Kenza Maher

Thermodynamics of Lithium-Ion Batteries

Abstract Li-ion batteries (LiB) of coin-cell form factor (2032) were subjected to three different accelerate aging modes; i. overcharge ageing up to 4.9 V end of charge cut-off voltages (COV), ii. thermal aging at 60 °C and 70 °C for a period of up to 8 weeks after being charged to 4.2 V at ambient temperatures , and iii. long cycling at a C/2 rate at ambient temperature for up to 1000 cycles. We found the entropy (ΔS) and enthalpy (ΔH) profiles varied with aging conditions much more readily than open-circuit potential (OCP) profiles. We also found that aged batteries keep a thermodynamic fingerprint of the process by which they aged. This memory effect can be revealed by measuring the entropy and enthalpy of aged cells at two particular OCP of 3.87 V and OCP = 3.94 V, which are specific to LiB based on graphite anode and lithiated cobalt oxide (LCO) cathode material. In fact 3.87 V and 3.94 V are potentials of phase transition in the anode and cathode materials, respectively. Another aspect of the thermodynamics memory effect is a characteristic change of entropy and enthalpy profiles of cells having incurred the same capacity loss under different aging modes as compared to almost identical OCP and discharge profiles.

Quality Decision for Overcharged Li-Ion Battery from Reliability and Safety Perspective

During charging of Lithium-ion battery (LiB), the charging cutoff voltage (COV) may exceed the manufacturers’ specification because of incorrect monitoring of the charging control circuit, either due to the aging of the control circuit or the design/manufacturing errors of the control circuit. In fact, it is found that overcharging LiB cell is a common abuse. This work shows the effect of excessive COV on cell’s discharging ability, and the use of a novel nondestructive method to evaluate if the damage made in the cell by the excessive COV is rendering the cell from further safe usage or it is still acceptable with minor degradation in reliability and safety, thus providing a basis for quality consideration of the cell. The method also enables battery manufacturers to identify the internal components for their cells that are most vulnerable to the excessive COV so that quality improvement of their batteries can be designed and produced. This method also alerts electric vehicles user on the hidden safety issues of their battery pack, and enables battery management system to perform reliability balancing, a new patented technique to ensure the safe and reliable operation of battery pack.