27.01.2012

Promising New Material for Lithium Batteries

DESY light source reveals structure of mesoporous hematite

Researchers from Germany and the United States have identified a promising new material for use in lithium ion batteries. The scientists from Gießen University, DESY, HRL Laboratories, and the University of California at Los Angeles have discovered that porous thin films of the iron oxide hematite, a-Fe2O3, can store a large number of lithium ions and do not lose much of their capacity over a great number of charging cycles. Both attributes are important when considering the use as electrodes in batteries. The research is presented today in the new Photon Science Highlights annual report of the German accelerator centre Deutsches Elektronen-Synchrotron DESY. A scientific paper has previously been published in the journal "small" (DOI: 10.1002/smll.201001333).

Synchrotron-based GISAXS pattern of a self-assembled hematit thin film heated to 250 degrees centigrade. Credit: Torsten Brezesinski/Gießen University

Lithium ion batteries are omnipresent in today’s modern world. They power our mobile phones, notebooks, and digital cameras. They help entertain our kids on-the-go in handheld video-game consoles. And they support our mobility on the road in electric and hybrid cars – an increasingly important aspect in the light of continually rising gas prices. With the upsurge of worldwide demand for battery-powered devices, research that targets the development of new cost-efficient, high-performance battery materials is of immense interest.

The research collaboration, led by Torsten Brezesinski and his group from Gießen University, found an unlikely candidate for a battery material: hematite. The iron oxide in its bulk form had been previously shown to be of little to no value for battery applications. However, the researchers did not look at the bulk material. Instead, they produced thin films that are riddled with pores of only 0.000 000 015 meters (15 nanometers) in diameter and whose pores are made of crystalline hematite. Scientists call such materials with pore sizes of 2–50 nanometers “mesoporous”. This transition from bulk to mesoporous thin films of hematite changes the properties of the material dramatically.

Two important quality factors for a lithium-ion battery are (1) the amount of lithium stored in the battery as well as (2) the number of charging cycles before the battery capacity drops, i.e. the lifetime of the battery. Using electrochemical methods, the researchers showed that both factors are greatly enhanced in mesoporous thin films as compared to bulk hematite or small particles of hematite. The mesoporous films store over thirteen times more lithium ions than hematite microcrystals (thousandths of a millimeter in size). In the voltage range used in the study, the thin films even surpass the specific capacities of other known lithium battery materials. The researchers were also able to perform an impressive 200 charging cycles without noticing a significant drop in charge storage.

Why do mesoporous films have excellent attributes that the bulk or small particles of the very same material lack? The insertion of lithium ions into hematite leads to tension within the material. “In microcrystals, the hematite undergoes an irreversible phase transformation when lithium is inserted, limiting lithium storage and leading to a poor cycling stability,” Brezesinski explains. “In the thin films, however, the pore architecture of the film allows the material to expand and to compensate for the tension within the film.” This explains why the thin films can successfully undergo so many charge cycles. Another advantage of the mesoporous film is that lithium can freely move into the film and access its entire volume. This increases the surface area for lithium insertion. 

In a rechargeable lithium-ion battery, mobility of the lithium ions is crucial. Lithium moves from the negative to the positive pole when the battery is discharged. It moves in the opposite direction when the battery is charged. “Due to the pore structure, the lithium can diffuse much faster into and out of the material. Compared to other materials, a battery with mesoporous hematite thin-film electrodes will be extremely fast to charge,” Brezesinski says.

The research team used a variety of state-of-the-art techniques to examine the structure of the mesoporous films, including microscopy, spectroscopy, and synchrotron methods. In the so-called GISAXS (grazing-incidence small-angle X-ray scattering) experiment at HASYLAB, the scientists made use of very intense X-rays to look at the inner structure of the thin films. “With the microscopic techniques we can only observe the sample surface,” DESY scientist Jan Perlich explains. “GISAXS allows us to actually look into the sample without destroying it and verify that the pores seen on the surface continue to the inside of the film.”

The researchers prepared the mesoporous films using an evaporation-induced self-assembly (EISA) process. In a first step the solution of a structure-directing agent, which promotes the periodic order, is mixed with a precursor compound, which is a building block for the pore walls. Evaporation of the volatile components in the mixture then leads to the formation of a composite. This composite in turn is heated to about 550 °C to form the desired porous architecture with crystalline walls.

Considering the comparatively low cost of iron oxide-based materials and modern society’s increasing demand for rechargeable batteries, the potential use of porous α-Fe2O3 in thin-film batteries is an exciting perspective.

Morphology of self-assembled α-Fe2O3 thin films heated to 550 °C in air. (a) CrosssectionalSEM image showing that the distorted cubic pore structure persists throughout the films. (b) Top view SEM image confirming the absence of a sealing layer at the solid-air interface. (c,d) Low- and high-magnification TEM images showing pores averaging 15 nm in diameter. Credit: Torsten Brezesinski/Gießen University