Facile Ultrasonic Synthesis of CoO Quantum Dot/Graphene Nanosheet Composites with High Lithium Storage Capacity

Abstract

In this paper, we report a facile ultrasonic method to synthesize well-dispersed CoO quantum dots (3-8 nm) on graphene nanosheets at room temperature by employing Co(4)(CO)(12) as cobalt precursor. The prepared CoO/graphene composites displayed high performance as an anode material for lithium-ion battery, such as high reversible lithium storage capacity (1592 mAh g(-1) after 50 cycles), high Coulombic efficiency (over 95%), excellent cycling stability, and high rate capability (1008 mAh g(-1) with a total retention of 77.6% after 50 cycles at a current density of 1000 mA g(-1), dramatically increased from the initial 50 mA g(-1)). The extraordinary performance arises from the structure advantages of the composites: the nanosized CoO quantum dots with high dispersity on conductive graphene substrates supply not only large quantity of accessible active sites for lithium-ion insertion but also good conductivity and short diffusion length for lithium ions, which are beneficial for high capacity and rate capability. Meanwhile, the isolated CoO quantum dots anchored tightly on the graphene nanosheets can effectively circumvent the volume expansion/contraction associated with lithium insertion/extraction during discharge/charge processes, which is good for high capacity as well as cycling stability. Moreover, regarding the anomalous behavior of capacity increase with cycles (activation effect) observed, we proposed a tentative hypothesis stressing the competition between the conductivity increase and the amorphorization of the composite electrodes during cycling in determining the trends of the capacity, in the hope to gain a fuller understanding of the inner working of the novel nanostructured electrode-based lithium-ion batteries.

Critical Insight into the Relentless Progression Toward Graphene and Graphene‐Containing Materials for Lithium‐Ion Battery Anodes

“…Regarding previously developed graphene-containing composites, many different semimetal-(e.g., Si [206][207][208][209] ), metal-(e.g., Sn [210][211][212][213] ) and metal-oxide-(e.g., NiO, [214][215][216] [ 225,226 ] or CoO [ 227,228 ] ) based graphene hybrids were further reported. However, despite some clever approaches to stabilize the structure of the active material (e.g., freeze-drying, [ 206 ] crumpling, [ 207 ] spin coating [ 209 ] or nanocabling [ 211 ] ), only few cases reported a successful minimization of the 1 st cycle irreversible capacity and improved delithiation voltage.…”

How to improve the stability and rate performance of lithium-ion batteries with transition metal oxide anodes

“…The reversible capacities are gradually increasing in the first dozens of cycles, which may be attributed to the formation of a polymeric surface film attached to the active material. 27 It gets a maximum value of 1027 mA h/g at the 36th cycle, which is about 99.5% of the initial discharge capacity, and it can still maintain 994 mA h/g even after 100 cycles. The value is even higher than the theoretical value of rGO/CoO composite (C theoretical 5 C CoO  mass percentage of CoO 1 C graphene  mass percentage of graphene 5 716  92% 1 744  8% 5 718.2 mA h/g).…”

“…What is more, it has high theoretical lithium storage of 744 mA h/g. 27,28 Thus, graphene can support another advanced anode materials meanwhile it also can act as a pathway for the flow of electrons from the active material to current collector. [29][30][31] Up to now, many nanoparticles (NPs) M x O y /graphene composites have been investigated as LIB anode materials, such as SnO 2 /graphene, [32][33][34] Mn 3 O 4 /graphene, 35 CuO/graphene, 36 and Fe 2 O 3 /graphene.…”

“…Such high specific capacity of rGO/CoO should be attributed to the following points: firstly, the rGO sheets have numerous significant disorder and defects, which can also contribute to the lithium storage capacity, except for the inherent capacity (formation of intercalation compounds Li 3 C). 27,40 Secondly, the reversible decomposition of the electrolyte with the formation of SEI and extra lithium-ion adsorption/desorption on the SEI during cycling may also lead to the high experimental lithium storage capacity. 18,41 For the pure CoO electrode, the initial capacity only is maintained for 18 cycles.…”

Recent Progress in Self‐Supported Metal Oxide Nanoarray Electrodes for Advanced Lithium‐Ion Batteries

“…Another representative Co‐based oxide is CoO, which has a theoretical capacity of 716 mAh g –1 as an LIB anode material 92, 93. Porous CoO nanowire arrays were prepared on Ti substrate via a hydrothermal process with subsequent pyrolysis 94.…”