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 stateof-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 À1 depending on experimental factors, which is a 3Â 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. Lithium ion batteriesThe need to have better energy storage for technological applications like consumer electronics, hybrid-electric-vehicles, and remote sensing applications is propelling electrochemical devices to the forefront of research goals. 1,2 In particular, battery

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.

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.

Germanium nanoparticles (Ge-NPs) were synthesized through a one-step chemical vapor deposition process and were included in a hybrid free-standing single-wall carbon nanotube (SWCNT) electrode. The Ge-NPs were characterized through scanning electron microscopy and Raman spectroscopy to confirm the presence of crystalline nanoparticles with average diameters of 60 nm. Electrochemical testing of the Ge-NPs shows high reversible lithium ion capacity up to 900 mAh g–1 and a Coulombic efficiency of 96% on the first cycle, with capacities realizing 1000 mAh g–1 and a Coulombic efficiency of 98% on the second cycle. The use of SWCNTs to provide a stable nanoscale electrical network to support Ge-NPs resulted in a hybrid three-dimensional free-standing electrode, which is an attractive alternative to the conventional composite-current collector approach. The Ge-NP:SWCNT hybrid electrode with thin film titanium contacts produced electrode capacities of 983 mAh g–1 versus Li/Li+ up to 3 V. The higher anode capacity for the hybrid is maintained at modest cycling rates up to 1C. The pairing of the hybrid electrode with a commerical LiFePO4 cathode showed excellent performance with anode capacities of 800 mAh g–1 over a 1 V discharge range. Even at higher discharge rates, up to 1C, the anode energy density changes by only 8.5%. Thus, this demonstrates the first full battery comprising a free-standing Ge-based anode with a high power cathode exhibiting improved energy and power density.

The electrochemical cycling performance of high purity single wall carbon nanotube (SWCNT) paper electrodes has been measured vs lithium metal for a series of electrolyte solvent compositions. The addition of propylene carbonate (PC) into the conventional ethylene carbonate (EC):dimethyl carbonate (DMC) cosolvent mixture enabled a reversible lithium ion capacity of 520 mAh/g for high purity SWCNTs. The free-standing SWCNT electrode (absent of polymer binder or metal substrate support) with this electrolyte combination demonstrates enhanced cycleability, retaining >95% of the initial capacity after 10 cycles. The first cycle hysteresis, common in these materials, is shown to vary dramatically with solvent selection and illustrates the importance of the solid-electrolyte-interface (SEI) formation on SWCNT capacity. The effect of galvanostatic charge rate (i.e., C-rate) on lithium ion capacity shows a 2× improvement [in capacity per current] over reported values for conventional graphite anode materials. These electrochemical results are complemented by a postmortem analysis of the purified SWCNT electrode after lithiation using scanning electron microscopy, X-ray diffraction, Raman, and optical absorption spectroscopy. The results show that the structural integrity and carbonaceous purity of individual SWCNTs is maintained during cycling, while the lithium insertion is accommodated by bundle channel expansion.