The Ink-jet Printing Preparation And Electrochemical Performance Of Lithium-ion Thin-film Electrodes And Composite Ultra-capacitor Materials

With the development of microelectromechanical systems (MEMS) and very large-scale integration (VLSI), there is an increasing requirement in the miniaturization and integration of power sources. The reduction in size and power requirement of electronic devices is the major driving force behind the development of all-solid-state thin-film batteries. Applications focus on the improvement of existing consumer and medical products, such as smart cards, sensors, portable electronic devices, as well as on the integration with electronic chips and microelectromechanical systems. With better integration compatibility and electrochemical performance, thin-film lithium ion battery becomes the optimal choice for miniaturization and integration of MEMS and VLSI power.Electrochemical (EC) capacitors, also called supercapacitors, are a kind of new-style energy-storing sources between conventional capacitors and batteries. They can provide high power densities and unusual cycleability and therefore are urgently needed for a number of technologically important systems. These systems include acceleration power for electric vehicles, electrical regenerative braking storage for electric drive systems, power assist to hybrid vehicles, starting power for fuel cells, pulse power for mobile telecommunication and other electronic devices which require high power to operate. In addition, when EC capacitors are coupled with batteries, they can reduce the peak power requirement, prolong the lifetime and reduce the energy requirement (or the size) of the battery. On the other hand, the big problem for the current supercapacitors such as carbon materials is their low energy density. Therefore, many researchers focus on the composite supercapacitor materials such as carbon-supported RuO₂ in order to increase the energy densities.This thesis includes two major parts. Firstly, thin-film electrodes including LiCoO₂ cathode, SnO₂ anode and Li₄Ti₅O₁₂ anode used for lithium ion batteries were successfully fabricated by a very novel and facile route of ink-jet printing technique. In addition, their structure, morphology and electrochemical performance were investigated in great detail. Secondly, RuO₂.xH₂O/MC composite materials obtained by loading small amount of amorphous hydrous ruthenium oxide nanoparticles on mesoporous carbon (MC) were fabricated and used for supercapacitors for the first time. Their electrochemical behaviors were also investigated.The main results are as follows.1. Nano-sized or single-crystalline materials including LiCoO₂, LiMn₂O₄, V₂O₅, SnO₂ and Li₄Ti₅O₁₂ were synthesized by using tri-block copolymer amphiphilic surfactant (EO₂₀PO₇₀EO₂₀, always abbreviated as P123) as a structure-directing agent through sol-gel process because nano-sized materials with good electrochemical activity are demanded during the subsequent ink-jet printing process. Among them, the nano-sized single-crystalline LiMn₂O₄ and single-crystalline V₂O₅ were synthesized for the first time by using this method. At the same time, the electrochemical performances of these materials were investigated.2. The key procedure for the ink-jet printing process is to obtain the stability of nano-sized materials in the dispersion system. The stable LiCoO₂, SnO₂ and Li₄Ti₅O₁₂"inks" containing conductive agent and binder were successfully prepared by employing both wet ball-milling technology and steric polymeric dispersant.3. Thin film LiCoO₂ electrode with the uniform thichness of 1.27μm was successfully prepared by using the novel facile and low-cost ink-jet printing technique onto the commercial Al substrate. The initial discharge capacity was 15μAh/cm².μm at charge current of 40 μA/cm² in the potential range of 3.0—4.2 V (vs. Li⁺/Li) . CV measurements showed the obvious phase transition and obvious capacity loss was also observed with respect to the as-printed thin film LiCoO₂ electrode without any post-annealing process. The reason for the capacity loss was attributed to both the crystalline structure change during the wet ball-milling process and the polymeric dispersant coating on the surface of the nano particles. Secondly, thin film LiCoO₂ was also ink-jet printed on the gold-coated Al foil and was then followed by a slight annealing process at 450°C for 30 min in order to improve the electrochemical performance. The electrochemical performance was obviously improved after this slight heatment. The initial discharge capacity of thin film LiCoO₂ electrode at a charge current of 20μA/cm² in the potential range of 3.0～ 4.2 V (vs. Li⁺/Li) was 20.31μAh/cm².μm (81 mAh.g^-1). The charge-discharge efficiency approached almost 100 % after 10 cycles. The discharge capacity was 18μAh /cm².μzm (71 mAh.g^-1), which is 87% of the initial capacity, after 50 charge-discharge cycles.Besides, the effects of three kinds of ball-milling processes on the crystalline structure and electrochemical performances of nano-sized LiCoO₂ were investigated in great detail. The ideal rock-salt structure of original nano-sized LiCoO₂ was obviously influenced by all these ball-milling processes employed in this paper. Thestructure of LiCoO₂ with smaller particle size was influenced more seriously.4. SnO₂ thin film electrodes on commercial Cu foil substrate as an anode for rechargeable lithium ion batteries were also successfully fabricatd by using ink-jet printing method for the first time. The distribution of as-printed thin film SnO₂ electrodes is smooth and uniform. The thickness can be adjusted by printing different layers. The thickness of monolayer is ca. 770-780 nm and the average thickness of the 10-layer film after compression is about 2.3 μm which was used for electrochemical measurements. The linear relationship between anodic peak current and the scan rate obtained by CV technique shows the characteristics of thin-film electrodes. High initial discharge capacity about 812.7 mAh/g was obtained at a constant discharge current density of 33 μA/cm² over a potential range of 0.05-1.2 V vs. Li⁺/Li and the cycle performance is improved because the conducting agent AB can also perform as a better matrix for nano-structured thin films. Aggregation and pulverization due to the large volume expansion/contraction during the alloy/dealloying process gives rise to capacity decay which can be concluded by comparison of SEM and TEM pictures of the thin films before and after the charge-discharge process.5. Thin-film Li₄Ti₅O₁₂ electrode, which can be used as an anode in lithium ion batteries, was successfully fabricated also employing the ink-jet printing technique for the first time.Firstly, the thin-film Li₄Ti₅O₁₂ electrode was ink-jet printed on commercial Cu foil without any post-annealling heat-treatment. The cross-sectional SEM image showed that the uniform thickness of monolayer ink-jet printing was about 700-800 nm. The sharp and symmetric reversible redox couples located at about 1.55V in CV curves corresponds to the spinel structure of Li₄Ti₅O₁₂. High initial discharge capacity about 172 mAh/g is obtained at a constant discharge current density of 20.8 μA/cm² over a potential range of 1.0-2.0 V vs. Li⁺/Li, which almost reaches the theoreticl capacity 175 mAh/g. Even at a very high current density of about 208 μA/cm², the initial discharge capacity was not decreased compared to that at the low current densities which showed the excellent rate capabilities of the thin-film electrodes. The capacity retention was about 62.7% after 50 cycles and the cycle stability was not so good.Secondly, thin-film Li₄Ti₅O₁₂ electrode was also ink-jet printed on Au plate and then followed by a post-annealing process at 550°C for 90min in order to improve theelectrochemical performance of thin film Li₄Ti₅O₁₂ electrodes. The cross-sectional SEM image showed that the uniform thickness of 10-layer ink-jet printing process was about 1.7～1.8μm. The linear relationship between peak current and the scan rate obtained by CV technique shows the characteristics of thin-film electrodes. Excellent cycle performance was observed at a constant discharge current density of 10.4 μA/cm² over a potential range of 1.0-2.0 V vs. Li⁺/Li. The capacity retention after 300 cycles was about 88% of the peak discharge capacity (172 mAh/g). At the same time, high discharge capacity was obtained. The high discharge capacity can be attributed to both the thin-film characteristics and the double-layer capacity due to the highly dispersed nano Li₄Ti₅O₁₂.6. Amorphous hydrous ruthenium oxide/mesoporous carbon powders (RuO₂.xH₂O/MC) were prepared by liquid adsorption method. The mesoporous characteristics of mesoporous carbon and the high specific capacitance and highly electrochemical reversibility of RuO₂·xH₂O play a dominant role in the electrochemical properties of amorphous hydrous ruthenium oxide/mesoporous carbon (RUO2.XH2O/MC) composites.Electrochemical measurements showed that the RUO2.XH2O/MC composites prepared by loading small amount of RuO₂.xH₂O nanoparticles (ranged from 0.9 to 5.4 wt % Ru) on MC not only have an enhanced specific capacitance but also retain the ideal capacitive performance such as highly reversibility, excellent rate capability, and good stability of MC. The RuO₂.xH₂O/MC composite (3.6 wt.% Ru) exhibited an increase of the specific capacitance of approximately 57% (from 115 to 181 F/g) at the scan rate of 25mVs^-1in 0.1 M H₂SO₄ aqueous electrolyte within the potential range from -0.2 to 0.8V vs. Ag/AgCl. The specific capacitance of RuO₂.xH₂O was estimated to be 1527 F/g by subtracting the contribution from MC in the composite in case of 3.6 wt.% Ru loaded electrode at the scan rate of 25 mVs^-1, which indicates the high utility of the active material RuO₂.xH₂O. Cycle performance tests derived by CV measurements showed that 97.2% of capacitance retention for the RuO₂.xH₂O/MC composite (3.6 wt.% Ru) was observed after 1000 cycles.