Investigation on performance of Li(Ni0.5Co0.2Mn0.3)1−xTixO2 cathode materials for lithium-ion battery
Introduction
Lithium-ion batteries are widely used throughout many domains such as 3C products, including computer, communication and consumer electronics due to its high energy density, long cycle life, small size, no memory effect, low self discharge and environment-friendly character [1], [2], [3], [4], [5]. LiCoO2 (LCO) is the main cathode material of current commercial lithium-ion battery [6], [7], [8]. However, high cost and lack of resource of Co element limit the application fields of lithium-ion batteries. Besides, LiNiO2 (LNO) and LiMnO2 (LMO) cathode materials, as the Co-free alternatives, still suffer from their own imperfection. LNO is difficult to be synthesized [9], and its unstable structure leads to rapid capacity fading and weak thermal stability [10]. LMO is composed of micro-sized particles reported in majority of existing literatures, and its capacity and cycle stability are unsatisfied [11], [12]. To this end, it renders researchers to make efforts to develop alternatives. LiNi1/3Co1/3Mn1/3O2 (NCM), a novel kind of layered structure cathode material using in lithium-ion battery, which has all advantages of LCO, LNO and LMO, has been developed in 2001 by Ohzuku [13].
NCM, in part, is a promising energy material on account of its high specific capacity and a moderately high rate performance with stable structure and good safety property, which may take the place of LCO in the future. Also it can be used in electric vehicle (EV) and hybrid electrical vehicle (HEV) as a consequence. Nevertheless, it still has some defects affecting its electrochemical performance such as undesirable ionic conductivity and cycle performance, low tap density and complicated preparation technique [14], [15], [16], [17], which are its main impediments to commercialization. Therefore, lots of reports have been focused on the modification of NCM cathode materials, including coating by certain metal oxides and doping with cations or/and anions. TiO2 [18], AlF3 [19], Al2O3 [20], Li3VO4 [21], Sb2O3 [22], Al3+ [23], Zr4+ [24], Mg2+ [25], Mo6+ [26], F- [27] have all been used as the modifications in order to improve the electrochemical performance of NCM layered cathode material. Besides, NCA (Li–Ni–Co–Al) as a cathode material has been modified on NCM in order to promote the capacity and lower the cost [28]. There are also a series of studies concentrating on Ti-substituted layered cathode materials, such as LiCo1−xTixO2 [29], LiNi0.5Mn0.5−xTixO2 [30], LiNi0.5−xMn0.5−xTi2xO2 [31], LiNi0.4Mn0.4Co0.2−xTixO2 [32] and so on.
However, almost all these above work focuses on the small amount of products under laboratory conditions, and few are put into use in the industry. It is of vital importance to combine industry and research together [33]. Besides, the method that they use to synthesize Ti modified electrodes is conventional co-precipitation, which is complicated to substitute Ti element on only one transition metal element. This work synthesizes the NCM cathode materials by Ti doping using the commercial precursor (Ni0.5Co0.2Mn0.3)(OH)2 with the ratio of transition metal ion invariable in bulk scale (>10 kg). Industrial TiO2 and Li2CO3 are used together on this basis to synthesize the final cathode materials via a ball-milling process. In addition, Ti element was doped on the whole precursors by a simple solid state method, which is different from other researches. Furthermore, the microscopic lattice structure and morphology of modified cathode materials and electrochemical performance of Li/Li(Ni0.5Co0.2Mn0.3)1−xTixO2 half cells are investigated. Furthermore, the results this work presents may play an active role in conducting the industry production.
Section snippets
Experimental
In this work, a series of Ti-doped LiNi0.5Co0.2Mn0.3O2 powders were synthesized by a simple conventional solid-state reaction using industrial raw materials in bulk scale (>10 kg) and use five different doping ratios of Ti, which were 0.0%, 0.2%, 0.4%, 0.8% and 3.0%.
Results and discussion
Fig. 1 shows the XRD Rietveld refinement results of the materials with different amounts of Ti-doping. LiNi0.5Co0.2Mn0.3O2 electrodes without Ti doping are also prepared and examined carefully with XRD measurement in Fig. 1 for comparison. It can be observed in Fig. 1 that all diffraction peaks are clear and sharp, which means that the as-prepared samples are well crystallized. It is well known that LiNi0.5Co0.2Mn0.3O2 has a hexagonal crystal structure of α-NaFeO2 with a space group of R-3m [35]
Conclusions
The typical α-NaFeO2 layered Ti-doped Li(Ni0.5Co0.2Mn0.3)1−xTixO2 cathode materials were synthesized via a simple solid state method using industrial raw materials in bulk scale (>10 kg). All samples have a pure phase hexagonal α-NaFeO2 layered structure with R-3m space group and there is no obvious impurity phase peaks observed. Ti was proved as being doped on NCM cathode materials successfully and homogeneously. The electrochemical performance of cathode material is increased by Ti-doping, and
Acknowledgment
This work acknowledges the National Natural Science Foundation of China (Grant no. 21273058), China Postdoctoral Science Foundation (Grant nos. 2012M520731 and 2014T70350), Heilongjiang Postdoctoral Financial Assistance (LBH-Z12089) for their financial support.
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