Positive Electrode For Li-ion Batteries Research Articles

(Invited) Multiscale Morphological and Electrical Characterization of Charge Transport and Charge Transfer Limitations to Power Performance of Positive Electrode for Li-Ion Batteries

Composite materials consisting of several interpenetrated and percolating phases are widely used as electrodes for a variety of electrochemical cells. Such electrodes are generally mixed ionic−electronic conductors, in which ions and electrons are confined to transport within the specific conducting network. In particular, for electrochemical storage and energy transformation devices, the transport properties play critical roles in determining their performance, which is pivotal to improve them for service as power sources. To reach this goal, an increased fundamental understanding of the factors that play on the transport properties within such composite materials is still required. The transport of the electrons and ions through Li-ion battery electrode and their transfer to the active matter depend on the electrode architecture (nano- and microstructure) through the interfacial areas, materials connectivity, and the transport length scales. Novel opportunity for progressing in this direction and exploring relationships between electrode architecture, charge transport and electrochemical performance is implementation of X-ray computed tomography (XRCT) and focused ion beam in combination with scanning electron microscopy (FIB-SEM) techniques to characterize and quantify the 3D architecture of composite electrodes. In particular, tortuosity, a critical parameter with respect to ionic transport, has been exhaustively studied. Comparatively, factors that play on the electrons transport and wiring to the active mass have been much less considered. Moreover, attempts to correlate quantified 3D geometries with measured electrochemical power performance or electrical transport properties of same real electrodes remain rare. We intend to fill this gap in this presentation. We have undertaken exhaustive characterizations of 3D geometries of LiNi1/3Mn1/3Co1/3O2 (NMC), LiFePO4 (LFP), and NMC/LFP blended electrodes and rationally interpreted their measured electrical properties and electrochemical performance. A combination of X-Ray tomography and FIB-SEM tomography are used for a multiscale analysis of electrodes 3D geometries, in particular the distribution of the conductive additive and the binder that perform as electronic wires, the tortuous pores distribution, which after electrolyte impregnation perform as ionic wires, and the inter-connectivity between the active matter phase and the electronic and the ionic wires [1]. The electrical properties (permittivity and conductivity) are measured from low (a few Hz) to microwave (a few GHz) frequencies by the broadband dielectric spectroscopy technique [2]. This one is very sensitive to the different scales of the electrode architecture involved in electronic transport, from interatomic distances to macroscopic sizes, as well as to the morphology at these scales, coarse or fine distribution of the constituents [2,3]. In addition to the geometry of the electrode architecture, the interfaces (adsorption of ions to the surface of active material and carbon nanoparticles), and interactions at such interfaces lead to a reciprocal influence of ionic and electronic transfers at the different scales of the electrode [4]. Finally, discharge rate performance is analyzed by simple, yet efficient methods. This approach allows discriminating between the contributions of the electronic and ionic wiring networks as the performance limiting factors, depending on the electrode geometry and discharge rate. [5] Acknowledgements Financial funding from the ANR Program No. ANR-15-CE05-0001-01 (PEPITE) is acknowledged. We also thank the CLYM (Centre Lyonnais de Microscopie) supported by the CNRS, the “Grand Lyon” and the Rhône-Alpes Region for use of the Zeiss NVision40 FIB/SEM. The IMD (Institut de la Mobilité Durable – Sustainable Mobility Institute) is also acknowledged for financial support.

Li2NiTiO4: A New Insertion type Pseudocapacitive Electrode for Aqueous Lithium-Ion Capacitors

Prabaharan et al first reported on the possibility of using a rocksalt type Li2NiTiO4 as a positive electrode for Li-Ion batteries and proven that the redox energy for the reaction Ni2+/4+ and Ti4+/3+ would favour facile redox reactions. We present here our first report on the possibility of using Li2NiTiO4 as pseudocapacitive electrode in Li based aqueous electrolyte and demonstrate the electrochemical properties of Li-Ion hybrid capacitor. Nanoparticles of Li2NiTiO4 have been synthesized by adopting a modified solvothermal process. The prepared Li2NiTiO4 nanoparticles were used as electrode material against a pure porous carbon electrode (AC) as Li-Ion capacitor couple, Li2NiTiO4//AC with a potential window of ∼1.6 V in aqueous electrolyte. Specific capacitance of Li2NiTiO4 was measured using a three electrode cell in Li+ alkaline electrolytes against Ag reference electrode. Electrochemical performance of the asymmetric capacitive couple was evaluated through constant current charge/discharge, cyclic voltammetry and ac impedance measurements. A hybrid Li-Ion capacitor Li2NiTiO4//AC was fabricated and studied for its capacitive performance. The electrochemical limitation of the potential window in aqueous medium < 1V is extended up to 1.6V by combining two different materials which predominantly enhances the energy density of the device. More specifically the asymmetric cell exhibits good cycling stability and high rate capability even at high scan rates.

Rock-Salt Growth-Induced (003) Cracking in a Layered Positive Electrode for Li-Ion Batteries

For the first time, (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor for the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode.

Impact of the Surface on the Delithiation Mechanism of LiNi0.8Co0.15Al0.05O2 positive Electrodes for Li-Ion Batteries

The occurrence of an apparent two-phase delithiation mechanism reported for LiNi0.8Co0.15Al0.05O2 (NCA)1 is not intrinsic to the material but is due to a surface layer. Ni-rich layered oxides like NCA are the most important class of positive electrodes for high-energy-density Li-ion batteries. These materials react with H2O and CO2 from air to form surface layers mainly composed of Li2CO3 2–5. The presence of such a surface layer is deleterious to the electrochemical performance of this class of material. We show, using in-situ synchrotron X-ray diffraction measurements, that this surface layer also induces the presence of a second phase during the 1st electrochemical delithiation. Such a mechanism is however not intrinsic to NCA. Indeed, a sample stored and handled in an argon atmosphere presents a typical solid solution behavior, as expected for this class of material. 1. R. Robert, C. Bunzli, E. J. Berg and P. Novák, Chem. Mater., 2015, 27, 526–536. 2. H. Liu, Y. Yang and J. Zhang, J. Power Sources, 2006, 162, 644–650. 3. G. V. Zhuang, G. Chen, J. Shim, X. Song, P. N. Ross and T. J. Richardson, J. Power Sources, 2004, 134, 293–297. 4. H. S. Liu, Z. R. Zhang, Z. L. Gong and Y. Yang, Electrochem. Solid-State Lett., 2004, 7, A190–A193. 5. K. Matsumoto, R. Kuzuo, K. Takeya and A. Yamanaka, J. Power Sources, 1999, 81–82, 558–561. Figure 1

Nanotechnology of Positive Electrodes for Li-Ion Batteries

This work presents the recent progress in nanostructured materials used as positive electrodes in Li-ion batteries (LIBs). Three classes of host lattices for lithium insertion are considered: transition-metal oxides V2O5, α-NaV2O5, α-MnO2, olivine-like LiFePO4, and layered compounds LiNi0.55Co0.45O2, LiNi1/3Mn1/3Co1/3O2 and Li2MnO3. First, a brief description of the preparation methods shows the advantage of a green process, i.e., environmentally friendliness wet chemistry, in which the synthesis route using single and mixed chelators is used. The impact of nanostructure and nano-morphology of cathode material on their electrochemical performance is investigated to determine the synthesis conditions to obtain the best electrochemical performance of LIBs.

Understanding Local Defects in Li-Ion Battery Electrodes through Combined DFT/NMR Studies: Application to LiVPO4F

In a recent study, we showed by solid-state NMR that LiVPO4F, which is a promising material as positive electrode for Li-ion batteries, often exhibits some defects that may affect its electrochemical behavior. In this paper, we use DFT calculations based on the projector augmented-wave (PAW) method in order to model possible defects in this (paramagnetic) material and to compute the Fermi contact shifts expected for Li nuclei located in their proximity. The advantage of the PAW approach versus FP-LAPW we have been previously using is that it allows considering large supercells suitable to model a diluted defect. In the first part of this paper, we aim to validate the Fermi contact shifts calculation using the PAW approach within the VASP code. Then we apply this strategy for modeling possible defects in LiVPO4F. By analogy with the already existing homeotypic LiVOPO4 phase, we first replace one fluoride ion, along the VO2F4 chains, by an oxygen one and consider, in a second step, an association with a lit...

Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A 7Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors

We present a 7Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO3 cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume ∼20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-li...

Flame-made Lithium Transition Metal Orthosilicates

Li2MSiO4 (M=Fe, Mn, Co) compounds have since their discovery gained increased attention as alternative, inexpensive and inherently safe positive electrodes for Li-ion batteries. To meet the required performance for an electrode, sophisticated, complex and time-consuming synthesis measures are required at present. Here, we present a time-efficient and scalable aerosol combustion method with subsequent annealing, leading to nanoscale and carbon-coated Li2FeSiO4 and Li2Fe0.5Mn0.5SiO4. Using liquid-feed flame spray pyrolysis, we demonstrate synthesis of orthosilicate materials, with phase purities exceeding 95wt. % according to Rietveld quantifications, in a relatively short time. The importance of the precursor concentration, in order to obtain loosely agglomerated nanoparticles, is discussed and the long-term performance is investigated. In the case of Li2FeSiO4, the optimised precursor concentration yielded particles of about 30nm, which delivered an initial discharge capacity of up to 150mAhg−1 at 60°C and C/20. Furthermore, over 50% of the capacity is retained at a high rate of 5C, and long-term cycling showed outstanding capacity retention of over 90% after 300 cycles at a moderate rate of C/2. Li2Fe0.5Mn0.5SiO4 on the other hand, was shown to suffer from a severe capacity fade, and upon prolonged cycling the redox activity can be attributed solely to Fe.

An Active Li-Mn-O Compound for High Energy Density Li-Ion Batteries

Due to their low weight, high energy densities and long cycle life, the battery scientific community is still trying to develop new high capacity cathodes materials in order to increase the energy density of storage devices [1]. After a composition screening, a new material family has been discovered in the Li-Mn-O system and has been investigated as potential new material for Li-ion batteries to replace conventional NMC materials (LiNi1/3Mn1/3Co1/3O2) [2]. In this study, we report for the first time the synthesis, structural and electrochemical characterizations of a new non-lamellar oxide with the highest capacity observed ever before in the Li-Mn-O system. This new patented [3] material with the composition of Li4Mn2O5 shows a discharge capacity of 300 mAh/g. It is a rock-salt type nanostructured material, prepared by a direct mechanochemical synthesis. The typical crystallite size is about 5-10nm (from Transmission electron microscopy TEM and electron diffraction ED). The ring ED pattern of this individual phase can be indexed on the cubic Fm-3m structure, with a rock-salt cell parameter of a=4.16Å, according to Rietveld refinement. The electrochemical performances were performed at a C/20 rate, and show a reversible discharge capacity about 300 mAh/g. A gradual increase in the cut-off voltage up to 4.8V activates the material and prevents the electrolyte degradations. During cycling, the derivative curves indicate at least 2 active redox couples [4]: Mn3+/ Mn4+ (around 3.3V), Mn4+/ Mn5+ and/or O2-/ O- (around 4.1V). In order to have a better insight of lithium deinsertion process, we used magnetic measurements at various charge states. We will show that magnetic measurements are perfect tools to access to the oxidation level of the material, and in agreement with the values obtained after iodometric redox titrations. We will namely show that this material is oxidized up to Mn5+oxidation state with a competition of the oxygen oxidation [4]. In short, in this work are reported the first results of a new electrochemically active compound as positive electrode for Li-ion batteries, with the active participation of both redox centers: Manganese and Oxygen [5]. [1] B.L. Ellis, et al., Chem. Mater., 2010, 22, 691 [2] H. Koga, et al., J. Electrochem. Soc., 2013, 160, A786 [3] Patent FR1258625 [4] M. Sathiya, et al., Nature Mat., 2013 , 12, 827 [5] M. Freire et al., Nature Mat. Letters, 2015, 14, DOI : 10.1038/NMAT4479

Characterization of Electronic and Ionic Transport in Li1-xNi0.33Mn0.33Co0.33O2 (NMC333) and Li1-xNi0.50Mn0.20Co0.30O2 (NMC523) as a Function of Li Content

Despite the extensive commercial use of Li1-xNi1-y-zMnzCoyO2 (NMC) as the positive electrode in Li-ion batteries, and its long research history, its fundamental transport properties are poorly understood. These properties are crucial for designing high energy density and high power Li-ion batteries. Here, the transport properties of NMC333 and NMC523 are investigated using impedance spectroscopy and DC polarization and depolarization techniques. The electronic conductivity is found to increase with decreasing Li-content (increasing state-of-charge) from ∼10−7 Scm−1 to ∼10−2 Scm−1 over Li concentrations x = 0.00 to 0.75, corresponding to an upper charge voltage of 4.8 V with respect to Li/Li+. The lithium ion diffusivity is at least one order of magnitude lower, and decreases with increasing x to at x = ∼0.5. The ionic conductivity and diffusivity obtained from the two measurements techniques (EIS and DC) are in good agreement, and chemical diffusion is limited by lithium transport over a wide state-of-charge range.

Electron Beam Curing of Composite Positive Electrode for Li-Ion Battery

Electron beam cured acrylated polyurethanes have been successfully used as novel binders for positive electrodes for Li-ion batteries. The cross-linked polymer after electron beam curing coheres active materials and carbon black together onto Al foil. Electrochemical tests demonstrate the stability of the polymer at a potential window of 2.0 V–4.6 V. The electrode is found to have similar voltage profiles and charge-transfer resistance compared to the conventional electrode using polyvinylidene fluoride as the binder. When the electrode is tested in full Li-ion cells, they exhibit excellent cycling performance, indicating promising use for this new type of binder in commercial Li-ion batteries in the future.

Chemical Delithiation Investigation of Olivine LiFePO4

LiFePO4 is currently one of the most attractive materials for positive electrode in Li-ion batteries due to its low ecological print and for safety issues. Currently, sodium-ion batteries show a great interest and LiFePO4 tends to be replaced by NaFePO4 for cost advantages. As the direct synthesis of NaFePO4 leads to the maricite, phase poorly electrochemically active, the only way to keep the olivine framework is to chemically delithiated LiFePO4. Mechanisms of delithiation of LiFePO4 have been previously studied to determine the model of Li ions deintercalation: Core-Shell [1-2] or Domino-Cascade [3]. Recent work [4] shows a Domino-Cascade mechanism in the case of electrochemical delithiation while the chemical delithiation will correspond to a Core-Shell process. However, the chemical delithiation have been investigated using mainly strong oxidizing agents like NO2BF4 or K2S2O8which create some defects due to the fast and violent reaction [1]. So it is interesting to understand the chemical delithiation mechanisms versus microstructure of the powders and the delithiation conditions. LiFePO4was synthesized by two different routes, hydrothermal (LFP-H) and precipitation (LFP-P). The synthesis process and the phosphate sources strongly impact the purity and the microstructure of the powders. LFP-H presents 2 types of particles, nano-rods and platelets (Fig. 1a) while LFP-P are constituted of agglomerated nano-spheroids with grain boundaries (Fig. 1b). The kinetic of the chemical delithiation was studied on these LiFePO4 samples by testing different delithiation conditions: oxidizing agents (I2, Br2 and NO2BF4), solvents (acetonitrile and chloroform) and reaction time (from 15 min to 96 h). The LiFePO4/FePO4 ratio was determined by Rietveld refinement and by elemental nuclear analysis to quantify precisely the very low remaining lithium amount after the complete delithiation. The LiFePO4morphology has a large impact on the chemical delithiation kinetic : LFP-H platelets are fully delithiated in less than one hour whereas LFP-P spheroids need a hundred of hours. The HR-TEM and STEM-EELS measurements on partially delithiated samples (Fig. 2) show for the first time a shrinking core mechanism with a core of LiFePO4 surrounded by a shell of FePO4. In particular the role of the grain boundaries in between the grains is shown in the mechanism of de chemical delithiation. Finally, FePO4phases were tested as positive electrode material in Li-ion and Na-ion batteries. The electrochemical intercalation/deintercalation is interpreted in the perspective of the previous chemical delihiation. References : [1] L. Laffont, C. Delacourt, P. Gibot, M. Y. Wu, P. Kooyman, C. Masquelier, J.M. Tarascon, Study of the LiFePO4/FePO4two-phase system by High Resolution Electron Energy Loss Spectroscopy, Chem. Mater. 18 (2006) 5520 [2] G. Chen, X. Song, T.J. Richardson, Electron Microscopy Study of the LiFePO4to FePO4 Phase Transition, J. Electrochem. Solid-State Lett., 9 (2006) A295-A298 [3] C. Delmas, M. Maccario, L. Croguennec, F. Le Cras, F. Weill, Lithium deintercalation in LiFePO4nanoparticles via a domino-cascade model, Nat. Mater. 7(2008) 665. [4] G. Brunetti, D. Robert, P. Bayle-Guillemaud ; J. L. Rouvières, E. F. Rauch, J. F. Martin, J. F. Colin, F. Bertin, C. Cayron, Confirmation of the Domino-Cascade Model by LiFePO4/FePO4 Precession Electron Diffraction, Chem. Mater. 23 (2011) 4515-4524. Figure 1

Characterization of Electronic and Ionic Transport in Li1-xNi0.8Co0.15Al0.05O2 (NCA)

Despite the extensive commercial use of Li1-xNi0.8Co0.15Al0.05O2 (NCA) as the positive electrode in Li-ion batteries, and its long research history, its fundamental transport properties are poorly understood. These properties are crucial for designing high energy density and high power Li-ion batteries. Here, the transport properties of NCA are investigated using impedance spectroscopy and dc polarization and depolarization techniques. The electronic conductivity is found to increase with decreasing Li-content from ∼10−4 Scm−1 to ∼10−2 Scm−1 over x = 0.0 to 0.6, while lithium ion conductivity is at least five orders of magnitude lower for x = 0.0 to 0.75. A surprising result is that the lithium ionic diffusivity vs. x shows a v-shaped curve with a minimum at x = 0.5, while the unit cell parameters show the opposite trend. This suggests that cation ordering has greater influence on the composition dependence than the Li layer separation, unlike other layered oxides. From temperature-dependent measurements in electron-blocking cells, the activation energy for lithium ion conductivity (diffusivity) is found to be 1.25 eV (1.20 eV). Chemical diffusion during electrochemical use is limited by lithium transport, but is fast enough over the entire state-of-charge range to allow charge/discharge of micron-scale particles at practical C-rates.

Influence of Local Structure on the Structural and Electronic Mechanisms Occurring in Li-Rich Layered Oxides Li1+XM1-XO2 during Cycling

Li-rich layered oxides Li1+xM1-xO2 with M = Mn, Co and Ni may be used as positive electrode in Li-ion batteries. Mechanisms at the origin of the large capacity exhibited by these Li-excess compounds compared to that obtained for stoichiometric ones (300 mAh/g vs 200 mAh/g, respectively) have already been extensively described in literature [1-2]. These materials can be viewed as an intergrowth between [LiO6]∞ and [MO6]∞ octahedral layers. Overlithiation induces a Li2MnO3-type order into [MO6]∞slabs which facilitates oxygen ions oxidation at high potential (>4.4 V), a phenomenon responsible for the extra-capacity observed.The average structure of these compounds is well-known but the complexity of the local structure needs further understanding. In fact, the local environments govern activation barriers and can affect notably Li ion transport. At the nanoscale, Li1+xM1-xO2 can be described as LiMO2-Li2MnO3nanocomposites with a complex microstructure which depends on the nominal composition. In addition, structural changes and redox processes occurring during cycling depend not only on the initial local structure but also on the cycling conditions (rate and temperature). Thus, a better understanding of the relationship between local structure and electrochemical properties is needed for improving performances of these materials.In this work two electrodes with different starting microstructures and activated up to 4.6 V at two different temperatures were studied. The electrochemical phenomena observed for each compound during galvanostatic cycling were found to be quite different, especially in discharge.Structural and redox mechanisms taking place during cycling were studied in detail and compared. In order to follow changes in electronic and crystal structures under battery operation, in-situ time-resolved experiments were performed at the synchrotron SOLEIL using x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) in operando mode. The evolution of microstructure (bulk and surface) during cycling was also monitored by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and x-ray photoelectron spectroscopy (XPS) in ex-situmode.Li2MnO3-type order, probably responsible for extra-capacity in these materials, is detected in both starting compounds; however its evolution is found to be highly dependent on the cycling conditions. During charge, there is a progressive attenuation of the Li2MnO3-type order at high potential, even disappearing at the end of the first charge under severe activation conditions. Many stacking faults were observed by HRTEM, especially after activation at high temperature. In addition, spinel-type defects are visible at the edges of crystals (see figure) and are probably the cause of the voltage decrease observed during cycling. Indeed, redox reactions associated with lithium ion insertion into “special” sites, like spinel defects, may require to overcome a larger energy barrier. The analysis of the surface composition by XPS shows a surface enrichment in manganese which is in good agreement with the formation of spinel-type defects (LiMn2O4 vs. LiMO2). Furthermore, we also monitored the effect of temperature and microstructure on the redox mechanisms involved during cycling, in ex-situ and operando mode. The evolutions of Mn-K, Co-K and Ni-K edges were studied by XAS. While the activation temperature has an influence on the Co-K and Ni-K edges, the influence of the microstructure is only noticeable on the Mn K-edge. Finally, in order to get information on redox processes at the nanoscale, O-K and Mn-L2,3 edges were studied by EELS at different states of charge. Modifications of the fine structures of these two edges were observed during the entire activation cycle and quantitative information was deduced from L3/L2ratios analyze.All these results will be discussed in detail.

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.

One-pot synthesis of LiFePO4–carbon mesoporous composites for Li-ion batteries

The synthesis of LiFePO4–carbon mesoporous composites by a one-pot soft-template approach is reported along with their performances as positive electrodes for Li-ion batteries. The synthesis is based on the multi-constituent co-assembly of triblock copolymer, resol and (Li, Fe, P) precursors followed by thermal treatment to in-situ generate the carbon matrix with embedded LiFePO4 particles. The effects of the Fe/Li precursor type and the thermal annealing temperature on the phase formation, mesostructural regularity, porosity and Fe oxidation state were systematically investigated by a large panel of analysis techniques. This new synthesis method allowed the successful synthesis of mesoporous LiFePO4–carbon composites with high surface area, defined pore size and small homogenously distributed LiFePO4 particles (20–50nm). These composites can be used as electrodes without extra carbon black additive and demonstrate high rate capability (up to 10C) with a significant capacity contribution from carbon.

Substitution Chemistry of Li2MSiO4: A Highly Flexible Framework

Lithium transition metal silicates, Li2MSiO4(M=Fe, Mn, Co)[1], have attracted a lot of interest recently as positive electrodes for Li-ion batteries due to their potentially high theoretical capacities and to their rich crystal chemistry as they undergo complex polymorphism. One of the drawbacks of the silicates is their modest rate capability, particularly at room temperature. The present work addressed this problem.The polymorphs of Li2MSiO4can be classified into low (β) and high (γ) temperature forms, which differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. β forms exhibit only corner sharing and γ polymorphs both corner and edge sharing. It has been shown by us and by others that Li2FeSiO4 exists in at least 3 different polymorphic forms and that. in the case of Li2MnSiO4two stable polymorphs are observed [1-3].The silicates are analogous to the LISICON materials (Lithium SuperIonic Conductors), of the form Li2+2xZn1-xGeO4, which adopt the γ structure with interstitial and highly mobile lithium. Therefore, stabilization of the γ polymorph to room temperature represents a potentially attractive route to enhancing the rate performance thus overcoming one of the major barriers to adoption of Li2MSiO4 cathodes. With appropriate substitutions we have been able to stabilise the γ polymorph for both Li2FeSiO4 and Li2MnSiO4systems. We will report the effects on structure and electrochemistry of the various substitutions. In the iron system good cycling stability and improved rate capability are observed (fig. 2).

Li9Mg3[PO4]4F3 and Li2Mg[PO4]F: Crystal Structures and Ionic Conductivities

In oxide electrolytes, crystalline phosphate with NASICON (NaSuper Ionic Conductor)-type structures such as Li1+xAlxTi2 −x(PO4)3 (LATP) and Li1+xAlxGe2 −x(PO4)3 (LAGP) are known as excellent Li ion conductors. Perovskite Li0.5 −3xLa0.5+xTiO3 (LLT) exhibits the high level of bulk conductivity of 10−3 S cm−1, but LATP and LLT electrolytes have Ti element in crystalline structure, and thus transition metal such as Ti is easily reduced by lithium metal negative electrode; it is thus difficult for these electrolytes to use in lithium metal batteries.Recently, we started the systematic study of crystal structure and the electrochemical properties of fluorophosphate phases which do not contain transition metal elements. Indeed, in the ternary phase diagram LiMgPO4-LiF-Li3PO4, two new lithium rich phases have been discovered (Li2Mg[PO4]F and Li9Mg3[PO4]4F3). Li2Mg[PO4]F crystallizes with the well-known Li2Ni[PO4]F-type structure, whereas the Li9Mg3[PO4]4F3 crystallizes with a new-type of structure. Li9Mg3[PO4]4F3 exhibits an ionic conductivity of 10-4 S cm-1 at 300 °C, which is much higher than that of Li3VO4 (σ 300 °C =1.6x10-6 S cm-1), γ-Li3PO4 (σ 300 °C =2.2 ´ 10-8 S cm-1) or Li2SO4 (σ 300 °C =2 ´ 10-7 S cm-1). This new phase would be also of great interest as positive electrode for Li-ion batteries if Mg2+ could be replaced by Fe2+ or Mn2+cations [1].[1] Crystal structures of the new fluorophosphates Li9Mg3[PO4]4F3 and Li2Mg[PO4]F and ionic conductivities of selected compositions, H. Ben Yahia, M. Shikano, T. Takeuchi, H. Kobayashi, M. Itoh, J. Mater. Chem. A (2014)DOI:10.1039/C3TA15264B.

Chemically engineered graphene oxide as high performance cathode materials for Li-ion batteries

The development of environment-friendly electrode materials is highly desired for the clean and sustainable Li-ion batteries (LIBs) system. Organic cathode materials that involve conducting polymers, organic carbonyl/sulfur compounds are expected to be promising candidates for future LIBs with a concept of “green and sustainable”. However, their battery performances are relatively worse than that of inorganic counterparts due to their low electronic conductivity and unwanted dissolution reactions occurring in electrolytes. Aimed to alter their performances, we herein focuses on the preparation of upgraded organic materials by chemical engineering of graphene oxide (GO) and the systematic study of their electrochemical performance as positive electrodes for LIBs. The obtained decarboxylated GO and carbonylated/hydroxylated GO electrodes show significantly enhanced electrochemical performance compared with that of the GO electrode. Our results demonstrate that the manipulation of oxygen functional groups on GO is an effective strategy to greatly improve the Li storage property of GO-based materials for advanced LIBs cathodes.

Effect of cation and anion doping on microstructure and electrochemical properties of the LiMn1.5Ni0.5O4−δ spinel

A detailed investigation of the structural and electrochemical impact of lithium and fluorine substitutions on the 4.7 V LiMn1.5Ni0.5O4−δ (LMNO) positive electrode for Li-ion batteries was accomplished. Impact of subsequent transition metal substitution on the tetrahedral 8a sites was also investigated relative to rate and elevated temperature cycling stability. Significantly improved cycling in excess of 600 cycles was established for an anion modified spinel relative to an optimized benchmark material at 60 °C.