The origin of the overcharge in the 5 V region observed in lithium-substituted LiM x Mn 2Ϫx O 4ϩ␦ spinels (M ϭ Cr, Ni, Cu; x Ϸ 0.2) prepared at 500°C was analyzed by using accurate analytical spectroscopic techniques ͑mass spectroscopy, nuclear magnetic resonance͒ to examine the electrolyte behavior. The spectra revealed organic solvents to be stable as no decomposition products were detected, thus excluding the electrolyte oxidation as a side reaction accounting for the cell overcharge. However, these spinels contain excess oxygen in an amount that was quantified from thermogravimetric data. The excess oxygen plays a prominent role in the electrochemical response of the spinel. The cyclic voltammetry and galvanostatic results support the assumption that the excess oxygen can be released above 4.5 V. The additional capacity obtained and that required to release the oxygen were quite consistent. This must be the origin of both the overcharge and the poor performance of the cells compared with spinels of similar composition but synthesized at higher temperatures ͑800°C͒, the excess of oxygen in which was smaller. The ability of some Li-Mn based spinels to exhibit high-voltage plateaus at about 5 V has opened up new prospects for lithium batteries such as the possibility of manufacturing high-voltage batteries capable of supplying highly specific energy.1 LiM to Mn 5ϩ or Mn 6ϩ hypothesized for the undoped spinel has not yet been confirmed. Two alternative side reactions can be considered: ͑i͒ electrolyte oxidation above 4.6 V and (ii) a redox process undergone by the oxygen lattice of the spinel framework involving release of oxygen. This latter model can be related with theoretical computations based on either first-principles calculations 10 or the DV-X␣ molecular orbital model.11 Based on such models, when M is substituted by Mn ions, a new O 2p band at a low energy is responsible for the high electrochemical cell voltage observed. Simultaneously, evolved oxygen may oxidize organic solvents. These unwanted side reactions can lead to a premature cell capacity loss. Three recent reports 12-14 have insisted on the origin of the electrochemical activity above 4.5 V. Thus, Wang et al. 12 claim that the high voltage capacity of Li 1ϩx Mn 2Ϫx O 4Ϯz originates from the extraction of Li ϩ at 16d sites, whereas Shin and Manthiram 13,14 ascribe it to the involvement of O 2Ϫ -2p in the redox process. The aim of this work was to help clarify the processes taking place in the high-potential region of 5 V in cation-substituted lithium manganese spinels. For this purpose, Cr-, Ni-, and Cusubstituted spinels containing the doping elements in various oxidation states and prepared at two different temperatures ͑500 and 800°C͒ were examined. The electrolyte decomposition was analyzed using mass spectrometry ͑MS͒ and nuclear magnetic resonance ͑NMR͒ measurements. These measurements, together with thermogravimetric ͑TG͒ data, confirmed the central role played by the excess oxygen ion in the spinel lattice. ExperimentalThree series ...
A comparative study of the electrochemical properties of two spinels of nominal composition LiM 0.5 Mn 1.5 O 4 ͑M ϭ Ni, Cu͒ was conducted. The doping element introduces significant structural changes in stoichiometry, cation distribution, and oxidation state of the elements. Whereas Ni spinels were virtually stoichiometric and exhibit mostly normal structure, the Cu samples were oxygen-deficient spinels with a greater contribution of the inverse structure. This was especially so in the sample obtained at 800°C, where X-ray photoelectron spectroscopy ͑XPS͒ revealed the presence of Cu ϩ . Its electron configuration favored the occupancy of tetrahedral positions and the resulting displacement of some Li ϩ ions to octahedral positions. All these features account for the increased Mn 3ϩ content of the Cu spinels, consistent with electrochemical measurements. Ex situ XPS measurements confirmed the oxidation processes of Ni and Cu ions on charging the cell at 5.0 V, and the reversibility of the electrochemical reaction undergone by Ni. However, the reversibility of the Cu 2ϩ Cu 3ϩ reaction could not be demonstrated because of the low capacity delivered by the cell in the 5.0-4.5 V region. The structural defects in the Cu spinels have an adverse impact on cell performance. Compared to the Ni spinel, the Cu spinel possesses a low discharge capacity that fades faster on cycling.The partial substitution of some manganese in LiMn 2 O 4 by various transition metals is known to endow it with the ability to deliver a substantial capacity at voltages above 4.5 V. 1-3 Ni-doped spinels have received much attention on account of their improved cycling behavior relative to the undoped spinel. 4-11 Thus, a highly pure LiNi 0.5 Mn 1.5 O 4 phase exhibits a flat operating voltage of ca. 4.7 V and an initial discharge capacity of 139 mAh/g, which is more than 90% of the theoretical capacity based on the Ni 2ϩ /Ni 4ϩ redox couple. 10,11 The origin of the difference between the theoretical and real capacity may be slight deviations from the previous composition resulting in the presence of Mn 3ϩ and the content which depends on the particular synthetic conditions. To the authors' knowledge, Cu-doped spinels have received comparatively little attention. Only several papers by the same research group have so far dealt with the electrochemical properties of LiCu 0.5 Mn 1.5 O 4 in lithium cells. [12][13][14][15] The copper spinel exhibits at least two significant differences relative to LiNi 0.5 Mn 1.5 O 4 , namely, ͑i͒ the capacity corresponding to the high-potential plateau is only 24 mAh/g ͑i.e., much lower than that calculated for the Cu 2ϩ → Cu 3ϩ process͒ and (ii) substantial capacity in the 3.5-4.5 V region ͑48 mAh/g͒ resulting from the Mn 3ϩ /Mn 4ϩ redox couple despite the formal absence of Mn 3ϩ from the spinel formula. Other possibilities such as the involvement of alternative oxidation states of Cu ͑particularly Cu ϩ or Cu 3ϩ ) require confirmation. 12 The aim of this paper is to facilitate understanding of the peculiar electrochemi...
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