Anode-Free Sodium Battery through in Situ Plating of Sodium Metal.

Sodium-ion batteries (SIBs) have been pursued as a more cost-effective and more sustainable alternative to lithium-ion batteries (LIBs), but these advantages come at the expense of energy density. In this work, we demonstrate that the challenge of energy density for sodium chemistries can be overcome through an anode-free architecture enabled by the use of a nanocarbon nucleation layer formed on Al current collectors. Electrochemical studies show this configuration to provide highly stable and efficient plating and stripping of sodium metal over a range of currents up to 4 mA/cm2, sodium loading up to 12 mAh/cm2, and with long-term durability exceeding 1000 cycles at a current of 0.5 mA/cm2. Building upon this anode-free architecture, we demonstrate a full cell using a presodiated pyrite cathode to achieve energy densities of ∼400 Wh/kg, far surpassing recent reports on SIBs and even the theoretical maximum for LIB technology (387 Wh/kg for LiCoO2/graphite cells) while still relying on naturally abundant raw materials and cost-effective aqueous processing.

Role of Nitrogen-Doped Graphene for Improved High-Capacity Potassium Ion Battery Anodes.

Potassium is an earth abundant alternative to lithium for rechargeable batteries, but a critical limitation in potassium ion battery anodes is the low capacity of KC8 graphite intercalation compounds in comparison to conventional LiC6. Here we demonstrate that nitrogen doping of few-layered graphene can increase the storage capacity of potassium from a theoretical maximum of 278 mAh/g in graphite to over 350 mAh/g, competitive with anode capacity in commercial lithium ion batteries and the highest reported anode capacity so far for potassium ion batteries. Control studies distinguish the importance of nitrogen dopant sites as opposed to sp3 carbon defect sites to achieve the improved performance, which also enables >6× increase in rate performance of doped vs undoped materials. Finally, in situ Raman spectroscopy studies elucidate the staging sequence for doped and undoped materials and demonstrate the mechanism of the observed capacity enhancement to be correlated with distributed storage at local nitrogen sites in a staged KC8 compound. This study demonstrates a pathway to overcome the limitations of graphitic carbons for anodes in potassium ion batteries by atomically precise engineering of nanomaterials.

Mechanism of potassium ion intercalation staging in few layered graphene from in situ Raman spectroscopy.

Recently emerging potassium ion (K-ion) batteries offer a lower-cost alternative to lithium-ion batteries while enabling comparably high storage capacity. Here, we leverage the strong Raman spectroscopic response of few-layered graphene to provide the first insight into the electrochemical staging sequence for K+ ions in graphitic carbons. Our analysis reveals the signature of a dilute stage I compound that precedes formation of ordered intercalation compounds transitioning from stage VI (KC72), stage II (KC24), and stage I (KC8) and correlates electrochemical responses to the stage formation. Overall, our study emphasizes a minimum barrier to transfer the general understanding acquired for lithium-ion battery anodes to cheaper, earth abundant K-ion battery systems ideally suited for grid-scale storage.

Interface strain in vertically stacked two-dimensional heterostructured carbon-MoS2 nanosheets controls electrochemical reactivity.

Two-dimensional (2D) materials offer numerous advantages for electrochemical energy storage and conversion due to fast charge transfer kinetics, highly accessible surface area, and tunable electronic and optical properties. Stacking of 2D materials generates heterogeneous interfaces that can modify native chemical and physical material properties. Here, we demonstrate that local strain at a carbon-MoS2 interface in a vertically stacked 2D material directs the pathway for chemical storage in MoS2 on lithium metal insertion. With average measured MoS2 strain of ∼0.1% due to lattice mismatch between the carbon and MoS2 layers, lithium insertion is facilitated by an energy-efficient cation-exchange transformation. This is compared with low-voltage lithium intercalation for unstrained MoS2. This observation implies that mechanical properties of interfaces in heterogeneous 2D materials can be leveraged to direct energetics of chemical processes relevant to a wide range of applications such as electrochemical energy storage and conversion, catalysis and sensing.

Ultrafast triggered transient energy storage by atomic layer deposition into porous silicon for integrated transient electronics.

Here we demonstrate the first on-chip silicon-integrated rechargeable transient power source based on atomic layer deposition (ALD) coating of vanadium oxide (VOx) into porous silicon. A stable specific capacitance above 20 F g(-1) is achieved until the device is triggered with alkaline solutions. Due to the rational design of the active VOx coating enabled by ALD, transience occurs through a rapid disabling step that occurs within seconds, followed by full dissolution of all active materials within 30 minutes of the initial trigger. This work demonstrates how engineered materials for energy storage can provide a basis for next-generation transient systems and highlights porous silicon as a versatile scaffold to integrate transient energy storage into transient electronics.

Ultrafast Solvent-Assisted Sodium Ion Intercalation into Highly Crystalline Few-Layered Graphene.

A maximum sodium capacity of ∼35 mAh/g has hampered the use of crystalline carbon nanostructures for sodium ion battery anodes. We demonstrate that a diglyme solvent shell encapsulating a sodium ion acts as a "nonstick" coating to facilitate rapid ion insertion into crystalline few-layer graphene and bypass slow desolvation kinetics. This yields storage capacities above 150 mAh/g, cycling performance with negligible capacity fade over 8000 cycles, and ∼100 mAh/g capacities maintained at currents of 30 A/g (∼12 s charge). Raman spectroscopy elucidates the ordered, but nondestructive cointercalation mechanism that differs from desolvated ion intercalation processes. In situ Raman measurements identify the Na(+) staging sequence and isolates Fermi energies for the first and second stage ternary intercalation compounds at ∼0.8 eV and ∼1.2 eV.

Ultrafine Iron Pyrite (FeS2) Nanocrystals Improve Sodium-Sulfur and Lithium-Sulfur Conversion Reactions for Efficient Batteries.

Nanocrystals with quantum-confined length scales are often considered impractical for metal-ion battery electrodes due to the dominance of solid-electrolyte interphase (SEI) layer effects on the measured storage properties. Here we demonstrate that ultrafine sizes (∼4.5 nm, average) of iron pyrite, or FeS2, nanoparticles are advantageous to sustain reversible conversion reactions in sodium ion and lithium ion batteries. This is attributed to a nanoparticle size comparable to or smaller than the diffusion length of Fe during cation exchange, yielding thermodynamically reversible nanodomains of converted Fe metal and NaxS or LixS conversion products. This is compared to bulk-like electrode materials, where kinetic and thermodynamic limitations of surface-nucleated conversion products inhibit successive conversion cycles. Reversible capacities over 500 and 600 mAh/g for sodium and lithium storage are observed for ultrafine nanoparticles, with improved cycling and rate capability. Unlike alloying or intercalation processes, where SEI effects limit the performance of ultrafine nanoparticles, our work highlights the benefit of quantum dot length-scale nanocrystal electrodes for nanoscale metal sulfide compounds that store energy through chemical conversion reactions.

Corrosion resistant three-dimensional nanotextured silicon for water photo-oxidation.

We demonstrate the ability to chemically transform bulk silicon into a nanotextured surface that exhibits excellent electrochemical stability in aqueous conditions for water photo-oxidation. Conformal defective graphene coatings on nanotextured silicon formed by thermal treatment enable over 50× corrosion resistance in aqueous electrolytes based upon Tafel analysis and impedance spectroscopy. This enables nanotextured silicon as an effective oxygen-evolution photoanode for water splitting with saturation current density measured near 35 mA cm(-2) under 100 mW cm(-2) (1 sun) illumination. Our approach builds upon simple and scalable processing techniques with silicon to develop corrosion resistant electrodes that can benefit a broad range of catalytic and photocatalytic applications.

All silicon electrode photocapacitor for integrated energy storage and conversion.

We demonstrate a simple wafer-scale process by which an individual silicon wafer can be processed into a multifunctional platform where one side is adapted to replace platinum and enable triiodide reduction in a dye-sensitized solar cell and the other side provides on-board charge storage as an electrochemical supercapacitor. This builds upon electrochemical fabrication of dual-sided porous silicon and subsequent carbon surface passivation for silicon electrochemical stability. The utilization of this silicon multifunctional platform as a combined energy storage and conversion system yields a total device efficiency of 2.1%, where the high frequency discharge capability of the integrated supercapacitor gives promise for dynamic load-leveling operations to overcome current and voltage fluctuations during solar energy harvesting.

On-chip high power porous silicon lithium ion batteries with stable capacity over 10,000 cycles.

We demonstrate the operation of a graphene-passivated on-chip porous silicon material as a high rate lithium battery anode with over 50 X power density, and 100 X energy density improvement compared to identically prepared on-chip supercapacitors. We demonstrate this Faradaic storage behavior to occur at fast charging rates (1-10 mA cm(-2)) where lithium locally intercalates into the nanoporous silicon, preventing the degradation and poor cycling performance attributed to deep storage in the bulk silicon. This device exhibits cycling performance that exceeds 10,000 cycles with capacity above 0.1 mA h cm(-2) without notable capacity fade. This demonstrates a practical route toward high power, high energy, and long lifetime all-silicon on-chip storage systems relevant toward integration into electronics, photovoltaics, and other silicon-based platforms.

Assessing the improved performance of freestanding, flexible graphene and carbon nanotube hybrid foams for lithium ion battery anodes.

We demonstrate the fabrication of three-dimensional freestanding foams of hybrid graphene-single-walled carbon nanotube nanomanufactured materials with reversible capacities of 2640 mA h g(-1) at 0.186 A g(-1) and 236 mA h g(-1) at 27.9 A g(-1). The Li storage behavior of this material is compared against other nanostructures in similar flexible foam platforms including graphene, ultra-thin graphite, and single-walled carbon nanotubes (SWNTs), and we elucidate the improved hybrid material performance due to the decoupling of lithium storage reaction energetics dictated by the SWNTs from the total storage capacity of the hybrid material. This work demonstrates a route to develop mechanically robust all-carbon electrodes with the potential for reversible Li-ion storage capacity approaching silicon, power capability of the best supercapacitors, and based on a material simultaneously usable as a charge collector and anode.