Li-rich oxides, Li(Li_xMe_1-x)O₂ (Me = Co, Ni, Mn), are promising positive electrode active materials for lithium-ion cells because of their ability to deliver capacities over 250 mAhg^-1.[1, 2]  However, the mechanisms responsible for their unique voltage profiles, such as the activation voltage plateau in the 1^st delithiation cycle and voltage hysteresis during cycling, are not fully understood. Electrochemical calorimetry is a powerful tool to observe reversible / irreversible reactions during charge and discharge of the electrode. In addition, the observed reversible heat flow can be described in terms of the change in configurational entropy of lithium ions (DS) in the structure, the heat of polarization, and side reactions. DS is also obtained using the temperature dependence of the open circuit voltage (dE/dT). Here, we compare the heat flow during charge and discharge, open circuit voltage (OCV), and dE/dT to evaluate reversible and irreversible reaction factors of the Li-rich oxides.

Experimental

All data reported here are obtained on the cathode material denoted as Li(Li_0.2Ni_0.15Mn_0.55Co_0.1)O₂= (0.5Li₂MnO₃ - 0.5LiNi_0.375Co_0.25Mn_0.375O₂). Coin-type half cells containing c.a. 10 mg cathode material with counter lithium metal were prepared, and the heat flow during charge and discharge was observed using an isothermal microcalorimeter. We also measured OCV and dE/dT at every 2.5 % state of charge (SOC), and correlated the values to the obtained heat flow. In the case of OCV assumption, the relaxation time of OCV (t) was also determined.

Results and Discussion

As is well known, the lithium-rich oxide material shows the wide voltage plateau only during the 1^st cycle.[3] We observed a large gap between the 1^st cycle charge voltage and OCV (Figure 1(a)) and also large exothermic heat in the corresponding region (capacity > 100 mAhg^-1) as shown in Figure 1(b). dE/dT values are negative (which means endothermic reaction during charge as shown in Figure 1(b); £). The estimated total heat flow (red dots), taking into consideration the heat of polarization from OCV (Figure 1(b); œ) is obviously lower than the observed heat flow (blue line). This observation suggests large irreversible exothermic heat flow for oxide capacities >100 mAhg^-1. In addition, the relaxation time constant (Figure 1(c); °) also drastically increases in the corresponding region, which suggests that the irreversible reactions are slower than the reversible reactions.

The heat flow during the 2^nd charge cycle is distinctly different from that of the 1^st cycle. However, the gap between observed heat flow (blue line in Figure 1 (e)) and the estimated one from dE/dT and heat of polarization (Figure 1 (e); œ) remained, which indicates the persistence of irreversible reactions. These irreversible heat flows may be associated with the energy required to induce structural changes within the oxide particles. In our presentation we will discuss the origins of the oxide voltage hysteresis using the heat flow, dE/dT, OCV, and the OCV relaxation times (t) obtained from the charge and discharge cycles.