Elettrochemical-thermal P2D aging model of a LiCoO2/graphite cell: capacity fade simulations

We simulate a simple model of aging in a lithium-ion cell that accounts for growth of a passivation film on the negative electrode, on top of an electrochemical-thermal pseudo-two-dimensional (P2D) description of “ideal” (i.e., ignoring aging effects) cell dynamics, using a finite-volume Matlab code. Capacity fade is assessed by evaluating at each instant in time the loss of cyclable lithium ions, which follows as a consequence of a single irreversible electrolyte decomposition reaction in the aging model. Assuming reasonable values for the side reaction rate and SEI film conductivity, we show an estimated 2.5% capacity loss after 1600 cycles at 1C. We also find a growth rate of about 0.64 nm/hr at 1C for the SEI film on the negative electrode, in a linear growth regime obtained by neglecting any diffusive transport limitations during new SEI formation. In addition, we show that for an “ideal” cell under cyclic loading conditions its energy dissipation (per cycle) is constant as a function of cycle number, while that same quantity keeps increasing with cycle number for an aging cell. Finally, we discuss the influence of the aging model as it affects surface cell temperature as a function of a (convective) heat transfer coefficient. Specifically, for values of h larger than about 1 W/(m2 K) we find essentially no effect of the aging model on surface cell temperature, which remains equal for all time to the surrounding ambient temperature. For smaller values of h, however, we find surface cell temperatures that are systematically larger than their “ideal” cell counterparts, with a strictly monotonic (increasing) behavior of the steady-state surface cell temperature as a function of the (reciprocal) heat transfer coefficient.