Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 407

Fig. 23 Theoretical energy
cost for Na (red curve) and Li
(blue curve) ions insertion
into carbon as a function of
carbon interlayer distance
[ 58 ]

to the different chemical and electrochemical reaction of carbonate solution on the
surface of the hard carbon material [ 59 ].
It was found that the charge/discharge profiles of the hard carbon anodes
commonly comprise two parts: a slope at the high potential region and a plateau at
the low potential region, as shown in Fig. 21 b[ 56 , 59 ]. Several spectroscopy
technologies have been used to investigate the specific mechanism, such as ex-situ
or in-situ X-ray diffraction, small-angle X-ray scattering (SAXS), [ 55 , 60 , 61 ] 23 Na
magic angle spinning nuclear magnetic resonance (MAS NMR) spectra, [ 57 ] and
Raman spectroscopy [ 59 ]. By in-situ SAXS investigation, Dahn et al. first dem-
onstrated that the slope at high potential region corresponded to Na intercalation/
deintercalation between the graphene layers and the plateau at the low potential
region corresponded to Na adsorption/desorption in the nanopores of the carbon
particles [ 60 ]. 23 Na MAS NMR spectra corresponding to the hard carbon micro-
spheres also clearly showed that two signal responses during charge and discharge,
which could be ascribed to Na insertion between misaligned graphene carbon
framework and in the nanocavities or on a surface solid film, respectively [ 57 ]. In
Raman spectroscopy, the shift of the G-band indicates the change of the state of
negatively charged graphenes, the redshift observed during the voltage-sloping
region corresponds to the Na insertion between the graphene layers. No shift of G-
band in lower potential region can be ascribed to the formation of a nano-sized
cluster of quasimetallic Na in the nanopores of the hard carbon [ 59 ].
It is obvious that the interplanar distances of the graphene layers in hard carbon
will play a vital role on the Na intercalation behavior. Cao et al. carried out a
theoretical simulation on the energy cost for the Na-ion insertion into carbon as a
function of the carbon interlayer distance (Fig. 23 )[ 58 ]. For comparison, the
energy cost curve for Li-ion insertion into carbon was also calculated (Fig. 23 ).
For graphite with an interlayer spacing of 0.335 nm, the energy cost for Li-ion and
Na-ion insertion are 0.03 and 0.12 eV, respectively. In consideration that the
energy of the thermal fluctuations at room temperature is 0.00257 eV, Li-ion
intercalation into graphite layers

is permissible while Na-ion is prohibitive.