Rescaling of metal oxide nanocrystals for energy storage having high capacitance and energy density with robust cycle life

Abstract

Nanocrystals are promising structures, but they are too large for achieving maximum energy storage performance. We show that rescaling 3-nm particles through lithiation followed by delithiation leads to high-performance energy storage by realizing high capacitance close to the theoretical capacitance available via ion-to-atom redox reactions. Reactive force-field (ReaxFF) molecular dynamics simulations support the conclusion that Li atoms react with nickel oxide nanocrystals (NiO-n) to form lithiated core-shell structures (Ni:Li 2 O), whereas subsequent delithiation causes Ni:Li 2 O to form atomic clusters of NiO-a. This is consistent with in situ X-ray photoelectron and optical spectroscopy results showing that Ni 2+ of the nanocrystal changes during lithiation-delithiation through Ni 0 and back to Ni 2+ . These processes are also demonstrated to provide a generic route to rescale another metal oxide. Furthermore, assembling NiO-a into the positive electrode of an asymmetric device enables extraction of full capacitance for a counter negative electrode, giving high energy density in addition to robust capacitance retention over 100,000 cycles.rescaled atomic clusters | metal oxide nanocrystals | energy storage | molecular dynamic simulation | in situ electrochemical spectroscopy T he most critical challenge in energy storage is maximizing capacitance along with high power density and long cycle life. High-power capacitors (1-4) are candidates to meet this challenge, and can be classified into two categories: (i) energy storage systems where charge is stored in electrochemical double layers (EDLs) (5, 6) and (ii) pseudocapacitors that store charge by redox reactions (7-14). Unfortunately, EDLs have low capacitance, whereas metal oxide pseudocapacitors lead to short cycle life. Furthermore, typical capacitors have low energy density (5, 10). In principle, the capacitances of metal oxide crystals can be fully obtained via ion-by-atom surface redox reactions. A capacitor that enables high capacitance with high energy density and long cycle life thus would represent a major breakthrough in energy storage.We synthesized metal oxide nanocrystals at a size of several nanometers on graphene, but found that rapid charging-discharging achieves only about 15% of their full capacitance. We hypothesized that reducing their sizes to the atomic clusters of subnanometer scales less than 1 nm, combined with conducting flexible graphene, would allow full redox reactions over entire constituents. Here we report that lithiation of 3-nm nickel oxide nanocrystals on graphene (NiO-n/gr) causes them to rescale down to subnanometer-scale Ni:Li 2 O-a/gr core-shell clusters and that subsequent delithiation of Ni:Li 2 O core-shell clusters leads to NiO (NiO-a/gr). We established the sequence as follows:NiO-n=gr → Ni : Li 2 O-a=gr → NiO-a=gr.We then verified this using a combination of experimental characterization with complementary reactive molecular dynamics.Moreover, we show that loading a positive electrode with NiO-a particles into an...

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“…The approach that we chose to create interparticle spacings is by electrochemical lithiation of CuO x nanoparticles. This process involves electrochemical reaction of Li + cations with CuO x , which creates atomic‐scale fractures in the particles . CuO x nanoparticles having an average size of 50 nm were selected for lithiation.…”

“…CuO x (EPRUI Nanoparticles & Microspheres Co. Ltd.) was cleaned by annealing under argon at 150 °C for 3 h before the experiments. The lithiated CuO x nanoparticles were prepared using the following procedures described in our previous work . At first, CuO x nanoparticles were mixed with 5 wt% polyvinylidene fluoride (PVDF) dispersed in N‐methyl‐2‐pyrrolidinone (NMP).…”

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