Preparation of Water-Resistant Surface Coated High-Voltage LiNi0.5Mn1.5O4 Cathode and Its Cathode Performance to Apply a Water-Based Hybrid Polymer Binder to Li-Ion Batteries
Graphical abstract
Introduction
Recently, water-soluble and aqueous polymers (water-based polymers) have attracted much attention as binders for lithium ion batteries (LIBs) because of the need for low-cost materials and environmentally compatible electrode fabrication processes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. N-methyl-2-pyrrolidone (NMP), which is listed as a carcinogenic chemical with reproductive toxicity [1], [14], [15], is often used as a solvent to prepare a binder slurry. The slurry is composed of cathode material particle, conducting carbon additive, conventional polyvinylidene difluoride (PVdF) binder and NMP solvent, and is casted on an aluminum current collector and finally is dried to evaporate the NMP. The NMP solvent should be recycled without releasing it to the atmospheric environment. A reduction in costs of the LIBs is severely constrained because of facility investments for the process used currently. Therefore, shift of a nonaqueous solution-based fabrication process of LIBs to an aqueous solution-based one is widely investigated. For graphite anodes, styrene-butadiene rubber has already been employed as a water-based polymer binder in fabricating some commercially available Li-ion batteries [16], [17]. Applying water-based polymer binders to the cathode is a next target to develop the low-cost and environmentally friendly fabrication process for LIBs. Some companies have produced prototype models using water-based polymer binders, and their battery test results have been reported to be comparable with those obtained with the conventional PVdF polymer binders [18], [19]. We also have applied a water-based hybrid polymer binder composed of acrylic polymer and fluoropolymer, TRD202A (JSR, Japan) to high-voltage Li-rich solid-solution cathode, in which the water-based polymer binder slurry was used immediately after its preparation to prepare the cathode films [20]. Uniform cathode films were prepared with a Li-rich solid-solution (Li[Li0.2Ni0.18Co0.03Mn0.58]O2) cathode material and water-based hybrid polymer binder (TDR202A), carboxymethyl cellulose (CMC), and conducting carbon additive. The films exhibited stable charge/discharge cycle performances (average discharge capacity: 260 mAh g−1) when cycled between 4.8 and 2.0 V for 80 cycles. The cathode film prepared with the water-based hybrid polymer binder showed longer-term reliability as well as higher electrochemical resistance when compared with that prepared using the conventional PVdF binder. Through our researches so far carried out concerning cathode/water-based hybrid polymer binders, we have understood that the water-based polymers binders cannot be applied to some cathode materials because of solubility of the cathode material surfaces in water (not shown in this paper). Many papers on the cathode materials/water-based polymers binders have been published [18], [19], [20], but in these cases, the water-based polymer binder slurries were used immediately after their preparation. In a practical production level, however, the cathode materials are put in water-based binder slurry solution at least for one week and therefore they are required to keep a “water-resistant” property for one week (“water-resistant” means slowing the penetration of water (but is not water-proof)).
In this study, we tried to modify the cathode surfaces with carbon material and water-stable metal oxides to isolate them from the aqueous solutions of the water-based slurry and to obtain stable charge/discharge cycle and rate performance even after the prepared cathode materials are exposed to the aqueous solutions of the cathode slurry for one week. The surface coating of the cathode material surfaces with carbon [21], [22] and metal oxides [23], [24] has been reported and the improvement of the charge/discharge cycle durability [24], [25], [26] and the rate performance [27], [28] have been achieved with the aid of the surface coatings. The surface coating should not prevent the intercalation/deintercalation of Li+ ions to/from the cathode material layers although it is required to isolate the cathode material surface from the water-based polymer binder slurry during the cathode fabrication process. The surface coatings with a unique property, i.e., they do pass Li+ ion, but not H2O molecule, were investigated using carbon, aluminum oxide and niobium oxide. A LiNi0.5Mn1.5O2 spinel cathode material was selected for the present surface coating. The LiNi0.5Mn1.5O4 has attracted a lot of attention from many research groups in the field of energy storage, owing to its high specific energy of 658 Wh kg−1 [29], [30], [31], which is much higher than commercially available cathode materials such as LiCoO2 (518 Wh kg−1), LiMn2O4 (400 Wh kg−1), LiFePO4 (495 Wh kg−1), and LiCo1/3Ni1/3Mn1/3O2 (576 Wh kg−1). In addition, the upper potential applicable for the charge/discharge reaction of LiNi0.5Mn1.5O4 is around 4.7 V (vs. Li/Li+) and thus it is suitable to test the electrochemical oxidation resistance of the water-based polymer binders which are required to possess a high resistance to electrochemical oxidation. Furthermore, metal oxide cathodes containing a high percentage of Ni2+ ions such as LiNi0.5Mn1.5O4 tend to suffer from chemical damage which is caused by the contact with water, i.e., Ni3+ ion on the cathode material surface is reduced with H2O to form Ni2+ ion. As a result of this reduction, lithium carbonate and lithium hydroxide are formed on the cathode material surface and dissolved into aqueous solutions. This leads to the corrosion of aluminum current collectors, especially in the case of water-based binders. Pieczonka et al. [32] reported self-Mn and Ni dissolution behaviors. The self-discharge reaction of LiNi0.5Mn1.5O4 causes a decomposition of electrolyte, and the resulting HF can accelerate Mn and Ni dissolution from LiNi0.5Mn1.5O4, and consequently various reaction products, such as LiF, MnF2, NiF2, and polymerized organic species, are found on the surface of LiNi0.5Mn1.5O4 electrode. So, the cathode surface coating is important for inhibiting the degradation of cathode performance.
Therefore, the present study on the coating process to inhibit the chemical dissolution of the LiNi0.5Mn1.5O4 containing a high percentage of Ni and Mn ions is considered to be suitable for realizing the degree of target achievement about the water-resistant property of cathode materials which could use a so-called water-based slurry binder solution in the practical fabrication process of LIBs.
Section snippets
Preparation of carbon and metal oxide-coated cathode materials
A LiNi0.5Mn1.5O4 particle sample was purchased from Hohsen Corp. (Japan). The particle was used as a cathode material without any purification. Surface coatings with carbon, AlOx and NbOx were conducted as mentioned below.
Carbon coating: Sucrose (Wako Pure Chemicals Co. Ltd. (Wako), Japan) as a source of carbon layer for coating was weighted with a proper amount to prepare 0.5, 1 and 10 wt% carbon-coated LiNi0.5Mn1.5O4 particle samples (0.5 wt% carbon-coated LiNi0.5Mn1.5O4 means that the sample
Characterization of surface-coated cathode materials
Fig. 1 shows (A, B) SEM images and (C, D) histograms of particle size of (A, C) pristine and (B, D) AlOx-coated LiNi0.5Mn1.5O4 cathode particles. In both cases, LiNi0.5Mn1.5O4 particles are of polyhedral shape and exhibit smooth surfaces with sharp edges. The distribution of size of the cathode particles was evaluated on the basis of approximately 100 particles in the SEM images. The average diameters of the pristine and AlOx-coated LiNi0.5Mn1.5O4 cathode particles were calculated as 1.23 ± 0.05
Conclusions
In this study, in order to establish the protocol to prepare water-resistant cathode materials of LIBs which can be used in the cathode preparation with water-based binders, carbon- and metal oxide-coated LiNi0.5Mn1.5O4 cathode materials were prepared and the performance of the batteries prepared from these cathode materials with and without a water-resistant property was compared in the viewpoint of charge/discharge cycleability and discharge rate performance. The carbon and metal oxides (Al2O3
Acknowledgements
This work was financially supported by the Strategic Research Base Development Program for Private Universities of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors are grateful to Dr. Keiichi Osaka, JASRI, for his help in XRD measurements at BL19B2 of SPring-8 (Proposal No. 2015B1618, 2016A1560). We thanks K. Shinoda for his help in STEM measurements at National Institute for Materials Science (NIMS) Battery Research Platform.
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