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Anisotropic disorder along the c-axis results from static disorder.

Anisotropic disorder along the c-axis results from static disorder.

Residual impurities such as lithium carbonate and hydroxide are a major concern for accelerating parasitic reactions at the cathode electrolyte interface of lithium‐ion batteries. Removal of these lithium‐bearing species becomes a necessity for high‐performance nickel‐rich cathode materials. Instead of directly removing these impurities through washing steps, a wet impregnation process is employed to convert these detrimental surface impurities into beneficial surface coating on nickel‐rich cathode materials. Specifically, the pristine cathode material is treated with Al(H2PO4)3 solution to convert undesired compounds into Li3PO4 and AlPO4, both of which are considered positive surface coating materials for high‐voltage cathodes. It is found that the introduced modification greatly suppresses the interfacial impedance hike and improves the capacity retention of the cathode material after repeating charging/discharging. It is believed that these benefits are realized through the modification of the surface chemistry of the cathode material, which helps to slow down the parasitic reactions and reduce the damage to the cathode material. [ABSTRACT FROM AUTHOR]

Lithium vanadyl phosphate (LiVOPO4) is a next-generation multielectron battery cathode that can intercalate up to two Li-ions per V-ion through the redox couples of V4+/V3+ and V5+/V4+. However, its rate capacity is undermined by the sluggish Li-ion diffusion in the high-voltage region (4 V for V5+/V4+ redox). Nb substitution was used to expand the crystal lattice to facilitate Li-ion diffusion. In this work, Nb substitution was achieved via hydrothermal synthesis, which resulted in a new, lower symmetry β′-LiVOPO4 phase with preferential Nb occupation of one of the two V sites. This phase presents complex defect chemistries, including cation vacancies and hydrogen interstitials, characterized by a combination of X-ray and neutron diffraction, elemental and thermogravimetric analyses, X-ray absorption spectroscopy, and magnetic susceptibility measurements. The Nb-substituted samples demonstrated improved capacity retention and rate capabilities in the high-voltage region, albeit an enlarged voltage hysteresis related to a partial V4+/V3+ redox reaction, as evidenced by ex situ X-ray absorption spectroscopy and pair distribution function analysis. This work highlights the importance of understanding the complex defect chemistry and its consequence on electrochemistry in polyanionic intercalation compounds. [ABSTRACT FROM AUTHOR]

LiVOPO4 is a promising next-generation multi-electron cathode material, boasting a theoretical capacity of 305 mA h g−1, significantly higher than any commercially used Li-ion battery cathode material. However, the material still faces several limitations, including the difficulty in attaining the full theoretical capacity at higher rates and capacity fade over several cycles. In this paper, we show that Ti- and Nb-substitution can be used to improve the thermal stability and electrochemical performance of LiVOPO4. We show through in situ heating with XRD and a novel gradient heating technique that both Ti- and Nb-substitution cause β-LiVOPO4 to be stabilized relative to ε-LiVOPO4. This is due to transition-metal substitution, which increases the O-vacancy formation energies, pushing the β → ε transition to higher temperatures. We show that it is still possible to synthesize pure-phase ε-LiVOPO4 through the use of high temperatures to generate these O-vacancies. We show that even 1% of Ti- or Nb-substitution can improve the initial capacity and long term cycling capability of LiVOPO4 by improving the high-voltage capacity and reducing the capacity fade in both the high- and low-voltage regions. This is due to the an overall improved Li+ ion diffusion which is caused by an improved charge-transfer resistance during cycling. [ABSTRACT FROM AUTHOR]

e-VOPO4 has the potential to be the next high energy density cathode material for lithium-ion batteries due to its high thermal stability and its ability to reversibly intercalate two full Li+, giving a high discharge capacity of 305 mAh g-1. However, vanadyl phosphate materials typically experience poor Li+ kinetics that impedes the high-rate capability at the high voltage plateau. In this work, we applied niobium substitution to improve the high-rate performance of the 4.0 V plateau of e-VOPO4. Elemental analysis and lattice parameter refinements determined that up to 10% Nb can be substituted into e-VOPO4 without any impurities. TEM and Synchrotron-based EXAFS confirmed that all the substituted samples have Nb in the e-VOPO4 structure. Electrochemical tests revealed that 1% Nb substitution can deliver a high discharge capacity of ~300 mAh g-1 without any degradation in cycling behavior by maintaining its nanosized morphology. [ABSTRACT FROM AUTHOR]

Increasing demands for higher energy density batteries have inspired multi-electron cathodes which can double the energy density per transition metal cation. In Li-ion batteries, vanadyl phosphates have been shown to intercalate 2 Li+ per V center by activating V4+/5+ and V3+/4+ redox couples, resulting in a gravimetric capacity of 305 mAh g-1. In order to employ the VOPO4 structure in earth abundant and cheaper alkali-ion batteries, channels must be expanded to allow the diffusion of larger cations. This can be achieved by pillaring the VOPO4 framework with large cations. KVOPO4 has been shown to be a high capacity cathode in Na-ion batteries, however not all of the K could be removed from the structure, limiting the accessible capacity. NH4VOPO4 is similar to KVOPO4 where the KTP-type VOPO4 framework is enlarged and distorted due to the presence of large cations. In this study, we investigate the electrochemical performance of NH4VOPO4. While the pristine material suffers greatly from parasitic reactions, we show that electrochemistry and thermal stability can be improved by exchanging some of the NH4 + with Na+. [ABSTRACT FROM AUTHOR]

LiVOPO4 (LVP) is a promising next generation multi-electron material with a theoretical capacity of 305 mA h g−1, higher than any commercially used cathode material. LVP still faces significant commercialization challenges, including difficulty obtaining full theoretical capacity at fast rates and capacity fading over several cycles. In this paper, we show that a low temperature microwave-assisted solvothermal synthesis yields a pure LVP material that can achieve high capacity at fast rates with decent cycle stability. There is, however, evidence of side reactions occurring during cycling starting at 4.1 V that leads to a slight degradation of capacity over time. In this paper, we also compare the effects of different substituents on the electrochemical performance of LVP. Structurally, we show the 5% Nb-substituted material adopts the beta phase, and the Ti-substituted LVP exhibits roughly 50/50 epsilon/beta phase, while all other substituents tend towards the epsilon phase. Ti-, Nb-, and Mo-substitution noticeably improved the electrochemical performance, while the other substituents either worsened or did not affect the material's electrochemical performance. [ABSTRACT FROM AUTHOR]

Vanadyl phosphates comprise a class of multielectron cathode materials capable of cycling two Li+, about 1.66 Na+, and some K+ ions per redox center. In this review, structures, thermodynamic stabilities, and ion diffusion kinetics of various AxVOPO4 (A = Li, Na, K, NH4) polymorphs are discussed. Both the experimental data and first‐principle calculations indicate kinetic limitations for alkali metal ions cycling, especially between for 0 ≤ x ≤ 1, and metastability of phases with x > 1. This creates challenges for multiple‐ion cycling, as the slow kinetics call for nanosized particles, which being metastable and reactive with organic electrolytes are prone to side reactions. Thus, various synthesis approaches, surface coating, and transition metal ion substitution strategies are discussed here as possible ways to stabilize AxVOPO4 structures and improve alkali metal ion diffusion. The role of advanced characterization techniques, such as X‐ray absorption spectroscopy, diffraction, pair distribution function analysis and 7Li and 31P NMR, in understanding the reaction mechanism from both structural and electronic points of view is emphasized. [ABSTRACT FROM AUTHOR]

The lithium-ion battery cathode material β-VOPO4 is capable of intercalating more than one Li ion per transition metal ion due to the accessibility of both the V5+/V4+ and V4+/V3+ redox couples at ∼4.5 V and ∼2.3 V vs. Li, respectively, giving a theoretical capacity greater than ∼300 mA h g−1. The ability to perform full and reversible two Li-ion intercalation in this material, however, has been a matter of debate and the poor crystallinity of the fully lithiated phase has thus far precluded its complete structural characterisation by conventional diffraction-based methods. In this work, 7Li and 31P NMR spectroscopy, in combination with first principles DFT calculations, indicate that chemical lithiation results in a single phase β-Li2VOPO4 exhibiting a complex Li ordering scheme with lithium ions occupying multiple disordered environments. 2D NMR 7Li correlation experiments were used to deduce the most likely Li ordering for the β-Li2VOPO4 phase from amongst several DFT optimised structures. In contrast, electrochemically lithiated β-Li2−xVOPO4 discharged to 1.6 V exhibits, in addition to β-Li2VOPO4, a β-Li1.5VOPO4 phase. The existence of β-Li1.5VOPO4 is not reflected in the flat galvanostatic charge and discharge curves nor is evident from diffraction-based methods due to the very close structural similarity between the β-Li1.5VOPO4 phase and β-Li2VOPO4 phases. We demonstrate that solid state NMR spectroscopy, in combination with DFT results, provides a powerful tool for identifying intermediate states formed during charge/discharge of these complex phosphates as these phases can be distinguished from the end member phases primarily by the nature of the lithium ordering. [ABSTRACT FROM AUTHOR]