Publications

The thermal stability of Li-ion cells with intercalating carbon anodes and metal oxide cathodes was measured as a function of state of charge and temperature for two advanced cell chemistries. Cells of the 18650 design with Li{sub x}CoO{sub 2} cathodes (commercial Sony cells) and Li{sub x}Ni{sub 0.8}Co{sub 0.2}O{sub 2} cathodes were measured for thermal reactivity. Accelerating rate calorimetry (ARC) was used to measure cell thermal runaway as a function of state of charge (SOC), microcalorimetry was used to measure the time dependence of thermal output, and differential scanning calorimetry (DSC) was used to study the thermal reactivity of the individual components. Thermal decomposition of the anode solid electrolyte interphase (SEI) layer occurred at low temperatures and contributes to the initiation of thermal runaway. Low temperature reactions from 40 C--70 C were observed during the ARC runs that were SOC dependent. These reactions measured in the microcalorimeter decayed over time with power-law dependence and were highly sensitive to SOC and temperature. ARC runs of aged and cycled cells showed complete absence of these low-temperature reactions but showed abrupt exothermic spikes between 105--135 C. These results suggest that during aging the anode SEI layer is decomposing from a metastable state to a stable composition that is breaking down at elevated temperatures.

Electrical characteristics of hybrid power sources consisting of Li-ion cells and double-layer capacitors were studied at 25 C and {minus}20 C. The cells were initially evaluated for pulse performance and then measured in hybrid modes of operation where they were coupled with the high-power capacitors. Cells manufactured by Panasonic measured at 25 C delivered full capacities of 0.76 Ah for pulses up to 3A and cells from A and T delivered full capacities of 0.73 Ah for pulses up to 4A. Measured cell resistances were 0.15 ohms and 0.12 ohms, respectively. These measurements were repeated at {minus}20 C. Direct coupling of the cells and capacitors (coupled hybrid) using 10F Panasonic capacitors in a 8F series/parallel combination extended the full capacity pulse limits (3.0V threshold) to 5.6A for the Panasonic cells and to 9A for the A and T cells. A similar arrangement using 100F capacitors from Elna in a 60F combination increased the Panasonic cell limit to 10 A. Operation in an uncoupled hybrid mode using uncoupled cell/capacitor discharge allowed fill cell capacity usage at 25 C up to the capacitor discharge limit and showed a factor of 5 improvement in delivered capacity at {minus}20 C.

Differential scanning calorimetry (DSC) analysis was used to identify thermal reactions in Sony-type lithium-ion cells and to correlate these reactions with interactions of cell constituents and reaction products. An electrochemical half-cell was used to cycle the anode and cathode materials and to set the state-of-charge (SOC). Three temperature regions of interaction were identified and associated with the SOC (degree of Li intercalation) of the cell. Anodes were shown to undergo exothermic reactions as low as 80 C involving decomposition of the solid electrolyte interphase (SEI) layer. The LiPF{sub 6} salt in the electrolyte (EC:PC:DEC/1M LiPF{sub 6}) was seen to play an essential role in this reaction. DSC analysis of the anodes from disassembled Sony cells showed similar behavior to the half-cell anodes with a strong exotherm beginning in the 80 C--90 C range. Exothermic reactions were also observed in the 200 C--300 C region between the intercalated lithium anodes, the LiPF{sub 6} salt, and the PVDF binder. These reactions were followed by a high-temperature reaction region, 300 C--400 C, also involving the PVDF binder and the intercalated lithium anodes. Cathode exothermic reactions with the PVDF binder were observed above 200 C and increased with the SOC (decreasing Li content in the cathode). No thermal reactions were seen at lower temperatures suggesting that thermal runaway reactions in this type of cell are initiated at the anode. An Accelerating Rate Calorimeter (ARC) was used to perform measurements of thermal runaway on commercial Sony Li-ion cells as a function of SOC. The cells showed sustained thermal output as low as 80 C in agreement with the DSC observations of anode materials but the heating rate was strongly dependent on the SOC.