Lithium Ion Repulsion-Enrichment Synergism Induced by Core-Shell Ionic Complexes to Enable High-Loading Lithium Metal Batteries

Recent Advances on the Functionalities of Polyoxometalate-Based Ionic Liquids

Polyoxometalate (POM)-based ionic liquids (POM-ILs) are gaining increasing attention due to their diverse structures and functionalities. POMs in POM-ILs not only act as essential structural building blocks but also play a crucial role in their functional performance. With the incorporation of POMs, POM-ILs find applications in various fields such as chemical catalysis, energy science, materials science, sensors, and more. The abundant availability of POMs and other building blocks in POM-ILs, along with their versatile combination possibilities, present promising opportunities for the future. Rather than focusing solely on discovering new structures of POM-ILs, current developments in this field emphasize exploring their functions, leading to the emergence of numerous new applications. Summarizing these advancements aids in understanding the latest trends and facilitates rapid evolution. This review examines the recent five years' worth of results to analyze the new functions of POM-ILs, categorizing them based on their unique characteristics.

Dynamic and Reversible Blending Interface on Polyoxovanadate Electrode for High-Performance Lithium-Ion Batteries

Solid electrolyte interphase (SEI) plays a critical role in the performance of lithium-ion batteries (LIBs). In contrast to the clear interface between the traditional consecutive electrode materials and SEI, ionic polyoxometalates (POMs) as electrode could bilaterally diffuse with SEI and form a blending interface for superior electrochemical performance. POMs have recently aroused much interest as electrode materials in LIBs due to their structural flexibility, high capacity, and cycling stability. However, the interface evolution between POM-based electrodes and SEI, which is critical for Li+ ion transportation, has rarely been explored. Herein, we choose Li10[V12B18O60H6] (LVB) as an example to investigate the formation and structural evolution of the electrode-electrolyte interface. Time-of-flight secondary ion mass spectrometry together with X-ray photoelectron spectroscopy demonstrates the evolution of a blending layer at the interface containing typical SEI components, a polyanion from LVB and a phosphate anion from decomposition products of LiPF6. In the blending layer, ion migration takes place between the P-related inorganic species and the polyanion during the Li+ insertion/extraction reaction. Such a compatible blending layer favors Li+ transportation and the reversibility of the redox reactions, as supported by a series of electrochemical analyses. This work provides detailed insights into understanding the interface evolution of the LVB electrode and demonstrates the importance of interfacial engineering to induce proper interface layers in the development of high-performance POM-based electrodes for LIBs.

Lithium-Ion Charged Polymer Channels Flattening Lithium Metal Anode

The concentration difference in the near-surface region of lithium metal is the main cause of lithium dendrite growth. Resolving this issue will be key to achieving high-performance lithium metal batteries (LMBs). Herein, we construct a lithium nitrate (LiNO3)-implanted electroactive β phase polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) crystalline polymorph layer (PHL). The electronegatively charged polymer chains attain lithium ions on the surface to form lithium-ion charged channels. These channels act as reservoirs to sustainably release Li ions to recompense the ionic flux of electrolytes, decreasing the growth of lithium dendrites. The stretched molecular channels can also accelerate the transport of Li ions. The combined effects enable a high Coulombic efficiency of 97.0% for 250 cycles in lithium (Li)||copper (Cu) cell and a stable symmetric plating/stripping behavior over 2000 h at 3 mA cm-2 with ultrahigh Li utilization of 50%. Furthermore, the full cell coupled with PHL-Cu@Li anode and LiFePO4 cathode exhibits long-term cycle stability with high-capacity retention of 95.9% after 900 cycles. Impressively, the full cell paired with LiNi0.87Co0.1Mn0.03O2 maintains a discharge capacity of 170.0 mAh g-1 with a capacity retention of 84.3% after 100 cycles even under harsh condition of ultralow N/P ratio of 0.83. This facile strategy will widen the potential application of LiNO3 in ester-based electrolyte for practical high-voltage LMBs.

Polyoxometalate Li3 PW12 O40 and Li3 PMo12 O40 Electrolytes for High-energy All-solid-state Lithium Batteries

Solid-state lithium (Li) batteries promise both high energy density and safety while existing solid-state electrolytes (SSEs) fail to satisfy the rigorous requirements of battery operations. Herein, novel polyoxometalate SSEs, Li3 PW12 O40 and Li3 PMo12 O40 , are synthesized, which exhibit excellent interfacial compatibility with electrodes and chemical stability, overcoming the limitations of conventional SSEs. A high ionic conductivity of 0.89 mS cm-1 and a low activation energy of 0.23 eV are obtained due to the optimized three-dimensional Li+ migration network of Li3 PW12 O40 . Li3 PW12 O40 exhibits a wide window of electrochemical stability that can both accommodate the Li anode and high-voltage cathodes. As a result, all-solid-state Li metal batteries fabricated with Li/Li3 PW12 O40 /LiNi0.5 Co0.2 Mn0.3 O2 display a stable cycling up to 100 cycles with a cutoff voltage of 4.35 V and an areal capacity of more than 4 mAh cm-2 , as well as a cost-competitive SSEs price of $5.68 kg-1 . Moreover, Li3 PMo12 O40 homologous to Li3 PW12 O40 was obtained via isomorphous substitution, which formed a low-resistance interface with Li3 PW12 O40 . Applications of Li3 PW12 O40 and Li3 PMo12 O40 in Li-air batteries further demonstrate that long cycle life (650 cycles) can be achieved. This strategy provides a facile, low-cost strategy to construct efficient and scalable solid polyoxometalate electrolytes for high-energy solid-state Li metal batteries.

A Forceful "Dendrite-Killer" of Polyoxomolybdate with Reusability Effectively Dominating Dendrite-Free Lithium Metal Anode

In this work, a series of Mo-containing polyoxometalates (POMs) modified separators to inhibit the growth of lithium dendrites, and thus improving the lifespan and safety of the cells is proposed. When the deposited lithium forms dendrites and touches the separator, the optimized Dawson-type POM of (NH4 )6 [P2 Mo18 O62 ]·11H2 O (P2 Mo18 ) with the stronger oxidizability, acts like a "killer", is more inclined to oxidize Li0 into Li+ , thus weakening the lethality of lithium dendrites. The above process is accompanied by the formation of Lix [P2 Mo18 O62 ] (x = 6-10) in its reduced state. Converting to the stripping process, the reduced state Lix [P2 Mo18 O62 ] (x = 6-10) can be reoxidized to P2 Mo18 , which achieves the reusability of P2 Mo18 functional material. Meanwhile, lithium ions are released into the cell system to participate in the subsequent electrochemical cycles, thus the undesired lithium dendrites are converted into usable lithium ions to prevent the generation of "dead lithium". As a result, the Li//Li symmetrical cell with P2 Mo18 modified separator delivers exceptional cyclic stability for over 1000 h at 3 mA cm-2 and 5 mAh cm-2 , and the assembled Li-S full cell maintains superior reversible capacity of 600 mAh g-1 after 200 cycles at 2 C.

Designing a Stable Solid Electrolyte Interphase on Lithium Metal Anodes by Tailoring a Mg Atom Center and the Inner Helmholtz Plane for Lithium-Sulfur Batteries

Arising from the extraordinary theoretical energy density, rechargeable lithium-sulfur (Li-S) batteries have been reputed as one of the most appealing options for next-generation high-performance energy storage and conversion devices. Unfortunately, their industrial implementation has been strongly governed by the formation of Li dendrites caused by the unstable solid electrolyte interphase (SEI) film. Herein, we report a novel electrolyte by introducing the Mg(NO₃)₂ additive to suppress the growth of Li dendrites, further improving the cycling lifetime of Li-S batteries. On the one hand, Mg²⁺ can rapidly react with Li atoms to generate Mg atoms, replacing the Li atoms on the top surface of Li metal and forming the Mg center simultaneously. On the other hand, NO₃^- can be adsorbed in the inner Helmholtz plane and reduced as an inorganic-rich SEI film for stabilizing the Li metal anode when the electrolyte comes in contact with Li metal, effectively mitigating the formation of Li dendrites. Combining the experimental results and theoretical calculations, we confirm that the Mg atom center and the inorganic-rich SEI film are both beneficial for enhancing the electrochemical performance of Li-S batteries. This work provides a new insight into the electrolyte additive and a possible alternative for the design of high-performance Li-S batteries beyond the LiNO₃ additive.

Controlling Li Dendritic Growth in Graphite Anodes by Potassium Electrolyte Additives for Li-Ion Batteries

Fast charging promotes Li dendrite formation and its growth on graphite anodes, which affects cell performance in Li-ion batteries (LIBs). This work reports the formation of a robust SEI layer by introducing a KPF₆ inorganic additive into the electrolyte. An optimal concentration of 0.001 M KPF₆ effectively inhibits the growth of Li dendrites at 2C charging rates, compared with a commercial electrolyte. Electrolytes containing a KPF₆ additive are shown here to deliver dual effects to mitigate the growth of dendrites. A thin LiF-rich SEI layer is formed on graphite, which blocks the electron leakage pathways. Additionally, K⁺ resides at defect sites (such as particle boundaries) due to its faster diffusion rate and blocks the incoming Li⁺ and restricts the growth of Li dendrites. The electrolyte with optimum concentration of KPF₆, i.e., 0.001 M, effectively directs Li⁺ transport through the thin, durable, and low resistance LiF-rich SEI layer. This has implications for fast charging through optimization of the electrode/electrolyte interphase by controlling additive concentrations.

Revisiting the Role of Physical Confinement and Chemical Regulation of 3D Hosts for Dendrite-Free Li Metal Anode

Lithium metal anode has been demonstrated as the most promising anode for lithium batteries because of its high theoretical capacity, but infinite volume change and dendritic growth during Li electrodeposition have prevented its practical applications. Both physical morphology confinement and chemical adsorption/diffusion regulation are two crucial approaches to designing lithiophilic materials to alleviate dendrite of Li metal anode. However, their roles in suppressing dendrite growth for long-life Li anode are not fully understood yet. Herein, three different Ni-based nanosheet arrays (NiO-NS, Ni₃N-NS, and Ni₅P₄-NS) on carbon cloth as proof-of-concept lithiophilic frameworks are proposed for Li metal anodes. The two-dimensional nanoarray is more promising to facilitate uniform Li⁺ flow and electric field. Compared with the NiO-NS and the Ni₅P₄-NS, the Ni₃N-NS on carbon cloth after reacting with molten Li (Li-Ni/Li₃N-NS@CC) can afford the strongest adsorption to Li⁺ and the most rapid Li⁺ diffusion path. Therefore, the Li-Ni/Li₃N-NS@CC electrode realizes the lowest overpotential and the most excellent electrochemical performance (60 mA cm^-2 and 60 mAh cm^-2 for 1000 h). Furthermore, a remarkable full battery (LiFePO₄||Li-Ni/Li₃N-NS@CC) reaches 300 cycles at 2C. This research provides valuable insight into designing dendrite-free alkali metal batteries.

Enflurane Additive for Sodium Negative Electrodes

Development of sodium anodes, both hard carbon (HC) and metallic, is dependent on the discovery of electrolyte formations and additives able to stabilize the interphase and support Na⁺ transport. Halogen salt additives are known to lower the energy barrier for the Na-ion charge transfer at the interface and facilitate stable Na plating/stripping in a symmetric cell configuration. Here, a halogen-rich additive for the sodium-ion battery electrolyte, 2-chloro-1,1,2-trifluoroethyl difluoromethyl ether (enflurane), is reported. Enflurane offers a simple molecular alternative to salt-based additives. The additive is also shown to improve the cycling performance of sodium metal electrodes. Our analysis demonstrates that enflurane is preferentially reduced at the HC electrode over propylene carbonate and is incorporated into the solid electrolyte interphase (SEI). The result is a thin, halogen-rich SEI that offers better charge transport properties and stability during cycling compared to that formed in the additive-free electrolyte. Additionally, enflurane inhibits polarization of metallic sodium electrodes, and when included in HC half-cells at 10 v/v %, it improves the reversible specific capacity and stability.

Long-Life Lithium-Metal All-Solid-State Batteries and Stable Li Plating Enabled by In Situ Formation of Li3 PS4 in the SEI Layer

An ultrastable and kinetically favorable interface is constructed between sulfide-poly(ethylene oxide) (PEO) composite solid electrolytes (CSEs) and lithium metal, via in situ formation of a solid electrolyte interphase (SEI) layer containing Li₃ PS₄ . A specially designed sulfide, lithium polysulfidophosphate (LPS), can distribute uniformly in the PEO matrix via a simple stirring process because of its complete solubility in acetonitrile solvent, which is advantageous for creating a homogeneous SEI layer. The CSE/Li interface with high Li⁺ transportation capability is stabilized quickly through in situ formation of a Li₃ PS₄ /Li₂ S/LiF layer via the reaction between LPS and lithium metal to inhibit lithium dendrite growth. A Li/Li symmetric cell with the LPS-integrated CSE exhibits constant and small CSE/Li resistance of 10 Ω cm²  during cycling, delivering stable cycling for 3475 h at a current density of 0.2 mA cm^-2  and a high critical current density of 0.9 mA cm^-2  at 60 °C. Impressive electrochemical performance is also demonstrated for LiFePO₄ /CSE/Li all-solid-state batteries with capacity of 127.6 mAh g^-1  after 1000 cycles at 1 C.

Lithium Oxalate as a Lifespan Extender for Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries (AFLMBs) have been extensively studied due to their intrinsic high energy and safety without a metallic Li anode in cell design. Yet, the dendrite and dead-Li buildup continuously consumes the active Li upon cycling, leading to the poor lifespan of AFLMBs. Here, we introduce lithium oxalate into the cathode as an electrode additive providing a Li reservoir to extend the lifespan of AFLMBs. The AFLMB using 20% lithium oxalate and a LiNi0.3Co0.3Mn0.3O2 composite cathode exhibits >80 and 40% capacity retention after 50 and 100 cycles, respectively, outperforming the poor cycle life of fewer than 20 cycles obtained from the cell using a pure LiNi0.3Co0.3Mn0.3O2 cathode. Surprisingly, the average Coulombic efficiency of AFLMBs is found to improve as the amount of lithium oxalate increases in the composite cathode. This abnormal phenomenon could be attributed to the as-formed carbon dioxide after the first activation cycle forming a Li2CO3-rich solid-electrolyte interphase and improving the Li deposition and stripping efficiency. The findings in this work provide a new strategy to delay the capacity roll-over of AFLMBs from an electrode engineering perspective, which can be coupled with other approaches such as functional electrolytes synergistically to further improve the cycle life of AFLMBs for practical application.

Achieve Stable Lithium Metal Anode by Sulfurized-Polyacrylonitrile Modified Separator for High-Performance Lithium Batteries

To develop a high-energy-density lithium battery, there still are several severe challenges for Li metal anode: low Coulombic efficiency caused by its high chemical reactivity, Li dendrite formation, and "dead" Li accumulation during repeated plating/stripping processes. Especially, lithium dendrite growth imposes inferior cycling stability and serious safety issues. Herein, we propose a facile but effective strategy to suppress lithium dendrite growth through an artificial inorganic-polymer protective layer derived from sulfurized polyacrylonitrile on a polyethylene separator. Benefiting from the lithiated sulfurized polyacrylonitrile and poly(acrylic acid), the flexible and ion-conductive protective layer could regulate Li⁺ flux and facilitate dendrite-free lithium deposition. Consequently, lithium metal with the meritorious protective layer can achieve a long-term cycling with negligible overpotential rise in Li-Li symmetric cells, even at a high areal capacity of 5 mAh cm^-2. Remarkably, such a protective layer enables stable cycling performance of Li-S cell with a high areal capacity (∼9 mAh cm^-2). This work provides a valuable exploration strategy for potential industrial applications of high-performance lithium metal batteries.