In this paper, we successfully synthesized Ti-coated Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode and microstructure and electrochemical performances for high-energy lithium-ion battery are investigated. The TiO₂-coated LiNi_0.8Co_0.1Mn_0.1O₂ delivers excellent cation mixing and there is no big difference in morphology between pristine and TiO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂. The TiO₂-coated LiNi_0.8Co_0.1Mn_0.1O₂ has higher electrochemical performances (initial discharge capacity : 201.3 mAh g^-1, rate capability : 90.5% at 4.0 C and cyclability : 79.4% after 100 cycles). It can be explained by the positive role of TiO₂ layer on the surface of LiNi_0.8Co_0.1Mn_0.1O₂

Keywords: Ti-coated Ni-rich layered LiNi_0.8Co_0.1Mn_0.1O₂, microstructure, cation mixing, higher electrochemical performances, TiO₂ layer

Recently, lithium ion batteries (LIBs) are used to electric vehicles (EVs), hybrid electric vehicles (HEVs) and energy storage systems [1-3]. LIBs are required to meet high energy, long cycle life, specific capacity, stability and low cost [4-5]. Until now, LiCoO₂ (LCO) is used to the cathode materials of LIBs due to stable discharge plateau and excellent reversibility. However, LCO have some shortcomings such as poor thermal stability, high costs and environmental pollution [6-7].
Hence, LCO need to be replaced by LiNi_xCo_yMn_zO₂. Compared with LCO, NCM have various advantages of good thermal stability, low cost and eco-friendly [6-7]. Also, high-Ni content of LiNi_xCo_yMn_zO₂ has excellent reversible specific capacity compared to low-Ni content LiNi_xCo_yMn_zO₂, which is helpful for commercialization [8-9]. Unfortunately, high-Ni content of NCM (Ni-rich NCM) have poor cycle performance and low coulombic efficiency. This is because, i) the cation mixing is caused by similar radius between Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å) and ii) structural instability due to oxygen release from the crystal lattice [10-11]. To solve the problems, effective strategies such as coating and doping are widely used [12-15].
In this paper, we successfully prepared TiO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ and investigated the electrochemical performances. The TiO₂ coated LiNi_0.8Co_0.1Mn_0.1O₂ can improve electrochemical performance and structure stability.

Spherical LiNi_0.8Co_0.1Mn_0.1O₂ powders were synthesized by using co-precipitation method. The Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor was prepared using NiSO₄·6H₂O, CoSO₄·7H₂O, MnSO₄·H₂O, Na₂CO₃ and NH₃·H₂O.Precipitating agentwas prepared using the NaOH and NH₄OH solution. The spherical Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor mixedwith LiOH·H₂O in a molar ratio 1.05 : 1. Subsequently, the mixture was preheated at 500 ^oC for 5 h and then calcined750 C for 15 h in tube furnace under the oxygen flow. The TiO₂ coating layer wasformed on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ by the hydroxylation of titanium tetrabutoxide (TBOT) in a mole ratio of Ti:NCM = 0.03 : 1. After that, the sample was sintered at 600 ^oC for 3 h in air.
The cathode was fabricated by cathode powder (96 wt.%), conductive carbon black (2 wt.%) and poly- vinylidene fluoride (PVDF) (2 wt.%). After that, to form slurry, N-methyl-pyrrolidinone (NMP) solvent was added. Aluminum foil coated on the prepared slurry and then dried at 120 ^oC for 10 h in a vacuum oven. The CR 2032 coin cells were assembled using lithium foil as an anode in glove box filled with argon gas. 1 M LiPF₆ in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (with a volumetric ratio 1 : 1 : 1) were used to electrolyte.
The X-ray diffraction (XRD, X'pert MPD DY1219) was used to measure the structural properties of samples. The field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) was prepared to confirm the morphology of the TiO₂ coated sample. An equipment (TOSCAT-3100, Toyo system) was adopted to measure the electrochemical performances.

Fig. 1 presents the XRD patterns recorded on TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂. It was observed that there is no significant change in peak position and shape of the XRD via TiO₂ coating. This is because the coating does not affect the LiNi_0.91Co_0.06Mn_0.03O₂ structure [16]. The XRD pattern of sample demonstrates that TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ can be identified by a typical structure of layered hexagonal α-NaFeO₂ structure with the space group R-3m. The clear peak splits corresponding to Miller indices of (006)/(102) and (108)/(110) are observed and it indicates the formation of well-crystallized hexagonal layered structure of LiNi_0.91Co_0.06Mn_0.03O₂ [17]. The I₍₀₀₃₎/I₍₁₀₄₎ ratio of the TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ is 1.56, which is sufficiently higher than 1.2, which is the standard for cation mixing degree. It means superior structure stability and cation ordering of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂.
Fig. 2 illustrates the (a) FESEM and (b) TEM images of the TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂. We can confirm that the TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ shows spherical granule shape with an average size of 12.0 μm, consisting of numerous primary particles of approximately 150 -450 nm. The primary particles contacted each other directly, which can reduce the electrical resistance, thus, it has a positive effect on the electrochemical performance. From FESEM image, it is clearly seen that TiO₂ coating does not change the spherical shape of pristine LiNi_0.91Co_0.06Mn_0.03O₂ [18]. Fig. 2(b) exhibits the TiO₂ coating layer. The average thickness of TiO₂ coating layer is approximately 12 nm. Based on structural properties of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂, we can infer that TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ could deliver excellent electrochemical performances.
To investigate the electrochemical tests of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂, the loading level of the sample was adjusted about 14.6  mg/cm² because the high areal capacity is necessary for practical application of lithium ion batteries [19]. Fig. 3 shows the initial charge-discharge curves of TiO₂ coated LiNi_0.91Co_0.06- Mn_0.03O₂ in a potential range of 3.0 - 4.3 V at 0.5 C. The TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ has a typical charge-discharge behavior. There is no noticeable additional plateau. It indicates that the TiO₂ coating does not affect the electrochemical activity of the LiNi_0.91Co_0.06Mn_0.03O₂. The TiO₂ coated LiNi_0.91Co_0.06- Mn_0.03O₂ has a discharge capacity of 201.3 mAh g^-1, which is higher value compared to pristine sample, resulting from electronically conductive TiO₂ layer [20]. It can be explained by the enhanced conductivity of LiNi_0.91Co_0.06Mn_0.03O₂.
Fig. 4 shows the rate performance of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ at various C-rates from 0.1 C to 4.0 C. It is obvious that the capacity retention of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ proportionally decreases with increasing the C-rate. The capacity retention slightly decrease at low C-rates (0.5 and 1.0 C) while the retention of sample is decreased relatively largely at high C-rates (2.0 and 4.0 C). It is associated with the low electrical resistance of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂, leading to rapid lithium ion transfer [21].
Fig. 5 shows the long-term cycle performance of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ at 0.5 C rate at 25 ^oC in the voltage range 3.0 and 4.3 V. The capacity is inversely proportional to the cycle number. However, the TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ shows the excellent cycle retention than previously reported pristine sample. The capacity retention of TiO₂ coated LiNi_0.91Co_0.06- Mn_0.03O₂ is 79.4% after 100 cycles. It is closely associated with the contribution of TiO₂ layer, suppressing the formation of a resistive solid electrolyte interphase (SEI) layer [22]. It causes the increase in charge transfer resistance at the interface between electrolyte and TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ cathode. More importantly, TiO₂ layer can alleviate the transition metal elution of LiNi_0.91Co_0.06Mn_0.03O₂ cathode to maintain the original layered structure/stable surface chemistry during electro- chemical cycling. It was reported that crystalline/amorphous TiO₂ layer can be formed on the surface of LiNi_0.91Co_0.06Mn_0.03O₂. Therefore, the protective/con- ductive TiO₂ layer can suppress the negative unwanted reaction between electrolyte and LiNi_0.91Co_0.06Mn_0.03O₂, leading to structural degradation and gassing [23, 24]. It is resulting from O₂ release and electrolyte decay. These phenomenon can be explained by strong bond dissociation energy of Ti-O compared to those of other transition metal ions-O. Moreover, it is well known that coating layer has a positive effect on phase transition (H2→H3) and micro-cracking, which are considered important factors of capacity fading [22, 25].

We successfully formed TiO₂ coating layer on the surface of LiNi_0.91Co_0.06Mn_0.03O₂ cathode via solid-state reaction. The TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ cathode shows not only good structural stability but also superior electrochemical performances. The well-crystallized TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ cathode maintain original XRD pattern and microstructure of pristine sample. The electrochemical performances of TiO₂ coated LiNi_0.91Co_0.06Mn_0.03O₂ cathode surpass pristine sample. It is elucidated that TiO₂ layer could decreases the cation disordering and plays a crucial role in physical on the surface of LiNi_0.91Co_0.06Mn_0.03O₂. As a result, it can be concluded that TiO₂ coating layer can be regarded as a breakthrough for high performance and stability LiNi_0.91Co_0.06Mn_0.03O₂ cathode.