Surface engineering of inorganic solid-state electrolytes via interlayers strategy for developing long-cycling quasi-all-solid-state lithium batteries

Lithium-silver alloys in anode-less batteries: comparison in liquid- and solid-electrolytes

This study comprehensively investigates the phase evolution of silver-carbon composite (Ag/C) layers in anode-less batteries with both liquid and solid electrolytes. The results of in situ X-ray diffraction and cross-sectional electron microscopy analyses reveal that the alloying reaction of Ag and Li is more homogeneous in solid-electrolyte-based cells compared to liquid-electrolyte-based cells. This homogeneity is attributed to diffusional Coble creep across the heterogeneous interfaces of Ag/C layers and solid electrolytes.

High Young's Modulus Li6.4La3Zr1.4Ta0.6O12-Based Solid Electrolyte Interphase Regulating Lithium Deposition for Dendrite-Free Lithium Metal Anode

Artificial solid electrolyte interphase (SEI) layers have been widely regarded as an effective protection for lithium (Li) metal anodes. In this work, an artificial SEI film consisting of dense Li6.4La3Zr1.4Ta0.6O12 (LLZTO) nanoparticles and polymerized styrene butadiene rubber is designed, which has good mechanical and chemical stability to effectively prevent Li anode corrosion by the electrolyte. The LLZTO-based SEI film can not only guide Li to uniformly deposit at the interface but also accelerate the electrochemical reaction kinetics due to its high Li+ conductivity. In particular, the high Young's modulus of the LLZTO-based SEI will regulate e- distribution in the continuous Li plating/stripping process and achieve uniform deposition of Li. As a consequence, the Li anode with LLZTO-based SEI (Li@LLZTO) enables symmetric cells to demonstrate a stable overpotential of 25 mV for 600 h at a current density of 1 mA cm-2 for 1 mA h cm-2. The Li@LLZTO||LFP (LiFePO4) full cell exhibits a capacity of 106 mA h g-1 after 800 cycles at 5 C with retention as high as 90%. Our strategy here suggests that the artificial SEI with high Young's modulus effectively inhibits the formation of Li dendrites and provides some guidance for the design of higher performance Li metal batteries.

Synergistic Evolution of Alloy Nanoparticles and Carbon in Solid-State Lithium Metal Anode Composites at Low Stack Pressure

Solid-state batteries with Li metal anodes can offer increased energy density compared to Li-ion batteries. However, the performance of pure Li anodes has been limited by morphological instabilities at the interface between Li and the solid-state electrolyte (SSE). Composites of Li metal with other materials such as carbon and Li alloys have exhibited improved cycling stability, but the mechanisms associated with this enhanced performance are not clear, especially at the low stack pressures needed for practical viability. Here, we investigate the structural evolution and correlated electrochemical behavior of Li metal composites containing reduced graphene oxide (rGO) and Li-Ag alloy particles. The nanoscale carbon scaffold maintains homogeneous contact with the SSE during stripping and facilitates Li transport to the interface; these effects largely prevent interfacial disconnection even at low stack pressure. The Li-Ag is needed to ensure cyclic refilling of the rGO scaffold with Li during plating, and the solid-solution character of Li-Ag improves cycling stability compared to other materials that form intermetallic compounds. Full cells with sulfur cathodes were tested at relatively low stack pressure, achieving 100 stable cycles with 79% capacity retention.

Interlayer Engineering and Prelithiation: Empowering Si Anodes for Low-Pressure-Operating All-Solid-State Batteries

Silicon (Si) anodes, free from the dendritic growth concerns found in lithium (Li) metal anodes, offer a promising alternative for high-energy all-solid-state batteries (ASSBs). However, most advancements in Si anodes have been achieved under impractical high operating pressures, which can mask detrimental electrochemo-mechanical issues. Herein, we effectively address the challenges related to the low-pressure operation of Si anodes in ASSBs by introducing an silver (Ag) interlayer between the solid electrolyte layer (Li6PS5Cl) and anode and prelithiating the anodes. The Si composite electrodes, consisting of Si/polyvinylidene fluoride/carbon nanotubes, are optimized for suitable mechanical properties and electrical connectivity. Although the impact of the Ag interlayer is insignificant at an exceedingly high operating pressure of 70 MPa, it substantially enhances the interfacial contacts under a practical low operating pressure of 15 MPa. Thus, Ag-coated Si anodes outperform bare Si anodes (discharge capacity: 2430 vs 1560 mA h g-1). The robust interfacial contact is attributed to the deformable, adhesive properties and protective role of the in situ lithiated Ag interlayer, as evidenced by comprehensive ex situ analyses. Operando electrochemical pressiometry is used effectively to probe the strong interface for Ag-coated Si anodes. Furthermore, prelithiation through the thermal evaporation deposition of Li metal significantly improves the cycling performance.

Rigid-flexible coupling network solid polymer electrolytes for all-solid-state lithium metal batteries

Poor mechanical strength at working temperature and low ionic conductivity seriously hinder the application of poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) in high performance all-solid-state lithium metal batteries (LMBs). Here, we design and prepare a series of rigid-flexible coupling network SPEs (RFN-SPEs) with soft poly(ethylene glycol) (PEG) chains and rigid crosslinkers containing the benzene structure. Compared with soft crosslinkers, rigid crosslinkers provide the same amount of active crosslinking points with smaller molecular weight, and meanwhile enhance the mechanical strength of the network. Therefore, based on the rigid crosslinkers, RFN-SPEs exhibit synchronously improved ionic conductivity and mechanical strength. With these RFN-SPEs, symmetrical cells can be cycled for over 2100 h at 0.5 mA cm-2. Meanwhile, stable cycling and high-rate capability could be achieved for LMBs, revealing that SPEs with the rigid-flexible coupling network are promising electrolyte systems for all-solid-state LMBs.

Near-strain-free anode architecture enabled by interfacial diffusion creep for initial-anode-free quasi-solid-state batteries

Anode-free (or lithium-metal-free) batteries with garnet-type solid-state electrolytes are considered a promising path in the development of safe and high-energy-density batteries. However, their practical implementation has been hindered by the internal strain that arises from the repeated plating and stripping of lithium metal at the interlayer between the solid electrolyte and negative electrode. Herein, we utilize the titanium nitrate nanotube architecture and a silver-carbon interlayer to mitigate the anisotropic stress caused by the recurring formation of lithium deposition layers during the cycling process. The mixed ionic-electronic conducting nature of the titanium nitrate nanotubes effectively accommodates the entry of reduced Li into its free volume space via interfacial diffusion creep, achieving near-strain-free operation with nearly tenfold volume suppressing capability compared to a conventional Cu anode counterpart during the lithiation process. Notably, the fabricated Li6.4La3Zr1.7Ta0.3O12 (LLZTO)-based initial-anode-free quasi-solid-state battery full cell, coupled with an ionic liquid catholyte infused high voltage LiNi0.33Co0.33Mn0.33O2-based cathode with an areal capacity of 3.2 mA cm-2, exhibits remarkable room temperature (25 °C) cyclability of over 600 cycles at 1 mA cm-2 with an average coulombic efficiency of 99.8%.

Direct imaging of dynamic heterogeneous lithium-gold interaction at the electrochemical interface during the charging/discharging processes

Lithium can smoothly plate on certain lithium alloys in theory, such as the Li-Au alloy, making the alloy/metal films promising current collectors for high energy density anode-free batteries. However, the actual performance of the batteries with alloy film electrodes often rapidly deteriorates. It remains challenging for current imaging approaches to provide sufficient details for fully understanding the process. Here, a "see-through" operando optical microscopic approach that allows direct imaging of Li-Au interaction with high spatiotemporal and chemical resolution has been developed. Through this approach, a two-step Li-Au alloying process that exhibits interesting complementary spatiotemporal evolution paths has been discovered. The alloying process regulates the nucleation of further Li deposition, while the Li nucleation sites generate pores on the electrode film. After several cycles, film rupture occurs due to the generation of an increased number of pores, thus explaining the previously unclear mechanism of poor cycling stability. We have also elucidated the deterioration mechanism of silver electrodes: the growth of defect pores in size, independent of the alloying process. Overall, this new imaging approach opens up an effective and simple way to monitor the dynamic heterogeneity of metal-metal interaction at the electrochemical interface, which could provide helpful insight for designing high-performance batteries.

Electrodeposition Stability Landscape for Solid-Solid Interfaces

As solid-state batteries (SSBs) with lithium (Li) metal anodes gain increasing traction as promising next-generation energy storage systems, a fundamental understanding of coupled electro-chemo-mechanical interactions is essential to design stable solid-solid interfaces. Notably, uneven electrodeposition at the Li metal/solid electrolyte (SE) interface arising from intrinsic electrochemical and mechanical heterogeneities remains a significant challenge. In this work, the thermodynamic origins of mechanics-coupled reaction kinetics at the Li/SE interface are investigated and its implications on electrodeposition stability are unveiled. It is established that the mechanics-driven energetic contribution to the free energy landscape of the Li deposition/dissolution redox reaction has a critical influence on the interface stability. The study presents the competing effects of mechanical and electrical overpotential on the reaction distribution, and demarcates the regimes under which stress interactions can be tailored to enable stable electrodeposition. It is revealed that different degrees of mechanics contribution to the forward (dissolution) and backward (deposition) reaction rates result in widely varying stability regimes, and the mechanics-coupled kinetics scenario exhibited by the Li/SE interface is shown to depend strongly on the thermodynamic and mechanical properties of the SE. This work highlights the importance of discerning the underpinning nature of electro-chemo-mechanical coupling toward achieving stable solid/solid interfaces in SSBs.

Effect of depth of discharge (DOD) on cycling in situ formed Li anodes

Lithium-metal solid-state batteries (LMSSBs) have garnered immense interest due to their potential to enhance safety and energy density compared to traditional Li-ion batteries. The anode-free approach to manufacturing Li-metal anodes could provide the additional benefit of reducing cost. However, a lack of understanding of the mechano-electrochemical behavior related to the cycling of in situ formed Li anodes remains a significant challenge. To bridge this knowledge gap, this work aims to understand the cycling behavior of in situ formed Li anodes on garnet Li7La3Zr2O12 (LLZO) solid-electrolyte as a function of the depth of discharge (DOD). The results of this study show that cycling in situ formed Li of 3 mA h cm-2 with a DOD of 66% leads to unstable cycling, while cycling with a DOD of 33% exhibits stable cycling. Furthermore, we observed interfacial deterioration and inhomogeneity of in situ formed Li anodes during cycling with a DOD of 66%. This study provides mechanistic insight into the factors that affect stable cycling that can help guide approaches to improve the cycling behavior of in situ formed Li anodes.

Towards lithium-free solid-state batteries with nanoscale Ag/Cu sputtered bilayer electrodes

Enhancing the reversible Li growth efficiency in "Li-free" solid-state batteries is key for the deployment of this technology. Here, we demonstrate a nanoscale material design path that enables the reversible cycling of a lithium-free solid-state battery, using Li7La3Zr2O12 (LLZO) electrolyte. By means of nanometric Ag-Cu bilayers, directly sputtered onto the LLZO, we can effectively control Li deposition. The robust thin film bilayer, which is compatible with LLZO, enables stable cycling, accommodating the volume changes without the need for extra external pressure.

Uniform Densification of Garnet Electrolyte for Solid-State Lithium Batteries

Highly uniformly dense garnet type solid-state electrolyte plays a significant role in determining the performance of solid-state lithium batteries. Herein, a rational powder-covering sintering strategy is proposed and demonstrated, in which narrow-particle-size-distribution fine powder and uniform sintering temperature distribution are considered as very significant factors. It is suggested that powder materials with wider particle size distribution dramatically decrease the densified level of electrolytes. Slow temperature elevating rate and the overhead structure of bearing table are found to be beneficial to uniform densification. Moreover, the uniform densification process of sintering solid-state electrolyte is studied both microscopically and macroscopically, which can be divided into three phases according to the grain growing evolution and linear shrinkage patterns. The ionic conductivity of the as-prepared Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) garnet electrolyte is determined to be 0.73 mS cm-1 at 303 K with an activation energy of 0.37 eV. The Li/LLZTO/Li symmetric cell exhibits a small interfacial impedance of 8.49 Ω cm2 and a high apparent critical current density of 2.15 mA cm-2 and also can be cycled for 1000 h continuously without short-circuit. Such results indicate the good feasibility of as-proposed sintering strategy to prepare uniformly dense garnet type solid-state electrolytes for solid-state lithium batteries.

Design Strategies for Anodes and Interfaces Toward Practical Solid-State Li-Metal Batteries

Solid-state Li-metal batteries (based on solid-state electrolytes) offer excellent safety and exhibit high potential to overcome the energy-density limitations of current Li-ion batteries, making them suitable candidates for the rapidly developing fields of electric vehicles and energy-storage systems. However, establishing close solid-solid contact is challenging, and Li-dendrite formation in solid-state electrolytes at high current densities causes fatal technical problems (due to high interfacial resistance and short-circuit failure). The Li metal/solid electrolyte interfacial properties significantly influence the kinetics of Li-metal batteries and short-circuit formation. This review discusses various strategies for introducing anode interlayers, from the perspective of reducing the interfacial resistance and preventing short-circuit formation. In addition, 3D anode structural-design strategies are discussed to alleviate the stress caused by volume changes during charging and discharging. This review highlights the importance of comprehensive anode/electrolyte interface control and anode design strategies that reduce the interfacial resistance, hinder short-circuit formation, and facilitate stress relief for developing Li-metal batteries with commercial-level performance.