Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi₂(PO₄)₂/C for Li-Ion Batteries

In this work, we first report the composite of CuNi₂(PO₄)₂/C (CNP/C) can be employed as the high-capacity conversion-type cathode material for rechargeable Li-ion batteries (LIBs), delivering a reversible capacity as high as 306 mA h g^–1 at a current density of 20 mA g^–1. Furthermore, CNP/C also presents good rate performance and reasonable cycling stability based on a nontraditional conversion reaction mode. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterizations show that CNP is reduced to form Cu/Ni and Li₃PO₄ during the discharging process, which is reversed in the following charging process, demonstrating that a reversible conversion reaction mechanism occurs. X-ray absorption spectroscopy (XAS) discloses that Ni²⁺/Ni⁰ exhibits a better reversibility compared to Cu²⁺/Cu during the conversion reaction process, while Cu⁰ is more difficult to be reoxidized during the recharge process, leading to capacity loss as a consequence. The fundamental understanding obtained in this work provides some important clues to explore the high-capacity conversion-type cathode materials for rechargeable LIBs

Novel 3.9 V Layered Na₃V₃(PO₄)₄ Cathode Material for Sodium Ion Batteries

A new compound Na₃V₃­(PO₄)₄ is successfully synthesized for sodium ion batteries using a sol–gel method. Composition analysis through ICP-OES confirms the stoichiometry of Na₃V₃(PO₄)₄. Structural analysis based on XRD reveals that the new material crystallizes in a monoclinic system with a <i>C</i>2/<i>c</i> space group. The new compound exhibits a layered structure containing 3D Na⁺ ion channels allowing excellent cycling and rate performance. Even at a high current rate of 3C (1C = 45 mA/g), it still delivers 82% of the theoretical capacity. Meanwhile, 92% of its capacity is retained after 100 electrochemical cycles. The voltage profiles of Na₃V₃­(PO₄)₄ show that it can reversibly uptake nearly one Na⁺ ion with a 3.9 V voltage plateau, which is the highest value among Na-containing V-based orthophosphates ever reported

Exploring Highly Reversible 1.5-Electron Reactions (V³⁺/V⁴⁺/V⁵⁺) in Na₃VCr(PO₄)₃ Cathode for Sodium-Ion Batteries

The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in Na₃VCr­(PO₄)₃. Combining a series of advanced characterization techniques such as ex situ ⁵¹V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance