Activation of 2D MoS2 electrodes induced by high-rate lithiation processes

Abstract

The energy storage processes of a 2D MoS 2 electrode evolve over cycling from battery (I) to pseudocapacitive (II) and to double layer capacitive (III) processes. Physical activation processes (1 and 2) are at the origin. • 2D MoS 2 nanosheets are synthesized by a scalable liquid-phase exfoliation method. • 2D MoS 2 film electrodes were produced by a scalable spray deposition technique. • Conversion processes of nanostructured 2D MoS 2 electrodes are reversible. • High-rate lithiation processes induced activation of 2D MoS 2 electrodes. • Activation induced a conversion-to-capacitive type electrode transformation. • Activated 2D MoS 2 electrodes have superior cycling stability and rate performance. MoS 2 is a highly promising material for application in lithium-ion battery anodes due to its high theoretical capacity and low cost. However, problems with a fast capacity decay over cycling, especially at the first cycles, and poor rate performance have deterred its practical implementation. Herein, electrodes comprised solely of few-layers 2D MoS 2 nanosheets have been manufactured by scalable liquid-phase exfoliation and spray deposition methods. The long-standing controversy questioning the reversibility of conversion processes of MoS 2 -based electrodes was addressed. Raman studies revealed that, in 2D MoS 2 electrodes, conversion processes are indeed reversible, where nanostructure played a key role. Cycling of the electrodes at high current rates revealed an intriguing phenomenon consisting of a continuously increasing capacity after ca. 100–200 cycles. This phenomenon was comprehensively addressed by a variety of electrochemical and microscopy methods that revealed underlying physical activation mechanisms that involved a range of profound electrode structural changes. Activation mechanisms delivered a capacitive electrode of a superior rate performance and cycling stability, as compared to the corresponding pristine electrodes, and to MoS 2 electrodes previously reported. Herein, we have devised a methodology to overcome the problem of cycling stability of 2D MoS 2 electrodes. Moreover, activation of electrodes constitutes a methodology that could be applied to enhance the energy storage performance of electrodes based on other 2D nanomaterials, or combinations thereof, strategically combining chemistries to engineer electrodes of superior energy storage properties.