Status and Prospect of Two-Dimensional Materials in Electrolytes for All-Solid-State Lithium Batteries.

Replacing liquid electrolytes and separators in conventional lithium-ion batteries with solid-state electrolytes (SSEs) is an important strategy to ensure both high energy density and high safety. Searching for fast ionic conductors with high electrochemical and chemical stability has been the core of SSE research and applications over the past decades. Based on the atomic-level thickness and infinitely expandable planar structure, numerous two-dimensional materials (2DMs) have been exploited and applied to address the most critical issues of low ionic conductivity of SSEs and lithium dendrite growth in all-solid-state lithium batteries. This review introduces the research process of 2DMs in SSEs, then summarizes the mechanisms and strategies of inert and active 2DMs toward Li+ transport to improve the ionic conductivity and enhance the electrode/SSE interfacial compatibility. More importantly, the main challenges and future directions for the application of 2DMs in SSEs are considered, including the importance of exploring the relationship between the anisotropic structure of 2DMs and Li+ diffusion behavior, the exploitation of more 2DMs, and the significance of in situ characterizations in elucidating the mechanisms of Li+ transport and interfacial reactions. This review aims to provide a comprehensive understanding to facilitate the application of 2DMs in SSEs.

Building K-C Anode with Ultrahigh Self-Diffusion Coefficient for Solid State Potassium Metal Batteries Operating at -20 to 120 °C.

Solid state potassium (K) metal batteries are intriguing in grid-scale energy storage, benefiting from the low cost, safety, and high energy density. However, their practical applications are impeded by poor K/solid electrolyte (SE) interfacial contact and limited capacity caused by the low K self-diffusion coefficient, dendrite growth, and intrinsically low melting point/soft features of metallic K. Herein, a fused-modeling strategy using potassiophilic carbon allotropes molted with K is demonstrated that can enhance the electrochemical performance/stability of the system via promoting K diffusion kinetics (2.37 × 10-8 cm2 s-1 ), creating a low interfacial resistance (≈1.3 Ω cm2 ), suppressing dendrite growth, and maintaining mechanical/thermal stability at 200 °C. A homogeneous/stable K stripping/plating is consequently implemented with a high current density of 2.8 mA cm-2 (at 25 °C) and a record-high areal capacity of 11.86 mAh cm-2 (at 0.2 mA cm-2 ). The enhanced K diffusion kinetics contribute to sustaining intimate interfacial contact, stabilizing the stripping/plating at high current densities. Full cells coupling ultrathin K-C composite anodes (≈50 µm) with Prussian blue cathodes and ß/ß″-Al2 O3 SEs deliver a high energy density of 389 Wh kg-1 with a retention of 94.4% after 150 cycles and fantastic performances at -20 to 120 °C.

Roles of Trimethyl Borate in Constructing an Interphase on Li Anode: Angel or Demon?

Although coupling a lithium metal anode with a Ni-rich layer cathode is a promising approach for high-energy lithium metal batteries, both electrodes are plagued by their intrinsic unstable interfaces which trigger electrolyte decomposition, lithium dendritic growth, and transition metal dissolution during cycling. Making use of electrolyte additives is one of the most effective solutions to address this issue. In this paper, we explore the roles of trimethyl borate (TMB)─a common film-forming additive to protect high-nickel-ratio ternary cathodes─in suppressing lithium dendrite growth. It is found that, on the one hand, the borate-containing solid electrolyte interphase (SEI) derived from the decomposition of TMB facilitates Li+ transport, homogenizing the deposition of Li ions. On the other hand, TMB as an anion receptor provokes LiPF6 decomposition, prompting the formation of SEI with superfluous LiF. As a result, it is imperative to raise awareness of this double-edge additive when using it to be immune to lithium dendrite and cathodic degradation.

Insight into Reversible Conversion Reactions in SnO2 -Based Anodes for Lithium Storage: A Review.

Various anode materials have been widely studied to pursue higher performance for next generation lithium ion batteries (LIBs). Metal oxides hold the promise for high energy density of LIBs through conversion reactions. Among these, tin dioxide (SnO2 ) has been typically investigated after the reversible lithium storage of tin-based oxides is reported by Idota and co-workers in 1997. Numerous in/ex situ studies suggest that SnO2 stores Li+ through a conversion reaction and an alloying reaction. The difficulty of reversible conversion between Li2 O and SnO2 is a great obstacle limiting the utilization of SnO2 with high theoretical capacity of 1494 mA h g-1 . Thus, enhancing the reversibility of the conversion reaction has become the research emphasis in recent years. Here, taking SnO2 as a typical representative, the recent progress is summarized and insight into the reverse conversion reaction is elaborated. Promoting Li2 O decomposition and maintaining high Sn/Li2 O interface density are two effective approaches, which also provide implications for designing other metal oxide anodes. In addition, some in/ex situ characterizations focusing on the conversion reaction are emphatically introduced. This review, from the viewpoint of material design and advanced characterizations, aims to provide a comprehensive understanding and shed light on the development of reversible metal oxide electrodes.

Tin-Based Anode Materials for Stable Sodium Storage: Progress and Perspective.

Because of concerns regarding shortages of lithium resources and the urgent need to develop low-cost and high-efficiency energy-storage systems, research and applications of sodium-ion batteries (SIBs) have re-emerged in recent years. Herein, recent advances in high-capacity Sn-based anode materials for stable SIBs are highlighted, including tin (Sn) alloys, Sn oxides, Sn sulfides, Sn selenides, Sn phosphides, and their composites. The reaction mechanisms between Sn-based materials and sodium are clarified. Multiphase and multiscale structural optimizations of Sn-based materials to achieve good sodium-storage performance are emphasized. Full-cell designs using Sn-based materials as anodes and further development of Sn-based materials are discussed from a commercialization perspective. Insights into the preparation of future high-performance Sn-based anode materials and the construction of sodium-ion full batteries with a high energy density and long service life are provided.

Boosting Reversibility and Stability of Li Storage in SnO2 -Mo Multilayers: Introduction of Interfacial Oxygen Redistribution.

Among the promising high-capacity anode materials, SnO2 represents a classic and important candidate that involves both conversion and alloying reactions toward Li storage. However, the inferior reversibility of conversion reactions usually results in low initial Coulombic efficiency (ICE, ≈60%), small reversible capacity, and poor cycling stability. Here, it is demonstrated that by carefully designing the interface structure of SnO2 -Mo, a breakthrough comprehensive performance with ultrahigh average ICE of 92.6%, large capacity of 1067 mA h g-1 , and 100% capacity retention after 700 cycles can be realized in a multilayer Mo/SnO2 /Mo electrode. Furthermore, high capacity retentions are also achieved in pouch-type Mo/SnO2 /Mo||Li half cells and Mo/SnO2 /Mo||LiFePO4 full cells. The amorphous SnO2 /Mo interfaces, which are induced by redistribution of oxygen between SnO2 and Mo, can precisely adjust the reversible capacity and cycling stability of the multilayers, while the stable capacities are parabolic with the interfacial density. Theoretical calculations and in/ex situ investigation reveal that oxygen redistribution in SnO2 /Mo heterointerfaces boosts Li-ion transport kinetics by inducing a built-in electric field and improves the reaction reversibility of SnO2 . This work provides a new understanding of interface-performance relationship of metal-oxide hybrid electrodes and pivotal guidance for creating high-performance Li-ion batteries.

Multiscale Observations of Inhomogeneous Bilayer SEI Film on a Conversion-Alloying SnO2 Anode.

SnO2 , storing Li through conversion and alloying reactions, has been regarded as one of the most typical anode materials and has been widely studied for both mechanism exploring and performance tuning. However, the structure of the solid electrolyte interphase (SEI) formed on the SnO2 electrode and its evolution process are rarely focused and still poorly understood. Herein, time of flight secondary ion mass spectrometry is used to observe the bilayer hybrid structure of SEI formed on a SnO2 film. Multiscale observations reveal the SEI accumulation after alloying reactions and distinct dissolving of the organic layer at potentials above de-conversion reactions, which results in the inorganic layer being directly exposed to the electrolyte and thus becoming thick and inhomogeneous. The broken and thick SEI causes rapid capacity decay and low Coulombic efficiencies (CEs) of 97.5%. Accordingly, it is demonstrated that, as the SnO2 is precoated with LiF or Li2 CO3 , a robust and thin SEI layer is induced to form and is stabilized in the continuous cycles, resulting in enhanced cycling stability and promoted CEs to 99.5%. This work adds new insights to the SEI evolution mechanisms on SnO2 -based anodes and suggests an effective strategy to create high performance metal oxide anode for Li-ion batteries.

Adding Metal Carbides to Suppress the Crystalline Li15Si4 Formation: A Route toward Cycling Durable Si-Based Anodes for Lithium-Ion Batteries.

In addition to large volume change and sluggish kinetics, the capacity decay of silicon anodes is also related to the formation of a crystalline Li15Si4 phase during cycling. Herein, we have demonstrated that refining cheap coarse-grained Si by ball milling with metal carbides (Mo2C, Cr2C3, etc.) can reduce the Si crystallite size significantly and can thus suppress the formation of the crystalline Li15Si4 during cycling, which increases the life of Si-based anode materials significantly. Si-Cr3C2@few-layer graphene (SC@G) composite anode materials were designed and prepared by plasma milling (P-milling) to achieve a considerable capacity of 881.8 mA h g-1 after 300 cycles at 1 A g-1. A study of the microstructure of the SC@G indicated that the refined amorphous-nanocrystal Si grains were distributed uniformly around multiscale Cr3C2 particles, which were covered by few-layer graphenes. The rigid Cr3C2 skeleton, which acts as a good conductive material, can increase the conductivity of the SC@G composite, avoid the agglomeration of refined Si, and regenerate Si nanosized grains during lithiation and delithiation. These results showed that the SC@G anode material exhibited an excellent overall performance based on its high capacity and long cycle stability, as well as excellent lithium-ion diffusion kinetics for lithium storage.