Long Cycle Life Rutile TiO2 Inverse Opal Anodes for Li-Ion Batteries
Authors: McNulty David , Carroll Elaine , O'Dwyer Colm 
Published: February 2020

With the ever advancing improvements in electronics and display technologies, it is crucial that Li-ion batteries will be able to rise to the challenge of powering future consumer electronics. Consumers are all too aware of the challenges facing next generation Li-ion batteries with issues such as poor cycle life and the requirement to charge devices everyday becoming commonplace. Consequently, in recent years there has been a tremendous amount of research into developing materials which are capable of delivering long cycle-life with suitable stability in terms of capacity values, capacity retention and voltage. (1-3) In order to significantly advance Li-ion battery technology, it is crucial to identify electrode materials that are capable of delivering stable capacities over a large number of cycles, instead of materials which offer high initial capacities and then significantly fade over the course of a relatively low number of cycles.
In this work we detail the preparation of rutile TiO2 inverse opal (IO) structured samples and their application as an anode material for Li-ion batteries. We present X-ray diffraction, electron diffraction and Raman analysis to confirm that the IOs are pure rutile TiO2. The electrochemical performance of the TiO2 IOs is evaluated via cyclic voltammetry, rate capability testing and long cycle life galvanostatic tests. We demonstrate stable cycling of TiO2 IOs over 1000 and 5000 cycles, at specific currents of 75 and 450 mA g-1, respectively. The capacity values obtained when cycled at a specific current of 75 mA g-1 are greater than previously reported values for other TiO2 nanostructures, achieving an impressive capacity of ~ 140 mAh g-1 after 1000 cycles. (4-6) We demonstrate that our TiO2 IO anodes are capable of delivering high specific capacities with stable capacity rentention, achieving a capacity retention of ~ 82.4% between the 100th and 1000th cycles.
We present SEM images of TiO2 IO samples after cycling showing that the IO morphology is retained after thousands of galvanostatic cycles. Previous reports for anode materials such as Si and Ge nanowires have shown that volume expansion during cycling can lead to substantial capacity fading due to significant changes in nanostructure morphology.(7, 8) However, the porosity of the IO structure is able to accommodate volume changes to the TiO2 nanoparticles that make up the walls of the IO material. The 3D porous structure and morphology is retained via expansion of the IO walls into the pores. The level of structural integrity retained after 5000 cycles is not only a first for TiO2 nanostructures but also for nanostructured oxide anode materials in general.
Acknowledgements
This work was also supported by Science Foundation Ireland (SFI) through an SFI Technology Innovation and Development Award under contract no. 13/TIDA/E2761. This publication has also emanated from research supported in part by a research grant from SFI under Grant Number 14/IA/2581.
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