Justin G. Connell

Lithium metal protected by atomic layer deposition metal oxide for high performance anodes

Lithium metal is a highly desirable anode material for lithium batteries due to its extremely high theoretical capacity (3860 mA h g−1), low potential (−3.04 V versus standard hydrogen electrode), and low density (0.534 g cm−3). However, dendrite growth during cycling and low coulombic efficiency, resulting in safety hazards and fast battery fading, are huge barriers to commercialization. Herein, we used atomic layer deposition (ALD) to prepare conformal, ultrathin aluminum oxide coatings on lithium. We investigated the growth mechanism during Al2O3 ALD on lithium by in situ quartz crystal microbalance and found larger growth than expected during the initial cycles. We also discovered that the ALD Al2O3 enhances the wettability of the Li surface towards both carbonate and ether electrolytes, leading to uniform and dense SEI formation and reduced electrolyte consumption during battery operation. Scanning electron microscopy verified that the bare Li surfaces become rough and dendritic after electrochemical cycling, whereas the ALD Al2O3 coated Li surfaces remain smooth and uniform. Analysis of the Li surfaces after cycling using X-ray photoelectron spectroscopy and in situ transmission electron microscopy revealed that the ALD Al2O3 coating remains intact during electrochemical cycling, and that Li ions diffuse through the coating and deposit on the underlying Li. Coin cell testing demonstrated more than two times longer cycling life for the ALD Al2O3 protected Li, and a coulombic efficiency as high as ∼98% at a practical current rate of 1 mA cm−2. More significantly, when the electrolyte volume was reduced from 20 to 5 μL, the stabilizing effect of the ALD coating became even more pronounced and the cycling life was around four times longer. These results indicate that ALD Al2O3 coatings are a promising strategy to stabilize Li anodes for high performance energy storage devices such as Li–S batteries.

Molecular understanding of polyelectrolyte binders that actively regulate ion transport in sulfur cathodes

Polymer binders in battery electrodes may be either active or passive. This distinction depends on whether the polymer influences charge or mass transport in the electrode. Although it is desirable to understand how to tailor the macromolecular design of a polymer to play a passive or active role, design rules are still lacking, as is a framework to assess the divergence in such behaviors. Here, we reveal the molecular-level underpinnings that distinguish an active polyelectrolyte binder designed for lithium–sulfur batteries from a passive alternative. The binder, a cationic polyelectrolyte, is shown to both facilitate lithium-ion transport through its reconfigurable network of mobile anions and restrict polysulfide diffusion from mesoporous carbon hosts by anion metathesis, which we show is selective for higher oligomers. These attributes allow cells to be operated for >100 cycles with excellent rate capability using cathodes with areal sulfur loadings up to 8.1 mg cm–2.Polymer binders in battery electrodes can affect their performance, however design rules are still lacking. Here, the authors reveal why polyelectrolyte binders outperform charge-neutral alternatives in lithium–sulfur batteries, showing how cationic polyelectrolytes can regulate ion transport selectively.

Electrocatalytic transformation of HF impurity to H2 and LiF in lithium-ion batteries

AbstractThe formation of solid electrolyte interphase on graphite anodes plays a key role in the efficiency of Li-ion batteries. However, to date, fundamental understanding of the formation of LiF as one of the main solid electrolyte interphase components in hexafluorophosphate-based electrolytes remains elusive. Here, we present experimental and theoretical evidence that LiF formation is an electrocatalytic process that is controlled by the electrochemical transformation of HF impurity to LiF and H2. Although the kinetics of HF dissociation and the concomitant production of LiF and H2 is dependent on the structure and nature of surface atoms, the underlying electrochemistry is the same. The morphology, and thus the role, of the LiF formed is strongly dependent on the nature of the substrate and HF inventory, leading to either complete or partial passivation of the interface. Our finding is of general importance and may lead to new opportunities for the improvement of existing, and design of new, Li-ion technologies.Despite the central role that the solid electrolyte interphase plays on the efficiency of Li-ion batteries, little is known about its formation mechanism. It is now shown that LiF forms on graphite anodes as a result of the electrocatalytic transformation of HF impurities present in the electrolyte.

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact with Lithium Metal

Understanding ionic transport across interfaces between dissimilar materials and the intrinsic chemical stability of such interfaces is a fundamental challenge spanning many disciplines and is of particular importance for designing conductive and stable solid electrolytes for solid-state Li-ion batteries. In this work, we establish a surface science-based approach for assessing the intrinsic stability of oxide materials in contact with Li metal. Through a combination of experimental and computational insights, using Nb-doped SrTiO3 (Nb/STO) single crystals as a model system, we were able to understand the impact of crystallographic orientation and surface morphology on the extent of the chemical reactions that take place between surface Nb, Ti, and Sr upon reaction with Li. By expanding our approach to investigate the intrinsic stability of the technologically relevant, polycrystalline Nb-doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Nb0.5O12) system, we found that this material reacts with Li metal through the reduction of Nb, similar to that observed for Nb/STO. These results clearly demonstrate the feasibility of our approach to assess the intrinsic (in)stability of oxide materials for solid-state batteries and point to new strategies for understanding the performance of such systems.

Early Stage Anodic Instability of Glassy Carbon Electrodes in Propylene Carbonate Solvent Containing Lithium Hexafluorophosphate

Irreversible changes to the morphology of glassy carbon (GC) electrodes at potentials between 3.5 and 4.5 V vs Li/Li+ in propylene carbonate (PC) solvent containing lithium hexafluorophosphate (LiPF6) are reported. Analysis of cyclic voltammetry (CV) experiments in the range of 3.0 to 6.0 V shows that the capacitance of the electrochemical double-layer increased irreversibly beginning at potentials as low as 3.5 V. These changes resulted from nonfaradaic interactions, and were not due to oxidative electrochemical decomposition of the electrode and electrolyte, anion intercalation, nor caused by the presence of water, a common impurity in organic electrolyte solutions. Atomic force microscopy (AFM) images revealed that increasing the potential of a bare GC surface from 3.0 to 4.5 V resulted in a 6× increase in roughness, in good agreement with the changes in double-layer capacitance. Treating the GC surface via exposure to trichloromethylsilane vapors resulted in a stable double-layer capacitance between 3.0 and 4.5 V, and this treatment also correlated with less roughening. These results inform future efforts aimed at controlling surface composition and morphology of carbon electrodes.

Improved performance through tight coupling of redox cycles of sulfur and 2,6-polyanthraquinone in lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries offer high theoretical capacity and energy density; however, complex nanoengineered cathodes have been required to suppress the unwanted polysulfide shuttling. The textural complexity of such electrodes hinders detailed understanding of their function, impeding the development of new materials. In this report, the redox-active polymer 2,6-polyanthraquinone (PAQ) was incorporated into the cathode. The presence of this polymer improves capacity retention in galvanostatic cycling and inhibits Li corrosion and S deposition. We show that redox reactions of this polymer are strongly coupled to the S redox cycle and hypothesize that the observed improvements in the performance originate from the electrocatalytic inhibition of polysulfide shuttling in this system.