Gary Henriksen

An accelerated calendar and cycle life study of Li-ion cells

Abstract The accelerated calendar and cycle life of lithium-ion cells was studied. Useful cell life was strongly affected by temperature, time, state-of-charge (SOC) and change in state-of-charge (ΔSOC). In calendar life experiments, useful cell life was strongly affected by temperature and time. Temperature accelerated cell performance degradation. The rates of area specific impedance (ASI) increase and power fade followed simple laws based on a power of time and Arrhenius kinetics. The data have been modeled using these two concepts and the calculated data agree well with the experimental values. The calendar life ASI increase and power fade data follow (time) 1/2 kinetics. This behavior may be due to solid electrolyte interface layer growth. From the cycle life experiments, the ASI increase data follow (time) 1/2 kinetics also, but there is an apparent change in overall power fade mechanism when going from 3 to 6% ΔSOC. Here, the power of time drops to below 1/2, which indicates that the power fade mechanism is more complex than layer growth.

Factors responsible for impedance rise in high power lithium ion batteries

Abstract High-power, 18,650 lithium-ion cells have been designed and fabricated in order to understand the factors limiting the calendar life of the lithium-ion system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a blend of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1:1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elevated temperatures, the cells experienced a significant impedance rise and loss of power. The fade rate of power in these cells was dependent of the state of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfacial resistance at the positive electrode was the main reason behind the impedance rise in the high power cell.

Modeling thermal management of lithium-ion PNGV batteries

Batteries were designed with the aid of a computer modeling program to study the requirements of the thermal control system for meeting the goals set by the Partnership for a New Generation of Vehicles (PNGV). The battery designs were based upon the lithium-ion cell composition designated Gen-2 in the US Department of Energy Advanced Technology Development Program. The worst-case cooling requirement that would occur during prolonged aggressive driving was estimated to be 250 W or about 5 W per cell for a 48-cell battery. Rapid heating of the battery from a very low startup temperature is more difficult than cooling during driving. A dielectric transformer fluid is superior to air for both heating and cooling the battery. A dedicated refrigeration system for cooling the battery coolant would be helpful in maintaining low temperature during driving. The use of ample insulation would effectively slow the battery temperature rise when parking the vehicle in warm weather. Operating the battery at 10 °C during the first several years when the battery has excess power would extend the battery life.

Effect of cathode composition on capacity fade, impedance rise and power fade in high-power, lithium-ion cells ☆

We tested the effect of Al concentration on the performance of lithium-ion cells. One set of cells contained a LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} cathode and the other, LiNi{sub 0.8}Co{sub 0.10}Al{sub 0.10}O{sub 2}. The cells were calendar- and cycle-life tested at several temperatures, with periodic interruptions for reference performance tests that were used to gauge capacity and power fade as a function of time. The C{sub 1}/25 capacity fade in the cells displayed t{sup 1/2} dependence. The capacity fade of the 10% Al-doped cells tested at 45 {sup o}C was similar to that of the 5% Al-doped cells at 25 {sup o}C. The impedance rise and power fade were also sensitive to the Al concentration. For the one common temperature investigated (i.e., 45 {sup o}C), the 10% Al-doped cells displayed higher impedance rise and power fade than the 5% Al-doped cells. Additionally, the time dependence of the impedance rise displayed two distinct kinetic regimes; the initial portion depended on t{sup 1/2} and the final, on t. On the other hand, the 10% Al-doped cells depended on t{sup 1/2}2 only.

Design modeling of lithium-ion battery performance

A computer design modeling technique has been developed for lithium-ion batteries to assist in setting goals for cell components, assessing materials requirements, and evaluating thermal management strategies. In this study, the input data for the model included design criteria from Quallion, LLC for Gen-2 18650 cells, which were used to test the accuracy of the dimensional modeling. Performance measurements on these cells were done at the electrochemical analysis and diagnostics laboratory (EADL) at Argonne National Laboratory. The impedance and capacity related criteria were calculated from the EADL measurements. Five batteries were designed for which the number of windings around the cell core was increased for each succeeding battery to study the effect of this variable upon the dimensions, weight, and performance of the batteries. The lumped-parameter battery model values were calculated for these batteries from the laboratory results, with adjustments for the current collection resistance calculated for the individual batteries.

Nonaqueous electrolytes for wide-temperature-range operation of Li-ion cells

Abstract Nonaqueous electrolytes play a key role in extending the operating temperature range of Li-ion batteries. In developing electrolytes for wide temperature operations, we adopted an approach of starting with thermally stable lithium tetrafluoroborate (LiBF4) and lithium bis(oxalato)borate (LiB(C2O4)2, or LiBOB) salts. We have demonstrated that the capacity of Li-ion cells fades much slower in electrolytes using LiBF4 or LiBOB than in electrolytes using LiPF6. For low temperatures applications, suitable solvent systems for LiBF4 and LiBOB were explored. We found that the charge transfer resistance (Rct) is smaller in Li-ion cells in electrolytes based on LiBF4 in selected solvent systems than that based on LiPF6 and results in better capacity utilization at low temperatures. We also found that the electrolytes based on LiBOB in PC-based solvent system would allow Li-ion cells with graphite anode to be cycled. By comparing the properties of LiBF4 and LiPF6 in the propylene carbonate and diethyl carbonate (PC–DEC) solvent system, we found that it is possible to formulate proper solvent mixtures for enhanced conductivity for LiBF4 and LiBOB salts at low temperatures. It is concluded that nonaqueous electrolytes for wide-temperature-range operations of Li-ion cells are achievable.

High temperature lithium/sulfide batteries

Bipolar LiAl/FeS and LiAl/FeS2 batteries are being developed for electric vehicle (EV) applications by Argonne National Laboratory. Current technology employs a two-phase Li alloy negative electrode, low melting point LiCl—rich LiClLiBrKBr molten salt electrolyte, and either an FeS or an upper-plateau (UP) FeS2 positive electrode. These components are assembled in an “electrolyte-starved” bipolar cell configuration. Use of the two-phase Li alloy (α + β LiAl and Li5Al5Fe2) negative electrode provides in situ overcharge tolerance that renders the bipolar design viable. Employing LiCl rich LiClLiBrKBr electrolyte in “electrolyte-starved” cells achieves low-burdened cells that possess low area-specific impedance; comparable to that of flooded cells using LiClLiBrKBr eutectic electrolyte. The combination of dense U.P. FeS2 electrodes and low-melting electrolyte produces a stable and reversible couple, achieving over 1000 cycles in flooded cells, with high power capabilities. In addition, a family of stable chalcogenide ceramic/sealant materials was developed that produce high-strength bonds between a variety of metals and ceramics, which renders lithium/iron sulfide bipolar stacks practical. Bipolar LiAl/FeS and LiAl/FeS2 cells and four-cell stacks using these seals are being built and tested in the 13 cm diameter size for EV applications. To date, LiAl/FeS cells have achieved 240 W kg−1 power at 80% depth of discharge (DOD) and 130 Wh kg−1 energy at the 25 W kg−1 rate. LiAl/FeS2 cells have attained 400 W kg−1 power at 80% DOD and 180 Wh kg−1 energy at the 30 W kg−1 rate. When cell performance characteristics are used to model full-scale EV and hybrid vehicle (HV) batteries, they are projected to meet or exceed the performance requirements for a large variety of EV and HV applications.

Advanced Technology Development Program for Lithium-Ion Batteries: Gen 2 Performance Evaluation Final Report

The Advanced Technology Development Program has completed performance testing of the second generation of lithium-ion cells (i.e., Gen 2 cells). The 18650-size Gen 2 cells, with a baseline and variant chemistry, were distributed over a matrix consisting of three states-of-charge (SOCs) (60, 80, and 100% SOC), four temperatures (25, 35, 45, and 55°C), and three life tests (calendar-, cycle-, and accelerated-life). The calendar- and accelerated-life cells were clamped at an open-circuit voltage corresponding to the designated SOC and were subjected to a once-per-day pulse profile. The cycle-life cells were continuously pulsed using a profile that was centered around 60% SOC. Life testing was interrupted every four weeks for reference performance tests (RPTs), which were used to quantify changes in cell degradation as a function of aging. The RPTs generally consisted of C1/1 and C1/25 static capacity tests, a low-current hybrid pulse power characterization test, and electrochemical impedance spectroscopy. The rate of cell degradation generally increased with increasing test temperature, and SOC. It was also usually slowest for the calendar-life cells and fastest for the accelerated-life cells. Detailed capacity-, power-, and impedance-based performance results are reported.

Lithium-aluminum/iron sulfide batteries

High-temperature rechargeable batteries are under development for electric-vehicle propulsion and for stationary energy-storage applications. These cells utilize a molten salt electrolyte such as LiCl-KC1 eutectic (mp, 352°C), negative electrodes of either Li-Si or Li-Al alloy, and positive electrodes of either FeS or FeS2. Over the past seven years, several hundred engineering cells with different designs have been tested, some of which have achieved specific energies of >100 W-hr/kg and specific powers of >80 W/kg at 50% depth of discharge, other cells have operated for >1000 cycles. During 1979, Eagle-Picher Industries completed fabrication of the first full-scale (40 kW-hr) Li/MS battery, which consisted of two 20 kW-hr modules (60 cells each) housed in a thermally insulated case. Testing of this battery has been indefinitely delayed due to the unexpected short-circuit of one of the modules during heat up. The fabrication of other full-scale batteries is planned in the coming years.

Olivine electrode engineering impact on the electrochemical performance of lithium-ion batteries

High energy and power density lithium iron phosphate was studied for hybrid electric vehicle applications. This work addresses the effects of porosity in a composite electrode using a four-point probe resistivity analyzer, galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The four-point probe result indicates that the porosity of composite electrode affects the electronic conductivity significantly. This effect is also observed from the cells pulse current discharge performance. Compared to the direct current (dc) methods used, the EIS data are more sensitive to electrode porosity, especially for electrodes with low porosity values.