Active Interphase Enables Stable Performance for an All-Phosphate-Based Composite Cathode in an All-Solid-State Battery

Impact of Composite Cathode Architecture Engineering on the Performance of All-Solid-State Sodium Batteries

The electrode/electrolyte interfacial contact, ionic percolation pathways, and charge transfer resistance within the cathode significantly impact the performance and lifespan of all-solid-state sodium batteries (AS3Bs). Addressing these issues requires optimization of the composite cathode architecture and the electrode/electrolyte interface in AS3Bs. One major challenge in developing composite cathodes with oxide solid electrolytes is selecting the appropriate thermal processing temperature to ensure intimate contact between the cathode active material and solid electrolyte while ensuring a sufficient ionic percolation pathway inside the composite cathode. In this study, we present an approach for fabricating a composite cathode by cofiring sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF) and the sodium superionic conductor (Na3Zr2Si2PO12, NZSP) at 700 °C using an optimized weight ratio. This method ensures reduced interfacial resistance between NVPF and NZSP while establishing an efficient ionic percolation pathway. To further enhance ionic percolation within the composite cathode, residual voids are filled with a polymer electrolyte composed of PEO/NaClO4. Benefits of the dense composite cathode structure, a stable NVPF/NZSP interface, negligible pores in the composite cathode, and a three-dimensional electronic and ionic percolation network facilitate the greater utilization of cathode active material with almost no capacity degradation upon long-term cycling. The full cell, with the optimized composite cathode, delivers an initial discharge capacity of 114 mA h g-1 at 0.1 C, retaining 85% of its capacity after 500 cycles with a 99% Coulombic efficiency and excellent rate capability at 1 C.

Formation Mechanism and New Function of Cathode Electrolyte Interphase/Solid Electrolyte Interphase in Lithium-Ion Battery with LiPF6 + LATP Composite Electrolyte

The inorganic components of the cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) play an important role in the cycle stability of lithium batteries. The electrolyte additive can modify CEI and SEI simultaneously. In this work, Li1.3Al0.3Ti1.7(PO4)3(LATP) is used as an electrolyte additive to form CEI and SEI layers with abundant LiF and Li3PO4. LTAP not only modulates the embedding/de-embedding process of Li+ by lowering the oxidation potential and forming CEI during the battery charge/discharge cycling process but also enhances the thermal stability and self-discharge. More importantly, during the charging and discharging processes, LATP participates in the electrochemical reaction, resulting in an enrichment of Li+ on the CEI surface, which increases the concentration difference of lithium ions in the electrolyte and enhances their migration speed during charging and discharging. When assembling the LiFePO4/Li coin cells, the experimental results indicate that the cell with LiPF6 + LATP composite electrolyte can discharge 119.12 mA h g-1 (2 C, 500 cycle), approximately 2 times that with pure LiPF6 electrolyte. Meanwhile, it can discharge 128.9 mA h g-1 (0.1 C) after being stored for 400 h, which is about 4 times higher than that with pure LiPF6, indicating that LATP improves the self-discharge and discharge capacity of LFP. Furthermore, the formation mechanism and function of CEI/SEI in lithium-ion batteries with LiPF6 + LATP composite electrolyte are discussed, which offers a new perspective for constructing composite electrolytes with superior energy density, high-rate capability, safety, and support for fast charging and discharging in the future.

Enabling High-Performance Hybrid Solid-State Batteries by Improving the Microstructure of Free-Standing LATP/LFP Composite Cathodes

The phosphate lithium-ion conductor Li1.5Al0.5Ti1.5(PO4)3 (LATP) is an economically attractive solid electrolyte for the fabrication of safe and robust solid-state batteries, but high sintering temperatures pose a material engineering challenge for the fabrication of cell components. In particular, the high surface roughness of composite cathodes resulting from enhanced crystal growth is detrimental to their integration into cells with practical energy density. In this work, we demonstrate that efficient free-standing ceramic cathodes of LATP and LiFePO4 (LFP) can be produced by using a scalable tape casting process. This is achieved by adding 5 wt % of Li2WO4 (LWO) to the casting slurry and optimizing the fabrication process. LWO lowers the sintering temperature without affecting the phase composition of the materials, resulting in mechanically stable, electronically conductive, and free-standing cathodes with a smooth, homogeneous surface. The optimized cathode microstructure enables the deposition of a thin polymer separator attached to the Li metal anode to produce a cell with good volumetric and gravimetric energy densities of 289 Wh dm-3 and 180 Wh kg-1, respectively, on the cell level and Coulombic efficiency above 99% after 30 cycles at 30 °C.

Cathode Interface Construction by Rapid Sintering in Solid-State Batteries

Solid-state batteries (SSBs) are poised to replace traditional organic liquid-electrolyte lithium-ion batteries due to their higher safety and energy density. Oxide-based solid electrolytes (SEs) are particularly attractive for their stability in air and inability to ignite during thermal runaway. However, achieving high-performance in oxide-based SSBs requires the development of an intimate and robust SE-cathode interface to overcome typically large interfacial resistances. The transition interphase should be both physically and chemically active. This study presents a thin, conductive interphase constructed between lithium aluminum titanium phosphate and lithium cobalt oxide using a rapid sintering method that modifies the interphase within 10 s. The rapid heating and cooling rates restrict side reactions and interdiffusion on the interface. SSBs with thick composite cathodes demonstrate a high initial capacity of ≈120 mAh g-1 over 200 cycles at room temperature. Furthermore, the rapid sintering method can be extended to other cathode systems under similar conditions. These findings highlight the importance of constructing an appropriate SE-cathode interface and provide insight into designing practical SSBs.

All-Solid-State Garnet-Based Lithium Batteries at Work-In Operando TEM Investigations of Delithiation/Lithiation Process and Capacity Degradation Mechanism

Li7 La3 Zr2 O12 (LLZO)-based all-solid-state Li batteries (SSLBs) are very attractive next-generation energy storage devices owing to their potential for achieving enhanced safety and improved energy density. However, the rigid nature of the ceramics challenges the SSLB fabrication and the afterward interfacial stability during electrochemical cycling. Here, a promising LLZO-based SSLB with a high areal capacity and stable cycle performance over 100 cycles is demonstrated. In operando transmission electron microscopy (TEM) is used for successfully demonstrating and investigating the delithiation/lithiation process and understanding the capacity degradation mechanism of the SSLB on an atomic scale. Other than the interfacial delamination between LLZO and LiCoO2 (LCO) owing to the stress evolvement during electrochemical cycling, oxygen deficiency of LCO not only causes microcrack formation in LCO but also partially decomposes LCO into metallic Co and is suggested to contribute to the capacity degradation based on the atomic-scale insights. When discharging the SSLB to a voltage of ≈1.2 versus Li/Li+ , severe capacity fading from the irreversible decomposition of LCO into metallic Co and Li2 O is observed under in operando TEM. These observations reveal the capacity degradation mechanisms of the LLZO-based SSLB, which provides important information for future LLZO-based SSLB developments.