[Show abstract][Hide abstract] ABSTRACT: The epsilon polymorph of vanadyl phosphate ε-VOPO4 is a promising cathode material for high-capacity Li ion batteries, owing to its demonstrated ability to reversibly incorporate two lithium ions per redox center. As lithium is inserted into the nanosized particles within the cathode, the electrochemical reaction can be largely affected by the interfacial chemistry at the nanoparticle surface. We performed X-ray photoelectron spectroscopy using both soft (XPS) and hard (HAXPES) X-rays to chemically distinguish and depth-resolve the interfacial phase transitions in ε-VOPO4 electrodes as a function of electrochemical discharge. Our analysis shows that the second lithium reaction begins before the full incorporation of the first lithium. This results in a pronounced lithium gradient within the nanoparticles, with the ε-Li2VOPO4 phase only forming near the surface. These results indicate that a disruption of the kinetics are limiting the realized capacity in our hydrothermally synthesized ε-VOPO4. Moreover, from inspection of the valence band region, we were able to monitor the evolution of ε-VOPO4 to ε-Li2VOPO4 at the surface of our nanoparticles. These assignments are confirmed by hybrid density functional theory of the three end phases.
Full-text · Article · Nov 2015 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: Mixed-anion oxyfluorides (i.e., FeOxF2-x) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability via oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO0.7F1.3 at length scales spanning from single particles to the bulk electrode, via a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO0.7F1.3 nanoparticles retain a FeF2-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li-Fe-O(-F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries.
[Show abstract][Hide abstract] ABSTRACT: Using atomic layer deposition of Al2O3 coating, improved high-voltage cycling stability has been demonstrated for the layered nickel-manganese-cobalt pseudoternary oxide, LiNi0.4Mn0.4Co0.2O2. To understand the effect of the Al2O3 coating, we have utilized electrochemical impedance spectroscopy, operando synchrotron-based X-ray diffraction, and operando X-ray absorption near edge fine structure spectroscopy to characterize the structure and chemistry evolution of the LiNi0.4Mn0.4Co0.2O2 cathode during cycling. Using this combination of techniques, we show that the Al2O3 coating successfully mitigates the strong side reactions of the active material with the electrolyte at higher voltages (>4.4 V), without restricting the uptake and release of Li ions. The impact of the Al2O3 coating is also revealed at beginning of lithium deintercalation, with an observed delay in the evolution of oxidation and coordination environment for the Co and Mn ions in the coated electrode due to protection of the surface. This protection prevents the competing side reactions of the electrolyte with the highly active Ni oxide sites, promoting charge compensation via the oxidation of Ni and enabling high-voltage cycling stability. (Figure Presented).
Full-text · Article · Sep 2015 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: Much groundbreaking research in the field of lithium batteries occurred in the 1970s. Some of “the first” rechargeable lithium cells for commercial applications were fabricated by the Exxon Enterprises Battery Division in New Jersey. A small collection of 1978-era 25 mAh and 100 mAh button cells were preserved in the personal collections of the original researchers. This presented a unique opportunity to evaluate lithium cells after 35 years of storage. Cells were characterized for capacity, cycling, rate and impedance. Results were compared with original data as recovered from historical documents.
No preview · Article · Apr 2015 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: Electrochemical performance of hybrid supercapacitor (HSC) utilizing surface sculpted Li4Ti5O12 (LTO) insertion electrode having nanoplatelet-like morphology and activated carbon (AC) electrode is investigated for energy storage application. Cyclic voltammetry (CV) at variable scan rates 0.5 to 60 mV.s-1 in the 0-3.2 V range show pseusocapacitive behavior and fast rate of current change indicating rapid Faradaic kinetics. Nyquist impedance study show charge transfer resistance due to kinetic effects of electron transfer and Li+ de-intercalation process at the LTO anode. Low capacity (0.2 C-1C) charge-discharge (CD) curves show high Coulomb efficiency with marginal reduction at high 5-10 C rates due to irreversibility of adsorbed PF6 anions at the electrolyte-AC interface. Galvanostatic CD cycling tests over 50 cycles at different C-rates show decline in storage capacity due to electrode polarization effects. Reduction, broadening and shift of the Raman line at 678 cm-1 from Ti-O bonds in TiO6 octahedra after cycling indicates Li insertion reactions in functioning of hybrid supercapacitor. The hybrid supercapacitor cells have shown energy density, 29 Wh.kg-1 and power density, 350 W.kg-1.
No preview · Article · Jan 2015 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Dec 2014 · Advanced Energy Materials
[Show abstract][Hide abstract] ABSTRACT: This work attempts to understand the rate capability of layered transition metal oxides LiNiyMnyCo1−2yO2 (0.33 ≤ y ≤ 0.5). The rate capability of LiNiyMnyCo1−2yO2 increase with increasing Co in the compounds and with increasing amount of carbon additives in the electrodes. The lithium diffusion coefficients and electronic conductivities of LixNiyMnyCo1−2yO2 are investigated and compared. The 333 compound has higher diffusivity and electronic conductivity and thus better rate performance than 550. Chemical diffusion coefficients for both delithiation and lithiation of LixNiyMnyCo1−2yO2 investigated by GITT and PITT experiments are calculated to be around 10−10 cm2 s−1, lower than that of LixCoO2. The electronic conductivity of LixNiyMnyCo1−2yO2 is inferior compared to LixCoO2 at same temperature and delithiation stage. However, the LixNiyMnyCo1−2yO2 are able to deliver 55%–80% of theoretical capacity at 5 C with good electronic wiring in the composite electrode that make them very promising candidates for electric propulsion in terms of rate capability.
No preview · Article · Dec 2014 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: Fe-P alloys with high phosphorous content have been targeted as promising anode materials because of their high theoretical capacity. However, the synthesis and cycling performance remain great challenges. Hereby FePy (3 <= y <= 4) nanoparticles are facilely synthesized through a dry mechanochemical method by reacting iron and red phosphorus powders in an inert atmosphere. The morphology and crystal structure of this material are characterized by SEM and XRD, respectively, while the electrochemical performance is evaluated by a number of different techniques. The 1st and 2nd discharge capacity of FePy reaches 1984 mAh g(-1) and 1486 mAh g(-1), respectively, and after 10 cycles at 0.03 mA cm(-2) (20 mA g(-1) 0.03C), the capacity remains 1089 mAh g(-1) with a coulombic efficiency of 97%, much higher than the reported results to date. The cyclability of this material becomes fairly better at a higher current density, but the specific capacity is lower compared to that of the smaller current density. By adding fluoroethylene carbonate (FEC) to the electrolyte, the cycling performance of this material was improved. The EIS analysis has also been performed in order to better understand the capacity fade mechanism of FePy.
Full-text · Article · Dec 2014 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: In order to address lithium dendrite formation and low cycling efficiency issues, Pulse Plating (PP) and Reverse Pulse Plating (RPP) have been systematically investigated for lithium electrodeposition with a modified button cell device. Compared with Direct Current (DC) electrodeposition, PP waveforms with short and widely spaced pulses improve lithium deposition morphology and cycling efficiency under diffusion-controlled conditions. While RPP waveforms with high current density anodic pulses further improve lithium cycling efficiency, no obvious improvement in morphology was seen under the conditions tested. This study suggests that PP and RPP could be powerful tools for utilizing lithium metal anodes in high energy density rechargeable battery systems, especially when high instant power is required.
No preview · Article · Nov 2014 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: Understanding the phase transformation behavior of electrode materials for lithium ion batteries is critical in determining the electrode kinetics and battery performance. Here, we demonstrate the lithiation/delithiation mechanism and electrochemical behavior of the simferite compound, LiMg0.5Fe0.3Mn0.2PO4. In contrast to the equilibrium two-phase nature of LiFePO4, LiMg0.5Fe0.3Mn0.2PO4 undergoes a one-phase reaction mechanism as confirmed by ex situ X-ray diffraction at different states of delithiation and electrochemical measurements. The equilibrium voltage measurement by galvanostatic intermittent titration technique shows a continuous change in voltage at Mn3+/Mn2+ redox couple with addition of Mg2+ in LiMn0.4Fe0.6PO4 olivine structure. There is, however, no significant change in the Fe3+/Fe2+ redox potential.
No preview · Article · Nov 2014 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: There is a growing demand for energy storage, for intermittent renewable energy such as solar and wind power, for transportation, and for the myriad portable electronic devices. The presence of carbon leads creates a reducing atmosphere, preventing the oxidation of the ferrous formed to ferric. The transient change of lattice constant during the two-phase reaction is clearly observed by the time-resolved X-ray diffraction measurement. Once the lithium is disordered, then the reaction will proceed by a single phase so long as the ordering time is longer than the reaction time. The wider the single-phase regions, the narrower is the miscibility gap and the smaller is the lattice mismatch between the lithium-rich and lithium-poor materials. In addition, once the lithium disorder is generated, then if the time it takes to order is greater than the reaction time, phase separation will not occur and the system will stay disordered throughout the reaction.
No preview · Article · Oct 2014 · Chemical Reviews
[Show abstract][Hide abstract] ABSTRACT: Lithium cobalt germanate (Li2CoGeO4) has been synthesized for the first time by a hydrothermal method at 150 °C. Elemental composition, morphology, and crystal structure of this compound were characterized by various analytical techniques including SEM, TEM, ICP, and XRD analyses. Structure refinement and DFT calculation suggests the resulting Li2CoGeO4 from hydrothermal synthesis has a crystal structure isostructural with Li2ZnGeO4, significantly different from previous reports. Electrochemical studies confirmed Li2CoGeO4 as an active catalyst for oxygen evolution reaction (OER). In alkaline electrolyte (0.1 M NaOH), rotating disk electrodes made with Li2CoGeO4 as catalyst have a Tafel slope of c.a. 67 mV/dec and an overpotential of 330 mV at 50 μA/cm2cat, about 90 mV less than electrodes containing another known OER catalyst, Co3O4. Further characterization with cyclic voltammetry, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and elemental analyses revealed the oxidation of cobalt from 2+ to 3+ during the reaction, along with significant surface amorphization and loss of Li and Ge from the catalyst.
[Show abstract][Hide abstract] ABSTRACT: VOPO4 is an example of a Li-ion battery cathode that can achieve over 300 Ah/kg when two Li-ions are intercalated. A two phase beta-VOPO4/epsilon-VOPO4 composite was found to improve the cycling capacity of epsilon-VOPO4 from tetragonal H2VOPO4, particularly as the rate is increased. In the potential range of 2.0-4.5 V, this composite showed an initial electrochemical capacity of 208 mAh/g at 0.08 mA/cm(2), 190 mAh/g at 0.16 mA/cm(2), and 160 mAh/g at 0.41 mA/cm(2).
No preview · Article · Sep 2014 · Electrochemistry Communications
[Show abstract][Hide abstract] ABSTRACT: We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350–450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability.
[Show abstract][Hide abstract] ABSTRACT: We have successfully prepared nanoribbons of blue potassium molybdenum bronze (K0.3MoO3) from hydrogen molybdenum bronze (H0.3MoO3) by using crystallite splitting induced by solid-phase transformation and subsequent selective growth under hydrothermal conditions. The obtained nanoribbons were single crystals with a size of ca. 100 mu m x 100 nm x 10 nm, well-elongated to the crystal b axis that corresponds to the conduction direction as a quasi-one-dimensional metal. A simple. deaeration procedure necessary for preparing a single phase of blue potassium bronze is also given.
No preview · Article · Dec 2013 · Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: High-voltage cathode material LiNi0.5Mn1.5O4 has been prepared with a novel organic coprecipitation route. The as-prepared sample was compared with samples produced through traditional solid state method and hydroxide coprecipitation method. The morphology was observed by SEM and the spinel structures were characterized by XRD and FTIR. Besides the ordered/disordered distribution of Ni/Mn on octahedral sites, the confusion between Li and transition metal is pointed out to be another important factor responsible for the corresponding performance, which is worthy further investigation. Galvanostatic cycles, CV and EIS are employed to characterize the electrochemical properties. The organic coprecipitation route produced sample shows superior rate capability and stable structure during cycling.