Synthesis and properties of nickel-doped Li4Ti5O12/C nano-composite: an anode for lithium ion batteries

Composite of nickel-doped Li4Ti5O12 and carbon (designated as Ni–Li4Ti5O12/C) is synthesized by solid-state reaction with sucrose added as the conductive carbon source. For comparison, nickel-doped Li4Ti5O12 (designated as Ni–Li4Ti5O12) is obtained under similar conditions. Both materials are characterized by x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, charge–discharge cycling and cyclic voltammetry. Analysis of SEM images shows that Ni–Li4Ti5O12/C consists of smaller particles than Ni–Li4Ti5O12. Charge/discharge cycling as well as cyclic voltammetry studies prove that Ni–Li4Ti5O12/C electrodes perform much better than those made from Ni–Li4Ti5O12. The specific capacities of the Ni–Li4Ti5O12/C composite are close to the theoretical capacity of Li4Ti5O12 even at high current rates—the discharge capacity of Ni–Li4Ti5O12/C at 2C is equal to 164 mA h g−1.


Introduction
Lithium-ion batteries are considered to be promising energy storage devices and are widely applied in e.g. portable electronic instruments or electric vehicles [1][2][3] because of their high energy and power density as well as long cycle-life [4][5][6]. The properties of electrode materials are crucial for the performance of the battery. Therefore, study of these materials is very important. Regarding anodes, only materials based on carbon (especially graphite) or Li 4 Ti 5 O 12 (LTO) are commercialized. Carbonic materials possess excellent electrochemical properties. However, they exhibit volume change during charge/discharge cycles which cause problems with safe usage of the battery. LTO is an attractive alternative to anodes based on carbon [7,8]. LTO has a high working potential (~1.55 V versus Li/Li + ) [9,10]. LTO is considered to be a zero-strain material, because during lithium intercalation and deintercalation the lattice parameter does not change almost at all [11]. Although the high working potential limits significantly the energy density, the operating potential occurs within the thermodynamic stability window of electrolytic solutions, so that it is not necessary to form a solid electrolyte interphase (SEI) layer for proper functioning of the electrode. Anodes based on Li 4 Ti 5 O 12 exhibit long cycle-life, are resistant to overcharge and can be used in a wide temperature range [10]. Theoretical capacity of non-doped LTO is equal to 175 mA h g −1 , because of insertion of 3 moles of lithium ion into LTO structure. Li  nano-materials [7,[14][15][16] doping with metal cations [17][18][19] or composing LTO with carbon.
In this study Li 4 Ti 5 O 12 was doped with 3 wt.% Ni 2+ ions to enhance electronic conductivity and reduce the electrode polarization. One of the samples was additionally composed with sucrose as a carbon source. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). What is more, the electrochemical properties were tested by charge-discharge cycling and cyclic voltammetry.

Experimental
Ni-Li 4 Ti 5 O 12 was obtained by a solid state method using Li 2 CO 3 (Aldrich, 99.8%), TiO 2 -anatase (Acros, 99 + %) and Ni(NO 3 ) 2 · 6H 2 O (Aldrich, 99.8%). The starting materials were mixed at the Li:Ti:Ni molar ratio of 4:5:0.24. The mixed reactants were ball-milled for 20 min with a propanol. After drying, powders were calcined at 800 °C for 4 h in the air. The Ni-Li 4 Ti 5 O 12 /C composite was obtained by using a similar solid-state method mentioned above. In this case, however, the sample was prepared from a mixture of Li 2 CO 3 , TiO 2anatase, Ni(NO 3 ) 2 · 6H 2 O and saccharose (Aldrich, 99.8%). Dried powder was calcinated in the flowing argon. Powder x-ray diffraction was performed using panalytical empyrean diffractometer using Cu-K α radiation in an angular range of 10-110° (2θ) with a 0.02° (2θ) step (λ = 1.5406 Å). Rietveld analysis of XRD data were done using GSAS/ EXPGUI set of software [20,21]. Morphology and structure of samples were analysed by NanoSEM 200 FEI scanning electron microscope equipped with low vacuum detector (LVD). The particle size was obtained using SEM analysis.
The electrochemical measurements were done using CR2032 coin-type cells. The working electrodes were prepared by making a black slurry containing 80 wt.% of active mat erial, 10 wt.% of carbon black and 10 wt.% of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone. The slurry was spread on an aluminium foil and dried at 70 °C for 1 h in a vacuum drying oven. After that disks with 8 mm diameter were punched out of the foil and roll-pressed. Metal lithium foil served as a counter electrode. 1 M LiPF 6 solution in the 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as an electrolyte. Batteries were prepared in the glove-box (UNILAB, M. Braun) under argon atmosphere with controlled oxygen and water vapor pressure (<0.1 ppm). The electrochemical properties of the samples were measured by galvanostatic charge/discharge cycles at different rates over a voltage range of 1.3-2.2 V. The C rate was calculated based on the weight of the electrode and theoretical capacity of LTO. Cyclic voltammetry measurements were carried out at different scanning rates in the range of 0.01-5 mV s −1 and in 1.3-2.0 V voltage range. Cells were tested at a computer-controlled galvanostat (KEST 32k multichannel) and on the electrochemical test instrument (ATLAS). All the electrochemical tests were carried out at room temperature.

Structure
Powders were examined by room-temperature x-ray diffraction in order to detect phase composition in the sintered material (figure 1). The study shows that Ni-Li 4        in the figure 2(a) clearly indicates that the average particle size of the Ni-Li 4 Ti 5 O 12 sample is in the range of 125.14-585.89 nm. The crystallites have irregular shape with fuzzy edges and the grains are sintered. Ni-Li 4 Ti 5 O 12 /C has much lower particle size which is in the range of 70.90-215.48 nm. Application of carbon from saccharose influenced calcination so that smaller particles were obtained. EDS spectra (figures 2(c) and (d)) further confirms the presence of Ti, O, C and Ni elements.
The sol-gel method is very well known, practically displaced method of solid phase synthesis. Mu et al [23] prepared main anode material with the use tetrabutyl titanate and anhydrous ethanol. Their composite particles obtained by sol-gel are sized 100-200 nm. However, smaller particles size (figure 2(b)) may be obtained by a solid state which is simpler and less harmful to the environment. The discharge capacity of Ni-Li 4 Ti 5 O 12 composite is quite high at lower current rates 0, it reaches 157 mA h g −1 at 0.2 C and 148 mA h g −1 at 0.5 C. However it decreases suddenly for higher current rates exhibiting only 53 mA h g −1 at 5 C and 22 mA h g −1 at 10 C. Ni-Li 4 Ti 5 O 12 /C gives much better electrochemical performance. Its discharge capacity at 0.2 C is equal to 170 mA h g −1 , it is about 97% of theoretical capacity. Ni-Li 4 Ti 5 O 12 /C exhibits high discharge capacity even at high current rates-e.g. its capacity at 2 C is equal to 164 mA h g −1 , only 6 mA h g −1 less than the capacity at 0.2 C and at 10 C is equal to 140 mA h g −1 (about 80% of theoretical capacity). At the end Ni-Li 4 Ti 5 O 12 /C was cycled again at 0.2 C and specific capacity  had the same value as it had before (170 mA h g −1 ). It can be clearly seen that the addition of carbon from saccharose to Ni-Li 4 Ti 5 O 12 /C greatly enhances its electrochemical performance.

Electrochemical and transport properties
For comparison Mu et al [23] studied ultrafast charge/ discharge nano-sized Li 4 Ti 5 O 12 /C anode material without Ni. The sample were prepared by sol-gel method with the use of tetrabutyl titanate (TBT), anhydrous ethanol, lithium hydroxide monohydrate and sucrose. The authors concluded that their composite anode displays a distinguished electrochemical charge/discharge performance, especially, quite high rate capability along with a stable cyclability. It delivered the initial discharge specific capacities of 156.7 and 142.1 mA h g −1 at 40 C and 60 C, respectively, and remained the values of 114.2 and 98.1 mA h g −1 after 200 cycles.    In order to check the stability and reversibility of process, the cell was tested three times at the 0.2 mV s −1 scan rate. It can be seen that the anodic peaks occur at 1.77 and 1.62 V for the Ni-Li 4 Ti 5 O 12 and Ni-Li 4 Ti 5 O 12 /C sample, respectively, while the corresponding cathodic peaks are 1.40 and 1.49 V. The cathodic/anodic peaks can correspond to both intercalation/deintercalation of lithium ions at the empty 16c octa hedral sites and the Ti 3+ /Ti 4+ redox couple. It can be clearly seen that Ni-Li 4 Ti 5 O 12 /C has higher lithiation and lower delithiation potential than Ni-Li 4 Ti 5 O 12 at every scan rate. It proves that Ni-Li 4 Ti 5 O 12 /C has better reversibility of the Li-ion insertion/extraction process.
Mu et al [23] studied cyclic voltammograms of the as-prepared LTO/C and LTO electrodes. All plots demonstrated a pair of oxidation/reduction peaks similarly and symmetrically in shape. The course of the curves confirmed lithium ion insertion and extraction accompanying with electrochemical reaction. The increase of the scanning rate from 0.1 to 5 mV s −1 leads to the oxidation and reduction peaks of every curve move away from each other in opposite direction, along with broader peaks and large peak current. The same effect was observed for the studied in this work composite with nickel. These results suggest that a change in polarization may adversely affect the reversibility and the cyclical performance of the electrode.

Conclusion
Two powders-Ni-Li 4 Ti 5 O 12 and Ni-Li 4 Ti 5 O 12 /C-were obtained by solid-state method. The crystal structures of the materials were investigated using XRD and SEM. Our findings suggest that Ni-Li 4 Ti 5 O 12 consists of micrometric grains with irregular shape, while the crystallites of Ni-Li 4 Ti 5 O 12 /C are partially nano-sized, with regular shape and wide Li-diffusion pathways. Decreasing sizes of grains improved electrochemical performance of the material. Nano-sized Ni-Li 4 Ti 5 O 12 /C composite exhibits the best electrochemical properties and reversibility during Li intercalation/deintercalation process.