(Revised 7 November 2005; published 10 February 2006) A two-dimensional thermal model is developed to establish a standard for the simulation of spirally wound cells. It properly deals with the geometric characteristics and the boundary conditions to avoid the distorted simulation results due to improper approximation of the spiral geometry. Furthermore, the flexible architecture makes it possible that the precision of the numerical solutions can be elevated by spending more time on calculation. According to the simulation of lithium batteries under natural convection, the hottest temperatures are in a circular region near the liquid-filled hollow core but not at the exact center. Furthermore, radiation contributes as much as 53.6% to the heat dissipation if the surface emissivity approaches unity. Adopting an air flow parallel to the cylinder axis is effective to suppress the surface temperature, but the hottest temperature inside a battery remains high if a battery has a large radius. The heat dissipation rate of an air flow perpendicular to the cylinder axis is slightly lower than that of a parallel flow, and a battery case with high thermal conductivity is suggested to maintain the temperature uniformity of a battery.

(Revised 12 October 2005; published 30 December 2005) A uniform ZrO2 coating on LiCoO2 cathode materials for rechargeable lithium batteries was applied by a spray coating technique. The cells showed improved cycle performance and better durability of storing the cell (calendar life) under a high-voltage charging condition (4.2 V-313 K). X-ray diffraction and calorimetric study revealed that no marked change was observed in the bulk properties, such as crystal structure and phase transition, in the cathode during charge and discharge. The suppression of the increase of cathode/electrolyte interfacial impedance was observed by ZrO2 coating. Thus, the improved electrochemical performance in the higher voltage region (>4.2��V) is ascribed to the stabilization of the interface between the cathode and electrolyte materials.

Two- and three-electrode impedance measurements were made on 18650 Li-ion cells at different temperatures ranging from 35[deg]C to -40[deg]C. The ohmic resistance of the cell is nearly constant in the temperature range studied although the total cell impedance increases by an order of magnitude in the same temperature range. In contrast to what is commonly believed, we show from our three-electrode impedance results that, the increase in cell impedance comes mostly from the cathode and not from the anode. Further, the anode and cathode contribute to both the impedance loops (in the NyQuist plot).

All-solid-state thin film micro-batteries comprised of a lithium anode, lithium phosphorus oxy-nitride (LiPON) solid electrolyte and LixCoO2 cathode were evaluated at different temperatures from -50 to 80 [deg]C for electrical behavior and impedance raise. The cell dimensions were 2 cm long, 1.5 cm wide and 15 [mu]m thick. The rated capacity of the cells was about 400 [mu]Ah. The cells were cycled (charge/discharge) at room temperature over 100 times at a 0.25C rate. The charge and discharge cut-off voltages were 4.2 and 3.0 V, respectively. The cells did not show any capacity decay over 100 cycles. The measured capacity was 400 [mu]Ah. The coulombic efficiency was 1, which suggests that the cell reaction is free from any parasitic side reactions and the lithium intercalation and de-intercalation reaction is completely and totally reversible. These cells also have good high-rate performance at room temperature. For example, these cells discharged at a 2.5C rate delivered 90% of the capacity at a 0.25C rate. However, the delivered capacities even at a 0.25C rate at 80 and -50 [deg]C were much lower than the room temperature capacity. Cells soaked at -50 [deg]C were not damaged permanently as seen by the near normal behavior when returned to room temperature. However, cells heated to 80 [deg]C were permanently damaged as seen by the lack of normal performance back at room temperature. Cell impedance was measured before and after cycling at different temperatures. The high-frequency resistance (generally ascribed to the electrolyte and other resistances in series with the electrolyte resistance) decreased with decreasing temperature. However, the interfacial resistance increased significantly with decreasing temperature. Further, the electrolyte resistance accounted for 2% of the total cell resistance. The cycled cells showed higher impedance than the uncycled cells.

Battery life is an important, yet technically challenging, issue for battery development and application. Adequately estimating battery life requires a significant amount of testing and modeling effort to validate the results. Integrated battery testing and modeling is quite feasible today to simulate battery performance, and therefore applicable to predict its life. A relatively simple equivalent-circuit model (ECM) is used in this work to show that such an integrated approach can actually lead to a high-fidelity simulation of a lithium-ion cell?s performance and life. The methodology to model the cell?s capacity fade during thermal aging is described to illustrate its applicability to battery calendar life prediction.

The seminal research by Wright et al. on polyethylene oxide (PEO) solid polymer electrolyte (SPE) generated intense interest in all solid-state rechargeable lithium batteries. Following this a number of researchers have studied the physical, electrical and transport properties of thin film PEO electrolyte containing Li salt. These studies have clearly identified the limitations of the PEO electrolyte. Chief among the limitations are a low cation transport number (t+), high crystallinity and segmental motion of the polymer chain, which carries the cation through the bulk electrolyte. While low t+ leads to cell polarization and increase in cell resistance high Tg reduces conductivity at and around room temperatures. For example, the conductivity of PEO electrolyte containing lithium salt is -7 S cm-1 at room temperature. Although modified PEO electrolytes with lower Tg exhibited higher conductivity ( 10-5 S cm-1 at RT) the t+ is still very low 0.25 for lithium ion. Numerous other attempts to improving t+ have met with limited success. The latest approach involves integrating nano domains of inorganic moieties, such as silcate, alumosilicate, etc. within the polymer component. This approach yields an inorganic-organic component (OIC) based polymer electrolyte with higher conductivity and t+ for Li+. This paper describes the improved electrical and electrochemical properties of OIC-based polymer electrolyte and cells containing Li anode with either a TiS2 cathode or Mag-10 carbon electrode. Several solid polymer electrolytes derived from silicate OIC and salt-in-polymer constituent based on Li triflate (LiTf) and PEO are studied. A typical composition of the SPE investigated in this work consists of 600 kDa PEO, lithium triflate (LiTf, LiSO3CF3) and 55% of silicate based on (3-glycidoxypropyl)trimethoxysilane and tetramethoxysilane at molar ratio 4:1 and 0.65 mol% of aluminum(tri-sec-butoxide) (GTMOS-Al1-900k-55%). Several pouch cells consisting of Li/OIC-based-SPE/cathode containing OIC-based-SPE-LiTf binder were fabricated and tested, these cells are called modified cells. The charge/discharge and impedance characteristics of the new cells (also called modified cells) are compared with that of the pouch cells containing the conventional PEO-LiTf electrolyte as the cathode binder, these cells are called non-modified cells. The new cells can be charged and discharged at 70 [deg]C at higher currents. However, the old cells can be charged and discharged only at 80 [deg]C or above and at lower currents. The cell impedance for the new cells is much lower than that for the old cells.

A statistically designed accelerated aging experiment was conducted to investigate the effects of aging time, temperature, and state-of-charge (SOC) on the performance of lithium-ion cells. In this experiment, a number of cells were stored in a variety of static aging environments ranging from 25 [deg]C and 60% SOC to 55 [deg]C and 100% SOC. The power output of each cell was monitored regularly over the course of 44 weeks via a low current level hybrid pulse power characterization test. A single empirical model of power fade, involving two concurrent degradation processes, was found to be applicable over a wide range of experimental conditions. The first degradation process is relatively rapid (nearly complete within 4 weeks) and is accelerated by temperature with unknown kinetics. The second degradation process (accelerated by temperature and SOC) is less rapid and exhibits time3/2 kinetics.

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.