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 + O2 ↔ Li2O2, 1675 mAh per gram of oxygen), many challenges currently impede development of Li/O2 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/O2 battery using a lithium anode, a molten nitrate-based electrolyte (e.g., LiNO3-KNO3 eutectic) and a porous carbon O2 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 Li2O2 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 Li2O2, which eventually becomes electrically disconnected from the O2 electrode.

Predicting the electrochemical behavior of lithium nitrite in acetonitrile with quantum chemical methods.

Electrolyte stability is an essential prerequisite for the successful development of a rechargeable organic electrolyte Li-O2 battery. Lithium nitrate (LiNO3) salt was employed in our previous work because it was capable of stabilizing a solid-electrolyte interphase on the Li anode. The byproduct of this process is lithium nitrite (LiNO2), the fate of which in a Li-O2 battery is unknown. In this work, we employ density functional theory and coupled-cluster calculations combined with an implicit solvation model for neutral molecules and a mixed cluster/continuum model for single ions to understand the chemical and electrochemical behavior of LiNO2 in acetonitrile (AN). The redox potentials of oxygenated nitrogen compounds predicted in this study are in excellent agreement with the experimental results (the average accuracy is 0.10 V). Theoretical calculations suggest that the reaction between the nitrite ion and its first oxidation product, nitrogen dioxide (NO2), in AN solution proceeds via the initial formation of a trans-ONO-NO2 dimer that is subject to autoionization and the subsequent reaction of produced nitrosyl ion (NO(+)) with NO2(-). Good agreement between experimental and simulated cyclic voltammograms for electrochemical oxidation of LiNO2 in AN provides support to the proposed mechanism of coupled electrochemical and chemical reactions. The results suggest a possible mechanism of regeneration of LiNO3 in electrolyte in the presence of oxygen, which is uniquely possible under charging conditions in a Li-O2 battery.

A rechargeable Li-O2 battery using a lithium nitrate/N,N-dimethylacetamide electrolyte.

A major challenge in the development of rechargeable Li-O(2) batteries is the identification of electrolyte materials that are stable in the operating environment of the O(2) electrode. Straight-chain alkyl amides are one of the few classes of polar, aprotic solvents that resist chemical degradation in the O(2) electrode, but these solvents do not form a stable solid-electrolyte interphase (SEI) on the Li anode. The lack of a persistent SEI leads to rapid and sustained solvent decomposition in the presence of Li metal. In this work, we demonstrate for the first time successful cycling of a Li anode in the presence of the solvent, N,N-dimethylacetamide (DMA), by employing a salt, lithium nitrate (LiNO(3)), that stabilizes the SEI. A Li-O(2) cell containing this electrolyte composition is shown to cycle for more than 2000 h (>80 cycles) at a current density of 0.1 mA/cm(2) with a consistent charging profile, good capacity retention, and O(2) detected as the primary gaseous product formed during charging. The discovery of an electrolyte system that is compatible with both electrodes in a Li-O(2) cell may eliminate the need for protecting the anode with a ceramic membrane.

Predicting solvent stability in aprotic electrolyte Li-air batteries: nucleophilic substitution by the superoxide anion radical (O2(•-)).

There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.