Higher‐capacity lithium batteries can be fabricated by using α‐MnO2 nanowires as the catalyst in a porous, composite oxygen cathode. In the depicted scanning electron micrograph of a composite positive electrode, deposits of Li2O2, which accumulates during discharge, can be seen within a porous matrix consisting of α‐MnO2 and carbon.

Eine höhere Kapazität von Lithiumbatterien lässt sich mithilfe von α‐MnO2‐Nanodrähten als Katalysator in einer porösen Sauerstoff‐Kompositkathode erreichen. Im rasterelektronenmikroskopischen Bild einer positiven Kompositelektrode sind Li2O2‐Ablagerungen, die sich während des Entladevorgangs bilden, in einer porösen Matrix aus α‐MnO2 und Kohlenstoff erkennbar.

The conversion that occurs from layered LiMnO2 to spinel on electrochemical cycling has been studied by neutron diffraction and NMR. Neutron diffraction results indicate that the tetrahedral sites in the Li layers that share faces with octahedral sites in the transition metal layers are occupied even following the first charge to 4.6 V. NMR results are consistent with the conversion from the monoclinic, Jahn−Teller distorted, to the rhombohedral, layered phase on charging. On subsequent discharging, clear evidence for the monoclinic phase is seen by NMR indicating the presence of Jahn−Teller distorted domains in the material at 3.5 V and below. The fraction of monoclinic phase decreases gradually as a function of cycle number and disappears by 35 charge−discharge cycles. Diffraction patterns obtained as a function of cycle number were refined with a structural model that included both a layered phase (with octahedral and tetrahedral site occupancy) and a spinel phase, with the fraction of spinel increasing from 0.12 (5 cycles) to 0.93 following 92 cycles. Both diffraction and NMR results indicate that the spinel phase that nucleates from the layered material is stoichiometric and does not contain Li occupancy on the Mn sites. An additional site is seen by NMR which reaches a maximum in cycles 25−50, which is assigned to tetrahedrally coordinated Li in a local environment intermediate between that of the spinel and the layered phase. The observation of this site by NMR is associated with two characteristic peaks in the incremental capacity plot at 3.75 and 3.9 V on charge and discharge. The data indicate that the mechanism for Li insertion and removal into these local environments is complex and involves simultaneous structural rearrangements. The intermediate environment decreases in concentration on subsequent cycling as the concentration of the spinel phase continues to grow.

Nonstoichiometric layered Li x Mn y O2 compounds with the O3 structure (α-NaFeO2 type), space group R3̄m, were synthesized by ion exchange from the sodium precursors Na x Mn y O2. Such lithium intercalation compounds are important in the context of rechargeable lithium batteries since they offer the key advantages of lower cost, lower toxicity, and higher safety when compared with LiCoO2, which is used presently as the positive electrode in these devices. By the varying of the synthesis conditions of the precursor and the ion-exchange process, significant variations of the vacancy content on the transition metal sites could be induced. The variations in composition and defect structure were reflected in differences in the lattice parameters and hkl dependent peak broadening, observed in the X-ray diffraction data. Structural details were confirmed by Rietveld refinement using neutron powder diffraction. Lithium intercalation/deintercalation proved very sensitive to the composition and defect structure. High discharge capacities of 190−200 mAhg-1 at a rate of C/7 (corresponding to complete discharge in 7 h) could be obtained with a capacity fade of only 0.12% per cycle, at room temperature. This is significantly better than results reported previously for stoichiometric LiMnO2. All of the materials described in this work convert to a spinel-like phase on repeated intercalation/deintercalation of lithium. Such conversion involves the generation of a nanostructured spinel-like phase within each particle. The nanostructure plays a key role in accommodating, at the domain wall boundaries, stresses which normally accompany the first-order Jahn−Teller driven phase transition occurring on cycling in the 3 V region.