Enhanced Room-Temperature Ionic Conductivity of NaCB11H12 via High-Energy Mechanical Milling

The body-centered cubic (bcc) polymorph of NaCB₁₁H₁₂ has been stabilized at room temperature by high-energy mechanical milling. Temperature-dependent electrochemical impedance spectroscopy shows an optimum at 45-min milling time, leading to an rt conductivity of 4 mS cm^-1. Mechanical milling suppresses an order-disorder phase transition in the investigated temperature range. Nevertheless, two main regimes can be identified, with two clearly distinct activation energies. Powder X-ray diffraction and ²³Na solid-state NMR reveal two different Na⁺ environments, which are partially occupied, in the bcc polymorph. The increased number of available sodium sites w.r.t. ccp polymorph raises the configurational entropy of the bcc phase, contributing to a higher ionic conductivity. Mechanical treatment does not alter the oxidative stability of NaCB₁₁H₁₂. Electrochemical test on a symmetric cell (Na|NaCB₁₁H₁₂|Na) without control of the stack pressure provides a critical current density of 0.12 mA cm^-2, able to fully charge/discharge a 120 mA h g^-1 specific capacity positive electrode at the rate of C/2.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na₄(B₁₂H₁₂)(B₁₀H₁₀) with 3 V-class cathode active materials: NaCrO₂, NaMnO₂, and NaFeO₂. Among these layered sodium transition metal oxide cathodes, NaCrO₂ shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na_0.5CrO₂) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na₄(B₁₂H₁₂)(B₁₀H₁₀) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Selective Functionalization of Carbonyl Closo-Decaborate [2-B10H9CO]- with Building Block Properties via Grignard Reagents

A green, fast and selective approach for the synthesis of mono-substituted closo-decaborate derivatives [2-B10H9COR]2- has been established via a nucleophilic addition reaction between the carbonyl derivative of closo-decaborate [2-B10H9CO]- and the corresponding Grignard reagent RMgX, where R is the ethyl, iso-propyl, pentyl, allyl, vinyl and propynyl groups. This approach is accomplished under mild conditions with 70-80% yields. The significance of these derivative is their ability to constitute building blocks for polymeric integration via the allyl, vinyl and propynyl substituents. All products were characterized by 11B, 1H and 13C NMR, elemental analysis and mass spectrometry.