New Anode-Less Li-metal Cell Maintains 80% Capacity After 90 Cycles

A team led by Professor Jeff Dahn at Dalhousie University with colleagues from Tesla Canada R&D and the University of Waterloo, demonstrated an anode-less Li-metal cell fabricated using dual-salt electrolyte (1 M lithium difluoro(oxalate) borate (LiDFOB) and 0.2 M lithium tetrafluoroborate (LiBF4) in a fluoroethylene carbonate (FEC): diethyl carbonate (DEC) solvent). Replacing the conventional graphite anode with lithium metal is one of the most popular approaches (to improve energy density), as this can increase the cell energy density by 40–50%.

Although this method can significantly improve the energy density of the battery, it can only be realized when the lithium metal anode is extremely thick containing over 10 times the amount of lithium actually being cycled, and in practice, a very thick anode cannot be used at all, As a result, researchers have called for limiting the lithium excess to less than 50μm.

new insights for design of lithium anode architecture image

The realization of this method is very difficult because lithium metal is prone to form dendrites with high surface area, which reduces cycling efficiency by increasing the reactivity of the anode with the electrolyte and forming isolated metallic lithium. In an anode-less Li-metal cell, such low cycling stability of lithium metal is especially apparent, where cells are built with a bare copper anode and the lithium is plated directly from the cathode on the first charge cycle. Since there is no excess lithium built into the cell, the volume is minimized and energy density is maximized, but performance may be very poor since there is no reservoir of fresh lithium to replenish the cell during cycling.

To improve the cycle stability of the electrolyte, the Canadian researchers used electrolytes made of LiDFOB and LiBF4, which retained 80% capacity after 90 charge-discharge cycles - the longest life demonstrated to date for cells with zero excess lithium. Since the liquid electrolyte enables smooth dendrite-free lithium morphology comprising densely packed columns even after 50 charge-discharge cycles. Besides, dual-salt electrolyte composite performs better at different voltages than the single salt electrolyte complex and does not rely on external pressure to achieve good cycle performance.

overview of characteristics of the Li metal battery family and challenges facing Li metal anodes image

With the new anode-less Li-metal cell, lithium ions are extracted from the cathode and electrodeposited as metallic Li onto the current collector (Cu), during the initial charging process, lithium ions are stripped from the current collector and intercalated back into the cathode.

 

 

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