F-Doping effects on carbon-coated Li3V2(PO4)3 as a cathode for high performance lithium rechargeable batteries: combined experimental and DFT studies

Three-Dimensional Holey Graphene Enwrapped Li3 V2 (PO4 )3 /N-Doped Carbon Cathode for High-Rate and Long-Life Li-Ion Batteries

Monoclinic Li₃ V₂ (PO₄ )₃ is a promising cathode material for high-power Li-ion batteries. Herein, a three-dimensional holey graphene enwrapped Li₃ V₂ (PO₄ )₃ /N-doped carbon (LVPNCHG) nanocomposite has been successfully synthesized. The holes could be in-situ and directly introduced in graphene through H₂ O₂ chemical etching in the synthesis process, which could remarkably enhance the ion and electron transport and greatly improve the electrochemical performance of the LVPNCHG electrode: 78 mAh g^-1 at 150 C, 86.1 % capacity retention over 2000 cycles at 10 C, and 96 % capacity retention over 500 cycles at 1 C under -20 °C. Moreover, in-situ distribution of relaxation time analysis was used to study LVPNCHG cathode during charge/discharge at 3.0-4.8 V, combined with in-situ X-ray diffraction measurement, and the results showed that a two-phase reaction mechanism was involved during the insertion of Li⁺ in the discharge process. Further demonstration of graphite//LVPNCHG full cell indicated great potential of the as-synthesized materials for practical application.

Ru- and Cl-Codoped Li3 V2 (PO4 )3 with Enhanced Performance for Lithium-Ion Batteries in a Wide Temperature Range

Li₃ V₂ (PO₄ )₃ (LVP) is a promising cathode material for lithium-ion batteries, especially when used in a wide temperature range, due to its high intrinsic ionic mobility and theoretical capacity. Herein, Ru- and Cl-codoped Li₃ V₂ (PO₄ )₃ (LVP-Ru_x -Cl_{3 x} ) coated with/without a nitrogen-doped carbon (NC) layer are synthesized. Among them, the optimized sample (LVP-Ru_0.05 -Cl_0.15 @NC) delivers remarkable performances at both room temperature and extreme temperatures (-40, 25, and 60 °C), indicating temperature adaptability. It achieves intriguing capacities (49 mAh g^-1 at -40 °C, 128 mAh g^-1 at 25 °C, and 123 mAh g^-1 at 60 °C, all at 0.5 C), long cycle life (94% capacity retention after 2000 cycles at 25 °C and 5 C), and high-rate capabilities (up to 20 C). The structural evolution features and capacity loss mechanisms of LVP-Ru_0.05 -Cl_0.15 @NC are further investigated using in situ X-ray diffraction (XRD) at different temperatures (-10, 25, and 60 °C) during redox reactions. Theoretical calculations elucidate that Ru- and Cl-codoping can greatly improve the intrinsic diffusion coefficient of LVP by reducing its bandgap energy and lowering the energy barrier of lithium-ion diffusion. In "all-weather" conditions, the dual-element co-doping strategy is critical for increasing electrochemical performance.

Systematic evaluation of lithium-excess polyanionic compounds as multi-electron reaction cathodes

Polyanion cathodes with multi-electron redox always facilitate wider application in a metal ion-based battery system because of their high capacity and safety. However, the irreversible phase transformation and interfacial deterioration remain major impediments. Herein, using monoclinic Li₃V₂(PO₄)₃ as a model, the impact of excess lithium on its electrochemical properties are demonstrated. It was determined that a maximum of 5% excess lithium could be incorporated into the monoclinic structure, and a further overdose of lithium led to the formation of secondary phase Li₃PO₄. The excess Li⁺ ions are located at both octahedral and interstitial sites, which enable enhanced redox kinetics that are mainly attributed to accelerated ionic movement induced by alternate diffusion behavior of Li⁺ ions in a three-dimensional permeation path. Moreover, Li-excess local configurations can stabilize the lattice oxygen and provide a favorable cathode-electrolyte interface, which synergistically relieves the structural degradation during electrochemical cycling, thus guaranteeing exceptional cycling stability (e.g., 82.5% after 1000 cycles at 1000 mA g^-1). These findings provide a comprehensive understanding of defect/electronic structure/ion transport and the intrinsic properties of polyanionic Li₃V₂(PO₄)₃ and may help to pave the way for other highly stable electrodes for rechargeable batteries.

Defects and dopant properties of Li3V2(PO4)3

Polyanion phosphate based Li₃V₂(PO₄)₃ material has attracted considerable attention as a novel cathode material for potential use in rechargeable lithium ion batteries. The defect chemistry and dopant properties of this material are studied using well-established atomistic scale simulation techniques. The most favourable intrinsic defect process is the Li Frenkel (0.45 eV/defect) ensuring the formation of Li vacancies required for Li diffusion via the vacancy mechanism. Long range lithium paths via the vacancy mechanism were constructed and it is confirmed that the lowest activation energy of migration (0.60 eV) path is three dimensional with curved trajectory. The second most stable defect energy process is calculated to be the anti-site defect, in which Li and V ions exchange their positions (0.91 eV/defect). Tetravalent dopants were considered on both V and P sites in order to form Li vacancies needed for Li diffusion and the Li interstitials to increase the capacity respectively. Doping by Zr on the V site and Si on the P site are calculated to be energetically favourable.