The researchers report that the problem may be an incorrect interpretation of how such batteries fail. The new study, which could open new avenues for developing lithium batteries with solid electrolytes, was led by Yet-Ming Chiang, the Kyocera Professor of Ceramics at MIT and W. Craig Carter, the POSCO Professor of Materials Science and Engineering at MIT.

The problem, according to this study, is that researchers have been focusing on the wrong properties in their search for a solid electrolyte material. The prevailing idea was that the material’s firmness or squishiness (a property called shear modulus) determined whether dendrites could penetrate into the electrolyte. The new analysis showed that it’s the smoothness of the surface that matters most. Microscopic nicks and scratches on the electrolyte’s surface can provide a toehold for the metallic deposits to begin to force their way in, the researchers found.

Four types of ion-conducting, inorganic solid electrolytes are tested: Amorphous 70/30 mol% Li₂S-P₂S₅, polycrystalline β-Li₃PS₄, and polycrystalline and single-crystalline Li₆La₃ZrTaO₁₂ garnet. The nature of lithium plating depends on the proximity of the current collector to defects such as surface cracks and on the current density. Lithium plating penetrates/infiltrates at defects, but only above a critical current density. Eventually, infiltration results in a short circuit between the current collector and the Li-source (anode).

These results do not depend on the electrolytes shear modulus and are thus not consistent with the Monroe–Newman model for “dendrites.” The observations suggest that Li-plating in pre-existing flaws produces crack-tip stresses which drive crack propagation, and an electrochemomechanical model of plating-induced Li infiltration is proposed. Lithium short-circuits through solid electrolytes occurs through a fundamentally different process than through liquid electrolytes. The onset of Li infiltration depends on solid-state electrolyte surface morphology, in particular the defect size and density.

—Porz et al.

On the solid surfaces, lithium from one of the electrodes begins to be deposited, through an electrochemical reaction, onto any tiny defect that exists on the electrolyte’s surface, including tiny pits, cracks, and scratches. Once the initial deposit forms on such a defect, it continues to build—and, surprisingly, the buildup extends from the dendrite’s tip, not from its base, as it forces its way into the solid, acting like a wedge as it goes and opening an ever-wider crack.

The study suggests that achieving smoother surfaces on a solid electrolyte could eliminate or greatly reduce the problem of dendrite formation. Courtesy of the researchers. Click to enlarge.

This suggests, Chiang says, that simply focusing on achieving smoother surfaces could eliminate or greatly reduce the problem of dendrite formation in batteries with a solid electrolyte. In addition to avoiding the flammability problem associated with liquid electrolytes, this approach could make it possible to use a solid lithium metal electrode as well. Doing so could potentially double a lithium-ion battery’s energy capacity.

The formation of dendrites, leading to eventual short-circuit failures, has been the main reason that lithium-metal rechargeable batteries have not been possible, Chiang said.

I believe that this high-quality and novel work will reset the thinking about how to engineer practical lithium metal solid-state batteries. The authors have shown that a different mechanism governs lithium metal shorting in lithium solid-state batteries than in liquid or polymer lithium metal batteries where dendrites form. … This implies that if lithium metal solid-state batteries are ever to have practical current densities, then careful minimization of all structural defects at the lithium metal and electrolyte interface is essential. I consider it to be an extremely important contribution to the goal of developing practical and safe all solid-state batteries.

—Alan Luntz, a consulting professor for metal-air battery research at Stanford University, who was not involved in this research

The research team included Lukas Porz, Tushar Swamy, Daniel Rettenwander, and Harry Thomas at MIT; Stefan Berendts at the Technical University of Berlin; Reinhard Uecker at the Leibnitz Institute for Crystal Growth in Berlin; Brian Sheldon at Brown University; and Till Fromling at the Technical University of Darmstadt, Germany.

Resources

• L. Porz, T. Swamy, B. W. Sheldon, D. Rettenwander, T. Frömling, H. L. Thaman, S. Berendts, R. Uecker, W. C. Carter, Y.-M. Chiang (2017) Adv. Energy Mater. 1701003 doi: 10.1002/aenm.201701003