Mechanistic insights of Li+ diffusion within doped LiFePO4 from Muon Spectroscopy

Abstract

The Li+ ion diffusion characteristics of V- and Nb-doped LiFePO4 were examined with respect to undoped LiFePO4 using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering DLi values in the range 1.8–2.3 × 10−10 cm2 s−1, this implied the improvement in electrochemical performance observed within doped LiFePO4 was not a result of increased local Li+ diffusion. This unexpected observation was made possible with the µSR technique, which can measure Li+ self-diffusion within LiFePO4, and therefore negated the effect of the LiFePO4 two-phase delithiation mechanism, which has previously prevented accurate Li+ diffusion comparison between the doped and undoped materials. Therefore, the authors suggest that µSR is an excellent technique for analysing materials on a local scale to elucidate the effects of dopants on solid-state diffusion behaviour.

Lithium diffusion in LiMnPO4 detected with μ±SR

“…In fact, according to the first attempt on Li x CoO 2 with x = 0.53 and 0.73 (the most common cathode material in the present Li-ion batteries), values of D Li estimated with μ + SR [7] were comparable to those predicted by first principles calculations [10]. Since then, many battery materials have been studied with μ + SR in order to extract their intrinsic D Li [4,[11][12][13][14][15][16][17][18][19][20][21][22] and D Na [23][24][25][26].…”

Local mobility in electrochemically inactive sodium in hard carbon anodes after the first cycle

“…The intrinsic diffusion dynamics of battery materials have been extensively studied using muon spin rotation (mSR) spectroscopy for lithium ion batteries [11][12][13] and cathodes in sodium ion batteries, 14,15 and have revealed activation energies for diffusion in the range of 225-571 meV for sodium ions in transition metal oxides and sodium jump frequencies of $0.15-0.2 ms À1 at room temperature. Combining this with jump distances determined by PXRD, diffusion constants can be derived, which for Na x CoO 2 are in the range 4 Â 10 À11 to 5 Â 10 À12 cm 2 s À1 depending on x.…”

Multiple diffusion pathways in LixNi0.77Co0.14Al0.09O2 (NCA) Li-ion battery cathodes

“…Arguably, the most important of these processes remains the diffusion of lithium through the host insertion electrode, as lithium diffusion is ultimately responsible for cell operation. Many complementary techniques have been used to probe the diffusion mechanics; electrochemical (cyclic voltammetry, 15 titration techniques [16][17][18] and impedance spectroscopy 19 ), physical (nuclear magnetic resonance 20 and muon spin relaxation [21][22][23] ) and computational methods, [24][25][26] each with their own associated benets and limitations.…”

“…Muon spin relaxation (mSR) has previously been employed to investigate Li + diffusion in many LIB materials including cathodes and solid-state electrolytes. [21][22][23][27][28][29][30][31] As mSR is a local probe technique, atomic scale diffusion can be probed as a local average within the material. This allows the measurement of diffusion processes independent from contributions to diffusion encountered in bulk measurements, for instance intercalation-based processes arising from grain boundary and/ or surface interactions.…”

“…34 It is important to note that computational calculations and experimental methods have regularly overestimated E a of Li + diffusion when compared to local mSR measurements. 21,23 All of these studies have been based on a single contribution to Li + diffusion, however computational investigations on the parent material LiCoO 2 suggest two possible diffusion routes, namely the Oxygen Dumbbell Hop (ODH) and Tetrahedral Site Hop (TSH). 24 A representation of the two mechanisms can be seen in Fig.…”