The work was done in the laboratory of Julia R. Greer, professor of materials science and mechanics in Caltech’s Division of Engineering and Applied Science.

Despite over 40 y of research, overcoming the uncontrollable dendrite growth during cycling has remained an insurmountable obstacle for Li-based components. Among multiple attempted approaches to eliminate or even reduce the dendrite growth, mechanical suppression has emerged as one of the most promising routes. In their pioneering theoretical work, Monroe et al. considered a solid polymer electrolyte (SPE) in contact with a Li metal electrode and performed a linear stability analysis of the deformation at the interface. They used linear elasticity to compute the stresses generated at the interface due to small deformations. They found that the dendrite growth decays with time if the shear modulus of the SPE is higher than about twice the shear modulus of Li through compressive forces. This led to enormous interest in demonstrating cells with polymer electrolytes, inorganic solid-state Li ion conductors, and ceramic thin films. Ferrese et al. further demonstrated that the elastic modulus of the separator also affects dendrite growth because it causes the stress in the separator to build up to beyond the yield strength of Li, which causes the anode to plastically deform and flatten out. Applying external pressure higher than the yield strength of bulk polycrystalline Li in a direction perpendicular to the cell stack has also been shown to limit dendrite growth and prolong cycle life.

These approaches have had limited success, and many un- solved questions regarding mechanical suppression remain. … A key reason for the lack of solutions to overcome the Li dendrite growth challenge may be that the mechanical properties of Li at small scales are expected to drastically differ from those in its bulk form; most single-crystalline metals at the micrometer and submicrometer scales have been shown to be up to an order of magnitude stronger compared with their bulk form. … In this work, we aim to fill the gaps in the existing, severely incomplete understanding of the mechanical properties of Li.

—XU et al.

Using a special vacuum chamber (Li oxidizes and immediately turns black upon contact with air) at Caltech, the team, led by former graduate student Chen Xu formed pillars of single-crystal lithium a few micrometers tall and some nanometers to micrometers in diameter. Each of these single crystalline lithium pillars was extracted from a larger piece of lithium, and thus each had a particular crystallographic orientation—a particular angle with respect to the original sample.

The researchers performed uniaxial compression experiments on the single-crystalline Li pillars with diameters of 980 nm to 9.45 μm at 298 and 363 K (25 and 90 ˚C) in an in situ nanomechanical instrument.

The researchers discovered that at this size, lithium is up to 100 times stronger than previous measurements indicated. Additionally, collaborators at Carnegie Mellon University calculated how the stiffness of lithium dendrites varied with the crystallographic orientation and discovered that it could be as different as a factor of four.

The discovered yield strength of 1-μm-sized Li of 105 MPa at room temperature represents a two-orders of magnitude increase over what is believed to be the bulk strength of Li, 0.41– 0.89 MPa, and exposes serious shortcomings of the current mechanical methods of dendrite suppression. The observed threefold decrease in yield strength at 363 K (operating temperature of many SPEs) is substantial compared with the marginal decrease in shear modulus, which indicates that, at high temperatures, dendrite suppression via inducing plastic deformation will be much more effective than finding SPEs with higher shear moduli.

The high elastic anisotropy warrants the move to beyond the simple isotropic treatment of most existing theoretical efforts. More attention needs to be paid to the variation of elastic and shear moduli in the polycrystalline anode when designing SPEs with high shear modulus or when fabricating Li anodes rich in compliant orientations. Based on our experimental data and theoretical analysis, a Li metal anode operating under elevated temperature and/or having an interfacial orientation with a low shear modulus, for example <111>, in contact with a solid electrolyte with high elastic modulus will be less prone to dendrite formation. The rational design guidelines and the high-fidelity data provided will rapidly accelerate the development of a reversible Li metal anode, paving the way for higher energy density Li ion batteries or “beyond-Li ion” chemistries such as Li–S or Li–O₂.

—Xu et al.

Resources

• Chen Xu, Zeeshan Ahmad, Asghar Aryanfar, Venkatasubramanian Viswanathan and Julia R. Greer (2016) “Enhanced strength and temperature dependence of mechanical properties of Li at small scales and its implications for Li metal anodes.” PNAS doi: 10.1073/pnas.1615733114