Understanding Origin of Voltage Hysteresis in Conversion Reaction for Na Rechargeable Batteries: The Case of Cobalt Oxides

Abstract

Conversion reaction electrodes offer a high specific capacity in rechargeable batteries by utilizing wider valence states of transition metals than conventional intercalation‐based electrodes and have thus been intensively studied in recent years as potential electrode materials for high‐energy‐density rechargeable batteries. However, several issues related to conversion reactions remain poorly understood, including the polarization or hysteresis during charge/discharge processes. Herein, Co3O4 in Na cells is taken as an example to understand the aforementioned properties. The large hysteresis in charge/discharge profiles is revealed to be due to different electrochemical reaction paths associated with respective charge and discharge processes, which is attributed to the mobility gap among inter‐diffusing species in a metal oxide compound during de/sodiation. Furthermore, a Co3O4–graphene nanoplatelet hybrid material is demonstrated to be a promising anode for Na rechargeable batteries, delivering a capacity of 756 mAh g−1 with a good reversibility and an energy density of 96 Wh kg−1 (based on the total electrode weight) when combined with a recently reported Na4Fe3(PO4)2(P2O7) cathode.

Recent advances in graphene based materials as anode materials in sodium-ion batteries

“…Sun's group combined graphene with mesoporous Co 3 O 4 particles and the hybrid anode ensured a good capacity (523.5 mAh g −1 ) and good cycle stability at 25 mA g ( Fig. 8 c) [190] . Their Co 3 O 4 -graphene nanoplatelet hybrid anode delivered a high capacity of 756 mAh g −1 with good reversibility ( Fig.…”

The State and Challenges of Anode Materials Based on Conversion Reactions for Sodium Storage

“…The mismatch in ionic mobility between the two species and the lack of a thermodynamic driving force to redistribute metal ions possibly lead to asymmetrical reaction pathways (different equilibrium potentials), which are the source of large voltage hysteresis . As shown in Figure c, Kim et al found that, after initial cycling, Co 3 O 4 nanocrystals were directly converted into a Co and Na 2 O composite on discharging, while the composite changed back into Co 3 O 4 nanocrystals via an intermediate phase (CoO 1− x ).…”

“…c) Schematic illustration of conversion reaction pathways for Co 3 O 4 in Na rechargeable batteries. Reproduced with permission . Copyright 2016, Wiley‐VCH.…”

Chemical Structures of Specific Sodium Ion Battery Components Determined by Operando Pair Distribution Function and X‐ray Diffraction Computed Tomography

“…The thermodynamic sodiation route, from P nanoclusters to NaP chains to layered Na 3 P, involves the breaking and forming of many bonds and should be significantly slower than deintercalation. A similar scheme with kinetic and thermodynamic routes for charge and discharge, respectively, has been described for the Na/Co 3 O 4 system …”