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In an article recently published in the open access journal "Frontiers in Energy Research", researchers from China have created a new layered composite anode material for lithium-ion batteries. 

Research: Easily construct layered TiNb2O7/rGO nanoflowers with robust charge storage characteristics for lithium-ion batteries through esterification reactions. Image source: Neon_dust 

The composite material is made of titanium niobium oxide and reduced graphene oxide through an esterification reaction. They found that under the same conditions, the storage performance of this new composite material is higher than that of pure titanium niobium oxide.

Carbon-based materials such as graphite are the most commonly used anode materials in commercial lithium-ion batteries (LiBs). However, it has disadvantages such as lower electrochemical potential, lower theoretical capacity, easy formation of a solid electrolyte interface (SEI) film due to electrolyte consumption, and safety issues related to its low melting point. Titanium niobium oxide TiNb2O7 (TNO) has a high theoretical capacity of 388 mAh g-1, a high discharge voltage of 1.65 V and a long life. It is a better substitute for graphite-based anode materials. However, the electrochemical performance of TNO is affected by its low ion diffusivity and poor conductivity.

Schematic diagram of the formation process of TNO/NC/GO composite material. Image source: Hu, L et al., Frontiers of Energy Research

Carbon coating can effectively solve the problem of poor conductivity of TNO, but due to the lack of sufficient electrostatic driving force and contact surface area, it is difficult to achieve a uniform TNO/graphene composite material. Another method is to shorten the lithium ion diffusion path by reducing the particle size or constructing a different structure.

The researchers synthesized a layered TNO by controlling the degree of hydrolysis of the esterification reaction between acid and alcohol. The flower-shaped layered structure of the prepared layered TNO promotes a short and effective Li+ ion migration path and excellent rate capability.

Due to two factors, the growth of flower-like layered nanostructures is possible. First, the alcoholysis of niobium chloride (NbCl5) in the presence of ethanol inhibits the hydrolysis during the esterification process, thereby preventing accidental precipitation and promoting the uniform nucleation and growth of complex structures. Secondly, polyacrylamide (PAM) as a binder provides strong adhesion between the TNO and graphene oxide (GO) interface and overcomes the problem of insufficient electrostatic force.

High purity TNO is synthesized by solvothermal method, in which NbCl5 and titanium butoxide (Ti(OC4H9)4) are added to a solution of ethanol and acetic acid, and then heated and precipitated. GO powder is synthesized through an improved Hummer process.

Finally, pure TNO powder was dispersed into graphene, and then PAM was added dropwise to the above suspension to initiate the crosslinking reaction. The high temperature annealing/calcination of the suspension removes the solvent and reduces the graphene oxide to form the TNO-NC-GO composite material. The working electrode is made by mixing TNO-NC-GO composite material, Super P carbon black and polyvinylidene fluoride (PVDF) and coating it on the copper foil.

(A) SEM image of original TNO, (B) magnified SEM image of original TNO, (C) SEM image of TNO/NC/GO, (D) EDS mapping of different elements. Image source: Hu, L et al., Frontiers of Energy Research

X-ray diffraction (XRD) patterns confirmed the formation of pure TNO and the ideal reduction of GO to graphene after the annealing process. They also indicate the presence of disordered amorphous carbon in ordered graphene, which is the ionic conductivity of the composite material.

Scanning electron microscopy (SEM) shows that the uniform distribution of the TNO layered nanoflower structure can prevent the agglomeration of the TNO microstructure, form a better dispersion on the graphene, and provide low capacity loss during cycling. It can be seen from the cyclic voltammetry (CV) curve that the TNO-NC-GO composite material shows a larger response current than pure TNO and TNO-NC, and the higher electrochemical response is due to the increase in conduction.

In addition, the presence of GO enhances the pseudocapacitance Li+ charge storage in the entire potential range. The constant current charge and discharge curves of TNO-NC-GO as an anode under different current densities show that the capacity retention rate after 200 cycles is 85%, which is almost twice that of pure TNO. The volume expansion of the composite anode after discharge after 1500 cycles is 8.9%, which is much lower than that of the silicon-based anode (300%).

The contact resistance between phases obtained from the Nyquist diagram shows that compared with pure TNO (66Ω) and TNO-NC (69 Ω), the TNO-NC-GO composite has a lower surface charge transfer resistance of 97Ω, indicating that due to The effect of carbon coating.

(A) Comparison of cyclic voltammetry curves at a scan rate of 0.1 mV s−1; (B) CV of LIB's TNO/NC/GO electrode. (C) b value obtained from the relationship between peak current and scan rate; (D) CV curve of TNO/NC/GO, total current and capacitance current are separated at 1 mV s−1; (E) different samples The capacitance contribution at different scan rates. Image source: Hu, L et al., Frontiers of Energy Research

In this work, the researchers prepared a new type of flower-shaped layered TNO nanostructure through esterification. The TNO-NC-GO composite material is constructed by a simple cross-linking reaction using PAM as a flocculant.

Under the same conditions, the new composite anode exhibits a higher storage performance than pure TNO. It has a high specific capacity of 264.2 mAh g-1 and excellent cycle performance. After 1500 cycles, the capacity loss is extremely small. It has proven to be an ideal negative electrode material that can replace graphite in future LiB manufacturing.

Hu, L., Yang, X., Chen, Y., Wang, L, Li, J., Tang, Y. and Zhang, H., hierarchical TiNb2O7/rGO nanoflowers with strong lithium charge storage characteristics It is easy to construct an ion battery through the esterification reaction. Frontier Energy Research, 9, 794527. https://www.frontiersin.org/articles/10.3389/fenrg.2021.794527/full

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Bismay is a technical writer living in Bhubaneswar, India. His academic background is engineering, and he has extensive experience in content writing, journal review, and mechanical design. Bismay holds a master's degree in materials engineering and a bachelor's degree in mechanical engineering, and is passionate about science, technology and engineering. Outside of work, he likes online games and cooking.

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Prakash defeated, Bismay. 2021. Improve the electronic and ionic conductivity of graphene-encapsulated new two-dimensional nanomaterials through esterification and flocculation. AZoM, viewed on December 6, 2021, https://www.azom.com/news.aspx?newsID=57541.

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