Localized high-concentration electrolytes for the simultaneous stabilization of lithium-metal anodes and high-nickel cathodes
There are several routes available for improving the energy density of lithium-based batteries. One possibility is to increase the nickel content of the layered-oxide cathodes, LiNi [subscript x] Mn [subscript y] Co [subscript 1-x-y] O₂. However, higher nickel contents lead to multiple complications, including reduced cycle life due to surface reactivity and bulk instability, and inferior safety due to more facile heat and gas release. Replacing the graphite anode with a pure lithium-metal anode can also provide a substantial boost in energy density. However, lithium-metal anodes also suffer from high reactivity, poor cycle life, and serious safety hazards. Both electrodes can thus benefit from electrolytes with greater stability and safety. This dissertation explores a new class of electrolyte which can simultaneously stabilize both the high-nickel cathode and the lithium-metal anode. First, the performance of a baseline electrolyte is characterized with a high-nickel cathode and a lithium-metal anode. In particular, anode-to-cathode and cathode-to-anode crossovers are examined. It is found that the lithium-metal anode is not substantially affected by transition-metal ions from the cathode, unlike a typical graphite anode. Conversely, the cathode is negatively affected by salt decomposition species originating at the lithium-metal anode. Second, a new class of electrolyte is introduced, a localized high-concentration electrolyte (LHCE). It is demonstrated that this electrolyte confers a 5x improvement in the stability of a high-nickel cathode compared to the baseline electrolyte, due to the fluorine-rich interface. Improved thermal and storage stabilities are also demonstrated. Third, crossover effects in the new electrolyte are explored. It is found that salt decomposition and crossover are even more substantial than in the baseline electrolyte. The crossover causes cathodes paired with lithium metal to lose capacity three times faster than cathodes paired with graphite. However, the lithium-metal anode paired with the cathode loses active lithium three times slower than when paired with another lithium-metal electrode. Finally, operando gas generation in the LHCE is measured and compared to that in the baseline electrolyte. While the LHCE has only half the gas generation with a 4.4 V upper cutoff voltage, it becomes sensitive to higher voltages. In both electrolytes, the cathode has a critical role in gas generation.