Lithiated transition metal oxides

Many applications require the preparation of composite electrodes in which the active materials are in a powder form which may be nonconductive. Hence, the electrode must include a rigid current collector, a binder and some electrically conducting additive, in addition to the active substance. Such electrodes are important for electrocatalysis and as cathodes for batteries. For instance, many cathode materials for rechargeable Li and Li ion batteries are lithiated transition metal oxides, which appear as a nonconductive powder. 

Nonaqueous solvents can form electrolyte solutions, using the appropriate electrolytes. The evaluation of nonaqueous solvents for electrochemical use is based on factors such as -> dielectric constant, -> dipole moment, - donor and acceptor number. Nonaqueous electrochemistry became an important subject in modern electrochemistry during the last three decades due to accelerated development in the field of Li and Li ion - batteries. Solutions based on ethers, esters, and alkyl carbonates with salts such as LiPF6, LiAsly, LiN(S02CF3)2, LiSOjCFs are apparently stable with lithium, its alloys, lithiated carbons, and lithiated transition metal oxides with red-ox activity up to 5 V (vs. Li/Li+). Thereby, they are widely used in Li and Li-ion batteries. Nonaqueous solvents (mostly ethers) are important in connection with other battery systems, such as magnesium batteries (see also -> nonaqueous electrochemistry). 

The lithiated transition metal oxide LiVMoOe has been synthesized by solid state reaction. This is the first report of this compound to be studied as an anode material. The synthesized LiVMo06 powder has been studied by means of X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) spectroscopy. The electrochemical characteristics of the prepared electrodes assembled in coin cells were also investigated in terms of half-cell performance. It is observed that the cell exhibits three stages of discharge plateaus in the ranges 2.1-2.0 V, 0.6-0.5 V and 0.2-0.01 V, respectively. 

If the battery is to be rechargeable, the reactions must be reversible. The importance of lithiated transition-metal oxides (Li cM02) is that they are capable of accommodating large quantities of lithium per formula unit and have low relative molecular masses, giving rise to high power and energy densities. 

Lithiated transition metal oxides These compounds can be classified via their structure layered compounds (e.g., LiCo02, LilMnNiCoJOa) and spinel (e.g., LiMn204, LiNio sMni 5O4) [3]. 

The case of air cathodes is complicated, as is presented later, and definitely deserves a separate discussion in this chapter. Therefore, the most interesting and important discussion on the surface chemistry of the positive electrodes in Li batteries relates to lithiated transition metal oxides and Li olivine compounds. 

This section aims at demonstrating the importance and relevance of the surface chemistry developed on cathodes for Li-ion battery systems to their performance. The topics selected for discussion are the effect of nano-size, surface chemical aspects of lithiated transition metal oxide cathodes and a comparison with the surface chemistry of L1MPO4 olivine-type cathodes. 

There are obvious reactions between Li MOj, compounds and acidic species such as HF. These reactions form MF surface species, M=Li, or transition metal. Li jMOj, compounds can also interact with trace PF5 that can be formed by the thermal decomposition of LiPFg (LiPFg LiF+PFs). The products should include MF species (always detected on the surface of lithiated transition metal oxides aged/cycled in standard electrolyte solutions). 

In this chapter we deal with four major electrode surfaces active metals, carbons, non-active metals (e.g., noble metals), and composite electrodes comprising lithiated transition metal oxide powders as the active mass, plus polymeric binder and conductive additives (usually carbon black or graphite powders at low percentage). In terms of general surface chemistry, we find that the surface reactions on lithium, lithiated carbons, carbon, and noble metals polarized to low potentials in non-aqueous Li salt solutions are very similar. All of these electrodes are covered by surface films comprising insoluble Li salts, which are formed by reduction of solution species. Upon anodic polarization of carbon or noble metal electrodes in non-aqueous solutions, solution species are oxidized. Here, the impact of the cations is negligible. It seems that the species that determine the anodic stability of non-aqueous solutions are the solvents. For instance, ether may be oxidized at potentials below 4 V, while alkyl carbonates may apparently be stable up to 5 V (Li/Li ). However, it should be noted that some minor oxidation reactions of alkyl carbonate solvents on noble metal electrodes (e.g., Pt, Au) can be detected even at a potential below 4 V. The... 

Hence, due to the above-described passivation phenomena, aluminum current collectors are apparently stable in nonaqueous solutions, even at potentials above 5 V (Li/Li" ). In the case of cathodes for Li batteries where the active mass constitutes lithiated transition metal oxides, we discovered that there is a possibility for a variety of spontaneous reactions between LixMOy (M=Co, Ni, Mn, V, etc.) compounds and electrolyte solutions comprising alkyl carbonate solutions (strong electrophiles) and Li salts such as LiPFe, which form surface films. ... 

The removal and insertion of the lithium ion for lithiated transition metal oxides (M) are ... 

TABLE 34.7 Diffusion Coefficients of Li-Ion in Lithiated Transition Metal Oxides... 

LIBs use the Li" ion intercalation materials for both the cathode and anode, between which the Li" ions are shuttled across the electrolyte absorbed in the separator. Figure 2 depicts the potential and specific capacity of typical cathode and anode materials suitable for the LIBs [2]. In order to assemble the battery in discharged state, the cathode materials are lithiated transition metal oxides and the anode materials are carbons or compounds capable of intercalating Li" ions or alloying with metallic Li. Research focuses have been on the increase of the battery energy/ power density, reduction of material cost, and the improvement of battery safety, which are outlined below. 

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