Zhenpeng Yao

Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism

There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.

Kinetically-Driven Phase Transformation during Lithiation in Copper Sulfide Nanoflakes

Two-dimensional (2D) transition metal chalcogenides have been widely studied and utilized as electrode materials for lithium ion batteries due to their unique layered structures to accommodate reversible lithium insertion. Real-time observation and mechanistic understanding of the phase transformations during lithiation of these materials are critically important for improving battery performance by controlling structures and reaction pathways. Here, we use in situ transmission electron microscopy methods to study the structural, morphological, and chemical evolutions in individual copper sulfide (CuS) nanoflakes during lithiation. We report a highly kinetically driven phase transformation in which lithium ions rapidly intercalate into the 2D van der Waals-stacked interlayers in the initial stage, and further lithiation induces the Cu extrusion via a displacement reaction mechanism that is different from the typical conversion reactions. Density functional theory calculations have confirmed both the thermodynamically favored and the kinetically driven reaction pathways. Our findings elucidate the reaction pathways of the Li/CuS system under nonequilibrium conditions and provide valuable insight into the atomistic lithiation mechanisms of transition metal sulfides in general.

Origin of Fracture-Resistance to Large Volume Change in Cu-Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu-substituted Co3 O4 supplemented by first-principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube-on-cube orientation relationship with Li2 O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube-on-cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li2 O in the discharge cycle and underpins the catalytic activity of Li2 O decomposition in the charge cycle.

Interplay of cation and anion redox in Li4Mn2O5 cathode material and prediction of improved Li4(Mn,M)2O5 electrodes for Li-ion batteries

High–energy density cathode materials for Li-ion batteries leverage oxygen and transition metal redox activity with reduced cost. Significant research effort has focused on improving the specific energy of lithium-ion batteries for emerging applications, such as electric vehicles. Recently, a rock salt–type Li4Mn2O5 cathode material with a large discharge capacity (~350 mA·hour g−1) was discovered. However, a full structural model of Li4Mn2O5 and its corresponding phase transformations, as well as the atomistic origins of the high capacity, warrants further investigation. We use first-principles density functional theory (DFT) calculations to investigate both the disordered rock salt–type Li4Mn2O5 structure and the ordered ground-state structure. The ionic ordering in the ground-state structure is determined via a DFT-based enumeration method. We use both the ordered and disordered structures to interrogate the delithiation process and find that it occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: (i) an initial metal oxidation, Mn3+→Mn4+ (LixMn2O5, 4 > x > 2); (ii) followed by anion oxidation, O2−→O1− (2 > x > 1); and (iii) finally, further metal oxidation, Mn4+→Mn5+ (1 > x > 0). This final step is concomitant with the Mn migration from the original octahedral site to the adjacent tetrahedral site, introducing a kinetic barrier to reversible charge/discharge cycles. Armed with this knowledge of the charging process, we use high-throughput DFT calculations to study metal mixing in this compound, screening potential new materials for stability and kinetic reversibility. We predict that mixing with M = V and Cr in Li4(Mn,M)2O5 will produce new stable compounds with substantially improved electrochemical properties.

Multistep Lithiation of Tin Sulfide: An Investigation Using in Situ Electron Microscopy

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds via a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using in situ electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS2 corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The in situ observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, in situ delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the ex situ analysis. This work reveals the full lithiation characteristic of SnS2 and sheds light on the understanding of complex multistep reactions in 2D materials.

Quaternary Pavonites A1+xSn2-xBi5+xS10 (A+ = Li+, Na+): Site Occupancy Disorder Defines Electronic Structure

The field of mineralogy represents an area of untapped potential for the synthetic chemist, as there are numerous structure types that can be utilized to form analogues of mineral structures with useful optoelectronic properties. In this work, we describe the synthesis and characterization of two novel quaternary sulfides A1+xSn2-xBi5+xS10 (A = Li+, Na+). Though not natural minerals themselves, both compounds adopt the pavonite structure, which crystallizes in the C2/m space group and consists of two connected, alternating defect rock-salt slabs of varying thicknesses to create a three-dimensional lattice. Despite their commonalities in structure, their crystallography is noticeably different, as both structures have a heavy degree of site occupancy disorder that affects the actual positions of the atoms. The differences in site occupancy alter their electronic structures, with band gap values of 0.31(2) eV and 0.07(2) eV for the lithium and sodium analogues, respectively. LiSn2Bi5S10 exhibits ultralow thermal conductivity of 0.62 W m-1 K-1 at 723 K, and this result is corroborated by phonon dispersion calculations. This structure type is a promising host candidate for future thermoelectric materials investigation, as these materials have narrow band gaps and intrinsically low thermal conductivities.

Revealing the Effects of Electrode Crystallographic Orientation on Battery Electrochemistry via the Anisotropic Lithiation and Sodiation of ReS2

The crystallographic orientation of battery electrode materials can significantly impact electrochemical performance, such as rate capability and cycling stability. Among the layered transition metal dichalcogenides, rhenium disulfide (ReS2) has the largest anisotropic ratio between the two main axes in addition to exceptionally weak interlayer coupling, which serves as an ideal system to observe and analyze anisotropy of electrochemical phenomena. Here, we report anisotropic lithiation and sodiation of exfoliated ReS2 at atomic resolution using in situ transmission electron microscopy. These results reveal the role of crystallographic orientation and anisotropy on battery electrode electrochemistry. Complemented with density functional theory calculations, the lithiation of ReS2 is found to begin with intercalation of Li-ions, followed by a conversion reaction that results in Re nanoparticles and Li2S nanocrystals. The reaction speed is highly anisotropic, occurring faster along the in-plane ReS2 layer than along the out-of-plane direction. Sodiation of ReS2 is found to proceed similarly to lithiation, although the intercalation step is relatively quicker. Furthermore, the microstructure and morphology of the reaction products after lithiation/sodiation show clear anisotropy along the in-plane and out-of-plane directions. These results suggest that crystallographic orientation in highly anisotropic electrode materials can be exploited as a design parameter to improve battery electrochemical performance.

In-situ Electron Diffraction Studies of Sodium Electrochemistry in MoS2

MoS2 is a promising electrode materials for sodium-ion batteries. In the structure of MoS2, there is adequate “space” between the MoS2 layers interconnected with weak van der Waals force to accommodate Na ions during charging. It turns out that MoS2 allows Na ions to intercalate therein without a significant volume expansion (1); which enables MoS2 to be a promising electrode material for rechargeable batteries (2). However, the number of the electrons can be accommodated in the S-MoS layer is limited while the structural framework remains stable. It has been shown that up to 1.5 electrons can be stored per unit formula in MoS2 before the layered structure collapses (3). Meanwhile, there is a structural transition between trigonal 2Hand octahedral 1T-AMoS2 (A = Li, Na, K, etc.) accompanied by an electronic state change from semiconducting to metallic observed upon alkali-metal ion’s intercalation (4). Recent developments of in-situ transmission electron microscopy (TEM), as one unique tool to conduct real time structural measurements under the dynamic electrochemical reaction processes. (5) Such in-situ or in-operando measurements make it possible to analyze and tackle the intricacies of the sodiation mechanism in electrode materials during charge/discharge cycles.

Kinetics of Sodium and Selenium Reactions in Sodium Ion Batteries

Selenium and sulfur, both as chalcogen elements, show similar volumetric capacity as cathode material for both lithium and sodium ion batteries.[1] Additionally selenium has notable higher electrical conductivity than sulfur.[2] In this work, we have investigated the kinetics of sodiation reaction in selenium nanotube as the cathode material for sodium ion battery. We have monitored the microstructure evolution and interface dynamics using in situ TEM during sodiation process. A three steps reaction mechanism appears to explain the sodiation process (Figure 1). In the first step, single crystalline selenium nanotube rapidly transforms to an amorphous NaxSe alloy phase. In the second step with continued charging, the amorphous phase recrystallizes to a polycrystal Na2Se2 phase. In the final step near full sodiation, polycrystalline Na2Se2 appears to completely transform into Na2Se phase with high content of Na. Intriguingly, the reaction front region movement is found to be quite different in the different sodiation stages. The solid-state amorphization process quickly finishes due to the high diffusion of sodium ions inside Se nanotube, with the highest nominal speed of ~2.8 nm/s, and the recrystallization processes has a speed of ~1.0 nm/s (Figure 2). Moreover the speed of solid-state amorphization process is nearly 10 times higher than lithation process when selenium nanotube were tested in lithiation reaction. Molecule Dynamics (MD) calculation shows all the intermediate phases produced in sodiation are good conductor of both electrons and ions. These observations can not only reveal the reaction mechanism and reaction process, but also to provide insights to design novel nanostructure of electrodes with excellent electrochemical performance.

Atomic-resolution in-situ TEM Studies of Lithium Electrochemistry in Co3O4-Carbon Nanotube Nanocomposite

In typical charge and discharge cycling, electrodes may exhibit metastable phases with unusual ordering to repeatedly host and extract lithium-ion and electrons. This is a clear consequence of not enough time and/or energy available for such metastable phases to relax and transform into their equilibrium counterparts (1). Such metastable structures may exist only for a short duration, which makes it extremely difficult to measure or even identify them experimentally. Yet, they clearly play important roles in the battery figures of merit, such as cycling stability, voltage hysteresis, to capacity. The recent developments in in-situ transmission electron microscopy (TEM) (2-6) has enabled us to observe de/lithiation processes at atomic resolution, identify metastable phases and monitor their continuous phase transformation, with the gradual addition of lithium-ions and electrons into the battery electrodes. Structural models of these metastable phases are derived from full DFT simulations, and seem to corroborate with high resolution phase contrast simulated images as compared to corresponding experimental ones. Li-Co-O system represents one of the most important materials for lithium-ion battery with rich chemistry and structures, i.e. LiCoO2 has been used as intercalated cathode in the first commercialized battery by SONY, while Co3O4 and CoO are found to be high-capacity anode materials with conversion reaction: MxOy + 2y Li+ + 2y e= xM0 + yLi2O. Here, Co3O4 nanoparticles grown on highly conductive multi-wall carbon nanotubes (CNT) are employed as a model material system to study the structural evolutions with different amount of lithium inserted or reacted during electrochemical activation. The in-situ electrochemical lithiation experiments were followed until cobalt oxide nanoparticles are completely reduced into Co nanoparticles along with the formation of Li2O by conversion reaction and reverse de-lithiation until return back to cobalt oxide. When lithium-ions are introduced at lower rate, metastable lithium-inserted LixCo3O4 (x=1 to 5) crystalline phases are observed prior to formation of Co0 and Li2O clusters as the product of conversion reaction. At higher rate, lithium-ions can occupy any empty sites simultaneously that instantly break the Co3O4 spinel lattice bypassing the metastable crystalline phases. The amount of lithium-ions intake in a low rate is larger than that of the high rate, which provides insight on the charge/discharge rate and capacity relation. The presentation will cover intricacies of such metastable structures and the overall dynamics of electrochemical processes, as monitored by in-situ TEM imaging, spectroscopy and diffraction.