Performance and cycle life test results of a PEVE first-generation prismatic nickel/metal-hydride battery pack

A first-generation, prismatic, nickel/metal-hydride battery pack from Panasonic EV Energy Company Ltd. (PEVE) was characterized following the standard PNGV test procedures and then cycle life tested at 25 °C. The pack met, or exceeded, PNGV power and energy goals at the beginning of life. After more than 500,000 cycles, the data for capacity and discharge pulse power capability showed no measurable fade; similarly, discharge pulse resistance at 60% DOD also showed no measurable change. After the same pack was tested with two size factors, it still met or exceeded the PNGV goals.     

Capacity fade of Sony 18650 cells cycled at elevated temperatures: Part I. Cycling performance

The capacity fade of Sony 18650 Li-ion cells increases with increase in temperature. After 800 cycles, the cells cycled at RT and 45 °C showed a capacity fade of 30 and 36%, respectively. The cell cycled at 55 °C showed a capacity loss of about 70% after 490 cycles. The rate capability of the cells continues to decrease with cycling. Impedance measurements showed an overall increase in the cell resistance with cycling and temperature. Impedance studies of the electrode materials showed an increased positive electrode resistance when compared to that of the negative electrode for cells cycled at RT and 45 °C. However, cells cycled at 50 and 55 °C exhibit higher negative electrode resistance. The increased capacity fade for the cells cycled at high temperatures can be explained by taking into account the repeated film formation over the surface of anode, which results in increased rate of lithium loss and also in a drastic increase in the negative electrode resistance with cycling.     

Cycle-life testing of 100-Ah class lithium-ion battery in a simulated geosynchronous-Earth-orbit satellite operation

In this paper, we review our work on cycle-life testing of a 100-Ah class lithium-ion battery in a simulated geosynchronous-Earth-orbit (GEO) satellite operation. The battery consists of ten 100-Ah lithium-ion (10) cells in a series, with a high energy density exceeding 100 Wh kg⁻¹ at the battery level. We simulate the eclipse period in real-time testing with five depth-of-discharge (DOD) patterns at an ambient temperature of 15 °C. We also simulate a sun-shine period in 8-day thermally accelerated full-charge storage at an ambient temperature of 25 °C, which in our experience corresponds to full-charge storage of a half-year operation at 0 °C. Eighteen eclipse seasons have presently been completed, corresponding to 9 years of GEO operation. The battery maintained a high voltage near 3.4 V at the end of the discharge, even when the DOD was set at 70%. The voltage dispersion of 10 cells was also sufficiently small in the range of 48 mV. The cell temperature reached a maximum of 29 °C and maintained minimal dispersion smaller than 4 °C even when the battery was discharged at a high DOD of 70%.     

Power and thermal characterization of a lithium-ion battery pack for hybrid-electric vehicles

A 1D electrochemical, lumped thermal model is used to explore pulse power limitations and thermal behavior of a 6 Ah, 72 cell, 276 V nominal Li-ion hybrid-electric vehicle (HEV) battery pack. Depleted/saturated active material Li surface concentrations in the negative/positive electrodes consistently cause end of high-rate (∼25 C) pulse discharge at the 2.7 V cell⁻¹ minimum limit, indicating solid-state diffusion is the limiting mechanism. The 3.9 V cell⁻¹ maximum limit, meant to protect the negative electrode from lithium deposition side reaction during charge, is overly conservative for high-rate (∼15 C) pulse charges initiated from states-of-charge (SOCs) less than 100%. Two-second maximum pulse charge rate from the 50% SOC initial condition can be increased by as much as 50% without risk of lithium deposition. Controlled to minimum/maximum voltage limits, the pack meets partnership for next generation vehicles (PNGV) power assist mode pulse power goals (at operating temperatures >16 °C), but falls short of the available energy goal.In a vehicle simulation, the pack generates heat at a 320 W rate on a US06 driving cycle at 25 °C, with more heat generated at lower temperatures. Less aggressive FUDS and HWFET cycles generate 6–12 times less heat. Contact resistance ohmic heating dominates all other mechanisms, followed by electrolyte phase ohmic heating. Reaction and electronic phase ohmic heats are negligible. A convective heat transfer coefficient of h = 10.1 W m⁻² K⁻¹ maintains cell temperature at or below the 52 °C PNGV operating limit under aggressive US06 driving.     

High-energy,high-power Pulses Plus™ battery for long-term applications

Pulses Plus™ batteries were developed at Tadiran and introduced into the market few years ago. These batteries combine a primary high-energy bobbin type Li/SOCl₂ cell with a hybrid layer capacitor (HLC). The HLC is a battery-like capacitor consisting of lithium intercalation compounds as electrodes with pseudo capacitance of 785 F for standard AA size.The Pulses Plus™ battery can deliver very high energy at very high-power pulses (above 15 A at RT). Under low temperature conditions, its power capabilities are also very good, as 2 A, 1 s pulses above 2.5 V at −40 °C can be obtained. The stability of performance after elevated temperature storage and its low self-discharge rate make this a battery to operate under high pulse power over 20 years.     

Experimental characterization of hybrid power systems under pulse current loads

Lithium-ion batteries, ultracapacitors, and parallel combinations of these devices were characterized with respect to their ability to meet the power demands of pulsed loads. Data are presented in the form of Ragone plots that relate the impact of current amplitude and pulse duty to the specific power and energy storage capacities. Adding a 50 F ultracapacitor in parallel with the battery exhibited up to a 20.3% increase in energy capacity as compared to a continuous discharge of the battery alone. The peak current capacity of the hybrid system was limited to 10 A, to prevent exceeding the maximum safe current of 2.4 A for the battery alone. The hybrid systems also suffered less voltage droop during the pulse ‘on’ time when compared to the battery alone. However, when considered on a per mass basis, the energy and power densities were lower for the hybrids than for the battery alone.     

Bipolar lead–acid battery for hybrid vehicles

Within the framework of the European project bipolar lead–acid power source (BILAPS), a new production route is being developed for the bipolar lead–acid battery. The performance targets are 500 W kg⁻¹, 30 Wh kg⁻¹ and 100 000 power-assist life cycles (PALCs). The operation voltage of the battery can be, according to the requirements, 12, 36 V or any other voltage. Tests with recently developed 4 and 12 V prototypes, each of 30 Ah capacity have demonstrated that the PALC can be operated using 10 C discharge and 9 C charge peaks. The tests show no overvoltage or undervoltage problems during three successive test periods of 16 h with 8 h rest in between. The temperature stabilizes during these tests at 40–45 °C using a thermal-management system. The bipolar lead acid battery is operated at an initial 50% state-of-charge. During the tests, the individual cell voltages display only very small differences. Tests are now in progress to improve further the battery-management system, which has been developed at the cell level, during the period no PALCs are run in order to improve the hybrid behaviour of the battery. The successful tests show the feasibility of operating the bipolar lead–acid battery in a hybrid mode. The costs of the system are estimated to be much lower than those for nickel–metal-hydride or Li-ion based high-power systems. An additional advantage of the lead–acid system is that recycling of lead–acid batteries is well established.     

Electrochemical properties of lithium manganese oxides with different surface areas for lithium ion batteries

Two lithium manganese oxides, Li_1.03Mn_1.96O₄, with different surface areas of 3.55 and 0.68 m²/g were prepared and their electrochemical properties were studied as positive electrodes for lithium ion batteries. Cycle performance tests gave capacity losses of 9 and 18% at 25 °C, and 28 and 33% at 55 °C for the samples with larger and smaller surface areas, respectively. The recovery of capacity losses have been found on the addition of the conductor after cycles. The defect of the conductivity between the active materials and the conductors was found mostly responsible for the capacity loss in the smaller surface sample at 25 °C. Cycle performance tests in each region of the charge states divided into five regions show that larger capacity losses are observed in rather lower potential states. Storage performances show the largest capacity loss at 10 and 20% of SOC and the less capacity losses at both 0 and 50–100% of SOC in both of the samples. However, in every region, the capacity loss is much smaller in the larger surface sample than in the smaller surface sample. The maximum Mn dissolution is observed to occur at 100% of SOC (4.5 V) and the next is found around 10–20% of SOC in either sample.     

Uneven temperature and voltage distributions due to rapid discharge rates and different boundary conditions for series-connected LiFePO4 batteries

This paper presents the surface temperature and voltage distributions on a prismatic lithium-ion battery pack at 1C, 2C, 3C, and 4C discharge rates and 5 °C, 15 °C, 25 °C, and 35 °C boundary conditions (BCs) for water cooling and ~ 22 °C for air cooling methods. It provides quantitative data regarding thermal behaviour of lithium-ion batteries for designing thermal management systems and developing reliable thermal models. In this regard, three large LiFePO₄ 20 Ah capacity, prismatic batteries are connected in series with four cold plates used between cells and eighteen thermocouples are placed at distributed locations on the principle surface of all three cells: the first six for the first cell, the second six for the second cell, and the third six for the third cell, and the average and peak surface temperatures as well as voltage distributions are measured and presented in this study. In addition, the simulated heat generation rate, temperature and voltage distributions are validated with an experimental data for the above mentioned C-rates and BCs. The present study shows that increasing discharge rates and BCs results in increase in the maximum and average surface temperatures at the three locations (near the anode, cathode, and mid surface of the body). The highest value of the average surface temperature is obtained for 4C and 35 °C BC (36.36 °C) and the lowest value is obtained for 1C and 5 °C BC (7.22 °C) for water cooling method.     

Development of true prismatic lithium-ion cells for high rate and low temperature applications

Lithium-ion cells are presently being considered for use in a wide range of aerospace applications. Cells for these aerospace applications, such as F-16 and JSF aircraft, are required to operate at rates up to 15 C and at temperatures from −40 to 71°C. To address these requirements, a series of experiments has been undertaken to empirically determine those factors that limit performance. The first experiment compares three different electrode weight loadings and two different anode particle sizes. A chemistry identified from this experiment was able to increase room temperature rate capability by >500%. Pulse discharge rates as high as 70 C and continuous discharge rates of 20 C were demonstrated. Furthermore, cell performance of 1 C at −40°C and 4 C at −30°C has been demonstrated.A second experiment evaluated the use of non-solid/electrolyte interface (SEI) forming conductive diluents in the anode. This experiment did not identify any advantages to the conductive diluent at temperatures above −20°C. However, at a discharge rate of 1 C at −40°C, the group with the highest level of diluent offers 300% more capacity than the baseline experimental group with no non-SEI forming diluent.     

Ionic liquid and plastic crystalline phases of pyrazolium imide salts as electrolytes for rechargeable lithium-ion batteries

A new member of the plastic crystal, pyrazolium imide family, N,N′-diethyl-3-methylpyrazolium bis-(trifluoromethanesulfonyl)imide (DEMPyr123) was prepared. It showed a single, plastic crystalline phase that extends from 4.2 °C to its melting at 11.3 °C. When 10 mol% LiTFSI salt was added, the mixture showed ionic conductivities reaching 1.7 × 10⁻³ S cm⁻¹ at 20 °C, in the liquid state and 6.9 × 10⁻⁴ S cm⁻¹ at 5 °C, in the solid, plastic phase. A wide electrochemical stability window's of 5.5 V was observed by cyclic voltammetry of the molten salt mixture. Batteries were assembled with LiFePO₄/Li₄Ti₅O₁₂ electrodes and the salt mixture as an electrolyte. They showed a charge/discharge efficiency of 93% and 87% in the liquid and the plastic phase, respectively. The capacity retention was very good in both states with 90% of the initial capacity still available after 40 cycles. In general, the batteries showed good rate capability and cycle life performance in the ionic liquid phase that were sustained when the electrolyte transformed to the plastic phase. Comparison of the battery results with those of a classic (non-plastic crystal) ionic liquid has proven the advantage of the dual state of matter character in this electrolyte.     

Solid-state Li/LiFePO4 polymer electrolyte batteries incorporating an ionic liquid cycled at 40 °C

The cycle behaviour and rate performance of solid-state Li/LiFePO₄ polymer electrolyte batteries incorporating the N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₃TFSI) room temperature ionic liquid (IL) into the P(EO)₂₀LiTFSI electrolyte and the cathode have been investigated at 40 °C. The ionic conductivity of the P(EO)₂₀LiTFSI + PYR₁₃TFSI polymer electrolyte was about 6 × 10⁻⁴ S cm⁻¹ at 40 °C for a PYR₁₃⁺/Li⁺ mole ratio of 1.73. Li/LiFePO₄ batteries retained about 86% of their initial discharge capacity (127 mAh g⁻¹) after 240 continuous cycles and showed excellent reversible cyclability with a capacity fade lower than 0.06% per cycle over about 500 cycles at various current densities. In addition, the Li/LiFePO₄ batteries exhibited some discharge capability at high currents up to 1.52 mA cm⁻² (2 C) at 40 °C which is very significant for a lithium metal-polymer electrolyte (solvent-free) battery systems. The addition of the IL to lithium metal-polymer electrolyte batteries has resulted in a very promising improvement in performance at moderate temperatures.     

Studies of the energy and power of current commercial prismatic and cylindrical Li-ion cells

We studied the specific energy, energy density, specific power, and power density of current commercial 18650 cylindrical and 103450 prismatic Li-ion cells. It was found that the specific energy, energy density, specific power, and power density have been increased dramatically since 1999. The highest specific energy obtained in this study is 193 Wh/kg, which is 90% more than that reported in 1999 and is only 5% lower than 200 Wh/kg, the long-term DOE goal [The International Energy Agency Implementing Agreement for Electric Vehicle Technologies and Programs, Annex V, Outlook Document, 1996–1997, p. 16.]. The cell energy density has also doubled since 1999 and is as much as about 70% more than 300 Wh/l, the long-term DOE goal. The cells studied here can deliver over 80% of their designed energy at the specific power 200 W/kg while the 18650 cell studied previously could only deliver 10% of their designed energy at the same specific power. Various kinds of the factors in the cell-specific energy and energy density were studied. It seems that the geometric difference can cause as much as a 9% difference in the specific energy and a 12% difference in the energy density between 18650 cylindrical and 103450 prismatic cells. Use of an aluminum can seems to lead to about a 16% improvement in the specific energy of 103450 cells compared with steel can. The decrease in the cell discharge voltage can cause as much as a 9% decrease in the cell energy at the 2 C rate while it has a relatively small effect on the cell energy or specific energy at the 0.2 C rate. Compared with what has been obtained at room temperature, there are 17–35% at −20 °C, 43–76% at −30 °C, and 78–100% decreases at −40 °C, respectively, in the cell discharge energy and specific energy depending on the cell manufacturer. The decrease in the cell average discharge voltage during the cycling test can contribute as much as a 6% decrease in the cell energy at the 1 C rate after 300 cycles, which is 21% of the total energy loss.     

Development and demonstration of a higher temperature PEM fuel cell stack

Research and development was conducted on a proton exchange membrane (PEM) fuel cell stack to demonstrate the capabilities of Ionomem Corporation's composite membrane to operate at 120 °C and ambient pressure for on-site electrical power generation with useful waste heat. The membrane was a composite of polytetrafluoroethylene (PTFE), Nafion^®, and phosphotungstic acid. Studies were first performed on the membrane, cathode catalyst layer, and gas diffusion layer to improve performance in 25 cm², subscale cells. This technology was then scaled-up to a commercial 300 cm² size and evaluated in multi-cell stacks. The resulting stack obtained a performance near that of the subscale cells, 0.60 V at 400 mA cm⁻² at near 120 °C and ambient pressure with hydrogen and air reactants containing water at 35% relative humidity. The water used for cooling the stack resulted in available waste heat at 116 °C. The performance of the stack was verified. This was the first successful test of a higher-temperature, PEM, fuel-cell stack that did not use phosphoric acid electrolyte.     

Comparison and improvement of the high rate performance of different types of LiMn2O4 spinels

The rate capacity especially the high rate discharge performance is another important aspect for the application of Mn-based spinel cathodes for EV/HEV power sources besides the cycling performance that is now intensively investigated. In this paper, spinel materials differing in chemical composition and thermal processing history were investigated by discharging at constant current rates from C/10 to 4 C at ambient temperature. It was found that the high-rate discharge capability of Mn-based spinels is very excellent if prepared at temperatures below 850 °C, no matter cation doping or not. In contrast, spinels synthesized over 950 °C showed much poorer high rate performance, and some kinds of impurities were proposed to be responsible for the deteriorated behavior. Annealing at lower temperatures was found to be useful for the significant improvement of the high rate discharge capability of Mn spinels.     

Solar hybrid systems with thermoelectric generators

The possibility of using of thermoelectric generators in solar hybrid systems has been investigated. Four systems were examined, one working without radiation concentration, of the traditional PV/Thermal geometry, but with TEGs between the solar cells and heat extractor, and three other using concentrators, namely: concentrator – TEG ? heat extractor, concentrator ? PV cell ? TEG ? heat extractor, and PV cell – concentrator – TEG – heat extractor. The TEGs based on traditional semiconductor material Bi₂Te₃ and designed for temperature interval of 50–200 °C were studied experimentally. It was found that the TEG’s efficiency has almost linear dependence on the temperature difference ΔT between its plates, reaching 4% at ΔT = 155 °C (hot plate at 200 °C) with 3 W of power generated over the matched load. The temperature dependencies of current and voltage are also linear; accordingly, the power generated has quadratic temperature dependence. The experimental parameters, as well as parameters of two advanced TEGs taken from the literature, were used for estimation of performance of the hybrid systems. The conclusions are drawn in relation to the efficiency at different modes of operation and the cost of hybrid systems, as well as some recommendations in relation to optimal solar cells for applications in these systems.     

Increasing the operation temperature of polymer electrolyte membranes for fuel cells: From nanocomposites to hybrids

Among the possible systems investigated for energy production with low environmental impact, polymeric electrolyte membrane fuel cells (PEMFCs) are very promising as electrochemical power sources for application in portable technology and electric vehicles. For practical applications, operating FCs at temperatures above 100 °C is desired, both for hydrogen and methanol fuelled cells. When hydrogen is used as fuel, an increase of the cell temperature produces enhanced CO tolerance, faster reaction kinetics, easier water management and reduced heat exchanger requirement. The use of methanol instead of hydrogen as a fuel for vehicles has several practical benefits such as easy transport and storage, but the slow oxidation kinetics of methanol needs operating direct methanol fuel cells (DMFCs) at intermediate temperatures. For this reason, new membranes are required. Our strategy to achieve the goal of operating at temperatures above 120 °C is to develop organic/inorganic hybrid membranes. The first approach was the use of nanocomposite class I hybrids where nanocrystalline ceramic oxides were added to Nafion. Nanocomposite membranes showed enhanced characteristics, hence allowing their operation up to 130 °C when the cell was fuelled with hydrogen and up to 145 °C in DMFCs, reaching power densities of 350 mW cm⁻². The second approach was to prepare Class II hybrids via the formation of covalent bonds between totally aromatic polymers and inorganic clusters. The properties of such covalent hybrids can be modulated by modifying the ratio between organic and inorganic groups and the nature of the chemical components allowing to reach high and stable conductivity values up to 6.4 × 10⁻² S cm⁻¹ at 120 °C.     

Electrochemical characteristics of needle coke refined by molten caustic leaching as an anode material for a lithium-ion battery

Needle coke, the remaining material after refining petroleum, is used as an anode of a lithium-ion secondary battery. Sulfur is separated from the needle coke to below 0.1 wt.% using the molten caustic leaching (MCL) method developed at the Korea Institute of Energy Research. The needle coke with high-purity is carbonized at various temperatures, namely 0, 500, 700 and 900 °C. The coke treated at 700 °C gives a first and second discharge capacity of more than 560 and 460 mAh g⁻¹, respectively, between 0 and 2.0 V. By contrast, the first and second discharge capacity of untreated coke is over 420 and 340 mAh g⁻¹, respectively, between 0.05 and 2.0 V.The first discharge capacity of 560 mAh g⁻¹ is beyond the theoretical maximum capacity of 372 mAh g⁻¹ for LiC₆. Though the cycle efficiency is not consistent, the needle coke heat-treated at 700 °C persistently maintains an efficiency of over 90% until the 50th cycle, except on the first cycle. This study demonstrates that the needle coke with high-purity could be a good candidate for an anode material in fabricating high-capacity lithium-ion secondary batteries.     

Synthesis of olivine-type LiFePO4 by emulsion-drying method

Olivine-type, orthorhombic, LiFePO₄ powders with small particle size have been successfully synthesized by the emulsion-drying method. The electronic and crystal structure is analyzed by X-ray absorption spectroscopy (XAS) and X-ray diffraction Rietveld refinement. The powder calcined at 750 °C shows the highest discharge capacity of 125 mAh g⁻¹ with excellent cycle stability. The discharge capacity of this powder increases to 154 mAh g⁻¹ on increasing the addition of carbon black as a conductive agent up to 40 wt.%. In a rate capability test, the discharge capacity is completely recovered and retained up to the 700th cycle.     

LiFePO4/carbon cathode materials prepared by ultrasonic spray pyrolysis

Small crystallites LiFePO₄ powder with conducting carbon coating can be synthesized by ultrasonic spray pyrolysis. Cheaper trivalent iron ion is used as the precursor. The pure olivine phase can be prepared with the duplex process of spray pyrolysis (synthesized at 450, 550 or 650 °C) and subsequent heat-treatment (at 650 °C for 4 h). The results indicate that the pyrolysis temperature of 450 °C is appropriate for best results. The carbon coating on the LiFePO₄ surface is critical to the electrochemical performance of LiFePO₄ cathode materials of the lithium secondary battery, since the carbon coating does not only increase the electronic conductivity via carbon on the surface of particles, but also enhance the ion mobility of lithium ion due to prohibiting the grain growth during post-heat-treatment. The carbon of 15 wt.% evenly distributed on the final LiFePO₄ powders can get the highest initial discharge capacity of 150 mA h g⁻¹ at C/10 and 50 °C.