Highly Stable 4.6 V LiCoO2 Cathodes for Rechargeable Li Batteries by Rubidium-Based Surface Modifications.

Among extensively studied Li-ion cathode materials, LiCoO2 (LCO) remains dominant for portable electronic applications. Although its theoretical capacity (274 mAh g-1 ) cannot be achieved in Li cells, high capacity (≤240 mAh g-1 ) can be obtained by raising the charging voltage up to 4.6 V. Unfortunately, charging Li-LCO cells to high potentials induces surface and structural instabilities that result in rapid degradation of cells containing LCO cathodes. Yet, significant stabilization is achieved by surface coatings that promote formation of robust passivation films and prevent parasitic interactions between the electrolyte solutions and the cathodes particles. In the search for effective coatings, the authors propose RbAlF4 modified LCO particles. The coated LCO cathodes demonstrate enhanced capacity (>220 mAh g-1 ) and impressive retention of >80/77% after 500/300 cycles at 30/45 °C. A plausible mechanism that leads to the superior stability is proposed. Finally the authors demonstrate that the main reason for the degradation of 4.6 V cells is the instability of the anode side rather than the failure of the coated cathodes.

Stabilizing High-Voltage LiNi0.5 Mn1.5 O4 Cathodes for High Energy Rechargeable Li Batteries by Coating With Organic Aromatic Acids and Their Li Salts.

Here, three types of surface coatings based on adsorption of organic aromatic acids or their Li salts are applied as functional coating substrates to engineer the surface properties of high voltage LiNi0.5 Mn1.5 O4 (LNMO) spinel cathodes. The materials used as coating include 1,3,5-benzene-tricarboxylic acid (trimesic acid [TMA]), its Li-salt, and 1,4-benzene-dicarboxylic acid (terephthalic acid). The surface coating involves simple ethanol liquid-phase mixing and low-temperature heat treatment under nitrogen flow. In typical comparative studies, TMA-coated (3-5%) LNMO cathodes deliver >90% capacity retention after 400 cycles with significantly improved rate performance in Li-coin cells at 30 °C compared to uncoated material with capacity retention of ≈40%. The cathode coating also prevents the rapid drop in the electrochemical activity of high voltage Li cells at 55 °C. Studies of high voltage full cells containing TMA coated cathodes versus graphite anodes also demonstrate improved electrochemical behavior, including improved cycling performance and capacity retention, increased rate capabilities, lower voltage hysteresis, and very minor direct current internal resistance evolution. In line with the highly positive effects on the electrochemical performance, it is found that these coatings reduce detrimental transition metal cations dissolution and ensure structural stability during prolonged cycling and thermal stability at elevated temperatures.

Studies of Nickel-Rich LiNi0.85Co0.10Mn0.05O2 Cathode Materials Doped with Molybdenum Ions for Lithium-Ion Batteries.

In this work, we continued our systematic investigations on synthesis, structural studies, and electrochemical behavior of Ni-rich materials Li[NixCoyMnz]O2 (x + y + z = 1; x ≥ 0.8) for advanced lithium-ion batteries (LIBs). We focused, herein, on LiNi0.85Co0.10Mn0.05O2 (NCM85) and demonstrated that doping this material with high-charge cation Mo6+ (1 at. %, by a minor nickel substitution) results in substantially stable cycling performance, increased rate capability, lowering of the voltage hysteresis, and impedance in Li-cells with EC-EMC/LiPF6 solutions. Incorporation of Mo-dopant into the NCM85 structure was carried out by in-situ approach, upon the synthesis using ammonium molybdate as the precursor. From X-ray diffraction studies and based on our previous investigation of Mo-doped NCM523 and Ni-rich NCM811 materials, it was revealed that Mo6+ preferably substitutes Ni residing either in 3a or 3b sites. We correlated the improved behavior of the doped NCM85 electrode materials in Li-cells with a partial Mo segregation at the surface and at the grain boundaries, a tendency established previously in our lab for the other members of the Li[NixCoyMnz]O2 family.

Modification of Li- and Mn-Rich Cathode Materials via Formation of the Rock-Salt and Spinel Surface Layers for Steady and High-Rate Electrochemical Performances.

We demonstrate a novel surface modification of Li- and Mn-rich cathode materials 0.33Li2MnO3·0.67LiNi0.4Co0.2Mn0.4O2 for lithium-ion batteries (high-energy Ni-Co-Mn oxides, HE-NCM) via their heat treatment with trimesic acid (TA) or terephthalic acid at 600 °C under argon. We established the optimal regimes of the treatment-the amounts of HE-NCM, acid, temperature, and time-resulting in a significant improvement of the electrochemical behavior of cathodes in Li cells. It was shown that upon treatment, some lithium is leached out from the surface, leading to the formation of a surface layer comprising rock-salt-like phase Li0.4Ni1.6O2. The analysis of the structural and surface studies by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy confirmed the formation of the above surface layer. We discuss the possible reactions of HE-NCM with the acids and the mechanism of the formation of the new phases, Li0.4Ni1.6O2 and spinel. The electrochemical characterizations were performed by testing the materials versus Li anodes at 30 °C. Importantly, the electrochemical results disclose significantly improved cycling stability (much lower capacity fading) and high-rate performance for the treated materials compared to the untreated ones. We established a lower evolution of the voltage hysteresis with cycling for the treated cathodes compared to that for the untreated ones. Thermal studies by differential scanning calorimetry also demonstrated lower (by ∼32%) total heat released in the reactions of the materials treated with fluoroethylene carbonate (FEC)-dimethyl carbonate (DEC)/LiPF6 electrolyte solutions, thus implying their significant surface stabilization because of the surface treatment. It was established by a postmortem analysis after 400 cycles that a lower amount of transition-metal cations dissolved (especially Ni) and a reduced number of surface cracks were formed for the 2 wt % TA-treated HE-NCMs compared to the untreated ones. We consider the proposed method of surface modification as a simple, cheap, and scalable approach to achieve a steady and superior electrochemical performance of HE-NCM cathodes.

Horizons for Li-Ion Batteries Relevant to Electro-Mobility: High-Specific-Energy Cathodes and Chemically Active Separators.

Li-ion batteries (LIBs) today face the challenge of application in electrified vehicles (xEVs) which require increased energy density, improved abuse tolerance, prolonged life, and low cost. LIB technology can significantly advance through more realistic approaches such as: i) stable high-specific-energy cathodes based on Li1+ x Niy Coz Mnw O2 (NCM) compounds with either Ni-rich (x = 0, y → 1), or Li- and Mn-rich (0.1 < x < 0.2, w > 0.5) compositions, and ii) chemically active separators and binders that mitigate battery performance degradation. While the stability of such cathode materials during cell operation tends to decrease with increasing specific capacity, active material doping and coatings, together with carefully designed cell-formation protocols, can enable both high specific capacities and good long-term stability. It has also been shown that major LIB capacity fading mechanisms can be reduced by multifunctional separators and binders that trap transition metal ions and/or scavenge acid species. Here, recent progress on improving Ni-rich and Mn-rich NCM cathode materials is reviewed, as well as in the search for inexpensive, multifunctional, chemically active separators. A realistic overview regarding some of the most promising approaches to improving the performance of rechargeable batteries for xEV applications is also presented.