Diagnosing the SEI Layer in a Potassium Ion Battery Using Distribution of Relaxation Time

Deciphering the structural and kinetic factors in lithium titanate for enhanced performance in Li+/Na+ dual-cation electrolyte

The widespread application of Li4Ti5O12 (LTO) anode in lithium-ion batteries has been hindered by its relatively low energy density. In this study, we investigated the capacity enhancement mechanism of LTO anode through the incorporation of Na+ cations in an Li+-based electrolyte (dual-cation electrolyte). LTO thin film electrodes were prepared as conductive additive-free and binder-free model electrodes. Electrochemical performance assessments revealed that the dual-cation electrolyte boosts the reversible capacity of the LTO thin film electrode, attributable to the additional pseudocapacitance and intercalation of Na+ into the LTO lattice. Operando Raman spectroscopy validated the insertion of Li+/Na+ cations into the LTO thin film electrode, and the cation migration kinetics were confirmed by ab initio molecular dynamic (AIMD) simulation and electrochemical impedance spectroscopy, which revealed that the incorporation of Na+ reduces the activation energy of cation diffusion within the LTO lattice and improves the rate performance of LTO thin film electrodes in the dual-cation electrolyte. Furthermore, the interfacial charge transfer resistance in the dual-cation electrolyte, associated with ion de-solvation processes and traversal of the cations in the solid-electrolyte interphase (SEI) layer, are evaluated using the distribution of relaxation time, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Our approach of performance enhancement using dual-cation electrolytes can be extrapolated to other battery electrodes with sodium/lithium storage capabilities, presenting a novel avenue for the performance enhancement of lithium/sodium-ion batteries.

In Situ Optical Observation of Lithium Dendrite Pattern in Solid Polymer Electrolytes

Solid polymer electrolytes (SPEs) have been treated as a viable solution to build high-performance solid-state lithium metal batteries (SSLMBs) at the industrial level, bypassing the safety and energy density dilemmas experienced by today's lithium-ion battery technology. To promote a wider application of SPEs-based SSLMBs, the chemical and electrochemical characteristics of lithium metal (Li°) electrode in SPEs have to be clearly elucidated. In this work, the morphological evolution of Li° electrode in the SPEs-based SSLMBs is comprehensively investigated, via a customized electrochemical cell allowing optical microscopic analyses. The results demonstrate that differing from inorganic solid electrolytes, the elastic feature of SPEs eliminates the "memory effect" of the dendrite formation, in which the previously formed dendrites can be dissolved and the resulting space can be simultaneously occupied by electrolyte components, instead of leaving for a second-round growth of Li° dendrites. Furthermore, the largely increased electronic conductivities of the as-formed interphases between Li° electrode and SPEs are found to be responsible for the notoriously soft short-circuit behavior observed during cycling. These findings bring a fresh understanding of the formation and evolution of lithium dendrites in SPE-based cells, which are vital for improving the long-term stability of SSLMBs and other related high-energy battery systems.

Enabling the Transport Dynamics and Interfacial Stability of Porous Si Anode Via Rigid and Flexible Carbon Encapsulation for High-Energy Lithium Storage

The stable electrode/electrolyte interface and fast electron/ion transport channel play important roles in boosting the rate performance and cycling life of lithium-ion batteries. Herein, a porous silicon/carbon composite (pSi@PC@MC) is presented by integrating hollow porous silicon (pSi) with pitch-derived carbon (PC) and dopamine-derived mesoporous carbon (MC), employing microporous zeolite as the silicon source. The finite element simulation first reveals the stress release effect of rigid and flexible carbon encapsulation on the hollow Si anode for lithium-ion storage. In situ and ex situ characterization results further elucidate that hybrid sp2/sp3 carbon coating greatly enhances the liquid/solid interface stability and the compatibility with the electrolyte, as well as facilitates the electron/ion transmission dynamics, achieving a uniform, stable, and LiF-rich SEI film, ultimately improving the lithium storage performance. As expected, the as-designed pSi@PC@MC anode delivers an impressive rate capability (756.6 mAh g-1 at 6 A g-1) and excellent cycling stability with a capacity of 1650 mAh g-1 after 300 cycles at 0.2 A g-1. Meanwhile, the pSi@PC@MC//NCM811 full-cell exhibits an outstanding cycling stability (75.8% capacity retention after 100 cycles). This study highlights the significance of rational porous design and effective hybrid carbon encapsulation for the development of fast-charging Si/carbon anodes.

Sulfolane-Based Flame-Retardant Electrolyte for High-Voltage Sodium-Ion Batteries

Sodium-ion batteries hold great promise as next-generation energy storage systems. However, the high instability of the electrode/electrolyte interphase during cycling has seriously hindered the development of SIBs. In particular, an unstable cathode-electrolyte interphase (CEI) leads to successive electrolyte side reactions, transition metal leaching and rapid capacity decay, which tends to be exacerbated under high-voltage conditions. Therefore, constructing dense and stable CEIs are crucial for high-performance SIBs. This work reports localized high-concentration electrolyte by incorporating a highly oxidation-resistant sulfolane solvent with non-solvent diluent 1H, 1H, 5H-octafluoropentyl-1, 1, 2, 2-tetrafluoroethyl ether, which exhibited excellent oxidative stability and was able to form thin, dense and homogeneous CEI. The excellent CEI enabled the O3-type layered oxide cathode NaNi1/3Mn1/3Fe1/3O2 (NaNMF) to achieve stable cycling, with a capacity retention of 79.48% after 300 cycles at 1 C and 81.15% after 400 cycles at 2 C with a high charging voltage of 4.2 V. In addition, its nonflammable nature enhances the safety of SIBs. This work provides a viable pathway for the application of sulfolane-based electrolytes on SIBs and the design of next-generation high-voltage electrolytes.

Development of Inverse-Opal-Structured Charge-Deficient Co9S8@nitrogen-Doped-Carbon to Catalytically Enable High Energy and High Power for the Two-Electron Transfer I+/I- Electrode

The iodine (I) electrode involving two-electron transfer chemistry by converting between I+ and I-, has the potential to deliver theoretically doubled capacity and higher working voltage platforms, thus achieving higher energy density. However, owing to the slow kinetics of the cascade two-electron transfer reactions, the system suffers from large overpotentials and low power density, especially at high working currents and low temperatures. Here, an inverse-opal-structured cobalt sulfide@nitrogen-doped-carbon (Co9S8@NC) catalyst with unique charge-deficient states is developed to promote the reaction kinetics of the I-/I+ electrode. The charge-deficient Co9S8@NC catalyst not only enables strong physicochemical adsorption with the iodine species but also significantly reduces the activation energy and interfacial charge transfer resistance of the cascade I+/I0/I- conversion reaction. Consequently, the prototypical Zn‖I+/I0/I- battery equipped with the Co9S8@NC catalyst can deliver a high energy density of 554 Wh kg-1 and a stable cycle life of 5000 cycles at 30 °C. Moreover, at a subzero temperature of -30 °C, the battery can exhibit enhanced kinetics and a high power density of 1514 W kg-1, high energy density of 485 Wh kg-1.

Space-Confined Guest Synthesis to Fabricate Sn-Monodispersed N-Doped Mesoporous Host toward Anode-Free Na Batteries

Severe issues including volume change and dendrite growth on sodium metal anodes hinder the pursuit of applicable high-energy-density sodium metal batteries. Herein, an in situ reaction approach is developed that takes metal-organic frameworks as nano-reactor and pore-former to produce a mesoporous host comprised of nitrogen-doped carbon fibers embedded with monodispersed Sn clusters (SnNCNFs). The hybrid host shows outstanding sodiophilicity that enables rapid Na infusion and ultralow Na nucleation overpotential of 2 mV. Its porous structure holds a high Na content and guides uniform Na deposition. Such host provides favorable Na plating/stripping with an average Coulombic efficiency of 99.96% over 2000 cycles (at 3 mA cm-2 and 3 mA h cm-2 ). The Na-infused SnNCNF anode delivers extreme Na utilization of 86% in symmetric cells (at 10 mA cm-2 and 10 mA h cm-2 ), outstanding rate capability and cycle life in Na-SnNCNF||Na3 V2 (PO4 )3 full cells (at 1 A g-1 for over 1000 cycles with capacity retention of 92.1%). Furthermore, high-energy/power-density anode-less and anode-free Na cells are achieved. This work presents an effective heteroatom-doping approach for fabricating multifunctional porous carbon materials and developing high-performance metal batteries.

Halide Layer Cathodes for Compatible and Fast-Charged Halides-Based All-Solid-State Li Metal Batteries

Insertion-type compounds based on oxides and sulfides have been widely identified and well-studied as cathode materials in lithium-ion batteries. However, halides have rarely been used due to their high solubility in organic liquid electrolytes. Here, we reveal the insertion electrochemistry of VX₃ (X=Cl, Br, I) by introducing a compatible halide solid-state electrolyte with a wide electrochemical stability window. X-ray absorption near-edge structure analyses reveal a two-step lithiation process and the structural transition of typical VCl₃ . Fast Li⁺ insertion/extraction in the layered VX₃ active materials and favorable interface guaranteed by the compatible electrode-electrolyte design enables high rate capability and stable operation of all-solid-state Li-VX₃ batteries. The findings from this study will contribute to developing intercalation insertion electrochemistry of halide materials and exploring novel electrode materials in viable energy storage systems.

Three-Dimensional Holey Graphene Modified Na4 Fe3 (PO4 )2 (P2 O7 )/C as a High-Performance Cathode for Rechargeable Sodium-Ion Batteries

Polyanion-type Na₄ Fe₃ (PO₄ )₂ (P₂ O₇ ) (NFPP) is a promising cathode material for sodium-ion batteries due to its low cost and high safety. Herein, a three-dimensional (3D) holey graphene (HG) modified NFPP/C material (NFPPCHG) has been successfully prepared by a simple and scalable ball milling strategy with sodium phytate and ferrous oxalate as precursors. The introduction of HG can obviously improve the specific surface area, electronic conductivity, and ions transport performance of NFPPCHG and largely enhance its electrochemical properties. The prepared NFPPCHG delivers a high reversible capacity of 118 mAh g^-1 at 0.2 C and keeps a considerable capacity of 53 mAh g^-1 even at an ultrahigh rate of 100 C. NFPPCHG also shows excellent performance at 55 °C and -20 °C. Moreover, in situ distribution of relaxation time analysis further demonstrates NFPPCHG has superior electrochemical kinetics. In addition, the HC//NFPPCHG full cell displays good performance, suggesting great potential of the prepared material for practical applications.

Three-Dimensional Holey Graphene Enwrapped Li3 V2 (PO4 )3 /N-Doped Carbon Cathode for High-Rate and Long-Life Li-Ion Batteries

Monoclinic Li₃ V₂ (PO₄ )₃ is a promising cathode material for high-power Li-ion batteries. Herein, a three-dimensional holey graphene enwrapped Li₃ V₂ (PO₄ )₃ /N-doped carbon (LVPNCHG) nanocomposite has been successfully synthesized. The holes could be in-situ and directly introduced in graphene through H₂ O₂ chemical etching in the synthesis process, which could remarkably enhance the ion and electron transport and greatly improve the electrochemical performance of the LVPNCHG electrode: 78 mAh g^-1 at 150 C, 86.1 % capacity retention over 2000 cycles at 10 C, and 96 % capacity retention over 500 cycles at 1 C under -20 °C. Moreover, in-situ distribution of relaxation time analysis was used to study LVPNCHG cathode during charge/discharge at 3.0-4.8 V, combined with in-situ X-ray diffraction measurement, and the results showed that a two-phase reaction mechanism was involved during the insertion of Li⁺ in the discharge process. Further demonstration of graphite//LVPNCHG full cell indicated great potential of the as-synthesized materials for practical application.

Integrated Design from Microstructural Engineering to Binder Optimization Enabling a Practical Carbon Anode with Ultrahigh ICE and Efficient Potassium Storage

Potassium-ion batteries (PIBs) are emerging as a powerful alternative to lithium-ion battery systems in large-scale energy storage owing to plentiful resources. Nevertheless, pursuing high-yield anode materials with high initial Coulombic efficiency (ICE) and superior rate capability is still one of the most critical challenges in practical application. Herein, an integrated electrode (PC-x) derived from a petroleum coke precursor (carbon residue rate as high as 89%) is regulated from microstructural engineering to binder optimization devoting to high ICE and efficient potassium storage. Excitingly, with a strong assist from a sodium carboxymethyl cellulose (CMC) binder, the PC-900 anode displays an ultrahigh ICE of 80.5%, one of the highest values reported for PIB carbon anodes. Simultaneously, the PC-900 anode submits a high capacity (304.3 mAh g^-1), superb rate (138.2 mAh g^-1 at 10C), and excellent stability. Furthermore, the full cell exhibits an outstanding rate and cycling performance (210.7 mAh g^-1 at 0.5C), confirming its large-scale application prospects. The ultrahigh ICE and excellent performance are mainly attributable to the beneficial microstructures (low surface area, functional group content, and larger interlayer spacing) created by microstructural engineering. Meanwhile, binder optimization also plays a crucial role in reducing the irreversible capacity and interface impedance, further improving the ICE and rate capability. Importantly, mechanism analysis confirms two-stage K⁺ storage behavior: reversible adsorption at edges and defects (>0.25 V) and intercalation into crystalline layers (<0.25 V). This work provides an efficient and easily scalable electrode design strategy for future practical applications of PIBs.

In-Depth Mechanism Understanding for Potassium-Ion Batteries by Electroanalytical Methods and Advanced In Situ Characterization Techniques

The advancement of potassium ion batteries (PIBs) stimulated by the dearth of lithium resources is accelerating. Major progresses on the electrochemical properties are based on the optimization of electrode materials, electrolytes, and other components. More significantly, the prerequisites for optimizing these key compositions are in-depth and comprehensive exploration of electrochemical reaction processes, including the evolution of morphology and structure, phase transition, interface behaviors, and K⁺ movement, etc. As a result, the obtained K⁺ storage mechanism via analyzing aforementioned reaction processes sheds light on furthering practical application of PIBs. Typical electrochemical analysis methods are capable of obtaining physical and chemical characteristics. The advent of in situ electrochemical measurements enables dynamic observation and monitoring, thereby gaining extensive insights into the intricate mechanism of capacity degradation and interface kinetics. By coupling with these powerful electrochemical characterization techniques, inspiring works in PIBs will burgeon into wide realms of energy storage fields. In this review, some typical electroanalytical tests and in situ hyphenated measurements are described with the main concentration on how these techniques play a role in investigating the potassium storage mechanism for PIBs and achieving encouraging results.

Critical CO2 Concentration for Practical Lithium-Air Batteries

The Li-air battery is expected to become the next generation of the energy storage system because of its high theoretical energy density of 3500 Wh/kg (based on Li₂O₂ formation at the cathode). CO₂ (∼300 ppm) in the air is regarded as an impurity for cathode reactions, because it can lead to the formation of Li₂CO₃, which increases the overcharge potentials, decreases energy efficiency, and gives rise to the serious decomposition of battery components. However, the impact of a low concentration of CO₂ (<1000 ppm) on cell performance has not been addressed. In this work, we quantitatively characterized and analyzed the impact of a low concentration of CO₂ on the electrochemical performance of Li-air batteries to investigate the tolerance of Li-air batteries to CO₂. The discharge capacities and cyclability of the batteries with CO₂ below 100 ppm are similar to those without CO₂. The batteries with 0, 50, and 100 ppm of CO₂ delivered 85, 88, and 83 cycles, respectively. At the same time, the critical byproduct Li₂CO₃ was quantified, and its effect on batteries is analyzed by in situ electrochemical impedance spectroscopy (EIS) with a distribution of relaxation time (DRT) calculation. This study promises a theoretical basis for developing CO₂ removal materials and devices for Li-air batteries in the future.