While LiCoO2 has been widely studied in the past 15 years as a promising positive electrode material in lithium‐ion batteries, suprisingly, many questions are still unanswered concerning the electrochemical characteristics of the lithium intercalation material. Among these is the existence of an end member CoO2 phase on complete lithium deintercalation. The use of dry plastic lithium‐ion battery technology has allowed the construction of an in situ x‐ray diffraction cell which allows structural characterization of LixCoO2 at x values at and close to 0 for the first time. Instead of the expected destruction of the core structure of LiCoO2 by a drastic increase in structural disorder, an increase in crystallographic quality occurred as x approached 0. For the first time, the end member CoO2 phase was isolated. This phase is a hexagonal single‐layered phase (O1) believed to be isostructural with CdI2 and has lattice parameters of a = 2.822 Å and c = 4.29 Å. The phase converted immediately back to a three‐layer (O3) delithiated LixCoO2 type phase on lithium reinsertion. Electrochemical studies show that 95% of lithium can be reinserted back into the structure on complete delithiation and reversible cycling properties are maintained when cycled back to 4.2 V.

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LiZnN has been isolated by way of an electrochemical conversion reaction of Zn3N2 with Li. We show that Zn3N2 reversibly reacts with lithium electrochemically, exhibiting a large reduction capacity of 1325 mAh/g corresponding to the insertion of 3.7 Li per Zn. Of this initial capacity, 555 mAh/g were found to be reversible. Through the use of extensive in situ and ex situ X-ray diffraction, the reaction mechanism with lithium was identified as a conversion reaction of Zn3N2 into LiZn and a matrix of βLi3N, the high pressure form of Li3N. Upon oxidation, LiZn transformed into metallic Zn, while βLi3N contributed to the transformation into LiZnN. This is the first identification of a reversible Li3N conversion mechanism. The formation of LiZnN as the new end member of the electrochemical reaction with lithium was identified as the cause of the irreversible loss observed during the first cycle. The βLi3N and LiZn oxidation into LiZnN was found to be reversible upon subsequent cycles. Poor cycle life was mainly attributed to the electromechanical grinding of the Li-Zn alloying reaction. Cu3N is also introduced as a material utilizing a similar conversion reaction but exhibiting improved cycle life. © 2002 Telcordia Technologies. All rights reserved.