Sustainable Dual-Ion Batteries beyond Li

The limitations of resources used in current Li-ion batteries may hinder their widespread use in grid-scale energy storage systems, prompting the search for low-cost and resource-abundant alternatives. "Beyond-Li cation" batteries have emerged as promising contenders; however, they confront noteworthy challenges due to the scarcity of suitable host materials for these cations. In contrast, anions, the other crucial component in electrolytes, demonstrate reversible intercalation capacity in specific materials like graphite. The convergence of anion and cation storage has given rise to a new battery technology known as dual-ion batteries (DIBs). This comprehensive review presents the current status, advancements, and future prospects of sustainable DIBs beyond Li. Notably, most DIBs exhibit similar cathode reaction mechanisms involving anion intercalation, while the distinguishing factor lies in the cation types functioning at the anode. Accordingly, the review is organized into sections by various cation types, including Na-, K-, Mg-, Zn-, Ca-, Al-, NH4 + -, and proton-based DIBs. Moreover, a perspective on these novel DIBs is presented, along with proposed protocols for investigating DIBs and promising future research directions. It is envisioned that this review will inspire fresh concepts, ideas, and research directions, while raising important questions to further tailor and understand sustainable DIBs, ultimately facilitating their practical realization.

Azacyclic Anchor-Enabled Cohesive Graphite Electrodes for Sustainable Anion Storage

Advanced energy-storage devices are indispensable for expanding electric mobility applications. While anion intercalation-type redox chemistry in graphite cathodes has opened the path to high-energy-density batteries, surpassing the limited energy density of conventional lithium-ion batteries , a significant challenge remains: the large volume expansion of graphite upon anion intercalation. In this study, a novel polymeric binder and cohesive graphite cathode design for dual-ion batteries (DIBs) is presented, which exhibits remarkable stability even under high voltage conditions (>5 V). The innovative binder incorporates an acrylate moiety ensuring superior oxidative stability and self-healing features, along with an azide moiety, which allows for azacyclic covalent bonding with graphite and interchain crosslinking. A simple 1-h ultraviolet treatment is sufficient for binder fixation within the electrode, leading to the covalent bond formation with graphite and the creation of a robust three-dimensional network. This modification facilitates deeper and more reversible anion intercalation, leading to improved capacity, extended lifespan, and sustainable anion storage. The binder design, exhibiting exceptional adhesive properties and effective stress mitigation, enables the construction of ultrathick graphite cathodes. These findings provide valuable insights for the development of advanced binders, paving the way for high-performance DIBs.

Anion Storage of MXenes

Recently, anion storage materials have gained significant attention owing to the widened cell voltage and additional anion storing capacity for a large energy density. MXenes are considered as the emerging anion storing materials owing to their sufficient interlayer spacing, rich surface chemistries, tunable structures, remarkable electrochemical properties, and mechanical integrity. Herein, a comprehensive review on the anion storage of MXenes covering their anion storage mechanism and state-of-the-art chemical strategies for the improved anion storage performances is reported. The recent progress of MXenes on aluminum ion batteries, metal halogen batteries, halogen ion batteries, and electrochemical electrode deionization is addressed. The scientific and technical challenges and the research direction into the anion storage of MXenes are also addressed and finally the authors' perspective on anion storage of MXenes is provided. Therefore, this review offers an insight into the rational design of MXenes for anion storage materials and the correlation of surface chemistries and structural modifications with anion storage properties for the applications into electrochemical energy storage and water purification.

Successive Storage of Cations and Anions by Ligands of π-d-Conjugated Coordination Polymers Enabling Robust Sodium-Ion Batteries

The oxidation of π-d-conjugated coordination polymers (CCPs) accompanied with anion insertion has the merits of increasing the capacity and elevating the discharge voltages. However, previous reports on this mechanism either required more investigations or showed low capacity and poor cyclablity. Herein, triphenylene-catecholate-based two-dimensional CCPs are constructed by employing inactive transition-metal ions (Zn²⁺ ) as nodes, forming Zn-HHTP. Substantial characterizations and theoretical calculations indicate the successive storage of cations and anions by redox reactions of only ligands, leading to a high reversible capacity of ≈150 mAh g^-1 at 100 mA g^-1 and a remarkable capacity retention of 90 % after 1000 cycles. On the contrary, as a control experiment, the analogous CCPs (Cu-HHTP) with Cu²⁺ nodes, where both ligands and metal ions undergo redox reactions, accompanied by the storage of only Na⁺ cations, show a much poorer cyclability. These results highlight the importance of redox reactions of only ligands for long-term cycle life and the insight into the storage mechanisms deepens our understanding on CCPs for the further design of CCPs with high performance.

Boost Anion Storage Capacity Using Conductive Polymer as a Pseudocapacitive Cathode for High-Energy and Flexible Lithium Ion Capacitors

Current lithium ion capacitors (LICs) have been severely plagued by the insufficient anion storage capacity of porous carbon. This work reports the exploration of conductive polyaniline (PANi) as an anion intercalation cathode to enhance the PF₆^- storage via fast doping/undoping reactions. The PANi is electrodeposited on an electrospun carbon nanofiber (CNF) textile (denoted as PANi@CNF), which not only provides a robust support for PANi to increase its pseudocapacity but also renders a free-standing architecture for flexible devices. The PANi@CNF composite with a dominant capacitive storage characteristic reveals high specific capacities of 158.5 mAh g_PANi^-1 at 1 A g^-1 and 118.5 mAh g_PANi^-1 even at 20 A g^-1, which significantly surpasses state-of-the-art porous carbons. First-principle calculations revealed the coordination of PF₆^- anions with -NH groups of PANi⁺ via F atoms through ion-dipole electrostatic interaction, which are accompanied by electron transfer. By pairing with CNF as an anode, a thin and flexible LIC was assembled, which achieves maximum energies of 106.5 Wh kg^-1 under 769.0 W kg^-1 and 64.5 Wh kg^-1 under a super high power of 15087.1 W kg^-1, together with an impressive cycling stability of 70.3% after 9000 cycles at 10 A g^-1. These findings provide a facile strategy for high-energy and flexible LICs via anion storage pseudocapacitive materials.

Toward Higher Capacities of Hydrocarbon Cathodes in Dual-Ion Batteries

We report the electrochemical anion storage properties of a group of molecular solids of polycyclic aromatic hydrocarbons (PAHs): coronene, perylene, and triphenylene. We discover an interesting trend of progressively lower potentials for these three molecular solids. Our DFT calculations reveal that the inserted PF₆^- anions preferably bind with the edge sites of the coronene molecules as opposed to being sandwiched between two coronene molecular planes. For smaller PAHs, the more edge sites in the solids may facilitate higher capacity values. However, small PAHs do face a greater challenge of dissolution in the nonaqueous electrolyte, which affects the cycling stability.