Lithium-rich Oxide Research Articles

Tris(trimethylsilyl)borate as an electrolyte additive for improving interfacial stability of high voltage layered lithium-rich oxide cathode/carbonate-based electrolyte

Tris(trimethylsilyl)borate (TMSB) is used as an electrolyte additive for high voltage lithium-rich oxide cathode of lithium ion battery. The interfacial natures of Li[Li0.2Mn0.54Ni0.13Co0.13]O2/carbonate-based electrolyte are investigated with a combination of electrochemical measurements and physical characterizations. Charge/discharge tests show that the cyclic performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 in a mixed carbonate electrolyte is significantly improved by using TMSB. After 200 cycles between 2 V and 4.8 V (vs. Li/Li+) at 0.5 C rate, the capacity retention of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is only 19% in the blank electrolyte, while it is improved to 74% when 0.5% TMSB is applied. The results from physical characterizations demonstrate that this excellent cyclic performance is attributed to the improved interfacial stability of Li[Li0.2Mn0.54Ni0.13Co0.13]O2/electrolyte due to the thin and protective film generated by TMSB.

Polyethylene glycol-assisted synthesis of hierarchically porous layered lithium-rich oxide as cathode of lithium ion battery

A hierarchically porous layered lithium-rich oxide, 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2, is synthesized by co-precipitation of metal oxalates with an assistance of a moderate polyethylene glycol (PEG2000). The morphology and crystal structure of the product are characterized by scanning electron microscope, transmission electron microscopy and X-ray diffraction, and its performance as cathode of lithium ion battery is evaluated with charge/discharge tests. It is found that the as-synthesized oxide exhibits excellent rate capability and cyclic stability: delivering an initial discharge capacity of 262 mAh g−1 at 0.1C (1C = 250 mA g−1) and 135 mAh g−1 at 4C, and possessing a capacity retention of 83% after 200 cycles at 4C. These performances can be attributed to the unique structure of the as-synthesized oxide: uniform secondary microspheres of about 10 μm, which is composed of uniform primary microparticles of about 2 μm, and hierarchically porous structure with pores distributed among primary and secondary particles. The hierarchically porous structure provides large reaction sites for lithium ion insertion/extraction and large space to buffer the volume change during cycling, leading to the excellent rate capability and cyclic stability of the as-synthesized oxide.

Reversible Li-Intercalation through Oxygen Reactivity in Li-Rich Li-Fe-Te Oxide Materials

Lithium-rich oxides are a promising class of positive electrode materials for next generation lithium-ion batteries, and oxygen plays a prominent role during electrochemical cycling either by forming peroxo-like species and/or by irreversibly forming oxygen gas during first charge. Here, we present Li-Fe-Te-O materials which show a tremendous amount of oxygen gas release. This oxygen release accounts for nearly all the capacity during the first charge and results in vacancies as seen by transmission electron microscopy. There is no oxidation of either metal during charge but significant changes in their environments. These changes are particularly extreme for tellurium. XRD and neutron powder diffraction both show limited changes during cycling and no appreciable change in lattice parameters. A density functional theory study of this material is performed and demonstrates that the holes created on some of the oxygen atoms upon oxidation are partially stabilized through the formation of shorter O-O bonds, i.e. (O2)n– species which on further delithiation show a spontaneous O2 de-coordination from the cationic network and migration to the now empty lithium layer. The rate limiting step during charge is undoubtedly the diffusion of oxygen either out along the lithium layer or via columns of oxygen atoms.

Porous layered lithium-rich oxide nanorods: Synthesis and performances as cathode of lithium ion battery

A layered lithium-rich oxide, Li[Li0.19Mn0.32Co0.49]O2, is synthesized by introducing manganese and cobalt via oxalates co-crystallization in reverse micellar microemulsion. The physical and electrochemical performances of the as-synthesized oxide are evaluated as cathode of lithium ion battery. The physical characterizations, from X-ray diffraction, scanning electron microscope and transmission electron microscope, indicate that the as-synthesized oxide takes a nanorod morphology of up to 1μm in length and 200nm in diameter, which is composed of about 20nm subunit nanoparticles, and possesses a hierarchical pore structure. Electrochemical measurements demonstrate that the as-synthesized oxide exhibits improved charge/discharge performances: less polarization, larger discharge capacity, higher rate capability, and better cyclic stability, compared to the sample synthesized by introducing the transition metals in solid-state reaction.

Solid State NMR Studies of Li2MnO3 and Li-Rich Cathode Materials: Proton Insertion, Local Structure, and Voltage Fade

High energy densities of lithium-rich transition metal oxides cannot be sufficiently maintained on cycling due to high-voltage first-cycle activation and the subsequent structural changes. These changes can be seen as a continuous decrease of the average voltage with cycling, known as voltage fade. Electrochemical and chemical insertion of protons has been reported suggesting that protons generated in the electrolyte could be involved in electrochemical cycling which could play a similar role in the “Li2MnO3 component” of lithium rich transition metal oxides. Here, electrochemical insertion of structural protons, changes in lithium occupancy at various states of charge, and changes in local structure have been investigated via a combination of local probes including solid state NMR, X-ray absorption spectroscopy and first principle calculations. While significant evidence is found for the deposition of non-structural proton-bearing species on electrodes, which accumulate with extensive cycling, structural proton insertion is not found to be a significant process directly effecting voltage fade. The electrochemical activity of disordered Li2MnO3, synthesized at low temperature, is also investigated and its Li removal/insertion properties measured quantitatively with NMR. Major reordering of Li sites and subsequent local structural transitions are observed by NMR and are found to be synchronous with voltage fade.

Raman Microscopy of Lithium-Manganese-Rich Transition Metal Oxide Cathodes

Lithium-rich and manganese-rich (LMR) layered transition metal (TM) oxide composites with general formula xLi2MnO3·(1-x)LiMO2 (M = Ni, Co, Mn) are promising cathode candidates for high energy density lithium ion batteries. Lithium-manganese-rich TM oxides crystallize as a nanocomposite layered phase whose structure further evolves with electrochemical cycling. Raman spectroscopy is a powerful tool to monitor the crystal chemistry and correlate phase changes with electrochemical behavior. While several groups have reported Raman spectra of lithium rich TM oxides, the data show considerable variability in terms of both the vibrational features observed and their interpretation. In this study, Raman microscopy is used to investigate lithium-rich and manganese-rich TM cathodes as a function of voltage and electrochemical cycling at various temperatures. No growth of a spinel phase is observed within the cycling conditions. However, analysis of the Raman spectra does indicate the structure of LMR-NMC deviates significantly from an ideal layered phase. The results also highlight the importance of using low laser power and large sample sizes to obtain consistent data sets.

An Li-rich oxide cathode material with mosaic spinel grain and a surface coating for high performance Li-ion batteries

Lithium-rich layered oxides are considered to be one of the most promising cathode materials for lithium ion batteries due to their extremely high reversible capacity. Here, we report the design of a novel heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material with mosaic spinel nanograins and a surface coating, which is synthesized by a facile and green one-step solid-phase surface-modification process. We propose that the chemical Li leaching from Li2MnO3 simultaneously induces the formation of a fluorite coating and the layer-to-spinel phase transformation at high temperatures. The fluorite coating protects the lithium-rich oxides from direct exposure to the highly active electrolyte. The spinel phase provides an efficient path for Li+ mobility and also facilitates the suppression of the initial irreversible capacity loss. This unique heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material thus exhibits an outstanding initial Coulombic efficiency, superior rate capability and excellent cyclability. The design concept and facile synthetic strategy can be applied to both advanced lithium ion batteries and other high-performance energy storage devices.

Direct In situ Observation of Li2O Evolution on Li-Rich High-Capacity Cathode Material, Li[NixLi(1–2x)/3Mn(2–x)/3]O2 (0 ≤ x ≤0.5)

High-capacity layered, lithium-rich oxide cathodes show great promise for use as positive electrode materials for rechargeable lithium ion batteries. Understanding the effects of oxygen activating reactions on the cathode's surface during electrochemical cycling can lead to improvements in stability and performance. We used in situ surfaced-enhanced Raman spectroscopy (SERS) to observe the oxygen-related surface reactions that occur during electrochemical cycling on lithium-rich cathodes. Here, we demonstrate the direct observation of Li2O formation during the extended plateau and discuss the consequences of its formation on the cathode and anode. The formation of Li2O on the cathode leads to the formation of species related to the generation of H2O together with LiOH and to changes within the electrolyte, which eventually result in diminished performance. Protection from, or mitigation of, such devastating surface reactions on both electrodes will be necessary to help realize the potential of high-capacity cathode materials (270 mAhg(-1) versus 140 mAhg(-1) for LiCoO2) for practical applications.

Electrochemical Synthesis of a Lithium-Rich Rock-Salt-Type Oxide Li5W2O7 with Reversible Deintercalation Properties

Starting from the ribbon structure Li2W2O7, the lithium-rich phase Li5W2O7 with an ordered rock-salt-type structure has been synthesized, through a topotactic irreversible reaction, using both electrochemistry and soft chemistry. In contrast to Li2W2O7, the lithium-rich oxide Li5W2O7 shows reversible deintercalation properties of two lithium molecules per formula unit: a stable reversible capacity of 110 mAh/g at 1.70 V is maintained after 10 cycles. The exploration of the lithium mobility in this system shows that Li2W2O7 is a cationic conductor with σ = 4.10(-4) S/cm at 400 °C and Ea = 0.5 eV.

Structural transformation of a lithium-rich Li1.2Co0.1Mn0.55Ni0.15O2 cathode during high voltage cycling resolved by in situ X-ray diffraction

Lithium-rich layered oxides having compositions of Li1+yM1−yO2 (M = Co, Mn, and Ni) have become attractive cathode materials for high energy density and high voltage lithium ion batteries for electric vehicle (EV) applications. However, their utility in EVs suffers from both voltage and capacity fade. The voltage fade is related to structural transformation in these lithium-rich oxides and must be thoroughly understood. In this work, we have utilized in situ X-ray diffraction in order to monitor these structural transformations during high voltage (4.8 V) cycling of a lithium- and manganese-rich Li1.2Co0.1Mn0.55Ni0.15O2 oxide cathode, which has not been reported previously. The lattice parameters of the cathode were monitored for first cycle and compared with the subsequent cycles. Based on our results, the c-lattice parameter increases during the course of initial charging and eventually decreases upon charging beyond 4.4 V, which verifies lithium extraction occurs from transition metal layers due to activation of Li2MnO3 phase at high-voltage. The fact that the a-lattice parameter remains constant at the first cycle plateau region indicates oxygen loss from the structure during first cycle charging which is attributed to irreversible capacity obtained from first cycle. For first and subsequent cycles, the c-lattice parameter increases during discharge up to 3.5 V and below 3.5 V, the decrease in those values was observed. After subsequent cycling, (440) cubic spinel reflections were observed during low voltage discharge process, which reveals a layer to spinel-like phase transformation in the lattice and is thought to be the reason for the observed voltage fade. A significant decrease in monoclinic phase was observed after subsequent cycles and is believed to contribute to the structural instability and capacity fade after repeated cycling.

In situ Raman spectroscopy of layered solid solution Li2MnO3–LiMO2 (M = Ni, Mn, Co)

Lithium rich manganese oxide based materials are emerging as possible high energy density cathode materials for lithium ion batteries. In the present research, we have investigated the first charge/discharge profile behavior of Li1.2Ni0.175Co0.1Mn0.52O2 via in situ Raman spectroscopy. X-ray diffraction studies confirm the presence of cation ordering in the transition metal layers. Scanning electron microscopy shows the homogeneous distribution of highly dense spherical agglomerates of ∼5 μm size. Steady decrease in impedance up to ∼4.5 V followed by a less steep increase as the cell voltage traversed the first charge cycle has been confirmed by electrochemical impedance spectroscopy. Raman spectroscopy shows that the extraction of lithium takes place from both the transition metal layers as well as lithium layers in the voltage range of 4.1–4.4 V. The oxygen removal becomes severe in the voltage range of 4.55–4.6 V, after which the diffusion of the transition metal ions to the vacant sites takes place.

Investigation of the Irreversible Capacity Loss in the Lithium-Rich Oxide Li[Li1/5Ni1/5Mn3/5]O2

The lithium-rich transition metal oxides show a larger first charge capacity and larger cycling capacities than the non-lithium-rich transition metal oxides. The disadvantages of the lithium-rich transition metal oxides include relatively poor rate capabilities and relatively large irreversible capacities. In this report, the irreversible capacity loss of the lithium rich oxide Li[Li1/5Ni1/5Mn3/5]O2 was investigated. Stepwise traverse of the oxygen-release plateau increased the cycling capacity of Li/Li[Li1/5Ni1/5Mn3/5]O2 cells and gave evidence that lithium was removed from the transition metal layer at the start of the oxygen release plateau. The irreversible capacity loss was attributed to the diffusion of transition metals into the lithium vacancies in the transition metal layer and the subsequent inability for lithium reinsertion into the transition metal layer. Isothermal calorimetry of Li/Li[Li1/5Ni1/5Mn3/5]O2 cells cycled from 2.5 to 4.4 V (no oxygen loss) supported the view that lithium is not deintercalated from the transition metal layer at the start of charge.

Oxygen-Release Characteristics of Lithium-Rich Oxides of Varying Size

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Lithium diffusion in aluminum-lithium alloy 2090 clad with 7072

The loss of lithium during solution heat treatment of alloy 2090 clad with approximately 100 μm of 7072 was investigated. Lithium concentration profiles in the alloy were measured using a nuclear reaction analysis technique after samples had been subjected to various solution heat treatments. The results showed that the cladding-base metal interface provided no impediment to lithium diffusion and that the rate of lithium diffusion in the cladding was similar to its rate of diffusion in the base metal. A lithium-rich oxide was formed on the external surface, but the concentration profiles showed that the effective lithium concentration at the surface was near zero. Thus, lithium in the bulk experienced a maximum concentration gradient and diffused to the surface as rapidly as possible. Despite the absence of any lithium diffusion barrier in the cladding, the initial rate of lithium loss is much less than in bare material. The cladding provides effective protection from lithium loss, and the rate of loss in clad materials only approaches that of bare materials after approximately 4 hours.

Thermal oxidation behavior of an Al-Li-Cu-Mg-Zr alloy

The chemical composition of oxide films formed during thermal treatments of an Al-Li-Cu-Mg-Zr alloy has been studied by means of Auger electron spectroscopy and X-ray photoelectron spectroscopy. The oxide layers formed after oxidation of 2.5 minutes to 30 minutes at 530 °C in lab air have been characterized. In the early stages of oxidation the surface is composed of both the lithium rich oxides and magnesium rich oxides. However, after longer oxidation times the oxidation of lithium becomes predominant and the air/oxide interface is completely covered by lithium compounds. Oxidation products formed on the alloy surface have been studied by X-ray diffraction analysis. The following three phases, namely, Li2CO3, α-Li5AlO4, and γ-LiAlO2, were identified. During heat treatment in lab air at 530 °C and at atmospheric pressure the dominating reaction product is Li2CO3. Due to the selective oxidation of lithium a soft surface layer is developed. The width of the soft layer formed during solution heat treatments carried out in lab air and in salt bath environments has been determined by microhardness measurements. The lithium concentration profiles were calculated from a diffusion equation. The depletion of alloying elements from the near surface region during heat treatments has been investigated using energy dispersive X-ray analysis. The oxide morphology was examined using scanning electron microscopy and optical microscopy.