Examining the Performance of Implantable-Grade Lithium-Ion Cells after Overdischarge and Thermally Accelerated Aging

For implanted medical devices containing rechargeable batteries, maximizing battery lifetime is paramount as surgery is required for battery replacement. In non-life-sustaining applications (e.g., spinal cord stimulators or sacral nerve modulation), these implants may be left unused and unmaintained for extended periods, according to patient preference or in the case of unexpected life events. In this study, we examine the performance of two commercial lithium-ion cells intended for implantable neurostimulators (using lithium titanium oxide (LTO) and graphite as the negative electrode) when subjected to repeated deep overdischarge and to aging at a high state of charge (SOC). The graphite-based cells exhibited significant performance decline and swelling after overdischarge and became unable to store a charge after 42 days at 0 V. In contrast, the LTO-based cells exhibited minimal changes in performance even after 84 days (the length of the study) at 0 V. When subjected to an accelerated aging protocol at 100% SOC, the graphite-based cells were found to age more rapidly than the LTO cells, which exhibited minimal aging over the course of the study period. These results show that practical LTO-based lithium-ion cells are much more tolerant of abuse as a result of neglect and misuse and are worth considering for use in high-value applications where battery replacement is difficult or impossible.