Investigations On Discharge Mechanism And Cyclic Performance For Li/MnO₂ Batteries

Lithium-manganese dioxide primary batteries (Li/MnO2) are widely used around the world due to its high power density, stable working voltage, superior safety and nontoxicity. As the cathode material of Li/MnO2 batteries, manganese dioxide is still highly appreciated by many researchers worldwide for applications in energy storage because of its high specific capacity (308 mAh/g), abundance, low toxicity and low cost. As a kind of semiconductor materials, manganese dioxide shows low conductivity also the instability during discharging and charging. Therefore, its utilization has been restricted in the practical Li/MnO2 batteries, which causes a waste of manganese dioxide in the electrode and energy. In addition, as the active material in the Li/MnO2 batteries, it has been reported that MnO2 has poor rechargeability. Thus, manganese dioxide has not been used in lithium secondary batteries directly, resulting in huge waste of raw materials and energy. Therefore, in the present work, the electrochemical kinetic behaviors and the discharge mechanism of MnO2 during discharging in Li/MnO2 batteries was investigated by electrochemical impedance spectroscopy (EIS) under a nonequilibrium state. Furthermore, the cyclic performance and the crystal structure evolution of manganese dioxide cathodes during cycles were also studied in this work.Firstlly, the kinetic behaviors of heat-treated electrolytic manganese dioxide (HEMD) was investigated using EIS at a discharge current of 0.1 mA/cm2. The kinetic parameters were obtained under real-world nonequilibrium conditions, indicating that the charge-transfer step and the diffusion of lithium ions proceeded together with the controlled rate of lithium insertion, except in the reduction range of 0.5≤x≤0.55, in which the charge-transfer step is the limiting step for lithium ion insertion during the discharge of HEMD. In addition, a second semicircle in the intermediate-frequency region was detected in the EIS experiment, which demonstrates that a closed phase interface has been formed in the two-phase reaction region. Additionally, our observations show that the phase boundary has blocking effects on the diffusion of lithium ions. Based on the analysis of the structural evolution and the variation in the dynamic behaviors, we presented a detailed core/shell-model mechanism to explain the discharge mechanism of HEMD.In addition, a number of significant issues were discovered when studying the cyclic performance of heat-treated electrolytic manganese dioxide (HEMD) in lithium secondary batteries. The investigations verified that HEMD has poor reversible behavior due to the significant irreversible phase conversion that occurs after the first discharging/charging process. Essentially, all detailed work was focused on this new phase to investigate its electrochemical behavior and phase evolution. Notably, the resultant new phase (spinel-related phase) revealed faultless cycling performance and stability. The stable spinel-related phase maintained the discharge capacity of 120 mAh/g for over 100 cycles without capacity loss between potential limits of 2.4 and 3.6 V at a current density of 0.1 mA/cm2. In addition, the present studies demonstrated that the spinel-related phase is a promising cathode material for lithium secondary batteries, where the structure collapse during cycles is the primary reason for fading capacity and loss of cycling ability.