Formation Of Discharge Products Research Articles

Reconfiguration of the charge density difference of nitrogen-doped graphene by covalently bonded Cu-N4 active sites boosting thermodynamics and performance in aprotic Li-CO2 battery

The slow kinetics of the CO2 reduction and evolution reactions in the Li-CO2 battery result in a high overpotential, low energy efficiency and undesired life. Exploring the durable electrocatalysts with high activity for CO2 reduction and evolution processes in aprotic Li-CO2 batteries is of great significance for CO2 capture and utilization. Herein, single-atom copper uniformly anchored on nitrogen-doped graphene (SA-Cu-NG) was demonstrated as a durable catalyst for the rechargeable Li-CO2 battery. The resulting Li-CO2 battery shows a remarkable specific capacity of 29033 mAh g−1 at 100 mA g−1, an ultra-long life up to 538 cycles (over 2730 h), and a low overpotential of 1.47 V (1000 mA g−1), outperforming the reported Li-CO2 batteries. The X-ray absorption fine structure analysis of SA-Cu-NG unravels that the covalent effect between Cu and N, which exists in the form of Cu-N4 in nitrogen-doped graphene. Further, it is theoretically elucidated that the covalent effect of Cu-N4 leads to the reconfiguration of the charge density difference on nitrogen-doped graphene, thereby improving the adsorption of CO2 and weakening the decomposition barrier of the discharge products on the surface single-atom copper, thus optimizing the nucleation decomposition process. In conclusion, the exceptional performances of Li-CO2 battery are attributed to the superior catalytic activity on Cu-N4 sites and the excellent electronic conductivity of nitrogen-doped graphene, activating the reversible process of discharge product formation and decomposition.

Freeze-Drying-Assisted ZIF-67 Template-Derived Co@NCS Porous Composite as Sulfur Cathode Host for Improved Li-S Battery Performance: Deconvolution of Diffusive and Capacitive Li+ Storage.

A zeolitic imidazole framework (ZIF-67) template-derived cobalt@nitrogen-doped carbon-sulfur composite (ZIF-Co@NCS) was synthesized using a freeze-drying method and explored for lithium-sulfur (Li-S) batteries. Material characterizations confirmed the formation of the ZIF-Co@NCS composite. To fabricate the Li-S battery in a coin-type CR-2032 cell, the as-synthesized ZIF-Co@NCS composite having a sulfur content of ∼65% was used as a cathode material and coupled with a lithium metal anode. This battery demonstrated exceptional cycling stability over 600 charge-discharge cycles with a steady capacity of 550 mAh g-1 at 0.05 C-rate. The good electrochemical performance of the cathode was ascribed to the nanofeatures associated with the freeze-dried porous carbon structure, which offered enough space for the sulfur cathode. The enhanced trapping of polysulfide facilitated by the presence of the Co-N sites in the ZIF-Co@NCS composite led to excellent cycling stability. The derived cathode composite showed a high specific capacity, outstanding rate capability, and excellent cyclic stability rendering it a promising candidate for high-performance Li-S batteries. Detailed Li+ ion charge storage examined by means of Dunn's method revealed a significant capacitive mode of charge storage both at peak currents and nonpeak currents. The post-mortem analysis of the cycled cathode using X-ray diffraction and scanning electron microscopy at different depth-of-discharge (DOD) values revealed the stepwise formation of discharge products.

Influence of the Mechanism of Discharge Product Formation on the Electrochemical Performance and Cyclability of Aprotic Na–O2 Batteries

Influence of the Mechanism of Discharge Product Formation on the Electrochemical Performance and Cyclability of Aprotic Na–O₂ Batteries

DFT Studies of Catalytic Activity of Rutile Typed Metal Oxide (110) Surface in Sodium-Air Batteries

Metal-air batteries, such as Li-air, have been extensively studied owing to their high theoretical energy density, which is up to three times that of the conventional to Li-ion batteries and interest in Na-air is now emerging [1,2, 3]. The current study is providing a better understanding of the catalytic activity of rutile typed MO2 (MnO2, TiO2 and VO2) for the oxygen reduction and oxygen evolution reactions in Na-air batteries, using density functional methods. When oxygen is adsorbed at the Na/MO2 (110) surface different clusters, such as superoxide, peroxo on Na/M, and dissociated configurations are formed. Our current study has demonstrated that MnO2, as a catalyst, promotes nucleation and growth of both Na2O and Na2O2. On the contrary, other metal oxides such as TiO2 and VO2 do not depict this catalytic effect towards the formation of discharge products in Na-O2 batteries. Furthermore, it can be deduced from our calculated electronic properties, in the form of an intercept of the density of states (DOS) by the Fermi level, that Mn catalyzed configurations are more stable than those based on Ti and V. It can therefore be surmised that the MnO2 catalyst supports the formation of NaO2 which is the most stable discharge product in Na-air batteries. Furthermore, the surface NaO2 appears to have bond lengths comparable to those of the bulk and monomer NaO2.

Conformal Lithium Peroxide Growth Kinetically Driven by MoS2/MoN Heterostructures Towards High‐Performance Li−O2 Batteries

AbstractThe morphological control and spatial accommodation of Li2O2 discharge products on the oxygen cathode are crucial to bolstering both the Coulombic and round‐trip efficiencies of Li−O2 batteries. Herein, MoS2/MoN heterostructures are constructed on carbon cloth to serve as the freestanding and binder‐free oxygen cathode for Li−O2 batteries, lending a low charge/discharge polarization of 0.79 V, a large specific capacity of 9.04 mAh cm−2, and a superb cycling stability beyond 800 h. Through comprehensive microscopic, electroanalytical and density function theory analyses, the superb electrochemical performance is attributed to the formation of a conformal and amorphous Li2O2 layer, which is kinetically driven in virtue of the higher electric conductivity of MoN and stronger LiO2 intermediate adsorption on the heterostructures. This work advocates the merits of interfacial engineering to enhance the performance of Li−O2 batteries by manipulating the discharge product formation.

Effect of O2 flow in discharge products and performance of Li-O2 batteries

Many studies on Li-O2 batteries have addressed the chemical and morphological evolution of the device as a function of electrode material, electrolyte, and discharge current density. Here we report that simple operating conditions can also affect discharge product formation. Results show the formation of Li2O2 and LiOH in experiments using continuous O2 flow (open system) and without it (closed system), and an additional more complex chemistry including LiOOH•H2O, Li2O and LiO2 in the closed system. This unexpected difference was also examined during charging of the Li-O2 cell, in which the LiOOH•H2O is preferably reversed in comparison to LiOH. Beside the influence in the reaction routes, the O2 flow and pressure impact on the cell performance. The total discharge capacity varies from 1,459 mA g − 1 to 2,460 mA g − 1 decreasing the O2 flow. In contrast, when using the closed pressurized system, the discharge capacity increased up to 5,851 mA g − 1 with the same electrode in the best result, as the electrolyte loss due to evaporation was avoided.

Driving the sodium-oxygen battery chemistry towards the efficient formation of discharge products: The importance of sodium superoxide quantification

Sodium-oxygen batteries (SOBs) have the potential to provide energy densities higher than the state-of-the-art Li-ion batteries. However, controlling the formation of sodium superoxide (NaO2) as the sole discharge product on the cathode side is crucial to achieve durable and efficient SOBs. In this work, the discharge efficiency of two graphene-based cathodes was evaluated and compared with that of a commercial gas diffusion layer. The discharge products formed at the surface of these cathodes in a glyme-based electrolyte were carefully studied using a range of characterization techniques. NaO2 was detected as the main discharge product regardless of the specific cathode material while small amounts of Na2O2⋅2H2O and carbonate-like side-products were detected by X-ray diffraction as well as by Raman and infrared spectroscopies. This work leverages the use of X-ray diffraction to determine the actual yield of NaO2 which is usually overlooked in this type of batteries. Thus, the proper quantification of the superoxide formed on the cathode surface is widely underestimated; even though is crucial for determining the efficiency of the battery while eliminating the parasitic chemistry in SOBs. Here, we develop an ex-situ analysis method to determine the amount of NaO2 generated upon discharge in SOBs by transmission X-ray diffraction and quantitative Rietveld analysis. This work unveils that the yield of NaO2 depends on the depth of discharge where high capacities lead to very low discharge efficiency, regardless of the used cathode. We anticipate that the methodology developed herein will provide a convenient diagnosis tool in future efforts to optimize the performance of the different cell components in SOBs.

Modeling of Hierarchical Cathodes for Li-Air Batteries with Improved Discharge Capacity

The non-aqueous Li-air battery is considered to be a promising energy source for electric-vehicles owing to its ultrahigh theoretical energy density. However, its commercialization is limited by the attained lower energy density value, which is mainly due to pore blockage and passivation which requires a more strategic design of the cathode. In this work, we have developed and validated a detailed one-dimensional continuum model of Li-Air battery that helps in examining the potential of hierarchical cathodes in guiding and enhancing the efficiency of ions transport and discharge product formation inside microstructures. The obtained results reveal the importance of reducing the tortuosity (shorten the path of oxygen transport) and increasing porosity at the airside of the hierarchical cathode, which improved discharge capacity at approximately 20.9 and 56%, respectively. The improved capacity is due to enhanced effective oxygen transport, impregnation of electrolyte, alignment of pores, and formation of permeable and low crystalline aggregates of Li2O2. Hence, strategies considering these insights can help in the design and fabrication of non-aqueous Li-air batteries with enhanced energy density and capacity.

In Situ Stress Evolution on an Au Thin Film Cathode during Charging/Discharging of a Lithium-Oxygen Battery

The theoretical energy density of lithium-oxygen batteries is by far the closest competitor to gasoline compared to other battery configurations. Unfortunately, the commercial implementation of this technology is hindered by the poor cycling performance, mostly due to the slow oxygen evolution/reduction kinetics. The sluggish oxygen evolution kinetics are typically more severe and cause the slow oxidation of lithium peroxide. This can cause the irreversible formation of a passivating lithium peroxide layer on the cathode surface. The interfacial dynamics due to the growth of lithium peroxide and the formation of cathode-electrolyte interface (CEI) layers remains largely unknown. It is crucial to elucidate how the nucleation of discharge products on the cathode surface affects the electrochemical performance in order to design new electrolytes and cathodes for lithium-oxygen batteries.In this study, we propose to investigate the surface stress generation associated with the formation of discharge products and CEI layers on the cathode. Thin Au films were deposited via e-beam evaporation onto a borosilicate glass substrate. These substrates were used as a cathode material. The electrolyte used was 1 M LiTFSI in DMSO, and it was saturated with oxygen prior to performing electrochemical measurements. In situ stress measurements were conducted using a multiple beam optical sensor (MOS). A custom cell was designed to monitor the physical response of the cathode during cycling while enabling optical access to the back of the substrate for stress measurements.The in situ curvature evolution on an Au coated borosilicate glass cantilever is monitored during one discharge/charge cycle at 10 µA/cm2. Tensile stress generation is observed during the formation of the lithium peroxide discharge product on the Au cathode. A large stress relaxation into the compressive regime is observed at the onset of charging. From our results, the nucleation of lithium peroxide onto the cathode surface causes tensile stress, while the oxidation of lithium peroxide causes compressive stress. Revealing the mechanical response of the charge/discharge products on the cathode will shed light into the formation mechanisms of these products, which is crucial to design both new electrolytes and cathode materials for lithium-oxygen batteries.Acknowledgement: This work was supported by the Binational Science Foundation (#2018327), and we are thankful to both Dr. Malachi Noked and Rosy for fruitful discussions.

Probing and Interpreting the Porosity and Tortuosity Evolution of Li-O2 Cathodes on Discharge through a Combined Experimental and Theoretical Approach.

Li-O2 batteries offer a high theoretical discharge capacity due to the formation of light discharged species such as Li2O2, which fill the porous positive electrode. However, in practice, it is challenging to reach the theoretical capacity and completely utilize the full electrode pore volume during discharge. With the formation of discharge products, the porous medium evolves, and the porosity and tortuosity factor of the positive electrode are altered through shrinkage and clogging of pores. A pore shrinks as solid discharge products accumulate, the pore clogging when it is filled (or when access is blocked). In this study, we investigate the structural evolution of the positive electrode through a combination of experimental and computational techniques. Pulsed field gradient nuclear magnetic resonance results show that the electrode tortuosity factor changes much faster than suggested by the Bruggeman relation (an equation that empirically links the tortuosity factor to the porosity) and that the electrolyte solvent affects the tortuosity factor evolution. The latter is ascribed to the different abilities of solvents to dissolve reaction intermediates, which leads to different discharge product particle sizes: on discharging using 0.5 M LiTFSI in dimethoxyethane, the tortuosity factor increases much faster than for discharging in 0.5 M LiTFSI in tetraglyme. The correlation between a discharge product size and tortuosity factor is studied using a pore network model, which shows that larger discharge products generate more pore clogging. The Knudsen diffusion effect, where collisions of diffusing molecules with pore walls reduce the effective diffusion coefficients, is investigated using a kinetic Monte Carlo model and is found to have an insignificant impact on the effective diffusion coefficient for molecules in pores with diameters above 5 nm, i.e., most of the pores present in the materials investigated here. As a consequence, pore clogging is thought to be the main origin of tortuosity factor evolution.

Role of Discharge-Product Morphology and Formation Rates in Li–O2 Battery Performance

Lithium–oxygen batteries have unprecedented theoretical capacity. Achieving the theoretical capacity in practice remains a challenge – even during low-power operation – because of higher-order phenomena that accompany the formation of discharge products, such as surface passivation and clogging of the cathode’s pores. In the last few years, Li–O2 battery research has moved towards systems that preferentially form large discharge-product particles, which can be achieved by using high-donicity electrolytes and including mediators as additives.1 In this study we examine the implications of discharge-product growth and morphology on performance using numerical simulations.A continuum model is developed which incorporates multicomponent mass transfer2 described by Onsager-Stefan-Maxwell laws. Including more details about liquid-phase transport is particularly important when modelling solution-mediated discharge processes, because the diffusion of soluble reaction intermediates can control the battery’s apparent discharge capacity. Detailed reaction models, which include precipitation and film-growth kinetics are also necessary, to elucidate the competition between discharge-product formation pathways3: thin-film formation can slow interfacial reaction kinetics, whereas pore clogging raises overpotentials by hindering the transport of reactive species.Large discharge products form through a solution-phase mechanism, which is favoured by electrolytes that sustain the presence of an LiO2 intermediate for a relatively long time. These intermediate forms the final Li2O2 discharge product either by electrochemically reacting with a lithium ion or by chemically disproportionating, thereby contributing to discharge-product-particle growth. When LiO2 reaction intermediates are long-lived, they can build up in the electrolyte, particularly near the electrode/electrolyte interface. Thus, the system is locally pushed from the dilute into the concentrated regime, and pairwise interactions between species can become significant to device performance.(1) Torayev, A.; Magusin, P. C. M. M.; Grey, C. P.; Merlet, C.; Franco, A. A. Text Mining Assisted Review of the Literature on Li-O2 Batteries . J. Phys. Mater. 2019, 2, 044004.(2) Liu, J.; Khaleghi Rahimian, S.; Monroe, C. W. Capacity-Limiting Mechanisms in Li/O2 batteries. Phys. Chem. Chem. Phys. 2016, 18, 22840–22851.(3) Yin, Y.; Torayev, A.; Gaya, C.; Mammeri, Y.; Franco, A. A. Linking the Performances of Li–O2 Batteries to Discharge Rate and Electrode and Electrolyte Properties through the Nucleation Mechanism of Li 2 O 2. J. Phys. Chem. C 2017, 121, 19577–19585.

In-situ formed free-standing Ir nanocatalysts as carbon- and binder-free cathode for rechargeable nonaqueous Li–O2 batteries

Abstract The design of carbon- and binder-free cathodes to replace the vulnerable carbon cathodes has become very imperative to further enhance the cycling performance of Li–O2 battery. Herein, free-standing Ir nanocatalyst as a novel carbon- and binder-free cathode is first proposed for rechargeable nonaqueous Li–O2 batteries. Impressively, the Li–O2 battery with freestanding iridium cathodes can maintain more than 67 stable cycles with a limited capacity of 800 mAh g−1 at a current density of 400 mA g−1. Greatly improved energy efficiency and rate capability are also successfully achieved. The iridium nanocatalyst is revealed to promote the discharge product formation via a newly desirable surface growth coupled with solution driven mechanism. This study sheds a new light on the role of iridium nanocatalyst, and provides a rational approach for the design of carbon- and binder-free cathodes of Li–O2 batteries.

Curtailing the Overpotential of Li-CO2 Batteries with Shape-Controlled Cu2 O as Cathode: Effect of Illuminating the Cathode.

Li-air batteries are limited to lab-scale research owing to the uninterrupted formation of discharge products. In the case of Li-CO2 batteries, the increase in overpotential caused by Li2 CO3 formation results in cell death. In this study, Cu2 O crystals having three different types of shapes (i.e., cubic, octahedral, and rhombic) were synthesized to compare their catalytic activity toward CO2 reactions. The full-cycle and long-term stability test revealed that rhombohedral Cu2 O facilitates Li2 CO3 decomposition more efficiently than that of cubic and octahedral Cu2 O. The cycle was extended to investigate the photocatalytic activity of the rhombic Cu2 O by illuminating the cell. The repeated cycles to 1 h showed a maximum overpotential of 1.5 V, which is 0.5 V lower than that of the cell without illumination. A postmortem analysis of the cell after dividing the cycles into segments demonstrated interesting results concerning the role of light and Cu2 O during the cell cycle.

Self-assembling RuO2 nanogranulates with few carbon layers as an interconnected nanoporous structure for lithium-oxygen batteries.

Electrocatalysis for cathodic oxygen is of great significance for achieving high-performance lithium-oxygen batteries. Herein, we report a facile and green method to prepare an interconnected nanoporous three-dimensional (3D) architecture, which is composed of RuO2 nanogranulates coated with few layers of carbon. The as-prepared 3D nanoporous RuO2@C nanostructure can demonstrate a high initial specific discharge capacity of 4000 mA h g-1 with high round-trip efficiency of 95%. Meanwhile, the nanoporous RuO2@C could achieve stable cycling performance with a fixed capacity of 1500 mA h g-1 over 100 cycles. The terminal discharge and charge potentials of nanoporous RuO2@C are well maintained with minor potential variation of 0.14 and 0.13 V at the 100th cycle, respectively. In addition, the formation of discharge products is monitored by using in situ high-energy synchrotron X-ray diffraction (XRD).

In situ AFM visualization of Li-O2 battery discharge products during redox cycling in an atmospherically controlled sample cell.

The in situ observation of electrochemical reactions is challenging due to a constantly changing electrode surface under highly sensitive conditions. This study reports the development of an in situ atomic force microscopy (AFM) technique for electrochemical systems, including the design, fabrication, and successful performance of a sealed AFM cell operating in a controlled atmosphere. Documentation of reversible physical processes on the cathode surface was performed on the example of a highly reactive lithium–oxygen battery system at different water concentrations in the solvent. The AFM data collected during the discharge–recharge cycles correlated well with the simultaneously recorded electrochemical data. We were able to capture the formation of discharge products from correlated electrical and topographical channels and measure the impact of the presence of water. The cell design permitted acquisition of electrochemical impedance spectroscopy, contributing information about electrical double layers under the system’s controlled environment. This characterization method can be applied to a wide range of reactive surfaces undergoing transformations under carefully controlled conditions.

Aprotic Li/O2 Batteries: Reactions and Products in Electrolytes with and without a Redox Mediator

The interest in secondary Li/O2 batteries has grown rapidly over the past two decades, as they exhibit a practically achieveable specific energy of about 1,700 Wh/kg, which equals that of gasoline and is well beyond those of conventional lithium ion, Ni metal hydride and Zn/air batteries.1,2 The large energy density is due to the very light-weight cell components: a Li metal anode, a porous C cathode (gas diffusion electrode, GDE) and gaseous O2 as active cathode material. During discharge LiO2 and Li2O2 (as well as Li2O) are formed via the oxygen reduction reaction (ORR) in the porous carbon matrix. These products are oxidized again upon recharge. This seemingly simple system, however, has some major drawbacks due to the decomposition of the organic electrolytes during battery cycling – which is caused by the reaction of the respective electrolyte with either the Li anode, different components of the GDE and/or even the desired ORR products. The decomposition products are deposited in the GDE pores, leading to a minimized active electrode surface, decreasing capacities and ultimately cell failure upon continued cycling.3 There are two major reaction mechanisms leading to the formation if Li2O2, and the prevailing mechanism depends on the electrolyte. Li2O2 toroids are a common discharge product of the so-called solution growth pathway proceeding via an EC mechanism4: O2 is reduced to O2 -, which is dissolved and forms LiO2. The solvated LiO2 intermediates subsequently disproportionate to solid Li2O2 particles with different morphologies (which depend on the nature of the electrolyte)5. The ORR proceeds primarily via this mechanism when high DN electrolyte solvents are used and/or discharge parameters, such as low discharge current densities and low overpotentials, are applied6. With low DN solvents and/or high current densities and overpotentials, the discharge reaction proceeds via the surface growth pathway. In this work, galvanostatic cycling and CV measurements were conducted with 1 M mixtures of LiTFSI/DMSO and LiTFSI/TEGDME, with and without additions of the redox mediator dimethylphenazine, DMPZ7, to analyze the electrolyte influence on the cycling behavior. At different states of charge, the GDE surface and pores were investigated by SEM and XRD to obtain information on the nature, the morphology and the distribution of the product deposits on and in the carbon structure. Galvanostatic cycling measurements show a larger discharge capacity and lower discharge overpotential with LiTFSI/DMSO as well as a better recharge behavior in the first cycle, although both electrolytes exhibit very high recharge potentials. The TEGDME-based electrolyte on the other hand exhibits a higher stability over the first five cycles, whereas the discharge capacity of LiTFSI/DMSO decreases drastically already upon second discharge. CV measurements confirm these absolute and relative trends of the discharge capacities. A SEM investigation of the GDE surfaces after first discharge reveals a considerable difference in discharge product morphology depending on the electrolyte. This is in agreement to previously published results: with DMSO as solvent the product consists of toroidal particles with diameters of approximately 200 nm, whereas discharge with the TEGDME-based electrolyte yields a product layer on the porous cathode surface.5,8 Differential capacity analysis confirmed the difference in discharge product formation: with LiTFSI/TEGDME two reduction processes were detected to contribute to the discharge capacity, whereas discharge with LiTFSI/DMSO contains only one reduction process. X-ray diffraction after the first discharge with LiTFSI/DMSO shows the presence of crystalline Li2O2 in the GDE and the absence of crystalline LiOH. The discharge with the TEGDME-based electrolyte, on the other hand, did not yield any crystalline Li2O2 (or LiOH). FIB-SEM measurements after the first discharge with each electrolyte furthermore showed that the GDE pores were almost completely blocked by bulk product. The addition of the DMPZ significantly reduced the charging potential, both for the DMSO and the TEGDME electrolyte. The cycling performance was, however, only improved for DMSO, and not for TEGDME. Bruce et al., Nat. Mater. (2012) 11, 19-29.Yao et al., Angew. Chem. Int. Ed. (2016) 55, 11344-11353.Sharon et al., Isr. J. Chem. (2015) 55, 508-520.Johnson, L. et al., Nat. Chem. 6, 1091–1099 (2014).Mitchell, R. R., et al., J. Phys. Chem. Lett. 4, 1060–1064 (2013).Aurbach, D., et al., Nat. Energy 1, 16128 (2016).Lim et al., Nature Energy (2016) 1, 16066.Aetukuri et al., Nat. Chem. (2014) 7, 50-56.

Understanding the Role of Lithium Ions and Electron Transport in a Multi-Electrode System Designed for Lithium-Oxygen Batteries

The formation and decomposition of the discharge products (Li2O2) during the charging and discharging processes in Li-oxygen batteries are the main reactions that drive this technology, initiated at a solid-liquid-gas phase boundary involving Li+, O2 and e- in the cathode compartment. However the mechanistic contribution of each of these component to the electrochemical reactions in a given cell remains unknown. In this study, we seek to understand the role of Li+ and e- at the phase boundary by developing a dual-cathode system containing a lithium conducting electrode(Li3N) and an electron conducting electrode(LiI) separated by nickel mesh. It was observed that, the LiI has the tendency to form reduced oxygens on the electrode surface via I-/I3 - redox couple, while Li-ions migrate to the electrode surface by the Li-ion conductive effect of the Li3N in the reactions leading to the discharge product formation. The discharge products on the Li3N electrode were found to be oxygen-rich, while those on the LiI electrode surface were found to be oxygen-deficient. Electrochemical analyses revealed that the cells could discharge up to a capacity of 3.74mAh/cm2 at a current density of 0.1mA/cm2 with an over potential of 0.59V upon charging. At the same current density the cells were able to discharge-charge up to 184 cycles at a cut-off capacity of 0.2mAh/cm2.

Aprotic Li/O2 Batteries: Reactions and Products in Different Electrolytes

The interest in secondary Li/O2 batteries has grown rapidly over the past two decades, as they exhibit a practically achieveable specific energy of about 1,700 Wh/kg, which equals that of gasoline and is well beyond those of conventional lithium ion, Ni metal hydride and Zn/air batteries.1,2 The large energy density is due to the very light-weight cell components: a Li metal anode, a porous C cathode (gas diffusion electrode, GDE) and gaseous O2 as active cathode material. During discharge LiO2 and Li2O2 (as well as Li2O) are formed via the oxygen reduction reaction (ORR) in the porous carbon matrix. These products are re-oxidized upon recharge. The major drawback of this seemingly simple system, however, is the reactivity of the organic electrolytes with almost all battery components and hence several decomposition reactions. The resulting side products are deposited in the GDE pores, leading to a minimized active electrode surface and ultimately cell failure upon continued cycling.3 Therefore, information on the electrolyte-dependent ORR and/or decomposition products deposited in the GDE during discharge is needed for an in-depth investigation of the causes for early battery failure. In order to achieve this, galvanostatic cycling and CV measurements were conducted with 1 M mixtures of LiTFSI/DMSO and LiTFSI/TEGDME to analyze the electrolyte influence on the cycling behavior. At different states of charge, the GDE surface and pores were investigated by SEM and XRD to obtain information about the product deposits on and in the carbon structure. Galvanostatic cycling measurements show a larger discharge capacity and lower discharge overpotential with LiTFSI/DMSO as well as a better recharge behavior in the first cycle (see Fig. 1.a). The TEGDME-based electrolyte on the other hand exhibits a higher long-term stability, whereas the discharge capacity of LiTFSI/DMSO decreases drastically already upon the second discharge (see Fig. 1.b). CV measurements confirm these absolute and relative trends of the discharge capacities (see inset in Fig. 1.b). A SEM investigation of the GDE surfaces after first discharge reveals a considerable difference in discharge product morphology depending on the electrolyte. This is in agreement to previously published reports:4,5 with DMSO as solvent the product consists of toroidal particles with diameters of approximately 200 nm (see Fig. 1.c), whereas discharge with the TEGDME-based electrolyte yields a product layer on the porous cathode surface (see Fig. 1d). Differential capacity analysis confirmed the difference in discharge product formation: with LiTFSI/TEGDME two reduction processes were detected to contribute to the discharge capacity, whereas discharge with LiTFSI/DMSO contains only one reduction process. X-ray diffraction after the first discharge with LiTFSI/DMSO shows the presence of crystalline Li2O2 as well as a minor amount of LiOH in the GDE (see Fig. 1.e). The discharge with the TEGDME-based electrolyte, on the other hand, did not yield crystalline Li2O2 but only a small amount of LiOH. FIB-SEM measurements after the first discharge with each electrolytes furthermore showed that the GDE pores were almost completely blocked by bulk product. Despite the absence of crystalline Li2O2 a comparison of SEM images after recharge with both electrolytes revealed a better cathode surface as well as pore recovery when using LiTFSI/TEGDME. This fact is most probably the reason for the significantly higher cycling stability with this electrolyte. In order to obtain a more complete picture on the obtained results, the replacement of LiTFSI by other Li+ ion conducting salts, e.g., LiClO4 and LiNO3, as well as the use of electrolyte additives, such as the promising tetrathiafulvalene and dimethylphenazine,6 will be investigated and compared to LiTFSI/DMSO and LiTFSI/TEGDME. Acknowledgments The Research Council of Norway is acknowledged for the financial support within the framework of the research project "Optimized electrode-electrolyte interfaces for Li-air batteries" (contract number 240 866). The Research Council of Norway is also acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50. References Bruce et al., Mater. (2012) 11, 19-29.Yao et al., Chem. Int. Ed. (2016) 55, 11344-11353.Sharon et al., J. Chem. (2015) 55, 508-520.Aetukuri et al., Chem. (2014) 7, 50-56.Mitchell et al., Phys. Chem. Lett. (2013) 4, 1060-1064.Lim et al., Nature Energy (2016) 1, 16066. Figure 1

Self-catalyzed decomposition of discharge products on the oxygen vacancy sites of MoO3 nanosheets for low-overpotential Li-O2 batteries

The efficient reversible formation of discharge products for Li-O2 batteries is still challenging. Meanwhile, the question of the nature of the discharge products and their decomposition mechanism are still remain. Implanting oxygen vacancies on the metal oxides can create negative-charge surface to provide strong adsorption of active oxygen and at the same time, the exposed metal sites can serve as an efficient substrate for decomposed reaction of discharge products. In this work, we apply the graphene-like MoO3 ultrathin nanosheets as a matrix. By controlling the reduction time, the MoO3 nanosheets with different concentration of oxygen vacancy are obtained. Experimental results reveal that the MoO3 nanosheets with the high-concentration oxygen vacancies can significantly decrease the overpotential and get enhanced electrochemical response, namely, a low overpotential of ~0.5V can be delivered with ultra-stable cycles (over 60 cycles). Moreover, the Li-O2 batteries demonstrate an interesting four-step discharge and charge process. X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and electron energy-loss spectroscopy analyses are carried out for the four typical states of the cathode to reveal the reaction mechanism for the unique electrochemical behavior.

Understanding the charge/discharge mechanisms and passivation reactions in Na-O 2 batteries

Sodium-oxygen batteries are becoming of increasing interest in the research community as they are able to overcome some of the difficulties associated with lithium-oxygen batteries. The interpretation of the processes governing the discharge and charge of these batteries, however, has been under debate since their early development. In this work we combine different electrochemical methods to build up a model of the discharge product formation and decomposition. We initially analyze the formation and decomposition of the discharge products by means of electrochemical impedance spectroscopy. After that, and for the first time, oxygen electrode processes in Na-O2 cells are analyzed by means of electrochemical quartz crystal microbalance experiments. Based on the combination of these two techniques it is possible to evidence the stabilization of the discharge products in the electrolyte prior to their precipitation. The deposition of passivating products that cannot be stripped off during charge is also demonstrated. Cyclic voltammetry experiments at different potential limits further confirm these passivation reactions. In conclusion, this work provides an accurate picture of the mechanism of the Na-O2 cell reactions by combining different electrochemical techniques.