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<p>Li/CF_x is one of the highest-energy-density primary batteries; however, poor rate capability hinders its practical applications in high-power devices. Here we report a preparation of fluorinated graphene (GF_x) with superior performance through a direct gas fluorination method. We find that the so-called “semi-ionic” C-F bond content in all C-F bonds presents a more critical impact on rate performance of the GF_x in comparison with sp² C content in the GF_x, morphology, structure, and specific surface area of the materials. The rate capability remains excellent before the semi-ionic C-F bond proportion in the GF_x decreases. Thus, by optimizing semi-ionic C-F content in our GF_x, we obtain the optimal x of 0.8, with which the GF_0.8 exhibits a very high energy density of 1,073 Wh kg⁻¹ and an excellent power density of 21,460 W kg⁻¹ at a high current density of 10 A g⁻¹. More importantly, our approach opens a new avenue to obtain fluorinated carbon with high energy densities without compromising high power densities.</p

Toward Understanding the Lithium Transport Mechanism in Garnet-type Solid Electrolytes: Li⁺ Ion Exchanges and Their Mobility at Octahedral/Tetrahedral Sites

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity, reaching up to ∼10^–3 S/cm at room temperature. Fully understanding the Li⁺ transport mechanism including Li⁺ mobility at different sites is a key topic in this field, and Li_{7–2<i>x</i>–3<i>y</i>}Al_<i>y</i>La₃Zr_2–<i>x</i>W_<i>x</i>O₁₂ (0 ≤ <i>x</i> ≤ 1) are selected as target electrolytes. X-ray and neutron diffraction as well as ac impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li⁺ ion jumping, but it does noticeably vary the distribution of Li⁺ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ ions. For the first time, high-resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well-resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d–96h–48g–96h–24d. The lithium mobility at the 24d sites is found to dominate the total ionic conductivity of the samples, with diffusion coefficients of 10^–9 m² s^–1 and 10^–12 m² s^–1 at the octahedral and tetrahedral sites, respectively

Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na₃V₂(PO₄)₂F₃

Na₃V₂(PO₄)₂F₃ is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na₃V₂(PO₄)₂F₃ as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution ²³Na and ³¹P solid-state nuclear magnetic resonance (NMR). The ²³Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na₃V₂(PO₄)₂F₃ electrode upon charge have been observed, where <i>a</i> (= <i>b</i>) continues to increase and <i>c</i> decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the ²³Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t_2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The ³¹P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V⁴⁺) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na₃V₂(PO₄)₂F₃ when used in a NIB

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

Copper Phosphate as a Cathode Material for Rechargeable Li Batteries and Its Electrochemical Reaction Mechanism

In the search for new cathode materials for rechargeable lithium batteries, conversion-type materials have great potential because of their ability to achieve high specific capacities via the full utilization of transition metal oxidation states. Here, we report for the first time that copper phosphate can be used as a novel high-capacity cathode for rechargeable Li batteries, capable of delivering a reversible capacity of 360 mAh/g with two discharge plateaus of 2.7 and 2.1 V at 400 mA/g. The underlying reaction involves the formation as well as the oxidation of metallic Cu. The solid-state NMR, <i>in situ</i> XAFS, HR-TEM, and XRD results clearly indicate that Cu can react with Li₃PO₄ to form copper phosphate and Li_<i>x</i>Cu_<i>y</i>PO₄ during the charging process, largely determining the reversibility of Cu₃(PO₄)₂. This new reaction scheme provides a new venue to explore polyanion-type compounds as high-capacity cathode materials with conversion reaction processes