High Performance of Salt-Modiﬁed–LTO Anode in LiFePO 4 Battery

: Highly crystalline “zero-strain” Li 4 Ti 5 O 12 (LTO) has great potential as an alternative material for the anodes in a lithium ion battery. In this research, highly crystalline LTO with impressive electrochemical characteristics was synthesized via a salt-assisted solid-state reaction using TiO 2 , LiOH, and various amounts of NaCl as a salt additive. The LTO particles exhibited a cubic spinel structure with homogenous micron-sized particles. The highest initial speciﬁc discharge capacity of LTO was 141.04 mAh / g with 4 wt % NaCl addition, which was tested in a full-cell (LTO / LiFePO 4 ) battery. The battery cell showed self-recovery ability during the cycling test at 10 C-rate, which can extend the cycle life of the cell. The salt-assisted process a ﬀ ected the crystallinity of the LTO particles, which has a favorable e ﬀ ect on its electrochemical performance as anodes.


Introduction
Lithium titanate (Li 4 Ti 5 O 12 ), which is usually referred to as LTO, has become a promising candidate for anode material that could substitute for carbon. LTO operates on a stable voltage of ≈1.5 V vs. Li+/Li, which avoid electrolyte decomposition and the safety issues presented by the use of carbon [1][2][3] that tends to form lithium dendrite and causes the occurrence of a surface passivation film. The surface passivation film could cause an initial loss in capacity, safety issues, loss of critical electrode performance, and cell failure [4,5]. In the midst of many promising candidates for anodes such as Si, SiO x , SiO 2 , SnO 2 , Fe 2 O 3 , Co 3 O 4 , etc. [6][7][8][9][10][11][12][13], LTO has superior structural stability with no structural or volume changes during the lithiation/de-lithiation process. Hence, LTO is classified as a zero-strain material that has 175 mAh/g of theoretical capacity [14][15][16].
As a lithium-intercalation material, the high crystallinity of LTO is a key for great capacity and cyclability [17]. Shen et al. 2013 studied the effect of crystallinity on the electrochemical performance

Materials Li 4 Ti 5 O 12
LiOH (Merck, Darmstadt, Germany) and TiO 2 (Merck, Darmstadt, Germany) were utilized as Li and Ti sources of Li 4 Ti 5 O 12 . Analytical grade NaCl (Merck, Darmstadt, Germany) was used as the salt addition, while CH 3 OH or methanol (Merck, Darmstadt, Germany) was used as the dispersant during solid-state mixing. All of the starting materials were used without any further purification steps.

Li 4 Ti 5 O 12 Preparation
The Li 4 Ti 5 O 12 (LTO) powders were prepared via solid-state reaction. TiO 2 and LiOH were used as titanium and lithium sources in a Li:Ti molar ratio of 4:5. In order to investigate the effect of salt addition on LTO powder, three different amounts of NaCl (0, 2, 4 and 6 wt %) were added. TiO 2 , LiOH, NaCl, and methanol were mixed in a ball mill for 4 h. Methanol acted as a dispersant to homogenize the precursors. The precursors were dried in an oven to evaporate the methanol and then sintered at 800 • C for 12 h under O 2 (Samator, Jakarta, Indonesia) atmosphere. After the sintering process, the LTO powders were ground and screened using a 100 mesh. The flowchart of Synthesis Process of Li 4 Ti 5 O 12 Powder is explained in Figure A1.

Li 4 Ti 5 O 12 /LiFePO 4 Cylindrical Cell Assembly
The electrochemical characteristics were analyzed using a full cell (cylindrical battery of 18650-type), which is used in commercial LiFePO 4 /LFP (MTI, Richmond, CA, USA) as a cathode. The LTO powders were mixed with conductive carbon (acetylene black/AB, MTI, Richmond, CA, USA), and water-based binders (carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR)) at a weight ratio of 80:10:2:8 in a water to make a slurry that could be coated onto copper foil using the doctor s blade technique, followed by drying to evaporate the water. Then, the electrode was fabricated to the full-cell in a glove box under Ar atmosphere. LiPF 6 (in 1:1 v/v of EC/DMC) and cellgard (MTI, Richmond, CA, USA) were used as an electrolyte and separator (MTI, Richmond, CA, USA). The charge/discharge performance of the LTO/LFP cell were tested using MTI BST-CH8-3A battery analyzer (1.0 V to 3.0 V potential range). The anode was selected as the limiting electrode. Therefore, the calculation of the specific capacity was referred to the weight of the anode material. To assure the consistency of the results, four cells of each LTO sample with consistent results were assembled.

Structural Analysis of LTO
In order to investigate the structural properties of LTO materials samples obtained via the salt-assisted solid state method, the material was characterized using XRD. Figure 1 presents the XRD patterns of the LTO powders with various additions of NaCl with 2θ (deg.) range of 10-70 • . The XRD patterns of all samples show that all peaks of LTO are detected and corresponding to the JCPDS Card No. 42-0207 with the structure of cubic spinel and the space group of F3dm [43][44][45]. Based on the XRD results, no slight impurities of NaCl were detected in the XRD pattern of LTO-2 wt % NaCl and LTO-4 wt % NaCl. However, with an addition of 6 wt % NaCl, tiny peaks of NaCl appear in the XRD pattern. The peaks of NaCl were easily detected at 2θ of 31 • and 45 • (JCPDS No. 5-0628).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 14 using thermogravimetric and differential scanning calorimeter (TG-DSC) with a heating rate of 10 °C/min.

Li4Ti5O12/LiFePO4 Cylindrical Cell Assembly
The electrochemical characteristics were analyzed using a full cell (cylindrical battery of 18650-type), which is used in commercial LiFePO4/LFP (MTI, Richmond, CA, USA)) as a cathode. The LTO powders were mixed with conductive carbon (acetylene black/AB, MTI, Richmond, CA, USA), and water-based binders (carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR)) at a weight ratio of 80:10:2:8 in a water to make a slurry that could be coated onto copper foil using the doctor′s blade technique, followed by drying to evaporate the water. Then, the electrode was fabricated to the full-cell in a glove box under Ar atmosphere. LiPF6 (in 1:1 v/v of EC/DMC) and cellgard (MTI, Richmond, CA, USA) were used as an electrolyte and separator (MTI, Richmond, CA, USA). The charge/discharge performance of the LTO/LFP cell were tested using MTI BST-CH8-3A battery analyzer (1.0 V to 3.0 V potential range). The anode was selected as the limiting electrode. Therefore, the calculation of the specific capacity was referred to the weight of the anode material. To assure the consistency of the results, four cells of each LTO sample with consistent results were assembled.

Structural Analysis of LTO
In order to investigate the structural properties of LTO materials samples obtained via the salt-assisted solid state method, the material was characterized using XRD. Figure 1 presents the XRD patterns of the LTO powders with various additions of NaCl with 2θ (deg.) range of 10-70°. The XRD patterns of all samples show that all peaks of LTO are detected and corresponding to the JCPDS Card No. 42-0207 with the structure of cubic spinel and the space group of F3dm [43][44][45]. Based on the XRD results, no slight impurities of NaCl were detected in the XRD pattern of LTO-2 wt % NaCl and LTO-4 wt % NaCl. However, with an addition of 6 wt % NaCl, tiny peaks of NaCl appear in the XRD pattern. The peaks of NaCl were easily detected at 2θ of 31° and 45° (JCPDS No. 5-0628).  For further analysis of the XRD patterns, the calculation of lattice constants, Full Width at Half Maximum (FWHM), and crystallite size were calculated and are presented in Table 1. The values of lattice constants were approximately 8.384-8.362 Å for the various samples of LTO, which correspond to those of the LTO sample reported in the literature (8.395-8.3538 Å), [34,[46][47][48] and indicates that LTO is a zero-strain material with good structural stability [48,49]. Based on these results, it can be predicted that the as-obtained LTO exhibits no volume expansion during the lithiation/delithiation process. The lattice constant values are decreased with NaCl additions ranging from 0-4 wt % but increased with an NaCl addition of 6 wt %. Since LTO with 4 wt % NaCl has the lowest lattice constant, it is predicted that the sample will have better electrode performance compared to the other samples during electrochemical testing, as reported by Huang et al. [50]. Surprisingly, the XRD results revealed the effect of NaCl addition on the crystallinity of LTO material. The crystallinity of LTO can be evaluated based on the FWHM (β) of (111), (311), and (400) peaks. Table 1 shows the FWHM values of LTO samples decrease from the range of 0 to 6 wt % NaCl addition. It means that the higher NaCl addition will result in the sharper peak on XRD and smaller values of FWHM, indicating the higher crystallinity of the material [51]. Therefore, it can be concluded that the increasing salt addition during the solid-state reaction had a good influence on LTO crystal formation. Salt exerts a medium reaction that accelerates the mass transfer of a precursor during the sintering process. During the reaction process, the NaCl (salt) enlarges and provides heat flux, which improves crystal growth [39]. Therefore, a highly crystalline product can be produced rapidly.
The crystallite sizes of the samples with 0-6 wt % NaCl additions were calculated using the Debye-Scherrer equation [51][52][53]. The crystallite sizes of LTO on (111), (311), and (400) peaks increase as the NaCl additions increase, which is in agreement with a study reported by Cai et al. [51].

Morphology Analysis of LTO
In order to observe the morphology, the LTO materials were characterized using SEM. Figure 2 shows different magnifications of SEM images for LTO powder with various amounts of NaCl addition via the solid-state reaction. The LTO particles exhibit micro sizes ranging from 1.83 to 2.28 µm, and all samples display a cubic morphology with a smooth surface. Moreover, previous studies reported that at certain concentrations, salt can prevent the agglomeration of particles during the sintering process [37,38,54]. Based on SEM images, the agglomeration of LTO particles is occurred without the NaCl addition ( Figure 2a). With the NaCl addition of 2-4 wt %, only a slight agglomeration is formed, indicating an effective inhibition of agglomeration using salt. However, the increasing NaCl addition to 6 wt % lead to more agglomeration of LTO particles as referred by Kong et al. [54]. Based on SEM images, it is predicted that LTO with a slight agglomeration will have better electrochemical performances.

Electrochemical Performance Measurement of LTO in LTO/LFP Battery
Then, for large-scale application, the electrochemical performance of LTO material is important to be analyzed using a full-cell battery (LTO vs. LFP). The LTO/LFP battery was tested using NEWARE Battery analyzer and BTS software. First, in order to investigate the best C-rate for the formation of an LTO-LFP battery, the various C-rates were used for testing. Figure A2 presents the charge-discharge formation curve at various C-rates of an LTO-0 wt % NaCl/LFP battery with a voltage range of 1.0-3.0 V.
The optimum initial specific capacity was operated at 0.3 C-rate, which obtained the specific discharge capacity of 121.1 mAh/g. Hence, the 0.3 C-rate was chosen for further testing of the LTO material with NaCl addition.

Electrochemical Performance Measurement of LTO in LTO/LFP Battery
Then, for large-scale application, the electrochemical performance of LTO material is important to be analyzed using a full-cell battery (LTO vs. LFP). The LTO/LFP battery was tested using NEWARE Battery analyzer and BTS software. First, in order to investigate the best C-rate for the formation of an LTO-LFP battery, the various C-rates were used for testing. Figure A2 presents the charge-discharge formation curve at various C-rates of an LTO-0 wt % NaCl/LFP battery with a voltage range of 1.0-3.0 V. The optimum initial specific capacity was operated at 0.3 C-rate, which obtained the specific discharge capacity of 121.1 mAh/g. Hence, the 0.3 C-rate was chosen for further testing of the LTO material with NaCl addition. Figure 3 presents the electrochemical performance (1.0-3.0 V) of LTO with various additions of NaCl at 0.3 C-rate. The LTO/LFP battery displayed a plateau voltage at ≈1.8 V, which agrees with the values found in the literature [42,55]. The initial specific discharge capacities at 0.3 C-rate of LTO with NaCl additions of 0, 2, 4, and 6 wt % were 121.1, 132.4, 145.6, and 113.5 mAh/g, respectively. The LTO-4 wt % NaCl shows the highest initial discharge capacity in this study, which confirms the predictions [50] that the highly crystalline LTO performs at great capacity [17]. However, LTO-6 wt % NaCl has the highest level of crystallinity, but it also has the lowest initial discharge capacity. It means that the addition of 6 wt % NaCl cannot be ignored on LTO material, which is confirmed by the appearance of NaCl peaks in the XRD pattern. In addition, based on the SEM result, the NaCl addition of 6 wt % can cause higher agglomeration on LTO morphology, resulting in the lowest initial specific discharge capacity in this study. It can be concluded that the higher NaCl addition (>6 wt %) causes the NaCl to act as impurities on the LTO material, which will have a poor effect on the electrochemical performance. The initial Coulombic efficiency (CE) of LTO with NaCl additions of 0, 2, 4, and 6 wt % were 84.46%, 90.99%, 91.55%, and 97.84%, respectively. Based on this finding, it can be concluded that a high level of crystallinity results in better structural properties and caused better CE, which is consistent with previous research reported by Shen et al. [18].   [42,55]. The initial specific discharge capacities at 0.3 C-rate of LTO with NaCl additions of 0, 2, 4, and 6 wt % were 121.1, 132.4, 145.6, and 113.5 mAh/g, respectively. The LTO-4 wt % NaCl shows the highest initial discharge capacity in this study, which confirms the predictions [50] that the highly crystalline LTO performs at great capacity [17]. However, LTO-6 wt % NaCl has the highest level of crystallinity, but it also has the lowest initial discharge capacity. It means that the addition of 6 wt % NaCl cannot be ignored on LTO material, which is confirmed by the appearance of NaCl peaks in the XRD pattern. In addition, based on the SEM result, the NaCl addition of 6 wt % can cause higher agglomeration on LTO morphology, resulting in the lowest initial specific discharge capacity in this study. It can be concluded that the higher NaCl addition (≥6 wt %) causes the NaCl to act as impurities on the LTO material, which will have a poor effect on the electrochemical performance. The initial Coulombic efficiency (CE) of LTO with NaCl additions of 0, 2, 4, and 6 wt % were 84.46%, 90.99%, 91.55%, and 97.84%, respectively. Based on this finding, it can be concluded that a high level of crystallinity results in better structural properties and caused better CE, which is consistent with previous research reported by Shen et al. [18].
In order to investigate the rate ability of LTO without and with NaCl addition, the LTO/LFP battery was charged and discharged at various higher current rates. Figure 4 compares the rate ability of LTO-0 wt % NaCl and LTO-4 wt % NaCl, as tested at 0.3, 1, 3, 5, 7, and 10 C charge-discharge rate. LTO-4 wt % NaCl shows a discharge capacity that ranged from 145.6 mAh/g at 0.3 C to 118.6 mAh/g at 10 C. Meanwhile, LTO-0 wt % NaCl showed a discharge capacity that ranged from 123.8 mAg/g at 0.3 C to 86.3 mAh/g at 10 C. It can be seen that the specific capacity of LTO-4 wt % only slightly decreases from 0.3 to 10 C-rate. Compared to LTO-0 wt % NaCl, LTO-4 wt % NaCl has a higher specific capacity at various higher current rates in this study. The capacity retentions of the LTO/LFP battery at 0.3-10 C-rate are presented in Table 2. The LTO-4 wt % NaCl has a smaller drop-specific capacity (18.6%) than LTO-0 wt % NaCl (30.8%) from 0.3 to 10 C-rate. It proves that the crystallinity of the LTO material can improve the rate ability performance compared with the previous studies [17,18].
However, both samples have a good reversibility while returning to the first rate at 0.3 C (99.9% for LTO-4 wt % NaCl and 99.7% for LTO-0 wt % NaCl).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 14 Figure 3. Specific initial discharge capacities of LTO with various NaCl additions at 0.3 C-rate.
In order to investigate the rate ability of LTO without and with NaCl addition, the LTO/LFP battery was charged and discharged at various higher current rates. Figure 4 compares the rate ability of LTO-0 wt % NaCl and LTO-4 wt % NaCl, as tested at 0.3, 1, 3, 5, 7, and 10 C chargedischarge rate. LTO-4 wt % NaCl shows a discharge capacity that ranged from 145.6 mAh/g at 0.3 C to 118.6 mAh/g at 10 C. Meanwhile, LTO-0 wt % NaCl showed a discharge capacity that ranged from 123.8 mAg/g at 0.3 C to 86.3 mAh/g at 10 C. It can be seen that the specific capacity of LTO-4 wt % only slightly decreases from 0.3 to 10 C-rate. Compared to LTO-0 wt % NaCl, LTO-4 wt % NaCl has a higher specific capacity at various higher current rates in this study. The capacity retentions of the LTO/LFP battery at 0.3-10 C-rate are presented in Table 2. The LTO-4 wt % NaCl has a smaller drop-specific capacity (18.6%) than LTO-0 wt % NaCl (30.8%) from 0.3 to 10 C-rate. It proves that the crystallinity of the LTO material can improve the rate ability performance compared with the previous studies [17,18]. However, both samples have a good reversibility while returning to the first rate at 0.3 C (99.9% for LTO-4 wt % NaCl and 99.7% for LTO-0 wt % NaCl).   In order to investigate the rate ability of LTO without and with NaCl addition, the LTO/LFP battery was charged and discharged at various higher current rates. Figure 4 compares the rate ability of LTO-0 wt % NaCl and LTO-4 wt % NaCl, as tested at 0.3, 1, 3, 5, 7, and 10 C chargedischarge rate. LTO-4 wt % NaCl shows a discharge capacity that ranged from 145.6 mAh/g at 0.3 C to 118.6 mAh/g at 10 C. Meanwhile, LTO-0 wt % NaCl showed a discharge capacity that ranged from 123.8 mAg/g at 0.3 C to 86.3 mAh/g at 10 C. It can be seen that the specific capacity of LTO-4 wt % only slightly decreases from 0.3 to 10 C-rate. Compared to LTO-0 wt % NaCl, LTO-4 wt % NaCl has a higher specific capacity at various higher current rates in this study. The capacity retentions of the LTO/LFP battery at 0.3-10 C-rate are presented in Table 2. The LTO-4 wt % NaCl has a smaller drop-specific capacity (18.6%) than LTO-0 wt % NaCl (30.8%) from 0.3 to 10 C-rate. It proves that the crystallinity of the LTO material can improve the rate ability performance compared with the previous studies [17,18]. However, both samples have a good reversibility while returning to the first rate at 0.3 C (99.9% for LTO-4 wt % NaCl and 99.7% for LTO-0 wt % NaCl).   Then, in order to study the performance stability of the LTO material, a cycle performance test at 10 C was conducted. Figure 5a shows the cycling test of LTO-0 wt % NaCl and LTO-4 wt % NaCl at the 10 C-rate. In the 1st cycle, the specific capacities of LTO-0 wt % NaCl and LTO-4 wt % NaCl were 86.3 and 118.6 mAh/g, respectively. After the 1065th cycle, the specific capacities of LTO-0 wt % NaCl and LTO-4 wt % NaCl decreased to 74.2 and 114.5 mAh/g, respectively. The cycling test shows that the drop capacity percentages of LTO-0 wt % NaCl and LTO-4 wt % NaCl after 1065 cycles are 13.9% and 3.4%, respectively. It exhibits that the LTO-4 wt % NaCl provides superior cyclability, which has only a 0.003% drop capacity per cycle. The result confirm that highly crystalline LTO shows not only good performance in rate ability but also cycling stability as reported by Sha et al. and Shen et al. [17,18]. On comparing with previous studies on LTO synthesis, it was found that employing a simple modification of NaCl addition during the synthesis resulted in high capacity, good rate ability, and good cyclability. Different from other studies listed in Table 3, the electrochemical performance of LTO samples was tested in an LFP/LTO 18650 cylindrical cell that is often found in the market; thus, the result provides better proof for large-scale application.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 14 % NaCl is not significant compared to that of an LTO 4 wt % NaCl. Since the gradual decrease and increase of specific capacity significantly occurred in LTO 4 wt % NaCl, it can be concluded that the cell exhibits a self-recovery ability that can extend the cycle life of the cell due to the presence of NaCl. In addition, the cells were tested using a cylindrical (18,650 type) full cell, and it is safe to say that the results provide superior evidences for the mass production of LTO/LFP considerations with superior performance. The average capacity of LTO 4 wt % NaCl is 83.6 mAh/g, and based on 1st cycle and 8000th cycles, the capacity retention is 98%.

Post Cycle Analysis of LTO
The post cycle of the LTO structure was analyzed to evaluate the volume expansion of the LTO material after 1060 cycles. Figure 6a displays the XRD pattern of LTO-4 wt % NaCl after 1060 cycles compared to the initial XRD pattern. The XRD patterns demonstrate that there are no changes on the peaks of LTO before and after the cycling test. However, new peaks are observable at 2θ of 27.42°, 33.08°, and 36.15°, which strongly correspond to the peaks of residual acetylene black (AB) [61], Li2CO3 [62], and LiF [62,63], respectively. The presence of Li2CO3 originated from the side reaction between lithium and alkyl carbonates (electrolyte solvent) during the intercalation/de-intercalation process [62]. Then, the existence of LiF was due to the decomposition of LiPF6 to LiF and PF5 [62]. Moreover, the formation of PF5 can also react with Li2CO3 to LiF6, as reported by He et al. [63].
Based on Figure 6b, the SEM image of LTO-4 wt % NaCl is still similar to the initial SEM image (Figure 2c) with cubic morphology. However, the cubic-shaped particles of LTO-4 wt % NaCl are covered by a thin film. It is predicted that the film consists of residual electrolyte and a solid electrolyte interface as referred by previous studies [62,63].   Since an LTO/LFP battery has an impressive cyclability, we perform a further cycle analysis using different cells and extended the cycles to 8000 in Figure 5b. During high rate charge/discharge using a 10 C-rate current, the capacity drop can be observed; however, at a certain point, the specific discharge capacity increases to around ≈120 mAh/g. The phenomenon occurred constantly and consistently up to 8000 cycles and consistent with every cell testing, which can be seen in Figure A3. Technically, this can be caused by a continuous cycling process with no significant resting time [59,60] and due to the temperature fluctuation of the surrounding areas, which is not tightly controlled [18]. However, based on Figure 5a, the capacity fluctuation that occurred in an LTO 0 wt % NaCl is not significant compared to that of an LTO 4 wt % NaCl. Since the gradual decrease and increase of specific capacity significantly occurred in LTO 4 wt % NaCl, it can be concluded that the cell exhibits a self-recovery ability that can extend the cycle life of the cell due to the presence of NaCl. In addition, the cells were tested using a cylindrical (18,650 type) full cell, and it is safe to say that the results provide superior evidences for the mass production of LTO/LFP considerations with superior performance. The average capacity of LTO 4 wt % NaCl is 83.6 mAh/g, and based on 1st cycle and 8000th cycles, the capacity retention is 98%.

Post Cycle Analysis of LTO
The post cycle of the LTO structure was analyzed to evaluate the volume expansion of the LTO material after 1060 cycles. Figure 6a displays the XRD pattern of LTO-4 wt % NaCl after 1060 cycles compared to the initial XRD pattern. The XRD patterns demonstrate that there are no changes on the peaks of LTO before and after the cycling test. However, new peaks are observable at 2θ of 27.42 • , 33.08 • , and 36.15 • , which strongly correspond to the peaks of residual acetylene black (AB) [61], Li 2 CO 3 [62], and LiF [62,63], respectively. The presence of Li 2 CO 3 originated from the side reaction between lithium and alkyl carbonates (electrolyte solvent) during the intercalation/de-intercalation process [62]. Then, the existence of LiF was due to the decomposition of LiPF 6 to LiF and PF 5 [62]. Moreover, the formation of PF 5 can also react with Li 2 CO 3 to LiF 6 , as reported by He et al. [63].
that the results provide superior evidences for the mass production of LTO/LFP considerations with superior performance. The average capacity of LTO 4 wt % NaCl is 83.6 mAh/g, and based on 1st cycle and 8000th cycles, the capacity retention is 98%.

Post Cycle Analysis of LTO
The post cycle of the LTO structure was analyzed to evaluate the volume expansion of the LTO material after 1060 cycles. Figure 6a displays the XRD pattern of LTO-4 wt % NaCl after 1060 cycles compared to the initial XRD pattern. The XRD patterns demonstrate that there are no changes on the peaks of LTO before and after the cycling test. However, new peaks are observable at 2θ of 27.42°, 33.08°, and 36.15°, which strongly correspond to the peaks of residual acetylene black (AB) [61], Li2CO3 [62], and LiF [62,63], respectively. The presence of Li2CO3 originated from the side reaction between lithium and alkyl carbonates (electrolyte solvent) during the intercalation/de-intercalation process [62]. Then, the existence of LiF was due to the decomposition of LiPF6 to LiF and PF5 [62]. Moreover, the formation of PF5 can also react with Li2CO3 to LiF6, as reported by He et al. [63].
Based on Figure 6b, the SEM image of LTO-4 wt % NaCl is still similar to the initial SEM image (Figure 2c) with cubic morphology. However, the cubic-shaped particles of LTO-4 wt % NaCl are covered by a thin film. It is predicted that the film consists of residual electrolyte and a solid electrolyte interface as referred by previous studies [62,63].  Based on Figure 6b, the SEM image of LTO-4 wt % NaCl is still similar to the initial SEM image (Figure 2c) with cubic morphology. However, the cubic-shaped particles of LTO-4 wt % NaCl are covered by a thin film. It is predicted that the film consists of residual electrolyte and a solid electrolyte interface as referred by previous studies [62,63].
The structural parameter of LTO-4 wt % NaCl can be evaluated using XRD data, and the results are displayed in Table 4. The lattice constant of LTO-4 wt % NaCl after cycles is non-significantly different compared to the initial LTO-4 wt % NaCl, which provides strong evidence of a zero-strain material [48,49]. The FWHM analysis indicates that the crystallinity of LTO is slightly decreased after 1060 cycles, which is correlated with the bigger size of LTO crystallite.

TG-DSC Study of Salt-Assisted LTO Formation
The thermogravimetric and differential scanning calorimeter (TG-DSC) analysis of LTO-0 wt % NaCl and LTO-4 wt % NaCl are depicted in Figure 7. It is clearly observed that both curves show a similar trend. At temperatures between 0 and 100 • C, there are weight losses around 12.06 and 10.10 wt % of LTO-0 wt % NaCl and LTO-4 wt % NaCl, respectively, which indicated the removal of absorbed water in the LTO precursor, according to the endothermic peaks at 107.5 and 95.8 • C [64]. At temperatures between 100 and 415 • C, both curves show a flat straight line, which indicates that no mass change is occurred. However, at temperatures of 411.1 and 406.8 • C, the endothermic peak is detected, which indicates the melting point of LiOH [65]. Then, the small weight losses of LTO-0 wt % NaCl and LTO-4 wt % NaCl (4.35 and 4.11 wt %) that occurred at 454.6 and 441.5 • C, respectively, can be interpreted as the hydroxylation [66]. While the temperature increases to 612.8 and 587.2 • C of both samples, a small weight loss is observed in each curve (3.13 and 3.08 wt %) corresponding to tiny endothermic peaks at 612.8 and 587.2 • C, respectively, which is attributed the reaction of LTO. The formation of spinel LTO-0 wt % NaCl and LTO-4 wt % NaCl occurred completely at temperatures of 717.4 and 624.6 • C, respectively, indicating no mass change occurred. However, a new endothermic peak is observed at 796.1 • C in LTO-4 wt % NaCl, which corresponds to the melting point of NaCl [67]. This phenomenon confirmed the XRD analysis that the addition of NaCl alters the sintering behavior. The crystal growth of LTO-4 wt % NaCl initiates earlier at 624.6 • C, whereas LTO-0 wt % NaCl initiates at 717.4 • C, which clarifies the higher crystallite size of LTO-4 wt % NaCl than LTO-0 wt % NaCl.
are displayed in Table 4. The lattice constant of LTO-4 wt % NaCl after cycles is non-significantly different compared to the initial LTO-4 wt % NaCl, which provides strong evidence of a zero-strain material [48,49]. The FWHM analysis indicates that the crystallinity of LTO is slightly decreased after 1060 cycles, which is correlated with the bigger size of LTO crystallite.

TG-DSC Study of Salt-Assisted LTO Formation
The thermogravimetric and differential scanning calorimeter (TG-DSC) analysis of LTO-0 wt % NaCl and LTO-4 wt % NaCl are depicted in Figure 7. It is clearly observed that both curves show a similar trend. At temperatures between 0 and 100 °C, there are weight losses around 12.06 and 10.10 wt % of LTO-0 wt % NaCl and LTO-4 wt % NaCl, respectively, which indicated the removal of absorbed water in the LTO precursor, according to the endothermic peaks at 107.5 and 95.8 °C [64]. At temperatures between 100 and 415 °C, both curves show a flat straight line, which indicates that no mass change is occurred. However, at temperatures of 411.1 and 406.8 °C, the endothermic peak is detected, which indicates the melting point of LiOH [65]. Then, the small weight losses of LTO-0 wt % NaCl and LTO-4 wt % NaCl (4.35 and 4.11 wt %) that occurred at 454.6 and 441.5 °C, respectively, can be interpreted as the hydroxylation [66]. While the temperature increases to 612.8 and 587.2 °C of both samples, a small weight loss is observed in each curve (3.13 and 3.08 wt %) corresponding to tiny endothermic peaks at 612.8 and 587.2 °C, respectively, which is attributed the reaction of LTO. The formation of spinel LTO-0 wt % NaCl and LTO-4 wt % NaCl occurred completely at temperatures of 717.4 and 624.6 °C, respectively, indicating no mass change occurred. However, a new endothermic peak is observed at 796.1°C in LTO-4 wt % NaCl, which corresponds to the melting point of NaCl [67]. This phenomenon confirmed the XRD analysis that the addition of NaCl alters the sintering behavior. The crystal growth of LTO-4 wt % NaCl initiates earlier at 624.6 °C, whereas LTO-0 wt % NaCl initiates at 717.4 °C, which clarifies the higher crystallite size of LTO-4 wt % NaCl than LTO-0 wt % NaCl.

Conclusions
The synthesis of highly crystalline Li 4 Ti 5 O 12 material that shows high electrochemical performance was successful using a salt-assisted solid-state reaction with LTO made up of micron-sized, homogenous particles. The XRD pattern results showed that the NaCl addition enhanced the crystallinity of the LTO material. Increasing the NaCl addition from 0 to 6 wt % improved the crystal formation of the LTO material. However, the optimal amount of NaCl addition proved to be 4 wt %. The excellent charge-discharge capacity of LTO with addition of NaCl at 4 wt % was 145.6 mAh/g, and the compound showed good stability during a subsequent cycling test. The favorable benefits of this method are its facile synthesis and low cost for large-scale production, as well as a performance that features a high discharge capacity and good stability. This compound shows promise for use as an anode material in lithium ion batteries.   % was 145.6 mAh/g, and the compound showed good stability during a subsequent cycling test. The favorable benefits of this method are its facile synthesis and low cost for large-scale production, as well as a performance that features a high discharge capacity and good stability. This compound shows promise for use as an anode material in lithium ion batteries.