In Situ Observation of Random Solid Solution Zone in LiFePO₄ Electrode

Abstract

Nanostructured LiFePO4 (LFP) electrodes have attracted great interest in the Li-ion battery field. Recently there have been debates on the presence and role of metastable phases during lithiation/delithiation, originating from the apparent high rate capability of LFP batteries despite poor electronic/ionic conductivities of bulk LFP and FePO4 (FP) phases. Here we report a potentiostatic in situ transmission electron microscopy (TEM) study of LFP electrode kinetics during delithiation. Using in situ high-resolution TEM, a Li-sublattice disordered solid solution zone (SSZ) is observed to form quickly and reach 10-25 nm × 20-40 nm in size, different from the sharp LFP|FP interface observed under other conditions. This 20 nm scale SSZ is quite stable and persists for hundreds of seconds at room temperature during our experiments. In contrast to the nanoscopically sharp LFP|FP interface, the wider SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. Our findings suggest that the disordered SSZ could dominate phase transformation behavior at nonequilibrium condition when high current/voltage is applied; for larger particles, the SSZ could still be important as it provides out-of-equilibrium but atomically wide avenues for Li(+)/e(-) transport.

Phase evolution in single-crystalline LiFePO4 followed by in situ scanning X-ray microscopy of a micrometre-sized battery

“…Scanning transmission X-ray microscopy (STXM) is one of the eligible techniques due to its favourable combination of chemical resolution, spatial resolution, field of view, low dose and connected favourable signal-to-noise ratio and measurement time, clean UHV environment and X-ray beam transmission, enabling highresolution imaging of LFP/FP phase propagation. While in situ XRD measurements 21,30 refer to ensemble averages and the forte of in situ TEM measurements 31,32 lies in probing smaller-length scales, STXM allows to observe the single-particle delithiation/ lithiation mechanism with high chemical, spatial and time resolution on a battery-relevant overall length scale comparable to a single particle in an electrode.…”

Atomic‐Scale Monitoring of Electrode Materials in Lithium‐Ion Batteries using In Situ Transmission Electron Microscopy

“…Using in situ high resolution transmission electron microscopy (HRTEM), Niu et al found that a Li-sublattice disordered SSZ formed preferentially at the surface of LiFePO 4 during electrochemical delithiation. [52] Figure 3a,c shows that pristine LiFePO 4 nanowires exhibit good crystallinity. However, when a voltage was applied, a relatively large and disordered SSZ appeared.…”

“…Such local structural changes, electron transport, and ion migration processes are inextricably linked and determine the macroscopic performance of LIBs. [52] Copyright 2014, American Chemical Society.…”

Rate-dependent Reversal of Lithium Concentration During Intercalation into LixFePO4 Nanoparticles

“…Yet, how Li intercalation occurs locally in a single LFP nanoparticle remains unresolved since real-time observation at such a fine scale is still lacking. High-resolution transmission electron microscopy (HRTEM) is capable of exceptional spatial resolution, and it has recently been developed for in situ tracking of the phase evolution [6,7]. However, without the formation of distinctive phase boundaries during a monophasic solid-solution transformation in Li x FePO 4 , the subtle change in lattice spacing, at sub-Ångstrom scale, cannot be directly resolved by in situ HRTEM [6].…”

“…High-resolution transmission electron microscopy (HRTEM) is capable of exceptional spatial resolution, and it has recently been developed for in situ tracking of the phase evolution [6,7]. However, without the formation of distinctive phase boundaries during a monophasic solid-solution transformation in Li x FePO 4 , the subtle change in lattice spacing, at sub-Ångstrom scale, cannot be directly resolved by in situ HRTEM [6].Herein, we report advancement in in situ techniques enabling real-time tracking of lithium intercalation in a single nanoparticle at different rates. Geometric phase analysis (GPA) was developed for processing time-and position-resolved lattice images, allowing for direct correlation of local displacement with the Li concentration within a single solid-solution phase.…”