Abstract
Abstract 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.
Highlights
In order to understand the operational processes of battery materials, it is crucial to understand the diffusive nature of ions in these materials, because the transport of ions is the fundamental mechanism behind their operation
“weak” means the field is comparable to the field distribution width formed by surrounding nuclear magnetic moments at the muon site, which typically ranges between 5 and 10 Oe, and “longitudinal field” means the magnetic field is parallel to the initial muon spin polarization
In order to extract the μ+SR parameters, the wTF-μ+SR spectrum is fitted by a combination of an exponentially relaxing cosine oscillation due to wTF and an exponentially relaxing non-oscillatory signal, due to the 1/3 tail arising from the distribution of static electronic components in the powder sample when in an AF ordered state
Summary
In order to understand the operational processes of battery materials, it is crucial to understand the diffusive nature of ions in these materials, because the transport of ions is the fundamental mechanism behind their operation. “weak” means the field is comparable to the field distribution width formed by surrounding nuclear magnetic moments at the muon site, which typically ranges between 5 and 10 Oe, and “longitudinal field” means the magnetic field is parallel to the initial muon spin polarization. Such weak LFs decouple the contribution from a nuclear magnetic field on the μ+ spin, while the effect of weak LF on the contribution from the electron magnetic field is negligibly small. This is an attractive feature for studying Li+ [11,12,13,14,15,16,17,18,19,20,21], Na+ [10, 22, 23], and K+ [24] diffusion in cathode materials for an ion battery, because these materials always contain transition metal ions to keep charge neutrality during ion insertion and extraction reactions
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