To achieve long-lasting, high-performance lithium-ion batteries, local inhomogeneity in current density must be avoided. One driver of such inhomogeneity is the spinodal decomposition of active materials into Li-rich and Li-poor phases during charge and discharge, respectively. Whether or not such phase separation occurs is linked to the dynamics of lithium (Li) at the active material surface and within its bulk. Here, we show that particle surface coatings can be designed to inhibit phase separation. Using LiFePO4 as a model system, we prepare particles with different coatings in order to tune Li dynamics and electronic structure and thereby induce or prevent phase separation, which we verify through experiment and ab initio calculations. We then leverage our findings to create a mixed ceramic-carbon coating that mitigates phase separation down to rates as low as 0.1C and simultaneously enables fast (dis)charge up to 5C.

Understanding ion diffusion mechanisms in layered materials is critical for advancing next-generation battery technologies. Using the well-characterized NaxCoO2 (NCO) system as a model platform, we investigated the temperature-dependent diffusion properties across a broad compositional range (x=0.33−0.89) using muon spin relaxation (μ+SR). Unexpected low-temperature internal magnetic field fluctuations were observed, systematically varying with Na content and appearing well before the onset of long-range diffusion. These fluctuations are attributed to phonon-assisted local Na motion, as suggested by a systematic increase in A with x, concurrent with a decreasing activation energy. The diffusion coefficient was calculated based on the crystal structure using a tailored diffusion model accounting for two inequivalent Na sites, yielding values consistent with those found in other layered battery materials. This work highlights the crucial role of phonon-coupled diffusion mechanisms in enabling ion transport at the microscopic scale, providing new insights into ion dynamics in layered solid-state conductors and their relevance to sodium-ion battery technology.

The assertion of intrinsic material properties based on measured experimental data is being challenged by emerging sample synthesis protocols, which opens new avenues for discovering novel functionalities. In this study, we revisit one of the most widely studied strongly correlated materials of the early 2000s, NaxCoO2 (NCO). Leveraging the sensitivity of muon spin rotation and relaxation (μ+SR) measurements, we discern significant differences between NCO samples synthesized via conventional solid-state reaction (SSR) and our electrochemical reaction (ECR) approach. Contrary to SSR-synthesized Na0.7CoO2, which exhibits a nonmagnetic ground state, our ECR-derived sample showcases an antiferromagnetic (AF) order from x≥0.7, challenging established phase boundaries. We attribute the observed magnetic phenomena in ECR-NCO to long-range order of Na-ions and/or vacancies, as well as the inherent flexibility of the crystal framework. Our study holds implications for tailoring and optimization of next-generation devices based on layered materials.

In this study, we investigate the influence of Cr-Cr distances on the magnetic properties of triangular lattice antiferromagnets through the lens of the recently synthesized Cr compounds LiCrSe2, 2 , LiCrTe2, 2 , and NaCrTe2. 2 . Our comprehensive analysis integrates existing magnetic structure data and new insights from muon spin rotation measurements, revealing a striking mutual influence between strongly correlated electrons and structural degrees of freedom in systems possessing very different magnetic properties despite having the same crystal symmetry. In particular, we delineate how Cr-Cr distances specifically dictate the magnetic behaviors of the triangular lattice antiferromagnets LiCrSe2, 2 , LiCrTe2, 2 , and NaCrTe2. 2 . By crafting phase diagrams based on these distances, we establish a clear correlation between the structural parameters and the magnetic ground states of these materials together with a wide variety of trivalent Cr triangular lattice layered magnets. Our analysis uncovers a transition range for in-plane and out-of-plane Cr-Cr distances that demarcates distinct magnetic behaviors, highlighting the nuanced role of lattice geometry in the spin-lattice interaction and electron correlation dynamics.

A Muon Spin Rotation (mu+SR) study was conducted to investigate the magnetic properties of SrCu2(BO3)(2) (SCBO) as a function of temperature/pressure. Measurements in zero field and transverse field confirm the absence of long range magnetic order at high pressures and low temperatures. These measurements suggest changes in the Cu spin fluctuations characteristics above 21 kbar, consistent with the formation of a plaquette phase as previously suggested by inelastic neutron scattering measurements. SCBO is the only known realisation of the Shatry-Sutherland model, thus the ground state mediating the dimer and antiferromagnetic phase is likekly to be a plaquette state.

LiFePO4 (LFPO) is an archetypical and well-known cathode material for rechargeable Li-ion batteries. However, its quasi-one-dimensional (Q1D) structure along with the Fe ions, LFPO also displays interesting low-temperature magnetic properties. Our team has previously utilized the muon spin rotation (mu+SR) technique to investigate both magnetic spin order as well as Li-ion diffusion in LFPO. In this initial study we extend our investigation and make use of high-pressure mu+SR to investigate effects on the low-T magnetic order. Contrary to theoretical predictions we find that the magnetic ordering temperature as well as the ordered magnetic moment increase at high pressure (compressive strain).

In an all solid state Li-ion battery, it is crucial to reduce ionic resistivity at the interface between the electrode and the electrolyte in order to enhance Li+ mobility across the interface. Recent first principles calculations predict the presence of a space-charge layer (SCL) at the interface due to the difference in the Li+ chemical potential at the interface between two different materials, as in the metal-semiconductor junction in electronic devices. However, the presence of SCL has never been experimentally observed. Our first attempt in a fresh multilayer sample, Cu(10 nm)/Li3PO4(50 nm)/LiCoO2(100 nm) on a sapphire substrate, with low-energy mu+SR (LE mu+SR) revealed a gradual change in the nuclear magnetic field distribution width as a function of implantation depth even across the interface between Li3PO4 and LiCoO2. This implies that the change in the field distribution width at SCL of the sample is too small to be detected by LE mu+SR.

The internal magnetic field in a ferromagnetic compound, Y2Co12P7 with T-C = 150 K, was studied with mu(+) SR using a powder sample down to 2 K. The wTF-mu(+) SR measurements revealed the presence of a sharp magnetic transition at T-C = 151 K, and the ZF-mu(+) SR measurements clarified the formation of static magnetic order below T-C. The presence of two muon spin precession signals in the ZF-mu(+) SR spectrum below TC indicates the existence of the two different muon sites in the lattice. By considering the muon sites and local spin densities at the muon sites predicted with DFT calculations, the ordered magnetic moments of Co were successfully determined.

For quantum systems or materials, a common procedure for probing their behaviour is to tune electronic/magnetic properties using external parameters, e.g. temperature, magnetic field or pressure. Pressure application as an external stimuli is a widely used tool, where the sample in question is inserted into a pressure cell providing a hydrostatic pressure condition. Such device causes some practical problems when using in Muon Spin Rotation/Relaxation (µ+SR) experiments as a large proportion of the muons will be implanted in the pressure cell rather than in the sample, resulting in a higher background signal. This issue gets further amplified when the temperature dependent response from the sample is much smaller than that of the pressure cell,which may cause the sample response to be lost in the background and cause difficulties in aligning the sample within the beam. To tackle this issue, we have used pySRIM [1] to construct a practical and helpful simulation tool for calculating muon stopping fractions, specifically for the pressure cell setup at the µE1 beamline using the GPD spectrometer at the Paul Scherrer Institute, with the use of TRIM simulations. The program is used to estimate the number of muon stopping in both the sample and the pressure cell at a given momentum. The simultion tool is programmed into a GUI, making it accessible to user to approximate prior to their experiments at GPD what fractions will belong to the sample and the pressure cell in their fitting procedure.

The diffusive behavior in a spinel-type Li+ ion battery material, Li[Ni1/2Mn3/2]O4, has been studied with positive and negative muon spin rotation and relaxation (μ±SR) measurements in the temperature range between 200 and 400 K using a powder sample. The implanted μ+ locates at an interstitial site near O2- ion so as to form a O-H like bond, while the implanted μ- is mainly captured by an oxygen nucleus, resulting in the formation of muonic oxygen. This means that local magnetic environments in Li[Ni1/2Mn3/2]O4 were investigated from the two different sites in the lattice, i.e., one is an interstitial site for μ+SR and the other is an oxygen site for μ-SR. Since both μ+SR and μ-SR detected an increase in the fluctuation rate of a nuclear magnetic field for temperatures above 200 K, the origin of this increase is clearly confirmed as Li diffusion. Assuming a random walk process with the hopping of thermally activated Li+ between a regular Li site and the nearest neighboring vacant octahedral sites, a self-diffusion coefficient of Li+ was found to range above 10-11 cm2/s at temperatures above 250 K with an activation energy of about 0.06 eV.