Super-Expanded Graphite Anodes Achieved By Employing Metallic Pillars for High Capacity Sodium-Ion Batteries
Authors: Kim You Jin , Ryu Won-Hee 
Published: December 2020

Na-ion battery is a promising energy storage systems for future large scale applications to replace the existing Li-ion batteries. Because of the similar structural configuration and electrochemical mechanisms to Li-ion batteries, Na-ion batteries could be potentially realized to the current relevant industries. Utilizing Na resources instead of Li can further alleviate problems regarding (i) localized distribution of lithium resources and (ii) expensive commodity price for Li-ion batteries.[1, 2, 4] However, larger radius of the sodium ion (0.98 Å) than lithium ion (0.69 Å) results in the low electrochemical performance (e.g. capacity, cycle life, and rate capability) for the electrode materials in Na-ion cell.[1] Previous researches to ensure the affordable sites in anode materials have been studied for Na-ion batteries, thereby enabling reversible electrochemical reaction in Na accommodation layer.
Graphite is an typical anode material with a theoretical capacity of 350 mAh/g for commercialized lithium ion batteries.[3] However, the graphite materials often suffer from poor capacity issue in Na-ion cells due to low stability and unfavorable intercalation of large Na ion into the graphite layers.[4, 5] In this regard, non-graphitic materials (i.e. hard carbon) exhibiting high capacity around 300 mAh/g have been suggested as an alternative instead of the graphite.[5] However, their expensive material cost and low structural stability resulted from low crystallinity become critical reason to seek better carbon alternatives for Na-ion cells.
Expanding the interlayer of graphite has been emerged as a favorable solution to develop carbon based anode material for Na-ion batteries. Extended interlayer space and stable long range order of the expanded graphite anodes enable the excellent electrochemical performance with Na ions.[6] Although there are many advantages of the expanded graphite anodes, parasitic reactions easily occurs between Na ions and a lot of anion pillars (e.g., epoxide and hydroxyl) in the carbon layers easily deteriorate the cell performances. High performance of expanded graphite anodes should be accomplished by developing the strategy to apply proper pillar components between the expanded layers.
In this work, we present a simple and powerful strategy to fabricate the expanded graphite anode materials for Na-ion batteries by introducing trivalent (Al3+) and hexavalent (W6+) pillar species between graphene layers.[7] Negatively charged graphene oxide (GO) layers can be self-assembled owing to the electrostatic attractive interaction with positively charged cations in the dispersing solution of GO sheets. As a result, we obviously find out the crystalline planes in regard to super-expanded layers (~7 Å) in contrast with the pristine distance for graphite without any pillars. We examined its morphology and structures of the expanded graphite anode materials and successfully achieved improved electrochemical properties of Na-ion batteries employing the expanded graphite. We will also explain concentration effects of the metal pillars on Na intercalation kinetics in the expanded graphitic anodes for Na-ion batteries.

KEYWORDS

Na-ion battery, Anode, Expanded graphite, Metallic pillar

References

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[7] Seongji Pyo, Wonsik Eom, You Jin Kim, Sang Hoon Lee, Tae Hee Han, Won-Hee Ryu (in press). Super-Expansion of Assembled Reduced Graphene Oxide Interlayers by Segregation of Al Nanoparticle Pillars for High-Capacity Na-Ion Battery Anodes., ACS Appl. Mater. Interfaces., doi: 1021/acsami.0c00659