Probing the Energy Storage Mechanism of Quasi‐Metallic Na in Hard Carbon for Sodium‐Ion Batteries

Abstract

Nevertheless, the research on SIBs was barely conducted after the successful commercialization of LIBs in 1990s, and this situation continued until the end of the 20th century. An obstacle toward the development of SIBs is the lack of suitable anode materials with acceptable performance. The early work conducted by Dahn et al. [5] suggested that hard carbon (HC) has a reversible capacity of 300 mAh g −1 for sodium, approaching the lithium storage capacity in graphite. Extensive attention has been focused on the development of SIBs recently, with a variety of materials being considered as potential anodes for SIBs, which includes alloys, [6-8] organic materials, [9-11] and carbonaceous materials. [12-14] Because of high sodium-ion storage capacity, appropriate working potential, excellent cycling stability, and natural abundance, HC represents the most promising anode for SIBs. Nowadays, increasing interests have been concentrated on revealing sodium intercalation process in HC, [15-19] but the steady state of sodium stored in HC still remains unexplored, which leads us to investigate the steady state of sodium ions in HC from thermodynamic and kinetic aspects. Heretofore, the steady state of sodium in HC has been incidentally proposed, but remains a controversial issue. Specifically, Stevens and Dahn [20] originally revealed the metallic nature of sodium absorbed in nanopores at the voltage plateau as the adsorption potential approaching the deposition potential of sodium metal. Meanwhile, the formation of metallic sodium was also confirmed with operando 23 Na solid-state nuclear magnetic resonance (NMR) and in situ Raman at the plateau region. [21,22] Moreover, Ji and co-workers [23] suggested that sodium adsorbed onto the pore surface is atomic even close to the cutoff potential (0.05-0 V). On the other hand, Liu et al. [19] demonstrated that neither metallic nor quasi-metallic sodium is presented in the whole discharge region over 0 V, based on the results of ex situ 23 Na NMR and electron paramagnetic resonance (EPR). Recently, Guo et al. [13] claimed that sodium stored in the HC is in the ionic state above 0.1 V, whereas metallic sodium clusters form in the nanopores at the plateau voltage, with the methods of EPR, XRD, and Raman. Apparently, scattered efforts have been devoted to uncover the state of sodium stored in hard carbon recently, nevertheless, systematic investigations are Hard carbon (HC) is the most promising anode material for sodium-ion batteries (SIBs), nevertheless, the understanding of sodium storage mechanism in HC is very limited. As an important aspect of storage mechanism, the steady state of sodium stored in HC has not been revealed clearly to date. Herein, the formation mechanism of quasi-metallic sodium and the quasi-ionic bond between sodium and carbon within the electrochemical reaction on the basis of theoretical calculations are disclosed. The presence of quasi-metallic sodium is further confirmed with the assistance of a specific reaction between the sodiated HC electrode and eth...

Sodiation Of Hard Carbon: How Separating Enthalpy And Entropy Contributions Can Find Transitions Hidden In The Voltage Profile

“…Both groups attributed this shift to charge transfer from the metal atoms to the carbon matrix [13,38] . DFT calculations combined with Bader charge transfer analysis also suggest changes in the charge transfer from sodium to carbon dependent on the amount of inserted sodium [39–41] …”

Enabling Fast Na+ Transfer Kinetics in the Whole‐Voltage‐Region of Hard‐Carbon Anodes for Ultrahigh‐Rate Sodium Storage

“…[13,67] The appearance of metalloid sodium peak in ex situ XPS also supports this observation (Figure 4e). In the low-voltage region, the quasimetalloid sodium peak of C−Na appears and shifts to the characteristic peak of Na metal with the intercalation of sodium into the interlayer, [74] and as the discharge reaches 0.10 V, sodium initiates to fill the nanopores, resulting in the significant upsurge of the quasi-metallic sodium peak but the position does not change. It is worth mentioning that until the voltage is close to 0 V, there is still no sodium metal peak, which indicates that no sodium metal deposition has eventually occurred.…”

Hard Carbon Anodes for Next‐Generation Li‐Ion Batteries: Review and Perspective

“…[ 12,31,38–41 ] Beyond LIBs, hard carbons have also been demonstrated to be potential anodes in other alkali metal batteries such as sodium/potassium ion batteries. [ 42–55 ] The latest research progress of carbonaceous anode materials for sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs) has been reviewed many times. [ 34,36–41 ] However, not all issues in LIBs have been covered in these literatures.…”