Facile molten salt synthesis of Li2NiTiO4 cathode material for Li-ion batteries
© Wang et al.; licensee Springer. 2014
Received: 24 January 2014
Accepted: 22 April 2014
Published: 1 May 2014
Well-crystallized Li2NiTiO4 nanoparticles are rapidly synthesized by a molten salt method using a mixture of NaCl and KCl salts. X-ray diffraction pattern and scanning electron microscopic image show that Li2NiTiO4 has a cubic rock salt structure with an average particle size of ca. 50 nm. Conductive carbon-coated Li2NiTiO4 is obtained by a facile ball milling method. As a novel 4 V positive cathode material for Li-ion batteries, the Li2NiTiO4/C delivers high discharge capacities of 115 mAh g-1 at room temperature and 138 mAh g-1 and 50°C, along with a superior cyclability.
The growing demand for high-energy Li-ion batteries in the development of portable electronic devices and electric vehicles has stimulated great research interest in advanced cathode materials with high voltage and specific capacity. Li2MSiO4 (M = Fe and Mn) has recently attracted particular attention owing to their high theoretical capacities (>330 mAh g-1) and good thermal stability through strong Si-O bond [1–3]. However, the practical discharge capacity is mainly achieved below 3.5 V, resulting in a lower cell energy density. Substituting Si atom for Ti atom leads to another attractive cathode material of Li2MTiO4 (M = Fe, Mn, Co, Ni) with high theoretical capacity (approximately 290 mAh g-1) . The titanate family has a cubic cation disordered rock salt structure, in which the strong Ti-O bond could stabilize the M3+/M2+ and M4+/M3+ transition [5, 6].
Recently, Küzma et al.  synthesized the carbon-coated Li2FeTiO4 and Li2MnTiO4 by a citrate-precursor method, which showed the reversible capacity of 123 and 132 mAh g-1 at 60°C, respectively. In addition, the reported Li2CoTiO4/C presented a high discharge capacity of 144 mAh g-1 at rate of 10 mA g-1. In comparison with Fe, Mn and Co analogues, Li2NiTiO4 provides much higher discharge voltage plateau near 4.0 V. The electrochemical characterization of Li2NiTiO4 was initially published in 2004 . In a LiBOB/EC-DMC electrolyte, Li2NiTiO4 could deliver a charge capacity of 182 mAh g-1; however, more than 50% of this capacity was lost after 1 cycle . Kawano et al.  reported that Li2NiTiO4 demonstrated a discharge capacity of 153 mAh g-1 at the extremely low rate of 0.32 mA g-1 but showed an inferior cycling stability. Li2NiTiO4 suffers from poor electrode kinetics caused by its intrinsically low ionic and electronic conductivity, leading to a poor electrochemical activity.
In this work, well-dispersed Li2NiTiO4 nanoparticles are successfully prepared by a molten salt process with a short reaction time. To enhance the surface electronic conductivity and reinforce the structural stability, Li2NiTiO4 nanoparticles are carbon-coated by ball milling with carbon black. The whole processes are facile and high-yielding, which are promising for industrial application.
An equal molar ratio of NaCl and KCl with a melting point of 658°C was used as a molten salt flux. Li2CO3, Ni (CH3COO)2 · 4H2O, TiO2 (5 to 10 nm) and NaCl-KCl (Aladdin, Shanghai, China) in a molar ratio of 1:1:1:4 were well mixed with a mortar and pestle. The mixture was decomposed at 350°C for 2 h, followed by treatment at 670°C for 1.5 h under air. The product was washed with deionized water to remove any remaining salt and dried under vacuum. The as-prepared Li2NiTiO4 powder was ball-milled with 20 wt.% acetylene black to obtain the Li2NiTiO4/C composite.
The as-prepared Li2NiTiO4 was studied by X-ray diffraction (XRD, Rigaku D/Max-2200, Rigaku Corporation, Tokyo, Japan) analysis using Cu Ka radiation (40 kV, 30 mA). The morphologies of the Li2NiTiO4 and Li2NiTiO4/C samples were observed by scanning electron microscope (SEM, JEOL JSM-7401 F, Ltd., Akishima, Tokyo, Japan) with an accelerating voltage of 5.0 kV and transmission electron microscope (TEM, JEOL JEM-2100, Ltd., Akishima, Tokyo, Japan) operating at 200 kV. The chemical valence states of transition metals was analyzed by X-ray photoelectron spectroscopy (XPS) acquired with a Kratos Axis Ultra spectrometer (Axis Ultra DLD, Kratos, Japan) using a monochromatic Al Ka source (1,486.6 eV). The amount of carbon was determined from PE 2400II elemental analyzer (Perkin Elmer, USA). The metal content (lithium, nickel, and titanium) of the as-prepared Li2NiTiO4 was analyzed using an inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements (iCAP6300, Thermo, USA).
Electrochemical tests were performed with CR2016-type coin cells using Li foil as anode. The cathode consisted of 85 wt.% Li2NiTiO4/C, 5 wt.% Super P carbon black, and 10 wt.% polyvinylidene difluoride binder. An aluminum disk with Ø = 1.2 cm was used as current collector in the cathode side, and the pure Li2NiTiO4 loading is 1.5 mgcm-2. The electrolyte was 1 M LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v). Galvanostatic charge-discharge measurements were carried out on a LAND CT2001A battery tester (Wuhan, China) in a potential range of 2.4 to 4.9 V at room temperature and 2.4 to 4.8 V at 50°C. The cyclic voltammogram (CV) was measured between 2.4 and 5.1 V using a CHI660D electrochemical workstation (Shanghai, China) with a scan rate of 0.1 mV s-1. The specific capacity was calculated based on the mass of pure Li2NiTiO4 active material.
Results and discussion
Figure 5b shows the charge-discharge curves of the Li2NiTiO4/C nanocomposite at 50°C. It delivers a high initial charge capacity of 203 mAh g-1 at 0.05 C rate, corresponding to 1.4 lithium extraction per formula unit. Also, the discharge capacity of 138 mAh g-1 is much higher than that tested at room temperature, demonstrating its enhanced electrode kinetics at high temperature. Figure 5c compares the cycling performances of the Li2NiTiO4/C nanocomposite at room temperature and 50°C. Li2NiTiO4/C exhibits a stable cycle life after several cycles, and its capacity retentions after 50 cycles are 86% at room temperature and 83% at 50°C. At the end of 80 cycles, Li2NiTiO4/C retains 82% of its initial capacity with typical coulombic efficiency of 95% at room temperature, displaying a high electrochemical reversibility and structural stability during cycling. Figure 5d presents the rate capability of the Li2NiTiO4/C nanocomposite at room temperature. The charge rate remains constant at 0.1 C to insure identical initial conditions for each discharge. The Li2NiTiO4/C retains about 63% of its capacity from 0.05 to 1 C rate. The nanoparticles may reduce Li+ diffusion length and improve the ionic conductivity. Moreover, the highly conductive carbon coated on the surface of Li2NiTiO4 nanoparticles facilitates the rapid electrical conduction and electrode reactions, thus gives rise to capacity delivery and high rate performance.
Nanostructured Li2NiTiO4/C composite has been successfully prepared by a rapid molten salt method followed by ball milling. Cyclic voltammetry together with the ex situ XRD analysis indicate that Li2NiTiO4 exhibits reversible extraction/insertion of lithium and retains the cubic structure during cycling. This Li2NiTiO4/C nanocomposite exhibits relatively high discharge capacities, superior capacity retentions, and rate performances at room temperature and 50°C. The improved electrochemical performances can be ascribed to the nanoscale particle size, homogeneous carbon coating, and phase retention upon cycling.
This work was supported by the Anhui Provincial Natural Science Foundation, China (No. 1308085QB41) and Special Foundation for Outstanding Young Scientists of Anhui Province, China (No. 2012SQRL226ZD).
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