Synthesis of LTO Nanorods with AC/Nano-Si Composite as Anode Material for Lithium-ion Batteries

In this study, the synthesis of lithium titanate (LTO) composite with 3 wt% activated carbons (AC) and 10 wt%, 15 wt%, as well as 20 wt% of nano silicon (nano-Si) are carried out. LTO has zero-strain characteristics and has a long life cycle. However, its capacity is limited, and it has poor electrical conductivity. The addition of nano-Si aims to enhance its capacity, while the AC aims to provide a large specific surface area to increase electrical conductivity. The nanorod templates are made from titanium dioxide (TiO₂), which is obtained from titanium (IV) butoxide using the sol–gel method. Nanorod structures are achieved by a hydrothermal process in a 10 M sodium hydroxide (NaOH) solution. However, needle-like structures are also observed, and the Li₂TiO₃ phase is finally formed. Battery performance is determined by CV, CD, and EIS tests. EIS results show that the highest electrical conductivity is found in LTO only; the CV test results show that the highest specific capacity is found in LTO–AC/15% nano-Si, at 140.7 mAh/g, as well as a charge–discharge (CD) capacity at a current rate of 0.2 to 20 C.

Activated carbon; Li2TiO3; Lithium-ion battery; Lithium titanate; Nano silicon

The lithium-ion battery is a promising electric energy resource. It has several advantages over other energy storage methods, including its high cell voltage, the fact that it does not contain a dangerous substance, and its high volumetric and gravimetric density (Cai et al., 2010). The lithium-ion battery has a theoretical energy density of ~400 Whkg^-1, higher than that of a conventional lead–acid battery, which has an energy density of 30–40 Whkg^-1, or nickel–cadmium (Ni-Cd) battery, which has an energy density of 40–60 Whkg^-1 (Girishkumar et al., 2010). This means a lithium-ion battery could be used as an energy resource for electric vehicles, enabling a driving distance of ~400 km per charge (Yoon et al., 2010).

Graphite is the material that is commonly used as an anode for a lithium-ion battery. However, a lithium dendrite structure is formed on the surface of the graphite anode because of short circuits after long-term charge–discharge (CD), especially when the battery is operated under 0.2 V vs. Li/Li⁺ (Kuo & Lin, 2014). In the first cycle of CD, a passivating solid electrolyte interface (SEI) layer is formed on the graphite surface because of the low Li⁺ insertion potential (<1 V vs. Li⁺/Li) (Aurbach et al., 2002). This SEI layer causes bad battery performance.

Therefore, lithium titanate (LTO) is used as the graphite substitute. LTO has a zero-strain characteristic because, during the intercalation and de-intercalation process, the changes of volume that happen are only ~0.2%. This is because the Li⁺ ion size is the same as the place in the crystal structure, meaning there is almost no expansion or diminution when an ion enters or leaves the crystal structure (Lu et al., 2007). Nevertheless, LTO has a low theoretical capacity (175 mAh/g) and a low conductivity (10^-8 – 10^-13 S/cm) (Zhao et al., 2015). These problems can be solved by reducing the size of the LTO particles and combining them with other materials.

The reduction of LTO particles into nano-size particles will increase the contact area between electrode and electrolyte and shorten the diffusion path between the two. This means the LTO rate capability will increase (Bresser et al., 2012). Previous research found that nanorods’ structure provides more electrons channel, has a high surface-to-volume ratio, and has a short diffusion path that will increase the electrochemical performance (Li et al., 2009; Zhou et al., 2015).

Activated carbon (AC) is added to LTO to increase its conductivity and thermal stability. With its addition, the surface area and lithium diffusion increase, resulting in good electrical conductivity (Wang et al., 2007). Silicon has an excellent theoretical capacity of 4,200 mAh/g (Zhou et al., 2014). However, volume changes of ~300% occur during the lithiation and delithiation process, because the silicon cracks during cycling, resulting in irreversible capacity loss (Beaulieu at al., 2001; Maranchi et al., 2003; Zhang et al., 2012). Silicon also has low conductivity (6.7×10^-4 S cm^-1) (Chen et al., 2011; Li et al., 2012;), and an oxide layer forms on the surface (Li et al., 2007; Gao et al., 2011). This problem can be solved by reducing the particle size and combining the silicon with a conductive element such as carbon (Stankulov et al., 2009). This experiment will conduct the synthesis of LTO nanorods with the addition of AC and nano silicon (nano-SI) to increase the electrochemical performance of LTO.

The result of the synthesis process was the formation of nanorods and needle-like structures, proven by SEM, and the formation of the Li₂TiO₃ phase, proven by XRD. Agglomeration in the LTO–AC/nano-Si can be seen from the SEM and EIS results. The AC that was added did not affect the composite because the surface area of the composite was lower than the AC surface area, proven by BET; the electrical conductivity did not improve with the addition of AC, proven by EIS. Also, it was found that the higher the nano-Si percentage in the composite, the higher the charge transfer resistance and the lower the conductivity of the composite was. Further, the higher nano-Si percentage in the composite was, the higher the resistivity was found to be. The CV test results showed that LTO–AC/15% nano-Si had the highest specific capacity, at 140.7 mAh/h. It also showed that the LTO peak was not formed, because of the formation of Li₂TiO₃ phase. However, the potential of the peak is around 1.55 V, which is the theoretical potential of LTO. The CD test results showed that LTO and LTO–AC/15% nano-Si can perform up to 20 C, and both have a high coulombic efficiency.

The authors would like to thank the Directorate Research and Public Services Universitas Indonesia for their financial support to do this research under HIBAH PITTA 2018 with contract No: 2377/UN2.R3.1/HKP.05.00/2018.

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