Critical Advances of Aqueous Rechargeable Ammonium Ion Batteries

Aqueous rechargeable batteries have drawn wide attention owing to the advantages of low cost, high safety, and rapid kinetic process. In recent years, the use of NH4+ ions and other nonmetallic ions as carrier batteries has come into researchers’ view. The storage of NH4+ shows fast diffusion kinetics and the ability to achieve highly reversible redox processes by forming hydrogen bonds between NH4+ and electrode materials. Designing and exploration of advanced materials for NH4+ storage are of high significance in building high‐performance aqueous battery systems. This review summarizes the latest advances of critical materials, including Prussian blue analogs, transition metal oxides, and organic compounds for NH4+ batteries. Comparison of properties among different materials is discussed in detail. Different NH4+ storage behaviors according to several kinds of materials are demonstrated. Finally, the challenges and valuable perspectives for the further development of aqueous NH4+ batteries are also provided.


Introduction
In order to reduce the impact of energy shortage, rapidly promoting energy transition and low-carbon energy, green energy sources such as solar, wind, and tidal energy have attracted wide research attention.Aqueous rechargeable batteries have the advantages of low price, less flammability, high safety, and higher ionic conductivity.At present, the research of aqueous rechargeable batteries mainly focuses on metal carriers such as Li þ , [1][2][3][4][5] Na þ , [6][7][8] K þ , [9][10][11] Zn 2þ , [12][13][14] and Al 3þ , [15][16][17] while there are relatively few studies on nonmetal carriers such as NH 4  þ and H þ .The electrochemical behavior of nonmetallic carriers is different from metal ions.The type of charge carriers significantly determines the electrochemical performance of aqueous rechargeable batteries.NH 4 þ is in rich source and has many advantages as carriers, e.g., environmental friendliness, and no dendrite growth behavior, which makes ammonium ion battery (AIB) an attractive energy storage system in recent years with bright prospects for development (Figure 1a).First, it shows excellent rate capability due to the high ionic conductivity of aqueous electrolyte.Second, although it has a large ionic radius, NH 4 þ owns the smallest hydrated ionic size of 3.31 Å (Figure 1b), which favors rapid diffusion.Third, low molar mass (only 18 g mol À1 ) makes it easier to deliver high energy density in a battery.In addition, the neutral or weak acidic electrolyte condition is less corrosive to devices and causes less side reactions.Finally, the rich source of NH 4 þ makes it a low-cost system. [18,19]he first study on the electrochemistry of NH 4 þ was performed on Prussian blue electrode and the stable redox reaction laid the research foundation of NH 4 þ storage. [20]Later, Cui and co-workers investigated the electrochemical properties of CuHCF and NiHCF in Li þ , Na þ , K þ , and NH 4 þ , in which the highest reaction potentials were observed in NH 4 þ . [21]Gogotsi et al. discovered the electrochemical insertion behavior of NH 4 þ in Mxenes. [22]Afterward, Ji et al. reported the first ammonium ion rocking chair battery.In their study, Prussian white analogue [(NH 4 ) 1.47 Ni[Fe(CN) 6 ] 0.88 ] served as the cathode; organic solid 3,4,9,10-pery-lenetetracarboxylic dimide (PTCDI) worked as the anode. [23]Electrode material is the center of electrode redox reaction, which largely determines the performance of the battery. [24]][27][28] For example, in 2019, our group assembled a zinc-ammonium double-ion cell, in which CuHCF and zinc metal were used as the positive and negative electrode, respectively, with a high cell voltage of 1.9 V and excellent rate capability was obtained. [29]Recently, Wang et al. constructed a flexible ammonium ion full battery, in which NH 4 V 4 O 10 and polyaniline grown on carbon fiber were employed as positive and negative electrodes, respectively. [30]Zhou et al. reported an aqueous ammonium ion hybrid supercapacitor with a positive electrode of δ-MnO 2 and a negative electrode of activated carbon. [28]Last year, Su and co-workers contributed a comprehensive review to demonstrate the basic configuration, operating mechanism and recent state-of-the-art electrode materials and corresponding electrolytes for NH 4 þ storage. [31]anwhile, critical achievements in aqueous nonmetallic ion batteries were summarized and evaluated in detail, [32] which showed valuable directions in developing high-performance ammonium ion batteries.So far, electrode materials for AIBs include inorganic host materials with 1D diffusion channels, 2D diffusion channels, 3D diffusion channels, and organic materials with bonding mechanisms. [31,32]The electrode material structure of 1D diffusion channel is tunneling structure, which mainly includes tetragonal diffusion channel and hexagonal diffusion channel, such as h-MoO 3 [33] and h-WO 3 . [28]Such electrode materials show excellent stability to NH 4  þ , but the metal oxide materials have low conductivity, so the rate performance is not excellent.The electrode materials of 2D diffusion channels are typical layered materials such as V 2 O 5 , [25] NH 4 V 4 O 10 , [30] and δ-MnO 2 . [19]It is possible to expand the layer spacing of such layered materials through intercalation engineering to improve their storage capacity.Because of the wide spacing between the layers of such materials, NH 4 þ with a large ionic radius can also be inserted into the layered material, and the way NH 4 þ moves in such materials can be described by the "monkey-swinging" model.3D structured materials to store NH 4 þ are represented by Prussian blue materials, including (NH 4 ) 1.47 Ni[Fe(CN) 6 ] 0.88 , [23] NH 4• Fe 4 [Fe (CN) 6 ] 3 , [34] (NH 4 )  [35] Cu 2.95 [Fe(CN) 6 ] 1.69 , [36] and so on.Besides, some organic materials like poly(1,4,5,8-naphthalenetetracarboxylic anhydride naphthylamine) imine (PNNI), [37] alloxazine (ALO), [38] etc. can also serve as 3D host material due to that NH 4 þ adsorbed on organic groups such as conjugated carbonyls and conjugated amine groups, and during the discharge process, NH 4 þ forms hydrogen bonds with oxygen or nitrogen atoms.This review summarizes the latest advances of the research on AIBs.First of all, the operation mechanism of AIB is discussed.Subsequently, the electrochemical behavior of the critical materials including Prussian blue analogues (PBAs), transition metal oxides, and organic compounds in NH 4 þ batteries is introduced in detail.A schematic of the different types of host materials is presented in Figure 1c.In this section, the NH 4 þ storage mechanism of different types of host materials is discussed, including the diffusion pathway and relevant structural evolution.In the third part, the common electrolyte for AIB and how the electrolyte influences the battery performance are elaborated.Finally, the challenges and valuable perspectives for the further development of aqueous NH 4 þ batteries are also demonstrated.

Operation Mechanism of AIBs
Typically, the construction of an AIB includes the cathode, anode, and electrolyte.At present, most AIBs are "rocking-chair" styles, in which the ammonium ion acts as a carrier between the positive and negative electrodes (Figure 1d). [39]During the discharge/charge process, NH 4 þ ions are inserted into/extracted from the lattice of electrode materials.Common positive materials include PBAs, transition metal oxides, and other materials with high working potentials, while the negative electrode is mainly organic and other materials with low working potential.To be specific, AIBs operate on the principle of the creation and breaking of hydrogen bonds with the host material during the insertion/deinsertion of ammonium ions. [40]For example, NH 4 þ is oxidized and diffused from the anode to the cathode during the discharge process.At the same time, electrons of the same charge also migrate through the external circuit, which realizes the transformation of the chemical energy stored in the AIBs into electrical energy upon cycling.During the subsequent charge process, the reaction is reversed and the electric energy is converted into chemical energy stored in the electrode materials. [24]Therefore, the ability and manner of ammonium ions inserted into the host material highly influence the electrochemical performance of the cell.
Apart from NH 4 þ insertion and deinsertion, some other storage mechanism, including H þ insertion and NH 4 þ /H þ coinsertion, also occurs in aqueous NH 4 þ batteries.For example, ionizing solvents such as water and acids can provide H þ .H þ can be stored in transition metal oxides by binding to terminal oxygen atoms in them. [41]In monoclinic VO 2 , NH 4 þ can provide H 3 O þ by deprotonation and hydrolysis with water molecules.Both NH 4 þ and H þ can form hydrogen bonds with transition metal oxides, which facilitates the diffusion of ions. [42]In the ammonium acetate electrolyte, NH 4 þ adsorbed on the surface of manganese phosphate not only formed hydrogen bonds with oxygen atoms in the vicinity of Mn, but also with oxygen atoms of acetate, which facilitated the adsorption of NH 4 þ .X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy showed that NH 4 þ and H þ coinserted into the electrode material. [43]

Electrode Materials
Electrode materials are the key part that affects the battery performance.To date, there have been several main types of host materials under investigation, including PBAs, transition metal oxides, organic compounds, and so on.Some representative electrode materials and the corresponding electrochemical performances are listed in Table 1 to give a brief summary of them.
In the subsequent part, a thorough discussion on the relationship between the characteristics of host material and their NH 4 þ storage capability will be provided.

PBAs
44] The representative formula of PBAs is A x L y [M(CN) 6 ] z ⋅nH 2 O, where A represents alkali metal ions, and L and M represent transition metal ions.A can be partially or completely occupied by many different ions. [35]PBAs have many advantages, such as being easy to synthesize and being independent in aqueous solutions.Moreover, the volume of PBAs hardly changes during carrier insertion/deinsertion.There are many substituted elements of M, making a huge variety of PBAs.During the discharge, NH 4 þ ions are inserted into PBA and the diffraction peaks move to higher angles, indicating lattice contraction. [13,23,34,35]During the charging process, NH 4 þ ions deinsert from the PBA and the diffraction peak moves to a lower angle, indicating the lattice expansion (Figure 2a,b).The lattice contraction that occurs during the discharge process is due to the contraction of Fe-C distance after the insertion of NH 4 þ into the lattice of PBA. [13]After charging, the angle returns to the initial state, indicating that PBAs have excellent reversibility with little strain after NH 4 þ ion insertion/deinsertion.Similar PBAs show a similar phenomenon where the lattice shrinkage caused by NH 4 þ ions intercalation is due to the shrinkage of Fe-C distance (Figure 2c).
Ji et al. reported the first aqueous "rocking-chair" AIB. [23]russian white analogue (NH 4 ) 1.47 Ni[Fe(CN) 6 ] 0.88 (Ni-APW) were used as the anode material and organic solid PTCDI acted as the negative electrode (Figure 2d).The CV curves of Ni-APW showed a broad cathodic current peak at 0.63 V (vs Ag/AgCl) and two anodic peaks at 0.83 and 0.62 V, respectively (Figure 2e).At a current density of 150 mA g À1 , the initial discharge capacity was 62.6 mAh g À1 with a Coulombic efficiency of 85.5%; the discharge capacity was maintained at 60 mAh g À1 in subsequent cycles and the Coulombic efficiency increased to 95% (Figure 2f ).The phenomenon that the Coulombic efficiency value increased after the initial cycle of the aqueous cell was attributed to the completion of the side reactions.FTIR spectroscopy showed a new peak at 2164 cm À1 after charging, and disappeared after discharge.It was attributed to the Fe 3þ -CN-Ni 2þ vibration, indicating a reversible redox reaction (Figure 2g).Although the insertion/deinsertion of ammonium ions in the Prussian white analogue Ni-APW is highly reversible, it produces large polarization in PTCDI, which affects the full cell.The capacity retention rate was 67% and the energy density was about 43 Wh kg À1 at 3 C after 1000 cycles.
Shu et al. found that Fe 4 [Fe(CN) 6 ] 3 could be used for ammonium ion storage, and its large framework structure facilitated the rapid intercalation/deintercalation of ammonium ions.More importantly, the interactions between N, C, and Fe form strong covalent bonds, resulting in a stable structure with good reversibility.Under the three-electrode test, a pair of distinct redox peaks can be observed in the CV curves, attributed to the redox reaction of the high-spin N-type coordination Fe 2þ / Fe 3þ couple, and the electrochemical polarization does not increase with the increase of the sweep speed (Figure 3a).The capacity retention rate was 88.9% after 2000 cycles at 30 C. It shows dynamics dominated by diffusion control behavior.Unlike other compounds, the lattice contraction due to the intercalated of cations in this process is associated with the reduction of the Fe─C bond distance in Fe 4 [Fe(CN) 6 ] 3 during the reduction.The battery delivered a retention rate of 89.8% after 300 cycles at 1 C (Figure 3b). [34]HCF, like other PBAs, has large ion diffusion channels, and shows a highly reversible intercalated/deintercalated of NH 4 þ ions with little polarization. [35]Benefited from the stable cubic structure and large ion transport channel, N-CuHCF showed excellent high-rate performance.After 20 000 cycles at an ultrahigh C-rate of 180 C, the capacity retention rate was 86.5%.Besides, the electrolyte additive Cu(NO 3 ) 2 inhibited the dissolution of Cu based on the co-ion effect, which enabled the battery output a capacity retention of 95.5% after 700 cycles (Figure 3c).Even at 50 C, the specific capacity remains unchanged at 120 mAh g À1 (Figure 3d).
Their team also synthesized a PBA (CuHCF), which had a polarization of only 0.01 V due to the highly reversible redox Reproduced with permission. [34]Copyright 2021, Elsevier.Electrochemical properties of N-CuHCF in a three-electrode cell: c) long-term cycling performance at 1 C over 700 cycles.d) Rate performance.Reproduced with permission. [35]Copyright 2021, Elsevier.e) Changes of diffusion activation energy during ammoniation/deammoniation progresses.f ) Schematic illustration of NH 4 þ diffusion from 48 g site to another.g) Detailed view of NH 4 þ diffusion.Reproduced with permission. [36]Copyright 2021, Springer Nature.
reaction of Fe 3þ /Fe 2þ within its structure. [13]The cycling performance of CuHCF was better than that of other PBAs, with 100% capacity retention after 1000 cycles.In the process of NH 4 þ ions intercalating in CuHCF, most of Fe 3þ are reduced to Fe 2þ .And during the extraction process, Fe 2þ is oxidized to Fe 3þ .NH 4 þ tends to form hydrogen bonds with N atoms at the 48 g site in CuHCF.During the diffusion process, the total energy of the system increases and the hydrogen bond breaks when the activation energy reaches its maximum value (Figure 3e).NH 4 þ moves forward until a new hydrogen bond forms and the total energy decreases (step 1).When the hydrogen bonds are completely broken, NH 4 þ rotates and the activation energy increases to 0.49 eV (step 2).Then the hydrogen bonds are formed at the new 48 g site, the same as step 1 (Figure 3f,g).
CuHCF with large open channels facilitates the intercalation/ deintercalation of NH 4 þ ions, and the hydrogen bond formed between them promotes the rapid charge transport, resulting in a capacity retention of 72.5% after 30 000 cycles at 50 C rate.When assembled with PANI negative electrode with an electrochemical window of 0.9 V, the initial discharge capacity of 55.3 mAh g À1 at 2 A g À1 was achieved.
As mentioned above, the PBA has a stable large space structure.It has good reversibility after NH 4 þ ion insertion/deinsertion and thus exhibits excellent cycling performance.However, the battery with PBA-based cathode usually showed undesirable specific capacity and low bulk capacity.Therefore, composite materials are expected to improve the drawback.

Transition Metal Oxide Materials
Although PBAs show stable structure and good reversibility, their specific capacity in storing NH 4 þ is not satisfactory.Its relatively small mass density also results in an uncompetitive low volume capacity. [24,45]The specific capacity of transition metal oxides is high, making them remarkable ammonium storage materials.NH 4 þ ions can twist and rotate in transition metal oxides to form hydrogen bonds with neighboring oxygen atoms.The movement of NH 4 þ ions in transition metal oxides can be described by the "monkey-swinging" model: the NH 4 þ ion twists and stretches to the previous oxygen atom of the transition metal oxide, breaking the connection with the following oxygen atom and forming a new hydrogen bond. [25]

Vanadium (V)-Based Oxides
Similar to PBAs, vanadium oxides also have large ion channels and open frame structures that facilitate storage of NH 4 þ .48][49] Wang et al. proposed a flexible fibrous battery configuration consisting of an urchin-like NH 4 V 4 O 10 cathode and polyaniline anode to storage ammonium ion.NH 4 V 4 O 10 as host material has suitable layer spacing and can provide certain ammonium ions.The discharge specific capacity of CF@NH 4 V 4 O 10 was 103 mAh g À1 at 0.1 A g À1 and the structure and morphology were well preserved after 1000 cycles.During the discharge process, NH 4 þ ions were intercalated between the layers of NH 4 V 4 O 10 and hydrogen bonds were formed between the ammonium ions and the V-O layer, evidenced by the increasement of the interlayer spacing.After further discharge, the diffraction angle was significantly lower than the initial state, indicating that the lattice parameters increased significantly after the intercalated/deintercalated of NH 4 þ ions (Figure 4a).The intercalation/deintercalation of ammonium ions in NH 4 V 4 O 10 is calculated to be an intercalation pseudocapacitance process, which enabled its excellent rate capability.However, the low voltage window of 1 V and low capacity retention limited its further application. [12]n ammonium vanadium oxide was discovered as a host material for storing ammonium ions accompanied by a PVA/ NH 4 Cl gel electrolyte.During the charge-discharge cycle, the PVA chains get close to the electrode and form a highly polymerized film after stimulation of current, which can alleviate the dissolution of NVO.NH 4 Cl instead of ammonium sulfate was chosen as electrolyte due to that Cl À has less charge than SO 4 2À , so the ammonium ion transfer number will be higher.At the same time, the smaller radius of Cl À can improve the storage performance.The capacitance was larger in the NH 4 Cl/PVA electrolyte compared to pure NH 4 Cl (Figure 4b).Even at the high current density of 12 A g À1 , the capacitance of NVO can still reach 82 F g À1 .The in situ XRD showed that during the charging process, ammonium ions were removed from the VO layer, leading to the decreased stability of NVO and the increased lattice space caused by repulsive force of the VO layer.During the discharge process, hydrogen bond was formed between ammonium ion and VO layer, which stabilized the structure of NVO and reduced the lattice space.The storage mechanism of NVO was demonstrated as both the intercalation pseudocapacitance and double-layer capacitance.The specific capacity was 169 mAh g À1 at 0.5 A g À1 .The device assembled by ammonium vanadium oxide and activated carbon achieved 324 mF cm À2 at 1 mA cm À2 and had a capacity retention of 67% after 12 000 cycles (Figure 4c). [18]ong et al. first found an oxygen-deficient vanadium dioxide (d-VO 2 ) as negative material for storing ammonium ions, whose tunnel structure provided a suitable transport path (Figure 4d,e).The migration energy barrier of d-VO 2 is only one-third of that of VO, indicating that the introduction of the oxygen defect remarkably reduces the migration energy barrier and facilitates the transportation of ammonium ions.The anisotropy of the d-VO structure variation between the c-axis and the ab-plane depends on the state of the charge.Expansion along the ab-plane dominated in the early state of charge (SOC).The contraction or expansion along the c-axis and ab-plane was consistent in the deep SOC, and the structural expansion along the c-axis dominates in the shallow charge state.When it was assembled with tetragonal phase manganese dioxide (Figure 4f ), a high energy density of 96.1 Wh kg À1 was achieved, with a lifespan more than 10 000 cycles and a capacity retention rate of 70% (Figure 4g). [50]

Manganese (Mn)-Based Oxides
As a typical layered material, manganese oxides have also shown their superiority in serving as host materials because of their rich natural abundance and nontoxicity. [51]Manganese oxides have multiple chemical valences and can form various oxides such as MnO, Mn 2 O 3 , MnO 2 , Mn 3 O 4 , etc.Moreover, the multiple structures also provide conditions for carrier insertion and diffusion. [52,53]Manganese oxide also forms hydrogen bonds with ammonium ions to accomplish ion storage (Figure 5a).
The first study of NH  5b).Experimental and theoretical results all indicated the storage of NH 4 þ in MnO x through H-bond formation/breaking (Figure 5a).Meanwhile, the interaction in NH 4 þ -MnO 2 system was different from other ionelectrode system (e.g., alkali metal ion and the electrode hosts).Besides, the total energy of the product after discharge for NH 4 þ insertion (À160.42eV) was lower than that of K þ (À118.97eV), indicating more favorable NH 4 þ insertion in MnO x than that of K þ .Experimental data proved the theoretical analysis.The specific capacity of NH 4 þ in MnO x -40 was 176 mAh g À1 at the current density of 0.5 A g À1 , much higher than that of K þ storage (126 mAh g À1 ) (Figure 5b).These achievements not only broadened the material family of NH 4 þ storage but also provided valuable insights in directing deep investigation of this area, e.g., electrolyte concentration. [54]ecently, the NH 4 þ storage behavior in δ-MnO 2 was investigated by Zhou and co-workers.The Coulombic efficiency of manganese dioxide was lower in the first few times (Figure 5c), which resulted from a portion of the NH 4 þ ions intercalated between the MnO 2 layers failed to come back during the initial cycles.These NH 4 þ ions served to stabilize the lamellar structure of MnO 2 , giving assistance to the subsequent completion of the reversible intercalated/deintercalated process.In addition, water molecules were also found insert into δ-MnO 2 to stabilize the structure, but it did not enhance the capacitance.The authors attributed to the storage behavior as pseudocapacitance characteristics because the CV curve showed a rectangular shape with no obvious redox peaks.A hybrid supercapacitor composed with activated carbon cloth and δ-MnO 2 delivered a voltage window of The in situ XRD patterns of CF@NH 4 V 4 O 10 electrode at ten selected points during cycling with the galvanostatic discharge/charge profiles on the left.Reproduced with permission. [30]Copyright 2020, Elsevier.b) Specific capacitance of NVO versus current densities.c) Cycling performance and columbic efficiency at 2 A g À1 , inserting a schematic illustration of the fabrication of the HSC device.Reproduced with permission. [18]Copyright 2022, Elsevier.d-f ) The crystal structure of d-VO.Vanadium, purple; oxygen, red.Electrochemical evaluation of MnO 2 //d-VO full cell: g) the diagrammatic sketch of the electrochemical reaction process during discharge.h) Capacity retention and Coulombic efficiency on 10 000 cycles at a current density of 1 A g À1 .Reproduced with permission. [50]Copyright 2022, Wiley-VCH.
up to 2 V and an area capacitance of up to 1550 mF cm À2 .However, δ-MnO 2 may dissolve in weakly acidic (NH 4 ) 2 SO 4 electrolyte; the capacitance retention was a bit unsatisfactory. [19]o better improve the ammonium ion storage performance, a manganese phosphate/exfoliated graphene composite structure with large specific surface area and high electrical conductivity was prepared.The small nanoparticles formed during charging and discharging acted as nucleation sites to induce subsequent electrochemical reactions (Figure 5d).FTIR analysis proved that NH 4 þ ions form hydrogen bonds with O in the manganese phosphate host material.When NH 4 Ac was used as the electrolyte, a specific capacity of 299.6 mAh g À1 at 1 A g À1 was achieved.Benefited from the high conductivity of the composite, the capacity reached up to 148.2 mAh g À1 at 10 A g À1 , indicating its good rate capability (Figure 5e).And the charge transfer impedance in NH 4 Ac was also lower than that in (NH 4 ) 2 SO 4 (Figure 5f ).FTIR results showed the coinsertion of ammonium ions and protons during charging in manganese phosphate, and the contribution of protons accounted for 30% indicated by the results of XPS and energy-dispersive X-ray spectroscopy (EDX).The adsorbed ammonium ions not only formed hydrogen bonds with oxygen atoms near Mn, but also with oxygen atoms in acetic acid ions to improve the electrochemical performance (Figure 5g).MoO x negative electrode was assembled together with the as-prepared manganese phosphate electrode to form a full battery, and an energy density of 83.6 Wh kg À1 and a capacity retention rate of 90.2% after 6000 cycles were obtained. [43]

Molybdenum (Mo)-Based Oxides
Molybdenum trioxide (MoO 3 ) has attracted wide research attention in the battery-related areas because of its abundant resources, low cost, and high electrochemical activity.There are three forms of molybdenum trioxide: orthogonal α-MoO 3 , monoclinic β-MoO 3 , and hexagonal h-MoO 3 . [55]Similar to other metal oxides, MoO 3 also could serve as potential host materials in ammonium storage.Among these three different phases, h-MoO 3 shows the best NH 4 þ migration behavior because of the tunneling structure with larger spatial location than other phases.
According to the research result from Zhi and co-workers, h-MoO 3 exhibited higher capacity and rate capability than α-MoO 3 (Figure 6a,b).A specific capacity of 115 mAh g À1 at a current density of 0.1 A g À1 and a specific capacity of Reproduced with permission. [54]Copyright 2021, Wiley-VCH.Electrochemical properties of the δ-MnO 2 cathode in a three-electrode system with 1.0 m (NH 4 ) 2 SO 4 electrolyte: c) selected GCD profiles from the initial 12 cycles at 2 mA cm À2 .Reproduced with permission. [19]Copyright 2022, Wiley-VCH.d) Schematic illustration of the fabrication process for the manganese phosphate electrode.e) Galvanostatic chargedischarge curves of MP-20 at different current densities.f ) Nyquist plots of the MP-20 electrodes in NH 4 Ac and (NH 4 ) 2 SO 4 electrolytes.g) Schematic illustration of the charge storage mechanism of MP-20 in NH 4 Ac electrolyte.Reproduced with permission. [43]Copyright 2022, Wiley-VCH.respectively (Figure 6c).Ex situ XRD provided evidence that NH 4 þ experienced reversible intercalation and deintercalation from the lattice of h-MoO 3 and the host material maintained structural stability during the charge and discharge processes.Apart from the experimental analysis, calculational method was used to investigate the transport pathway of NH 4 þ and the ammoniation dynamics.The interaction behavior between NH 4 þ and h-MoO 3 host framework were claimed to be a "monkey-swimming" model where NH 4 þ proceeded forward inside the h-MoO 3 tunnel.Besides, during the process, hydrogen bonds were form between hydrogen from NH 4 þ and the adjoining oxygen atom in h-MoO 3 (Figure 6d-f ).To fully evaluate the NH 4 þ storage properties of h-MoO 3 , the authors assembled a full cell by uniting a polyacrylamide gel electrolyte, a CuFe PBA cathode, and the as-obtained h-MoO 3 anode.Results showed a 92.4% capacity retention after 2000 cycles at 1 A g À1 . [33]ater, Xu et al. proposed an oxygen-deficient α-MoO 3 and modified it with carbon core-shell structure for ammonium ion storage.The presence of oxygen defect increased electronic conductivity and promoted the formation of hydrogen bonds between NH 4 þ ions and the host material (Figure 6g).In addition, the core-shell structure of the carbon not only protected MoO 3 from collapse but also facilitated the transportation of NH 4 þ .Based on the theoretical analysis, the adsorption energy of NH 4 þ ions on both Mo site and O vacancy of oxygen-deficient MoO 3 is lower than that on pristine MoO 3 , indicating that the introduction of oxygen vacancies in MoO 3 can promote the NH 4 þ adsorption.As a result, the oxygen-deficient MoO 3 showed a specific capacitance of 208.8 F g À1 even at 20 A g À1 and a capacity retention rate of 92.7% after 5000 cycles. [56]
Among them, h-WO 3 possesses both triangular and hexagonal tunneling structures, which provides mobile path for NH 4 þ transportation and is favor to the insertion/deinsertion of NH 4 þ . [28,58]

Tang et al. preinserted NH 4
þ into h-WO 3 to stabilize the structure and enable the rapid storage of ammonium ions, which was named as (NH 4 ) x WO 3 .Preinsertion of NH 4 þ in the h-WO 3 tunnel structure led to a binding energy of À3.78 eV, which provided a superior adsorption capacity for NH 4 þ and achieved rapid ion diffusion (Figure 7a), so the capacity of (NH 4 ) x WO 3 remained 85.8% after 5000 cycles (Figure 7b).Due to the large tunnel structure of the (NH 4 ) x WO 3 electrode, it exhibited pseudocapacitive behavior.Ex-FTIR showed that the peak at 2855 cm À1 is due to the stretching of N-H with WO and 2926 cm À1 is due to the stretching of nonbonded N-H (Figure 7c,d).It showed relatively weak signal in the original state because of the preintercalated NH 4 þ ions.The discharge signal was enhanced during discharging and weakened during charging, suggesting that the formation/breakage of hydrogen bonds was related to the intercalated/deintercalated of NH 4 þ ions.By coupling with the α-MnO 2 positive electrode (Figure 7e), the voltage window reached 1.8 V.It provided an energy density of 1010.1 μWh cm À2 and maintained a capacity retention of 78.6% after 13 600 cycles. [59]imilar to h-MoO 3 , Dong et al. found that tungsten trioxide with hexagonal phase could also serve as host to storage NH 4 þ .The authors reported a capacity retention rate of 68% after 200 000 cycles even at a high current density of 20 A g À1 with the h-WO 3 electrode (Figure 7f ).In contrast to the diffusion behavior exhibited by metal ion Li þ , h-WO 3 presented a pseudocapacitance behavior in storing NH 4 þ .The water molecules in the h-WO 3 structure facilitated the formation of hydrogen bonds and promoted the transport of ammonium ions, as evidenced by the fact that the capacity decreased when using annealed m-WO 3 with no water molecules inside (Figure 7g).FTIR A-HSCs at 8 mA cm À2 .d) FT-IR spectrum and magnification of the dashed region of the (NH 4 ) x WO 3 anode at different charge/discharge states.e) CV curves of the (NH 4 ) x WO 3 anode and the a-MnO 2 cathode at a scan rate of 5 mV s À1 .Reproduced with permission. [59]Copyright 2022, The Royal Society of Chemistry.f ) GCD curves of h-WO 3 in 1 m (NH 4 ) 2 SO 4 or 1 m LiClO 4 electrolytes, along with the GCD curve of m-WO 3 in 1 m (NH 4 ) 2 SO 4 .g) Cycling performance of h-WO 3 and m-WO 3 in 1 m (NH 4 ) 2 SO 4 at 20 A g À1 .Ex situ h) FTIR characterization of the h-WO 3 electrodes at the selected state of charge.Reproduced with permission. [28]Copyright 2022, Wiley-VCH.
verified the building/breakage of the hydrogen bonds between NH 4 þ ions and h-WO 3 in the process of discharging and charging (Figure 7h).The hydrogen bond between NH 4 þ ions and h-WO 3 can provide a channel for electron transfer.To further demonstrate the property of the h-WO 3 , a full battery with h-WO 3 as the negative electrode and (NH 4 ) 0.5 V 2 O 5 as the positive electrode was assembled, and the capacity remained 100% after 5000 cycles at 5 A g À1 . [28]These abovementioned findings further broadened the scope of the study on anode material for NH 4 þ storage.Transition metal oxides have large 2D or 3D channels, and the valence state of vanadium can change from þ2 to þ5, thus achieving high capacity.NH 4 þ rotates forward in the transition metal oxide, and hydrogen bonds are constantly formed/broken, which improves the migration kinetics of NH 4 þ .

Layered Double Hydroxide
Layered double hydroxide (LDH) is an anionic layered material with the advantages of easy synthesis, large specific surface area, and easy insertion of interlayer anions. [60]During electrochemical activation, the anions in the interlayer are extracted to provide vacancies for the insertion of cations. [61]The structure is characterized by the presence of two or more variable metal cations and exchanged anions in the layers of the compound. [62]LDHs are widely used in aqueous zinc-ion battery systems with good electrochemical activity and high discharge voltage.The H vacancies in the LDH can improve the conductivity and increase the cation storage performance. [63]LDHs offer more possibilities for the development of ammonium ions.For instance, Hu et al. first reported LDHs (Mn x Al y OH 12 CO 3 ) to store NH 4 þ ions.In their study, MnAl underwent a rapid amorphization conversion from 2D nanosheets to a pleated nanoparticle-like morphology during the initial charging, which facilitated ammonium ion transport.It delivered a specific capacity of 183.7 mAh g À1 at 0.1 A g À1 .The capacity retention rate of the prepared LDH electrode was 81% after 400 cycles.With the rapid amorphous phase transition, LDH gradually changed into microspheres composed of folded nanosheets.And these microspheres maintained their shape during the following cycles (Figure 8a).The authors assembled a full battery by combining MnAl with PTCDI negative electrode, and a lifetime of over 100 cycles and an energy density of 45.8 Wh kg À1 were achieved. [64]DHs feature unique layered structure and large specific surface area.The interlayer anions of LDH are easy to insert or remove, which brings more possibilities for the physicochemical properties of LDHs.In the future, LDHs will promote the further development of ion batteries.

Organic Compounds
Organic electrode materials possess the advantages of abundant resources and easy synthesis.They have specific functional groups and can interact with charge carriers. [37,38]In AIBs, ammonium ions bond with carbonyl oxygen atoms in organic combines with C═O to form C─O─NH 4 probably as a parasubstitution.Due to the mutual repulsion of cations, the C═O functional group on the same side will be retained (Figure 8b). [37]ovalent organic framework (COF) materials are 2D or 3D porous polymers linked by covalent bonds and formed by the thermodynamic reversible polymerization of organic small molecule monomers. [65]COFs materials are commonly used as lithium storage materials. [66]Husam N. Alshareef et al. synthesized QA-COF from cyclohexane-1,2,3,4,5,6-hexaone and 2,3,5,6tetraaminocyclohexa-2,5-diene-1,4-dione, which was used for the first time to store ammonium ions.QA-COFs have better thermodynamic stability due to the coordination of NH 4 þ with hydrogen bonds.Compared to monomer, the frame structure avoids the stacking and poor conductivity at monomer and improves the cycle stability.In the initial discharge state, NH 4 þ forms a sixcoordinated structure with QA-COF; in the subsequent discharge process, the diagonal position will continue to be coordinated with ammonium ions, and the final discharge product is a 12coordinated structure (Figure 8c).NH 4 þ ions exhibit higher capacity and better rate performance than other metal ions (Figure 8d), probably because hydrogen bonds are weaker and more flexible than metal bonds.The weaker hydrogen bonding makes the solvation structure looser and also accelerates the desolvation process, making the desolvation energy barrier low, so NH 4 þ has a higher redox potential (Figure 8e).QA-COF had lower capacity at high current densities due to the low conductivity of COFs.And the Coulombic efficiency was also lower at low current densities due to side reactions (Figure 8f ). [67]ao et al. synthesized ALO as a negative material for storing NH 4 þ ions, which showed both strong pseudocapacitance effect and rapid diffusion kinetics.As a result, it displayed a specific capacity of 120 mAh g À1 at 40 C (10 A g À1 ).More interestingly, the galvanostatic charge/discharge (GCD) profiles showed that the charging and discharging platforms were almost the same under different current densities, and there was no obvious polarization.To improve the Coulombic efficiency at low current density, ALO was assembled with Ni-APW cathode to form a full battery.Results showed that the specific capacity remained at 110 mAh g À1 and Coulombic efficiency stabilized at 100% after 10 000 cycles, demonstrating excellent cycle stability. [38]rganic materials owe the advantage of being easy to synthesize, and NH 4 þ ions form hydrogen bonds with oxygen or nitrogen atoms in organic compound, completing the charge storage process.When the organic compound contains multiple carbonyl and amine groups, the reaction is divided into multiple steps.Organic materials have low electrical conductivity and need to add a large number of conductive additives, resulting in poor weight and volume energy density.

Electrolyte
As an essential component in a battery, electrolyte undertakes the transmission of NH 4 þ between anode and cathode, which undoubtedly plays an important role in influencing the rate and cyclic capability of the battery.In this section, the currently used electrolyte and how the electrolyte characteristics, e.g., concentration, types affect the NH 4 þ storage behavior are discussed.

Aqueous Liquid Electrolyte
Dilute electrolytes show high ionic conductivity and low cost.Therefore, most of the current electrolytes used for NH 4 þ ion batteries/supercapacitors are dilute electrolytes, such as 1 M (NH 4 ) 2 SO 4 , [19,23,28,30,38,50] 1 M CH 3 COONH 4 , [37,43] and 2 M NH 4 NO 3 . [13,35]However, the electrochemical activity of the diluted electrolyte is high, and the hydrolysis phenomenon is inevitable, resulting in a narrow electrochemical window, which further limits the energy density.Nevertheless, electrolyte of high concentration can inhibit electrochemical activity, widen voltage window, and improve Coulombic efficiency.For example, Du et al. investigated the ammonium storage properties of VS 2 / VO x heterostructure in 5 M (NH 4 ) 2 SO 4 with an electrochemical window up to 1.7 V.At a current density of 1 A g À1 , the VS 2 /VO x heterostructure maintained 43% capacity after 1000 cycles. [68]hu et al. used saturated (NH 4 ) 2 SO 4 as an electrolyte; the capacity Reproduced with permission. [64]Copyright 2022, Wiley-VCH.b) The ideal insertion model of NH 4 þ in PNNI electrode.Reproduced with permission. [37]Copyright 2023, The American Chemical Society.Reproduced with permission. [67]Copyright 2021, The American Chemical Society.
retention rate of Fe 4 [Fe(CN) 6 ] 3 was 88.9% after 2000 cycles at 30 C. [34] In spite of this, high concentration electrolyte does not necessarily enhance the electrochemical performance.MnO x -40 synthesized by Liu et al. showed the highest specific capacity at 0.5 M NH 4 Ac than at other concentrations.When the concentration is 8 M, its structure will collapse and the specific capacity decay rapidly. [54]Besides, recent researches have showed that some certain electrolyte additives stabilized the cycling performance of NH 4 þ .Shu et al. slowed the dissolution of N-CuHCF in electrolytes by adding 0.01 M Cu(NO 3 ) 2 to 2.0 M NH 4 NO 3 due to the co-ionic effect. [35]

Hydrogel Electrolytes
Hydrogel is an extremely hydrophilic 3D network structure gel. [69]The polymer chain contains a large number of hydrophilic functional groups, so it can swell rapidly in water but does not dissolve, retaining a large amount of water. [45,69]Meng et al. adopted an NH 4 Cl/PVA gel electrolytes because PVA had abundant oxygen-containing functional groups that could form hydrogen bonds with ammonium ions, which facilitated a rapid hydrogen bonding conduction mechanism. [18]Zhi et al. developed an NH 4 Cl-containing PAM polymer electrolytes and employed it in a full cell consisting of CuFe PBA cathodes and h-MoO 3 .Results showed that this battery design could maintain excellent cycling performance under different bending angles. [33]

Summary and Perspectives
Aqueous rechargeable batteries possess the advantages of environment friendly and safety.In aqueous systems, NH 4 þ as a carrier has some special properties different from other metal ions.The small hydrated ionic radius of NH 4 þ facilitates the rapid insertion/deinsertion from the electrode material.Milder acid and alkaline environments slow down the corrosion of devices.Moreover, NH 4 þ also exhibits fast diffusion kinetics in aqueous solutions, and its unique storage mechanism provides ideas for the study of other nonmetallic cells.
PBAs are widely used in a variety of batteries because of their large channel structure, which can accommodate ion insertion/ deinsertion with little change in volume.Since the pioneering test of the electrochemical properties of CuHCF and NiHCF in NH 4 þ by Cui et al, [21] it has been found that a variety of PBAs perform better in aqueous solutions of NH 4 þ ions.Prussian white and Berlin green have also used as electrode materials for storing NH 4 þ , which also possessed a stable large space structure.However, the specific capacity of PBAs is unsatisfactory and the volume capacity is low.
Transition metal oxides, which have large 2D channels and are adapted to volume changes during ion insertion, have also been used as materials for storing NH 4 þ ions.It has a high theoretical specific capacity and has a laminar structure that facilitates the diffusion of active substances, thus promoting electron transfer.The storage of NH 4 þ in the transition metal oxide is accomplished by forming hydrogen bonds with the oxygen atoms in the transition metal oxide, resulting in ultrafast kinetics.The movement of NH 4 þ conforms to the "monkey-swinging" model: [39] the NH 4 þ ion twists and stretches to the previous oxygen atom in the transition metal oxide, breaking the connection with the following oxygen atom and forming a new hydrogen bond.When the prepared cathode material does not contain NH 4 þ ions, it is usually necessary to improve its cycling performance by preinsertion of NH 4 þ ions.Organic compounds have specific functional groups that show certain advantages in storing NH 4 þ , such as PANI, [27] ALO, [38] PNNI, [37] PTCDI, [23] and PI. [70]However, the conductivity of organic compounds is poor, and in order to improve the conductivity, Yang et al. proposed to introduce rGO into TCNQ to promote electron transfer and also to inhibit the dissolution during cycling. [71]hen choosing electrode materials, we should adhere to the following rules: 1) inorganic materials with large layer spacing to store NH 4 þ , such as PBAs, transition metal oxides, transition metal sulfides, MXenes, LDHs, and so on; 2) organic materials with suitable functional groups (e.g., carbonyl groups); 3) electrochemically stable in aqueous electrolyte solutions and not easy to dissolve; and 4) low cost, easy to preparation, nontoxic, etc.
The electrochemical window of the electrolyte needs to be as wide and stable as possible.(NH 4 ) 2 SO 4 , [19,23,30,34,38,50,56,64,67] NH 4 Cl, [18,33] CH 3 COONH 4 , [37,43,54,59] NH 4 NO 3 , [13,35] etc. are used as electrolytes for aqueous ammonium ion (NH 4 þ ) batteries.Based on the common ion effect, researchers also inhibit the dissolution of electrode materials by adding additive into electrolyte to improve cycling performance.Shu et al. used 0.01 M Cu(NO 3 ) 2 as an additive of electrolyte, which contributed significantly to improve the cycling performance of N-CuHCF. [35]With the development of flexible devices, the research on hydrogel electrolytes is also increasing.Usually, ammonium salt is combined with hydrogel.Zhi et al. used NH 4 Cl/PAM as electrolyte; [33] Meng et al. used NH 4 Cl/PVA as electrolyte, which promoted the development of flexible NH 4 þ ionic energy storage. [18]he electrolyte also plays a vital role in the performance of the battery; we should choose: 1) a large potential window, less prone to side reactions; 2) acid and alkali conditions are mild and reduce the dissolution of the electrode material; and 3) low cost and environmentally friendly.
Most current aqueous NH 4 þ batteries use a single carrier: NH 4 þ , which is the "rocking chair" battery.NH 4 þ ions can also be combined with other ions to form two-ion batteries.For example, our team proposed the aqueous zinc-ammonium hybrid battery, which has a higher battery voltage (1.8 V) and low self-discharge, providing reference for the future formation of NH 4 þ and other ions hybrid battery. [29]he aqueous AIB also faces some challenges; although the hydrated ion radius of NH 4 þ is small, it has a large ionic radius.When NH 4 þ ions are inserted into the material, it needs to be desolvated.And the large ionic radius leads to the lack of a suitable host, which makes the lower specific capacity and has a negative effect on the migration rate in the electrode material.The reason why the Coulombic efficiency of ammonium ion batteries is inferior to lithium-ion battery is the irreversible change of electrode and some side reactions. [12,67]Apart from this, although a variety of cathode materials with large framework structures such as PBAs have been reported, the lack of suitable anode materials restricts the performance of the full battery.The relatively slow redox kinetics of NH 4 þ and the poor conductivity of the electrodes result in a higher polarization and poor rate capability of the aqueous NH 4 þ batteries. [30]IBs are inherently safe and enable large-scale storage.Ammonium ions have a theoretically unlimited feedstock and are less costly than metal ions.Moreover, it has a near-neutral pH and is less corrosive to devices.The assembly environment is not harsh, which is conducive to scale up applications.Therefore, ammonium ion batteries will bring more options for wind and solar energy storage, and will be promising in wearable devices and grid-scale applications (Figure 9).At present, the electrolyte of AIB is mostly aqueous electrolyte, and the application of organic electrolyte in AIB will bring more possibilities in the future.With broad development prospect and research value based on the unique advantages, the limitations of aqueous ammonium ion will be solved in the future.

Figure 1 .
Figure 1.a) Advantages of aqueous AIB.b) Comparison of the ionic radius, hydrated ionic radius, and ionic weight of different charge carriers.c) Schematic of the different types of host materials for aqueous ammonium ion batteries.d) Schematic diagram of an aqueous AIB.

Figure 4 .
Figure 4. a)The in situ XRD patterns of CF@NH 4 V 4 O 10 electrode at ten selected points during cycling with the galvanostatic discharge/charge profiles on the left.Reproduced with permission.[30]Copyright 2020, Elsevier.b) Specific capacitance of NVO versus current densities.c) Cycling performance and columbic efficiency at 2 A g À1 , inserting a schematic illustration of the fabrication of the HSC device.Reproduced with permission.[18]Copyright 2022, Elsevier.d-f ) The crystal structure of d-VO.Vanadium, purple; oxygen, red.Electrochemical evaluation of MnO 2 //d-VO full cell: g) the diagrammatic sketch of the electrochemical reaction process during discharge.h) Capacity retention and Coulombic efficiency on 10 000 cycles at a current density of 1 A g À1 .Reproduced with permission.[50]Copyright 2022, Wiley-VCH.

Figure 5 .
Figure 5. a) Schematic diagram of NH 4 þ transport in layered MnO 2 .b) Charge/discharge curves of MnO x electrodes charged and discharged after 40 cycles in different NH 4 Ac electrolytes.Reproduced with permission.[54]Copyright 2021, Wiley-VCH.Electrochemical properties of the δ-MnO 2 cathode in a three-electrode system with 1.0 m (NH 4 ) 2 SO 4 electrolyte: c) selected GCD profiles from the initial 12 cycles at 2 mA cm À2 .Reproduced with permission.[19]Copyright 2022, Wiley-VCH.d) Schematic illustration of the fabrication process for the manganese phosphate electrode.e) Galvanostatic chargedischarge curves of MP-20 at different current densities.f ) Nyquist plots of the MP-20 electrodes in NH 4 Ac and (NH 4 ) 2 SO 4 electrolytes.g) Schematic illustration of the charge storage mechanism of MP-20 in NH 4 Ac electrolyte.Reproduced with permission.[43]Copyright 2022, Wiley-VCH.

Figure 6 .
Figure 6.a) Tunnel structure of h-MoO 3 .b) Rate performance of h-MoO 3 and for α-MoO 3 , respectively.c) GCD profiles h-MoO 3 electrode of at different current densities.Investigation of ammoniation/deammoniation behavior.d-f ) Evolution of three stages during NH 4 þ diffusion process from state a to b

Figure 7 .
Figure 7. a) Schematic diagram of the three bonding states and adsorption energy in the WO 3 tunnel structure: single NH 4 þ , an H 2 O and an NH 4 þ , and two NH 4 þ intercalation.b) Cycling performance of WO 3 •0.33(H 2 O), h-WO 3 , and (NH 4 ) x WO 3 at 20 mA cm À2 .c) GCD curves of (NH 4 ) x WO 3 //a-MnO 2 compounds to form hydrogen bonds to accomplish charge storage.Cao et al. reported PNNI for the storage of NH 4 þ .In the presence of aqueous solution of NH 4 þ , the carbonyl functional groups of PNNI can be reversibly oxidized and reduced.The multilayer structures of PNNI provide a large contact area to facilitate the diffusion of NH 4 þ ions.During the discharge process, NH 4 þ

Figure 8 .
Figure8.a) Schematic illustration of the ammonium-ion storage mechanism in the MnAl-LDH host.Reproduced with permission.[64]Copyright 2022, Wiley-VCH.b) The ideal insertion model of NH 4 þ in PNNI electrode.Reproduced with permission.[37]Copyright 2023, The American Chemical Society.

c) The NH 4 þ
ion storage mechanism in QA-COF.d) Rate performance in the four different cation-based electrolytes.e) Redox potential of P O1 (oxidation) and P R1 (reduction) in different cation-based electrolytes.f ) Rate performance and relevant Coulombic efficiency of QA-COF in 0.5 m (NH 4 ) 2 SO 4 .

Table 1 .
Summary of the host materials and corresponding electrochemical performances of aqueous ammonium ion batteries.
4 þ storage chemistry in MnO x was performed by Liu et al.Electrodeposited amorphous MnO x was selected as the host material.The concentration of ammonium acetate (NH 4 Ac) electrolyte is demonstrated to have obvious influence in NH 4 þ storage for electrodeposited MnO x .MnO x