Annealing effect on structural and electrochemical performance of Ti-doped LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode materials

NMC 111 cathode materials exhibit engaging properties in high energy density and low cost, making it great potential for the next generation of high-energy lithium-ion batteries. However, it still faces challenges such as fast capacity fade, especially at high C rates. Herein, we implement the novel Ti-doped cathode material, LiNi 0.3 Mn 0.3 Co 0.3 Ti 0.1 O 2 (NMCT) synthesized via the combustion method. It was discovered that NMCT can effectively improve capacity delivery at high C rates. The T80 material demonstrated superior electrochemical annealed at 800 ˚C for 72 h, with an exceptional specific discharge capacity of 148.6 mAh g -1 and excellent cycle stability (capacity retention 96.8 %) after 30th cycles at 3 C. The results demonstrated that Ti-doped NMC had superior advantages for LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC 111) material at the optimum temperature of 800 °C for 72 h. It is one of the potential cathode materials for Li-ion batteries.


Introduction
The world's fastest-growing battery technology is Li-ion batteries (LIBs), which are widely used in the application of portable electronics, mobile phones, power tools, and electric vehicles (EVs).In LIBs, it is important to secure stable cathode materials as they govern the capacity of the whole cell.LiNi1-x-yMnxCoyO2 (NMC), LiNi1-x-yCoxAlyO2 (NCA) and LiFePO4 (LFP) are currently three cathode active materials that constitute a majority use in LIBs [1].Among the most widely investigated cathode materials, NMC is well-investigated, showing improved structural/thermal stability and cycling performance [2].
NMC is an attractive cathode material, as it combines the advantages of LiCoO2 (high energy density), LiNiO2 (high reversible capacity), and LiMnO2 (high thermal stability) [3].This material has better characteristics such as high capacity, less toxicity, and low cost compared to commercial LiCoO2 material [4].No wonder that two NMC compositions which are LiNi1/3Mn1/3Co1/3O2 (NMC 111) and LiNi0.5Mn0.3Co0.2O2(NMC 532), already used in the commercialization of lithium batteries [5].Although there are a large number of reports covering the properties of specific NMC composition, the challenges remain.It still has limitations, especially for EVs applications, due to poor rate capability and higher degradation at high voltage [6], which are its main impediments to comercialization.Therefore, batteries researchers have proposed some methods for the modification of NMC cathode, such as morphology control, use of electrolyte additives, ion doping, surface modification, etc., to remedy this drawback so that the properties, including high power and high energy densities, can be improved.
As is widely known, different methods and process parameters play a vital role in the crystal structure, morphology, crystallite size, and electrochemical performance of the cathode materials [7].Many alternative methods have been used to prepare the NMC powders, such as the solid-state method [8], sol-gel method [9], co-precipitation method [10], spray pyrolysis [11], and so on.It is difficult to synthesize homogenous NMC materials, so selecting a suitable preparation method is important to obtain pure and single-phase final products that will give the optimum electrochemical performance.One of the effective methods that can be used is the combustion method.This method is simple, low cost eco-friendly, giving high crystalline phases and better control of the microstructures, which will influence the electrochemical performance [12].Zhao et al. [13] reported that NMC111 synthesized via combustion can achieve a high discharge capacity of 179.2 mAh g -1 due to the high crystallinity and small particle sizes.
Element doping plays a critical role in stabilizing the structure and each transition metal added to the crystal structure of NMC has a distinct role.Changes in the oxidation state of Ni from Ni 2+ to Ni 3+ and to Ni 4+ help in improving the capacity [14].After the Ni 2+ oxidation, Co will oxidize from Co 2+ to Co 3+ to prevent the Ni from drifting to the Li sites, which causes less cation mixing between Li + /Ni 2+ ions [15].Mn 4+ ions are responsible for maintaining structural integrity and reducing production costs by replacing cobalt with cheaper manganese [16].There are several elements used as dopants, such as aluminium (Al), iron (Fe), tin (Sn), chromium (Cr), zirconium (Zr), copper (Cu), magnesium (Mg), and titanium (Ti).Eilers-Rethwisch et al. [17] reported that capacity fading is reduced when Al, Fe, and Sn were used as dopants.Zheng et al. [18] proved that doping with Al shows a higher specific discharge capacity of NMC 111, which is 154.9 mAh g -1 .This is due to the good crystallinity and homogeneous distributions of smaller particles size that provide higher surface area.However, Samarasingha et al. [19] reported that the X-Ray diffraction pattern of Al dopant showed the existence of secondary phase impurity with lower specific discharge capacities of 120 mAh g -1 .Thus, it is very important to choose the right dopant to improve the structural integrity, leading to excellent electrochemical performance.
Wang et al. reported Ti doping improved the cycling performance of the Li1.2Mn0.54-xTixNi0.13Co0.13O2cathode material by enhancing the framework between electrode/electrolyte interface stability [20].Ti used as a dopant in NMC 811 also showed much better cycling and rate property with a cut-off potential of 4.3 V [21].Table 1 summarizes the electrochemical properties of NMC from the previous works.To the best of our knowledge, no study of Ti dopant in NMC 111 prepared using the combustion method.Therefore, in this paper, we aim to understand the impact of novel partial substitution of Ti in LiNi1/3Mn1/3Co1/3O2 material on the phase, structure, morphology, size of the crystallite, element distribution, and electrochemical performances of the materials with increasing the annealing temperature that synthesizes using combustion method.

Synthesis of cathode materials
In a typical process, raw materials employed in the experiment were lithium nitrate, LiNO3, nickel(II) nitrate hexahydrate (Ni(NO3)26H2O), manganese(II) nitrate hexahydrate, Mn(NO3)2H2O, cobalt(II) nitrate hexahydrate, Co(NO3)26H2O and titanium(IV) methoxide, Ti(OCH3)4.All starting materials were weight according to the stoichiometric proportion of LiNi0.3Mn0.3Co0.3Ti0.1O2(NMCT).LiNO3, Ni(NO3)26H2O, Mn(NO3)2H2O, and Co(NO3)26H2O were first dissolved in an appropriate amount of deionized water and Ti(OCH3)4, was dissolved in nitric acid, HNO3 to form titanium nitrate.The equation for the reaction is given by the following expression, equation (1): After that, all starting raw materials were mixed in a 1 L beaker.The citric acid, C6H8O7, was added to the solution as a fuel and the mixture was uniformly mixed by stirring for 1 hour.The solution was heated slowly at 200 °C on a hot plate to give auto-combusted powders.Finally, the powders were calcined under various temperatures for 72 hours to obtain the target NMCT cathode material and then subjected to further analyses.Finally, NMCT materials annealed at 650, 700, 750, 800, 850 and 900 °C for 72 hours were denoted as T65, T70, T75, T80, T85, and T90, respectively.The synthesis process is illustrated in Figure 1.

Characterization
The phase composition and structural characteristics of all samples were studied with X-ray diffraction (XRD, PANalytical Xpert Pro diffractometer) with Cu Kα radiation and X'celerator detector.The measurement data were collected over the range of 10° < 2 < 90° with a step size of 0.02°.The crystal structure was analyzed by the Rietveld refinement using X'pert Highscore Plus Software.The XRD pattern for Rietveld refinement was carried out at a higher count up to 10,000 at 2  range 10 to 151°.
Field emission scanning electron microscope (FESEM, JEOL JSM-7600F) equipped with an energy dispersive X-ray spectroscopy (EDX, Oxford INCA X-Max 0021) was used for analyzing the morphology, element distribution, and composition mapping.The microstructural characteristics of the material were assessed by high-resolution transmission electron microscopy (HRTEM, TEM JEOL 2100) at 200 kV.For the preparation, the sample was mixed with methanol and treated with ultrasound for 1 hour.The suspension was deposited onto standard C/Cu grids and heated in the oven for 1 hour at 80 °C.

Electrochemical measurements
Active LiNi0.3Mn0.3Co0.3Ti0.1O2(NMCT) material, polytetrafluoroethylene (PTFE) binder, and Super P with the mass ratio of 8:1:1 were well mixed to prepare the working electrode.The mixture was pressed onto a stainless-steel grid and dried in an oven at 200 °C for 24 hours.The cell was assembled in a dry glove box (Unilab Mbraun) under an Ar atmosphere (O2 < 0.1 ppm, H2O < 0.1 ppm) with pure lithium foil as the counter electrode and Celgard 2400 microporous as the separator.The electrolytes are 1 M LiPF6 dissolved in a 1:1:1 (v/v/v) mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC).The cyclic voltammetry (CV) was performed between 0.5 -5.6 V with a scan rate of 3 mV s -1 .Galvanostatic charge-discharge tests were conducted using Wonatech (WBCS 3000) battery tester at 43.5 mA g-1 current density in the voltage range of 2.5 -4.2 V.

Phase and structural studies
The effect of calcination temperature on the formation of NMCT is investigated using XRD patterns shown in Figure 2. The main peaks of all samples are well indexed and isostructural with a typical hexagonal layered α-NaFeO2 structure (ICDD pattern number 01-070-2685 for lithium cobalt oxide) except for T85 and T90 materials.The presence of cobalt oxide (Co3O4) peak at 30.9° made both samples cannot be used for further other characterization and also electrochemical measurement [28].T65 material shows low-intensity peaks, which indicate low crystallinity, but with the increase in the calcination temperature, the intensities of all peaks increase and the peaks become sharper, especially for T80 material that reflects the increase of the crystallinity that expected will give good electrochemical performance [29].
To further investigate the effect of temperatures on the crystal structure, we calculated the ratio of (003) and (104) peaks called RIR in Table 2, which indicates the degree of "layeredness".All samples show an RIR value over 1.2 and slightly increase with increasing calcination temperature, indicating a good crystallinity and lower cation mixing in NMCT samples.RIR > 1.2 is important for layered cathode material with a well-ordered structure with limited Li/Ni mixing [2].T80 material gives the positive sign of good cation ordering and crystallinity when demonstrated with the highest I(003)/I(104) ratio [30].It also causes a clear split between (006) and (102) peaks and good symmetry of the (108) and ( 110) peaks (in the circle zone of Figure 2(d)), which indicates this material has a well-defined layered structure.The R-factor, which is defined as (I(006) + I(102))/ I(101), should be at a minimum in a system with good hexagonal ordering [31].These results prove that the crystal structure of NMC 111 did not change with Ti incorporation and increased calcination temperature promotes the formation of a well-defined layered structure.
To further analyze the effect of temperature on NMCT materials, all materials except T85 and T90 were analyzed by Rietveld refinement.The XRD pattern for Rietveld refinement, as shown in Figure 3, was carried out at a higher count up to 10,000 at 2  range 10 to 151˚.The summary of crystallographic values is listed in Table 2.The confidence profile factor, Rwp shows the refinement results of NMCT were credible (less than 10 %), indicating the results of the Rietveld refinements have a good fitting degree [32].All the materials show the ratios of the lattice constants, c/a values are larger than 4.9, representing that the NMCT materials have a good hexagonal layered structure [33].T65 material shows a c/a value of 4.9743, as the temperature increase, the crystallographic values of c/a ratio and volume (V) gradually enhance, corroborating that Ti was incorporated into the NMC crystal lattice [21].It was found that T65 has the largest c-axis compared to other samples, which supposedly gives an advantage for Li ions to diffuse easier [34], but unfortunately, this material has the highest cation mixing, which about 10 % of Ni 2+ takes place at the Li + 3a site.Refinement reveals that the c-axis was decreased when temperature increased except for the T80 sample.It was also found that, as the temperature increased, the cation mixing decreased.Interestingly, the T80 sample possesses a larger c-axis and the lowest cation mixing of 6.8 % as compared to other samples.By having good structural integrity, it is believed that this material will have excellent electrochemical performance.Refinements results prove that increasing annealing temperature can help promote the formation of a well-defined layered structure that allows Li+ ions to move randomly between the layered cathode materials, subsequently improving the cathode's electrochemical performance materials [35].
The atomic distance of Li-O, Li-TM, and TM-O obtained from refinements is shown in Table 2, where TM indicates transition metal.Based on the outcome, increasing temperature shows the materials had a shorter bond length of Li-O, a larger atomic distance of Li-TM, and a longer bond length of TM-O.It can see that T65 has a larger atomic distance and longer bond length, but the material has a higher level of cation mixing than shown by the RIR value.This may be one of the reasons why T65 does not show good electrochemical charge-discharge, as seen later.Interestingly, the T80 sample has a larger atomic distance of Li-O and TM-O, and in addition to that, this material has the smallest cation mixing as compared to other samples.By having a larger atomic distance of Li-O, less energy is needed for Li-ions to break free from their position in the crystal structure, which could promote more Li-ions diffusion, which later gives a higher specific capacity for the material to hold [36].Therefore, T80 is estimated to exhibit better electrochemical performance due to atomic distance and RIR value.

Morphology
Morphology and particle size are one of the main effects that can affect the electrochemical performance of cathode materials.The SEM images of the NMCT materials at 30000 magnification are shown in Figure 4(a-d).The materials have a spherical shape with some polyhedral-type and smooth crystals.As the calcination increase, the average crystallite sizes become larger, significant to the crystal growth, and the distribution of particle size more uniform when increasing the temperature.The average crystallite size for all the samples was measured via FESEM images by randomly measuring 80 crystallites, as shown in Table 3. From Figure 4(e), we can see that the crystallite size distribution of the T80 sample approximates to the normal distribution and agrees with the average crystallite size of 155.57nm shown in Table 3.The EDX spectra in Due to the excellent structural integrity possessed by the T80 sample (Table 2), this material was then subjected to the TEM studies and the results are shown in Figure 5.In Figure 5(a), it is clear that the morphology and crystallite size of the T80 sample from the TEM micrograph agree with the SEM results.A high-resolution image of the T80 nanocrystals is shown in Figure 5(b) for lattice studies, where the selected spot used can be seen from the marked circle in Figure 5(a).The higher magnification image clearly shows the T80 nano crystal's lattice, demonstrating that the T80 sample is crystalline in nature with a single crystalline structure, indicating atomic distribution with high ordering.

Figure 5. TEM image of (a) T80 (b) HRTEM image of T80 in the circle zone of and corresponding element mapping of a T80 material
Good crystal structure and orderly atomic distribution are speculated to facilitate the electrochemical performance of the NMCT [23].The HRTEM image of the T80 nano crystal marked by the rectangle zone in Figure 5(b) displays an interplanar distance of 0.47 nm, which corresponds to the (003) plane spacing of NMC [37].The (003) crystal plane is identified using the selected area FFT results (inset image in Figure 5(a)).The existence of (003) crystal plane also confirms the hexagonal layered α-NaFeO2 structure of the T80 nanocrystal as revealed by the XRD data.
EDS mapping of T80 material manifested in Figure 5 reveals the presence of nickel (blue), cobalt (green), manganese (red), and titanium (grey), respectively.Overall, the area distribution of all elements was fully uniform on the surface of the material and the effective incorporation in the particles.

Electrochemical performance of cathode materials
Cyclic voltammetry (CV) was a measure to study the redox potential of the transition metal ions in NMCT. Figure 6(a) shows the CV curves in which lithium metal is used as the counter and reference electrodes.The broad peak between 4.78 V to 5.60 V on the charge curve has been ascribed to the oxidation of Co ions (Co 3+ /Co 4+ ) and Ni ions (Ni 2+ / Ni 4+ ), accounting for the Li + extraction from the cathode material.The reduction peak between 2.71 V to 3.26 V on the discharge curve illustrates the reduction of Co ions (Co 4+ /Co 3+ ) and Ni ions (Ni 4+ / Ni 2+ ), which means that Li + insert back into the layered material [38].The potential difference, ΔE (Eoxidation -Ereduction) was calculated to explain the cycle reversibility of the electrode of NMCT.ΔE value was increased with the increasing annealing temperature but not for T80 and the details are revealed in Table 3.This material shows smaller ΔE and higher peak currents indicating better electrode reaction kinetics and rate performance [39].This may be one of the reasons why T80 material shows good electrochemical performance, as seen later.
Within the voltage range, it is worth pointing out that the performance of NMCT materials is comparable to the reported in the literature [40,41].As shown in Figure 6(b), the best material, T80 exhibited the highest 1 st discharge specific capacity of about 148.6 mAh g -1 .This seemed to be consistent with the good cation ordering shown by XRD analysis.Overall, NMCT materials demonstrated superior electrochemical after 30 th cycles shown in Table 4 and Figure 6(c), especially T80 material with a higher discharge capacity of 143.7 mAh g -1 with 96.8 % capacity retention.This is probably due to the well-formed layered structure, as confirmed by the XRD result (Table 2), low cation mixing, and also uniform particle distribution.
In addition, to improve the rate capability of T80 (Figure 6(d)), a current density corresponding to a rate from 1 C to 15 C was applied for the 5 th cycles.With increasing current, the capacity was decreased progressively and the capacity of T80 at 15 C was only 23 mAh g -1 after the 5 th cycle.However, almost no capacity decay was observed when the current rate returned to 3 C, suggesting the capacity decrease at high rates was mainly due to the kinetic barrier [42].Therefore, in this study, we used the 3C rate as the official scan rate for the electrochemical measurements.

Conclusion
In summary, NMCT materials calcined at different temperatures were successfully prepared via the combustion method.Rietveld refinements reveal that annealing temperature has a significant effect on the structural parameters of cathode materials.Refinements revealed that T80 material has the best structural integrity by having larger parameters of the c-axis, the lowest cation mixing, and a larger atomic distance of Li-O, which makes this material capable of possessing excellent electrochemical performance behavior compared to the other samples.The SEM result shows the crystallite size increasing with increasing calcination temperatures, indicating that high temperatures may promote the formation of more ordered layered structures, and we can see the distribution of the particles is more uniform for T80 material.Furthermore, T80 material shows a smaller ΔE value, indicating better electrode reaction kinetics and rate performance, promising superior electrochemical performance.So, in this case, this material demonstrated excellent cycling stability, with a higher initial discharge capacity of 148.6 mhA g -1 and capacity retention of 96.8 % after 30 th cycles, as well as excellent rate capability.The above-mentioned results indicate that T80 material can promote electrochemical performance tremendously and become a potential cathode for Li-ion battery applications.This work also demonstrates that different annealing temperatures have a synergistic effect on the structure, improving the stability and electrochemical performance of NMCT.

Figure 1 .
Figure 1.Summary of combustion method

Table 1 .
A comparison of electrochemical performance of previously reported NMC materials