Although numerous rechargeable battery technologies have been developed and successfully demonstrated, Lithium-ion batteries still remain at the forefront, both in terms of its performance, and in its span of applications. However, despite their commercialization, the Li-ion industry is still limited by the choices available for anode materials. Graphite, with its high specific capacity, good rate capability, low irreversible capacity, low volume expansion during lithiation, and good electronic conductivity, still remains an ideal choice as an anode material for Li-ion batteries.^[1] Nevertheless, owing to the safety issues with graphite,^[2] an alternative for its use as an anode material has always been sought after. A considerable effort has already gone into the development of Li-alloys and other intermetallic alloys such as tin, antimony, aluminium  etc. with their attractive gravimetric capacity.^[3] However, with the huge volume expansion issues, alloying materials have their own limitation that prevented their widespread application and commercialization. Further, research efforts began on metal oxides of the type MO as anodes for Li-ion batteries,^[4]  owing to their widespread availability, ease in synthesis and only moderate safety concerns in comparison to the graphite materials. Among these metal oxides, Cobalt oxide, CoO is of particular interest because of its high theoretical capacity of 715 mAh g^-1.^[5] Several morphologies of CoO have been shown to have a high specific capacity and excellent cyclic stability against lithium, and there is a continued interest in developing innovative morphologies for their successful application in Li-ion batteries. However, as is the case with several other materials, Cobalt oxide suffers from particle aggregation and huge volume change upon Li-ion insertion and extraction during the conversion reactions.^[6]This consequentially leads to deterioration of electrical contact between CoO particles and eventually causes the capacity to fade. Therefore, it becomes important to develop appropriate methods and composite materials that improve conduction between the cobalt oxide particles, and thereby enable them to be considered as ideal candidates for Li-ion battery anodes.

Numerous carbonaceous composites of cobalt oxides have been shown to improve the capacity and maintain the cyclic stability of cobalt oxide particles. Graphene hydrogel is a new class of material that possesses inherent mechanical stability and still retains the superior conducting properties of graphene.^[7] This would act as a three-dimensional support for materials embedded in it thereby addressing the volume expansion issue in addition to providing improved conductivity. Herein, we report the synthesis of a new 3D networked completely interconnected hybrid material with flower-like cobalt oxide (CoO) embedded on graphene hydrogel matrix, by using hydrothermal technique, and its subsequent application as anodes for Li-ion batteries. The electrochemical studies were carried out in the voltage range of 0.01–3.0 V versus Li/Li⁺ by using galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical performance of this hybrid material has been compared with the pristine cobalt oxide flowers. The 3D CoO–graphene hydrogel sample exhibited excellent electrochemical properties when studied as an anode for LIB, with high reversible specific capacity (1010 mAh g⁻¹ over 100 cycles), long cycling lifetime, and good rate capability (300 mAh g⁻¹ at 1.6 A g⁻¹). The unique flower-like morphology and its cross-linking with the rGO sheets enable enhanced interfaces leading to shorter Li⁺ diffusion and buffering against volume expansion upon lithiation/delithiation, thus resulting in improved electrochemical performances for such CoO microstructures. The obtained electrochemical results are promising and the synthesis strategy could easily be extended to form 3D networked macrostructures with other metal oxides.

References:

[1] B. Luo, L. Zhi, Energy Environ. Sci. 2015, 8, 456.

[2] J. B. Goodenough, Y. Kim, Chem. Mater. 2010, 22, 587.

[3] M. N. Obrovac, V. L. Chevrier, Chem. Rev. 2014, 114, 11444.

[4] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 2000, 407, 496.

[5] H. J. Liu, S. H. Bo, W. J. Cui, F. Li, C. X. Wang, Y. Y. Xia, Electrochim. Acta 2008, 53,6497.

[6] Y. Dong, S. Liu, Z. Wang, Y. Liu, Z. Zhao, J. Qiu, RSC Adv. 2014, 5, 8929.

[7] H. P. Cong, X. C. Ren, P. Wang, S. H. Yu, ACS Nano 2012, 6, 2693.