W. Peter Kalisvaart

High Density Sodium and Lithium Ion Battery Anodes from Banana Peels

Banana peel pseudographite (BPPG) offers superb dual functionality for sodium ion battery (NIB) and lithium ion battery (LIB) anodes. The materials possess low surface areas (19-217 m(2) g(-1)) and a relatively high electrode packing density (0.75 g cm(-3) vs ∼1 g cm(-3) for graphite). Tested against Na, BPPG delivers a gravimetric (and volumetric) capacity of 355 mAh g(-1) (by active material ∼700 mAh cm(-3), by electrode volume ∼270 mAh cm(-3)) after 10 cycles at 50 mA g(-1). A nearly flat ∼200 mAh g(-1) plateau that is below 0.1 V and a minimal charge/discharge voltage hysteresis make BPPG a direct electrochemical analogue to graphite but with Na. A charge capacity of 221 mAh g(-1) at 500 mA g(-1) is degraded by 7% after 600 cycles, while a capacity of 336 mAh g(-1) at 100 mAg(-1) is degraded by 11% after 300 cycles, in both cases with ∼100% cycling Coulombic efficiency. For LIB applications BPPG offers a gravimetric (volumetric) capacity of 1090 mAh g(-1) (by material ∼2200 mAh cm(-3), by electrode ∼900 mAh cm(-3)) at 50 mA g(-1). The reason that BPPG works so well for both NIBs and LIBs is that it uniquely contains three essential features: (a) dilated intergraphene spacing for Na intercalation at low voltages; (b) highly accessible near-surface nanopores for Li metal filling at low voltages; and (c) substantial defect content in the graphene planes for Li adsorption at higher voltages. The <0.1 V charge storage mechanism is fundamentally different for Na versus for Li. A combination of XRD and XPS demonstrates highly reversible Na intercalation rather than metal underpotential deposition. By contrast, the same analysis proves the presence of metallic Li in the pores, with intercalation being much less pronounced.

Anodes for Sodium Ion Batteries Based on Tin–Germanium–Antimony Alloys

Here we provide the first report on several compositions of ternary Sn-Ge-Sb thin film alloys for application as sodium ion battery (aka NIB, NaB or SIB) anodes, employing Sn50Ge50, Sb50Ge50, and pure Sn, Ge, Sb as baselines. Sn33Ge33Sb33, Sn50Ge25Sb25, Sn60Ge20Sb20, and Sn50Ge50 all demonstrate promising electrochemical behavior, with Sn50Ge25Sb25 being the best overall. This alloy has an initial reversible specific capacity of 833 mAhg(-1) (at 85 mAg(-1)) and 662 mAhg(-1) after 50 charge-discharge cycles. Sn50Ge25Sb25 also shows excellent rate capability, displaying a stable capacity of 381 mAhg(-1) at a current density of 8500 mAg(-1) (∼10C). A survey of published literature indicates that 833 mAhg(-1) is among the highest reversible capacities reported for a Sn-based NIB anode, while 381 mAhg(-1) represents the optimum fast charge value. HRTEM shows that Sn50Ge25Sb25 is a composite of 10-15 nm Sn and Sn-alloyed Ge nanocrystallites that are densely dispersed within an amorphous matrix. Comparing the microstructures of alloys where the capacity significantly exceeds the rule of mixtures prediction to those where it does not leads us to hypothesize that this new phenomenon originates from the Ge(Sn) that is able to sodiate beyond the 1:1 Na:Ge ratio reported for the pure element. Combined TOF-SIMS, EELS TEM, and FIB analysis demonstrates substantial Na segregation within the film near the current collector interface that is present as early as the second discharge, followed by cycling-induced delamination from the current collector.

Activation with Li Enables Facile Sodium Storage in Germanium

Germanium is a promising sodium ion battery (NIB, NAB, SIB) anode material that is held back by its extremely sluggish kinetics and poor cyclability. We are the first to demonstrate that activation by a single lithiation-delithiation cycle leads to a dramatic improvement in the practically achievable capacity, in rate capability, and in cycling stability of Ge nanowires (GeNWs) and Ge thin film (GeTF). TEM and TOF-SIMS analysis shows that without activation, the initially single crystal GeNWs are effectively Na inactive, while the 100 nm amorphous GeTF sodiates only partially and inhomogeneously. Activation with Li induces amorphization in GeNWs reducing the barrier for nucleation of the NaxGe phase(s) and accelerates solid-state diffusion that aids the performance of both GeNWs and GeTF. Low rate (0.1C) Li activation also introduces a dense distribution of nanopores that lead to further improvements in the rate capability, which is ascribed to the lowered solid-state diffusion distances caused by the effective thinning of the Ge walls and by an additional Na diffusion path via the pore surfaces. The resultant kinetics are promising. Tested at 0.15C (1C = 369 mA/g, i.e. Na/Ge 1:1) for 50 cycles the GeNWs and GeTF maintain a reversible (desodiation) capacity of 346 and 418 mAh/g, respectively. They also demonstrate a capacity of 355 and 360 mAh/g at 1C and 284 and 310 mAh/g at 4C. Even at a very high rate of 10C the GeTF delivers 169 mAh/g. Preliminary results demonstrate that Li activation is also effective in promoting cycling stability of Sb blanket films.

Silicon nanowire core aluminum shell coaxial nanocomposites for lithium ion battery anodes grown with and without a TiN interlayer

We investigated the effect of aluminum coating layers and of the support growth substrates on the electrochemical performance of silicon nanowires (SiNWs) used as negative electrodes in lithium ion battery half-cells. Extensive TEM and SEM analysis was utilized to detail the cycling induced morphology changes in both the Al-SiNW nanocomposites and in the baseline SiNWs. We observed an improved cycling performance in the Si nanowires that were coated with 3 and 8 wt.% aluminum. After 50 cycles, both the bare and the 3 wt.% Al coated nanowires retained 2600 mAh/g capacity. However beyond 50 cycles, the coated nanowires showed higher capacity as well as better capacity retention with respect to the first cycle. Our hypothesis is that the nanoscale yet continuous electrochemically active aluminum shell places the Si nanowires in compression, reducing the magnitude of their cracking/disintegration and the subsequent loss of electrical contact with the electrode. We combined impedance spectroscopy with microscopy analysis to demonstrate how the Al coating affects the solid electrolyte interface (SEI). A similar thickness alumina (Al2O3) coating, grown via atomic layer deposition (ALD), was shown not to be as effective in reducing the long-term capacity loss. We demonstrate that an electrically conducting TiN barrier layer present between the nanowires and the underlying stainless steel current collector leads to a higher specific capacity during cycling and a significantly improved coulombic efficiency. Using TiN the irreversible capacity loss was only 6.9% from the initial 3581 mAh/g, while the first discharge (lithiation) capacity loss was only 4%. This is one of the best combinations reported in literature.

Sn–Bi–Sb alloys as anode materials for sodium ion batteries

In this work, the performance and electrochemical charge/discharge behavior of Sn–Bi–Sb alloy films were examined, as well as pure Sn, Bi, and Sb films, as anodes for sodium ion batteries (SIBs). Alloying was utilized as an approach to modify the morphology and active phases in an effort to improve the cycling stability of elemental anodes of Sn or Sb, while maintaining a high capacity. The films were prepared via sputtering, which enabled study of a broad swath of compositional space. The cycling performance of the Sb-rich compositions surpassed that of all other alloys tested as anodes for SIBs. The best performing alloy had a composition of 10 at% Sn, 10 at% Bi, and 80 at% Sb (called Sn10Bi10Sb80, here), and maintained 99% of its maximum capacity during cycling (621 mA h g−1) after 100 cycles. Stability of these anodes dropped as the quantity of Sb decreased; to contrast, Sn20Bi20Sb60, Sn25Bi25Sb50 and Sn33Bi33Sb33 were increasingly less stable as anodes in SIBs as the molar quantity of Sb in the films dropped to 60%, 50%, and 33%, respectively. The Sn10Bi10Sb80 electrode was found to possess a single phase as-deposited microstructure of Sn and Bi in substitutional solid solution with the Sb lattice and the sodiation sequence was found to be significantly different from pure Sb. Numerous possible mechanisms for the improvement in capacity retention were discussed, where modification and material response to internal stresses by changes in the Sb chemical potential and solid solution strengthening were found to be the most likely.

Array geometry dictates electrochemical performance of Ge nanowire lithium ion battery anodes

Scientific literature shows a substantial study-to-study variation in the electrochemical lithiation performance of “1-D” nanomaterials such as Si and Ge nanowires or nanotubes. In this study we varied the vapor–liquid–solid (VLS) growth temperature and time, resulting in nanowire arrays with distinct mass loadings, mean diameters and lengths, and thicknesses of the parasitic Ge films that are formed at the base of the array during growth. When all the results were compared, a key empirical trend to emerge was that increasing active material mass loading drastically degraded the electrochemical performance. For instance, GeNWs grown for 2 minutes at 320 °C (0.12 mg cm−2 mass loading, 34 nm mean nanowire diameter, 170 nm parasitic film thickness) had a reversible capacity of 1405 mA h g−1, a cycle 50 coulombic efficiency (CE) of 99.9%, a cycle 100 capacity retention of 98%, and delivered ∼1200 mA h g−1 at 5 C. In contrast, electrodes grown for 10 minutes at 360 °C (0.86 mg cm−2, 115 nm, 1410 nm) retained merely 5.6% of their initial capacity after 100 cycles, had a CE of 96%, and delivered ∼400 mA h g−1 at 5 C. Using TOF-SIMS we are the first to demonstrate marked segregation of Li to the current collector interface in planar Ge films that are 300 and 500 nm thick, but not in the 150 nm specimens. FIB analysis shows that the cycled higher mass loaded electrodes develop more SEI and interfacial cracks near the current collector. A comparison with the state-of-the-art scientific literature for a range of Ge-based nanostructures shows that our low mass loaded GeNWs are highly favorable in terms of the reversible capacity at cycle 1 and cycle 100, steady-state cycling CE and high-rate capability.