Michal Leskes

Cycling Li-O2 batteries via LiOH formation and decomposition

Solving the problems with Li-air batteries Li-air batteries come as close as possible to the theoretical limits for energy density in a battery. By weight, this is roughly 10 times higher than conventional lithium-ion batteries and would be sufficient to power cars with a range comparable to those with gasoline engines. But engineering a Li-air battery has been a challenge. Liu et al. managed to overcome the remaining challenges: They were able to avoid electrode passivation, turn limited solvent stability into an advantage, eliminate the fatal problems caused by superoxides, achieve high power with negligible degradation, and even circumvent the problems of removing atmospheric water. Science, this issue p. 530 An efficient, high-capacity, safe Li-air battery system is realized. The rechargeable aprotic lithium-air (Li-O2) battery is a promising potential technology for next-generation energy storage, but its practical realization still faces many challenges. In contrast to the standard Li-O2 cells, which cycle via the formation of Li2O2, we used a reduced graphene oxide electrode, the additive LiI, and the solvent dimethoxyethane to reversibly form and remove crystalline LiOH with particle sizes larger than 15 micrometers during discharge and charge. This leads to high specific capacities, excellent energy efficiency (93.2%) with a voltage gap of only 0.2 volt, and impressive rechargeability. The cells tolerate high concentrations of water, water being the dominant proton source for the LiOH; together with LiI, it has a decisive impact on the chemical nature of the discharge product and on battery performance.

In situ solid-state NMR spectroscopy of electrochemical cells: batteries, supercapacitors, and fuel cells.

Electrochemical cells, in the form of batteries (or supercapacitors) and fuel cells, are efficient devices for energy storage and conversion. These devices show considerable promise for use in portable and static devices to power electronics and various modes of transport and to produce and store electricity both locally and on the grid. For example, high power and energy density lithium-ion batteries are being developed for use in hybrid electric vehicles where they improve the efficiency of fuel use and help to reduce greenhouse gas emissions. To gain insight into the chemical reactions involving the multiple components (electrodes, electrolytes, interfaces) in the electrochemical cells and to determine how cells operate and how they fail, researchers ideally should employ techniques that allow real-time characterization of the behavior of the cells under operating conditions. This Account reviews the recent use of in situ solid-state NMR spectroscopy, a technique that probes local structure and dynamics, to study these devices. In situ NMR studies of lithium-ion batteries are performed on the entire battery, by using a coin cell design, a flat sealed plastic bag, or a cylindrical cell. The battery is placed inside the NMR coil, leads are connected to a potentiostat, and the NMR spectra are recorded as a function of state of charge. (7)Li is used for many of these experiments because of its high sensitivity, straightforward spectral interpretation, and relevance to these devices. For example, (7)Li spectroscopy was used to detect intermediates formed during electrochemical cycling such as LixC and LiySiz species in batteries with carbon and silicon anodes, respectively. It was also used to observe and quantify the formation and growth of metallic lithium microstructures, which can cause short circuits and battery failure. This approach can be utilized to identify conditions that promote dendrite formation and whether different electrolytes and additives can help prevent dendrite formation. The in situ method was also applied to monitor (by (11)B NMR) electrochemical double-layer formation in supercapacitors in real time. Though this method is useful, it comes with challenges. The separation of the contributions from the different cell components in the NMR spectra is not trivial because of overlapping resonances. In addition, orientation-dependent NMR interactions, including the spatial- and orientation-dependent bulk magnetic susceptibility (BMS) effects, can lead to resonance broadening. Efforts to understand and mitigate these BMS effects are discussed in this Account. The in situ NMR investigation of fuel cells initially focused on the surface electrochemistry at the electrodes and the electrochemical oxidation of methanol and CO to CO2 on the Pt cathode. On the basis of the (13)C and (195)Pt NMR spectra of the adsorbates and electrodes, CO adsorbed on Pt and other reaction intermediates and complete oxidation products were detected and their mode of binding to the electrodes investigated. Appropriate design and engineering of the NMR hardware has allowed researchers to integrate intact direct methanol fuel cells into NMR probes. Chemical transformations of the circulating methanol could be followed and reaction intermediates could be detected in real time by either (2)H or (13)C NMR spectroscopy. By use of the in situ NMR approach, factors that control fuel cell performance, such as methanol cross over and catalyst performance, were identified.

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.

Ab initio structure search and in situ 7Li NMR studies of discharge products in the Li-S battery system.

The high theoretical gravimetric capacity of the Li–S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li–S phase diagram using computational techniques and complement this with an in situ 7Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li2S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li+-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

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.

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence.

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.

Solid Electrolyte Interphase Growth and Capacity Loss in Silicon Electrodes

The solid electrolyte interphase (SEI) of the high capacity anode material Si is monitored over multiple electrochemical cycles by (7)Li, (19)F, and (13)C solid-state nuclear magnetic resonance spectroscopies, with the organics dominating the SEI. Homonuclear correlation experiments are used to identify the organic fragments -OCH2CH2O-, -OCH2CH2-, -OCH2CH3, and -CH2CH3 contained in both oligomeric species and lithium semicarbonates ROCO2Li, RCO2Li. The SEI growth is correlated with increasing electrode tortuosity by using focused ion beam and scanning electron microscopy. A two-stage model for lithiation capacity loss is developed: initially, the lithiation capacity steadily decreases, Li(+) is irreversibly consumed at a steady rate, and pronounced SEI growth is seen. Later, below 50% of the initial lithiation capacity, less Si is (de)lithiated resulting in less volume expansion and contraction; the rate of Li(+) being irreversibly consumed declines, and the Si SEI thickness stabilizes. The decreasing lithiation capacity is primarily attributed to kinetics, the increased electrode tortuousity severely limiting Li(+) ion diffusion through the bulk of the electrode. The resulting changes in the lithiation processes seen in the electrochemical capacity curves are ascribed to non-uniform lithiation, the reaction commencing near the separator/on the surface of the particles.

Bimodal Floquet description of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear (1)H-(1)H dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SW(f)-TPPM) sequence. It is shown that the experimental robustness of SW(f)-TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments.

Formation of Ti28Ln Cages, the Highest Nuclearity Polyoxotitanates (Ln=La, Ce)

The solvothermal reactions of Ti(OEt)(4) with LnCl(3) (Ln = La, Ce) produced new Ti(28) Ln cages, in which the Ln(3+) ions are coordinated within a metallocrown arrangement, which represents the highest nuclearity cages of this type (see figure).

Paramagnetic electrodes and bulk magnetic susceptibility effects in the in situ NMR studies of batteries: application to Li1.08Mn1.92O4 spinels.

To date, in situ nuclear magnetic resonance (NMR) studies of working batteries have been performed in static mode, i.e., in the absence of magic angle spinning (MAS). Thus, it is extremely challenging to apply the method to paramagnetic systems such as the cathodes spinels Li(1+x)Mn(2-x)O4 primarily due to three factors: (1) the resonance lines are broadened severely; (2) spectral analysis is made more complicated by bulk magnetic susceptibility (BMS) effects, which depend on the orientation and shape of the object under investigation; (3) the difficulty in untangling the BMS effects induced by the paramagnetic and metallic components on other (often diamagnetic) components in the system, which result in additional shifts and line broadening. Here we evaluate the orientation-dependence of the BMS effect of Li1.08Mn1.92O4, analyzing the experimental results by using a simple long-distance Li-electron dipolar coupling model. In addition, we discuss the shape and packing density dependence of the BMS effect and its influence on the observed frequencies of other components, such as the Li metal and the electrolyte in the battery. Finally, we show that by taking these effects into account we are able to minimize the BMS induced shift by orienting the cell at a rotation angle, αi=54.7° which facilitates the interpretation of the in situ NMR spectra of a working battery with the paramagnetic Li1.08Mn1.92O4 cathode.