There is an ever-increasing demand for electrochemical power sources for electronics, electric vehicles and stationary storage, where currently Li-ion batteries (LIBs) dominate the market.  However, the scarcity of lithium minerals causes a serious concern on LIBs’ sustainability in the decades to come, which motivates researchers to look for alternative technologies.  To date, several Earth-abundant metal-ion batteries are under intense scrutiny, including Na-ion batteries (NIBs), Mg-ion batteries (MIBs) and Al-ion batteries (AIBs), where these technologies may provide sustainable solutions.  However, exceedingly little attention has been paid to K-ion batteries (KIBs) despite the fact that potassium is nearly 1000 times more abundant than lithium in Earth’s crust.  For KIBs, besides low cost, another advantage is its excellent adaptability to the existing carbon industry already developed for Li-ion batteries.  Recently, we reported that both graphite and soft carbon can be used as anodes for KIBs with remarkable properties, where such materials exhibit very poor performance in NIBs.  This is a significant advantage for KIBs over NIBs, which makes it very much urgent for the KIB field to develop more suitable cathode materials for a full cell design, where PTCDA is highly promising in filling up this gap.

We demonstrate that PTCDA is a promising cathode for K-ion batteries with a high capacity of about 122 mAh/g at 20 mA/g and moderate rate/cycling performance.  Upon potassiation, PTCDA crystals experience a high level of amorphization, while upon depotassiation, the crystal structure is partially restored, revealed by ex situ XRD and IR.  We will discuss the mechanism for the different performance between KIBs and NIBs.  As shown in Figure 1, greater polarization between the discharge and charge profiles of PTCDA is observed in KIB than in NIB, which may be attributed to larger strain when intercalating of K ions into PTCDA and the associated slower diffusion of K ions.

Fig. 1. The comparison of galvanostatic charge-discharge potential profiles of the PTCDA electrode in KIBs and NIBs, (a) the first cycle and (b) the second cycle.

We hope to present our results to bring attention to organic crystals that have intrinsic large interstitial sites for applications in Earth-abundant KIBs.  The very different results of PTCDA in KIBs from in NIBs bring insights as well as questions that may attract further attention on this topic.