Anomalous Impact of Mechanochemical Treatment on the Na-ion Conductivity of Sodium Closo-Carbadodecaborate Probed by X-Ray Raman Scattering Spectroscopy

Solid-state sodium ion conductors are crucial for the next generation of all-solid-state sodium batteries with high capacity, low cost, and improved safety. Sodium closo-carbadodecaborate (NaCB11H12) is an attractive Na-ion conductor owing to its high thermal, electrochemical, and interfacial stability. Mechanical milling has recently been shown to increase conductivity by five orders of magnitude at room temperature, making it appealing for application in all-solid-state sodium batteries. Intriguingly, milling longer than 2 h led to a significant decrease in conductivity. In this study, X-ray Raman scattering (XRS) spectroscopy is used to probe the origin of the anomalous impact of mechanical treatment on the ionic conductivity of NaCB11H12. The B, C, and Na K-edge XRS spectra are successfully measured for the first time, and ab initio calculations are employed to interpret the results. The experimental and computational results reveal that the decrease in ionic conductivity upon prolonged milling is due to the increased proximity of Na to the CB11H12 cage, caused by severe distortion of the long-range structure. Overall, this work demonstrates how the XRS technique, allowing investigation of low Z elements such as C and B in the bulk, can be used to acquire valuable information on the electronic structure of solid electrolytes and battery materials in general. Mechanochemical milling is a common technique to achieve high ionic conductivity in solid electrolytes. In this work, X-ray Raman scattering (XRS) spectroscopy reveals that the structural defects introduced by milling are beneficial to the ionic conductivity of sodium closo-carbadodecaborate (NaCB11H12). However, extended milling destroys the structure by increasing the proximity of Na+ to the CB11H12- cage, leading to a profound decrease in the Na-ion conductivity.image