(NH₄)₂V₇O₁₆ as a Cathode Material for Rechargeable Calcium-Ion Batteries: Structural Transformation and Co-Intercalation of Ammonium and Calcium Ions

Abstract

Calcium-ion batteries (CIBs) are viable alternatives to lithium-ion batteries. However, few cathode materials can reversibly intercalate Ca ions in anhydrous electrolytes. Most highcapacity materials contain crystal water, causing unwanted reactions on the anode. Herein, we report a crystal-water-free ammonium vanadate, (NH 4 ) 2 V 7 O 16 , as a CIB host material. Synthesized via a microwave-assisted hydrothermal method, (NH 4 ) 2 V 7 O 16 exhibits a layered structure with stacked V 7 O 16 layers and interlayer ammonium ions hydrogen-bonded to adjacent oxygen atoms. We demonstrate the reversible electrochemical intercalation of Ca 2+ ions into (NH 4 ) 2 V 7 O 16 , achieving a reversible capacity of 89 mA h g −1 and an average discharge voltage of ∼3.21 V vs Ca/Ca 2+ . Although (NH 4 ) 2 V 7 O 16 displays poor rate capability and cycling performance, we reveal a unique reaction mechanism. During the initial charge, an irreversible structural change occurs, removing all ammonium ions and inserting a small amount of Ca ions, forming Ca 0.37 V 7 O 16 . This suggests an ion-exchange reaction between calcium and ammonium ions. Subsequent cycles exhibit the reversible coinsertion and coextraction of calcium and ammonium ions. We observe that V 7 O 16 lacks structural stability without interlayer cations. Our findings offer insight into electrochemical reaction processes in crystal-water-free layered materials containing interlayer ammonium ions, highlighting the importance of cointercalation between ammonium and carrier ions for reversible cycling.

Ammonium Ion-Pre-Intercalated MnO2 on Carbon Cloth for High-Energy Density Asymmetric Supercapacitors

“…From the spectrum, we can observe that compared with CC@MnO2, there is a characteristic peak at 1409 cm −1 for CC@NMO, which belongs to the N-H bending mode. The peak around 3200 cm −1 , corresponding to the hydrogen bonding of N-H, indicates the characteristic N-H•••O stretching vibration [32]. All the above demonstrates how NH4 + ions Fourier-Transform Infrared Spectroscopy (FTIR) was used to further investigate how NH 4 + intercalates into MnO 2 .…”

Unraveling Polymorphic Crystal Structures of Li4SiS4 for All-Solid-State Batteries: Enhanced Ionic Conductivity via Aliovalent Sb Substitution

“…For HT-Li 4 SiS 4 , the initial crystal structural model used in the Rietveld refinement contained the Si and S atoms, in accordance with the literature . The lithium ions were located later using ab initio structure determination techniques by combining GSAS II, the single-crystal structure refinement software CRYSTALS, and the Fourier electron density map visualization software MCE, as previously described by our research group. ,− For structural determination of LT-Li 4 SiS 4 , we utilized the crystal structure of Li 4 GeS 4 as the initial model, following the aforementioned procedure. Finally, all the atomic positions and isotropic thermal displacements, peak shape profile functions, background functions, lattice parameters, and atomic occupancies were refined and determined.…”