Revealing the Kinetic Limits of Sodiation and Lithiation at Hard Carbon Using the Diluted Electrode Method

Abstract

Electrochemical sodium- and lithium-insertion into hard carbon (HC) relies on two main reactions: adsorption/intercalation and pore-filling. The rates of these two reactions are key to attaining high power densities and fast charging in batteries, but distinguishing the rate-limitations can be challenging due to their overlap and issue with Na⁺ and Li⁺ transport in conventional composite electrodes. Herein, we focus on the usage of the diluted electrode method to better evaluate the kinetics of electrochemical sodiation and lithiation at HC. Through galvanostatic charge/discharge testing, cyclic voltammetry and potential step analysis performed on diluted HC-electrodes in aprotic Na cells, we confirm that the sodium-insertion rate into HC is faster than lithium-insertion when we consider both adsorption/intercalation and pore-filling reactions with apparent diffusion coefficients, D_app, on the order of 10⁻¹⁰ −10⁻¹¹ and 10⁻¹⁰ −10⁻¹² cm² s⁻¹ for sodium- and lithium-insertion of HC, respectively. Additionaly, the sodiation into diluted HC-electrode showed comparable rate-capability and D_app of adsorption/intercalation to lithium intercalation into diluted graphite-electrodes. We further evaluated temperature dependence using potential-step and electrochemical impedance methods, finding that activation energies, E_a, were ~55 and ~65 kJ mol⁻¹ for sodiation and lithiation, respectively. We find reactions in solid-state, i.e., nucleation of pseudo-metallic cluster can limit the rate-performance in diluted HC-electrodes as well as the charge-transfer at the electrolyte/HC interface, at the lower potential region.