$LiMn_2O_4$ has been studied for cathode material for Li ion secondary battery because of its low cost and environmental affinity. But in spite of these merits, the big obstacle to its commercialization was the poor calendar life. Two different mechanisms for capacity fading of $LiMn_2O_4$ have been suggested. The first one is Jahn-Teller distortion which explains the structrual instability of $LiMn_2O_4$ and the other is the decomposition of electrolyte caused by the catalytic effect of $LiMn_2O_4$.
Jahn-Teller distortion is caused by $Mn^{3+}$ ion and it is evident when the average oxidation state of Mn is lower than 3.5. There are two methods to reduce Jahn-Teller distortion. The first one is to make manganese deficient non-stoichimetric $LiMn_{2-x}O_4$ and the other one is to dope with another materials which has oxidation state lower than 3 in the Mn spinel. Both methods has the effect of reducing the relative concentration of $Mn^{3+}$. However, it is very difficult to make the non-stoichiomeric oxides so the latter, doping, is regarded as the more realistic way to reduce the Jahn-Teller distortion.
With regard to the catalytic effect of $LiMn_2O_4$, there has not been any suggestion to overcome this problem. But this could also be handled with the doping method because the catalytic effect of oxides depends strongly on their composition (although it generally increases with the complexity of the compositon).
To improve the cyclic property of $LiMn_2O_4$ electrode for Li secondary battery, we substituted $Mn^{3+}$ with Fe aiming to reduce the Jahn-Teller distortion. Fe was chosen to be the doping element considering the stress, which might arise from doping and so depends greatly on the relative ionic sizes of the host ion and the dopant ; Fe ion is very similar to Mn ion in its ionic radius. Sol-gel process was adopted for the homogeneity of the doped oxides and chelating-route was also used to guarantee its full homogeneity.
The calendar life of $LiMn_2O_4$ is very poor at room temperature, much worse at higher temperature above 55℃. This high temperature durability of Li secondary battery is very important, because frequently the working temperature of many electronic devices rises as high as 75℃. Fe-doped $LiFe_xMn_{2-x}O_4$ exhibited only one phase (spinel phase) up to x=0.5 and the electrode showed much enhanced cyclic property at both room temperature and high temperature.
As the concentration of Fe increases, the cycling property was also enhanced but the specific capacity decreased. And up to 700℃, the specific capacity increased but the cyclability decreased somewhat in proportion to the sintering temperature. So considering both the specific capacity and the cyclabiltiy, we found the optimum composition to be $LiFe_{0.05}Mn_{1.95}O_4$ and the optimum calcination temperature to be 600℃.
To understand the effects of Fe contributing to the enhanced cycling durability, electrochemical and physical analyses were performed. From XPS analysis, we found that Fe exists as $Fe^{3+}$ ion in the spinel, and from Rietveld structural analysis, that the Fe atoms occupied 8a sites of spinel phase, which were originally occupied by $Li^+$ ions. XRD analysis of the electrode before and after cycling showed that the enhanced cyclic property of Fe-doped electrode ''at room temperature'' was principally due to the enhanced structural stability caused by Fe. ICP and EIS analyses showed that the enhanced cyclic property ''at high temperature'' was due to the diminishment of electrolyte decomposition, which implies that Fe lowered the catalytic effect of $LiMn_2O_4$.
The structure-stabilization effect of Fe can be explained with the well-known Jahn-Teller distortion theorem and the catalysis-lowering effect of Fe can be explained with Tanabe's model which can predict the intensity of catalysis of an oxide. This is the first attempt to explain the effect of dopants on the catalytic effect of $LiMn_2O_4$ and from this we can suggest a rule for dopant element selection to enhance the cycling properties of $LiMn_2O_4$.
In short, we improved the cyclability of $LiMn_2O_4$ at both room temperature and high temperature by Fe doping and suggested a rule for selecting new dopants to enhance the cyclbility of $LiMn_2O_4$ at high temperature.'
