Quantification of Charge Transport and Mass Deprivation in Solid Electrolyte Interphase for Kinetically-Stable Low-Temperature Lithium-Ion Batteries.

Graphite (Gr)-based lithium-ion batteries with admirable electrochemical performance below -20 °C are desired but are hindered by sluggish interfacial charge transport and desolvation process. Li salt dissociation via Li+-solvent interaction enables mobile Li+ liberation and contributes to bulk ion transport, while is contradictory to fast interfacial desolvation. Designing kinetically-stable solid electrolyte interphase (SEI) without compromising strong Li+-solvent interaction is expected to compatibly improve interfacial charge transport and desolvation kinetics. However, the relationship between physicochemical features and temperature-dependent kinetics properties of SEI remains vague. Herein, we propose four key thermodynamics parameters of SEI potentially influencing low-temperature electrochemistry, including electron work function, Li+ transfer barrier, surface energy, and desolvation energy. Based on the above parameters, we further define a novel descriptor, separation factor of SEI (SSEI), to quantitatively depict charge (Li+/e-) transport and solvent deprivation processes at Gr/electrolyte interface. A Li3PO4-based, inorganics-enriched SEI derived by Li difluorophosphate (LiDFP) additive exhibits the highest SSEI (4.89×103) to enable efficient Li+ conduction, e- blocking and rapid desolvation, and as a result, much suppressed Li-metal precipitation, electrolyte decomposition and Gr sheets exfoliation, thus improving low-temperature battery performances. Overall, our work originally provides visualized guides to improve low-temperature reaction kinetics/thermodynamics by constructing desirable SEI chemistry.

A Fast-Charge Graphite Anode with a Li-Ion-Conductive, Electron/Solvent-Repelling Interface.

Graphite has been serving as the key anode material of rechargeable Li-ion batteries, yet is difficultly charged within a quarter hour while maintaining stable electrochemistry. In addition to a defective edge structure that prevents fast Li-ion entry, the high-rate performance of graphite could be hampered by co-intercalation and parasitic reduction of solvent molecules at anode/electrolyte interface. Conventional surface modification by pitch-derived carbon barely isolates the solvent and electrons, and usually lead to inadequate rate capability to meet practical fast-charge requirements. Here we show that, by applying a MoOx-MoNx layer onto graphite surface, the interface allows fast Li-ion diffusion yet blocks solvent access and electron leakage. By regulating interfacial mass and charge transfer, the modified graphite anode delivers a reversible capacity of 340.3 mAh g-1 after 4000 cycles at 6 C, showing promises in building 10-min-rechargeable batteries with a long operation life.

Formulating the Electrolyte Towards High-Energy and Safe Rechargeable Lithium-Metal Batteries.

Rechargeable lithium-metal batteries with a cell-level specific energy of >400 Wh kg-1 are highly desired for next-generation storage applications, yet the research has been retarded by poor electrolyte-electrode compatibility and rigorous safety concerns. We demonstrate that by simply formulating the composition of conventional electrolytes, a hybrid electrolyte was constructed to ensure high (electro)chemical and thermal stability with both the Li-metal anode and the nickel-rich layered oxide cathodes. By employing the new electrolyte, Li∥LiNi0.6 Co0.2 Mn0.2 O2 cells show favorable cycling and rate performance, and a 10 Ah Li∥LiNi0.8 Co0.1 Mn0.1 O2 pouch cell demonstrates a practical specific energy of >450 Wh kg-1 . Our findings shed light on reasonable design principles for electrolyte and electrode/electrolyte interfaces toward practical realization of high-energy rechargeable batteries.

Preparation of a porous graphite felt electrode for advance vanadium redox flow batteries.

Rapid mass transfer and great electrochemical activity have become the critical points for designing electrodes in vanadium redox flow batteries (VRFBs). In this research, we show a porous graphite felt (GF@P) electrode to improve the electrochemical properties of VRFBs. The generation of pores on graphite felt electrodes is based on etching effects of iron to carbon. The voltage and energy efficiencies of VRFB based on the GF@P electrode can reach 72.6% and 70.7% at a current density of 200 mA cm-2, respectively, which are 8.3% and 7.9% better than that of untreated GF@U (graphite felt). Further, the VRFBs based on GF@P electrodes possess supreme stability after over 500 charge-discharge cycles at 200 mA cm-2. The high-efficiency approach reported in this study offers a new strategy for designing high-performance electrode materials applied in VRFBs.

Tuning wettability of molten lithium via a chemical strategy for lithium metal anodes.

Metallic lithium affords the highest theoretical capacity and lowest electrochemical potential and is viewed as a leading contender as an anode for high-energy-density rechargeable batteries. However, the poor wettability of molten lithium does not allow it to spread across the surface of lithiophobic substrates, hindering the production and application of this anode. Here we report a general chemical strategy to overcome this dilemma by reacting molten lithium with functional organic coatings or elemental additives. The Gibbs formation energy and newly formed chemical bonds are found to be the governing factor for the wetting behavior. As a result of the improved wettability, a series of ultrathin lithium of 10-20 µm thick is obtained together with impressive electrochemical performance in lithium metal batteries. These findings provide an overall guide for tuning the wettability of molten lithium and offer an affordable strategy for the large-scale production of ultrathin lithium, and could be further extended to other alkali metals, such as sodium and potassium.

Phosphorus and oxygen co-doped composite electrode with hierarchical electronic and ionic mixed conducting networks for vanadium redox flow batteries.

The hierarchical electronic and ionic mixed conducting networks build in graphite felt electrodes possess excellent electrocatalytic activity and faster electronic and ionic conduction, resulting in an enhanced energy efficiency of vanadium redox flow batteries with durable life for 1000 cycles and a high discharge capacity of 10.1 A h L-1 at a current density of 350 mA cm-2.

High-Performance Lithiated SiOx Anode Obtained by a Controllable and Efficient Prelithiation Strategy.

Silicon-based electrodes are promising and appealing for futuristic Li-ion batteries because of their high theoretical specific capacity. However, massive volume change of silicon upon lithiation and delithiation, accompanied by continual formation and destruction of the solid-electrolyte interface (SEI), leads to low Coulombic efficiency. Prelithiation of Si-based anode is regarded as an effective way for compensating for the loss of Li+ in the first discharging process. Here, a high-performance lithiated SiOx anode was prepared by using a controllable, efficient, and novel prelithiation strategy. The lithiation of SiOx is homogeneous and efficient in bulk due to well-improved Li+ diffusion in SiOx. Moreover, the in situ formed SEI during the process of prelithiation reduces the irreversible capacity loss in the first cycle and thus improves the initial Coulombic efficiency (ICE). Half-cells and full cells based on the as-prepared lithiated SiOx anode prominently increase the ICE from 79 to 89% and 68 to 87%, respectively. It is worth mentioning that the homogeneously lithiated SiOx anode achieves stable 200 cycles in NCM622//SiOx coin full cells. These exciting results provide applicable prospects of lithiated SiOx anode in the next-generation high-energy-density Li-ion batteries.

Nitriding-Interface-Regulated Lithium Plating Enables Flame-Retardant Electrolytes for High-Voltage Lithium Metal Batteries.

Safety concerns are impeding the applications of lithium metal batteries. Flame-retardant electrolytes, such as organic phosphates electrolytes (OPEs), could intrinsically eliminate fire hazards and improve battery safety. However, OPEs show poor compatibility with Li metal though the exact reason has yet to be identified. Here, the lithium plating process in OPEs and Li/OPEs interface chemistry were investigated through ex situ and in situ techniques, and the cause for this incompatibility was revealed to be the highly resistive and inhomogeneous interfaces. Further, a nitriding interface strategy was proposed to ameliorate this issue and a Li metal anode with an improved Li cycling stability (300 h) and dendrite-free morphology is achieved. Meanwhile, the full batteries coupled with nickel-rich cathodes, such as LiNi0.8 Co0.1 Mn0.1 O2 , show excellent cycling stability and outstanding safety (passed the nail penetration test). This successful nitriding-interface strategy paves a new way to handle the incompatibility between electrode and electrolyte.

Synthesis of rare-earth metal and rare-earth metal-fluoride nanoparticles in ionic liquids and propylene carbonate.

Decomposition of rare-earth tris(N,N'-diisopropyl-2-methylamidinato)metal(III) complexes [RE{MeC(N(iPr)2)}3] (RE(amd)3; RE = Pr(III), Gd(III), Er(III)) and tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium(III) (Eu(dpm)3) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][NTf2]) and in propylene carbonate (PC) yield oxide-free rare-earth metal nanoparticles (RE-NPs) in [BMIm][NTf2] and PC for RE = Pr, Gd and Er or rare-earth metal-fluoride nanoparticles (REF3-NPs) in the fluoride-donating IL [BMIm][BF4] for RE = Pr, Eu, Gd and Er. The crystalline phases and the absence of significant oxide impurities in RE-NPs and REF3-NPs were verified by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED) and high-resolution X-ray photoelectron spectroscopy (XPS). The size distributions of the nanoparticles were determined by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to an average diameter of (11 ± 6) to (38 ± 17) nm for the REF3-NPs from [BMIm][BF4]. The RE-NPs from [BMIm][NTf2] or PC showed diameters of (1.5 ± 0.5) to (5 ± 1) nm. The characterization was completed by energy-dispersive X-ray spectroscopy (EDX).

Critical Role of the Crystallite Size in Nanostructured Li4Ti5O12 Anodes for Lithium-Ion Batteries.

Lithium titanate Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages, such as the sluggish lithium-ion diffusion and poor electronic conductivity. To modify the performance of LTO as an anode material, nanosizing constitutes a promising approach and the impact is studied here by a systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies, including high-resolution transmission electron microscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry, and electrochemical impedance spectroscopy, a crystallite size of approx. 7 nm is identified as the optimum particle size. Such NPs exhibit as good reversible capacity as the ones with larger crystallite sizes but with a more pronounced interfacial charge storage. By decreasing the crystallite size to about 4 nm, the interfacial charge storage increases remarkably, however resulting in a loss of reversible capacity. An in-depth structural characterization using the PDF obtained from synchrotron XRD data indicates an enrichment in Ti for NPs with the small crystallite sizes, and this Ti-rich structure enables a higher Li storage. The electrochemical characterization confirms this result and furthermore points to a plausible reason as to why a higher Li storage in very small nanoparticles (4 nm) results in a loss in the reversible capacity.

Synthesis of metal-fluoride nanoparticles supported on thermally reduced graphite oxide.

Metal-fluoride nanoparticles, (MF x -NPs) with M = Fe, Co, Pr, Eu, supported on different types of thermally reduced graphite oxide (TRGO) were obtained by microwave-assisted thermal decomposition of transition-metal amidinates, (M{MeC[N(iPr)]2} n ) or [M(AMD) n ] with M = Fe(II), Co(II), Pr(III), and tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium, Eu(dpm)3, in the presence of TRGO in the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]). The crystalline phases of the metal fluorides synthesized in [BMIm][BF4] were identified by powder X-ray diffraction (PXRD) to be MF2 for M = Fe, Co and MF3 for M = Eu, Pr. The diameters and size distributions of MF x @TRGO were from (6 ± 2) to (102 ± 41) nm. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used for further characterization of the MF x -NPs. Electrochemical investigations of the FeF2-NPs@TRGO as cathode material for lithium-ion batteries were evaluated by galvanostatic charge/discharge profiles. The results indicate that the FeF2-NPs@TRGO as cathode material can present a specific capacity of 500 mAh/g at a current density of 50 mA/g, including a significant interfacial charge storage contribution. The obtained nanomaterials show a good rate capacity as well (220 mAh/g and 130 mAh/g) at a current density of 200 and 500 mA/g, respectively.

Corrigendum: Synthesis of Metal Nanoparticles and Metal Fluoride Nanoparticles from Metal Amidinate Precursors in 1-Butyl-3-Methylimidazolium Ionic Liquids and Propylene Carbonate.

[This corrects the article DOI: 10.1002/open.201600105.].

Synthesis of Metal Nanoparticles and Metal Fluoride Nanoparticles from Metal Amidinate Precursors in 1-Butyl-3-Methylimidazolium Ionic Liquids and Propylene Carbonate.

Decomposition of transition-metal amidinates [M{MeC(NiPr)2} n ] [M(AMD) n ; M=MnII, FeII, CoII, NiII, n=2; CuI, n=1) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (triflate) ([BMIm][TfO]), and 1-butyl-3-methylimidazolium tosylate ([BMIm][Tos]) or in propylene carbonate (PC) gives transition-metal nanoparticles (M-NPs) in non-fluorous media (e.g. [BMIm][Tos] and PC) or metal fluoride nanoparticles (MF2-NPs) for M=Mn, Fe, and Co in [BMIm][BF4]. FeF2-NPs can be prepared upon Fe(AMD)2 decomposition in [BMIm][BF4], [BMIm][PF6], and [BMIm][TfO]. The nanoparticles are stable in the absence of capping ligands (surfactants) for more than 6 weeks. The crystalline phases of the metal or metal fluoride synthesized in [BMIm][BF4] were identified by powder X-ray diffraction (PXRD) to exclusively Ni- and Cu-NPs or to solely MF2-NPs for M=Mn, Fe, and Co. The size and size dispersion of the nanoparticles were determined by transmission electron microscopy (TEM) to an average diameter of 2(±2) to 14(±4) nm for the M-NPs, except for the Cu-NPs in PC, which were 51(±8) nm. The MF2-NPs from [BMIm][BF4] were 15(±4) to 65(±18) nm. The average diameter from TEM is in fair agreement with the size evaluated from PXRD with the Scherrer equation. The characterization was complemented by energy-dispersive X-ray spectroscopy (EDX). Electrochemical investigations of the CoF2-NPs as cathode materials for lithium-ion batteries were simply evaluated by galvanostatic charge/discharge profiles, and the results indicated that the reversible capacity of the CoF2-NPs was much lower than the theoretical value, which may have originated from the complex conversion reaction mechanism and residue on the surface of the nanoparticles.