With the ever-growing technology industry and the need for more sustainable energy, the race to develop better, more efficient, renewable materials for lithium ion batteries is at the forefront of research. Metal oxide materials for use as electrodes in lithium batteries have been promising in recent years, transition metal oxides are favourable due to their low cost and nanostructured metal oxides exhibit encouraging electrochemical properties.^1-2 Photonic crystal structures, more specifically inverse opal (IO) structures, are porous, highly interconnected structures which have proven to have advantageous benefits such as increased surface area and lower ion diffusion distances.³

In this work we combine TiO₂ and GeO₂ in solution phase in two ratios, 2:1 and 5:1 in favour of TiO₂. Titanium oxides have high working potentials, higher than graphite which is most commonly used today. TiO₂ has previously shown excellent long life cyclability and has shown little structural strain during the lithiation/delithiation process.⁴ Germanium has a high theoretical capacity of 1384 mAh/g and high electrical conductivity however it is expensive for use as an anode.⁵ Previous reports have shown that by nano-structuring GeO₂ into an inverse opal structure, it can behave like pure germanium during cycling beyond the initial reduction and lithitation cycles.⁶ By combining both materials, we were able to achieve high capacity values for both ratios, 3 times the capacity of titanium IOs on their own. Work is ongoing to investigate the optimal cost to benefit ratio of the TiO₂/GeO₂ composites. Initial capacity results showcase high capacity retention and voltage stability.

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3. Armstrong, E.; O'Dwyer, C., Artificial opal photonic crystals and inverse opal structures – fundamentals and applications from optics to energy storage. Journal of Materials Chemistry C 2015, 3 (24), 6109-6143.
4. McNulty, D.; Carroll, E.; O'Dwyer, C., Rutile TiO2 Inverse Opal Anodes for Li-Ion Batteries with Long Cycle Life, High-Rate Capability, and High Structural Stability. Advanced Energy Materials 2017, 7 (12), 1602291.
5. Zhang, Q.; Chen, H.; Luo, L.; Zhao, B.; Luo, H.; Han, X.; Wang, J.; Wang, C.; Yang, Y.; Zhu, T.; Liu, M., Harnessing the concurrent reaction dynamics in active Si and Ge to achieve high performance lithium-ion batteries. Energy & Environmental Science 2018, 11 (3), 669-681.
6. McNulty, D.; Geaney, H.; Buckley, D.; O'Dwyer, C., High capacity binder-free nanocrystalline GeO2 inverse opal anodes for Li-ion batteries with long cycle life and stable cell voltage. Nano Energy 2018, 43, 11-21.

Figure 1. (a) SEM image of a TiO₂/GeO₂ inverse opal (b) SEM image of TiO₂/GeO₂ IOs with EDX mapping overlay indicating position of titanium (green) and germanium (red). (c) and (d) Charge discharge profiles at 150 mA/g for the first 100 cycles for the 2:1 ratio and 5:1 ratio respectively.(e) Rate capability test for both ratios with specific currents ranging from 75 mA/g-450 mA/g.