Polarizable Molecular Dynamics and Experiments of 1,2-Dimethoxyethane Electrolytes with Lithium and Sodium Salts: Structure and Transport Properties.

The structure and transport properties of electrolyte solutions of 1,2-dimethoxyethane (DME) having salts of Li+ with bis(trifluoromethanesulfonyl)imide ([TFSI]-) or Na+ with [TFSI]- are investigated with polarizable molecular dynamics and experiments. Polarizable force fields for Li+ and Na+ with DME and [TFSI]- were developed based on quantum chemistry calculations, ab initio molecular dynamics simulations, and thermodynamic liquid-state properties. Simulation results for density, viscosity, self-diffusion coefficient, and conductivity of the electrolytes all agree well with the trends and magnitudes of available experimental data for a wide range of salt concentrations. As the concentration of salt increases, the electrolytes become more viscous and molecular species become less mobile. Ionic conductivity does not change monotonically with salt concentration and exhibits a maximum between 0.5 and 1.0 M for both Li[TFSI] and Na[TFSI] electrolytes. Comparatively, both cations are solvated by 5-6 DME or [TFSI]- oxygen atoms and exhibit similar diffusivities and conductivities. The solvation shell of Na+ is found to be more weakly bound and to have a lower binding residence time than that of Li+. The transport of Li+ therefore is more vehicular, through the motion of the solvation shell, while the transport of Na+ is based more on exchange, through the replacement of solvating species. The atomistic insight provided by this work can be used as the basis for future rational design of improved electrolyte solvents for lithium-oxygen, sodium-oxygen, and lithium-sulfur batteries.

Poly(vinylidene fluoride) (PVDF) Binder Degradation in Li-O2 Batteries: A Consideration for the Characterization of Lithium Superoxide.

We show that a common Li-O2 battery cathode binder, poly(vinylidene fluoride) (PVDF), degrades in the presence of reduced oxygen species during Li-O2 discharge when adventitious impurities are present. This degradation process forms products that exhibit Raman shifts (∼1133 and 1525 cm-1) nearly identical to those reported to belong to lithium superoxide (LiO2), complicating the identification of LiO2 in Li-O2 batteries. We show that these peaks are not observed when characterizing extracted discharged cathodes that employ poly(tetrafluoroethylene) (PTFE) as a binder, even when used to bind iridium-decorated reduced graphene oxide (Ir-rGO)-based cathodes similar to those that reportedly stabilize bulk LiO2 formation. We confirm that for all extracted discharged cathodes on which the 1133 and 1525 cm-1 Raman shifts are observed, only a 2.0 e-/O2 process is identified during the discharge, and lithium peroxide (Li2O2) is predominantly formed (along with typical parasitic side product formation). Our results strongly suggest that bulk, stable LiO2 formation via the 1 e-/O2 process is not an active discharge reaction in Li-O2 batteries.

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.

Mechanistic insights for the development of Li-O2 battery materials: addressing Li2O2 conductivity limitations and electrolyte and cathode instabilities.

The Li-air battery has received significant attention over the past decade given its high theoretical specific energy compared to competing energy storage technologies. Yet, numerous scientific challenges remain unsolved in the pursuit of attaining a battery with modest Coulombic efficiency and high capacity. In this Feature Article, we provide our current perspective on challenges facing the development of nonaqueous Li-O2 battery cathodes. We initially present a review on our understanding of electrochemical processes occurring at the nonaqueous Li-O2 cathode. Electrolyte and cathode instabilities and Li2O2 conductivity limitations are then discussed, and suggestions for future materials research development to alleviate these issues are provided.

Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li-O2 battery capacity.

Among the "beyond Li-ion" battery chemistries, nonaqueous Li-O2 batteries have the highest theoretical specific energy and, as a result, have attracted significant research attention over the past decade. A critical scientific challenge facing nonaqueous Li-O2 batteries is the electronically insulating nature of the primary discharge product, lithium peroxide, which passivates the battery cathode as it is formed, leading to low ultimate cell capacities. Recently, strategies to enhance solubility to circumvent this issue have been reported, but rely upon electrolyte formulations that further decrease the overall electrochemical stability of the system, thereby deleteriously affecting battery rechargeability. In this study, we report that a significant enhancement (greater than fourfold) in Li-O2 cell capacity is possible by appropriately selecting the salt anion in the electrolyte solution. Using (7)Li NMR and modeling, we confirm that this improvement is a result of enhanced Li(+) stability in solution, which, in turn, induces solubility of the intermediate to Li2O2 formation. Using this strategy, the challenging task of identifying an electrolyte solvent that possesses the anticorrelated properties of high intermediate solubility and solvent stability is alleviated, potentially providing a pathway to develop an electrolyte that affords both high capacity and rechargeability. We believe the model and strategy presented here will be generally useful to enhance Coulombic efficiency in many electrochemical systems (e.g., Li-S batteries) where improving intermediate stability in solution could induce desired mechanisms of product formation.