Investigation of Rechargeable Poly(ethylene oxide)-Based Solid Lithium–Oxygen Batteries

Liquid-free solid polymer electrolyte (SPE) Li–O₂ batteries are considered advantageous power sources for multiple applications, albeit their cycle performance is far from being acceptable. A most challenging SPE stability in Li–O₂ battery operating at 80 °C is described here, presenting possible directions for this battery type future development. Hereby, we investigated poly­(ethylene oxide) (PEO) stability in Li–O₂ batteries after cycling and determined that the polymer instability is originated from an accumulation of formate-based species, which required high decomposition potential and showed low decomposition efficiency. This poses a key challenging issue of unfavorable round-trip efficiency, dictating a poor cycle performance

Voltage Dependent Solid Electrolyte Interphase Formation in Silicon Electrodes: Monitoring the Formation of Organic Decomposition Products

The solid electrolyte interphase (SEI) passivating layer that grows on all battery electrodes during cycling is critical to the long-term capacity retention of lithium-ion batteries. Yet, it is inherently difficult to study because of its nanoscale thickness, amorphous composite structure, and air sensitivity. Here, we employ an experimental strategy using ¹H, ⁷Li, ¹⁹F, and ¹³C solid-state nuclear magnetic resonance (ssNMR) to gain insight into the decomposition products in the SEI formed on silicon electrodes, the uncontrolled growth of the SEI representing a major failure mechanism that prevents the practical use of silicon in lithium-ion batteries. The voltage dependent formation of the SEI is confirmed, with the SEI growth correlating with irreversible capacity. By studying both conductive carbon and mixed Si/C composite electrodes separately, a correlation with increased capacity loss of the composite system and the low-voltage silicon plateau is demonstrated. Using selective ¹³C labeling, we detect decomposition products of the electrolyte solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) independently. EC decomposition products are present in higher concentrations and are dominated by oligomer species. Lithium semicarbonates, lithium fluoride, and lithium carbonate products are also seen. Ab initio calculations have been carried out to aid in the assignment of NMR shifts. ssNMR applied to both rinsed and unrinsed electrodes show that the organics are easily rinsed away, suggesting that they are located on the outer layer of the SEI

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

Ion Dynamics in Li₂CO₃ Studied by Solid-State NMR and First-Principles Calculations

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO₂ looping, where the material reacts reversibly with CO₂ to form Li₂CO₃, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li₂CO₃ has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li₂CO₃ over the temperature range of 293–973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li₂CO₃. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations

Comprehensive Study of the CuF₂ Conversion Reaction Mechanism in a Lithium Ion Battery

Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF₂, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal–fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF₂ and NiF₂, good capacity retention has not, however, been achieved for CuF₂. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF₂ are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF₂ conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu¹⁺ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF₂. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery

Unraveling the Complex Delithiation Mechanisms of Olivine-Type Cathode Materials, LiFe_<i>x</i>Co_1–<i>x</i>PO₄

The delithiation mechanisms occurring within the olivine-type class of cathode materials for Li-ion batteries have received considerable attention because of the good capacity retention at high rates for LiFePO₄. A comprehensive mechanistic study of the (de)­lithiation reactions that occur when the substituted olivine-type cathode materials LiFe_<i>x</i>Co_1–<i>x</i>PO₄ (<i>x</i> = 0, 0.05, 0.125, 0.25, 0.5, 0.75, 0.875, 0.95, 1) are electrochemically cycled is reported here using in situ X-ray diffraction (XRD) data and supporting ex situ ³¹P NMR spectra. On the first charge, two intermediate phases are observed and identified: Li_1–<i>x</i>(Fe³⁺)_<i>x</i>(Co²⁺)_1–<i>x</i>PO₄ for 0 < <i>x</i> < 1 (i.e., after oxidation of Fe²⁺ to Fe³⁺) and Li_2/3Fe_<i>x</i>Co_1–<i>x</i>PO₄ for 0 ≤ <i>x</i> ≤ 0.5 (i.e., the Co-majority materials). For the Fe-rich materials, we study how nonequilibrium, single-phase mechanisms that occur discretely in single particles, as observed for LiFePO₄ at high rates, are affected by Co substitution. In the Co-majority materials, a two-phase mechanism with a coherent interface is observed, as was seen in LiCoPO₄, and we discuss how it is manifested in the XRD patterns. We then compare the nonequilibrium, single-phase mechanism with the bulk single-phase and coherent interface two-phase mechanisms. Despite the apparent differences between these mechanisms, we discuss how they are related and interconverted as a function of Fe/Co substitution and the potential implications for the electrochemistry of this system

Highly Reversible Conversion-Type FeOF Composite Electrode with Extended Lithium Insertion by Atomic Layer Deposition LiPON Protection

High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the LiPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction

Identifying the Critical Role of Li Substitution in P2–Na_<i>x</i>[Li_<i>y</i>Ni_<i>z</i>Mn_{1–<i>y</i>–<i>z</i>}]O₂ (0 < <i>x</i>, <i>y</i>, <i>z</i> < 1) Intercalation Cathode Materials for High-Energy Na-Ion Batteries

Li-substituted layered P2–Na_0.80[Li_0.12Ni_0.22Mn_0.66]­O₂ is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different states of charge/discharge to track Li-ion site occupation changes. Surprisingly, Li is found to be mobile, some Li ions migrate from the TM layer to the Na layer at high voltage, and yet this process is highly reversible. Novel design principles for Na cathode materials are proposed on the basis of an atomistic level understanding of the underlying electrochemical processes. These principles enable us to devise an optimized, high capacity, and structurally stable compound as a potential cathode material for high-energy Na-ion batteries

Identifying the Structure of the Intermediate, Li_2/3CoPO₄, Formed during Electrochemical Cycling of LiCoPO₄

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO₄ (space group <i>Pnma</i>) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li_2/3(Co²⁺)_2/3(Co³⁺)_1/3PO₄ phase. Two resonances are observed for Li_2/3CoPO₄ with intensity ratios of 2:1 and 1:1 in the ³¹P and ⁷Li NMR spectra, respectively. An ordering of Co²⁺/Co³⁺ oxidation states is proposed within a (<i>a</i> × 3<i>b</i> × <i>c</i>) supercell, and Li⁺/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co³⁺ ions and Li vacancies are found to order along the <i>b</i>-axis. Two other low energy Li⁺/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO₄–CoPO₄ convex hull and they may be readily interconverted by Li⁺ hops along the <i>b</i>-direction

A Mechanistic Study of Phase Transformation in Perovskite Nanocrystals Driven by Ligand Passivation

Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX₃ nanocrystals to rhombohedral Cs₄PbX₆ nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs₄PbX₆ reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX₆ octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs₄PbX₆ nanoparticles with controlled sizes. Also, depending on the final size of the Cs₄PbX₆ nanoparticles as tuned by the reaction time, the back reaction yields CsPbX₃ nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites