NCA, NCM811, and the Route to Ni-Richer Lithium-Ion Batteries

The aim of this article is to examine the progress achieved in the recent years on two advanced cathode materials for EV Li-ion batteries, namely Ni-rich layered oxides LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi0.8Co0.1Mn0.1O2 (NCM811). Both materials have the common layered (two-dimensional) crystal network isostructural with LiCoO2. The performance of these electrode materials are examined, the mitigation of their drawbacks (i.e., antisite defects, microcracks, surface side reactions) are discussed, together with the prospect on a next generation of Li-ion batteries with Co-free Ni-rich Li-ion batteries.

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 0.33Li₂MnO₃·0.67LiNi_0.4Co_0.2Mn_0.4O₂ 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 Li_0.4Ni_1.6O₂. 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, Li_0.4Ni_1.6O₂ 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.

Comparison of electrochemical performance of LiNi1−xCoxO2 cathode materials synthesized from coated (1−x)Ni(OH)2@xCo(OH)2 and doped Ni1−xCox(OH)2 precursors

LiNi_1−xCo_xO₂ cathode materials were successfully synthesized from coated (1−x)Ni(OH)₂@xCo(OH)₂ and doped Ni_1−xCo_x(OH)₂ precursors, and the effects of the Co site and content in the precursor and final cathode material on the structure, morphology, and electrochemical performance of the cathodes were investigated using X-ray diffraction, scanning electron microscopy, and charge–discharge tests. The electrochemical performance of the materials prepared from the coated precursor was generally better than that of the materials prepared from the doped precursor. However, with increasing Co content, the performance difference gradually decreased. Among the as-prepared samples, the sample coated with 12 mol% Co delivered an excellent reversible capacity of 213.8 mA h g⁻¹ at 0.1C and the highest capacity retention of 88.5% after 100 cycles at 0.2C in the voltage range of 2.75–4.3 V. High-performance LiNi_1−xCo_xO₂ materials were successfully synthesized, and our findings clearly reveal the differences in the electrochemical properties of the materials prepared from the two different precursors with increasing Co content, thereby providing a valuable reference for the synthesis of high-performance Ni-rich layered cathode materials for Li-ion batteries.

The effects of Co site and content on electrochemical performance of LiNi_1−xCo_xO₂ cathodes materials were investigated.

Ab Initio Modeling of Electrolyte Molecule Ethylene Carbonate Decomposition Reaction on Li(Ni,Mn,Co)O2 Cathode Surface

Electrolyte decomposition reactions on Li-ion battery electrodes contribute to the formation of solid electrolyte interphase (SEI) layers. These SEI layers are one of the known causes for the loss in battery voltage and capacity over repeated charge/discharge cycles. In this work, density functional theory (DFT)-based ab initio calculations are applied to study the initial steps of the decomposition of the organic electrolyte component ethylene carbonate (EC) on the (101̅4) surface of a layered Li(Ni_x,Mn_y,Co_1-x-y)O₂ (NMC) cathode crystal, which is commonly used in commercial Li-ion batteries. The effects on the EC reaction pathway due to dissolved Li⁺ ions in the electrolyte solution and different NMC cathode surface terminations containing adsorbed hydroxyl -OH or fluorine -F species are explicitly considered. We predict a very fast chemical reaction consisting of an EC ring-opening process on the bare cathode surface, the rate of which is independent of the battery operation voltage. This EC ring-opening reaction is unavoidable once the cathode material contacts with the electrolyte because this process is purely chemical rather than electrochemical in nature. The -OH and -F adsorbed species display a passivation effect on the surface against the reaction with EC, but the extent is limited except for the case of -OH bonded to a surface transition metal atom. Our work implies that the possible rate-limiting steps of the electrolyte molecule decomposition are the reactions on the decomposed organic products on the cathode surface rather than on the bare cathode surface.

Self-assembly of core–shell structures driven by low doping limit of Ti in LiCoO2: first-principles thermodynamic and experimental investigation

The structure of an LiCoO₂ core and a Ti-rich shell is obtained from a simple sol–gel route.