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The high charge-state dopant Zr4+ improves the structural stability and electrochemical behavior of the lithiated transition metal oxide LiNi0.6Co0.2Mn0.2O2.

One of the major hurdles of Ni‐rich cathode materials Li1+x(NixCozMnz)wO2, y > 0.5 for lithium‐ion batteries is their low cycling stability especially for compositions with Ni ≥ 60%, which suffer from severe capacity fading and impedance increase during cycling at elevated temperatures (e.g., 45 °C). Two promising surface and structural modifications of these materials to alleviate the above drawback are (1) coatings by electrochemically inert inorganic compounds (e.g., ZrO2) or (2) lattice doping by cations like Zr4+, Al3+, Mg2+, etc. This paper demonstrates the enhanced electrochemical behavior of Ni‐rich material LiNi0.8Co0.1Mn0.1O2 (NCM811) coated with a thin ZrO2 layer. The coating is produced by an easy and scalable wet chemical approach followed by annealing the material at ≥700 °C under oxygen that results in Zr doping. It is established that some ZrO2 remains even after annealing at ≥800 °C as a surface layer on NCM811. The main finding of this work is the enhanced cycling stability and lower impedance of the coated/doped NCM811 that can be attributed to a synergetic effect of the ZrO2 coating in combination with a zirconium doping.

We report herein on the synthesis of “layered-layered” integrated xLi2MnO3⋅(1−x)LiMn1/3Ni1/3Co1/3normalO2 materials ( x=0.3 , 0.5, and 0.7) using the self-combustion reaction in solutions containing metal nitrates and sucrose. The nanoparticles of these materials were obtained by further annealing of the as-prepared product in air at 700°C for 1 h and submicrometric particles were obtained by further annealing at 900°C for 22 h. The effect of composition on the electrochemical performance was explored in this work. By a rigorous study with high resolution transmission electron microscopy (HRTEM), it became clear that the syntheses with the above stoichiometries produce two-phase materials comprising nanodomains of both rhombohedral LiNiO2 -like and monoclinic Li2MnO3 structures, which are closely integrated and interconnected with one another at the atomic level. Stable reversible capacities ∼220mAh/g were obtained with composite electrodes containing submicrometer particles of 0.5Li2MnO3⋅0.5LiMn1/3Ni1/3Co1/3normalO2 . Structural aspects, activation of the monoclinic component, and stabilization mechanisms are thoroughly discussed using Raman spectroscopy, solid-state NMR, HRTEM, and X-ray diffraction (including Rietveld analysis) in conjunction with electrochemical measurements. This work provides a further indication that this family of integrated compounds contains the most promising cathode materials for high energy density Li-ion batteries.

The work reported herein is an important continuation of our recent experimental and computational studies on Li­[Ni x Co y Mn z ]­O2 (x + y + z = 1) cathode materials for Li-ion batteries, containing minor amounts of multivalent cationic dopants like Al3+, Zr4+, W6+, Mo6+. On the basis of DFT calculations for LiNi0.8Co0.1­Mn0.1O2, it was concluded that Mo6+ cations preferably substitute Ni cations in the layered structure due to the lowest substitution energy compared to Li, Co, and Mn. It was established that the electrochemical behavior of LiNi0.8Co0.1­Mn0.1O2 as a positive electrode material for Li-ion batteries can be substantially improved by doping with 1–3 mol % of Mo6+, in terms of lowering the irreversible capacity loss during the first cycle, increasing discharge capacity and rate capability, decreasing capacity fade upon prolonged cycling, and lowering the voltage hysteresis and charge-transfer resistance. The latter is attributed to the presence of additional conduction bands near the Fermi level of the doped materials, which facilitate Li-ions and electron transfer within the doped material. This is expressed by a lower charge-transfer resistance of Mo-doped electrodes as shown by impedance spectroscopy studies. We also discovered unique segregation phenomena, in which the surface concentration of the transition metals and dopant differs from that of the bulk. This near surface segregation of the Mo-dopant seems to have a stabilization effect on these cathode materials.

This paper is dedicated to studies of the electrochemical behavior, the structural and thermal features of the Ni-rich LiNi 0.5 Co 0.2 Mn 0.3 O 2 undoped and Al-doped (∼0.01 at.%) materials for positive electrodes of lithium batteries. We have found that structural characteristics of these materials are quite similar from the crystallographic point of view. It was demonstrated that Al substitution in the doped LiNi 0.5 Co 0.2 Mn 0.3 O 2 is preferred at Ni sites over Co sites, and the thermodynamic preference for Al 3+ substitutions follows the order: Ni>Co>Mn. The lower capacity fading of the Al-doped electrodes upon cycling and aging of the cells in a charged state (4.3 V) at 60 • C, as well as more stable mean voltage behavior, are likely due to the chemical and structural modifications of the electrode/solution interface. The Al-doped LiNi 0.5 Co 0.2 Mn 0.3 O 2 electrodes demonstrate also lower resistances of the surface film and charge-transfer as well as lower activation energies for the discharge process. From XPS studies we conclude that the modified stable and less resistive interface on the Al-doped particles comprises the Li + -ion conducting nano-sized centers like LiAlO 2 , AlF 3 , etc., which promote, to some extent, the Li + ionic transport to the bulk. A partial layered-to-spinel transformation was established upon cycling of LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathodes.One of the major challenges in lithium batteries technology is, undoubtedly, the further improvement of battery components -electrodes, solutions, and separators. 1-7 Among several modern strategies to improve electrochemical performance and structural characteristics of materials for positive electrodes, doping has attracted the attention of scientists over the years. This is due to the effectiveness of dopants in stabilizing the structure of materials (even in minute amounts) and thus to increase the electrochemical cycling activity and to diminish the heat evolution of the electrodes in a charged state. A variety of dopant ions, like Co 2+ , Al 3+ , Ti 4+ , Zr 4+ , Zn 2+ , Fe 3+ , Cu 2+ , and Cr 3+ , has been used to improve the stability, morphology and microstructure of cathode materials, to enhance the electrode cycleability and rate capability, and to reduce capacity fading upon cycling. 8-13 For instance, doping of LiNi 0.5 Mn 0.5 O 2 with Co, Al, Ti resulted in decrease of the irreversible capacity loss and in almost no capacity fading of the doped electrodes. 14,15 In a systematic study of the Al-doped Ni-rich electrodes (LiNi 0.8 Co 0.15 Al 0.05 O 2 ), which are promising materials for use in batteries for electromotive applications, the authors have shown high cycling stability of these electrodes upon accelerated testing. 16 Several other doping metals, such as silver, magnesium, cobalt, gallium, lanthanum, bismuth, 17-19 as well as non-metallic ions (boron, fluorine), 20,21 were also explored in an attempt to increase the electrochemical cycling behavior of cathodes (both of layered and spinel structures) and to reduce their in...

Doping LiNiCoMnO (NCM523) cathode material by small amount of Mo ions, around 1 mol %, affects pronouncedly its structure, surface properties, and electronic and electrochemical behavior. Cathodes comprising Mo-doped NCM523 exhibited in Li cells higher specific capacities, higher rate capabilities, lower capacity fading, and lower charge-transfer resistance that relates to a more stable electrode/solution interface due to doping. This, in turn, is ascribed to the fact that the Mo ions tend to concentrate more at the surface, as a result of a synthesis that always includes a necessary calcination, high-temperature stage. This phenomenon of the Mo dopant segregation at the surface in NCM523 material was discovered in the present work for the first time. It appears that Mo doping reduces the reactivity of the Ni-rich NCM cathode materials toward the standard electrolyte solutions of Li-ion batteries. Using density functional theory (DFT) calculations, we showed that Mo ions are preferably incorporated at Ni sites and that the doping increases the amount of Ni ions at the expense of Ni ions, due to charge compensation, in accord with X-ray absorption fine structure (XAFS) spectroscopy measurements. Furthermore, DFT calculations predicted Ni-O bond length distributions in good agreement with the XAFS results, supporting a model of partial substitution of Ni sites by molybdenum.

Li-rich electrode materials of the family xLi 2 MnO 3 •(1-x)LiNi a Co b Mn c O 2 (a + b + c = 1) suffer a voltage fade upon cycling that limits their utilization in commercial batteries despite their extremely high discharge capacity, ca. 250 mAhg -1 . We exposed Li-rich, 0.35Li 2 MnO 3 •0.65LiNi 0.35 Mn 0.45 Co 0.20 O 2 , to NH 3 at 400 °C , producing materials with improved characteristics: enhanced electrode capacity and a limited average voltage fade during 100 cycles in half cells vs. Li. We established three main changes caused by NH 3 treatment. First, a general bulk reduction of Co and Mn was observed via XPS and XANES. Next, a structural rearrangement lowered the coordination number of Co-O and Mn-O bonds, as well as formation of a surface spinel-like structure. Additionally, Li + removal from the bulk causes the formation of surface LiOH, Li 2 CO 3 , and Li 2 O. These structural and surface changes can enhance the voltage and capacity stability of the Li-rich material electrodes after moderate NH 3treatment times of 1 -2 hours.