Assembling Iron Oxide Nanoparticles into Aggregates by Li3PO4: A Universal Strategy Inspired by Frogspawn for Robust Li-Storage

The poor ionic conductivity of transition metal oxides (TMOs) is a huge obstacle to their practical application as anodes for lithium-ion batteries (LIBs). Although good performance can be harvested by constructing nanostructures, some other foundmental issues including low tap density and serious electrolyte consumption come along. Herein, inspired by frogspawn, we propose a universal strategy of using lithium salts to assemble TMO nanoparticles into large aggregates to improve their Li+ conductivity. In such a frogspawn-like structure, lithium salt networks can not only realize the rapid transmission of Li+ but also alleviate the volume change during the charging/discharging process. When Li3PO4 is applied to assemble iron oxides nanoparticles, aggregates with size over 1 μm and tap density up to 1.33 g cm-3 can be obtained, which even hasve an ionic conductivity up to 9.61 × 10-5 S cm-1. Fe3O4 was also introduced through reduction to boost electron transfer. Consequently, this carbon-free composite delivered a capacity up to 896 mA h g-1 even after 1000 cycles at 5 A g-1, which can also be maintained under high mass loading. When using lithium salts such as Li2SO4, Li2CO3, LiBO2, and LiCl, the corresponding composites also showed similar performance. This strategy is also effective for TMOs such as NiO, Co3O4, and ZnO, demonstrating the universality of this frogspawn-inspired design.