Enhanced electrochemical performance of La2O3-modified Li4Ti5O12 anode material for Li-ion batteries

1.0 wt.% La2O3-modified Li4Ti5O12 sample is prosperous synthesized through a polymeric method followed by calcination at 500 °C for 5 hours in air. The 1.0 wt.% La2O3-modified Li4Ti5O12 is characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to confirm the structure and morphology. After the modification process, the lattice structure of Li4Ti5O12 is not changed and a La2O3 coating layer has formed on the Li4Ti5O12 particles. The 1.0 wt.% La2O3-modified Li4Ti5O12 exhibits a superior rate capacity, with charge capacity between 0 and 3 V of 235.2, 224.8, 214.1 202.2, 194.5 and 181.9 mAh g-1 at rates of 0.2, 0.5, 1, 3, 5, and 10 C (1 C = 250 mAh g-1), respectively.


Introduction
In recent years, many efforts have been devoted to develop the lithium ion batteries (LIBS) because of their potential applications in electrical vehicles, hybrid electric vehicles and electrical energy systems [1][2][3]. Spinel Li 4 Ti 5 O 12 (LTO) has been regarded as an ideal anode material for long-life LIBS due to its negligible volumetric change during the insertion/extraction of the lithium ion. Furthermore, in comparison with commercial carbon-based materials, LTO is a safer material because of its high thermal stability and high discharge plateau (at about 1.5V vs Li + ), which prohibits the formation of metallic lithium dendrites on the electrodes' surfaces. Although LTO has the above-mentioned advantages, it still has some problems in the power LIBS industries with large-scale applications. The gas generation is a severe drawback of LTO when using LTO as anode material in LIBS. The main reason for the gassing behavior may be the interfacial reactions between LTO and electrolyte [4,5].
Surface modification has been demonstrated to be an effective way to prevent LTO from reacting with electrolyte. For example, carbon [6][7][8][9], metal [10][11][12] and metal oxide [13][14][15][16] all have been used to modify the LTO to suppress the electrolyte decomposition and enhance the electrochemical performance. To the best of our knowledge, La 2 O 3 with excellent thermal stability has been used in various applications. In particular, when La 2 O 3 is used as an additive, it greatly decreased the overpotential in the ternary alkaline metal carbonate system and protected the transition metal ions from dissolving into the electrolyte [17][18][19]. In this present work, we use La 2 O 3 to modify pure LTO. The structure and electrochemical performance of the La 2 O 3 modified LTO samples are studied and reported below.

Synthesis and characterization
Pure LTO powder (Shenzhen beiterui new energy Limited by Share Ltd), La(NO 3 ) 3 · 6H 2 O (Aladdin) and polyvinyl alcohol (PVA, degree of polymerization is 1500) were purchased for this work. In brief, The 3 g of LTO was dispersed in deionized water by 0.5 hour sonication followed by 2 hours stirring. The calculated 1.0 wt.% of La(NO 3 ) 3 · 6H 2 O to form La 2 O 3 and 1.0 wt.% polyvinyl alcohol were dissolved in warm deionized water and added dropwise to the dispersed LTO. The mixture was continuously stirred for 3 hours at 25 °C and continuously stirred at 100 °C to evaporate the water, then the mixed solid powder was calcined at 500 °C for 5 hours in air to obtain the 1.0 wt.% La 2 O 3 modified LTO sample.
Powder X-ray diffraction (XRD, Ultima IV, Riguku) with Cu K α radiation was used to characterize the phase composition and crystal structures of all the samples. The diffraction patterns were collected at room temperature by step scanning in the range of 10-90° at a scanning rate of 0.02• per 10 s. The morphology of the materials was characterized by SEM (Hitachi S-4800, Japan).

Electrochemical measurements
The electrochemical properties of LTO samples were measured using a CR2032 coin-type half-cell in which the cathode and Li metal anode were separated by a porous polypropylene film (Celgard 2400, Celgard Inc., USA). The cathode slurry was prepared by homogeneously mixing the active material (LTO materials), Supper-P, and a polyvinylidene fluoride (PVDF) in a mass ratio of 90:5:5 in N-methyl-2-pyrrolidone (NMP) solvent. Then the slurry was cast onto a Cu foil and dried for 12 h in vacuum at 105 °C. Finally, the electrode laminate was punched into disks (10 mm in diameter) and dried in a vacuum oven at 105 °C for 24 h. The coin cell was assembled entirely in an argon-filled glovebox. The electrolyte (Capchem Technology (Shenzhen) Co., Ltd.) was a solution of 1 mol L -1 LiPF 6 in ethylene carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1, in volume).
Galvanostatic charge-discharge tests were carried out on an automatic galvanostatic charge-discharge unit (Land 2001A, Wuhan, China) between 0 and 3 V at C-rates of 0.2C, 0.5C, 1C, 3C, 5C and 10C (1C = 250 mAh g -1 ) at 25 °C. Figure 1 shows the XRD patterns of pure LTO and 1.0 wt.% La 2 O 3 -modified LTO sample. All the XRD diffraction patterns of 1.0 wt.% La 2 O 3 -modified LTO are indexed to the standard spinel structure of LTO (card NO. 49-0207) with Fd3m space group and no peaks of La 2 O 3 are present due to low quantity. The XRD patterns prove that the structure of 1.0 wt.% La 2 O 3 -modified LTO sample is not destroyed after La 2 O 3 modification process. Figure 2 shows the SEM images of pure LTO and 1.0 wt.% La 2 O 3 -modified LTO sample. It can be observed that pure LTO has a smooth surface as clearly shown in Figure 2a

Conclusion
After La 2 O 3 -modification process, the results indicated that a La 2 O 3 coating layer was formed on the surface of LTO. The electrochemical results show that 1.0 wt.% La 2 O 3 -modified LTO displays a superior rate capacity between 0 and 3 V. The improvement in electrochemical properties is attributed to the La 2 O 3 coating layer on the surface of LTO, which suppresses the electrolyte reduction decomposition. Therefore, application of the La 2 O 3 modification process has potential to be an efficient strategy for improving the electrochemical performance of pure LTO for lithium ion batteries.