Asymmetric rate behavior of Si anodes for lithium-ion batteries: Ultrafast de-lithiation versus sluggish lithiation at high current densities

The combined effect of lithium-ion diffusion, potential-concentration gradient, and stress plays a critical role in the rate capability and cycle life of Si-based anodes of lithium-ion batteries. In this work, Si nanofilm anodes are shown to exhibit an asymmetric rate performance: around 72% of the total available capacity can be delivered during de-lithiation under a high current density of 420 A g^-1 (100C where C is the charge-rate) in 22 s; in striking contrast, only 1% capacity can be delivered during lithiation. A mathematical model of single-ion diffusion is established to elucidate the asymmetric rate performance, which can be mainly attributed to the potential-concentration profile associated with the active material and the ohmic voltage shift under high currents; the difference in chemical diffusion coefficients during lithiation and de-lithiation also plays a role. This clarifies that the charge and discharge rates of lithium-ion-battery electrodes should be evaluated separately due to the asymmetric effect in the electrochemical system.