Solid State Battery Cells for in Operandi Diffusion Measurements Using µ+sr Spectroscopy

Abstract

Solid state lithium ion batteries (SSBs) have been widely tipped to revolutionise energy storage technology. The combination of increased safety, longer lifetime, larger electrochemical potential window and higher energy density offered has led to sustained global research into their development. One important aspect of that development is the dynamics of ion diffusion, particularly through the bulk solid electrolyte but also at the electrode/electrolyte interface. Ion diffusion rates can limit how fast batteries can be charged/discharged, with charge movement across the electrode/electrolyte boundary especially key in SSBs, which can experience poor interfacial contact. An increased understanding of microscopic level ion behaviour at these boundaries will prove valuable, but has so far been elusive with techniques such as NMR and impedance spectroscopy yielding inconsistent results.1,2 Muon spin relaxation (µ+SR) spectroscopy is a relatively new technique used to investigate Li+ migration mechanisms, using spin-polarised positive muons created using a synchrotron. The muons can be controlled to stop at any point within a battery, acting as an atomic scale probe of charge movement in a particular area within the crystal structure. This occurs as an implanted muon’s spin is perturbed by the magnetic field of nearby diffusing ions. After an average lifetime of 2.2 µs the muon will decay into a positron, emitted preferentially along the direction of the muon spin at the instant of decay. Detection of the asymmetry of emitted positrons gives a time evolution of the muon ensemble’s spin polarisation, yielding information on ion diffusion dynamics and activation energies. µ+SR has been proven a successful technique to study in situ batteries, producing accurate measurements that are consistent with first-principle calculations.2,3 This research will focus on the study of the promising solid-state battery electrolyte material Al-doped LLZO (Li7La3Zr2O12), using µ+SR spectroscopy as a reliable and reproducible probe. The anode and cathode materials used will be Li4Ti5O12 and LiCoO2 to complete the cell. This cell will be tested in situ from room temperature to around 80ºC on the EMU beamline at ISIS Pulsed Neutron and Muon Source. Any insights gained will be translated back to design principles to optimise battery performance. Power Source, 2014, 268, 153–162Mater. Chem. A, 2017, 5, 8653Mater. Chem. A, 2016, 4, 1729Phys. Scr., 2013, 88, 068509