Improved Capacity Retention Research Articles

Improved Performance of Si Nanowire Lithium Ion Battery Anodes By Au Catalyst Nanodot Formation on TiN.

While silicon (Si) is considered to be an excellent candidate for next-generation lithium ion battery (LIB) anodes due to its abundance and high gravimetric capacity, Si suffers from large volume expansion as a result of lithiation. Nanowires (NWs) allow for radial expansion and strain release. Due to this volume expansion, the formation of bulk-like Si structures during the NWs growth process significantly degrades battery performance. Here we report suppression of bulk-like Si formation by interfacial energy contrast between the NWs nucleation catalysts (spontaneously formed gold (Au) nanodots) and titanium nitride (TiN) films. The TiN film promoted a narrow distribution of Si NWs’ diameters and a direct electrical connection to the stainless steel current collector. The characteristics of the solid electrolyte interphase and lithiation/charge transfer mechanisms in Si NWs were also investigated using electrochemical impedance spectroscopy (EIS). The EIS data suggest that the impedance to lithium diffusion within the NWs is approximately 40 % smaller in the NWs grown on TiN than the NWs grown directly on stainless steel. LIB anodes fabricated with the Au-catalyzed Si NWs on TiN films displayed enhanced specific gravimetric capacities (>30 %) under diverse charge/discharge rates and significantly improved capacity retention over 500 cycles.

2D Molybdenum Disulfide Nanosheets As Functional Electroactive Materials and Additives for Lithium and Lithium Sulfur Based Batteries

The 2D structured dichalcogenide molybdenum(IV) sulfide (MoS2) has generated significant interest for Li ion batteries due to its high theoretical capacity, good rate capability, and minimal volume changes during cycling. To enable full realization of MoS2 as an electroactive material, an important and continuing challenge is to understand the many polymorphs of MoS2 that can be (de)stabilized by electrochemical lithiation and nanosizing. Characteriation of bulk MoS2 and nanosheet-type MoS2 as solids, both structurally (X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS)) and compositionally (X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectroscopy (ICP-OES)) will be described to rationalize observed differences in capacity and efficiency in lithium based cells. X-ray analysis compatible lithium cells (in situ XRD and ex situ XAS) measurements of the specific phase changes to rationalize the significant difference in electrochemistry between the nanosheet and bulk materials upon (de)lithiation will also be discussed. The application of 2D MoS2 nanosheets for lithium sulfur batteries (Li−S) will also be described. Significantly improved Li−S electrochemistry in an environmentally friendly electrolyte solvent is demonstrated through implementation of MoS2 nanosheet materials as cathode additives, leading to improved capacity retention upon extended cycling in lithium sulfur cells. The findings will be discussed in light of the unique attributes of the MoS2 atomic level (2D) and mesoscale (3D) heterostructure.

Higher Voltage, Wider Voltage Plateau, Longer Cycle Life, and Faster Kinetics Via Thermally Modulated Interfaces between Ramsdellite and Pyrolusite MnO2 for Lithium-Ion Battery Cathodes

The cathode for Lithium-ion batteries is dominated by transition metal oxides that exist in numerous polymorphic structures. To emphasize generality and the underpinnings of thermodynamics in the stability, activity, and kinetics of lithiation, we utilize the nomenclature of the native structure and non-native structure for referring to different transition metal oxides polymorphic structures. The native structure is the thermodynamic ground state (bulk phase) structure and various non-native structures have different discrete translational symmetry in their sub-surface regions compared to the native structure. Typically, non-native structures have higher (i.e. less negative) bulk formation energy and more open structures than the native structure. Hence, non-native structures are expected to generate higher discharge potential and better lithium mobility as a positive electrode material. However, non-native structures may be limited in stability, which can be increased by scaffolding them with the native structure. To explore these hypotheses, we synthesize pure Ramsdellite MnO2 (r/NN1-MnO2) and thermally phase transform it gradually to Pyrolusite MnO2 (β/N-MnO2) and intimately interfaced (r/NN1-MnO2)/(β/N-MnO2) intergrowth structures are obtained. As the temperature increases, the ratio of r/NN1-MnO2 to β/N-MnO2 in the intergrowth structure decreases; so does the discharge potential and lithium mobility in the lithium-ion coin cell. In addition, the galvanostatic discharge profile shows a wider discharge plateau for intergrowth structures as compared to the pure native and non-native structure. We rationalize these trends in discharge potential using formation energies computed via density functional theory followed by a statistical averaging to account for the different types of lithium intercalation sites in the intergrowth structures. The wider discharge plateau in the intergrowth structure is rationalized with the presence of additional interfacial sites of higher stability than the site available in single phase native and non-native structure. The capacity retention is higher for the intergrowth structure, which is ~64% compared to ~42% and ~32% as obtained for pure native and non-native structures after 100 cycles at a rate of 0.1C. The improved capacity retention for intergrowth structure is due to higher structural stability and available free volume, where the presence of native structure acts as a scaffold to stabilize the non-native structure. The electrochemical impedance spectroscopy suggests that the larger percentage of non-native structure helps in better transport due to greater free volume present in r/NN1-MnO2 structure with wider 2×1 channel compared to the native structure with the narrow 1×1 channel. This study demonstrates how a systematic control of the relative percentage of each phase, without change in chemical composition, helps in designing cathodes with improved capacity and cycle life, thereby expanding the available material space.

Electrochemical Performance of BiF3‐BaF2 Solid Solution with Three Different Phases on a Fluoride Shuttle Battery System

AbstractSolid solution of bismuth(III) fluoride (BiF3) and barium(II) fluoride (BaF2) with a variety of crystal structures was prepared, and their electrochemical performances were evaluated on a fluoride shuttle battery (FSB) system. BiF3‐BaF2 based solid solution (Bi1‐xBaxF3‐x (x=0) with orthorhombic phase, x=0.2 with hexagonal phase, and x=0.4 with cubic phase) were successfully obtained with the mechanical alloy method. The change from Bi1‐xBaxF3‐x (x=0) (orthorhombic) to x=0.2 (hexagonal) improved the initial reversible capacity and the capacity retention during cycling. The change from x=0.2 (hexagonal) to 0.4 (cubic) led to further improvement in capacity retention during cycling; however, the initial reversible capacity decreased due to the substitution of excess amount of barium that did not contribute to the redox reaction. For the BiF3‐based compound, controlling the crystal structure and substitution of inactive element affected the FSB performance.

Ultrathin Conductive Interlayer with High‐Density Antisite Defects for Advanced Lithium–Sulfur Batteries

AbstractLithium–sulfur (Li–S) batteries are promising next‐generation rechargeable batteries due to thier high energy density, low cost, and environmental friendliness. However, the extremely low electrical conductivity of sulfur and the dissolution of polysulfides limit their actual electrochemical performances, especially in the case of high sulfur mass loading. Here, a new strategy based on intrinsic point defects of materials is proposed to simultaneously enhance the electrical conductivity of active material and regulate the migration of polysulfides. Taking advantage of ultrathin and lightweight Bi2Te2.7Se0.3 (BTS) interlayers with high‐density antisite defects on the separator surface, the Li–S battery with BTS interlayer shows a capacity of 756 mAh g−1 at 2C and a low capacity decay rate of 0.1% over 300 cycles. The BTS interlayer can not only enhance the active material utilization but also improve capacity retention. The defect engineering strategy accompanied with facile method is promising for the development of advanced Li–S batteries for practical application.

Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3–LiMnO2 binary system

A binary system of Li2TiO3–LiMnO2 is systematically examined by joint experimental and theoretical studies as electrode materials for Li storage applications. Increase in a fraction of Li2TiO3 effectively activates anionic redox, and thus holes are reversibly formed on oxygen by electrochemical oxidation. Such holes are energetically stabilized through π-type interaction with Mn t2g orbital as suggested by theoretical calculation. However, excess enrichment of Li2TiO3 fractions in this binary system results in the oxygen loss as an irreversible process on delithiation because of a non-bonding character for Ti–O bonds coupled with the formation of O–O dimers, which are chemically and electrochemically unstable species. Additionally, detailed electrochemical study clearly shows that Li migration kinetics is relatively slow, presumably coupled with low electronic conductivity. Nevertheless, nanosizing of primary particles is an effective strategy to overcome this limitation. The nanosized sample prepared by mechanical milling delivers a large reversible capacity, ∼300 mA h g−1, even at room temperature and shows much improved capacity retention. Formation and stabilization of holes for the nanosized sample are also directly evidenced by soft X-ray absorption spectroscopy. From these results, factors affecting the reversibility of anionic redox as emerging new chemistry and its possibility for energy storage applications are discussed in more details.

Protective coatings on silicon particles and their effect on energy density and specific energy in lithium ion battery cells: A model study

Abstract Protective coating on silicon particles is a strategy reported in literature to improve capacity retention of Si-containing lithium ion batteries. Up to date, the impact of the coating on the cell energy density and specific energy is not considered and guidelines for coating design are missing. In this paper a model is proposed to fill this gap. The model depicts how energy density and specific energy of lithium ion cells based on a Si-graphite composite electrode change in function of coating type, thickness and silicon weight fraction in the negative electrode. Volume changes during lithiation-delithiation and corresponding electrolyte displacement are also considered. Energy density depends on the ratio of coating thickness to silicon particle dimension and weight fraction of silicon in the electrode. Specific energy depends - marginally - also on the coating type. As a case study silicon spherical particles of 200 nm diameter are considered. For a 10 nm coating, the maximum energy density gain vs. state of art graphite negative electrodes is 13%, obtained with 40% weight fraction of silicon in the negative electrode. Above 60 nm thickness no improvement can be obtained vs. state of art graphite negative electrodes.

High Efficient and Environment Friendly Plasma-Enhanced Synthesis of Al2O3-Coated LiNi1/3Co1/3Mn1/3O2 With Excellent Electrochemical Performance.

PLA-1-Al2O3@LNCM synthesized using an efficient and facile plasma-enhanced method exhibits markedly improved capacity retention of 98.6% after 100 cycles, which is much larger than that of LNCM at 80% after 100 cycles. What is more, it also exhibits significantly enhanced cyclicity compared to that of 1-Al2O3@LNCM cathodes prepared using the normal solid state method, which further illustrates the efficiency and superiority of this plasma-enhanced method. More importantly, the rate performance of PLA-1-Al2O3@LNCM is improved because of the better electrolyte storage of the assembled hierarchical architecture of the Al2O3 coating layer according to unimpeded Li+ diffusion from electrode to electrolyte. When cycling at 55°C, the PLA-1-Al2O3@LNCM shows 93.6% capacity retention after 100 cycles, which is greatly enhanced due to the uniform Al2O3 layer. Further, growth of polarization impedance during cycling can be effectively suppressed by the Al2O3 layer, which can further confirm the effect the Al2O3 layer coated on the surface of the LNCM. The enhanced cycling performance and thermal stability illustrates that this facile surface modification, using the plasma-enhanced method, can form an effective structured coating layer, which indicates its prospects as an application in the modification of other electrode materials.

Fabrication of an LiMn2O4@LiMnPO4 composite cathode for improved cycling performance at high temperatures

ABSTRACT The cyclability of LiMn2O4 (LMO), a spinel cathode used in Li-ion batteries (LIBs), at high temperatures (>50°C) should be improved. In this study, we use an olivine-type LiMnPO4 (LMP) cathode with a composite structure as the surface material to improve the cyclability of LMO. The fabrication strategy includes mechanical coating of the LMO core with shell precursor particles and their subsequent transformation into LMP rod particles via hydrothermal vapor conversion. The core-shell precursor particles (composing NH4MnPO4·H2O (AmMP) plates) are successfully converted into LMO@LMP composite particles with a rod-like surface layer. The composite cathode fabricated with 7 mass% AmMP exhibits improved capacity retention of 72% after 30 cycles at 60°C, while that of the uncoated LMO cathode decreases to 63%. Further structural arrangement of the surface particles can lead to the development of a suitable cathode material for large-scale applications of LIBs.

Inter-layer-calated Thin Li Metal Electrode with Improved Battery Capacity Retention and Dendrite Suppression.

An inter-layer-calated thin Li metal (ILC-Li) electrode using nondelaminated 2D Ti3C2Tx MXene stacks (15 μm) coated on a thin Li host (30 μm) was developed. The excellent electrical conductivity and expanded interlayer space of the MXene provide a fast e-/Li+ transport while the layer limits the Li growth along the perpendicular direction, thus largely mitigating the dendrite growth. The highly reversible Li deposition/extraction greatly reduces the dead lithium and electrolyte consumption by forming a thin solid-electrolyte-interphase (SEI) layer. A small overpotential of less than 135 mV in symmetric cells was achieved after >1050 cycles at 10 mA cm-2 and 10 mAh cm-2. In a full cell, the battery exhibited an improved capacity retention when compared with Li foil, particularly with lean electrolyte of 2.5 μL mAh-1, thus leading to a high energy density up to 366.6 Wh/kg. The current approach is manufacture scalable, which displays promising potentials in lithium ion batteries.

Agglomerated nickel–cobalt layered double hydroxide nanosheets on reduced graphene oxide clusters as efficient asymmetric supercapacitor electrodes

Abstract

Higher voltage, wider voltage plateau, longer cycle life, and faster kinetics via thermally modulated interfaces between Ramsdellite and Pyrolusite MnO2 for lithium-ion battery cathodes

Non-native structures have more open structure with less stable bulk formation energy compared to their native structure and hence, non-native structured cathodes are expected to generate higher discharge potential and better lithium mobility. However, non-native structures are limited in stability, which can be enhanced by scaffolding with native structure. We synthesize highly crystalline non-native Ramsdellite MnO2 (r/NN1–MnO2) and gradually thermal phase transform to native Pyrolusite MnO2 (β/N–MnO2), intimately interfaced (r/NN1–MnO2)/(β/N–MnO2) intergrowth intermediate structures are realized. As the temperature increases, the ratio of r/NN1–MnO2 to β/N–MnO2 in intergrowth structure decreases, so does the discharge potential and lithium mobility. The wider discharge plateau for intergrowth structures is rationalized using density functional simulations followed by statistical averaging to account for interfacial intercalation sites with greater stability. Improved capacity retention for intergrowth structure is due to the higher structural stability and available free volume, where the presence of native structure acts as a stabilizing element. Electrochemical impedance spectroscopy correlates faster transport in the intergrowth structure with higher percentage of non-native phase with wider 2 × 1 channel compared to native phase with narrow 1 × 1 channel. The insights generated from this study have broad implications for interface engineering in multicomponent electrodes such as Li-rich-NMC oxides.

Dendrite-free lithium metal anode with lithiophilic interphase from hierarchical frameworks by tuned nucleation

In the development of rechargeable Li metal batteries, current collectors with high porosity, rational pore structure and lithiophilic skeleton are essentially needed to suppress Li dendrite and accommodate the volume variation. Herein, 3D hierarchical porous collector activated from chemical dealloying of brass mesh is elaborately decorated with highly lithiophilic Sn layer via electroless plating method for dendrite-free Li metal anode. During Li plating process, the nucleation overpotential is remarkably reduced with the in-situ electrochemically generated Li–Sn alloy, realizing uniform Li nucleation/growth. Meanwhile, unique hierarchical porous structure with a high porosity of 60.2% ensures well dispersed current density for homogeneous Li deposition and accommodation. As expected, a high coulombic efficiency above 98% for 240 cycles at 1 ​mA ​cm−2 and a prolonged lifespan of symmetrical cells for 800 ​h ​at 1 ​mA ​cm−2 with lower polarization are achieved, which further renders the LiFePO4 based full cells with improved capacity retention up to 92.9% after 200 cycles at 0.5C. This work proposes a highly effective strategy to realize dendrite-free Li metal anode.

Analysis on pulse charging–discharging strategies for improving capacity retention rates of lithium-ion batteries

The capacity fade of lithium-ion batteries (LIBs) are intimately dependent upon charging–discharging strategies. In this work, a pseudo-two-dimensional model coupled with thermal effects was developed to investigate the effects of pulse current charging–discharging strategies on the capacity fade for LIBs, in which the growth of solid electrolyte interphase (SEI) and the lithium ion migration process are highlighted. LiFePO4/graphite (LFP) batteries were taken as samples. The capacity fading processes of the LFP batteries under different pulse current charging–discharging strategies were studied by numerical simulations. The impacts of the growth of the SEI layer at the negative electrode on the capacity degradation were studied by using eleven charging–discharging cases. The results show that, compared with the constant current-constant voltage (CC-CV) strategy, the pulse current-constant voltage (PC-CV) strategies can effectively alleviate the capacity fade of the LFP batteries due to the relaxation process in the pulse charging process. Under the PC-CV strategies, the relaxation duration and the peak current were altered. The relaxation process renders the decrease of the current density of side reactions in the batteries, which is conducive to the recovery of the aged batteries, thus improving the capacity retention rates of the batteries. The advantages of the PC-CV strategies were demonstrated in this work. The results will provide theoretical guidance for the rapid charging–discharging strategies of LIBs.

Decoration by dual-phase Li2ZrO3 islands with core–shell structures enhances the electrochemical performance of high-voltage LiNi0.5Mn1.5O4

A high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode was modified by Li2ZrO3 (LZO), a fast ion conductor with a unique core–shell crystalline-amorphous structure. The electrochemical results indicated a greatly improved capacity retention for LNMO-1LZO compared to LNMO. Moreover, the rate performance (100 mAh·g−1) of LNMO-1LZO at a high current density of 10 C was superior to those of pristine LNMO and other modified samples. The enhanced electrochemical performance was ascribed to the generation of dual-phase island-shaped LZO with an interior crystalline phase, which accelerated Li+ diffusion, and an exterior amorphous shell, which enhanced interfacial compatibility and stability without influencing the intrinsic spinel structure of bulk LNMO. Thus, modification with this hybrid material has the remarkable synergetic effect of enhancing interfacial Li+ diffusion and stabilizing the interfacial structure during cycling.

Stabilization of Lithium Cobalt Phosphate Cathodes via Artificial Interphases

Olivine LiCoPO4 (LCP) exhibits a rare combination of high theoretical capacity (167 mAh g−1), excellent thermal stability, and high redox potential (4.8 V vs vs Li/Li+), making it a promising candidate for high voltage lithium batteries. Despite these attractive properties, practical implementation of this electrode chemistry has been limited by stability issues at the cathode-electrolyte interface, including parasitic electrolyte reactions, surface decomposition of the electrode material, and Co dissolution. Carbon coating and substitutions of Co by Fe and other cations improve the performance, however the cycling stability needs further improvement. In an effort to address these issues, we deposited thin, conformal metal oxide surface coatings on substituted LCP powder and investigated the effects of these coatings on the performance of carbon-coated substituted LCP/MCMB graphite full cells with a standard carbonate electrolyte. Remarkably, some of these coatings clearly improved operation of carbon-coated substituted LCP cells as compared to the as-prepared cathode powder. Observed improvements in capacity retention relate to stabilization of the cathode-electrolyte interface and suppression of electrolyte oxidation, as measured by online electrochemical mass spectroscopy (OEMS) of evolved gases within the cell. Together, these results suggest artificial interphases are a viable pathway toward stabilizing LCP and achieving commercial viability.

Fluorinated Acetic Anhydrides as Electrolyte Additives to Improve Cycling Performance of the Lithium Metal Anode

The investigation of novel fluorinated electrolyte additives for lithium metal anodes has been conducted. Two acetic anhydride derivatives, difluoroacetic anhydride (DFAA) and trifluoroacetic anhydride (TFAA), were investigated in electrolytes composed of LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The addition of either DFAA or TFAA results in a significant improvement in capacity retention and reversibility of lithium plating. Ex situ surface analysis (XPS, IR-ATR) suggests that incorporation of either TFAA or DFAA results in a lithium carboxylate rich SEI which in turn inhibits SEI degradation resulting in superior cycling performance.

A Low-Cost Instrument for Dry Particle Fusion Coating of Advanced Electrode Material Particles at the Laboratory Scale

Surface coating is an approach used to improve capacity retention of electrode materials for lithium-ion batteries. Dry particle fusion is a relatively new approach for applying coatings on particles. In this work, we introduce a low-cost dry particle fusion instrument that was constructed in house. The operation and performance of the machine is demonstrated by dry particle fusion coating of alumina on Ni(OH)2 and alumina and LiFePO4 on LiNi0.8Co0.15Al0.05O2, respectively. The hammer temperature vs time during dry particle fusion is used to monitor the process. Particle size distribution results demonstrate that the original core particles are not fractured by the coating process. SEM images show the morphology of particles before and after dry particle fusion coating and cross-sectional SEM/EDS images show the uniformity of the coating. Coin cell testing results show that dry particle fusion with suitable coating materials, at the laboratory scale using this instrument, is effective in improving capacity retention.

Communication—Solid-State Synthesis of Lithium Silicide

Lithium silicide (Li-Si) compounds were synthesized by a reaction between ball-milled commercially available micron sized precursor powders of lithium hydride and silicon. The reaction occurs at ≤500 °C, with the composition controlled by the LiH-to-Si ratio. The products are crystalline equilibrium phases found in the Li-Si phase diagram. The synthesized Li-Si material has a particle size akin to that in the precursor Si, thus eliminating the need for attrition processing before electrode fabrication. Furthermore, the crack-free Li-Si particles may reduce gassing and improve capacity retention in cells. Synthesized Li-Si was also found to be relatively stable during handling in ambient air.

Tuning Low Concentration Electrolytes for High Rate Performance in Lithium-Sulfur Batteries

While lithium-sulfur (Li-S) batteries promise very high gravimetric energy densities, they typically suffer from short cycle life and low power densities, particularly at high electrode loadings. Highly concentrated electrolytes (2M-5M) may demonstrate improved capacity retention, but at the expense of added weight and cost of electrolyte salts and reduced rate performance. Here we report our counter-intuitive findings on very promising performance of electrolyte compositions with salt concentrations below 0.3M. Such a low concentration enables effective use of unconventional salts and electrolyte additives which do not dissolve in the ether-based solvents at higher concentrations typically used in experimental and commercial batteries, thus substantially increasing cell design freedom. The significantly lower viscosity of the low concentration electrolytes also allows for faster electrolyte permeation. This report opens a new and promising field of studying low concentration electrolytes for Li-S and likely other chemistries.