Safety and capacity retention of lithium ion cells after long periods of storage

Abstract

This paper provides the results of simple preliminary tests on the storage characteristics of lithium ion cells in relation to their use for UPS or BPS. Commercial cylindrical 18 650 size cells with a discharge capacity of around 1200 mAh were used in the experiments. The cells consisted of an amorphous carbon anode, a LiCoO₂ cathode and an organic electrolyte. Cells were stored for 1–12 months and then their capacity was measured after constant voltage charging (similar to trickle charging) at 4.1 or 4.2 V and 21 or 60 °C. After measuring the capacity, the cells were crushed with a round 15 mm bar in diameter as an example of a fundamental abuse test. The residual cell capacity after 10 years of 4.2 V constant charging at 20 °C was predicted to be approximately 65%. This exceeds our tentative target of 50%. We also found that no cells smoked, ignited or exploded when crushed. We also measured the cell capacity after simple storage (i.e., after self-discharge).