Lithiation-delithiation Cycles Research Articles

Bio-Inspired Electrodes with Rational Spatiotemporal Management for Lithium-Ion Batteries.

Lithium-ion batteries (LIBs) are currently the predominantenergy storage power source. However, the urgent issues of enhancing electrochemical performance, prolonging lifetime, preventing thermal runaway-caused fires, and intelligent application are obstacles to their applications. Herein, bio-inspired electrodes owning spatiotemporal management of self-healing, fast ion transport, fire-extinguishing, thermoresponsive switching, recycling, and flexibility are overviewed comprehensively, showing great promising potentials in practical application due to the significantly enhanced durability and thermal safety of LIBs. Taking advantage of the self-healing core-shell structures, binders, capsules, or liquid metal alloys, these electrodes can maintain the mechanical integrity during the lithiation-delithiation cycling. After the incorporation of fire-extinguishing binders, current collectors, or capsules, flame retardants can be released spatiotemporally during thermal runaway to ensure safety. Thermoresponsive switching electrodes are also constructed though adding thermally responsive components, which can rapidly switch LIB off under abnormal conditions and resume their functions quickly when normal operating conditions return. Finally, the challenges of bio-inspired electrode designs are presented to optimize the spatiotemporal management of LIBs. It is anticipated that the proposed electrodes with spatiotemporal management will not only promote industrial application, but also strengthen the fundamental research of bionics in energy storage.

Mechanistic insights into structural parameters maximizing energy storage density in Si mesoporous electrodes for Li-ion batteries

Mesoporous Si electrodes have an interesting set of structural parameters, which, when carefully optimized, can lead to ultrahigh energy density Si electrodes for Li-ion batteries. We present here details of a systematic research leading to the discovery of an “ideal” structure of mesoporous Si electrode, which results in exceptional cracking/damage resistance, while simultaneously having very high specific (>2000 mAh g-1) and total (>1.5 mAh cm-2) capacities for large number of cycles. The electrodes near the “ideal” value of the characteristic structural parameter (the ratio of Si wall thickness to pore diameter) are free from first-cycle capacity degradation, and are efficient in accommodating the volume changes during lithiation by uniformly filling up the porous space between Si walls. Interestingly, these electrodes are also found to be structurally damage-resistant, surviving through many lithiation-delithiation cycles. Using charge-discharge cycling and electron microscopy we show that ideal structure is the key for achieving ultrahigh energy storage density in Si mesoporous electrodes for Li-ion batteries.

In-Operando Neutron Diffraction Investigation of Structural Transitions during Lithiation of Si Electrode in Li-Ion Battery

A major challenge for Si negative electrodes in Li-ion batteries is how to accommodate the large lithiation-induced volume expansion and prevent electrode fragmentation, such that impressive Li storage capacity of Si can be exploited in practice. Electrochemically etched Si mesoporous electrodes have significant potential in this context. This research is focused on an optimum mesoporous Si electrode structure that shows a very high energy storage density, electrochemically cycling well without cracking or fragmentation. To explore the factors causing the superior performance, this study performed in-operando neutron diffraction experiments on optimized electrode during lithiation-delithiation cycles in a simple in-situ electrochemical cell. It is shown that an unusual diffraction phenomenon arises from lithiation-induced expansion of Si leading to the development of mosaic structure in Si. This new phenomenon appears to arise from the increased contribution of kinematic diffraction from the lithiated Si, relative to the conventional dynamic diffraction. This is also supported by changes in diffraction intensities directly synchronizing with the volume changes in Si during lithiation-delithiation cycle. The in-operando experiments explain why the optimum mesoporous Si electrode possesses a high specific capacity without electrode fragmentation. The present findings can help to improve Si electrode designs for high energy density Li-ion batteries.

Effects of carbon impurities on the performance of silicon as an anode material for lithium ion batteries: An ab initio study

Silicon is widely used in the semiconductor industry and has recently become very attractive as a lithium ion battery anode due to its high capacity. However, volume changes associated with repeated lithiation–delithiation cycles expose fresh silicon surfaces to the electrolyte, causing irreversible side reactions. Moreover, silicon suffers from a poor electronic conductivity at a low lithium content. Carbon impurities originating at synthesis or resulting from subsequent contact with other electrode components are often neglected. However, atomistic simulations reveal that dissolved carbon decreases the local potential energy surface by drawing the electron density from silicon to form polar covalent C–Si bonds that are stronger than the non-polar covalent Si–Si bonds they replace. This leads to a higher density and elastic stiffness, regardless of the interstitial lithium concentration. Substitutional carbon also reduces the mobility of silicon self-vacancies and interstitial lithium by increasing their diffusion barriers by 24.7 and 27.3 kJ mol−1, respectively. Moreover, the [carbon, silicon vacancy] complex is basically stable, while the [carbon, lithium] complex is found to become stable against both single defects at a spacing of 4.72 Å. The minimum energy paths ultimately demonstrate that both the interstitialcy and dissociative mechanisms are mainly responsible for carbon diffusion in silicon.

Electrochemical characterization of bi-layered graphite anodes combining high and low porosity in lithium-ion cells to improve cell performance

Lithium-ion batteries have been implemented worldwide in portable devices. Furthermore, they are strong candidates to facilitate the upcoming revolution associated with the electrification of vehicles. Nowadays, these cells are based on transition metal oxides in the positive electrode and graphite in the negative electrode. Despite focusing many efforts on developing novel materials for enhancing the performance of lithium-based batteries, there is still room for improvement by working on the electrode characteristics with widely industry-implemented and trustable state-of-the-art materials. In this work, we developed and characterized graphite electrodes with bi-layered structure consisting of two layers with different porosity. These electrodes were tested both in half and full coin cell configuration, the latter using LiNi0.6Mn0.2Co0.2O2 as the positive electrode. Regular single-layer graphite electrodes with the same average porosity and loading were also elaborated and tested for comparison. Galvanostatic cycling, power tests and impedance spectroscopy were combined with post-mortem characterization to understand the influence of the multi-layer structure onto the electrochemical response. Results show that the bi-layered structure provides enhanced capacity retention during the beginning of life of cells. However, this advantage is lost when the repeated lithiation-delithiation cycles of graphite affect the pore size of the lower porosity layer. In any case, it is evidenced that by modulating the coating method and therefore the macroscopic properties of graphite, it is possible to have a significant impact on its electrochemical characteristics. This should be of high interest for battery manufacturers facing the uncertainties associated with the processing and implementation of novel materials.

In situ implanting fine ZnSe nanoparticles into N-doped porous carbon nanosheets as an exposed highly active and long-life anode for lithium-ion batteries

Owing to the high theoretical specific capacity, transition metal selenides (TMSs) have emerged as promising anode materials for lithium-ion batteries (LIBs). However, the application of TMSs still confronts fast capacity decay and sluggish reaction kinetics associated to their inherent poor conductivity and huge volume expansion during lithiation-delithiation cycles. Herein, the fine ZnSe nanoparticles are implanted into N-doped porous carbon nanosheets through synchronous pyrolysis/selenization of a leaf-like zeolitic imidazolate framework mixed with selenium powder. The highly dispersed fine ZnSe nanoparticles (NPs) coupled with conductive porous carbon nanosheets (denoted as ZnSe/NC) endow the resultant composite with superior structural merits for facilitating electron/ion transfer, thus alleviating the volume fluctuation. The systematic electrochemical investigation illustrates that the as-prepared ZnSe/NC-700 electrode can obtain a high reversible capacity (426.5 mAh g−1 at 1 A g−1 after 1000 cycles), excellent cycling stability (368.9 mAh g−1 after 1800 cycles at 2 A g−1) and nearly 100％ columbic efficiency. In all, our strategy provides a new idea for synthesis of unique anode materials in LIBs.

Ammonium Intercalation Induced Expanded 1T-Rich Molybdenum Diselenides for Improved Lithium Ion Storage

Transition metal dichalcogenides (TMDs), particularly molybdenum diselenides (MoSe2), have the merits of their unique two-dimensional (2D) layered structures, large interlayer spacing (∼0.64 nm), good electrical conductivities, and high theoretical capacities when applied in lithium-ion batteries (LIBs) as anode materials. However, MoSe2 remains suffering from inferior stability as well as unsatisfactory rate capability because of the unavoidable volume expansion and sluggish charge transport during lithiation-delithiation cycles. Herein, we develop a simultaneous reduction-intercalation strategy to synthesize expanded MoSe2 (e-MoSe2) with an interlayer spacing of 0.98 nm and a rich 1T phase (53.7%) by rationally selecting the safe precursors of ethylenediamine (NH2C2H4NH2), selenium dioxide (SeO2), and sodium molybdate (Na2MoO4). It is noteworthy that NH2C2H4NH2 can effectively reduce SeO2 and MoO42- forming MoSe2 nanosheets; in the meantime, the generated ammonium (NH4+) efficiently intercalates between MoSe2 layers, leading to charge transfer, thus stabilizing 1T phases. The obtained e-MoSe2 exhibits high capacities of 778.99 and 611.40 mAh g-1 at 0.2 and 1 C, respectively, together with excellent cycling stability (retaining >90% initial capacity at 0.2 C over 100 charge-discharge cycles). It is believed that the material design strategy proposed in this paper provides a favorable reference for the synthesis of other transition metal selenides with improved electrochemical performance for battery applications.

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.

In Situ Focused Ion Beam-Scanning Electron Microscope Study of Crack and Nanopore Formation in Germanium Particle during (De)Lithiation

Germanium (Ge) has been considered a promising anode active material, due to its high capacity, low voltage, fast lithium ion diffusion, and high electrical conductivity. However, Ge based anode materials suffer from pulverization due to a large volume change, up to 300%. Although pulverization issues can be alleviated by nano-structuring the Ge-based anode materials, recent studies have shown that nanometer-sized pores are still formed in the nano-structured electrode due to vacancy-mediated diffusion, which could lead to pulverization during repeated cycling. In recent years, the reaction mechanism of the Ge anode has been studied by using different in situ and operando characterization methods such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). For instance, Lim et al. proposed the phase transformations of Ge that occur during lithiation and delithiation.1 They also studied the effect of cycling rate on Ge phase transformation during the charge/discharge process and proposed a mechanism to explain the C‐rate dependence phase transformation phenomenon. However, it is still necessary to correlate the microstructural evolution of the Ge electrode and its electrochemical performance. To this end, some advanced imaging technologies have been used to monitor the microstructural evolution of the Ge electrode during cycling such as transmission electron microscopy (TEM)2 and transmission X-ray microscopy (TXM).3 In situ TEM studies have shown the volume expansion during lithiation and formation of nanopores during delithiation in Ge nanowires in real time. But due to the small size of the sample used in the TEM, it is difficult to control the current rate. In situ TXM can obtain two dimensional (2D) or three dimensional (3D) microstructures of Ge particles during cycling with a large field of view. However, the resolution of TXM is not sufficient for monitoring nanometer-sized structures, such as nanopores. In this study, we developed an approach to build a single particle battery in the chamber of focused-ion beam-scanning electron microscope (FIB-SEM) to monitor the microstructural evolution of a single Ge micro-particle during cycling. The experiment was performed within a ZEISS Nvision FIB-SEM at the Center for Nanoscale Materials, Argonne National Laboratory. A Ge particle was attached to the tungsten probe by carbon coating using FIB deposition as positive electrode. The lithium metal was placed on the top of SEM stage as negative electrode. One drop of ionic liquid electrolyte (ILE) was placed on the top of lithium metal. The ILE was made by dissolving the lithium salt, lithium bis (trifluoromethylsulfonyl) imide (LiTFSI), in a solvent of 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl -sulfonyl) imide (P14TFSI). The tungsten probe and the SEM stage were connected to a Keithley 6430 sub-femtoamp remote sourcemeter. Galvanostatic mode was used in all electrochemical cycling. The particle was immersed in the ILE drop during cycling and lifted out for imaging at different states of charge. The particle was polished by FIB before imaging to remove ILE on the surface. This study reveals that germanium electrodes with low and high cycling rates have better microstructure integrity, which leads to better cycling performance. The nanopores tend to aggregate into large porous structures during cycling which leads to particle pulverization and capacity fading of the electrode. 1. Lim, L. Y.; Fan, S.; Hng, H. H.; Toney, M. F., Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate. Advanced Energy Materials 2015, 5 (15), 1500599. 2. Liu, X. H.; Huang, S.; Picraux, S. T.; Li, J.; Zhu, T.; Huang, J. Y., Reversible Nanopore Formation in Ge Nanowires during Lithiation-Delithiation Cycling: An In Situ Transmission Electron Microscopy Study. Nano Letters 2011, 11 (9), 3991-3997. 3.Weker, J. N.; Liu, N.; Misra, S.; Andrews, J. C.; Cui, Y.; Toney, M. F., In situ nanotomography and operando transmission X-ray microscopy of micron-sized Ge particles. Energy & Environmental Science 2014, 7 (8), 2771-2777.

Cu-Sn alloy microtube assembly fabricated by electroless plating on polyester nonwoven fabric and its lithiation-delithiation performance

The 300-nm-thick Cu6Sn5 microtube assemblies with a Cu inner layer and with a polyester fiber core were fabricated and their lithiation-delithiation performances as the anode of lithium ion batteries were evaluated. The Cu inner layer was obtained by electroless plating on nonwoven polyester fabrics. Cu was uniformly deposited on the fibers in a fabric treated with HNO3-O3, by which stable hydroxy groups were formed on the fiber surface, and the Pd catalyst adsorption was enhanced. The Cu6Sn5 microtubes were fabricated by the contact immersion deposition process of Sn on the Cu-coated fibers. The deposited Sn formed a dense Cu6Sn5 layer without heat treatment, and its thickness agreed with the theoretical value estimated from the solid-state diffusion coefficient of Sn. Voltage profiles of the microtube assembly electrode during the lithiation-delithiation cycling show the typical features of Cu6Sn5. Capacities were stable during the initial 40 cycles and were paralleled with the Sn loadings. The capacities began to fade after 50 cycles without the removal of Sn, but with the morphological and compositional changes, that is, the trapping of Sn in the degradation products of electrolyte.

Minimized Volume Expansion in Hierarchical Porous Silicon upon Lithiation.

Silicon (Si) remains one of the most promising anode materials for next-generation lithium-ion batteries (LIBs). The key challenge for Si anodes is the huge volume change during lithiation-delithiation cycles that leads to electrode pulverization and rapid capacity fading. Here, we report a hierarchical porous Si (hp-Si) with a tailored porous structure [tunable primary pores (20-200 nm) and secondary nanopores (∼3-10 nm)] that can effectively minimize the volume expansion. An in situ transmission electron microscopy (TEM) study revealed that the hp-Si material with the same porosity but larger primary pores can more effectively accommodate lithiation-induced volume expansion, giving rise to a much reduced apparent volume expansion on both material and electrode levels. Chemomechanical modeling revealed that because of the different relative stiffnesses of the lithiated and unlithiated Si phases, the primary pore size plays a key role in accommodating the volume expansion of lithiated Si. The higher structural stability of the hp-Si materials with larger primary pores also maintains the fast diffusion channels of the connective pores, giving rise to better power capability and capacity retention upon electrochemical cycling. Our findings point toward an optimized hp-Si material with minimal volume change during electrochemical cycling for next-generation LIBs.

In−situ preparation of ultrathin graphdiyne layer decorated aluminum foil with improved cycling stability for dual−ion batteries

Dual−ion batteries (DIBs) are a new kind of battery operated across a high voltage range with safer performance and low cost. However, the relative low capacity and poor cyclic stability restricts the application when aluminium (Al) anode was used to increase the capacity of the DIBs. Graphdiyne (GDY) as a new kind of carbonaceous allotropeis provides a facile and controllable strategy by in−situ preparation of ultrathin GDY layer on Al surface to promote the formation of stable SEI layer and solve the volume expansion of Al anode. Therefore, we report the fabrication of ultrathin GDY layer decorated on Al foil for both anode and current collector in DIB devices. The butadiyne linkages with large triangle−like intramolecular pores in GDY molecular are expected to provide efficient pathways for the orderly lithium intercalation to buffer the massive volume expansion and pulverization of Al and improve the formation of stable SEI layer during lithiation–delithiation cycles. As a result, the assembled Al−GDY based DIBs could deliver an improved reversible capacity of approximately 125 mAh g−1 at 1 C and show an improved cycling stability of 88% capacity retention more than 450 cycles than commercial Al foil, suggesting remarkable improvement on cycling stability.

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.

Hollow Amorphous MnSnO3 Nanohybrid with Nitrogen-Doped Graphene for High-Performance Lithium Storage

Tin-based metal oxides usually suffer from severe capacity fading resulting from aggregation and considerable volume variation during the charge/discharge process in lithium ion batteries. In this work, a novel nanocomposite (MTO/N-RGO) of hollow amorphous MnSnO3 (MTO) nanoparticles and nitrogen-doped reduced graphene oxide (N-RGO) has been designed and synthesized by a two-step method. Firstly, the nitrogen-doped graphene nanocomposite (MTO/N-RGO-P) with MnSn(OH)6 crystal nanoparticles was synthesized by a facile solvothermal method. Subsequently, the MTO/N-RGO nanocomposite was obtained through the post heat treatment of MTO/N-RGO-P. The designed heterostructure and well-combination of the hollow amorphous MTO and N-RGO matrix can accelerate the ionic and electronic transport, and simultaneously accommodate the aggregation and volume variation of MTO nanoparticles during the lithiation–delithiation cycles. The as-prepared hybrid of MTO and N-RGO (MTO/N-RGO) exhibits a high reversible capacity of 707mAhg−1 after 110 cycles at 200mAg−1, superior rate capability, and long-term cyclic life with high capacity of 610mAhg−1 over 1000 cycles at 400mAg−1. Superior capacity retention of 97.3% over 1000 cycles was obtained. This work opens new opportunities to fabricate the high-performance electrode materials with heterostructure for lithium storage systems, especially for novel multi-metal oxide based nanocomposites with high cycling stability.

Free‐Standing Graphene‐Encapsulated Silicon Nanoparticle Aerogel as an Anode for Lithium Ion Batteries

AbstractSilicon is a remarkable candidate for lithium‐ion battery anodes, due to the highest theoretical specific capacity, low working potential, and abundance on earth. However, the volume of Si changes ≈300 % during the lithiation–delithiation cycling, leading to pulverization and poor electrical contact between Si and the current collector, resulting in fast fading capacity. To overcome these problems, we develop a novel free‐standing anode by encapsulating Si nanoparticles into a lightweight, flexible, and conductive graphene network by utilizing a facile freeze‐drying strategy. The Si nanoparticles are tightly anchored on the ultrathin and flexible graphene sheets, which can serve as mechanical support, electrical network, and buffer layer for Si. This free‐standing aerogel exhibits superior electrochemical performance, and the specific capacity is 617 mA h g−1 after 50 cycles at a current density of 0.8 A g−1, 4 times that of a traditional Si electrode on Cu foil fabricated by a typical slurry method.

A Phase-Field Model and Simulation of Kinetically Asymmetric Ternary Conversion-Reconversion Transformation in Battery Electrodes

Electrochemical processes in high-energy electrode materials often involve diffusion of multiple species and solid-state phase transformations. Some of these phase transformations involve breaking and rearranging ionic bonds and are referred to as conversion reactions (e.g., the lithium and iron difluoride conversion reaction: 2Li+ + 2e− + FeF2 → 2LiF + Fe). The phase transformations during conversion processes are governed by fundamental thermodynamics and kinetics in a similar manner to metallurgical systems. In this work, we developed a phase-field model that tracks atomic fractions of three constituent species to simulate the morphological evolution of different phases. The simulations demonstrate that conversion proceeds via a two-stage process consisting of lithiation and decomposition stages, whereas the reconversion process consists of a single-stage delithiation. This asymmetry in evolution paths of conversion and reconversion is likely responsible for the voltage hysteresis commonly observed during lithiation-delithiation cycling of conversion materials.

Ultrafast and reversible electrochemical lithiation of InAs nanowires observed by in-situ transmission electron microscopy

The electrochemical lithiation/delithiation processes of InAs nanowires (NWs) are studied by in-situ transmission electron microscopy. Our results indicate that InAs NWs have a fast lithiation speed of 275nm/s and a high lithium ion (Li-ion) diffusion coefficient of 2.49×10−8cm2/s at room temperature. Upon lithiation, the Li-ion insertion firstly results in severe lattice distortions of InAs NWs, and the formation of Li3As and LixIn through the conversion and alloying processes take place on further lithiation. A small volume expansion of 157% is observed in full lithiation and is attributed to the naturally formed surface oxide layer. During the delithiation process, volume contraction and the dealloying of LixIn take place. Induced by the alloying and dealloying of LixIn, the dark and bright strips along the basal plane of InAs NWs appear and disappear alternately during the lithiation–delithiation cycling. Our results provide important insights into the lithiation/delithiation mechanism of III–V group nanomaterials and are envisaged to be helpful for designing lithium ion battery anode materials with fast lithiation speed, small volume expansion and reversible lithiation/delithiation processes.

In Situ Transmission Electron Microscopy Observation of the Lithiation-Delithiation Conversion Behavior of CuO/Graphene Anode.

The electrochemical conversion behavior of metal oxides as well as its influence on the lithium-storage performance remains unclear. In this paper, we studied the dynamic electrochemical conversion process of CuO/graphene as anode by in situ transmission electron microscopy. The microscopic conversion behavior of the electrode was further correlated with its macroscopic lithium-storage properties. During the first lithiation, the porous CuO nanoparticles transformed to numerous Cu nanograins (2-3 nm) embedded in Li2O matrix. The porous spaces were found to be favorable for accommodating the volume expansion during lithium insertion. Two types of irreversible processes were revealed during the lithiation-delithiation cycles. First, the nature of the charge-discharge process of CuO anode is a reversible phase conversion between Cu2O and Cu nanograins. The delithiation reaction cannot recover the electrode to its pristine structure (CuO), which is responsible for about ∼55% of the capacity fading in the first cycle. Second, there is a severe nanograin aggregation during the initial conversion cycles, which leads to low Coulombic efficiency. This finding could also account for the electrochemical behaviors of other transition metal oxide anodes that operate with similar electrochemical conversion mechanism.

The Evolution of a High Capacity Electrode

Silicon can store Li+ at a capacity 10 times as high as that of graphite anodes. However, to harness this remarkable potential for electrical energy storage, one must address the multifaceted challenge of volume change inherent to high capacity electrode materials. In this talk, we present novel chemical strategies to address this challenge. Based on fundamental understanding of nanostructure interface, we have synthesized non-cracking silicon-carbon nanotube composite electrodes that electrically connect amorphous silicon beads. This synthesis is made possible by a propagative chemistry which creates regular bands of alkylcarboxyl groups along the nanotubes. The functional bands anchor the beads such that they exhibit a symmetric “radial breathing” around the CNT string, remaining crack-free and electrically connected throughout lithiation-delithiation cycling. We further show that, solely by chemical tailoring of the Si-C interface with atomic oxygen, the cycle life of the electrodes can be substantially improved, by 300%, even at high mass loadings. The evolution of this work culminates in the design and fabrication of a high capacity composite electrode that is weavable. The engineered andoe simultaneously attains high areal capacity (3.86 mAh/cm2), high specific capacity (922 mAh/g based on the mass of the entire electrode), and excellent cyclability (80% retention of capacity after 160 cycles), which are among the highest reported. This work was mainly supported as part of Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. We acknowledge cumulative contributions from many collaborators particularly K. Karki, M. Okada, H. Liao, H. Zhu, Z. Jia, E. Baker, Y. Zhang, K. Gaskell, A. Ghemes, P. Wang, J. Cumings, L. Hu, Y. Inoue, T. Li, Y. Qi, and G. W. Rubloff.