Page 275 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposite materials in energy storage: Properties and applications 247

material was also an exception even at 150°C [85]. Zhou et al. [86] studied the effect
of addition of functionalized multiwall carbon nanotube (MWCNT) onto the PEO-
LiClO 4 electrolyte. Enhancement by a factor of 3.3 was observed. The enhancement
in the ionic conductivity is attributed to the functionalized MWCNT-induced decrease
in the crystallinity of PEO and increase in the salt dissociation due to the Lewis acid-
base interaction of the functionalized MWCNTs with PEO and LiClO 4 . The addition
of functionalized MWCNTs can also effectively improve the mechanical properties of
PEO films. Recently, Liu et al. reported the effect of addition of Y 2 O 3 -doped ZrO 2
(YSZ) nanowires on the PAN-LiClO 4 nanocomposite. It was shown that 7 mol% of
Y 2 O 3 doped in the nanowire increased the ionic conductivity to 1.7 10 5 from
3.62 10 7 at room temperature. The positively charged oxygen vacancies on the sur-
face of nanowires could associate with the anions, thereby improving the transfer of Li
cations [87].

9.3.1.2 PNCs as cathode material for the Li ion batteries
It has been mentioned earlier that a Li-ion battery has two electrodes: positive elec-
trode is called cathode, and negative electrode is called anode. In fact, the electrode
materials selected are critical to the performance of the Li-ion battery as they gener-
ally determine the energy density, power density, cyclability, and cell voltage [88–
90]. As far as cathodes are concerned, they are very important; they account for
40% of the cost of the entire battery [91]. A cathode material should show high free
energy of reaction with Li so that it can show high cathode voltage, should incorporate
large amount of Li, should have high electronic conductivity, should be inert toward
the nonaqueous liquid electrolyte, and should be nontoxic and environmentally
benign. State-of-the-art cathode material used in commercial batteries is lithium
cobalt oxide, LiCoO 2 [92,93]. Lithium cobalt oxide was first reported by Mizushima
et al. in 1980 [94]. This material caught the attention of the researcher because of its
excellent cyclability (>500 cycles), high energy density, ease of manufacturing, and
good stability in air [95–97]. The theoretical and practical capacities for batteries with
1
LiCoO 2 are 274 and 140 mAh g , respectively. Their high cost, structural instability,
and decaying capacity, presence of toxic Co, and safety concern raised in the recent
times are some of the major disadvantages of the LiCoO 2 cathode materials
[89,98,99]. Apart from these, LiCoO 2 has the Li diffusion coefficient of 5 10 9 -
2 1
cm s and very low electronic conductivity of 10 3 Scm 1 [100]. In the last two
decades, wide varieties of cathode materials have been explored as a replacement
for the LiCoO 2 . These materials include (1) 4 V-layered compounds, LiMO 2
(M¼Co, Ni, Mn, etc), which has two-dimensional (2D)-layered structure, and (2)
spinel-type LiM 2 O 4 (M¼Mn), (3) Olivine compounds LiMPO 4 (M¼Fe, Mn, Ni,
Co, etc.) show 5 volt potential [89,101]. Apart from these materials, monoclinic
Li 3 M 2 (PO 4 ) 3 , borates LiMBO 3 (M¼Mn, Fe, and Co) [102], tavorite fluorosulfates
(LiMSO 4 F) [103], tavorite fluorophosphates (LiVPO 4 F) [104], and orthosilicates
Li 2 MSiO 4 /C (M¼Mn and Fe) [105] are investigated. In general, cathode materials
with high Vage are preferred for their high stored energy. But, high voltage stability
of the liquid and polymer electrolytes remains a challenge, and cathode materials