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The specific surface area of samples CMD-120, CMD-275, and
CMD-375 are 25.4, 26.8, and 36.1 m2 g−1, respectively, which sug-
gests that the electrochemical performances of our ␥-MnO2 and its
annealed forms are not determined by specific surface area but by
the crystal structure and the structural water of these samples. In
other words, those samples, removed the W2-type ͑rather than W3
type͒ structural water, can maintain their ␥ phase and, consequently,
have good electrochemical performance. By contrast, removing W3-
type structural water results in phase transformation from ␥ into  in
the structure of MnO2, which goes against fast lithium-ion diffusion
upon discharging. As a result, 275°C is an appropriate temperature
to remove the water from the sample but preserve the ␥ phase,
evolving to pyrolusite. Accordingly, sample CMD-275 showed very
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charge performance of sample CMD-275 may also be associated
with its nanosize and the smaller amount of Pr and Tw defects.
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The ␥-MnO2 nanomaterial was hydrothermally synthesized at
180°C based on the reaction between NaMnO4 and MnSO4. The
hydrated Na+ plays a crucial role in the hydrothermal formation of
␥-MnO2. Only in a narrow range of added NaOH ͑e.g., 0.08–0.09 M
in our case͒ into the hydrothermal system could ␥-MnO2 be ob-
tained. It is likely that such an adding range of NaOH leads the
hydrated Na+ to have a suitable size for template growth of
␥-MnO2, associated with the dissolution–recrystallization mecha-
nism.
Thermal analysis of such a ␥-MnO2 suggests that there are two
types of structural water which significantly affect the structure and
electrochemical performance of ␥-MnO2. When the as-prepared
␥-MnO2 was calcined at 275°C, about 51% of structural water ͑type
W2͒ was removed; the resulting product, however, remained as
␥-MnO2. Such an annealed sample delivered high capacities ͑e.g.,
296.5 mAh g−1 at 18 mA g−1͒ and good rate performance. Thus,
this ␥-MnO2 nanomaterial is a promising cathode material for
lithium battery.
38. H. J. Yue, X. K. Huang, D. P. Lv, and Y. Yang, Electrochem. Solid-State Lett., 11,
A163 ͑2008͒.
39. X. K. Huang, Q. S. Zhang, H. T. Chang, J. L. Gan, H. J. Yue, and Y. Yang, J.
Acknowledgment
The financial support from the National Natural Science Foun-
dation of China ͑grant no. 20433060, no. 20473068, and no.
29925310͒ and the National Basic Research Program ͑973 Program͒
is gratefully acknowledged.
Electrochem. Soc., 156, A162 ͑2009͒.
40. J. Luo, Q. Zhang, J. Garcia-Martinez, and S. L. Suib, J. Am. Chem. Soc., 130, 3198
͑2008͒.
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42. G. G. Xia, W. Tong, E. N. Tolentino, N. G. Duan, S. L. Brock, J. Y. Wang, S. L.
Suib, and T. Ressler, Chem. Mater., 13, 1585 ͑2001͒.
Xiamen University assisted in meeting the publication costs of this ar-
ticle.
43. Q. Feng, T. Horiuchi, L. H. Liu, K. Yanagisawa, and T. Mitsushio, Chem. Lett.,
2000, 284.
44. See EPAPS supplementary material at E-JESOAN-156–013911 K-S samples using
KMnO4.
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