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

Abstract

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...

Synthesis and electrochemical characterizations of Nano-SiO2-coated LiMn2O4 cathode materials for rechargeable lithium batteries

“…Fig. 5c shows the XPS spectra of Mn 2p, the peak observed around 642.2 eV at the surface and 50 nm depth may be assigned to characterize the binding energy value of Mn 2p spectra in LiMn 2 O 4 [35][36][37]. 50 nm depth profile has higher intensity peak compared to surface level.…”

LiNi0.5Mn1.5O4 Spinel and Its Derivatives as Cathodes for Li-Ion Batteries

“…It is common that after precursors are obtained by wet method, less energy or lower reaction temperature are needed to turn the precursors into final products. Wet chemical methods include coprecipitation method [57][58][59][60], polymerpyrolysis method [61,62], ultrasonic-assisted co-precipitation (UACP) method [63,64], solgel method [65][66][67], radiated polymer gel method [68], sucrose-aided combustion method [69], spray-drying method [70], emulsion drying method [71], composite carbonate process [72], molten salt method [73,74], mechanochemical process [75], poly (methyl methacrylate) (PMMA)-assisted method [76] ultrasonic spray pyrolysis [77], polymer-assisted synthesis [78], combinational annealing method [79], pulsed laser deposition [80], electrophoretic deposition [81], spin-coating deposition [82], carbon combustion synthesis [83], soft combustion reaction method [84], pulsed laser deposition [85], spray drying and postannealing [86], rheological method [87], polymer-mediated growth [88], self-reaction method [89], internal combustion type spray pyrolysis [90,91], a chloride-ammonia coprecipitation method [92], a novel carbon exo-templating method [93], flame type spray pyrolysis [94], self-combustion reaction (SCR) …”

Effect of the impurity LixNi1−xO on the electrochemical properties of 5V cathode material LiNi0.5Mn1.5O4

“…So far, nearly all studies have focused on pure phase LiNi 0.5 Mn 1.5 O 4 and its derivatives, and great efforts have been made to avoid impurity phase such as Li x Ni 1−x O [1][2][3]. It is generally believed that the impurity Li x Ni 1−x O can deteriorate the electrochemical properties of products [4][5][6].The impurity Li x Ni 1−x O results from oxygen deficiency in high temperature process of synthesizing LiNi 0.5 Mn 1.5 O 4 [7]. It is a common phenomenon that there is impurity Li x Ni 1−x O in product when solid-state reaction method is used.…”