Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 390

3.1 Metal Oxides
Li metal oxide materials (LiCoO 2 , LiNi x Co y Mn 1-x-y O 2 , LiMn 2 O 4 , etc.) have been
widely investigated as Li-ion intercalation hosts. These materials exhibit good
capacity and structural stability during repeated Li-ion insertion/extraction cycles.
Analogously, Na-based metal oxides also have similar formula and crystal struc-
ture with Li-based oxides. Indeed, these metal oxides have been first considered as
Na-ion hosts. Some typical crystal structures of Na-based metal oxides are illus-
trated in Fig. 3 . Three types of layered structures (Fig. 3 a-c) can be denoted as On
or Pn (n = 1, 2, 3,…), depending on the insertion environment of alkali atoms and
the repeat period (n) of the transition metal oxide (MO) sheets forming MO 6 edge-
sharing octahedra [ 3 ]. Na atoms lie in the octahedral sites in O3 structure (Fig. 3 a)
while occupy the trigonal prismatic sites in P2 and P3 structures (Fig. 3 b, c).
Therefore, phase transitions between O3 and P3 will occur due to the movement of
Na atoms from the octahedral sites to the trigonal prismatic sites along with the
gliding of MO sheets. The phase transitions cause the multi-voltage plateaus of the
layered Na x MO 2 material during charge/discharge. However, the phase transitions
between O3 and P2 cannot occur at room temperature because of the high cleavage
energy of the M-O bonds [ 2 ]. In general, O3-type metal oxides can extract more
Na atoms (hence discharge capacity) from the crystal structure; P2-type metal
oxides have higher structural stability during repeated charge/discharge. Some
other metal oxides have a three-dimensional tunnel structure such as orthorhombic
Na 0.44 MnO 2 shown in Fig. 3 d. Na 0.44 MnO 2 is made of MnO 5 square pyramids and
MnO 6 octahedra, which are arranged to form two types of tunnels: large S-shaped
tunnels and smaller pentagon tunnels (Fig. 3 d). Na atoms can intercalate into these
tunnels. 3D tunnels usually can not only offer stable structure to allow free
movement of Na ions inside the crystal structure, but also provide fast channels for
Na ion moving in/out of the crystal structure [ 4 - 7 ].
3.1.1 Na x CoO 2
In the early 1980s, Na x CoO 2 has been studied as cathode materials for Na batteries
[ 8 ]. Braconnier et al. reported that Na x CoO 2 has four different phases, O3
(0.9 \ x \ 1), O 0 3(x= 0.75), P3 (0.55 \ x \ 0.6), and P2 (0.64 \ x \ 0.74)
depending on the concentration of Na ions. Among them, P2-Na 0.7 CoO 2 (Fig. 1 b)
shows the largest energy density (260 Wh kg -1 ). Recently, Delmas et al. used Na
batteries to convey a thorough investigation of the P2-Na x CoO 2 phase diagram for
x C 0.50 (Fig. 4 ). It showed a succession of single phases or two-phase domains
on Na ion intercalation [ 9 ]. Harharan et al. investigated the effect of synthesis
methods on the electrochemical performance of P2-Na 0.7 CoO 2 [ 10 ]. They found
the sample prepared by sol-gel method has the highest discharge capacity (10 and
50 % higher than those prepared by solid state reaction and high-energy ball
milling methods), due to the smaller particular size and larger surface area.