Low Initial Coulombic Efficiency Research Articles

Synergistic effect of interlayered doping and surface modification to improve the performance of Li-rich manganese-based cathode materials

Li-rich manganese-based oxides (LRMOs) with high energy density have been intensively studied, but their practical use as cathode materials for lithium-ion batteries is still impeded by the shortcomings such as low initial coulombic efficiency, voltage decay and poor rate capability. To address these issues, element Nb is selected to bulk dope and simultaneously surface modify the hierarchical Li1.20Mn0.54Co0.13Ni0.13O2 spheres in this work. Synergistic effect of Nb doping and surface modification on the crystalline structure and electrochemical performance of the resultant LRMOs is characterized in detail. The results suggest that Nb5+ ions are doped into the interlayer sites within the crystals while the LiMn2O4 and LiNb3O8 double layers are coated on the primary nanoparticles. The galvanostatic charge-discharge measurements reveal that the optimal LRMO-Nb2 sample can deliver the capacities of 286 and 167 mAh g−1 at the rates of 0.1 C and 5 C, respectively, and retains a capacity of 191 mAh g−1 at 1 C rate after 200 cycles. Ex-situ XRD patterns prove that the layered structure is strongly pillared by the interlayer-doped Nb5+ during the delithiation/lithiation processes. Especially, some lithium vacancies formed around Nb5+ ions can accelerate the diffusion of Li+ ions, which was proved by DFT calculations and experimental results. Our research demonstrates that cooperating of the interlayered doping of Nb5+ with surface coating of LiNb3O8 layer can improve rate capability and cyclability of the resultant LRMOs significantly.

Efficient Chemical Prelithiation with Modificatory Li+ Solvation Structure Enabling Spatially Homogeneous SEI toward High Performance SiOx Anode†

Comprehensive SummaryChemical prelithiation is widely proven to be an effective strategy to address the low initial coulombic efficiency (ICE) of promising SiOx anode. Though the reagent composition has been widely explored, the Li+ solvation structure, which practically plays the cornerstone role in the prelithiation ability, rate, uniformility, has rarely been explored. A novel environmentally‐friendly reagent with weak solvent cyclopentyl methyl ether (CPME) is proposed that enables both improved ICE and spatial homogeneous solid electrolyte interphase (SEI). And the prelithiation behavior and mechanism were explored focused on the Li+ solvation structure. Both theoretical investigation and spectroscopic results suggest that weak solvent feature of CPME reduces the solvent coordination number and decreases the Li+ desolvation energy. The optimized Li+ solvation structure enables high‐efficiency prelithiation that ensures the horizontal homogenization and mechanical properties of SEI. Moreover, the accompanied CPME molecules preferentially occupy positions in initial SEI, reducing the likelihood of LiPF6 decomposition and promoting longitudinal homogenization of SEI. Consequently, the efficient and homogenous prelithiation enables impressive ICE of 109.52% and improved cycling performance with 80.77% retained after 300 cycles via just 5 min soaking. Furthermore, the full cells with LiNi0.83Co0.12Mn0.05O2 (NCM831205) cathode display an enhancement in the energy density of 179.74% and up to 648.35 Wh·kg–1.

Consummating ion desolvation in hard carbon anodes for reversible sodium storage

Hard carbons are emerging as the most viable anodes to support the commercialization of sodium-ion (Na-ion) batteries due to their competitive performance. However, the hard carbon anode suffers from low initial Coulombic efficiency (ICE), and the ambiguous Na-ion (Na+) storage mechanism and interfacial chemistry fail to give a reasonable interpretation. Here, we have identified the time-dependent ion pre-desolvation on the nanopore of hard carbons, which significantly affects the Na+ storage efficiency by altering the solvation structure of electrolytes. Consummating the pre-desolvation by extending the aging time, generates a highly aggregated electrolyte configuration inside the nanopore, resulting in negligible reductive decomposition of electrolytes. When applying the above insights, the hard carbon anodes achieve a high average ICE of 98.21% in the absence of any Na supplementation techniques. Therefore, the negative-to-positive capacity ratio can be reduced to 1.02 for full cells, which enables an improved energy density. The insight into hard carbons and related interphases may be extended to other battery systems and support the continued development of battery technology.

Lithium Pre-Storage Enables High Initial Coulombic Efficiency and Stable Lithium-Enriched Silicon/Graphite Anode.

Application of silicon-based anode is significantly challenged by low initial Coulombic efficiency (ICE) and poor cyclability. Traditional pre-lithiation reagents often pose safety concerns due to their unstable chemical nature. Achieving a balance between water-stability and high ICE in prelithiated silicon is a critical issue. Here, we present a lithium-enriched silicon/graphite material with an ultra-high ICE of ≥110% through a high-stable lithium pre-storage methodology. Lithium pre-storage prepared a nano-drilled graphite material with surficial lithium functional groups, which can form chemical bonds with adjacent silicon during high-temperature sintering. This results in an unexpected O-Li-Si interaction, leading to in-situ pre-lithiation of silicon nanoparticles and providing high stability characteristics in air and water. Additionally, the lithium-enriched silicon/graphite materials impart a combination of high ICE, high specific capacity (620 mAh g-1), and long cycling stability (> 400 cycles). This study opens up a promising avenue for highly air and water-stable silicon anode prelithiation methods.

Lithium Pre‐Storage Enables High Initial Coulombic Efficiency and Stable Lithium‐Enriched Silicon/Graphite Anode

Application of silicon‐based anode is significantly challenged by low initial Coulombic efficiency (ICE) and poor cyclability. Traditional pre‐lithiation reagents often pose safety concerns due to their unstable chemical nature. Achieving a balance between water‐stability and high ICE in prelithiated silicon is a critical issue. Here, we present a lithium‐enriched silicon/graphite material with an ultra‐high ICE of ≥110% through a high‐stable lithium pre‐storage methodology. Lithium pre‐storage prepared a nano‐drilled graphite material with surficial lithium functional groups, which can form chemical bonds with adjacent silicon during high‐temperature sintering. This results in an unexpected O‐Li‐Si interaction, leading to in‐situ pre‐lithiation of silicon nanoparticles and providing high stability characteristics in air and water. Additionally, the lithium‐enriched silicon/graphite materials impart a combination of high ICE, high specific capacity (620 mAh g‐1), and long cycling stability (> 400 cycles). This study opens up a promising avenue for highly air and water‐stable silicon anode prelithiation methods.

Enhancing the lithium storage properties of SiO@NC anode by MnNb2O6 decoration

The scalable application of SiO anode in lithium ion batteries is still limited by its low initial Coulombic efficiency and inferior cycle stability. Compositing SiO with other substances that own high structural stability, fast Li ion diffusion rate and good Li ion storage capacity has been proven of effectiveness to tackle the aforementioned problems. Herein, manganese niobate (MnNb2O6) that possesses 3-dimensional Li ion diffusion channels is used to modify SiO anode in conjunction with N-doped carbon coating to improve the Li-ion storage performance. Ex situ XRD measurements reveal that the orthorhombic MnNb2O6 is converted to amorphous LixMnNb2O6 during lithiation and reversed back to MnNb2O6 in the following delithiation process. The crystal structure of MnNb2O6 is well preserved after a long-term cycle test, suggesting its high structural stability. With such stable structure and ultrafast Li ion diffusion kinetics, the optimized SiO@MnNb2O6@NC composite anode exhibits obviously improved rate performance and cycle stability as compared to SiO@NC. A reversible capacity of 835 mAh g−1 is maintained at a relatively high current density of 3.0 A g−1, and a discharge capacity of 799 mAh g−1 is preserved after 200 cycles at 1.0 A g−1. This work provides a facile and promising strategy to modify SiO-based anode with Columbite-type Nb-based metal oxides towards better Li-ion storage properties.

Porous Si/Ca-Si-O nanocomposite lithium storage materials by calciothermic reduction of Si with CaH2

Silicon monoxide (SiO)-based anode materials are one of the most intensively investigated class of materials for use as high-capacity anodes in lithium-ion batteries. However, their low initial Coulombic efficiency (ICE), resulting from the irreversible electrochemical reaction of the amorphous SiO2 (a-SiO2) phase in the SiO, restricts the wide-spread adoption of SiO-based anode materials in lithium-ion batteries. Pre-emptive formation of an irreversible phase that converts a-SiO2 to a non-reactive state with lithium prior to cell assembly has received much attention for enhancing ICE. However, this process has the drawback of not ensuring long-term cycle performance. Herein, the pre-emptive formation of irreversible phase combined with the high-energy mechanical milling (HEMM) process is proposed to simultaneously improve both the ICE and long-term cycle performance. The resulting porous Si/Ca-Si-O nanocomposite electrode exhibits an impressive ICE of 86.4 % and maintains 55 % of its capacity over 200 cycles. The key factor in the significant ICE improvement is the irreversible Ca-Si-O phase, which is formed by the calciothermic reduction of Si with CaH2. In addition, the microstructural changes induced by the HEMM process effectively mitigate the volume expansion of Si during cycling, resulting in improved long-term cycle stability.

Realization of high-capacity coulombic efficiency in sodium alginate/carbon nanotube double network coated Si-anode for lithium-ion batteries

Silicon (Si) is considered as one of the most promising anodes for the next-generation lithium-ion batteries (LIBs) owing to its ultra-high specific capacity, low redox potential and the second abundance of elements in the earth's crust. However, drastic volume change will directly cause electrode pulverization, thus leading to low initial coulombic efficiency (ICE) and terrible cycling stability. In this work, a sodium alginate (SA)‑carbon nanotube (CNT) derived double carbon-coated Si composite (SA-CNT@Si) was prepared through freeze-drying technique followed by high-temperature carbonization, where SA was utilized to encase Si nanospheres, with CNT serving as the conductive framework that interconnects Si spheres. These composites exhibit improved electrical conductivity and stability throughout the charge and discharge cycles when employed as the anodes in LIBs, demonstrating superior electrochemical properties. The SA-CNT@Si electrode with a mass ratio of Si: SA: CNT = 5: 1: 0.75, achieved a first discharge specific capacity of 2200.8 mAh g−1 at a current density of 500 mA g−1 (with the initial three cycles at 100 mA g−1), along with an ICE of 86.2%. Even after 500 cycles, it maintained a capacity of 607 mAh g−1. This study presents a novel approach to designing Si-based anode materials characterized by both high electrical conductivity and structural stability.

Non-destructive chemical prelithiation of VO2(B) anodes constructing a multifunctional interphase layer for advanced Li-Ion batteries

VO2(B) stands as a promising anode material, comparable to the disordered rock-salt phase Li3+xV2O5, owning to its ease of preparation, suitable redox potential, and remarkable rate capability. However, the practical application of VO2(B) anodes faces substantial limitations due to its low initial coulombic efficiency (ICE) and instability of solid electrolyte interface (SEI) layer during cycling. In this study, we introduce a highly efficient, controllable, and non-destructive chemical prelithiation method utilizing a reductive Li-arene reagent, formed by dissolving Li-metal to THF solutions containing biphenyl derivatives. This method enables the VO2(B) anode to achieve an ICE of nearly 100 %. The investigation on the chemical composition and mechanical properties of SEI layers reveals that the prelithiated VO2(B) electrode contains a more favorable LiF component, fewer adverse byproducts, and exhibits a thinner thickness with high structural strength. Kinetic analysis demonstrates that the prelithiation treatment accelerates the diffusion of lithium ions, reduces interfacial reaction resistances, and maintains interface stability during cycling. To demonstrate the practical application, we assemble a well-designed Prelithiation VO2(B)||NCM811 full cell, achieving an enhancement in the energy density of at least 20 %. This work underscores the potential of prelithiated VO2(B) electrodes for developing lithium-ion batteries (LIBs) with high-safety and high-energy/power density.

Improved Performance of High‐Entropy Disordered Rocksalt Oxyfluoride Cathode by Atomic Layer Deposition Coating for Li‐Ion Batteries

Lithium‐excess cation‐disordered rocksalt materials are a promising class of transition metal‐based cathodes that exhibit high specific capacity and energy density. The exceptional performance is achieved through participation of anionic redox in addition to cationic redox reactions in the electrochemistry. However, anionic redox reactions accompanied by oxygen evolution, accelerated electrolyte breakdown, and structural evolution lead to voltage hysteresis and low initial Coulombic efficiency. Herein, an Al2O3 layer with varying thickness has been coated onto a high‐entropy disordered rocksalt oxyfluoride cathode through atomic layer deposition to enhance battery performance. The results indicate that the utilization of a uniform Al2O3 coating improves the capacity retention and rate capability of the cathode, with the performance being strongly dependent on the layer thickness. Further investigation into cathode–electrolyte interfacial reactions reveals that the thin protecting Al2O3 coating can reduce the decomposition of electrolyte on the cathode surface but cannot prevent bulk phase degradation during prolonged cycling. These findings highlight the need for optimized coating design on the disordered rocksalt cathode to improve battery performance.

Overcoming low initial coulombic efficiencies of Si anodes through prelithiation in all-solid-state batteries

All-solid-state batteries using Si as the anode have shown promising performance without continual solid-electrolyte interface (SEI) growth. However, the first cycle irreversible capacity loss yields low initial Coulombic efficiency (ICE) of Si, limiting the energy density. To address this, we adopt a prelithiation strategy to increase ICE and conductivity of all-solid-state Si cells. A significant increase in ICE is observed for Li1Si anode paired with a lithium cobalt oxide (LCO) cathode. Additionally, a comparison with lithium nickel manganese cobalt oxide (NCM) reveals that performance improvements with Si prelithiation is only applicable for full cells dominated by high anode irreversibility. With this prelithiation strategy, 15% improvement in capacity retention is achieved after 1000 cycles compared to a pure Si. With Li1Si, a high areal capacity of up to 10 mAh cm–2 is attained using a dry-processed LCO cathode film, suggesting that the prelithiation method may be suitable for high-loading next-generation all-solid-state batteries.

Unraveling the influence of Sb dopant to reversible capacity and initial Coulombic efficiency of SnO2-graphene as anodes for sodium storage

Sodium storage materials based on conversion and alloying reactions often suffer from low initial Coulombic efficiency (ICE). Most of literature shows that the SnO2-based anode commonly reveals an ICE below 50 %, and the reasons could be attributed to the partial reversibility in conversion reaction by forming an intermediate phase. This work focuses on using an element doping strategy to suppress the formation of an inert intermediate phase and improves the ICE for SnO2-based anodes in sodium storage applications. Mechanisms of performance enhancement are also carefully investigated. The 3 % Sb doped sample is demonstrated with enhanced electrochemical performance. Specifically, it delivers an initial discharge capacity of 847 mAh g−1 at 50 mA g−1, combining an average enhanced ICE of 57.1 % (37.8 % for the undoped comparison), and it also presents a good rate performance of 206 mAh g−1 at a high current density of 1600 mA g−1. It is found that the Sb dopant reduces the specific capacity of active materials, but it enhances reaction reversibility; improves the overall conductivity of active materials, but does not directly enhance the Na+ diffusion coefficient; helps to restrain the formation of inert phase (the SnO phase) in sodium storage reactions to improve reversibility and initial Coulombic efficiency. Therefore, reversible capacities and initial Coulombic efficiencies can be effectively enhanced after introducing the Sb dopant at an optimized percentage. This work could provide valuable inspiration for designing conversion and alloying-type anodes for sodium storage applications via ion doping methods.

On the Practicability of the Solid‐State Electrochemical Pre‐Sodiation Technique on Hard Carbon Anodes for Sodium‐Ion Batteries

AbstractHard carbon (HC) with low cost and high specific capacity is considered the appropriate anode material for sodium‐ion batteries (SIBs), but the low initial coulombic efficiency (ICE) caused by solid electrolyte interface (SEI) formation and inherent active defects impede its practical battery application. Here, the practicability of solid‐state electrochemical (SSE) pre‐sodiation technique for hard carbon anode is assessed to conquer such challenges. The uniformly pre‐sodiated HC (Pre‐HC) can be fabricated through the SSE reaction between the HC and preloaded sodium metal film without introducing a liquid electrolyte. After being immerged in the electrolyte, a thin artificial SEI with abundant inorganic species is formed on the surface of Pre‐HC due to the spontaneous chemical reaction, and the ICE of HC is improved from 76.0% to 107.9%. Full cell paired with Na3V2(PO4)3 cathode exhibits a high ICE of 94.0% with 70% of energy density augment (from 126.5 to 214.4 Wh kg−1) after anode pre‐sodiation. Pre‐HC anode still retains the pre‐sodiation capacity of 671.1 mAh gNa−1 after being stored in the dry air for 2 h. This work demonstrates the practical applicability of this SSE pre‐sodiation strategy to enhance the energy density of SIBs.

Molecular‐Level Design of High Flash Point Solvents Enables High‐Safety and Dual‐Function Chemical Presodiation of Hard Carbon and Alloy Anodes for High‐Performance Sodium‐Ion Batteries

AbstractHard carbon (HC) is subjected to low initial Coulombic efficiency (ICE) and unsteady solid electrolyte interphase (SEI), which limits the energy density and cycling performance. Meanwhile, studies related to emerging chemical presodiation have specifically focused on proper redox potential and overlooked its safety hazard. To address these drawbacks of HC and chemical presodiation, a series of high‐safety chemical presodiation solutions based on tetraethylene glycol dimethyl ether (TEGDME) are proposed for uniform and fast presodiation of HC and Bi anodes. Among them, Na‐4‐methylbiphenyl in TEGDME solution exhibits the lowest redox potential (0.146 V vs Na+/Na), which achieves the inhibition of initial irreversible sodium uptake. Meantime, the potential‐driven decomposition of fluoroethylene carbonate endows presodiated HC (pNa‐HC) a fast‐ion conducting and robust F‐rich SEI. Accordingly, pNa‐HC delivers an ideal ICE of 99.1% compared to HC (65.28%). Meanwhile, pNa‐HC exhibits significantly enhanced rate performance and cycling life (193.39 mAh g−1 after 2300 cycles at 1000 mA g−1) benefiting from reduced kinetic energy barriers. When pNa‐HC pairs with Na3V2(PO4)3 cathode, the full cell demonstrates a desirable ICE of 91.25%. This work provides a novel and universal solvent design strategy to realize high‐safety chemical pre‐metallation.

An In Situ Near‐Surface Reconstruction Strategy Endowing Lithium‐Rich Oxides with Li/O Dual Vacancies and Spinel‐Carbon Dual Coating Layers Toward High Energy Density Cathode

AbstractLi‐rich cathode materials (LRMs) are regarded as the key cathode candidates for next‐generation lithium‐ion batteries(LIBs) because of their high specific capacity and environmental friendliness. However, LRMs encounter poor cyclability and low initial coulombic efficiency (ICE) hindering their practical application. Herein, a general near‐surface in situ reconstruction strategy is proposed of constructing the Li/O dual vacancies and spinel‐carbon dual coating layers on the surface of LRMs concurrently to improve Li+ storage performance. The as‐prepared LRMs exhibit a greatly strengthened specific capacity of 283 mAh g−1 with an enhanced ICE of 94% and long‐term cyclability of 91% retention after 200 cycles compared with the pristine LRMs (212 mAh g−1 with an ICE of 65%, 76% retention after 200 cycles). Furthermore, it is theoretically revealed that O vacancies (Ov) prefer to occur at the interface of the C2/m and Rm phases to mitigate lattice stress, rather the O sites in individual C2/m or Rm phase with more coordinated atoms. Besides, Li ions exhibit lower migration energy from C2/m phase to Rm phase with the Ov located at the lattice interface. Therefore, this strategy opens a new avenue in the design perspective of the LRMs’ near‐surface for high‐energy‐density LIBs.

MOF-derived cobalt nanoparticles in silicon suboxide-based anodes for enhanced lithium storage

The ultra-high theoretical capacity of silicon-based (Si) materials makes them promising anode materials for high energy density lithium-ion batteries. Unfortunately, the dramatic volume change (∼300%) and low electrical conductivity of silicon have severely hindered the commercial use of silicon anodes. Silicon suboxide (SiOx) is one of the ideal candidates for high energy density batteries due to its reduced swelling and lower cost as compared to silicon. However, it remains a huge challenge to address the low conductivity, low (initial) coulombic efficiency and apparent volume effects of SiOx. In this paper, SiOx/Co@C composite anode materials loaded with metal Co nanoparticles were uniformly and efficiently prepared by using 3-aminopropyl triethoxysilane (APTES) as silicon source and Co-MOF as Co source via molecular self-assembly strategy. The experimental results and density functional theory (DFT) calculation showed that the catalytic action of embedded Co nanoparticles had activated the silico-oxygen bond of SiOx and the irreversible product Li2O, thereby improving the initial coulombic efficiency (ICE) and enhancing the reversible capacity. At the same time, the metal Co with mechanical rigidity and electrical conductivity alleviated the volume fluctuation during alloying and improved the charge transfer capacity and ion transport rate of the composite. When compared with SiOx@C anodes, SiOx/Co@C-600 composites showed higher initial CE, superior cyclic stability and good rate performance. In particular, the successful matching of the anode with the lithium iron phosphate (LFP) cathode in full cell systems validated the possibility of its practical application. This work provides important insights into the application of Si-based materials in lithium-ion battery electrode materials and the development of high energy density batteries.

A functional cathode sodium compensation agent for stable sodium-ion batteries

Hard carbon (HC) is widely used in sodium-ion batteries (SIBs), but its performance has always been limited by low initial Coulombic efficiency (ICE) and cycling stability. Cathode compensation agent is a favorable strategy to make up for the loss of active sodium ions consumed by HC anode. Yet it lacks agent that effectively decomposes to increase the active sodium ions as well as regulate carbon defects for decreasing the irreversible sodium ions consumption. Here, we propose 1,2-dihydroxybenzene Na salt (NaDB) as a cathode compensation agent with high specific capacity (347.9 mAh g−1), lower desodiation potential (2.4–2.8 V) and high utilization (99%). Meanwhile, its byproduct could functionalize HC with more CO groups and promotes its reversible capacity. Consequently, the presodiation hard carbon (pHC) anode exhibits highly reversible capacity of 204.7 mAh g−1 with 98% retention at 5 C rate over 1000 cycles. Moreover, with 5 wt% NaDB initially coated on the Na3V2(PO4)3 (NVP) cathode, the capacity retention of NVP + NaDB|HC cell could increase form 36%–89% after 1000 cycles at 1 C rate. This work provides a new avenue to improve SIBs reversible capacity and cycling performance through designing functional cathode compensation agent.

Surface structure regulation toward anionic redox activation of Li1.20Mn0.533Ni0.133Co0.133O2 cathodes with high initial coulombic efficiency

Li-rich layered oxides cathodes (LLOs) as the promising next-generation cathode materials can provide ultrahigh capacity and energy density due to their distinctive anionic redox chemistry. Unfortunately, severe interfacial side reactions, surface structural degradation and sluggish Li+ kinetics have resulted in low initial coulombic efficiency (ICE), capacity decay and poor rate performance, restricting their practical applications for high-energy–density lithium-ion batteries. Herein, Surface structure regulation strategy used as surface modified agent is proposed to activate the anionic redox chemistry via ammonium tungstate treatment. Experimental results showcase that dual coating layer spinel-like structure LiMn2O4 and Li2WO4 have been successfully constructed on the surface of LLOs. The surface spinel-like structure providing 3 D Li+ diffusion channels together with fast-ion conductive layer decrease the interfacial Li+ diffusion barrier and boost the fasting Li+ kinetics. In addition, the in-situ reconstruction layer can further alleviate the interfacial side reactions and reinforce the surface structural stability. As a result, the ICE of modified LLOs can be precisely increased from 74.71 % to 107.42 % with the adjustment of ammonium tungstate usage. Moreover, it delivers a high reversible capacity of 279.5 mAh/g at 0.1 C, as well as excellent rate capability with capacity of 147.2 mAh/g at 5 C. This work provides a significant reference for designing high-energy–density LLOs via surface structure regulation strategy.

Solution-Based Deep Prelithiation for Lithium-Ion Capacitors with High Energy Density.

Lithium-ion capacitors (LICs) exhibit superior power density and cyclability compared to lithium-ion batteries. However, the low initial Coulombic efficiency (ICE) of amorphous carbon anodes (e.g., hard carbon (HC) and soft carbon (SC)) limits the energy density of LICs by underutilizing cathode capacity. Here, a solution-based deep prelithiation strategy for carbon anodes is applied using a contact-ion pair dominant solution, offering high energy density based on a systematic electrode balancing based on the cathode capacity increased beyond the original theoretical limit. Increasing the anode ICE to 150% over 100%, the activated carbon (AC)capacity is doubled by activating Li+ cation storage, which unleashes rocking-chair LIC operation alongside the dual-ion-storage mechanism. The increased AC capacity results in an energy density of 106.6Wh kg-1 AC+SC , equivalent to 281% of that of LICs without prelithiation. Moreover, this process lowers the cathode-anode mass ratio, reducing the cell thickness by 67% without compromising the cell capacity. This solution-based deep chemical prelithiation promises high-energy LICs based on transition metal-free, earth-abundant active materials to meet the practical demands of power-intensive applications.

A facile graft-repair strategy: Subsurface Na2Sx heterostructures in hard carbon anodes for high-capacity and ultrafast sodium-ion storage

Hard carbon (HC) anodes hold great promise for sodium-ion batteries (SIBs), yet they still suffer from insufficient rate capability and low initial coulombic efficiency (ICE). Herein, an innovative approach based on the synergistic graft-repair mechanism is proposed to create the multifunctional Na2Sx heterostructure within the subsurface of HC via a facile ball-milling and calcination method. This structure with specific disulfide bonds enlarges the carbon layer for fast ion transfer, offers extra S active sites to boost capacity, and catalytically reduces electrolytes to form an inorganic-dominated and thinner solid electrolyte interface (∼7.1 nm). The introduced Na+ ions compensate for the irreversible Na uptake at intrinsic defects and oxygen-containing groups. These effects are validated by GITT, EIS, Raman, and depth-profiling XPS measurements. The optimized hard carbon delivers an ultrahigh reversible capacity with superior ICE (514.8 mAh g−1 at 0.05 A g−1 with ICE of 84.1%) and excellent rate capability (87.5 mAh g−1 at 40 A g−1) simultaneously. DFT calculations reveal that the moderate S/Na ratio yields suitable adsorption energy, maintaining the balance of rate performance and ICE. The revealed mechanism of ion transport based on graft-repair effects contributes to boosting the key SIB performance indicators required for practical applications.