MXene as a Charge Storage Host

ConspectusThe development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)­intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., Ti₂CT_<i>x</i>, Ti₃C₂T_<i>x</i>, V₂CT_<i>x</i>, and Nb₂CT_<i>x</i>, where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins.MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes for advanced EES applications is warranted

Electrode Properties of P2–Na_2/3Mn_<i>y</i>Co_1–<i>y</i>O₂ as Cathode Materials for Sodium-Ion Batteries

The development of high-performance Na-ion intercalation electrodes has been required recently because Na-ion batteries hold much promise for inexpensive and efficient energy storage, which can be deployed in a power grid. For both optimization and better understanding of the electrode materials, it is indispensable to clarify the relationship between the electronic state and electrochemical properties systematically. In this work, we studied the electrochemical properties of P2–Na_2/3Mn_<i>y</i>Co_1–<i>y</i>O₂ in detail. A series of the P2 phases was successfully synthesized by the conventional solid-state reaction. The solid solution P2 compounds showed that the redox potential of Co⁴⁺/Co³⁺ and Mn⁴⁺/Mn³⁺ shifts systematically by the transition-metal substitution. The charge–discharge cycle tests revealed that with increasing <i>y</i> the initial specific capacity increases while the cycle stability degrades. The origin for the cycle degradation was analyzed by the electrochemical impedance spectroscopy, which evidenced that the substitution of Co for Mn accelerates the formation of the passivating layer at the electrode surface

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

Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte

Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes in lithium-ion battery (LIB). Here we theoretically investigated low (LC) and high (HC) concentration systems of LiN­(SO₂CF₃)₂ (Li-TFSA) salt in acetonitrile (AN) solution, to elucidate the mechanism of improving the low reduction stability of AN at the HC condition, by density functional theory based molecular dynamics (DFT-MD) sampling of the solvation character with extra electron(s). We demonstrated that TFSA anions sacrificially accept the reductive electron at the HC condition, which is ascribed to formation of specific network structure and the resulting shift of electron affinity of the anions. We also found that, even in the LC condition, TFSA eventually decomposes with one electron reduction. This sacrificial anion reduction hinders two electron reductive decomposition of AN, leading to improved electrochemical stability. The mechanism may give a guiding principle for the design of better LIB electrolytes

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

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