(630g) Carbonization of Poly(vinylpyrrolidone) Thin Films for Fabricating Lithium Ion Anodes Via Initiated Chemical Vapor Deposition

Traditional lithium ion batteries face a tradeoff in improving gravimetric capacity and specific power; increasing one typically decreases the other. This is due to the planar sandwich structure of the battery electrodes, where increasing the electrode loading with a thicker electrode to increase capacity decreases power output as a result of increasing the lithium ion transport distance (and vice versa). A proposed solution to overcome this tradeoff is the 3D battery architecture, where the third dimension (height) of the battery’s geometry is taken advantage of to increase electrode loading (increase cell capacity) while maintaining thinner electrodes (increase power output). However, several proposed 3D geometries call for processing that is capable of depositing a conformal, uniform electrode material onto a 3D substrate that acts as the current collector.¹ In this work, we demonstrate the use of initiated chemical vapor deposition (iCVD) to deposit (poly)vinylpyrrolidone (PVP) onto a substrate, followed by preoxidation and subsequent carbonization to fabricate a carbon anode. iCVD is capable of growing conformal, uniform, pristine polymer films onto a wide variety of substrates without using liquid solvents. Polymers have been grown onto flexible substrates such as metal foils and nanoporous substrates such as solar cell electrodes², making iCVD a promising candidate for fabricating a 3D battery anode. While polyacrylonitrile (PAN) is a popular carbon precursor for fabricating carbon anodes, PVP was chosen as it is non-toxic, environmentally-friendly, and significantly cheaper than PAN. Moreover, its significantly lower volatility allows iCVD, which is typically an adsorption-limited deposition process, to achieve reasonable polymer deposition rates. Most work on carbon anodes have focused on electrospinning free-standing polymer fibers and carbonizing them to study their gravimetric capacity. However, the deposition of PVP that is affixed onto a substrate presents novel questions on the effect of the substrate on the carbonized film, particularly in understanding the role of the substrate-polymer interface on preoxidation and carbonization, and its impact on the final carbonized film geometry and properties.

Method and Results

This work studies the effect of the underlying substrate on the final carbonized PVP film deposited by iCVD. The monomer N-vinylpyrrolidone (VP) and initiator tert-butyl peroxide (TBPO) were fed to an iCVD reactor at 1 and 0.5 sccm (standard cubic centimeters per minute) respectively to achieve a 2:1 monomer to initiator flow ratio. The substrate, of which both a silicon wafer and titanium foil were used, was kept cool at 25 ºC. The reactor pressure was set to 130 mTorr to maintain a monomer saturation ratio (P_m/P_sat) of 0.7 to achieve a deposition rate of 120 nm/min. After 10 min, a film of 1.2 µm thickness was obtained, which was confirmed by a stylus profilometer, and the mass of the deposited film on a 25 mm square surface was approximately 0.6 mg. Following the iCVD deposition, the samples were preoxidized in air in a tube furnace. The ramp rate was set to 60 ºC/min as a fast preoxidation ramp was found to increase the internal BET surface area.³ The preoxidation temperature was set to 360 ºC and held for 1–4 h. The furnace was allowed to cool back to room temperature before the sample was removed. Upon evaluating the preoxidized carbon film, the shrinkage of the film in the planar (xy) direction was found to be fairly small (approximately 2 mm shrinkage for each side) on both the silicon and titanium substrates. The film mass decreased from 0.6 mg to 0.1 mg, and the profilometry analysis revealed that the preoxidation step shrunk the film thickness (z) from 1.2 µm to 120 nm. Following the preoxidation step, the substrates were carbonized under an inert atmosphere with nitrogen gas flowing at 100 mL/min. The ramp rate was set to 20 ºC/min, and the carbonization temperature was set to 800 ºC and held for 1–4 h. Upon evaluating the carbonized film, the shrinkage of the film in the planar direction was still insignificant at approximately 2 mm in all four directions. The film thickness shrunk from 120 nm to 12 nm and the carbon mass was approximately 0.02 mg. The duration of the preoxidation and carbonization steps (i.e. 1 hour or 4 hours long for each) had no impact on both the mass loss and the film shrinkage in all three dimensions.

Implications and Future Work

This work shows that iCVD can be used to fabricate a carbon anode for both 2D and 3D batteries. At the current scale, the carbon anodes produced by iCVD are suitable for miniature devices such as pacemakers. In such devices, the battery comprises of the majority of the device’s mass and volume. 3D batteries allow for greater capacity and power output to be contained within a smaller volume, which allows the battery to be shrunk without compromising performance. In turn, this allows miniature devices to be shrunk even further. More broadly, the development of 3D batteries can further aid in the development of electric vehicles and grid storage, where high capacity, high power output, and fast charging are simultaneously sought after. Additional work is currently underway to characterize the produced carbon to determine its morphology and electrical conductivity, and to study the effect of the preoxidation temperature on the final carbon film. In addition to depositing conformal and uniform polymer films onto 2D and 3D substrates, iCVD has been previously shown to selectively deposit polymer films onto patterned metal surfaces based on the monomer wettability differences among the surfaces. Future work will involve directly growing 3D carbon anodes using this patterning process, thereby eliminating the need for a pre-fabricated 3D substrate, which in turn will further decrease the size and mass of the battery.

References

1. Arthur, T. S.; Bates, D. J.; Cirigliano, N.; Johnson, D. C.; Malati, P.; Mosby, J. M.; Perre, E.; Rawls, M. T.; Prieto, A. L.; Dunn, B. Three-Dimensional Electrodes and Battery Architectures. MRS Bulletin 2011, 36 (7), 523–531. https://doi.org/10.1557/mrs.2011.156.

2. Nejati, S.; Lau, K. K. S. Integration of Polymer Electrolytes in Dye Sensitized Solar Cells by Initiated Chemical Vapor Deposition. Thin Solid Films 2011, 519 (14), 4551–4554. https://doi.org/10.1016/j.tsf.2011.01.292.

3. Wang, P.; Zhang, D.; Ma, F.; Ou, Y.; Chen, Q. N.; Xie, S.; Li, J. Mesoporous Carbon Nanofibers with a High Surface Area Electrospun from Thermoplastic Polyvinylpyrrolidone. Nanoscale 2012, 4 (22), 7199. https://doi.org/10.1039/c2nr32249h.