Electro-chemo-mechanical effects of lithium incorporation in zirconium oxide

Abstract

Understanding the response of functional oxides to extrinsic ion insertion is important for technological applications including electrochemical energy storage and conversion, corrosion, and electronic materials in neuromorphic computing devices. Decoupling the complicated chemical and mechanical effects of ion insertion is difficult experimentally. In this work, we assessed the effect of lithium incorporation in zirconium oxide as a model system, by performing first-principles based calculations. The chemical effect of lithium is to change the equilibria of charged defects. Lithium exists in ZrO 2 as a positively charged interstitial defect, and raises the concentration of free electrons, negatively charged oxygen interstitials, and zirconium vacancies. As a result, oxygen diffusion becomes faster by five orders of magnitude, and the total electronic conduction increases by up to five orders of magnitude in the low oxygen partial pressure regime. In the context of Zr metal oxidation, this effect accelerates oxide growth kinetics. In the context of electronic materials, it has implications for resistance modulations via ion incorporation. The mechanical effect of lithium is in changing the volume and equilibrium phase of the oxide. Lithium interstitials together with zirconium vacancies shrink the volume of the oxide matrix, release the compressive stress that is needed for stabilizing the tetragonal phase ZrO 2 at low temperature, and promote tetragonal-to-monoclinic phase transformation. By identifying these factors, we are able to mechanistically interpret experimental results in the literature for zirconium alloy corrosion in different alkali-metal hydroxide solutions. These results provide a mechanistic and quantitative understanding of lithium-accelerated corrosion of zirconium alloy, as well as, and more broadly, show the importance of considering coupled electro-chemo-mechanical effects of cation insertion in functional oxides.

The accommodation of lithium in bulk ZrO2

“…The Brouwer diagrams presented in this investigation that used Fermi-Dirac statistics gave subtle differences, with enhanced accuracy but do not fundamentally alter the picture previously reported in literature when considering no lithium [23]. The ability to stipulate a defect species concentration is novel to the Brouwer diagram script created by Murphy et al [30].…”

“…The Brouwer diagrams indicate that the oxygen vacancy concentration is increased by the presence of lithium. It has been previously reported that lithium, (𝐿𝐿𝐿𝐿 𝑖𝑖 • ), can reduce the volume of both monoclinic and the tetragonal polymorphs [23]. By combining the volume changes from all defect concentrations included in the Brouwer diagrams with constant pressure calculations that allow for volumetric relaxation, a small reduction in volume for the monoclinic polymorph was observed with a minimal effect to the tetragonal polymorph.…”

“…This investigation into the bulk monoclinic and tetragonal polymorphs of ZrO2 probes the impact of lithium at a range of concentrations and identifies subsequent defect concentrations throughout the oxide layers. Brouwer diagrams are created to highlight defect concentrations and are compared to previous investigations [23]. In addition, the solubility of lithium is calculated.…”