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As a promising candidate for the flame‐retardant electrolyte, triethyl phosphate (TEP)/potassium bis(fluorosulfonyl)amide (KFSI)‐based electrolyte has drawn much attention in the K‐ion battery community. Although the TEP/KFSI formula at a moderate main salt concentration (normally, <3 m) enables the compatibility of the reactive K metal anode, the long‐standing oxidative instability of KFSI salt remains unsolved. Here, an additive strategy is reported to address the high‐voltage issue in the TEP/KFSI electrolyte, and generalize it to the other KFSI‐based electrolytes. The addition of potassium nitrate changes the surface charge distribution and effectively suppresses the decomposition of KFSI toward the high‐voltage cathode. The nitrate‐containing electrolyte enables superior stability of a 4.3 V‐class K‐ion battery, as evidenced by its 80% capacity retention over 2000 cycles (≈6 months) at the 1 C rate. Moreover, the long‐cycling stability of the graphite‐based full cell with Prussian Blue cathode is demonstrated. [ABSTRACT FROM AUTHOR]

Rechargeable aluminum ion batteries (AIBs) are one of the most promising battery technologies for future large‐scale energy storage due to their high theoretical volumetric capacity, low‐cost, and high safety. However, the low capacity of the intercalation‐type cathode materials reduces the competitiveness of AIBs in practical applications. Herein, a conversion‐type FeF3‐expanded graphite (EG) composite is synthesized as a novel cathode material for AIBs with good conductivity and cycle stability. Combined with the introduction of a single‐wall carbon nanotube modified separator, the shuttle effect of the intermediate product, FeCl2, is significantly restrained. Moreover, enhanced coulombic efficiency and reversible capacity are achieved. The AIB exhibits a satisfying reversible specific capacity of 266 mAh g−1 at 60 mA g−1 after 200 cycles, and good Coulombic efficiency of nearly 100% after 400 cycles at a current density of 100 mA g−1. Ex situ X‐ray diffraction and X‐ray photoelectron spectroscopy are applied to explore the energy storage mechanism of FeF3 in AIBs. The results reveal that the intercalation of Al3+ species and the reduction of Fe3+ species occurrs in the discharge process. These findings are meaningful for the fundamental understanding of the FeF3 cathode for AIBs and provide unprecedented insight into novel conversion type cathode materials for AIBs. [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]

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x-ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K-edge absorption spectroscopy and density functional theory. [ABSTRACT FROM AUTHOR]

Understanding the reaction mechanism of olivine compounds as electrode materials for lithium lithium-ion batteries have has received much attention recently. The question whether olivine LiFePO4 undergoes two-phase or non-nonequilibrium single-phase reaction during electrochemical processes has taken center stage in the understanding of the faster reaction kinetics observed in this material. Here, the lithiation/delithiation mechanism of Mg Mg-substituted LiFePO4 using high high-resolution X-ray diffraction(XRD), transmission electron microscopy(TEM), and electrochemical measurements is reported. Ex situ partially (de)lithiated olivine- LiMg0.2Fe0.8PO4 show the existence of stable equilibrium intermediate phases as characterized by the presence of more than two phases and broadness of diffraction peaks. Electron energy loss spectroscopy profiles across individual nanoparticles further confirm uniform lithiation with a constant Fe-L3 energy measured across each nanoparticle, suggestive of solid solution behavior in individual particles. In addition, a continuous shift in the diffraction peak position is observed even in the 'two-phase' region in the ex situ electrochemical (de)lithiated electrodes. [ABSTRACT FROM AUTHOR]

Understanding the reaction mechanism of olivine compounds as electrode materials for lithium lithium-ion batteries have has received much attention recently. The question whether olivine LiFePO4 undergoes two-phase or non-nonequilibrium single-phase reaction during electrochemical processes has taken center stage in the understanding of the faster reaction kinetics observed in this material. Here, the lithiation/delithiation mechanism of Mg Mg-substituted LiFePO4 using high high-resolution X-ray diffraction(XRD), transmission electron microscopy(TEM), and electrochemical measurements is reported. Ex situ partially (de)lithiated olivine- LiMg0.2Fe0.8PO4 show the existence of stable equilibrium intermediate phases as characterized by the presence of more than two phases and broadness of diffraction peaks. Electron energy loss spectroscopy profiles across individual nanoparticles further confirm uniform lithiation with a constant Fe-L3 energy measured across each nanoparticle, suggestive of solid solution behavior in individual particles. In addition, a continuous shift in the diffraction peak position is observed even in the 'two-phase' region in the ex situ electrochemical (de)lithiated electrodes. [ABSTRACT FROM AUTHOR]

Thelayered structure of molybdenum (oxy)pyrophosphate (δ-(MoO2)2P2O7) was synthesized byheating MoO2HPO4·H2O precursorat 560 °C. The synthesis temperature was selected using in situhigh-temperature X-ray diffraction (XRD) depicting phase transformationsof the precursor from room temperature up to 800 °C. Electrochemicalevaluation reveals that up to four Li ions per formula unit can beintercalated into δ-(MoO2)2P2O7upon discharge to 2 V. Three voltage plateaus are observedat 3.2, 2.6, and 2.1 V, lower than the theoretical predictions. Thefirst plateau corresponds to the intercalation of 1.2 Li forming δ-Li1.2(MoO2)2P2O7,the same structure formed upon chemical lithiation with LiI. In-situXRD indicates two-phase reaction upon the first lithium insertionand expansion of the lithiated phase unit cell in the adirection. Intercalation of the second lithium results in a differentlithiated structure, which is also reversible, giving the capacityof about 110 mAh/g between 2.3 and 4 V. More lithium-ion intercalationleads to loss of crystallinity and structural reversibility. The Moreduction upon lithiation is consistent with the amount of Li intercalatedas confirmed by the X-ray absorption fine structure. [ABSTRACT FROM AUTHOR]

Advancements in portable electronic devices and electric powered transportation has drawn more attention to high energy density batteries, especially lithium–sulfur batteries due to the low cost of sulfur and its high energy density. However, the lithium–sulfur battery is still quite far from commercialization mostly because of incompatibility between all major components of the battery—the cathode, anode, and electrolyte. Here a methodology is demonstrated that shows promise in significantly improving battery stability by multilayer encapsulation of sulfur particles, while using conventional electrolytes, which allows a long cycle life and an improved Coulombic efficiency battery at low electrolyte feeding. The multilayer encapsulated sulfur battery demonstrates a Coulombic efficiency as high as 98%, when a binder‐free electrode is used. It is also shown that the all‐out self‐discharge of the cell after 168 h can be reduced from 34% in the regular sulfur battery to less than 9% in the battery with the multilayer encapsulated sulfur electrode. A stable lithium sulfur battery is enabled by a multilayer encapsulated sulfur nanocomposite electrode. The inner coating layer is a metal‐oxide capable of anchoring long‐chain polysulfides and the outer coating layer is a conductive porous polymer resulting in a battery with more than 500 cycles. [ABSTRACT FROM AUTHOR]