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Management of reversible lithium is an advantageous approach to design lithium ion cells that are tolerant to near zero volt (NZV) storage under fixed resistive load towards highly controllable, enhanced user-inactive safety. Presently, the first cycle loss from a high energy density Li-rich HE5050 cathode is used to provide excess reversible lithium when paired with an appropriately capacity matched mesocarbon microbead (MCMB) anode. Cells utilizing 1.2 M LiPF6 3:7 v/v ethylene carbonate:ethyl methyl carbonate electrolyte and a lithium reference were used for 3-electrode testing. After conditioning, a fixed resistive load was applied to 3-electrode cells for 72 or 168-h during which the anode potential and electrode asymptotic potential (EAP) remained less than the copper dissolution potential. After multiple storage cycles (room temperature or 40 °C), the NZV coulombic efficiency (cell reversibility) exceeded 97% and the discharge capacity retention was >98%. Conventional 2-electrode HE5050/MCMB pouch cells stored at NZV or open circuit for 3 days had nearly identical rate capability (up to 5C) and discharge performance stability (for 500 cycles under a 30% depth of discharge low-earth-orbit regime). Thus, lithium ion cells with appropriately capacity matched HE5050/MCMB electrodes have excellent tolerance to prolonged NZV storage, which can lead to enhanced user-inactive safety.
The high-frequency induction heating method is an attractive way to synthesize LiFePO4/C within a few minutes. We optimize the heating conditions by a carbon crucible for the homogeneous heating of a precursor pellet from both sides. Compared with our previous samples, the lattice parameters of the LiFePO4/C synthesized in this work (an optimized sample) are improved and are much closer to the values reported by Padhi et al. Although the primary particle size of the optimized sample is slightly larger than that of the previous samples, it is sufficiently small to fully utilize the electrochemical performances of LiFePO4. We reduced the internal charge-transfer resistance of the optimized sample by improving the crystal structure because the Nyquist plots of the electrodes indicate decreased resistance, even though the optimized sample's electronic conductivity is almost the same as that of the previous sample. The electrode based on the optimized sample shows a specific discharge capacity of 168.0 mAh g−1, which achieves 99% theoretical specific capacity of the LiFePO4 phase. Moreover, its charge–discharge rate performance is superior to that of the previous sample.
Extensive studies have already been performed in the past to integrate more than one ‘green’ energy source, e.g., solar, wind and hydrogen, for power generation. For actual operation in a realistic environment, such a hybrid process must be fully functional despite random fluctuations in energy supplies and power demands. A common option for accommodating the uncertain disturbances and their cumulative effects is to introduce battery into a properly structured system. However, by using an ad hoc approach, these schemes may be either overdesigned or inoperable. A generic mathematical programming model is thus adopted in the present study to compute a so-called temporal flexibility index for use as a performance measure. In order to demonstrate the usefulness of this assessment criterion, a large collection of photovoltaic–fuel cell (PVFC) systems can be configured for a specific application and then compared accordingly so as to identify the best combination of energy supply ratio and battery capacity. A MATLAB/Simulink simulation program has also been developed in this work to validate these design decisions.
In this study, synchrotron X-ray nano-computed tomography at Advanced Photon Source in Argonne National Laboratory has been employed to reconstruct real 3D active particle morphology of a LiMn2O4 (LMO) electrode commonly used in lithium-ion batteries (LIBs). For the first time, carbon-doped binder domain (CBD) has been included in the electrode structure as a 108nm thick uniform layer using image processing technique. With this unique model, stress generated inside four LMO particles with a uniform layer of CBD has been simulated, demonstrating its strong dependence on local morphology (surface concavity and convexity), and the mechanical properties of CBD such as Young’s modulus. Specifically, high levels of stress have been found in vicinity of particle’s center or near surface concave regions, however, much lower than the material failure limits even after discharging at the rate as high as 5C. On the other hand, the stress inside CBD has reached its mechanical limits when discharged at 5C, suggesting that it can potentially lead to failure by plastic deformation. The findings in this study highlight the importance of modeling LIB active particles with CBD and its appropriate compositional design and development to prevent the loss of electrical connectivity of the active particles from the percolated solid network and power losses due to CBD failure.
Nanocrystalline Co3O4 thin-film anodes were deposited on Pt-coated silicon and 304 stainless steel by radio frequency (RF) magnetron sputtering. The as-deposited and annealed cobalt oxide thin films showed smooth and crack-free morphologies. Both the as-deposited and annealed films exhibited spinel Co3O4 phase with nanocrystalline structure. High-temperature annealing enhanced the crystallinity of RF-sputtered cobalt oxide films due to rearrangement of cobalt and oxygen atoms. Electrochemical characterization of RF-sputtered films was carried out by cyclic voltammetry and charge/discharge tests in the voltage range of 0.3–3.0V. Cyclic voltammetry plots showed that the RF-sputtered Co3O4 thin films were electrochemically active. X-ray photoelectron spectrometer (XPS) showed that the fresh cobalt oxide films had two peaks of Co3O4. In addition to the binding energy of cobalt oxide, the XPS spectrum of discharged film presented two additional binding energies correspond to Co metal. The first discharge capacities of as-deposited, 300, 500, and 700°C-annealed films were 722.8, 772.5, 868.4, and 1059.9μAhcm−2 μm−1, respectively. High-temperature annealing could enhance the capacity and cycle retention obviously. After 25 cycles discharging, the annealed films showed better cycle retention than as-deposited film. The 700°C-annealed film exhibited excellent discharge capacity approximated to the theoretical capacity.
Cobalt hydroxide nanowires network grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with a specific capacitance of 610 F/g is successfully synthesized by a chemical precipitation method. Asymmetric-type pseudocapacitors are fabricated with Co(OH)2/NMEG and polypyrrole (PPy)/rG-O applied as positive and negative electrodes respectively. The electrochemical properties of the electrodes in three electrode and two electrode systems are systematically investigated in 1 M KOH electrolyte. Various supercapacitor devices, such as rG-O//Co(OH)2/NMEG, NG//NG, rG-O//rG-O, and PPy/rG-O//PPy/rG-O are assembled with electrochemical performance evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. The Co(OH)2/NMEG//PPy/rG-O asymmetric supercapacitor cells can achieve a high cell voltage of 1.6 V and an energy density up to 24.9 Wh/kg with an active materials loading of ∼5 mg/cm2, significantly higher than that of rG-O//Co(OH)2/NMEG (19.3 Wh/kg), NG//NG (16.4 Wh/kg), rG-O//rG-O (15.3 Wh/kg) and PPy/rG-O//PPy/rG-O (9.4 Wh/kg) supercapacitor devices under the same measurement environment. The PPy/rG-O is a superior negative electrode to match cobalt/nickel oxides/hydroxides based positive electrodes for supercapacitor devices.
ABSTRACT Two types of sodium-air cells, i.e., nonaqueous and mixed aqueous-nonaqueous (abbreviated as ‘aqueous’) cells, have been compared to elucidate factors limiting performances of nonaqueous air cells and how the aqueous electrolyte is effective to reliving these limitation. The two cells have the same configuration consisting of a nanoporous gold (NPG) air electrode and a ceramic separator of fast sodium ion conductor, NASICON. Only the selection of catholyte, either alkyl carbonate-based nonaqueous solution of NaClO4 or aqueous solution of NaOH, is different. All performances are demonstrated to be better for the aqueous one. Lower overpotential of aqueous cell leads to better round-trip efficiency. A large resistance relevant to oxygen reduction reaction in the nonaqueous cell is relieved by changing the catholyte to the aqueous electrolyte, affording higher rate capability and power density. The aqueous electrolyte is also effective to remove the limitation of the discharge capacity defined by the volumetric amount of air electrode, as has been claimed in previous studies on aqueous Na-air cells. Furthermore, the aqueous cell is demonstrated to be robust and less sensitive and to atmosphere. The NPG electrode works reversibly in the half-cell reaction of the aqueous cell, while an electrodeposition of metallic Na on the anode during charging requires an improvement.
Solid-state batteries have recently attracted great interest as potentially safe and stable high-energy storage systems. However, key issues remain unsolved, hindering full-scale commercialization.
Volunteers associated with the North Carolina Adult Asthma and Environment Study (NCAAES) participated in an investigation of personal daily exposures to coarse and fine particulate matter size fractions (PM10–2.5, PM2.5). Data from these personal measurements were then compared to community-based measures that might typically represent surrogate measurements of exposure often used in epidemiological assessments. To determine personal exposures to various particulate matter (PM) size fractions, a recently evaluated personal PM monitor capable of direct PM10–2.5 size fraction collection was used. Participants living in the central region of North Carolina and enrolled in the NCAAES were asked to wear the monitor attached to a supporting backpack for 24-h collection periods. These volunteers were monitored for 2 to 4days with subsequent gravimetric analysis of their PM samples. Personal PM10–2.5 mass concentrations were observed to be highly variable and ranged from 7.6 to 40.2μg/m3 over an 8-month period. The median for this measurement from all participants (50th percentile) was 13.7μg/m3. A coefficient of determination (r 2) of 0.02 was established for community-based PM10–2.5 mass concentrations versus personal exposures. Similar coefficients established for PM2.5 mass revealed only a modest improvement in agreement (r 2 =0.12). Data from the exposure findings are reported here.
The aging stability of several Li2FeSiO4 (LFS) polymorphs in LiPF6-based organic carbonate solvent electrolyte at elevated temperature was investigated. The different LFS polymorphs were obtained by hydrothermal reaction and subsequent annealing at different temperatures. In addition, a carbon-coated sample was prepared. The samples were aged separately from each other at 60°C in LiPF6-based organic electrolyte for 10 days and analyzed afterwards. The characterization of the residual powder by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) reveals a different aging behavior depending on the structure of the LFS polymorph. This influence of the polymorph on the stability of LFS powder against HF was confirmed by investigations of the aged electrolyte by nuclear magnetic resonance (NMR) spectroscopy and ion chromatography (IC).
The rational design of cathode host materials is significant in fulfilling high-efficiency sulfur electrochemistry as well as boosting the energy density of lithium–sulfur (Li–S) batteries. Herein, we develop a stringed “tube on cube” nanohybrid (CPZC) with a ternary hierarchical architecture, which contains a fibrous carbon skeleton, highly porous carbon cube filler, and abundant CNT tentacles as an advanced matrix for sulfur electrodes. The as-developed CPZC delivers excellent conductivity, abundant active interfaces, and strong confinement to polysulfide, and thus is capable of significantly expediting the sulfur redox kinetics and promoting battery durability. The fabricated sulfur electrode achieves a superb rate capability up to 10C, outstanding cyclability over 2000 cycles, and more importantly, excellent performance under high a sulfur loading and sparing electrolyte with a high energy density of 348.8 W h kg−1 and 327.6 W h L−1 at the system level, which reveals its potential in promoting the practical application of Li–S batteries.
Highly efficient electrocatalysts with high intrinsic activity for oxygen and sulfur redox reactions are strongly required for sustainable energy systems. Generally, cations serve as the real active sites in transition metal compound electrocatalysts, whose electrocatalytic activity is regulated by the surrounding anionic structure. Herein, an electrochemical reaction assisted by an anionic regulation strategy is proposed for precise construction of advanced electrocatalysts with extraordinary electrocatalytic activity. The electrochemical anionic regulation process ensures general release of the regulation reagents for precise substitution of sulfur anions in pristine hydroxide. The as-obtained hydroxysulfide electrocatalyst exhibits a desired electronic structure to afford superb electrocatalytic activity regarding reduced overpotential of 286 mV at 10 mA cm−2 for electrocatalytic oxygen evolution and improved polysulfide redox electrocatalytic activity. This contribution not only renders an emerging strategy for precise regulation of the anionic structure for improved electrocatalytic activity, but also provides information for the rational design of advanced electrocatalysts for sustainable energy applications.
As a popular commercialized cathode material in lithium-ion batteries, the cycle life, practical reversible capacity, and rate capabilities of LiCoO2 (LCO) at high voltage are limited owning to structural irreversibility and surface side reactions. Here, we employ a binary Ba and Ti-based hybrid surface treatment on LiCoO2 (LCO@BT) by facile wet chemical routes. This strategy integrates the advantages of both interface particle doping and surface coating as the layered structure of LCO is stabilized by a binary hybrid surface treatment, and the modified layer promotes the surface Li+ diffusivity and protects the LCO cathode from steady corrosion induced by surface side reaction. On the basis of in-situ NMR characterizations, it is found that after the first electrochemical delithiation/lithiation the phase reversibly changes from O3–I-type in LCO@BT and back to O3–I-type LxCO@BT (0.98 ≤ x＜1) rather than to a two-phase domain of O3–I and O3-II phase in uncoated LxCO (x＜0.98). The target material is achievable in displaying an initial discharge capacity of 190.5 mAh g−1 and delivers a high capacity retention of 90.29% (172 mAh g−1) at 0.2C after 100 cycles which is superior to most LiCoO2 cathodes that are operated at high cut-off voltage of 4.5V.
The Li2MnO3 coated LiNi0.8Mn0.1Co0.1O2 electrode has been sucessfully synthesized through a PVP-chelation and syn-lithiation strategy. X-ray diffraction (XRD) and X-ray photoelectron Spectrometer (XPS) characterizations are applied to verify the existence of the Li2MnO3 surface layer. The complete and nanoscale Li2MnO3 coating layer strongly adheres to the host material because of the layered/layered homostyructure, improves the content of Mn element on the surface of the electrode and has three-dimensional path for Li+-ion diffusion. Due to the numerous unique and dramatic advantages of the Li2MnO3 surface layer, the layered/layered homostyructure electrode exhibits superior cycle stability, rate capability and other electrochemical properties. The newly developmental coating material and versatile nanocoating strategy can also be popularized and applied in other electrode materials.
A redox-active organic molecular electrode is prepared for supercapacitors. Benzo[1,2-b:4,5-b']dithiophene-4,8-dione (BDTD), planar molecule with fused heteroaromatic structure, has been adsorbed on conductive reduced graphene oxide (rGO) by π–π interactions to form a 3D interconnected and functionalized xerogel (BDTD-rGO). Because the five-membered aromatic heterocycle is more electron-rich than the six-membered aromatic ring, which enlarges the electronic interaction between the BDTD molecule and the conjugated graphene network and accordingly reduce the solubility of BDTD molecule in electrolyte. As a result, the optimized BDTD-rGO electrodes achieve specific capacitance of 360 F g−1 at 1 A g−1, with ultra-long cycle life of 96.4% after 10,000 cycles, and even 80% after 50,000 cycles at 5 A g−1 in 1 M H2SO4. Furthermore, the asymmetric supercapacitor (ASC), which is assembled by using the BDTD-rGO as the negative electrode and lamellar holey graphene hydrogel (LGH) as the positive electrode respectively, exhibits better energy storage performance.
The charge-compensation mechanism and host stability of a cathode play the central role in determining its reversible capacity. Here, first-principle calculations are presented to study the charge compensation and its effect on the stability of orthosilicates, Li2−x TMSiO4 (TM = Fe, Mn), as the promising high-capacity cathode materials for Li-ion batteries. The charge compensation in Li2−x TMSiO4 upon delithiation is found to be achieved first by a combined reversible TM and oxygen redox process, originating from the dynamic response of their electronic structures to the Li ions (or electrons) removal and the associated charge transfer from the O to Fe ions, and then by the irreversible formation of O vacancy (VO) that destroys the host stability of these materials. Whether the formation of VO in these materials upon delithiation would occur is demonstrated to be essentially determined by the energy level of their highest occupied electronic states and can be understood by the defect charge transition mechanism which provides a quantitative way to estimate to what extent the oxygen redox could be reversibly used in a cathode that being important for the future design of high-capacity cathode materials.
Porous and multi-layer network of interconnected silver particles is deposited by galvanic displacement on a technologically relevant substrate, silicon with an aluminum/copper film. The mean particle diameter is approximately 200nm and the particle density in a single layer is 109 particles per cm2. Cyclic voltammetry and electrochemical impedance spectroscopy reveal that capacitance normalized to the electrode geometric area reaches a value of 1.7±0.2mF/cm2, which is about two orders of magnitude higher than that observed on a smooth silver/electrolyte interface. The specific surface area of silver particles, which are assumed to be spherical, is 2.7m2/g. The electrolyte accessible surface area is slightly larger (3.5m2/g) due to the surface roughness of silver particles. The frequency response of the porous network of silver particles is analyzed using the transmission line model. The “knee” frequency is determined to be around 200Hz. The described capacitor could find applications for special electronic circuits where a high-frequency response is needed.
Organic-inorganic hybrid perovskites have been representing a scientific breakthrough in the photovoltaic field since 2009 when they were applied to replace photoactive dyes in hybrid solar cells. Further development has been highly boosted by a large and enthusiastic effort up to a current maximum efficiency of 24.2%. The exceptionality of this class of materials resides in their soft character combined with long diffusion lengths of the photo-generated carriers, a wide absorption range and direct tunable bandgap. Nonetheless, the low structural stability of the hybrid perovskites, primary MAPbI3, risks to severely retard their wide-range applications in low-cost/high-yield devices. Focused research is currently relating instability sources and degradation mechanisms with the operation conditions, including temperature, illumination, humidity, contaminants and interfacing materials. Although the overall scenario is brighter than years ago, reliable and long-lasting solutions to avoid back-reaction of perovskites to the starting byproducts and indeed to extend cell durability are under spotlight. For the market uptake, moreover, device architectures to be produced via simple and sequential steps, free of contaminants and at low environmental impact, are warmly encouraged to catch the interest of investors. The paper will thus frame strengths and weaknesses of hybrid perovskites for next-generation photovoltaics in view of their extended use and dissemination in daily life.
The ability to shift between repetitive and goal-directed actions is a hallmark of cognitive control. Previous studies have reported that adaptive shifts in behavior are accompanied by changes of neural activity in frontal cortex. However, neural and behavioral adaptations can occur at multiple time scales, and their relationship remains poorly defined. Here we developed an adaptive sensorimotor decision-making task for head-fixed mice, requiring them to shift flexibly between multiple auditory–motor mappings. Two-photon calcium imaging of secondary motor cortex (M2) revealed different ensemble activity states for each mapping. When adapting to a conditional mapping, transitions in ensemble activity were abrupt and occurred before the recovery of behavioral performance. By contrast, gradual and delayed transitions accompanied shifts toward repetitive responding. These results demonstrate distinct ensemble signatures associated with the start versus end of sensory-guided behavior and suggest that M2 leads in engaging goal-directed response strategies that require sensorimotor associations.
LiFePO4 has been a promising cathode material for rechargeable lithium ion batteries. Different secondary or impurity phases, forming during either synthesis or subsequent redox process under normal operating conditions, can have a significant impact on the performance of the electrode. The exploration of the electronic and chemical structures of impurity phases is crucial to understand such influence. We have embarked on a series of synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy studies for the element speciation in various impurity phase materials relevant to LiFePO4 for Li ion batteries. In the present report, soft-X-ray XANES spectra of Li K-edge, P L2,3-edge, O K-edge and Fe L2,3-edge have been obtained for LiFePO4 in crystalline, disordered and amorphous forms and some possible “impurities”, including LiPO3, Li4P2O7, Li3PO4, Fe3(PO4)2, FePO4, and Fe2O3. The results indicate that each element from different pure reference compounds exhibits unique spectral features in terms of energy position, shape and intensity of the resonances in its XANES. In addition, inverse partial fluorescence yield (IPFY) reveals the surface vs. bulk property of the specimens. Therefore, the spectral data provided here can be used as standards in the future for phase composition analysis.