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Na‐ion batteries based on abundant and sustainable materials might become one of the leading alternative technologies especially suitable for large‐scale stationary storage. Various (mixed)phosphate framework materials are attracting much interest mainly due to their high structural stability and diversity. In this study, we report on the successful synthesis of mixed phosphate‐pyrophosphate Na7V4(PO4)(P2O7)4, Na4Fe3(PO4)2P2O7, and Na4Mn3(PO4)2P2O7. The electrochemical properties of these materials are comprehensively characterized in different organic and aqueous electrolytes. The findings reveal that Na7V4(PO4)(P2O7)4 and Na4Fe3(PO4)2P2O7 exhibit very good cycling performance and rate capability in organic solvent‐based electrolytes. However, their performance deteriorates significantly even in 'water‐in‐salt' aqueous electrolytes due to the rapid electrochemical degradation. Na4Mn3(PO4)2P2O7 demonstrates limited electrochemical activity in organic electrolytes and virtually no activity in 'water‐in‐salt' electrolytes, likely due to degradation processes resulting in blocking interphasial layers on electrode particles. These results underscore the need for further research to optimize the performance of these materials and identify potential strategies for enhancing their stability and activity in different electrolytes. [ABSTRACT FROM AUTHOR]

Due to their stability and structural freedom, NASICON-structured materials such as NaTi2(PO4)3 show a lot of promise as active electrode materials for aqueous batteries and deionization cells. However, due to their low intrinsic electronic conductivity, they must usually be composited with carbon to form suitable electrodes for power applications. In this work, two series of NaTi2(PO4)3–carbon composite structures were successfully prepared by different approaches: postsynthetic pyrolytic treatment of citric acid and surface polymerized dopamine. The latter route allows for a superior carbon loading control and yields more uniform and continuous particle coatings. The homogeneity of the polydopamine derived core–shell carbon layer is supported by FTIR, TEM, and XPS analysis. Combustion elemental analysis also indicates significant nitrogen doping in the final carbonaceous structure. The galvanostatic charge and discharge cycling results show similar initial capacities and their retention, but at only half of the carbon loading in polydopamine derived samples. The overall results indicate that careful nanostructure engineering could yield materials with superior properties and stability suitable for various electrochemical applications such as aqueous Na-ion batteries and deionization cells. [ABSTRACT FROM AUTHOR]

Aqueous Na-ion batteries are deemed to be among the most suitable candidates for future large-scale stationary energy storage systems. However, there are still a number of problems to be solved before their full potential can be utilized such as instability of the aqueous electrolyte/electrode interface towards electrolyte decomposition, chemical dissolution, and other side reactions. The mechanism of electrochemically induced degradation in NASICON-structured NaTi2(PO4)3 is studied in detail using various chemical and electrochemical techniques. The results unambiguously show that irreversible capacity fade, especially severe at low charging rates, is an electrochemically induced chemical dissolution of the active material. The oxygen reduction induced self-discharge reaction leading to a local pH increase is indicated as the main culprit for the material degradation and capacity loss during charge–discharge cycling. Although the degradation products form an insoluble Ti-rich interphasial layer similar to non-aqueous solid-electrolyte interphases, in this case it is shown to be insufficient for providing even kinetic stability of the electrochemical interface. Artificial protective layers applied using atomic layer deposition of Al2O3 directly on the electrode surface are shown to be a simple and scalable approach to significantly stabilize the electrodes towards oxygen attack and limit the rate of negative electrode degradation. The new understanding of irreversible degradation and its mitigation techniques presented in this work offer a pathway to viable and scalable strategies that enable and exploit the high-performance potential of this and many other aqueous battery materials. [ABSTRACT FROM AUTHOR]

Aqueous Na-ion based batteries are considered promising candidates for replacing Li-ion technologies, especially in the area of stationary energy storage. Research and development of novel electrode materials remain a central research effort in this field. NASICON-structured NaTi2(PO4)3 phosphate framework has attracted a great deal of attention and remains the most studied negative electrode material for aqueous Na-ion batteries. The active material preparation and carbon composite property tailoring play a critical role in the electrode materials engineering process which governs the performance of an entire electrochemical system. We present a study of a series of aqueous sol-gel synthesized NaTi2(PO4)3--carbon composites using three different solgel approaches and three distinct pyrolysis/calcination strategies. The structural and morphological analysis shows that both the sol-gel route as well as the thermal treatment strategy have significant effects on active material purity and degree of crystallinity as well as the content and properties of the carbonaceous phase. Electrochemical investigations of these composites also show strong effects of the materials structure and carbon matrix morphology on the electrochemical performance and degradation of these materials when employed as aqueous Na-ion battery negative electrodes. [ABSTRACT FROM AUTHOR]

Due to their stability and structural freedom, NASICON-structured materials such as NaTi 2 (PO 4 ) 3 show a lot of promise as active electrode materials for aqueous batteries and deionization cells. However, due to their low intrinsic electronic conductivity, they must usually be composited with carbon to form suitable electrodes for power applications. In this work, two series of NaTi 2 (PO 4 ) 3 -carbon composite structures were successfully prepared by different approaches: postsynthetic pyrolytic treatment of citric acid and surface polymerized dopamine. The latter route allows for a superior carbon loading control and yields more uniform and continuous particle coatings. The homogeneity of the polydopamine derived core-shell carbon layer is supported by FTIR, TEM, and XPS analysis. Combustion elemental analysis also indicates significant nitrogen doping in the final carbonaceous structure. The galvanostatic charge and discharge cycling results show similar initial capacities and their retention, but at only half of the carbon loading in polydopamine derived samples. The overall results indicate that careful nanostructure engineering could yield materials with superior properties and stability suitable for various electrochemical applications such as aqueous Na-ion batteries and deionization cells., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Aqueous Na-ion batteries are among the most discussed alternatives to the currently dominating Li-ion battery technology, in the area of stationary storage systems because of their sustainability, safety, stability, and environmental friendliness. The electrochemical properties such as ion insertion kinetics, practical capacity, cycling stability, or Coulombic efficiency are strongly dependent on the structure, morphology, and purity of an electrode material. The selection and optimization of materials synthesis route in many cases allows researchers to engineer materials with desired properties. In this work, we present a comprehensive study on size- and shape-controlled hydro(solvo)thermal synthesis of NaTi 2 (PO 4 ) 3 nanoparticles. The effects of different alcohol/water synthesis media on nanoparticle phase purity, morphology, and size distribution are analyzed. Water activity in the synthesis media of different alcohol solutions is identified as the key parameter governing the nanoparticle phase purity, size, and shape. The careful engineering of NaTi 2 (PO 4 ) 3 nanoparticle morphology allows control of the electrochemical performance and degradation of these materials as aqueous Na-ion battery electrodes., Competing Interests: The authors declare no competing financial interest., (© 2023 American Chemical Society.)

NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for "rocking chair" type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na 1+2 x Mn x Ti 2- x (PO 4 ) 3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition-temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na 3 MnTi(PO 4 ) 3 ( x = 1.0) and NaTi 2 (PO 4 ) 3 for x < 1.0 or NaMnPO 4 for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol-gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na 3 MnTi(PO 4 ) 3 (space group C 2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn-O bond as compared to Ti-O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)