Dissolution kinetics of spinel lithium manganate and its relation to capacity fading in lithium ion batteries
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The dissolution behavior and kinetics of spinel lithium manganate LiMn2O4 with different particle sizes have been investigated in this study. The dissolution of manganese cations from LiMn2O4 is confirmed to occur when LiMn2O4 particles are immersed in the electrolytes. The amount of dissolved manganese ions markedly increases with a rise in temperature and a decrease in particle size, which implies that the capacity fading of LiMn2O4 at elevated temperatures is associated with manganese dissolution. On the basis of the isothermal analysis of reaction kinetics, the rate of manganese dissolution from LiMn2O4 is dominated by the rate of dissolution reaction. Smaller particles exhibit a larger reaction rate constant and higher activation energy of the dissolution process than the larger ones. Therefore, an increase in temperature has a more pronounced effect on the dissolution reaction of small particles than on that of large particles.
I. INTRODUCTION
In the last decade, the spinel lithium manganate (LiMn2O4) has been intensively investigated as one of the potential cathode materials for lithium ion secondary batteries.1– 4 In view of the low material cost and the acceptable environmental characteristics, LiMn2O4 is more applicable to commercial manufacturing than LiCoO2 and LiNiO2. On the other hand, LiMn2O4 suffers significant capacity fading during cycling at elevated temperatures, which becomes a major drawback in the practical utilization.5–8 Since Tarascon et al. reported in 1993 that LiMn2O4 possesses inferior cyclic stability at 55 °C compared to that at room temperature,9 several research groups have focused on investigating the electrochemical characteristics of LiMn2O4 at high temperatures.10–13 Different types of mechanisms such as the slow dissolution of cathode materials into the electrolytes, the coexistence of an unstable two-phase structure in the high-voltage region, decomposition of electrolyte solution, protonic ion-exchange reaction, and an increase in polarization voltage have been proposed to explain the fading behavior of LiMn2O4.3,6,10,12 Although the proposed mechanisms are not consistent, several research groups have confirmed the dissolution of manganese cations into the electrolytes and concluded that the dissolution of
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J. Mater. Res., Vol. 17, No. 6, Jun 2002 Downloaded: 12 Feb 2015
manganese cations from LiMn2O4 is a major problem for the application of LiMn2O4 to lithium batteries at high temperatures. The dissolution of manganese cations is considered to result in the formation of lattice defects in the spinel phase and lead to a disordered structure, thereby blocking the lithium insertion–extraction paths and leading to capacity fading.14 To overcome this cumbersome problem of manganese dissolution, use of various electrolytes, surface modification of powders, and doping of suitable cations in the original spinel phase have been proposed.11,13,15 It was also reported that the fading of LiMn2O4 at
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