Investigating the grain boundary features of lithium titanium phosphate as an electrolyte for all-solid-state lithium-ion batteries and their optimization by boron doping

This work reports a detailed analysis of the grain boundary electrochemical properties of lithium titanium phosphate for application in all-solid-state lithium-ion batteries and their improvement by boron addition, Li1+xTi2_xBx(PO4)3 (x = 0, 0.2). The results demonstrate that highly resistive grain boundaries exist in the undoped material, which preclude its ability to attain a high overall Li-ion conductivity. Conversely, boron addition provides a significant improvement of both bulk and grain boundary conductivities, resulting from the Li-enrichment associated with charge compensation for the boron doping. A detailed analysis using the brick layer model and a space charge analysis reveals a lower depletion of Li+ species at the space charge layers of the grain boundaries of the boron-doped sample, which allow a higher intrinsic grain boundary conductivity to be offered by this material. This factor permits a much higher overall conductivity to be attained in the boron-containing sample, despite a finer microstructure. Overall, this work provides new insights regarding the elec-trochemical nature of the grain boundary of NASICON-based solid-state Li-electrolytes, underscoring the effec-tiveness of composition-driven grain boundary engineering for performance improvement; a factor that is currently understudied for this category of material.