Attainable Gravimetric and Volumetric Energy Density of Li–S and Li Ion Battery Cells with Solid Separator-Protected Li Metal Anodes

As a result of sulfur’s high electrochemical capacity (1675 mA h/g_s), lithium–sulfur batteries have received significant attention as a potential high-specific-energy alternative to current state-of-the-art rechargeable Li ion batteries. For Li–S batteries to compete with commercially available Li ion batteries, high-capacity anodes, such as those that use Li metal, will need to be enabled to fully exploit sulfur’s high capacity. The development of Li metal anodes has focused on eliminating Coulombically inefficient and dendritic Li cycling, and to this end, an interesting direction of research is to protect Li metal by employing mechanically stiff solid-state Li⁺ conductors, such as garnet phase Li₇La₃Zr₂O₁₂ (LLZO), NASICON-type Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃ (LATP), and Li₂S–P₂S₅ glasses (LPS), as electrode separators. Basic calculations are used to quantify useful targets for solid Li metal protective separator thickness and cost to enable Li metal batteries in general and Li–S batteries specifically. Furthermore, maximum electrolyte-to-sulfur ratios that allow Li–S batteries to compete with Li ion batteries are calculated. The results presented here suggest that controlling the complex polysulfide speciation chemistry in Li–S cells with realistic, minimal electrolyte loading presents a meaningful opportunity to develop Li–S batteries that are competitive on a specific energy basis with current state-of-the-art Li ion batteries

Quantification of Surface Oxygen Depletion and Solid Carbonate Evolution on the First Cycle of LiNi_0.6Mn_0.2Co_0.2O₂ Electrodes

By combining differential electrochemical mass spectrometry (DEMS) with titrations of electrochemically modified LiNi0.6Mn0.2Co0.2O2 (NMC622), we find that coinciding with the onset of CO2 evolution above ∼3.9 V vs Li/Li+ anodic cutoff potentials are several phenomena: (i) degradation of the native surface Li2CO3, (ii) degradation of the electrolyte evolving CO2, (iii) formation of a film of carbonate-like electrolyte degradation products on charge which are (iv) largely reduced and desorb on discharge, (v) near-surface oxygen charge compensation during charge, and (vi) irreversible formation of a transition metal-reduced, oxygen-depleted layer on the surface of NMC622 that persists after discharge. CO2 stemming from electrolyte degradation and Li2CO3 decomposition begins to evolve above ∼3.9 V on charge, discharge, and rest and results from a corrosion-like process involving NMC622, which appears to be distinct from the process that evolves O2. As measured using titrations that quantify surface peroxo-like character, the disordered surface layer that forms during cycling extends deeper into the oxide bulk than would be anticipated simply from the total O2 evolved. The analyses we report here could be used to quantify the role of the electrolyte, surface contaminants, and transition metal oxide composition on outgassing, electrolyte decomposition, and transition metal oxide surface degradation

The Sudden Death Phenomena in Nonaqueous Na–O₂ Batteries

Metal–air (O₂) batteries have been intensely studied over the past decade as potential high-energy alternatives to current state-of-the-art Li-ion batteries. Although Li–O₂ batteries possess higher theoretical specific energies, Na–O₂ cells have been reported to achieve higher capacities on discharge and exhibit much lower overpotentials on charge than analogous Li–O₂ cells. Nevertheless, sudden and large overpotential increases (“sudden deaths”) occur in Na–O₂ cells on both discharge and charge, substantially limiting achievable capacity on discharge and increasing the average charge voltage, thereby reducing round-trip energy efficiency. In this work, we unravel the origins of these sudden death phenomena, which have been previously linked to the electrochemistry occurring at the cathode. On discharge, the maximum capacity was limited by pore clogging at low current densities and by surface passivation at high current densities, with concentration polarization playing only a small role in limiting the achievable capacity. On charge, the discharge and charge current densities were both found to influence the attainable capacity prior to sudden death. We propose a charge mechanism consistent with our data, where a concerted surface oxidation mechanism and a dissolution–oxidation mechanism both contribute to the observed overpotentials. Sudden death on charge is proposed to occur when these two pathways cannot support the applied current rate

Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries

High concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) have emerged as promising candidates to enable higher energy density Li-ion batteries due to their advantageous interfacial properties that result from their unique solvent structures. Using electrophoretic NMR and electrochemical techniques, we characterize and report full transport properties, including the lithium transference numbers (t+) for electrolytes ranging from the conventional ∼1 M to HCE regimes as well as for LHCE systems. We find that compared to conventional electrolytes, t+ increases for HCEs; however the addition of diluents to LHCEs significantly decreases t+. Viscosity effects alone cannot explain this behavior. Using Onsager transport coefficients calculated from our experiments, we demonstrate that there is more positively correlated cation–cation motion in HCEs as well as fast cation–anion ligand exchange consistent with a concerted ion-hopping mechanism. The addition of diluents to LHCEs results in more anticorrelated motion indicating a disruption of concerted cation-hopping leading to low t+ in LHCEs

Counterion Transport and Transference Number in Aqueous and Nonaqueous Short-Chain Polyelectrolyte Solutions

Nonaqueous polyelectrolyte solutions have recently been proposed as potential battery electrolytes due to their unique ability to tune the mobility of the anion relative to that of the electrochemically active lithium ion. This could potentially be used to study the effect of concentration polarization during battery charge, a major limiting factor in achieving fast charge rates that is caused by high anion mobility. An important consideration in the design of polyelectrolyte solutions for battery applications is the solubility of the polymer in battery-relevant carbonate blend solvents. Little is understood from a transport perspective, however, about the importance of designing the polymer to be solvophillic or if it is sufficient to obtain solubility through the incorporation of appended ions alone (as with polystyrene sulfonate in water). Using a model polysulfone-based system without added salt, we investigate the conductivity, viscosity, and diffusion of polyelectrolyte solutions over a range of concentrations and molecular weights in dimethyl sulfoxide (DMSO) and water. In both solvents, sulfonated polysulfone is readily soluble and the charged group is known to dissociate, but the neutral backbone polymer is only soluble in DMSO. We find marked differences in the transport behavior of polymer solutions prepared from the two solvents, particularly at high concentrations. Comparing this transport behavior to that of the monomer in solution demonstrates a larger decrease in lithium motion in DMSO than in water, even though the bulk viscosity in water increases far more rapidly. This study sheds light on the important parameters for optimizing polyelectrolyte solution transport in different solvents

Altering Surface Contaminants and Defects Influences the First-Cycle Outgassing and Irreversible Transformations of LiNi_0.6Mn_0.2Co_0.2O₂

By altering the surface of LiNi0.6Mn0.2Co0.2O2 (NMC622) we show that surface defects and contaminants dominate the outgassing and irreversible surface transformations during the first electrochemical cycle. To alter the surface defects and contaminants without changing the bulk structure of the NMC622, we perform mild methanol and water rinses, a water soak, a water rinse and subsequent heat treatment, as well as purposeful increase of the surface Li2CO3. By combining isotopic labeling; gas analysis; and peroxide, hydroxide, and carbonate titrations we observe that these alterations change the surface Li2CO3, surface hydroxides, and the local defects, which in turn alter the nature and extent of the outgassing to O2 and CO2. Our results highlight that outgassing of Li-ion cathode materials is highly dependent on the synthesis and storage routes and comparison of varying compositions must take into account these differences to make any meaningful conclusions. We also show that simple rinsing procedures may be an effective route to controlling interfacial reactivity of Li-ion active materials

Lithium-Ion Transport and Exchange between Phases in a Concentrated Liquid Electrolyte Containing Lithium-Ion-Conducting Inorganic Particles

Understanding Li+ transport in organic–inorganic hybrid electrolytes, where Li+ has to lose its organic solvation shell to enter and transport through the inorganic phase, is crucial to the design of high-performance batteries. As a model system, we investigate a range of Li+-conducting particles suspended in a concentrated electrolyte. We show that large Li1.3Al0.3Ti1.7P3O12 and Li6PS5Cl particles can enhance the overall conductivity of the electrolyte. When studying impedance using a cell with a large cell constant, the Nyquist plot shows two semicircles: a high-frequency semicircle related to ion transport in the bulk of both phases and a medium-frequency semicircle attributed to Li+ transporting through the particle/liquid interfaces. Contrary to the high-frequency resistance, the medium-frequency resistance increases with particle content and shows a higher activation energy. Furthermore, we show that small particles, requiring Li+ to overcome particle/liquid interfaces more frequently, are less effective in facilitating Li+ transport. Overall, this study provides a straightforward approach to study the Li+ transport behavior in hybrid electrolytes

Chemical and Electrochemical Differences in Nonaqueous Li–O₂ and Na–O₂ Batteries

We present a comparative study of nonaqueous Li–O₂ and Na–O₂ batteries employing an ether-based electrolyte. The most intriguing difference between the two batteries is their respective galvanostatic charging overpotentials: a Na–O₂ battery exhibits a low overpotential throughout most of its charge, whereas a Li–O₂ battery has a low initial overpotential that continuously increases to very high voltages by the end of charge. However, we find that the inherent kinetic Li and Na–O₂ overpotentials, as measured on a flat glassy carbon electrode in a bulk electrolysis cell, are similar. Measurement of each batteries’ desired product yield, <i>Y</i>_NaO2 and <i>Y</i>_Li2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na–O₂ battery during the first Galvanostatic discharge–charge cycle. We therefore postulate that reactivity differences (Li₂O₂ being more reactive than NaO₂) between the major discharge products lead to the observed charge overpotential difference between each battery

Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation

Perfluorosulfonic acid (PFSA) dispersions are used as components in a variety of electrochemical technologies, particularly in fuel-cell catalyst-layer inks. In this study, we characterize dispersions of a common PFSA, Nafion, as well as inks of Nafion and carbon. It is shown that solvent choice affects a dispersion’s measured pH, which is found to scale linearly with Nafion loading. Dispersions in water-rich solvents are more acidic than those in propanol-rich solvents: a 90% water versus 30% water dispersion can have up to a 55% measured proton deviation. Furthermore, because electrostatic interactions are a function of pH, these differences affect how particles aggregate in solution. Despite having different water contents, all inks studied demonstrate the same particle size and surface charge trends as a function of pH, thus providing insights into the relative influence of solvent and pH effects on these properties

Oxygen Pressure Influences Spatial NaO₂ Deposition and the Sudden Death Mechanism in Na–O₂ Batteries

Over the past decade, metal–O₂ batteries have been intensely studied as potential high energy density alternatives to current state-of-the-art Li-ion batteries. Of these, nonaqueous Na–O₂ batteries offer high stability, improved full-cycle efficiency, and lower overpotentials, particularly on charge, when compared to the higher-energy-density Li–O₂ system. However, Na–O₂ batteries exhibit sudden and large overpotential increases or “sudden deaths” on discharge, substantially limiting the achievable capacity. In this work, we examine the influence of O₂ pressure effects in Na–O₂ batteries and the mechanism of sudden death at different O₂ pressures and current density regimes. We observe that at a given current density, there exists a transition between failure mechanisms with O₂ pressure as a result of different phenomena related to the deposition of the solid discharge product, sodium superoxide (NaO₂). Cells operated at a lower O₂ pressure are more susceptible to failure due to surface passivation resulting from thin NaO₂ film coverage, whereas cells operated at a higher O₂ pressure achieve higher capacities but are increasingly subject to failure due to pore clogging from substantial solid NaO₂ deposition. We associate the transition between these failure mechanisms with a combination of electron and mass transfer effects, leading to dramatic differences in the spatial deposition of NaO₂ through the cathode