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The emergence of K‐ion batteries (KIBs) heralds a promising frontier in energy storage technology, offering the potential for high specific energy density, long cycle life, and robust power capabilities, all while utilizing the abundant resources of potassium. In response to the challenges posed by synthetic intricacies related to K‐based cathodes, the effort is directed toward employing soft chemistry (chimie douce) method to unveil a hitherto‐unknown P2‐type K1/3Co1/3Mn2/3O2 (KCM) layered oxide cathode for KIBs. Comprehensive analysis using diffraction, microscopy, and spectroscopy tools reveals the ion exchange reaction proceeds through overlay ordered structure formation mechanism. The as‐prepared KCM material serves as a ≈2.9 V positive K+ insertion host. Further, it showcases an exceptional structural reversibility, robust cycling performance with ≈100% coulombic efficiency even after 100 cycles, and maintaining electrochemical stability even at elevated temperature (c.a. 40° and 50 °C). The KCM cathode exhibits in‐plane Co–Mn ordering and solid‐solution redox mechanism during (de)potassiation. Combining first‐principles calculations with experimental tools, this research demonstrates the efficacy of ambient ion‐exchange route in stabilizing promising cathode materials for KIBs. This innovative synthetic approach not only streamlines synthetic complexities, but also holds significant implications for the advancement of KIB technology for stationary energy storage. [ABSTRACT FROM AUTHOR]

To improve the cycle life of sodium‐ion batteries, it is essential to understand the microscopic processes that lead to cell degradation. The mismatched response time of anode and cathode has profound but poorly understood impact on cycle life. In this work, we combine electrochemical and materials characterization along with electrochemical modeling to investigate the root cause of degradation in sodium‐ion full cells made from Na4Fe3(PO4)2P2O7 (NFPP) cathodes and hard carbon (HC) anode. Our results pinpoint to the slow diffusion of Na in HC as the main cause of diffusional polarization that leads to cathode experiencing high local potentials and ultimately to active material loss over cycling. We demonstrate that by reducing the anode particle size, the diffusional timescales in anode can be matched with that of cathode to improve both extractable capacity as well as cycle life. These observations shed light on non‐intuitive and intricate ways in which cathode and anode can interact with each other to cause degradation in Na‐ion batteries and how microscopic understanding of these cause and effects can help design long lasting batteries. [ABSTRACT FROM AUTHOR]

Aqueous zinc-ion batteries form a key post-Li-ion batteries to cater the rising demand for grid storage. Fe-based compounds can be used as economical cathodes for zinc-ion batteries. Herein, we explored iron-based flourophosphate as a potential polyanionic cathode. Involving the Fe3+/2+ redox process, it can reversibly intercalate Zn2+ yielding a capacity of ∼80 mA h g−1, involving a solid-solution mechanism. Polyanionic Fe-based phosphate frameworks can be harnessed as potential low-cost cathodes for secondary zinc-ion batteries. [ABSTRACT FROM AUTHOR]

Cost, safety, and cycle life have emerged as prime concerns to build robust batteries to cater to the global energy demand. These concerns are impacted by all battery components, but the realizable energy density of lithium‐ion batteries (LIBs) is limited by the performance of cathodes. Thus, cathode materials have a significant role to play in advancing the performance and economics of secondary batteries. To realize next generation Li‐ion and post Li‐ion batteries, a variety of cathode insertion materials have been explored, but finding a cost effective and stable cathode material that can deliver high energy density has been a daunting task. Oxide cathode materials are ubiquitous in commercial applications, as they can deliver high capacity. In comparison, polyanionic insertion materials can offer tuneable (high) redox potential, operational safety, and structural as well as thermal stability. Indeed, a wide range of polyanionic materials like phosphates, borates, sulfates, and their complexes have been reported. In this article, the alkali metal fluorophosphates class of polyanionic cathodes for secondary batteries is discussed. The various reported fluorophosphate insertion materials are discussed in terms of their electrochemical and electrocatalytic properties. The historical overview, recent progress, and remaining challenges for polyanionic fluorophosphates are presented along with suggested future research directions and potential application. [ABSTRACT FROM AUTHOR]

Rechargeable batteries have emerged as ubiquitous and indispensable technologies of the 21st century, propelling myriads of consumer electronics and ushering a new era of electric vehicles and stationary grid storage. Since the commercialization of Li‐ion batteries by SONY (approximately in the year 1991), the secondary battery sector has seen unprecedented growth with diverse applications touching global populations across socioeconomic strata. There is a steady quest to develop a wide range of batteries to cater diverse global demands from milliwatt‐scale electronics to megawatt‐scale grid‐storage applications. In this journey, Li‐ion batteries are complemented by various post‐Li‐ion chemistry (e.g., Na+, K+, Mg2+, Ca2+, and Al3+‐ion batteries), conversion mechanism‐based systems (e.g., Li–S, Na–S, and Li–O2) as well as renewed development of pre‐Li‐ion era technologies like aqueous batteries. Cathodes sit at the core with command over the net cost and performance of batteries. Various polyanionic cathodes have been developed to date, often guided by structure of naturally occurring minerals. One such mineral system is alluaudite, named after French geologist François Alluaud. The current article portrays the discovery and development of the alluaudite class of polyanionic cathode materials for rechargeable batteries. The structure and electrochemical properties of various alluaudite insertion materials are gauged along with possible future perspectives. [ABSTRACT FROM AUTHOR]

P3-type layered K0.48Mn0.4Co0.6O2 was synthesized using a solid-state method. By stabilising into a rhombohedral structure [s.g. R3m (#160)], it delivers a reversible capacity of 64 mA h g−1 with a nominal voltage of ∼3.0 V (vs. K/K+) and it has good cycling stability. It involves a solid-solution redox mechanism, and forms an economical and stable oxide insertion material for potassium-ion batteries. [ABSTRACT FROM AUTHOR]

Exploring stable and economic cathodes for aqueous sodium‐ion batteries, we have investigated iron‐based fluorophosphate (Na2FePO4F) as a compound for applications in aqueous battery systems. Solution combustion synthesized Na2FePO4F is found to act as an efficient cathode for aqueous Na‐ion batteries delivering a reversible capacity over 85 mAh g−1 (at a rate of 1 mA cm−2) with excellent rate kinetics. Further, this layered Na2FePO4F has been successfully implemented to design a full aqueous cell with NASICON‐type NaTi2(PO4)3 as anode, delivering a reversible capacity of 90 mAh g−1. In conclusion, Na2FePO4F is proposed as a potential cathode material for aqueous Na‐ion batteries. The iron‐based fluorophosphate Na2FePO4F is explored as cathode material for aqueous sodium‐ion batteries. The material showed promising performance in Na‐ion half‐ and full‐cell configuration with excellent reversible capacity and rate kinetics. [ABSTRACT FROM AUTHOR]

Abstract: Efficient energy storage is a driving factor propelling myriads of mobile electronics, electric vehicles and stationary electric grid storage. Li‐ion batteries have realized these goals in a commercially viable manner with ever increasing penetration to different technology sectors across the globe. While these electronic devices are more evident and appealing to consumers, there has been a growing concern for micro‐to‐mega grid storage systems. Overall, the modern world demands energy in ‘terawatt’ scale. It needs a multipronged approach with alternate technologies complementing the Li‐ion batteries. One such viable approach is to design and implement Na‐ion batteries. With the uniform geographical distribution, abundance and materials economy of Na resources as well as a striking operational similarity to Li‐ion batteries, Na‐ion batteries have commercial potential, particularly for applications unrestricted by volumetric/gravimetric energy density. In pursuit of the development of Na‐ion batteries, suites of oxides, sulfides, fluorides, and polyanionic materials have been reported in addition to several organic complexes. This article gives an overview of recent progress in polyanionic framework compounds, with emphasis on high‐voltage candidates consisting of earth abundant elements. Guided by ternary phase diagrams, recently discovered and potential cathode candidates will be discussed gauging their performance, current status, and future perspectives. [ABSTRACT FROM AUTHOR]

Reversible electrochemical potassium-ion intercalation in P2-type NaxCoO2 was examined for the first time. Hexagonal Na0.84CoO2 platelets prepared by a solution combustion synthesis technique were found to work as an efficient host for K+ intercalation. They deliver a high reversible capacity of 82 mA h g−1, good rate capability and excellent cycling performance up to 50 cycles. [ABSTRACT FROM AUTHOR]

Rechargeable batteries have been the torchbearer electrochemical energy storage devices empowering small-scale electronic gadgets to large-scale grid storage. Complementing the lithium-ion technology, sodium-ion batteries have emerged as viable economic alternatives in applications unrestricted by volume/weight. What is the best performance limit for new-age Na-ion batteries? This mission has unravelled suites of oxides and polyanionic positive insertion (cathode) compounds in the quest to realize high energy density. Economically and ecologically, iron-based cathodes are ideal for mass-scale dissemination of sodium batteries. This Perspective captures the progress of Fe-containing earth-abundant sodium battery cathodes with two best examples: (i) an oxide system delivering the highest capacity (∼200 mA h/g) and (ii) a polyanionic system showing the highest redox potential (3.8 V). Both develop very high energy density with commercial promise for large-scale applications. Here, the structural and electrochemical properties of these two cathodes are compared and contrasted to describe two alternate strategies to achieve the same goal, i.e., improved energy density in Fe-based sodium battery cathodes. [ABSTRACT FROM AUTHOR]

Pyrophosphate cathodes have been recently reported as a competent family of insertion compounds for sodium-ion batteries. In the current study, we have investigated the binary Na 2 (Fe 1 − y Mn y )P 2 O 7 (0 ≤ y ≤ 1) pyrophosphate family, synthesized by the classical solid-state method. They form a continuous solid solution maintaining triclinic P -1 (#2) symmetry. The local structural coordination differs mainly by different degrees of Na site occupancy and preferential occupation of the Fe2 site by Mn. The structural and magnetic properties of these mixed-metal pyrophosphate phases have been studied. In each case, complete Fe 3 + /Fe 2 + redox activity has been obtained centered at 3 V vs. Na. The Fe 3 + /Fe 2 + redox process involves multiple steps between 2.5 and 3 V owing to Na-cation ordering during electrochemical cycling, which merge to form a broad single Fe 3 + /Fe 2 + redox peak upon progressive Mn-doping. [ABSTRACT FROM AUTHOR]

Vying for newer sodium-ion chemistryfor rechargeable batteries,Na2FeP2O7pyrophosphate has beenrecently unveiled as a 3 V high-rate cathode. In addition to its lowcost and promising electrochemical performance, here we demonstrateNa2FeP2O7as a safe cathode withhigh thermal stability. Chemical/electrochemical desodiation of thisinsertion compound has led to the discovery of a new polymorph ofNaFeP2O7. High-temperature analyses of the desodiatedstate NaFeP2O7show an irreversible phase transitionfrom triclinic (P1̅) to the ground state monoclinic(P21/c) polymorph above560 °C. It demonstrates high thermal stability, with no thermaldecomposition and/or oxygen evolution until 600 °C, the upperlimit of the present investigation. This high operational stabilityis rooted in the stable pyrophosphate (P2O7)4–anion, which offers better safety than other phosphate-basedcathodes. It establishes Na2FeP2O7as a safe cathode candidate for large-scale economic sodium-ionbattery applications. [ABSTRACT FROM AUTHOR]

Aspiring for better cathodes beyond the LiFePO4 system, recently LiFeBO3 has been reported as a high-capacity (ca. 220 mAh/g) intercalation material involving the lowest reported volume change (ca. 2%) during its operation. It was obtained by conventional solid-state synthesis at 600°C for 12 h and careful cathode optimization. Pursuing an alternate energy-savvy synthetic route, here we report a high-throughput aqueous synthesis forming one-step carbon-coated nanoscale LiFeIIBO3 product, starting with the low-cost FeIII precursor. Known as solution combustion synthesis, this two-step synthetic method can yield the LiFeIIBO3 phase by a quick annealing at 400-600°C for just ≤1 minute. The resulting borate cathode delivers near 1-electron discharge capacity in excess of 170 mAh/g with excellent reversibility, the first such achievement using any solution based synthesis. Various synthesis aspects, physical and electrochemical properties of combustion prepared LiFeIIBO3 cathode have been presented, showcasing the versatility of this eco-efficient method to produce other metal borates and various polyanionic insertion materials for secondary batteries. [ABSTRACT FROM AUTHOR]

The crystal and magnetic structure and properties of the Na2CoP207 Na+-ion battery cathode material have been characterized by magnetic susceptibility, specific heat, and variable-temperature neutron powder diffraction measurements. Na2CoP207 crystallizes in the orthorhombic space group Pno21 with a = 15.4061(3) Å, b = 10.28854(9) Å, and c = 7.70316(15) Å, having a layered structure with slabs of [CoP207]∞ separated by Na cations. The magnetic property measurements and neutron diffraction data analysis reveal that the material undergoes long-range ordering to a noncollinear antiferromagnetic G-type structure below TN ≈ 6.5 K. The magnetic structure is rationalized as a result of sup ersup exexchan ge between Co2+ atoms linked by phosphate groups. [ABSTRACT FROM AUTHOR]

Abstract: Extending the pyrophosphate chemistry for rechargeable Na-ion batteries, here we report the synthesis and electrochemical characterization of Na2FeP2O7, a novel Fe-based cathode material for sodium batteries. Prepared by conventional solid-state as well as solution-combustion synthesis (at 600°C), the Na2FeP2O7 adopts a triclinic structure (space group: P-1) with three-dimensional channels running along [100], [−110] and [01-1] directions. With no further optimization, the as-synthesized Na2FeP2O7 cathode was found to be electrochemically active, delivering a reversible capacity of 82mAh·g−1 with the Fe3+/Fe2+ redox potential centered around 3V (vs. Na/Na+). With its theoretical capacity of ~100mAh·g−1 and potential high rate-capability, Na2FeP2O7 forms a promising novel cathode material, with the distinction of being the first ever pyrophosphate-class of cathode for sodium-ion batteries. [Copyright &y& Elsevier]

The cathode plays a key role in sodium‐ion batteries leading to the exploration of layered and three‐dimensional framework insertion materials. While oxides deliver high capacity, polyanionic hosts offer structural and operational stability and tunable redox potentials. They can be further exploited in 'mixed polyanion' cathodes combining more than one kind of polyanion. In article number 1800253 Baskar Senthilkumar, Prabeer Barpanda and co‐workers delve into the world of mixed polyanionic cathode materials for sodium‐ion batteries. [ABSTRACT FROM AUTHOR]

"Building better batteries" remains an ongoing process to cater diverse energy demands starting from small‐scale consumer electronics to large‐scale automobiles and grid storage. While Li‐ion batteries have carried this burden over the last three decades, the ever‐growing and highly diverse applications (based on size, energy‐density, and stationary vs mobile usages) have led to an era of "beyond lithium‐ion batteries." In this postlithium‐battery era, sodium‐ion batteries (NIBs) have emerged as a pragmatic option particularly for large‐scale applications. They attract attention due to the abundance and uniform geographic distribution of sodium‐based minerals, materials/process economy, and well‐known (de)intercalation mechanisms, in particular for stationary applications independent of size/weight restriction. Parallel to the Li‐ion batteries, the cathode (positive electrode) plays a key role in overall performance, leading to the exploration of various layered and 3D framework insertion materials. While layered oxides deliver high capacity, polyanionic hosts offer structural stability, operational safety, and tunable redox potentials. It can be further exploited in "mixed polyanion" cathode materials combining more than one kind of polyanion units. This article focuses on mixed polyanionic cathode materials for NIBs. It renders a sneak‐peek on suites of mixed polyanionic insertion materials discussing their structure, overall electrochemical performance, and future perspectives. The polyanionic family of battery electrodes offers structural stability, operational safety, and tunable redox potentials. Going a step further, "mixed polyanionic cathode materials" form a niche sector combining more than one kind of polyanion units. This article renders a sneak‐peek on suites of mixed polyanionic cathode materials for sodium‐ion batteries discussing their structure, overall electrochemical performance, and future perspectives. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A novel P3-type layered K 0.45 Mn 0.5 Co 0.5 O 2 has been synthesized using conventional sol-gel method. • It exhibits excellent cyclic stability with 80% capacity retention up to 50 cycles. • When coupled with commercial AC, the KIC could provide a very high energy and power density of 43 Wh kg−1 and 30 kW kg−1. • The KIC retains 88% of its energy density with an ultrahigh stability of up to 30,000 cycles at 10 A g−1. Abstract Herein, we demonstrate a new non-aqueous potassium-ion hybrid capacitor (KIC) using novel P3-K 0.45 Mn 0.5 Co 0.5 O 2 and commercial activated carbon (CAC) as the cathode and anode, respectively. A simple sol–gel method is used to synthesize the P3-K 0.45 Mn 0.5 Co 0.5 O 2 cathode nanoplatelets. The structural and morphological studies are performed using various characterization techniques, and their electrochemical performances are studied in half-cell configurations against metallic K. The P3-K 0.45 Mn 0.5 Co 0.5 O 2 nanoplatelets can reversibly host K+ ions delivering a high capacity of 140 mAh g−1 in the wide voltage window of 1.2–3.9 V. Exhibiting smooth voltage profiles, it offers reasonable rate capability and cyclability, retaining over 80% capacity after 50 cycles. Involving a two-phase (P3–O3) redox mechanism, P3-K 0.45 Mn 0.5 Co 0.5 O 2 forms robust cathode material for potassium-ion batteries. With Activated Carbon, the capacitor could provide very high energy and power densities of 43 Wh kg−1 and 30 kW kg−1, respectively, in the voltage range of 0–3.0 V. Even at a 3-s charge–discharge rate (10 A g−1), an energy density of 14.5 Wh kg−1 could be retained (corresponding to a power of 15 kW kg−1). Also it could retain 88% of its energy density with a substantially high stability up to 30,000 cycles at 10 A g−1. [ABSTRACT FROM AUTHOR]

Review: structural and electrochemical properties of two types of cathodes, i.e., an oxide system delivering the highest capacity and a polyanionic system having the highest redox potential; 25 refs. [ABSTRACT FROM AUTHOR]

Mixed-metal Li2MxFe1-xP2O7 (M: Mn, Co, Mg) cathode materials for lithium ion batteries with a monoclinic structure (space group P21/c) exhibit unusually high Fe3+/Fe2+ redox potentials close to 4.0 V vs Li/Li+. [ABSTRACT FROM AUTHOR]

NaFeP2O7, the charged state of the title compound, shows high thermal stability up to at least 600 °C with no thermal decomposition and/or oxygen evolution. [ABSTRACT FROM AUTHOR]

Li2FeP2O7pyrophosphateis thelatest phosphate-based polyanionic cathode material operating at 3.5V (vs Liﲸ). Capable of two-dimensional Li+ion diffusion,the pyrophosphate has a complex three-dimensional crystal structure,rich in Li–Fe antisite defects. The electrochemical (de)lithiationof pristine Li2FeP2O7involves permanentstructural rearrangement, as reflected by the voltage drop betweenthe first and subsequent charging segments. The current article presentsthe structural analysis of the electrochemical redox mechanism ofLi2FeP2O7cathode coupling in situand ex-situstructural characterization.Contrary to previous reports, it involves a single-phase redox reactionduring (de)lithiation cycles involving a minimal <2% volume expansion.Further, it forms a rare example of cathode showing positive expansionupon delithiation similar to LiCoO2. The mechanism of single-phase(de)lithiation and related (ir)reversible structural arrangement iselucidated. [ABSTRACT FROM AUTHOR]

Common preparation methods of manganese-based Li-rich layered oxides include co-precipitation, combustion, spray pyrolysis, and molten salt synthesis. Herein, we present a facile new reactive-templating route to prepare manganese-based lithium-rich Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 using β−MnO 2 nanorod, which plays a dual role as reactive template as well as Mn-source. Rietveld refinement and electron diffraction patterns confirm the formation of phase pure, highly crystalline Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 with two integrated layered components of 0.5(Li 2 MnO 3).0.5(LiMn 1/3 Ni 1/3 Co 1/3 O 2). Electron microscopy studies reveal the formation of anisotropic rod-like crystals of 0.8–1.0 μm in length and ∼200 nm in thickness. The Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 nanorod as cathode for Li-ion battery, delivers an impressive reversible capacity of 223 mAh.g−1 at 0.1C rate after 150 cycles and 161 mAh.g−1 at 1 C rate after 300 cycles. Rapid structural transition from layered to spinel-like phase at high temperature (55 °C) leads to gradual decay in discharge capacity upon cycling, whereas low Li-ion diffusivity, cell resistance, and high viscosity hampers the performance at low temperature (5 °C). Impedance spectroscopy along with (dis)charge differential plots corroborate that activation of Li 2 MnO 3 and Li-ion de(intercalation) into the MnO 2 phase is very facile at high temperature (55 °C), which leads to high specific capacity, high coulombic efficiency, and high-power capability. Herein, we demonstrate a facile solid-state template conversion strategy for the synthesis of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 nanorods, and was studied as cathode for Li ion batteries. The influence of testing temperature on the structural and electrochemical behavior of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 nanorods were analysed. Image 1 • Facile reactive template synthesis of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 nanorods using β-MnO 2 nanorods without any stabilizing or structure directing agents. • The Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 nanorods were 0.8–1.0 μm in length and 200 nm in width. • Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 delivered an impressive reversible capacity of 223 mAh.g−1 at 0.1C rate after 150 cycles and 161 mAh.g−1 at 1 C rate after 300 cycles. • The significant capacity decay observed at 55 °C is attributed mostly to layered to spinel-like structural changes. • The low capacity realized at 5 °C was related to high cell resistance and low Li+ diffusion kinetics. [ABSTRACT FROM AUTHOR]

Transition metal pyrophosphate material forms a robust polyanionic cathode family for sodium-ion batteries. Here, binary Na 2 (Fe 1-y Mn y)P 2 O 7 (0 ≤ y ≤ 1) system has been recently investigated by different groups, as Na 2 FeP 2 O 7 is reported as a low-cost cathode with promising electrochemical performance and thermal stability. While the isostructural Na 2 FeP 2 O 7 and Na 2 MnP 2 O 7 assume triclinic P 1 ‾ (#2) framework, pyrophosphate system shows structural diversity/polymorphism. Considering this, we have investigated the binary Na 2 (Fe 1-x Zn x)P 2 O 7 (0 ≤ x ≤ 1) pyrophosphate family with anisostructural end members Na 2 FeP 2 O 7 (P 1 ‾ , #2) and Na 2 ZnP 2 O 7 (P 4 2 / n , #86). The current study reports solution combustion as well as solid-state preparation of novel Na 2 (Fe 1-x Zn x)P 2 O 7 (0 ≤ x ≤ 1) family of materials, their structural and electrochemical characterizations. The degree of solid-solution formation and effect of Zn on Fe-redox activity in Na 2 (Fe 1-x Zn x)P 2 O 7 (x = 0, 0.25) cathodes has been examined using electrochemical titration techniques such as galvanostatic intermittent titration (GITT) and potentiostatic intermittent titration (PITT) mode. The binary Na 2 FeP 2 O 7 and Na 2 ZnP 2 O 7 pyrophosphate system has been explored to gauge their structure and electrochemical properties. Image 1 [ABSTRACT FROM AUTHOR]

Abstract The energy-demanding nature of solid-state synthesis can be circumvented by using solvothermal synthesis routes involving milder heat treatment steps. Herein, we exploit one such route, ultrasonic sonochemical method, for the synthesis of Na 0.44 MnO 2 insertion material. Using simple oxide and carbonate precursors, the target compound can be successfully prepared by restricting the final annealing (at 900 °C) to within 2 h. Rietveld analysis confirms the purity of final product assuming an orthorhombic framework (s.g. Pbam). Involving a step-wise voltage profile with the average Mn4+/Mn3+ redox potential ∼3 V (vs. Na/Na+), it delivers reversible capacity over 110 mAh.g−1 at a rate of C/10 with decent rate kinetics and cycling stability. Ultrasonic sonochemical synthesis can be employed for synthesis and rapid screening of variety of insertion materials for sodium-ion batteries. Highlights • Ultrasonic sonochemical synthesis Na 0.44 MnO 2 Na-insertion material. • Quick synthesis involving restricted annealing duration of just 1–2 h. • Homogeneous morphology having reversible Mn4+/Mn3+ activity at 3 V. • Reversible capacity exceeding 110 mAh g−1 with good cycling stability. [ABSTRACT FROM AUTHOR]

Potassium-ion batteries (KIBs) can offer high-voltage performance and energy density similar to lithium-ion batteries with the added advantages of elemental abundance, materials economy, and efficient K+ intercalation due to smaller radius of solvated ions. This nascent field offers ample room to exploit open framework polyanionic compounds as efficient cathode materials. Due to similar ionic size, Na-based compounds can be employed as suitable cathodes for KIBs. In this work, iron-based mixed phosphate Na 4 Fe 3 (PO 4) 2 P 2 O 7 is demonstrated as a robust 3.0 V cathode for potassium-ion batteries. The in-situ carbon coated nanoscale Na 4 Fe 3 (PO 4) 2 P 2 O 7 cathode delivers a discharge capacity of ~120 mAh g−1 (i.e., 94% of its theoretical capacity) with excellent capacity retention and rate kinetics. With its three-dimensional open framework having multiple alkali sites, Na 4-x Fe 3 (PO 4) 2 P 2 O 7 undergoes a solid-solution Fe3+/Fe2+ redox mechanism acting as an efficient host for K+ (de)insertion. It forms the best Fe-based phospho-polyanionic cathode for potassium-ion batteries. A full cell comprising Na 4-x Fe 3 (PO 4) 2 P 2 O 7 cathode and graphite anode demonstrates the potential future application in KIBs. It marks the first demonstration of Na-based mixed polyanionic phosphates as insertion hosts for KIBs, which can be extended to various polyanionic insertion materials. Image 1 • Na-based mixed polyanion as robust intercalation host for K+-ions. • Reversible K+-ion intercalation in NASICON-type NaFe 3 (PO 4) 2 P 2 O 7 was demonstrated. • In-situ carbon coated Na 4 Fe 3 (PO 4) 2 P 2 O 7 delivered a capacity of ~118 mAh g−1 (~3e−). • Presence of 3D K-ion intercalating pathways and multiple K-ion sites in the structure. • Demonstration of full cell KIBs with NaK 3 Fe 3 (PO 4) 2 P 2 O 7 cathode and graphite anode. [ABSTRACT FROM AUTHOR]