Tien Q. Duong

A capacity and power fade study of Li-ion cells during life cycle testing

We tested three lithium-ion cells to evaluate capacity and power fade during cycle life testing of a hybrid electric vehicle (HEV) cell with varying state of charge (ΔSOC). Test results showed that the cells had sufficient power and energy capability to meet the Partnership for a New Generation of Vehicles (PNGV), now called FreedomCAR, goals for Power Assist at the beginning of life and after 120,000 life cycles using 48 cells. The initial static capacity tests showed that the capacity of the cells stabilized after three discharges at an average of 14.67 Ah. Capacity faded as expected over the course of 120,000 life cycles. However, capacity fade did not vary with ΔSOC. The hybrid pulse power characterization (HPPC) tests indicated that the cells were able to meet the power and energy goals at the beginning of testing and after 120,000 life cycles. The rate of power fade of the lithium-ion cells during cycle life testing increased with increasing ΔSOC. Capacity fade is believed to be due to lithium corrosion at the anode, and power fade suggested a buildup of the SEI layer or a decrepitation of the active material.

Lumped parameter modeling as a predictive tool for a battery status monitor

The Advanced Technology Development Program is currently evaluating the performance of the second generation of lithium-ion cells (i.e., Gen 2 cells). Both the Gen 2 Baseline and Variant C cells are tested in accordance with the cell-specific test plan, and are removed at roughly equal power fade increments and sent for destructive diagnostic analysis. The diagnostic laboratories did not need all test cells for analysis, and returned five spare cells to the Idaho National Engineering and Environmental Laboratory (INEEL). INEEL used these cells for special pulse testing at various duty cycles, amplitudes, and durations to investigate the usefulness of the lumped parameter model as a predictive tool in a battery status monitor. The lumped parameter model is a simplified linear model that accurately predicts the voltage response during certain pulse conditions. A database of parameter trends should enable dynamic predictions of state-of-charge and state-of-health conditions during in-vehicle pulsing. This information could be used by the battery status monitor to provide accurate information to the vehicle control system.

An overview of DOE's advanced technology development program

Through its FreedomCAR and Vehicle Technologies Office, the U.S. Department of Energy is conducting a R&D program at its national laboratories to support the commercial development of high-power lithium ion (Li-Ion) batteries for hybrid electric vehicle applications. It focuses on understanding the factors that limit the calendar life, abuse tolerance, and low-temperature performance of high-power Li-Ion cells and seeks to assist Li-Ion battery industrial developers by improving the performance of Li-Ion cells in these three areas, through the identification and development of lower-cost and more stable cell materials and components. This paper provides a brief summary of the organization, activities, and status of this R&D program.

U.S. DOE FreedomCAR and Vehicle Technologies Advanced Technology Development Program for Lithium-Ion Batteries: Gen 2 Performance Evaluation Interim Report

The Advanced Technology Development Program is currently evaluating the performance of the second generation of Lithium-ion cells (i.e., Gen 2 cells). The 18650-size Gen 2 cells consist of a baseline chemistry and one variant chemistry. These cells were distributed over a matrix consisting of three states-of-charge (SOC) (60, 80, and 100% SOC), four temperatures (25, 35, 45, and 55°C), and three life tests (calendar-, cycle-, and accelerated-life). The calendar-life cells are clamped at an opencircuit voltage corresponding to 60% SOC and undergo a once-per-day pulse profile. The cycle-life cells are continuously pulsed using a profile that is centered around 60% SOC. The accelerated-life cells are following the calendar-life test procedures, but using the cycle-life pulse profile. Life testing is interrupted every four weeks for reference performance tests (RPTs), which are used to quantify changes in capacity, resistance, and power. The RPTs consist of a C1/1 and C1/25 static capacity tests, a low-current hybrid pulse power characterization test, and electrochemical impedance spectroscopy at 60% SOC. Capacity-, power-, and electrochemical impedance spectroscopy-based performance results are reported.