Crucially, they elucidated the mechanism behind the measured results, concluding that the BaTiO3 nanodots are able to create a special interface through which Li ions can circulate easily, even at very high charge/discharge rates.
While LIBs have proven to be very useful, it is not possible to charge them quickly enough with high currents without running into problems such as sudden decreases in cyclability and output capacity owing to their intrinsic high resistance and unwanted side reactions.
The negative effects of such unwanted reactions hinder LIBs using LiCoO2 (LCO) as a cathode material. One of them involves the dissolution of Co4+ ions into the electrolyte solution of the battery during charge/discharge cycles. Another effect is the formation of a solid electrolyte interface between the active material and the electrode in these batteries, which hinders the movement of Li ions and thus degrades performance.
In a previous research, scientists reported that using materials with a high dielectric constant, such as BaTiO3 (BTO), enhanced the high-rate performance of LCO cathodes. However, the mechanism behind the observed improvements was unclear. To shed light on this promising approach, a team of scientists from Tokyo Tech, led by Prof. Mitsuru Itoh, Dr. Shintaro Yasui and Sou Yasuhara, studied LCO cathodes with BTO applied in different ways to find out what happened at the BTO-LCO interface in more detail.
The team created three different LCO cathodes: a bare one, one coated with a layer of BTO, and one covered with BTO nanodots (below).
The team also modelled an LCO cathode with a single BTO nanodot and predicted that, interestingly, the current density close to the edge of the BTO nanodot was very high. This particular area is called the triple phase interface (BTO-LCO-electrolyte), and its existence greatly enhanced the electrical performance of the cathode covered with microscopic BTO nanodots.
As expected, after testing and comparing the three cathodes they prepared, the team found that the one with a layer of BTO dots exhibited a much better performance, both in terms of stability and discharge capacity. "Our results clearly demonstrate that decorating with BTO nanodots plays an important role in improving cyclability and reducing resistance," states Prof Itoh. Realising that the BTO dots had a crucial effect on the motility of Li ions in the cathode, the team looked for an explanation.
After examining their measurements results, the team concluded that BTO nanodots create paths through which Li ions can easily intercalate/de-intercalate, even at very high charge/discharge rates (below). This is so because the electric field concentrates around materials with a high
dielectric constant. Moreover, the formation of a solid electrolyte interface is greatly suppressed near the triple phase interface, which would otherwise result in poor cyclability. "The mechanism by which the formation of a solid electrolyte interface is inhibited near the triple phase interface is still unclear," remarks Prof Itoh.
While still much research on this topic needs to be done, the results obtained by the team are promising and might hint at a new way of greatly improving LIBs. This could be a significant step for meeting the demands of modern and future devices.
