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 S2- 3p band. For
example, Fig. 1 displays that Co2+/3+ 3d band is overlapping with S2-
3p band. The overlapping causes introduction of holes or formation of molecular
ions (S22-).
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 O2- 2p band will be
higher, so no more overlapping problem. This is much obvious in Co2+/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)
