Magnetic and diffusive nature of LiFePO4investigated by muon spin rotation and relaxation

Abstract

In order to elucidate the magnetism and Li diffusion in LiFePO 4 , we have measured muon-spin rotation and relaxation (μ + SR) spectra for the polycrystalline LiFePO 4 sample in the temperature range between 1.8 and 500 K. Below T N ∼ 52 K, two oscillatory signals together with one fast relaxation signal were clearly found in the zero-field (ZF) μ + SR spectrum. The three signals are reasonably explained using an antiferromagnetic (AF) spin structure proposed by neutron measurements, because electrostatic potential calculations suggests multiple different muon sites in the LiFePO 4 lattice. However, the AF ordered moment estimated from μ + SR was about 3/4 of that reported by neutron, probably due to a different time window between the two techniques. In the paramagnetic state, ZF and longitudinal-field (LF) μ + SR spectra exhibited a dynamic nuclear field relaxation. From the temperature dependence of the field fluctuation rate, a diffusion coefficient of Li + ions (D Li) at 300 K was estimated about 3.6 × 10 −10 cm 2 /s, assuming that diffusing Li + ions jump between the regular site and interstitial sites.

Diffusive behavior in LiMPO4withM=Fe, Co, Ni probed by muon-spin relaxation

“…6,7 As a result, D Li estimated by 7 Li-NMR for LiCoO 2 and LiNiO 2 8 is three or four orders of magnitude smaller than the D Li predicted by first principles calculations, 9 while μ + SR yields higher D Li for the related compounds LiNiO 2 and LiCrO 2 , more in line with the theoretical predictions. 10 Very recent μ + SR work on the olivine-type lithium iron phosphate LiFePO 4 , which is heavily investigated as a positive electrode material for the near-future Li-ion battery, 11,12 showed that D Li ∼ 3.6 × 10 −10 cm 2 /s at 300 K, 13 a result confirmed by another group. 14 Regarding the reliability of the estimation, the D Li value obtained by μ + SR is consistent with recent electrochemical simulations using the chronoamperometric response data, in which D Li ∼ 7.6 × 10 −11 cm 2 /s for Li 0.999 FePO 4 at ambient temperature (T ), 15 while first-principles calculations predicted D Li ∼ 10 −8 cm 2 /s for the Li 7/8 FePO 4 case.…”

“…23,24 However, there is, to our knowledge, no systematic electrochemical work on LiMPO 4 from M = Mn to Ni through Fe and Co, although their magnetic nature has been extensively investigated by several techniques, [25][26][27][28][29][30][31][32][33][34][35] including our μ + SR work at low T . 13,36 In particular, all four compounds exhibit a magnetic transition from a Curie-Weiss paramagnetic phase to an antiferromagnetic (AF) ordered phase at T N = 23 to 53 K.…”

Ion Diffusion in Solids Probed by Muon-Spin Spectroscopy

“…This is a highly unsatisfactory situation, since D Li is one of the primary parameters that govern the charge and discharge rate of a Li-ion battery, particularly in the case of future solid state batteries. We have, therefore, attempted to measure D Li for layered lithium-transition-metal-oxides with µ + SR since 2005 [14][15][16][17][18][19][20]. Muons do not feel fluctuating magnetic moments at high T , but instead sense the change in nuclear dipole field due to Li diffusion.…”

Macroscopic Electrochemical Properties Clarified by Microscopic Measurements; Present and Future

“…This method gives a unique direct access to one of the most important material properties, namely the intrinsic Li-ion self-diffusion constant D Li . We have shown from our extensive measurements of Li-ion cathode materials [3][4][5][6][7][8]11] how the knowledge of such vital information will be crucial for the understanding of solid state ionics in these compounds. This is an important first step towards the materials development needed for future solid state batteries (SSB).…”

“…Recently, it was found that µ + SR detects a slight variation of an internal magnetic field (H int ) due to Li diffusion even for materials containing magnetic ions [3][4][5][6][7][8][9][10], since the implanted muons sense H int with a zero applied field (ZF) in contrast to Li-NMR. Furthermore, making comparison between µ + SR and neutron scattering for detecting D Li from a viewpoint of time window, µ + SR is a very good complementary technique to quasi-elastic neutron scattering (QENS) in the research of Li diffusion in solids (see Fig.…”