전고상 리튬이온 박막전지의 성능향상을 위한 borophosphate계 비정질 전해질 및 LiCoO2 전극의 구조 제어 연구

Abstract

The ongoing improvements in the microelectronics industry and the miniaturization of electronic devices have reduced the current and power requirements of some devices to extremely low levels. Therefore, there is an increasing need for new lightweight batteries with long life, and high energy density. In addition, unlike conventional batteries, these batteries have to be deposited directly onto chips or chip package in any shape or size. One approach to fulfilling this need is the development of thin film batteries. In this study, to establish the fabrication techniques for the all-solid-state thin film batteries with its improved electrochemical performances, structural modifications of cathode and electrolyte thin film materials was investigated. First, LiCoO2 thin films are deposited by RF-sputtering with controlling the lattice orientations to maximize lithium ion diffusivity in film textures. due to the anisotropy of structural and electrochemical properties, LiCoO2 cathode film needs to be aligned to the crystallographically preferred orientation to improve the interfacial resistance, especially, with the solid state electrolytes. The nano-sized crystalline grains grow up with the (003) preferred orientation parallel to the substrates at room temperature due to the lowest surface energy of this atomic plane. However, because the surface energy difference of atomic planes of LiCoO2 reduces with increasing substrate temperature, the influence of surface energy becomes weaker at high temperature. As a consequence, the LiCoO2 thin films with the (110) preferred orientation are obtained at 400 ��C by dominant influence from the lowest the volume strain energy of this orientation. Also, to take advance of this orientation effect in full cell, the influences of the uppermost metallic current collector layer on the structural properties of sputtered cathode are investigated. It turns out that the Li2O buffer layers between the cathode films and the metallic current collector layers can suppress the formation of lithium-deficient phase, Co3O4, and the growth of (003) plane by reducing the lattice match between of LiCoO2 (003) plane and Al (111) plane. The LiCoO2 films with the controlled orientation show enhanced rate performance owing to improved interfacial resistance and lithium-ion conductivity. On the other hand, as solid-state electrolytes for thin film batteries, many inorganic glass electrolytes were investigated with high ionic conductivity, thermal stability, high power density, and good chemical property. In this thesis, Li2O-B2O3-P2O5 glass systems were prepared with a wide range of chemical composition by co-sputtering method with multi-targets. The maximum ionic conductivity at room temperature was 1.22 �~ 10-6 S/cm and the activation energy of this specimen was 0.54 eV when enhanced by the mixed former effect and the high network modifier content. The increased ion conductivity of the films appeared to be associated with the formation of stable tetrahedral borate at high lithium concentration and this structural change facilitated faster ion migrations. The relationship between the glass structure and electrical performance of the mixed former glasses was investigated by the infrared spectra analysis of borate networks. The conductivity of the mixed former electrolytes increased gradually with increasing the RF power on Li2O target, and the maximum conductivity was obtained at 100 W. The structural role of the network modifier in xLi2O-(1-x)(B2O3-P2O5) glasses was analyzed by the O1s spectra of X-ray Photoelectron Spectroscopy. A quantitative deconvolution of O1s spectra suggests that the increased conductivity with increasing lithium content is due to the formation of non-bridging oxygen. And then, this mixed former effect of boron and phosphorus demonstrating its improved stability and electrochemical property applied to the established LiPON electrolyte. This new composition thin films named �gLiBPON�h were deposited by using a RF magnetron sputtering method with double targets of Li3PO4 and Li3BO4 targets. When the boron content is varied by controlling the RF power for Li3PO4 and Li3BO4 to investigate the B-P mixed former effect, it can be demonstrated that all additive boron ions successfully replaced the phosphorus sites in the B / B+P range of 0 󰠏 0.35. The Raman spectra and the XPS spectra for N1s revealed that three-coordinated nitrogen atoms (P󰠏N<P) were dominant in these new structures, completely opposite to the result for the LiPON films without the B-P mixed former effect. As expected from the structural analysis, the mixed former LiBPON film deposited with the RF powers of 50 W : 20 W showed the maximum conductivity of 6.88 �~ 10-7 S/cm. It is required further increases of the nitridation in the films by the controls of another deposition conditions such as the RF power, the chamber pressure and the gas flow rate. Also, through the exposure to humid ambient conditions, the improved chemical stability of LiBPON films could be demonstrated. All-solid-state thin film cells LiCoO2 / LiBPON / Li exhibited excellent cycling performance with the initial capacity of 34.5 ��Ah/cm2 and the cycling efficiency and 95.3 % of its initial capacity.