Thermodynamic Analysis of LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) Voltage Hysteresis Induced through High Voltage Charge

Abstract

High voltage charge of NMC materials can extend the initial capacity but also impair the long-term cyclability. This investigation probes NMC622 with charge limits of 4.3 and 4.7 V using operando isothermal microcalorimetry. A higher voltage charge to 4.7 V brings ∼17% increase in capacity yet a doubling of the heat dissipated compared to 4.3 V. Additionally, the thermodynamic paths of (dis)charge visualized by enthalpy potential show path-dependent (de)lithiation in the 3.0−4.7 V window. More evidence is provided explaining the long-term energy fading of NMC622 induced by high voltage, including increased thermal energy waste and asymmetric (de)lithiation thermodynamics.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC – progress and outlook

“…76 The enlarged voltage hysteresis physically represents an increased amount of electrical energy loss, and thermal dissipation could be a major contributing factor to this energy loss. 32 For the cell cycled in 3–4.7 V, an intense heat flow spike could be spotted at a high state-of-charge (>4.3 V) at every C-rate (Fig. S15b, ESI†).…”

“…In the d Q /d V plot, two distinct peaks at ∼3.75 V and ∼4.20 V are observed that are associated with the H1 → H2 and H2 → H3 transitions, respectively. The enthalpy potential upon the delithiation (charge) of NMC811 was determined with the IMC data based on eqn (2), 32 where V and I are loaded voltage and current of the cycling cell and q̇ is the instantaneous heat flow.The calculated enthalpy potential ( U H ) curves at different C-rates varying from C/10, C5, and C/2 were superimposed on each other (Fig. 13b), which indicates that the enthalpy potential is a thermodynamic parameter and independent of kinetics.…”

“…13,29,31 Moreover, the degradation of the NMC811 material due to high voltage charge to 4.7 V may also result in additional heat generation. In a previous isothermal microcalorimetric study on LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), 32 charging to high voltage resulted in irreversible energy conversion from chemical energy to heat. The increased thermal output not only wastes energy but is also a potential safety hazard.…”

“…In the dQ/dV plot, two distinct peaks at B3.75 V and B4.20 V are observed that are associated with the H1 -H2 and H2 -H3 transitions, respectively. The enthalpy potential upon the delithiation (charge) of NMC811 was determined with the IMC data based on eqn (2), 32 where V and I are loaded voltage and current of the cycling cell and : q is the instantaneous heat flow.…”

“…The path-dependent thermodynamic path, suggesting thermodynamically irreversible electrochemical processes, could be an origin of voltage hysteresis triggering energy conversion inefficiency. 32,76 Additionally, parasitic reactions, including the surface reconstruction revealed by the sXAS, at high voltages can be another critical contributing factor to the significantly increased heat dissipation. In summary, high-voltage and high SoC charge (delithiation) could cause irreversible energy loss through thermal dissipation due to thermodynamic factors.…”