Lithium recharge batteries have been used as a reliable energy source for portable electronic devices, Furthermore, Lithium batteries have also been expected to be a probable energy source for electric vehicles (EV). At the present, most commercial Lithium batteries have $LiCoO_2$ as a cathode material due to its high capacity and excellent cycle stability although $LiCoO_2$ is very expensive and has a relatively low thermal safety, which requires the expensive safety devices (PTC, vent cap etc.). However, the high cost and low thermal safety of $LiCoO_2$ has been the greatest obstacle to its use as a cathode material in large scaled batteries for electric vehicle and consequently to the commercialization of electric vehicle.
$LiMn_2O_4$ is the most favored because of its low cost and environmental friendly character. For the past ten years, the spinel $LiMn_2O_4$ has been studied extensively as a positive electrode material for rechargeable lithium and lithium ion cells. However, the $LiMn_2O_4$ electrodes have showed capacity fading during cycling. But origin of main capacity fading mechanism of $LiMn_2O_4$ electrodes has not confirmed up to now. Several possible reasons about capacity fading are suggested; such as an organic-based electrolyte in a high potential region, the dissolution of manganese into electrolyte, Jahn-Yeller distortion due to $Mn^{3+}$ ion , change in crystal uniformity with cycling, and so on.
In this work, $LiMn_2O_4$, a new alternative cathode material for large scaled batteries for electric vehicles, taking advantages of low cost and good thermal safety over $LiCoO_2$ was studied to elucidate the main capacity fading mechanism.
Part Ⅰ. capacity fading analysis as a function of Mn valence state.
un-doping
We have adapted the modified Pechini process to the synthesis of $LiMn_2O_4$. Using this method, it is possible to obtain phase-pure ultrafine crystalline spinel phases after firing the polymeric precursors at low temperatures. The powders were calcined in a furnace at 600 ~ 850℃ for 4h in air. The samples calcined at low temperature have a smaller lattice constant than the ones calcined at high temperature. The morphology is changed from round to facet shape with higher temperature. The sample calcined at 600℃ for 4h yielded the lowest specific capacity (110 mAh/g) for both charge and discharge. The sample calcined from 700℃ to 850℃ for 4h yielded similar high specific capacity (130 mAh/g) for both charge and discharge.
The cycle efficiency sharply decreases with increasing calcination temperature. This decay tendency is compatible to lowering of Mn valence state and growing of particle size. There appears to be a close relationship between Mn valence-state, powder size of cathode powder and their cathode cycleability.
doping→high Mn valence state
We investigated $LiMn_{2-x}M_xO_4$, where Mn is partly replaced by Co, Ni, Ga, prepared by the Pechini method in the range of 700~850℃ for 4h. This work aims to elucidate the effect whether high Mn valence state that is generated by doping plays major role in cycle-life.
The partial substitution of Mn by M (M=Co, Cu,Ga) can improve the cycling performance of spinel. The doped samples maintain high cycle efficiency regardless of calcination temperature if maintain single phase. The improvement in cycling properties resulting from the stabilization of spinel structure might attribute to have smaller spinel lattice constant and high Mn valence state in doped spinel powders.
Part Ⅱ. Improvement of electrochemical properties of $LiMn_2O_4$ at high current densities.
The high lithium-ion and electron conductivity is essential to the good performance of $LiMn_2O_4$ electrode in lithium ion secondary battery. It was reported that the electrical conductivity of $LiMn_2O_4$ electrode is about $2×10^{-6}S/cm$, while the ionic conductivity is approximately $10^{-4}S/cm$. These results suggest $LiMn_2O_4$ be a mixed ionic-electronic conductor with dominating lithium-ion conduction. Thus, an electronically conductive phase, such as carbon, may be necessary in order to form composite electrode with adequate electronic conductivity. However $LiMn_2O_4$ require much amount than that of $LiCoO_2$ electrode ($LiCoO_2$ : electrical conductivity of about $10^{-4 }$ ~ $10^{-5}$ S/cm). The carbon will reduce energy efficiency.
This study examines the effects of $LiMn_2O_4$ surface modification by chemically Ag-metal coating on the electrochemical performance that could have especially high current retention. This treatment of the surface coating is expected to affect the charge-discharge capacity and cycleability associated with formation of the porous metal film on the $LiMn_2O_4$ surface.
The surface treated $LiMn_2O_4$ cathode material has the advantage of higher charge-discharge capacity, lower cell polarization. The improved cycleability may attribute to enhanced electron conduction between $LiMn_2O_4$ particles, because of the low resistance of Ag.
