Marca M. Doeff

Effect of surface carbon structure on the electrochemical performance of LiFePO{sub 4}

The electrochemical performance of LiFePO 4 samples synthesized by sol-gel or solid-state routes varies considerably, although their physical characteristics are similar. Raman microprobe spectroscopic analysis indicated that the structure of the residual carbon present on the surfaces of the powders differs significantly and accounts for the performance variation. Higher utilization is associated with a larger ratio of sp 2 -coordinated carbon, which exhibits better electronic properties than disordered or sp 3 -coordinated carbonaceous materials. Incorporation of naphthalenetetracarboxylic dianhydride during synthesis results in a more graphitic carbon coating and improves utilization of LiFePO 4 in lithium cells, although the total carbon content is not necessarily higher than that of samples prepared without the additive. This result suggests that practical energy density need not be sacrificed for power density, provided that carbon coatings are optimized by carefully choosing additives.

Electrochemical performance of Sol-Gel synthesized LiFePO{sub 4} in lithium batteries

Electrochemical Performance of Sol-Gel Synthesized LiFePO 4 in Lithium Batteries Yaoqin Hu,* Marca M. Doeff,* Robert Kostecki, † and Rita Finones* *Materials Sciences Division and Environmental Energy Technologies Division Lawrence Berkeley National Laboratory University of California Berkeley, CA 94720, USA This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Y. H. would like to thank the Department of Chinese Education for financial support. Some of this work was previously presented as Abstract 121 at the 202nd Meeting of the Electrochemical Society, Salt Lake City, UT October 2002.

Factors Influencing the Quality of Carbon Coatings on LiFePO4

Several LiFePO4/C composites were prepared and characterized electrochemically in lithium half-cells. Pressed pellet conductivities correlated well with the electrochemical performance in lithium half-cells. It was found that carbon structural factors such as sp2/sp3, D/G, and H/C ratios, as determined by Raman spectroscopy and elemental analysis, influenced the conductivity and rate behavior strongly. The structure of the residual carbon could be manipulated through the use of additives during LiFePO4 synthesis. Increasing the pyromellitic acid (PA) content in the precursor mix prior to calcination resulted in a significant lowering of the D/G ratio and a concomitant rise in the sp2/sp3 ratio of the carbon. Addition of both iron nitrate and PA resulted in higher sp2/sp3 ratios without further lowering the D/G ratios, or increasing carbon contents. The best electrochemical results were obtained for LiFePO4 processed with both ferrocene and PA. The improvement is attributed to better decomposition of the carbon sources, as evidenced by lower H/C ratios, a slight increase of the carbon content (still below 2 wt. percent), and more homogeneous coverage. A discussion of the influence of carbon content vs. structural factors on the composite conductivities and, by inference, the electrochemical performance, is included.

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.

A High-Rate Manganese Oxide for Rechargeable Lithium Battery Applications

Li x MnO 2 made by ion exchange of glycine-nitrate combustion synthesis-processed (GNP) orthorhombic Na 0.44 MnO 2 (GNP-Li x MnO 2 ) has been cycled in lithium/liquid electrolyte cell configurations at room temperature and lithium/polymer cell configurations at 85°C over one hundred times without showing capacity fading or phase conversion to spinel. At 2.5 mA/cm 2 in liquid cells (5C rate) or I mA/cm 2 (1.5C rate) in polymer cells, 80-95% of the expected capacity is delivered. The remarkable stability is attributable to the unusual double tunnel structure, which cannot easily undergo rearrangement to spinel. The enhanced rate capability of GNP-Li x MnO 2 compared to conventionally prepared materials is attributable to the shorter particle length, which allows faster diffusion of lithium ions along the tunnels.

7Li and 31 P Magic Angle Spinning Nuclear Magnetic Resonance of LiFePO4-Type Materials

LiFePO 4 and LiMnPO 4 have been characterized using 7 Li and 3 1 P magic angle spinning (MAS) nuclear magnetic resonance spectroscopy. LiFePO 4 was synthesized by a hydrothermal route and LiMnPO 4 was synthesized at high temperature in an inert atmosphere. Both compositions give rise to single isotropic 7 Li resonances. The MAS isotropic peak linewidth for LiFePO 4 is considerably larger than that for LiMnPO 4 , suggesting the presence of local disorder in the Li coordination sphere for LiFePO 4 . In both samples, the isotropic peak is accompanied by a large, asymmetric spinning sideband manifold, arising from bulk magnetic susceptibility broadening and the paramagnetic interaction between the lithium nucleus and transition metal unpaired electrons.

Compatibility of Li x Ti y Mn1 − y O2 ( y = 0 , 0.11 ) Electrode Materials with Pyrrolidinium-Based Ionic Liquid Electrolyte Systems

The possibility of using electrolyte systems based on room-temperature ionic liquids (RTILs) in lithium-battery configurations is discussed. The nonflammability and wide potential windows of RTIL-based systems are attractive potential advantages, which may ultimately lead to the development of safer, higher energy density devices than those that are currently available. An evaluation of the compatibility of these electrolyte systems with candidate electrodes is critical for further progress. A comparison of the electrochemical behavior of Li/RTIL/Li x MnO 2 and Li x Ti 0.11 Mn 0.89 O 2 cells with those containing conventional carbonate solutions is presented and discussed in terms of the physical properties of two RTIL systems and their interactions with the cathodes. Strategies to improve performance and minimize cathode dissolution are presented.

Transport properties of a high molecular weight poly(propylene oxide)-LiCF{sub 3}SO{sub 3} system

Conductivities ({sigma}), salt diffusion coefficients (D{sub s}), and cationic transference numbers (t{sub +}{sup 0}) are reported for a high molecular weight polypropylene oxide (Parel{trademark})-LiCF{sub 3}SO{sub 3} polymer electrolyte system at 85 C. Transference numbers were determined as a function of salt concentration using a recently described electrochemical method based on concentrated solution theory. For the Parel-LiCF{sub 3}SO{sub 3} system, t{sub +}{sup 0} is slightly positive for electrolytes with O:Li ratios of 15 or 12:1 but decreases to negative values for more concentrated solutions. This implies that negatively charged ionic aggregates such as triplets are more mobile than free cations in this concentration range. Such behavior is commonly seen in binary salt/polymer electrolytes, which typically exhibit a high degree of nonideality. The nonunity transference numbers and microphase separation in the Parel-LiCF{sub 3}SO{sub 3} system strongly suggest that salt precipitation or phase separation in operating cells containing these electrolytes due to the development of large concentration gradients during passage of current.

Structure and Electrochemistry of LiNi1/3Co1/3-yMyMn1/3O2 (M=Ti, Al, Fe) Positive Electrode Materials

A series of materials based on the LiNi1/3Co1/3-yMyMn1/3O2 (M = Ti,Al,Fe) system has been synthesized and examined structurally and electrochemically. It is found that the changes in electrochemical performance depend highly on the nature of the substituting atom and its effect on the crystal structure. Substitution with small amounts of Ti4+ (y = 1/12) leads to the formation of a high-capacity and high-rate positive electrode material. Iron substituted materials suffer from an increased antisite defect concentration and exhibit lower capacities and poor rate capabilities. Single-phase materials are found for LiNi1/3Co1/3-yAlyMn1/3O2 when y<_ 1/4 and all exhibit decreased capacities when cycled to 4.3 V. However, an increase in rate performance and cycle stability upon aluminum substitution is correlated with an improved lamellar structure.

Effect of Electrolyte Composition on the Performance of Sodium/Polymer Cells

The dependence on Na/P(EO){sub n}NaX/Na{sub x}MnO{sub 2} (P(EO) = poly(ethylene oxide), X = CF{sub 3}SO{sub 3} or (CF{sub 3}SO{sub 2}){sub 2}N) cell cycle life and rate capability on polymer electrolyte composition is described. Transition time experiments and mathematical modeling indicate that failure due to salt precipitation occurs at it{sup 1/2} = 10.5 to 21.4 mA s{sup 0.5}/cm{sup 2}, when high initial concentrations of NaCF{sub 3}SO{sub 3} are used in operating cells. Evidence for large ionic clusters in concentrated PEO/NaCF{sub 3}SO{sub 3} solutions is also seen in the Raman spectroscopic data. Salt precipitation is a direct consequence of the concentration gradients that arise during operation, due to the negative cationic transference numbers (t{sub +}{sup 0}) of the binary salt/polymer electrolyte. By decreasing the initial salt concentration, t{sub +}{sup 0} is increased, cell rate capability is doubled, and the cycle life is enhanced nearly threefold. Similar improvements are obtained when PEO/NaN(CF{sub 3}SO{sub 2}){sub 2} electrolytes are used.