Nickel oxalate dihydrate nanorods attached to reduced graphene oxide sheets as a high-capacity anode for rechargeable lithium batteries

In the search for high-capacity anode materials, a facile hydrothermal route has been developed to synthesize phase-pure NiC2O4·2H2O nanorods, which were crystallized into the orthorhombic structure without using templates. To ensure the electrical conductivity of the nanorods, the produced NiC2O4·2H2O nanorods were attached to reduced graphene oxide (rGO) sheets via self-assembly layer-by-layer processes that utilize the electrostatic adsorption that occurs in a poly(diallyldimethylammonium chloride) solution. The high electrical conductivity aided by the presence of rGO significantly improved the electrochemical properties: 933 mAh g−1 for the charge capacity (oxidation), which showed 87.5% efficiency at the first cycle with a retention of approximately 85% for 100 cycles, and 586 mAh g−1 at 10 C-rates (10 A g−1) for the NiC2O4·2H2O/rGO electrode. The lithium storage processes were involved in the conversion reaction, which were fairly reversible via a transformation to Ni metal accompanied by the formation of a lithium oxalate compound upon discharge (reduction) and restoration to the original NiC2O4·2H2O upon charging (oxidation); this was confirmed via X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy and time-of-flight secondary ion mass spectroscopy. We believe that the high rate capacity and rechargeability upon cycling are the result of the unique features of the highly crystalline NiC2O4·2H2O nanorods assisted by conducting rGOs. The storage capacity of lithium-ion batteries may be tripled with an anode that combines rod-like nickel nanocrystals with graphene oxide. Graphite is commonly used as the anode in commercial lithium-ion batteries, but it can short-circuit when capacities are pushed too high. As an alternative, Seung-Taek Myung of Sejong University in Seoul and co-workers investigated nickel oxalate (NiC2O4) compounds that can bind significant quantities of lithium ions through reversible conversion reactions. To avoid the volume expansion problems that often reduce the cycling performance of oxalates, the team turned NiC2O4 into nanorod shapes using template-free hydrothermal synthesis and then sandwiched them between atom-thin sheets of graphene oxide. The high conductivity of graphene oxide greatly improved lithium ion movement to the volume-restricted nanorods, and prototypes retained 85% of their sizeable capacity after 100 cycles. Nanorod NiC2O4·2H2O/rGO composite exhibits good cyclability with high capacity and reasonable rate performance. These characteristics are originated from the high electric conductivity and buffering effect of rGO that promote the reversible lithiation and delithiation processes associated with conversion reaction.