RESUMO
The substitution of an organic liquid electrolyte with lithium-conducting solid materials is a promising approach to overcome the limitations associated with conventional lithium-ion batteries. These constraints include a reduced electrochemical stability window, high toxicity, flammability, and the formation of lithium dendrites. In this way, all-solid-state batteries present themselves as ideal candidates for improving energy density, environmental friendliness, and safety. In particular, all-solid-state configurations allow the introduction of compact, lightweight, high-energy-density batteries, suitable for low-power applications, known as thin-film batteries. Moreover, solid electrolytes typically offer wide electrochemical stability windows, enabling the integration of high-voltage cathodes and permitting the fabrication of higher-energy-density batteries. A high-voltage, all-solid-state lithium-ion thin-film battery composed of LiNi0.5Mn1.5O4 cathode, a LiPON solid electrolyte, and a lithium metal anode has been deposited layer by layer on low-cost stainless-steel current collector substrates. The structural and electrochemical properties of each electroactive component of the battery had been analyzed separately prior to the full cell implementation. In addition to a study of the internal solid-solid interface, comparing them was done with two similar cells assembled using conventional lithium foil, one with thin-film solid electrolyte and another one with thin-film solid electrolyte plus a droplet of LP30 liquid electrolyte. The thin-film all-solid state cell developed in this work delivered 80.5 mAh g-1 in the first cycle at C/20 and after a C-rate test of 25 cycles at C/10, C/5, C/2, and 1C and stabilized its capacity at around 70 mAh g-1 for another 12 cycles prior to the start of its degradation. This cell reached gravimetric and volumetric energy densities of 333 Wh kg-1 and 1,212 Wh l-1, respectively. Overall, this cell showed a better performance than its counterparts assembled with Li foil, highlighting the importance of the battery interface control.
RESUMO
Cobalt-free spinel LiNi0.5Mn1.5O4 is one of the most promising and environmentally friendly cathodes, based on its high specific theoretical capacity (147 mAh·g-1) and high electrochemical potential (4.7 V vs Li+/Li), as well as good electronic and Li-ion conductivities. In this work, we present the fabrication of LiNi0.5Mn1.5O4 thin-film cathodes deposited by the industrially scalable AC magnetron sputtering technique on functional and cost-effective stainless steel current collectors. This is the first step toward battery downscaling, envisioning the fabrication of compact microbatteries for low-power energy supply. The thin-film strategy is crucial also for solid electrolyte fabrication that will allow the integration of high-energy-density batteries while overcoming most of the current battery challenges. In this work, the effect of film thickness on the material's electrochemical performance is discussed, correlating the observed structural and morphological evolution with the final electrochemical response. Moreover, the effect of iron diffusion from the current collector substrate into the cathode film is analyzed. The addition of a stable CrN barrier layer in between the substrate and the film is proposed to prevent Fe diffusion, with a direct positive influence on the electrochemical behavior. All in all, the obtained results will facilitate the practical implementation of LiNi0.5Mn1.5O4 thin films as high-voltage cathodes in functional cost-effective microbatteries.