Propylene Carbonate Electrolyte Research Articles

Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries

Summary Zn-Sn interdigitated eutectic alloy (IdEA) foils are highly promising anodes for lithium-ion batteries. Synthesized via severe plastic deformation, a two-phase alloy is converted into a nanostructured foil composed of tin domains distributed throughout an electrically conductive zinc matrix. In addition to providing mechanical integrity, uncycled capacity in the zinc matrix eliminates the lithium-plating risk. These monolithic foil anodes offer an electrode-level volumetric capacity approximately double that of graphite (797 mA h cm−3), with a highly promising formation loss of

(Keynote) The Solvation Structure and Rational Design of Liquid Organic Electrolytes

Electrolytes with improved stability and transport properties have the potential to transform the current state-of-the-art Li-ion architectures and enable future technologies such as multivalent systems (e.g., Mg2+, Ca2+ and Zn2+), as well as Li-S and Li-O2 conversion systems. To address the need for novel and optimized electrolytes, in this work, we apply our high-throughput computational infrastructure1 to a range of systems – spanning both multivalent as well as monovalent charge carriers- and present our insights, highlighting the role of careful speciation for rational design of future, improved energy storage concepts. To improve our understanding of structural and dynamical properties of highly concentrated LIB electrolytes, we use classical molecular dynamics simulations with enhanced sampling techniques to allow for correct statistics in these more viscous systems. We describe the modes of charge transport in LiPF6 /propylene carbonate electrolyte in concentrated to superconcentrated regimes and elucidate the minority as well as majority species solvation structures and corresponding atomistic transport motifs. For multivalent systems, we find that the existence of multimeric species, such as contact-ion pairs are vitally important to account for the anodic stability of the electrolyte.2 - 4 The plating process inherently requires the solvated divalent ion to undergo a transient, partially charged valence state; e.g. Mg+ or Ca+, which is identified as the fundamental descriptor for electrolyte stability. We find most solvent molecules as well as ion-paired anion molecules to be highly susceptible to rapid decomposition when in close contact with the reactive partially charged cation, thereby precluding stable, reversible plating behavior. This insight provides a ready explanation why so few classes of solvents and salts have demonstrated reversible Mg and Ca metal plating as well as enabling a path forward for rapid, in silico rational design and testing of novel, highly functional electrolyte molecules. 5

Highly Durable Ester-Based Electrolyte Solutions for Higher-Voltage Lithium-Ion Batteries

The practical specific capacity of conventional positive electrodes, such as LiCoO2, remained less than the half of graphite negative-electrodes. Hence, increasing the specific capacity of positive-electrodes effectively improves the energy density of lithium-ion batteries (LIBs). A layer-structured LiNi0.5Co0.2Mn0.3O2 positive-electrode can deliver a high discharge capacity of ca. 200 mAh g-1 by charging to around 4.6 V. However, the severe decomposition of conventional electrolyte solutions is a problem at such a high potential. The stability of electrolyte solutions against oxidation can be improved by increasing the concentration, and there are three major problems to be solved toward the practical use; 1) high costs due to an extensive use of lithium salts, 2) high viscosities which make it virtually impossible to inject the electrolyte solutions into cylindrical and laminated cells in the production processes of practical LIBs, and 3) low rate capabilities due to the low ionic conductivity of highly viscous electrolyte solutions. These problems can be settled by diluting the highly concentrated electrolyte solution with an appropriate diluent. We previously reported that propylene carbonate (PC)- and γ-butyrolactone-based concentrated electrolyte solutions showed high stability against oxidation at LiNi0.5Co0.2Mn0.3O2 and 5-V class LiNi0.5Mn1.5O4 [1-3]; irreversible capacities due to the oxidative decomposition of electrolyte solution decreased with increasing the electrolyte concentration, while the discharge capacities increased. In particular, in the nearly saturated 7.25 mol kg-1 LiBF4 /PC electrolyte solution, LiNi0.5Co0.2Mn0.3O2 retained a high discharge capacity of ca. 186 mAh g-1 even after 50 charge/discharge cycles at a C/10 rate. The polarization on charge/discharge reactions remained small even at the very high concentration [2]. Considering the wide operating temperature range of LIBs, the electrolyte solution needs to be stable even at elevated temperatures. 7.25 mol kg-1 LiBF4 /PC allowed charge/discharge cycles of LiNi0.5Mn1.5O4 between 3.5 and 5.0 V at an elevated temperature of 50 °C, whereas not in the nearly saturated 4.27 mol kg-1 LiPF6 /PC [4]. In electrolyte solutions of moderate concentrations, the dissolution of active materials become more of a problem at elevated temperatures. The highly concentrated LiBF4/PC suppressed the dissolution of manganese ions from a LiNi0.5Mn1.5O4 electrode more effectively than LiPF6 /PC [4]. Highly concentrated LiBF4 /PC can be diluted with a diluent to reduce the high viscosity and concentration. Fluorinated solvents have relatively low donor number and high stability against oxidation because they have electron-withdrawing fluorine atoms, and are suitable as diluents for concentrated LiBF4 /PC systems. We explored various kinds of fluoroalkyl ethers [5,6] and esters as a diluent, of these, bis(2,2,2-trifluoroethyl) carbonate (TFEC) has a relatively low HOMO energy, and could hardly dissolve LiBF4. Unfortunately, however, the miscibility between TFEC and concentrated LiBF4 /PC was not high, and hence tris(2,2,2-trifluoroethyl) phosphate (TFEP) was introduced as a co-solvent of PC to prepare 2.03 mol kg-1 LiBF4 /PC + TFEP (1:2 by volume, PC/Li+ molar ratio = 1.35). This concentrated electrolyte solution was diluted with 50 vol.% TFEC to obtain 1.00 mol kg-1 LiBF4 /PC+TFEP+TFEC (1:2:3 by volume, PC/Li + molar ratio = 1.35). The initial discharge capacity reached 192 mhA g- 1 and 98.0% of it was retained even at the 50th cycle (Fig.1) [7]. Thus, the degradation of a LiNi0.5Co0.2Mn0.3O2 electrode was extremely suppressed even though the upper cut-off voltage was as high as 4.6 V. The initial Coulombic efficiency (84.2%) was low, while the average Coulombic efficiency from the 2nd (97.6%) to 50th cycle (99.1%) was high at 98.7%. Thus, the present electrolyte solution has a low viscosity (ca. 7 mPa s), moderate lithium salt concentration, and high stability against oxidation, and is therefore suitable for higher-voltage operating of LIBs. This research has been supported by “Advanced Research Program for Energy and Environmental Technologies” from NEDO, Japan. Reference [1] T. Doi et al., Electrochim. Acta, 209 (2016) 219-224. [2] T. Doi et al., J. Electrochem. Soc., 163 (2016) A2211-A2215. [3] T. Doi et al., ChemElectroChem, 4 (2017) 2398-2403. [4] T. Doi et al., ChemistrySelect, 2 (2017) 8824-8827. [5] T. Doi et al., J. Electrochem. Soc., 164 (2017) A6412-A6416. [6] T. Doi et al., Sustain. Energy Fuels, 2 (2018) 1197–1205. [7] T. Doi et al., Curr. Opin. Electrochem., 9 (2018) 49-55. Figure 1

Approaching the maximum capacity of nickel-rich LiNi0.8Co0.1Mn0.1O2 cathodes by charging to high-voltage in a non-flammable electrolyte of propylene carbonate and fluorinated linear carbonates.

We report a promising approach to achieve the maximum capacity (>230 mA h g-1) and high capacity retention (95% during 100 cycles) of a nickel-rich cathode of LiNi0.8Co0.1Mn0.1O2 (NCM811) by charging to 4.5 V in a non-flammable electrolyte of propylene carbonate and fluorinated linear carbonates. Our electrolyte permits the stabilization of the cathode-electrolyte interface and cathode structure at high-voltage, enabling stable and safe operation of the Ni-rich cathode for high-energy density and high-safety lithium-ion and lithium metal batteries.

Li-Ion solvation in propylene carbonate electrolytes determined by molecular rotational measurements.

Lithium-ion batteries are an attractive power source for a wide variety of applications. Expanding the performance limit of current Li-ion batteries requires ion-solvent interaction, which governs the ion transport behavior of electrolytes, to be fully understood. We herein examine the coordination number of the Li+ ion in LiPF6-PC solutions using femtosecond vibrational spectroscopy. Surprisingly, we found that the coordination number of PC in the first solvation shell of Li+ decreases from 4 to 2 as the salt concentration increases. At dilute salt concentrations, the Li(PC)4+ complex with a tetrahedral geometry is dominant, while at high salt concentrations, the presence of anions in the first solvation shell modifies the solvation structure, leading only 2 PC molecules to coordinate to Li+ directly. The variety of the solvation structure provides a rational explanation for the ionic conductivity changing as the salt concentration increases.

Low power stretchable active-matrix red, green, blue (RGB) electrochromic device array of poly(3-methylthiophene)/Prussian blue

We report the fabrication of a stretchable array of active-matrix (AM) electrochromic devices (ECDs). The ECD is made of poly(3-methylthiophene) and Prussian blue electrodes with a mixed gel electrolyte of acetonitrile, poly(methyl methacrylate), propylene carbonate, and lithium perchlorate. The ECD displays can express red, green, or blue (RGB) depending on the applied voltage and has a high stability in air. It exhibits low power consumption levels of 331 and 378 μW cm−2 (10 s duration) at −1.0 and 1.0 V, respectively, and a high coloration efficiency of 201.6 cm2 C−1 at 1.0 V. A 4 × 4 AM ECD array is integrated on a deformable Ecoflex substrate via a dry transfer method using patterned liquid metal interconnections. The display shows a mechanical stability under bending and biaxial stretching by 30%. Finite element method analysis of the strain distribution also confirms that strain is not applied to the ECDs, guaranteeing stable operation of the array under deformation. Furthermore, a stretchable 6 × 6 AM ECD array for higher resolution display also exhibits mechanical stability under 30% biaxial stretching. This work demonstrates the high potential of our stretchable ECD array in low power-consuming displays for stretchable electronics, wearable devices, and electronic skin.

Influence of solvent on linear polypyrrole–polyethylene oxide actuators

ABSTRACTIonic conductivity of polypyrrole (PPy) doped with dodecylbenzenesulfonate (DBS) was improved by loading of 10 wt % of polyethylene oxide (PEO). The linear actuation properties of PPy–PEO/DBS films were investigated and compared in polar (aqueous) and aprotic (propylene carbonate) solvents keeping the concentration of lithium bis(trifluoromethane)sulfonimide (LiTFSI) electrolyte the same. The results were compared to those of pristine PPy/DBS films. The actuation direction changed from pure cation‐driven in the aqueous electrolyte (LiTFSI(aq)) to anion‐driven in propylene carbonate electrolyte (LiTFSI(PC)). At the same time, the electro‐chemo‐mechanical deformation revealed that the strain of PPy–PEO/DBS increased from 5% in LiTFSI(aq) to 20% in LiTFSI(PC). The effect of solvent on the ionic conductivity was investigated using electrochemical impedance spectroscopy, showing a 27% increase in charge transfer kinetics and an increase in the electronic conductivity, resulting in significant increase in the strain rate, when propylene carbonate electrolyte was used. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46831.

Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene Carbonate Electrolytes

Expanding the performance limit of current Li-ion batteries requires ion–ion and ion–solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1–3.0 M LiPF6 and LiBF4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF6–PC and LiBF4–PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (<0.8 M), and SIP becomes dominant at intermediate concentrations (0.8–1.5 M). At higher concentrations (1.5–3.0 M), the solvent-shared dimer (SSD) and contact dimer (CD) are dominant in LiPF6–PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF4–PC. Ionic conduction in 0.1–1.5 M LiPF6–PC and LiBF4–PC is governed by the migration of free ions and SIP. Notably, above 1.5 M of the two PC electrolytes, SSD participates in ionic conduction via the migration mode as well. Furthermore, it is suggested that the large number of CIPs present in LiBF4–PC may contribute to ionic conduction via a Grotthuss-type mechanism.

Highly Stable Active Materials for Nonaqueous Redox Flow Batteries

The commercialization of truly practical nonaqueous redox flow batteries (RFBs) requires the development of compounds that are extremely stable in at least two oxidation states. Moreover, ideal RFB catholyte materials should possess high oxidation potentials (>4 V vs. Li/Li+), while their anolyte counterparts should possess relatively low reduction potentials. A series of promising RFB anolytes based on p-phenylene-bridged bis(pyridinium) compounds has been prepared and characterized. These compounds are highly soluble in polar organic solvents and undergo a chemically reversible net two-electron reduction. Because the para orientation allows the formation of a quinoidal neutral species upon 2e- reduction, the reduction product is expected to display good stability. The synthesis, characterization, and electrochemical properties of several RFB catholytes based on phenothiazine-5,5-dioxide will also be described. These compounds exhibit chemically and electrochemically reversible one-electron oxidation at very high potentials (4.2-4.3 V vs. Li/Li+) in polar organic solvents. By attaching suitable ether functional groups, solubilities exceeding 2 M in propylene carbonate electrolytes can be achieved. Optical spectroscopy reveals that the radical cations produced by bulk electrolysis are extremely stable, undergoing minimal loss even after 1500 hours.

Post-Cycling Surface Analysis of Li2s-FeS Composite Positive Electrode

High energy density batteries have been required for the next generation EV applications, triggering the search for sulfur-based positive electrode materials with high capacity. Recently, we have developed the Fe-containing Li2S-based materials (Li8FeS5), and found that Li8FeS5 cell showed relatively high initial discharge capacity (> 700 mAh g-1) [1]. However, a rapid capacity degradation was observed during the initial several cycles. To address this issue, we have focused on improving cycle performance and it was revealed that a surface coating by Titanium oxide was effective for suppressing the capacity degradation of Li8FeS5 positive electrode [2]. In particular, a capacity decay related to the sulfur redox were suppressed. This indicates that the surface reaction on the Li8FeS5 particles is one of the causes of capacity degradation and the surface reaction might be related to sulfur redox. In the present work, we analyzed the surface of Li8FeS5 positive electrode after cycle test to clarify the effect of coating on the cycle stability. The Li8FeS5 was prepared based on the previously reported method [1]. Because of the sensitivity of Li2S to moisture, all the material synthesis procedures were carried out in an argon atmosphere. The obtained Li8FeS5 and commercially available titanium tetrachloride (TiCl4) were mixed in solvent and filtered after stirring, and washed by dimethyl carbonate, according to a previously reported procedure [3]. Then this intermediate product was heated at 400 ºC to obtain the sample (hereafter noted as Li8FeS5-Ti). The electrochemical charge/discharge tests were carried out with 1 mol dm-3 lithium hexafluorophosphate in 10:90 (v/v) ethylene carbonate and propylene carbonate electrolyte at a current density of 98.6 mA g−1 (corresponding to 0.1 C) and between 2.6 and 1.0 V vs. Li/Li+. The surface of positive electrodes were characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) with Bi as the primary ion source. The cycle performance of discharge capacity related to sulfur redox were shown in Fig.1. The Li8FeS5 without coating exhibited a gradually capacity decay on the whole range of cycle test. On the other hand, Li8FeS5-Ti showed an almost constant discharge capacity after rapid decreasing at initial charge-discharge. The TOF-SIMS measurement was carried out after the first charge, the first discharge and after 10 cycles to determine the cause of different cycle performance. The Secondary ion images of Li8FeS5 electrodes surface were shown in Fig.2. The formation of Li2O particle was observed after the first charge at electrode surface without coating. Furthermore, the particle size increased after the first discharge, until eventually the electrode surface was fully covered by Li2O. Even though the Li2O formation was observed at Li8FeS5-Ti electrode surface after first charge, the particle size was significantly smaller than uncoated samples. In addition, the particle size didn’t increase after 10 cycles. It seemed that the formation of Li2O at the surface of positive electrode might be one of the causes of rapid capacity decay which was observed on uncoated electrode. Also it was revealed that the sulfite compounds formed uniformly only at the Li8FeS5-Ti electrode surface after the first charge and discharge. However the surface concentration of sulfite compound decreased after 10 cycles, and then the degradation products from LiPF6 covered the whole area of electrode surface. This could indicate that certain side reactions of the surface sulfur layer with the electrolyte occurred at first cycle. However, this undesirable reaction was suppressed in the subsequent cycles and therefore, the electrode surface could be stabilized. We have carried out an in-depth analysis of the electrode surface, and the results are presented in this conference. The work was financially supported by RISING2 Project of NEDO.

Wire-Shaped Supercapacitors with Organic Electrolytes Fabricated via Layer-by-Layer Assembly.

A wire-shaped supercapacitor (WSS) has structural advantages of high flexibility and ease of incorporation into conventional textile substrates. In this work, we report a thin reproducible WSS fabricated via layer-by-layer (LbL) assembly of multiwalled carbon nanotubes (MWCNTs), combined with an organic electrolyte of propylene carbonate (PC)-acetonitrile (ACN)-lithium perchlorate (LiClO4)-poly(methyl methacrylate) (PMMA) that extends the voltage window to 1.6 V. The MWCNTs were uniformly deposited on a curved surface of a thin Au wire using an LbL assembly technique, resulting in linearly increased areal capacitance of the fabricated WSS. Vanadium oxide was coated on the LbL-assembled MWCNT electrode to induce pseudocapacitance, hence enhancing the overall capacitance of the fabricated WSS. Both the cyclic stability of the WSS and the viscosity of the electrolyte could be optimized by controlling the mixing ratio of PC to ACN. As a result, the fabricated WSS exhibits an areal capacitance of 5.23 mF cm-2 at 0.2 mA cm-2, an energy density of 1.86 μ W h cm-2, and a power density of 8.5 mW cm-2, in addition to a high cyclic stability with a 94% capacitance retention after 10 000 galvanostatic charge-discharge cycles. This work demonstrates a great potential of the fabricated scalable WSS in the application to high-performance textile electronics as an integrated energy storage device.

Moderately concentrated electrolyte improves solid–electrolyte interphase and sodium storage performance of hard carbon

Hard carbon (HC) is a promising anode for sodium-ion batteries. The current hurdles for the HC electrodes are insufficient coulombic efficiency (CE), rate capability, and cyclic stability. This study reveals that an intelligent electrolyte design can effectively overcome these limitations. The sodium salt, concentration, and solvent of the electrolytes are systematically investigated. Incorporation of ethylene carbonate (EC) in propylene carbonate (PC) electrolyte can promote the formation of contact ion pairs and ion aggregates between Na+ and FSI–. At a moderate concentration, the 3 mol dm−3 NaFSI in PC:EC electrolyte with reasonable conductivity and viscosity can lead to the formation of a robust organic–inorganic balanced solid–electrolyte interphase, which is thoroughly examined by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. With this, the first-cycle and steady-state CE of the HC electrode is increased to 85% and>99.9%, respectively, and the reversible sodiation/desodiation capacities at high rates are markedly improved. In addition, 95% of the initial capacity can be retained after 500 charge–discharge cycles. The proposed electrolyte represents a huge step towards HC electrodes with high effectiveness and durability for electrochemical Na+ storage.

Cathode Coatings for Magnesium Ion Batteries

Over the past 25 years, significant effort has gone into perfecting lithium ion battery chemistries, including materials coatings, electrolytes, and electrolyte additives to improve safety, capacity, and cyclability. [1,2] By comparison, the area of non-lithium-based batteries is far less explored due to the relative novelty of the technology. Magnesium ion batteries in particular are promising as a next-generation energy storage technology due to the forecasted lower costs. The best cathode materials to date have been metal sulfides and have been demonstrated to achieve high cycleability; however, their capacity and voltage are relatively low compared to state of the art Li ion batteries. One of the challenges in demonstrating high voltage MV-based systems is identifying a cathode capable of desolvating Mg2+ at the cathode-electrolyte interface in order to allow Mg2+ insertion. [3] Additionally, magnesium electrolytes can become quite complicated, with highly solvated ions, large anions, and a variety of both solvents and ion pairs (i.e. MgxClx species). For most electrolyte/cathode pairs, the interaction between electrolyte and cathode surface is not well understood. In the case of the Chevrel Mo6S8 cathode and APC electrolyte, Wan and Prendergast calculated that Mg-Cl species actually interact quite favorably with Mo6S8, which facilitates breaking the Mg-Cl bond and Mg2+ insertion. [4] The cathode-electrolyte interface of magnesium cathode materials is what facilitates desolvation and Mg2+ insertion. By modifying the surface of these cathodes, improvements in performance and lifetime of magnesium ion batteries could be achieved. Materials such as metal sulfides and metal nanoparticles have been demonstrated to interact with the solvated Mg complex and improve the insertion rates of Mg2+into a host lattice. As the Chevrel Mo6S8 was calculated to favorably interact with magnesium electrolytes, it is possible that other transition metal sulfides will provide the same type of interactions and facilitate Mg2+ insertion into cathode materials. Here we focus on Prussian blue type cathode materials, as they have a relatively high voltage and have demonstrated issues with reaching their full theoretical capacity due to desolvation and ion pairing issues. [5] Coatings of Ag­2S and MnS were found to decrease the accessible capacity by about 40% (Figure 1), with similar behavior found in two different coating methods. As cathodes and electrolytes are highly particular in their interactions, coatings may also need to be tailored to the specific cathode/electrolyte system. Figure 1. Charge/discharge curves for Ni[Fe(CN)6] with Mg(TFSI)2 in propylene carbonate electrolyte, with Ag2S and MnS coatings. Cycled at 10 mA/g with a BP2000 carbon anode. References M. M. Thackeray, C. Wolverton and E. D. Isaacs, Energy & Environmental Science, 5 (2012).Z. Chen and J. R. Dahn, Electrochimica Acta, 49, 1079 (2004).P. Canepa, G. S. Gautam, D. C. Hannah, R. Malik, M. Lui, K. G. Gallagher, K. A. Persson, and G. Ceder, Chemical Reviews, 117, 4287 (2017).L. F. Wan, B. R. Perdue, C. A. Apblett and D. Prendergast, Chemistry of Materials, 27, 5932 (2015).A. L. Lipson, S.-D. Han, S. Kim, B. Pan, N. Sa, C. Liao, T. T. Fister, A. K. Burrell, J. T. Vaughey, and B. J. Ingram, Journal of Power Sources, 325, 646 (2016). Figure 1

Improved Charge/Discharge Performance of LiNi0.8Co0.1Mn0.1O2 positive Electrodes in Solvation-Controlled Electrolyte Solutions for Lithium-Ion Batteries

LiNi0.8Co0.1Mn0.1O2 positive-electrodes exhibit high discharge capacities about 200 mAh g-1 by charging up to high potentials of ca. 4.3 V vs. Li/Li+. However, the reversible capacities gradually decrease upon repeated charge/discharge cycling in conventional electrolyte solutions due to 1) their oxidative decomposition at high potentials, 2) cracks of LiNi0.8Co0.1Mn0.1O2 powder, and 3) the phase transition to spinel/rock salt phases. The charge/discharge cycle performance of LiNi0.8Co0.1Mn0.1O2 was greatly improved in highly concentrated LiBF4 propylene carbonate (PC) electrolyte solutions, which have low PC/Li+ molar ratios, such as nearly saturated 7.25 mol kg-1 LiBF4/PC (PC/Li+=1.35). However, the viscosity was much higher than that of conventional electrolyte solutions containing 1 mol dm-3 Li salt. To reduce the viscosity, the highly concentrated LiBF4/PC was diluted with fluorinated linear ether, 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (HFE). The dilution with HFE lowered both the viscosity and concentration of LiBF4. A part of PC was replaced with low-viscosity dimethyl carbonate (DMC) to reduce the viscosity further. The charge and discharge properties of LiNi0.8Co0.1Mn0.1O2 were investigated in the resultant electrolyte solutions with Li| LiNi0.8Co0.1Mn0.1O2 half cells. The cells were cycled galvanostatically at 0.1 C (1 C=275 mAh g-1) between 3.0 and 4.3 V at 30oC. In 2.50 mol kg-1 LiBF4/PC+HFE (2:1 by vol., PC/Li+=2.39), the discharge capacity significantly decreased in 50 charge/discharge cycles. On the other hand, in 4.00 mol kg-1 LiBF4/PC+DMC+HFE (1:3:2 by vol., (PC+DMC)/Li+=1.58), 96.5% of the initial discharge capacity (193 mAh g-1) was maintained even at the 50th cycle. The average Coulombic efficiency in the 50 cycles was also high at 99.7 %. Thus, the introduction of DMC into the PC+HFE system improved the charge/discharge cycle performance of LiNi0.8Co0.1Mn0.1O2 positive electrodes.

Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube electrodes prepared by defluorination

Nitrogen-doped vertically aligned multi-walled carbon nanotubes (N-VAMWCNTs) were prepared by reacting fluorinated VAMWCNTs with ammonia gas at temperatures of 300–600 °C. The N-VAMWCNTs were characterized using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. In addition, the electrochemical properties of capacitors with N-VAMWCNT electrodes were evaluated by cyclic voltammetry and AC impedance spectroscopy using a two-electrode coin-type cell in an electrolyte of propylene carbonate containing triethylmethylammonium tetrafluoroborate. All the samples were prepared without destroying the alignment structure of the nanotubes. The ratios between the concentration of fluorine, carbon, and nitrogen (F/C and N/F) and the R value (degree of crystallinity) of the samples indicate that the N-VAMWCNTs prepared at 500 °C (N500-VAMWCNTs) had the highest level of nitrogen doping and the best crystallinity among the samples. Nitrogen atoms were doped at a concentration of 5.26 at% into the nanotube frames, thus enriching the N500-VAMWCNTs with pyridinic nitrogen species. The average specific capacitance of the N500-VAMWCNT electrodes was 12.0 F/g at a scan rate of 100 mV/s, which is approximately 1.8 times the value obtained for the as-grown VAMWCNT electrodes (6.5 F/g).

High-voltage operation of binder-free CNT supercapacitors using ionic liquid electrolytes

Abstract

Optimizing solid electrolyte interphase on graphite anode by adjusting the electrolyte solution structure with ionic liquid

N-allyl-N-methylpiperidinium bis(trifuoromethansulfonyl)imide (PP13*TFSI) ionic liquid was adopted as a solute replacing partial of lithium salt in a concentrated propylene carbonate (PC) electrolyte. Varying the concentration of PP13*TFSI ionic liquid can change the electrolyte solution structure and thus optimize the solid electrolyte interphase (SEI) on the graphite anode. The number of PC molecules coordinated with lithium ion is significantly altered and this results in a change of the SEI mechanism at the electrode/electrolyte interface. At an optimum molar ratio of 2:1 between LiTFSI and PP13*TFSI, the first coulombic efficiency and the cycling capability of the graphite anode are significantly improved. Fourier transform infrared spectroscopy (FTIR) verified the SEI mechanism change from two-electron reduction to one-electron reduction in the presence of PP13*TFSI ionic liquid. SEM and TEM observations reveal a thin, homogenous and compact SEI film formed on the graphite surface in the 3 mol L−1 LiTFSI+PP13*TFSI(2:1)/PC concentrated electrolyte.

Correlation studies between pore structure and electrochemical performance of activated polymer-based hard carbon with various organic and aqueous electrolytes

In this study, activated polymer-based hard carbon (APHC) was first prepared using CO2 activation. Then electrochemical measurements of APHC-applied electrodes were used to evaluate these electrodes for application to electric double-layer capacitors. The APHCs were prepared under different conditions of activation temperature and time. The APHCs that were activated at 1000°C for 40 and 50 minutes showed good textural properties (specific surface area: 2440 and 2210 m2/g, mesopore fraction: 43.5% and 63.1%, respectively). These properties were observed using the N2/77 K adsorption isotherm, and APHCs have favorable pore structures for ion adsorption in electrolytes. Electrochemical techniques such as galvanostatic charge/discharge, cyclic voltammetry, and electrochemical impedance spectroscopy were used to assess the capacitive behaviors and rate performance of the APHCs in various electrolytes. The specific capacitance of the APHCs showed different results depending on the electrolytes used. APHC-10-50 showed 117 F/g of specific capacitance in propylene carbonate (PC), but only 106 F/g in acetonitrile (AcN), very different from APHC-10-40 (77 F/g in PC and 120 F/g in AcN). Although APHC-10-50 had a smaller specific surface area than that of APHC-10-40, it exhibited a high mesopore fraction with pore size optimal for PC electrolyte, which has larger ions and clusters than AcN does. This can lead to a relatively higher specific capacitance of APHC-10-50 in PC electrolyte than that of APHC-10-40 due to the high mesopore fraction.

Suppression of Mn-Ion-Dissolution of LiNi0.5 Mn1.5 O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures

Mn-based active materials, such as LiMn2O4, are widely used for positive electrodes in lithium ion batteries, and spinel LiNi0.5Mn1.5O4 is drawing much attention to realize 5-V class batteries. However, the oxidative decomposition of electrolyte solution at high voltages and Mn-dissolution of LiNi0.5Mn1.5O4 are serious problems to be solved. These two drawbacks are more marked at elevated temperatures, and should be caused by free solvent molecules in electrolyte solution. In this study, highly concentrated electrolyte solution, which contains few free solvent molecules, was investigated to solve the problems. LiNi0.5Mn1.5O4 electrodes worked at 50oC in nearly saturated 7.25 mol kg−1 LiBF4/ propylene carbonate (PC) electrolyte solution, whereas not in the nearly saturated 4.30 mol kg−1 LiPF6/PC. In addition, Mn-ion dissolution from LiNi0.5Mn1.5O4 was significantly suppressed in highly concentrated electrolyte solutions, and correlated to the fraction of free PC molecules in them.

Synthesis and characterization of d-glucose derived nanospheric hard carbon negative electrodes for lithium- and sodium-ion batteries

The electrochemical performance of glucose-derived hard carbon (GDHC) anode has been evaluated using Li- and Na-salts in ethylene carbonate and propylene carbonate electrolyte mixtures. The LiPF6/EC:PC (1:1) system exhibits high capacity at low current densities (400mAhg−1 at 25mAg−1) and also good power characteristics retaining 150mAhg−1 capacity at 2Ag−1 current density. The best overall performance was achieved with 1M NaPF6/EC:PC (1:1) electrolyte based system with capacities of 175mAhg−1 at 0.1V vs Na/Na+ and 330mAhg−1 at 1.5V vs Na/Na+. The electrode has been physically characterized ex-situ using SEM, Raman and TOF-SIMS methods TOF-SIMS analysis revealed that the solid electrolyte interphase is more inorganic on the negative electrode in the Na-cell than on the negative electrode the Li-cell. The positive ion-specific images established by TOF-SIMS analysis show the non-homogeneous distribution of various fragments from the pristine GDHC, which is caused by slightly inhomogeneous mixture of GDHC and conducting carbon black (Super P®) particles.