Oliver Pecher

Structural and Mechanistic Insights into Fast Lithium-Ion Conduction in Li4SiO4–Li3PO4 Solid Electrolytes

Solid electrolytes that are chemically stable and have a high ionic conductivity would dramatically enhance the safety and operating lifespan of rechargeable lithium batteries. Here, we apply a multi-technique approach to the Li-ion conducting system (1-z)Li4SiO4-(z)Li3PO4 with the aim of developing a solid electrolyte with enhanced ionic conductivity. Previously unidentified superstructure and immiscibility features in high-purity samples are characterized by X-ray and neutron diffraction across a range of compositions (z = 0.0-1.0). Ionic conductivities from AC impedance measurements and large-scale molecular dynamics (MD) simulations are in good agreement, showing very low values in the parent phases (Li4SiO4 and Li3PO4) but orders of magnitude higher conductivities (10(-3) S/cm at 573 K) in the mixed compositions. The MD simulations reveal new mechanistic insights into the mixed Si/P compositions in which Li-ion conduction occurs through 3D pathways and a cooperative interstitial mechanism; such correlated motion is a key factor in promoting high ionic conductivity. Solid-state (6)Li, (7)Li, and (31)P NMR experiments reveal enhanced local Li-ion dynamics and atomic disorder in the solid solutions, which are correlated to the ionic diffusivity. These unique insights will be valuable in developing strategies to optimize the ionic conductivity in this system and to identify next-generation solid electrolytes.

Investigating Sodium Storage Mechanisms in Tin Anodes: A Combined Pair Distribution Function Analysis, Density Functional Theory, and Solid-State NMR Approach

The alloying mechanism of high-capacity tin anodes for sodium-ion batteries is investigated using a combined theoretical and experimental approach. Ab initio random structure searching (AIRSS) and high-throughput screening using a species-swap method provide insights into a range of possible sodium-tin structures. These structures are linked to experiments using both average and local structure probes in the form of operando pair distribution function analysis, X-ray diffraction, and 23Na solid-state nuclear magnetic resonance (ssNMR), along with ex situ 119Sn ssNMR. Through this approach, we propose structures for the previously unidentified crystalline and amorphous intermediates. The first electrochemical process of sodium insertion into tin results in the conversion of crystalline tin into a layered structure consisting of mixed Na/Sn occupancy sites intercalated between planar hexagonal layers of Sn atoms (approximate stoichiometry NaSn3). Following this, NaSn2, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn3, but has no tin atoms between the layers. NaSn2 is broken down into an amorphous phase of approximate composition Na1.2Sn. Reverse Monte Carlo refinements of an ab initio molecular dynamics model of this phase show that the predominant tin connectivity is chains. Further reaction with sodium results in the formation of structures containing Sn-Sn dumbbells, which interconvert through a solid-solution mechanism. These structures are based upon Na5-xSn2, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, Na15Sn4, can store additional sodium atoms as an off-stoichiometry compound (Na15+xSn4) in a manner similar to Li15Si4.

[Ge2]4− Dumbbells with Very Short Ge−Ge Distances in the Zintl Phase Li3NaGe2: A Solid‐State Equivalent to Molecular O2

The novel ternary Zintl phase Li3NaGe2 comprises alkali-metal cations and [Ge2](4-) dumbbells. The diatomic [Ge2](4-) unit is characterized by the shortest Ge-Ge distance (2.390(1) Å) ever observed in a Zintl phase and thus represents the first Ge=Ge double bond under such conditions, as also suggested by the (8-N) rule. Raman measurements support these findings. The multiple-bond character is confirmed by electronic-structure calculations, and an upfield (6)Li NMR shift of -10.0 ppm, which was assigned to the Li cations surrounded by the π systems of three Ge dumbbells, further underlines this interpretation. For the unperturbed, ligand-free dumbbell in Li3NaGe2, the π- bonding py and pz orbitals are degenerate as in molecular oxygen, which has singly occupied orbitals. The partially filled π-type bands of the neat solid Li3NaGe2 cross the Fermi level, resulting in metallic properties. Li3NaGe2 was synthesized from the elements as well as from binary reactants and subsequently characterized crystallographically.

The Solid Solution Sr1−xBaxGa2: Substitutional Disorder and Chemical Bonding Visited by NMR Spectroscopy and Quantum Mechanical Calculations

Complete miscibility of the intermetallic phases (IPs) SrGa2 and BaGa2 forming the solid solution Sr(1-x)Ba(x)Ga2 is shown by means of X-ray diffraction, thermoanalytical and metallographic studies. Regarding the distances of Sr/Ba sites versus substitution degree, a model of isolated substitution centres (ISC) for up to 10% cation substitution is explored to study the influence on the Ga bonding situation. A combined application of NMR spectroscopy and quantum mechanical (QM) calculations proves the electric field gradient (EFG) to be a sensitive measure of different bonding situations. The experimental resolution is boosted by orientation-dependent NMR on magnetically aligned powder samples, revealing in first approximation two different Ga species in the ISC regimes. EFG calculations using superlattice structures within periodic boundary conditions are in fair agreement with the NMR spectroscopy data and are discussed in detail regarding their application on disordered IPs.

Unravelling Local Atomic Order of the Anionic Sublattice in M(Al1−xGax)4 with M=Sr and Ba by Using NMR Spectroscopy and Quantum Mechanical Modelling

The quasibinary section of the intermetallic phases MAl4 and MGa4 with M=Sr and Ba have been characterised by means of X-ray diffraction (XRD) studies and differential thermal analysis. The binary phases show complete miscibility and form solid solutions M(Al1-x Gax )4 with M=Sr and Ba. These structures crystallise in the BaAl4 structure type with four- and five-bonded Al and/or Ga atoms (denoted as Al(4b), Al(5b), Ga(4b), and Ga(5b), respectively) that form a polyanionic Al/Ga sublattice. Solid state 27 Al NMR spectroscopic analysis and quantum mechanical (QM) calculations were applied to study the bonding of the Al centres and the influence of Al/Ga substitution, especially in the regimes with low degrees of substitution. M(Al1-x Gax )4 with M=Sr and Ba and 0.925≤x≤0.975 can be described as a matrix of the binary majority compound in which a low amount of the Ga atoms has been substituted by Al atoms. In good agreement with the QM calculations, 27 Al NMR investigations and single crystal XRD studies prove a preferred occupancy of Al(4b) for these substitution regimes. Furthermore, two different local Al environments were found, namely isolated Al(4b1) atoms and Al(4b2), due to the formation of Al(4b)-Al(4b) pairs besides isolated Al(4b) atoms within the polyanionic sublattice. QM calculations of the electric field gradient (EFG) using superlattice structures under periodic boundary conditions are in good agreement with the NMR spectroscopic results.

Crystal Structures, Local Atomic Environments, and Ion Diffusion Mechanisms of Scandium-Substituted Sodium Superionic Conductor (NASICON) Solid Electrolytes

The importance of exploring new solid electrolytes for all-solid-state batteries has led to significant interest in NASICON-type materials. Here, the Sc3+-substituted NASICON compositions Na3ScxZr2–x(SiO4)2–x(PO4)1+x (termed N3) and Na2ScyZr2–y(SiO4)1–y(PO4)2+y (termed N2) (x, y = 0–1) are studied as model Na+-ion conducting electrolytes for solid-state batteries. The influence of Sc3+ substitution on the crystal structures and local atomic environments has been characterized by powder X-ray diffraction (XRD) and neutron powder diffraction (NPD), as well as solid-state 23Na, 31P, and 29Si nuclear magnetic resonance (NMR) spectroscopy. A phase transition between 295 and 473 K from monoclinic C2/c to rhombohedral R3c is observed for the N3 compositions, while N2 compositions crystallize in a rhombohedral R3c unit cell in this temperature range. Alternating current (AC) impedance spectroscopy, molecular dynamics (MD), and high temperature 23Na NMR studies are in good agreement, showing that, with a higher ...

Challenges and new opportunities of in situ NMR characterization of electrochemical processes

The shift in efficient and sustainable energy storage materials technologies has to be accompanied with a deeper understanding of the chemical reactions involving the multiple cell components. The application of a non-invasive analysis tool that can follow the reactions in operando is therefore highly desired. In this matter, we report on the use of in situ nuclear magnetic resonance (NMR) spectroscopy as a valuable tool for these investigations. We discuss recent applications, challenges, and new opportunities of this technique especially in the context of investigating paramagnetic cathode materials and new electrochemical cell designs.

Chapter 11:NMR Studies of Electrochemical Storage Materials

This chapter describes the application of solid-state NMR spectroscopy for investigating battery electrode materials at controlled state of charge. Magic-angle spinning NMR gives the highest possible chemical resolution, but only allows these often metastable electrode materials to be studied in an ex situ manner, i.e., outside the electrochemical cell, with a risk of oxidation and chemical relaxation. Complementary to the MAS NMR approach, we therefore explain the use of dedicated static NMR probes optimally designed for coupling to battery cyclers. This in situ approach allows electrode materials to be studied inside electrochemical cells during repeated charge and discharge cycles. As electrode materials are generally paramagnetic or conductive in, at least, certain charge states, one of the NMR challenges is to deal with the large line broadening and shifts. In conjunction with density functional theory computation described in this chapter, however, these paramagnetic and Knight shifts are, in fact, rich sources of detailed information about the underlying materials’ structure.

Research data supporting "Enhanced efficiency of solid-state NMR investigations of energy materials using an external Automatic Tuning/Matching (eATM) robot"

LiFePO4: 7Li and 31P solid-state MAS NMR data of LiFePO4 at 7.05 T. La2NiO4: 17O solid-state MAS variable-temperature NMR of La2NiO4 at 16.4 T, at 79°C and 148°C. B-Nb2O5: Raw XRD data (B-Nb2O5) as plotted in supplementary information. 93Nb solid-state (static) NMR data of B-Nb2O5 at 16.4 T. Calculations of NMR parameters of B-Nb2O5 as output from the solid-state density functional theory (DFT) code CASTEP, on both relaxed and ICSD structures. All experimental and computational parameters are given in the article and/or data files.

Research data supporting Zintl Phases K4–xNaxSi4 (1 ≤ x ≤ 2.2) and K7NaSi8: Synthesis, Crystal Structures, and Solid-State NMR Spectroscopic Investigations

FID and Fourier-transformed 29Si and 23Na solid-state NMR data for K3NaSi4 and K7NaSi8; CASTEP density functional theory (DFT) calculation txt file containing input and outputs, especially NMR shielding and nuclear quadrupole coupling values for K3NaSi4 and K7NaSi8