Additive-Free Branch-Structured Electrode for Maximizing Energy Density of Solid-State Lithium-Ion Batteries
Authors: Kim Minho , Kang Song Kyu , Park Gwan Hyeon , Ji Junhyuk , Kim Won Bae 
Published: December 2024

Solid-state lithium-ion batteries (LIBs) are expected as one of the promising alternatives due to their improved energy density and excellent safety features compared to conventional LIBs. In the present work, one-dimensional SnO2 nanowires (NWs) were directly grown on current collectors without the use of conductive enhancers and binding additives via chemical vapor deposition (CVD) to enlarge the surface area in contact with the solid electrolyte. Additionally, novel branch-shaped NWs electrodes were fabricated by applying an additional CVD process on the standard NWs electrodes. Two types of branched electrodes, characterized by long branches with a short backbone and short branches with a long backbone, were fabricated with the same weight by finely controlling the CVD time. These branched electrodes enhance interfacial contact with the solid electrolyte and increase charge transfer kinetics between lithium ions and electrons by increasing the surface area at the interface. A polymer-based composite solid electrolyte, consisting of polyethylene oxide and ceramic fillers of Al-doped Li7La3Zr2O12, was infiltrated into NWs. The branched electrodes showed significant potential in improving electrode dynamics, especially where electrochemical reaction kinetics could be hindered by poor interfacial characteristics with the solid electrolyte. The electrochemical performance of the branched NWs electrodes was compared with that of normal NWs electrodes, with the impact of the variation in length of NW backbones and branches. Notably, the long-branched electrodes exhibited superior specific and areal electrochemical rate capability and lifespan. The thicker normal NWs electrode hinders solid electrolyte permeation into the NWs, whereas the thinner branched NWs electrode significantly increase electrode density of LIBs, thus ensuring effective solid electrolyte penetration. Specifically, the long-branched electrode exhibited an enhanced density under the same electrode weight due to its lowest thickness. Furthermore, the branched structure also promoted spacing between NWs, facilitating solid electrolyte penetration. Cross-sectional SEM images of the electrode/electrolyte interface clearly present that spacing between NWs in the branched electrodes can lead to the effective permeation of LLZO/PEO into the current collector. These results show that the energy density of LIBs could be maximized using long-branched NWs electrodes by achieving fast kinetics through stable electrode/electrolyte contact. The ratio of pseudocapacitive and diffusion-controlled contributions to deliverable capacity for both normal and branched electrodes was estimated from the cyclic voltammetry profiles. The effective electrode/electrolyte interface tends to offer comparatively higher pseudocapacitive contributions, resulting in fast reaction kinetics at high rates. Simulations of lithium-ion distribution and local current density for normal and branched electrodes indicated that lithium-ion mobility is enhanced in long-branched electrodes due to the structural optimization. Therefore, this research shows how the branched NW electrode approach can improve the electrode/electrolyte interfacial property and the energy density of LIBs, providing beneficial functionality to electrochemically active materials, particularly in the solid-phase electrolyte systems challenged by poor electrode dynamics at the electrode interfaces.