Adam P. Cohn

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.

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.

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.

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.

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.

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.

Durable potassium ion battery electrodes from high-rate cointercalation into graphitic carbons

We report the first demonstration of potassium ion cointercalation into graphitic carbon electrodes including both natural graphite and multi-layered graphene in both diglyme and monoglyme based electrolytes. Contrary to conventional desolvation-based intercalation of potassium, we demonstrate excellent capacity retention of ∼80% at rates up to 10 A g−1 (30 second charge), with 95% capacity retention over 1000 cycles, and up to 100 mA h g−1 capacity. Raman and X-ray diffraction following 1000 cycles demonstrates no signature of defects, damage, or change to graphitic crystallinity compared to uncycled pristine materials that is attributed to weak ion–lattice interactions due to the solvated guest K ions. In situ Raman spectroscopy highlights the sequential formation of a stage 4, 3, 2, and 1 graphite intercalation compound (GIC) that occurs without the signature of dilute staging. In a charged stage 1 compound, we observe lattice expansion from 0.335 nm to 1.16 nm and measure the work function to be ∼3.4 eV. Overall, this system overcomes rate and durability bottlenecks that limit current K-ion battery electrodes, and gives promise to cointercalation for durable, fast, and low-cost storage systems.

Direct integration of a supercapacitor into the backside of a silicon photovoltaic device

We demonstrate a route to integrate active material for energy storage directly into a silicon photovoltaic (PV) device, and the synergistic operation of the PV and storage systems for load leveling. Porous silicon supercapacitors with 84% Coulombic efficiency are etched directly into the excess absorbing layer material in a commercially available polycrystalline silicon PV device and coupled with solid-state polymer electrolytes. Our work demonstrates the simple idea both that the PV device can charge the supercapacitor under an external load and that a constant current load can be maintained through periods of intermittent illumination, demonstrating the concept of an all-silicon integrated solar supercapacitor.

Tungsten diselenide (WSe2) as a high capacity, low overpotential conversion electrode for sodium ion batteries

Tungsten diselenide (WSe2) is demonstrated as an efficient electrode for sodium ion batteries for the first time. A high reversible capacity above 200 mA h g−1 is observed at 20 mA g−1 rate, with over 250 mA h g−1 capacity measured in the first sodium extraction. Assessment of electrolyte and binder materials on performance was examined and an EC/DEC electrolyte with CMC binder emerges to yield the highest capacity and cycling retention. Comparison between WS2 and WSe2 distinguishes WSe2 to exhibit superior performance due to more efficient energetics bearing a small overpotential <0.30 V. Ex situ analysis and imaging after cycling confirms a sodium-mediated conversion reaction that yields isolated domains of W metal or NaxSe and reformation of WSe2 upon sodium extraction, enabling insight into the chemical storage pathway. This work highlights the promise of WSe2 compared to other conversion-based transition metal dichalcogenides as a practical material for sodium ion batteries.

A Sugar-Derived Room-Temperature Sodium Sulfur Battery with Long Term Cycling Stability

We demonstrate a room-temperature sodium sulfur battery based on a confining microporous carbon template derived from sucrose that delivers a reversible capacity over 700 mAh/gS at 0.1C rates, maintaining 370 mAh/gS at 10 times higher rates of 1C. Cycling at 1C rates reveals retention of over 300 mAh/gS capacity across 1500 cycles with Coulombic efficiency >98% due to microporous sulfur confinement and stability of the sodium metal anode in a glyme-based electrolyte. We show sucrose to be an ideal platform to develop microporous carbon capable of mitigating electrode-electrolyte reactivity and loss of soluble intermediate discharge products. In a manner parallel to the low-cost materials of the traditional sodium beta battery, our work demonstrates the combination of table sugar, sulfur, and sodium, all of which are cheap and earth abundant, for a high-performance stable room-temperature sodium sulfur battery.