High energy-density lithium-ion batteries are in demand for portable electronic devices and electrical vehicles. Since the energy density of the batteries relies heavily on the cathode material used, major research efforts have been made to develop alternative cathode materials with a higher degree of lithium utilization and specific energy density. In particular, layered, Ni-rich, lithium transition-metal oxides can deliver higher capacity at lower cost than the conventional LiCoO2 . However, for these Ni-rich compounds there are still several problems associated with their cycle life, thermal stability, and safety. Herein the performance enhancement of Ni-rich cathode materials through structure tuning or interface engineering is summarized. The underlying mechanisms and remaining challenges will also be discussed.

A solid solution series of lithium nickel metal oxides, Li[Ni(1-x)M(x)]O2 (with M = Co, Mn, and Al) have been investigated intensively to enhance the inherent structural instability of LiNiO2. However, when a voltage range of Ni-based cathode materials was increased up to >4.5 V, phase transitions occurring above 4.3 V resulted in accelerated formation of the trigonal phase (P3m1) and NiO phases, leading to and pulverization of the cathode during cycling at 60 °C. In an attempt to overcome these problems, LiNi0.62Co0.14Mn0.24O2 cathode material with pillar layers in which Ni(2+) ions were resided in Li slabs near the surface having a thickness of ∼10 nm was prepared using a polyvinylpyrrolidone (PVP) functionalized Mn precursor coating on Ni0.7Co0.15Mn0.15(OH)2. We confirmed the formation of a pillar layer via various analysis methods (XPS, HRTEM, and STEM). This material showed excellent structural stability due to a pillar layer, corresponding to 85% capacity retention between 3.0 and 4.5 V at 60 °C after 100 cycles. In addition, the amount of heat generation was decreased by 40%, compared to LiNi0.70Co0.15Mn0.15O2.

Undesired electrode–electrolyte interactions prevent the use of many high-energy-density cathode materials in practical lithium-ion batteries. Efforts to address their limited service life have predominantly focused on the active electrode materials and electrolytes. Here an advanced three-dimensional chemical and imaging analysis on a model material, the nickel-rich layered lithium transition-metal oxide, reveals the dynamic behaviour of cathode interphases driven by conductive carbon additives (carbon black) in a common nonaqueous electrolyte. Region-of-interest sensitive secondary-ion mass spectrometry shows that a cathode-electrolyte interphase, initially formed on carbon black with no electrochemical bias applied, readily passivates the cathode particles through mutual exchange of surface species. By tuning the interphase thickness, we demonstrate its robustness in suppressing the deterioration of the electrode/electrolyte interface during high-voltage cell operation. Our results provide insights on the formation and evolution of cathode interphases, facilitating development of in situ surface protection on high-energy-density cathode materials in lithium-based batteries.

5799wileyonlinelibrary.com issues associated with energy security and environmental pollution. [1][2][3][4][5] Oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are the most crucial electrochemical reactions to realize energy storage and conversion in these technologies. Although Pt-, Ir-, and Ru-based materials exhibit the highest activity for these electrochemical reactions, these precious-metal catalysts cannot be largely used for these clean energy because of their scarcity on earth and high cost. [ 6,7 ] Therefore, high-active, low-cost, and durable precious-metalfree catalysts from earth-abundant elements have been attracted considerable attention since last decade. [8][9][10][11] Among studied nonprecious metal catalysts, [ 12 ] nickel and cobalt are earth-abundant, low-cost, and environment-friendly materials that have widely been explored as electrocatalysts for the oxygen or hydrogen reactions in energy conversion and storage devices. [13][14][15] However, the pure cobalt and nickel oxides usually show insuffi cient electrical conductivity and low reactive surface areas, resulting in limited kinetics during these electrochemical reactions such as ORR, OER. [ 16 ] Oppositely, standalone metallic Ni or Co has good electrical conductivity, but is less active than Pt, because the formation energies of Ni-H or Co-H is lower than that of Pt-H for the HER. [ 17 ] Furthermore, combining graphene with metal or metal oxide is an effective way to improve catalytic activities due to the high surface area and excellent electrical conductivity of graphene, [18][19][20] thereby increasing number of active sites and promoting the charge transfer in electrodes. [21][22][23] For example, Co 3 O 4 /graphene, [ 24 ] Ni/graphene fi lm, [ 25 ] and NiO/rGO [ 26 ] composite catalysts have been explored showing enhanced catalytic activity, relative to single metals or metal oxides. Based on previous studies on the nickel or cobalt electrocatalysts, in this work, we synthesized a new family catalyst including Co-CoO/N-rGO and Ni-NiO/N-rGO via a pyrolysis of graphene oxide-supported cobalt and nickel salts, respectively. The possible synergetic effect among transition metals, metal oxides, and graphene was systematically studied, making them simultaneously highly active for the OER, ORR, or HER. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional ElectrocatalystsXien Liu , Wen Liu , Minseong Ko , Minjoon Park , Min Gyu Kim , Pilgun Oh , Sujong Chae , Suhyeon Park , Anix Casimir , Gang Wu , * and

An epitaxy layer on the LiNi0.8Co0.1Mn0.1O2 cathode significantly suppressed the nickel-ion crossover, which enhanced the structural/electrochemical stability at high temperature.