Effect of ball milling conditions on microstructure and lithium storage properties of LiNi0.5Mn1.5O4 as cathode for lithium-ion batteries
Graphical abstract
Schematic representation of dry ball-milling (a), wet ball-milling (b) and cyclic performance at 10 C rate.
Introduction
Nowadays, researchers have been looking for the next generation cathode materials for high-performance lithium-ion batteries (LIBs) [1]. LiCoO2, LiMn2O4, Li[M]O2 (M: Mn, Ni, Co) and LiFePO4 as commercial cathode materials for LIBs have been widely used in the energy storage devices of electric vehicles (EV) and hybrid electric vehicles (HEV) due to their intrinsic advantages, such as high energy density, long cyclic life, and high operating voltage of single battery, and so on [2]. However, these cathode materials also have their own drawbacks. For example, spinel LiMn2O4, the cheapest cathode material due to the abundance of Mn sources, has been attractive for dozens of years [3]. The rapid capacity fading especially under high operation temperature (>55 °C), seriously hinders the large-scale application of LiMn2O4–based LIBs for EV and HEV, because of its intrinsic drawbacks of Jahn-Teller effect and the dissolution of Mn element [4], [5], [6].
Based on the studies of LiMn2O4, another spinel compound LiMn1.5Ni0.5O4 is considered as an advanced cathode material for LIBs, and it has been received major attention in recent years [7], [8], [9], [10]. LiMn1.5Ni0.5O4 offers high power density for its higher operating voltage (∼4.7 V), and thus it shows 20% and 30% higher energy density than that of LiCoO2 and LiFePO4, respectively. LiMn1.5Ni0.5O4 has been well known in two kinds of crystal structures, an ordered structure with a space group of P4332 in which Ni2+ and Mn4+ are appeared in the crystal lattice, and a disordered structure with a space group of Fd-3m in which Ni2+, Mn4+, minority Mn3+ and oxygen deficiency are observed in the lattice [11], [12], [13], [14].
According to the previous publications, the most popular opinion about the electrochemical performance of LiMn1.5Ni0.5O4 is that the spinel with minor disordered structure is better than the ordered one, because a proper degree of disorder can enhance the diffusion coefficient of Li+ [15], [16], [17]. An order-to-disorder transition has been reported by many researchers to occur at the step of reannealing [18], [19], [20]. The preparation method is one of the efficient way to tune the transition associated with minor loss of oxygen in the lattice structure, usually accompanied with minor disordered coordination of Ni/Mn [21], [22]. Recently, a variety of synthetic methods, such as emulsion [23], co-precipitation [24], [25], sol–gel [26], [27], spray pyrolysis [28], [29], and pulsed laser deposition [30], have been reported to prepare the precursor of LiMn1.5Ni0.5O4 before the final calcination. For example, Wang reported the synthesis of porous peanut-like LiNi0.5Mn1.5O4 through an ethylene glycol-assisted hydrothermal method using urea as a precipitant [31]. The preparation method making precursor plays an important role in crystal structure and the electrochemical properties of LiMn1.5Ni0.5O4.
Ball milling is a facile technique in reducing the particles size and mixing raw materials [32], [33], [34], [35], [36]. Ball milling includes dry ball-milling and wet ball-milling. Dry ball-milling has the advantage of leaving out the addition and removal of solvent, but the problem of the powder sticky on the ball and the wall of container hinder the complete removal of powder after milling. Wet ball-milling is to be added a certain amount of solvent (distilled water, anhydrous ethanol, acetone, etc.) into the milling system [33]. The advantage of wet ball-milling compared with the dry ball-milling is good slurry fluidity and homogeneity, especially for multiphase precursors. Zhou et al. prepared high performance electrode materials using wet-milled precursor of Sb powder and MWCNTs [35]. However, the influence of specific amount of solvent in wet ball-milling technique was few studied, especially, the solvent used in milling is important in crystal structure and properties but still unknown.
In this work, the precursors of LNMO are respectively synthesized by using dry and wet ball-milling methods. Adjusting the quantity of ethanol to mill the precursor, the as-synthesized samples present various crystal morphologies and electrochemical properties. It provides insights into the relationship among the precursor preparation, microstructure and performance of the samples.
Section snippets
Materials preparation
Li2CO3, MnCO3 and NiCO3 were used as the raw materials without any pretreatment. In this work, the parameters of ball milling include as follow: absolute ethanol was applied as dispersant for wet ball-milling, the precursor was milled by planetary ball mill (QM-1SP4-CL, Nanjing, China). Zirconia balls (size of balls 10 mm and 6 mm, respectively) are used to mill the starting materials in stoichiometric ratio, the weight ratio of the raw materials and the balls was 1:8. The milling was set as
Results and discussion
Fig. 1 shows the schematic route in preparation of LNM using dry and wet ball-milling. Under dry ball-milling, the precursors powder containing the carbonates of lithium, manganese and nickel, are pulverized under the collision of ZrO2 balls, while the samples can be mixed in micrometers level. One of the disadvantages of dry ball-milling is the powder sticky on the ball and on the wall of container. The residue that is difficult to remove after milling (as shown in Fig. 1a) results in the
Conclusions
In this work, the precursors of LNM are respectively synthesized by using two milling modes of dry and wet ball-milling. Under wet ball-milling, the sticky problem of reactants observed in dry ball-milling is effectively resolved, the three raw materials form homogeneous slurry and mix well in micrometers level. The amount and effect of ethanol as milling medium play the important role in crystal structure and the electrochemical properties of the final product. LNM-w12 presents a proper
Acknowledgement
This work was supported by Natural Science Foundation of Jiangsu Province (Grant Nos. BK20141229, BK20161267).
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