Yunhui Gong

Negating interfacial impedance in garnet-based solid-state Li metal batteries

Garnet-type solid-state electrolytes have attracted extensive attention due to their high ionic conductivity, approaching 1 mS cm-1, excellent environmental stability, and wide electrochemical stability window, from lithium metal to ∼6 V. However, to date, there has been little success in the development of high-performance solid-state batteries using these exceptional materials, the major challenge being the high solid-solid interfacial impedance between the garnet electrolyte and electrode materials. In this work, we effectively address the large interfacial impedance between a lithium metal anode and the garnet electrolyte using ultrathin aluminium oxide (Al2O3) by atomic layer deposition. Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) is the garnet composition of choice in this work due to its reduced sintering temperature and increased lithium ion conductivity. A significant decrease of interfacial impedance, from 1,710 Ω cm2 to 1 Ω cm2, was observed at room temperature, effectively negating the lithium metal/garnet interfacial impedance. Experimental and computational results reveal that the oxide coating enables wetting of metallic lithium in contact with the garnet electrolyte surface and the lithiated-alumina interface allows effective lithium ion transport between the lithium metal anode and garnet electrolyte. We also demonstrate a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode by applying the newly developed interface chemistry.

Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries

Significance This work describes a flexible, solid-state, lithium-ion–conducting membrane based on a 3D ion-conducting network and polymer electrolyte for lithium batteries. The 3D ion-conducting network is based on percolative garnet-type Li6.4La3Zr2Al0.2O12 solid-state electrolyte nanofibers, which enhance the ionic conductivity of the solid-state electrolyte membrane at room temperature and improve the mechanical strength of the polymer electrolyte. The membrane has shown superior electrochemical stability to high voltage and high mechanical stability to effectively block lithium dendrites. This work represents a significant breakthrough to enable high performance of lithium batteries. Beyond state-of-the-art lithium-ion battery (LIB) technology with metallic lithium anodes to replace conventional ion intercalation anode materials is highly desirable because of lithium’s highest specific capacity (3,860 mA/g) and lowest negative electrochemical potential (∼3.040 V vs. the standard hydrogen electrode). In this work, we report for the first time, to our knowledge, a 3D lithium-ion–conducting ceramic network based on garnet-type Li6.4La3Zr2Al0.2O12 (LLZO) lithium-ion conductor to provide continuous Li+ transfer channels in a polyethylene oxide (PEO)-based composite. This composite structure further provides structural reinforcement to enhance the mechanical properties of the polymer matrix. The flexible solid-state electrolyte composite membrane exhibited an ionic conductivity of 2.5 × 10−4 S/cm at room temperature. The membrane can effectively block dendrites in a symmetric Li | electrolyte | Li cell during repeated lithium stripping/plating at room temperature, with a current density of 0.2 mA/cm2 for around 500 h and a current density of 0.5 mA/cm2 for over 300 h. These results provide an all solid ion-conducting membrane that can be applied to flexible LIBs and other electrochemical energy storage systems, such as lithium–sulfur batteries.

All-wood, low tortuosity, aqueous, biodegradable supercapacitors with ultra-high capacitance

In energy storage devices, the critical demands for high energy/power density, low cost, long cycle lives and environmental friendliness have highlighted an urgent need for developing storage electrodes with low cost, large thickness, high mass loading, low tortuosity and high energy/power density. Here we demonstrate the design and construction of an all-wood-structured asymmetric supercapacitor (ASC) based on an activated wood carbon (AWC) anode, a wood membrane separator and a MnO2/wood carbon (MnO2@WC) cathode. The structural virtues of the all-wood-structured ASC device – desirable thickness (up to ∼1 mm), direct channels with low tortuosity, high electronic and ionic conductivity – enable ASC high areal mass loadings (up to 30 mg cm−2 for the anode and 75 mg cm−2 for the wood carbon/MnO2 composite cathode), a high energy density of 1.6 mW h cm−2 and a maximum power density of 24 W cm−2, representing the highest mass loading and areal energy/power densities among all reported MnO2-based supercapacitors. Moreover, all components in the all-wood-structured ASC are low-cost, environmentally friendly and biocompatible. With these unique features, the all-wood-structured ASC represents a promising energy storage device to realize high mass loading, high energy/power density, and biocompatibility for green and renewable energy storage.

Conformal, Nanoscale ZnO Surface Modification of Garnet-Based Solid-State Electrolyte for Lithium Metal Anodes

Solid-state electrolytes are known for nonflammability, dendrite blocking, and stability over large potential windows. Garnet-based solid-state electrolytes have attracted much attention for their high ionic conductivities and stability with lithium metal anodes. However, high-interface resistance with lithium anodes hinders their application to lithium metal batteries. Here, we demonstrate an ultrathin, conformal ZnO surface coating by atomic layer deposition for improved wettability of garnet solid-state electrolytes to molten lithium that significantly decreases the interface resistance to as low as ∼20 Ω·cm2. The ZnO coating demonstrates a high reactivity with lithium metal, which is systematically characterized. As a proof-of-concept, we successfully infiltrated lithium metal into porous garnet electrolyte, which can potentially serve as a self-supported lithium metal composite anode having both high ionic and electrical conductivity for solid-state lithium metal batteries. The facile surface treatment method offers a simple strategy to solve the interface problem in solid-state lithium metal batteries with garnet solid electrolytes.

Reducing Interfacial Resistance between Garnet‐Structured Solid‐State Electrolyte and Li‐Metal Anode by a Germanium Layer

Substantial efforts are underway to develop all-solid-state Li batteries (SSLiBs) toward high safety, high power density, and high energy density. Garnet-structured solid-state electrolyte exhibits great promise for SSLiBs owing to its high Li-ion conductivity, wide potential window, and sufficient thermal/chemical stability. A major challenge of garnet is that the contact between the garnet and the Li-metal anodes is poor due to the rigidity of the garnet, which leads to limited active sites and large interfacial resistance. This study proposes a new methodology for reducing the garnet/Li-metal interfacial resistance by depositing a thin germanium (Ge) (20 nm) layer on garnet. By applying this approach, the garnet/Li-metal interfacial resistance decreases from ≈900 to ≈115 Ω cm2 due to an alloying reaction between the Li metal and the Ge. In agreement with experiments, first-principles calculation confirms the good stability and improved wetting at the interface between the lithiated Ge layer and garnet. In this way, this unique Ge modification technique enables a stable cycling performance of a full cell of lithium metal, garnet electrolyte, and LiFePO4 cathode at room temperature.

Toward garnet electrolyte–based Li metal batteries: An ultrathin, highly effective, artificial solid-state electrolyte/metallic Li interface

Strategy to change the wettability of the solid-state electrolyte against Li and reduce interface resistance. Solid-state batteries are a promising option toward high energy and power densities due to the use of lithium (Li) metal as an anode. Among all solid electrolyte materials ranging from sulfides to oxides and oxynitrides, cubic garnet–type Li7La3Zr2O12 (LLZO) ceramic electrolytes are superior candidates because of their high ionic conductivity (10−3 to 10−4 S/cm) and good stability against Li metal. However, garnet solid electrolytes generally have poor contact with Li metal, which causes high resistance and uneven current distribution at the interface. To address this challenge, we demonstrate a strategy to engineer the garnet solid electrolyte and the Li metal interface by forming an intermediary Li-metal alloy, which changes the wettability of the garnet surface (lithiophobic to lithiophilic) and reduces the interface resistance by more than an order of magnitude: 950 ohm·cm2 for the pristine garnet/Li and 75 ohm·cm2 for the surface-engineered garnet/Li. Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) was selected as the solid-state electrolyte (SSE) in this work because of its low sintering temperature, stabilized cubic garnet phase, and high ionic conductivity. This low area-specific resistance enables a solid-state garnet SSE/Li metal configuration and promotes the development of a hybrid electrolyte system. The hybrid system uses the improved solid-state garnet SSE Li metal anode and a thin liquid electrolyte cathode interfacial layer. This work provides new ways to address the garnet SSE wetting issue against Li and get more stable cell performances based on the hybrid electrolyte system for Li-ion, Li-sulfur, and Li-oxygen batteries toward the next generation of Li metal batteries.

Three-dimensional bilayer garnet solid electrolyte based high energy density lithium metal–sulfur batteries

To simultaneously address the challenges of chemical/physical short circuits and electrode volume variation, we demonstrate a three-dimensional (3D) bilayer garnet solid-state electrolyte framework for advanced Li metal batteries. The dense layer is reduced in thickness to a few microns and still retains good mechanical stability, thereby enabling the safe use of Li metal anodes. The thick porous layer acts as a mechanical support for the thin dense layer which serves as a host for high loading of cathode materials and provides pathways for continuous ion transport. Results show that the integrated sulfur cathode loading can reach >7 mg cm−2 while the proposed hybrid Li–S battery exhibits a high initial coulombic efficiency (>99.8%) and high average coulombic efficiency (>99%) during the subsequent cycles. This electrolyte framework represents a promising strategy to revolutionize Li-metal batteries by transitioning to all-solid-state batteries and can be extended to other cathode materials.

Garnet Solid Electrolyte Protected Li-Metal Batteries

Garnet-type solid state electrolyte (SSE) is a promising candidate for high performance lithium (Li)-metal batteries due to its good stability and high ionic conductivity. One of the main challenges for garnet solid state batteries is the poor solid-solid contact between the garnet and electrodes, which results in high interfacial resistance, large polarizations, and low efficiencies in batteries. To address this challenge, in this work gel electrolyte is used as an interlayer between solid electrolyte and solid electrodes to improve their contact and reduce their interfacial resistance. The gel electrolyte has a soft structure, high ionic conductivity, and good wettability. Through construction of the garnet/gel interlayer/electrode structure, the interfacial resistance of the garnet significantly decreased from 6.5 × 104 to 248 Ω cm2 for the cathode and from 1.4 × 103 to 214 Ω cm2 for the Li-metal anode, successfully demonstrating a full cell with high capacity (140 mAh/g for LiFePO4 cathode) over 70 stable cycles in room temperature. This work provides a binary electrolyte consisting of gel electrolyte and solid electrolyte to address the interfacial challenge of solid electrolyte and electrodes and the demonstrated hybrid battery presents a promising future for battery development with high energy and good safety.

Transient Behavior of the Metal Interface in Lithium Metal–Garnet Batteries

The interface between solid electrolytes and Li metal is a primary issue for solid-state batteries. Introducing a metal interlayer to conformally coat solid electrolytes can improve the interface wettability of Li metal and reduce the interfacial resistance, but the mechanism of the metal interlayer is unknown. In this work, we used magnesium (Mg) as a model to investigate the effect of a metal coating on the interfacial resistance of a solid electrolyte and Li metal anode. The Li-Mg alloy has low overpotential, leading to a lower interfacial resistance. Our motivation is to understand how the metal interlayer behaves at the interface to promote increased Li-metal wettability of the solid electrolyte surface and reduce interfacial resistance. Surprisingly, we found that the metal coating dissolved in the molten piece of Li and diffused into the bulk Li metal, leading to a small and stable interfacial resistance between the garnet solid electrolyte and the Li metal. We also found that the interfacial resistance did not change with increase in the thickness of the metal coating (5, 10, and 100 nm), due to the transient behavior of the metal interface layer.

Rapid Thermal Annealing of Cathode-Garnet Interface toward High-Temperature Solid State Batteries

High-temperature batteries require the battery components to be thermally stable and function properly at high temperatures. Conventional batteries have high-temperature safety issues such as thermal runaway, which are mainly attributed to the properties of liquid organic electrolytes such as low boiling points and high flammability. In this work, we demonstrate a truly all-solid-state high-temperature battery using a thermally stable garnet solid-state electrolyte, a lithium metal anode, and a V2O5 cathode, which can operate well at 100 °C. To address the high interfacial resistance between the solid electrolyte and cathode, a rapid thermal annealing method was developed to melt the cathode and form a continuous contact. The resulting interfacial resistance of the solid electrolyte and V2O5 cathode was significantly decreased from 2.5 × 104 to 71 Ω·cm2 at room temperature and from 170 to 31 Ω·cm2 at 100 °C. Additionally, the diffusion resistance in the V2O5 cathode significantly decreased as well. The demonstrated high-temperature solid-state full cell has an interfacial resistance of 45 Ω·cm2 and 97% Coulombic efficiency cycling at 100 °C. This work provides a strategy to develop high-temperature all-solid-state batteries using garnet solid electrolytes and successfully addresses the high contact resistance between the V2O5 cathode and garnet solid electrolyte without compromising battery safety or performance.