(Invited) The Life and Death of Anode-Free Lithium Metal Cells

Abstract

Anode-free lithium metal cells store 60% more energy per volume than conventional lithium ion cells. Such high energy density cells will enable electric vehicles with increased range and decreased cost. The anode-free strategy of using zero excess lithium presents a number of advantages and challenges over conventional lithium metal cells. Anode-free cells exhibit ultra-high stack energy density >1200 Wh/L for the NMC532 and 811 cells investigated in this work. Lithium metal cells, on the other hand, with lithium foils thicker than 60 µm are actually less energy dense than lithium ion cells. Challenges associated with manufacturing thin lithium foils and their incorporation in cell production are avoided in anode-free cells. Safety is a concern for any cell with metallic lithium, although this often remains unaddressed in the literature. However, with no excess lithium, anode-free cells suffer from rapid capacity loss. Lifetimes of fewer than 40 cycles to 80% capacity are common. Increasing cycle life to hundreds of cycles is necessary to produce viable anode-free cells for electric vehicles.Recently, we demonstrated state-of-the-art cycle life for anode-free cells utilizing a dual-salt carbonate-based electrolyte.1 This liquid electrolyte enabled a pristine lithium morphology of tightly packed lithium grains forming a smooth lithium mosaic, shown in Fig. 1a-b. In this present work, we probe the lithium morphology at different states of charge and conclude that this morphology is composed of lithium columns (Fig. 1c-e). Incredibly, the columnar lithium enabled by dual-salt electrolyte forms a matrix of dead lithium which facilitates reversible lithium deposition.Unfortunately, this pristine morphology is not maintained. In this work, we characterize the degradation of anode-free cells with dual-salt electrolyte. We probe the deterioration of lithium morphology during aging with SEM and correlate this to electrolyte degradation. We observe increased lithium porosity via x-ray tomography and use acoustic transmission imaging to demonstrate electrolyte depletion. For safety characterization, we measure the temperature of cells during penetration with a nail and compare to cells made with other electrolytes promoted in the literature. Finally, we use the insights gained here to build anode-free cells that deliver a greater energy density than lithium ion cells for over 200 cycles. Although lifetime must still be increased, we believe anode-free lithium metal cells with liquid electrolyte present the most straightforward and lowest cost path towards viable high energy density lithium batteries. Reference s : [1] Rochelle Weber, Matthew Genovese, A. J. Louli, Samuel Hames, Cameron Martin, Ian G. Hill and J. R. Dahn, Nat. Energy, 4 (2019) 683-689. Figure 1