Daniel H. Doughty

Accelerated calendar and pulse life analysis of lithium-ion cells

Abstract Sandia National Laboratories has been studying calendar and pulse discharge life of prototype high-power lithium-ion cells as part of the Advanced Technology Development (ATD) Program. One of the goals of ATD is to establish validated accelerated life test protocols for lithium-ion cells in the hybrid electric vehicle application. In order to accomplish this, aging experiments have been conducted on 18650-size cells containing a chemistry representative of these high-power designs. Loss of power and capacity are accompanied by increasing interfacial impedance at the cathode. These relationships are consistent within a given state-of-charge (SOC) over the range of storage temperatures and times. Inductive models have been used to construct detailed descriptions of the relationships between power fade and aging time and to relate power fade, capacity loss and impedance rise. These models can interpolate among the different experimental conditions and can also describe the error surface when fitting life prediction models to the data.

Lithium battery thermal models

Thermal characteristics and thermal behavior of lithium batteries are important both for the batteries meeting operating life requirements and for safety considerations. Sandia National Laboratories has a broad-based program that includes analysis, engineering and model development. We have determined thermal properties of lithium batteries using a variety of calorimetric methods for many years. We developed the capability to model temperature gradients and cooling rates of high-temperature primary lithium thermal batteries several years ago. Work is now under way to characterize the response of ambient-temperature rechargeable lithium-ion batteries to thermal abuse. Once the self-heating rates of lithium cells have been established over a range of temperatures, the thermal response can be estimated under a variety of conditions. We have extended this process to isolate the behavior of individual battery components and have begun to understand the chemical nature of the species responsible for heat evolution within the cells. This enhanced level of understanding will enable more accurate modeling of cell thermal behavior and will allow model-based design of safer, more abuse-tolerant lithium batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the future. Progress toward this goal and key information still needed to reach it are discussed.

Characterization of chemically prepared PZT thin films

We have systematically varied processing parameters to fabricate PZT 53/47 thin films. Polycrystalline PZT thin films were fabricated by spin depositing Pt coated SiO{sub 2}/Si substrates with alkoxide solutions. Our study focused on two process parameters: (1) heating rate and (2) excess Pb additions. We used rapid thermal processing techniques to vary heating rates from 3{degree}C/min to 8400{degree}C/min. Films were characterized with the following excess Pb additions: 0, 3, 5, and 10 mol %. For all process variations, films with greater perovskite content had better ferroelectric properties. Our best films were fabricated using the following process parameters: an excess Pb addition of 5 mol %, a heating rate of 8400{degree}C/min and annealing conditions of 700{degree}C for 1 min. Films fabricated using these process conditions had a remanent polarization of 0.27 C/m{sup 2} and a coercive field of 3.4 MV/m. 12 refs., 4 figs.

Electrochemical impedance spectroscopy studies of lithium diffusion in doped manganese oxide

Cathode performance is critical to lithium ion rechargeable battery performance; effects of doping lithium manganese oxide cathode materials on cathode performance are being investigated. In this paper, Li diffusion in Al-doped LiMn{sub 2}O{sub 4} was studied and found to be controlled by the quantity of Al dopant. Electrochemical cycling was conducted at 0.5mA/cm{sub 2}; electrochemical impedance spectra were taken at open circuit potential, with impedance being measured at 65 kHz-0.01 Hz. As the Al dopant level was increased, the Li diffusion rate decreased; this was attributed to the decreased lattice parameter of the doped oxide.

Nanocrystalline LiCo1−xNixO2 (0≤x≤0.3) for Li-ion batteries

Nanocrystalline LiCo 1-x Ni x O 2 (0≤x≤0.3)-a promising cathode material for rechargeable lithium batteries has been successfully prepared by a novel soft chemical route. Both the formation of the metal-glycine complex and subsequent decomposition of the same at low temperatures under carefully controlled oxygen flow play a critical role in the formation of nanocrystalline material. The thermal history of the as-prepared gel is established by differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirm the formation of layered α-NaFeO 2 structure at temperature as low as 330 °C. The exothermic combustion reaction of the organic precursors, which generates high temperature, should be avoided as it results in the spontaneous growth of large crystals. High-resolution transmission electron microscopy (HRTEM) investigation reveals that the particle size of LiCo 0.7 Ni 0.3 O 2 heated at 400 °C is in the range of 10- 15 nm. Substitution of nickel retards the crystal growth. Solid state 6 Li-Magic Angle Spinning (MAS) NMR investigation reveals that the micro-structural short range ordering of nickel ions in LiCo 1-x Ni x O 2 (0≤x≤0.3) is minimum at lower processing temperatures. Li-MAS NMR studies show that considerable amount of short range ordering of nickel ions is observed when the calcination temperature is raised beyond 800 °C indicating that the upper limit for processing temperature is around 750 °C. These materials were fabricated into thin electrodes using polyvinylidene fluoride (PVDF) as polymer binder and the electrochemical properties such as charge/discharge and impedance were evaluated. The electrodes cycled well with a coulombic efficiency of close to one.

Preparation and characterization of chemically derived (Pb,La)TiO/sub 3/ thin films

Ferroelectric lead lanthanum titanate (PLT) thin films with composition varying from pure PbTiO/sub 3/ to PLT 25/100 (0 to 25 mol.% La) were prepared by spin-casting 0.25M solutions containing metallo-organic precursors of Pb, La, and Ti. The dielectric and ferroelectric properties of the thin (410-nm) films were characterized. The dielectric constants of the films varied from approximately 80 to approximately 690 for La contents varying from 0 to 25 mol%, respectively. Dissipation factors varied from approximately 0.03 to approximately 0.09 over the same compositional range. The temperature dependence of the dielectric properties was also studied to determine the effects of La content on the Curie point (T/sub c/). As expected, T/sub c/ was found to decrease with increasing La concentration. Coercive field and remanent polarization also decreased with increased La concentration.<<ETX>>

An alternative lithium cathode material: Synthesis, characterization, and electrochemical analysis of Li{sub 8}(Ni{sub 5}Co{sub 2}Mn)O{sub 16}

The authors have previously reported a nonaqueous solution route to LiMn{sub 2}O{sub 4} and LiCoO{sub 2} materials that have demonstrated acceptable characteristics for use as lithium battery cathode materials. Due to the flexibility of processes, complex formulations of cathode materials can be easily generated and studied. Utilizing this flexibility, the authors have investigated a complex lithium transition metal oxide material, Li{sub 8}(Ni{sub 5}Co{sub 2}Mn)O{sub 16}, for use as an alternative cathode material. This material was analyzed by X-ray diffraction and scanning electron microscopy techniques which indicated it was phase pure. Electrochemical investigations revealed a high capacity ({approximately}150 mAh/g) with a fade rate of {approximately}0.41 mAh/g/cycle (0.27%/cycle).

Advanced technology development program for lithium-ion batteries : thermal abuse performance of 18650 Li-ion cells.

Li-ion cells are being developed for high-power applications in hybrid electric vehicles currently being designed for the FreedomCAR (Freedom Cooperative Automotive Research) program. These cells offer superior performance in terms of power and energy density over current cell chemistries. Cells using this chemistry are the basis of battery systems for both gasoline and fuel cell based hybrids. However, the safety of these cells needs to be understood and improved for eventual widespread commercial application in hybrid electric vehicles. The thermal behavior of commercial and prototype cells has been measured under varying conditions of cell composition, age and state-of-charge (SOC). The thermal runaway behavior of full cells has been measured along with the thermal properties of the cell components. We have also measured gas generation and gas composition over the temperature range corresponding to the thermal runaway regime. These studies have allowed characterization of cell thermal abuse tolerance and an understanding of the mechanisms that result in cell thermal runaway.

Electrical and electrochemical performance characteristics of large capacity lithium-ion cells

Abstract We are currently evaluating large capacity (20–40 Ah) Bluestar (cylindrical) and Yardney (prismatic) lithium-ion cells for their electrical and electrochemical performance characteristics at different temperatures. The cell resistances were nearly constant from room temperature down to −20°C, but increased by over 10 times at −40°C. The specific energies and powers, as well as the energy densities and power densities are high and did not reach a plateau even at the highest discharge rates tested. For example, the prismatic lithium-ion cells gave close to 280 Wh dm−3 from a 4 A discharge and 249 Wh dm−3 at 20 A, both at room temperature. For the same current range the specific energy values were 102 Wh kg−1 and 91 Wh kg−1. Cycle life and other electrical and electrochemical properties of the cells will be presented.

Ionic modeling of lithium manganese spinel materials for use in rechargeable batteries

In order to understand and evaluate materials for use in Li ion rechargeable battery electrodes, we have modeled the crystal structures of various Mn oxide and Li Mn oxide compounds. We have modeled the MnO{sub 2} polymorphs and several spinels with intermediate compositions based on the amount of Li inserted into the tetrahedral site. 3-D representations of the structures provide a basis for identifying site occupancies, coordinations, Mn valence, order-disorder, and potentially new dopants for enhanced cathode behavior. XRD simulations of the crystal structures provide good agreement with observed patterns for synthesized samples. Ionic modeling of these materials consists of an energy minimization approach using Coulombic, repulsive, and van der Waals interactions. Modeling using electronic polarizabilities (shell model) allows a systematic analysis of changes in lattice energy, cell volume, and the relative stability of doped structures using ions such as Al, Ti, Ni, and Co.