Lithium rechargeable batteries have been used as a reliable energy source for portable electronic devices(3C products). 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. Moreover, although $LiCoO_2$ is the most suitable for a cathode material in small scaled battery for portable electronic devices owing to high capacity, its low thermal safety should be enhanced from the viewpoint of energy density and cost.
In this work, the first, a new alternative cathode material for large scaled batteries for electric vehicle, $LiMn_2O_4$, taking advantages of low cost and good thermal safety over $LiCoO_2$ was studied and the second, the thermal safety of $LiCoO_2$ was improved.
Part Ⅰ. Improvement of electrochemical properties of $LiMn_2O_4$
$LiMn_2O_4$ is a very promising material, especially, as the cathode material for the battery of electric vehicle. However, $LiMn_2O_4$ has a poor elevated temperature performance and rate capability. The poor performance of $LiMn_2O_4$ at high temperature is due to Mn dissolution into electrolyte and the poor rate capability of $LiMn_2O_4$ can be attributed to its low electric conductivity ($10^{-6} S/cm$) in comparison with that of $LiCoO_2 (10^{-2} S/cm)$.
In the first work, the surface of $LiMn_2O_4$ was coated with $LiNi_{1-X}Co_XO_2$(X = 0.2 and 1) particles in order to improve its high temperature performance and rate capability. Because $LiNi_{1-X}Co_XO_2$ exhibits an excellent elevated temperature performance and a high electric conductivity, it is a very suitable for a coating material.
The surface of $LiMn_2O_4$ was coated with a gel precursor of $LiNi_{1-X}Co_XO_2$ prepared by a chemical method using an aqueous solution of metal acetate containing glycolic acid as a chelating agent and calcined. When the surface of $LiMn_2O_4$ was coated with $LiNi_{1-X}Co_XO_2$(X = 0.2 and 1), the surface of $LiMn_2O_4$ was covered with fine particles. From XRD, EDAX, and TEM analyses, it was clarified that the fine particle on its surface was $LiNi_{1-X}Co_XO_2$(X = 0.2 and 1). The high temperature(65℃) storage and cyclic property of $LiMn_2O_4$ was notably improved by coating its surface with $LiNi_{1-X}Co_XO_2$. $LiNi_{1-X}Co_XO_2$ -coated $LiMn_2O_4$ showed no capacity loss after storing for 300 h at 65℃ although as-received $LiMn_2O_4$ showed 19% of initial capacity loss.
$LiNi_{1-X}Co_XO_2$-coated $LiMn_2O_4$ retained approximately 92 % of the initial capacity after 100 cycles at 65℃, while as-received $LiMn_2O_4$ retained 70 %. The improvement of elevated temperature performance can be attributed to the suppression of electrolyte decomposition on the surface of $LiMn_2O_4$ and the restraint of Mn dissolution which results from encapsulating the surface of $LiMn_2O_4$ with $LiNi_{1-X}Co_XO_2$. When as-received $LiMn_2O_4$ is cycled at 65℃, a new $δ'-MnO_2$ occurred. The reason for the formation of $δ'-MnO_2$ in $LiMn_2O_4$ is that $Li_2MnO_3$ formed on the surface of $LiMn_2O_4$ is decomposed to $δ'-MnO_2$ in the acidic electrolyte. $LiNi_{1-X}Co_XO_2$ (X = 0.2 and 1)-coated $LiMn_2O_4$ has a better rate capability than as-received $LiMn_2O_4$. EIS analysis explains that the improvement of rate capability of $LiNi_{1-X}Co_XO_2$ (X = 0.2 and 1) is due to the suppression of 1st arc and 2nd arc in EIS profile which results from the decrease of passivation layer that acts as an electronic insulating layer and higher electrical conductivity of $LiNi_{1-X}Co_XO_2$.
Part Ⅱ. Improvement of thermal safety of $LiCoO_2$
The exact mechanism of the violent chain reaction induced by the exothermic reaction of cathode is still not fully understood. It was speculated that the exothermic reaction from dissociation of the metal-oxygen bond in cathode ignited the electrolyte, which resulted in the exothermic decomposition(combustion reaction) of electrolyte and the increase of internal pressure induced by the evolution of oxygen. Therefore, there has been much effort to lower the tendency toward dissociation of the metal-oxygen bond. It was demonstrated that the substitution of a certain element such as Mg, Ti, and Al decreased the exothermic heat dissipation substantially. However, the substitution method decreased the capacity as well as the exothermic heat dissipation.
Recently, it was reported that the mechanisms of the exothermic heat dissipation(combustion reaction of electrolyte) of a battery using $LiCoO_2$ are related to the surface property of cathode material. However, the relation between the combustion reaction of electrolyte and the cathode material has not been clarified yet.
In the second work, the role of transition metal oxide in the combustion reaction of electrolyte was studied. The surface of $LiCoO_2$ was coated with $LiMn_2O_4$ in order to improve the heat dissipation property of the battery using $LiCoO_2$ as a cathode material.
DSC analysis shows that the combustion reaction of electrolyte(heat dissipation property) is related to the catalytic activity of surface of transition metal oxide(cathode material). The capacity of $LiMn_2O_4$-coated $LiCoO_2$ was 150 mAh/g and its rate capability was nearly maintained. When the surface of $LiCoO_2$ was coated with $LiMn_2O_4$, the combustion reaction of electrolyte was suppressed distinctively. The reason for the improved heat dissipation property is that the dense and thick passivation film on the surface of $LiMn_2O_4$ as a coating material restrains the electrolyte combustion reaction, which result from that the passivation film preserves the electrolyte from the catalytic surface of transition metal oxide.
