Deconvolving double-layer, pseudocapacitance, and battery-like charge-storage mechanisms in nanoscale LiMn2O4 at 3D carbon architectures

Abstract

Lithium manganese spinel, LiMn2O4 (LMO), is a well-established active material for batteries, in which charge is stored via a two-stage insertion of Li+ that manifests with coupled current peaks or voltage plateaus under voltammetric or galvanostatic conditions, respectively. The charge–discharge kinetics for LMO are significantly enhanced when the oxide is expressed in a nanoscale form, for example as an ultrathin coating of nanocrystallites at the surfaces of a 3D carbon substrate, such as fiber paper–supported carbon nanofoams (CNFs). In addition to expressing well-defined “battery-like” Li+-insertion peaks between ∼0.6 and 1 V vs. Ag/AgCl when cycled in aqueous lithium sulfate, LMO@CNF electrodes exhibit a pseudocapacitance envelop at more negative potentials (∼0.2–0.6 V). The pseudocapacitive character of LMO@CNF is further verified when cycled in aqueous sodium sulfate, where a broad capacitance envelop dominates the voltammetric response over the entire potential range at a current density an order of magnitude greater than expected for double-layer capacitance. Thus, the LMO@CNF electrode provides a prime example where multiple distinct charge-storage mechanisms are expressed in a single material. Herein, we apply voltammetry and impedance-based methods to deconvolve the respective contributions of double-layer capacitance, surface-based pseudocapacitance, and battery-like Li+ insertion. The use of power-law analysis provides a general assignment of transport dynamics, while projecting impedance data as 3D Bode-style representations provides key mechanistic information regarding the time/frequency–potential response of LMO@CNF electrodes.