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with open framework have shown several advantages as Na-ion host materials: (1)
high discharge voltage plateau; (2) excellent cycling stability due to their stable
frameworks during Na intercalation/deintercalation; (3) high reversible capacity
([120 mAh g -1 ); (4) abundant resources and low cost. So, Hexacyano-type
compounds should be promising candidates as cathode hosts for low-cost and
long-term Na-ion batteries.
4 Anode Materials for Na-Ion Batteries
Appropriate anode materials should meet the following requirements:
1. Na-ion intercalation/deintercalation potentials are close to Na/Na + , and less
influenced by the concentration of Na ions in the lattices, so as to guarantee
high output voltage in the full cells.
2. The reversible Na-ion intercalation capacity and the discharge/charge effi-
ciency are as high as possible to provide high capacity density.
3. Low volume change during Na-ion intercalation/deintercalation, good com-
patibility with electrolyte to obtain good cycling stability.
4. High chemical as well as thermal stability to ensure intrinsical safety.
5. High electron conductivity and Na-ion diffusion rate to offer high C rate at
charge/discharge.
6. Low cost, abundant resources, environmental benignity, and easy to mass
produce.
The possible anode materials for Na-ion batteries mainly include carbonaceous
materials, alloy, and some metal oxides.
4.1 Carbonaceous Materials
Graphite and graphite-like materials were most often surveyed for alkali-ion
intercalation anodes. The Na intercalation behavior in graphite was first investi-
gated in the late 1970s [ 49 , 50 - 52 ]. However, the intercalation amount of Na in
graphite is low (~NaC 64 ) compared to that of LiC 6 and KC 8 . Other carbon material
with microcrystalline graphitic structure and microporous domains were found to
have high Na storage capacity, such as carbon black (121 mAh g -1 ), [ 53 ] petro-
leum coke, [ 51 ] and carbon fibers (209 mAh g -1 )[ 52 , 54 , 55 ].
A variety of precursors were used to obtain different structured hard carbon
materials. Dahn et al. found that the hard carbon obtained by the glucose pyrol-
yzation showed reversible Na intercalation capacity of ~300 mAh g -1 [ 56 ].
Microspherical hard carbon particles with disordered nanostructure (Fig. 21 a)
synthesized by pyrolyzing resorcinol-formaldehyde compounds, [ 57 ] delivered an
initial capacity of 285 mAh g -1 , and over 255 mAh g -1
after seven cycles in
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