High-Performance Lithium Metal Rechargeable Battery Using an Ultrafine Porous Polyimide Separator with Three-Dimensionally Ordered Macroporous Structure

It is very important to control the compositional and morphological changes of the lithium metal anode in order to improve the cycle performance of the lithium metal battery (LMB). In this work, we report that the combination of an ultrafine porous polyimide (PI) separator with three-dimensionally ordered macroporous (3DOM) structure and an electrolyte composed of ethylene carbonate (EC) solvent with a high dielectric constant containing LiPF6 can improve the cycle performance of LMB using a Li4Mn5O12 cathode. In LMBs in which depositions of lithium with high reactivity are repeated on the anode at every cycle, undesirable side reactions generating deteriorated products are promoted at the interface between freshly deposited metallic lithium and electrolyte. The reaction of LiPF6 with water and subsequent undesirable side reactions hardly occur in EC solvent with a high dielectric constant. In addition, the size and shape of the deposited lithium particles on the cycled anode in the cell using EC solution are uniform, so EC solvent is suitable for LMBs. The 3DOM PI separator is superior to the conventional polypropylene separator in terms of the uniform current density distribution in addition to high permeability and reservation for electrolytes with low fluidity such as EC solution

A Si−O−C Composite Anode: High Capability and Proposed Mechanism of Lithium Storage Associated with Microstructural Characteristics

A blend of phenyl-substituted, branched polysilane, (Ph2Si)0.85(PhSi)0.15, and polystyrene (1:1 in weight) has been transformed into a composite material consisting of graphene layers, Si−O−C glasses, and micropores through a pyrolytic polymer-to-ceramic conversion. Several analytical techniques have been employed to characterize the Si−O−C composite material, demonstrating the presence of the three components in its host framework. The Si−O−C composite material performs well in electrochemical operations with a characteristic voltage plateau, offering a capacity of more than 600 mA h g−1. When polystyrene is not blended, the resulting comparative material is even less porous and shows a shorter voltage plateau in electrochemical operations. A broad resonance in the 7Li NMR spectrum recorded at low temperature can be deconvoluted into three components in the fully lithiated state of the Si−O−C composite material derived from the polymer blend. This result indicates that the Si−O−C composite material electrochemically stores lithium species in interstitial spaces or edges of the graphene layers, directly or indirectly the Si−O−C glass phase, and the micropores. However, both the Si−O−C glass phase and micropores are minor as electrochemically active sites for lithium storage, and interstitial spaces or edges of the graphene layers act as major electrochemically active sites in this composite material. Despite the excellent cyclability of the Si−O−C composite material, the voltage plateau disappeared over cycling. This phenomenon suggests that the microstructure is delicate for repetitive lithium insertion and extraction and that newly formed sites must generate the nearly equal capacity

The Effect of Insoluble Oxide Additives on a Magnesium Plating/Stripping Reaction in Mg(N(CF₃SO₂)₂)₂/Glyme Solutions

Mg­(N­(CF3SO2)2)2/glyme solutions are attractive as the electrolyte solution for Mg secondary batteries. However, the solutions are suffering from the poor reversibility and large overpotential of the Mg negative electrode. In the present study, we studied the effect of oxide particles on the Mg plating/stripping reaction, aiming at the use as insoluble additives for Mg secondary batteries. By placing Al2O3 particles on the substrate, the Coulombic efficiency was improved from 5% to 33%. In addition, the overpotential for Mg stripping reaction drastically decreased from 1.5 to 0.2 V. The open circuit potential of the plated Mg maintained 0.2 V for 24 h. We applied Al2O3 additives on the cell consisting of Mg/V2O5 and achieved the discharge profile over 1.7 V, which was about 0.7 V higher than the similar report without additives. From the results, we first confirmed that Al2O3 functioned as insoluble additives for the Mg electrode reaction

High-Rate Lithium Deintercalation from Lithiated Graphite Single-Particle Electrode

The electrochemical behavior of a lithiated graphite single-particle electrode during high-rate Li deintercalation in an organic electrolyte was investigated using a microelectrode technique. A Ni-plated metal filament (diameter: 10 μm) was attached to a mesocarbon microbead (MCMB) in the electrolyte under optical microscope observation, and galvanostatic charge−discharge tests were carried out. The discharge capacity of a lithiated MCMB particle (diameter: 18 μm) was 2.02 nA h in the potential range of 0.005−2.5 V vs Li/Li+. The fully lithiated MCMB particle showed an extremely high rate capability and released more than 98% of the accommodated Li at a constant discharge current of 1000 nA within 10 s. At discharge currents lower than 200 nA, the charge transfer process at the interface controlled the reaction of the single-particle electrode, and the Li diffusion process in the MCMB particle did not significantly affect the Li deintercalation rate. The charge transfer resistance for Li intercalation/deintercalation was in the range of 20−50 Ω cm2, and the apparent chemical diffusion coefficient of Li in the MCMB particle was estimated to be 8.3 × 10−8 cm2 s−1

Quantification of the Carbon-Coating Effect on the Interfacial Behavior of Graphite Single Particles

The effect of carbon coating on the interfacial charge transfer resistance of natural graphite (NG) was investigated by a single-particle measurement. The microscale carbon-coated natural graphite (NG@C) particles were synthesized by the simple wet-chemical mixing method using a phenolic resin as the carbon source. The electrochemical test results of NG@C using the conventional composite electrodes demonstrated desirable rate capability, cycle stability, and enhanced kinetic property. Moreover, the improvements in the composite electrodes were confirmed with the electrochemical parameters (i.e., charge transfer resistance, exchange current density, and solid phase diffusion coefficient) analyzed by a single-particle measurement. The surface carbon coating on the NG particles reduced the interfacial charge transfer resistance (Rct) and increased the exchange current density (i0). The Rct decreased from 81–101 (NG) to 49–67 Ω cm2 (NG@C), while i0 increased from 0.25–0.32 (NG) to 0.38–0.52 mA cm–2 (NG@C) after the coating process. The results suggested both electrochemically and quantitatively that the outer uniformly coated surface carbon layer on the graphite particles can improve the solid–liquid interface and other kinetic parameters, therefore enhancing the rate capabilities to obtain the high-power anode materials

Phosphoric Acid Diethylmethylammonium Trifluoromethanesulfonate-Based Electrolytes for Nonhumidified Intermediate Temperature Fuel Cells

The present study reports a new series of electrolytes for nonhumidified intermediate temperature fuel cells (IT-FCs). This series of new mixed electrolytes, composed of phosphoric acid (PA) and diethylmethylammonium trifluoromethanesulfonate ([dema]­[TfO]), was designed as nonhumidified IT-FC electrolytes. The mixed electrolytes show a higher thermal stability than pure PA, which is dehydrated at ITs. The thermal stability of the mixed electrolytes could be explained by the interaction between the triflate group in [dema]­[TfO] and PA, as indicated by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR) spectroscopies. On the other hand, the ionic conductivity and proton transference number of the mixed electrolytes were similar to those of the pure PA. However, the oxygen reduction reaction (ORR) activity of a platinum catalyst is significantly enhanced in the mixed electrolytes, which was due to the several orders of magnitude increase in oxygen solubility by the addition of [dema]­[TfO] to PA. Specifically, for the equimolar fraction mixed electrolyte, the diffusion coefficient and the solubility of oxygen were ca. 1.47 × 10–5 cm2 s–1 and ca. 1.28 mmol dm–3 at 150 °C, respectively. The addition of [dema]­[TfO] to PA could significantly enhance the ORR activity. Therefore, the PA_[dema]­[TfO] mixed electrolyte can be one of the solutions to develop nonhumidified intermediate FC electrolytes

Lithiation and Delithiation of Silicon Oxycarbide Single Particles with a Unique Microstructure

Single particles (11 and 13 μm diameter) of a silicon oxycarbide (Si–O–C) glass were electrochemically analyzed using a microelectrode technique. A micromanipulator-guided nickel-plated rhodium–platinum microfilament (25 μm diameter, 13 wt % rhodium) was used to maintain electrical contact to a single Si–O–C glass particle in an organic solution containing 1 mol dm–3 LiClO4. The cyclic voltammograms of a single Si–O–C glass particle (11 μm diameter) featured a characteristic sharp peak at ca. 0.1 V vs Li/Li+, along with a broad peak and a shoulder, in the anodic reaction. This result indicates that there are several electrochemically active sites for lithium storage in the single Si–O–C glass particle. The first lithiation and delithiation capacities of a single Si–O–C glass particle (13 μm diameter) were 1.67 nA h and 1.12 nA h, respectively, at 5 nA (4C rate) in the potential range 0.01–2.5 V vs Li/Li+, leading to a Coulombic efficiency of 67%. These results are in good agreement with those observed in typical porous composite electrodes. The 13 μm diameter particle gives 75% of the full-delithiation capacity even at 100 nA (80C rate), demonstrating that its intrinsic delithiation rate capability is suitable for practical purposes. Assuming that the Tafel equation is applicable to the delithiation of the single Si–O–C glass particle, the charge-transfer resistance tended to increase as lithium was released

Concerted Migration Mechanism in the Li Ion Dynamics of Garnet-Type Li₇La₃Zr₂O₁₂

The garnet-type Li7La3Zr2O12 (LLZO) belonging to cubic symmetry (space group Ia3̅d) is considered as one of the most promising solid electrolyte materials for all-solid state lithium ion batteries. In this study, the diffusion coefficient and site occupancy of Li ions within the 3D network structure of the cubic LLZO framework have been investigated using ab initio molecular dynamics calculations. The bulk conductivity at 300 K is estimated to be about 1.06 × 10–4 S cm–1 with an energy barrier of 0.331 eV, in reasonable agreement with experimental results. The complex mechanism for self-diffusion of Li ions can be viewed as a concerted migration governed by two crucial features: (i) the restriction imposed for occupied site-to-site interatomic separation, and (ii) the unstable residence of Li ion at the 24d site, which can serve as the trigger for ion mobility and reconfiguration of surrounding Li neighbors to accommodate the initiated movement. Evidence for Li ordering is also found at low temperature for the LLZO system

Phase Transition Mechanism of ZnMn₂O₄ Spinel Oxide with Electrochemical Magnesium-Ion Insertion

Spinel oxides with 3d transition metals are expected to be cathode materials with high energy density for magnesium rechargeable batteries. Although it is important to control their phase transitions in order to reduce the polarization during charge/discharge processes for practical use, the relationship between the electrochemical properties and the phase transition mechanism of spinel oxides is not well understood. In this study, we examined the electrochemical properties and the phase transition mechanism of Mg2+ insertion into ZnMn2O4 spinel oxide by using the galvanostatic intermittent titration technique (GITT), X-ray absorption spectroscopy (XAS), and synchrotron X-ray diffraction (XRD) measurements and compared them to those of MgMn2O4 spinel oxide. Compared to MgMn2O4, the polarization was relatively small in ZnMn2O4 in the early stage of the Mg2+ insertion process (0 ≤ x ≤ 0.3) because the ZnMn2O4 spinel phase has a larger solid-solution limit for Mg2+ insertion. On the other hand, in the late stage of the Mg2+ insertion process (0.3 x ≤ 0.58), the polarization of ZnMn2O4 was larger than that of MgMn2O4 due to the larger volume change between the spinel and rocksalt phases. The finding that the use of zinc stable at the tetrahedral configuration in spinel oxides can expand the solid-solution limit for Mg2+ insertion into the spinel phase and reduce the polarization is significant for the development of cathode materials in magnesium rechargeable batteries