Surfactant-Free Formate/O₂ Biofuel Cell with Electropolymerized Phenothiazine Derivative-Modified Enzymatic Bioanode

A formate (HCOO–) bioanode was developed by utilizing a phenothiazine-based electropolymerized layer deposited on sucrose-derived carbon. The electrode modified with NAD-dependent formate dehydrogenase and the electropolymerized layer synergistically catalyzed the oxidation of the coenzyme (NADH) and fuel (HCOO–) to achieve efficient electron transfer. Further, the replacement of carbon nanotubes with water-dispersible sucrose-derived carbon used as the electrode base allowed the fabrication of a surfactant-free bioanode delivering a maximum current density of 1.96 mA cm–2 in the fuel solution. Finally, a separator- and surfactant-free HCOO–/O2 biofuel cell featuring the above bioanode and a gas-diffusion biocathode modified with bilirubin oxidase and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) was fabricated, delivering a maximum power density of 70 μW cm–2 (at 0.24 V) and an open-circuit voltage of 0.59 V. Thus, this study demonstrates the potential of formic acid as a fuel and possibilities for the application of carbon materials in bioanodes

Function of Ammonium Polyacrylate Binders for Long-Life Lithium/Sulfur Cells

The long cycle life of lithium–sulfur cells is strongly desired to leverage the sulfur’s high specific capacity. In this study, we investigated the effects of the countercations of polyacrylate binders used in sulfur composite electrodes on the cycling performance of the lithium–sulfur cells. Lithium, sodium, and ammonium were compared as countercations. Sulfur electrodes were evaluated in terms of their mechanical strength, porous structure, and interaction with the polysulfide. The electrodes with an ammonium polyacrylate binder, which has high mechanical strength, could maintain a porous structure throughout cycling, resulting in the highest cycling performance. Moreover, the affinity of the ammonium polyacrylate with the polysulfide contributed to recovering some capacity by recapturing the polysulfide during the charge step

Three-Dimensionally Hierarchical Ni/Ni₃S₂/S Cathode for Lithium–Sulfur Battery

Lithium–sulfur (Li–S) batteries have attracted interest as a promising energy-storage technology due to their overwhelming advantages such as high energy density and low cost. However, their commercial success is impeded by deterioration of sulfur utilization, significant capacity fade, and poor cycle life, which are principally originated from the severe shuttle effect in relation to the dissolution and migration of lithium polysulfides. Herein, we proposed an effective and facile strategy to anchor the polysulfides and improve sulfur loading by constructing a three-dimensionally hierarchical Ni/Ni₃S₂/S cathode. This self-supported hybrid architecture is sequentially fabricated by the partial sulfurization of Ni foam by a mild hydrothermal process, followed by physical loading of elemental sulfur. The incorporation of Ni₃S₂, with high electronic conductivity and strong polysulfide adsorption capability, can not only empower the cathode to alleviate the shuttle effect, but also afford a favorable electrochemical environment with lower interfacial resistance, which could facilitate the redox kinetics of the anchored polysulfides. Consequently, the obtained Ni/Ni₃S₂/S cathode with a sulfur loading of ∼4.0 mg/cm² demonstrated excellent electrochemical characteristics. For example, at high current density of 4 mA/cm², this thick cathode demonstrated a discharge capacity of 441 mAh/g at the 150th cycle

Electrolyte Composition in Li/O₂ Batteries with LiI Redox Mediators: Solvation Effects on Redox Potentials and Implications for Redox Shuttling

The use of LiI as redox mediator for the charge reaction in nonaqueous Li/O₂ cells has been widely studied recently, as a possible means to fulfill the great promise of the Li/O₂ system as a high energy density “beyond Li-ion” battery. In this work, we highlight the importance of considering the redox potential for both the I^–/I₃^– and I₃^–/I₂ redox couples and how the electrolyte solvent (here tetraglyme (G4) and dimethyl sulfoxide (DMSO)) and concentration (here 1.0 and 2.8 M) have a profound influence on these potentials. Through a combination of galvanostatic cycling, electrochemical mass spectrometry, and cyclic voltammetry, we thus consider the influence of solvent and electrolyte concentration on both the redox mediation and redox shuttle processes and suggest that this important aspect must be considered for further studies with mediators in Li/O₂ and related systems. We demonstrate that, in our system, 100 mM LiI in 1.0 M Li­[TFSA]/DMSO provides the most effective redox mediation among the electrolytes we have studied but conversely exhibits the highest degree of redox shuttling (in the absence of O₂). The balance between effective limitation of redox shuttle and ease of mediator diffusion to discharge products is of great importance and should be considered in any future cell design utilizing a mediator

Tuning NaO₂ Cube Sizes by Controlling Na⁺ and Solvent Activity in Na–O₂ Batteries

Understanding the kinetics of electrochemical oxygen reduction reaction (ORR) and controlling the chemistry, morphology, and size of discharge products are critical to realize reversible operation of metal–air batteries. Here we show that increasing Na⁺ activity and free DME (not coordinated to Na⁺) activity in the solution increases the solubility of NaO₂ and size of NaO₂ cubes in Na–O₂ cells. With increasing Na salt concentration, Raman spectroscopy revealed that Na⁺ activity increased while free DME activity decreased. NaO₂ solubility and NaO₂ cube size were found to exhibit a maximum at a medium concentration of Na⁺, which was accompanied by the highest full discharge capacity. This trend was attributed to two competing effects that stabilize NaO₂ in solution; both higher Na⁺ activity and higher free DME activity can enhance NaO₂ solubility. These results highlight immense opportunities in the design of discharge/charge characteristics such as reaction product sizes and discharge capacity through the manipulation of the chemical physics of electrolytes as well as the solvation of reaction intermediates in the electrolytes

Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries

Solvent–ion and ion–ion interactions have significant effects on the physicochemical properties of electrolyte solutions for lithium batteries. The solvation structure of Li⁺ and formation of ion pairs in electrolyte solutions composed of triglyme (G3) and a hydrofluoroether (HFE) containing 1 mol dm^–3 Li­[TFSA] (TFSA: bis­(trifluoromethanesulfonyl)­amide) were analyzed using pulsed-field gradient spin–echo (PGSE) NMR and Raman spectroscopy. It was found that Li⁺ is preferentially solvated by G3 and forms a [Li­(G3)]⁺ complex cation in the electrolytes. The HFE scarcely participates in the solvation because of low donor ability and relatively low permittivity. The dissociativity of Li­[TFSA] decreased as the molar ratio of G3/Li­[TFSA] in the solution decreased. The activity of G3 in the electrolyte diminishes negligibly as the molar ratio approaches unity because G3 is involved in 1:1 complexation with Li⁺ ions. The negligible activity of G3 in the electrolyte solutions has significant effects on the electrochemical reactions in lithium batteries. As the activity of G3 diminished, the oxidative stability of the electrolyte was enhanced. The corrosion rate of the Al current collector of the positive electrode was suppressed as the activity of G3 diminished. The high oxidative stability and low corrosion rate of Al in the G3/Li­[TFSA] = 1 electrolyte enabled the stable operation of 4-V-class lithium batteries. The activity of G3 also has a significant impact on the Li⁺ ion intercalation reaction of the graphite electrode. The desolvation of Li⁺ occurs at the interface of graphite and the electrolyte when the activity of G3 in the electrolyte is significantly low, while the cointercalation of Li⁺ and G3 takes place in an electrolyte containing excess G3. The activity of G3 influenced the electrochemical reaction process of elemental sulfur in a Li–S battery. The solubility of lithium polysulfides, which are reaction intermediates of the sulfur electrode, decreased as the activity of G3 in the electrolyte decreased. In the G3/Li­[TFSA] = 1 electrolyte, the solubility of Li₂S_<i>m</i> is very low, and highly efficient charge/discharge of the Li–S battery is possible without severe side reactions

Mechanism of Li Ion Desolvation at the Interface of Graphite Electrode and Glyme–Li Salt Solvate Ionic Liquids

Li⁺ intercalation into graphite electrodes was investigated in electrolytes consisting of triglyme (G3) and Li­[TFSA] [TFSA = bis­(trifluoromethanesulfonyl)­amide]. Li⁺-intercalated graphite was successfully formed in an equimolar molten complex, [Li­(G3)₁]­[TFSA]. The desolvation of Li⁺ ions took place at the graphite/[Li­(G3)₁]­[TFSA] interface in the electrode potential range 0.3–0 V vs Li. In contrast, the cointercalation of G3 and Li⁺ (intercalation of solvate [Li­(G3)₁]⁺ cation) into graphite occurred in [Li­(G3)_<i>x</i>]­[TFSA] electrolytes containing excess G3 (<i>x</i> > 1). This cointercalation took place in the voltage range 1.5–0.2 V of the [Li|[Li­(G3)_<i>x</i>]­[TFSA]|graphite] cell. X-ray diffraction showed that the [Li­(G3)₁]⁺-intercalated graphite forms staged phases in the voltage range 1.5–0.3 V. However, exfoliation of the graphite is caused by further intercalation at voltages lower than 0.3 V. [Li­(G3)₁]⁺ intercalation was reversible in the voltage range 1.5–0.4 V. The cointercalation process was studied using cyclic voltammetry, and it was found that the electrode potential for cointercalation depends on the [Li­(G3)₁]⁺ activity, irrespective of the presence of free (uncoordinated) G3. In contrast, the electrode potential for the formation of Li⁺-intercalated graphite (desolvation of solvate [Li­(G3)₁]⁺ cation) changes greatly, depending on the activities of not only the solvate [Li­(G3)₁]⁺ cation but also free G3 in the electrolyte. In extremely concentrated electrolytes, the activity of the free solvent becomes very low. Raman spectroscopy confirmed a very low concentration of free G3 in [Li­(G3)₁]­[TFSA]. Consequently, the electrode potentials for the formation of Li⁺-intercalated graphite were higher than that for cointercalation, and the cointercalation of G3 was inhibited in [Li­(G3)₁]­[TFSA]

Stability of Glyme Solvate Ionic Liquid as an Electrolyte for Rechargeable Li−O₂ Batteries

A solvate ionic liquid (SIL) was compared with a conventional organic solvent for the electrolyte of the Li–O₂ battery. An equimolar mixture of triglyme (G3) and lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA]), and a G3/Li­[TFSA] mixture containing excess glyme were chosen as the SIL and the conventional electrolyte, respectively. Charge behavior and accompanying gas evolution of the two electrolytes was investigated by electrochemical mass spectrometry (ECMS). From the linear sweep voltammetry performed on an as-prepared cell, we demonstrate that the SIL has a higher oxidative stability than the conventional electrolyte and, furthermore, offers the advantage of lower volatility, which would benefit an open-type lithium-O₂ cell design. Moreover, CO₂ evolution during galvanostatic charge was less in the SIL, which implies less side reaction. However, O₂ evolution during charge did not reach the theoretical value in either of the two electrolytes. Several mass spectral fragments were generated during the charge process, which provided evidence for side reactions of glyme-based electrolytes. We further relate the difference in observed discharge product morphology for these electrolytes to the solubility of the superoxide intermediate, determined by rotating ring disk electrode (RRDE) measurements

Li⁺ Solvation and Ionic Transport in Lithium Solvate Ionic Liquids Diluted by Molecular Solvents

An equimolar mixture of lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA]) and either triglyme (G3) or tetraglyme (G4) yielded stable molten complexes: [Li­(G3)]­[TFSA] and [Li­(G4)]­[TFSA]. These are known as solvate ionic liquids (SILs). Glyme-based SILs have thermal and electrochemical properties favorable for use as lithium-conducting electrolytes in lithium batteries. However, their intrinsically high viscosities and low ionic conductivities prevent practical application. Therefore, we diluted SILs with molecular solvents in order to enhance their ionic conductivities. To determine the stabilities of the complex cations in diluted SILs, their conductivity and viscosity, the self-diffusion coefficients, and Raman spectra were measured. [Li­(G3)]⁺ and [Li­(G4)]⁺ were stable in nonpolar solvents, that is, toluene, diethyl carbonate, and a hydrofluoroether (HFE); however, ligand exchange took place between glyme and solvent when polar solvents, that is, water and propylene carbonate, were used. In acetonitrile (AN) mixed solvent complex cations [Li­(G3)­(AN)]⁺ and [Li­(G4)­(AN)]⁺ were formed. [Li­(G4)]­[TFSA] was more conductive than [Li­(G3)]­[TFSA] when diluted with nonpolar solvents due to the greater ionic dissociativity in [Li­(G4)]­[TFSA] mixtures. In view of the stability of the Li–glyme complex cations, the enhanced ionic conductivities, and the intrinsic electrochemical stabilities of the diluting solvents, [Li­(G4)]­[TFSA] diluted by toluene or HFE, can be a candidate for an alternative battery electrolyte

Oxygen Reduction Reaction in Highly Concentrated Electrolyte Solutions of Lithium Bis(trifluoromethanesulfonyl)amide/Dimethyl Sulfoxide

The performance of current Li–air batteries is greatly limited by critical obstacles such as electrolyte decomposition, high charging overpotentials, and limited cycle life. Thus, much effort is devoted to fundamental studies to understand the mechanisms of discharge/charge processes and overcome the above-mentioned obstacles. In particular, the search for new stable electrolytes is vital for long-lasting and highly cyclable batteries. The highly reactive lithium superoxide intermediate (LiO₂) produced during discharge process can react with the electrolyte and produce a variety of byproducts that will shorten battery life span. To study this degradation mechanism, we investigated oxygen reduction reaction (ORR) in highly concentrated electrolyte solutions of lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA])/dimethyl sulfoxide (DMSO). On the basis of rotating ring disk electrode measurements, we showed that LiO₂ dissolution can be limited by increasing lithium salt concentration over 2.3 mol dm^–3. Our Raman results suggested that this phenomenon can be related to lack of free DMSO molecules and increasing DMSO–Li⁺ interactions with higher Li⁺ concentration. X-ray diffraction measurements for the products of ORR suggested that the side reaction of DMSO with Li₂O₂ and/or LiO₂ could be suppressed by decreasing the solubility of LiO₂ in highly concentrated electrolytes