Highly Efficient Spatially-Temporally Synchronized Construction of Robust Li3 PO4 -rich Solid-Electrolyte Interphases in Aqueous Li-ion Batteries

Solid electrolyte interphase (SEI) makes the electrochemical window of aqueous electrolytes beyond the thermodynamics limitation of water. However, achieving the energetic and robust SEI is more challenging in aqueous electrolytes because the low SEI formation efficiency (SFE) only contributed from anion-reduced products, and the low SEI formation quality (SFQ) negatively impacted by the hydrogen evolution, resulting in a high Li loss to compensate for SEI formation. Herein, we propose a highly efficient strategy to construct Spatially-Temporally Synchronized (STS) robust SEI by the involvement of synergistic chemical precipitation-electrochemical reduction. In this case, a robust Li3 PO4 -rich SEI enables intelligent inherent growth at the active site of the hydrogen by the chemical capture of the OH- stemmed from the HER to trigger the ionization balance of dihydrogen phosphate (H2 PO4 - ) shift to insoluble solid Li3 PO4 . It is worth highlighting that the Li3 PO4 formation does not extra-consume lithium derived from the cathode but makes good use of the product of HER (OH- ), prompting the SEI to achieve 100 % SFE and pushing the HER potential into -1.8 V vs. Ag/AgCl. This energetic and robust SEI offers a new way to achieve anion/concentration-independent interfacial chemistry for the aqueous batteries.

Facile Solid-State Synthesis to In Situ Generate a Composite Coating Layer Composed of Spinel-Structural Compounds and Li3PO4 for Stable Cycling of LiCoO2 at 4.6 V

Due to its high energy density, high-voltage LiCoO2 is the preferred cathode material for consumer electronic products. However, its commercial viability is hindered by rapid capacity decay resulting from structural degradation and surface passivation during cycling at 4.6 V. The key to achieving stable cycling of LiCoO2 at high voltages lies in constructing a highly stable interface to mitigate surface side reactions. In this study, we present a facile in situ coating strategy that is amenable to mass production through a simple wet-mixing process, followed by high-temperature calcination. By capitalizing on the facile dispersion characteristics of nano-TiO2 in ethanol and the ethanol dissolubility of LiPO2F2, we construct a uniform precoating layer on LiCoO2 with nano-TiO2 and LiPO2F2. The subsequent thermal treatment triggers an in situ reaction between the coating reagents and LiCoO2, yielding a uniform composite coating layer. This composite layer comprises spinel-structured compounds (e.g., LiCoTiO4) and Li3PO4, which exhibit excellent chemical and structural stability under high-voltage conditions. The uniform and stable coating layer effectively prevents direct contact between LiCoO2 and the electrolyte, thereby reducing side reactions and suppressing the surface passivation of LiCoO2 particles. As a result, coated LiCoO2 maintains favorable electronic and ionic conductivity even after prolonged cycling. The synergistic effects of spinel-structured compounds and Li3PO4 contribute to the superior performance of LiCoO2, demonstrating a high capacity of 202.1 mA h g-1 (3.0-4.6 V, 0.5 C, 1 C = 274 mA g-1), with a capacity retention rate of 96.7% after 100 cycles.

Realizing Holistic Charging-Discharging for Dendrite-Free Lithium Metal Anodes via Constructing Three-Dimensional Li+ Conductive Networks

Lithium (Li) metal is a promising candidate for next-generation anode materials with high energy densities. However, Li dissolution/deposition processes are limited at the upper surface in contact with the electrolyte, which brings a locally high current density and then results in dendritic Li growth. This restraint of the local surface reaction during cycling has not been solved by commonly used modification strategies. In this study, a three-dimensional (3D) Li⁺ conductive skeleton is activated from atomic layer deposition (ALD) coating Li₃PO₄ (LPO) on the surface of the Ni foam (LPNF). Then, the skeleton is efficiently constructed in the Li metal anode by the lower-temperature Li infusion. Ionic conductor LPO layers and electronic conductor Ni fibers supply charge transport channels between the electrolyte and the internal Li. The mixed conductive network realizes holistic charge transfer, which is proved by in situ scanning electron microscopy experiments. In virtue of dispersive dissolution/deposition and optimized electrochemical kinetics brought by a Li⁺ conductive network, the composited Li electrode presents an excellent symmetric battery cycling stability (over 1200 h) and enhanced rate performances (stable cycling even at 10.0 mA cm^-2). When matching with a LiCoO₂ (LCO) cathode, LCO||Li@LPNF full batteries exhibit a capacity retention of 80.8% over 250 cycles. During cycling, there was no evidence of dendrite growth and the remaining Li in the composited anode showed a smooth, compact, and well-combined condition with LPNF. Through constructing a 3D Li⁺ conductive network, the composited Li metal anode breaks through the limit of the local surface reaction; this work proposes a novel insight of realizing holistic charging/discharging for the dendrite-free Li metal anode.

H3PO4-Induced Nano-Li3PO4 Pre-reduction Layer to Address Instability between the Nb-Doped Li7La3Zr2O12 Electrolyte and Metallic Li Anode

Solid-state batteries based on a metallic Li anode and nonflammable solid electrolytes (SEs) are anticipated to achieve high energy and power densities with absolute safety. In particular, cubic garnet-type Nb-doped Li₇La₃Zr₂O₁₂ (Nb-LLZO) SEs possess superior ionic conductivity, are feasible to prepare under ambient conditions, have strong thermal stability, and are of low cost. However, the interfacial compatibility with Li metal and Li dendrite hazards still hinder the applications of Nb-LLZO. Herein, a quick and efficient solution was applied to address this issue, generating a nano-Li₃PO₄ pre-reduction layer from the reaction of H₃PO₄ with the ion-exchanged passivation layer (Li₂CO₃/LiOH) on the surface of Nb-LLZO. A lithiophilic, electrically insulating interlayer is in situ created when the Li₃PO₄ modified layer interacts with molten Li, successfully preventing the reduction of Nb⁵⁺. The interlayer, which mostly consists of Li₃P and Li₃PO₄, also has a high shear modulus and relatively high Li⁺ conductivity, which effectively inhibit the growth of Li dendrites. The Li|Li₃PO₄|Nb-LLZO|Li₃PO₄|Li symmetric cells stably cycled for over 5000 h at 0.05 mA cm^-2 and over 1000 h at a high rate of 0.15 mA cm^-2 without any short circuits. The LiFePO₄ and S/C hybrid solid-state batteries using the modified Nb-LLZO electrolyte also demonstrated good electrochemical performances, confirming the practical application of this interfacial engineering in various solid-state battery systems. This work offers an efficient solution to the instability issue between the Nb-LLZO SE and metallic Li anode.

Important Impact of the Slurry Mixing Speed on Water-Processed Li4Ti5O12 Lithium-Ion Anodes in the Presence of H3PO4 as the Processing Additive

The aqueous processing of lithium transition metal oxides into battery electrodes is attracting a lot of attention as it would allow for avoiding the use of harmful N-methyl-2-pyrrolidone (NMP) from the cell fabrication process and, thus, render it more sustainable. The addition of slurry additives, for instance phosphoric acid (PA), has been proven to be highly effective for overcoming the corresponding challenges such as aluminum current collector corrosion and stabilization of the active material particle. Herein, a comprehensive investigation of the effect of the ball-milling speed on the effectiveness of PA as a slurry additive is reported using Li₄Ti₅O₁₂ (LTO) as an exemplary lithium transition metal oxide. Interestingly, at elevated ball-milling speeds, rod-shaped lithium phosphate particles are formed, which remain absent at lower ball-milling speeds. A detailed surface characterization by means of SEM, EDX, HRTEM, STEM-EDX, XPS, and EIS revealed that in the latter case, a thin protective phosphate layer is formed on the LTO particles, leading to an improved electrochemical performance. As a result, the corresponding lithium-ion cells comprising LTO anodes and LiNi_0.5Mn_0.3Co_0.2O₂ (NMC₅₃₂) cathodes reveal greater long-term cycling stability and higher capacity retention after more than 800 cycles. This superior performance originates from the less resistive electrode-electrolyte interphase evolving upon cycling, owing to the interface-stabilizing effect of the lithium phosphate coating formed during electrode preparation. The results highlight the importance of commonly neglected─frequently not even reported─electrode preparation parameters.

Atomically Well-Ordered Structure at Solid Electrolyte and Electrode Interface Reduces the Interfacial Resistance

Using synchrotron surface X-ray diffraction, we investigated the atomic structures of the interfaces of a solid electrolyte (Li₃PO₄) and electrode (LiCoO₂). We prepared two types of interfaces with high and low interface resistances; the low-resistance interface exhibited a flat and well-ordered atomic arrangement at the electrode surface, whereas the high-resistance interface showed a disordered interface. These results indicate that the crystallinity of LiCoO₂ at the interface has a significant impact on interface resistance. Furthermore, we reveal that the migration of Li ions along the interface and into grain boundaries and antiphase domain boundaries is a critical factor reducing interface resistance.