Heike Gabrisch

TEM Study of Fracturing in Spherical and Plate-like LiFePO4 Particles

An investigation of fracturing in LiFePO{sub 4} particles as a function of the particle morphology and history is presented. Two types of samples, one subjected to electrochemical cycling and another to chemical delithiation are compared. We observe the formation of micro fractures parallel to low indexed lattice planes in both samples. The fracture surfaces are predominantly parallel to (100) planes in the chemically delithiated powder and (100) and (010) planes in the electrochemically cycled powder. A consideration of the threshold stresses for dislocation glide shows that particle geometry plays an important role in the observed behavior.

Carbon Surface Layers on a High-Rate LiFePO4

Transmission electron microscopy (TEM) was used to image particles of a high-rate LiFePO4 sample containing a small amount of in situ carbon. The particle morphology is highly irregular, with a wide size distribution. Nevertheless, coatings, varying from about 5-10 nm in thickness, could readily be detected on surfaces of particles as well as on edges of agglomerates. Elemental mapping using Energy Filtered TEM (EFTEM) indicates that these very thin surface layers are composed of carbon. These observations have important implications for the design of high-rate LiFePO4 materials in which, ideally, a minimal amount of carbon coating is used.

Hexagonal to Cubic Spinel Transformation in Lithiated Cobalt Oxide

A transmission electron microscopy (TEM) investigation was performed on LiCoO2 before and after it had been subjected to charge/discharge cycling in electrochemical cells, as well as on delithiated Li(1–x)CoO2 before and after thermal aging. Turbostratic disorder involving small rotations of the O-Co-O slabs was found in as-received material, and in material subjected to a few cycles. In LiCoO2 subjected to extensive charge/discharge cycling, it was found that increasing amounts of the trigonal O3 phase had transformed to H1-3 phase and to the cubic spinel phase. The transformation appears to initiate on the surfaces of trigonal crystals. The orientation relationship between the trigonal and spinel phases was determined from diffraction patterns to be {0001}trigonal parallel {111}cubic and trigonal parallel cubic. The difference in unit cell dimensions leads to transformation stresses when spinel crystals are formed, and spallation of surface layers was observed. The formation of a spinel phase could suppress electrochemical performance of LiCoO2 cathodes in heavily cycled cells. Aging in the charged state also can alter particle surfaces and therefore the performance.

Transmission Electron Microscope Studies of LiNi1 / 3Mn1 / 3Co1 / 3O2 before and after Long-Term Aging at 70 ° C

LiNi1/3Mn1/3Co1/3O2 is a potential cathode material for high-power applications in lithium-ion batteries. While cation ordering on a sqrt(3)×sqrt(3) R30° in-plane superlattice was proposed for the layered structure, the experimental data do not fully support this model. Here, we present a systematic electron diffraction study of LiNi1/3Mn1/3Co1/3O2 in the pristine state and after aging. Our results show that a mixture of different phases in the starting material transforms to the O3-type phase and the cubic spinel phase after aging, accompanied by an increase in the percentage of polycrystals.

Electron Diffraction Studies of LiNi1 / 3Mn1 / 3Co1 / 3O2 After Charge–Discharge Cycling

The performance of LiNi 1/3 Mn 1/3 Co 1/3 O 2 depends largely on the distribution of transition-metal (TM) ions over the available lattice sites within the layered rocksalt structure. Despite predictions that the TM ions take on an ordered arrangement described by a √3 × √3R30° supercell, little experimental evidence is available to confirm this model. However, the observed cation distribution likely depends on synthesis conditions. Here, we present a study of commercially produced LiNi 1/3 Mn 1/3 Co 1/3 O 2 synthesized at 1000°C before and after charge-discharge cycling. Electron diffraction shows that in-plane ordering is observed to small extents. After charge-discharge cycling, a transformation toward the cubic spinel structure and significant morphology changes are observed.

Magnetic Properties of LixCoO2 and its Thermally Decomposed Phases

Stoichiometric LiCoO2 is nonmagnetic in nature. When lithium ion is removed from the lattice magnetic character is introduced in the lattice due to change of spin states of cobalt ions. These lithium deficient phases are not stable at elevated temperature eventually transform to a mixture of LiCoO2 and Co3O4 under evolution of oxygen. Here we report the magnetic behavior of LixCoO2 (x=1.03, 0.98, 0.76, 0.55) and its thermally decomposed products.

TEM Studies of Carbon Coated LiFePO4 After Charge Discharge Cycling

Carbon coating has proven to be a successful approach toimprove the rate capability of LiFePO4 used in rechargeable Li-ionbatteries. Investigations of the microstructure of carbon coated LiFePO4after charge discharge cycling shows that the carbon surface layerremains intact over 100 cycles. We find micro cracks in the cycledmaterial that extend parallel to low indexed lattice planes. Ourobservations differ from observations made by other authors. However thedifferences between the orientations of crack surfaces in both studiescan be reconciled considering the location of weak bonds in the unit celland specimen geometry as well as elastic stress fields ofdislocation.

Synthesis of LiNi1/3Mn1/3Co1/3O2 and Crystal Structure Studies by Electron Diffraction

The performance of LiNi1/3Mn1/3Co1/3O2 depends largely on the distribution of transition metal ions over the available lattice sites within the layered rocksalt structure. Despite predictions that the TM ions take on an ordered arrangement described by a 3 x 3 R30o supercell little experimental evidence is available to confirm this model. On the other hand the observed cation distribution likely depends on synthesis conditions. Here we compare LiNi1/3Mn1/3Co1/3O2 synthesized on a small scale in our laboratory at 850oC, subjected to chemical delithiation and annealing, to commercially produced LiNi1/3Mn1/3Co1/3O2 synthesized at 1000oC before and after charge discharge cycling. Electron diffraction shows that in plane ordering is observed to small extents at the higher synthesis temperature but not at 850oC. In both compounds a transformation towards the cubic spinel structure and significant changes towards a lamellar morphology are observed after annealing of the delithiated particles and after charge discharge cycling.