ORIGINAL PAPER Synthesis and application of nanostructured MCo 2 O 4 (M=Co, Ni) for hybrid Li-air batteries Audrey Tan 1 & M.V. Reddy 1 & S. Adams 1 Received: 30 September 2016 /Revised: 15 November 2016 /Accepted: 19 November 2016 # Springer-Verlag Berlin Heidelberg 2017 Abstract We report the synthesis of MCo 2 O 4 (M=Co, Ni) on Ni-mesh by a simple metal acetate decomposition method. Stability tests of the samples in aqueous acidified LiCl, LiOH and LiTFSI in H 2 O/DME showed that Co 3 O 4 /Ni and Co 3 O 4 - PVP/Ni are relatively stable in alkaline and neutral environ- ments, with Co 3 O 4 /Ni being relatively more stable. For NiCo 2 O 4 /Ni and NiCo 2 O 4 -PVP/Ni, the low weight percentage change of cobalt in LiTFSI in H 2 O/DME suggests that they are mostly stable in this electrolyte. The electrochemical perfor- mance of the Li-air cell was evaluated using Li anode and a LAGP ceramic separator with above mentioned electrolytes. Co 3 O 4 showed slightly higher catalytic activity for oxygen re- duction reaction (ORR) than for oxygen evolution reaction (OER) for the first three cycles. The cell with LiTFSI in H 2 O/ DME as aqueous catholyte showed that NiCo 2 O 4 is a better catalyst for the OER than for the ORR, while the reverse was observed when LiOH was used as the electrolyte. Keywords MCo 2 O 4 (M=Co, Ni) . Hybrid Li-air batteries . Electrochemical properties . Characterisation Introduction To date, the performance of a commonly used energy storage technology, lithium-ion batteries, has been maximised but cannot satisfy the energy storage needs in markets such as transportation or grid support. Currently, electric vehicles run- ning on Li-ion batteries have a limited driving range, and the goal is to achieve approximately 500 km [1] between each charge while retaining its affordability compared to gasoline vehicles and grid storage systems. This has sparked off an interest in research to identify new materials and electrochem- istry to meet to the growing energy storage demands. The lithium-air battery has a Li anode and utilises O 2 , ideally from the atmosphere, as the cathode. The aprotic Li-air battery has a theoretical specific energy of ∼3500 Wh kg -1 while the esti- mated practical specific energy is between 500 and 900 Wh kg -1 [ 2]. This is significantly larger than the 250 Wh kg -1 specific energy value that Li-ion batteries offer and relatively closer to 1750 Wh kg -1 for that of gasoline used in internal combustion engines. Upon discharge, solid Li an- ode is oxidised to Li + , and O 2 is reduced at the positive porous electrode surface. On charging, Li metal will be plated on the anode and O 2 is evolved. The different types of Li-air batteries are distinguished by the electrolytes used, which in turn deter- mines the electrochemical reaction of the cell. The mechanism behind the oxygen reduction reaction (ORR) and oxygen evo- lution reaction (OER) is complex, and the reactions for the respective systems have been suggested as follows [2, 3]: Aprotic LiÀair : 2Li þ 1 2 O 2 ↔Li 2 O and 2Li þ O 2 ↔Li 2 O 2 ð1Þ Aqueous=hybrid LiÀair with basic catholyte : 4Li þ O 2 þ 6H 2 O↔4LiOH:H 2 O ð2Þ Aqueous=hybrid LiÀair with acidic catholyte : 4Li þ O 2 þ 6HCl ↔4LiCl þ 2:H 2 O ð3Þ In the aprotic system, the electrolyte has to be stable in the presence of highly active reduced species, in particular * S. Adams mseasn@nus.edu.sg 1 Department of Materials Science and Engineering, National University of Singapore, Singapore 117546, Singapore Ionics DOI 10.1007/s11581-016-1913-9