Microwave-Assisted Solvothermal Synthesis of Spinel AV₂O₄ (M = Mg, Mn, Fe, and Co)

Lower-valent vanadium oxide spinels AV₂O₄ (A = Mg, Mn, Fe, and Co) consisting of A²⁺ and V³⁺ ions have been synthesized by a low-temperature microwave-assisted solvothermal (MW-ST) synthesis process in a tetraethylene glycol (TEG) medium. The oxides are formed within a short reaction time of 30 min at 300 °C. Subsequent postheat treatment of the oxides at elevated temperatures in inert or reducing atmospheres results in an instability of the spinel phase, especially CoV₂O₄ due to the ease of formation of metallic Co, demonstrating the advantage of the low-temperature MW-ST process in accessing these oxides. This MW-ST synthesis approach is attractive for synthesizing other lower-valent transition-metal oxides that are otherwise difficult to obtain by conventional synthesis methods and for subsequent study of their unique physical and chemical properties

Crystal-Chemical Guide for Understanding Redox Energy Variations of M^2+/3+ Couples in Polyanion Cathodes for Lithium-Ion Batteries

A crystal-chemical guide is provided for understanding how factors such as the crystal structure and covalency of the polyanion affect the M^2+/3+ redox energies in polyanion cathodes. Although there are more rigorous techniques available, our approach is precise in spite of being simple. We show that an accurate prediction can be made with regard to the voltages delivered based on a basic understanding of how the coordination of the transition-metal ion affects the covalency of the M-O bond. Additionally, a new method for assessing the covalency of the polyanion (beyond the electronegativity of the countercation) is presented and used to explain why the voltage delivered by Li₂MP₂O₇ cathodes is higher than that of LiMPO₄. Furthermore, a comparison of the silicate and phosphate structures reveals that edge sharing between transition metal polyhedra and other cation polyhedra has an opposite effect on the voltage delivered by these materials. For instance, edge sharing with LiO₄ polyhedra in the silicates raises the M^2+/3+ redox energy, whereas edge sharing with PO₄ polyhedra in the phosphates lowers the M^2+/3+ redox energy. This is due to a difference in the strength of the repulsive force exerted on the transition metal by the P⁵⁺ cation when compared to Li⁺. This observation is significant since edge sharing has generally been viewed as a structural feature that lowers the redox energy. Lastly, crystal field splitting consideration alone is not sufficient to understand the voltage trends for polyanion cathodes and one must consider the contributions of the structure and/or the inductive effect. Our analysis provides new insights that may prove useful in tuning the voltage of existing polyanion systems and in the design of new cathode materials

Microwave-Assisted Synthesis of NaCoPO₄ Red-Phase and Initial Characterization as High Voltage Cathode for Sodium-Ion Batteries

Transition metal-containing polyanion compounds are attractive for use as cathode materials in sodium-ion batteries (SIB) because they possess elevated higher intrinsic electrochemical potentials versus oxide analogs given the same M^{<i>n</i>+/(<i>n</i>+1)+} redox couple, which leads to higher energy densities. NaMPO₄ (M = transition metal) compounds have a driving force to form into the electrochemically inactive maricite phase when using conventional methods. Herein we report on the synthesis of a NaCoPO₄ (NCP) polymorph (“Red”-phase) by a microwave-assisted solvothermal process at 200 °C using tetraethylene glycol as the solvent. Ex situ XRD, XANES, and electrochemical data are used to determine the reversibility of the Co^2+/3+ redox center

High-Capacity, Aliovalently Doped Olivine LiMn_{1–3<i>x</i>/2}V_<i>x</i>□_<i>x</i>/2PO₄ Cathodes without Carbon Coating

A substantial amount of Mn²⁺ has been aliovalently substituted by V³⁺ in cation-deficient LiMn_{1–3<i>x</i>/2}V_<i>x</i>□_<i>x</i>/2PO₄ (0 ≤ <i>x</i> ≤ 0.20) by a low-temperature (<300 °C) microwave-assisted solvothermal (MW-ST) process. The necessity of a low-temperature synthesis to achieve higher levels of doping is demonstrated as the solubility of vanadium decreases with the formation of impurity phases on heating the samples to ≥575 °C. Soft X-ray absorption spectroscopy reveals enhanced Mn–O hybridization in the vanadium-doped samples, which is believed to facilitate an increase in capacity with increasing vanadium content in the lattice. For example, a high capacity of 155 mAh/g is achieved above a cutoff voltage of 3 V without any carbon coating for the <i>x</i> = 0.2 sample. The vanadium substitution enhances the overall kinetics of the material by lowering the charge-transfer impedance and increasing the lithium-diffusion coefficient