Nitin Muralidharan

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.

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.

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.

Polysulfide Anchoring Mechanism Revealed by Atomic Layer Deposition of V2O5 and Sulfur-Filled Carbon Nanotubes for Lithium–Sulfur Batteries

Despite the promise of surface engineering to address the challenge of polysulfide shuttling in sulfur-carbon composite cathodes, melt infiltration techniques limit mechanistic studies correlating engineered surfaces and polysulfide anchoring. Here, we present a controlled experimental demonstration of polysulfide anchoring using vapor phase isothermal processing to fill the interior of carbon nanotubes (CNTs) after assembly into binder-free electrodes and atomic layer deposition (ALD) coating of polar V2O5 anchoring layers on the CNT surfaces. The ultrathin submonolayer V2O5 coating on the CNT exterior surface balances the adverse effect of polysulfide shuttling with the necessity for high sulfur utilization due to binding sites near the conductive CNT surface. The sulfur loaded into the CNT interior provides a spatially separated control volume enabling high sulfur loading with direct sulfur-CNT electrical contact for efficient sulfur conversion. By controlling ALD coating thickness, high initial discharge capacity of 1209 mAh/gS at 0.1 C and exceptional cycling at 0.2 C with 87% capacity retention after 100 cycles and 73% at 450 cycles is achieved and correlated to an optimal V2O5 anchoring layer thickness. This provides experimental evidence that surface engineering approaches can be effective to overcome polysulfide shuttling by controlled design of molecular-scale building blocks for efficient binder free lithium sulfur battery cathodes.

Strain Engineering to Modify the Electrochemistry of Energy Storage Electrodes

Strain engineering has been a critical aspect of device design in semiconductor manufacturing for the past decade, but remains relatively unexplored for other applications, such as energy storage. Using mechanical strain as an input parameter to modulate electrochemical potentials of metal oxides opens new opportunities intersecting fields of electrochemistry and mechanics. Here we demonstrate that less than 0.1% strain on a Ni-Ti-O based metal-oxide formed on superelastic shape memory NiTi alloys leads to anodic and cathodic peak potential shifts by up to ~30 mV in an electrochemical cell. Moreover, using the superelastic properties of NiTi to enable strain recovery also recovers the electrochemical potential of the metal oxide, providing mechanistic evidence of strain-modified electrochemistry. These results indicate that mechanical energy can be coupled with electrochemical systems to efficiently design and optimize a new class of strain-modulated energy storage materials.

Role of carbon defects in the reversible alloying states of red phosphorus composite anodes for efficient sodium ion batteries

Here we report the first mechanistic study investigating the effect of carbon defects on the evolution of different sodium–red phosphorus (red P) alloy states for stable high capacity sodium ion battery anodes. Using tunable sp2/sp3 carbon composites containing controlled single-walled carbon nanotube (SWCNT) and single-walled carbon nanohorn (SWCNH) compositions, we identify potentials over which both stable and unstable alloying of red P occurs with sodium. Examination of the stable alloy region includes both NaP and Na5P4 formation that occurs between 0.40 and 0.15 V where alloying is mostly independent of the carbon composite matrix chemistry. However, an unstable region corresponding to Na3P formation below 0.15 V results in capacity degradation that directly correlates with the density of carbon defects. In the unstable region, defects are observed to initiate deep alloying and poor reversibility due to the formation of irreversible Na3P products that form over the carbon surface. Our results present a mechanistic roadmap to guide the design of red P–carbon composite anodes to approach high theoretical sodium ion capacity (2596 mA h g−1) while simultaneously addressing chemical interactions that compromise performance stability.

Tunable Mechanochemistry of Lithium Battery Electrodes

The interplay between mechanical strains and battery electrochemistry, or the tunable mechanochemistry of batteries, remains an emerging research area with limited experimental progress. In this report, we demonstrate how elastic strains applied to vanadium pentoxide (V2O5), a widely studied cathode material for Li-ion batteries, can modulate the kinetics and energetics of lithium-ion intercalation. We utilize atomic layer deposition to coat V2O5 materials onto the surface of a shapememory superelastic NiTi alloy, which allows electrochemical assessment at a fixed and measurable level of elastic strain imposed on the V2O5, with strain state assessed through Raman spectroscopy and X-ray diffraction. Our results indicate modulation of electrochemical intercalation potentials by ∼40 mV and an increase of the diffusion coefficient of lithium ions by up to 2.5-times with elastic prestrains of <2% imposed on the V2O5. These results are supported by density functional theory calculations and demonstrate how mechanics of nanomaterials can be used as a precise tool to strain engineer the electrochemical energy storage performance of battery materials.

Noncovalent Pi-Pi Stacking at the Carbon-Electrolyte Interface: Controlling the Voltage Window of Electrochemical Supercapacitors.

A key parameter in the operation of an electrochemical double-layer capacitor is the voltage window, which dictates the device energy density and power density. Here we demonstrate experimental evidence that π-π stacking at a carbon-ionic liquid interface can modify the operation voltage of a supercapacitor device by up to 30%, and this can be recovered by steric hindrance at the electrode-electrolyte interface introduced by poly(ethylene oxide) polymer electrolyte additives. This observation is supported by Raman spectroscopy, electrochemical impedance spectroscopy, and differential scanning calorimetry that each independently elucidates the signature of π-π stacking between imidazole groups in the ionic liquid and the carbon surface and the role this plays to lower the energy barrier for charge transfer at the electrode-electrolyte interface. This effect is further observed universally across two separate ionic liquid electrolyte systems and is validated by control experiments showing an invariant electrochemical window in the absence of a carbon-ionic liquid electrode-electrolyte interface. As interfacial or noncovalent interactions are usually neglected in the mechanistic picture of double-layer capacitors, this work highlights the importance of understanding chemical properties at supercapacitor interfaces to engineer voltage and energy capability.

Catalyst morphology matters for lithium–oxygen battery cathodes

The effectiveness of using catalyst nanoparticles to reduce the overpotential and energy efficiency of lithium-oxygen (or lithium-air) batteries (LOBs) is usually attributed to the inherent catalytic properties of individual nanoparticles. Here, we demonstrate that the morphology of the catalyst layer is equally important in maintaining integrity of the catalyst coating during product formation in LOBs. We demonstrate this by comparing the performance of smooth, conformal coated Mn2O3 catalyst nanoparticles prepared by electric field-assisted deposition, and more irregular coatings using conventional film assembly techniques both on three-dimensional mesh substrates. Smooth coatings lead to an improved overpotential of 50 mV during oxygen reduction and 130 mV during oxygen evolution in addition to a nearly 2X improvement in durability compared to the more irregular films. In situ electrochemical impedance spectroscopy combined with imaging studies elucidates a mechanism of morphology-directed deactivation of catalyst layers during charging and discharging that must be overcome at practical electrode scales to achieve cell-level performance targets in LOBs.

Electrically Conductive Hierarchical Carbon Nanotube Networks with Tunable Mechanical Response

Small diameter carbon nanotube (CNTs) are synthesized directly from a parent CNT forest using a floating catalyst chemical vapor deposition (CVD) method. To support a new CNT generation from an existing forest, an alumina coating was applied to the CNT forest using atomic layer deposition (ALD). The new generation of small diameter CNTs (8 nm average) surround the first generation, filling the interstitial regions. The hierarchical forests exhibit a 5-10-fold increase in stiffness, and the two generations are electrically addressable in spite of the interfacial alumina layer between them. This work enables the design of complex CNT architectures with hierarchical features that bring tailored properties such as high specific surface area and robust mechanical properties that can benefit a range of applications.