As a result of sulfur’s high electrochemical capacity (1675 mA h/gs), 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 Li1+x Al x Ti2–x (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.
Layered Li-rich Ni, Mn, Co (NMC) oxide cathodes in Li-ion batteries provide high specific capacities (>250 mAh/g) via O-redox at high voltages. However, associated high-voltage interfacial ...degradation processes require strategies for effective electrode surface passivation. Here, we show that an acidic surface treatment of a Li-rich NMC layered oxide cathode material leads to a substantial suppression of CO2 and O2 evolution, ∼90% and ∼100% respectively, during the first charge up to 4.8 V vs Li+/0. CO2 suppression is related to Li2CO3 removal as well as effective surface passivation against electrolyte degradation. This treatment does not result in any loss of discharge capacity and provides superior long-term cycling and rate performance in comparison to as-received, untreated materials. We also quantify the extent of lattice oxygen participation in charge compensation (“O-redox”) during Li+ removal by a novel ex situ acid titration. Our results indicate that the peroxo-like species resulting from O-redox originate on the surface at least 300 mV earlier than the activation plateau region at around 4.5 V. X-ray photoelectron spectra and Mn L-edge X-ray absorption spectra of the cathode powders reveal a Li+ deficiency and a partial reduction of Mn ions on the surface of the acid-treated material. More interestingly, although the irreversible oxygen evolution is greatly suppressed through the surface treatment, O K-edge resonant inelastic X-ray scattering shows that the lattice O-redox behavior is largely sustained. The acidic treatment, therefore, only optimizes the surface of the Li-rich material and almost eliminates the irreversible gas evolution, leading to improved cycling and rate performance. This work therefore presents a simple yet effective approach to passivate cathode surfaces against interfacial instabilities during high-voltage battery operation.
Single-ion conducting polymer electrolytes have been proposed to significantly enhance lithium ion battery performance by eliminating concentration gradients within the cell. Such electrolytes have ...universally suffered from poor conductivity at low to moderate temperatures. In an attempt to improve conductivity, numerous studies have sought to better understand the fundamental interplay of ion content and segmental motion, with typical analyses relying on a fit of temperature-dependent conductivity data using the Vogel–Tammann–Fulcher (VTF) equation to assist in separating these effects. In this study, we leverage the large accessible composition window of a newly synthesized, single ion conducting polysulfone–poly(ethylene glycol) (PSf-co-PEG) miscible random copolymer to more completely understand the interrelationship of glass transition temperature, ion content, and the polymer’s Li+ conductivity. It is demonstrated here that choice of fitting procedure and Vogel temperature plays a crucial role in the observed trends, and importantly, after optimization of the data fitting procedure, a strong positive correlation was observed between the VTF equation prefactor and apparent activation energy for polymers in this electrolyte class. This relationship, known as the compensation effect (among other names) for the related Arrhenius-type behavior of activated processes such as chemical kinetics and diffusion, is shown here to exist in several other polymer electrolyte classes. Given conductivity’s inverse exponential dependence on the apparent activation energy, maximum conductivity within an electrolyte class is achieved in samples where the activation energy is small. For a system in which the compensation effect exists, decreasing activation energy also decreases the prefactor, highlighting the limiting nature of the compensation effect and the importance of escaping from it. Blending of small molecules is shown to break the apparent trend within the PSf-co-PEG system, suggesting a clear route to high transference number, high conductivity electrolytes.
The role of residual lithium carbonate in the electrochemistry and outgassing of lithium transition-metal oxides (TMOs) has been largely overlooked. By combining in situ gas analysis, isotopic ...labeling, and a surface carbonate titration, we show that the presence of residual lithium carbonate (Li2CO3) on the surface of both Ni-rich Li-stoichiometric (specifically LiNi0.6Mn0.2Co0.2O2) and Li-rich (Li1.2Ni0.15Co0.1Mn0.55O2) TMOs has a direct correlation with the amount of CO2 and CO evolved and has a relationship with O2 evolved from the TMO lattice on the first charge. By selectively isotopically labeling the residual surface Li2CO3, which remains in small quantities (∼0.1 wt %) after synthesis, and not the carbonate electrolyte, we further show that, up to 4.8 V vs Li/Li+ on the first charge, carbonate electrolyte degradation negligibly contributes to gas evolution. These key conclusions warrant a reassessment of our notion of oxidative decomposition of carbonate electrolytes on TMO surfaces and, more generally, the reactivity of TMO surfaces. For the battery research community, our results highlight the importance of quantification of the surface contaminants and suggest that further research is needed to fully understand the long-term effects of trace surface Li2CO3.
The continued search for routes to improve the power and energy density of lithium ion batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell ...components, particularly in electrode materials design. In this Review, we highlight an often less noted route to improving energy density: increasing the Li+ transference number of the electrolyte. Turning to Newman’s original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li+ transference numbers compared to traditional carbonate-based liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than that of conventional electrolytes. Most current research in high transference number electrolytes (HTNEs) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems and suggest additional work on promising new HTNE systems, such as “solvent-in-salt” electrolytes, perfluorinated solvent electrolytes, nonaqueous polyelectrolyte solutions, and solutions containing anion-decorated nanoparticles.
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 ⁷Li 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.
Solid alkali metal carbonates are universal passivation layer components of intercalation battery materials and common side products in metal‐O2 batteries, and are believed to form and decompose ...reversibly in metal‐O2/CO2 cells. In these cathodes, Li2CO3 decomposes to CO2 when exposed to potentials above 3.8 V vs. Li/Li+. However, O2 evolution, as would be expected according to the decomposition reaction 2 Li2CO3→4 Li++4 e−+2 CO2+O2, is not detected. O atoms are thus unaccounted for, which was previously ascribed to unidentified parasitic reactions. Here, we show that highly reactive singlet oxygen (1O2) forms upon oxidizing Li2CO3 in an aprotic electrolyte and therefore does not evolve as O2. These results have substantial implications for the long‐term cyclability of batteries: they underpin the importance of avoiding 1O2 in metal‐O2 batteries, question the possibility of a reversible metal‐O2/CO2 battery based on a carbonate discharge product, and help explain the interfacial reactivity of transition‐metal cathodes with residual Li2CO3.
Knowledge is power: Lithium carbonate is ubiquitous in Li battery systems. Evidence is provided that highly reactive singlet oxygen forms when Li2CO3 is electrochemically decomposed beyond 3.8 V vs. Li/Li+, which suggests that most currently studied cathodes will be deleteriously affected by Li2CO3. Strategies to mitigate 1O2 formation or the presence of Li2CO3 during battery operation are therefore warranted.
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