Solid Electrolyte Interphase Layers by Using Lithiophilic and Electrochemically Active Ionic Additives for Lithium Metal Anodes

The use of role-assigned ionic additives with different adsorption energies and distinct electron-accepting abilities enables the construction of a multilayer solid electrolyte interphase (SEI) with a sequential structure of lithiophilic, mechanically robust, and ion-permeable layers on Li metal anodes. The uncontrollable Li dendrite formation, which is promoted by localized electric fields on the Li metal anode, is suppressed by the lithiophilic Ag-containing inner SEI and LiF + Li3N-enriched outer SEI with reduced overpotentials upon Li deposition

Cost-Effective Scalable Synthesis of Mesoporous Germanium Particles <i>via</i> a Redox-Transmetalation Reaction for High-Performance Energy Storage Devices

Nanostructured germanium is a promising material for high-performance energy storage devices. However, synthesizing it in a cost-effective and simple manner on a large scale remains a significant challenge. Herein, we report a redox-transmetalation reaction-based route for the large-scale synthesis of mesoporous germanium particles from germanium oxide at temperatures of 420–600 °C. We could confirm that a unique redox-transmetalation reaction occurs between Zn⁰ and Ge⁴⁺ at approximately 420 °C using temperature-dependent <i>in situ</i> X-ray absorption fine structure analysis. This reaction has several advantages, which include (i) the successful synthesis of germanium particles at a low temperature (∼450 °C), (ii) the accommodation of large volume changes, owing to the mesoporous structure of the germanium particles, and (iii) the ability to synthesize the particles in a cost-effective and scalable manner, as inexpensive metal oxides are used as the starting materials. The optimized mesoporous germanium anode exhibits a reversible capacity of ∼1400 mA h g^–1 after 300 cycles at a rate of 0.5 C (corresponding to the capacity retention of 99.5%), as well as stable cycling in a full cell containing a LiCoO₂ cathode with a high energy density (charge capacity = 286.62 mA h cm^–3)

A Phosphorofluoridate-Based Multifunctional Electrolyte Additive Enables Long Cycling of High-Energy Lithium-Ion Batteries

Ni-rich layered oxides are regarded as key components for realizing post Li-ion batteries (LIBs). However, high-valence Ni, which acts as an oxidant in deeply delithiated states, aggravates the oxidation of the electrolyte at the cathode, causing cell impedance to increase. Additionally, the leaching of transition metal (TM) ions from Ni-rich cathodes by acidic compounds such as Brønsted-acidic HF produced through LiPF6 hydrolysis aggravates the structural instability of the cathode and renders the electrode–electrolyte interface unstable. Herein, we present a multifunctional electrolyte additive, bis(trimethylsilyl) phosphorofluoridate (BTSPFA), to attain enhanced interfacial stability of graphite anodes and Ni-rich cathodes in Li-ion cells. BTSPFA eliminates the corrosive HF molecules by cleaving silyl ether bonds and enables the formation of a polar P–O- and P–F-enriched cathode electrolyte interface (CEI) on the Ni-rich cathode. It also promotes the creation of a solid electrolyte interphase composed of inorganic-rich species, which suppresses the reduction of the electrolyte during battery operation. The synergistic effect of the HF scavenging ability of BTSPFA and the stable BTSPFA-promoted CEI effectively suppresses the TM leaching from the Ni-rich cathode while also preventing unwanted TM deposition on the anode. LiNi0.8Co0.1Mn0.1O2/graphite full cells with 1 wt % BTSPFA exhibited an enhanced discharge capacity retention of 79.8% after 500 cycles at 1C and 45 °C. These unique features of BTSPFA are useful for resolving the interfacial deterioration issue of high-capacity Ni-rich cathodes paired with graphite anodes

Acid- and Gas-Scavenging Electrolyte Additive Improving the Electrochemical Reversibility of Ni-Rich Cathodes in Li-Ion Batteries

In view of their high theoretical capacities, nickel-rich layered oxides are promising cathode materials for high-energy Li-ion batteries. However, the practical applications of these oxides are hindered by transition metal dissolution, microcracking, and gas/reactive compound formation due to the undesired reactions of residual lithium species. Herein, we show that the interfacial degradation of the LiNi0.9CoxMnyAlzO2 (NCMA, x + y + z = 0.1) cathode and the graphite (Gr) anode of a representative Li-ion battery by HF can be hindered by supplementing the electrolyte with tert-butyldimethylsilyl glycidyl ether (tBS-GE). The silyl ether moiety of tBS-GE scavenges HF and PF5, thus stabilizing the interfacial layers on both electrodes, while the epoxide moiety reacts with CO2 released by the parasitic reaction between HF and Li2CO3 on the NCMA surface to afford cyclic carbonates and thus suppresses battery swelling. NCMA/Gr full cells fabricated by supplementing the baseline electrolyte with 0.1 wt % tBS-GE feature an increased capacity retention of 85.5% and deliver a high discharge capacity of 162.9 mAh/g after 500 cycles at 1 C and 25 °C. Thus, our results reveal that the molecular aspect-based design of electrolyte additives can be efficiently used to eliminate reactive species and gas components from Li-ion batteries and increase their performance

Designing Electrolytes for Stable Operation of High-Voltage LiCoO₂ in Lithium-Ion Batteries

High-voltage lithium cobalt oxide (LiCoO2) can be used to implement high-energy-density lithium-ion batteries (LIBs). However, the detrimental rock-salt phase-induced poor reversibility, lattice oxygen loss, Co leaching, and construction of a resistive cathode–electrolyte interface (CEI) by uncontrolled electrolyte decomposition at high voltages restrict the use of LiCoO2. Here, we discuss the rational design of an electrolyte for use in LIBs. We obtained this electrolyte using an ester-based solvent, without any severe evolution of CO2. The combined use of fluoroethylene carbonate and lithium fluoromalonato(difluoro)borate (LiFMDFB) constructs a LiF-rich solid–electrolyte interphase. Further, a 1,3,6-hexanetricarbonitrile (HTCN) and LiFMDFB-driven CEI prevent the structural collapse and improve the reversibility of the LiCoO2. Moreover, PF5 stabilization and HF scavenging by HTCN and tris(trimethylsilyl) phosphite limit the damage to interfacial layers and Co leaching. Our method for a rational electrolyte design may help in formulating more advanced electrolytes for practical application in high-voltage cell operations

Control of Interfacial Layers for High-Performance Porous Si Lithium-Ion Battery Anode

We demonstrate a facile synthesis of micrometer-sized porous Si particles via copper-assisted chemical etching process. Subsequently, metal and/or metal silicide layers are introduced on the surface of porous Si particles using a simple chemical reduction process. Macroporous Si and metal/metal silicide-coated Si electrodes exhibit a high initial Coulombic efficiency of ∼90%. Reversible capacity of carbon-coated porous Si gradually decays after 80 cycles, while metal/metal silicide-coated porous Si electrodes show significantly improved cycling performance even after 100 cycles with a reversible capacity of >1500 mAh g^–1. We confirm that a stable solid-electrolyte interface layer is formed on metal/metal silicide-coated porous Si electrodes during cycling, leading to a highly stable cycling performance

Ultraconcentrated Sodium Bis(fluorosulfonyl)imide-Based Electrolytes for High-Performance Sodium Metal Batteries

We present an ultraconcentrated electrolyte composed of 5 M sodium bis­(fluorosulfonyl)­imide in 1,2-dimethoxyethane for Na metal anodes coupled with high-voltage cathodes. Using this electrolyte, a very high Coulombic efficiency of 99.3% at the 120th cycle for Na plating/stripping is obtained in Na/stainless steel (SS) cells with highly reduced corrosivity toward Na metal and high oxidation durability (over 4.9 V versus Na/Na+) without corrosion of the aluminum cathode current collector. Importantly, the use of this ultraconcentrated electrolyte results in substantially improved rate capability in Na/SS cells and excellent cycling performance in Na/Na symmetric cells without the increase of polarization. Moreover, this ultraconcentrated electrolyte exhibits good compatibility with high-voltage Na4Fe3(PO4)2(P2O7) and Na0.7(Fe0.5Mn0.5)­O2 cathodes charged to high voltages (>4.2 V versus Na/Na+), resulting in outstanding cycling stability (high reversible capacity of 109 mAh g–1 over 300 cycles for the Na/Na4Fe3(PO4)2(P2O7) cell) compared with the conventional dilute electrolyte, 1 M NaPF6 in ethylene carbonate/propylene carbonate (5/5, v/v)

Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries

We present a promising electrolyte candidate, Mg­(TFSI)₂ dissolved in glyme/diglyme, for future design of advanced magnesium (Mg) batteries. This electrolyte shows high anodic stability on an aluminum current collector and allows Mg stripping at the Mg electrode and Mg deposition on the stainless steel or the copper electrode. It is clearly shown that nondendritic and agglomerated Mg secondary particles composed of ca. 50 nm primary particles alleviating safety concern are formed in glyme/diglyme with 0.3 M Mg­(TFSI)₂ at a high rate of 1C. Moreover, a Mg­(TFSI)₂-based electrolyte presents the compatibility toward a Chevrel phase Mo₆S₈, a radical polymer charged up to a high voltage of 3.4 V versus Mg/Mg²⁺ and a carbon–sulfur composite as cathodes

Fluoroethylene Carbonate-Based Electrolyte with 1 M Sodium Bis(fluorosulfonyl)imide Enables High-Performance Sodium Metal Electrodes

Sodium (Na) metal anodes with stable electrochemical cycling have attracted widespread attention because of their highest specific capacity and lowest potential among anode materials for Na batteries. The main challenges associated with Na metal anodes are dendritic formation and the low density of deposited Na during electrochemical plating. Here, we demonstrate a fluoroethylene carbonate (FEC)-based electrolyte with 1 M sodium bis­(fluorosulfonyl)­imide (NaFSI) salt for the stable and dense deposition of the Na metal during electrochemical cycling. The novel electrolyte combination developed here circumvents the dendritic Na deposition that is one of the primary concerns for battery safety and constructs the uniform ionic interlayer achieving highly reversible Na plating/stripping reactions. The FEC–NaFSI constructs the mechanically strong and ion-permeable interlayer containing NaF and ionic compounds such as Na2CO3 and sodium alkylcarbonates

Co-intercalation of Mg²⁺ and Na⁺ in Na_0.69Fe₂(CN)₆ as a High-Voltage Cathode for Magnesium Batteries

Thanks to the advantages of low cost and good safety, magnesium metal batteries get the limelight as substituent for lithium ion batteries. However, the energy density of state-of-the-art magnesium batteries is not high enough because of their low operating potential; thus, it is necessary to improve the energy density by developing new high-voltage cathode materials. In this study, nanosized Berlin green Fe₂(CN)₆ and Prussian blue Na_0.69Fe₂(CN)₆ are compared as high-voltage cathode materials for magnesium batteries. Interestingly, while Mg²⁺ ions cannot be intercalated in Fe₂(CN)₆, Na_0.69Fe₂(CN)₆ shows reversible intercalation and deintercalation of Mg²⁺ ions, although they have the same crystal structure except for the presence of Na⁺ ions. This phenomenon is attributed to the fact that Mg²⁺ ions are more stable in Na⁺-containing Na_0.69Fe₂(CN)₆ than in Na⁺-free Fe₂(CN)₆, indicating Na⁺ ions in Na_0.69Fe₂(CN)₆ plays a crucial role in stabilizing Mg²⁺ ions. Na_0.69Fe₂(CN)₆ delivers reversible capacity of approximately 70 mA h g^–1 at 3.0 V vs Mg/Mg²⁺ and shows stable cycle performance over 35 cycles. Therefore, Prussian blue analogues are promising structures for high-voltage cathode materials in Mg batteries. Furthermore, this co-intercalation effect suggests new avenues for the development of cathode materials in hybrid magnesium batteries that use both Mg²⁺ and Na⁺ ions as charge carriers