The MIT researchers found that inside this electrode during charging, a solid-solution zone (SSZ) forms at the boundary between lithium-rich and lithium-depleted areas—the region where charging activity is concentrated, as lithium ions are pulled out of the electrode. Professor Ju Li, one of the authors, noted that the SSZ “ has been theoretically predicted to exist, but we see it directly for the first time ” in transmission electron microscope (TEM) videos taken during charging.

New observations by researchers at MIT have revealed the inner workings of a lithium iron phosphate (LiFePO 4 ) cathode—a material widely used in lithium-ion batteries. The new findings, published in a paper in the ACS journal Nano Letters , explain the unexpectedly high power and long cycle life of such batteries, the researchers say.

Nanostructured LiFePO 4 (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 FePO 4 (FP) phases. … 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 non-equilibrium 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. —Niu et al.

The observations help to resolve a longstanding puzzle about LiFePO 4 : In bulk crystal form, both lithium iron phosphate and iron phosphate (FePO 4 , which is left behind as lithium ions migrate out of the material during charging) have very poor ionic and electrical conductivities. Yet when treated—with doping and carbon coating—and used as nanoparticles in a battery, the material exhibits an impressively high charging rate.

It was quite surprising when this [rapid charging and discharging rate] was first demonstrated. e directly observed a metastable random solid solution that may resolve this fundamental problem that has intrigued [materials scientists] for many years. —Prof. Li

The SSZ is a metastable state, persisting for at least several minutes at room temperature. Replacing a sharp interface between LiFePO 4 and FePO 4 that has been shown to contain many additional line defects called “dislocations,” the SSZ serves as a buffer, reducing the number of dislocations that would otherwise move with the electrochemical reaction front. “We don’t see any dislocations,” Li says. This could be important because the generation and storage of dislocations can cause fatigue and limit the cycle life of an electrode.



Schematic model of the phase transition during delithiation/lithiation process. a) and b) SSZ migration in ac plane during the delithiation process. The boundary propagation speed along a is closed to 2.8 nm/min. Yellow balls represent Li ions, purple facets represent FeO 6 octahedra, and green/blue facets represent PO 4 tetrahedra. c) Lithium ions are inserted/extracted into/out the particle (blue arrows) from the formed sharp interface boundary between full (LFP) and empty (FP) channels. d) Plenty of lithium ions are inserted/extracted into/out the particle from the surface of big solid solution zone. Credit: ACS, Niu et al. Click to enlarge.

Unlike conventional TEM imaging, the technique used in this work, developed in 2010 by Kushima and Li, makes it possible to observe battery components as they charge and discharge, which can reveal dynamic processes. A better understanding of these dynamic processes could improve the performance of an electrode material by allowing better tuning of its properties, Li says.

Despite an incomplete understanding to date, lithium iron phosphate nanoparticles are already used at an industrial scale for lithium-ion batteries. “The science is lagging behind the application,” Li says.

Chongmin Wang, a research scientist at the Pacific Northwest National Laboratory who was not involved in this research, said that this new research “provides convincing and direct evidence” of the mechanism at work.

The research was supported by the National Science Foundation.

Resources