Session D33: Physics of Batteries and Fuel Cells

Effect of Iron-based Impurities on the performance of nanostructured C-LiFePO$_{4}$ cathode materials for Li ion Batteries

We report synthesis of pure and C-LiFePO$_{4}$ nanoparticles in 20-30 nm size by sol-gel method. Three samples of C-LiFePO$_{4}$ were prepared by mixing 0.25M, 0.50M, and 1M lauric acid in the precursor solutions for carbon coating the particles. The samples were~characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), IR spectroscopy, SQUID magnetometery, Raman spectroscopy, and Fe$^{57}$ M\"{o}ssbauer spectroscopy. All the samples were thoroughly investigated for their electrochemical properties. The XRD measurements showed all the samples are single phase materials with no impurity phase. However, we identified at least three residual non crystalline impurity phases simultaneously using Fe$^{57 }$M\"{o}ssbauer spectroscopy, XPS, and the magnetic measurements. The elemental chemical states for Fe 2p, P 2p, and O 1s are analyzed using XPS for LiFePO$_{4}$ and compared with those of C-LiFePO$_{4}$ materials. SQUID magnetometery measurements suggest an antiferromagnetic transition $\sim $50 K in both pure LiFePO$_{4 }$and C-LiFePO$_{4}$ samples. The role of various phases, such as FeP, Fe$_{2}$P, $\alpha $-Fe and Fe$_{2}$O$_{3 }$identified and analyzed by Fe$^{57}$ M\"{o}ssbauer spectroscopy and XPS, will be discussed in relationship to the electrochemical properties of the cathode materials.   

Role of Ce and In doping in the performance of LiFePO$_{4}$ cathode material for Li ion Batteries

Recently, the olivine LiFePO$_{4}$ has attracted attention as a promising cathode material for Li ion batteries. However, its poor electronic conductivity is a major challenge for its industrial applications. Different approaches have been taken to address this problem. Here, we report a method of improving its conductivity by doping In and Ce ions at the Fe site. We prepared the samples by sol-gel method followed by annealing at 650 \r{ }C in Ar (95{\%}) +H2(5{\%}) atmosphere for 5 hrs. XRD and Raman spectroscopy confirm that the olivine structure remains unchanged upon doping with In and Ce up to 5 wt{\%}. XRD analysis shows the values of the lattice parameters increase with doping as the ionic radii of Ce and In ions are larger than that of the Fe$^{2+}$ ion. This observation also suggests that both Ce and In ions replace Fe ions and not the Li ions in the material. Upon doping, ionic conductivity was found to increase from 10$^{-9}$ to 10$^{-4}$ Ohm$^{-1}$cm$^{-1}$. Interestingly, Ce doped LiFePO$_{4}$ showed a higher conductivity than In doped LiFePO$_{4}$. SEM measurements show a bigger grain size of $\sim $300-500 nm in doped LiFePO$_{4}$ which decreased to $\sim $50 nm when the materials were synthesized using 0.25M lauric acid as a precursor. The electrochemical characteristics of the doped LiFePO4 along with conductivity and Raman data will be presented.   

Formation of small polarons in Li$_{2}$O$_{2}$ and implications for Li-air batteries

Lithium-air batteries (LABs) have recently been revitalized as a promising electrical energy storage system due to their exceptionally high theoretical energy density. However, its usage is limited by poor rate capability and large polarization in the cell voltage due primarily to the formation of Li$_{2}$O$_{2}$ in the air cathode. Here, using hybrid density functional theory, we found that the formation of small polarons in Li$_{2}$O$_{2}$ is the origin that limits the electron transport in Li$_{2}$O$_{2}$. Consequently, the low electron mobility contributes to the hysteresis in cell voltage and limits the power density of the LABs. We suggest that similar behavior should exist in other peroxides, and p-type doping in Li$_{2}$O$_{2}$ could significantly improve the performance of LABs at high current densities.   

The Li-induced Conversion Reaction of Ultra-Thin FeF2 Films

Iron (II) fluoride has recently gained interest as a possible cathode material in Li-ion conversion batteries. Conversion materials like this could potentially store 2-3 times more energy than conventional intercalation battery materials by utilizing the full range of charge states in their constituent transition metal ions, i.e., Fe$^{(2+)}$F$_{2}$ + 2Li$^{+}$ + 2e$^{-}$ $\rightarrow$ Fe$^{0}$ + 2LiF. Using surface science techniques, we are able to observe this reaction at the FeF$_{2}$-Li interface. We have grown 5nm films of high-purity polycrystalline FeF$_{2}$ in ultra-high vacuum and deposited atomic Li on the surface to simulate the conversion reaction in the absence of external contaminants. Using UV photoemission (UPS), x-ray photoemission (XPS), and inverse photoemission (IPE) spectroscopies, we have measured the composition and charge states of these materials. XPS of the FeF$_{2}$ sample after various Li exposures indicate a direct conversion from Fe$^{2+}$ to Fe$^{0}$, with no intermediary phases. The growth of a Fermi edge in UPS and IPE also indicates the formation of metallic Fe, while peaks characteristic of LiF can be seen in UPS after sufficient Li exposure. These results are consistent with those of recent TEM measurements on real electrochemical cells.   

Electrochemical Performance of Lithium Iron Phosphate Doped with Tungsten

Due to its high thermal stability, low cost and high theoretical charge capacity, LiFePO4 has emerged as one of the most promising cathode materials for large-scale lithium ion batteries. In this work, we systematically investigated the effect on structure and electrochemical properties brought by W doping on Fe site of LiFePO$_{4}$. LiFe$_{1-x}$W$_{x}$PO$_{4}$ (x= 0. 0.01, 0.02, 0.03) samples with and without carbon coating were prepared by using solid-state reaction. The phase and structure of as prepared powders were characterized by X-ray diffraction and scanning electron microscope. Cycling charge and discharge measurement at various C-rates and cyclic voltammetry were employed to reveal the electrochemical properties. Results showed that carbon coating dramatically improved the capacity at fast C-rate. 2 at.{\%} W doping was observed to have the highest charge capacity with 143 mAh/g at 0.1C and a 109 mAh/g for 1C.   

Model conversion reaction: Li reactivity on iron oxifluorides

Iron fluorides have gained interest as choice materials for conversion reaction-based batteries. Owing to their large band gaps and their ability to store up to three electrons per formula unit, batteries using these materials operate at high voltages and high energy densities. However the large band gap inhibits charge conduction and thus impedes efficient charging and discharging cycles. Two paths have been taken to overcome this limitation: 1) The use of nanoparticles embedded into a conducting carbon matrix improves both ionic and electronic conduction; and 2) The use of iron oxifluorides, which are characterized by a slightly reduced energy gap that facilitates electronic conduction. It is the latter point that that is central to this study as, curiously, relatively little is known about the electronic structure of iron oxifluorides and their interaction with Li, a key aspect of a storage cell's electrochemistry. Model conversion reactions have been studied by evaporating Li on FeF$_{2}$ and FeF$_{x}$O$_{y}$ samples. Using X-ray and UV photoemission as well as inverse photoemission spectroscopies, the occupied states and the unoccupied electronic states of the resulting samples have been probed. Transmission electron microscopy has been used in parallel to investigate phase analysis.   

Shape and volume changes in Lithiated Silicon anodes for batteries

Silicon is one of the most promising materials for use as an anode for Lithium-ion batteries due to its capacity to hold a large number of Li atoms (up to 4.4 Li per Si atom). One of the biggest challenges in using Silicon as anode material is mechanical failures due to large volume expansion during the lithiation process. Recently detailed experiments have been reported on the dependence of volume change on the crystal orientation, especially for nano-scale structures. We investigate the microscopic mechanisms for shape changes during lithiation of Si by comparing the reaction mechanisms of Li atoms on different surfaces of crystalline Si using first-principles calculations.   

Effect of carbon support on catalytic efficiency and durability in fuel cells

New nanomaterials already play a key role in several emerging technologies. For instance, in fuel cell technology, catalytic efficiency can be greatly enhanced due to the high surface area of nanomaterials. Improving the durability and efficiency of a platinum catalyst is an important step in increasing its utility when incorporated as the anode or cathode of a proton exchange membrane fuel cell (PEMFC). The authors have shown using Density Functional Theory methods [1] that doping the carbon support of the Pt catalyst can increase the durability and efficiency of the catalyst. This paper will present the effect of doping of the carbon support on the complete reaction path of the Oxygen Reduction Reaction using \textit{ab initio} structural methods as well as a complete \textit{ab initio} molecular dynamics characterization of the reaction. In addition, the electronic structure of the carbon support was shown to improve the metal/CO interaction for the development of a membrane to prevent catalyst poisoning [2]. The paper will also emphasize the effect of the solvent, which is experimentally shown to be crucial. [1] M. Groves, A. Chan, C. Malardier-Jugroot and M. Jugroot, \textit{Chem. Phys. Letters}, $481$(4-6), 214-219, 2009 [2] D. Durbin and C. Malardier-Jugroot, \textit{J. Phys. Chem. C}, $115$ (3), 808--815, 2011   

Probing the Reversibility Limit of Lithium Ion Transport in Solid Film Batteries

Time-resolved neutron depth profiling (TR-NDP) has been used to measure the lithium distribution in electrode layers of thin film batteries during charge/discharge cycles. TR-NDP data demonstrate quantitatively that ionic transport in electrodes follows the electric current in the external circuits under normal charge/discharge conditions whereas deviates upon sudden structural changes. The reversibility limit of ionic transport has been quantified to indicate the onset of battery failure.   

Energy Storage and Generation from Thermopower Waves

We have demonstrated through simulation and experiment that the nonlinear coupling between an exothermic chemical reaction in a fuel and a nanowire or nanotube with large axial heat conduction accelerates the thermal reaction wave along the nano-conduit. The thermal conduit rapidly transports energy to unreacted fuel regions, and the reaction wave induces a concomitant thermopower wave of high power density, producing electrical current in the same direction. At up to 14 W/g, this can be substantially larger than the power density offered by current micro-scale power sources (e.g. fuel cells, batteries) and even about seven times greater than that of commercial Li-ion batteries. MEMS devices and wireless sensor networks would benefit from such high power density sources to enable functions such as communications and acceleration hampered by present power sources.   

In-situ TEM observations of the nanoscale electrochemistry in a Li-ion cell

The lithiation-delithiation processes of anode materials in lithium ion batteries were observed by in-situ electron microscopy. The lithiation-delithiation was strongly materials, size, and orientation dependent. Upon charging of SnO$_{2}$ nanowires, we observed high density of dislocations in the reaction front, while in charging of ZnO nanowires, we observed discrete cracks in the reaction front. In charging Si nanowires, we found the volume expansion was highly anisotropic, resulting in a dumbbell-shaped cross-section and cracking, eventually splitting the single nanowire into sub-wires. Carbon coating not only increases rate performance but also alters the lithiation-induced strain of SnO$_{2}$ nanowires. The radial expansion of the coated nanowires was completely suppressed. The lithiation process of individual Si nanoparticles was strongly size-dependent, i.e., there exists a critical particle size with a diameter of $\sim $ 150 nm, below which the particles neither cracked nor fractured upon lithiation, above which the particles first formed cracks and then fractured.   

Gasoline-powered series hybrid cars cause lower life cycle carbon emissions than battery cars

Battery cars powered by grid electricity promise reduced life cycle green house gas (GHG) emissions from the automotive sector. Such scenarios usually point to the much higher emissions from conventional, internal combustion engine cars. However, today's commercially available series hybrid technology achieves the well known efficiency gains in electric drivetrains (regenerative breaking, lack of gearbox) even if the electricity is generated onboard, from conventional fuels. Here, we analyze life cycle GHG emissions for commercially available, state-of the-art plug-in battery cars (e.g. Nissan Leaf) and those of commercially available series hybrid cars (e.g., GM Volt, at same size and performance). Crucially, we find that series hybrid cars driven on (fossil) gasoline cause fewer emissions (126g CO2eq per km) than battery cars driven on current US grid electricity (142g CO2eq per km). We attribute this novel finding to the significant incremental emissions from plug-in battery cars due to losses during grid transmission and battery dis-/charging, and manufacturing larger batteries. We discuss crucial implications for strategic policy decisions towards a low carbon automotive sector as well as relative land intensity when powering cars by biofuel vs. bioelectricity.   

Design and Characterization of a Microcombustor for Thermophotovoltaic Devices

While batteries are currently used to provide power to devices in remote areas, their low energy density (.5MJ/kg) increases carrier weight, limiting the range of applications. Microcombustor systems, on the other hand, rely on hydrocarbon fuels which have a much greater energy density (40MJ/kg) and generate the same power at a fraction of the weight. The microcombustor when paired with a high-efficiency energy extracting device, such as a thermophotovoltaic cell, presents a complete micro-power system. This work describes the fabrication and characterization of a catalytic microcombustor designed specifically to optimize the power production of a thermophotovoltaic cell.   

Reverse electrowetting -- a new approach to high-power harvesting of mechanical energy

Over the last decade electrical batteries have emerged as a critical bottleneck in portable electronics development. High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries, but until now, its adoption has been hampered by the lack of an efficient mechanical-to-electrical energy conversion technology. In this talk a novel mechanical-to-electrical energy conversion method is discussed. The method is based on reverse electrowetting (REWOD) -- a novel microfluidic phenomenon. Electrical energy generation is achieved through the interaction of arrays of moving microscopic liquid droplets with novel nanometer-thick multilayer dielectric films. Advantages of this process include the production of high power densities, up to 1 KW sq. m; the ability to directly utilize a very broad range of mechanical forces and displacements; and the ability to directly output a broad range of currents and voltages, from several volts to tens of volts. We hope that the REWOD-based energy harvesting can provide a novel technology platform for a broad range of new electronic products and enable reduction of cost, pollution, and other problems associated with the wide-spread battery use.