Invited Presentation: 4.7 V Li-ion cells: Nonsense or Possibility

Abstract

One way to improve the energy density of Li-ion cells is to use high voltage positive electrode materials like LiNi0.5Mn1.5O4 [LNMO] or simply increase the upper cutoff potential when positive electrodes like Li[NixMnxCo1-2x]O2 [NMC] are used. This sounds simple, but there are numerous problems to overcome before high voltage Li-ion cells are a reality.Many literature reports of Li/LNMO and LTO/LNMO cells present very promising behavior. However, most of the reports show charge discharge cycling at high rates, where many cycles can be accumulated in a short period of time. In such experiments, authors are able to “beat the clock” on parasitic electrolyte oxidation reactions which occur in cells. We recently showed using storage experiments on LTO/LNMO cells that electrolyte oxidation, as evidenced by rapid self discharge, is a huge problem [1, 2] that is ignored in most literature reports.In this paper we discuss a number of the problems that occur in LTO/LNMO cells and high voltage graphite/NMC cells. These include:1. Impedance increase at the positive electrode as the potential increases due to parasitic reactions.2. Increases in parasitic heat as a function of cell voltage as measured using isothermal battery microcalorimetry3. Increases in gas evolution as a function of cell potential, even when fluorinated solvents are used.4. Decreases in coulombic efficiency as cell potentials increase.5. Etc.The results above paint a bleak picture about the future of Li-ion cells that can operate at 4.7 V. Next, partial solutions to the problems above will be presented:a) Electrolyte additives that can stabilize positive electrode impedance increases during high potential cycling will be discussed. b) Electrolyte additives that can dramatically reduce parasitic heats evolved at high potentials will be discussed. c) Materials that can dramatically lower electrolyte oxidation reactions at the surface of the positive electrode and are suitable as “shells” in core-shell materials will be discussed.d) Electrolyte additives that can reduce gas evolution at high potential will be discussed. e) Etc.These encouraging results provide hope that real solutions can be found.