Full Delithiation Research Articles

Unravelling lithium distribution in carbon fibre electrodes for structural batteries with atom probe tomography

Carbon fibres in structural batteries are multifunctional by acting both as structural reinforcement and as lithium (Li)-ion battery electrode. The relationship between the microstructure and mechanical capabilities of carbon fibres are well established, but much remains unexplored regarding their electrochemical properties. Specifically needed is a nanoscale understanding of how Li atoms distribute and interact in the carbon fibres. Atom probe tomography (APT) is uniquely positioned to provide subnanometre resolution in three dimensions. However, it has previously been hampered by undesirable Li migration during analysis. Here, we show that APT is successfully used to analyse electrochemically cycled polyacrylonitrile-based carbon fibres, through electrostatic shielding by means of conductive coating. We measure ∼1.5 at% Li in the carbon fibres after full delithiation, and thus identify trapped Li to constitute a substantial part of the initial capacity fade. After lithiation, Li accounts for ∼9 at% and according to frequency distribution analysis tend to agglomerate on the atomic scale. With nearest neighbour analysis, Li agglomeration is shown independent of heteroatom dopants such as nitrogen. Thus, the agglomeration is more likely induced by differing accessibility for Li in the crystalline and amorphous domains in the carbon fibre. The method used in this study can inform APT experiments on other type of Li-containing carbon electrodes. The findings of the study can be used to guide design of novel carbon fibres for structural batteries with enhanced electrochemical properties.

Nonflammable Fluorinated Ester-Based Electrolytes for High Voltage Li Batteries with LiCoO2 Positive Electrode

LiCoO2 is widely used as a positive electrode material for LIBs. The theoretical capacity of LiCoO2 is calculated to be 274 mA h g–1 upon full delithiation, but the practical reversible capacity is generally restricted to approximately 140 mA h g–1 to avoid capacity deterioration when cycled at higher voltages (>4.2 V vs Li/Li+). To solve this limitation, functional electrolytes that possess a high oxidative stability and form a stable and effective passivation layer on the LiCoO2, are desired.In this study, we report that the cyclability of LiCoO2 under high-voltage operation is drastically improved by using a fluorinated ester, methyl 3,3,3-trifluoropropionate (MTFP), as an electrolyte solvent.1 Notably, the combined use of MTFP-based electrolyte and mixed sodium carboxymethyl cellulose/styrene-butadiene rubber binder synergistically stabilize the LiCoO2 electrode surface, resulting in the almost no capacity fading for 50 cycles even with the upper cutoff voltage of 4.5 V. Note that, in general, LiCoO2 shows significant capacity deterioration when cycled at higher voltages over 4.2 V vs Li/Li+. Moreover, our study reveals that superior low-temperature performance and higher thermal stability is evidenced for the LiCoO2 with MTFP-based electrolyte when compared with that with conventional electrolyte. XPS analysis reveals that thinner and uniform passivation layer derived from PF6 – anion and MTFP is formed on the LiCoO2 electrode surface, which is the origin for improved cyclability and thermal stability of high-voltage LiCoO2 with MTFP-based electrolyte.Reference[1] Y. Ugata et al., and N. Yabuuchi, Chemistry of Materials, in press.

α-LiFe5O8: A promising iron-based anode material for lithium-ion batteries

Owing to the low-cost, environmental friendliness and high-power density, Fe-based materials attract great attention as promising anodes for Lithium-ion batteries (LIBs). Here, spinel-type α-LiFe5O8 powders are synthesised through a facile room-temperature reaction followed by a high-temperature calcination step. When test as an anode, α-LiFe5O8 shows an excellent electrochemical performance, with specific capacities of 840.12 (0.1C), 532.1 (0.2C) and 411 mAh g−1 (0.5C) after 50 cycles. Furthermore, we investigate the lithium storage mechanism in this material using a combination of techniques including in-situ PXRD, FESEM, TEM, and XPS. Combined data reveal that an irreversible phase change takes place during the first cycle. After the first full delithiation, the original α-LiFe5O8 phase is not recovered. Instead, the face-centred cubic FeO with a space group of Fm3¯m is detected which indicates an irreversible phase transition happened during the first cycle process.

Dynamic In‐Plane Heterogeneous and Inverted Response of Graphite to Fast Charging and Discharging Conditions in Lithium‐Ion Pouch Cells

Solutions for improving fast charging of lithium‐ion batteries have largely focused on alleviating through‐plane lithiation gradients while little is understood about in‐plane heterogeneities and how to resolve them. Herein, high‐speed synchrotron X‐ray diffraction (XRD) resolves graphite lithiation spatially and temporally during 6 C charging and 2 C discharging. At every point during operation, considerable differences in the state of lithiation across the pouch cell are present. Some regions are more responsive to operation than others, reaching full lithiation early during charge and full delithiation during discharge. Other regions within the cell never fully delithiate during discharge, despite a prolonged voltage hold at 2.8 V. Using time‐resolved XRD data, the calculated local current density (mA cm−2) at the graphite surface shows an unexpected occurrence of local inverted current densities where regions of graphite are observed to delithiate during charging and lithiate during discharging. A pseudo‐3D model is developed for the graphite electrode with spatially varying microstructural tortuosity to show how microstructural heterogeneity could influence spatial charge dynamics. The model could not predict the complex in‐plane charge behavior observed within the cell. Consequently physics‐based charging protocols based on homogeneous electrode assumptions may underestimate the local variations in charge dynamics and occurrence of lithium plating.

Feigned death induced by partial delithiation in silicon composite electrodes

This paper discovers and reveals a phenomenon of feigned death induced by partial delithiation in silicon composite electrodes. Unexpected extremely rapid capacity fading occurs in the initial dozens of the cycles with low upper cut-off voltages. By looking into the irreversibility, the crack, the elemental analysis and the thickness/weight of the electrode, it suggests that the rapid capacity fading is not induced by degradation. Additionally, by providing a reboot with full delithiation, the capacity can be recovered completely, which confirms the integrity of the electrode. This novel type of capacity fading is named by “feigned death” to distinguish it from the “true death” caused by degradation. It is further confirmed that the feigned death is caused by the asymmetric phase transformation between crystalline and amorphous phases in silicon electrodes. When the depth of lithiation is high enough to generate the c-Li3.75Si phase but the depth of delithiation is too low to eliminate the generated crystalline phase, the feigned death is triggered. The findings of this work provide an in-depth understanding of the electrochemical performance of Si-based electrodes.

Why is the O3 to O1 phase transition hindered in LiNiO2 on full delithiation?

The O1 phase transition is suppressed by Ni migration to tetrahedral sites in NiO2.

Empirical potential optimization for the investigation of lithiation-delithiation cycles of amorphous Si nanowires

The atomistic mechanisms during lithiation and delithiation of amorphous Si nanowires ($a$-SiNW) have been investigated over cycles by molecular dynamics simulations. First, the Modified Embedded Atom Method (MEAM) potential from Cui et al. [J. Power Sources. 2012, (207) 150] has been further optimized on static (Li$_x$Si alloy phases and point defect energies) and on dynamic properties (Li diffusion) to reproduce the lithiation of small crystalline Si nanowires calculated at the {\it ab initio} level. The lithiation of $a$-SiNW reveals a two-phase process of lithiation with a larger diffusion interface compared to crystalline Si lithiation. Compressive axial stresses are observed in the amorphous Si$_x$Li alloy outer shell. They are easily released thanks to the soft glassy behavior of the amorphous alloy. Conversely, in crystalline SiNW, the larger stress in the narrow crystalline lithiated interface is hardly released and requires a phase transformation to amorphous to operate, which delays the lithiation. The history of the charge-discharge cycles as well as the temperature appear as driving forces for phase transformation from amorphous Li$_x$Si alloy to the more stable crystalline phase counterpart. Our work suggest that a full delithiation could heal the SiNWs to improve the life cycles of Li-ion batteries with Si anode.

Unraveling Oxygen Evolution in Li-Rich Oxides: A Unified Modeling of the Intermediate Peroxo/Superoxo-like Dimers.

Peroxo/superoxo is a key intermediate in oxygen evolution/reduction reactions in (electro)catalysis. However, peroxo/superoxo analogues have aroused controversies relevant to the origin of oxygen-anion redox. Specifically, some characteristics such as the magnitude of the O-O bond length in bulk materials have been puzzling during oxygen oxidation, as has the relationship between the peroxo/superoxo intermediate and the release of oxygen. The latter is a major safety concern to the application of oxygen-anion redox in lithium ion batteries. Herein, we present a unified modeling of the full delithiation process for model system Li2MnO3 by using first-principles calculations. We find that the cationic antisite defects and the electron deficiency are two major limiting factors in the anionic oxidation whose state can evolve, as the degree of delithiation increases, from the electron/hole, through peroxo-like O2δ- dimer formation, to the eventual release of gas-phase oxygen molecule. During the delithiation process, the dangling oxygen (i.e., singly coordinated with Mn) pairs play a critical role in intermediate dimer formation. Meanwhile, we identify five generic binding patterns of O2δ- dimers with Mn ions for which the O-O bond length varies from 1.45 Å in the peroxo state to 1.22 Å in the gas-phase oxygen molecule. Moreover, the dominant features of the three molecular orbitals, σc, πa, and πb, are distinguished, with the corresponding energy levels being highly delocalized and mixed as a result of the interplay with the host lattice. This work provides a deep understanding of the intermediate states of the anionic redox and suggests new strategies that mitigate oxygen release for the design of highly efficient and safe Li-rich cathode materials.

Chemical and Electrochemical Lithiation of van der Waals Tetrel-Arsenides.

Lithiation of van der Waals tetrel-arsenides, GeAs and SiAs, has been investigated. Electrochemical lithiation demonstrated large initial capacities of over 950 mAh g-1 accompanied by rapid fading over successive cycling in the voltage range 0.01-2 V. Limiting the voltage range to 0.5-2 V achieved more stable cycling, which was attributed to the intercalation process with lower capacities. Ex situ powder X-ray diffraction confirmed complete amorphization of the samples after lithiation, as well as recrystallization of the binary tetrel-arsenide phases after full delithiation in the voltage range 0.5-2 V. Solid-state synthetic methods produce layered phases, in which Si-As or Ge-As layers are separated by Li cations. The first layered compounds in the corresponding ternary systems were discovered, Li0.9 Ge2.9 As3.1 and Li3 Si7 As8 , which crystallize in the Pbam (No. 55) and P2/m (No. 10) space groups, respectively. Semiconducting layered GeAs and SiAs accommodate the extra charge from Li cations through structural rearrangement in the Si-As or Ge-As layers and eventually by replacement of the tetrel dumbbells with sets of Li atoms. Ge and Si monoarsenides demonstrated high structural flexibility and a mild ability for reversible lithiation.

Phase Evolution and Degradation Modes of R3̅m LixNi1–y–zCoyAlzO2 Electrodes Cycled Near Complete Delithiation

Practical utilization of energy densities near the theoretical limit for R3m layered oxide positive electrode materials is dependent on the stability of the electrochemical performance of these materials at or near full delithiation. To develop new chemistries and novel approaches toward the improvement of the electrochemical performance of these materials at such high states of charge, a robust understanding of the failure mechanisms limiting current materials is necessary. Thorough analysis of LixCo1–yAlyO2 and LixNi1–yAlyO2 as well as LixNi0.8Co0.2O2 and LixNi0.8Co0.15Al0.05O2 (1 ≥ x ≥ 0 and 0.2 ≥ y ≥ 0) enabled the identification of key relationships between the transition metal chemistry of the electrode, its structural stability, and cycling characteristics at or near complete delithiation (4.75 V). Extensive characterization of these materials was achieved by a multitude of physical and electrochemical techniques to investigate the relative importance of surface vs bulk phenomena. The resulting in...

Buckling analysis and control of layered electrode structure at finite deformation

This work has proposed a coupled mechanochemical model to investigate the buckling performance of electrode and thereby enhance the mechanical integrity of lithium-ion batteries. This method based upon finite deformation theory is capable to simulate the state of charge (SOC) and path dependent buckling phenomenon of the layered structure. It is revealed that the buckling behavior of an electrochemical loaded electrode depends not only on the material properties and the electrode geometry but also on the charging/discharging process, SOC as well as the operation current rate. Moreover, with regarding to the lithiation stiffening active material, the results show that increasing the current density in lithiation while decreasing it upon delithiation can increase the relatively critical buckling load, and contribute the electrode to withstand buckling failure. As for the lithiation softening one, it is disclosed that the bucking resistance would be impaired in lithiation and recovered upon full delithiation. In addition, to eliminate the adverse effect of lithiation soften on the electrode buckling resistance, design insights into the current collector have been provided.

Improved Structural Reversibility and Cycling Stability of Li2MnSiO4 Cathode Material by the Pillar Effect of [TiOx] Polyanions

AbstractLi2MnSiO4 cathode material suffers from structural distortion caused by irreversible transition of [MnO4] tetrahedron to [MnO6] octahedron and layer exfoliation upon full delithiation, thus resulting in rapid capacity decay during cycling. In this study, Ti was introduced into the Mn site in Li2MnSiO4 as pillars to prevent the structural collapse. The results showed that [TiO4] tetrahedrons and [TiO6] octahedrons coexisted in Mn sites, and the proportion of [TiO6] octahedrons increased with increasing Ti. [TiOx] pillared Li2MnSiO4 samples presented greatly improved cycling stability, especially [TiO6] enriched Li2Mn0.85Ti0.15SiO4, which achieved the highest initial Coulumbic efficiency (86%) and the best capacity retention at C/20 rate. Moreover, Li2Mn0.85Ti0.15SiO4 exhibited excellent rate performance and retained 93% of initial discharge capacity after 50th cycles at C/2 rate. Ex‐situ XPS revealed [TiOx] pillars, especially [TiO6] pillars facilitated the reversible transition of [MnOx] during cycling. Ex‐situ XRD demonstrated better structure reversibility of [TiO6] enriched sample. The enhanced structural reversibility and cycling stability are ascribed to the pillaring effect of [TiOx], especially large [TiO6] octahedrons in structure, which ensure the reversible oxidation/reduction of Mn and alleviate the layer exfoliation during cycling.

Microstructure Evolution and Conversion Mechanism of Mn3O4 under Electrochemical Cyclings

Probing the microstructure evolution, phase change, and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation–delithiation cycles is important to gain insights into understanding how the electrode works and thus how it can be improved. The electrochemical reaction and phase evolution of Mn3O4 during lithiation–delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn3O4 during lithiation–delithiation cycles, a nanosized LIB was constructed inside a transmission electron microscope (TEM) using an individual Mn3O4/graphene moiety as the anode. Upon the first lithiation, Mn3O4 nanoparticles are lithiated into the crystallized Mn nanograins embedded within the Li2O matrix. However, Mn and Li2O cannot be recovered to the original Mn3O4 phase but to MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle ...

A study of evolution of residual stress in single crystal silicon electrode using Raman spectroscopy

Silicon is a promising anode material for lithium ion batteries. However, lithiation of silicon generates stress that is known to be the primary reason for the failure of the anode. This study explored the existence of residual stress in single crystalline silicon electrodes after full de-lithiation, i.e., under no mechanical or electrical load. The magnitude of residual stress and its evolution with the number of lithiation-delithiation cycles is measured by Raman spectroscopy and a simple mechanics based approach. It is shown that the residual stress is tensile in nature and increased from 69 ± 11 MPa after the 1st cycle to ∼291 ± 56 MPa after 50 cycles of lithiation and de-lithiation. Concurrently, microstructural studies were performed to demonstrate the consequence of the evolution of residual stress on failure by the fracture of the silicon anode. These results provide understanding on the progressive failure mechanism of single crystalline-based silicon electrodes upon lithiation-delithiation.

Electrolyte-Induced Surface Transformation and Transition-Metal Dissolution of Fully Delithiated LiNi0.8Co0.15Al0.05O2.

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[Ni0.8Co0.15Al0.05]O2 (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface Li2CO3 content on the magnitude and character of the dissolution reaction was studied.

Dynamic Transformations of Layered Compounds at Full Delithiation Due to Surface Triggered Reactions

Modern lithium-ion batteries using layered R-3m intercalation positive electrodes have insufficient practical capacity and energy density to meet the future demands of portable electronics, and electric vehicles. If the complete, reversible, theoretical capacity upon full delithiation of these materials could be accessed while retaining high voltages and a narrow voltage swing, a significant advancement in energy density could be achieved. Although, full delithiation of most layered compounds has been known for decades, the failure modes that prevent practical cycle life are still in question. This paper focuses on a model R-3m layered compound of Li[Ni0.8Co0.15Al0.05]O2 (NCA) and explores the failure modes at the surface and subsurface utilizing a unique combination of operando microcalorimetry / electrochemical impedance spectroscopy , high resolution transmission electron microscopy, electron energy loss spectroscopy, x-ray photo electron microscopy, x-ray absorption spectroscopy, hard x-ray photoelectron spectroscopy, and x-ray diffraction. The development of operando microcalorimetry (isothermal and DSC mode) enables simultaneous measurement of the heat flux, current, and impedance, while controlling the environmental temperature and the electrochemical testing parameters. Although surface changes are known in NCA, we explore the evolution of the surface and subsurface at extreme levels of delithiation and take efforts to isolate the failure modes induced by cycling vs. static transitions. The progression of the surface and subsurface structure and chemistry is found to be much more complex than previously identified and has direct implications to impedance development throughout the electrochemical cell. Specific attention was given to surface catalyzed triggers of the surface and subsurface failure modes, evolution of surface and subsurface phase transformations, and comments on the broad applicability of our findings to other layered compounds.

Damage of Solid Electrolyte Interphase (SEI) Layer By the Mechanical Stress Induced By Volume Change of Si Electrode

Introduction Even if silicon (Si) is tested as the alternative to graphite negative electrode, its practical use is still hindered due to a massive volume change during cycling. Repeated volume change imposes a large mechanical stress to Si particles to cause a crack formation and/or pulverization. As a result, Coulombic efficiency is poor due to irreversible charge consumption for electrolyte decomposition and Li trapping. Cycle performance is also poor due to solid electrolyte interphase (SEI) deposition and electrode polarization.1 Such a volume change also imposes a mechanical stress to the SEI layer covering Si particles. The SEI layer may be damaged by the mechanical stress, resulting in a loss of passivating ability and concomitant electrolyte decomposition. In this work, as a way to suppress the volume change of Si electrodes, the de-lithiation voltage cutoff was lowered. The expectation here was that the volume change is less severe when cycled within a narrower cut-off range. Less serious SEI damage is expected because mechanical stress is less significant. Meanwhile, the endurance against mechanical stress was examined for the SEI layer derived from fluoroethylene carbonate (FEC), which is known as an effective SEI former.2 Experimental The 2032 type coin-cells were assembled from nano-sized Si powder and Li metal foil. The used electrolyte was 1.3 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). 25 wt. % of FEC was added to examine the additive effect. The galvanostatic charge/discharge test was performed in two different cut-off ranges. The lithiation cut-off voltage was fixed at 0.1 V (vs. Li/Li+), whereas the de-lithiation cut-off was varied from 1.5 V to 0.5 V. The morphology of SEI layer was examined by using field-emission scanning electron microscope (FE-SEM). The X-ray photoelectron spectroscopy (XPS) was used to analyze the coverage of SEI layers. Results and discussion The XPS data obtained after the first lithiation/de-lithiation show that the Si surface is still covered by SEI layer when the de-lithiation cut-off is 0.5 V, whereas the surface film is removed after the full de-lithiation (1.5 V). This illustrates that the SEI layer is damaged by the mechanical stress being imposed, which is stronger with 1.5 V cut-off due to a larger volume change. The electrolyte continuously decomposes at the newly exposed Si surface to generate a thick SEI layer in the later cycles (Fig. 1a). When the cycling voltage range is narrower (de-lithiation cut-off = 0.5 V), however, the electrolyte decomposition/film deposition in the later cycles is less severe (Fig. 1b) because the initially generated SEI layer is not seriously damaged. The SEI damage is also insignificant even after the full de-lithiation (1.5 V) when FEC is added in the electrolyte. It is seemingly that the SEI layer derived from FEC has a higher endurance against mechanical stress. As a result, it is not damaged in the initial cycles. Additional electrolyte decomposition/film deposition is not serious (Fig. 1c) because the Si electrode is still passivated.

First-Cycle Evolution of Local Structure in Electrochemically Activated Li2MnO3

X-ray absorption spectroscopy is utilized to determine changes in the local structure of Li2MnO3 resulting from electrochemical extraction and insertion of lithium. Specially prepared electrodes allow for a first-cycle charge close to 100% of theoretical, even in crystalline Li2MnO3 prepared at 850 °C. After full delithiation to ∼5 V, the local Mn environments are similar to those of the freshly prepared Li2MnO3 electrodes but with increased disorder. On discharge, significant reduction of Mn is observed accompanied by rearrangement of Mn resulting in Jahn–Teller distorted, Mn3+ local environments. It is also shown, as expected, that the electrochemical and structural behavior of pure Li2MnO3 is markedly different than that observed for the structurally similar Li2MnO3 component of lithium- and manganese-rich composite cathodes.

Understanding the stability of MnPO4

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350–450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability.

Origin of Poor Cyclability in Li2MnSiO4 from First-Principles Calculations: Layer Exfoliation and Unstable Cycled Structure

Good cyclability is essential for the potential application of cathode materials. Here, we investigate the structural stability of two-dimensional (2D) Li-layered and three-dimensional (3D) structured polymorphs of Li2FeSiO4 and Li2MnSiO4 using the density functional theory calculations. We find that all 2D Li-layered polymorphs of both materials are unstable upon full delithiation owing to layer exfoliation, which can lead to an amorphous structure. However, in contrast to the fact that the amorphization of Li2FeSiO4 can be prevented by the formation of the 3D cycled structure that is energetically stable, the 3D cycled structure of Li2MnSiO4 is found to be unstable during delithiationlithiation cycling. As a result, Li2MnSiO4 easily undergoes amorphization and shows a poor cyclability.