Lithium Metal Oxide Research Articles

Examining the Electrochemical Impedance at Low States of Charge in Lithium- and Manganese-Rich Layered Transition-Metal Oxide Electrodes

Lithium- and manganese-rich layered transition-metal oxide (LMR-NMC) intercalation electrodes are projected to enable batteries with high energy density and low costs for energy. However, implementation of LMR-NMC materials are challenged by life limiting mechanisms as well as less than desired rate performance. Here-in, we use electrochemical characterization of LMR-NMC electrodes to examine the large magnitude of impedance and the asymmetric polarization between charge and discharge at low states of charge (SOC). The lithium metal oxide cathode materials examined were LMR-NMC and standard NMC both from Toda Kogyo (Japan). The LMR-NMC material, referred to as HE5050, had the composition 0.5Li2MnO3•0.5LiMn0.375Ni0.375Co0.25O2 (or in layered notation Li1.2Ni0.15Mn0.55Co0.10O2). The NMC material had composition of Li1+x(Ni1/3Mn1/3Co1/3)1-xO2 and can be considered to be the commercial standard stoichiometry for Li-ion transportation cells at the time of this publication. Full cells used A12 graphite (Conoco Phillips) and half cells used lithium metal (FMC Lithium) both with standard LiPF6based carbonate electrolyte. A three electrode pouch cell was utilized for the asymmetry polarization experiments in the study, with a composite LMR-NMC electrode as the working electrode, lithium metal was used as the reference and counter electrodes The area-specific impedance (ASI) of LMR-NMC in Fig. 1 displays a similar dependency as standard layered lithium metal oxides when compared as a function of voltage rather than SOC. The increasing and asymmetric behavior of the polarization in LMR-NMC measured at low SOC is hypothesized to be the result of the differing lithium contents and lithium mobilities in the heterogeneous, nano-composite metal oxide material. Transport of lithium within LMR-NMC is suggested to be governed by the relatively facile nickel- and cobalt-rich domains even as this domain appears to reach full lithiation. As the majority of capacity at low SOC in LMR-NMC is associated with the lithium- and manganese-rich domains, the observed high impedance presents a significant hurdle to utilizing this capacity in moderate power-to-energy ratio applications. Much work is needed to clarify the transport and structural mechanism in LMR-NMC. Other studies have shown the structural transformation known as voltage fade is less severe with lower lithium content. However, lowering the lithium stoichiometry also tends to lower the specific energy of the material at beginning of life. Optimizing the stoichiometry of the lithium and manganese to achieve a maximum specific energy over life at relevant discharge rates is a potentially viable path forward. Figure 1

Solid-state activation of Li2O2 oxidation kinetics and implications for Li–O2 batteries

Solid-state activation of Li2O2 oxidation is mediated by chemical conversion of Li2O2 with slow oxidation kinetics to a lithium metal oxide with generally faster kinetics.

Examining the Electrochemical Impedance at Low States of Charge in Lithium- and Manganese-Rich Layered Transition-Metal Oxide Electrodes

Lithium- and manganese-rich layered transition-metal oxide (LMR-NMC) intercalation electrodes are projected to enable batteries with high energy density and low costs for energy. However, implementation of LMR-NMC materials are challenged by life limiting mechanisms as well as less than desired rate performance. Here-in, we use electrochemical characterization of LMR-NMC electrodes to examine the large magnitude of impedance and the asymmetric polarization between charge and discharge at low states of charge (SOC). The area-specific impedance (ASI) of LMR-NMC displays a similar dependency as standard layered lithium metal oxides when compared as a function of voltage rather than SOC. Numerical physics-based modeling is used to analyze and simulate the potential response. The increasing and asymmetric behavior of the ASI in LMR-NMC at low SOC is suggested to be the result of the differing lithium diffusivities in the heterogeneous, nano-composite metal oxide material. Transport of lithium within LMR-NMC is governed by the relatively facile nickel- and cobalt-rich domains. Conversely, the mass transport within the lithium- and manganese-rich domains are characterized as comparatively sluggish. Lowering the stoichiometry of the lithium and manganese to achieve an optimal energy density at relevant discharge rates is suggested as a potentially viable path forward.

In Situ Solid State 7Li NMR Observations of Lithium Metal Deposition during Overcharge in Lithium Ion Batteries

Lithium metal deposition during overcharge in practical lithium ion cells composed of a lithium metal oxide (LiCoO2) positive electrode coated on Al foil, carbon (synthesized graphite and hard carbon) negative electrodes coated on Cu foil, polypropylene separator, and liquid electrolyte was observed using in situ solid state 7Li nuclear magnetic resonance (NMR) measurements with an original probe featuring a flattened solenoid coil. Li insertion and extraction in carbon electrodes were monitored during charge and discharge and the intensities of certain peaks were found to be proportional to the cell capacity change. The deposition of metallic Li commenced after the cell voltage exceeded the nominal value and almost saturated after 160% of charge at a low current rate. The measurements showed that the deposition of metallic Li was much easier on graphite compared to hard carbon. The metallic Li deposited on hard carbon was almost completely discharged, whereas that on graphite remained after discharging to 2.5 V.

In Situ Solid State 7Li NMR Observation of Lithium Metal Deposition during Overcharge in Lithium Ion Battery

Lithium metal deposition during overcharge in a full cell composed of lithium metal oxide (LiCoO2) positive electrodes and carbon negative electrodes (a synthesized graphite, a natural graphite and two types of hard carbon) was observed by in situ 7Li solid state NMR (nuclear magnetic resonance). Peak intensities corresponding to the GIC (Graphite Intercalation Compound, charged state) and dendritic Li (deposited metallic Li) were monitored at each 20% state of charge (SOC) during discharge (and charge) and each 17 minutes (or 1000 scans) after the cell capacity reached the expected overcharge. The GIC peak increased with charge and decreased with discharge during the normal voltage of operation. The Li metal deposition occurred after the cell was charged above its normal operational voltage (4.2 V) though intercalation into the carbon still simultaneously occured, suggesting that metallic Li can be formed when the negative electrode reaches a voltage below 0 V vs. Li. We also found that the dendritic Li is not inert but active to be oxidized again and intercalated into the carbon within a few hours. The dendritic Li formed on hard carbon was completely discharged, while that deposited on graphite was not, indicating that the electrochemical property of the dendritic Li varies with the nature of the negative electrode.

Density Functional Theory Based Study of the Electron Transfer Reaction at the Lithium Metal Anode in a Lithium–Air Battery with Ionic Liquid Electrolytes

Room temperature ionic liquids, which have unique properties such as a relatively wide electrochemical stability window and negligible vapor pressure, are promising candidates as electrolytes for developing lithium–air batteries with enhanced performance. The local current density, a crucial parameter in determining the performance of lithium–air batteries, is directly proportional to the rate constant of the electron transfer reaction at the surface of the anode that involves the oxidation of pure lithium metal into lithium ion (Li+). The electrochemical properties of ionic liquid based electrolytes, which can be molecularly tailored on the basis of the structure of their constituent cations and anions, play a crucial role in determining the reaction rate at the anode. In this paper, we present a novel approach, based on Marcus theory, to evaluate the effect of varying length of the alkyl side chain of model imidazolium based cations on the rates of electron transfer reaction at the anode. Density functi...

Composites of graphene and encapsulated silicon for practically viable high-performance lithium-ion batteries

A facile and scalable approach to synthesize silicon composite anodes has been developed by encapsulating Si particles via in situ polymerization and carbonization of phloroglucinol-formaldehyde gel, followed by incorporation of graphene nanoplatelets. As a result of its structural integrity, high packing density and an intimate electrical contact consolidated by the conductive networks, the composite anode yielded excellent electrochemical performance in terms of charge storage capability, cycling life and coulombic efficiency. A half cell achieved reversible capacities of 1,600 mAh·g−1 and 1,000 mAh·g−1 at 0.5 A·g−1 and 2.1 A·g−1, respectively, while retaining more than 70% of the initial capacities over 1,000 cycles. Complete lithium-ion pouch cells coupling the anode with a lithium metal oxide cathode demonstrated excellent cycling performance and energy output, representing significant advance in developing Si-based electrode for practical application in high-performance lithium-ion batteries.

Electrochemical and in-Situ Investigation of Quaternary Lithium Transition Metal Fluorides in Li-Ion Batteries

During the last decade Lithium metal oxides or polyanionic compounds containing two or more different transition metal ions (e.g. LiNi0.5Mn1.5O4 or LiNi1-x-yCoxMnyO2) are in the focus of research and commercial interest as positive electrodes for Li-ion batteries. Lithium transition metal fluorides are very promising materials compared to common oxide materials with corresponding electrochemically active cations because the more electronegative fluorine atoms increase the redox potential leading to a higher specific energy. However, no reports are given about the electrochemical properties of quaternary lithium transition metal fluorides as positive electrode. In this study a novel sol-gel synthesis route was applied to synthesize several quaternary lithium transition metal fluorides LiMFeF6 (M2+ = 3d transition metal) without toxic chemicals like HF, LiF or F2(gas). After the structural and morphological characterization the fundamental electrochemical properties are characterized. It could be shown that up to 1 eq. Lithium can be inserted fully reversible into the LiMFeF6 host structure with a notable cycling stability and a remarkable rate performance. Furthermore an in-situ x-ray powder diffraction and x-ray absorption investigation reveals that the host structure is an insertion material. The electrochemical active redox couple Fe3+/2+ was confirmed by Mößbauer analysis.

The Electrochemical Properties of Hyperbranched Polymer Nanocoated Cathode Materials

We introduce a novel type of surface nanostructured cathode materials. This approach relies on hyperbranched polymer beforehand directed nanoscale coating. The polymer coating successfully forms a uniform coverage onto the surface of the active materials. TEM photograph shows that lithium metal oxide particle is well decorated by polymer coating with 10 nm thickness. Cycling performance (Figure 1) of modified electrode is comparing with pristine electrode at 1C charge rate with cut-off voltage 4.4V. Even after 300 cycles, the cells with modified electrode maintain at higher capacity level. On the other hand, the cells with pristine electrode quickly fade out.These results demonstrate that the polymer coating effectively prevents the direct contact between electrodes and electrolytes that are highly vulnerable to electrochemical decomposition at high voltage conditions. Polymeric coating can also behave as interfacial protection layer that suppress the undesired interfacial reactions.

Analysis of Mechanical Degradation in Lithium-Ion Battery Electrodes

Lithium-ion battery (LIB) electrodes consist of four different phases; active particle, binder, conductive additive and electrolyte. During operation, the active particles (e.g. graphite anode, lithium metal oxide cathode) are responsible for hosting the Li ions. Conductive additives help in increasing the electronic conductivity of the electrode. The binder, typically PVDF, holds the active particles and conductive additives together and gives mechanical stability to the electrode. The electrolyte occupies the pore-space and forms the electrochemically active interfacial area with the active particles and provides pathways for lithium-ion transport. Lithium ions intercalate and deintercalate into the host material through diffusion process. This can cause significant swelling in the active particles and induce diffusion induced stress. For high-capacity active materials, the swelling can be significant. Because of swelling, the active particles squeeze the binders and nearby conductive additives to make space for themselves. During delithiation, the active particles shrink. Because of its inherent plasticity, the binders cannot restore its initial configuration. As a result, delamination may occur and the contact between the active particles and the conductive additives may be lost, which eventually would lead to capacity fade and deterioration in performance of the lithium ion battery. Diffusion induced stress may lead to mechanical degradation in terms of formation of microcracks in the active particles. Nucleation of these microcracks results in formation of spanning cracks which eventually leads to catastrophic failure of the material.We have demonstrated a random lattice spring based methodology to characterize the development of microscopic damage and their subsequent nucleation to form crack fronts. According the lattice spring method, breaking of each bond is simulated by solving the equilibrium equation. A discrete lattice based model will be developed to characterize the nonlinear deformation of binder materials. Delamination between the active particles and the binders will also be captured. Part (a), (b) and (c) in the figure provides a schematic demonstration of the delamination mechanism observed in lithium ion batteries. Part (d) of the figure shows mechanical degradation inside an active particle during delithiation. In this work, we will highlight the implications of mechanical degradation in terms of interfacial delamination and micro-crack formation in the LIB electrodes.

A REVIEW OF METAL OXIDE COMPOSITE ELECTRODE MATERIALS FOR ELECTROCHEMICAL CAPACITORS

With the emerging technology in the 21st century, which requires higher electrochemical performances, metal oxide composite electrodes in particular offer complementary properties of individual materials via the incorporation of both physical and chemical charge storage mechanism together in a single electrode. Numerous works reviewed herein have identified a wide variety of attractive metal oxide-based composite electrode material for symmetric and asymmetric electrochemical capacitors. The focus of the review is the detailed literature data and discussion regarding the electrochemical performance of various metal oxide composite electrodes fabricated from different configurations including binary and ternary composites. Additionally, projection of future development in hybrid capacitor coupling lithium metal oxides and carbonaceous materials are found to obtain significantly higher energy storage than currently available commercial electrochemical capacitors. This review describes the novel concept of lithium metal oxide electrode materials which are of value to researchers in developing high-energy and enhanced-cyclability electrochemical capacitors comparable to Li -ion batteries. In order to fully exploit the potential of metal oxide composite electrode materials, developing low cost, environment-friendly nanocomposite electrodes is certainly a research direction that should be extensively investigated in the future.

Titanium and Niobium Polysulfide Electrodes for Lithium/Metal Sulfide Secondary Batteries

Achieving high capacity by making use of multi electron processes has been desired to enhance the energy density of lithium-ion batteries. Recently, layered lithium metal oxides with more than one equivalent of lithium per transition metal have been actively researched and developed as new positive-electrode materials with capacities of more than 200 mAh g−1. However, it is still difficult to achieve reversible capacity of more than 300 mAh g−1. To achieve the high capacity, the redox reaction of anions in addition to that of transition metals is needed to be used; that is, the charge and discharge with processes involving more than two-electrons should be required in materials. Therefore, the challenging is development of new electrode active materials which can show redox reactions with more than two electron processes. Metal sulfides are attractive candidates for positive-electrode materials [1, 2]. Their high capacities originate from charge and discharge processes that involve more than one electron; for example, crystalline TiS3 charges and discharges with high reversible capacities of greater than 300 mAh g−1, which corresponds to ca.1.5 electron redox reaction.In this study, amorphous titanium polysulfides a-TiSx (x = 3, 4) and niobium polysulfides a-NbSy(y = 3, 4, 5) were prepared by a mechanical milling process and their electrochemical properties were examined using the cells with carbonate-based electrolytes.Amorphous titanium and niobium polysulfides were mechanochemically synthesized at room temperature using a planetary ball mill apparatus. Sulfur (S8) and titanium disulfide (TiS2) or niobioum disulfide (NbS2) were used as starting materials.The XRD measurements indicated that the diffraction peaks attributable to S8 and TiS2 or NbS2disappeared by mechanical milling, suggesting that the samples became amorphous.Fig. 1 shows charge-discharge curves of the cells using crystalline NbS2 reagent, amorphous NbS3, NbS4, and NbS5. The capacity increases with increasing sulfur/metal ratio and the cell using amorphous NbS5 shows a high capacity of ca 600 mAh g-1. The amorphous TiS4 discharged and charged with a high reversible capacity of ca. 700 mAh g-1in the 1.5–3.0 voltage range [3]. The cell using the amorphous metal polysulfides charged and discharged despite of the use of a carbonate-based electrolyte. Coulombic efficiency of the composite was higher than that of a sulfur electrode because the dissolution of the polysulfide into electrolytes was suppressed.Designing amorphous polysulfides is an effective way to develop novel sulfide-based electrode materials with high capacity.

Understanding the Redox Reaction Mechanism of Li2CoPO4f Cathode Material for Achieving Two Lithium Intercalation

Lithium ion batteries have attracted considerable attention as important energy storage and conversion systems for applications including electric vehicles (EVs) and hybrid electric vehicles (HEVs), owing to their high energy and power densities, as well as a long cycle life. Recently, lithium transition metal phosphates, such as LiFePO4, LiMnPO4, and Li3V2(PO4)3, have been considered as potential cathode materials for lithium ion batteries. It is well known that the phosphates display much better electrochemical and thermal stability compared to conventional lithium metal oxides [1]. However, recently, fluorophosphates are being developed as advanced cathode materials display higher operating potential, corresponding to the redox reaction of transition metal, when compared to the respective metal oxides and phosphates [2]. In addition, the highly electronegative fluoride ion helps to improve the cycle stability, which fluorophosphates make attractive materials for high energy batteries. Our current work is focused on maximizing the deliverable discharge capacity of Li2CoPO4F cathode material and to reach theoretical capacity by achieving more than one electron intercalation. The redox couple Co2+/3+ and Co3+/4+ was closely followed during galvanostatic charge-discharge test by x-ray photoemission spectroscopy and x-ray diffraction (XRD) analysis. It was found that the incomplete reduction of cobalt ions during discharge initiated the irreversible capacity loss rather than the electrolyte decomposition being solely responsible. A novel approach was carried out to activate the transition metal ions which resulted in a discharge capacity as high as 230 mAh g-1 at a current rate of 20 mA g-1 for Li/Li2CoPO4F cell. A long plateau at 4.8 V has been observed in the charge cycle with an additional voltage plateau at ~5.0 V, which can be attributed to the two lithium intercalation reaction and has been validated using cyclic voltammetry studies. The detrimental cycle stability observed [3] in Li2CoPO4F has been replaced with a stable cycle performance of > 90 % capacity retention as shown in figure 1. A quasi single phase reaction was confirmed using gravimetric intermittent titration technique in addition to the results from ex-situXRD studies. A discussion based on the obtained results will be presented in detail.

CO2 and O2 Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study

A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Li-ion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO2 or LiMn2O4 was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF6 in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO2, LiMn2O4, and LiNi(0.5)Mn(1.5)O4 were studied. We found that 1 M LiPF6 + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi(0.5)Mn(1.5)O4 is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF6 + EC/DEC electrolyte on a LiCoO2 cathode, as indicated by CO2 and O2 evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described.

The Configurational Space of Rocksalt‐Type Oxides for High‐Capacity Lithium Battery Electrodes

A unifying theory is presented to explain the lithium exchange capacity of rocksalt‐like structures with any degree of cation ordering, and how lithium percolation properties can be used as a guideline for the development of novel high‐capacity electrode materials is demonstrated. The lithium percolation properties of the three most common lithium metal oxide phases, the layered α‐NaFeO2 structure, the spinel‐like LT‐LiCoO2 structure, and the γ‐LiFeO2 structure, are demonstrated and a strong dependence of the percolation thresholds on the cation ordering and the lithium content is observed. The poor performance of γ‐LiFeO2‐type structures is explained by their lack of percolation of good Li migration channels. The spinel‐like structure exhibits excellent percolation properties that are robust with respect to off‐stoichiometry and some amount of cation disorder. The layered structure is unique, as it possesses two different types of lithium diffusion channels, one of which is, however, strongly dependent on the lattice parameters, and therefore very sensitive to disorder. In general it is found that a critical Li‐excess concentration exists at which Li percolation occurs, although the amount of Li excess needed depends on the partial cation ordering. In fully cation‐disordered materials, macroscopic lithium diffusion is enabled by ≈10% excess lithium.

Defect-induced plating of lithium metal within porous graphene networks.

Lithium metal is known to possess a very high theoretical capacity of 3,842 mAh g(-1) in lithium batteries. However, the use of metallic lithium leads to extensive dendritic growth that poses serious safety hazards. Hence, lithium metal has long been replaced by layered lithium metal oxide and phospho-olivine cathodes that offer safer performance over extended cycling, although significantly compromising on the achievable capacities. Here we report the defect-induced plating of metallic lithium within the interior of a porous graphene network. The network acts as a caged entrapment for lithium metal that prevents dendritic growth, facilitating extended cycling of the electrode. The plating of lithium metal within the interior of the porous graphene structure results in very high specific capacities in excess of 850 mAh g(-1). Extended testing for over 1,000 charge/discharge cycles indicates excellent reversibility and coulombic efficiencies above 99%.

Layered 2D Electrode Architecture for High-Power Lithium Ion Microbatteries

The power density of a lithium ion battery is predominantly limited by poor ionic and electronic pathways in the battery electrodes. To overcome these limitations, we have fabricated positive electrodes composed of a two-dimensional (2D) layer-by-layer electrode architecture using electrophoretic deposition. In this architecture, alternating layers of multi-walled carbon nanotubes (MWNT) and active lithium metal oxide nanoparticles are electrophoretically assembled onto a current collector. Each active metal oxide nanoparticle is precisely placed to make intimate contact with the conductive MWNT layers, eliminating all other electrode additives such as carbon black and polymeric binder (figure 1). Lithium manganese oxide nanoparticles (LiMn2O4, 300nm) are used as a model active material while MWNT are used as the scaffold. These multilayer electrodes demonstrate excellent rate capability up to 10C and cycling stability for over the 100 cycles tested (figure 2) in a Li/liquid electrolyte/LiMn2O4 cell. We observe that the electrode capacity per cm2increases linearly with the number of stacked layersat a rate of 0.06 mAh/per layer. Therefore, the capacity per footprint area can be increased by increasing the number of stacked layers. These multilayer electrodes are fabricated at room temperature and atmospheric pressure, and the assembly time per layer is less than 60 seconds.As the number of layers is increased, the dependence of the electrode morphology on rate performance and cycle life is studied though galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy and physical characterization techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). Multi-layer electrodes with higher capacity active materials such as the lithium-rich layered nickel-manganese-cobalt oxide will also be fabricated to further increase the capacity per footprint area of the microbattery [1, 2]. Acknowledgment: Financial support for this work was provided by the Department of Defense.1. Thackeray, M., Kang, SH., Johnson, CS., Vaughey, JT., Benedek, R., and Hackney, SA., Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) electrodes for lithium-ion batteries. Journal of Materials Chemistry, 2007. 17: p. 15.2. W.C. West, J.S., M.C. Smart, B. V. Ratnakumar, S. Firdosy, V. Ravi, M. S. Anderson, J. Hrbacek, E. S. Lee, and A. Manthiram, Electrochemical Behavior of Layered Solid Solution Li2MnO3-LiMO2 (M=Ni, Mn, Co) Li-Ion Cathodes with and without Alumina Coatings. Journal of the Electrochemical Society, 2011. 158: p. 7.

Editorial: Energy Storage Materials: A Special Issue of Energy Technology

Editorial: Energy Storage Materials: A Special Issue of <i>Energy Technology</i>

Electrochemical Characterization of LiMnFeF6 for Use as Positive Electrode in Lithium-Ion Batteries

To replace common commercial lithium metal oxides or polyanionic cathode materials, lithium transition metal fluorides are of particular interest, as they potentially provide a high energy density, safety, and cyclability. The quaternary fluoride LiMnFeF6 that crystallizes in the Na2SiF6-type structure was synthesized without toxic chemicals by a sol-gel process with trifluoroacetic acid as fluorine source. After structural and morphological characterization, the as-synthesized material was ball-milled with carbon. The novel composite cathode material produced in this way was cycled versus lithium. The electrochemical potential range, cycling ability, and rate performance were investigated and the electrochemically active redox couple Fe3+/Fe2+ was confirmed by Mössbauer spectroscopy. Structural changes of the LiMnFeF6 host structure after Li insertion were investigated by X-ray powder diffraction.

Remarkable enhancement of the electrode performance of nanocrystalline LiMn2O4 via solvothermally-assisted immobilization on reduced graphene oxide nanosheets

A facile solvothermal way to immobilize nanocrystalline LiMn2O4 on the surface of graphene nanosheets is developed to improve the functionality of lithium manganate as lithium intercalation electrode. A solvothermal treatment for the colloidal mixture of graphene oxide (GO) nanosheets and LiMn2O4 nanocrystals gives rise not only to the reduction of GO to reduced graphene oxide (RGO) but also to the immobilization of lithium manganate nanoparticles on the surface of RGO nanosheets. According to powder X-ray diffraction and electron microscopic analyses, the crystal structure and morphology of spinel lithium manganate remain intact upon the composite formation with the RGO nanosheets. The application of larger aldehyde molecule as a reductant leads to the increase of crystallinity and the lowering of Mn oxidation state for the pristine LiMn2O4 and its nanocomposite with the RGO nanosheets. The present LiMn2O4–RGO nanocomposites display promising cathode performances for lithium rechargeable batteries, which are much superior to those of the pristine LiMn2O4 nanocrystals. The observed enhancement of electrode performance upon the composite formation with the RGO nanosheets is attributable both to the improvement of the surface ion transport of nanocrystalline lithium manganate and to the increase of electrical conductivity. The present experimental findings demonstrate that the solvothermal treatment with RGO nanosheets provides an effective way of improving the electrochemical activity of nanocrystalline lithium metal oxides.