Electrolyte and Electrode Design for Aqueous Potassium-Ion Batteries.

Electrochemical Energy Storage with Mediator-Ion Solid Electrolytes

Sodium-ion batteries (SIB) require less critical materials and lead to a significant cost reduction compared to state-of-the-art lithium-ion batteries [1]. Although a large-scale commercialization of SIBs is within reach [2], the understanding of the degradation mechanisms of these batteries is still incomplete. The larger ionic radius of sodium in comparison to that of lithium can induce larger stress changes in the host material causing poor long-term stability. Minimizing degradation of positive electrode materials is of high importance for the advancement of SIB technology. Here, we report on degradation of the important structure types: layered oxides, polyanionic compounds, and Prussian blue analogues. Our results show that the polyanionic Na3V2(PO4)3 is a highly reliable cathode material, which can be used as a benchmark or reference material.During battery operation, mechanical stresses are caused by volume changes of the active materials, when sodium ions are inserted or extracted into/from them. We use operando methods such as the substrate curvature technique to measure stresses in the electrode and draw conclusions about electrochemical and physical processes in the materials during cycling [3]. To investigate phenomena like volume changes, crack formation and overall morphology, ex situ scanning electron microscopy (SEM) is used intermittently. Here, we compare the same site on the electrode in different states of charge and for different cycle numbers. Additionally, X-ray diffraction (XRD) measurements are conducted to study phase transitions.Na2/3Fe1/3Mn2/3O2 is a layered P2-type oxide that offers a high specific capacity and is composed of abundant elements. Due to its intrinsic sodium deficiency, diverse presodiation methods become important [4]. We investigate Na2C4O4 as sacrificial salt and its effects on electrode morphology during decomposition. Our goal is to understand the resulting reaction pathways and optimize the full cell performance of layered oxides.Sodium hexacyanoferrate NaxFe[Fe(CN)6] is a Prussian blue analogue and one of the most prominent candidates for commercial SIBs [2]. It also offers high specific capacities and a long cycle life, but its high moisture sensitivity and interstitial water demand appropriate electrode drying procedures to avoid side reactions during cell operation. Crack formation and growth were studied with SEM after and during cycling. Phase transitions are observed by operando XRD and operando substrate curvature measurements.Sodium vanadium phosphate Na3V2(PO4)3 (NVP) with its highly stable three-dimensional network of a NASICON-type structure and its large diffusion channels for sodium ions is another promising electrode material that deserves attention. NVP can be sodiated and desodiated at two different voltage plateaus and therefore can serve either as cathode or anode material. SEM observations at different states of charge show how preexisting cracks open/close during desodiation/sodiation due to volume shrinkage/expansion. We compare symmetric NVP−NVP cells to full cells with NVP cathodes and anodes of Na metal, hard carbon, Sb/C, and SnSb/C. Our data suggests that problems of Na and Na-ion cells frequently result from the anode. NVP itself is extremely reliable and balanced symmetric NVP−NVP cells exhibit very high cycling stability. More than one thousand cycles can be easily achieved. Due to its distinct plateaus and high stability, we suggest NVP as a reference material for cathode and anode materials. For example, NVP can serve as a reliable anode material for testing the stability of different cathode materials. NVP as a cathode may be used to test anode materials or as a benchmark for other cathode materials [5].Acknowledgements:This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence, EXC 2154).Literature:[1] C. Vaalma et al., Nature Reviews Materials 2018, 3, 4, 1-11.[2] M. A. Sawhney et al., ChemPhysChem 2022, 23, 5, 1-19.[3] Z. Choi et al., Journal of Power Sources 2013, 240, 245-251.[4] J. M. De Ilarduya et al. Electrochimica Acta 2019, 321, 134693.[5] T. Akçay et al., ACS Applied Energy Materials 2021, 4, 11, 12688-12695.

Introduction Na-ion batteries with aqueous electrolyte have attracted much attention, since it has 3 big advantages about the conductivity, non-flammability and cost. In addition, water is ideal solvent that can dissolve various salts in large amount. However, there is a severe restriction in the selection of the cathode and anode active materials, because of the narrow electrochemical window of water. Most recently, the expansion of the working voltage in aqueous cell have been tried either by judicious choice of active materials [1] and by increasing electrolyte concentration [2], which effectively prevents the electrochemical decomposition of water. Herein, new aqueous sodium-ion battery with inexpensive Na2MnFe(CN)6 (NMHCF) Prussian blue analogues as cathode and NASICON-type NaTi2(PO4)3 (NTP) as anode is introduced. Minor-metal free NMHCF and NTP are very attractive electrode active materials, because they have voltage plateaus near the upper/lower limit of electrochemical window of water. It has been reported that NTP anode can work in aqueous electrolyte reversibly at 2.1 V vs. Na/Na+ [3]. On the other hand, NMHCF has 2 high voltage plateaus at 3.5 V and 3.8 V vs. Na/Na+ and the higher voltage plateau is located above the electrochemical window of water. This is a reason that there are no reports about NMHCF cathode in aqueous electrolyte up to now. Here, to suppress the electrochemical decomposition of water, highly concentrated NaClO4 aqueous electrolyte was tried to realize high voltage aqueous sodium-ion battery with NMHCF cathode and NTP anode. Experimental NaxMn[Fe(CN)6]y·zH2O (NMHCF, 0<x<2, 0<y<1, 0<z) cathode material was obtained by co-precipitating method written in the previous reports [4]. NaTi2(PO4)3 anode material was prepared by conventional solid-state reaction of stoichiometric starting materials. NaxMn[Fe(CN)6]y·zH2O molecular formula was determined by inductive coupled plasma atomic emission spectroscopy (ICP-AES) and thermogravimetric analysis (TGA). Both NMHCF and NTP were mixed with acetylene black (AB) in a weight ratio of active material/AB = 70/25, respectively. In addition, NTP/AB was annealed in Ar atmosphere at 800 °C for 12 h. The cathode and anode pellets were fabricated with 5 wt % of polytetrafluoroethylene binder and punched into disks. And then these pellets were sandwiched by titanium mesh. A three-electrode electrochemical cell (half-cell) and two electrode cell (full-cell) with aqueous electrolyte were used for the galvanostatic charge/discharge test. An Ag-AgCl electrode with saturated KCl and NTP were used as the reference and counter electrodes, respectively. The cathode/anode weight balance for this ion-type cell is 2:3, and the cathode/anode capacity balance is approximately 2:3 (anode capacity excess condition). Here, 1 mol/kg and 17 mol/kg NaClO4 diluted/concentrated aqueous solution were used as aqueous electrolytes. Results and Discussion By the ICP-AES and TGA, the NMHCF cathode molecular formula and theoretical capacity were determined as Na1.27Mn[Fe(CN)6]0.76·1.39H2O and 126 mAh/g, respectively. Figure 1 compares the charge/discharge profiles of NMHCF//NTP full-cell with (a) 1 mol/kg and (b) 17 mol/kg NaClO4 aq. at the rate of 2.0 mA/cm2. In the case of 1 mol/kg electrolyte, the large irreversible capacity corresponding to the O2 gas evolution on cathode was observed on the 1st cycle between 0.5 and 2 V. On the other hand, the full-cell with 17 mol/kg electrolyte showed the reversible behavior in the wider voltage range than the electrochemical window of water. The drastic effect must be due to the suppression of the oxidation by the concentrated aqueous electrolyte. To confirm the Fe2+/Fe3+ and Mn2+/Mn3+ redox reaction of NMHCF cathode in concentrated aqueous electrolyte, X-ray photoelectron spectroscopy and X-ray diffraction were measured during 1st cycle. These results will be discussed on the day.

Aqueous aluminum-ion batteries have many advantages such as their safety, environmental friendliness, low cost, high reserves and the high theoretical specific capacity of aluminum. So aqueous aluminum-ion batteries are potential substitute for lithium-ion batteries. In this paper, the current research status and development trends of cathode and anode materials and electrolytes for aqueous aluminum-ion batteries are described. Aiming at the problem of passivation, corrosion and hydrogen evolution reaction of aluminum anode and dissolution and irreversible change of cathode after cycling in aqueous aluminum-ion batteries. Solutions of different research routes such as ASEI (artificial solid electrolyte interphase), alloying, amorphization, elemental doping, electrolyte regulation, etc and different transformation mechanisms of anode and cathode materials during cycling have been summarized. Moreover, it looks forward to the possible research directions of aqueous aluminum-ion batteries in the future. We hope that this review can provide some insights and support for the design of more suitable electrode materials and electrolytes for aqueous aluminum-ion batteries.

A Na-rich manganese hexacyanoferrate hollow nano-cube with low crystal water and few defects for efficient Zn ion storage in aqueous batteries

Calcium rechargeable batteries based on divalent charge carriers have the potential to meet the future demands for large-scale energy storage applications, due to the crustal abundance of Ca element and the high capacity and high safety of Ca metal anodes. The discernible progress in electrolyte and anode materials has put calcium battery technology a step closer to practice. However, the pursuit of high-voltage, high-capacity and stable cathode materials had been formidable because of the sluggish ion migration kinetics and the instability of host lattices during Ca2+ insertion and extraction. Unlocking the potential of Ca rechargeable batteries particularly hinges on the strategic identification of high-performance cathode materials. Herein, this review summarizes the representative strategies to develop novel cathode materials that allow reversible accommodation of Ca2+ ions for high energy output. The cathode materials can be classified into intercalation-type (layered structure, polyanionic compounds, and Prussian blue analogues) and conversion-type (organic materials, sulfur, and oxygen). The scrutinization of their performances and drawbacks sheds light on the current stage of cathode material advancement and provides informative suggestions for future studies to develop advanced calcium rechargeable batteries with competitive performance.

Aqueous aluminium-ion batteries (AAIBs) have emerged as a promising post-lithium energy storage technology due to their low cost, abundant resources, and inherent safety. This review provides a comprehensive summary of recent advances in AAIBs, focusing on three key aspects: cathode materials, anode engineering, and electrolyte innovation. Among cathode materials, manganese-based oxides, Prussian blue analogues, and organic compounds have shown notable capacities and cycling performance, with manganese dioxides standing out for its rich polymorphs and high electrochemical activity. However, structural instability remains a challenge, prompting the development of in situ electrochemical transformation, heteroatom doping, and electrolyte additive strategies. On the anode side, aluminium (Al) metal suffers from passivation and irreversible reactions in aqueous media, limiting its cycling life. Strategies such as surface pretreatment, amorphization, and alloying have been employed to improve reversibility and interfacial stability. Electrolyte development has progressed from traditional Al salt solutions to highly concentrated Al(OTF)3 systems, deep eutectic solvents, and gel-based formulations, effectively widening the electrochemical stability window and enhancing overall battery performance. Despite significant progress, challenges such as cathode structural degradation and Al anode instability persist. Continued advancements in interfacial engineering and electrolyte design will be crucial to realizing the practical deployment of AAIBs.

Aqueous sodium ion battery (ASIB) is gaining more attention with the growing concern in safety and environmental issues. However, the potential window of ASIB is limited to the decomposition voltage of water. Recently, the potential window of aqueous electrolyte has been reported that it can be extended in high-concentration electrolytes. Since then, researchers are dedicating to finding cathode materials with higher discharging potential which can increase the energy density of aqueous batteries. In this study, we demonstrate a new concept in cathode material design to reach an ultra-high rate capability by modifying the side group of a Prussian blue analogue, copper hexacyanoferrates (CuHCF). The modified cathode material was carefully analyzed by ex-situ XRD measurements, X-ray photoelectron spectroscopy, cyclic voltammetry (CV), and charge-discharge measurement to reveal their electrochemical performances. The diffusivity was also calculated by potentiostatic intermittent titration technique test and Randles-Sevcik equation.The results showed a great rate capacity for more than 80% from 1 C to 20 C accompanied with an outstanding cycle stability (~ 100% after 1000 cycle at 20 C). The high discharge plateau at about 0.7 V (vs Ag/AgCl) indicates that this material is promising in high-energy ASIB applications. This Prussian blue analogue cathode material is further tested in a full cell system with NaTi2(PO4)3 as anode material. This new concept may shed light on the field of increasing cycle stability and rate capability of Prussian blue analogues.

In the quest for sustainable and efficient energy storage solutions, aqueous calcium ion batteries emerge as a promising alternative to conventional lithium-ion technology, highlighting advantages such as abundance, low cost, and environmental friendliness[1]. Among the cathode materials explored for aqueous calcium-ion batteries, Metal Organic Framework (MOF), especially Prussian Blue Analogues (PBAs) have attracted attention due to their unique open-framework structure, which facilitates fast ion transport and enables high-rate capabilities. Moreover, the straightforward synthesis method and structural tunability of these materials further enhances their potential[2]. Prussian blue analogues have demonstrated promising performance for calcium ion batteries[3]. However, the diffusion mechanism of calcium ions in PBAs has not been extensively studied. Understanding the transport properties of calcium ions in PBAs is important for determining the optimal PBA structure.In this study, we used both electrochemical and computational techniques to investigate the transport mechanism of calcium ions in the PBAs aqueous system, particularly copper hexacyanoferrate (CuHCF). The diffusion coefficients at various states of charge were determined using galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). To further elucidate experimental findings, we employed the semi-empirical quantum mechanics method GFN-xTB to study calcium-ion transport properties. The energy-minimum transport pathway was identified by performing a mesh scan within a cavity in the CuHCF unit cell based on xTB single-point calculations[4]. The results revealed that calcium ions preferentially diffuse along cyanide groups due to attractive interactions between the positively charged calcium ions and the negatively charged cyanide groups. On the other hand, strong repulsive interactions between calcium ions and copper ions caused the calcium ions to shift away from copper sites during transport. Based on the defined transport path and calculated energy barrier from xTB, we predicted the diffusion coefficient using KineCluE program[5]. The predicted diffusion coefficient is 4.75 × 10⁻¹³ m²/s, which is in good agreement with the experimental value of (1.74 ± 0.12) × 10⁻¹² m²/s obtained from cyclic voltammetry. This work shows a way for understanding calcium-ion diffusion mechanisms in PBAs, which can also be applied to the design and optimization of other cathode materials for next-generation calcium-ion batteries. R. J. Gummow, G. Vamvounis, M. B. Kannan, and Y. He, Advanced Materials, 30, 1801702 (2018).Y. Yang et al., Nano Energy, 99, 107424 (2022).S. Gheytani et al., Adv. Sci., 4, 1700465 (2017).J. Nordstrand et al., ACS Appl. Mater. Interfaces, 14, 1102–1113 (2022).T. Schuler, L. Messina, and M. Nastar, Computational Materials Science, 172, 109191 (2020). Figure 1

Aqueous monovalent-ion batteries have been rapidly developed recently as promising energy storage devices in large-scale energy storage systems owing to their fast charging capability and high power densities. In recent years, Prussian blue analogues, polyanion-type compounds, and layered oxides have been widely developed as cathodes for aqueous monovalent-ion batteries because of their low cost and high theoretical capacity. Furthermore, many design strategies have been proposed to expand their electrochemical stability window by reducing the amount of free water molecules and introducing an electrolyte addictive. This review highlights the advantages and drawbacks of cathode and anode materials, and summarizes the correlations between the various strategies and the electrochemical performance in terms of structural engineering, morphology control, elemental compositions, and interfacial design. Finally, this review can offer rational principles and potential future directions in the design of aqueous monovalent-ion batteries.

Aluminum metal is considered an ideal candidate for aqueous metal batteries due to its abundant availability and high theoretical capacity (2980mAhg-1). However, the development of aqueous aluminum-metal batteries is significantly hindered by the detrimental side reactions (such as solvent decomposition, Al corrosion, and passivation) that occur when aluminum metal is in contact with aqueous electrolytes. In this paper, introducing trace amounts of lanthanum chloride (LaCl3) into the low-cost yet highly corrosive aqueous aluminum chloride (AlCl3) solution as an electrolyte for aqueous aluminum-metal batteries is proposed. The additional halide ions introduced into the electrolyte system modulate the solvation structure of Al3+, while the electrochemically inert La3+ induces a transformation of the aluminum metal interface from aggressive, localized penetration corrosion to more controlled, uniform corrosion, thereby enabling long-lasting and stable electrochemical reactions. In a full battery test using Prussian blue analogs (PBA) as the cathode material and aluminum metal as the anode material, the average Coulombic efficiency exceeded 97%, and the cycling stability is 74.4% at a current density of 250mAg-1 over 800 cycles.

The chemically self‐charging aqueous batteries are regarded as potential candidates for off‐grid energy storage devices due to their environmental independence and simple construction. Although tremendous research efforts have been recently made to design chemically self‐charging aqueous batteries, a comprehensive review about them is still absent. This review describes the design principles of chemically self‐charging aqueous batteries and their self‐charging mechanism. The advances in their cathode materials mainly include transition‐metal oxides or sulfides, Prussian blue analogues, and organic compounds. Subsequently, the strategies of enhancing chemically self‐charging kinetics are highlighted, including the design of materials, selection of oxidants, and the introduction of catalysts. In addition, various applications of chemically self‐charging batteries are also discussed in wearable electronic devices, low temperatures, and all‐pH scenarios. In the final section, the challenges and future perspectives are presented for designing high‐performance chemically self‐charging aqueous batteries. It will shed light on the R&amp;D of chemically self‐charging aqueous batteries.

A new perylene-based tetracarboxylate as anode and LiMn2O4 as cathode in aqueous Mg-Li batteries with excellent capacity

Efficient non-destructive recovery of LiFePO4 from spent lithium-Ion batteries for high-purity regeneration.

Recent progress and perspectives on aqueous Zn-based rechargeable batteries with mild aqueous electrolytes