From Current Collectors to Electrodes : Aluminium Rod Structures for Three-dimensional Li-ion Micro-battery Applications

The potential use of 3D aluminium nanorod structures as current collectors and negative electrodes for 3D Li-ion micro-batteries was studied based on the use of relatively simple and cost-effective electrochemical and sol-gel deposition techniques. Aluminium rod structures were synthesised by galvanostatic electrodeposition using commercial porous membranes as templates. It was shown that the use of a short (i.e., 50 ms long) potential pulse (i.e., -0.9 V vs. Al3+/Al) applied prior to a pulsed current electrochemical deposition gave rise to homogeneous deposits with more even rod heights. Electrophoretic and sol-gel deposition of TiO2 on the same substrates were also studied. The use of the sol-gel technique successfully resulted in a thin coating of amorphous TiO2 on the Al nanorod current collector, but with relatively small discharge capacities due to the amorphous character of the deposits. Electrophoretic deposition was, however, successful only on 2D substrates. Anodisation of titanium was used to prepare 3D TiO2 nanotube electrodes, with a nanotube length of 9 um and wall thickness of 50 nm. The electrodes displayed high and stable discharge capacities of 460 µAh/cm2 at a 0.1 C rate upon prolonged cycling with good rate capability. The 3D aluminium nanorod structures were tested as negative electrodes for Li-ion cells and the observed capacity fading was assigned to trapping of LiAl alloy inside the aluminium electrode caused by the diffusion of lithium into the electrode, rather than to pulverisation of the aluminium rods. The capacity fading effect could, however, be eliminated by decreasing the oxidation potential limit from 3.0 to 1.0 V vs. Li+/Li. A model for the alloying and dealloying of lithium with aluminium was also proposed. Finally, a proof-of-concept for a full 3D Li-ion micro-battery with electrodes of different geometries was demonstrated. The cell comprised a positive electrode, based on LiFePO4 deposited on a carbon foam current collector, with an area gain factor an order of magnitude larger than that for the Al nanorod negative electrode. This concept facilitates the balancing of 3D Li-ion cells as the positive electrode materials generally have significant lower specific energy densities than the negative electrodes