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.

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 based hollow nanostructures are receiving significant scientific attention as potential high energy density anodes for lithium ion batteries. However their cycling performance still requires further improvement. Here we explore the use of atomic layer deposition (ALD) of TiO 2 , TiN and Al 2 O 3 on the inner, the outer, or both surfaces of hollow Si nanotubes (SiNTs) for improving their cycling performance.We demonstrate that all three materials enhance the cycling performance, with optimum performance being achieved for SiNTs conformally coated on both sides with 1.5 nm of Li active TiO 2 . Substantial improvements are achieved in the cycling capacity retention (1700 mA h g À1 vs. 1287 mA h g À1 for the uncoated baseline, after 200 cycles at 0.2 C), steady-state coulombic efficiency ($100% vs. 97-98%), and high rate capability (capacity retention of 50% vs. 20%, going from 0.2 C to 5 C). TEM and other analytical techniques are employed to provide new insight into the lithiation cycling-induced failure mechanisms that turn out to be intimately linked to the microstructure and the location of these layers.

We demonstrate that nanometer-scale TiN coatings deposited by atomic layer deposition (ALD), and to a lesser extent by magnetron sputtering, will significantly improve the electrochemical cycling performance of silicon nanowire lithium-ion battery (LIB) anodes. A 5 nm thick ALD coating resulted in optimum cycling capacity retention (55% vs. 30% for the bare nanowire baseline, after 100 cycles) and coulombic efficiency (98% vs. 95%, at 50 cycles), also more than doubling the high rate capacity retention (e.g. 740 mA h g À1 vs.330 mA h g À1 , at 5 C). We employed a variety of advanced analytical techniques such as electron energy loss spectroscopy (EELS), focused ion beam analysis (FIB) and X-ray photoelectron spectroscopy (XPS) to elucidate the origin of these effects. The conformal 5 nm TiN remains sufficiently intact to limit the growth of the solid electrolyte interphase (SEI), which in turn both improves the overall coulombic efficiency and reduces the life-ending delamination of the nanowire assemblies from the underlying current collector. Our findings should provide a broadly applicable coating design methodology that will improve the performance of any nanostructured LIB anodes where SEI growth is detrimental.

We demonstrate that silicon nanowire (SiNW) Li-ion battery anodes that are conformally coated with TiO2 using atomic layer deposition (ALD) show a remarkable performance improvement. The coulombic efficiency is increased to ∼99%, among the highest ever reported for SiNWs, as compared to 95% for the baseline uncoated samples. The capacity retention after 100 cycles for the nanocomposite is twice as high as that of the baseline at 0.1 C (60% vs. 30%), and more than three times higher at 5 C (34% vs. 10%). We also demonstrate that the microstructure of the coatings is critically important for achieving this effect. Titanium dioxide coatings with an as-deposited anatase structure are nowhere near as effective as amorphous ones, the latter proving much more resistant to delamination from the SiNW core. We use TEM to demonstrate that upon lithiation the amorphous coating develops a highly dispersed nanostructure comprised of crystalline LiTiO2 and a secondary amorphous phase. Electron energy loss spectroscopy (EELS) combined with bulk and surface analytical techniques are employed to highlight the passivating effect of TiO2, which results in significantly fewer cycling-induced electrolyte decomposition products as compared to the bare nanowires.