Surface Structural Disordering in Graphite upon Lithium Intercalation/Deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in LixC (0 \leq x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li+). Steep Li+ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the LixC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.Comment: 12 pages, 5 figure

An electrochemical investigation of oxygen adsorption on Pt single crystal electrodes in a non-aqueous Li+ electrolyte

Cyclic voltammetry has been used to probe the initial stages of oxygen reduction and oxidation in lithium-containing dimethyl sulfoxide at well-defined Pt single crystal electrodes in order to elucidate any catalytic effects ascribable to surface structure. In contrast to previous work involving sodium-oxygen, lithium-oxygen studies did not yield any significant differences for reaction on the three basal planes of platinum. Rather, all three planes generated a similar voltammetric response. However, by judicious use of various potential sweep limits, the formation of superoxide together with both a “conformal” or surface adlayer of lithium peroxide (Li2O2) together with a “microcrystallite” surface Li2O2 phase was resolved. Voltammetric peak intensity versus sweep rate measurements confirmed that superoxide electrooxidation was diffusion limited whereas electrooxidation of the two Li2O2 phases displayed behaviour typical of a surface-confined process. Under steady-state conditions for the formation of superoxide, it was found that for both the conformal and microcrystallite Li2O2 phases, electrooxidation followed zero-order kinetics, pointing to the importance of free surface sites in facilitating these reactions. A marked change in the rate of Li2O2 formation was found to coincide with a coverage of 0.25 monolayers, as measured by the charge density of the conformal Li2O2 electrooxidation peak. We postulate that electron tunnelling through both the conformal Li2O2 layer and microcrystallites deposited on this surface layer coincides with this coverage and accounts for such behaviour. This phenomenon of electron tunnelling through single conformal and mixed conformal/microcrystallite structures should prove vitally important in governing the overall electrooxidation rate

Time-resolved SERS study of the oxygen reduction reaction in ionic liquid electrolytes for non-aqueous lithium-oxygen cells

We use the Raman active bands of O2˙− to probe its changing Lewis basicity through its interaction with various ionic liquid electrolytes at the electrode surface.</p

Growth and dissolution of NaO2 in an ether-based electrolyte as the discharge product in the Na-O-2 cell

The deposition and dissolution of sodium superoxide (NaO2) was investigated by atomic force microscopy.</p

Localised degradation within sulfide-based all-solid-state electrodes visualised by Raman mapping

The distribution of degradation products, before and after cycling, within common sulfide-based solid electrolytes (β-Li$_3$PS$_4$, Li$_6$PS$_5$Cl and Li$_{10}$GeP$_2$S$_{12}$) was mapped using Raman microscopy. All composite electrodes displayed the appearance of side reaction products after the initial charge-discharge cycle, located at the site of a LiNi$_{0.6}$Mn$_{0.2}$Co$_{0.2}$O$_2$ particle

In Situ Surface-Enhanced Infrared Spectroscopy to Identify Oxygen Reduction Products in Nonaqueous Metal-Oxygen Batteries

We report on the detection of metastable, solvated, and surface adsorbed alkali metal–oxygen (M–O₂) discharge species using in situ attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). Oxygen–oxygen stretching bands (ν_O–O) of superoxide species formed during M–O₂ battery discharge have been challenging to observe by conventional infrared (IR) techniques, and because of this, there has been limited use of IR techniques for in situ monitoring of the discharge products at the cathode in metal–O₂ batteries. We explore SEIRAS technique to investigate lithium–oxygen and sodium–oxygen electrochemistry in acetonitrile (MeCN; a low Gutmann donor number solvent) as well as dimethyl sulfoxide (DMSO; a high Gutmann donor number solvent) in order to demonstrate the feasibility of our approach in the ongoing efforts toward the realization of M–O₂ battery technology. In situ IR spectroscopy studies, together with a coupled-cluster method including perturbative triple excitations [CCSD­(T)] calculations, establishes that certain M–O and O–O stretching bands (ν_M–O and ν_O–O) of metal superoxide and peroxide molecular species are IR active, although these vibrational modes are silent or suppressed in their crystalline forms. An in situ IR spectroscopy based approach to distinguish between “solution mediated” and “surface confined” discharge pathways in nonaqueous M–O₂ batteries is demonstrated

Divalent Nonaqueous Metal-Air Batteries

In the field of secondary batteries, the growing diversity of possible applications for energy storage has led to the investigation of numerous alternative systems to the state-of-the-art lithium-ion battery. Metal-air batteries are one such technology, due to promising specific energies that could reach beyond the theoretical maximum of lithium-ion. Much focus over the past decade has been on lithium and sodium-air, and, only in recent years, efforts have been stepped up in the study of divalent metal-air batteries. Within this article, the opportunities, progress, and challenges in nonaqueous rechargeable magnesium and calcium-air batteries will be examined and critically reviewed. In particular, attention will be focused on the electrolyte development for reversible metal deposition and the positive electrode chemistries (frequently referred to as the “air cathode”). Synergies between two cell chemistries will be described, along with the present impediments required to be overcome. Scientific advances in understanding fundamental cell (electro)chemistry and electrolyte development are crucial to surmount these barriers in order to edge these technologies toward practical application.</jats:p