In this paper, we successfully fabricated Al-modified Li₄Ti₅O₁₂ in one-step, easily, simply, and quickly. The structural properties of Li₄Ti₅O₁₂ by Al modification were favorable to electrochemical activity compared to pristine Li₄Ti₅O₁₂, and thus, it was confirmed that electrochemical performances such as cell balancing and initial discharge capacitance were effectively improved. The optimized anode/cathode thickness was selected 70 μm/240 μm. Al modified Li₄Ti₅O₁₂ realized high discharge capacitance of 61 F/g. Therefore, Al modification can be considered as one of the effective methods for the electrochemical performances of Li₄Ti₅O₁₂ anodes for next-generation hybrid supercapacitors

Keywords: Li₄Ti₅O₁₂, Al modification, Optimized anode/cathode thickness, Hybrid supercapacitors

Currently, with interest in electric vehicles, electric motorcycles, uninterruptible power supplies, and new and renewable energy, research on energy storage devices such as lithium-ion batteries, supercapacitors, and fuel cells is being actively conducted [1, 2]. Until now, lithium-ion batteries have been applied in most places due to their high energy density, but studies are being conducted to prevent safety accidents such as low power density, short lifespan, and fire and explosion. However, there is a limit to reliably solving these shortcomings in terms of the energy storage mechanism of the lithium-ion battery. Therefore, although the energy density is low, studies on high power density, infinite lifespan, and safe supercapacitors are in progress. Recently, in order to improve the low energy density of the existing supercapacitor, interest in a hybrid supercapacitor using a lithium ion battery and an electrode of a supercapacitor one by one is increasing [3-5]. The hybrid supercapacitors are designed to combine the advantages of supercapacitors and lithium-ion batteries.
Among the various components, it is the anode that affects the electrochemical performances of hybrid supercapacitors. Among various anode candidates such as TiO₂, H₂Ti₁₂O₂₅, Li₄Ti₅O₁₂, Nb₂O₅, α-Fe₂O₃, LiCrTiO₄, MoO₂ and Li₄Ti₅O₁₂ is the most promising anode candidate because it possesses almost infinite lifetime by zero-strain [1-3]. However, since Li₄Ti₅O₁₂ is also an insulator with low conductivity, studies such as morphology control, doping, and coating are being conducted to improve it. Among them, doping is the easiest and most effective way to definitely improve electrochemical performances [3, 6]. Therefore, studies have been reported to improve the electrochemical performance by doping cations such as Cr, Ni, Sr, Ta and La and anions such as F and Br.
In this paper, an Al modified Li₄Ti₅O₁₂ anode was fabricated using Al₂O₃, one of the most economical materials among various dopants, to fabricate a hybrid supercapacitor.

Granuel-Li₄Ti₅O₁₂ powder was prepared via spray drying method with precursor slurry and afterwards sintered in air at 790 ^oC for 8 h. The precursor slurry was fabricated using Li₂CO₃ and TiO₂. The molar ratio of Li₂CO₃ and TiO₂ was 4:5. The powder was mixed in ethyl alcohol for 20 h with ZrO₂ ball for homogeneous mixing. The slurry was atomized at 205 ^oC via a two-fluid nozzle with atomizing pressure (3.8 kg/cm²). The commercial activated carbon was selected as a cathode of hybrid supercapacitors. For Al-modified Li₄Ti₅O₁₂, the Li₄Ti₅O₁₂ and Al₂O₃ are mixed in a weight ratio of 99.5 to 0.5, and heat treatment was performed at 790 °C for 3 h.
The Al-modified Li₄Ti₅O₁₂ anode was produced by the subsequent process: to prepare a slurry, anode powder, conductive Super P (carbon black) and polyvinylidene fluoride (PVDF) binder were blended with a weight ratio of 85:6:9. The N-Methyl pyrrolidone (NMP) was added for slurry formation. The weights of cathode and anode were 2.8 g and 3.2 g, respectively. The blended slurry was cast on Al foil with 110 mm thickness and then dried at 100 ^oC to eliminate the NMP. The Al foil was compressed with 75-85 mm thickness. After that, the hybrid supercapacitor was dried in a vacuum oven for 40 h to eliminate the moisture and impregnated with a 1.7 M LiBF₄ solution in dimethyl carbonate (DMC):ethylene carbonate (EC).
The crystallinity and microstructure of the pristine and Al modified Li₄Ti₅O₁₂ particles were measured via X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). The electrochemical performances were confirmed by an Arbin BT 2042 in the voltage range of 0-2.8 V.

Figure 1 shows the XRD patterns of Al-modified Li₄Ti₅O₁₂. The (111), (311), (400), (331), (511), (440) and (531) peaks with cubic spinel structure (Fd3m space group) are located at 18.8, 35.2 43.4, 47.7, 57.2, 62.9 and 66.3, respectively (JCPDS file no. 26-1198) [4]. This result is consistent with the typical Li₄Ti₅O₁₂ peaks. Also, no significant secondary peaks is confirmed. As a result, it can be inferred that Al-modified Li₄Ti₅O₁₂ is successfully synthesized. As shown in Fig. 1(b), we can confirm that the (111) peak of Al-modified Li₄Ti₅O₁₂ is shifted to a lower angle compared to pristine Li₄Ti₅O₁₂, which indicates successful modification of Al. The some of Al³⁺ (0.52 Å) of Al-modified Li₄Ti₅O₁₂ leads to reduction/oxidation of Ti⁴⁺/Ti³⁺, indicating charge compensation. It can change the lattice plane of Li₄Ti₅O₁₂ since the Ti⁴⁺ and Ti³ have different ionic radii of 0.68 Å and 0.76 Å, respectively. The bigger lattice parameters spacing via Al substitution, resulting in rapid lithium ion transfer.
Figure 2 shows the microstructure and morphology of Al-modified Li₄Ti₅O₁₂. Al-modified Li₄Ti₅O₁₂ has a spherical shape like typical Li₄Ti₅O₁₂, and the size of secondary particles is about 4.8 μm. The secondary particles consist of numerous primary particles with 300-500 nm. In general, such porous structure of Al-modified Li₄Ti₅O₁₂ in with numerous pores is thought to facilitate fast and smooth movement of lithium ions and electrons due to sufficient wetting of the electrolyte [5, 6].
In order to measure the electrochemical performances of the hybrid supercapacitor with Al-modified Li₄Ti₅O₁₂ anode, the discharge capacitance retention with the various electrode thickness, as shown in Table 1. The capacity retention tends to decrease as the electrode thickness increases. When the current density of 0.1 A/g, the capacitance is proportional to the electrode thickness. It can be explained by sufficient movement of lithium ions and electrons. However, it can be confirmed that the capacitance decreases at high current density (1.0 A/g and 2.0 A/g). It is closely related to the relatively slower kinetics of lithium ions in the anode than BF₄^- ions in the cathode, which move relatively faster. Therefore, only the upper part of the anode participates in the electrochemical reaction. In the cathode, capacitance retention increases with thickness regardless of current density. As a result, the 90 um anode and 240 um cathode have the highest capacitance at 0.1 A/g, but have low capacitance retention at high current density. Therefore, 70 um of anode and 240 um of cathode were selected as thicknesses showing the optimal electrochemical performance (72 % at 3.0 A/g) [1, 2].
Figure 3 shows the cyclic voltammetry (CV) curves at a scan rate of 10 mV/s in the voltage of 0-2.8 V since working voltage of activated carbon (cathode) is 4.3 V and Al-modified Li₄Ti₅O₁₂ (anode) is 1.5 V. The shape of CV curve is similar to the previously reported hybrid supercapacitors. The movement of lithium ions at the anode and BF₄^- ions at the cathode can be confirmed from the CV curve. Below a voltage of 1.5 V, a small and similar amount of charging and discharging occurs only by lithium ions. The BF₄^- ions at the cathode do not affect the capacitance. Therefore, it is called the “mirror effect” [7, 8]. Above 1.5 V, both lithium ions and BF₄^- ions participate in electrochemical activity. Among them, BF₄^- ion is a major factor and lithium ion is a minor factor for capacitance. The oxidation and reduction peaks of the hybrid supercapacitors are about 2.52 V and 2.35 V, respectively. Such small polarization of the oxidation and reduction peaks indicates the excellent reversibility of the hybrid supercapacitors [9, 10]. In the CV curve, the inner area is proportional to the total capacitance of hybrid supercapacitor. Therefore, it can be inferred that Al modified Li₄Ti₅O₁₂ can possess higher capacitance than pristine Li₄Ti₅O₁₂. This is because Al modified Li₄Ti₅O₁₂ has greater electrical conductivity compared to pristine Li₄Ti₅O₁₂.
Figure 4 shows the initial charge-discharge curves of the hybrid supercapacitor with Al modified Li₄Ti₅O₁₂ anode/activated carbon cathode. The specific capacitance of the hybrid supercapacitor can be calculated using following equation [11, 12]:



where ‘C’ is the capacitance, ‘q’ is the charge, ‘ΔV’ is the potential range, ‘m’ is the mass of the active materials, ‘i’ is the current, and ‘t’ is discharge time. The discharge capacitance of the hybrid supercapacitor with Al modified Li₄Ti₅O₁₂ anode/activated carbon cathode is 61 F/g at current density of 0.1 A/g. This capacitance is higher than that of conventional hybrid supercapacitors. This is related to the internal resistance called IR drops, which can be calculated as [13, 14]:



where ‘V_charge’ is the end charge voltage, ‘V_discharge’ is the starting discharge voltage and ‘I’ is the current. The IR drop value of hybrid supercapacitor is 0.19 Ω at 0.1 A/g. It is well known that IR drop can affect the electrochemical performances. This is because IR drop is related to the different mechanism of both Faradaic process on anode and non-Faradaic process on cathode [15]. It is judged that the electrochemical properties have a positive effect by the low IR drop of Al modified Li₄Ti₅O₁₂.

In this study, we successfully synthesized Al-modified Li₄Ti₅O₁₂. In addition, an asymmetric hybrid supercapacitor was fabricated using an Al-modified Li₄Ti₅O₁₂ anode and an activated carbon cathode, and electrochemical performances such as cell balancing and initial discharge capacity were measured. Al-modified Li₄Ti₅O₁₂ displayed the identical crystal structure and morphology as Li₄Ti₅O₁₂. To optimize of electrochemical performances, capacitance retention was calculated with various thickness of anode and cathode. In addition, high initial discharge capacity and low IR drop were obtained through Al modification. As a result, we can conclude the Al modification as one of the methods that can compensate for the shortcomings of Li₄Ti₅O₁₂ anode.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1F1A1055979).