High Rate Lithium Ion Battery with Niobium Tungsten Oxide Anode

Highly stable lithium-ion battery cycling of niobium tungsten oxide (Nb16W5O55, NWO) is demonstrated in full cells with cathode materials LiNi0.6Mn0.2Co0.2O2 (NMC-622) and LiFePO4 (LFP). The cells show high rate performance and long-term stability under 5 C and 10 C cycling rates with a conventional carbonate electrolyte without any additives. The degradation of the cell performance is mainly attributed to the increased charge transfer resistance at the NMC side, consistent with the ex situ XRD and XPS analysis demonstrating the structural stability of NWO during cycling together with minimal electrolyte decomposition. Finally, we demonstrate the temperature-dependent performance of this full cell at 10, 25 and 60 °C and confirm, using operando XRD, that the structural change of the NWO material during lithiation/de-lithiation at 60 °C is very similar to its behaviour at 25 °C, reversible and with a low volume change. With the merits of high rate performance and long cycle life, the combination of NWO and a commercial cathode represents a promising, safe battery for fast charge/discharge applications

Selective NMR observation of the SEI–metal interface by dynamic nuclear polarisation from lithium metal

Funder: Oppenheimer Foundation, Cambridge. Blavatnik Cambridge Fellowships.Abstract: While lithium metal represents the ultimate high-energy-density battery anode material, its use is limited by dendrite formation and associated safety risks, motivating studies of the solid–electrolyte interphase layer that forms on the lithium, which is key in controlling lithium metal deposition. Dynamic nuclear polarisation enhanced NMR can provide important structural information; however, typical exogenous dynamic nuclear polarisation experiments, in which organic radicals are added to the sample, require cryogenic sample cooling and are not selective for the interface between the metal and the solid–electrolyte interphase. Here we instead exploit the conduction electrons of lithium metal to achieve an order of magnitude hyperpolarisation at room temperature. We enhance the 7Li, 1H and 19F NMR spectra of solid–electrolyte interphase species selectively, revealing their chemical nature and spatial distribution. These experiments pave the way for more ambitious room temperature in situ dynamic nuclear polarisation studies of batteries and the selective enhancement of metal–solid interfaces in a wider range of systems

Electrolyte Oxidation Pathways in Lithium-Ion Batteries

&lt;p&gt;The mitigation of decomposition reactions of lithium-ion battery electrolyte solutions is of critical importance in controlling device lifetime and performance. However, due to the complexity of the system, exacerbated by the diverse set of electrolyte compositions, electrode materials, and operating parameters, a clear understanding of the key chemical mechanisms remains elusive. In this work, operando pressure measurements, solution NMR, and electrochemical methods were combined to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment LiCoO&lt;sub&gt;2&lt;/sub&gt;/Li cells were cycled with a lithium-ion conducting glass-ceramic separator so that the species formed at each electrode could be identified separately and further reactions of these species at the opposite electrode prevented. One principal finding is that chemical oxidation (with an onset voltage of ~4.7 V vs Li/Li&lt;sup&gt;+&lt;/sup&gt; for LiCoO&lt;sub&gt;2&lt;/sub&gt;), rather than electrochemical reaction, is the dominant decomposition process at the positive electrode surface in this system. This is ascribed to the well-known release of reactive oxygen at higher states-of-charge, indicating that reactions of the electrolyte at the positive electrode are intrinsically linked to surface reactivity of the active material. Soluble electrolyte decomposition products formed at both electrodes are characterised, and a detailed reaction scheme is constructed to rationalise the formation of the observed species. The insights on electrolyte decomposition through reactions with reactive oxygen species identified through this work have direct impact on understanding and mitigating degradation in high voltage/higher energy density LiCoO&lt;sub&gt;2&lt;/sub&gt;-based cells,&lt;sub&gt; &lt;/sub&gt;and more generally for cells containing nickel-containing cathode materials (e.g. LiNi&lt;sub&gt;x&lt;/sub&gt;Mn&lt;sub&gt;y&lt;/sub&gt;Co&lt;sub&gt;z&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;; NMCs), as they lose oxygen at lower operating voltages.&lt;/p&gt;</jats:p

Solution NMR Studies of Electrolyte Decomposition Pathways

One approach to increasing the energy density of lithium-ion batteries is to increase the operational upper voltage limit. However, higher cell voltages tend to lead to rapid capacity fade and shorter cycle life. This is typically ascribed to the loss of oxygen due to surface reconstructions of the cathode material, increased electrolyte oxidation rates, and transition metal dissolution. It is well-known that electrolyte decomposition can negatively impact cell behaviour and lifetime by causing gassing, electrochemical impedance growth, or slippage, and that extensive electrolyte oxidation at high cell voltages can induce eventual rapid capacity loss or ‘rollover’ failure.1,2 Whereas the reduction processes at the anode have been well-studied and are generally understood,3 the chemical pathways and mechanisms for oxidation at the cathode remain relatively unknown. It is desirable to develop a more detailed chemical understanding of electrolyte degradation in support of longer-lasting high voltage materials and cell designs. Herein, operando pressure measurements, liquid-state NMR, and electrochemical methods are combined to study electrolyte oxidation at multiple cell voltages. LiCoO2 (LCO)/Li cells were cycled with a lithium-ion conducting ceramic separator to identify the degradation products formed at the anode and cathode separately and to prevent further reactions of these species on the opposite electrode. The results are compared with the species generated by electrochemical oxidation of the electrolyte in a simple H-cell and with cells cycled without a separator. A major finding of the present work is that all degradation products from the cathode side of the LCO/Li cells can be explained by chemical oxidation of the electrolyte, involving oxygen evolved from the cathode surface, at least up to 4.9 V vs. Li/Li+. This result is consistent with and complementary to previously reported online electrochemical mass spectrometry results4 and has significance for nickel-containing cathode materials (e.g., NMC). Finally, a comprehensive summary of electrolyte degradation pathways was constructed by analysing a series of multinuclear and one- and two-dimensional NMR spectra. Reference s : [1] L.M Thompson, W. Stone, A. Eldesoky, N.K. Smith, C.R. M. Mcfarlane, J. S. Kim, M. B. Johnson, R. Petibon and J. R Dahn, J. Electrochem. Soc., 165 (2018), 2732-2740. [2] X. Ma, J. E. Harlow, J. Li, L. Ma, D. S. Hall, M. Genovese, M. Cormier, J. R. Dahn, S. Buteau, J. Electrochem. Soc., 166 (2019), A711-A724. [3] S. K. Heiskanen, J. Kim, B. L. Lucht, Joule, 3 (2019), 2322-2333. [4] R. Jung, M. Metzger, F. Maglia, C. Stinner, H. A. Gasteiger, J. Phys. Chem. Lett., 8 (2017), 4820-4825. </jats:p

Electrolyte Oxidation Pathways in Lithium-Ion Batteries

The mitigation of decomposition reactions of lithium-ion battery electrolyte solutions is of critical importance in controlling device lifetime and performance. However, due to the complexity of the system, exacerbated by the diverse set of electrolyte compositions, electrode materials, and operating parameters, a clear understanding of the key chemical mechanisms remains elusive. In this work, operando pressure measurements, solution NMR, and electrochemical methods were combined to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment LiCoO2/Li cells were cycled with a lithium-ion conducting glass-ceramic separator so that the species formed at each electrode could be identified separately and further reactions of these species at the opposite electrode prevented. One principal finding is that chemical oxidation (with an onset voltage of ~4.7 V vs Li/Li+ for LiCoO2), rather than electrochemical reaction, is the dominant decomposition process at the positive electrode surface in this system. This is ascribed to the well-known release of reactive oxygen at higher states-of-charge, indicating that reactions of the electrolyte at the positive electrode are intrinsically linked to surface reactivity of the active material. Soluble electrolyte decomposition products formed at both electrodes are characterised, and a detailed reaction scheme is constructed to rationalise the formation of the observed species. The insights on electrolyte decomposition through reactions with reactive oxygen species identified through this work have direct impact on understanding and mitigating degradation in high voltage/higher energy density LiCoO2-based cells, and more generally for cells containing nickel-containing cathode materials (e.g. LiNixMnyCozO2; NMCs), as they lose oxygen at lower operating voltages.</p

Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries

NMR and operando gas measurements show that at low potentials, EC is dehydrogenated to VC, whereas at high potentials, EC is chemically oxidised to CO2, CO and H2O, where the water that is formed induces secondary decomposition reactions.</jats:p

Two Electrolyte Decomposition Pathways at NMC Electrodes in Lithium-Ion Batteries

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNixMnyCo1-x-yO2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (LixFePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrode. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (&lt;80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (&gt;80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via 1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/LixFePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells. </jats:p

Identifying and preventing degradation in flavin mononucleotide-based redox flow batteries via NMR and EPR spectroscopy

Abstract While aqueous organic redox flow batteries (RFBs) represent potential solutions to large-scale grid storage, their electrolytes suffer from short lifetimes due to rapid degradation. We show how an understanding of these degradation processes can be used to dramatically improve performance, as illustrated here via a detailed study of the redox-active biomolecule, flavin mononucleotide (FMN), a molecule readily derived from vitamin B2. Via in-situ nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) we identify FMN hydrolysis products and show that these give rise to the additional plateau seen during charging of an FMN-cyanoferrate battery. The redox reactions of the hydrolysis product are not reversible, but we demonstrate that capacity is still retained even after substantial hydrolysis, albeit with reduced voltaic efficiency, FMN acting as a redox mediator. Critically, we demonstrate that degradation is mitigated and battery efficiency is substantially improved by lowering the pH to 11. Furthermore, the addition of cheap electrolyte salts to tune the pH results in a dramatic increase in solubility (above 1 M), this systematic improvement of the flavin-based system bringing RFBs one step closer to commercial viability

Electrolyte Oxidation Pathways in Lithium-Ion Batteries.

The mitigation of decomposition reactions of lithium-ion battery electrolyte solutions is of critical importance in controlling device lifetime and performance. However, due to the complexity of the system, exacerbated by the diverse set of electrolyte compositions, electrode materials, and operating parameters, a clear understanding of the key chemical mechanisms remains elusive. In this work, operando pressure measurements, solution NMR, and electrochemical methods were combined to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment LiCoO2/Li cells were cycled with a lithium-ion conducting glass-ceramic separator so that the species formed at each electrode could be identified separately and further reactions of these species at the opposite electrode prevented. One principal finding is that chemical oxidation (with an onset voltage of ∼4.7 V vs Li/Li+ for LiCoO2), rather than electrochemical reaction, is the dominant decomposition process at the positive electrode surface in this system. This is ascribed to the well-known release of reactive oxygen at higher states-of-charge, indicating that reactions of the electrolyte at the positive electrode are intrinsically linked to surface reactivity of the active material. Soluble electrolyte decomposition products formed at both electrodes are characterized, and a detailed reaction scheme is constructed to rationalize the formation of the observed species. The insights on electrolyte decomposition through reactions with reactive oxygen species identified through this work have a direct impact on understanding and mitigating degradation in high-voltage/higher-energy-density LiCoO2-based cells, and more generally for cells containing nickel-containing cathode materials (e.g., LiNixMnyCozO2; NMCs), as they lose oxygen at lower operating voltages.US Department of Energy, the Faraday Institution and the European Research Counci