Solid‐State NMR Study on the Structure and Dynamics of Graphite Electrodes in Sodium‐Ion Batteries with Solvent Co‐Intercalation

Abstract

The possibility to co-intercalate sodium ions together with various glymes in graphite enables its use as a negative electrode material in sodium-ion batteries (SIBs). However, the storage mechanism and local interactions appearing during this reaction still needs further clarification. 1 H, 13 C and 23 Na ex situ solid-state NMR (ss-NMR) experiments are performed to obtain insights into the storage mechanism depending on the state of charge (SOC) and the electrolyte solvent used. Distinct differences could be seen depending on the SOC, indicating a possible change of the solvation shell, differences in the mobility as well as a phase transition at the voltage plateau. Furthermore, exchange experiments reveal information on the sodium ion transport process in the graphitic lattice. The inferior cycling performance of triglyme (3G) (compared to diglyme (2G) and pentaglyme (5G)) is also reflected in the ss-NMR spectra, showing a reduced mobility and stronger interactions between sodium ions, 3G and graphite already at room temperature (RT).

Diglyme as a Promoter for the Electrochemical Formation of Quaternary Graphite Intercalation Compounds Containing two Different Types of Solvents

“…The signal at 123-126 ppm is assigned to the graphitic carbon adjacent to a solvated sodium ion layer in agreement with previous studies. [42,51] The decrease in line width for the THF/2G electrolyte compared to pure 2G indicates a loss in mobility for the THF/2G electrolyte, which corroborates its lower rate capability (see Figure S4). This is pushed to the extreme for pure THF, where only a very broad peak is visible in the same graphitic carbon region.…”

“…Several NMR studies have indicated that at full sodiation, there are both free and bound solvents inside the graphite galleries, and a recent paper by Escher et al indicate that there are 2-3 2G molecules per Na + in the graphite at full sodiation. [18,42] This would indicate that roughly 1.2-1.8 μl of 2G is needed per mg of graphite. Thus, an electrode with a radius of 12 mm and a loading of 9 mg/cm 2 requires in the range of 12-18 μl of 2G at full sodiation.…”

“…No indication of a separate 23 Na signal for co-intercalated sodium ions for pure THF and 2G is apparent, which is different from previous co-intercalation studies. [42,[51][52] Note that the weak intensity of the spectrum for the THF electrolyte sample is related to the high vapor pressure of THF.…”

Protective NaSICON Interlayer between a Sodium–Tin Alloy Anode and Sulfide-Based Solid Electrolytes for All-Solid-State Sodium Batteries

“…12 Recently, studies have shown a method of cointercalation with solvent molecules, which, however, in a solid electrolyte battery is not applicable. 13,14 For sodium ASSB studies, sodium metal is used most commonly as the anode material. 11,15 The use of sodium as anode is most attractive for maximizing the energy density of the cell, but its application is difficult for the same reasons that are known from lithium.…”

“…The Na + ion ( r (Na + ) = 102 pm) has a larger ion radius than Li + ( r (Li + ) = 76 pm), and its intercalation into the graphite layered structure has been perceived thermodynamically impossible . Recently, studies have shown a method of cointercalation with solvent molecules, which, however, in a solid electrolyte battery is not applicable. , For sodium ASSB studies, sodium metal is used most commonly as the anode material. , The use of sodium as anode is most attractive for maximizing the energy density of the cell, but its application is difficult for the same reasons that are known from lithium. In particular, side reactions with the solid electrolyte and the formation of dendrites during charging remain key challenges.…”