Elastic and Plastic Characteristics of Sodium Metal

Sodium metal holds promise as an anode material for rechargeable batteries due to its large theoretical charging capacity, low electrochemical potential, earth abundance, and low cost. However, a number of concerns remain that must be addressed prior to practical implementation, particularly Na's propensity to form unstable morphologies during electrochemical deposition. Given the importance of mechanical deformation in this process, unlocking the potential of sodium metal as an anode material requires a thorough understanding of its mechanical properties. To this end, we evaluate the mechanical properties of sodium metal at room temperature through a combination of bulk compression, microhardness, and nanoindentation tests. With regard to elastic properties, nanoindentation testing produced an elastic modulus of 3.9 +/- 0.5 GPa. With regard to plastic properties, bulk compression testing revealed the flow stress at 0.08 strain as varying between 102 and 254 kPa at strain rates between 10' and 10(-2) s(-1). Nanoindentation indicated a decrease in hardness from 26.6 to 2.3 MPa at target P/P = 0.05 s(-1) as the indentation depth increased from 0.25 to 10 pm, while microhardness testing indicated hardness values between 1.6 and 1.1 MPa at depths varying between SO and 130 mu m. We perform finite element simulations to relate length scales in these measurements (e.g., depth) to physical lengths scales relevant to battery applications. We also found that Na exhibits a marked strain -rate sensitivity, with a strainrate sensitivity exponent of m = 0.14 from nanoindentation and m = 0.20 from bulk compression. Likewise, indentation demonstrated that Na is even more susceptible to creep than is Li metal. Overall, our studies indicate that Na metal is extremely soft, readily creeps, and exhibits pronounced size effects. We discuss the implications of these properties relative to other candidate anode materials of rechargeable batteries. Most notably, Na's greater propensity to creep than Li, in turn, has implications for charging rate capability in solid-state batteries.