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

Increased Conductivity in the Metastable Intermediate in Li_<i>x</i>FePO₄ Electrode

With increasing concerns about energy and environmental issues, lithium ion batteries are now penetrating into large-scale applications such as electric vehicles. As an electrode reaction process, it is generally believed that two-phase reaction with structural rearrangement and large lattice mismatch impedes high-rate capability. However, Li_<i>x</i>FePO₄, with its two-phase reaction between LiFePO₄ and FePO₄, exhibits an exceptional high-rate performance. In this article, after confirming the existence of a single-phase reaction even under moderate rates, we demonstrate an approximately 2 orders of magnitude increase of the conductivity for the quenched intermediate Li_0.6FePO₄. In addition to the widely accepted strain relaxation effect at the two-phase interface, the dramatically increased conductivity due to polaron/lithium carrier density increase in the intermediate phase should be highlighted as an important factor to accelerate the electrode reaction of olivine Li_<i>x</i>FePO₄

Phase Diagram of Olivine Na_<i>x</i>FePO₄ (0 < <i>x</i> < 1)

The composition–temperature phase diagram of cathode material Na_<i>x</i>FePO₄ in the olivine phase for sodium-ion battery has been determined. Samples were prepared by solid-state synthetic, electrochemical and chemical oxidation/reduction methods. Powder X-ray diffraction and Mössbauer spectroscopies were employed to investigate the phase behavior. At room temperature, despite the resemblance to the Li_<i>x</i>FePO₄ structure, the solubility limit of alkali metal (sodium) vacancy in Na_<i>x</i>FePO₄ is found to be large. In the range 2/3 < <i>x</i> < 1 Na_<i>x</i>FePO₄ is found to be a solid-solution phase. Two distinguished Fe²⁺ sites in the solid-solution phase were found by Mössbauer spectroscopy. <i>Ab initio</i> calculations reproduce the quadrupole splitting constants and suggest large distortion of the Fe²⁺ octahedra. High-temperature <i>in situ</i> X-ray diffraction suggests a completely different phase diagram of sodium olivine with a much more stable nature of the intermediate at <i>x</i> = 2/3 in comparison to the lithium one

Kinetics of Nucleation and Growth in Two-Phase Electrochemical Reaction of Li_<i>x</i>FePO₄

The kinetics of a two-phase electrochemical reaction in Li_<i>x</i>FePO₄ was investigated by potential-step chronoamperometry under various experimental conditions: amplitude of potential step, direction of potential step, particle size, and thickness of composite electrodes. Only under a small potential step (10 mV) applied to large Li_<i>x</i>FePO₄ particles (203 nm), the chronoamperogram showed a momentary current increase, followed by gradual decline, indicating that the nucleation and growth governed the electrode kinetics. In that condition, the chronoamperogram was analyzed with the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model, which describes the kinetics of phase transition. The obtained Avrami exponent of ca. 1.1 indicates that the phase transition proceeds with a one-dimensional phase-boundary movement, which is consistent with the previously reported mechanism. From the temperature dependence of the obtained rate constant, the activation energy of the phase-boundary movement in Li_<i>x</i>FePO₄ was estimated to be 42 and 40 kJ mol^–1 in cathodic and anodic reactions, respectively

Pyrophosphate Chemistry toward Safe Rechargeable Batteries

We demonstrate that pyrophosphate anion can result in metal pyrophosphate cathode materials with high thermal stabilities. High temperature behaviors for the delithiated states of Li₂FeP₂O₇ and Li₂MnP₂O₇ in the <i>P</i>2₁/<i>c</i> symmetry are studied. Above 540 °C, the singly delithiated structure LiFeP₂O₇ undergoes an irreversible phase transformation to the ground state polymorph with a symmetry of <i>P</i>2₁. Intermediate delithiated compounds Li_2‑<i>x</i>FeP₂O₇ (0 < <i>x</i> < 1) convert to a mixture of LiFeP₂O₇ in the <i>P</i>2₁ symmetry and Li₂FeP₂O₇ in the <i>P</i>2₁/<i>c</i> symmetry. No decomposition is observed for both the singly and partially delithiated compounds until 600 °C showing the high thermal stabilities of the compounds. Analysis of phase stabilities reveals that LiFeP₂O₇ (<i>P</i>2₁/<i>c</i>) is intrinsically more stable than FePO₄ (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li_1.4MnP₂O₇. It decomposes to Li₂MnP₂O₇, Mn₂P₂O₇, LiPO₃, and O₂ at 450 °C, much higher than the olivine counterpart MnPO₄. The high stability of these metal pyrophosphates is rationalized by the stability of the P₂O₇^4‑ anion

High-Temperature Neutron and X‑ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate

Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na_2.5Fe_1.75(SO₄)₃ (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied high-temperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo-)­alluaudite system, forming emerging functional materials group for electrochemical applications

Unveiling the Origin of Unusual Pseudocapacitance of RuO₂·<i>n</i>H₂O from Its Hierarchical Nanostructure by Small-Angle X‑ray Scattering

Hydrous ruthenium oxide (RuO₂·<i>n</i>H₂O) has inherent proton–electron mixed-conductive nature and offers huge pseudocapacitance (>700 F g^–1), having attracted the attention of many capacitor engineers. However, the origin of the anomalous pseudocapacitance, exhibiting a strong maximum at a specific narrow optimum annealing temperature of ca. 150 °C, has yet to be understood. Here we show a long-awaited explanation for this mystery based on its hierarchical nanostructure unveiled by small-angle X-ray scattering (SAXS). The striking contrast in X-ray atomic scattering factors enables SAXS to exclusively probe heavy RuO₂ in subnano- to nanoscale, dispersed in confined water. We demonstrate that the surface area of the first aggregate of subnano primary RuO₂ particles dominates the accessible number of proton and hence pseudocapacitance, providing critical insights into the nanoarchitectural design of high-performance electrodes for electrochemical capacitors

Phase Separation of a Hexacyanoferrate-Bridged Coordination Framework under Electrochemical Na-ion Insertion

Phase separation and transformation induced by electrochemical ion insertion are key processes in achieving efficient energy storage. Exploration of novel insertion electrode materials/reactions is particularly important to unravel the atomic/molecular-level mechanism and improve the electrochemical properties. Here, we report the unconventional phase separation of a cyanide-bridged coordination polymer, Eu­[Fe­(CN)₆]·4H₂O, under electrochemical Na-ion insertion. Detailed structural analyses performed during the electrochemical reaction revealed that, in contrast to conventional electrochemical phase separation induced by the elastic interaction between nearest neighbors, the phase separation of Na_<i>x</i>Eu­[Fe­(CN)₆]·4H₂O is due to a long-range interaction, namely, cooperative rotation ordering of hexacyanoferrates. Kolmogorov-Johnson-Mehl-Avrami analysis showed that the activation energy for the phase boundary migration in Na_<i>x</i>Eu­[Fe­(CN)₆]·4H₂O is lower than that in other conventional electrode materials such as Li_1–<i>x</i>FePO₄