Layered Metal Oxide Cathodes

Recent studies have shown that partial substitution of manganese in LiNio.5Mno.5O2 not only provides high capacities ( 200 mAhg ), but also results in a significant improvement in thermal stability compared to LiNi02 [37]. The increase in capacity and thermal stability is associated with the substitution of chemically more stable Mn + ions for Ni +. Recently, the mixed layered oxide 

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Figure 1. Schematic description of a (lithium ion) rocking-chair cell that employs graphitic carbon as anode and transition metal oxide as cathode. The undergoing electrochemical process is lithium ion deintercalation from the graphene structure of the anode and simultaneous intercalation into the layered structure of the metal oxide cathode. For the cell, this process is discharge, since the reaction is spontaneous.

Formation mechanism of SEI layers on cathodes in Li-ion batteries, their thermal and electrochemical stabihty, and their roles in affecting the cycle life and safety characteristics are well documented by many researchers [43 6]. Here we present some recent data on identifying the surface layer generation and their composition on transition metal oxide cathodes like spinel and layered materials by various spectroscopic techniques. The structural changes and the reaction at the surface during the first delithiation process in Li-rich layered material are explained. The effects of additives and coatings on electrode materials to their electrochemical performance are also discussed at the end. 

Xiao J, Chernova NA, Whittingham MS (2008) Layered mixed transition metal oxide cathodes with reduced cobalt content for lithium ion batteries. Chem Mater 20 7454—7464... 

Where E)(Li ) is Fermi energy of the Li metal and Ef(M /M" ) is the Fermi energy of M /lVf redox couple x is lithimn eontent of cathode and (1-x) is lithimn content of anode F is Faraday s constant. It is clear that the voltage of the Lithium battery is related to the electron band structure, which is dominated by the erystal structure and the voltage deereasing is related to the chemical activity of Li intercalated layer metal oxides (eathode) and graphite... 

He P, Yu H, li D, Zhou H (2012) Layered lithium transition metal oxide cathodes towards high energy lithium-ion batteries. J Mater Chem 22 3680-3695. doi 10.1039/c2jml4305d... 

Each of these processes has an associated time constant. Diffusion of the lithium through the solids (carbon, hthiated metal oxide cathode, and SEI) is relahvely fast. The processes occurring at and in the boundary layer are the slowest and are rate-limiting. 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

Cathodic stripping voitammetry (CSV) is more rarely required than ASV, but is useful for the determination of halide(s), sulfide or other anions. Occasionally, cations of intermediate valency, such as or can be analysed by using CSV, e.g. by reducing cations within a layer of metal oxide on an electrode made of carbon. 

During the cathodic delamination process there are two important reactions which can occur at the cathode and which are catalyzed on the thin layer of metal oxide which covers the cathode surface. These reactions are ... 

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