum batteries. With the first-principle calculation of the electron distribution, molecular energy levels and theoretical specific capacity, the results suggest that the electron-deficient cyano groups in organic molecules can reversibly coordinate/dissociate with positively charged Al-complex ions (i.e. AlCl₂⁺). According to the molecule design and experiment results, tetracyanoquinodimethane (TCNQ) with high conductivity is identified as the optimized cyano-organic positive electrode material. The as-assembled cell delivers a high specific capacity of 180 mAh g⁻¹ at first cycle and a discharge plateau ~1.6 V, along with long-term operation beyond 2000 cycles and coulombic efficiency of ~100%. The design principle here opens a new platform to utilize stable cyano-organic molecules as positive electrode materials for high-energy-density aluminum batteries, offering opportunities to design and exploit high-performance rechargeable Al-organic batteries.

Introduction

Rechargeable aluminum batteries have been regarded as the most competitive candidates for low-cost and large-scale energy storage, owing to abundant Al resource, high specific capacity and safety guarantee [1,2]. Up to date, the promising aluminum batteries assembled with graphite-based positive electrode and ionic liquid electrolyte are based on the intercalation/deintercalation of aluminum-coordination ions (Al_xCl_y⁻) [1,3]. Although graphite-based positive electrodes exhibit considerable cycling stability, they still suffer from low specific capacity and large volume expansion, severely limiting the scalable application [2,[4], [5], [6]]. On the other hand, inorganic metal-based compounds, such as metal chalcogenide and phosphorus positive electrode materials, are also pursued for their high theoretical capacity [7], [8], [9], [10]. Unfortunately, the solid-state crystalline structures would be inevitably decomposed upon reversible conversion processes, leading to unexpected chemical/electrochemical dissolution into the ionic liquid electrolyte and poor cycling stability [11], [12], [13]. Apparently, these inorganic positive electrode materials (i.e. graphite and inorganic metal-based compounds) are facing critical challenges and cannot well meet the requirement of high-energy density aluminum batteries.

Alternatively, organic compounds possess diverse molecular structures and active functional groups, with expectation of offering sufficiently reactive sites for electrochemical reactions. In Li, Na, K and Mg batteries, a series of organic compounds were already confirmed as the effective electrode materials, which exhibit competitive electrochemical performance to the inorganic electrodes [14], [15], [16], [17], [18], [19], [20], [21]. In the reaction mechanism, the effective functional groups in organic compounds should be firstly reduced to bond metal cations, and then are reversibly oxidized to release cations [22], [23], [24]. In the alkali/alkaline earth metal batteries, for example, free alkali/alkaline earth metal cations mainly exist in the electrolytes and can easily combine with the reduced functional groups, such as C=O groups [17,21,[25], [26], [27], [28]]. Therefore, the reversible charging/discharging could be achieved based on the conversion mechanism.

In the aluminum batteries, however, serious issues should be addressed using organic electrode materials. Initially, the negatively charged aluminum-coordination ions with complex spatial structure mainly exist in the ionic liquid electrolytes and cannot be directly combined with the reduced functional groups [1,29]. Additional, organic compounds usually possess low intrinsic electrical conductivity, which would result in poor rate capability [30]. Therefore, it is essential to engineer organic molecule structures and to tailor electronic energy level of functional groups. As a typical strong polarity structure, cyano group (CN) can be easily reduced and regenerated by further oxidation [31,32]. The redox behavior and electronic energy levels of cyano groups can be effectively manipulated by changing the molecule structures [33,34]. Note that the intrinsic electron-deficient cyano groups exhibit greater electron affinities, which is prone to attack nucleophilic reagent. Particularly, the cyano groups are usually used as the substituents for nucleophilic reactions to replace halogen groups, which allows cyano-based organic molecules to coordinate with aluminum-chlorine coordination ions [35,36]. Therefore, cyano-based organic compounds offer the opportunity for serving as the positive electrode materials in aluminum batteries. Additionally, multi-cyano organic compounds, such as tetracyanoquinodimethane (TCNQ), have been widely recognized as the organic superconductors [37, 38] because of their high theoretical specific capacity and good conductivity. Although phenanthrenequinone with C=O group has been confirmed as the active materials for aluminum batteries, the effective specific capacity and stability was still unexpected for long-term operation [39].

In this contribution, molecule design based on density functional theory (DFT) calculation was initially applied for rationally understanding the electron distribution, molecular energy levels and theoretical specific capacity of cyano-based organic compounds in aluminum batteries. Specifically, three typical tetracyano-based organic compounds with different benzene ring numbers, i.e. tetracyanoethylene (TCNE), tetracyanoquinodimethane (TCNQ) and tetrakis(4-cyanophenyl)methane (TCPM), were proposed as the positive electrode materials. The positively charged Al-complex ions (i.e. AlCl₂⁺) were identified to bond the reduced cyano groups, followed by confirming the optimum tetracyano-based organic compounds (i.e. TCNQ) for reversible charging/discharging capability. As the positive electrode materials in aluminum batteries, the assembled cell with TCNQ delivered a reversible specific capacity of 180 mAh g⁻¹ at the first discharge and high discharging potential of 1.6 V (versus Al³⁺/Al). Even after 2000 cycles at 500 mA g⁻¹, the reversible capacity was still 115 mA h g⁻¹. The design principle and results provide a promising opportunity to develop effective cyano-organic positive electrodes for high-performance aluminum battery.