Research Express@NCKU Volume 4 Issue 8 - June 6, 2008 [ http://research.ncku.edu.tw/re/articles/e/20080606/2.html ]
Effect of chitosan on stabilization of
acetates-containing solution: A novel precursor for
LiMn
2
O
4
film deposition
Fu-Yun Shih, Kuan-Zong Fung
*Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101,Taiwan
E-mail address: [email protected] Journal of Power Sources 159 (2006) 1370–1376
R
ecently, thin-film solid-state batteries have attracted significant attention because they have been studied for various applications. One of these applications is to serve as an independent power supply for microsensors, microelectronics and high-temperature semiconductor electronics, etc. Therefore, to develop long-lasting and high-energy efficient microbatteries is very important.Spinel LiMn2O4 is a promising cathode material for microbatteries due to its high operating voltage, low cost and low toxicity. Wet chemical deposition has the most advantages, including simple processing, easy control of stoichiometry and high deposition rate. However, it has also been found that the quality of the prepared LiMn2O4 films is strongly dependent on the nature of the precursor solution used. For example, when the aqueous solution containing metal acetates was spin-coated on the platinumized silicon substrates, homogeneous films could not be obtained, and droplets were just left on the substrate. In order to eliminate the formation of the precipitates, the addition of an appropriate chelating agent was found to be effective in the stabilization of the ethanol solution containing lithium and manganese acetates and then beneficial to the deposition of LiMn2O4 films on a Pt-coated silicon substrate. For example, poly(vinylpyridine) (PVP) has been used as an additive to adjust the nature of the prepared precursor solutions. Thus, the addition of PVP in the precursor solution was able to enhance the deposition of LiMn2O4 films. Chitosan, a polysaccharide derived from crustacean and fungal chitin, is a non-toxic, tissue-compatible polymeric biomaterial. Chitosan can be dissolved in the aqueous solution with weak acid. Homogeneous polymeric membranes or films can be easily fabricated from the chitosan-containing aqueous solution by dipping or spin-coating.
Fig. 1. Schematic illustration of the chelate mechanism between chitosan and Li/Mn ions.
A large amount of precipitate was observed after the ethanol solution containing acetates was stirred continuously for 12 h. On the other hand, the chitosan-added precursor solution showed better stability with no formation of precipitate for 10 months. Thus, it is inferred that the enhanced stability of the precursor solution may be attributed to the addition of chitosan. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the –NH2 groups of chitosan. The structure for the lithium/manganese acetates–chitosan precursor is proposed and shown in Fig. 1.
Fig. 2. SEM micrographs of the films deposited from the acetates-containing precursor solutions (a) with and (b) without chitosan addition followed by heattreatment at 700 ˚C for 1 h.
Fig. 2(a) shows that a relatively dense LiMn2O4 film without cracking was formed when the chitosan-added precursor solution was used. However, the films deposited from the chitosan-free precursor solution showed numerous cotton-like precipitates(Fig. 2(b)). The addition of chitosan can indeed stabilize the lithium/manganese acetates-containing precursor solution and is also helpful for the deposition of a single-phase and crack-free LiMn2O4 film.
The electrochemical behavior for the deposited films calcined at 700 ˚C for 1 h was characterized by charge–discharge test. Electrochemical charge–discharge behaviors of the deposited LiMn2O4 films were
investigated using a metal Li/liquid electrolyte (LiPF6)/LiMn2O4 cell. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 134 mAh g-1 and about 9% capacity loss after 50 charge/discharge cycles, which is better than the one deposited from chitosan-free precursor solution with an initial discharge capacity of 108 mAh g-1 and 24% capacity loss after 50 cycles.
In this work, the addition of chitosan in the precursor solution was found to be an effective method for the deposition of LiMn2O4 films. Furthermore, the chitosan addition can eliminate the formation of the precipitates in the precursor solution containing lithium/manganese acetates. A chitosan-added precursor solution remained stable without any precipitation for 10 months. Due to the chemical bonding between chitosan and cations, Li/Mn ions can be homogeneously distributed in the precursor solution at a molecular scale. Such a precursor solution is beneficial for the deposition of a single-phase LiMn2O4 film on a Pt-coated silicon substrate. The charge–discharge test also showed that the prepared thin-film cathode deposited from the chitosan-added precursor solution exhibited a higher initial discharge capacity of 134 mAh g-1 and better capacity retention of 91% after 50 charge/discharge cycles in comparison with the performance of the film from the chitosan-free precursor solution. Thus, the lithium/manganese acetates-containing precursor solution with chitosan addition is a unique and appropriate way to prepare a dense and single-phase LiMn2O4 film.
Fig. 3. Cycling discharge curves of Li/liquid electrolyte (LiPF6)/LiMn2O4 cell using the films deposited
from the acetates-containing precursor solutions (a) with chitosan addition, (b) with PVP of MW~10,000 addition, (c) with PVP of MW~360,000 addition and (d) with no chitosan addition.
