Na2B11H13 and Na11(B11H14)3(B11H13)4 as potential solid-state electrolytes for Na-ion batteries

Abstract

Solid-state sodium batteries have attracted great attention owing to their improved safety, high energy density, large abundance and low cost of sodium compared to the current Li-ion batteries. Sodium-boranes have been studied as potential solid-state electrolytes and the search for new materials is necessary for future battery applications. Here, a facile and cost-effective solution-based synthesis of Na₂B₁₁H₁₃ and Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄ is demonstrated. Na₂B₁₁H₁₃ presents an ionic conductivity in the order of 10⁻⁷ S cm⁻¹ at 30 °C, but undergoes an order–disorder phase transition and reaches 10⁻³ S cm⁻¹ at 100 °C, close to that of liquids and the solid-state electrolyte Na-β-Al₂O₃. The formation of a mixed-anion solid-solution, Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄, partially stabilises the high temperature structural polymorph observed for Na₂B₁₁H₁₃ at room temperature and it exhibits Na⁺ conductivity higher than its constituents (4.7 × 10⁻⁵ S cm⁻¹ at 30 °C). Na₂B₁₁H₁₃ and Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄ exhibit an oxidative stability limit of 2.1 V vs. Na⁺/Na.