Cathode and Anode Materials for Na-Ion Battery - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 392

Fig. 4 Galvanostatic cycling curve of an Na/P2-Na x CoO 2 battery giving an overview of the
phase diagram [ 9 ]
successive cycles. It was considered that the intercalation/deintercalation of Na
ions can cause repeated gliding of the MO sheets, leading to collapse of the layered
structure into amorphous after the first eight cycles [ 12 ]. So, the structural stability
is still an issue on the layered Na x MnO 2 material.
Compared to layered Na x MnO 2 , orthorhombic Na 0.44 MnO 2 shows higher sta-
bility in structure, the MnO 6 octahedra, and MnO 5 square pyramids form a
framework possessing two types of Na-ion diffusion channels (Fig. 3 d): large S-
shaped tunnels and smaller pentagon tunnels. Na ions occupy three sites in this
structure: the Na(1) site in the small tunnels is fully occupied while the Na(2) and
Na(3) sites in the large S-shaped tunnels are half occupied. Na ions in the large S-
shaped tunnels can be extracted, while Na ions in the small tunnels are immovable
due to the small tunnel size and strong interaction with the surrounding oxygen
atoms. Sauvage et al. synthesized well-crystallized Na 0.44 MnO 2 with a rod shape
(500 nm in width and 5-10 lm in length) as shown in Fig. 5 a[ 5 ]. A specific
capacity of about 140 mAh g -1 was obtained at a slow rate (C/200) within the
potential range of 2-3.8 V (vs. Na/Na + ), indicating that the reversible insertion/
extraction of Na in Na x MnO 2 occurred with x varying between 0.18 and 0.64.
Notably, the evident multi-potential plateaus in charge/discharge curves and multi-
peaks in the incremental capacity curves suggest multiple phase transitions
occurring during the Na intercalation/deintercalation. In situ X-ray diffraction
measurements were carried out to detect the evolution of the multiphases. Six
biphasic transition processes were observed during charge and discharge, which
were consistent with the numbers of the potential plateaus on the charge/discharge
curves. Unfortunately, when the current was increased to 1/10 C, only
80 mAh g -1 of capacity was obtained and faded to 50 % over 50 cycles (Fig. 5 b).
This notable hysteresis in Na intercalation/deintercalation dynamics probably