Atomic Layer Deposition of Lithium Borate and Lithium Borophosphate for Lithium-Ion Batteries

Protective coatings on lithium-ion battery electrodes have proven to be an effective way to suppress detrimental side reactions, thereby improving the performance of lithium-ion batteries. Atomic layer deposition (ALD) provides conformal deposition of these layers with precise thickness control, ensuring optimized cathode protection. In this work, an ALD process is developed for the deposition of lithium borate coatings using lithiumbis(trimethylsilyl)amide (LiHMDS), H₂O and trimethylborate (TMB). The ionic conductivity varies with deposition temperature: a value of 1.17 × 10^–7 S cm^–1 at 25 °C is obtained, with an activation energy of 0.58 eV. Using a supercycle approach to combine lithium borate with the known Li₃PO₄ process, and varying the cycle ratio, allows for the deposition of borophosphate coatings with tunable B/P ratios. As-deposited Li₃PO₄ films are crystalline, whereas lithium borate films are amorphous. Interestingly, a small amount of B incorporation (<1 at. %) enhances the crystallinity of the Li₃PO₄ films, which was attributed to a lower amount of C contamination (from 9.3 to 4.4 at. %). To explore the electrochemical properties of these layers, 10 nm coatings were deposited on a LiMn₂O₄ electrode as a model 2D system, where good Li-kinetics were proven. Next to this, they have shown to provide protection at elevated potentials. This work demonstrates that lithium borate and lithium borophosphate coatings are promising materials for interfacial layers and solid-state electrolytes to be used in next-generation lithium battery technologies.