Exploring the Kinetics and Thermodynamics of Voltage Fade

Layered-layered, lithium-, manganese-rich, nickel-manganese-cobalt (LMR-NMC) oxides are commonly used as cathode materials for lithium-ion batteries.  They can be thought of as composite materials consisting of (monoclinic) Li₂MnO₃ and (layered) Li(Ni_1-a-bMn_aCo_b)O₂ (a>0; b<1) and written as xLi₂MnO₃▪(1-x) Li(Ni_1-a-bMn_aCo_b)O₂.  With a desirable combination of low-cost and high energy density, they have attracted much attention for transportation applications. 

During cycling, these materials display voltage fade; i.e., the voltage vs. normalized capacity curve tends to droop.  Voltage fade reduces the energy density of the material during its lifetime and, consequently, a significant research effort is being devoted to prevent it altogether.  In an alternative approach, it may be possible to retard the voltage fade process to the point where it has only a minor impact on battery performance.  However, this approach requires detailed understanding of the kinetics and thermodynamic driving force of voltage fade, of which there is scant information in the literature.  In order to bridge this gap, we explore the effects of cycling temperature and oxygen availability during synthesis on voltage fade.

We cycled half-cells containing 0.5Li₂MnO₃•0.5LiNi_0.375Mn_0.375Co_0.25O₂ (or Li_1.50Ni_0.1875Co_0.125Mn_0.6875O_2.50) cathodes at 25, 35, 45 and 55°C at a 20 mA/g rate. Analysis of the change in open-circuit voltage values from t=0 for each temperature showed that these data can be fit to a kinetic rate law containing t^1/2 and t terms.  Indeed, the dependence of the coefficients of these terms with temperature suggests two competing chemical processes.

 We explored the response of LMR-NMC oxides using Li_1.50Ni_0.15Co_0.167Mn_0.683O_2.50 (HL) and Li_1.30Ni_0.15Co_0.167Mn_0.683O_2.30 (LL) to changes in oxygen partial pressure during synthesis.  The oxygen partial pressures were 1.0, 2.0 × 10^-1, 2.0 × 10^-2, and 2.0 × 10^-3 atm.  There were several partial-pressure-dependent trends in the data.  For example, examining the dependence of the deviation from nominal lithium stoichiometry on pO₂ shows that it decreases with increasing pO₂ for both the HL and LL material.  These and other trends will be discussed.

This work was performed under the auspices of the DOE Office of Vehicle Technologies, Hybrid and Electric Systems, under Contract No. DE-AC02-06CH11357. 

 The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.