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continUous flow ReaCtor for Hierarchically desIgned Nanocomposites

Periodic Reporting for period 1 - URCHIN (continUous flow ReaCtor for Hierarchically desIgned Nanocomposites)

Reporting period: 2018-03-01 to 2020-02-29

Because of their high energy and power density, lithium ion batteries (LIBs) are currently the most promising energy storage technology for mobile electronic devices and electric vehicles. To increase the energy density of LIBs, relentless research efforts are invested in the development of new electrode materials and optimizing the electrode formulation. New anode materials that allow for alloying or conversion energy storage mechanisms (e.g. Si, Sn, GeO2, SnO2, Fe3O4, etc.) offer high capacities but have disadvantages including poor cycling stability, large volume change during Li+ insertion/extraction, high voltage hysteresis, poor rate performance, poor coulombic efficiencies (CE) and low electrical conductivity.

Conductive carbons additives such as graphene and carbon nanotubes (CNTs) have been applied as additive materials for high capacity materials. However, these materials tend to cause side reactions and phase segregate after slurry mixing. This phase segregation during the electrode coating and drying, compromises electron conduction and results in poor material utilization and high ohmic losses. The phase segregation is particularly pronounced with high areal loading electrodes pursued in industry, which dry slower. Further, several researchers have shown that the interface between carbon additives and the active material can degrade over time, which further accentuates the above problems. Industrially, time consuming kneading and high intensity mixing processes are used to reduce the agglomeration of carbon additives, and academically, methods are developed to anchor active battery materials on the surface of carbon additives (e.g. by synthesizing the active material in the presence of CNTs or graphene). However, to nucleate the active material on the carbon additives, the carbon surface typically requires oxidation or other chemical modifications, which in turns decreases their electronic conductivity and is poorly scalable.

In this work we address several of the above challenges by fabricating advanced Silicon / Iron Silicide anodes coated with CNTs. In particular, we developed a continuous fabrication process that structures nanoparticles into secondary micrometer sized particles on which CNTs are synthesized. Here, we use spray drying to pack our Si nanoparticles into micrometer-sized spheres. The electrical resistance of micrometer-sized pure Si secondary particles would be too high for good battery operation, but this work shows that CNTs can be synthesized inside the crevices of the secondary particles hereby providing a good internal short range electrical network. In addition, CNTs extending from the surface of the secondary particles can help with the inter-particle long-range conductivity, which is particularly important as industry shifts to thicker electrode coatings.
At the begining of this project, we tried to prepare different reactors and synthesis processes for Si-CNT composite structures. Two synthesis routes were followed. In the first approach, a batch process is used where the secondary particles are first spray dried on a commercial tool (ESDT1 Lab Spray Dryer) and are then transferred to a batch CVD furnace for CNT synthesis. This process allows for efficient screening of different material parameters. In a second approach, the continuous flow process is implemented.

In these processes, we initially form secondary clusters of iron nitrate and Si particles with an average diameter of ~ 3 µm. CNTs are synthesized both inside and outside the pores of the secondary particle and the length and amount of CNTs were controlled by changing the synthesis temperature and time. After CNT growth, some portion of the Si phase was converted to iron silicide, resulting in the formation of Silicon/iron silicide/CNT composites.

Using a semi-continuous flow synthesis process (spray drying + CVD treatment), we could synthezied Si-CNT particles in gram scale in a day. Spray drying in our set-up is highly scalable, resulting in the synthesis of cluster particles in kg scale in a day. By applying large sized CVD furnace, this process can achieve kg scale synthesis. The resulting particles (Si-CNT) were tested as anode materials in LIB system. Half-cell test using Si-CNT anode exhibited much higher charge-discharge efficiency, reversible capacity, and stable cycle stability.

We also used the Si-CNT particles as an additive to improve the performance of traditional graphite electrode. By mixing conventional graphite with Si-CNT particles, we successfully synthesized graphite/Si-CNT blending electrode. The target capacity was 500 mAh/g and electrode density was adjusted to 1.6 g/cc. To get same electrode capacity with graphite, which has 350 mAh/g, with same electrode density, our blending electrode needs only 7 mg/cm2. So, we can decrease the thickness of electrode by 30%. To emphasize scalable process, we used a pilot scale roll to roll coating to manufacture a 35 m long electrode. Compared with commercial graphite and commercial silicon nanoparticle blending electrode, graphite with silicon CNT electrode shows excellent cycle stability without increasing overpotential.
We reported a scalable process to fabricate Si-CNT secondary particles for LIB anodes that can be used either by themselves or blend with graphite. These particles are fabricated by spray-drying primary Si particles (~50 nm diameter) into spherical secondary particles (~3 µm diameter) to improve their packing density and reduce the amount of binder needed in the electrode formulation. Next, CNTs are grown directly from the surface of the Si particles using CVD and we found that the nanotubes are formed both inside and outside of the secondary particles leading to better electron transport. Pure SiFeCNT electrodes show a capacity of over 1150 mAh/g after 300 cycles at 1A/g and retain over 43% of their capacity at a rate of 5C. In addition, we present blend electrodes where SiFeCNT is mixed with graphite to obtain a 550 mAh/g anode. These were calendared to obtain industrially relevant loading levels (>6.5 mg/cm2 and >1.5 g/cc) and we demonstrated a large area, 35 meter long electrode manufactured using a continuous roll-to-roll coating tool.

This project devleoped synthetic process for complicated nanocomposite structures consisting of Si and CNT materials. The resulting materials showed interesting nanostructure and excellent battery performance as an anode in LIBs. The result in this project has been published in ACS Nano journal (ACS Nano, 2020, 14, 1, 698-707). Becuase we developed highly scalable process for nanocomposite structured microparticles, the methods can be applied in conventional synthesis reactors.

In addition, the processes learned in thiw project were also applied to structure other advanced battery materials which were published in ACS Appl. Mater. Interfaces 2020, 12, 18803-12. And they were applied in other advanced battery systems published in Nano Letters, 2020, doi.org/10.1021/acs.nanolett.0c01958
Schematic for this project
Microscopy images of materials in this project
charge discharge profiles of graphite-Si-CNT electrode and graphite-commercial Si electrode