Gillian R. Goward

Direct Detection of Discharge Products in Lithium–Oxygen Batteries by Solid‐State NMR Spectroscopy

A closer look: Solid-state (7) Li and (17) O NMR spectroscopy is a valuable tool in the characterization of products formed in the lithium-oxygen battery, a necessary step in the development of a viable cell. Since lithium peroxide, the desired discharge product, has a unique (17) O NMR signature, it can be clearly identified.

Monitoring the Electrochemical Processes in the Lithium-Air Battery by Solid State NMR Spectroscopy.

A multi-nuclear solid-state NMR approach is employed to investigate the lithium–air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by 17O solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. 13C ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium–oxygen battery.

Reorientation phenomena in imidazolium methyl sulfonate as probed by advanced solid-state NMR

Evidence for reorientation of imidazolium rings in imidazolium methylsulfonate is demonstrated using solid-state NMR. This material is a model system for exciting new proton-conducting materials based on imidazole. Two advanced NMR methods, including 1H-13C and 1H-15N recoupled polarization transfer with dipolar sideband pattern analysis and analysis of the coalescence of 13C lineshapes are used to characterize the ring reorientation. The process is found to occur at temperatures well below the melting point of the salt, between 240 and 380 K, and is described by a single activation energy, of 38+/-5 kJ/mol. This material is considered as a model system for quantifying the ring reorientation process, which is often proposed to be the rate-limiting step in proton transport in imidazole-based proton conducting materials.

Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging

Accurate modeling of Li-ion batteries performance, particularly during the transient conditions experienced in automotive applications, requires knowledge of electrolyte transport properties (ionic conductivity κ, salt diffusivity D, and lithium ion transference number t(+)) over a wide range of salt concentrations and temperatures. While specific conductivity data can be easily obtained with modern computerized instrumentation, this is not the case for D and t(+). A combination of NMR and MRI techniques was used to solve the problem. The main advantage of such an approach over classical electrochemical methods is its ability to provide spatially resolved details regarding the chemical and dynamic features of charged species in solution, hence the ability to present a more accurate characterization of processes in an electrolyte under operational conditions. We demonstrate herein data on ion transport properties (D and t(+)) of concentrated LiPF6 solutions in a binary ethylene carbonate (EC)-dimethyl carbonate (DMC) 1:1 v/v solvent mixture, obtained by the proposed technique. The buildup of steady-state (time-invariant) ion concentration profiles during galvanostatic experiments with graphite-lithium metal cells containing the electrolyte was monitored by pure phase-encoding single point imaging MRI. We then derived the salt diffusivity and Li(+) transference number over the salt concentration range 0.78-1.27 M from a pseudo-3D combined PFG-NMR and MRI technique. The results obtained with our novel methodology agree with those obtained by electrochemical methods, but in contrast to them, the concentration dependences of salt diffusivity and Li(+) transference number were obtained simultaneously within the single in situ experiment.

Accurate Characterization of Ion Transport Properties in Binary Symmetric Electrolytes Using In Situ NMR Imaging and Inverse Modeling

We used NMR imaging (MRI) combined with data analysis based on inverse modeling of the mass transport problem to determine ionic diffusion coefficients and transference numbers in electrolyte solutions of interest for Li-ion batteries. Sensitivity analyses have shown that accurate estimates of these parameters (as a function of concentration) are critical to the reliability of the predictions provided by models of porous electrodes. The inverse modeling (IM) solution was generated with an extension of the Planck-Nernst model for the transport of ionic species in electrolyte solutions. Concentration-dependent diffusion coefficients and transference numbers were derived using concentration profiles obtained from in situ (19)F MRI measurements. Material properties were reconstructed under minimal assumptions using methods of variational optimization to minimize the least-squares deviation between experimental and simulated concentration values with uncertainty of the reconstructions quantified using a Monte Carlo analysis. The diffusion coefficients obtained by pulsed field gradient NMR (PFG-NMR) fall within the 95% confidence bounds for the diffusion coefficient values obtained by the MRI+IM method. The MRI+IM method also yields the concentration dependence of the Li(+) transference number in agreement with trends obtained by electrochemical methods for similar systems and with predictions of theoretical models for concentrated electrolyte solutions, in marked contrast to the salt concentration dependence of transport numbers determined from PFG-NMR data.

NMR chemical shifts in proton conducting crystals from first principles

Abstract We compute the hydrogen 1 H NMR chemical shift spectra of proton conducting crystals using a recently developed density functional theory method for systems under periodic boundary conditions. Comparison with experimental spectra yields an excellent agreement. Thus, besides of unambiguously assigning the chemical shifts to individual atoms, the calculations can also characterize the microscopic hydrogen bonding structure of this class of materials. Apart from the example presented, the method can be applied to crystalline and amorphous insulators and semiconductors, as well as to isolated molecules using a supercell technique. It is implemented in CPMD, a state-of-the-art pseudopotential plane wave DFT package.

Detection of Electrochemical Reaction Products from the Sodium–Oxygen Cell with Solid-State 23Na NMR Spectroscopy

23Na MAS NMR spectra of sodium-oxygen (Na-O2) cathodes reveals a combination of degradation species: newly observed sodium fluoride (NaF) and the expected sodium carbonate (Na2CO3), as well as the desired reaction product sodium peroxide (Na2O2). The initial reaction product, sodium superoxide (NaO2), is not present in a measurable quantity in the 23Na NMR spectra of the cycled electrodes. The reactivity of solid NaO2 is probed further, and NaF is found to be formed through a reaction between the electrochemically generated NaO2 and the electrode binder, polyvinylidene fluoride (PVDF). The instability of cell components in the presence of desired electrochemical reaction products is clearly problematic and bears further investigation.

Studies of lithium ion dynamics in paramagnetic cathode materials using 6Li 1D selective inversion methods

The effectiveness of two different selective inversion methods is investigated to determine timescales of Li ion mobility in paramagnetic Li intercalation materials. The first method is 1D exchange spectroscopy, which employs a 90°-τ(1)-90° sequence for selective inversion of a Li resonance undergoing site exchange. The experiment is most easily applied when the first delay period, τ(1), is set to the frequency difference between two resonances undergoing ion exchange. This enables the determination of ion hopping timescales for single exchange pair systems only. To measure ion dynamics in systems having more than one exchange process, a second selective inversion method was tested on two paramagnetic Li intercalation materials. This second technique, replaces the 90°-τ(1)-90° portion of 1D EXSY with a long, selective shaped pulse (SP). Two paramagnetic solid-state materials, which are both cathode materials for lithion ion batteries, were chosen as model compounds to test the effectiveness of both the selective inversion methods. The first compound, Li(2)VPO(4)F, was chosen as it hosts two Li sites with 1-exchange process. The second model compound is a 3-site, 3-exchange process system, Li(2)VOPO(4). For the 2-site material, Li(2)VPO(4)F, the timescales of the single A-B exchange process were found to be within error of one another regardless of the inversion method. For the 3 Li-site material Li(2)VOPO(4), the three exchange processes, AB, BC, and AC, were found to be on the millisecond timescale as revealed using the SP method. These timescales were determined over a variable temperature range where activation energies extended from 0.6 ± 0.1 eV up to 0.9 ± 0.2 eV.

Spatially resolved surface valence gradient and structural transformation of lithium transition metal oxides in lithium-ion batteries

Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNixMnyCo1-x-yO2 (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni2+ (active) and Co3+ (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive 7Li nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R3[combining macron]m structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R3[combining macron]m → Fm3[combining macron]m), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.

Synthesis of Li4V(PO4)2F2 and 6,7Li NMR studies of its lithium ion dynamics

A single phase, well-crystallized Li4V(PO4)2F2/carbon nanocomposite has been prepared by an optimized solid-state route via oxidation of Li5V(PO4)2F2. The Li4V(PO4)2F2 composition exhibits lattice parameters close to those of the oxidized parent (a = 6.898 A, b = 10.673 A, c = 9.977 A; β = 87.84°; V = 734.0 A3). 1D 6Li solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) studies identified which of the six lithium ions are removed from the lattice of the parent Li5V(PO4)2F2. The results are in perfect accord with previous NMR-based predictions of which sites would be the most mobile and thereby most easily extracted upon cycling. Variable-temperature NMR studies and 2D exchange spectroscopy (EXSY) are used to probe the Li ion dynamics in Li4V(PO4)2F2. Importantly, our studies show that upon delithiation, the ion mobility was found to increase significantly vis a vis the parent Li5V(PO4)2F2. We ascribe this to the creation of lithium vacancies within the structure that open up pathways for ion transport.