Carbon Nanotube-Enabled 300 Wh/Kg Lithium Ion Batteries

Alternative active materials and electrode designs for lithium ion batteries are under investigation to meet increasing energy storage demands.  Lithium ion battery electrode designs employing carbon nanotubes (CNTs) have recently demonstrated increased battery energy densities through use as a conductive additive and as a current collector replacement.  CNTs as current collectors provide a flexible, lightweight, conductive structure to effectively support high capacity, nanostructured anode active materials like Si and Ge.  Since nanostructured anodes constructed with these materials exhibit large irreversible first cycle capacity loss, Stabilized Lithium Metal Powder has been effectively used to pre-lithiate both Si-CNT freestanding and Ge-NP anodes.  Application of these pre-lithiated anodes can allow for the intended capacity to be realized in full batteries when paired with a traditional cathode.  An example of Si-CNT freestanding anodes paired with NCA cathodes has shown successful capacity matching, and has achieved over 1800-90 minute cycles at 20% depth of discharge.  CNTs as conductive additives can enhance the loading (thickness and areal capacity) of composite coatings, rate capability, and thermal stability of electrodes with both traditional and experimental active materials.  An alternative candidate anode composite consisting of germanium nanoparticles (Ge-NP) and single wall carbon nanotube (SWCNT) conductive additives demonstrates significantly improved specific capacities (1200 mAh/g with 2% SWCNT) and rate performance (80% capacity retention at an effective 1C rate) over composites with traditional carbon black conductive additives.  Differential scanning calorimetry analysis also shows a 30% reduction in exothermic release when using SWCNT conductive additives, which demonstrates improved thermal stability for Ge-NP anodes with SWCNT conductive additives compared to those with traditional carbon black conductive additives.  SWCNT additives have also been used with high capacity lithium-rich cathode composites to achieve significant benefits in terms of rate capability (i.e. improving capacity retention at 1C by more than 50% over conventional carbon black additives).  Proper capacity matching of the high capacity Ge-NP anodes with a high capacity cathode has shown the ability to fabricate full batteries with increased energy density over current lithium ion batteries.  Since the high capacity lithium-rich cathode and Ge-NP anode active materials exhibit similar first cycle irreversible capacity losses of ~20%, direct matching was used to eliminate the need for pre-lithiation.  These results illustrate a novel demonstration of managing capacity loss as a viable pathway to combine active materials with first cycle loss.  Prototype coin cells using matched composites have exceeded 1450-90 minute cycles at 20% depth of discharge.  In addition, pouch cells comprising four electrode pairs (with electrode areal capacities of 14 mAh/cm²) have achieved cell energy densities of 307 Wh/kg.  Overall, advanced SWCNT additives provide benefits for electrode thermal stability and rate performance, which can also lead to energy density gains for batteries with high areal capacity electrodes.