Page 381 - Handbook of Battery Materials

352 12 Lithium Intercalation Cathode Materials for Lithium-Ion Batteries

LiMn 1/3 Ni 1/3 Co 1/3 O 2 has become an attractive cathode material because of its high
capacity, better thermal stability, and stable cycle performance [38, 39]. In these
4+
3+
mixed layered oxides, Ni, Mn, and Co exist as, respectively, Ni ,Mn , and Co .
2+
However, only Li 1−x CoO 2 becomes metallic on charging, because of the partially
filled t 2g band, while Li 1−x NiO 2 and Li 1−x MnO 2 remain as semiconductors during
charging as the e g band is redox active and not the t 2g band in the edge-shared
MO 6 lattice.

12.7
Layered LiMnO 2
Layered LiMnO 2 is attractive from an economical and environmental point of
view, since manganese is inexpensive and environmentally benign compared to
cobalt and nickel. However, LiMnO 2 synthesized at high temperatures adopts
an orthorhombic structure instead of the layered O3-type structure, resulting in
poor electrochemical performance [40]. Ion-exchange of Na + by Li + in layered
α-NaMnO 2 has been shown to form layered, monoclinic LiMnO 2 [41, 42]. The
reduction in crystal symmetry from trigonal in LiCoO 2 to monoclinic in LiMnO 2 is
attributed to the crystallographic Jahn–Teller distortion induced by the Mn 3+ ions.
However, the layered LiMnO 2 synthesized by the ion-exchange method exhibits
poor cycling performance because of the transformation of the charged Li 1−x MnO 2
into spinel LiMn 2 O 4 during electrochemical charge–discharge cycling. This is
because of the low OSSE value of Mn 3+ ions and the consequent easy migration of
the Mn 3+ ions from the octahedral sites of the Mn planes to the octahedral sites of
the Li planes via the neighboring tetrahedral sites [43].

12.8
Li[Li 1/3 Mn 2/3 ]O 2 - LiMO 2 Solid Solutions

Recently, solid solutions between Li[Li 1/3 Mn 2/3 ]O 2 (commonly known as Li 2 MnO 3 )
and LiMO 2 (M = Mn 0.5 Ni 0.5 , Co, Ni, and Cr) have become attractive as they
exhibit a high reversible capacity of around 250 mAh g −1 with lower cost and better
safety than LiCoO 2 cathodes [44–51]. Li 2 MnO 3 has the layered structure similar
+
to LiCoO 2 , but one third of the transition metal planes are occupied by Li ions.
Although Li 2 MnO 3 is electrochemically inactive at 3–4 V vs Li/Li , subsequent
+
studies showed that it can be made electrochemically active by acid leaching [52] or
charging to high voltages [53]. Although xLi[Li 1/3 Mn 2/3 ]O 2 –(1–x)LiMO 2 can be
considered as solid solutions on a macroscopic scale, more detailed investigations
with high-resolution transmission electron microscopy (TEM) and NMR have
shown nanodomains consisting of layered Li 2 MnO 3 -like phases and layered LiMO 2
phases [50, 51].
Figure 12.8a shows a typical first charge–discharge curve of the xLi[Li 1/3 Mn 2/3 ]
O 2 –(1–x)LiMO 2 cathodes. Following the initial sloping region corresponding
to the oxidation of the transition metal ions, a plateau region around 4.5 V