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Design and Fabrication of Microspheres with Hierarchical Internal Structure for Tuning Battery Performance

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The development of higher performance lithium ion batteries (LIBs) requires not only higher capacity materials but also their rational structuring for optimal function within the LIB... This is true for both existing intercalation compounds and next generation conversion compounds; in intercalating transition metal oxides, the particle internal structure can be used to tune the trade‐off between energy density and power. 1, 2 In conversion systems such as silicon and tin, such internal structure can be used to accommodate for their large volume expansion. 3, 4, 5 Structuring of intercalation compounds has already been the subject of extensive investigation... Materials with rather slow lithium ion diffusion, such as lithium titanate (LTO), exhibit enhanced performance with nanostructuring that does not significantly impact the electron transport in the material. 6, 7, 8, 9 Structuring generates a pore network that enables penetration of the liquid electrolyte into the particle, such that lithium ions are transported in the electrolyte and lithium ion have a shorter diffusion path in the solid... Assembling nanoparticles into micrometer‐sized spherical particles with a defined structure is a particularly attractive approach for fabricating active materials for LIBs both in terms of electrode manufacturing and electrochemical performance of the resulting cell. 10, 11, 12, 13 Such assembled nanostructured microparticles have a higher tap density than nanopowders, which results in a higher packing density of the particles in the electrodes and therefore higher volumetric energy density. 14, 15, 16 Furthermore, less polymeric binder and conductive agent is required to ensure an electrical path between the particles and the current collector compared to nanopowders... In addition, particle handling may be easier and safer in view of concerns associated with nanoparticles in industry. 17 Finally, spherically shaped microparticles are advantageous over platelets or ellipsoidal‐shaped particles in terms of decreased electrode tortuosity as shown by Ebner et al. 18 Every batch is characterized for frequency particle size distribution (PSD) using laser diffraction, specific surface area (SSA) using nitrogen adsorption, and morphology using scanning electron microscopy (SEM)... PSD and SSA results are given in Particles before calcination are shown in Figure S1 (Supporting Information)... The one hour calcination step at 750 °C does not alter the phase composition of the LTO, as shown by X‐ray diffraction (XRD) results in Figure S2 (Supporting Information)... To demonstrate the scalability of the template‐based spray drying approach, we also fabricate particles in a pilot‐scale spray dryer32, 33 capable of a production rate of 4 kg h... This is repeated until the axes do not intersect with neighboring ellipsoids or until the minimal axes size, defined beforehand by the user and corresponding to the smallest nanoparticles, is reached... Macropores or channels are also added by taking single ellipsoids away or moving them until they do not touch... Since template‐assisted spray drying is generalizable to a variety of starting nanoparticles or precursors, this approach could be applied to engineer a variety of battery materials ranging from intercalation compounds such as Li(Ni,Mn,Co)O2,25 Li(Ni,Co,Al)O2 to strongly expanding active materials such as silicon or sulfur.

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a) Experimentally measured capacities at different C‐rates for spray dried structures with no templates (red line), with 3 wt% cellulose (blue line), and 5 wt% cellulose (green line) as well as for the LTO nanoparticles (dashed gray line). b) Experimentally measured capacities at different C‐rates for spray dried structures with macroporous structures obtained with PS beads (orange line) and carbon fiber (purple line). Black points in panels (a) and (b) are from simulation of single micrometer‐sized particles with computer‐generated structures. c) Computer‐generated 3D structure of a single particle. d) Simulation of the lithium concentration in a dense (top row), nanoporous (middle row), and macroporous (bottom row) structure at the end of a galvanostatic cycles at 0.1C (left column), 1C (middle column), and 10C (right column). e) SOC distributions at the end of a galvanostatic 5C half‐cycle for a dense sphere and 1/8 of spheres with different nanoporosities. Note different SOC scale bars. Plot of final SOC versus porosity (red) and decrease in total energy density of particle (blue). e) Spatial visualization of lithiation in a structure with 41.4% porosity at different time steps during a 5C charging cycle.
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advs201500078-fig-0003: a) Experimentally measured capacities at different C‐rates for spray dried structures with no templates (red line), with 3 wt% cellulose (blue line), and 5 wt% cellulose (green line) as well as for the LTO nanoparticles (dashed gray line). b) Experimentally measured capacities at different C‐rates for spray dried structures with macroporous structures obtained with PS beads (orange line) and carbon fiber (purple line). Black points in panels (a) and (b) are from simulation of single micrometer‐sized particles with computer‐generated structures. c) Computer‐generated 3D structure of a single particle. d) Simulation of the lithium concentration in a dense (top row), nanoporous (middle row), and macroporous (bottom row) structure at the end of a galvanostatic cycles at 0.1C (left column), 1C (middle column), and 10C (right column). e) SOC distributions at the end of a galvanostatic 5C half‐cycle for a dense sphere and 1/8 of spheres with different nanoporosities. Note different SOC scale bars. Plot of final SOC versus porosity (red) and decrease in total energy density of particle (blue). e) Spatial visualization of lithiation in a structure with 41.4% porosity at different time steps during a 5C charging cycle.

Mentions: Figure3a shows the rate performance for microparticles fabricated with 0, 3, and 5 wt% cellulose in the slurry. At low cycling rates (0.1C), all materials exhibit a capacity close to the theoretical 175 mAh g−1. The improvements enabled by structuring become obvious at higher rates: at 10C, nanoporous microparticles fabricated with 5 wt% cellulose exhibit capacities of about 150 mAh g−1, while dense microparticles fabricated without a template (i.e., 0 wt% cellulose) exhibit only 100 mAh g−1. Comparing Figure 3a,b, we see that macroporous particles formed using 800 nm PS templates show similar rate performance to the nanoporous particles formed using 5% cellulose, both exhibiting capacities of about 150 mAh g−1 at 10C. The microparticles with single channels formed with carbon fiber templates also show higher capacities at higher C‐rates (140 mAh g−1 at 10C) than the dense microparticles (100 mAh g−1 at 10C). The trend in capacity improvement at high C‐rates correlates well with the measurements of SSA (Table 1): the larger the SSA of a microparticle, the greater the achieved capacity at elevated C‐rate.


Design and Fabrication of Microspheres with Hierarchical Internal Structure for Tuning Battery Performance
a) Experimentally measured capacities at different C‐rates for spray dried structures with no templates (red line), with 3 wt% cellulose (blue line), and 5 wt% cellulose (green line) as well as for the LTO nanoparticles (dashed gray line). b) Experimentally measured capacities at different C‐rates for spray dried structures with macroporous structures obtained with PS beads (orange line) and carbon fiber (purple line). Black points in panels (a) and (b) are from simulation of single micrometer‐sized particles with computer‐generated structures. c) Computer‐generated 3D structure of a single particle. d) Simulation of the lithium concentration in a dense (top row), nanoporous (middle row), and macroporous (bottom row) structure at the end of a galvanostatic cycles at 0.1C (left column), 1C (middle column), and 10C (right column). e) SOC distributions at the end of a galvanostatic 5C half‐cycle for a dense sphere and 1/8 of spheres with different nanoporosities. Note different SOC scale bars. Plot of final SOC versus porosity (red) and decrease in total energy density of particle (blue). e) Spatial visualization of lithiation in a structure with 41.4% porosity at different time steps during a 5C charging cycle.
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advs201500078-fig-0003: a) Experimentally measured capacities at different C‐rates for spray dried structures with no templates (red line), with 3 wt% cellulose (blue line), and 5 wt% cellulose (green line) as well as for the LTO nanoparticles (dashed gray line). b) Experimentally measured capacities at different C‐rates for spray dried structures with macroporous structures obtained with PS beads (orange line) and carbon fiber (purple line). Black points in panels (a) and (b) are from simulation of single micrometer‐sized particles with computer‐generated structures. c) Computer‐generated 3D structure of a single particle. d) Simulation of the lithium concentration in a dense (top row), nanoporous (middle row), and macroporous (bottom row) structure at the end of a galvanostatic cycles at 0.1C (left column), 1C (middle column), and 10C (right column). e) SOC distributions at the end of a galvanostatic 5C half‐cycle for a dense sphere and 1/8 of spheres with different nanoporosities. Note different SOC scale bars. Plot of final SOC versus porosity (red) and decrease in total energy density of particle (blue). e) Spatial visualization of lithiation in a structure with 41.4% porosity at different time steps during a 5C charging cycle.
Mentions: Figure3a shows the rate performance for microparticles fabricated with 0, 3, and 5 wt% cellulose in the slurry. At low cycling rates (0.1C), all materials exhibit a capacity close to the theoretical 175 mAh g−1. The improvements enabled by structuring become obvious at higher rates: at 10C, nanoporous microparticles fabricated with 5 wt% cellulose exhibit capacities of about 150 mAh g−1, while dense microparticles fabricated without a template (i.e., 0 wt% cellulose) exhibit only 100 mAh g−1. Comparing Figure 3a,b, we see that macroporous particles formed using 800 nm PS templates show similar rate performance to the nanoporous particles formed using 5% cellulose, both exhibiting capacities of about 150 mAh g−1 at 10C. The microparticles with single channels formed with carbon fiber templates also show higher capacities at higher C‐rates (140 mAh g−1 at 10C) than the dense microparticles (100 mAh g−1 at 10C). The trend in capacity improvement at high C‐rates correlates well with the measurements of SSA (Table 1): the larger the SSA of a microparticle, the greater the achieved capacity at elevated C‐rate.

View Article: PubMed Central - PubMed

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

The development of higher performance lithium ion batteries (LIBs) requires not only higher capacity materials but also their rational structuring for optimal function within the LIB... This is true for both existing intercalation compounds and next generation conversion compounds; in intercalating transition metal oxides, the particle internal structure can be used to tune the trade‐off between energy density and power. 1, 2 In conversion systems such as silicon and tin, such internal structure can be used to accommodate for their large volume expansion. 3, 4, 5 Structuring of intercalation compounds has already been the subject of extensive investigation... Materials with rather slow lithium ion diffusion, such as lithium titanate (LTO), exhibit enhanced performance with nanostructuring that does not significantly impact the electron transport in the material. 6, 7, 8, 9 Structuring generates a pore network that enables penetration of the liquid electrolyte into the particle, such that lithium ions are transported in the electrolyte and lithium ion have a shorter diffusion path in the solid... Assembling nanoparticles into micrometer‐sized spherical particles with a defined structure is a particularly attractive approach for fabricating active materials for LIBs both in terms of electrode manufacturing and electrochemical performance of the resulting cell. 10, 11, 12, 13 Such assembled nanostructured microparticles have a higher tap density than nanopowders, which results in a higher packing density of the particles in the electrodes and therefore higher volumetric energy density. 14, 15, 16 Furthermore, less polymeric binder and conductive agent is required to ensure an electrical path between the particles and the current collector compared to nanopowders... In addition, particle handling may be easier and safer in view of concerns associated with nanoparticles in industry. 17 Finally, spherically shaped microparticles are advantageous over platelets or ellipsoidal‐shaped particles in terms of decreased electrode tortuosity as shown by Ebner et al. 18 Every batch is characterized for frequency particle size distribution (PSD) using laser diffraction, specific surface area (SSA) using nitrogen adsorption, and morphology using scanning electron microscopy (SEM)... PSD and SSA results are given in Particles before calcination are shown in Figure S1 (Supporting Information)... The one hour calcination step at 750 °C does not alter the phase composition of the LTO, as shown by X‐ray diffraction (XRD) results in Figure S2 (Supporting Information)... To demonstrate the scalability of the template‐based spray drying approach, we also fabricate particles in a pilot‐scale spray dryer32, 33 capable of a production rate of 4 kg h... This is repeated until the axes do not intersect with neighboring ellipsoids or until the minimal axes size, defined beforehand by the user and corresponding to the smallest nanoparticles, is reached... Macropores or channels are also added by taking single ellipsoids away or moving them until they do not touch... Since template‐assisted spray drying is generalizable to a variety of starting nanoparticles or precursors, this approach could be applied to engineer a variety of battery materials ranging from intercalation compounds such as Li(Ni,Mn,Co)O2,25 Li(Ni,Co,Al)O2 to strongly expanding active materials such as silicon or sulfur.

No MeSH data available.


Related in: MedlinePlus