Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects.

Recent developments in electrode materials for dual-ion batteries: Potential alternatives to conventional batteries

The limited active sites of cathode materials in aluminum‐ion batteries restrict the storage of more large‐sized Al‐complex ions, leading to a low celling of theoretical capacity. To make the utmost of active sites, an alternate storage mechanism of opposite charges (AlCl4− anions and AlCl2+ cations) in multisites is proposed herein to achieve an ultrahigh capacity in Al–metal–organic framework (MOF) battery. The bipolar ligands (oxidized from 18π to 16π electrons and reduced from 18π to 20π electrons in a planar cyclic conjugated system) can alternately uptake and release AlCl4− anions and AlCl2+ cations in charge/discharge processes, which can double the capacity of unipolar ligands. Moreover, the high‐density active Cu sites (Cu nodes) in the 2D Cu‐based MOF can also store AlCl2+ cations for a higher capacity. The rigid and extended MOF structure can address the problems of high solubility and poor stability of small organic molecules. As a result, three‐step redox reactions with two‐electron transfer in each step are demonstrated in charge/discharge processes, achieving high reversible capacity (184 mAh g−1) and energy density (177 Wh kg−1) of the optimized cathode in an Al–MOF battery. The findings provide a new insight for the rational design of stable high‐energy Al–MOF batteries.

Alternate Storage of Opposite Charges in Multisites for High‐Energy‐Density Al–MOF Batteries

Fully aromatic conjugated covalent organic frameworks (FAC‐COFs) with excellent physicochemical stability have been emerging as active semiconductors for diverse potential applications. Developing efficient synthesis methods for fabricating FAC‐COFs will significantly facilitate the exploration over their material and photonic/electronic functionalities. Herein, a facile solvent‐free strategy is developed for the synthesis of 2D phthalocyanine‐based FAC‐COFs (FAC‐Pc‐COFs). Cyclopolymerization of benzo[1,2‐b:4,5‐b′]bis[1,4]benzodioxin‐2,3,9,10‐tetracarbonitrile (BBTC) and quinoxalino[2′,3′:9,10]phenanthro[4,5‐abc]phenazine‐6,7,15,16‐tetracarbonitrile (QPPTC) in ZnCl2 leads to the fast formation and isolation of BB‐FAC‐Pc‐COF and QPP‐FAC‐Pc‐COF, respectively. Powder X‐ray diffraction and electron microscopy analysis reveal their crystalline nature with sql topology and AA stacking configuration. Thermogravimetric analysis and immersion experiment indicate their excellent stability. The conductivity test demonstrates their high conductivity of 0.93–1.94 × 10−4 S cm−1 owing to the fully π‐conjugated electronic structural nature. In particular, the as‐prepared FAC‐Pc‐COFs show high‐performance K+ storage in potassium‐ion batteries due to their excellent conductivity, highly ordered and robust structure, and N/O‐rich framework nature. Impressively, QPP‐FAC‐Pc‐COF shows a large reversible capacity of 424 mA h g−1 after 100 cycles at 50 mA g−1 and a capacity retention of nearly 100% at 2000 mA g−1 for over 10 000 cycles.

Ionothermal Synthesis of Fully Conjugated Covalent Organic Frameworks for High‐Capacity and Ultrastable Potassium‐Ion Batteries

Na3 V2 (PO4 )2 F3 (NVPF) is a suitable cathode for sodium-ion batteries owing to its stable structure. However, the large radius of Na+ restricts diffusion kinetics during charging and discharging. Thus, in this study, a phosphomolybdic acid (PMA)-assisted hydrothermal method is proposed. In the hydrothermal process, the NVPF morphologies vary from bulk to cuboid with varying PMA contents. The optimal channel for accelerated Na+ transmission is obtained by cuboid NVPF. With nitrogen-doping of carbon, the conductivity of NVPF is further enhanced. Combined with crystal growth engineering and surface modification, the optimal nitrogen-doped carbon-covered NVPF cuboid (c-NVPF@NC) exhibits a high initial discharge capacity of 121 mAh g-1 at 0.2 C. Coupled with a commercial hard carbon (CHC) anode, the c-NVPF@NC||CHC full battery delivers 118 mAh g-1 at 0.2 C, thereby achieving a high energy density of 450 Wh kg-1 . Therefore, this work provides a novel strategy for boosting electrochemical performance by crystal growth engineering and surface modification.

Engineering Crystal Growth and Surface Modification of Na3 V2 (PO4 )2 F3 Cathode for High-Energy-Density Sodium-Ion Batteries.

Sodium-organic batteries utilizing natural abundance of sodium element and renewable active materials gain great attentions for grid-scale applications. However, the development is still limited by lack of suitable organic cathode materials with high electronic conductivity that can be operated stably in liquid electrolyte. Herein, we present 5,15-bis(ethynyl)-10,20-diphenylporphyrin (DEPP) and [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) as new cathodes for extremely stable sodium-organic batteries. The copper(II) ion partially contributes the charge storage and significantly stabilizes the structure of porphyrin complex for electrochemical energy storage. In situ electrochemical stabilization of organic cathode with a lower charging current density was identified which enables both improved high energy density and power density. An excellent long-term cycling stability up to 600 cycles and an extremely high power density of 28 kW kg−1 were achieved for porphyrin-based cathode. This observation would open new pathway for developing highly stable sodium-organic cathode for electrochemical energy storage.

/pdf/high-rate-and-long-lifespan-sodium-organic-batteries-using-357kl54pky.pdf

High Rate and Long Lifespan Sodium-Organic Batteries Using Pseudocapacitive Porphyrin Complexes-Based Cathode

Chloride ion batteries (CIBs) are regarded as promising energy storage devices due to their high theoretical energy density, sustainability, and abundant resources. However, the development of CIBs is still limited by the proper cathode materials and electrolyte due to the dissolution issue of active material during the electrochemical reaction. Herein, a stable room temperature rechargeable CIB coupled with CuCl cathode in a tributylmethylammonium chloride based liquid electrolyte is reported. Chloride ion shuttle behavior is evidenced in two different cells, in which lithium foil, and CuCl are used as the anode and cathode, respectively. The CuCl cathode delivers an initial discharge capacity of 278 mAh g−1 and a reversible capacity of 70 mAh g−1 is still retained after 100 cycles at a current density of 5 mA g−1. This study indicates that CuCl would be a promising cathode for CIBs with good cycling stability. 

Cuprous Chloride as a New Cathode Material for Room Temperature Chloride Ion Batteries

Organic cathode materials have recently attracted abundant attention due to their flexible structural tunability and recyclability. However, the low intrinsic electrical conductivity and high solubility in electrolytes of organic electrode materials have significantly limited their practical application. Herein, we present [5,15-bis(ethynyl)-10,20-difurylporphinato] copper(II) (CuDEOP) as a new cathode for rechargeable organic lithium batteries (ROLBs). The combination of both ethynyl and furyl groups of the CuDEOP cathode with a nanorod structure renders it with enhanced structural stability and an extended delocalized π-electron system to deliver excellent cycling stability (capacity retention of 76% after 6000 cycles) and a high power density (16 kW kg-1). The furyl electroactive groups participate in charge storage contribution to achieve a reversible six-electron-transfer redox reaction in a specific voltage range. The mechanism characterizations indicate that the nitrogen atoms on the porphyrin ring act as active sites to alternatively store both PF6- anions and Li+ cations, and the charge storage process is a pseudocapacitive-dominated reaction. This observation will offer a new avenue for designing functionalized molecules for electrochemical energy-storage (EES) systems.

Ethynyl and Furyl Functionalized Porphyrin Complexes as New Organic Cathodes Enabling High Power Density and Long-Term Cycling Stability.

A bipolar pyridine-functionalized porphyrin with hybrid charge-storage for dual-ion batteries.

A bipolar organic molecule toward a universal pseudocapacitive cathode for stable dual ion charge storage

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