Cathode Materials
Cathode materials are the most expensive part of lithium ion batteries. Cathode materials are found in different crystalline structures like layered structure, spinel structure, olivine structure and recently a series of superlattice structures.
Layered Structure
In the year of 1990, Sony (Japan) introduced the lithium ion battery. Sony used LiCoO2 as cathode material. This material is expensive and unsafe and some military application has disregarded this material to use. A considerable investment has been made in this battery technology that utilizes LiCoO2 with an operating voltage range of 4.2 to 2.75V. However, during operation at high temperature, LiCoO2 shows an exothermic reaction which eventually generates loose oxygen and can cause fire hazards.
Another promising material is LiNiO2, however, phase pure oxide is difficult to produce resulting in poor or low discharge capacity around 140-150 mAh/g. It has capacity degradation also due to the formation of NiO2 during intercalation and deintercalation of lithium ion. Toxicity and high cost are other issues for the cobalt and nickel based layered oxides.
New LiMn1/3Co1/3Ni1/3O2 layered structure is being used in lithium ion batteries considering its safety and non-toxic nature. The disadvantage is that the structure of cathode material is destroyed once discharge below 2.5V. The cost of the material is also higher than LiCoO2 considering the difficulties of process control.
Olivine Structure
Lithium iron phosphate, LiFePO4, is widely used and under investigation considering its low cost and safety. The oxygen is strongly bonded in phosphate form and thus avoids formation of loose oxygen. The disadvantage of this material is that it has a low operating voltage within the range of 3.4V to 2.9V and nominal voltage is 3.2V only.
Spinel Structure
Recently, manganese based oxides such as LiMn2O4 spinel and LiMnO2 layered oxides have been studied extensively. The reason was manganese is abundant in nature, less expensive and non-toxic. The problem encountered using manganese was significant capacity fading which is due to dissolution of manganese in the form of Mn+2.
Superlattice Structure
This crystal structure models were examined as [√3X√3] R30°, superlattice. In Figure 3, it can be observed that all transition metal occupy same layer in hexagonal structures and in layered structure they occupy alternative layers. In Superlattice structure some transition metals is substituted by lithium and it enhances the power density significantly. However, superlattice mixed oxide synthesis becomes difficult when starting materials are not mixed homogeneously. An innovative synthesis process is required when large scale production of superlattice cathode material is a major concern.
Figure 3: a) Superlattice Structure, b) Alternating Layers
Table 1: Comparative Electrochemical Properties
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