Probing Ion Mobility Mechanisms in Solid Electrolytes using Solid-State NMR Spectroscopy

Abstract

The increased use of portable electronic devices, the electrification of vehicles, and the need to store the energy generated by renewable energy sources are driving the search for safer and better performing batteries. The development of all-solid-state batteries, those containing a solid electrolyte, is vital in the search for safer alternatives to current commercial lithium-ion batteries. At present there are a number of inorganic materials with the potential to be solid electrolytes. However, a complete understanding of ion migration within these materials is necessary if they are to be improved and implemented commercially.

Here, a range of complementary techniques have been used to study ion migration in the sodium-doped lithium-stuffed garnet, LiLaNaxM2O12 (M = Nb, Ta, x = 0 - 0.4), a potential solid electrolyte. High-resolution 6,7Li and 23Na multiple-quantum magic-angle spinning (MQMAS) NMR data, paired with powder X-ray diffraction (PXRD) studies, indicate that sodium substitutes onto multiple sites within the lithium-stuffed garnet structure, including those believed to be involved in ion conduction. Impedance measurements, muon spin relaxation (MuSR) spectroscopy, variable-temperature (VT) 6Li NMR, and 23Na exchange spectroscopy (EXSY) studies, in conjunction with computational methods (molecular dynamics simulations), have been used to investigate the ion mobility of the system. The work presented suggests that the substitution of sodium ions into the structure results in a blocking of the lithium-ion conduction pathway, thereby reducing the ionic conductivity of the system. To fully understand the affects of sodium substitution on the structure and resulting ion mobility, 93Nb, 139La, and 17O NMR studies were also undertaken. Interestingly, the data presented here suggests that, although the structure is cubic on average (Ia-3d), locally, the system exhibits a lower symmetry due to the number of crystallographically distinct sites observed. This is of significant interest as the observed physical properties may vary depending on the local- and long-range order of the structure.

The complex spinel Li2NiGe3O8 is currently being investigated as a potential solid electrolyte for all-spinel all-solid-state batteries. Here, by combining VT solid-state NMR studies with MuSR, the ion mobility was investigated. 7Li MAS NMR studies indicate the occupation of the octahedral lithium site, associated with ion migration, at temperatures much lower than previously reported (260 K). Ion migration is observed at this temperature in both the VT 6,7Li NMR and MuSR data. Furthermore, the proximity of the tetrahedral and octahedral sites to the paramagnetic nickel centre has allowed valuable insight into the migration pathway, specifically the migration of lithium via the octahedral site, to be gained via NMR studies.

In addition to studying solid electrolytes, Sc2(WO4)3, a material that exhibits negative thermal expansion, has been investigated as a potential electrode material for potassium-ion batteries. Here, a Sc2(WO4)3 electrode discharged against potassium was studied using 45Sc and 39K MAS NMR spectroscopy, in conjunction with PXRD. The ability to observe intercalated potassium ions via 39K MAS NMR is extremely promising for future studies of potassium-discharged samples. Using VT PXRD, the thermal expansion behaviour of the discharged electrodes were also investigated. Between 923 and 1023 K, significant negative thermal expansion (alpha_V = -1.90x10^-4 K^-1) was observed, attributed to the formation of a alpha-K2WO4 phase. This study has highlighted the potential for solid-state NMR to assist in the characterisation of potassium-discharged batteries, which is particularly encouraging.