Letter Cite This: Nano Lett. 2019, 19, 228−234
pubs.acs.org/NanoLett
Self-Assembly of Hybrid Nanorods for Enhanced Volumetric Performance of Nanoparticles in Li-Ion Batteries Mohammad Hadi Modarres,† Simon Engelke,†,‡ Changshin Jo,† David Seveno,§ and Michael De Volder*,† †
Department of Engineering, University of Cambridge, 17 Charles Babbage Road, Cambridge, CB3 0FS, United Kingdom Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, United Kingdom § Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 - bus 2450, B-3001 Heverlee, Belgium Downloaded via IOWA STATE UNIV on January 9, 2019 at 13:36:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: The benefits of nanosize active particles in Li-ion batteries are currently ambiguous. They are acclaimed for enhancing the cyclability of certain electrode materials and for improving rate performance. However, at the same time, nanoparticles are criticized for causing side reactions as well as for their low packing density and, therefore, poor volumetric battery performance. This paper demonstrates for the first time that self-assembly can be used to pack nanoparticles into dense battery electrodes with up to 4-fold higher volumetric capacities. Furthermore, despite the dense packing of the self-assembled electrodes, they retain a higher volumetric capacity than randomly dispersed nanoparticles up to rates of 5 C. Finally, we did not observe substential degradation in capacity after 1000 cycles, and post-mortem analysis indicates that the self-assembled structures are maintained during cycling. Therefore, the proposed self-assembled electrodes profit from the advantages of nanostructured battery materials without compromising the volumetric performance. KEYWORDS: Li-ion Battery, self-assembly, alignment, nanorods, titanium dioxide
L
poly(methyl methacrylate) (PMMA) spheres into colloidal crystals to create porous inverse opal electrodes with controlled porosity,9,10 and surfactant and polymer directed self-assembly has been used to create mesoporous metal oxides11 and ordered stacks of metal oxide−graphene nanocomposites.12 Here, we do not use self-assembly to create controlled pore structures but to increase the volumetric performance. As a model system, this paper investigates the self-assembly of hybrid reduced graphene oxide (rGO)-TiO2 nanorods. TiO2 is an abundant, low-cost, and environmentally benign material that is structurally stable during cycling and has a theoretical capacity of 335 mAhg−1,13 comparable with commercial graphite anodes (372 mAhg−1).14 Furthermore, TiO2 (B) allows for fast charging and discharging of the electrode15 due to a pseudocapacitive mechanism for lithium insertion.16 Finally, TiO2 can be synthesized into high aspect ratio nanowires17 and nanotubes,18 which is important for our self-assembly process, as discussed further on. However, TiO2 suffers from poor electrical conductivity and therefore requires special attention to the conductive additives used in the electrode.19 This is challenging here because adding classic conductive additive powders to our electrodes would prevent
i-ion batteries (LIBs) are taking an increasingly prominent role in our everyday lives, with applications ranging from pocket-size consumer electronic devices to electric vehicles.1−4 As these applications advance, so do their requirements in terms of energy and power density. One strategy, which is pursued to address these needs, is to nanostructure the active battery materials. On the one hand, nanostructuring shortens the Li-diffusion path in the active material and thus improves the rate performance and the gravimetric power density.4,5 On the other hand, nanostructuring allows for a reduction in mechanical stress during cycling, which is important for the stability of high energy density alloying and conversion battery materials such as silicon anodes.6 While nanomaterials have fostered impressive advances in the gravimetric energy and power density, this comes at the price of low volumetric capacity, which is often not reported in literature.7 The challenge here is that nanosized particles are difficult to pack densely (
