Multilayer and Graded Thin-Film Electrodes: Paving the Way for High-Capacity and Long-Lasting Li-Ion Batteries

Li-ion batteries (LIBs) with thin-film electrodes are being widely explored owing to their potential as portable and miniaturized energy storage devices. In the present work, LiNi0.33Mn0.33Co0.33O2 (LNMC-333) and LiNi0.8Mn0.1Co0.1O2 (LNMC-811) thin-film electrodes have been prepared using radio frequency magnetron sputtering on gold-coated polished stainless-steel substrates. During the cycling test, the LIB with the Ni-rich LNMC-811 electrode showed high capacity, but its capacity decayed faster compared to the LIB with lower composition Ni LNMC (LNMC-333). We have subsequently tried to synergize the properties of both by depositing a thin-film bilayer structure mimicking a spherical core-shell structure with LNMC-333 as an outer layer offering better stability and LNMC-811 as an inner layer with high capacity. The LIB fabricated with such bilayer electrode showed elevated cyclability and stability relative to the LIB with only the LNMC-811 electrode and superior capacity relative to the LIB fabricated with only the LNMC-333 electrode up to 100 cycles. We further advanced our research by incorporating a compact multilayer design featuring five bilayers of LNMC-333/LNMC-811(M-LNMC) and a concentration-graded trilayer structure of LNMC-333/LiNi0.6Mn0.2Co0.2O2 (LNMC-622)/LNMC-811 (G-LNMC). It has been observed that LIBs with these thin-film structures demonstrated enhanced specific capacity compared to the bilayer design, and the graded cathodes exhibited superior capacity retention over 500 cycles despite the multilayer batteries initially having higher capacity. Thus, high specific capacity LIBs with superior stability, as reported in this study, have been achieved through an easily scalable and highly reproducible magnetron sputtering technique compared to complicated chemical synthesis routes. Hence, this thin-film approach has significant implications in LIB technology, especially for microbattery applications with solid electrolytes.