Rechargeable lithium batteries are key components of the telecommunication equipment and portable computer systems. The layered $LiCoO_2$ is one of the earliest developed cathode materials, and the good performance and the stability make it the main commercially used cathode material despite its toxicity and high cost. Since the commercialization of the $LiCoO_2$ by SONY in 1991, alternative cathode materials have been pursued to improve the battery performances and cut down the cost. Among the several cathode materials, $LiFePO_4$ of the phosphor-olivine family proposed by Goodenough appears particularly interesting due to the low cost and the environmental compatibility. For $LiFePO_4$, the charge/discharge voltage is about 3.4V vs. Li/Li+ and the capacity fade is very small even after several hundred cycles. Its theoretical capacity is 170mAh/g, which is as large as those of present cathode materials.
Although $LiFePO_4$ has good electrochemical properties, its utilization for cathode material in lithium secondary battery is limited by its low electronic conductivity. In this study, therefore, we tried to overcome drawback through Li substitution for lanthanide dopants.
$Li_{0.995}M_{0.005}FePO_4$ (M=La, Nd, Dy) were synthesized by solid state reaction. Their structures were studied using synchrotron X-ray diffraction, high resolution neutron diffraction and Rietveld refinement. The particle morphologies of these materials were observed by Scanning Electron Microscope. The electronic properties were characterized using 2-probe DC method and thermoelectric power. The electrochemical properties were characterized using galvanostatic cycling.
We studied $Li_{0.995}M_{0.005}FePO_4$ $(M=La^{3+}, Nd^{3+}, Dy^{3+})$ whose average particle size is 500nm by solid state reaction. The lattice parameters a, b and cell volume of doped $Li_{0.995}M_{0.005}FePO_4$ were increased with increasing ionic radius of M ion. On the other hand, the lattice parameter c and Fe-Fe distance were decreased with increasing ionic radius M ion. We found that the substitution of supervalent ions such as $La^{3+}$, $Nd^{3+}$ and $Dy^{3+}$ for Li site in $LiFePO_4$ system increased electronic conductivity. Moreover, as Fe-Fe distance was decreased, electronic conductivity was increased. During electro chemical operation doped $Li_{0.995}M_{0.005}FePO_4$ showed lower polarization than un-doped $LiFePO_4$ and the capacity loss was decreased. Therefore, the electrochemical properties of $Li_{0.995}M_{0.005}FePO_4$ are correlated with ionic radius of substituted ion and the substitution for Li site is a way to improve electronic conductivity of $LiFePO_4$. Consequently, we conclude that low electronic conductivity of $LiFePO_4$ can be improved through substitution for Li site.
최근 $LiFePO_4$ 전기화학적 특성을 개선시키기 위하여 supervalent cation을 도핑하는 연구가 활발히 진행되고 있다. 본 연구에서는 이러한 도핑 방법의 일환으로 Lanthanide의 도핑을 시도하였는데, 이는 Lanthanide가 Li이나 Fe보다 산화수가 높을 뿐만 아니라 이온반경도 더 큰 원소이기 때문이다. 즉, 이온 반경이 더 큰 물질이 도핑 되었을 때의 구조적인 격자의 변화와 supervalent cation에 의한 defect의 변화가 물질의 전기전도도와 전기화학적 특성에 미치는 영향에 대한 분석을 시도하였다.
Lanthanide는 이온 반경에 따라 La,Nd,Dy를 사용하였고 도핑된 $LiFePO_4$ 의 합성은 수소분위기에서 고상반응법을 이용해 합성하였고, dopant로써 oxalate hydrate를 사용하였다. XRD, SEM 분석을 통해 상 합성여부와 morphology를 비교하였으며, 보다 정확한 구조적인 분석을 위하여 중성자 회절을 통한 Rietveld refinement를 시행하였다.
전기화학적 분석은 beaker cell을 제작하여 half cell 측정으로 하였으며, 양극 cell 제조를 위해 활물질과 도전체(carbon black계통의 Vulcan XC-72), 그리고 binder (PVdF)를 75 : 20 : 5의 질량비로 혼합하였다. 전해질로서는 1M LiPF6 EC/DEC (1/1 vol.%)를 사용하였고, 음극은 리튬금속을 사용하였다.
본 연구 결과를 통해 도핑에 따른 격자 구조의 변화를 측정하였고, 격자 구조의 변화와 전기전도도, 전기화학적 특성과의 상관관계에 대해 고찰해보았다.
