The phase diagram of the Li−Mn−Ni-oxide pseudoternary system 1 was used as a starting point to understand the influence of transition metal composition and lithium content on the structures and electrochemistry of materials with compositions: Li 1+x (Ni y Mn 1−y ) 1−x O 2 (y = 0.2, 0.33, 0.4, 0.5, 0.6, and 0.7, 0 ≤ x ≤ 0.34). Mixed transition metal hydroxide precursors were synthesized via coprecipitation using a continuously stirred tank reactor. Powder X-ray diffraction results showed that additional spinel, rocksalt and/ or layered phases were observed when the lithium content was less than that required for a single layered phase in agreement with ref 1. Additionally, residual Li 2 CO 3 was detected in the backscattered scanning electron microscopy images for samples with relatively high lithium content. The boundaries of the single phase region were therefore defined and the contour plots of the lattice constants versus composition in the single phase region were generated. Electrochemical measurements showed that phase separation should be avoided and single phase samples should be prepared in order to obtain the highest capacity. The maximum reversible capacity to 4.6 V was found near the middle of the single phase region when 0.5 < y < 0.7, whereas it was at the top (smallest lithium content) of the single phase region when y ≤ 0.5.
Lithium-rich layered Ni–Mn–Co oxide materials have been intensely studied in the past decade. Mn-rich materials have serious voltage fade issues, and the Ni-rich materials have poor thermal stability and readily oxidize the organic carbonate electrolyte. Core–shell (CS) strategies that use Ni-rich material as the core and Mn-rich materials as the shell can balance the pros and cons of these materials in a hybrid system. The lithium-rich CS materials introduced here show much improved overall electrochemical performance compared to the core-only and shell-only samples. Energy dispersive spectroscopy results show that there was diffusion of transition metals between the core and shell phases after sintering at 900 °C compared to the prepared hydroxide precursors. A Mn-rich shell was still maintained whereas the Co which was only in the shell in the precursor was approximately homogeneous throughout the particles. The CS samples with optimal lithium content showed low irreversible capacity (IRC), as well as high capacity and excellent capacity retention. Sample CS2-3 (the third sample in the 0.67Li1+x (Ni0.67Mn0.33)1–x O2·0.33Li1+y (Ni0.4Mn0.5Co0.1)1–y O2 CS2 series) had a reversible capacity of ∼218 mAh/g with 12.3% (∼30 mAh/g) irreversible capacity (IRC) and 98% capacity retention after 40 cycles to 4.6 V at 30 °C at a rate of ∼C/20. Differential capacity versus potential (dQ/dV versus V) analysis confirmed that cells of the CS samples had stable impedance as well as a very stable average voltage. Apparently, the Mn-rich shell can effectively protect the Ni-rich core from reactions with the electrolyte while the Ni-rich core renders a high and stable average voltage.
Core–shell structure positive electrode materials, based on layered Li–Ni–Mn–Co oxides, could be the next generation of positive electrode materials for high energy density lithium-ion batteries. Diffusion of the transition metal cations between the core and shell phases occurs during sintering, which can significantly affect the final core–shell (CS) properties. However, the interdiffusion constants have never been measured. Laminar pellets of the pure core phase and pure shell phase were pressed in contact and then heated to measure the interdiffusivity of the transition metals at various temperatures. The diffusion couples Ni3+/Co3+, Co3+/Mn4+, and Ni3+/Mn4+ were measured by analyzing composition versus position, with respect to the initial interface between the core and shell phase pellets. The transition metal composition profiles were measured with energy dispersive spectroscopy (EDS) line scans. This is the first time interdiffusion constants have been reported in the layered cathode materials to our knowledge. At 900 °C, Ni3+/Co3+ has the highest interdiffusivity, D, of ∼4.7 × 10–12 cm2/s, while Ni3+/Mn4+ has the lowest of ∼0.1 × 10–12 cm2/s. The activation energy barriers for Ni3+/Co3+, Co3+/Mn4+, and Ni3+/Mn4+ interdiffusion were determined from Arrhenius plots of D vs 1/T. Simulations of diffusion in spherical core–shell materials were performed to show how knowledge of the interdiffusion constants can guide rational design of practical core–shell materials.
The Li–Mn–Ni-O system has received much attention for potential positive electrode materials in lithium-ion batteries. Recent work mapping the phase diagrams of the entire pseudo-ternary system showed that the layered solid-solution region extends to compositions with both less and more lithium than the well-known lithium-rich layered composition line that joins Li2MnO3 to LiNi0.5Mn0.5O2. The part of this solid-solution region that is lithium deficient has a “bump” feature in the single-phase boundary, which could not be explained until now. The current study explores this part of the phase diagram with the use of X-ray diffraction, helium pycnometry measurements, redox titrations, and a Monte Carlo simulation. Results show that metal site vacancies are present in the structures in increasing amounts as the lithium content of the samples decreases. A Ni2+ ion and a vacancy can replace two Li+ ions in Li[Li1/3Mn2/3]O2 to make the solid solution series Li[Li(1/3)–x Ni x/2□ x/2Mn2/3]O2 with 0 < x < 1/3. The most lithium-deficient structures contain sufficient vacancies to allow manganese to form on two-thirds (2/3) of the transition-metal layer, such that the ordering of manganese on two √3 × √3 lattices yields a structure with low internal energy and sharp superlattice peaks in XRD patterns. The material with the maximum theoretical vacancy fraction that still has two-thirds of the transition-metal layer filled with manganese, Li[Ni1/6□1/6Mn2/3]O2, was also synthesized. Both XRD and electrochemical data regarding this new material are presented.
Positive electrode materials which do not react with electrolyte at high potentials (≥ 4.6 V vs. Li/Li + ) are essential for developing Liion batteries with high energy densities and long cycle lives. Reactions with electrolyte can be detected using precise measurements of coulombic efficiency (CE) and charge end point capacity slippage. Three single-phase layered compositions in the Li-Mn-Ni-O system, Li[Li 0.16 Ni 0.12 Mn 0.65 0.07 ]O 2 , Li[Li 0.12 Ni 0.32 Mn 0.56 ]O 2 , and Li[Li 0.09 Ni 0.46 Mn 0.45 ]O 2 were studied by high precision coulometry at upper potential limits of 4.6 V and 4.8 V. When cycled to 4.6 V, Li[Li 0.16 Ni 0.12 Mn 0.65 0.07 ]O 2 had a reversible capacity of 225 mAh g −1 after 50 cycles, and maintained a substantially higher CE and a lower charge end point capacity slippage per cycle than Li[Li 0.12 Ni 0.32 Mn 0.56 ]O 2 , Li[Li 0.09 Ni 0.46 Mn 0.45 ]O 2 , and industry standard Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (cycled to only 4.2 V). Overall, these results highlight the inherent "inertness" of Li[Li 0.16 Ni 0.12 Mn 0.65 0.07 ]O 2 and its suitability as a thin protective shell in a core-shell particle configuration.
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