Cathode Design for High Energy Molten Salt Lithium-Oxygen Batteries

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support. We begin by describing the effect of using a molten salt electrolyte in a lithium-oxygen battery. In particular, we focus on how the electrochemical performance and discharge product, lithium peroxide, differ from that of a traditional organic electrolyte. In addition, we discuss the enhanced peroxide solubility in a molten salt and its implications for lithium peroxide growth and coulombic efficiency. Finally, we address the cell death of a galvanostatically cycled battery. We then introduce a similar phase-forming conversion chemistry, whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts were previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate reduces to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of micron-scale lithium oxide crystals in this system. After introducing these molten salt lithium batteries, we address the effect of cathode geometry on electrochemical performance. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area, comparing it with the more typical geometries of thin films and nanoparticles.</p

Rechargeable-battery chemistry based on lithium oxide growth through nitrate anion redox

Next-generation lithium-battery cathodes often involve the growth of lithium-rich phases, which enable specific capacities that are 2−3 times higher than insertion cathode materials, such as lithium cobalt oxide. Here, we investigated battery chemistry previously deemed irreversible in which lithium oxide, a lithium-rich phase, grows through the reduction of the nitrate anion in a lithium nitrate-based molten salt at 150 °C. Using a suite of independent characterization techniques, we demonstrated that a Ni nanoparticle catalyst enables the reversible growth and dissolution of micrometre-sized lithium oxide crystals through the effective catalysis of nitrate reduction and nitrite oxidation, which results in high cathode areal capacities (~12 mAh cm^(–2)). These results enable a rechargeable battery system that has a full-cell theoretical specific energy of 1,579 Wh kg^(–1), in which a molten nitrate salt serves as both an active material and the electrolyte

A Molten Salt Lithium-Oxygen Battery

Despite the promise of extremely high theoretical capacity (2Li + O_2 ↔ Li_2O_2, 1675 mAh per gram of oxygen), many challenges currently impede development of Li/O_2 battery technology. Finding suitable electrode and electrolyte materials remains the most elusive challenge to date. A radical new approach is to replace volatile, unstable and air-intolerant organic electrolytes common to prior research in the field with alkali metal nitrate molten salt electrolytes and operate the battery above the liquidus temperature (>80 °C). Here we demonstrate an intermediate temperature Li/O_2 battery using a lithium anode, a molten nitrate-based electrolyte (e.g., LiNO_3–KNO_3 eutectic) and a porous carbon O_2 cathode with high energy efficiency (∼95%) and improved rate capability because the discharge product, lithium peroxide, is stable and moderately soluble in the molten salt electrolyte. The results, supported by essential state-of-the-art electrochemical and analytical techniques such as in situ pressure and gas analyses, scanning electron microscopy, rotating disk electrode voltammetry, demonstrate that Li_2O_2 electrochemically forms and decomposes upon cycling with discharge/charge overpotentials as low as 50 mV. We show that the cycle life of such batteries is limited only by carbon reactivity and by the uncontrolled precipitation of Li_2O_2, which eventually becomes electrically disconnected from the O_2 electrode

The Role of Cathode Architecture in Conversion Reaction Chemistries

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support. Consider a similar phase-forming conversion chemistry[1], whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts have been previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate reduces to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of large (several micron) lithium oxide crystals in this system, as substantiated by SEM, XRD, and TEM. After introducing this molten salt lithium battery, we address the effect of cathode geometry on the electrochemical performance of this model system. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. For instance, it is not just the surface area of the catalyst support that determines the discharge capacity, but also the amount of usable pore volume which can accommodate this solid phase discharge product. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area, comparing it with the more typical geometries of thin films and nanoparticles. These electrode design principles can be extended to other phase-forming conversion chemistries. [1] Addison, D.; Bryantsev, V.; Chase, G. V.; Giordani, V.; Uddin, J.; Walker, W Rechargeable Batteries Employing Catalyzed Molten Nitrate Positive Electrodes. US Patent 2016/0204418, August 8, 2013. </jats:p

Cathode Design for High Energy Molten Salt Lithium-Oxygen Batteries

State of the art commercial lithium ion batteries use cathodes such as lithium cobalt oxide which rely on insertion and removal of lithium ions from a host material. However, insertion cathode materials are limited in their capacity, and replacing them with a cathode that employs growth and dissolution of new phases could significantly increase a battery’s energy density. For example, oxygen and sulfur cathodes have been widely researched to this end, with both cases involving the growth of a lithium-rich compound on a current collector/catalyst support. While the lithium-oxygen battery is promising with regard to its energy density, there are many practical challenges that remain to be solved. For instance, traditional organic electrolytes decompose in the presence of superoxide anions, intermediates in the growth of the lithium peroxide discharge product. However, by replacing the organic electrolyte with a molten salt, we can inherently avoid this organic electrolyte decomposition. In addition, the use of a molten salt electrolyte results in a system operating at elevated temperature with a large concentration of lithium ions, encouraging faster diffusion and kinetics. The morphology of the lithium peroxide grown in this molten salt lithium-oxygen battery is notably different from that previously observed in literature. While previous works have observed thin films, platelets, and “toroids” on the order of several hundred nanometers, we observe much larger (several micron) structures which appear to be stacks of hexagonal layers. We believe these stacks to be a new morphology of lithium peroxide growth. These new lithium peroxide morphologies are characterized with SEM, EDS, and XRD. In addition to the lithium-oxygen system described above, we introduce another phase-forming chemistry, whereby a molten nitrate salt serves as both an active material and the electrolyte. Molten nitrate salts have been previously studied as an active material in a primary lithium battery where lithium oxide irreversibly forms as nitrate converts to nitrite. We will describe how the use of a nanoparticle heterogeneous catalyst allows the reversible growth and dissolution of large (several micron) lithium oxide crystals in this system, as substantiated by SEM, XRD, and TEM. After introducing these molten salt lithium batteries, we address the effect of cathode geometry on the discharge capacity. In particular, we note that the growth of such large, solid phase species on the surface of the catalyst support imposes new design restrictions when optimizing a cathode for energy density. For instance, it is not just the surface area of the catalyst support that determines the discharge capacity, but also the amount of usable pore volume to allow this solid phase discharge product to form. As a proof of concept, we design and implement an architected electrode with large pore volume and relatively small surface area. Such cathode design principles could be extended to other phase-forming chemistries.</jats:p

Cathode Design for a Molten Salt Lithium-Oxygen Battery

The rechargeable lithium-oxygen battery has attracted attention due to its large theoretical energy density compared to modern lithium-ion batteries. This large energy density is attributed to the reaction of lithium with molecular oxygen to form lithium peroxide, which grows on the surface of the cathode. While this is a promising chemistry, there are many practical challenges that remain to be solved, such as the decomposition of organic electrolyte in the presence of superoxide anions and large overpotentials on charge. Additionally, the mechanism of lithium peroxide growth and its resulting morphologies is not fully understood. Here we propose a system which inherently avoids many of the issues associated with organic electrolyte decomposition, while also forming lithium peroxide with a unique morphology. By using a molten salt (Li/K nitrate) in place of a conventional solvent/salt electrolyte, solvent decomposition is obviated. In addition, the elevated temperature of the molten salt as well as the large concentration of lithium ions encourage faster diffusion and kinetics. In literature, the three commonly observed morphologies of electrochemically grown lithium peroxide are thin films, platelets, and “toroids” which are small stacks of platelets. While we do observe the platelet style growth of lithium peroxide, we also see much larger structures which appear to be stacks of hexagonal layers (see attached figure). We believe these stacks to be a new morphology of lithium peroxide growth. To substantiate this claim, we note that the Wulff construction for lithium peroxide is a short hexagonal prism, while also confirming the reaction product using XRD. This new morphology could be attributed to the fact that our cell operates with elevated temperature and large concentration of lithium and superoxide ions, making it easier to achieve an equilibrium (Wulff) structure. We have shown a lithium-oxygen battery chemistry that produces a new morphology of lithium peroxide, and begun to develop a mechanism for why it forms. In addition, we have explored the effect of various cell parameters such as discharge rate on the morphology of the resulting lithium peroxide. Figure 1 <jats:p /

Intermediate Temperature Molten Salt Lithium Batteries, New Chemistries and Beyond

A current focus in battery research, particularly for electric vehicles and stationary energy storage applications, is identifying new chemistries that could enable a higher energy density and lower cost battery than currently available Li-ion batteries. Li-ion electrode materials, which dictate attainable battery energy densities, used in these batteries have changed little in the past decades, and therefore physical limits in Li-ion battery pack energy density are being approached given the enormous development efforts in the field.  In this work, we take a close look at molten salt lithium batteries and in particular alkali metal nitrate/nitrite molten salt electrolytes which exhibit low melting point temperature, high ionic conductivity, high thermal stability and good interfacial properties against lithium metal anode. More specifically, we investigated the O2 electrode electrochemical behavior as well as the reduction of nitrates on metal and metal oxide nanoparticles to form lithium oxide and nitrites. The results reported herein provide a potentially transformative approach to enable a rechargeable Li-O2 battery chemistry through the use of inorganic molten salts as electrolytes. Building on our previous foundational research, in which we identified that all organic electrolytes are incompatible with the reversible Li-O2 electrochemistry, the electrolyte compositions reported here do not contain unstable organic solvents. As a result, the reversibility of the Li-O2 electrochemistry reported here is unprecedented.  Another approach developed at Liox Power, Inc. is the use of nitrates as electroactive material, or in other words, as both the electrolyte and the cathode material. In this case the battery discharge reaction proceeds as the electrochemical reduction of nitrate anions NO3 - present in the electrolyte phase. High Li+ and NO3 -concentration and ionic conductivity allow extremely fast kinetics at 150 °C resulting in very low discharge overpotentials. Furthermore, we demonstrate that such electrochemical reaction is reversible and lithium oxide particles (octahedron equilibrium Wulff shape) can be recharged with energy efficiencies greater than 97%. Figure 1. Top, voltage profile (first 2 cycles) of a cell employing a Li metal anode, a LiNO3-KNO3 eutectic and a Ni cathode, cycling at 150 °C at 0.32 mA/cm2, under Ar gas. Bottom, voltage profiles of (black) organic electrolyte Li-O2 cell and (red) LiNO3-KNO3 molten salt Li-O2 cell; both cells used a Li metal anode, a Super P carbon black cathode (5% PTFE binder) and cycled at 0.32 mA/cm2 under pure O2 gas. Figure 1 <jats:p /

Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide

Graphene oxide (GO) has drawn tremendous interest as a tunable precursor in numerous areas, due to its readily manipulable surface. However, its inhomogeneous and nonstoichiometric structure makes achieving chemical control a major challenge. Here, we present a room-temperature based, controlled method for the stepwise reduction of GO, with evidence of sequential removal of each organic moiety. By analyzing signature infrared absorption frequencies, we identify the carbonyl group as the first to be reduced, while the tertiary alcohol takes the longest to be completely removed from the GO surface. Controlled reduction allows for progressive tuning of the optical gap from 3.5 eV down to 1 eV, while XPS spectra show a concurrent increase in the C/O ratio. This study is the first step toward selectively enhancing the chemical homogeneity of GO, thus providing greater control over its structure, and elucidating the order of removal of functional groups and hydrazine-vapor reduction