Li2O Matrix Research Articles

Reversible conversion–alloying of cobalt–bismuth oxide nanoneedles for long-life lithium storage anodes

Materials that undergo combined conversion and alloying reactions are promising as anodes for lithium storage application because they can accommodate multiple lithium ions. However, irreversible conversion reactions, large voltage hysteresis, poor rate capabilities, and low initial Coulombic efficiencies during continuous discharge–charge cycling make practical applications of conversion–alloying materials challenging. Herein, we present cobalt–bismuth oxide (CBO) nanoneedles, a new bimetallic material in which Li+-ion uptake proceeds stepwise, thus effectively suppressing volume expansion. During the initial lithiation stage, the CBO nanoneedles undergo a conversion reaction to form nanodomains of low-oxidation-state Coδ+ (δ ≤ 2) in a Li2O matrix. At the next potential platform, the Bi phase attracts Li+ ions via an alloying reaction, while the Coδ+/Li2O phase hinders excessive volume expansion. Consequently, the CBO electrode delivers a high reversible discharge capacity of 392 mAh g−1 at 50 mA g−1 for up to 100 cycles. Further, an ultrastable long-term capacity of 300 mAh g−1 at 250 mA g−1 is realized over 1000 cycles. The stepwise lithiation process decreases volume expansion significantly (by ∼10%), which leads to good cyclability. Owing to their ease of preparation and excellent performance characteristics, CBO nanoneedles are an attractive long-life anode material for lithium-ion batteries.

A facile and scalable Fe-Cr decorating strategy to boost the lithium storage of SiOx anode

SiOx is regarded as a promising anode material for the next-generation lithium-ion batteries with high energy density. However, the reduplicative volume changes during cycling cause the pulverizations of Si, leading to decreased utilization of active Si component and poor cycling performance. Herein, a facile and scalable Fe-Cr decorating strategy is reported to boost the lithium storage of SiOx anode. Our comprehensive characterizations indicate that there are strong interactions between Fe-Cr and SiOx during a facile ball milling process. After cycling, Fe-Cr nanoparticles are dispersed in the in-situ formed lithium silicates and Li2O matrix, which can maintain superior conductivity of lithium ion and electron for the active components through point-to-point connections. This unique structure boosts the electrochemical de-alloying process of the low-valent LixSi (0 ≤ x ≤ 2), and increases effective utilization of the active Si component during cycling, resulting in improvement of reversible capacity and cycling stability. This bm SiOx-FeCr electrode can deliver high reversible specific capacity (1370 m Ah g−1 at 200 mA g−1), good rate capability (653.8 m Ah g−1 at 2000 mA g−1), and superior cycling stability. This work provides a promising route for improving the utilization of active Si component and the structural stability of SiOx-based anodes.

Controlling the Voltage Window for Improved Cycling Performance of SnO₂ as Anode Material for Lithium-Ion Batteries.

Transition metal oxide materials with high theoretical capacities have been studied as substitutes for commercial graphite in lithiumion batteries. Among these, SnO₂ is a promising alloying reaction-based anode material. However, the problem of rapid capacity fading in SnO₂ due to volume variation during the alloying/dealloying processes must be solved. The lithiation of SnO₂ results in the formation of a Li₂O matrix. Herein, the volume variation of SnO₂ was suppressed by controlling the voltage window to 1 V to prevent the delithiation reaction between Li₂O and Sn. Using this strategy the unreacted Li₂O matrix was enriched with metallic Sn particles, thereby providing a pathway for lithium ions. The specific capacity decay in the voltage window of 0.05-3 V was 1.8% per cycle. However, the specific capacity decay was improved to 0.04% per cycle after the voltage window was restricted (in the range of 0.05-1 V). This strategy resulted in a specific capacity of 374.7 mAh g-1 at 0.1 C after 40 cycles for the SnO₂ anode.

The Effect of Crystalline Structure and Iron Doping on the Electrochemical Behavior of Germanium Oxide Anodes in Lithium-Ion Batteries

The need for alternative anode materials for lithium-ion batteries (LIBs) is one of the decisive steps towards the electrification of our public and private transportation, since the state-of-the-art anode material graphite intrinsically limits the energy and power density of current LIBs.1,2 As a potential alternative, alloying metal-oxides such as ZnO and SnO2 have been widely studied, mostly due to their high theoretical specific capacities of 988 mAh g-1 and 1494 mAh g-1, respectively. Nevertheless, the pure oxides reveal poor electrochemical performance as a result of the eventually irreversible formation of the Li2O matrix and the pronounced volume variation, causing active material exfoliation.3–5 When doped with transition metals, however, the Li2O formation turns very reversible and the theoretically possible capacities can be achieved.6–9 As an example, Co-doped ZnO offers substantially higher specific capacities than pure ZnO (i.e., 966 mAh g-1 vs 330 mAh g-1). Recently, the class of TM-doped alloying metal oxides has been extended to Fe-doped GeO2, offering even higher specific capacities than transition metal doped SnO2 (theoretically 2152 mAh g-1).10 Within this study we enlighten the beneficial effects of iron doping on the structural evolution and electrochemical performance of GeO2. For this purpose, we combined a series of highly complementary in situ/operando techniques, including in situ high temperature XRD, operando dilatometry, and operando XRD. The results shed light on a commonly overlooked impact factor for the electrochemical performance of such materials and, in fact, metal oxides in general. 1. D. Bresser et al., J. Power Sources, 382, 176–178 (2018).2. Y. Yamada, Y. Iriyama, T. Abe, and Z. Ogumi, Langmuir, 25, 12766–12770 (2009).3. I. A. Courtney and J. R. Dahn, J. Electrochem. Soc., 144, 2045–2052 (1997).4. I. A. Courtney, W. R. Mckinnon, and J. R. Dahn, J. Electrochem. Soc., 146, 59–68 (1999).5. F. Mueller, D. Geiger, U. Kaiser, S. Passerini, and D. Bresser, ChemElectroChem, 3, 1311–1319 (2016).6. D. Bresser, S. Passerini, and B. Scrosati, Energy Environ. Sci., 9, 3348–3367 (2016).7. G. Giuli et al., Inorg. Chem., 54, 9393–9400 (2015).8. G. Giuli et al., Materials, 11, 49 (2017).9. Y. Ma et al., Sustain. Energy Fuels, 2, 2601–2608 (2018).10. J. Wu, N. Luo, S. Huang, W. Yang, and M. Wei, J. Mater. Chem. A, 7, 4574–4580 (2019).

Elucidating the Interfacial Reactions for Conversion-Alloying Materials Towards the Realization of High-Performance Lithium-Ion Full-Cells

The unique combination of high energy and power density made Li-ion batteries the state-of-the-art electrochemical energy storage technology for powering small consumer electronics as well as large-scale applications like electric vehicles (EVs).[1] This increasing diversity of potential applications, however, also results in a greater variety of required characteristics. For the use in EVs, in fact, two of the major issues of the current lithium-ion chemistry based on graphite anodes are the sluggish lithium transport across the solid electrolyte interphase (SEI) upon lithium intercalation into the graphite host structure and the very low lithiation potential – in combination intrinsically limiting the fast charging capability.[2] This intrinsic kinetic limitation triggered researchers to find alternative anodes, following, for instance, an alloying (e.g., Si) or a (multi-phase) conversion (e.g., Fe2O3) mechanism. Both alternatives commonly show higher capacities and frequently superior rate capabilities. Despite steady improvements, though, these alternatives still suffer from capacity fading due to extensive volume variations (especially alloying materials) and low energy efficiencies due to a significant voltage hysteresis (in particular for conversion materials) as well as an improvable coulombic efficiency. Conversion/alloying-materials (CAMs) are a rather new class of electrode compounds that combine the alloying and conversion mechanism in one single material.[3] Upon lithiation, nanograins of the non-alloying element are formed in situ and build a percolating electron conductive network. Besides allowing for the reversible formation of the simultaneously formed Li2O matrix, this metallic nano-network enables fast lithiation and delithiation kinetics, rendering CAMs a promising candidate for high-power applications. Nevertheless, there is still a lack of knowledge which parameters eventually determine the overall performance and cycling stability.Herein, we show that the reactions occurring at the electrode/electrolyte interface play a decisive role and that these reactions are largely dependent on the (surface) chemistry of the active material. The techniques used include amongst others ex situ synchrotron soft X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and in situ isothermal microcalorimetry. It is shown that the stabilization of the interface is essential when targeting long-term stable performance – in both half-cells and full-cells. The latter provides a specific energy of 284 Wh kg-1 accompanied by an excellent energy efficiency of >93%.[1] N. Nitta, F. Wu, J. T. Lee, G. Yushin, Mater. Today 2015, 18, 252–264.[2] N. Loeffler, D. Bresser, S. Passerini, Johnson Matthey Technol. Rev. 2015, 59, 34–44.[3] D. Bresser, S. Passerini, B. Scrosati, Energy Environ. Sci. 2016, 9, 3348–3367.

Metal/LiF/Li2O Nanocomposite for Battery Cathode Prelithiation: Trade-off between Capacity and Stability.

Lithium-ion batteries (LIBs) are currently dominating the portable electronics market and supplying power for electric vehicles and grid-level storage. However, lithium loss in the formation cycle at the anode side reduces the energy density of state-of-the-art LIBs with carbon anode materials. This situation will be even more severe for future LIBs using high-capacity Si-based anode materials. In this study, a transition metal-based nanocomposite with built-in lithium source was synthesized, featuring Fe nanodomains with a size of ∼5 nm uniformly dispersed in a hybrid Li2O and LiF matrix with intimate contact between them. The Fe/LiF/Li2O nanocomposite released a high Li-ion capacity of 550 mA h/g based on a multielectron inverse conversion reaction during the first-cycle charge process and exhibited better ambient stability than the counterpart with a pure Li2O matrix and also a lower lithium-extraction voltage and faster reaction kinetics than the counterpart with a pure LiF matrix. Serving as an additive to various cathodes (e.g., LiCoO2, LiFePO4, and LiNi1-x-yCoxMnyO2), the Fe/LiF/Li2O nanocomposite showed excellent lithium compensation effect. Using 4.8 wt % Fe/LiF/Li2O additive based on the total mass of the electrodes, a LiNi0.8Co0.1Mn0.1O2|SiO-graphite full cell with a high cathode mass loading of 20 mg/cm2 exhibited a high reversible capacity of 2.9 mA h/cm2 at 0.5 C after 100 cycles which is a 15% increase in comparison to the counterpart without the prelithiation additive. After the Fe/LiF/Li2O nanocomposite was immersed into the electrolyte and rested for 72 h, the content of iron metal in the electrolyte was negligible, indicating that this prelithiation additive was stable in the electrolyte and would not cause any side reactions, such as the shuttle of iron ions during cycling. The high "donor" Li-ion capacity, good ambient stability, and its compatibility with existing cathode materials and battery fabrication processes make the Fe/LiF/Li2O nanocomposite a promising cathode prelithiation additive to offset the initial lithium loss and improve the energy density of LIBs.

The Critical Size of Sn Nanograins for Achieving High Round-Trip Efficiency of Reversible Conversion Reaction in Lithiated SnO2 Nanocrystals

SnO2 has been considered one of the most promising anode materials for Li-ion batteries. However, the realization of the full capacity of SnO2 anode has been hindered mainly by the inferior reversibility of conversion (Sn+Li2O→SnO2+Li+), and especially the large initial capacity loss characterized by low initial Coulombic efficiency (ICE). In the past two decades, although many works have focused on enhancing the cycle performance of SnO2 based anodes, the achieved ICEs were still very low (<60%). Few works were devoted to understanding the fundamental reason and explore a strategy to solve this problem. Recently, we have revealed that the coarsening of Sn in the Sn/Li2O mixture, mainly induced by recrystallization, is responsible for the limited conversion of Sn/Li2O back into SnO2 during Li extraction, and achieved highly reversible conversion between Li2O and SnO2 with ICEs >86.2%, by suppressing the Sn coarsening in Li2O matrix in a sputtered SnO2 film[1]. However, it was found that the reversibility declined as the cycles increased due mainly to the gradually Sn coarsening, which was induced by the cyclic electrochemical stress (σ) imposed on the electrode[2]. Furthermore, combining nanosized transition metal with SnO2 can help to reduce the surface/interface energy of Sn nanograins and dramatically suppress the Sn coarsening, which however also affect the Coulombic efficiency and energy efficiency of the Li-SnO2 cells[3-4]. Accordingly, uncovering the critical size of Sn nanograins in lithiated SnO2 nanocrystals is of great important for enabling high round-trip efficiency of reversible conversion reaction, ensuring large stable capacity in the electrode. In this work, we report our finding that unlike the previous reports, even a pure SnO2 film consisting of nanocrystals with size around 15 nm, stable high capacity above 900mAh/g can be remained throughout 100cycles. The capacity decay in the initial dozens of cycles of SnO2 anode is mainly induced by the gradually degradation of reversible conversion reaction (Sn+Li2O←→SnO2) due to the thermal and tress driven Sn coarsening along cycles. Furthermore, as shown in Figure 1, the coarsening of Sn have reduced the reversible capacity, Coulombic efficiency, energy efficiency and Li+ ion diffusion kinetics of the SnO2 nanostructured film. The grain size of coarsening Sn (x) and the degree of irreversibility (y) monotonically increase along cycles, which is quantitatively expressed with a linear equation (y=0.0236x-0.266). It is determined that the Sn nanograins with diameters less than 11.3 nm will ensure fast interdiffusion kinetics among interfaces of Sn/Li2O for completely reversible conversion reactions, enabling full application of the high capacity of lithiated SnO2[5]. We firmly believe that these results will provide a valuable insight into designing new conversion-type electrode materials with high stable capacity for next generation rechargeable batteries. Figure 1 (a) Degree of reversibility vs. cycle number for the SnO2 film electrodes cycled among potential ranges of 0.01–3.00 and 0.01–2.00 V, respectively. (b) Average grain size and degree of irreversibility vs. cycle number. (c) Relationship between degree of irreversibility and grain size of Sn. (d) Schematic of the partially reversible conversion reaction with different grain sizes of Sn.

In Situ Studies of 30% Li-Doped Bi25FeO40 Conversion Type Lithium Battery Electrodes.

One of the important discharge mechanisms for lithium batteries is the conversion reaction mechanism, where a metal oxide (fluoride) can decompose into metallic nanoparticles embedded in a Li2O (LiF) matrix. Here, 30% Li-doped Bi25FeO40 is successfully synthesized and displays an electrochemical discharge capacity of ∼300 mAh/g above 1.5 V (vs Li/Li+). During the electrochemical cycling process, 30% Li-doped Bi25FeO40 is decomposed into metallic Bi. During the subsequent charging process, the metallic bismuth can be first converted into an amorphous bismuth oxide phase, which contributed to the electrochemical discharge activities observed between 2 and 2.5 V. At a higher charging voltage between 3.5 and 5 V, metallic Bi can be oxidized to BiOx2–O3–2x–, which contributes to the discharge activities observed above 2.5 V. Using graphite as current collectors can prevent the corrosion from O– species and the discharge capacity is greatly enhanced at the voltage region between 1.5 and 2.5 V. This work provides a deeper understanding over the role of oxygen ions during the conversion reaction process and is beneficial for the future design of battery systems based on the conversion reaction.

One-step hydrothermal reduction synthesis of tiny Sn/SnO2 nanoparticles sandwiching between spherical graphene with excellent lithium storage cycling performances

The reunion of Sn is always a troublesome issue when it’s used as the energy storage materials, on the one hand it comes from the preparation process due to its low melting point, and on the other hand it comes from the Li-Sn alloying process because of its natural tendency of migration. The Sn/SnO2/spherical graphene composite prepared by one-step low temperature hydrothermal reduction method can effectively solve this problem. In the composite, Sn/SnO2 tiny nanoparticles with average diameter of 5 nm distribute evenly between multilayers of graphene sheets presenting a hollow spherical structure. The introduction of SnO2 can effectively restrain the agglomeration of Sn nanoparticles during alloying process since an amorphous Li2O matrix is formed to separate the adjacent active particles. The sandwich graphene hollow sphere skeleton effectively buffers the volume expansion of Sn/SnO2 and further restricts their migration and agglomeration. Due to the above advantages, the nano-Li2O generating from the decomposition of SnO2 can contact closely with excessive nano-Sn in restricted area, promoting facile conversed conversion reaction. Therefore, the composite exhibits high reversible capacity and excellent cycle performance. A stable and high specific capacity of 843.8 mAh g−1 is obtained after 100 cycles at 0.1 C.

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 ...

Incorporation of Alloy-de-Alloy Phase with Conversion Based Manganese Oxide to Enable High and Stable Capacity and Density Functional Theory Study of CdMn2O4

In this work, incorporation of alloying-dealloying based element i.e. Cd due to its high ductility and good wear resistance in manganese based spinel oxides i.e. CdMn2O4 has been studied as Li-ion battery (LIB) anode. This study demonstrates that the Li-alloying phases in such type of compound provides extra assistances of dissemination of these intermetallics element in Li2O matrix which stabilizes the conversion reaction and significantly buffer the accompanying volume variation. As a result, CdMn2O4 exhibits high-capacity (810 (±5) mAh g−1, 100 mA g−1) and -rate capability at different C-rates that signifies the introduction of an alloying element over pure conversion based materials. Further, reduced particle size of ∼ 10 nm supports mechanical integrity of electrode by transferring the developed stress to voids/gaps in spinel CdMn2O4 enabling high performance as LIB anodes. Based on the galvanostatic, cyclic voltammetry data and ex situ –X-ray diffraction,-transmission electron microscope, -selected area electron diffraction studies, a plausible reaction mechanism is also proposed. The reversibility of proposed reaction is further complemented by electrochemical impedance spectroscopy. Furthermore, diffusion of Li ion into spinel CdMn2O4 and associated structural and electronically changes are studied by Density Functional Theory (DFT) and discussed in detail.

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.

Dynamic observation of reversible lithium storage phenomena in hybrid supercapacitor devices

Carbonaceous composites have attracted much attention as electrode materials for hybrid supercapacitors. Additionally, transition metal oxides, such as Co3O4, have high specific capacitance in the charging/discharging process. Here, we investigated the lithium storage mechanism of Co3O4/CNTs material via in situ transmission electron microscopy (TEM). Additionally, we analyzed the structure and composition of the anode material by high-resolution TEM, electron diffraction, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Using our unique in situ experimental setup that employs colloidal electrolyte, we elucidate two different mechanisms during operation, including the electrochemical reaction (battery-type) and ions intercalation (supercapacitor-type) of the electrode material. The cube-like Co3O4 nanoparticles were converted to Co nanograins dispersed in the Li2O matrix after the first charging cycle. Subsequent cycles presented a reversible reaction between Co/Li2O and CoO/Li2O. Furthermore, the porous structure of the CNTs and conservation of the Li2O matrix allow for the excellent ability to accommodate tremendous volume expansion, which enhances the life of hybrid supercapacitors. Our observations not only provide direct evidence of the electrochemical behavior but also improve the structure to promote enhanced performance for the application of hybrid supercapacitors.

Exploring Electrochemical Energy Storage of SnO2, Sn3N4, and SnOxNy through Ald

As conversion electrodes in lithium ion batteries, Sn and SnO2 have been well studied in the literature, while the properties of Sn3N4 and SnOxNy have yet to be been explored in any detail. To study the differences in the electrochemical performance of SnO2, Sn3N4, and SnOxNy, an ALD process was developed which allows for a controlled N/O ratio. This process uses tetrakis(dimethylamido)tin (TDMAS) as a precursor with remote nitrogen plasma and H2O. As with many nitride ALD processes, obtaining Sn3N4 films with low O and C contamination is challenging due to the greater reactivity of Sn with O, and incorporation of precursor ligands. Optimized process parameters for the pulse times and substrate temperature were obtained for the nitride process, which were incorporated into the oxynitride process. SnOxNy films were grown by periodically pulsing water into the reactor after several cycles of the nitride process. By varying the number of nitride cycles between water pulses, the relative concentration of O and N in the film can be controlled between 0% N and 95% N. The Li2O matrix formed on conversion of SnO2 is expected to be more chemically stable than the Li3N matrix formed from Sn3N4, but at the cost of lower ionic conductivity. Solid-state half cells were constructed using SnO2, Sn3N4, and SnOxNy thin films versus thermally evaporated Li with ALD LiPON as the electrolyte. The electrochemical properties of the anodes were tested by cyclic voltammetry and galvanostatic cycling. Only a single narrow conversion reaction was observed, regardless of film composition, but the voltage at which the reaction occurred shifted as a function of the N/O ratio. At voltages below 0.4 V vs Li/Li+ the half cells shorted, possibly due to mechanical breakdown of the LiPON layer from significant volume expansion of the anodes during the alloying reaction with Li. However, the stability of the cells was improved by varying the composition and thickness of the anodes. Galvanostatic intermittent titration and impedance spectroscopy were used to analyze the ionic conductivity of the anodes before and after the initial conversion reaction and as the N/O ratio was varied. The high energy storage capacity of SnOxNy electrodes, which can be deposited conformally by ALD, makes them attractive materials for applications in thin film, solid state devices, where the particle aggregation and delamination effects are limited by the physical constraints of the device.

A general prelithiation approach for group IV elements and corresponding oxides

Prelithiation of anode materials is an important strategy to compensate for lithium loss as a result of the formation of a solid electrolyte interphase (SEI) at the surface of anodes in lithium-ion batteries. Conventional prelithiation reagents often present serious safety concerns due to the high flammability and unstable chemical nature. Here, we successfully developed a general one-pot metallurgical process to prelithiate group IV elements and their corresponding oxides, yielding prelithiation capacity approaching the theoretical specific capacity. As-synthesized Li22Z5 alloys and Li22Z5-Li2O composites (Z = Si, Ge, Sn etc) can serve as prelithiation reagents to increase the first cycle Coulombic efficiency of both graphite and alloy-type anode materials. Among all lithiated group IV alloys, LixGe exhibits the best stability under ambient-air conditions, consistent with the simulation results showing the large binding energy between Li and Ge atoms in Li22Ge5 crystal. Metallurgical lithiation of ZO2 results in composites with homogeneously dispersed reactive LixZ nanodomains embedded in a robust Li2O matrix which effectively suppresses the oxidation process. Li22Z5-Li2O composites further improve the ambient-air stability because the strong binding between O atoms in Li2O and Li atoms in Li22Z5 stabilizes the reactive Li22Z5 nanodomains. These results allow us to identify the prelithiation reagents with the optimal stability in air, thereby simplifying the requirement on the industrial electrode fabrication environment.

A Robust and Conductive Black Tin Oxide Nanostructure Makes Efficient Lithium‐Ion Batteries Possible

SnO2 -based lithium-ion batteries have low cost and high energy density, but their capacity fades rapidly during lithiation/delithiation due to phase aggregation and cracking. These problems can be mitigated by using highly conducting black SnO2-x , which homogenizes the redox reactions and stabilizes fine, fracture-resistant Sn precipitates in the Li2 O matrix. Such fine Sn precipitates and their ample contact with Li2 O proliferate the reversible Sn → Li x Sn → Sn → SnO2 /SnO2-x cycle during charging/discharging. SnO2-x electrode has a reversible capacity of 1340 mAh g-1 and retains 590 mAh g-1 after 100 cycles. The addition of highly conductive, well-dispersed reduced graphene oxide further stabilizes and improves its performance, allowing 950 mAh g-1 remaining after 100 cycles at 0.2 A g-1 with 700 mAh g-1 at 2.0 A g-1 . Conductivity-directed microstructure development may offer a new approach to form advanced electrodes.

Growth of ultrafine CuCo2O4 nanoparticle on graphene with enhanced lithium storage properties

Nanosized anode materials have been shown to be successful in addressing the problems of metal oxide anodes during electrochemical reactions. However, issues with low Coulombic efficiencies and poor cyclability of nanostructured metal oxide anodes still need to be resolved. Ultrafine CuCo2O4 nanoparticles of 6∼9nm uniformly anchored on graphene have been synthesized. CuCo2O4/graphene (CuCo2O4/G) composites show superior lithium-storage performances. A high reversible capacity of 1040mAhg−1 is achieved at 0.1C after 80 cycles, and can retain 211mAhg−1 at a current density of 2C. Furthermore, the lithiation process of CuCo2O4/G was recorded by TEM. It suggests the Li+ diffuse through an individual CuCo2O4 nanoparticle consists of three stages: (1) the insertion of Li+ without crystalline structure change, (2) the nanoparticle’s crystalline lattice collapses, and (3) nanograins grow and reform from the collapsed nanoparticle. The lithium conversion resulting in the formation of a network of many ultrafine nanograins (2nm) embedded in the Li2O matrix; also these ultrafine nanograins are confined in a layer of crystal, which is beneficial for the improved electrochemical performance. The macroscopic electrochemical performance of CuCo2O4/G was further correlated with the microcosmic in situ TEM results.

Investigation of the Electronic and Local Structural Changes during Li Uptake and Release of Nanocrystalline NiFe2O4 By X-Ray Absorption Spectroscopy

As the most promising power sources for smart phones, laptops, electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium ion batteries have attracted much attention in past decades. In order to meet the ever-increasing demand for high energy density, high capacity and long service life, various transition metal oxides and compounds (TMOs) have been investigated because of their much higher theoretical capacity based on the conversion reactions [1-3]. During Li uptake, TMOs are firstly reduced to extremely small metallic nanoparticles dispersed in a Li2O matrix. And during subsequent charging, these metallic particles are oxidized. The reaction mechanism can be described using the following general equation [1]: TM x O y + 2yLi+ +2ye- → xTM + yLi2O (1) Among these TMOs, iron based compounds are particularly interesting candidates to investigate as they are less expensive and more environmental friendly than cobalt contained oxides. As a member of iron based compounds, NiFe2O4 comes into notice since it contains electrochemically active nickel and displayed a high theoretical capacity of 914 mAh g-1. However, these conversion reaction electrodes showed a fast capacity fading in few cycles because of the formation of an insulating polymeric layer around the particles and poor integrity caused by the drastic volume changes upon cycling [4-5]. Although the conversion reaction 1 of simple monoxides (CoO, CuO, FeO or NiO)[1] is thermodynamically favorable and has been widely investigated by ex situ/in situ X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Mössbauer spectroscopy during discharge and charge. Lots of detailed information for the reaction is still not well understood yet due to the intriguing nature of amorphous Li2O and ultrafine metal particles and limitations for the above techniques. A better understanding of the reactions occurring during Li uptake and release is crucially important to search for new high performance electrode materials. In this work, nanocrystalline NiFe2O4 particles were successfully synthesized and used as active electrode material for lithium ion battery. X-ray absorption spectroscopy (XAS) was applied to investigate the local structural changes and electrochemical reactions of NiFe2O4 in the cell during 1st discharge and charge process. As lithium inserted into the structure, Fe3+ ions are firstly reduced to Fe2+, but Ni2+ ions are unaffected which can be seen in fig.1. The matrix spinel structure collapses and transform to a rock salt monoxide phase with further lithiation. Along with the conversion reaction, the formation of SEI on the electrode consumed a large amount of lithium ions. The rock salt monoxides are reduced to metallic products at the end of discharging with a small particle size and highly disorder. The refined EXAFS results show that these metallic Fe/Ni nanoparticles are not well refined with a nearest number of neighbors of 6 and 8 for Fe and Ni respectively. All these ultrafine metal particles are reoxidized to Fe2O3and NiO phase instead of pristine spinel structure during the following charging. Reference [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496-499. [2] R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, J.C. Jumas, J. Olivier-Fourcade, Electrochemistry Communications 5 (2003) 16-21. [3] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Advanced Materials 22 (2010) E170-E192. [4] X. Guo, X. Lu, X. Fang, Y. Mao, Z. Wang, L. Chen, X. Xu, H. Yang, Y. Liu, Electrochemistry Communications 12 (2010) 847-850. [5] J.-M. Tarascon, S. Grugeon, M. Morcrette, S. Laruelle, P. Rozier, P. Poizot, Comptes Rendus Chimie 8 (2005) 9-15. Figure 1

Electrochemical Performance of Carbon-Coated Cauliflower-like WO3 for Li-Ion Batteries

Metal oxides have been considered and investigated as alternative anodes for use in lithium ion batteries. The research focus is on two different mechanism groups: intercalation–deintercalation reaction and conversion (redox) reaction. The intercalation–deintercalation mechanism is that transition metal oxides and other compounds with a multidimensional layer structure can reversibly intercalate lithium ions into the lattice without destroying their crystal structure. While conversion mechanism usually produces metallic nanoparticles embedded in an insulating Li2O matrix and the shows generally much higher theoretical capacity than that of already-commercialized graphite. Among the metal oxides, tungsten oxides (WO2, WO3, and WO3-x H2O) as one of the important metal oxides with respect to physicochemical properties, have been extensively investigated for such applications as gas sensors, photocatalysis, electrochromic devices, electronic devices, dye-sensitized solar cells, supercapacitors, and lithium ion batteries. Interestingly, the tungsten oxides have better electronic conductivity (10–10-6 S cm-1) than some of the other oxides; in addition, they also rely on designing a nanostructured particle with a small radius and right crystalline phase for fast lithium-ion insertion and superior electrochemical performance. From this point of view, a wide variety of approaches have been pursued over the years for the synthesis of tungsten oxides, such as nanoparticles, nanowires, nanorods, and nanoflowers using various approaches which include a hydrothermal reaction, combustion process, sol–gel process, solution-phase reaction, template directed method, mechanochemical activation, and hot-wire chemical vapor deposition (HWCVD) method. However, most of the previous approaches led to the formation of monoclinic W18O49, hexagonal WO3, and WO3-xH2O nanostructures. There have been very few studies on nanostructured monoclinic WO3 (m-WO3). In this study, we will present a facile hydrothermal process that obtains cauliflower-like carbon-coated WO3 oxide with a high rate-capability and good cycling performance for lithium-ion batteries. The motivation in this study is to explore the possibility of using m-WO3 as an anode material for lithium-ion batteries and to elucidate its reaction mechanism with lithium during the charge–discharge process.

Study on the Electrochemical Reaction Mechanism of ZnFe2O4 by In Situ Transmission Electron Microscopy.

A family of mixed transition–metal oxides (MTMOs) has great potential for applications as anodes for lithium ion batteries (LIBs). However, the reaction mechanism of MTMOs anodes during lithiation/delithiation is remain unclear. Here, the lithiation/delithiation processes of ZnFe2O4 nanoparticles are observed dynamically using in situ transmission electron microscopy (TEM). Our results suggest that during the first lithiation process the ZnFe2O4 nanoparticles undergo a conversion process and generate a composite structure of 1–3 nm Fe and Zn nanograins within Li2O matrix. During the delithiation process, volume contraction and the conversion of Zn and Fe take place with the disappearance of Li2O, followed by the complete conversion to Fe2O3 and ZnO not the original phase ZnFe2O4. The following cycles are dominated by the full reversible phase conversion between Zn, Fe and ZnO, Fe2O3. The Fe valence evolution during cycles evidenced by electron energy–loss spectroscopy (EELS) techniques also exhibit the reversible conversion between Fe and Fe2O3 after the first lithiation, agreeing well with the in situ TEM results. Such in situ TEM observations provide valuable phenomenological insights into electrochemical reaction of MTMOs, which may help to optimize the composition of anode materials for further improved electrochemical performance.