Matthew J. Ganter

Carbon nanotubes for lithium ion batteries

Lithium ion batteries are receiving considerable attention in applications, ranging from portable electronics to electric vehicles, due to their superior energy density over other rechargeable battery technologies. However, the societal demands for lighter, thinner, and higher capacity lithium ion batteries necessitate ongoing research for novel materials with improved properties over that of state-of-the-art. Such an effort requires a concerted development of both electrodes and electrolyte to improve battery capacity, cycle life, and charge–discharge rates while maintaining the highest degree of safety available. Carbon nanotubes (CNTs) are a candidate material for use in lithium ion batteries due to their unique set of electrochemical and mechanical properties. The incorporation of CNTs as a conductive additive at a lower weight loading than conventional carbons, like carbon black and graphite, presents a more effective strategy to establish an electrical percolation network. In addition, CNTs have the capability to be assembled into free-standing electrodes (absent of any binder or current collector) as an active lithium ion storage material or as a physical support for ultra high capacity anode materials like silicon or germanium. The measured reversible lithium ion capacities for CNT-based anodes can exceed 1000 mAh g−1 depending on experimental factors, which is a 3× improvement over conventional graphite anodes. The major advantage from utilizing free-standing CNT anodes is the removal of the copper current collectors which can translate into an increase in specific energy density by more than 50% for the overall battery design. However, a developmental effort needs to overcome current research challenges including the first cycle charge loss and paper crystallinity for free-standing CNT electrodes. Efforts to utilize pre-lithiation methods and modification of the single wall carbon nanotube bundling are expected to increase the energy density of future CNT batteries. Other progress may be achieved using open-ended structures and enriched chiral fractions of semiconducting or metallic chiralities that are potentially able to improve capacity and electrical transport in CNT-based lithium ion batteries.

Prelithiation of Silicon–Carbon Nanotube Anodes for Lithium Ion Batteries by Stabilized Lithium Metal Powder (SLMP)

Stabilized lithium metal powder (SLMP) has been applied during battery assembly to effectively prelithiate high capacity (1500-2500 mAh/g) silicon-carbon nanotube (Si-CNT) anodes, eliminating the 20-40% first cycle irreversible capacity loss. Pressure-activation of SLMP is shown to enhance prelithiation and enable capacity matching between Si-CNT anodes and lithium nickel cobalt aluminum oxide (NCA) cathodes in full batteries with minimal added mass. The prelithiation approach enables high energy density NCA/Si-CNT batteries achieving >1000 cycles at 20% depth-of-discharge.

Economic and environmental characterization of an evolving Li-ion battery waste stream.

While disposal bans of lithium-ion batteries are gaining in popularity, the infrastructure required to recycle these batteries has not yet fully emerged and the economic motivation for this type of recycling system has not yet been quantified comprehensively. This study combines economic modeling and fundamental material characterization methods to quantify economic trade-offs for lithium ion batteries at their end-of-life. Results show that as chemistries transition from lithium-cobalt based cathodes to less costly chemistries, battery recovery value decreases along with the initial value of the raw materials used. For example, manganese-spinel and iron phosphate cathode batteries have potential material values 73% and 79% less than cobalt cathode batteries, respectively. A majority of the potentially recoverable value resides in the base metals contained in the cathode; this increases disassembly cost and time as this is the last portion of the battery taken apart. A great deal of compositional variability exists, even within the same cathode chemistry, due to differences between manufacturers with coefficient of variation up to 37% for some base metals. Cathode changes over time will result in a heavily co-mingled waste stream, further complicating waste management and recycling processes. These results aim to inform disposal, collection, and take-back policies being proposed currently that affect waste management infrastructure as well as guide future deployment of novel recycling techniques.

Recycling single-wall carbon nanotube anodes from lithium ion batteries

Large scale incorporation of nanomaterials into industrial systems and commercial products is relatively new, and therefore little attention has been given for options when these products reach their end-of-life. During the course of this study, the ability to recycle end-of-life (EOL) single-wall carbon nanotubes (SWCNTs), recovered from lithium ion battery electrodes, was investigated. Specifically, SWCNT–Li+ coin cells were forced to their EOL though extended cycling at high charge rates and recycled using a series of acid and thermal treatments originally developed for the purification of as-produced SWCNT material. The recycling treatments were successful in removing the EOL byproducts (e.g. solid electrolyte interphase, lithium) and upgrading the SWCNT material to its pre-cycling functionality. The material was characterized at each step in the recycling process through a combination of scanning electron microscopy, thermogravimetric analysis, Raman spectroscopy, and optical absorption spectroscopy. The energy required for each of the recycling procedures was measured and compared to the energy of SWCNT synthesis. The recycled-SWCNT material was successfully incorporated into Li+ battery coin cells with insertion and extraction capacities of 650 mA h g−1, comparable to the virgin pure-SWCNT electrodes. Therefore, the ability to refunctionalize “used” SWCNTs from a device, through chemical processing, to their initial purity and functionality has been demonstrated. The direct energy required to refunctionalize the SWCNTs was measured and is less than half of the direct energy required to synthesize new material. Thus, the ability to preserve the nanoscale properties of SWCNTs with reduced impact offers new opportunities for end-of-life management.

Germanium–single-wall carbon nanotube anodes for lithium ion batteries

High-capacity thin-film germanium was coupled with free-standing single-wall carbon nanotube (SWCNT) current collectors as a novel lithium ion battery anode. A series of Ge–SWCNT compositions were fabricated and characterized by scanning electron microscopy and Raman spectroscopy. The lithium ion storage capacities of the anodes were measured to be proportional to the Ge weight loading, with a 40 wt% Ge–SWCNT electrode measuring 800 mAh/g. Full batteries comprising a Ge–SWCNT anode in concert with a LiCoO2 cathode have demonstrated a nominal voltage of 3.35 V and anode energy densities 3× the conventional graphite-based value. The higher observed energy density for Ge–SWCNT anodes has been used to calculate the relative improvement in full battery performance when capacity matched with conventional cathodes (e.g., LiCoO2, LiNiCoAlO2, and LiFePO4). The results show a >50% increase in both specific and volumetric energy densities, with values approaching 275 Wh/kg and 700 Wh/L.

A life-cycle energy analysis of single wall carbon nanotubes produced through laser vaporization

The energy consumed to produce and purify a kilogram of laser vaporization SWCNTs at the laboratory scale was found to be 0.13–0.19 GWh/kg under standard production conditions, with 0.114 GWh/kg coming from electrical energy. Most of the energy consumption results from thermal and resistive losses of the laser and single zone furnace in use. The time and temperature of final oxidation of the material was found to greatly affect purity while only slightly affecting the energy consumed. Although the energy consumption for laser vaporization is comparable to other synthesis processes, scale-up of production from laboratory to manufacturing rates may result in substantial efficiency gains. Additional energy savings may be realized from the capture and recovery of gases, solvents and other materials that are currently discarded.

Advanced germanium nanoparticle composite anodes using single wall carbon nanotube conductive additives

The morphology, thermal stability, impedance, and rate performance of germanium nanoparticle (Ge-NP) based lithium ion battery electrodes that incorporate single-walled carbon nanotube (SWCNT) conductive additives has been systematically studied for varying SWCNT loadings (1–3% w/w SWCNT) and electrode areal capacities (4–12 mA h cm−2). Scanning electron microscopy (SEM) was used to characterize the surface coverage for carbon black and SWCNT conductive additives. Differential scanning calorimetry (DSC) analysis shows a 30% reduction in exothermic release with SWCNT conductive additives, which demonstrates improved thermal stability for Ge-NP electrodes. Electrochemical impedance spectroscopy (EIS) indicates that the charge transfer impedance can be reduced roughly 2.5× when comparing 5% carbon black to ≤3% SWCNT conductive additive. Electrochemical cycling and rate testing demonstrate that SWCNT conductive additives provide significantly improved specific capacities (1100 mA h g−1 with 1% SWCNT) and rate performance (80% capacity retention at effective 1 C rate) over traditional carbon black conductive additives when using Ge-NP active material. In addition to the benefits for thermal stability, impedance, and rate performance, predicted energy density gains from Ge-NP anodes can be up to 20–25% in full batteries.

Rechargeable lithium-ion cell state of charge and defect detection by in-situ inside-out magnetic resonance imaging

When and why does a rechargeable battery lose capacity or go bad? This is a question that is surprisingly difficult to answer; yet, it lies at the heart of progress in the fields of consumer electronics, electric vehicles, and electrical storage. The difficulty is related to the limited amount of information one can obtain from a cell without taking it apart and analyzing it destructively. Here, we demonstrate that the measurement of tiny induced magnetic field changes within a cell can be used to assess the level of lithium incorporation into the electrode materials, and diagnose certain cell flaws that could arise from assembly. The measurements are fast, can be performed on finished and unfinished cells, and most importantly, can be done nondestructively with cells that are compatible with commercial design requirements with conductive enclosures.The development of noninvasive methodology plays an important role in advancing lithium ion battery technology. Here the authors utilize the measurement of tiny magnetic field changes within a cell to assess the lithiation state of the active material, and detect defects.

Single Wall Carbon Nanotube – LiCoO 2 Lithium Ion Batteries

The electrochemical cycling performance of high purity single wall carbon nanotube (SWCNT) paper electrodes has been measured for a series of electrolyte solvent compositions. The effects of varying the galvanostatic charge rate and cycling temperature on lithium ion capacity have been evaluated between 25-100 °C. The measured reversible lithium ion capacities for SWCNT anodes range from 600-1000 mAh/g for a 1M LiPF 6 electrolyte, depending on solvent composition and cycling temperature. The solid-electrolyte-interface (SEI) formation and first cycle charge loss are also shown to vary dramatically with carbonate solvent selection and illustrate the importance of solvent alkyl chain length and polarity on SWCNT capacity. SWCNT anodes have also been incorporated into full battery designs using LiCoO 2 cathode composites. An electrochemical pre-lithiation sequence, prior to battery assembly, has been developed to mitigate the first cycle charge loss of SWCNT anodes. The pre-lithiated SWCNT anodes show reversible cycling at varying charge rates and depths of discharge with the cathode system. The summary of data shows that the structural integrity of individual SWCNTs is preserved after cycling, and that free-standing SWCNT paper electrodes represent an attractive material for lithium ion batteries.

Carbon nanotube wires with continuous current rating exceeding 20 Amperes

A process to fabricate carbon nanotube (CNT) wires with diameters greater than 1 cm and continuous current carrying capability exceeding 20 A is demonstrated. Wires larger than 5 mm are formed using a multi-step radial densification process that begins with a densified CNT wire core followed by successive wrapping of additional CNT material to increase the wire size. This process allows for a wide range of wire diameters to be fabricated, with and without potassium tetrabromoaurate (KAuBr4) chemical doping, and the resulting electrical and thermal properties to be characterized. Electrical measurements are performed with on/off current steps to obtain the maximum current before reaching a peak CNT wire temperature of 100 °C and before failure, yielding values of instantaneous currents in excess of 45 A for KAuBr4 doped CNT wires with a diameter of 6 mm achieved prior to failure. The peak temperature of the wires at failure (∼530 °C) is correlated with the primary decomposition peak observed in thermal gravimetric analysis of a wire sample confirming that oxidation is the primary failure mode of CNT wires operated in air. The in operando stability of doped CNT wires is confirmed by monitoring the resistance and temperature, which remain largely unaltered over 40 days and 1 day for wires with 1.5 mm and 11.2 mm diameters, respectively. The 100 °C continuous current rating, or ampacity, is measured for a range of doped CNT wire diameters and corresponding linear mass densities ρL. To describe the results, a new form of the fuse-law, where the critical current is defined as I∝ρL3/4, is developed and shows good agreement with the experimental data. Ultimately, CNT wires are shown to be stable electrical conductors, with failure current densities in excess of 50 A in the case of a convectively cooled 11.2 mm doped CNT wire, and amenable for use in applications that have long-term, high-current demands.A process to fabricate carbon nanotube (CNT) wires with diameters greater than 1 cm and continuous current carrying capability exceeding 20 A is demonstrated. Wires larger than 5 mm are formed using a multi-step radial densification process that begins with a densified CNT wire core followed by successive wrapping of additional CNT material to increase the wire size. This process allows for a wide range of wire diameters to be fabricated, with and without potassium tetrabromoaurate (KAuBr4) chemical doping, and the resulting electrical and thermal properties to be characterized. Electrical measurements are performed with on/off current steps to obtain the maximum current before reaching a peak CNT wire temperature of 100 °C and before failure, yielding values of instantaneous currents in excess of 45 A for KAuBr4 doped CNT wires with a diameter of 6 mm achieved prior to failure. The peak temperature of the wires at failure (∼530 °C) is correlated with the primary decomposition peak observed in thermal gra...