DEVELOPING ANODE-FREE LITHIUM METAL CELLS WITH LIQUID ELECTROLYTES

Abstract

Anode-free lithium metal cells store 60% more energy than lithium-ion cells. Such high energy density can increase the range of electric vehicles by over 200 km and will be critical for enabling electrified urban aviation. This is made possible by discarding the conventional graphite anode and harnessing the capacity delivered by the positive electrode in the form of a lithium metal anode formed in-situ. Using this cell design with a liquid electrolyte, anode-free cells are a drop-in solution compatible with today’s manufacturing infrastructure.

However, anode-free cells are plagued with short lifetime. Inefficient lithium plating and stripping results in rapid capacity loss—lifetimes fewer than 20 cycles are typical. This is mostly a result of the lithium microstructure which usually forms in liquid electrolytes. Instead of plating as a flat metal sheet, lithium tends to deposit in a wild mossy structure. Unwanted reactions which deplete lithium capacity are exacerbated by this mossy structure, and high surface area lithium is also more volatile and less safe.

In this work, we investigate the degradation modes of anode-free cells that are necessary to overcome: microstructural degradation via scanning electron microscopy and x-ray tomography, resistance growth via electrochemical impedance spectroscopy, and electrolyte degradation via nuclear magnetic resonance spectroscopy. We also characterize the safety of anode-free cells with nail penetration tests. We show how cell chemistry affects performance using different electrolytes and different positive electrode materials (NMC532, NMC811, LCO and LFP). The impact of different cycling conditions—temperature, mechanical pressure, depth of discharge, and cycling rate are also studied. Finally, we use the insights gained in this work to extend the lifetime of anode-free cells to 200 cycles. Although cycle life must still be increased, we believe anode-free lithium metal cells with liquid electrolyte present one of the most straightforward paths toward unlocking the highest energy density cells for the next generation of energy storage.