A 3.2 V Binary Layered Oxide Cathode for Potassium-Ion Batteries.

Potassium-ion batteries (KIBs) can offer high energy density, cyclability, and operational safety while being economical due to the natural abundance of potassium. Utilizing graphite as an anode, suitable cathodes can realize full cells. Searching for potential cathodes, this work introduces P3-type K0.5Ni1/3Mn2/3O2 layered oxide as a potential candidate synthesized by a simple solid-state method. The material works as a 3.2 V cathode combining Ni redox at high voltage and Mn redox at low voltage and exhibits highly reversible K+ ion (de)insertion at ambient and elevated (40-50 °C) temperatures. First-principles calculations suggest the ground state in-plane Mn-Ni ordering in the MO2 sheets is strongly correlated to the K-content in the framework, leading to an interwoven and alternative row ordering of Ni-Mn in K0.5Ni1/3Mn2/3O2. Postmortem and electrochemical titration reveal the occurrence of a solid solution mechanism during K+ (de)insertion. The findings suggest that the Ni addition can effectively tune the electronic and structural properties of the cathode, leading to improved electrochemical performance. This work provides new insights in the quest to develop potential low-cost Co-free KIB cathodes for practical applications in stationary energy storage.

Role of Co Content on the Electrode Properties of P3-Type K0.5Mn1-xCoxO2 Potassium Insertion Materials.

Potassium-ion batteries are widely being pursued as potential candidates for stationary (grid) storage, where energy dense K+ insertion cathodes are central to economic and energy efficient operation. To develop robust K-based cathodes, it is key to correlate their underlying electronic states to the final electrochemical performance. Here, we report the synthesis and structure-electrochemical property correlation in P3-type K0.5Mn1-xCoxO2 binary layered oxide cathodes. Spectroscopic analyses revealed a random distribution of Mn and Co in transition metal layers in the oxygen anion framework. In this solid-solution family, Co substitution improved the electronic conductivity and structural stability of P3 phases by minimizing local lattice distortion. Co substitution led to a systematic shift of the Co4+/Co3+ and Mn4+/Mn3+ redox potentials. Galvanostatic cycling showed that the Co substitution reduced the initial capacity while improving the cycling stability. The role of Co on final electrochemical properties of P3-layered oxides has been elucidated as a design tool to develop practical potassium-ion batteries.

Evaluation of P3-Type Layered Oxides as K-Ion Battery Cathodes.

Given the increasing energy storage demands and limited natural resources of Li, K-ion batteries (KIBs) could be promising next-generation systems having natural abundance, similar chemistry, and energy density. Here, we have investigated the P3-type K0.5TMO2 (where TM = Ti, V, Cr, Mn, Co, or Ni) systems using density functional theory calculations as potential positive intercalation electrodes (or cathodes) for KIBs. Specifically, we have identified ground-state configurations and calculated the average topotactic voltages, electronic structures, on-site magnetic moments, and thermodynamic stabilities of all P3-K0.5TMO2 compositions and their corresponding depotassiated P3-TMO2 frameworks. Additionally, we evaluated the dynamic stability and K-mobility in select P3 structures. We find that K adopts the honeycomb or zig-zag configuration within each K-layer of all P3 structures considered, irrespective of the transition-metal (TM). In terms of voltages, we find the Co- and Ti-based compositions to exhibit the highest (4.59 V vs. K) and lowest (2.24 V) voltages, respectively, with the TM contributing to the redox behavior upon K (de-)intercalation. We observe all P3-K0.5TMO2 to be (meta)stable and hence experimentally synthesizable according to our 0 K convex hull calculations, while all depotassiated P3-TMO2 configurations are unstable and may appear during electrochemical cycling. Also, we verified the stability of the prismatic coordination environment of K compared to octahedral coordination at the K0.5TMO2 compositions using Rouxel and cationic potential models. Finally, combining our voltage and stability calculations, we find P3-KxCoO2 to be the most promising cathode composition, while P3-KxNiO2 is worth exploring. We also find P3-KxMnO2 to be worth pursuing given its dynamic stability and facile migration of K+ at both potassiated and depotassiated compositions. Our work should contribute to the exploration of strategies and materials required to make practical KIBs.

Zinc-Substituted Cobalt Phosphate [ZnCo2(PO4)2] as a Bifunctional Electrocatalyst.

Development of highly efficient, earth-abundant, and stable bifunctional electrocatalysts is pivotal for designing viable next-generation metal-air batteries. Cobalt-based phosphates provide a treasure house to design electrocatalysts, with a wide range of cation substitutions to further enhance their electrocatalytic activity. In particular, phosphates with distorted geometry show favorable binding efficiency toward water molecules with low overpotential. In the present work, zinc-substituted cobalt phosphate ZnCo2(PO4)2 was investigated. Its crystal structure was solved to a monoclinic framework built with CoO6 octahedra and distorted CoO5/ZnO5 trigonal bipyramid leading to efficient bifunctional electrocatalytic activity. It offers robust structural stability with onset potential values of 0.87 V (vs reversible hydrogen electrode (RHE)) and 1.50 V (vs RHE) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, respectively, comparable to the precious metal catalysts. The origin and stability of the bifunctional activity were probed by combining ex situ diffraction and electron microscopy corroborated by ab initio calculations. Overall, zinc-substituted cobalt phosphate [ZnCo2(PO4)2] forms a potential bifunctional electrocatalyst with tunable local cobalt coordination that can be harnessed for metal-air batteries.

Manganese-Based Tunnel-Type Cathode Materials for Secondary Li-Ion and K-Ion Batteries.

The rational design of novel cathode materials remains a key pursuit in the development of (post) Li-ion batteries. Considering the relative ionic and Stokes radii and open frameworks with large tunnels, Na-based compounds can act as versatile cathodes for monovalent Li-ion and post-Li-ion batteries. Here, tunnel-type sodium insertion material Na0.44MnO2 is demonstrated as an intercalation host for Li-ion and K-ion batteries. The rod-shaped Na0.44MnO2 was synthesized by a solution combustion method assuming an orthorhombic structure (space group Pbam), which led to Na0.11K0.27MnO2 (NKMO) and Na0.18Li0.51MnO2 (NLMO) cathodes for K-ion batteries and Li-ion batteries, respectively, via facile electrochemical ion exchange from Na0.44MnO2. These new compositions, NKMO and NLMO, exhibited capacities of ∼74 and 141 mAh g-1, respectively (at a rate of C/20), with excellent cycling stability. The underlying mechanistic aspects (structural changes and charge storage mechanism) in these cathode compositions were probed by combining ex situ structural, spectroscopy, and electrochemical tools. Tunnel-type Na0.44MnO2 forms a versatile cathode material for non-aqueous alkali-ion batteries.

First principles investigation of anionic redox in bisulfate lithium battery cathodes.

The search for an alternative high-voltage polyanionic cathode material for Li-ion batteries is vital to improve the energy densities beyond the state-of-the-art, where sulfate frameworks form an important class of high-voltage cathode materials due to the strong inductive effect of the S6+ ion. Here, we have investigated the mechanism of cationic and/or anionic redox in LixM(SO4)2 frameworks (M = Mn, Fe, Co, and Ni and 0 ≤ x ≤ 2) using density functional calculations. Specifically, we have used a combination of Hubbard U corrected strongly constrained and appropriately normed (SCAN+U) and generalized gradient approximation (GGA+U) functionals to explore the thermodynamic (polymorph stability), electrochemical (intercalation voltage), geometric (bond lengths), and electronic (band gaps, magnetic moments, charge populations, etc.) properties of the bisulfate frameworks considered. Importantly, we find that the anionic (cationic) redox process is dominant throughout delithiation in the Ni (Mn) bisulfate, as verified using our calculated projected density of states, bond lengths, and on-site magnetic moments. On the other hand, in Fe and Co bisulfates, cationic redox dominates the initial delithiation (1 ≤ x ≤ 2), while anionic redox dominates subsequent delithiation (0 ≤ x ≤ 2). In addition, evaluation of the crystal overlap Hamilton population reveals insignificant bonding between oxidized O atoms throughout the delithiation process in the Ni bisulfate, indicating robust battery performance that is resistant to irreversible oxygen evolution. Finally, we observe that both GGA+U and SCAN+U predictions are in qualitative agreement for the various properties predicted. Our work should open new avenues for exploring lattice oxygen redox in novel high voltage polyanionic cathodes, especially using the SCAN+U functional.

Cobalt Metaphosphates as Economic Bifunctional Electrocatalysts for Hybrid Sodium-Air Batteries.

Bifunctional electrocatalysts are pre-eminent to achieve high capacity, cycling stability, and high Coulombic efficiency for rechargeable hybrid sodium-air batteries. The current work introduces metaphosphate (Na)KCo(PO3)3 nanostructures as noble metal-free bifunctional electrocatalysts suitable for the rechargeable aqueous sodium-air battery. Prepared by the scalable solution combustion method, the metaphosphate class of (Na)KCo(PO3)3 with spherical morphology exhibited robust oxygen reduction as well as evolution activity similar to the state-of-the-art catalysts. NaCo(PO3)3 metaphosphate, when employed as an air cathode in hybrid sodium-air batteries, delivered reasonably low overpotential along with excellent cycling stability with a round-trip energy efficiency of 78%. Cobalt metaphosphates thus form a new class of economical bifunctional catalysts to develop hybrid sodium-air batteries.

Crystal and Magnetic Structures of Monoclinic FeOHSO4.

The crystal and magnetic structures and properties of the monoclinic form of the iron hydroxysulfate FeOHSO4 were investigated by magnetometry and neutron powder diffraction. The space group C2/c was confirmed, and the proton position was located close to that predicted by ab initio calculations. The collinear antiferromagnetic k(0,0,0) structure forming below the Néel temperature TN ∼ 125 K is described by the C2'/c' (No. 15.89) magnetic space group, with the moments along the b axis. Overall, FeOHSO4 is isostructural to FeSO4F in terms of both the crystal and magnetic structures.

An overview of hydroxy-based polyanionic cathode insertion materials for metal-ion batteries.

Rechargeable batteries based on Li-ion and post Li-ion chemistry have come a long way since their inception in the early 1980s. The last four decades have witnessed steady development and discovery of myriads of cathode materials taking into account their processing, economy, and performance along with ecological sustainability. Though oxides rule the battery sector with their high energy and power density, polyanionic insertion compounds work as gold mines for designing insertion compounds with rich structural diversity leading to tuneable redox potential coupled with high structural/chemical/thermal stability. The scope of polyanionic compounds can be taken a step further by combining two or more different types of polyanions to get suites of mixed polyanionic materials. While most cathodes are built with metal polyhedra constituted by oxygen (MOm|XOm, M = 3d metals, X = P, S, Si, B, W, etc., m = 3-6), in some cases, selected oxygen sites can form bonding with hydrogen to form OH/H2O ligands. It can lead to the family of hydroxy-based mixed-polyanionic cathode materials. The presence of hydroxy components can affect the crystal structure, local chemical bonding, and electronic, magnetic, diffusivity and electrochemical properties. Employing a mineralogical survey, the current review renders a sneak peek on various hydroxy-based polyanionic cathode materials for Li-ion and post Li-ion batteries. Their crystal structure, and electrochemical properties have been overviewed to outline future research focus and scope for real-life application.

Polymorphism and Temperature-Induced Phase Transitions of Na2CoP2O7.

Polymorphism and temperature-induced phase transitions of Na2CoP2O7 were studied by in situ neutron powder diffraction and complemented by ab initio calculations to reconcile previous reports of its three polymorphs. We show that the "blue" form prepared at 873 K exists at room temperature in the orthorhombic Pna21 (= P21cn) phase, which transforms via a first-order transition to the tetragonal form at the temperature close to room temperature (∼335 K). Just above the transition, the tetragonal form is likely incommensurately modulated with the modulation vanishing at ∼423 K. Above that temperature the phase remains in the unmodulated tetragonal state (P42/mnm) until melting at ∼900 K. Upon cooling after melting, Na2CoP2O7 crystallizes into the "rose" triclinic P1 form which persists while it cools to room temperature, apparently stabilized by the barrier of the reconstructive "rose"-"blue" transition. We also discuss the relationship between the tetragonal and orthorhombic structures, the driving forces of the orthorhombic distortion, and similarity to Na2ZnP2O7 and the melilite-type structural family.