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.

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

Solid electrolytes (SEs) are widely considered as an ‘enabler’ of lithium anodes for high-energy batteries. However, recent reports demonstrate that the Li dendrite formation in Li7La3Zr2O12 (LLZO) and Li2S–P2S5 is actually much easier than that in liquid electrolytes of lithium batteries, by mechanisms that remain elusive. Here we illustrate the origin of the dendrite formation by monitoring the dynamic evolution of Li concentration profiles in three popular but representative SEs (LiPON, LLZO and amorphous Li3PS4) during lithium plating using time-resolved operando neutron depth profiling. Although no apparent changes in the lithium concentration in LiPON can be observed, we visualize the direct deposition of Li inside the bulk LLZO and Li3PS4. Our findings suggest the high electronic conductivity of LLZO and Li3PS4 is mostly responsible for dendrite formation in these SEs. Lowering the electronic conductivity, rather than further increasing the ionic conductivity of SEs, is therefore critical for the success of all-solid-state Li batteries. Despite its importance in lithium batteries, the mechanism of Li dendrite growth is not well understood. Here the authors study three representative solid electrolytes with neutron depth profiling and identify high electronic conductivity as the root cause for the dendrite issue.

High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes

Review on solid electrolytes for all-solid-state lithium-ion batteries

All-solid-state lithium batteries (ASSLBs) have the potential to revolutionize battery systems for electric vehicles due to their benefits in safety, energy density, packaging, and operable temperature range. As the key component in ASSLBs, inorganic lithium-ion-based solid-state electrolytes (SSEs) have attracted great interest, and advances in SSEs are vital to deliver the promise of ASSLBs. Herein, a survey of emerging SSEs is presented, and ion-transport mechanisms are briefly discussed. Techniques for increasing the ionic conductivity of SSEs, including substitution and mechanical strain treatment, are highlighted. Recent advances in various classes of SSEs enabled by different preparation methods are described. Then, the issues of chemical stabilities, electrochemical compatibility, and the interfaces between electrodes and SSEs are focused on. A variety of research addressing these issues is outlined accordingly. Given their importance for next-generation battery systems and transportation style, a perspective on the current challenges and opportunities is provided, and suggestions for future research directions for SSEs and ASSLBs are suggested.

Promises, Challenges, and Recent Progress of Inorganic Solid-State Electrolytes for All-Solid-State Lithium Batteries

Among the contenders in the new generation energy storage arena, all-solid-state batteries (ASSBs) have emerged as particularly promising, owing to their potential to exhibit high safety, high energy density and long cycle life. The relatively low conductivity of most solid electrolytes and the often sluggish charge transfer kinetics at the interface between solid electrolyte and electrode layers are considered to be amongst the major challenges facing ASSBs. This review presents an overview of the state of the art in solid lithium and sodium ion conductors, with an emphasis on inorganic materials. The correlations between the composition, structure and conductivity of these solid electrolytes are illustrated and strategies to boost ion conductivity are proposed. In particular, the high grain boundary resistance of solid oxide electrolytes is identified as a challenge. Critical issues of solid electrolytes beyond ion conductivity are also discussed with respect to their potential problems for practical applications. The chemical and electrochemical stabilities of solid electrolytes are discussed, as are chemo-mechanical effects which have been overlooked to some extent. Furthermore, strategies to improve the practical performance of ASSBs, including optimizing the interface between solid electrolytes and electrode materials to improve stability and lower charge transfer resistance are also suggested.

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New horizons for inorganic solid state ion conductors

Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte

As the most promising candidate of the solid electrolyte materials for future lithium batteries, oxide electrolytes with high–lithium-ion conductivity have experienced a rapid development in the past few decades. Existing oxide electrolytes are divided into two groups, i.e., crystalline group including NASICON, perovskite, garnet, and some newly developing structures, and amorphous/glass group including Li2O–MOx (M = Si, B, P, etc.) and LiPON-related materials. After a historical perspective on the general development of oxide electrolytes, we try to give a comprehensive review on the oxide electrolytes with high–lithium-ion conductivity, with special emphasis on the aspect of materials selection and design for applications as solid electrolytes in lithium batteries. Some successful examples and meaningful attempts on the incorporation of oxide electrolytes in lithium batteries are also presented. In the conclusion part, an outlook for the future direction of oxide electrolytes development is given.

Oxide Electrolytes for Lithium Batteries

Achieving high capacity in bulk-type solid-state lithium ion battery based on Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 electrolyte: Interfacial resistance

Cubic garnet-type Al-contained Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (Al-LLZTO) ceramics were prepared by conventional solid-state reaction using either Li 2 CO 3 or LiOH·H 2 O as Li source. The effects of Li source on the lithium ionic conductivity of Al-LLZTO ceramics were systematically investigated by comparing the synthesis processes and characteristics of the mother powders and the microstructures of the sintered pellets made with different Li sources. Different calcination processes of the raw materials mixtures with different Li sources are revealed by thermogravimetry-differential scanning calorimetry investigation. The mother powder derived from LiOH·H 2 O is of higher sintering activity than those made from Li 2 CO 3 at the same calcination temperature. For the LiOH·H 2 O-derived pellets, higher relative density and enhanced lithium-ion conductivity is achieved at a much lower sintering temperature of 1100 °C as compared with Li 2 CO 3 -derived pellets. The highest conductivity obtained at room temperature is 9.28 × 10 −4  S cm −1 , which is the highest value reported to date for the same Ta doping level.

Yaoyu Ren

Papers

Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte

Oxide Electrolytes for Lithium Batteries

Achieving high capacity in bulk-type solid-state lithium ion battery based on Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 electrolyte: Interfacial resistance

Effects of Li source on microstructure and ionic conductivity of Al-contained Li6.75La3Zr1.75Ta0.25O12 ceramics

Chemical compatibility between garnet-like solid state electrolyte Li6.75La3Zr1.75Ta0.25O12 and major commercial lithium battery cathode materials