XU ET AL. VOL. 9 ’ NO. 6 ’ 5876–5883 ’ 2015 www.acsnano.org 5876 May 07, 2015 C 2015 American Chemical Society Three-Dimensional Au Microlattices as Positive Electrodes for LiÀO 2 Batteries Chen Xu, * ,† Betar M. Gallant, ‡ Phillip U. Wunderlich, § Timm Lohmann, § and Julia R. Greer † † Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States, ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States, and § Research and Technology Center, Robert Bosch LLC, Palo Alto, California 94304, United States R echargeable LiÀO 2 batteries have been the subject of significant re- search efforts because of their poten- tial as efficient energy storage devices in electric vehicles. A “practical” LiÀO 2 cell may have a gravimetric specific energy 2 to 5 times higher than that of the current state-of-the-art Li-ion battery, 1,2 allowing a vehicle powered by this device to travel more than 300 miles between charges. 3 Among many challenges, a lack of chemical stability of the positive electrode mate- rial, 4À6 along with poor catalytic activity toward oxygen reduction reaction (ORR and OER), 3 remain key obstacles that pre- vent the utilization of LiÀO 2 batteries in commercial applications. Initial investiga- tions 5,7À9 utilized mostly carbon-based po- sitive electrodes because of their low cost and weight, high electrical conductivity, high surface areas, and structural versatility. Oxidation of carbon upon contact with Li 2 O 2 and/or battery operation in a highly oxidizing environment during charge 4À6,10 has been proposed to explain the widely observed coformation of Li 2 CO 3 , which ac- cumulates with cycling and causes rapid de- gradation in performance. Select oxides, 11 carbides, 12 and noble metals 13 can signifi- cantly improve the chemical stability against oxidation when compared to car- bon. For example, Thotiyl et al. 12 found that in cells using dimethyl sulfoxide (DMSO) as electrolyte solvent, a TiC nanoparticle cath- ode produced 40 times less Li 2 CO 3 at the end of the fifth discharge compared to a carbon electrode, which ultimately led to 98% capacity retention after 100 cycles. Nanoporous Au foils have also been re- ported as chemically stable electrodes cap- able of high cyclability. 13 The discharge product morphology for these reportedly stable electrode materials has not been systematically investigated, which is critical for developing design principles for using these materials as structured electrodes. Early efforts in this area focused on elec- trode structure design to maximize useable * Address correspondence to chenxu@caltech.edu. Received for review January 21, 2015 and accepted May 7, 2015. Published online 10.1021/acsnano.5b00443 ABSTRACT We demonstrate the feasibility of using a 3-dimensional gold microlattice with a periodic porous structure and independently tunable surface composition as a LiÀO 2 battery cathode. The structure provides a platform for studying electrochemical reactions in architected LiÀO 2 electro- des with large (300 μm) pore sizes. The lack of carbon and chemical binders in these Au microlattices enabled the investigation of chemical and morpholo- gical processes that occur on the surfaces of the microlattice during cycling. LiÀO 2 cells with Au microlattice cathodes were discharged in 0.5 M lithium- bis(trifluoromethane)sulfonamide (LiTFSI) in a 1,2-dimethoxyethane (DME) electrolyte, with lithium metal foil as the anode. SEM analysis of microlattice cathodes after first discharge revealed the presence of toroidal-shaped 500À700 nm particles covering the surface of the electrode, which disappeared upon subsequent charging. Raman and FTIR spectroscopy analysis determined these particulates to be Li 2 O 2 . The morphology of discharge products evolved with cycling into micrometer-sized clusters of arranged “platelets”, with a higher amount of side reaction products such as Li 2 CO 3 and LiOH. This work shows that properly designed 3-dimensional architected materials may provide a useful foundation for investigating fundamental surface electrochemistry while simultaneously enabling mechanical robustness and enhancing the surface area over a factor of 30 compared with a thin film with the same foot print. KEYWORDS: LiÀO 2 battery . microlattice . polycrystalline Au . toroids ARTICLE