Role of Co Content on the Electrode Properties of P3-Type K_0.5Mn_1–<i>x</i>Co_<i>x</i>O₂ 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

Probing Capacity Trends in MLi₂Ti₆O₁₄ Lithium-Ion Battery Anodes Using Calorimetric Studies

Due to higher packing density, lower working potential, and area specific impedance, the MLi2Ti6O14 (M = 2Na, Sr, Ba, and Pb) titanate family is a potential alternative to zero-strain Li4Ti5O12 anodes used commercially in Li-ion batteries. However, the exact lithiation mechanism in these compounds remains unclear. Despite its structural similarity, MLi2Ti6O14 behaves differently depending on charge and size of the metal ion, hosting 1.3, 2.7, 2.9, and 4.4 Li per formula unit, giving charge capacity values from 60 to 160 mAh/g in contrast to the theoretical capacity trend. However, high-temperature oxide melt solution calorimetry measurements confirm strong correlation between thermodynamic stability and the observed capacity. The main factors controlling energetics are strong acid–base interactions between basic oxides MO, Li2O and acidic TiO2, size of the cation, and compressive strain. Accordingly, the energetic stability diminishes in the order Na2Li2Ti6O14 > BaLi2Ti6O14 > SrLi2Ti6O14 > PbLi2Ti6O14. This sequence is similar to that in many other oxide systems. This work exhibits that thermodynamic systematics can serve as guidelines for the choice of composition for building better batteries

Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batteries

Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batterie

Electrochemical Redox Mechanism in 3.5 V Li_2‑<i>x</i>FeP₂O₇ (0 ≤ <i>x</i> ≤ 1) Pyrophosphate Cathode

Li₂FeP₂O₇ pyrophosphate is the latest phosphate-based polyanionic cathode material operating at 3.5 V (vs Li+/Li). 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)­lithiation of pristine Li₂FeP₂O₇ involves permanent structural rearrangement, as reflected by the voltage drop between the first and subsequent charging segments. The current article presents the structural analysis of the electrochemical redox mechanism of Li₂FeP₂O₇ cathode coupling <i>in situ</i> and <i>ex-situ</i> structural characterization. Contrary to previous reports, it involves a single-phase redox reaction during (de)­lithiation cycles involving a minimal <2% volume expansion. Further, it forms a rare example of cathode showing positive expansion upon delithiation similar to LiCoO₂. The mechanism of single-phase (de)­lithiation and related (ir)­reversible structural arrangement is elucidated

Fe³⁺/Fe²⁺ Redox Couple Approaching 4 V in Li_2–<i>x</i>(Fe_1–<i>y</i>Mn_<i>y</i>)P₂O₇ Pyrophosphate Cathodes

Li-metal pyrophosphates have been recently reported as novel polyanionic cathode materials with competent electrochemical properties. The current study presents a detailed analysis of inherent electrochemical properties of mixed-metal pyrophosphates, Li₂(Fe_1–<i>y</i>Mn_<i>y</i>)­P₂O₇, synthesized by an optimized solid-state route. They form a complete solid solution assuming a monoclinic framework with space group <i>P</i>2₁/<i>c</i>. The electrochemical analysis of these single-phase pyrophosphates shows absence of activity associated with Mn, where near-theoretical redox activity associated with Fe metal center was realized around 3.5 V. We noticed a closer look revealed the gradual substitution of Mn into parent Li₂FeP₂O₇ phase triggered a splitting of Fe³⁺/Fe²⁺ redox peak and partial upshifting in Fe³⁺/Fe²⁺ redox potentials nearing 4.0 V. Introduction of Mn into the pyrophosphate structure may stabilize the two distinct Fe³⁺/Fe²⁺ redox reactions by Fe ions in octahedral and trigonal-bipyramidal sites. Increase of the Gibb’s free energy at charged state by introducing Li⁺–Fe³⁺ and/or Li vacancy–Mn²⁺ pairs can be the root cause behind redox upshift. The underlying electrochemical behavior has been examined to assess these mixed-metal pyrophosphates for usage in Li-ion batteries

General Observation of Fe³⁺/Fe²⁺ Redox Couple Close to 4 V in Partially Substituted Li₂FeP₂O₇ Pyrophosphate Solid-Solution Cathodes

Exploring the newly unveiled Li₂<i>M</i>P₂O₇ pyrophosphate cathode materials for lithium-ion batteries, the current study reports the general observation of an unusually high Fe³⁺/Fe²⁺ redox potential close to 4.0 V vs Li/Li⁺ in mixed-metal Li₂<i>M</i>_<i>x</i>Fe_1–<i>x</i>P₂O₇ (<i>M</i> = Mn, Co, Mg) phases with a monoclinic structure (space group <i>P</i>2₁/<i>c</i>). Such a high voltage Fe³⁺/Fe²⁺ operation over 3.5 V has long been believed to be possible only by the existence of much more electronegative but hygroscopic anions such as SO₄^2– or F^–. Thereby, this is the first universal confirmation of >3.5 V operation by stable, simple phosphate material. High voltage (close to 4 V) operation of the Fe³⁺/Fe²⁺ couple was stabilized by all dopants, either by larger Mn²⁺ or smaller Co²⁺ and Mg²⁺ ions, where Mg²⁺ is redox inactive, revealing that the high voltage is induced neither by reduced Fe–O bond covalency nor by contamination by the redox couple of other transition metals. The cause of higher Fe³⁺/Fe²⁺ redox potential is argued and rooted in the stabilized edge-sharing local structural arrangement and the associated larger Gibbs free energy in the charged state

Magnetic Structure and Properties of the Rechargeable Battery Insertion Compound Na₂FePO₄F

The magnetic structure and properties of sodium iron fluorophosphate Na₂FePO₄F (space group <i>Pbcn</i>), a cathode material for rechargeable batteries, were studied using magnetometry and neutron powder diffraction. The material, which can be described as a quasi-layered structure with zigzag Fe-octahedral chains, develops a long-range antiferromagnetic order below ∼3.4 K. The magnetic structure is rationalized as a super-exchange-driven ferromagnetic ordering of chains running along the <i>a</i>-axis, coupled antiferromagnetically by super-super-exchange via phosphate groups along the <i>c</i>-axis, with ordering along the <i>b</i>-axis likely due to the contribution of dipole–dipole interactions

Magnetic Structures of NaFePO₄ Maricite and Triphylite Polymorphs for Sodium-Ion Batteries

The magnetic structure and properties of polycrystalline NaFePO₄ polymorphs, maricite and triphylite, both derived from the olivine structure type, have been investigated using magnetic susceptibility, heat capacity, and low-temperature neutron powder diffraction. These NaFePO₄ polymorphs assume orthorhombic frameworks (space group No. 62, <i>Pnma</i>), built from FeO₆ octahedral and PO₄ tetrahedral units having corner-sharing and edge-sharing arrangements. Both polymorphs demonstrate antiferromagnetic ordering below 13 K for maricite and 50 K for triphylite. The magnetic structure and properties are discussed considering super- and supersuperexchange interactions in comparison to those of triphylite-LiFePO₄

Electrochemical and Diffusional Investigation of Na₂Fe^IIPO₄F Fluorophosphate Sodium Insertion Material Obtained from Fe^III Precursor

Sodium iron fluorophosphate (Na₂Fe^IIPO₄F) was synthesized by economic solvothermal combustion technique using Fe^III precursors, developing one-step carbon-coated homogeneous product. Synchrotron diffraction and Mössbauer spectroscopy revealed the formation of single-phase product assuming an orthorhombic structure (s.g. <i>Pbcn</i>) with Fe^II species. This Fe^III precursor derived Na₂Fe^IIPO₄F exhibited reversible Na⁺ (de)­intercalation with discharge capacity of 100 mAh/g at a rate of C/10 involving flat Fe^III/Fe^II redox plateaus located at 2.92 and 3.05 V (vs Na/Na⁺). It delivered good cycling stability and rate kinetics at room temperature. The stability of Na₂FePO₄F cathode was further verified by electrochemical impedance spectroscopy at different stages of galvanostatic analysis. Bond valence site energy (BVSE) calculations revealed the existence of 2-dimensional Na⁺ percolation pathways in the <i>a–c</i> plane with a moderate migration barrier of 0.6 eV. Combustion synthesized Na₂Fe^IIPO₄F forms an economically viable sodium battery material. Although the capacity of this cathode is relatively low, this study continues systematic work, which attempts to broaden the scope of reversible sodium insertion materials

Na₂FeP₂O₇: A Safe Cathode for Rechargeable Sodium-ion Batteries

Vying for newer sodium-ion chemistry for rechargeable batteries, Na₂FeP₂O₇ pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na₂FeP₂O₇ as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP₂O₇. High-temperature analyses of the desodiated state NaFeP₂O₇ show an irreversible phase transition from triclinic (<i>P</i>1̅) to the ground state monoclinic (<i>P</i>2₁/<i>c</i>) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P₂O₇)^4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na₂FeP₂O₇ as a safe cathode candidate for large-scale economic sodium-ion battery applications