Nancy J. Dudney

Electrochemically-Driven Solid-State Amorphization in Lithium-Silicon Alloys and Implications for Lithium Storage

Lithiated metal alloys such as Li-Si are of great interest as high energy density anodes for future rechargeable battery technology. We show that the mechanism of electrochemical alloying is electrochemically-driven solid-state amorphization, a process closely analogous to the diffusive solid-state amorphization of thin films. X-ray diffraction and HREM experiments reveal that the crystallization of equilibrium intermetallic compounds is circumvented during lithiation at room temperature, and that formation of highly lithiated Li-Si glass instead occurs. This glass is shown to be metastable with respect to the equilibrium crystalline phases. Similar behavior is observed in the diffusive reaction of Li and Si bilayer films, suggesting that lithium-metal alloys in general are likely candidates for solid-state amorphization.

Nanoscale mapping of ion diffusion in a lithium-ion battery cathode

The movement of lithium ions into and out of electrodes is central to the operation of lithium-ion batteries. Although this process has been extensively studied at the device level, it remains insufficiently characterized at the nanoscale level of grain clusters, single grains and defects. Here, we probe the spatial variation of lithium-ion diffusion times in the battery-cathode material LiCoO(2) at a resolution of ∼100 nm by using an atomic force microscope to both redistribute lithium ions and measure the resulting cathode deformation. The relationship between diffusion and single grains and grain boundaries is observed, revealing that the diffusion coefficient increases for certain grain orientations and single-grain boundaries. This knowledge provides feedback to improve understanding of the nanoscale mechanisms underpinning lithium-ion battery operation.

Fabrication and characterization of amorphous lithium electrolyte thin films and rechargeable thin-film batteries

Amorphous oxide and oxynitride lithium electrolyte thin films were synthesized by r.f. magnetron sputtering of lithium silicates and lithium phosphates in Ar, Ar + O2, Ar + N2, or N2. The composition, structure, and electrical properties of the films were characterized using ion and electron beam, X-ray, optical, photoelectron, and a.c. impedance techniques. For the lithium phosphosilicate films, lithium ion conductivities as high as 1.4 × 10−6 S/cm at 25 °C were observed, but none of these films selected for extended testing were stable in contact with lithium. On the other hand, a new thin-film lithium phosphorus oxynitride electrolyte, synthesized by sputtering Li3PO4 in pure N2, was found to have a conductivity of 2 × 10-6 S/cm at 25 °C and excellent long-term stability in contact with lithium. Thin-films cells consisting of a 1 μm thick amorphous V2O5 cathode, a 1 μm thick oxynitride electrolyte film, and a 5 μm thick lithium anode were cycled between 3.7 and 1.5 V using discharge rates of up to 100 μA/cm2 and charge rates of up to 20 μA/cm2. The open-circuit voltage of 3.6 to 3.7 V of fully-charged cells remained virtually unchanged after months of storage.

Preferred Orientation of Polycrystalline LiCoO2 Films

Polycrystalline films of deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700°C. For films thicker than about 1 μm, more than 90% of the grains were oriented with their (101) and (104) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 μm, the percentage of (003)‐oriented grains increased until at a thickness of about 0.05 μm, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures, and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)–(104) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of (101)‐ and (104)‐oriented grains than through cathodes with predominately (003)‐oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte‐cathode interface. Nonetheless, cells with cathodes thicker than about 2 μm could deliver more than 50% of their maximum energies at discharge rates of or higher.

Electrical properties of amorphous lithium electrolyte thin films

The impedance of xLi2O·ySiO2·zP2O5 thin films deposited by RF-magnetron sputtering was analyzed using two models in which the frequency dependence of the bulk response was represented by: (1) a Cole-Cole dielectric function and (2) a constant phase angle element. Increases in the conductivity with Li2O concentration and with addition of SiO2 to Li2O-P2O5 compositions are attributed to an increase in Li+ mobility caused by changes in the film structure. A new amorphous oxynitride electrolyte, Li3.3PO3.9N0.17, prepared by sputtering Li3PO4 in N2, has a conductivity at 25°C of 2×10−6S/cm and is stable in conta with lithium.

Anomalous High Ionic Conductivity of Nanoporous β-Li3PS4

Lithium-ion-conducting solid electrolytes hold promise for enabling high-energy battery chemistries and circumventing safety issues of conventional lithium batteries. Achieving the combination of high ionic conductivity and a broad electrochemical window in solid electrolytes is a grand challenge for the synthesis of battery materials. Herein we show an enhancement of the room-temperature lithium-ion conductivity by 3 orders of magnitude through the creation of nanostructured Li(3)PS(4). This material has a wide electrochemical window (5 V) and superior chemical stability against lithium metal. The nanoporous structure of Li(3)PS(4) reconciles two vital effects that enhance the ionic conductivity: (1) the reduction of the dimensions to a nanometer-sized framework stabilizes the high-conduction β phase that occurs at elevated temperatures, and (2) the high surface-to-bulk ratio of nanoporous β-Li(3)PS(4) promotes surface conduction. Manipulating the ionic conductivity of solid electrolytes has far-reaching implications for materials design and synthesis in a broad range of applications, including batteries, fuel cells, sensors, photovoltaic systems, and so forth.

Lithium superionic sulfide cathode for all-solid lithium-sulfur batteries.

This work presents a facile synthesis approach for core-shell structured Li2S nanoparticles with Li2S as the core and Li3PS4 as the shell. This material functions as lithium superionic sulfide (LSS) cathode for long-lasting, energy-efficient lithium-sulfur (Li-S) batteries. The LSS has an ionic conductivity of 10(-7) S cm(-1) at 25 °C, which is 6 orders of magnitude higher than that of bulk Li2S (∼10(-13) S cm(-1)). The high lithium-ion conductivity of LSS imparts an excellent cycling performance to all-solid Li-S batteries, which also promises safe cycling of high-energy batteries with metallic lithium anodes.

Real Space Mapping of Li-Ion Transport in Amorphous Si Anodes with Nanometer Resolution

The electrical bias driven Li-ion motion in silicon anode materials in thin film battery heterostructures is investigated using electrochemical strain microscopy (ESM), which is a newly developed scanning probe microscopy based characterization method. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for studies on the sub-20 nm scale, and allows the relationship between Li-ion flow and microstructure to be established. The evolution of Li-ion transport during the battery charging is directly observed.

Electrochemically-driven solid-state amorphization in lithium–metal anodes

Abstract As lithiated–metal alloys such as Li–Si or Li–Sn are of great interest as high energy density anodes for Li-ion rechargeable batteries, a fundamental understanding on how the metals behave upon lithiation is important. X-ray diffraction and HREM experiments in this work reveal that the crystallization of equilibrium inter-metallic compounds (e.g. Li–Si) is inhibited during lithiation at room temperature, and that formation of highly lithiated glass instead occurs. This glass is shown to be metastable with respect to the equilibrium crystalline phases. We show that the mechanism of electrochemical alloying is electrochemically-driven solid-state amorphization (ESA), a process closely analogous to the diffusive solid-state amorphization (SSA) of thin films. Experimental results on the diffusive reaction of Li and Si bilayer films support the proposed mechanism.

Lithium polysulfidophosphates: a family of lithium-conducting sulfur-rich compounds for lithium-sulfur batteries.

Sulfur-rich lithium polysulfidophosphates (LPSPs) act as an enabler for long-lasting and efficient lithium-sulfur batteries. LPSPs have ionic conductivities of 3.0×10(-5)  S cm(-1) at 25 °C, which is 8 orders of magnitude higher than that of Li2S. The high lithium ion conductivity imparts excellent cycling performance, and the batteries are configured in an all-solid state, which promises safe cycling with metallic lithium anodes.