Electrical, Mechanical, and Capacity Percolation Leads to High-Performance MoS₂/Nanotube Composite Lithium Ion Battery Electrodes

Abstract

Advances in lithium ion batteries would facilitate technological developments inareas from electrical vehicles to mobile communications. While 2-dimensional systems like MoS2 are promising electrode materials due to their potentially high capacity, their poor ratecapability and low cycle-stability are severe handicaps. Here we study the electrical, mechanical and lithium storage properties of solution-processed MoS2/carbon nanotube anodes.Nanotube addition gives up to ×10 10 and ×40 increases in electrical conductivity and mechanical toughness respectively. The increased conductivity results in up to a ×100 capacity enhancement to ~1200 mAh/g (~3000 mAh/cm 3 ) at 0.1 A/g, while the improved toughness significantly boosts cycle stability. Composites with 20 wt% nanotubes combined high reversible capacity with excellent cycling stability (e.g. ~950 mAh/g after 500 cycles at 2 A/g) and high-rate capability (~600 mAh/g at 20 A/g). The conductivity, toughness and capacity scaled with nanotube content according to percolation theory while the stability increased sharply at the mechanical percolation threshold. We believe the improvements in conductivity and toughness obtained after addition of nanotubes can be transferred to other electrode materials such as silicon nanoparticles.Keywords: percolating, network, anode, mechanical 2 In recent years, lithium ion batteries (LIBs) have become the most common rechargeable power sources for portable electronic devices and electric vehicles. 1, 2 Nevertheless, they still suffer from several problems; their energy and especially power densities have not fulfilled their ultimate potential while their safety record is not unblemished. 3 A significant problem is that graphite, the dominant anode material used in LIBs, is limited by a relatively low theoretical capacity of 372 mAh/g. 4 As such, the development of the next-generation of LIBs, is expected to see the replacement of graphite-based anodes with alternative materials having higher capacity at similarly low cost. While a range of materials, including silicon, have been envisaged as future LIB anode materials, 4 of particular interest are 2-dimensional (2D) nanomaterials 5 such as graphene 6 and MoS2. 7 Over the last decade, 2D nano-materials have generated much excitement in the nanomaterials science community. [8][9][10] They come in many types including graphene, transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs). These materials consist of covalently bonded monolayers which can stack via van der Waals interactions to form layered crystals. 8,9 Such 2D nanomaterials are often found as nanosheets with lateral size ranging from 10s of nm to microns and thickness of ~nm. 9 These materials have shown potential for applications 5 in both energy generation 11 and storage. 12 In the context of LIBs, exfoliated TMDs have received significant attention as prospective anode materials. 13,14 While bulk MoS2 was proposed 15 as a Li ion battery electrode material as early as 1980 due to its hi...

Liquid Exfoliated Co(OH)2 Nanosheets as Low‐Cost, Yet High‐Performance, Catalysts for the Oxygen Evolution Reaction

“…Fitting gives the mechanical percolation threshold and exponent to be c,m=4.8vol% and nm=0.6 respectively, similar to previous reports. [52] We note that the reinforcement mechanism is in-part associated with the fact that cracking is suppressed by bridging with nanotubes (figure 3B inset).…”

“…Previously, the toughness, which is a measure of the volumetric fracture energy, has been linked with the cycling stability of battery electrodes. [52] The toughness, T, is plotted in figure 3B versus SWNT volume fraction and shows a 1000-fold improvement, characterised by a sharp increase at ~5vol%. It has been suggested [52] that such an increase coincides with the formation of a fully-formed nanotube network with the toughness increase subsequently described by percolation theory:…”

“…In general, electrochemical systems such as electrocatalysts, battery or supercapacitor electrodes experience performance saturation at high electrode thickness due to diffusion or charge transport limitations or possibly a reduction in mechanical integrity. [19,20,52] Despite the linear scaling, this work is indeed limited by problems at high electrode thickness. We found t=9 μm to be the highest thickness where we could make Co(OH)2 nanosheet films reliably without spontaneous cracking during film drying or transfer to GC.…”

“…[1] However, while graphene will certainly improve electrode conductivity, for geometrical reasons it is unlikely to enhance the mechanical properties as nanotubes do. [52] To test the effect of SWNTs on s-Co(OH)2 films, we prepared a range of SWNT/Co(OH)2 composites films. For mechanical measurements, thick, free-standing composites were made while for electrical and electrochemical measurements, thinner films were prepared and transferred onto glass and GC respectively.…”

“…[19,20,52] However, we note that in catalytic electrodes, the technologically relevant parameter is the wet (in the presence of electrolyte) out-of-plane conductivity. Based on previous results, [21] we would expect this increase in in-plane conductivity to translate to a roughly 100 increase in wet out-of-plane conductivity for the 10 wt% composite relative to the pure Co(OH)2 electrode (in the presence of electrolyte).…”