Search

Development of cathode active materials (CAM) with higher energy density has been desired in the development of next-generation batteries. In this respect, organic CAMs are probable candidates because of their lightweight frameworks without heavy elements. Herein, we examined the characteristics of two possible organic crystals─a naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) lithium salt dimer fused by the dithiin ring (DNP) and phenazinetetrone (PTO)─by density functional theory (DFT)-based first-principles calculations. Based on the pristine crystals of fully oxidized molecules carrying no Li, we computationally explored the low-energy crystal structures at different states of charge (SOCs) during the lithiation (discharging), Lin(DNP), and Lin(PTO)2(n= 0–14), with the DFT energetics. We then calculated the voltage (Li vacancy formation energy) profile and confirmed that our calculations can reasonably explain the experimental discharge curve, indicating the validity of our structure models. It is also demonstrated that deeper discharge destabilizes the lithiated structure. Calculated unit cell shapes suggested that the volume change is significant for early n(stage I), ∼10% for Lin(DNP), and ∼15% for Lin(PTO)2. On the other hand, the volume over this stage is almost kept by the adjustment of the DNP/PTO stacking manner according to the Li insertion, which is advantageous for the usage as CAMs. Li in Lin(DNP) mainly has fourfold coordination to the framework anions, while threefold coordination is dominant in Lin(PTO)2, implying that Lin(DNP) is more stable than Lin(PTO)2. We also evaluated the electronic density of states and the partial electron distributions of both materials with selected SOCs and demonstrated that the electronic conductivities of both materials seem similar. On the other hand, the calculated migration barriers of Li indicated that the PTO crystal has faster Li migration and thus higher rate capability than the DNP. These results suggest that Lin(PTO)2exhibits better cathode performance. The present computational predictive approach reveals the voltage and structural as well as electronic characteristics of the potential organic CAMs, and suggests useful aspects for the material selection.

Lithium–oxygen batteries (LOBs) are promising next-generation rechargeable batteries due to their high theoretical energy densities. The optimization of the porous carbon-based positive electrode is a crucial challenge in the practical implementation of LOB technologies. Although numerous studies have been conducted regarding the relationships between LOB performance and the physicochemical properties of carbon electrodes, most of these studies evaluate the performances of the electrodes under unrealistic conditions with inappropriate technological parameters. In this study, we prepared carbon gel-based self-standing membranes as positive electrodes and evaluated their performances in LOBs under lean-electrolyte, high-areal-capacity conditions. We clarified the following three crucial points: (1) The nanometer-sized pores exhibited limited effects in improving the cycle performance, although they contributed in enhancing the discharge capacity. (2) The macro-sized pores displayed positive effects in enhancing the discharge capacity. (3) The crystallinity and/or surface functional groups influence the discharge potential and cycle life. The results of this study suggest the significance of controlling the physicochemical properties of a porous carbon-based positive electrode in preparing a LOB with a practically high energy density and an extended cycle life.

Aprotic lithium–oxygen (Li–O2) batteries are promising candidates for next-generation energy storage devices because of their much higher potential energy density than Li-ion batteries. However, the practical application of rechargeable Li–O2batteries has been limited by poor cycle performance, especially the side reactions that lower the oxygen reaction efficiency at the positive electrode. The present study demonstrated that when the triethylphosphate-based electrolyte contains lithium nitrate and triethylphosphate forms a solvated complex by coordinating with Li ions, the O2evolution rate could reach almost 100 % of that of the ideal two-electron reaction (O2/e–= 0.5) during the most part of the charging process, with the total oxygen evolution yield exceeding 90 %. These results are useful for designing electrolytes for rechargeable Li–O2batteries with high energy densities.

Aprotic lithium–oxygen (Li–O2) batteries are promising devices for use in sustainable energy management systems as they have the potential to achieve significantly higher energy densities than current state-of-the-art Li-ion batteries. However, the low electrical conductivity of the main discharge product, lithium peroxide (Li2O2), which forms on the positive electrode, gradually suppresses the electrochemical reactions involved in the discharge process, thereby lowering the energy capacity of these systems. Herein, we demonstrate that the energy capacity of Li–O2batteries can be significantly improved by simply adding chloride ions to the electrolyte. Scanning electron microscopy analysis revealed that thick chloride (Cl)-incorporated Li2O2films formed on the positive electrode as the discharge product. Using conductive atomic force microscopy, the Cl-incorporated Li2O2films were shown to exhibit much higher electric conductivity than pristine Li2O2. Taken together, the present findings suggest that modulation of the electrical conductivity of the discharge product by the incorporation of heteroatoms is an effective approach for constructing Li–O2batteries with high volumetric energy density.

As lithium–oxygen batteries (LOBs) possess extremely high theoretical energy densities that exceed those of lithium-ion batteries, they are regarded as promising candidates for portable energy applications, including as power sources for next-generation electrical devices. However, physical degradation factors affecting the prolonged cycle of the LOBs, such as electrode volume changes and electrolyte displacement, have been rarely explored thus far, owing to the technical difficulty of applying non-destructive analytical methods to LOBs during cycling. In the present study, we employed ex situX-ray computerized tomography (XCT), in situinternal pressure monitoring, and in situconfocal microscopy to study the physical states of the electrodes and the electrolyte in high energy density LOBs. The analysis results reveal that large irreversible volume changes occurred in both the positive electrode and the negative electrode of the LOB cells. Furthermore, it was also confirmed that the electrolyte was displaced from the pores of the positive electrode associated with the formation of Li2O2in these pores during the discharge process. As these results present new fundamental insights into the physical behavior of LOBs during cycling conditions, the methodologies demonstrated here are therefore effective means to gain a new understanding of complex physical processes previously unexamined in LOBs, which shed light on the future design strategies of LOBs with high energy density and long cycle life.

Lithium oxygen batteries (LOBs) have attracted considerable research interest as promising candidates for next-generation rechargeable batteries. However, their cell-level performance remains unsatisfactory, and the reaction efficiency must be improved further for practical implementation of this technology. Although the combination of LiNO3and LiBr was recognized as an effective redox mediator for improving performance of LOBs, details of the reaction mechanism remain unknown. In this study, several ex situ and in situ techniques were used to comprehensively analyze the complex chemical reactions at the positive electrode of a LOB containing LiNO3and LiBr under high areal capacity and lean electrolyte conditions. Our analysis revealed a synergetic effect of the Br–/NO3–redox mediator on suppressing side reactions, such as the decomposition of the electrolyte and redox mediator itself. In particular, the formation of bromoform and other Br-containing organic compounds such as 1-bromo-2-methoxyethane and 1-bromo-2,2-methoxyethoxyethane was confirmed, suggesting complex side reactions involving Br–. We believe that the methodology presented herein for the comprehensive analysis will provide an accurate understanding of the complex chemical reactions in LOBs.

Since the oxygen evolution reaction (OER) in aprotic Li ion electrolytes is a crucial reaction in the charging process of nonaqueous aprotic Li–air batteries, there is a strong demand for decreasing the overpotential by developing more efficient OER catalysts. Herein, we investigated the effect of addition of transition metal complexes with macrocyclic ligands, such as porphyrins and phthalocyanines, for OER in aprotic Li ion electrolytes. Electrochemical experiments using a three-electrode system revealed that such complexes functioned as efficient OER catalysts, in which the center metal in the complex played an essential role in the catalytic process. Among the metal complexes studied, cobalt tert-butylphthalocyanine was found to be the best catalyst: the charging potential was lowered from 4.1 V to about 3.4 V at 1 μA/cm2by addition of 1 mM catalyst

Since the oxygen reduction reaction (ORR) in aprotic Li ion electrolytes accompanied by Li2O2formation is a crucial reaction for the discharge of nonaqueous aprotic Li–air batteries, there is a strong demand for a reduction in the overpotential of the reaction in order to improve the discharge performance. In the present work, we investigated the effect of the addition of quinone derivatives for ORR on carbon materials in aprotic Li+–electrolytes. Detailed electrochemical analysis revealed that the semiquinone species catalyze the aprotic ORR, resulting in the efficient Li2O2formation. Among the quinone derivatives, benzoquinone exhibited the best catalytic performance, with an overpotential for the Li2O2formation of less than 100 mV.

Porous architecture is key in the nonaqueous lithium–oxygen (Li–O2) electrode process, which is attracting huge interest because of its application in reversible energy storage with high theoretical energy density. However, it is still challenging to understand the optimal porous structure to obtain high reversibility of the reaction. One main reason is because of instability and undefined porous structures of standard electrodes consisting of carbonaceous materials, and this issue hinders from unveiling the fundamental mechanism in the complicated electrode process. Here, we developed a new synthetic strategy to design monolithic electrodes of pure metallic nickel with controlled bimodal porous structures. The present work aims to investigate the fundamental effects of the bimodal macroporous structure in the Li–O2electrode process under carbon-/binder-free stable model electrodes. As the result, we found that, depending on the multiscale structural configurations, the bimodal macroporous structure gave significant influences to key properties, such as the efficiency of redox-mediators, discharge overpotential, and cycling life. This work indicates that the rational design of stable and conductive porous materials is one of the promising approaches to investigate highly complicated electrochemical reactions in porous electrodes and suggest new guidelines for further development of hierarchically structured electrodes toward advanced electrochemical systems.