Artificial Interphase Design Employing Inorganic-Organic Components for High-Energy Lithium-Metal Batteries

To increase the energy density of today's lithium batteries, it is necessary to develop an anode with higher energy density than graphite or carbon/silicon composites. Hence, research on metallic lithium has gained a steadily increasing momentum. However, the severe safety issues and poor Coulombic efficiency of this highly reactive metal hinder its practical application in lithium-metal batteries (LMBs). Herein, the development of an artificial interphase is reported to enhance the reversibility of the lithium stripping/plating process and suppress the parasitic reactions with the liquid organic carbonate-based electrolyte. This artificial interphase is spontaneously formed by an alloying reaction-based coating, forming a stable inorganic/organic hybrid interphase. The accordingly modified lithium-metal electrodes provide substantially improved cycle life to symmetric Li||Li cells and high-energy Li||LiNi_0.8Co_0.1Mn_0.1O₂ cells. For these LMBs, 7 μm thick lithium-metal electrodes have been employed while applying a current density of 1.0 mA cm^-2, thus highlighting the great potential of this tailored interphase.

Enabling High-Stability of Aqueous-Processed Nickel-Rich Positive Electrodes in Lithium Metal Batteries

Lithium batteries occupy the large-scale electric mobility market raising concerns about the environmental impact of cell production, especially regarding the use of poly(vinylidene difluoride) (teratogenic) and N-methyl-2-pyrrolidone (NMP, harmful). To avoid their use, an aqueous electrode processing route is utilized in which a water-soluble hybrid acrylic-fluoropolymer together with sodium carboxymethyl cellulose is used as binder, and a thin phosphate coating layer is in situ formed on the surface of the nickel-rich cathode during electrode processing. The resulting electrodes achieve a comparable performance to that of NMP-based electrodes in conventional organic carbonate-based electrolyte (LP30). Subsequently, an ionic liquid electrolyte (ILE) is employed to replace the organic electrolyte, building stable electrode/electrolyte interphases on the surface of the nickel-rich positive electrode (cathode) and metallic lithium negative electrode (anode). In such ILE, the aqueously processed electrodes achieve high cycling stability with a capacity retention of 91% after 1000 cycles (20 °C). In addition, a high capacity of more than 2.5 mAh cm^-2 is achieved for high loading electrodes (≈15 mg cm^-2 ) by using a modified ILE with 5% vinylene carbonate additive. A path to achieve environmentally friendly electrode manufacturing while maintaining their outstanding performance and structural integrity is demonstrated.

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.

Concentrated Electrolytes Enabling Stable Aqueous Ammonium-Ion Batteries

Rechargeable aqueous batteries are promising devices for large-scale energy-storage applications because of their low-cost, inherent safety, and environmental friendliness. Among them, aqueous ammonium-ion (NH₄ ⁺ ) batteries (AAIB) are currently emerging owing to the fast diffusion kinetics of NH₄ ⁺ . Nevertheless, it is still a challenge to obtain stable AAIB with relatively high output potential, considering the instability of many electrode materials in an aqueous environment. Herein, a cell based on a concentrated (5.8 m) aqueous (NH₄ )₂ SO₄ electrolyte, ammonium copper hexacyanoferrate (N-CuHCF) as the positive electrode (cathode), and 3,4,9,10-perylene-bis(dicarboximide) (PTCDI) as the negative electrode (anode) is reported. The solvation structure, electrochemical properties, as well as the electrode-electrolyte interface and interphase are systematically investigated by the combination of theoretical and experimental methods. The results indicate a remarkable cycling performance of the low-cost rocking-chair AAIB, which offers a capacity retention of ≈72% after 1000 cycles and an average output potential of ≈1.0 V.

Determination of the Volume Changes Occurring for Conversion/Alloying-Type Li-Ion Anodes upon Lithiation/Delithiation

High-capacity lithium-ion anodes such as alloying-, conversion-, and conversion/alloying-type materials are subjected to extensive volume variation upon lithiation/delithiation. However, a careful examination of these processes at the particle and electrode level as well as the impact of the kind of lithium-ion uptake mechanism is still missing. Herein, we investigated the volume variation upon lithiation/delithiation for a series of conversion/alloying materials with a varying relative contribution of the alloying and conversion reaction, i.e., carbon-coated ZnFe₂O₄, Zn_0.9Fe_0.1O, and Sn_0.9Fe_0.1O₂ by operando dilatometry and ex situ scanning electron microscopy of the electrode cross section. While the theoretical estimation at the particle level indicates a rather large volume expansion of 113% (ZnFe₂O₄) and more, the true volume variation on the electrode level reveals very limited changes of only around 11% (ZnFe₂O₄). Combining the experimental findings with some theoretical considerations highlights the (to a certain extent unexpected) impact of the initial electrode porosity.

Scalable Synthesis of Microsized, Nanocrystalline Zn0.9 Fe0.1 O-C Secondary Particles and Their Use in Zn0.9 Fe0.1 O-C/LiNi0.5 Mn1.5 O4 Lithium-Ion Full Cells

Conversion/alloying materials (CAMs) are a potential alternative to graphite as Li-ion anodes, especially for high-power performance. The so far most investigated CAM is carbon-coated Zn0.9 Fe0.1 O, which provides very high specific capacity of more than 900 mAh g-1 and good rate capability. Especially for the latter the optimal particle size is in the nanometer regime. However, this leads to limited electrode packing densities and safety issues in large-scale handling and processing. Herein, a new synthesis route including three spray-drying steps that results in the formation of microsized, spherical secondary particles is reported. The resulting particles with sizes of 10-15 μm are composed of carbon-coated Zn0.9 Fe0.1 O nanocrystals with an average diameter of approximately 30-40 nm. The carbon coating ensures fast electron transport in the secondary particles and, thus, high rate capability of the resulting electrodes. Coupling partially prelithiated, carbon-coated Zn0.9 Fe0.1 O anodes with LiNi0.5 Mn1.5 O4 cathodes results in cobalt-free Li-ion cells delivering a specific energy of up to 284 Wh kg-1 (at 1 C rate) and power of 1105 W kg-1 (at 3 C) with remarkable energy efficiency (>93 % at 1 C and 91.8 % at 3 C).

Co-Crosslinked Water-Soluble Biopolymers as a Binder for High-Voltage LiNi0.5 Mn1.5 O4 |Graphite Lithium-Ion Full Cells

The use of water-soluble, abundant biopolymers as binders for lithium-ion positive electrodes is explored because it represents a great step forward towards environmentally benign battery processing. However, to date, most studies that employ, for instance, carboxymethyl cellulose (CMC) as a binder have focused on rather low electrode areal loadings with limited relevance for industrial needs. This study concerns the use of natural guar gum (GG) as a binding agent for cobalt-free, high-voltage LiNi_0.5 Mn_1.5 O₄ (LNMO), which realizes electrodes with substantially increased areal loadings, low binder content, and greatly enhanced cycling stability. Co-crosslinking GG through citric acid with CMC allows for an enhanced rate capability and essentially maintains the beneficial impact of using GG as a binder rather than CMC only. Lithium-ion full cells based on water-processed LNMO and graphite electrodes provide a remarkably high cycling stability with 80 % capacity retention after 1000 cycles at 1 C.

Overcoming the Interfacial Limitations Imposed by the Solid-Solid Interface in Solid-State Batteries Using Ionic Liquid-Based Interlayers

Li-garnets are promising inorganic ceramic solid electrolytes for lithium metal batteries, showing good electrochemical stability with Li anode. However, their brittle and stiff nature restricts their intimate contact with both the electrodes, hence presenting high interfacial resistance to the ionic mobility. To address this issue, a strategy employing ionic liquid electrolyte (ILE) thin interlayers at the electrodes/electrolyte interfaces is adopted, which helps overcome the barrier for ion transport. The chemically stable ILE improves the electrodes-solid electrolyte contact, significantly reducing the interfacial resistance at both the positive and negative electrodes interfaces. This results in the more homogeneous deposition of metallic lithium at the negative electrode, suppressing the dendrite growth across the solid electrolyte even at high current densities of 0.3 mA cm^-2 . Further, the improved interface Li/electrolyte interface results in decreasing the overpotential of symmetric Li/Li cells from 1.35 to 0.35 V. The ILE modified Li/LLZO/LFP cells stacked either in monopolar or bipolar configurations show excellent electrochemical performance. In particular, the bipolar cell operates at a high voltage (≈8 V) and delivers specific capacity as high as 145 mAh g^-1 with a coulombic efficiency greater than 99%.

Toward Stable Electrode/Electrolyte Interface of P2-Layered Oxide for Rechargeable Na-Ion Batteries

The electrochemical properties of P2-Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ layered oxide, which is a promising cathode material for rechargeable Na-ion batteries (NIBs), are evaluated with an optimized in-house ionic liquid (IL)-based electrolyte, and its performance is compared with that using carbonate-based electrolyte. The IL-based system reveals better electrochemical performance at room temperature than the carbonate electrolyte-based one at 0.1C and 1C, especially in terms of cycling stability, with a 97% capacity retention after 100 deep cycles (0.1C). The electrode/electrolyte interface is thoroughly studied in both systems by means of X-ray photoelectron spectroscopy and scanning electron microscopy so as proof that the formed interface is crucial to optimizing the electrochemical performance of NIBs. The carbonate-based system shows a thin, inhomogeneous, and unstable interface layer, while the IL-based one exhibits an even thinner but homogeneous and more stable interface, which may result in safer and longer-lasting NIBs.

Complementary Strategies Toward the Aqueous Processing of High-Voltage LiNi0.5 Mn1.5 O4 Lithium-Ion Cathodes

Increasing the environmental benignity of lithium-ion batteries is one of the greatest challenges for their large-scale deployment. Toward this end, we present herein a strategy to enable the aqueous processing of high-voltage LiNi_0.5 Mn_1.5 O₄ (LNMO) cathodes, which are considered highly, if not the most, promising for the realization of cobalt-free next-generation lithium-ion cathodes. Combining the addition of phosphoric acid with the cross-linking of sodium carboxymethyl cellulose by means of citric acid, aqueously processed electrodes with excellent performance are produced. The combined approach offers synergistic benefits, resulting in stable cycling performance and excellent coulombic efficiency (98.96 %) in lithium-metal cells. Remarkably, this approach can be easily incorporated into standard electrode preparation processes with no additional processing step.