Alireza Kohandehghan

Interconnected carbon nanosheets derived from hemp for ultrafast supercapacitors with high energy.

We created unique interconnected partially graphitic carbon nanosheets (10-30 nm in thickness) with high specific surface area (up to 2287 m(2) g(-1)), significant volume fraction of mesoporosity (up to 58%), and good electrical conductivity (211-226 S m(-1)) from hemp bast fiber. The nanosheets are ideally suited for low (down to 0 °C) through high (100 °C) temperature ionic-liquid-based supercapacitor applications: At 0 °C and a current density of 10 A g(-1), the electrode maintains a remarkable capacitance of 106 F g(-1). At 20, 60, and 100 °C and an extreme current density of 100 A g(-1), there is excellent capacitance retention (72-92%) with the specific capacitances being 113, 144, and 142 F g(-1), respectively. These characteristics favorably place the materials on a Ragone chart providing among the best power-energy characteristics (on an active mass normalized basis) ever reported for an electrochemical capacitor: At a very high power density of 20 kW kg(-1) and 20, 60, and 100 °C, the energy densities are 19, 34, and 40 Wh kg(-1), respectively. Moreover the assembled supercapacitor device yields a maximum energy density of 12 Wh kg(-1), which is higher than that of commercially available supercapacitors. By taking advantage of the complex multilayered structure of a hemp bast fiber precursor, such exquisite carbons were able to be achieved by simple hydrothermal carbonization combined with activation. This novel precursor-synthesis route presents a great potential for facile large-scale production of high-performance carbons for a variety of diverse applications including energy storage.

Carbon Nanosheet Frameworks Derived from Peat Moss as High Performance Sodium Ion Battery Anodes

We demonstrate that peat moss, a wild plant that covers 3% of the earths surface, serves as an ideal precursor to create sodium ion battery (NIB) anodes with some of the most attractive electrochemical properties ever reported for carbonaceous materials. By inheriting the unique cellular structure of peat moss leaves, the resultant materials are composed of three-dimensional macroporous interconnected networks of carbon nanosheets (as thin as 60 nm). The peat moss tissue is highly cross-linked, being rich in lignin and hemicellulose, suppressing the nucleation of equilibrium graphite even at 1100 °C. Rather, the carbons form highly ordered pseudographitic arrays with substantially larger intergraphene spacing (0.388 nm) than graphite (c/2 = 0.3354 nm). XRD analysis demonstrates that this allows for significant Na intercalation to occur even below 0.2 V vs Na/Na(+). By also incorporating a mild (300 °C) air activation step, we introduce hierarchical micro- and mesoporosity that tremendously improves the high rate performance through facile electrolyte access and further reduced Na ion diffusion distances. The optimized structures (carbonization at 1100 °C + activation) result in a stable cycling capacity of 298 mAh g(-1) (after 10 cycles, 50 mA g(-1)), with ∼150 mAh g(-1) of charge accumulating between 0.1 and 0.001 V with negligible voltage hysteresis in that region, nearly 100% cycling Coulombic efficiency, and superb cycling retention and high rate capacity (255 mAh g(-1) at the 210th cycle, stable capacity of 203 mAh g(-1) at 500 mA g(-1)).

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.

Peanut shell hybrid sodium ion capacitor with extreme energy–power rivals lithium ion capacitors

This is the first report of a hybrid sodium ion capacitor (NIC) with the active materials in both the anode and the cathode being derived entirely from a single precursor: peanut shells, which are a green and highly economical waste globally generated at over 6 million tons per year. The electrodes push the envelope of performance, delivering among the most promising sodiation capacity–rate capability–cycling retention combinations reported in the literature for each materials class. Hence the resultant NIC also offers a state-of-the-art cyclically stable combination of energy and power, not only in respect to previously but also as compared to Li ion capacitors (LICs). The ion adsorption cathode based on Peanut Shell Nanosheet Carbon (PSNC) displays a hierarchically porous architecture, a sheet-like morphology down to 15 nm in thickness, a surface area on par with graphene materials (up to 2396 m2 g−1) and high levels of oxygen doping (up to 13.51 wt%). Scanned from 1.5–4.2 V vs. Na/Na+ PSNC delivers a specific capacity of 161 mA h g−1 at 0.1 A g−1 and 73 mA h g−1 at 25.6 A g−1. A low surface area Peanut Shell Ordered Carbon (PSOC) is employed as an ion intercalation anode. PSOC delivers a total capacity of 315 mA h g−1 with a flat plateau of 181 mA h g−1 occurring below 0.1 V (tested at 0.1 A g−1), and is stable at 10 000 cycles (tested at 3.2 A g−1). The assembled NIC operates within a wide temperature range (0–65 °C), yielding at room temperature (by active mass) 201, 76 and 50 W h kg−1 at 285, 8500 and 16 500 W kg−1, respectively. At 1.5–3.5 V, the hybrid device achieved 72% capacity retention after 10 000 cycles tested at 6.4 A g−1, and 88% after 100 000 cycles at 51.2 A g−1.

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.

Origin of non-SEI related coulombic efficiency loss in carbons tested against Na and Li

Partially ordered but not graphitized carbons are widely employed for sodium and lithium ion battery (NIB and LIB) anodes, either in their pure form or as a secondary supporting phase for oxides, sulfides and insertion electrodes. These “pseudographitic” materials ubiquitously display a poor initial coulombic efficiency (CE), which has been historically attributed to solid electrolyte interface (SEI) formation on their large surface areas (up to ∼2500 m2 g−1). Here we identify the other sources CE loss by examining a pseudographitic carbon with a state-of-the-art capacity (>350 mA h g−1 for NIB, >800 mA h g−1 for LIB), but with a purposely designed low surface area (14.5 m2 g−1) that disqualifies SEI from having a substantial role. During the initial several (<5) cycles both Na and Li are irreversibly trapped in the bulk, with the associated CE loss occurring at higher desodiation/delithiation voltages. We measure a progressively increasing graphene interlayer spacing and a progressively increasing Raman G band intensity, indicating that the charge carriers become trapped not only at the graphene defects but also between the graphene planes hence causing them to both dilate and order. For the case of Li, we also unambiguously detected irreversible metal underpotential deposition (“nanoplating”) within the nanopores at roughly below 0.2 V. It is expected that in conventional high surface area carbons these mechanisms will be a major contributor to CE loss in parallel to classic SEI formation. Key implications to emerge from these findings are that improvements in early cycling CE may be achieved by synthesizing pseudographitic carbons with lower levels of trapping defects, but that for LIBs the large cycle 1 CE loss may be unavoidable if highly porous structures are utilized.

Si nanotubes ALD coated with TiO2, TiN or Al2O3 as high performance lithium ion battery anodes

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 TiO2, TiN and Al2O3 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 TiO2. Substantial improvements are achieved in the cycling capacity retention (1700 mA h g−1vs. 1287 mA h g−1 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.

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.

Sodiation vs. lithiation phase transformations in a high rate – high stability SnO2 in carbon nanocomposite

We employed a glucose mediated hydrothermal self-assembly method to create a SnO2–carbon nanocomposite with promising electrochemical performance as both a sodium and a lithium ion battery anode (NIBs NABs SIBs, LIBs), being among the best in terms of cyclability and rate capability when tested against Na. In parallel we provide a systematic side-by-side comparison of the sodiation vs. lithiation phase transformations in nano SnO2. The high surface area (338 m2 g−1) electrode is named C–SnO2, and consists of a continuous Li and Na active carbon frame with internally imbedded sub-5 nm SnO2 crystallites of high mass loading (60 wt%). The frame imparts excellent electrical conductivity to the electrode, allows for rapid diffusion of Na and Li ions, and carries the sodiation/lithiation stresses while preventing cycling-induced agglomeration of the individual crystals. C–SnO2 employed as a NIB anode displays a reversible capacity of 531 mA h g−1 (at 0.08 A g−1) with 81% capacity retention after 200 cycles, while capacities of 240, 188 and 133 mA h g−1 are achieved at the much higher rates of 1.3, 2.6 and 5 A g−1. As a LIB anode C–SnO2 maintains a capacity of 1367 mA h g−1 (at 0.5 A g−1) after 400 cycles, and 420 mA h g−1 at 10 A g−1. Combined TEM, XRD and XPS prove that the much lower capacity of SnO2 as a NIB anode is due to the kinetic difficulty of the Na–Sn alloying reaction to reach the terminal Na15Sn4 intermetallic, whereas for Li–Sn the Li22Sn5 intermetallic is readily formed at 0.01 V. Rather, with applied voltage a significant portion of the material effectively shuffles between SnO2 and β-Sn + NaO2. The conversion reaction proceeds differently in the two systems: LiO2 is reduced directly to SnO2 and Li, whereas the NaO2 to SnO2 reaction proceeds through an intermediate SnO phase.

Silicon nanowire lithium-ion battery anodes with ALD deposited TiN coatings demonstrate a major improvement in cycling performance

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−1vs. 330 mA h g−1, 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.