Sub-millimetre scale Van der Waals single-crystal MoTe2 for potassium storage: Electrochemical properties, and its failure and structure evolution mechanisms

Abstract Potassium-ion batteries (KIBs) are competent candidates for next-generation energy-storage systems due to the source abundance, cost efficiency, and high energy density from comparable electrode potential to lithium. However, developing practical electrode materials for KIBs is still in its infancy, and the electrochemical reaction mechanisms for a specific material are far from clear. Here, MoTe2, due to the merits of sizeable ion-intercalation interlayer spacing of Van der Waals-type material, superior electron conductivity of its metal-semimetal characteristics to wildly studied MoS2, was for the first time investigated as the working electrode for potassium storage. The potassiation/depotassiation mechanisms were unravelled, combining electrochemical analysis, ex-situ scanning electron microscope (SEM), ex-situ transmission electron microscope (TEM) and in-situ X-ray diffraction. Sub-millimetre single-crystal MoTe2 displayed a high volumetric capacity of 792.4 mAh cm−3 at initial depotassiation and maintained a gravimetric specific capacity of 210 mAh g−1 at 100 mA g−1. It decayed rapidly after 25 cycles caused by the deactivation of active electrode material from the irreversible crystalline cracking and structure evolution during the electrochemical cycling. At initial potassiation, 2H-MoTe2 was irreversibly converted to 1T-MoTe2 and then further converted to potassium telluride. And at initial depotassiation over 2.5 V (vs. K/K+), a tentative K-Mo-Te compound with R-3H Cs4Mo18Te20 structure was formed and soon irreversibly converted to K2Te3 under following potassiation. Meanwhile, the stepwise reversible conversion of K2Te3-KTe-K5Te3 predominates the continuous electrochemical processes after the initial discharge/charge. Apart from the potential application for potassium-ion batteries, the conversion mechanisms amongst potassium tellurides also provide instructions for upcoming potassium-tellurium batteries.