Synthesis, Characterization And Properties Of Co₃O₄ -based Anode Materials

Lithium-ion batteries are considered as the most promising power sources in the 21th century for the wide application ranging from microbatteries for small-size electronic devices to power sources for electrical vehicles. Co₃O₄, an intrinsic p-type semiconductor (direct optical band gaps at 1.48 and 2.19 eV), excellent cycle reversibility and high specific capacity, has received a considerable amount attention over the last few years as one of the promising potential electrode materials for lithium-ion batteries. On the basis of reviewing the developments of lithium ion battery and relative materials in detail, with cobalt oxide-based anode materials as objects of the research, focusing studies on the specific capacity, cyclability and rate capability of materials, the cobalt oxide-based materials were characterized by various electrochemical methods in combination with powder X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Brunauer-Emmer-Teller (BET) surface area measurement and electrochemical measurements. The effects of structure, impurity, particle size, specific surface area of materials on the electrochemical characteristics are studied. The relationship between morphologies and electrochemical performances are discussed.Co₃O₄ microspheres were synthesized by a simple hydrothermal reaction in mass production via a mixture of cobalt nitrate (Co(NO₃)₂·6H2O) as precursor, hexamethylenetetramine (HMT, C6H12N4) as precipitator and trisodium citrate (C₆H₅Na₃O₇·2H₂O) as template. 1μm-sized spherical particles with well-crystallization could be obtained by XRD and SEM. Higher specific surface area (93.4 m2·g^-1) and larger pore volume (78.4 cm3·g^-1) by BET measurements offered more interfacial bondings for extra sites of Li+ insertion, which resulted in the anomalous large initial irreversible capacity and capacity cycling loss due to SEI film formation. The capacity retention of Co₃O₄ microspheres involved first forming acted as Li-ion anode material is almost above 90% from 12th cycle and it retain lithium storage capacity of 550.2 mAh·g^-1 after 25 cycles, which show good long-life stability. The electrochemical impedance spectroscopy (EIS) tests before and after cyclic voltammetry measurements and charge-discharge experiments were carried out and the corresponding DLi values were also calculated. The relationship of the AC impedance spectra and the cycling behaviors was discussed. It is found that the decrease of capacity result from the larger Li+ charge-transfer impedance and the lower lithium-diffusion processes on cycling, which is in very good agreement with the electrochemical behaviors of Co₃O₄ electrode.In order to investigate the effects of the initial HMT concentrations on the morphology and electrochemical properties of Co₃O₄ materials, we prepared four different Co(NO₃)₂/HMT mole ratios ranging from 2:1 to 1:4. By changing the initial HMT concentrations, the prepared Co₃O₄ were readily regulated in its morphologies, which could vary from microsphere to urchin-like hollow microsphere, and finally to collapsed porous structure. The growth mechanism was also studied. HMT could play a key role in the formation of four Co₃O₄ materials. It is found that the capacity retention of Sample 2 with an original Co(NO₃)₂: HMT mole ratio of 1:1 is almost above 94% from 5th cycle at different current densities of 40 and 60 mA·g^-1, which shows good long-life stability and favorable electrochemical behaviors. In addition, Sample 2 possessed the higher specific surface area (97.1 m2·g^-1) and the uniform urchin-like hollow structure.Sample 2 with an original Co(NO₃)₂: HMT mole ratio of 1:1 were calcinated at different temperatures ranging from 300 to 600 oC and characterized by XRD and SEM. The structure became agglomerative and collapsed with the increase of calcination temperature. Evaluation of the electrochemical performance in combination with SEM and BET analysis suggests that there is an optimum calcination temperature for Co₃O₄. It is found that the capacity retention of well crystallized Co₃O₄ hollow microspheres with larger specific surface area at 300 oC is almost above 94% from 5th cycle at different current densities of 40 and 60 mA·g^-1, which shows good long-life stability and favorable electrochemical behaviors. Using EIS analysis, we demonstrated that lithium-ion conduction inside the SEI layers and charge transfer at the electrode/electrolyte interface became hindered with the increase of calcination temperature, which was in good agreement with the electrochemical behaviors of three Co₃O₄ electrodes. It is proposed that drastic capacity fading and the variation of resistive components (SEI layers and charge transfer) can be influenced by morphologies due to the calcination temperature.A simple approach to synthesize Co₃O₄ in mass production by using surfactant (CTAB) and cosurfactants (C₅H₁₂O and C₈H₁₂O) via the microemulsion treatment has been developed. The samples were characterized by XRD and SEM. By changing the reaction time (4, 6, 8 h), the prepared Co₃O₄ were readily regulated in its morphologies varying from the chrysanthemum-like microspheres in bud to in full bloom. The sample reacted in 6h maintained 565.5 mAh·g^-1 after 30 cycles, and 495.1 mAh·g^-1 after 40 cycles at different cycling rates of 60 and 80 mA·g^-1. Although the cycling performance at current density of 50 mA·g^-1 started to show some fall off in the initial 20 cycles, the capacities were still comparable to the theoretical capacity of graphite (372 mAh·g^-1) after more than 100 cycles. The nearly 100% capacity retention after 20 cycles is believed to benefit from the unique structural features, particularly clusters of nanofibers. The chrysanthemum-like nanostructures with larger BET specific surface area create an easy and shorter diffusion pathway for ionic and electronic diffusion, which results in good power performance.Co₃O₄/C materials were prepared via a hydrothermal synthesis method, in which glucose and MWCNTs were carbon sources, respectively. The morphologies and crystal structures of three samples were characterized by XRD and SEM. SEM observations revealed that MWCNTs were highly dispersed among Co₃O₄ materials, especially between the nano-sheets. When applied as anodes in lithium-ion cells, Co₃O₄/MWCNTs electrode exhibited the best electrochemical performance, which maintained 706.1 mAh·g^-1 after 30 cycles. Although the cycling performance at current density of 50 mA·g^-1 started to show some fall off, the capacity retention maintained nearly 100% after 20 cycles. The lithium storage capacities in 10th, 20th, 30th, 40th, 50th were 897.8, 804.1, 777.6, 710.9 and 672.5 mAh·g^-1, which demonstrated good electrochemical cycling stability. Further EIS analyses showed the relationship between the AC impedance and the electrochemical properties. The improved cycle performance of Co₃O₄/MWCNTs composite can be attributed to the MWCNTs network that cannot only buffer the large volume change of Co₃O₄ during the cycling process but also provide excellent electronic and ionic conduction pathway for the electrochemical processes.The Co₃O₄ material was carried out the initial discharge/charge cyclic test between -0.6 and 0.6 V vs. SCE in LiOH solution. The pristine, discharged and recharged specimens were further characterized by XRD, SEM, Raman spectroscopy measurements. CV curves of Co₃O₄ at various scan rates and concentrations in LiOH solution were also investigated. Co₃O₄ was shown to undergo reversible insertion of Li+ and H+ ions from LiOH solution. The appearance of the two pairs of redox peaks indicated that two sets of faradaic reactions were involved for the redox reactions of Co₃O₄ to LiCoO2 and LiHCoO2. Various scan rate experiments revealed a linear relationship between the peak current and the square root of scan rate for the redox peak pairs between Co₃O₄ and LiCoO2, indicating that the Li+ deintercalation/intercalation processes occurred in Co₃O₄ were diffusion-controlled. The diffusion coefficients (DLi+) in anodic and cathodic processes are 2.15×10 ^-10 and 0.91×10 ^-11 cm2·s^-1, which were in agreement with the CV curves.