Due to higher gravimetric and volumetric energy density than other battery technologies, Lithium-ion batteries (LIBs) have assumed an important role as portable energy systems to power electronic devices such as mobiles and laptops. However, implementation of LIBs to power automotive vehicles are limited due to their high cost, low cycle life and capacity loss at high discharge rates. For successful commercialization of electric- (EVs) and hybrid-electric (HEVs) vehicles, battery technology in terms of novel electrode materials, electrolytes and fabrication techniques needs to be further developed so that it is able to deliver better discharge capacity with improved capacity retention and cycle life. LiMn₂O₄ (LMO) has been used as a promising cathode material in battery systems to power EVs and HEVs due to its abundance, low cost, environmentally benign nature, minimum safety and thermal runaway issues^1-3. Along with these exceptional advantages, LMO suffers from poor rate capability and cycling stability due to the imperative manganese (Mn²⁺) dissolution through disproportionation reaction (2Mn³⁺=Mn²⁺+Mn⁴⁺), structural changes and electrolyte decomposition which act as hindrance to high power applications^4-5. Research has been directed towards elucidating novel avenues to improve the cycling performance of LMO cathode for better capacity retention through investigating its electrochemical performance experimentally as well numerically^3-7. However, a comprehensive numerical analysis that imitates experimental measurements towards an improved understanding of LMO degradation is missing in the literature. Consequently, galvanostatic charge/discharge, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and potentiostatic intermittent titration technique (PITT) has been employed in this work to quantify the performance of LMO half-cell (against Li foil anode). This experimental data has been used as an input to the numerical model to explain the phenomenon of degradation caused by manganese dissolution. Figure 1 (a) shows the experimentally and numerically obtained capacity derived from cycling two 2016 LMO coin cells at 1C rate for the first 100 cycles. The experimental and simulated results are in good agreement which indicates that capacity decreases gradually as cycle number increases. This loss in capacity, which we regard as having been caused by Mn²⁺ dissolution in electrolyte, induces a reduction in volume fraction of solid matrix of LMO cathode. Figure 1 (b) shows the variation of cell potential with discharge capacity for the 1^st and 100^th cycles, which also indicates 16.5% loss in discharge capacity due to reduction in volume fraction of solid matrix. Through this study, the rate of dissolution and loss in cyclic lithium with cycling can be quantified, and could be applied to develop novel methods of synthesis of LMO for better cycling performance and capacity retention.

References

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