The formation of inclusion complexes between cucurbit[7]uril (CB[7]) and ferrocene and its derivatives has been investigated. The X-ray crystal structure of the 1:1 inclusion complex between ferrocene and CB[7] revealed that the guest molecule resides in the host cavity with two different orientations. Inclusion of a set of five water-soluble ferrocene derivatives in CB[7] was investigated by 1H NMR spectroscopy and calorimetric and voltammetric techniques. Our data indicate that all neutral and cationic guests form highly stable inclusion complexes with CB[7], with binding constants in the 10(9)-10(10) M(-)(1) and 10(12)-10(13) M(-1) ranges, respectively. However, the anionic ferrocenecarboxylate, the only negatively charged guest among those surveyed, was not bound by CB[7] at all. These results are in sharp contrast to the known binding behavior of the same guests to beta-cyclodextrin (beta-CD), since all the guests form stable inclusion complexes with beta-CD, with binding constants in the range 10(3)-10(4) M(-1). The electrostatic surface potentials of CB[6], CB[7], and CB[8] and their size-equivalent CDs were calculated and compared. The CD portals and cavities exhibit low surface potential values, whereas the regions around the carbonyl oxygens in CBs are significantly negative, which explains the strong affinity of CBs for positively charged guests and also provides a rationalization for the rejection of anionic guests. Taken together, our data suggest that cucurbiturils may form very stable complexes. However, the host-guest interactions are very sensitive to some structural features, such as a negatively charged carboxylate group attached to the ferrocene residue, which may completely disrupt the stability of the complexes.

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The unique structural characteristics and their effect on the electrochemical performances of the layered-spinel composite cathode system xLi[Li 0.2 Mn 0.6 Ni 0.17 Co 0.03 ]O 2 −(1 − x)Li[Mn 1.5 Ni 0.425 Co 0.075 ]O 4 (0 ≤ x ≤ 1) have been investigated by a systematic analysis of the X-ray diffraction (XRD) data, neutron diffraction data (ND), electrochemical charge−discharge profiles, and electrochemical differential−capacity measurements. In the 0.5 ≤ x < 1 samples, the capacity and energy density of the composite cathodes gradually increase during 50 cycles with a change in the shape of the charge−discharge profiles. Ex situ X-ray diffraction data reveal two important findings, which account for the superior cycle performance: (i) the layered phase in the composite cathodes (x = 0.5 and 0.75) undergoes an irreversible phase transformation to a cubic spinel phase during extended electrochemical cycling, and the newly formed spinel phase exhibits only a 3 V plateau without any 4 or 4.7 V plateau as both Mn and Ni are present in the 4+ state; (ii) the parent 5 V cubic spinel phase undergoes a cubic to tetragonal transition during discharge, but the volume change is small (∼5%) for the x = 0.5 and 0.75 compositions. Both the small volume change associated with the cubic to tetragonal transition and the excellent stability of the newly evolved 3 V spinel-like phase lead to remarkable cycle life despite a wide voltage range (2−5 V) involving phase transitions. ■ INTRODUCTIONLithium-ion batteries are being intensively pursued as a power source for vehicle applications as they offer much higher energy density compared to other rechargeable systems like the Ni-MH batteries. However, the energy density of the current lithium-ion technology is limited by the cathode capacity, and there is immense interest to develop new cathodes with higher capacity or higher operating voltages. 1 In this regard, lithiumrich layered oxides Li[Li,Mn,Ni,Co]O 2 , which are solid solutions between layered Li[Li 1/3 Mn 2/3 ]O 2 and LiMO 2 (M = Mn, Ni, and Co), have become attractive since they exhibit high capacities of ∼250 (mA h)/g when charged above 4.5 V. 2−7 However, they exhibit inferior rate capability and a large first cycle irreversible capacity loss. On the other hand, the spinel LiMn 1.5 Ni 0.5 O 4 with an operating voltage of ∼4.7 V exhibits high rate capability due to the three-dimensional diffusion of lithium ions. 8−16 Moreover, since most of the manganese ions are in the 4+ state, LiMn 1.5 Ni 0.5 O 4 does not suffer from Jahn− Teller distortion or drastic Mn dissolution. However, LiMn 1.5 Ni 0.5 O 4 exhibits a limited capacity of ∼130 (mA h)/g.One approach to realizing a combination of high energy and power density is to use a composite that consists of the highcapacity lithium-rich layered oxide Li [Li,Mn,Ni,Co]O 2 and the high-power spinel oxide LiMn 1.5 Ni 0.5 O 4 . The cubic closepacked oxygen arrays in both the layered and the spinel oxides are structurally compatible. Two layered-spinel composite systems have been invest...

In order to achieve consistent electrochemical properties essential for the commercialization of the high-voltage spinel cathode LiMn 1.5 Ni 0.5 O 4 , a deeper fundamental understanding of the factors contributing to capacity fade is required. Specifically, the relationship between cation ordering, impurity phases present, and particle morphology must be elucidated. We present here a comparison of stoichiometric LiMn 1.5 Ni 0.5 O 4 cathodes with a 3:1 Mn/Ni ratio prepared by different methods with varying morphologies and degrees of cation ordering. Careful structural, chemical, and electrochemical characterizations illuminate the relative influence of the various factors on the electrochemical cycling stability and high-rate performance. It is found that although an increase in the degree of cation ordering decreases the rate capability, the crystallographic planes in contact with the electrolyte have a dominant effect on the electrochemical properties.

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