Lithium Intercalation Reaction Research Articles

(Invited) Using a Chemo-Mechanical Model to Investigate Electrolyte Infiltration and Surface Reconstruction within Cracked Cathode Material

One of the main goals in modeling lithium-ion batteries is to improve/predict longevity and resilience of new chemistries. To that end, this talk investigates the formation of stress-induced fracture within polycrystalline cathode particles and the impact on capacity loss. The model captures anisotropic Li diffusion within a single polycrystalline particle comprised of hundreds to thousands of randomly oriented grains. Fracture is primarily due to non-ideal grain interactions with slight dependence on high-rate charge demands. Essentially, when neighboring grains are misaligned, they expand a different rates relative to one another leading to high stresses and ultimately the formation of intraparticle cracks. A previous study showed that small particle with large or single grain geometry tend to crack less and retain the most capacity.Cracks within the cathode particle that have access to the surface can potentially fill with electrolyte. When this happens, the cracks can be considered new active surface area where the lithium intercalation reaction can occur. Under sufficient conditions, this could lead to better performance because the grains of the cracked particle start to operate more like a network of single-grain particles. However, the newly open surface area is also subject to side reactions and surface reconstruction resulting in diminished transport properties and potentially trapped lithium. The goal of this talk is to investigate this balance between improved lithium transport pathways caused by lithium infiltration and the reduced diffusion due to surface reconstruction.

Electrical Double Layer Formation at Intercalation Cathode–Organic Electrolyte Interfaces During Initial Lithium‐Ion Battery Reactions

AbstractInformation on the cathode/organic–electrolyte interface structure provides clues regarding the rate and reversibility of lithium intercalation reactions in lithium‐ion batteries. Herein, structural changes within the LiCoO2 electrode, throughout the interphase region, and in the LiPF6/propylene carbonate electrolyte are observed concurrently by in situ neutron reflectometry. The formation of an electrical double layer (EDL) during the early stages of charging and discharging is investigated and compared with that at an intercalation‐inactive Nb:SrTiO3 electrode. At the intercalation‐inactive interface between Nb:SrTiO3 and the electrolyte, a voltage‐dependent ionic distribution corresponding to the EDL forms on the electrolyte side without the formation of a cathode/electrolyte interphase (CEI) layer. In contrast, at the intercalation‐active LiCoO2/electrolyte interface, a CEI layer forms immediately after cell construction, and the ionic distribution in the electrolyte formed outside the CEI layer scarcely changes upon voltage application. The CEI/electrolyte interface is shielded from potential changes by the electronically insulating CEI; therefore, structural changes in the EDL are restricted. This supports the prevailing understanding that the CEI layer defines the rates of solvation/de‐solvation and adsorption/desorption reactions of lithium ions.

Nano-Architectural Design of V2O5 for High-Performance Lithium-Ion Battery Cathode

Recently, the quest for lithium-ion batteries (LIBs) with higher energy densities, power densities, and low cost has gained great attention due to the increasing demands of high-power equipment such as electric vehicles. One of the most promising approaches to enhance the energy densities of LIBs is to couple the anode with a high-capacity cathode, since the capacity of LIBs is typically limited by the cathode. Among the high-capacity cathode materials, vanadium pentoxide (V2O5) is receiving attention due to its exceptional theoretical capacity (~294 mAh g-1) [2]. Also, since the V2O5 has no lithium in the chemical, it can increase the specific energy in the cell design of the lithium-metal battery (LMB, ~700 Wh kg-1). Moreover, it needs no first charge enhancing the initial cycling stability of the LMB.However, it suffers from a poor cycling stability and rate capability due to its structural transformation during cycling and low electrons/lithium-ion conductivities [2]. To overcome this issues, numerous efforts have been focused on shortening the lithium ion (Li+) diffusion length through the development of nanostructured V2O5 such as nanorods, nanobelts, nanosheet, and hollow spheres. Particularly, 1D nanostructured V2O5 along the [010] orientation, which is the fastest growth direction of V2O5 [4], has been suggested in many studies because of its simplicity in synthesis [1]. In nanostructures, however, structural collapse and particles agglomeration could emerge during Li+ ion penetration [3]. Moreover, the [010] direction of V2O5 corresponds to the energy-preferred diffusion pathway of Li+ ion [5]. Accordingly, 1D nanostructured V2O5 provides a long distance for the Li+ ion diffusion along [010] direction. Therefore, it is assumed that to achieve the high kinetics and structural stability, the structure for V2O5 as cathode materials should be re-designed to satisfy the following conditions, favorable to Li-ion accommodation and its pathway : ⅰ) secondary structures consisted of primary nanostructured units to prevent agglomeration. ii) a short [010] length with highly exposed (010) facets to shorten a Li+ ion diffusion pathway and facilitate the Li+ ion intercalation reactions.In this context, we introduce a facile lithium-treatment method for synthesis of nanoplates-stacked structured V2O5 (Li-treated VO) with highly exposed (010) facets and shorter [010] growth-length (0.7-1.4 um) compared to a V2O5 nanobelt (~20 um) (pristine VO). In lithium-treatment method, Li+ ions from lithium nitrate (LiNO3), which is added to precursor solution, can inhibit the complete crystallization of V2O5 during hydrothermal process, hence evolving xerogel/crystal composite. Afterward, during heat treatment process, the cleavage and oriented attachment mechanism are accompanied, generating Li-treated V2O5 with nanoplates-stacked structure having short [010] length and highly exposed (010) facets (Figure 1). The robust nanoplates-stacked structure can facilitate the fast and efficient transportation of Li+ ion into the [010] channel. In Li//V2O5 half-cell, the Li-treated VO electrodes can achieve a reversible capacity of 252 mAh g-1 at 50 mA g-1 in the voltage range of 2.05–4.0 V (vs. Li/Li+) (Figure 2). Notably, the Li-treated VO electrodes also exhibit a higher rate performance (140 mAh g-1 at 1 A g-1 (Figure 2)) and cycling capability (79 % capacity retention after 100 cycles (Figure 3)) compared to the untreated V2O5 nanobelt (VO) electrodes. In addition, the reversible lithium intercalation reaction and structural stability of Li-treated VO will be discussed in detail.

Nanoplates-Stacked Vanadium Oxide Cathode Employing Higher Kinetics and Stability in a Lithium-Ion Battery

In recent years, the popularity of high-power equipment such as electric vehicles demands a higher the energy density, higher the power density, and low cost in lithium-ion batteries (LIBs). The capacity of LIBs is typically limited by the cathode. For example, in LIBs based on LiCoO2//graphite system, which have achieved great success in commercialization, the reversible capacity of LiCoO2 (~137 mAh g-1) is lower than that of graphite (~370 mAh g-1) [1]. Hence, it is necessary to improve the capacity of the cathode in order to improve the performance of LIBs. Among the cathode materials, Vanadium pentoxide (V2O5) is considered as one of the ideal next-generation electrode materials due to its high theoretical capacities (294 mAh g-1), high energy densities, good safety and low cost [2]. Nevertheless, its cycle performance and poor rate-capability caused by severe structural changes during the repeated lithiation/de-lithiation, slow diffusion of lithium ion (Li+), and low electrical conductivity hinders its commercialization [2]. Various attempts to fabricate the nanostructured V2O5 such as nanorods, nanobelts, nanosheet, and hollow spheres have been pursued to remedy the issues. The nanostructures are used to overcome their kinetic limitation by reduction of Li+ ion diffusion distance and increase in the electrode/electrolyte contact area. In nanostructures, however, structural collapse and particles agglomeration could emerge during Li+ ion penetration procedure [3]. In particular, many studies have focused on the 1D nanostructured V2O5 in length along [010], the fastest growth direction of V2O5, because of its simplicity in synthesis [4]. However, 1D-nanostructured V2O5 has long Li+ ion diffusion distance along [010] direction, which is corresponding to the energy preferred Li+ ion diffusion path [5]. 1D diffusion along the short [010] direction in V2O5 can be the fast Li+ ion diffusion path for cathode in LIBs. Therefore, to design an optimal structure for V2O5 as cathode materials, the following conditions should be satisfied: ⅰ) structures consisted of primary nanostructured units to prevent agglomeration. ii) a short [010] length with high percentage of exposed (010) facets to shorten a Li+ ion diffusion path and facilitate the Li+ ion intercalation reactions.In this context, we fabricated nanoplates–stacked V2O5 (Li-treated VO) through Li-treatment method in hydrothermal synthesis followed by heat treatment. Li+ ions from lithium nitrate (LiNO3), which is added to precursor solution, can inhibit the completely phase transition from xerogel to crystal during hydrothermal process, leading to formation of xerogel/crystal composite. During following heat treatment process, the cleavage and oriented attachment mechanism are accompanied, evolving nanoplates–stacked V2O5 with short [010] length and highly exposed (010) facets (Figure 1). The Li-treated VO electrodes could facilitate the fast and efficient transportation of Li+ ion into the [010] channel. In particular, the Li-treated VO electrodes can achieve a reversible capacity of 252 mAh g-1 at 50 mA g-1 in the voltage range of 2.05–4.0 V (vs. Li/Li+) (Figure 2). Notably, the Li-treated VO electrodes also exhibit a higher rate performance (140 mAh g-1 at 1 A g-1 (Figure 2)) and cycling capability (79 % capacity retention after 100 cycles (Figure 3)) compared to untreated V2O5 nanobelt (VO) electrodes. In addtion, the reversible lithium intercalation reaction and structure stability of Li-treated VO are confirmed by further analyses. Figure 1

Reconciling the Volcano Trend with the Butler-Volmer Model for the Hydrogen Evolution Reaction.

The volcano trend has been widely utilized to forecast new optimum catalysts in computational chemistry while the Butler-Volmer relationship is the norm to explain current-potential characteristics from cyclic voltammetry in analytical chemistry. Herein, we develop an electrochemical model for hydrogen evolution reaction exchange currents that reconciles device-level chemistry, atomic-level volcano trend, and the Butler-Volmer relation. We show that the model is a function of the easy-to-compute hydrogen adsorption energy invariably obtained from first-principles atomic simulations. In addition, the model reproduces with high fidelity the experimental exchange currents for elemental metal catalysts over 15 orders of magnitude and is consistent with the recently proposed analytical model based on a data-driven approach. Our findings based on fundamental electrochemistry principles are general and can be applied to other reactions including CO2 reduction, metal oxidation, and lithium (de)intercalation reactions.

The lithiophobic-to-lithiophilic transition on the graphite towards ultrafast-charging and long-cycling lithium-ion batteries

The extensive use of electric vehicles has raised the requirement of fast-charging for lithium-ion batteries. However, conventional graphite anode suffers from lithium dendrites and unacceptable capacity fade during fast charging. The limited ion channels and lithiophobic nature hinder the application of graphite in fast charging. Besides, the formation and accumulation of “dead lithium” accelerate the capacity decay during long-term cycling at high current density. Here, lithiophilic hard carbon-riveted graphite (HCRG) anode is delicately designed and fabricated to overcome the influence of “dead lithium” and meet the requirement of fast charging. The riveted heterostructure plays a crucial role in the lithiophobic-to-lithiophilic transition of graphite surface and the lithiophilic surface of HCRG can attract more Li-ions and facilitate re-intercalation of Li-ions into graphite bulk in relaxation time at low potential rather than plating on the surface, which effectively decreases the polarization and increases the proportion of reversible lithium at high current density. As a result, it exhibits excellent rate performance with 98.2% capacity retention when rate increasesing from 1 C to 15 C and fast-charging performance with 90.1% energy retention after 4000 cycles at an ultrahigh rate of 10 C in LiFePO4/HCRG full-cell. This strategy of constructing riveted heterostructure efficiently boosts the lithium intercalation reaction. It achieves fast-charging performance, highlighting the comprehensive understanding of interfacial kinetics and structure optimization for other fast-charging anodes.

Electrochemical model analysis of bulk-type all-solid-state lithium-ion battery

This paper presents a numerical analysis of bulk-type all-solid-state lithium-ion batteries. The electrochemical modeling for analyzing all-solid-state batteries (ASSBs) is described, in which a single-ion conducting electrolyte theory is presented and coupled with the pseudo-two-dimensional (P2D) battery model used for lithium intercalation reactions in the porous electrodes. The benchmark study for an all-solid-state cell of Li/Li9.6P3S12/LiNbO3-coated LiCoO2 is performed using COMSOL Multiphysics®. The calculated charge-discharge curves agree well with the available experimental data. The modeling for the lithium diffusion coefficient in electrode active material particles, which is represented as a function of lithium stoichiometry, is also examined.

(Invited) Novel Disordered Rocksalt Electrodes for Safe, Fast Charging Lithium-Ion Batteries

Rechargeable lithium-ion batteries with high energy density that can be safely charged and discharged at high rates are desired for electrified transportation and other applications. In this talk, I will report the discovery of novel disordered rocksalt oxides (DRSOs) electrodes with near-optimal intercalation potentials for safe, fast-charging batteries. In particular, we will demonstrate that the outstanding electrochemical performance of these DRSO electrodes can be attributed to a hitherto undiscovered redistributive lithium intercalation reaction with low energy barriers. Beyond the immediate practical applications, the discovery of this novel high-rate intercalation reaction opens a completely new space for the search of metal oxide electrodes for fast-charging, long-life lithium-ion batteries.

Interfacial and Electronic Modulation via Localized Sulfurization for Boosting Lithium Storage Kinetics.

Structural modulation endows electrochemical hybrids with promising energy storage properties owing to their adjustable interfacial and/or electronic characteristics. For MXene-based materials, however, the facile but effective strategies for tuning their structural properties at nanoscale are still lacking. Herein, 3D crumpled S-functionalized Ti3 C2 Tx substrate is rationally integrated with Fe3 O4 /FeS heterostructures via coprecipitation and subsequent partial sulfurization to induce a highly active and stable electrode architecture. The unique heterostructures with tuned electronic properties can induce improved kinetics and structural stability. The surface engineering by S terminations on the MXene further unlocks extra (pseudo)capacitive lithium storage. Serving as anode for lithium storage, the optimized electrode delivers an excellent long-term cycling stability (913.9 mAh g-1 after 1000 cycles at 1 A g-1 ) and superior rate capability (490.4 mAh g-1 at 10 A g-1 ). Moreover, the (de)lithiation pathways associated with energy storage mechanisms are further revealed by operando X-ray diffraction, in situ electroanalytical techniques, and first-principles calculations. The hybrid electrode is proved to undergo stepwise phase transformations during discharging but a relatively uniform reconversion during charging, suggesting an asymmetric conversion mechanism. This work provides a novel strategy for designing high-performance hybrids and paves the way for in-depth understanding of complex lithium intercalation and conversion reactions.

Entropy Change Characteristics of the LiNi0.5Mn1.5O4 Cathode Material for Lithium-Ion Batteries.

Lithium-ion batteries are widely used in the field of new energy vehicles and energy storage. Understanding the electrode reaction of lithium-ion batteries is the key to improve their cycle life and safety. Direct measurement of thermodynamic data of the electrode reaction is a practical, economical, and nondestructive method for electrode characterization. In this paper, the open-circuit voltage of the LiNi0.5Mn1.5O4/Li half-cell is measured at different discharge states and different temperatures. The dE/dT–SOD (state of discharge) relation curves are fitted linearly by the least square method, and the entropy change values of different SODs are calculated. Finally, the Gibbs free energy and enthalpy change of different SODs are obtained. The electrode reaction of LiNi0.5Mn1.5O4 in different SODs was discussed by the entropy change in different SODs. According to the evolution trend of ΔS, the lithium intercalation reaction of LiNi0.5Mn1.5O4 may be a single-phase solid solution reaction rather than a two-phase reaction. Finally, the reversible heat generation at different current values and SODs are calculated.

Charge Density and Redox Potential of LiNiO2 Using Ab Initio Diffusion Quantum Monte Carlo

Electronic structure of layered LiNiO2 has been controversial despite numerous theoretical and experimental reports regarding its nature. We investigate the charge densities, lithium intercalation potentials and Li diffusion barrier energies of LixNiO2 (0.0 < x < 1.0) system using a truly ab-initio method, diffusion quantum Monte Carlo (DMC). We compare the charge densities from DMC and density functional theory (DFT) and show that local and semi-local DFT functionals yield spin polarization densities with incorrect sign on the oxygen atoms. SCAN functional and Hubbard-U correction improves the polarization density around Ni and O atoms, resulting in smaller deviations from the DMC densities. DMC accurately captures the p-d hybridization between the Ni-O atoms, yielding accurate lithium intercalation voltages, polarization densities and reaction barriers.

Selenium-Carbon Nanotubes Composite Cathode with Long Cycle Life for Rechargeable Li-Se Batteries

In the past two decades, lithium-ion batteries (LIBs) have been extensively utilized in portable electronic devices as well as electric vehicles (EVs). However, conventional LIBs depending on the lithium intercalation reactions of anode and transitional metal oxide cathode materials have limited capacities and low energy densities, which prevent their broader deployment as power sources for prolonged usage. High energy density and durable electrical energy storage devices are the key factors to meet the crucial demands for high-performance portable electronic devices and EVs. Selenium (Se), as a congener of sulfur in the periodic table, has been proposed as an alternative cathode material for rechargeable lithium batteries. Although Se has lower theoretical capacity (678 mAh g-1) than sulfur, it can provide a high volumetric capacity of 3253 mAh cm-3 because of its high mass density (4.8 g cm-3). In addition, selenium has a much higher electronic conductivity (1×10-3 S m-1), which implies that Se could provide better electrochemical activity. However, Se cathode is still facing similar challenges as sulfur such as poor cycling life and low Coulombic efficiency, which are attributed to the dissolution and shuttle effect of the formed high-order polyselenides as redox intermediates in the ether-based electrolyte. In addition, the significant volume change increases the difficulty for Li-Se batteries to be used in practical applications. During the last decade, lots of effort have been put into developing approaches to address these problems, thereby increasing energy density and cycle life of Li-Se batteries. In this contribution, we synthesized Se nanowires in the presence of carbon nanotubes (CNTs) by a facile method. The experimental process for preparing the Se/CNT composite consists of dissolving selenium oxide (SeO2) and beta-cyclodextrin (β-cyclodextrin) and dispersing the CNT bundles in an ethanolic mixture with de-ionized water. The vigorously stirring and ultrasonication procedure in the initial mixture cause fine dispersion of CNTs. In addition, the ethanol extenuates the hydrophobic activity of CNTs, and hence accelerates the dispersion-weave process by ultrasonication. The addition of ascorbic acid solution ensures the product of selenium nanowires. Upon another ultrasonication for selenium nanowires and CNTs to be perfectly woven, and then filtered and dried, the Se/CNT composite electrode was obtained. The filter paper has a diameter of 7 cm, which decides the overall size of the composite electrode. Se nanowires are weaved with CNTs to form a uniform binder-free composite electrode, which has superior electrical conductivity. This synthesis method provides a path for fabricating the Se cathodes with controllable mass loadings and thicknesses. At initial state, Se is in the form of branched nanowires with a diameter of <150 nm and length of 1-2 µm, interweaving with CNTs. By studying the composites electrodes with different mass loadings of active materials and different thicknesses, it is found that the electrode thickness has influence on the accumulation of redox products upon repeated cycling. The structural and morphological changes are analyzed by scanning electron microscope (SEM). Finally, the electrochemical phenomena are revealed experimentally, and enhanced cycling performance is achieved. The composite electrode with 23 µm thickness and 60% Se loading shows a high initial capacity of 537 mAh g-1, followed by stable cycling performance with a capacity of 401 mAh g-1 after 500 cycles at 1C rate. This study reports a synthesis strategy to obtain Se/CNT composite cathode with long cycle life for rechargeable Li-Se batteries. Figure 1

Characterization of Layered Rocksalt Type Cathode/Sulfide Solid-Electrolyte Interface Using a Model Film Battery and in Situ X-Ray Diffraction

Sulfide based ionic conductors are attractive solid electrolytes for all solid-state batteries owing to their high ionic conductivity and high reliability. One of remaining issues is to stabilize the lithium (de)intercalation reaction at the cathode/sulfide electrolyte interface in high potential regions for the development of all solid state batteries with high energy density. Here we investigated the interfacial reactions between LiMO2 (M= Ni, Mn, Co) and amorphous Li3PS4 using a model film battery. LiMO2 cathode/LiNbO3 buffer layer/Li3PS4 films were fabricated by pulsed laser deposition, and Li metal anode was deposited by vapor deposition. The thin-film battery had a well defined interface suitable for elucidating electrochemical reactions. The relationship between electrochemical properties and crystal structural changes were discussed based on crystal structures determined by in situ X-ray diffraction (XRD) measurements conducted under high voltage operation.

Direct Synthesis of Nanotubular Si-TiO2for Lithium-Ion Batteries Anodes By Anodizing

Industry and the scientific community have been concentrating efforts on the production and development of energy storage systems ESS [1].By the energy density improvements of lithium ion battery electrodes for electronic devices (EDs) and energy vehicles (EVs), and cost of manufacturing must be taken into account, so that new developed materials can with the currently used batteries[2-4].Silicon is an important candidate as anode material for the next generation of lithium ion batteries(LIBs) because of its extremely high specific capacity (4200 mAh/g). However, Si particles degrade during the volumetric change related to lithiation/delithiation processes.Efforts have been made in the last decade in an attempt to minimize the instabilities caused by the large volumetric variation [5,6]. The most effective approaches in this regard include: (a) reduction of particle size to the nanometer size range to lower mechanical stress, (b) formation of a porous structure promoting a stable solid electrolyte interface SEI (c) dispersing Si nanoparticles in a conductive matrix to accommodate the volumetric variations and preserve the mechanical integrity of the electrode, (d) formation of amorphous SiO2 structure in small dimensions, and the(e) formation of intermetallic composite with structure layers to accommodate Si particles[7]. In the other hand, TiO2also incorporates Li(330 mAh/g)and shows a fast lithium intercalation reaction, without major volumetric effects during Li intercalation/deintercalation. In this work the aim is to produce Si nanoparticles embedded in a nanotubular TiO2 matrix, avoiding mechanical stress problems, due to volumetric changes and the formation of SEI directly between Si nanoparticles(SiNP) and the electrolyte. For this, Ti-Si alloys with 2 and 4 at.%Si were prepared by arc melting in an Ar atmosphere. A Si-Ti solid solution phase was obtained by furnace heating to 1200 ºC and rapidly cooling to a hard martensitic phase.Thin sheets of these alloys were cut and anodized in different electrolytes aiming to avoid the dissolution of the Si Particles. The structure and morphology of the Si rich TiO2 nanotubes were investigated by TEM, XPS, SEMand GID/DRX. Cell voltage transients of the anodizing of Ti2Si and Ti4Si indicate different levels of SiNP accumulation. Different electrolyte compositions, organic and inorganic, with and without fluoride were used for the growth of the porous structures. The effect of lithium addition on electrolytes is observed in favor of porous structure. SiNPs embedded in TiO2 were observed by SEM, TEM and characterized by GID/DRX.1% at. Si was measured by EDS onregular tubular TiO2 detached from Ti-2 at.%Si anodized in a fluoride based electrolyte, indicating that around 50% of the Si was not lost to the electrolyte during anodization. In the other hand, metallic Si phase was detected by grazing angle diffraction indicating that Si NP are formed. The developed process shows to be an efficient way for the direct fabrication of self-organized titania nanotubes containing silicon nanoparticles. [1]S.Goriparti, E.Miele, F. De Angelis, E. Di Fabrizio, R. P.Zaccaria, C. Capiglia, Journal of Power Sources, 257 (2014) 421-443. [2]Naoki Nitta, Feixiang Wu, Jung Tae Lee and GlebYushin. Materials Today Volume 18, Number 5 June (2015) [3] Peter G. Bruce, Bruno Scrosati, Jean-Marie TarasconAngewandteChemieAngew. Chem. Int. Ed. (2008), 47, 2930 – 2946 [4] Bruno Scrosati, Jürgen Garche, Lithium batteries: Status, prospects and future, Journal of Power Sources 195 (2010) 2419–2430 [5] Weitian Lia,1, Xianwei Guoa,1, YueLub, Lin Wanga, Ailing Fana, ManlingSuib, HaijunYua, Amorphous nanosized silicon with hierarchically porous structure for highperformance lithium ion batteries. Energy Storage Materials 7 (2017) 203–208 [6] Lucie Leveau , Barbara Laïk , Jean-Pierre Pereira-Ramos, AurelienGohier , Pierre Tran-Van ,Costel-SorinCojocaru. Silicon nano-trees as high areal capacity anodes for lithium-ion batteriesJournal of Power Sources 316 (2016) 1-7. [7] HuajunTian, FengxiaXin, Xiaoliang Wang, Wei He ,Weiqiang HanHigh capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries. Journal of Materiomics 1 (2015) 153-169 Figure 1

Conversion reaction of vanadium sulfide electrode in the lithium-ion cell: Reversible or not reversible?

With the increasing interest in transition metal chalcogenides, sulfide minerals containing the disulfide unit (S22-) have gained intensive attention for potential applications in energy storage devices, such as lithium-ion batteries (LIBs). Vanadium tetrasulfide (VS4) possesses a unique linear-chain structure with a Peierls distortion and shows great promise for application in LIBs. However, its electrochemical reaction mechanism is still controversial, mainly due to the amorphous nature of the intermediates and final products. Here, by applying multiple X-ray spectroscopies, we reveal that VS4 undergoes lithium intercalation and conversion reactions sequentially during the first discharge process, which are partially reversible in the subsequent charge process. However, an anomalous intercalation/conversion mixed reaction mechanism is dominant for the second cycle, mainly owing to the amorphization of the VS4 electrode during the first cycle. In addition, the sulfur atoms are also involved in the redox reaction during cycling, with the anionic contribution of S22-↔ 2S2- transformation. Furthermore, we find that the formation process of the solid electrolyte interphase is highly dynamic during the discharge and charge processes. The present study provides deeper insights into the complex reaction mechanism of VS4. This knowledge can accelerate the development of high-performance VS4-based electrode materials for LIBs.

Tracking the Chemical and Structural Evolution of the TiS2 Electrode in the Lithium-Ion Cell Using Operando X-ray Absorption Spectroscopy.

As the lightest and cheapest transition metal dichalcogenide, TiS2 possesses great potential as an electrode material for lithium batteries due to the advantages of high energy density storage capability, fast ion diffusion rate, and low volume expansion. Despite the extensive investigation of its electrochemical properties, the fundamental discharge-charge reaction mechanism of the TiS2 electrode is still elusive. Here, by a combination of ex situ and operando X-ray absorption spectroscopy with density functional theory calculations, we have clearly elucidated the evolution of the structural and chemical properties of TiS2 during the discharge-charge processes. The lithium intercalation reaction is highly reversible and both Ti and sulfur are involved in the redox reaction during the discharge and charge processes. In contrast, the conversion reaction of TiS2 is partially reversible in the first cycle. However, Ti-O related compounds are developed during electrochemical cycling over extended cycles, which results in the decrease of the conversion reaction reversibility and the rapid capacity fading. In addition, the solid electrolyte interphase formed on the electrode surface is found to be highly dynamic in the initial cycles and then gradually becomes more stable upon further cycling. Such understanding is important for the future design and optimization of TiS2 based electrodes for lithium batteries.

(Invited) Tracking Ionic Transport and Electrochemical Reactions in Low-Dimensional Nanomaterialsfor Energy Storage

Lithium-ion batteries are one of the most popular rechargeable energy storage devices for portable electronics and electrical vehicles. To design safe, high-energy electrodes with long-cycle life, we need a better understanding of how electrode materials function in real-time at nanoscale. In this presentation, I will show a method we developed for performing in-situ electron microscopy and spectroscopy to track lithium transport and electrochemical reactions. Three examples will be given. One is on lithium conversion reaction in individual iron fluoride nanoparticles and the other is on lithium intercalation reaction in spinel titanate nanoplatelets. The third one is on lithium diffusion in doped hollandite nanorods which exhibit intriguing 1D tunnel structure that can be manipulated for charge transfer. The microscopy investigations were integrated with bulk x-ray and electrochemical measurements as well as theoretical calculations at atomic scale based on DFT and meso scale based on phase-field theory. The kinetics of ionic transport and the role of defects in the lithiation process will be discussed. The author would like to thank F. Wang, L. Wu, W. Zhang, F. Xu, Q. Meng, M. Hybertsen, AC. Marschilok and ES. Takeuchi for their collaborations. The work was supported by the U.S. DOE/BES under contract DE-SC0012704.

Dynamic imaging of metastable reaction pathways in lithiated cobalt oxide electrodes

Understanding how lithium-ion batteries function down to the atomic level during charge and discharge cycling can provide valuable guidance to optimize structure-property relationships and to design and understand new electrode materials. Lithium insertion and reactions with the electrodes during charge and discharge cycling can occur via metastable structures with complex ordering and related non-equilibrium phenomena. Remarkably, these processes remain still poorly understood despite their significance in the operation of lithium battery systems in critical technologies. In this communication, we present the dynamics of lithium insertion into Co3O4 and the evolution of metastable phases as probed by in-situ transmission electron microscopy, in concert with first principles density functional theory calculations. We show that the initial lithium intercalation reaction occurs with the formation of several metastable and intermediate phases, followed by a sequence of conversion reactions that perturb and expand the cubic-close-packed oxygen array, ultimately generating an end-product of finely dispersed cobalt metal clusters within a Li2O matrix. The calculated non-equilibrium lithiation pathways corroborate with the experimental lithiation voltages, and explain the significant hysteresis that occurs during electrochemical cycling. The data provide new insights into the complexity of solid state lithium electrochemistry in metal oxides that are relevant to advancing lithium battery technology.

Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries

Lithium-ion batteries based on intercalation compounds have dominated the advanced portable energy storage market. The positive electrode materials in these batteries belong to a material group of lithium-conducting crystals that contain redox-active transition metal and lithium. Materials without lithium-conducting paths or lithium-free compounds could be rarely used as positive electrodes due to the incapability of reversible lithium intercalation or the necessity of using metallic lithium as negative electrodes. These constraints have significantly limited the choice of materials and retarded the development of new positive electrodes in lithium-ion batteries. Here, we demonstrate that lithium-free transition metal monoxides that do not contain lithium-conducting paths in their crystal structure can be converted into high-capacity positive electrodes in the electrochemical cell by initially decorating the monoxide surface with nanosized lithium fluoride. This unusual electrochemical behaviour is attributed to a surface conversion reaction mechanism in contrast with the classic lithium intercalation reaction. Our findings will offer a potential new path in the design of positive electrode materials in lithium-ion batteries. Positive electrode materials for lithium-ion batteries feature lithium element and lithium-ion conduction paths. Here the authors report transition metal monoxides that contain neither the intrinsic lithium nor conduction channels for high-capacity positive electrode materials.

Consecutive Phase Transition of Lattice Strain Controlled LiFePO4 for High-Power Li-Ion Batteries

INTRODUCTION Lithium iron phosphate, LiFePO4, becomes conspicuous as a commercially important cathode material due to low cost, high safety, and non-toxic nature. Lithium-ion intercalation proceeds through a two-phase reaction between two compositions close to the endmembers LiFePO4 (LFP-phase) and FePO4 (FP-phase). The first-order phase transformation accompanied with a large volume change of 6.8% hinders moving phase boundaries faster than ever. Co-substituted LiFePO4, represented in Li(Fe1-xZrx)(P1-2xSi2x)O4 or Z2S, which decreases the lattice volume change between two phases, shows six times longer cycle life than undoped LiFePO4 (ref. 1). Here we focus on the Z2S, consisting of moderate two-phase interfaces, and investigate kinetics and mechanisms for the two-phase reaction with reduced lattice mismatch using time-resolved X-ray diffraction. EXPERIMENTAL Undoped LiFePO4 and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4, just called hereafter Z2S, were synthesized in the same manner as reported1. The ex situ X-ray diffraction (XRD) measurements were performed at BL02B2, SPring-8 with a wavelength of 0.699292(4) Å using a Debye-Scherrer camera and an imaging plate detector. The cathode materials for the electrochemical tests were prepared by mixing 80% active material, 10% carbon black, and 10% polyvinylidene fluoride (PVDF) with 1-methyl-2-pyrrolidinone solvent. The composite electrodes were placed in original laminate-type cells in an Ar-filled glovebox with lithium metal as the counter and reference electrodes. LiPF6 (1 M) in a 3:7 volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used as the electrolyte. The in situ XRD measurements were performed at BL28XU, SPring-8 with a wavelength of 0.619862(2) Å using a 1D detector, MYTHEN 1K. The data were collected in the 2θ range of 10° to 13° with an exposure time of 1 s every 60 s(1C) and 15 s(10C). RESULTS AND DISCCUSION Both Undoped LiFePO4 and Z2S are the same particle size as about 150 nm, indicating independent of nano-sized effects2 and allowing the comparison of the difference of the lattice volume change. Galvanostatic charge/discharge tests were performed. At a low rate of 1C, both of them show the same capacity as 120 mAh/g and the similar shape of charge/discharge curves. At a high rate of 10C, the capacity of Z2S keeps still 90 mAh/g although that of undoped LiFePO4 falls to 75 mAh/g. The difference of both capacities are lager with increasing rates, which indicates the enhancement of the rate capability in Z2S. In order to get information on the static state, the ex-situ XRD and the open-circuit voltage (OCV) measurements were performed. The ex situ XRD patterns show a typical two-phase characteristics in both cathodes as emerged FP-phase peaks from 0.1Li during a charge process. The OCV curves show the flat voltage plateau in a wide range, which also indicates the coexistence of two phases in the reaction. From these results based on an equilibrium system, there is no significant difference in phase transition behavior between undoped LiFePO4 and Z2S. To try to understand what is going on in lithium intercalation reaction indeed and to achieve high power lithium-ion batteries, time-resolved in-situ XRD measurements were performed. At a low rate of 1C, undoped LiFePO4 proceeds via two phase reaction similarly in the results of the ex situ experiments. In Z2S, the peaks belonging to each LFP-phase and FP-phase simply increase or decrease although accompanying with peak broadening. At a high rate of 10C, on the other hand, the peak shift mainly occurs in Z2S compared with undoped LiFePO4. This suggests the quasi-single phase reaction proceeds in Z2S and explains the enhancement of rate capability in Z2S, which is the small lattice volume change. REFERENCES 1. M. Nishijima, T. Ootani, Y. Kamimura, T. Sueki, S. Esaki, S. Murai, K. Fujita, K. Tanaka, K. Ohira, Y. Koyama and I. Tanaka, Nat. Commun., 5 (2014). 2. N. Meethong, H. Y. S. Huang, W. C. Carter and Y. M. Chiang, Electrochemical and Solid State Letters, 10, A134 (2007).