High-Energy and High-Power Lithium-Ion Cells Enabled by Electrochemically Derived Carbon Nanotubes

Abstract

A common approach to increasing packaged cell level energy density is to increase the mass fraction of active materials and decrease that of inactive materials such as conductive and polymeric additives in the electrodes, current collectors, and separators. To increase active materials, thick electrodes have been proposed and studied in the literature as a method for increasing the mass ratio of active materials to current collector foils. However, there are many challenges associated with increasing the thickness of cathodes. Notably, most cathode materials are inherently insulating and therefore require conductive additives to maintain electrical conductivity to facilitate charge transfer from the electrolyte-cathode interface back to the current collector. The length of this pathway increases with thicker cathodes and requires reconsideration of the conductive materials used to form this network. One dimensional (1D) CNTs have been demonstrated recently as superior conductive additives due to their line-to-line contact method and superior active-material surface coverage, which allows them to create a 3D conductive network at a low mass fraction of the electrode. In extreme conditions of battery operations, performance is limited by the inactive materials, rather than the active energy-storing materials in a cell. This holds true for thick coatings, as well as high rates of charge and discharge. Switching from traditional carbon blacks (CB) such as acetylene or furnace black to CNT additives allows for simultaneous improvements in thick electrodes which increase packaged cell-level energy density, as well as increased performance in extreme fast charging environments. SkyNano is commercializing a highly tunable molten-salt electrolysis process to make low-cost carbon nanotubes (CNTs) using electrochemical capture and conversion of atmospheric CO2 utilizing several highly scalable processes. This technology can be harnessed to produce carbon additives with precisely tuned properties to enable ultra-thick cathode coatings with high areal loadings that exhibit superior rate performance.Enabling the widespread commercialization of high-energy, advanced batteries with low cost for transportation electrification, renewable energy storage, and portable electronics will result in substantially reduced GHG emissions, increased mobility, and ubiquitous opportunity for personal connectivity. In addition SkyNano produces its CNTs through the consumption of ambient or flowing CO2 streams, which will contribute to slowing the increase in CO2 emissions and ultimately reducing the current atmospheric content of 420 ppm. CNTs provide many commercial advantages in advanced batteries: 1) using thicker, higher-energy electrodes by minimizing electronic conductivity losses; 2) higher power density (i.e., higher capacity at high discharge rates); 3) integrating multilayer electrode architectures; 4) higher loadings of Si at the anode by forming a more mechanically stable composite with graphite during electrochemical cycling; and 5) faster charging rates approaching 6C. In addition, SkyNano CNTs will be less expensive than current chemical vapor deposited CNTs and optimized for a both multilayer (thick) electrode and all-solid-state battery approaches. The proposed electrode designs rely on the use of conventional electrolytes, widely available active materials, and conventional electrode manufacturing approaches and require no CapEx to integrate into current manufacturing lines.Electrochemical performance data (rate capability, AC impedance, capacity retention during cycling, etc.) is discussed from both molten-salt electrolysis reactor and LIB operation points of view. Single unit-cell (“single-layer”) pouch cells were constructed (five replicates for each cell configuration) using three distinct cathode coatings, which had an average rated capacity of 134 ± 10 mAh. The cathode areal capacities were 3.1 mAh/cm2, and the anodes were 3.4 mAh/cm2, giving an N:P ratio of 1.1. It was found that both formulations F7 and F8 outperformed the Control 2 at all discharge rates above 0.5C. The scaled F7 pouch cells performed better than their half coin cell counterparts and significantly outperformed Control 2 (20.5%, 62.7%, and 46.7% capacity improvements at 2C, 3C, and 5C discharge rates, respectively), showing that only a small amount of CNTs is required to realize large improvements in cathode rate capability.