With the aim of developing a better understanding of the capacity fading mechanisms of spinel manganese oxides, the percent capacity fade of a number of singly substituted LiMn 2Ϫy M y O 4 (M ϭ Li, Al, Ti, Co, Ni, and Cu and 0 р y р 0.2) and doubly substituted LiMn 2ϪyϪz M y Li z O 4 (M ϭ Al, Ti, Fe, Co, Ni, Cu, and Ga, 0 р y р 0.1, and 0 р z р 0.1) oxides is correlated to the initial lattice parameter, initial manganese valence, degree of manganese dissolution, electrode resistance, irreversible capacity ͑IRC͒ loss in the first cycle, and lattice parameter difference ⌬a between the two cubic phases formed in the two-phase region during the charge-discharge process. The doubly substituted LiMn 2ϪyϪz Ni y Li z O 4 samples exhibit a combination of superior capacity retention and rate capability with lower IRC compared to LiMn 2 O 4 despite a similar amount of manganese dissolution due to a low ⌬a and the consequent low microstrain. The samples with a low ⌬a are also characterized by a lower initial lattice parameter and a higher initial Mn valence of Ͼ3.58ϩ for the as-prepared samples. The percent capacity fade, IRC, and ⌬a all exhibit a sharp decrease as the initial Mn valence in the as-prepared sample exceeds 3.58ϩ.Lithium-ion batteries have become attractive for portable electronic devices. However, the high cost and toxicity of the currently used LiCoO 2 cathodes hinder the use of lithium-ion cells for electric vehicle applications. In this regard, the spinel LiMn 2 O 4 cathode has become appealing as Mn is inexpensive and environmentally benign, but it is plagued by severe capacity fade at elevated temperatures. Several mechanisms have been proposed over the years to account for the capacity fade. The mechanisms include ͑i͒ occurrence of lattice ͑Jahn-Teller͒ distortion on the surface of LiMn 2 O 4 due to inhomogeneity in discharge and formation of tetragonal Li 2 Mn 2 O 4 , 1 ͑ii͒ manganese dissolution into the electrolyte, 2,3 ͑iii͒ formation of oxygen defects, 4 ͑iv͒ formation of new phases, 5 ͑v͒ loss of crystallinity, 6,7 and ͑vi͒ instability arising from the existence of two cubic phases during the charge-discharge process. 8,9 Among the various mechanisms proposed, manganese dissolution is generally thought to be the most important factor. However, it has been suggested that the manganese dissolution could account for only some portion of the total capacity loss and that other factors may play a significant role. 3,8,10 Additionally, monitoring of the oxygen content of the chemically delithiated samples with lithium content has revealed that the Li 1Ϫx Mn 2 O 4 system does not lose any oxygen from the lattice, ruling out the role of formation of oxygen defects in the capacity fade. 11 We showed recently by investigating a series of singly substituted LiMn 2Ϫy M y O 4 (M ϭ Li, Al, Ti, Co, and Ni and 0 р y р 0.2) and doubly substituted LiMn 2ϪyϪz M y Li z O 4 (M ϭ Ti, Co, and Ni, 0 р y р 0.1, and 0 р z р 0.1) oxides that the samples that exhibit poor cyclability exhibit significant broadening of the diffr...

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With an objective of understanding the differences in the capacity retention behavior of LiMn 2 O 4 and the cation-substituted LiMn 2Ϫy M y O 4 ͑M ϭ Li, Co, and Ni͒ spinel oxides, the peak broadening in the X-ray diffraction patterns on extracting lithium has been analyzed. An analysis of the LiMn 2 O 4 and LiMn 2Ϫy M y O 4 samples before and after treating at room temperature with acid or an oxidizing agent (NO 2 BF 4 ) or at 55°C with electrolyte reveals that the LiMn 2 O 4 system experiences a larger amount of microstrain during these treatments ͑lithium extraction͒ compared to the LiMn 2Ϫy M y O 4 systems. Additionally, the two cubic phases formed at lower lithium contents have a larger difference in lattice parameters in Li 1Ϫx Mn 2 O 4 compared to Li 1Ϫx Mn 2Ϫy M y O 4 . The observation of nearly the same amount of manganese dissolution in LiMn 2 O 4 and LiMn 2Ϫy M y O 4 suggests that the faster capacity fade in the former is due to the development of microstrain and the larger difference in lattice parameters between the two cubic phases.The toxicity and high cost of LiCoO 2 cathodes that are presently used in lithium-ion cells have prompted the development of alternate cathodes. In this regard, LiMn 2 O 4 spinel oxide has become appealing, but it is confronted with capacity fade, particularly at elevated temperatures. Several mechanisms have been proposed for the capacity fade in LiMn 2 O 4 , such as ͑1͒ formation of tetragonal Li 2 Mn 2 O 4 on the surface of LiMn 2 O 4 and the associated Jahn-Teller distortion, 1 ͑2͒ manganese dissolution into the electrolyte, 2-5 ͑3͒ formation of oxygen vacancies, 6 ͑4͒ cation mixing between lithium and manganese, 7 and ͑5͒ loss of crystallinity during cycling. 8 Although manganese dissolution is an important factor, it has been shown to account for only some portion of capacity loss. 3,9 Recent studies on chemically delithiated Li 1Ϫx Mn 2 O 4 have also shown that the spinel manganese oxide does not experience oxygen loss from the lattice. 10 Several strategies have been followed over the years to suppress the capacity fade in spinel manganese oxides. For example, substitutions of other cations for manganese in LiMn 2Ϫy M y O 4 ͑M ϭ Li, Cr, Co, Ni, Mg, and Al͒ have been shown to improve the capacity retention. 8,9,11,12 However, the reason for the improved capacity retention is not clear in the literature. We present in this paper a systematic investigation of the strain developed on chemically extracting lithium from LiMn 2Ϫy M y O 4 ͑M ϭ Li, Ni, and Co͒ and a correlation of the strain to capacity fade. ExperimentalLiMn 2Ϫy M y O 4 ͑M ϭ Li, Co, and Ni͒ samples were synthesized by firing a mixture of Li 2 CO 3 , Mn 2 O 3 , Co 3 O 4 , and/or NiO at 800°C for 48 h in air. Chemical extraction of lithium was carried out by stirring the LiMn 2Ϫy M y O 4 powder either with 2.5 N sulfuric acid 13 at room temperature for 1 h followed by washing the products with water or with an acetonitrile solution of the oxidizing agent NO 2 BF 4 at room temperature for 2 days under argon a...

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