The electrochemistry of Zn3N2 and LiZnN - A lithium reaction mechanism for metal nitride electrodes

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 betaLi(3)N, the high pressure form of Li3N. Upon oxidation, LiZn transformed into metallic Zn, while betaLi(3)N 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 betaLi(3)N 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. (C) 2002 Telcordia Technologies.