Oxygen Lattice Instability as a Capacity Fading Mechanism for 5 V Cathode Materials

Abstract

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 ...

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis

“…They attributed these redox peaks at about 4.9 V vs. Li/Li + to decomposition of the electrolyte. Caballero et al observed an oxidation peak at about 5.1 V and ascribed this peak to the release of oxygen from the spinel framework [72]. According to previous studies [9,17,24,73], the features of the obtained cyclic voltammogram indicates that the as-prepared LNMO exhibited dominantly the disordered structure, which is in accordance to the SAED result.…”

Crystallinity Control of a Nanostructured LiNi0.5Mn1.5O4 Spinel via Polymer‐Assisted Synthesis: A Method for Improving Its Rate Capability and Performance in 5 V Lithium Batteries

“…The presence of impurities such as traces of organic components seems to play a minor role here. The unresolved peak above 4.8 V, which was specially significant for the PEG 400 sample but barely visible for the PEG 800 sample, can be assigned to either electrolyte oxidation [40] or release of oxygen from the spinel lattice. [41] The electrochemical performance of the two spinels was assessed from galvanostatic measurements made over a wide range of charge/discharge rates (from C/4 to 15C, where C represents 1 Li + ion exchanged in 1 h equivalent to 148 mA g -1 ) and cycling over the voltage range 3.5-5.0 V. Figure 7 shows the variation of the discharge capacity of the two spinels obtained from the PEG-based synthesis recorded at two rates, C/4 and 4C, as a function of cycle number.…”

“…The coulombic efficiency of the electrode made from the spinel obtained in the absence of PEG is also included for comparison. The first four cycles were excluded owing to the overcharge introduced by the charging process as the potential result of some oxygen being released above 4.8 V [40] and/or electrolyte oxidation. [41] Although most values exceeded 95 %-which suggests good coulombic efficiency of both electrodes-there were small differences that are worth some comment.…”

Oxygen Deficiency as the Origin of the Disparate Behavior of LiM[sub 0.5]Mn[sub 1.5]O[sub 4] (M = Ni, Cu) Nanospinels in Lithium Cells

“…21 The Cu spinels, particularly sample Cu-800, exhibit the most marked differences between experimental and theoretical values. One salient feature is the good consistency between the experimental capacity and the content in Cu 2ϩ at octahedral positions; this suggests that Cu 2ϩ ions at tetrahedral positions are electrochemically inactive below 5.0 V regardless of their origin.…”