From the first intercalating cathode (TiS2) to discovery of LiCoO2

Hello everyone, are you ready for an exciting journey to Li-ion batteries? Let’s start! 

As you know before Li-ion batteries, we used to use aqueous electrolytes. This limits the operation voltage, basically we cannot cycle the electrodes above 2 V. To cycle the electrodes at higher voltage, researchers suggested to use non-aqueous electrolytes in rechargeable batteries. By non-aqueous electrolytes, the battery safety improved because there is no water evaporation/moisture issue anymore.

In 1970s, for the first time, layered TiS₂ cathode was used as an intercalating electrode with Li metal anode and non-aqueous electrolyte. Layered cathode brought good reversibility during charge-discharge of the battery. Li ions (Li⁺) could flow from anode to cathode and intercalate into cathode 2D layers, which was accompanied by reduction of Ti⁺⁴ to Ti³⁺ ions. However, researchers realized that operating voltage is still low (<2.5 V). But why? The reason was behind the Density of States (DOS) theory.

Now, we need to know what the DOS is. If we define DOS; it is a way to show electronic configuration of molecular orbitals. If we consider the case in TiS₂ – Li system, DOS demonstrates that we have overlapping between bands. Here, transition metal (Ti ^3+/4+) 3d band is overlapping the S^2- 3p band. For example, Fig. 1 displays that Co^2+/3+ 3d band is overlapping with S^2- 3p band. The overlapping causes introduction of holes or formation of molecular ions (S₂^2-).

Later, Goodenough’s (still is not good enough to be awarded by Nobel Prize (!), why??) groups tried to find a solution to this problem. What they did is use of oxide based cathodes instead of sulphide based one. This is so reasonable that the gap between transition metal (Co) 3d band and O^2- 2p band will be higher, so no more overlapping problem. This is much obvious in Co^2+/3+ case that there is no overlapping. This research also opened a new road to layer-oxides world by discovery of LiCoO₂ (LCO), which will be explained in the next chapter.

Reference: “Handbook of Battery Materials” but the story and the way of describing the information belongs to me. (no cheating/no coping)