Anion-Induced Uniform and Robust Cathode-Electrolyte Interphase for Layered Metal Oxide Cathodes of Sodium Ion Batteries

Layer metal oxides demonstrate great commercial application potential in sodium-ion batteries, while their commercialization is extremely hampered by the unsatisfactory cycling performance caused by the irreversible phase transition and interfacial side reaction. Herein, trimethoxymethylsilane (TMSI) is introduced into electrolytes to construct an advanced cathode/electrolyte interphase by tuning the solvation structure of anions. It is found that due to the stronger interaction between ClO4- and TMSI than that of ClO4- and PC/FEC, the ClO4--TMSI complexes tend to accumulate on the surface of the cathode during the charging process, leading to the formation of a stable cathode/electrolyte interface (CEI). In addition, the Si species with excellent electronic insulation ability are distributed in the TMSI-derived CEI film, which is conducive to inhibiting the continuous side reaction of solvents and the growth of the CEI film. As a result, under a current density of 250 mA g-1, the capacity retention of the NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode after 200 cycles in the TMSI-modified electrolyte is 74.4% in comparison to 51.5% of the bare electrolyte (1 M NaClO4/PC/5% FEC). Moreover, the NFM cathode shows better kinetics, with the specific discharge capacity increasing from 22 to 67 mAh g-1 at 300 mA g-1. It also demonstrates greatly improved rate capability, cycling stability, and Coulombic efficiency under various operating conditions, including high temperature (55 °C) and high cutoff voltage (2.0-4.3 V vs Na+/Na).

Constructing Matching Cathode-Anode Interphases with Improved Chemo-mechanical Stability for High-Energy Batteries

Coupling Ni-rich layered oxide cathodes with Si-based anodes is one of the most promising strategies to realize high-energy-density Li-ion batteries. However, unstable interfaces on both cathode and anode sides cause continuous parasitic reactions, resulting in structural degradation and capacity fading of full cells. Herein, lithium tetrafluoro(oxalato) phosphate is synthesized and applied as a multifunctional electrolyte additive to mitigate irreversible volume swing of the SiOx anode and suppress undesirable interfacial evolution of the LiNi0.83Co0.12Mn0.05O2 (NCM) cathode simultaneously, resulting in improved cycle life. Benefiting from its desirable redox thermodynamics and kinetics, the molecularly tailored additive facilitates matching interphases consisting of LiF, Li3PO4, and P-containing macromolecular polymer on both the NCM cathode and SiOx anode, respectively, modulating interfacial chemo-mechanical stability as well as charge transfer kinetics. More encouragingly, the proposed strategy enables 4.4 V 21700 cylindrical batteries (5 Ah) with excellent cycling stability (92.9% capacity retention after 300 cycles) under practical conditions. The key finding points out a fresh perspective on interfacial optimization for high-energy-density battery systems.

Probing the Formation of Cathode-Electrolyte Interphase on Lithium Iron Phosphate Cathodes via Operando Mechanical Measurements

Interfacial instabilities in electrodes control the performance and lifetime of Li-ion batteries. While the formation of the solid-electrolyte interphase (SEI) on anodes has received much attention, there is still a lack of understanding the formation of the cathode-electrolyte interphase (CEI) on the cathodes. To fill this gap, we report on dynamic deformations on LiFePO4 cathodes during charge/discharge by utilizing operando digital image correlation, impedance spectroscopy, and cryo X-ray photoelectron spectroscopy. LiFePO4 cathodes were cycled in either LiPF6, LiClO4, or LiTFSI-containing organic liquid electrolytes. Beyond the first cycle, Li-ion intercalation results in a nearly linear correlation between electrochemical strains and the state of (dis)-charge, regardless of the electrolyte chemistry. However, during the first charge in the LiPF6-containing electrolyte, there is a distinct irreversible positive strain evolution at the onset of anodic current rise as well as current decay at around 4.0 V. Impedance studies show an increase in surface resistance in the same potential window, suggesting the formation of CEI layers on the cathode. The chemistry of the CEI layer was characterized by X-ray photoelectron spectroscopy. LiF is detected in the CEI layer starting as early as 3.4 V and LixPOyFz appeared at voltages higher than 4.0 V during the first charge. Our approach offers insights into the formation mechanism of CEI layers on the cathode electrodes, which is crucial for the development of robust cathodes and electrolyte chemistries for higher-performance batteries.

Manganese and Cobalt-Free Ultrahigh-Ni-Rich Single-Crystal Cathode for High-Performance Lithium Batteries

Current commercial nickel (Ni)-rich Mn, Co, and Al-containing cathodes are employed in high-energy-density lithium (Li) batteries all around the globe. The presence of Mn/Co in them brings out several problems, such as high toxicity, high cost, severe transition-metal dissolution, and quick surface degradation. Herein, a Mn/Co-free ultrahigh-Ni-rich single-crystal LiNi_0.94Fe_0.05Cu_0.01O₂ (SCNFCu) cathode with acceptable electrochemical performance is benchmarked against a Mn/Co-containing cathode. Despite having a slightly lower discharge capacity, the SCNFCu cathode retaining 77% of its capacity across 600 deep cycles in full-cell outperforms comparable to a high-Ni single-crystal LiNi_0.9Mn_0.05Co_0.05O₂ (SCNMC; 66%) cathode. It is shown that the stabilizing ions Fe/Cu in the SCNFCu cathode reduce structural disintegration, undesirable side reactions with the electrolyte, transition-metal dissolution, and active Li loss. This discovery provides a new extent for cathode material development for next-generation high-energy, Mn/Co-free Li batteries due to the compositional tuning flexibility and quick scalability of SCNFCu, which is comparable to the SCNMC cathode.

Mg2+-Doping Constructed a Continuous and Homogeneous Cathode-Electrolyte Interphase Film on Na3.12Fe2.44(P2O7)2 with Superior and Stable High-Temperature Performance for Sodium-Ion Storage

Sodium-ion batteries (SIBs) are on the verge of achieving practical applications, and the key is to find suitable electrode materials. The polyanionic iron-based material Na_3.12Fe_2.44(P₂O₇)₂ (NFPO) possesses an open three-dimensional framework structure with good thermal stability and is regarded as an outstanding cathode material for SIBs. Nevertheless, its poor electrical conductivity, problems with erosion of electrolytes, and structural deterioration during cycling still need to be urgently addressed. Here, we first design a Mg²⁺-doped NFPO (NFPO-Mg) material with a dual-action effect. On the one hand, Mg²⁺ improves the intrinsic conductivity of the NFPO material, and on the other hand, Mg²⁺ promotes the formation of a homogeneous and stable cathode-electrolyte interphase film during the cycling process, which results in a superior rate performance and cycling stability. A capacity of 68.6 mAh g^-1 was achieved at 50C (1C = 117.4 mAh g^-1), and a capacity retention of 79.1% was maintained after 3000 cycles at 20C. More impressively, NFPO-Mg exhibits outstanding high-temperature electrochemical performance, with a capacity retention of 95.3% after 400 cycles at 10C at 60 °C (much higher than the 54.2% for the NFPO). This paper explores an effective method for improving the electrochemical performance of cathode materials, which may prove instrumental in guiding the design of more high-performance cathode materials in the future.

Stable Room-Temperature Sodium-Sulfur Batteries in Ether-Based Electrolytes Enabled by the Fluoroethylene Carbonate Additive

Because of its high energy density and low cost, the room-temperature sodium-sulfur (RT Na-S) battery is a promising candidate to power the next-generation large-scale energy storage system. However, its practical utilization is hampered by the short life span owing to the severe shuttle effect, which originates from the "solid-liquid-solid" reaction mechanism of the sulfur cathode. In this work, fluoroethylene carbonate is proposed as an additive, and tetraethylene glycol dimethyl ether is used as the base solvent. For the sulfurized polyacrylonitrile cathode, a robust F-containing cathode-electrolyte interphase (CEI) forms on the cathode surface during the initial discharging. The CEI prohibits the dissolution and diffusion of the soluble intermediate products, realizing a "solid-solid" reaction process. The RT Na-S cell exhibits a stable cycling performance: a capacity of 587 mA h g^-1 is retained after 200 cycles at 0.2 A g^-1 with nearly 100% Coulombic efficiency.

Improved Sodiation Additive and Its Nuances in the Performance Enhancement of Sodium-Ion Batteries

The abundance of the available sodium sources has led to rapid progress in sodium-ion batteries (SIBs), making them potential candidates for immediate replacement of lithium-ion batteries (LIBs). However, commercialization of SIBs has been hampered by their fading efficiency due to the sodium consumed in the formation of solid-electrolyte interphase (SEI) when using hard carbon (HC) anodes. Herein, Na₂C₃O₅ sodium salt is introduced as a highly efficient, cost-effective, and safe cathode sodiation additive. This sustainable sodium salt has an oxidation potential of ∼4.0 V vs Na⁺/Na°, so it could be practically implemented into SIBs. Moreover, for the first time, we have also revealed by X-ray photoelectron spectroscopy (XPS) that in addition to the compensating Na⁺ ions spent in the SEI layer, the high specific capacity and capacity retention observed from electrochemical measurements are due to the formation of a thinner and more stable cathode-electrolyte interphase (CEI) on the P2-Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ while using such a cathode sodiation additive. Half-cell studies with P2-Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ cathodes show a 27% increase in the specific capacity (164 mAh g_P2^-1) with cathode sodiation additives. Full-cell studies with the HC anode show a 4 times increase in the specific capacity of P2-Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂. This work provides notable insights into and avenues toward the development of SIBs.

Realizing Ultralong-Term Cyclicability of 5 Volt-Cathode-Material Graphite Flakes by Uniformly Comodified TiO2/Carbon Layer Inducing Stable Cathode-Electrolyte Interphase

A common issue the high-voltage cathode materials of secondary batteries suffered from is oxidative electrolyte decomposition inducing rapid capacity fading with discharge/charge cycling. Herein, a highly efficient strategy realizing stable cathode-electrolyte interphase (CEI) and ultralong-term cyclicability of 5 volt-cathode-material graphite flakes (GFs) for dual-ion batteries is demonstrated. The TiO₂/carbon-comodified GF (TO/GF) cathode material with uniform distribution and tight bonding of the nanosized TiO₂/carbon layer on the GF surface is synthesized, in which the GF surface is partitioned into nanodomains by the uniformly distributed TiO₂ nanoparticles. Meanwhile, the amorphous carbon layer acts as a gummed tape bonding tightly the TiO₂ nanoparticles on the graphite flake surface. Serial electrochemical impedance spectroscopy and structural/chemical analyses demonstrate that these unique structural characteristics of the TiO₂/carbon comodification endow the TO/GF cathode material with a stable CEI layer coupled with much reduced electrolyte decomposition. Consequently, extremely high cyclicability of 10,000 stable discharge/charge cycles with an extremely low capacity fading rate of 0.0021% for anion PF₆^- storage is realized. This efficient strategy has a potential to be extended to other high-voltage cathode materials and further scaled to the industrial level.

Cathode-Electrolyte Interphase in a LiTFSI/Tetraglyme Electrolyte Promoting the Cyclability of V2O5

V₂O₅, one of the earliest intercalation-type cathode materials investigated as a Li⁺ host, is characterized by an extremely high theoretical capacity (441 mAh g^-1). However, the fast capacity fading upon cycling in conventional carbonate-based electrolytes is an unresolved issue. Herein, we show that using a LiTFSI/tetraglyme (1:1 in mole ratio) electrolyte yields a highly enhanced cycling ability of V₂O₅ (from 20% capacity retention to 80% after 100 cycles at 50 mA g^-1 within 1.5-4.0 V vs Li⁺/Li). The improved performance mostly originates from the V₂O₅ electrode itself, since refreshing the electrolyte and the lithium electrode of the cycled cells does not help in restoring the V₂O₅ electrode capacity. Electrochemical impedance spectroscopy (EIS), post-mortem scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the origin of the improved electrochemical behavior. The results demonstrate that the enhanced cyclability is a consequence of a thinner but more stable cathode-electrolyte interphase (CEI) layer formed in LiTFSI/tetraglyme with respect to the one occurring in 1 M LiPF₆ in EC/DMC (1:1 in weight ratio, LP30). These results show that the cyclability of V₂O₅ can be effectively improved by simple electrolyte engineering. At the same time, the uncovered mechanism further reveals the vital role of the CEI on the cyclability of V₂O₅, which can be helpful for the performance optimization of vanadium-oxide-based batteries.

In Situ Interfacial Tuning To Obtain High-Performance Nickel-Rich Cathodes in Lithium Metal Batteries

Nickel-rich layered oxides are currently considered the most practical candidates for realizing high-energy-density lithium metal batteries (LMBs) because of their relatively high capacities. However, undesired nickel-rich cathode-electrolyte interactions hinder their applicability. Here, we report a satisfactory combination of an antioxidant fluorinated ether solvent and an ionic additive that can form a stable, robust interfacial structure on the nickel-rich cathode in ether-based electrolytes. The fluorinated ether 1,1,2,2-tetrafluoroethyl-1H,1H,5H-octafluoropentyl ether (TFOFE) introduced as a cosolvent into ether-based electrolytes stabilizes the electrolytes against oxidation at the LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cathode while simultaneously preserving the electrochemical performance of the Li metal anode. Lithium difluoro(bisoxalato)phosphate (LiDFBP) forms a uniform cathode-electrolyte interphase that limits the generation of microcracks inside secondary particles and undesired dissolution of transition metal ions such as nickel, cobalt, and manganese from the cathode into the electrolyte. Using TFOFE and LiDFBP in ether-based electrolytes provides an excellent capacity retention of 94.5% in a Li|NCM811 cell after 100 cycles and enables the delivery of significantly increased capacity at high charge and discharge rates by manipulating the interfaces of both electrodes. This research provides insights into advancing electrolyte technologies to resolve the interfacial instability of nickel-rich cathodes in LMBs.

Unraveling the Rapid Performance Decay of Layered High-Energy Cathodes: From Nanoscale Degradation to Drastic Bulk Evolution

Lithium-rich layered oxides are promising cathode candidates because of their exceptional high capacity. The commercial application of these high-energy cathodes, however, is thwarted by the undesired rapid performance decay during cycling. Surface degradation has been widely considered to correlate with the performance decay of the cathodes, whereas, in this work, we demonstrate that the degradation of Li-rich high-energy Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (HENMC) cathode material not only takes place at surfaces but also proceeds from its internal structure. In addition to demonstrating the surface reconstruction and the formation of a cathode-electrolyte interphase (CEI) layer of cycled HENMC cathode, this study uncovers the irreversible bulk phase transition from a Li-excess monoclinic ( C2/ m) solid solution into a conventional "layered" ( R3̅ m) phase, accompanied by complete loss of Li⁺ from the TM layers during cycling. Furthermore, the internal grains of HENMC bear lattice distortions, leading to the formation of "nano-defect" domains, which could limit the Li⁺ diffusion inside the grains. More prominently, the layered-to-spinel transition in the form of large spinel grains ( Fd3̅ m), hundreds of nanometers across, is discovered, and their detailed atomic arrangement is studied. The findings suggest that, instead of attributing the overall capacity fade to the surface degradation, these drastic bulk evolutions would be the main degradation mechanisms at the source of the rapid failure of Li-rich cathodes.