Diffusion and migration in polymer electrolytes

Mixtures of neutral polymers and lithium salts have the potential to serve as electrolytes in next-generation rechargeable Li-ion batteries. The purpose of this review is to expose the delicate interplay between polymer-salt interactions at the segmental level and macroscopic ion transport at the battery level. Since complete characterization of this interplay has only been completed in one system: mixtures of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI), we focus on data obtained from this system. We begin with a discussion of the activity coefficient, followed by a discussion of six different diffusion coefficients: the Rouse motion of polymer segments is quantified by Dseg, the self-diffusion of cations and anions is quantified by Dself,+ and Dself,−, and the build-up of concentration gradients in electrolytes under an applied potential is quantified by Stefan-Maxwell diffusion coefficients, D0+, D0-, and D+-. The Stefan-Maxwell diffusion coefficients can be used to predict the velocities of the ions at very early times after an electric field is applied across the electrolyte. The surprising result is that D0- is negative in certain concentration windows. A consequence of this finding is that at these concentrations, both cations and anions are predicted to migrate toward the positive electrode at early times. We describe the controversies that surround this result. Knowledge of the Stefan-Maxwell diffusion coefficients enable prediction of the limiting current. We argue that the limiting current is the most important characteristic of an electrolyte. Excellent agreement between theoretical and experimental limiting current is seen in PEO/LiTFSI mixtures. What sequence of monomers that, when polymerized, will lead to the highest limiting current remains an important unanswered question. It is our hope that the approach presented in this review will guide the development of such polymers

Enhanced efficiency of solid-state NMR investigations of energy materials using an external automatic tuning/matching (eATM) robot.

We have developed and explored an external automatic tuning/matching (eATM) robot that can be attached to commercial and/or home-built magic angle spinning (MAS) or static nuclear magnetic resonance (NMR) probeheads. Complete synchronization and automation with Bruker and Tecmag spectrometers is ensured via transistor-transistor-logic (TTL) signals. The eATM robot enables an automated "on-the-fly" re-calibration of the radio frequency (rf) carrier frequency, which is beneficial whenever tuning/matching of the resonance circuit is required, e.g. variable temperature (VT) NMR, spin-echo mapping (variable offset cumulative spectroscopy, VOCS) and/or in situ NMR experiments of batteries. This allows a significant increase in efficiency for NMR experiments outside regular working hours (e.g. overnight) and, furthermore, enables measurements of quadrupolar nuclei which would not be possible in reasonable timeframes due to excessively large spectral widths. Additionally, different tuning/matching capacitor (and/or coil) settings for desired frequencies (e.g. $^{7}$Li and $^{31}$P at 117 and 122MHz, respectively, at 7.05 T) can be saved and made directly accessible before automatic tuning/matching, thus enabling automated measurements of multiple nuclei for one sample with no manual adjustment required by the user. We have applied this new eATM approach in static and MAS spin-echo mapping NMR experiments in different magnetic fields on four energy storage materials, namely: (1) paramagnetic $^{7}$Li and $^{31}$P MAS NMR (without manual recalibration) of the Li-ion battery cathode material LiFePO$_{4}$; (2) paramagnetic $^{17}$O VT-NMR of the solid oxide fuel cell cathode material La$_{2}$NiO$_{4+δ}$; (3) broadband $^{93}$Nb static NMR of the Li-ion battery material BNb$_{2}$O$_{5}$; and (4) broadband static $^{127}$I NMR of a potential Li-air battery product LiIO$_{3}$. In each case, insight into local atomic structure and dynamics arises primarily from the highly broadened (1-25MHz) NMR lineshapes that the eATM robot is uniquely suited to collect. These new developments in automation of NMR experiments are likely to advance the application of in and ex situ NMR investigations to an ever-increasing range of energy storage materials and systems.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655444 (O.P.). D.M.H. acknowledges funding from the Cambridge Commonwealth Trusts. J.L. gratefully acknowledges Trinity College, Cambridge (UK) for funding. K.J.G. gratefully acknowledges funding from the Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. M.B. is the CEO of NMR Service GmbH (Erfurt, Germany), which manufactures the eATM device; M.B. acknowledges funding of the Central Innovation Programme for small and medium-sized enterprises (SMEs; Zentrales Innovationsprogramm Mittelstand, ZIM) of the German Federal Ministry of Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie, BMWi) under the Grant No. KF 2845501UWF. DFT calculations were performed on (1) the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council and (2) the Center for Functional Nanomaterials cluster, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886

Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via ¹⁷O NMR Spectroscopy

Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, 17O NMR spectroscopy is used to study CeO2–SrTiO3 VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the 17O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from 17O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO2–SrTiO3 VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies

Surface-selective direct 17^{17}O DNP NMR of CeO2_{2} nanoparticles

Surface-selective direct $^{17}$O DNP has been demonstrated for the first time on CeO$_{2}$ nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.We are grateful for financial support by the Oppenheimer Foundation (M. A. H.), the Cambridge Commonwealth Trusts (D. M. H.), the National Natural Science Foundation of China (NSFC) (21573103 and 21661130149) and the Royal Society Newton Fund (L. P.). The DNP experiments were performed at the DNP MAS NMR Facility at the University of Nottingham, with thanks to the EPSRC for funding of pilot studies (EP/L022524/1)

Study of Defect Chemistry in the System La<inf>2- x</inf>Sr<inf>x</inf>NiO<inf>4+δ</inf> by ¹⁷O Solid-State NMR Spectroscopy and Ni K-Edge XANES

The properties of mixed ionic-electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution17O (I = 5/2) solid-state NMR spectroscopy of La2-xSrxNiO4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) materials. In particular, the intensity of the17O resonance arising from mobile interstitial defects decreases, and then disappears, at x = 0.5, consistent with reports of lower bulk ionic conductivity in Sr-substituted phases. Second, local distortions among the split axial oxygen sites diminish, even on modest incorporation of Sr (x < 0.1), which is also accompanied by faster spin-lattice (T1) relaxation of the interstitial17O resonances, indicating increased mobility of the associated sites. Finally, the hyperfine shift of the equatorial oxygen resonance decreases due to conversion of Ni2+(d8) to Ni3+(d7) by charge compensation, a mechanism associated with improved electronic conductivity in the Sr-substituted phases. Valence and coordination changes of the Ni cations are further supported by Ni K-edge X-ray absorption near-edge structure (XANES) measurements, which show a decrease in the Jahn-Teller distortion of the Ni3+sites and a Ni coordination change consistent with the formation of oxygen vacancies. Ultimately, these insights into local atomic and electronic structure that rely on17O solid-state NMR spectroscopy should prove relevant for a broad range of aliovalently substituted functional paramagnetic oxides

Understanding the Impact of Multi-Chain Ion Coordination in Poly(ether-Acetal) Electrolytes

Performant solid polymer electrolytes for battery applications usually have a low glass transition temperature and good ion solvation. Recently, to understand the success of PEO for solid-sate battery applications and explore alternatives, we have studied a series of polyacetals along with PEO, both from an experimental and a computational standpoint. We observed that even though the mechanism of transport may be more optimal in polyacetals, the lower glass transition temperature of the PEO-salt electrolyte system still makes it the best option, in this class of polymers, for battery applications. In this work, we explored the free-energy landscape of PEO and P(EO-MO) at various compositions and temperatures using metadynamics simulations to gain deeper insights into the various factors that affect the glass transition temperatures in these systems. In particular, we study the competition between intra- and inter-chain coordination of the cation in these systems that we had hypothesized in our previous work was responsible for the differences in the glass transition temperature. We observe that in PEO, the single-chain binding motif is thermodynamically more stable than the multi-chain binding motif, unlike P(EO-MO), where the opposite is true. We also show that multi-chain coordination, and the associated higher glass transition temperature, in P(EO-MO) is due to a larger strain energy for single-chain coordination that originates in the introduced OCO linkages (relative to PEO's consistent OCCO linkages). Furthermore, the type of pathways to move from one transition state to another in the various systems do not change at higher concentrations though the relative probability of cation-anion coordinated states increases. Calculations at different temperatures to understand the entropic effect on the stability of these coordination environments reveal that as we increase the temperature, single-chain coordination becomes relatively more stable due to the entropic cost of multi-chain coordination, reducing the number of accessible states for the polymer. The various insights into the factors that affect glass transition temperature in these systems suggest design principles for polymer electrolyte systems with lower glass transition temperatures that need further research to compete with PEO at the same absolute battery working temperatures

Diffusion and migration in polymer electrolytes

Mixtures of neutral polymers and lithium salts have the potential to serve as electrolytes in next-generation rechargeable Li-ion batteries. The purpose of this review is to expose the delicate interplay between polymer-salt interactions at the segmental level and macroscopic ion transport at the battery level. Since complete characterization of this interplay has only been completed in one system: mixtures of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI), we focus on data obtained from this system. We begin with a discussion of the activity coefficient, followed by a discussion of six different diffusion coefficients: the Rouse motion of polymer segments is quantified by Dseg, the self-diffusion of cations and anions is quantified by Dself,+ and Dself,−, and the build-up of concentration gradients in electrolytes under an applied potential is quantified by Stefan-Maxwell diffusion coefficients, D0+, D0-, and D+-. The Stefan-Maxwell diffusion coefficients can be used to predict the velocities of the ions at very early times after an electric field is applied across the electrolyte. The surprising result is that D0- is negative in certain concentration windows. A consequence of this finding is that at these concentrations, both cations and anions are predicted to migrate toward the positive electrode at early times. We describe the controversies that surround this result. Knowledge of the Stefan-Maxwell diffusion coefficients enable prediction of the limiting current. We argue that the limiting current is the most important characteristic of an electrolyte. Excellent agreement between theoretical and experimental limiting current is seen in PEO/LiTFSI mixtures. What sequence of monomers that, when polymerized, will lead to the highest limiting current remains an important unanswered question. It is our hope that the approach presented in this review will guide the development of such polymers

Diffusion and migration in polymer electrolytes

Mixtures of neutral polymers and lithium salts have the potential to serve as electrolytes in next-generation rechargeable Li-ion batteries. The purpose of this review is to expose the delicate interplay between polymer-salt interactions at the segmental level and macroscopic ion transport at the battery level. Since complete characterization of this interplay has only been completed in one system: mixtures of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI), we focus on data obtained from this system. We begin with a discussion of the activity coefficient, followed by a discussion of six different diffusion coefficients: the Rouse motion of polymer segments is quantified by Dseg, the self-diffusion of cations and anions is quantified by Dself,+ and Dself,−, and the build-up of concentration gradients in electrolytes under an applied potential is quantified by Stefan-Maxwell diffusion coefficients, D0+, D0-, and D+-. The Stefan-Maxwell diffusion coefficients can be used to predict the velocities of the ions at very early times after an electric field is applied across the electrolyte. The surprising result is that D0- is negative in certain concentration windows. A consequence of this finding is that at these concentrations, both cations and anions are predicted to migrate toward the positive electrode at early times. We describe the controversies that surround this result. Knowledge of the Stefan-Maxwell diffusion coefficients enable prediction of the limiting current. We argue that the limiting current is the most important characteristic of an electrolyte. Excellent agreement between theoretical and experimental limiting current is seen in PEO/LiTFSI mixtures. What sequence of monomers that, when polymerized, will lead to the highest limiting current remains an important unanswered question. It is our hope that the approach presented in this review will guide the development of such polymers

Dissolution of lithium metal in poly(ethylene oxide)

We demonstrate that lithium metal is sparingly soluble in poly(ethylene oxide) (PEO). 7Li NMR shows that when a PEO sample is placed in contact with lithium metal at elevated temperatures, a lithium species dissolves and diffuses into the bulk polymer. A lithium/PEO/lithium electrochemical cell, containing no lithium salts, shows increasing conductivity over time when annealed at 120 °C. Chronoamperometry shows that the annealed cell obeys Ohm's law, implying that conduction occurs without the development of concentration gradients. To explain the results, it is proposed that atomic lithium dissolves into PEO, where it exists as a lithium cation and free electron. The dissolution of lithium also affects the phase behavior of block copolymer electrolytes. These observations explain the strong adhesion between lithium metal and PEO and have important implications for lithium metal battery systems that contain PEO-based electrolytes