Ionically Conductive Tunnels in h-WO3 Enable High-Rate NH4 + Storage

'Beyond Li-ion technology'-a status review

Li-ion battery is currently considered to be the most proven technology for energy storage systems when it comes to the overall combination of energy, power, cyclability and cost. However, there are continuous expectations for cost reduction in large-scale applications, especially in electric vehicles and grids, alongside growing concerns over safety, availability of natural resources for lithium, and environmental remediation. Therefore, industry and academia have consequently shifted their focus towards 'beyond Li-ion technologies'. In this respect, other non-Li-based alkali-ion/polyvalent-ion batteries, non-Li-based all solid-state batteries, fluoride-ion/ammonium-ion batteries, redox-flow batteries, sand batteries and hydrogen fuel cells etc. are becoming potential cost-effective alternatives. While there has been notable swift advancement across various materials, chemistries, architectures, and applications in this field, a comprehensive overview encompassing high-energy 'beyond Li-ion' technologies, along with considerations of commercial viability, is currently lacking. Therefore, in this review article, a rationalized approach is adopted to identify notable 'post-Li' candidates. Their pros and cons are comprehensively presented by discussing the fundamental principles in terms of material characteristics, relevant chemistries, and architectural developments that make a good high-energy 'beyond Li' storage system. Furthermore, a concise summary outlining the primary challenges of each system is provided, alongside the potential strategies being implemented to mitigate these issues. Additionally, the extent to which these strategies have positively influenced the performance of these 'post-Li' technologies is discussed.

Coordination and Hydrogen Bond Chemistry in Tungsten Oxide@Polyaniline Composite toward High-Capacity Aqueous Ammonium Storage

Aqueous ammonium ion batteries (AAIBs) have garnered significant attention due to their unique energy storage mechanism. However, their progress is hindered by the relatively low capacities of NH4 + host materials. Herein, the study proposes an electrodeposited tungsten oxide@polyaniline (WOx@PANI) composite electrode as a NH4 + host, which achieves an ultrahigh capacity of 280.3 mAh g-1 at 1 A g-1, surpassing the vast majority of previously reported NH4 + host materials. The synergistic interaction of coordination chemistry and hydrogen bond chemistry between the WOx and PANI enhances the charge storage capacity. Experimental results indicate that the strong interfacial coordination bonding (N: →W6+) effectively modulates the chemical environment of W atoms, enhances the protonation level of PANI, and thus consequently the conductivity and stability of the composites. Spectroscopy analysis further reveals a unique NH4 +/H+ co-insertion mechanism, in which the interfacial hydrogen bond network (N-H···O) accelerates proton involvement in the energy storage process and activates the Grotthuss hopping conduction of H+ between the hydrated tungsten oxide layers. This work opens a new avenue to achieving high-capacity NH4 + storage through interfacial chemistry interactions, overcoming the capacity limitations of NH4 + host materials for aqueous energy storage.

Enhancing ammonium-ion storage in Mo-doped VO2 (B) nanobelt-bundles anode for aqueous ammonium-ion batteries

The recent surge in interest in aqueous ammonium ion rechargeable batteries (AAIBs) has been fueled by their eco-friendliness, efficiency, safety, and sustainability. However, finding the optimal anode material for effective ammonium ion (NH4+) storage remains a nascent and significant challenge. The research presented here focuses on the enhancement of aqueous ammonium rechargeable batteries by incorporating Mo atoms into VO2 (B) (denote as MVO), a material that has shown promise as an anode for NH4+ storage. The introduction of Mo ions was found to optimize the electronic structure and morphology of pristine VO2 (B) (label as PVO), resulting in the transformation of its nanobelts into thin nanobelt-bundles. This alteration exposes more active sites and increases oxygen vacancies, which in turn improve the conductivity and diffusion rate of NH4+ ions, thereby enhancing the overall electrochemical performance of the material. The MVO material demonstrates a high initial capacity of 283.5 mA h g-1 at 0.3 A g-1, and maintained 86.7% of its capacity after 4500 cycles, indicating excellent long-term stability. To further validate the practical application, a full cell was garnered utilizing MVO as the anode and Cu3[Fe(CN)6]2 (CuHCF) as the cathode. The resulting AAIB displays remarkable cycling stability, with 81.5% capacity preservation after 1000 cycles and large energy density of 57.9 W h kg-1. The study reveals that the doping of Mo ions can significantly improve both the stability and NH4+ storage capacity of PVO, offering a promising new direction for the exploitation of efficacious and sustainable NH4+ host materials for rechargeable batteries.

Investigations on Tunnel-Structure MnO2 for Utilization as a High-Voltage and Long-Life Cathode Material in Aqueous Ammonium-Ion and Hybrid-Ion Batteries

Recently, nonmetal NH4 + ions have attracted extensive attention for use as charge carries in the field of energy storage due to their abundant resources, environmental friendliness, and low cost. However, the development of aqueous ammonium-ion batteries (AAIBs) is constrained by the absence of high-voltage and long-life materials. Herein, different tunnel-structure MnO2 materials (α-, β-, and γ-MnO2) are utilized as cathodes for AAIBs and hybrid-ion batteries and compared, and α-MnO2 is demonstrated to exhibit the most remarkable electrochemical performance. The α-MnO2 cathode material delivers the highest discharge capacity of 219 mAh g-1 at a current density of 0.1 A g-1 and the best cyclability with a capacity retention of 95.4% after 10 000 cycles at 1.0 A g-1. Moreover, aqueous ammonium-ion and hybrid-ion (ammonium/sodium ions) full batteries are successfully constructed using α-MnO2 cathodes. This work provides a novel direction for the development of aqueous energy storage for practical applications.

An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries

Ammonium ion batteries are promising for energy storage with the merits of low cost, inherent security, environmental friendliness, and excellent electrochemical properties. Unfortunately, the lack of anode materials restricts their development. Herein, we utilized density functional theory calculations to explore the V2CTx MXene as a promising anode with a low working potential. V2CTx MXene demonstrates pseudocapacitive behavior for ammonium ion storage, delivering a high specific capacity of 115.9 mAh g-1 at 1 A g-1 and excellent capacity retention of 100% after 5000 cycles at 5 A g-1. In-situ electrochemical quartz crystal microbalance measurement verifies a two-step electrochemical process of this unique pseudocapacitive storage behavior in the ammonium acetate electrolyte. Theoretical simulation reveals reversible electron transfer reactions with [NH4+(HAc)3]···O coordination bonds, resulting in a superior ammonium ion storage capacity. The generality of this acetate ion enhancement effect is also confirmed in the MoS2-based ammonium-ion battery system. These findings open a new door to realizing high capacity on ammonium ion storage through acetate ion enhancement, breaking the capacity limitations of both Faradaic and non-Faradaic energy storage.

Percolating Network of Anionic Vacancies in Prussian Blue: Origin of Superior Ammonium-Ion Storage Performance

Emerging aqueous ammonium-ion batteries (AIBs) are considered inexpensive, highly safe, ecofriendly, and sustainable energy storage systems. Although some high-performance electrode materials have been reported for AIBs, a comprehensive understanding of the origin of the high ammonium-ion storage performance is still lacking. Herein, the percolating network of anionic vacancies is determined to be the origin of the superior ammonium-ion storage properties of the Prussian blue analogues based on ab initio molecular dynamics simulation and electrochemical kinetic analyses. Fe[Fe(CN)6] with a percolating anionic vacancy network delivers an outstanding rate of 64.7 mAh g-1 at 2000 mA g-1 in addition to a capacity retention of 94.5% after 10 000 cycles. The low-strain intercalation ammonium-ion storage mechanism of highly deficient Fe Prussian blue with Fe as the redox center is revealed by in situ X-ray diffraction and ex situ X-ray absorption fine structure analysis. The results provide insights into the mechanism of ammonium-ion storage in Prussian blue analogues and guidance in the development of aqueous AIBs.

Vanadium Hexacyanoferrate Prussian Blue Analogs for Aqueous Proton Storage: Excellent Electrochemical Properties and Mechanism Insights

The significant attraction toward aqueous proton batteries (APBs) is attributable to their expedited kinetics, elevated safety profile, and economical feasibility. Nevertheless, their practical implement is significantly blocked by the unsatisfactory energy density due to the limited cathode materials. Herein, vanadium hexacyanoferrate Prussian blue analog (VOHCF) is introduced as a potentially favorable cathode material for APBs. The findings demonstrate that this VOHCF electrode exhibits a notable reversible capacity of 102.7 mAh g-1 and exceptional cycling stability, with 95.4% capacity retention over 10 000 cycles at 10 A g-1 . It is noteworthy that this is the detailed instance of VOHCF being proposed as a cathode for APBs. Combining the in situ characterization techniques and theoretical simulations, the origins of excellent proton storage performance are revealed as the multiple redox mechanisms with double active centers of ─C≡N group and V═O bond in VOHCF as well as the robust structure stability. A proton full cell with excellent performance is further achieved by coupling the VOHCF cathode and diquinoxalino[2,3-a:2',3'-c] phenazine (HATN) anode, demonstrating the great potential of VOHCF in practical applications. This work could provide fundamental understanding to the development of feasible cathode materials for proton storage device.

Oxygen Vacancy-Enriched Bi2SeO5 Nanosheets with Dual Mechanism for Ammonium-Ion Batteries

Ammonium ions feature a light molar mass and small hydrated radius, and the interesting interaction between NH4+ and host materials has attracted widespread attention in aqueous energy storage, while few studies focus on high-performance NH4+ storage anodes. Herein, we present a high-performance inset-type anode for aqueous ammonium-ion batteries (AIBs) based on Bi2SeO5 nanosheets. A reversible NH4+/H+ co-intercalation/deintercalation accompanied by hydrogen bond formation/breaking and a conversion reaction mechanism in layered Bi2SeO5 is proposed according to ex situ characterizations. Accordingly, the optimized Bi2SeO5 anode has a high reversible capacity of 341.03 mAh g-1 at 0.3 A g-1 in 1 M NH4Cl electrolyte and an impressive capacity retention of 86.7% after 7000 cycles at 3 A g-1, which is related to the existence of oxygen vacancies that enhance ion/electron transfer and promote the formation of hydrogen bonds between NH4+ and the host material. When the rocking-chair ammonium-ion battery is assembled using a MnO2 cathode, the device delivers an ultrahigh capacity of 140.73 mAh g-1 at 0.15 A g-1 and energy density of 207.13 Wh kg-1 at the power density of 2985.07 W kg-1. This work provides a promising strategy for designing high-performance anodes for next-generation AIBs.

A salt-concentrated electrolyte for aqueous ammonium-ion hybrid batteries

The development of aqueous ammonium-ion batteries (AAIBs) is currently attracting great attention because of the interesting electrochemical features induced by the charge carrier NH4+. One possible way to improve the performance of AAIBs is increasing the salt concentration in the electrolyte. Yet, few studies focus on the complex electrode-electrolyte interface behaviors in highly concentrated electrolytes, which affect the electrochemical performance of AAIBs significantly. Herein, we aim to understand the impact of CH3COONH4 electrolyte concentration on the NH4+ storage performance of a bimetallic hydroxide material. Experimental and theoretical simulation results indicate that the acetate anion will participate in the construction of the solvated NH4+ in a highly concentrated electrolyte, facilitating the adsorption of the solvated NH4+ cluster on the electrode surface. Besides, a new partial de-solvation model is also proposed, demonstrating an energy favorable de-solvation process. Finally, an ammonium-ion hybrid battery is designed, which provides a high average discharge voltage of 1.7 V and good energy density of 368 W h kg(cathode)-1, outperforming most of the state-of-the-art aqueous batteries. This work provides new understanding about the electrode's interfacial chemistry in different concentrated CH3COONH4 electrolytes, establishes a correlation between the electrolyte concentration and the electrode's performances, and demonstrates the superiority of the hybrid ammonium-ion battery design.

2D Conjugated Metal-Organic Frameworks Embedded with Iodine for High-Performance Ammonium-Ion Hybrid Supercapacitors

Ammonium ions (NH4 + ) are emerging non-metallic charge carriers for advanced electrochemical energy storage devices, due to their low cost, elemental abundance, and environmental benignity. However, finding suitable electrode materials to achieve rapid diffusion kinetics for NH4 + storage remains a great challenge. Herein, a 2D conjugated metal-organic framework (2D c-MOF) for immobilizing iodine, as a high-performance cathode material for NH4 + hybrid supercapacitors, is reported. Cu-HHB (HHB = hexahydroxybenzene) MOF embedded with iodine (Cu-HHB/I2 ) features excellent electrical conductivity, highly porous structure, and rich accessible active sites of copper-bis(dihydroxy) (Cu─O4 ) and iodide species, resulting in a remarkable areal capacitance of 111.7 mF cm-2 at 0.4 mA cm-2 . Experimental results and theoretical calculations indicate that the Cu─O4 species in Cu-HHB play a critical role in binding polyiodide and suppressing its dissolution, as well as contributing to a large pseudocapacitance with adsorbed iodide. In combination with a porous MXene anode, the full NH4 + hybrid supercapacitors deliver an excellent energy density of 31.5 mWh cm-2 and long-term cycling stability with 89.5% capacitance retention after 10 000 cycles, superior to those of the state-of-the-art NH4 + hybrid supercapacitors. This study sheds light on the material design for NH4 + storage, enabling the development of novel high-performance energy storage devices.

Unlocking High-Performance Ammonium-Ion Batteries: Activation of In-Layer Channels for Enhanced Ion Storage and Migration

Ammonium-ion batteries, leveraging non-metallic ammonium ions, have arisen as a promising electrochemical energy storage system; however, their advancement has been hindered by the scarcity of high-performance ammonium-ion storage materials. In this study, an electrochemical phase transformation approach is proposed for the in situ synthesis of layered VOPO4 ·2H2 O (E-VOPO) with predominant growth on the (200) plane, corresponding to the tetragonal channels on the (001) layers. The findings reveal that these tetragonal in-layer channels not only furnish NH4 + storage sites but also enhance transfer kinetics by providing rapid cross-layer migration pathways. This crucial aspect has been largely overlooked in previous studies. The E-VOPO electrode exhibits exceptional ammonium-ion storage performance, including significantly increased specific capacity, enhanced rate capability, and robust cycling stability. The resulting full cell can be stably operated for 12 500 charge-discharge cycles at 2 A g-1 for over 70 days. The proposed approach offers a new strategy for meticulously engineering electrode materials with facilitated ion storage and migration, thereby paving the way for developing more efficient and sustainable energy storage systems.

An All-Printed, Fast-Response Flexible Humidity Sensor Based on Hexagonal-WO3 Nanowires for Multifunctional Applications

The utilization of printing techniques for the development of high-performance humidity sensors holds immense significance for various applications in the fields of the Internet of Things, agriculture, human healthcare, and storage environments. However, the long response time and low sensitivity of current printed humidity sensors limit their practical applications. Herein, a series of high-sensing-performance flexible resistive-type humidity sensors is fabricated by the screen-printing method, and hexagonal tungsten oxide (h-WO3 ) is employed as the humidity-sensing material due to its low cost, strong chemical adsorption ability, and excellent humidity-sensing ability. The as-prepared printed sensors exhibit high sensitivity, good repeatability, outstanding flexibility, low hysteresis, and fast response (1.5 s) in a wide relative humidity (RH) range (11-95% RH). Furthermore, the sensitivity of humidity sensors can be easily adjusted by altering the manufacturing parameters of the sensing layer and interdigital electrode to meet the diverse requirements of specific applications. The printed flexible humidity sensors possess immense potential in various applications, including wearable devices, non-contact measurements, and packaging opening state monitoring.

MoS2 @Polyaniline for Aqueous Ammonium-Ion Supercapacitors

Ammonium-ion aqueous supercapacitors are raising notable attention owing to their cost, safety, and environmental advantages, but the development of optimized electrode materials for ammonium-ion storage still lacks behind expectations. To overcome current challenges, here, a sulfide-based composite electrode based on MoS2 and polyaniline (MoS2 @PANI) is proposed as an ammonium-ion host. The optimized composite possesses specific capacitances above 450 F g-1 at 1 A g-1 , and 86.3% capacitance retention after 5000 cycles in a three-electrode configuration. PANI not only contributes to the electrochemical performance but also plays a key role in defining the final MoS2 architecture. Symmetric supercapacitors assembled with such electrodes display energy densities above 60 Wh kg-1 at a power density of 725 W kg-1 . Compared with Li+ and K+ ions, the surface capacitive contribution in NH4 + -based devices is lower at every scan rate, which points to an effective generation/breaking of H-bonds as the mechanism controlling the rate of NH4 + insertion/de-insertion. This result is supported by density functional theory calculations, which also show that sulfur vacancies effectively enhance the NH4 + adsorption energy and improve the electrical conductivity of the whole composite. Overall, this work demonstrates the great potential of composite engineering in optimizing the performance of ammonium-ion insertion electrodes.

Coupling Engineering of NH4 + Pre-Intercalation and Rich Oxygen Vacancies in Tunnel WO3 Toward Fast and Stable Rocking Chair Zinc-Ion Battery

Herein, for the first time, a pre-intercalated non-metal ion (NH₄ ⁺ ) with rich oxygen vacancies stabilized tunnel WO₃ is proposed as a new intercalation anode to construct Zn-metal-free rocking-chair ZIBs. With the ethylene glycol additive in the aqueous electrolyte, the Zn²⁺ solvation structure can be regulated and the side reaction of hydrogen evolution can also be suppressed. Owing to the integrated synergetic modification, a high-rate and ultra-stable aqueous Zn-(NH₄ )_x WO₃ battery can be constructed, which exhibits an improved specific capacity (153 mAh g^-1 at 0.1 A g^-1 ), excellent rate performance (when the current density increases to 3 A g^-1 , the specific capacitance is still 86 mAh g^-1 ), and a high cycle stability with 100% capacity retention after 2,200 cycles under 5 A g^-1 . Ex situ X-ray diffraction and XPS reveal the reversible insertion/extraction of Zn²⁺ in (NH₄ )_x WO₃ . The assembled (NH₄ )_x WO₃ //MnO₂ rocking-chair ZIBs delivers excellent capacity of 82 mAh g^-1 at 0.1 A g^-1 , impressive cyclic stability. Additionally, the flexible (NH₄ )_x WO₃ //MnO₂ ZIBs can power the electrochromic device-based PANI/WO₃ with high color contrast and fast response time. This study provides new insight for developing high-performance rechargeable aqueous ZIBs.

Valence Engineering Enhancing NH4 + Storage Capacity of Manganese Oxides

Ammonium ions (NH₄ ⁺ ), as non-metallic charge carriers, are attracting attention in aqueous batteries due to its low molar mass, element sufficiency, and non-toxicity. However, the host materials for NH₄ ⁺ storage are still limited. Herein, an oxygen defects-rich manganese oxide (MnO_2-x ) for NH₄ ⁺ storage are reported. The oxygen defects can endow the MnO_2-x sample with improved electric conductivity and low interface activation energy. The electrochemical reaction mechanism is also verified by using ex situ X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FT-IR), demonstrating the insertion and extraction of NH₄ ⁺ in the MnO_2-x by formation/breaking of a hydrogen bond. As a result, MnO_2-x delivers a high capacity of 109.9 mAh g^-1 at the current density of 0.5 A g^-1 and retention of 24 mAh g^-1 after 1000 cycles at the current density of 4 A g^-1 , outperforming the pristine MnO₂ sample.

Flexible Ammonium-Ion Pouch Cells Based on a Tunneled Manganese Dioxide Cathode

Aqueous ammonium-ion (NH₄⁺) batteries are becoming the competitive energy storage candidate on account of their safety, affordability, sustainability, and intrinsically peculiar properties. Herein, an aqueous NH₄⁺-ion pouch cell is investigated based on a tunneled manganese dioxide (α-MnO₂) cathode and a 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) anode. The MnO₂ electrode possesses a high specific capacity of ∼190 mA h g^-1 at 0.1 A g^-1 and displays excellent long cycling performance after 50,000 cycles in 1 M (NH₄)₂SO₄, which outperforms the most reported ammonium-ion host materials. Besides, a solid-solution behavior is revealed about the migration of NH₄⁺ in the tunnel-like α-MnO₂. The battery displays a splendid rate capacity of 83.2 mA h g^-1 even at 10 A g^-1. It also exhibits a high energy density of ∼78 W h kg^-1 as well as a high power density of ∼8212 W kg^-1 (based on the mass of MnO₂). What is more, the flexible MnO₂//PTCDA pouch cell based on the hydrogel electrolyte shows excellent flexibility and good electrochemical properties. The topochemistry results of MnO₂//PTCDA point to the potential practicability of ammonium-ion energy storage.

Electrodepositing amorphous molybdenum oxides for aqueous NH4+ storage

The limited choice of anode materials always challenges the development of high performance aqueous ammonium-ion batteries (AAIBs). Herein, we fabricate amorphous molybdenum oxide (MoO_x) materials and study the NH₄⁺ storage performances. The results indicate that the optimized electrode exhibits high gravimetric/areal capacities of 175 mA h g^-1/1.30 mA h cm^-2, outperforming state-of-the-art anode materials for AAIBs. Our findings indicate that the valence state of Mo and the Mo-O-H content in MoO_x synergistically control the NH₄⁺ storage performances, offering new understanding for rational design of MoO_x materials for energy storage applications.