Why is sodium-intercalated graphite unstable?

Abstract

Na-ion batteries offer an attractive low-cost alternative to Li-ion batteries. Although graphite is used as the negative electrode in conventional Li-ion batteries, attempts to use it in Na-ion batteries have been hampered by the inability of Na to form graphite intercalation compounds (GICs) under moderate conditions. It is generally considered that this is due to the size mismatch between Na and the graphite interlayer spacings, but here we show with detailed first-principles calculations of Li, Na, K, Rb, and Cs GICs that the major reason is the change in chemical bonding between alkali metal (AM) ion and C atoms. van der Waals correction terms are introduced to better reproduce the layered graphite structure, and calculated formation energies of GICs AMC 6 , where AM ¼ Li, Na, K, Rb, and Cs, are found to become less negative (less stable) as ion size decreases from Cs to Na as a result of weakening ionic bonding, until the formation energy of NaC 6 becomes positive. The much smaller Li ion represents an exception to this trend, as its bonds with C atoms contain a covalent component, resulting in a negative formation energy. These subtle differences in staging for the different alkali metals explain why graphite is a good intercalation material for Li and K but not for Na.

Charged Carbon Nanomaterials: Redox Chemistries of Fullerenes, Carbon Nanotubes, and Graphenes

“…202 All stable alkali metals have been shown to be capable of forming GIC/nanotubide intercalants, 167,203 although graphite intercalation of sodium typically leads to low stoichiometries and high-stage GICs attributed variously to the small size of the metal cations preventing intergraphitic distances reaching energy minima, 204 the less reducing nature versus lithium, and the weakness of the subsequent ionic bond between the graphenide sheets and sodium cations. 205,206 Pure stage-1 sodium GICs can be synthesized from decomposition of the so-called "superdense" GIC 204 NaC 2 (formed at 40 kPa), and ternary stage-1 sodium GICs may be formed through use of a cointercalating solvent, 207 heavy group 1 metal, 208 or trace oxygen. 209 The group 2 alkali earth metals calcium, strontium, and barium readily intercalate graphite, 210 while magnesium is more challenging (analogously to sodium) but readily forms ternary intercalation compounds alongside a group 1 metal.…”

Understanding Fundamentals and Reaction Mechanisms of Electrode Materials for Na‐Ion Batteries

“…Although Li and Na have similar chemical properties, it is difficult for Na + to intercalate into the layered graphene structure of graphite, which is unlike Li + intercalation in graphite. This is possibly due to thermodynamic restrictions . As a result, graphite is reported to have a very low capacity of <20 mA h g −1 for Na storage with conventional electrolytes .…”

“…This is possibly due to thermodynamic restrictions. [98] As a result, graphite is reported to have a very low capacity of <20 mA h g −1 for Na storage with conventional electrolytes. [4] This limitation can be circumvented by using cointercalation phenomena in an etherbased electrolyte.…”

Carbon Anodes for Nonaqueous Alkali Metal‐Ion Batteries and Their Thermal Safety Aspects

“…Quantum mechanical calculations by Liu et al revealed that this results from competing energetic trends in ionization energy and ion-substrate coupling, causing unfavorable formation energy. [59] This also explains the reason for the lower voltages obtained in SIBs and magnesium-ion batteries as compared to the other alkali and alkaline earth metals. [59] This also explains the reason for the lower voltages obtained in SIBs and magnesium-ion batteries as compared to the other alkali and alkaline earth metals.…”

“…[58] These energetic changes modify the chemical bonding nature between the alkali metal and carbon, wherein weakening ionic bonding is observed from cesium to sodium due to the decrease in ionic size, culminating with lithiumcarbon bonds containing a covalent component and a negative formation energy. [59] This also explains the reason for the lower voltages obtained in SIBs and magnesium-ion batteries as compared to the other alkali and alkaline earth metals. [58] This www.advancedsciencenews.com inability of sodium to intercalate has often been attributed to the insufficient interlayer spacing for graphite, [60] as reversible intercalation of sodium in expanded RGO has been demonstrated.…”