Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material
Introduction
Although LiCoO2 has formed the basis of cathode electrode in commercial lithium-ion batteries (LIBs), its relatively high cost, toxic properties, practically reachable capacity of only 50% of the theoretical capacity and safety problem inhibit its further use in price-sensitive and large-scale applications, such as electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1], [2]. So researchers worldwide are searching for high-capacity, safe, and inexpensive replacement for LiCoO2. Nowadays, a layered transition-metal oxide LiNi1/3Mn1/3Co1/3O2 (named LNMCO hereafter) with hexagonal structure, first introduced by Ohzuku's group in 2001 as a candidate of cathode materials [3], [4], has attracted significant interest, because the combination of nickel, manganese, and cobalt can provide many advantages compared with LiCoO2, such as lower cost, less toxicity, milder thermal stability at charged state, better stability during cycling even at elevated temperature, and higher reversible capacity [5], [6], [7]. It has been demonstrated that the charge compensation is achieved by stabilization of Ni2+, Mn4+ and Co3+ ions [8], [9]. The electrochemical reaction of lithium extraction/insertion takes place by oxidation/reduction of Ni2+/Ni4+ and Co3+/Co4+ ions depending on different cut-off voltages, while Mn4+ remains inactive but maintains the structural stability [10]. Therefore, LNMCO might be a promising candidate of cathode material for the next generation of high-power and high-energy LIBs [11].
However, the cation mixing between lithium and nickel ions on the crystallographic 3b site of the LNMCO lattice, which is known to deteriorate the electrochemical performance of layered oxides, still remains a severe problem [12]. Since the ionic radius of Li+ (0.76 Å) is close to that of Ni2+ (0.69 Å), a partial occupation of Ni2+ lattice sites by Li+ (by interchange between a Li+ ion and a nearest neighbor Ni2+ ion to insure local charge neutrality and minimize Coulomb energy) generates a cation mixing in the structure, which blocks the pathway of lithium diffusion [13], [14].
The aim of this work is the optimization of electrode materials for Li-ion batteries by adjusting one parameter of the synthesis, namely the lithium/transition-metal ratio, so as to minimize the cation mixing. Two independent experimental procedures are used to estimate the cation mixing: (i) magnetic measurements, which are powerful tools to check the quality of samples and structural properties at nanoscopic scale [15], especially in the case of transition-metal oxides; (ii) structural refinements of XRD patterns made by Rietveld method, which accurately determine the transition-metal occupancy between the slab of this layered material [16]. It is well-established that the presence of Ni2+ ions onto the 3b site of the lithium sublattice (noted Ni2+(3b) hereafter) can generate a ferromagnetic interaction with the Mn4+ ions nearest neighbors on 3a sites, and this Ni2+(3b)–Mn4+(3a) ferromagnetic interaction results in the formation of a ferromagnetic cluster centred on the Ni2+(3b) defect [17], [18]. The magnetic moment of these ferromagnetic clusters can be accessed by the magnetic measurements, from which the concentration of the Ni2+(3b) defects can be deduced and compared with the concentration deduced from Rietveld refinement [19]. This paper is then focussed on the investigation of these important structural features and their correlation with the magnetic of Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 samples as a function of the synthetic conditions. The electrochemical behaviours of the Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 electrode materials synthesized by co-precipitation method are investigated in Li cells and data are discussed in detail.
Section snippets
Sample preparation
The synthesis of Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 powders were performed by a hydroxide route using transition-metal hydroxide and lithium carbonate as starting materials. First, the synthesis of transition-metal hydroxide of composition (Ni1/3Mn1/3Co1/3)(OH)2 was realized using NiSO4·6H2O, MnSO4·6H2O, CoSO4·6H2O, NaOH and NH4OH (analysis grade) as raw materials dissolved in distilled water. An aqueous solution prepared with a concentration of 2 mol L−1 was pumped into a continuously stirred tank
Structure and morphology
Elemental titration of samples A and B synthesized with nominal Li/M ratio η = 1.05 and 1.10, respectively, have been evaluated by inductively coupled plasma (ICP) spectroscopy. The results, reported in Table 1, show that the technological composition for each metal in Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 is close to the stoichiometry and we obtained lithium-rich samples Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 with x = 0.02 and 0.04 for samples A and B, respectively. Note that the Li/M ratios of the sample are (1 + x)/(1 −
Conclusion
The layered Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 compound was synthesized via co-precipitation method with different Li/M molar ratios. XRD, SQUID and charge–discharge characterizations show that the structural, magnetic and electrochemical profiles are sensitive to the synthetic conditions. The optimized performance of the Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 electrode was obtained from the lithium-rich sample B. Concentrations of Ni2+ located onto 3b sites, which account for the cation mixing, estimated from
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