Investigation of lithium metal stabilization through electrolyte and alloy optimization

Li-ion batteries are a widely utilized energy storage tool found within many consumer and commercial devices. To meet the energy and power density demands of these sectors and allow for the continued practical use of Li-ion battery technology, there must be improvements upon currently utilized Li-ion battery materials. At present, graphite negative electrodes and lithium intercalation positive electrodes fail to meet accelerating energy density demands. The incorporation of Li-metal as an electrode material offers high theoretical capacity (~3860mAh/g), low redox potentials, and low density, allowing for significant improvements in gravimetric and volumetric energy densities when paired with high energy density cathode materials. Practical utilization of Li-metal as an electrode material, however, is hindered by heterogenous Li plating behavior, manifesting in dendrite formation, and resulting in poor performance and safety concerns. There are several strategies investigated to address this shortcoming, where emphasis is placed on altering electrode materials and electrolyte compositions in order to induce favorable Li-metal nucleation, plating and morphological evolution with cycling. In this work, a novel dual-salt fluoroganosiyl (FOS) based electrolyte was developed, based on the investigation of the initial formation reactions of the SEI, and found to enable high efficiency in-situ formed Li-metal batteries (<3% initial loss at 4.0mAh/cm2). Ultra-low areal plating capacities (0.08mAh/cm2) were utilized within Li-metal half cells, to amplify these initial SEI formation reactions, and correlate SEI formation efficiency with a given electrolyte modification. This amplification of SEI formation reactions was found to be predictive of capacity fade and dendrite formation under higher areal plating capacities (2.5, 4.0, 6.5 mAh/cm2), and thus can be utilized as a tool for relating initial SEI formation efficiency and longer-term performance. Further evaluation of this novel electrolyte using XPS characterization in the pre-SEI and early Li-plating environments revealed that contributions to the SEI from this novel electrolyte may include Li-C-N, Li3N and B-N components. Further work looking to stabilize the Li-plating interface involved the investigation of low voltage and high-capacity Li-alloying materials, such as Ag (<2150mAh/g) and Mg (<670mAh/g). The incorporation of Mg and Ag as negative electrode materials may allow for more favorable Li-plating behavior, where a continuum between Li metal plating and Li alloying reactions may be established. Here, a novel composition 7:1:0.125 Li:Mg:Ag has been found to enable low irreversible loss and stable cyclability vs Li-containing LCO and non-Li containing Nb0.99Ta0.1PO5 cathodes. Here, Li2AgMg and Li0.9Mg0.1 phases formed within this composition were shown to enable this observed high performance, where a mechanistic relationship between these phases is established.