Negative electrode materials types

As mentioned above, the typical positive electrode material is LiCo02, and there are typically two types of negative electrode materials, such as coke and graphite. The characteristics of lithium-ion batteries constructed using these electrode materials are discussed below. 

Only rare-earth system (AB5-type) and zirconium-titanium-vanadium system (AB2 Laves phase-type) hydrogen storage alloys have been used as negative electrode materials for the commercial production of Ni-MH batteries [3, 7, 8], However, these materials have a low hydrogen storage capacity resulting in a low electrode energy density. 

The Zn negative electrode material, or anode, and electrolyte solution are similar to other primary alkaline battery types, like zinc-air and zinc-silver oxide (Zn/ Ag20). Zinc powder is mixed with a gelling agent like polyacrylic acid and a KOH-Zn0-H20 electrolyte. 

Figure 1.2. Electrochemical mechanisms of different negative electrode materials, of insertion-type (above), or alloys and conversion-type (below). For a color version of the figure, see www.iste.co.uk/dedryvere/electrodes.zip...

In this case, we have an electrolyte identical to that which is present in lithium-polymer batteries, made of poly(ethylene oxide) (or PEO) in the presence of a lithium salt, solid at ambient temperature, and which needs to be heated above ambient temperature in order for the battery to work (T > 65°C for PEO). Thus, the electrolyte, in its molten state, exhibits sufficient ionic conductivity for the lithium ions to pass. This type of electrolyte can be used on its own (without a membrane) because it ensures physical separation of the positive and negative electrodes. This type of polymer electrolyte needs to be differentiated from gelled or plasticized electrolytes, wherein a polymer is mixed with a lithium salt but also with a solvent or a blend of organic solvents, and which function at ambient temperature. In the case of a Li-S battery, dry polymer membranes are often preferred because they present a genuine all solid state at ambient temperature, which helps limit the dissolution of the active material and therefore self-discharge. Similarly, in the molten state (viscous polymer), the diffusion of the species is slowed, and there is the hope of being able to contain the lithium polysulfides near to the positive electrode. In addition, this technology limits the formation of dendrites on the metal lithium... 

A commercial MmNi5-type hydrogen storage alloy was used for the negative electrode material. A slurry containing 96% alloy powders, 4% nickel powder and a proper amount of binders styrene-butadiene rubber and hydroxypropyl methyl cellulose was pasted onto the nickel coated stainless steel strip substrates, and then dried and compressed to obtain the metal hydride electrode. The dimensions of the pasted metal hydride electrode plate were 125 mm X 42 mm X 0.28 mm. 

We have so far described the research and development for nickel-metal hydride batteries in which a hydrogen storage alloy (metal hydride) is used as a negative electrode material. This type of battery has been attracting a great deal of attention as a new non-military battery because it is characterized by several advantages over the conventional secondary batteries ... 

Figure 2.16 shows the charge-discharge cycle characteristics of alloys in which part of the nickel component was replaced with cobalt. Misch metal (Mm), which is a mixture of rare earth elements such as lanthanum, cerium, praseodymium, and neodymium, was used in place of lanthanum. It was found that the partial replacement of nickel with cobalt and the substitution of the lanthanum content with Mm was very useful in improving the charge-discharge cycle life. However, such alloys have insufficient capacity, as shown in Figure 2.17 [18]. From study of the effect that their compositions had on the charge-discharge capacity, it was concluded that the best alloy elements were Mm(Ni-Co-Al-Mn)This alloy led to the commercialization of sealed nickel-M H batteries. All the battery manufacturers who use a rare earth-nickel-type alloy for the negative electrode material employ similar alloys with slightly different compositions.

Other metals can also form compounds X-C or Li-X-C (X includes Zn, Ag, Mg, Cd, In, Pb, and Sn), which can be used as negative electrode materials for lithium-ion batteries. There is an evident improvement in the electrochemical performance, mainly because the introduced metals favor Li diffusion. Other kinds or two or more types of heteroatoms can also be introduced. 

Observations based on the alloy-type mechanism have shown that the reversible capacity of composite Sn oxides fades with cycling. In contrast, with the ionic-type mechanism, only slow capacity fading was observed with cycling. Using Li NMR (with LiCl solution as a reference), it was shown that the ionic nature of the inserted lithium is more than that for other negative electrode materials (Table 8.1). This indicates that ionic-type mechanisms may be possible. 

Similar to Sn, oxides such as PbO and Pb02 can also be used as negative electrode materials for lithium-ion batteries based on an alloy-type mechanism. It is also possible to improve their electrochemical performance by modification. 

Cycling behavior of 30650-type lithium-ion batteries with nominal capacities of 3 Ah and 10 Wh based on a LiNio7Coo.302 positive electrode material and a mixture of graphite and coke (weight ratio, 4 1) as the negative electrode material The charge and discharge current is 1190 mA. (Adapted from Kida, Y. et al., Electmchim. Acta, 47,2002.)... 

The irreversible capacity results from formation of a surface-electrolyte interface (SEI) layer, and is believed to be caused by decomposition of the electrolyte on the surface of active material during few first charge cycles [3-5]. The values of irreversible capacity and the SEI are functions of the type of active material and the electrolyte. Also, the safety issue, which is believed to be associated with stability of SEI, has been identified as a major parameter in the equation [6-7]. The contribution of the negative electrode to the thermal runaway is believed to be related to the nature and also to the surface area of the active material [8-9]. 

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