Ion Transport In Electrolytes Research Articles

Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation Studies of Nonaqueous Lithium Ion Battery Electrolytes.

Lithium ion battery (LIB) technology is undoubtedly indispensable to modern life. However, despite enormous and extended effort to improve LIB performance, our understanding of the underlying principles and mechanisms of lithium ion transport in nonaqueous LIB electrolytes remained limited until recently. There is a particular lack of knowledge of the microscopic solvation structures and fluctuation dynamics around charge carriers in real electrolytes. Typical electrolytes found in commercially available LIBs consist of lithium salts and mixed carbonate solvents, with the latter playing an essential role in promoting lithium ion transport and forming an electrically stable solid electrolyte interphase. Although a number of linear spectroscopic studies of LIB electrolytes aiming at understanding the complex nature of lithium ion solvation processes have been reported, the notion that each lithium ion is strongly solvated by carbonate molecules to form a long-lasting solvation sheath structure has remained the subject of intense debate. Here, we present the results of FTIR, fs IR pump-probe, two-dimensional IR spectroscopy, and molecular dynamics simulations reported by us and others and discuss the possible interplay of picosecond solvation dynamics and macroscopic ion transport processes within the framework of the fluctuation-dissipation relationship. Further, by measuring the time-dependent fluctuations and spectral diffusions of carbonate carbonyl stretch modes that act as excellent infrared probes for the local electrostatic environment, we show that lithium cations are not only solvated by carbonate molecules but also interact with counteranions at equilibrium depending on solvent composition. Molecular dynamics simulations support the notion that rapid chemical exchanges between carbonate solvent molecules in the first and outer solvation shells are critical for describing mobile lithium ion transport phenomena. We thus anticipate that time-resolved coherent multidimensional vibrational spectroscopy is capable of providing decisive evidence on the ultrafast solvent dynamics of various electrolytes, which is potentially helpful for designing improved and more efficient LIB electrolytes in the future.

Low content of Fe3C anchored on Fe,N,S-codoped graphene-like carbon as bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

A novel kind of non-precious metal Fe3C@Fe,N,S-GCM nanocomposite with very low metal content is obtained by a simple one-pot pyrolysis method using abundent, low cost and environmental friendly resouces. In the Fe3C@Fe,N,S-GCM nanocomposite, the Fe3C nanoparticles are uniformly anchored on the Fe,N,S-codoped graphene-like carbonaceous matrix. Synergetic multiple electrocatalytic active sites of Fe3C, Fe-Nx and N,S-co-doped carbon are identified for Fe3C@Fe,N,S-GCM. The Fe3C@Fe,N,S-GCM nanocomposite also possesses the high BET surface area and meso/macropore volume from the hierarchical porous superstructure, which may benefit the storing and transport of electrolyte ions. Attributed to the rational content and microstructure, the Fe3C@Fe,N,S-GCM presents a high bifunctional electrocatalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with the ΔE (= EOER - EORR) of only 0.750 V. The bifunctional electrocatalytic activity is not only better than the commercial precious metal couples of RuO2 (OER) and Pt/C (ORR), but also represents the superiority among the non-precious metal ferrous bifunctional electrocatalysts.

High-Energy Flexible Supercapacitor-Synergistic Effects of Polyhydroquinone and RuO2· xH2O with Microsized, Few-Layered, Self-Supportive Exfoliated-Graphite Sheets.

An effective and straightforward route for tailoring the self-supporting, exfoliated flexible graphite substrate (E-FGS) using electrochemical anodization is proposed. E-FGS has essential features of highly exfoliated, few-layered, two-dimensional graphite sheets with the size of several tens of micrometers, interconnected along the axis of the substrate surface. The novel hierarchical porous structural morphology of E-FGS enables large active sites for efficient electrolyte ion and charge transport when used as electrode material for a supercapacitor. In order to effectively utilize this promising E-FGS electrode for energy storage purpose, a ternary composite is further synthesized with pseudocapacitive polyhydroquinone (PHQ) and hydrous RuO2 (hRO). hRO is synthesized via a sol-gel route, while electropolymerization is utilized for the electrodeposition of PHQ over E-FGS. Ultimately, the fabricated self-supporting E-FGS-based flexible supercapacitor is capable of delivering areal specific capacitance values as high as 378 mF cm-2 at a current density of 1 mA cm-2. Addition of the pseudocapacitive component to the E-FGS texture leads to ∼10 times increase of the electrochemical charge storage capability. The imposition of mechanical forces to this flexible supercapacitor device results in trivial changes in electrochemical properties and is still capable of retaining 91% of the initial specific capacitance after 10 000 cycles. Alongside, the fabricated symmetrical solid-state flexible device exhibited a high energy density of 8.4 μWh cm-2. The excellent performance along with the ease of synthesis and fabrication process of the flexible solid-state supercapacitor device using PHQ/hRO/E-FGS holds promise for large-scale production.

(Invited) Self-Assembly Synthesis and Interfacial Control of Electrode Architectures

Enhancement in electrochemical energy storage resides in tailored nanotextures resulting from making electrode materials hierarchically nanoporous. The complex interplay of solvent and solute structure and dynamics at the charged interface, the transport of electrolyte ions into and out of the pores, the solvation/desolvation processes occurring in pores approaching bare-ion dimensions, and formation of interfaces via chemical reactions are all important parameters. Herein, several self-assembly synthesis methods for carbon and oxide composites as electrode materials for energy storage will be discussed. The objective of this talk is to demonstrate that mesoporous carbons and oxides derived from soft-template synthesis not only entail a high storage capacity but also, most importantly, can be made through self-assembly synthesis to have a significantly enhanced electronic conductivity and storage capacity. This enhanced electronic conductivity in 3D architectures is the key to providing high rate capability for the corresponding energy storage systems.

Na+ Ion Transport in Glassy Electrolytes for Solid-State Sodium Ion Batteries: A Reactive Molecular Dynamics Study

All solid-state sodium ion battery is a promising technology for electrical grid energy storage. The low ionic conductivity of solid-state electrolytes is a major challenge towards commercialization of solid-state sodium ion batteries. Sulfide glasses are potential electrolytes possessing excellent ionic conductivity at room temperature for high performance sodium ion batteries. Understanding the Na+ ion transport in these glasses is critical towards developing novel electrolytes for these all solid-state sodium batteries. In-situ characterization of Na+ ion transport through experiments is extremely challenging. Molecular dynamics (MD) simulations offer an economical route for investigating the local structure of these glasses as well as Na+ ion transport mechanisms. Classical MD simulations employ empirical forcefields to define pairwise interactions between constituent ions in these glasses. These empirical forcefields captures the short-range order accurately, but cannot capture the medium-range order. Additionally, these empirical forcefields are not to account for the complex chemistry in sulfide glasses makes it extremely challenging to model these glasses. While ab initio MD simulations are able to overcome these limitations, the maximum system size that can be realistically modeled using ab initio MD is limited (400-500 atoms) and cannot account for the bulk properties of these glasses. Reactive force fields, such as ReaxFF, provide an effective route to accurately capture bond breakage and formation, while simulating considerably large systems (~ 100,000 atoms). These reactive force fields (ReaxFF) are based on bond-order formalism in conjunction with charge equilibration scheme. ReaxFF can more accurately capture the complex electrode-electrolyte interfacial reactions as compared to both classical and ab initio MD. Understanding these interfacial reactions is extremely crucial in developing a stable interface for high performance sodium ion batteries. ReaxFF forcefields are highly system specific and need to be parameterized for an individual system. In the present study, we employed Monte Carlo Force Fields (MCFF) optimizer to parameterize ReaxFF for x Na2S – (1-x) SiS2 glasses. We utilized ReaxFF for Li/Al/Si/O as a starting forcefield which was expanded for the x Na2S – (1-x) SiS2 glasses. Parameterization was performed by fitting against formation energies, atomic configurations and charge distributions for a range of representative clusters and equations of state obtained through DFT calculations. The parameterized reactive forcefields were validated by comparing the pair distribution functions (PDF’s) obtained through MD simulations to those obtained through X-ray scattering experiments. Na+ ion transport was investigated in these glasses characterizing the changes in the local structure with composition. Interfacial reactions between pure sodium metal and glassy electrolyte were further analyzed using these parameterized forcefields.

Template-assisted synthesized MoS2/polyaniline hollow microsphere electrode for high performance supercapacitors

Herein we report a template-assisted method to synthesize molybdenum disulfide/polyaniline (MoS2/PANI) hollow microspheres with excellent electrochemical performances for supercapacitors applications. The hollow MoS2 microspheres are assembled by layered MoS2 nanosheets, which provide substantial ion channels and large surface area for electrolyte ion transport. PANI is grown on the surface of the hollow MoS2 microspheres by oxidative polymerization with different mass. The MoS2/PANI-1:2 hollow microspheres electrode possesses the maximum specific capacitance of 364 F/g at a scan rate of 5 mV/s. In addition, the specific capacitance achieves 299 F/g with a current density of 10 A/g and 84.3% of the specific capacitance is retained after 8000 cycles at 10 A/g. The energy density can be as high as 32 Wh/kg at a power density of 320 W/kg. Even at a very high power density of 8320 W/kg, the energy density still maintains 23.1 Wh/kg, showing a large power density range for supercapacitor applications. We assemble a symmetric supercapacitor by using two identical MoS2/PANI microspheres electrodes, and it shows high specific capacitances of 231, 139 F/g at 0.2 and 0.5 A/g, respectively. Our results suggest that the MoS2/PANI hollow microspheres electrode have good cycle stability and significant potential for supercapacitor applications.

S-doping coupled with pore-structure modulation to conducting carbon black: Toward high mass loading electrical double-layer capacitor

The conductive carbon black (CB) was modified by the pore-structure modulation and S doping in the presence of pore-modulation agent and S dopant via the one-step calcination methodology, resulting in the S-doped and pore-structure modified CB (labeled as SPCB). The conductivity and contact angle tests prove that the function of S doping can not only increase the electrical conductivity but also enhance the hydrophilicity. As electrode materials for symmetric electrical double-layer capacitor (EDLC), the influence of different mass loading of the electrode on the capacitive behavior is investigated. The areal mass loading of active materials for each electrode in the symmetric EDLC can reach up to 10 mg cm−2, comparable to the values of commercial devices. Without obvious sacrifice of the gravimetric capacitance is observed with the increase of mass loading, illustrating the small electrolyte ion transport resistance in electrode material and quick charge transfer property as well as the great compatibility for the gravimetric, areal and total capacitances. Additionally, the symmetric EDLC configured with SPCB affords the larger capacitance, better rate capability and higher capacitance retention as compared to the commercial activated carbon.

Vertically-aligned nanostructures for electrochemical energy storage

Energy storage devices with high energy and power densities are highly attractive for various applications ranging from portable electronics to electric vehicles and grid-level energy storage, such as rechargeable batteries and supercapacitors. One limiting factor in power density is the ion transport in electrolyte, particularly in tortuous electrode materials with low porosity. A viable approach to enhance ion transport in electrolyte is to create vertically aligned structures and thus reduce electrode tortuosity. In the past decades, various methods have been explored to develop vertically aligned structures. This review summarizes battery kinetics to illustrate the importance of low tortuosity in electrodes, and then introduces various methods to create vertically aligned nanostructures, such as direct growth, templating and microfabrications. The electrochemical performance of electrodes or electrolytes created by each method is presented. At the end, this paper discusses challenges with these structures and the directions these technologies can be taken in the future.

Mesoporous carbon nanotube microspheres supported microporous pyrolytic carbon for high-performance supercapacitors

An ideal carbon-based supercapacitor electrode requires abundant micropores that are favorable for charge accumulation, and suitable mesopores that are beneficial for fast electrolyte ions transport. Herein, 3D porous carbon microspheres (carbon nanotube-ketjen black/activated carbon, CNT-KB/AC), where the CNT-KB microspheres (CKMS) as the framework providing mesopores and the activated carbon as the core providing micropores, have been constructed by spray drying and KOH activation. The obtained carbon composites exhibit a homogeneous spherical structure and high specific surface area (up to 1751m2g−1). The CNT-KB/AC electrode with an areal density of 3mgcm−2 can achieve a high areal capacitance of 506.9mFcm−2 at the current density of 0.2Ag−1 in 6M KOH solution, which is much larger than that of the CNT/AC electrode (232.6mFcm−2) and CNT-AB/AC electrode (179.5mFcm−2). Besides, when the areal density is increased up to 9mgcm−2, the areal capacitance of CNT-KB/AC is increased up to 998.9mFcm−2 at 0.2Ag−1 and 724.9mFcm−2 at 20Ag−1. Furthermore, it exhibits excellent long-term cycling stability (achieving capacitance retention of 94% after 10,000cycles). These results indicate that the CNT-KB/AC composites are promising electrode materials for high-areal-density supercapacitors.

Interplay between grain boundary diffusion and annealing temperature on the conductivity of NiO coated Samarium doped ceria

Elemental diffusion in ceramics can modulate the conductivity as well as fuel cell performance. Here, we report the effect of diffusion on NiO coated samarium doped ceria (SDC) with temperature and correlated with conductivity. NiO thin film was deposited using electron beam physical vapor deposition over the pre-sintered SDC substrate and subsequently treated at 800 °C, 1000 °C, and 1200 °C. Depth profiling analysis across NiO-SDC hetero-interface using FESEM-EDAX revealed that the width of the diffusion zone increased with temperature. All the constituent ions like Ni, Sm and Ce diffuse mutually without forming any impurity phases at the NiO-SDC interface. The estimated diffusivity values demonstrated that the nickel predominantly diffuses through the grain boundary of SDC. The constituent cations were found to diffuse several hundred nanometers through grain boundaries at a temperature of 1200 °C. An increase in the electrical conductivity was observed in proportion to the width of the diffusion layer. The diffusion of nickel predominantly along the grain boundary reduced the grain boundary resistance by altering the space charge potential at the NiO-SDC interface. Our study demonstrates that the modification of grain boundary through the controlled diffusion of metal ions reduce the resistance for oxygen ion transport in NiO-SDC electrolyte.

Fabrication of composite probe electrode used for localized impedance analysis of solid-state electrolyte LATP

A composite probe combining manganese-based active material with a tipless probe has been developed using a simplified dip-pen process for the localized impedance measurement (LIM) of a solid-state phosphate electrolyte. The active material is found to be composed of a spinel LiMn2O4 crystal phase, while the crystal structure of the solid-state electrolyte is indicated as lithium aluminum titanium phosphate (LATP). Compared with the impedance measurement of the LATP through a platinum/LATP/platinum (Pt/LATP/Pt) metal-insulator-metal (MIM) cell, in which only the overall bulk electrolyte resistance can be recognized, a LIM performed with a tipless probe enables discernment of the grain boundary resistance from the bulk electrolyte. It is seen that ion transport in the bulk electrolyte can be facilitated by applying external DC bias. The composite probe further allows us to detect the charge transfer resistance in the LiMn2O4/LATP interface in addition to the grain boundary resistance. It is also found that the charge transfer and grain boundary resistances increase with a similar trend when a specific high bias voltage (>1.5 V) is applied. This can be attributed to more ion extraction from LiMn2O4 into the LATP under the bias voltage. Consequently, the excess ions accumulated in the LATP would either inhibit further ion transport into LATP or retard ion transport across the electrolyte grain boundary.

Ion transport and association study of glyme-based electrolytes with lithium and sodium salts

Glyme solvents are a promising avenue of research in novel electrolytes, due to their thermal and chemical stability, large electrochemical window, and tunable properties. This work investigates the transport properties of LiPF6 and NaPF6 salts in Monoglyme (G1), Diglyme (G2), and Tetraglyme (G4). Self-Diffusion coefficients, and degrees of ion association were found through spectroscopic techniques. Raman and Fourier-Transform Infrared (FTIR) spectroscopy exhibited an absorbance peak at 740 cm−1, corresponding to the a1g mode; known to signify the presence of solvent-separated and contact ion pairs. The increased intensity of this peak for G1 compared to G4 suggests that the electrolyte exhibits stronger association with decreasing solvent size. Nuclear Magnetic Resonance Diffusometry measurements yield self-diffusion coefficients for 7Li, 23Na, and 19F across all samples to be on the order of 10−9 m2s−1 in G1, to 10−11 m2s−1 in G4. Comparison of conductivities calculated from the measured diffusivities and the Nernst-Einstein equation with conductivity measurements deduced from Electrochemical Impedance Spectroscopy (EIS) determined the ion association degree; the electrolytes were shown to exhibit stronger ion pairing with increased temperature which is attributed to a decrease in the dielectric constant of the solvents with increasing temperature. Additionally, the ion association was shown to decrease with increasing solvent molecular size, consistent with FTIR findings.

Magazine‐Bending‐Inspired Architecting Anti‐T of MXene Flakes with Vertical Ion Transport for High‐Performance Supercapacitors

AbstractMXenes have shown distinctive advantages as active materials for supercapacitors due to their 2D open structure and high electronic conductivity. However, the horizontal stack and random orientation of MXene flakes on a current collector lead to the limited accessible surface area and transport pathways. Blocking the ion transport is largely deteriorated for the electrochemical performance of an MXene electrode. Herein, a vacuum filtration process, affording with the entwined mental mesh as filter membranes, is employed for constructing an “anti‐T‐shape” MXene film electrode. The resultant vertical ionic tunnels improve the kinetics of electrolyte ion transport. Consequently, a lower interfacial charge transfer impedance and an increased pseudo‐capacity are achieved in the sulfuric acid electrolyte. This work may inspire further investigations on anisotropic properties of MXenes beyond electrochemical energy storage.

Surface activated polyethylene separator promoting Li+ ion transport in gel polymer electrolytes and cycling stability of Li-metal anode

This paper proposes a strategy to fabricate surface activated polyethylene (PE)-supported gel polymer electrolyte (GPE) with high ion transport ability, excellent electrolyte retention and mechanical properties to stabilize lithium (Li)-metal anodes. The inert outer and inner pore surface activation of polyethylene is demonstrated by coating an ultrathin zirconium oxide nanocrystal (ZrO2)/polyhedral oligomeric silsesquioxane (POSS) composite layer through a simple layer by layer (LBL) assembly method prior to the in situ polymerization. It is found that the activation layer may improve the Li+ ion transference number and induce the formation of GPE with a gradient structure by the interaction with the initiator system, giving rise to higher ion transport ability of final GPE. On the other hand, the GPE using the activated PE separator as support improves the Li/electrolyte interfacial stability during storage and repeated lithium plating/stripping cycling. A stable voltage profile with cycling for more than 800 h in a Li/Li symmetric cell was obtained by using surface activated PE-supported GPE. When it is assembled into the cells with metallic lithium anodes and lithium cobalt oxide (LiCoO2) cathodes, the cells show excellent rate capability and cycling performance, as well as effective dendrite inhibition.

Synergistically reinforced capacitive performance from a hierarchically structured composite of polyaniline and cellulose-derived highly porous carbons

Polyaniline/cellulous-derived highly porous activated carbons (PANI/C-ACs) composites have been synthesized for high-performance supercapacitors via a simple and economical method. The obtained three-dimensional and hierarchical porous structure of PANI/C-ACs, not only exhibited high specific surface area and electron conductivity, but also promoted electrolyte ion transportation, facilitated fast and stable surface redox reactions, hence synergistically improved the overall capacitive performance. The as-designed PANI/C-ACs composites exhibited a specific capacitance of 765 F/g at 1 A/g and capacitance retention of two-electrode cell was as high as 91% after 5000 cycles. Moreover, evident pseudocapacitive behavior can be well maintained at high current densities, indicating superior rate capability for these hierarchically structured composites, making them promising electrode materials for high performance supercapacitors.

Supercapacitive Properties of Micropore- and Mesopore-Rich Activated Carbon in Ionic-Liquid Electrolytes with Various Constituent Ions.

Ionic-liquid (IL) electrolytes, characterized by large potential windows, intrinsic ionic conductivity, low environmental hazard, and high safety, are used for micropore- and mesopore-rich activated-carbon (ACmicro and ACmeso ) supercapacitors. IL electrolytes consisting of various cations [1-ethyl-3-methylimidazolium (EMI+ ), N-propyl-N-methylpyrrolidinium (PMP+ ), and N-butyl-N-methylpyrrolidinium (BMP+ )] and various anions [bis(trifluoromethylsulfonyl)imide (TFSI- ), BF4 - , and bis(fluorosulfonyl)imide (FSI- )] are investigated. The electrolyte conductivity, viscosity, and ion transport properties at the ACmicro and ACmeso electrodes are studied. In addition, the capacitance, rate capability, and cycling stability of the two types of AC electrodes are systematically examined and post-mortem material analyses are conducted. The effects of IL composition on the charge-discharge capacitances of the ACmicro electrodes are more pronounced than those for the ACmeso electrodes. The FSI-based IL electrolytes, for which electrochemical properties are cation dependent, are found to be promising. Incorporating EMI+ with FSI- results in a low electrolyte viscosity and a fast ion transport, giving rise to optimized electrode capacitance and rate capability. Replacing EMI+ with PMP+ increases the cell voltage (to 3.5 V) and maximum energy density (to 42 Wh kg-1 ) of the ACmicro cell at the cost of cycling stability.

Composite Aerogels of Carbon Nanocellulose Fibers and Mixed-Valent Manganese Oxides as Renewable Supercapacitor Electrodes.

Bio-waste derived nanocelluloses show excellent mechanical flexibility and self-aggregated capability, which enable them to be good supporting substrates for the synthesis of electroactive materials. Herein, we present a facile route for fabricating composite aerogels consisting of carbonized nanocellulose fibers (CNF) and mixed-valent manganese oxide (MnOx), toward supercapacitor applications. Mixed solutions of nanocellulose and manganese acetate with different ratios were prepared and freeze-dried into hybrid aerogels. The hybrid aerogels were then transformed into CNF/MnOx composites by a calcination process. The CNF membranes served as porous carbon nano-reservoirs for MnOx and electrolyte. The CNF/MnOx composites also kept a 3D porous aerogel structure with hierarchical pores, which enabled stable transport of both electrolyte ions and electrons to the electrode surface, leading to low a charge-transfer impedance and good electrochemical kinetics. The CNF/MnOx-based symmetric supercapacitor showed a satisfied energy density and power density of 37.5 Wh kg−1 and 2.75 kW kg−1, respectively. All the above results demonstrate the feasibility of using sustainable nanocellulose as a nanoscale carbon substrate for the synthesis of hybrid composite electrodes toward renewable supercapacitor applications.

Ab initio calculations for crystalline PEO6:LiPF6 polymer electrolytes

Dry polymer electrolytes have been studied extensively because of their enhanced mechanical properties. Experimental studies have demonstrated that crystalline PEO6:LiPF6 has higher ionic conductivity than that of the amorphous phase, which seemingly contradicts the prevailing theory that amorphous polymer electrolytes have higher conduction performance. Also, the underlying mechanism at the atomic scale has remained elusive. In this work, the atomic and electronic structures of crystalline PEO6:LiPF6 are studied using ab initio density-functional theory. The results agree with experimental measurements. Additionally, the atomic-scale mechanisms of lithium ion transport in the polymer electrolytes are investigated with the climbing nudged elastic band method. The diffusion pathway and activation energy (∼0.87 eV) of lithium ions in crystalline PEO6:LiPF6 are determined.

A finite strain electro-chemo-mechanical theory for ion transport with application to binary solid electrolytes

An electro-chemo-mechanical formulation of ion transport in solid electrolytes, in particular for binary systems, is presented. Starting with conservation laws and the second law of thermodynamic, we state a consistent Helmholtz-energy-based framework taking electrostatics, component transport and nonlinear elastic mechanical interaction into account. With the help of finite strain continuum mechanics, we include the effect of geometry changes on ion transport. Changes of local concentration cause swelling and shrinkage and hence stress assisted diffusion. Further coupling originates via an osmotic pressure. Since binary systems are of special interest in battery applications, we formulate both, a fully resolved and an electroneutral model for ion transport. The latter turns out to be an extended version of Newman’s concentrated solution theory taking mechanical effects into account. We demonstrate the importance of these mechanical effects by means of double layers adjacent to blocking electrodes and concentration profiles during galvanostatic charging. Further, we investigate the effect of external deformation as, e.g. found in dendrite growth.

Visualizing percolation and ion transport in hybrid solid electrolytes for Li–metal batteries

Hybrid solid state electrolyte structure was imaged using synchrotron nanotomography. Accessible surface area of ceramic particles in the electrolytes governs ion transport.