Li Ti 5 O 12 Coated by Biomass-Derived Carbon Quantum Dots as Anode Material with Enhanced Electrochemical Performance for Lithium-Ion Batteries

: Li 4 Ti 5 O 12 (LTO) is a promising anode material for lithium-ion batteries (LIBs) due to its stable reversibility, high-rate cyclability, and high operational potential. On the other hand, it suffers from poor electronic conductivity and low capacitance. To overcome these disadvantages, modiﬁcation of the LTO surface is frequently undertaken. Considering this idea, the production of a biomass-derived carbon-coated LTO material (LTO/C) and its application as an anode in LIBs is described in this work. The carbon precursor was obtained from commercial carrot juice, which was degraded using microwaves. According to the UV studies, the carbon precursor revealed similar properties to carbon quantum dots. Then, it was deposited on LTO synthetized through a sol-gel method. The LTO/C electrode exhibited a high speciﬁc capacity of 211 mAhg − 1 at 0.1 C. Capacity retention equal to 53% of the initial value was found for the charge–discharge rate increase from 0.1 C to 20 C. The excellent electrochemical performance of LTO/C was caused by the carbon coating, which provided (i) short diffusion pathways for the Li + ions into the LTO structure and (ii) enhanced electronic conductivity. The obtained results indicated that biomass-derived carbon quantum dot-coated LTO can be considered as a promising anode for LIBs. material. The TGA curve for the pristine LTO material did not signiﬁcantly change up to 700 ◦ C. Only one very small mass loss was observed in the temperature range between 250 to 400 ◦ C and this could correspond to the decomposition of some impurities undetectable by XRD, FTIR, and Raman spectroscopy. In turn, the TGA curve for the LTO/C material revealed the ﬁrst mass drop of about 0.4% up to 100 ◦ C. This corresponded to a release of adsorbed water molecules, which were also detected with the FTIR method. The second mass loss started at about 200 ◦ C and ﬁnished at 450 ◦ C. This mass change (~2.6% in relation to the initial mass) was related to a combustion reaction of the carbon coating with oxygen coming from the experimental atmosphere.


Introduction
In the era of electromobility, rechargeable energy storage devices play a very important role. Thus far, lithium-ion batteries (LIBs) have been achieved success in the market and have become the most widely used rechargeable energy storage systems due to their excellent rechargeability, suitable power density, and outstanding energy density, as well as long cycling life [1,2]. LIBs are typically composed of three primary elements, i.e., a positive electrode (cathode), a negative electrode (anode), and an electrolyte. To improve the electrochemical performance of this type of batteries, investigations on each of these elements are important. However, the search for new or the optimization of already known anode materials that would constitute a good alternative for the commercial graphite electrodes is a crucial issue [3,4]. In general, anode materials with long life, thermodynamic stability, enhanced safety, excellent electrochemical performance, and high-rate cyclability are highly appreciated. Among these, a spinel lithium titanium oxide (Li 4 Ti 5 O 12 ; LTO) is considered as one of the most promising candidates to satisfy the features mentioned above [4][5][6].
Thus far, the surface modification of LTO electrode material has received particular attention. This strategy is usually realized by the deposition of various coatings on the LTO surface. Among many successfully deposited coatings, carbon coatings are one of the most frequently studied because they aim to improve the interphase chemistry, electrical conductivity, and mechanical integration [3,6]. In addition, the presence of carbon coating provides fast lithium-ion diffusion. Carbon coatings are commonly obtained by mixing the various carbon precursors with LTO or its precursors and their subsequent annealing at high temperatures in an inert or reducing atmosphere [8,[18][19][20][24][25][26]. Recently, a new trend to transform biomass into useful biocarbons has been appeared in the scientific community. Such a strategy has been successfully used by K.Q. Lia et al. [27] to obtain a carbon coating from phoenix tree leaves for LTO material. Their carbon-coated LTO electrode has revealed a great electrochemical performance, i.e., a reversible capacity of 201 mAh g −1 after 500 cycles measured at charge/discharge current density of 175 mA g −1 (1 C). This finding shows that biomass can be considered as a cheap, abundant, and environmentally friendly carbon source for the formation of coating layers on the electrode materials. Therefore, following this trend, this publication describes the preparation of a biomass-derived carboncoated LTO material and its application as an anode for LIBs. The carbon precursor for coating was obtained from a commercial carrot juice through microwave degradation and was deposited on the LTO material prepared with a well-known sol-gel method. Moreover, the carbon deposition on the LTO was carried out without post-synthesis annealing at high temperatures in an inert or reducing atmosphere, which greatly decreases the time and cost of material manufacturing. The carbon-coated electrode exhibited greatly enhanced electrochemical performance even at high charge/discharge rates in comparison with the pristine LTO electrode.

Preparation of Biomass-Derived Carbon Precursor
Commercially available carrot juice was chosen as a source of carbon. It was transferred to a glass container and then heated in a microwave oven (Panasonic Model NN-GT231 M, Osaka, Japan) for 20 min at maximum power. The resulting char product was stirred for 1 h at 1000 rpm with a diluted solution of 0.01 M NaOH of pH 7.4, which acted as a passivating agent. After that, the char carbon was separated from the solution by centrifugation. It was also rinsed several times with water and then sonicated and filtered with Whatman filter paper (Little Chalfont, UK). The biomass-derived carbon precursor was stored in ethanol at room temperature.

Preparation of Pristine and Carbon-Coated LTO Materials
Pristine LTO material was synthetized through a sol-gel method. In the process, 10 solution containing a citric acid (99.5%, Acros), which acted as a chelating agent, was poured into the previously obtained solution. The resulting sol-gel precursor solution was composed of CH 3 COOLi:Ti(OC 4 H 9 ) 4 :citric acid at a molar ratio of 4:5:1.25. It was stirred at room temperature for 30 min and then heated at 80 • C for 6 h to obtain a gel precursor, which was further dried at 60 • C for 8 h to remove the remaining solvent. Afterwards, it was preheated at 350 • C for 4 h and cooled to room temperature; then, it was thermally treated with a heating rate of 10 • C min −1 up to 800 • C. The gel was left at 800 • C for 12 h under air atmosphere to obtain the pristine LTO material.
The pristine LTO material was used to obtain the biomass-derived carbon-coated LTO labeled as LTO/C. In general, the as-synthesized LTO powder was dispersed in ethanol solution containing the previously prepared biomass-derived carbon precursor. The solution was sonicated for 20 min. After that, it was dried at 60 • C for 8 h to evaporate the ethanol and obtain the dry LTO/C material.

Characterization of Materials
The morphological properties and particle size of the LTO and LTO/C materials were determined with a scanning electron microscope (SEM; JSM-7800F, JEOL, Akishima, Japan) and a high-resolution transmission electron microscope (HR-TEM; Philips Tecnai G2, Eindhoven, The Netherlands). The structural properties of both LTO samples were analyzed using an X-ray diffractometer (XRD; 6000 Shimadzu Corporation, Kyoto, Japan) operating at 40 kV and 30 mA and equipped with a Cu K α lamp (λ XRD = 1.5406 Å), a UV-VIS spectrometer (Agilent 8453, Santa Clara, CA, USA) with ethanol as the reference, a Fourier transform infrared spectrometer (FTIR, JASCO FT/IR 6700, Tokyo, Japan), and a Raman spectrometer (iHR550, Horiba, λ RS = 532 nm, Kyoto, Japan). The surface chemical composition of samples was characterized using X-ray photoelectron spectroscopy (XPS, VG ESCA Scientific Theta Probe, Chanhassen, MN, USA). The content of carbon in the LTO/C was estimated by a thermogravimetric analyzer (TGA, Pyris 1, PerkinElmer, Waltham, MA, USA). The TGA measurement was carried out from room temperature to 700 • C with a heating rate of 10 • C min −1 in air atmosphere. The specific surface areas and pore sizes of the investigated LTO materials were determined with nitrogen adsorption-desorption measurements performed with an ASAP 2020-Micromeritics analyzer.

Characterization of Materials
The pristine and carbon-coated LTO materials as active electrode materials, acetylene black (UBIQ Technology Co., San Francisco, CA, USA) as a conductive material, and polyvinylidene fluoride (PVDF) (Kynar HSV 900, Avon Lake, OH, USA) as a binder were mixed at a weight ratio of 83:10:7, were dissolved in N-methyl-2-pyrrolidone (NMP, 99%, Alfa Aesar, Haverhill, MA, USA), and then stirred for 2 h. The obtained slurry was deposited on a current collector, in this work, a copper foil (UBIQ Technology Co.). After that, the current collector with electrode was dried at 120 • C for 12 h in vacuum in order to evaporate the residual solvent.
The electrochemical cells composed of the investigated electrodes, lithium metal, and separator soaked with the 1.0 M LiPF 6 in ethylene carbonate (EC):diethyl carbonate (DEC) (1:1, v/v) electrolyte (Formosa Plastics Corporation Co., Kaohsiung, Taiwan) were assembled in an argon-filled glove box. The electrochemical tests were prepared using a CR2032 coin-type cell.
The galvanostatic charge and discharge (GCD) and rate capability analyses were performed by a commercial battery auto test system (NEWARE CT-4000, Shenzhen, China) at a potential window between 1.0 and 2.5 V (vs. Li + /Li) at varied current densities at room temperature. The cyclic voltammetry (CV) experiments were collated at a voltage of 1.0-2.5 V (vs. Li + /Li) at various rates with VMP-300 (BioLogic, Seyssinet-Pariset, France). The electrochemical impedance spectroscopy (EIS) analysis was carried out by using a Zahner Zennium (Kronach-Gundelsdorf, Germany) in the frequency range from 0.1 mHz to 100 kHz with an amplitude of 5 mV. The EIS spectra were collected at a constant potential of 1.587 V.

Results and Discussion
The structural properties of the LTO and LTO/C samples were initially analyzed using the XRD technique. The obtained results are shown in Figure 1a. The XRD patterns of both samples were almost the same, indicating that the crystal structure of the pristine LTO material was not influenced by the deposition of the carbon coating. The positions of the peaks matched well to the structure of cubic spinel Li 4 Ti 5 O 12 with an Fd3m space group (JCPDS card no. 49-0207). The calculated unit-cell parameter of the LTO/C (a~8.3535 Å) was in line with the unit-cell parameter of the pristine LTO (a~8.35053 Å). No signal related to impurities such as rutile and anatase, which usually exist in LTO materials prepared by sol-gel reactions, was found in the XRD patterns. It is also worth noting that no evidence of carbon was observed for the LTO/C sample. This means that the deposited carbon was amorphous or its content was too low to detect with the XRD technique. Hence, the presence of carbon in the LTO/C was additionally verified by the performance of UV, FTIR, Raman spectroscopy, and XPS investigations.
Energies 2022, 15, x FOR PEER REVIEW 4 2.5 V (vs. Li + /Li) at various rates with VMP-300 (BioLogic, Seyssinet-Pariset, France). electrochemical impedance spectroscopy (EIS) analysis was carried out by using a Zah Zennium (Kronach-Gundelsdorf, Germany) in the frequency range from 0.1 mHz to kHz with an amplitude of 5 mV. The EIS spectra were collected at a constant potenti 1.587 V.

Results and Discussion
The structural properties of the LTO and LTO/C samples were initially analyzed ing the XRD technique. The obtained results are shown in Figure 1a. The XRD pattern both samples were almost the same, indicating that the crystal structure of the pris LTO material was not influenced by the deposition of the carbon coating. The position the peaks matched well to the structure of cubic spinel Li4Ti5O12 with an Fd3m space gr (JCPDS card no. 49-0207). The calculated unit-cell parameter of the LTO/C (a~8.353 was in line with the unit-cell parameter of the pristine LTO (a~8.35053 Å). No signa lated to impurities such as rutile and anatase, which usually exist in LTO materials pared by sol-gel reactions, was found in the XRD patterns. It is also worth noting tha evidence of carbon was observed for the LTO/C sample. This means that the depos carbon was amorphous or its content was too low to detect with the XRD techni Hence, the presence of carbon in the LTO/C was additionally verified by the perform of UV, FTIR, Raman spectroscopy, and XPS investigations. The UV-VIS spectra of carbon precursor and LTO/C dispersed in ethanol are sented in Figure 1b. The spectrum recorded for the carbon precursor reveals a broad The UV-VIS spectra of carbon precursor and LTO/C dispersed in ethanol are presented in Figure 1b. The spectrum recorded for the carbon precursor reveals a broad and strong absorption peak at 260-320 nm. In fact, this peak is characteristic for carbon quantum dots (CQDs) and typically corresponds to π-π* and n-π* transitions of C=C and C-O bonds [28,29]. In turn, two broad overlapping peaks were visible for the LTO/C sample. The maxima of the first peak matches well to the position of the absorption peak observed for the carbon precursor, whereas the second peak was associated with the LTO material [30][31][32]. It is also worth noting that no absorption edge at approximately 435 nm, corresponding to the second-phase Li 2 TiO 3 , was observed [32]. This is in good agreement with the XRD results and confirmed that the LTO material was quite pure.
The FTIR results are presented in Figure 1c. The spectrum of the biomass-derived CQDs revealed three bands whose maxima were 3315, 1640, and 1400 cm −1 corresponding to O-H, C=O, and C=C vibrational modes, respectively [28,29]. The peak associated with O-H came from adsorbed water, whereas the two latter peaks were related to carbon material. All these three peaks were not visible in the spectrum of pristine LTO, but were distinguishable for the LTO/C sample. Their intensities were much lower, which means that the amount of carbon deposited on the LTO was not high.
The Raman spectra shown in Figure 1d [33][34][35]. All these bands were very sharp and well defined in the case of LTO material. In turn, they were much lower and somewhat suppressed for the LTO/C sample. Moreover, the three additional bands located at 1100, 1357, and 1592 cm −1 appeared in the spectrum of carboncoated LTO. The origin of the peak at 1100 cm −1 is associated with the disordered sp 3 carbon [36,37]. In turn, the peaks at 1357 and 1592 cm −1 referred to the D and G bands, respectively, and they are typically observed for graphitic carbon [34,35,38]. Nevertheless, they were very broad and partially overlapping, which suggests that the carbon coating was amorphous [39].
The successful deposition of the carbon coating on the LTO was also confirmed by XPS investigations. The C 1s XPS spectra recorded for the biomass-derived CQDs and the LTO/C material are shown in Figure 2. In fact, they were different. The spectrum of the biomass-derived CQDs was deconvoluted into four bands located at 283.5, 284.4, 286.6, and 291.5 eV corresponding to C-C, C=C, C-O/C=O, and π-π* bonds, respectively [40][41][42][43][44]. In turn, the C 1s XPS spectrum of the LTO/C material was split into four bands located at 281.5, 282.9, 284.4, and 286.5 eV associated with Ti-C, C-C, C=C, and C-O/C=O bonds [38,[40][41][42][43][44]. The XPS data were in good agreement with the previously discussed UV-VIS measurements in which the characteristic π-π* transition was observed for the CQDs, whereas this signal was greatly weaker for the LTO/C material. tum dots (CQDs) and typically corresponds to π-π* and n-π* transitions of C=C and C-O bonds [28,29]. In turn, two broad overlapping peaks were visible for the LTO/C sample. The maxima of the first peak matches well to the position of the absorption peak observed for the carbon precursor, whereas the second peak was associated with the LTO material [30][31][32]. It is also worth noting that no absorption edge at approximately 435 nm, corresponding to the second-phase Li2TiO3, was observed [32]. This is in good agreement with the XRD results and confirmed that the LTO material was quite pure.
The FTIR results are presented in Figure 1c. The spectrum of the biomass-derived CQDs revealed three bands whose maxima were 3315, 1640, and 1400 cm −1 corresponding to O-H, C=O, and C=C vibrational modes, respectively [28,29]. The peak associated with O-H came from adsorbed water, whereas the two latter peaks were related to carbon material. All these three peaks were not visible in the spectrum of pristine LTO, but were distinguishable for the LTO/C sample. Their intensities were much lower, which means that the amount of carbon deposited on the LTO was not high.
The Raman spectra shown in Figure 1d [33][34][35]. All these bands were very sharp and well defined in the case of LTO material. In turn, they were much lower and somewhat suppressed for the LTO/C sample. Moreover, the three additional bands located at 1100, 1357, and 1592 cm −1 appeared in the spectrum of carboncoated LTO. The origin of the peak at 1100 cm −1 is associated with the disordered sp 3 carbon [36,37]. In turn, the peaks at 1357 and 1592 cm −1 referred to the D and G bands, respectively, and they are typically observed for graphitic carbon [34,35,38]. Nevertheless, they were very broad and partially overlapping, which suggests that the carbon coating was amorphous [39].
The successful deposition of the carbon coating on the LTO was also confirmed by XPS investigations. The C 1s XPS spectra recorded for the biomass-derived CQDs and the LTO/C material are shown in Figure 2. In fact, they were different. The spectrum of the biomass-derived CQDs was deconvoluted into four bands located at 283.5, 284.4, 286.6, and 291.5 eV corresponding to C-C, C=C, C-O/C=O, and π-π* bonds, respectively [40][41][42][43][44]. In turn, the C 1s XPS spectrum of the LTO/C material was split into four bands located at 281.5, 282.9, 284.4, and 286.5 eV associated with Ti-C, C-C, C=C, and C-O/C=O bonds [38,[40][41][42][43][44]. The XPS data were in good agreement with the previously discussed UV-VIS measurements in which the characteristic π-π* transition was observed for the CQDs, whereas this signal was greatly weaker for the LTO/C material.  The thermogravimetric curves recorded for the LTO and LTO/C samples are shown in Figure 3. In fact, they were collected in order to estimate the content of carbon in the LTO/C Energies 2022, 15, 7715 6 of 13 material. The TGA curve for the pristine LTO material did not significantly change up to 700 • C. Only one very small mass loss was observed in the temperature range between 250 to 400 • C and this could correspond to the decomposition of some impurities undetectable by XRD, FTIR, and Raman spectroscopy. In turn, the TGA curve for the LTO/C material revealed the first mass drop of about 0.4% up to 100 • C. This corresponded to a release of adsorbed water molecules, which were also detected with the FTIR method. The second mass loss started at about 200 • C and finished at 450 • C. This mass change (~2.6% in relation to the initial mass) was related to a combustion reaction of the carbon coating with oxygen coming from the experimental atmosphere.
The thermogravimetric curves recorded for the LTO and LTO/C samples are in Figure 3. In fact, they were collected in order to estimate the content of carbon LTO/C material. The TGA curve for the pristine LTO material did not significantly up to 700 °C. Only one very small mass loss was observed in the temperature ra tween 250 to 400 °C and this could correspond to the decomposition of some imp undetectable by XRD, FTIR, and Raman spectroscopy. In turn, the TGA curve LTO/C material revealed the first mass drop of about 0.4% up to 100 °C. This sponded to a release of adsorbed water molecules, which were also detected with th method. The second mass loss started at about 200 °C and finished at 450 °C. Th change (~2.6% in relation to the initial mass) was related to a combustion reaction carbon coating with oxygen coming from the experimental atmosphere. The morphologies and particle sizes of LTO and LTO/C materials were dete with SEM. The collected data is presented in Figure 4. According to the SEM ima size of the pristine LTO particles was larger than that of LTO/C. The estimated a sizes equaled about 245 and 202 nm for LTO and LTO/C, respectively. Besides t particle size distribution of the LTO/C sample was more homogeneous and the s were rougher due to the deposition of carbon coating. The HR-TEM results confirm observation (cf. Figure 5a), but they also showed that some of the grain edges exh flat continuous thin coating (cf. Figure 5b). The thickness of both types of observed coatings did not exceed 3 nm. The well adherent carbon cover of the LTO mate constitute a good conductive network for electrons passing through the insulatin surface. This, in turn, could be beneficial to enhance the electronic conductivity LTO/C material, as well as provide a shorter diffusion pathway for the lithium ion the electrolyte, resulting in superior rate performance and cycling stability. The morphologies and particle sizes of LTO and LTO/C materials were determined with SEM. The collected data is presented in Figure 4. According to the SEM images, the size of the pristine LTO particles was larger than that of LTO/C. The estimated average sizes equaled about 245 and 202 nm for LTO and LTO/C, respectively. Besides that, the particle size distribution of the LTO/C sample was more homogeneous and the surfaces were rougher due to the deposition of carbon coating. The HR-TEM results confirmed this observation (cf. Figure 5a), but they also showed that some of the grain edges exhibited a flat continuous thin coating (cf. Figure 5b). The thickness of both types of observed carbon coatings did not exceed 3 nm. The well adherent carbon cover of the LTO material can constitute a good conductive network for electrons passing through the insulating LTO surface. This, in turn, could be beneficial to enhance the electronic conductivity of the LTO/C material, as well as provide a shorter diffusion pathway for the lithium ions from the electrolyte, resulting in superior rate performance and cycling stability. The thermogravimetric curves recorded for the LTO and LTO/C samples are shown in Figure 3. In fact, they were collected in order to estimate the content of carbon in the LTO/C material. The TGA curve for the pristine LTO material did not significantly change up to 700 °C. Only one very small mass loss was observed in the temperature range between 250 to 400 °C and this could correspond to the decomposition of some impurities undetectable by XRD, FTIR, and Raman spectroscopy. In turn, the TGA curve for the LTO/C material revealed the first mass drop of about 0.4% up to 100 °C. This corresponded to a release of adsorbed water molecules, which were also detected with the FTIR method. The second mass loss started at about 200 °C and finished at 450 °C. This mass change (~2.6% in relation to the initial mass) was related to a combustion reaction of the carbon coating with oxygen coming from the experimental atmosphere. The morphologies and particle sizes of LTO and LTO/C materials were determined with SEM. The collected data is presented in Figure 4. According to the SEM images, the size of the pristine LTO particles was larger than that of LTO/C. The estimated average sizes equaled about 245 and 202 nm for LTO and LTO/C, respectively. Besides that, the particle size distribution of the LTO/C sample was more homogeneous and the surfaces were rougher due to the deposition of carbon coating. The HR-TEM results confirmed this observation (cf. Figure 5a), but they also showed that some of the grain edges exhibited a flat continuous thin coating (cf. Figure 5b). The thickness of both types of observed carbon coatings did not exceed 3 nm. The well adherent carbon cover of the LTO material can constitute a good conductive network for electrons passing through the insulating LTO surface. This, in turn, could be beneficial to enhance the electronic conductivity of the LTO/C material, as well as provide a shorter diffusion pathway for the lithium ions from the electrolyte, resulting in superior rate performance and cycling stability.  The results coming from the nitrogen adsorption-desorption measurements are shown in Figure 6. The collected isotherms exhibited type IV isotherms with an H1-type hysteresis loop, indicating a mesoporous nature. The values of pore size and specific surface area were estimated using Barret-Joyner-Halenda (BJH) [45] and Brunauer-Emmett-Teller (BET) [46] models, respectively. According to BJH model, the average pore size was 58.7 and 37.7 nm for the pristine and carbon-coated LTO materials, respectively. The specific surface area calculated based on the BET model was 1.434 and 2.076 m 2 g −1 for the pristine and carbon-coated LTO materials, respectively. The obtained results are in good agreement with the electron microscopy observations. Namely, the larger specific surface area and smaller size of pores found for the LTO/C sample resulted from the roughness and partial filling of the pores caused by the carbon deposition. It is important that the reduction of particle size usually provides access to higher surface area. This, in turn, is beneficial for the high Li-ion flux across electrolyte/electrode interface. Cyclic voltammetry (CV) curves for the electrochemical cells with LTO and LTO/C anode materials are in Figure 7. They were recorded with scan rates of 100, 300, and 500 mV s −1 at a potential window from 1.0 to 2.5 V (vs. Li + /Li). The voltammograms for both LTO electrodes revealed only one pair of redox peaks, which corresponded to oxidation and reduction processes of the Ti 3+ /Ti 4+ couple [25,27,35]. No other redox peaks were observed, which indicates that the carbon coating did not alter the electrochemical reaction The results coming from the nitrogen adsorption-desorption measurements are shown in Figure 6. The collected isotherms exhibited type IV isotherms with an H1-type hysteresis loop, indicating a mesoporous nature. The values of pore size and specific surface area were estimated using Barret-Joyner-Halenda (BJH) [45] and Brunauer-Emmett-Teller (BET) [46] models, respectively. According to BJH model, the average pore size was 58.7 and 37.7 nm for the pristine and carbon-coated LTO materials, respectively. The specific surface area calculated based on the BET model was 1.434 and 2.076 m 2 g −1 for the pristine and carbon-coated LTO materials, respectively. The obtained results are in good agreement with the electron microscopy observations. Namely, the larger specific surface area and smaller size of pores found for the LTO/C sample resulted from the roughness and partial filling of the pores caused by the carbon deposition. It is important that the reduction of particle size usually provides access to higher surface area. This, in turn, is beneficial for the high Li-ion flux across electrolyte/electrode interface. The results coming from the nitrogen adsorption-desorption measurements are shown in Figure 6. The collected isotherms exhibited type IV isotherms with an H1-type hysteresis loop, indicating a mesoporous nature. The values of pore size and specific surface area were estimated using Barret-Joyner-Halenda (BJH) [45] and Brunauer-Emmett-Teller (BET) [46] models, respectively. According to BJH model, the average pore size was 58.7 and 37.7 nm for the pristine and carbon-coated LTO materials, respectively. The specific surface area calculated based on the BET model was 1.434 and 2.076 m 2 g −1 for the pristine and carbon-coated LTO materials, respectively. The obtained results are in good agreement with the electron microscopy observations. Namely, the larger specific surface area and smaller size of pores found for the LTO/C sample resulted from the roughness and partial filling of the pores caused by the carbon deposition. It is important that the reduction of particle size usually provides access to higher surface area. This, in turn, is beneficial for the high Li-ion flux across electrolyte/electrode interface. Cyclic voltammetry (CV) curves for the electrochemical cells with LTO and LTO/C anode materials are in Figure 7. They were recorded with scan rates of 100, 300, and 500 mV s −1 at a potential window from 1.0 to 2.5 V (vs. Li + /Li). The voltammograms for both LTO electrodes revealed only one pair of redox peaks, which corresponded to oxidation and reduction processes of the Ti 3+ /Ti 4+ couple [25,27,35]. No other redox peaks were observed, which indicates that the carbon coating did not alter the electrochemical reaction Cyclic voltammetry (CV) curves for the electrochemical cells with LTO and LTO/C anode materials are in Figure 7. They were recorded with scan rates of 100, 300, and 500 mV s −1 at a potential window from 1.0 to 2.5 V (vs. Li + /Li). The voltammograms for both LTO electrodes revealed only one pair of redox peaks, which corresponded to oxidation and reduction processes of the Ti 3+ /Ti 4+ couple [25,27,35]. No other redox peaks were observed, which indicates that the carbon coating did not alter the electrochemical reaction process in the given potential range. Nevertheless, the peaks obtained for the LTO/C electrode were much sharper than those of the pristine LTO anode. This can be explained by the faster Li + ion insertion/extraction in the case of the LTO/C electrode [47]. In addition, the carbon-coated LTO electrode showed a smaller potential difference ∆E between two redox peaks than the pristine LTO for all scan rates. This suggests that the carbon coating could efficiently diminish polarization behavior, as well as enhance the rate capability of the LTO/C electrode. process in the given potential range. Nevertheless, the peaks obtained for the LTO/C electrode were much sharper than those of the pristine LTO anode. This can be explained by the faster Li + ion insertion/extraction in the case of the LTO/C electrode [47]. In addition, the carbon-coated LTO electrode showed a smaller potential difference ΔE between two redox peaks than the pristine LTO for all scan rates. This suggests that the carbon coating could efficiently diminish polarization behavior, as well as enhance the rate capability of the LTO/C electrode. The EIS data recorded for LTO and LTO/C electrodes and their corresponding equivalent circuit models are shown in Figure 8. The shapes of the EIS spectra were similar and were composed of a semicircle at high frequency and one slope line at low frequency. An intercept at high frequency corresponded to the ohmic resistance (Rs) between the electrolyte and electrode. The large semicircle at high frequency was associated with the charge transfer resistance (Rct) and the constant phase element (CPE) corresponding to double layer capacitance at the electrode/electrolyte interface. The inclined line at low frequency was related to the Warburg impedance (W), which referred to a solid state diffusion of Li + ions into the LTO structure. The fitted Rct for the LTO and LTO/C electrodes was 40.81 and 19.37 Ω, respectively. The lower value found for the LTO/C material was associated with the carbon coating. which facilitated the electrochemical kinetics of the Ti 3+ and Ti 4+ redox reaction by the increase of the surface electrical conductivity. The presence of a carbon layer reduced the electrode polarization of the LTO, which agreed with the CV investigations. Furthermore, the Li-ion diffusion coefficient was determined for the pristine and carbon-coated LTO electrodes based on the equations: where DLi is the Li + diffusion coefficient, R is the gas constant (8.314 J mol −1 K −1 ), T is the room temperature (298 K), A is the surface area of the electrode (0.785 cm 2 ), n is the number of electrons during the half reaction of the redox couple (equal to 1), F is the Faraday constant (96,500 C mol −1 ), C is the molar concentration of Li + ions, σ is the Warburg impedance coefficient, and ω is the angular frequency in the low-frequency region. The σ was delivered from the relationship between the real axis of resistance (Z') and the reciprocal square root of the frequency (ω −0.5 ) in the low-frequency region shown in Figure 8b. The performed calculations allowed estimating the diffusion coefficient, which equaled 3.4 × 10 −12 and 1.0 × 10 −11 cm 2 s −1 for the pristine LTO and LTO/C electrodes, respectively.
The obtained values confirmed that the carbon coating provided the shorter diffusion pathways for the Li + ion into the LTO structure. The EIS data recorded for LTO and LTO/C electrodes and their corresponding equivalent circuit models are shown in Figure 8. The shapes of the EIS spectra were similar and were composed of a semicircle at high frequency and one slope line at low frequency. An intercept at high frequency corresponded to the ohmic resistance (R s ) between the electrolyte and electrode. The large semicircle at high frequency was associated with the charge transfer resistance (R ct ) and the constant phase element (CPE) corresponding to double layer capacitance at the electrode/electrolyte interface. The inclined line at low frequency was related to the Warburg impedance (W), which referred to a solid state diffusion of Li + ions into the LTO structure. The fitted R ct for the LTO and LTO/C electrodes was 40.81 and 19.37 Ω, respectively. The lower value found for the LTO/C material was associated with the carbon coating. which facilitated the electrochemical kinetics of the Ti 3+ and Ti 4+ redox reaction by the increase of the surface electrical conductivity. The presence of a carbon layer reduced the electrode polarization of the LTO, which agreed with the CV investigations. Furthermore, the Li-ion diffusion coefficient was determined for the pristine and carbon-coated LTO electrodes based on the equations: where D Li is the Li + diffusion coefficient, R is the gas constant (8.314 J mol −1 K −1 ), T is the room temperature (298 K), A is the surface area of the electrode (0.785 cm 2 ), n is the number of electrons during the half reaction of the redox couple (equal to 1), F is the Faraday constant (96,500 C mol −1 ), C is the molar concentration of Li + ions, σ is the Warburg impedance coefficient, and ω is the angular frequency in the low-frequency region.
The σ was delivered from the relationship between the real axis of resistance (Z ) and the reciprocal square root of the frequency (ω −0.5 ) in the low-frequency region shown in Figure 8b. The performed calculations allowed estimating the diffusion coefficient, which equaled 3.4 × 10 −12 and 1.0 × 10 −11 cm 2 s −1 for the pristine LTO and LTO/C electrodes, respectively. The obtained values confirmed that the carbon coating provided the shorter diffusion pathways for the Li + ion into the LTO structure. The electrochemical performance of the investigated LTO materials as anodes for LIBs was evaluated via room temperature galvanostatic discharge/charge tests. The initial charge-discharge profiles of LTO and LTO/C electrodes recorded with 0.1 C, 2 C, 5 C, 10 C, and 20 C rates (1 C = 175 mA g −1 ) are shown in Figure 9. Analyzing them, one can see that the flat discharge and charges plateaus at approximately 1.5 and 1.6 V, respectively, were well defined. They were associated with the reversible Ti 3+ /Ti 4+ redox reactions corresponding to the insertion/extraction of Li + ions into the spinel structure. It is also worth noting that additional plateaus for charge and discharge profiles appeared for the LTO/C electrodes cycled at high C-rates above 2 C. Their origin was associated with the pseudocapacitive effect induced by the presence of the carbon coating [47][48][49][50]. Thus far, it has been reported that this pseudocapacitive behavior improves the capacity and high-rate capability [47]. This is in good agreement with the obtained results for the LTO/C electrode investigated in this work. It is important that an interval of the middle sections of the charge and discharge plateaus observed for the LTO/C sample was distinctly lower than that of the pristine LTO electrode. This observation was related to lower electrode polarization. The initial discharge capacity recorded for LTO and LTO/C electrodes at 0.1 C was about 177 and 213 mAh g −1 , respectively. Undoubtedly, the lower polarization and the higher capacity observed for LTO/C sample resulted from the presence of carbon, which shortened the Li + diffusion pathways and enhanced the electronic conductivity of LTO material. Moreover, the carbon-coated LTO electrode had the higher surface area, which offered more active sites for the storage of Li + ions. It was also evident that the increase of cycling rate up to 10 C and 20 C caused a more significant drop of initial capacity for the pristine LTO.  The electrochemical performance of the investigated LTO materials as anodes for LIBs was evaluated via room temperature galvanostatic discharge/charge tests. The initial charge-discharge profiles of LTO and LTO/C electrodes recorded with 0.1 C, 2 C, 5 C, 10 C, and 20 C rates (1 C = 175 mA g −1 ) are shown in Figure 9. Analyzing them, one can see that the flat discharge and charges plateaus at approximately 1.5 and 1.6 V, respectively, were well defined. They were associated with the reversible Ti 3+ /Ti 4+ redox reactions corresponding to the insertion/extraction of Li + ions into the spinel structure. It is also worth noting that additional plateaus for charge and discharge profiles appeared for the LTO/C electrodes cycled at high C-rates above 2 C. Their origin was associated with the pseudocapacitive effect induced by the presence of the carbon coating [47][48][49][50]. Thus far, it has been reported that this pseudocapacitive behavior improves the capacity and highrate capability [47]. This is in good agreement with the obtained results for the LTO/C electrode investigated in this work. It is important that an interval of the middle sections of the charge and discharge plateaus observed for the LTO/C sample was distinctly lower than that of the pristine LTO electrode. This observation was related to lower electrode polarization. The initial discharge capacity recorded for LTO and LTO/C electrodes at 0.1 C was about 177 and 213 mAh g −1 , respectively. Undoubtedly, the lower polarization and the higher capacity observed for LTO/C sample resulted from the presence of carbon, which shortened the Li + diffusion pathways and enhanced the electronic conductivity of LTO material. Moreover, the carbon-coated LTO electrode had the higher surface area, which offered more active sites for the storage of Li + ions. It was also evident that the increase of cycling rate up to 10 C and 20 C caused a more significant drop of initial capacity for the pristine LTO. The electrochemical performance of the investigated LTO materials as anodes for LIBs was evaluated via room temperature galvanostatic discharge/charge tests. The initial charge-discharge profiles of LTO and LTO/C electrodes recorded with 0.1 C, 2 C, 5 C, 10 C, and 20 C rates (1 C = 175 mA g −1 ) are shown in Figure 9. Analyzing them, one can see that the flat discharge and charges plateaus at approximately 1.5 and 1.6 V, respectively, were well defined. They were associated with the reversible Ti 3+ /Ti 4+ redox reactions corresponding to the insertion/extraction of Li + ions into the spinel structure. It is also worth noting that additional plateaus for charge and discharge profiles appeared for the LTO/C electrodes cycled at high C-rates above 2 C. Their origin was associated with the pseudocapacitive effect induced by the presence of the carbon coating [47][48][49][50]. Thus far, it has been reported that this pseudocapacitive behavior improves the capacity and high-rate capability [47]. This is in good agreement with the obtained results for the LTO/C electrode investigated in this work. It is important that an interval of the middle sections of the charge and discharge plateaus observed for the LTO/C sample was distinctly lower than that of the pristine LTO electrode. This observation was related to lower electrode polarization. The initial discharge capacity recorded for LTO and LTO/C electrodes at 0.1 C was about 177 and 213 mAh g −1 , respectively. Undoubtedly, the lower polarization and the higher capacity observed for LTO/C sample resulted from the presence of carbon, which shortened the Li + diffusion pathways and enhanced the electronic conductivity of LTO material. Moreover, the carbon-coated LTO electrode had the higher surface area, which offered more active sites for the storage of Li + ions. It was also evident that the increase of cycling rate up to 10 C and 20 C caused a more significant drop of initial capacity for the pristine LTO.   The cycling performance tests of LTO and LTO/C electrodes are shown in Figure 10. The obtained results indicate that both investigated materials had stable cycling life during 50 consecutive discharge-charge cycles. The Coulombic efficiency was established at the level of approximately 100%, indicating that the electrochemical Li + lithiation-delithiation processes are entirely reversible. However, it should be noted that the LTO/C electrode exhibited significantly higher capacity of about 211 mAh g −1 after 50 cycles, whereas the capacity of the LTO was around 171 mAh g −1 . The capacity for LTO/C did not change even after 100 cycles at 0.1 C. In turn, the increase of charge-discharge rate up to 10 C and 20 C caused a drop of capacity equal to 144 and 106 mAh g −1 after 100 cycles, respectively. The performed tests indicated that the cycling performance of the LTO/C electrode was very stable during the cycling, even at high rates. The cycling performance tests of LTO and LTO/C electrodes are shown in Figure 10. The obtained results indicate that both investigated materials had stable cycling life during 50 consecutive discharge-charge cycles. The Coulombic efficiency was established at the level of approximately 100%, indicating that the electrochemical Li + lithiation-delithiation processes are entirely reversible. However, it should be noted that the LTO/C electrode exhibited significantly higher capacity of about 211 mAh g −1 after 50 cycles, whereas the capacity of the LTO was around 171 mAh g −1 . The capacity for LTO/C did not change even after 100 cycles at 0.1 C. In turn, the increase of charge-discharge rate up to 10 C and 20 C caused a drop of capacity equal to 144 and 106 mAh g −1 after 100 cycles, respectively. The performed tests indicated that the cycling performance of the LTO/C electrode was very stable during the cycling, even at high rates. To investigate the effect of carbon coating on electrochemical performance, the rate capabilities of LTO, and LTO/C electrodes were tested at different current densities from 0.1 to 20 C, as shown in Figure 11. According to the obtained results, the LTO/C electrode exhibited better cycling response to continuously varied current densities than the pristine LTO electrode. This observation was based on the specific capacity for the LTO significantly decreasing with the increasing C-rate. The LTO and LTO/C electrodes achieved capacities of 173 and 203 mAh g −1 , respectively, at the initial 0.1 C rate. As the current rate increased from 0.1 to 2, 5, 10, and 20 C, the capacity decreased to 152 (88% retention), 112 (65% retention), 74 (43% retention), and 27 (16% retention) mAh g −1 for the LTO electrode and 191 (94% retention), 153 (75% retention), 135 (67% retention), and 107 (53% retention) mAh g −1 for the LTO/C electrode. In turn, when the current rate came back to 0.1 C, the capacities returned to their initial values. This indicates that the capacity fade for the LTO/C was reduced by the presence of carbon coating. which provided (i) shorter distance for solid-state diffusion of the Li + ions during the charge and discharge cycling, and (ii) higher surface area and conductivity, confirmed by BET and EIS measurements, respectively. To investigate the effect of carbon coating on electrochemical performance, the rate capabilities of LTO, and LTO/C electrodes were tested at different current densities from 0.1 to 20 C, as shown in Figure 11. According to the obtained results, the LTO/C electrode exhibited better cycling response to continuously varied current densities than the pristine LTO electrode. This observation was based on the specific capacity for the LTO significantly decreasing with the increasing C-rate. The LTO and LTO/C electrodes achieved capacities of 173 and 203 mAh g −1 , respectively, at the initial 0.1 C rate. As the current rate increased from 0.1 to 2, 5, 10, and 20 C, the capacity decreased to 152 (88% retention), 112 (65% retention), 74 (43% retention), and 27 (16% retention) mAh g −1 for the LTO electrode and 191 (94% retention), 153 (75% retention), 135 (67% retention), and 107 (53% retention) mAh g −1 for the LTO/C electrode. In turn, when the current rate came back to 0.1 C, the capacities returned to their initial values. This indicates that the capacity fade for the LTO/C was reduced by the presence of carbon coating. which provided (i) shorter distance for solid-state diffusion of the Li + ions during the charge and discharge cycling, and (ii) higher surface area and conductivity, confirmed by BET and EIS measurements, respectively.

Conclusions
This work presents the successful preparation of carbon-coated LTO material and its application as the anode material for the Li-ion batteries. The pristine LTO material was synthetized through the sol-gel process and then was covered with a thin carbon coating

Conclusions
This work presents the successful preparation of carbon-coated LTO material and its application as the anode material for the Li-ion batteries. The pristine LTO material was synthetized through the sol-gel process and then was covered with a thin carbon coating obtained from carrot juice. The LTO/C electrode revealed enhanced conductivity, cyclic stability, and excellent high-rate capability in comparison with the pristine LTO electrode. The LTO/C exhibited a high reversible specific capacity of 212 mAh g −1 at 0.1 C at the potential window between 1.0 and 2.5 V, even after 100 cycles. Moreover, the capacity retention reached 53% of the initial value when the charge-discharge rate increased from 0.1 C to 20 C. Its excellent electrochemical performance was attributed to the presence of carbon coating, which provided (i) shorter diffusion pathways for the Li + ions into the LTO structure during the charge and discharge cycling, and (ii) higher surface area and conductivity, confirmed by BET as well as EIS measurements, respectively. The obtained results indicate that the investigated LTO/C electrode fulfils the necessary requirements to be the anode material for the future high-rate lithium-ion batteries.