Co-written by PhD candidate in Materials Science and Chemical Engineering Mikaela Dunkin
Batteries for implantable medical devices must be small, reliable, safe, and long lasting. These characteristics are desirable for other battery applications as well. Thus, the insights gained from extended lifetime medical batteries are providing inspiration to design and improve other types of batteries for multiple types of uses.
Implantable cardiac pacemakers are powered by the lithium/iodine-polyvinylpyridine (PVP) battery system, which was first patented in 1972 and continues to be used to the present day due to its high energy density, safety, and reliability. As the cell discharges, the reaction product, LiI, grows. While this change provides a safe battery and allows the battery’s status to be determined while still inside a patient, it ultimately contributes to the decline of the primary battery.
Recent research has transformed the use of the lithium iodide layer into a secondary (rechargeable) battery through the addition of lithium iodide-(3-hydroxypropionitrile)2. We have expanded upon this concept to create a fully self-forming battery where the lithium anode and iodine cathode are generated during the first charge step. This mitigates the risk of self-discharge, allowing the inactivated battery to be stored over long time periods. Further work on decreasing the overall impedance, particularly due to interfacial effects, has been inspired by the formation of a liquid electrolyte phase when PVP reacts with lithium and iodine. By studying this battery system under the rigorous conditions needed by implantable medical devices, a variety of issues important for many different applications are addressed.
Since the 1980s, implantable cardiac defibrillators (ICDs) have been implemented and powered by the lithium/silver vanadium oxide (SVO) battery, a versatile system which is capable of providing a constant low current to monitor a heart and then producing current pulses in the 2 – 3 A range to shock the heart back into a proper rhythm. The Li/SVO system has remained in use over many years due to this capability and its overall reliability. Battery life can be limited by trace amounts of vanadium species dissolution into the electrolyte and subsequent deposition onto the lithium anode, thereby forming a passivation layer, increasing internal resistance and decreasing pulse power. Similar dissolution issues are seen in lithium/sulfur and aqueous battery systems. Understanding the underlying processes by which cathodic dissolution occurs, allows us to address the issue by intelligently mitigating, overcoming, or utilizing the materials differently. Li/SVO, and analogues such as silver vanadium phosphorous oxide (SVOP), have been examined in dissolution studies utilizing inductively coupled plasma—optical emission spectroscopy (ICP-OES) to better understand the kinetics at work. Quantifying the role of cathode dissolution, has provided insight into multiple systems in a variety of ways.
Other passivating reactions that are common within secondary batteries are the formation of a solid electrolyte interphase (SEI) layer that often forms at electrode surfaces due to the decomposition of electrolytes. This often occurs because liquid electrolytes are thermodynamically unstable at the working potential of the battery’s active materials. While the SEI can serve to protect the electrodes from further dissolution, it can also remove active materials from the battery and increase cell resistance. These mechanisms are difficult to study under a battery’s operating conditions in a sealed cell and the vital importance of unaltered reaction conditions, which are critical in specially designed cells. Isothermal microcalorimetry (IMC) can be utilized as an operando tool to probe the reactivity within a working battery by detecting the evolved heat from parasitic or decomposition reactions and correlating them with specific reactions within a cell. Indeed, it is often the parasitic reactions taking place inside a battery that limit its lifetime rather than consumption of the active materials.
Esther Takeuchi, pictured above, is the 2019 Sigma Xi Walston Chubb Award for Innovation recipient and will be a plenary speaker at the 2019 Annual Meeting and Student Research Conference on November 15 in Madison, Wisconsin. She is a SUNY Distinguished Professor and holds the William and Jane Knapp Chair in Energy and the Environment in the Departments of Materials Science and Chemical Engineering and Chemistry at Stony Brook University. She holds a joint appointment at Brookhaven National Laboratory as chief scientist in the Energy and Photon Sciences Directorate.