Li4Ti5O12 Surface Research Articles

Effective Upcycling of Degraded NCM Cathode Materials Assisted by Surface Engineering for High‐Performance Lithium‐Ion Batteries

AbstractLithium‐ion batteries (LIBs) with ternary oxide cathode materials are the prevalent energy storage devices for electric vehicles, and the huge amounts of spent LIBs pose severe challenges in terms of environmental impact and resource management. Particularly, the proper handling of degraded cathode materials is of central importance for the sustainable and closed‐loop development of LIBs industry. In this context, direct regeneration of degraded ternary oxides toward reusable high‐performance cathode materials is environmentally and economically favorable in contrast to present metallurgical recycling methods. In this work, a simple and effective two‐step method is demonstrated to regenerate the degraded NCM 622 (LiNi0.6Co0.2Mn0.2O2) materials by elemental compensation and structural restoration. Moreover, a multi‐functional LTO (Li4Ti5O12) surface coating is simultaneously designed to guarantee rapid Li+ diffusion and stable surface of the regenerated product. Therefore, the regenerated LTO‐coated NCM materials show excellent electrochemical performance; specifically, the initial discharge capacity (183.0 mAh g−1 at 0.1 C), rate capability (90.0 mAh g−1 at 10 C), and cycling stability (79.3% capacity retention after 200 cycles) are even comparable with those of fresh materials. The as‐established upcycling strategy may shed light on the value‐added recycling of degraded cathode materials and thereby a virtuous cycle of LIBs.

Electrochemical impedance analysis of the charge transfer mechanism on Li4Ti5O12 surface in the presence or absence of the solid/electrolyte interface film

A novel three-electrode type cell is developed for highly reliable electrochemical impedance spectroscopy (EIS). A pair of Li4Ti5O12 (LTO) electrodes are placed at both end of the cell, and a lithium counter electrode is inserted between them to assemble a three-electrode-type LTO half-cell. The half-cell is initially discharged to either 0.3 or 1.4 V to prepare the pair of LTO electrodes with or without a solid/electrolyte interface (SEI) film, respectively. The half-cells are reassembled to LTO symmetric cells by removing the central Li electrode without damaging the LTO electrodes. EIS measurements of the symmetric cell indicate impedances associated with charge transfer (RCT) and the SEI film (RSEI) in distinctly different frequency ranges. The temperature dependences of RCT and RSEI are analyzed to clearly understand the influence of the SEI film on the charge transfer mechanism and to explicitly support the common understanding that lithium ions are desolvated at the film/electrolyte interface or inside the film through a different consideration than the conventional one.

The effects of fluorine doping on the structure, interface and electrochemical properties of lithium titanate

Enhancing the electronic and ionic conductivity of Li4Ti5O12 electrode materials, as well as suppressing the interface reaction between Ti4+ on the surface of Li4Ti5O12 and the electrolyte, is one of the key factors in preparing high-power and long-life Li4Ti5O12 lithium-ion batteries. To tackle these challenges, fluorine ions with strong electronegativity was chosen as dopants to engineer fluorine-doped Li4Ti5O12 electrode materials. Fluorine ion doping changes the surface state of Li4Ti5O12, increases the interface compatibility, suppresses the reactivity between Li4Ti5O12 and the electrolyte, and forms a uniform SEI film during cycling. Moreover, fluorine ion doping induces the generation of oxygen vacancies and improves the crystallinity of Li4Ti5O12, thereby fostering improved electronic and ionic transport kinetics. The above appealing features enables the prepared fluorine-doped Li4Ti5O12 material demonstrates exceptional high-rate performance, delivering specific capacities of 175 mAh·g−1/0.5C, 159 mAh·g−1/10C, and 138 mAh·g−1/50C. In essence, the fluoride ion doping strategy holds profound implications for enhancing electrode interface characteristics, transport kinetics, and the fabrication of high-power lithium-ion batteries.

Three-dimensional printed Li4Ti5O12@VSe2 composites as high-performance anode material in full 3D-printed lithium-ion batteries with three-dimensional -printed LiFePO4@AC/rGO cathode

Currently, the development of high-performance lithium-ion batteries (LIBs) with improved areal capacity is still challenging. The common strategy to improve areal capacity is to increase the thickness of electrode materials. However, the application of thick electrodes remains challenge due to poor mechanical properties, slow charge and ion transport, and poor electrolyte infiltration. In this study, thick electrodes are constructed by 3D printing a Li4Ti5O12@VSe2-based ink. The highly electrical conductor VSe2 on the Li4Ti5O12 surface improves the ion and charge transport and eases the internal resistance of the 3D-printed electrode during charge and discharge. As a result, LIBs employing these thick electrodes show a high-rate capability of 128.9 mAh/g at 10 C, improved areal capacities up to 6.2 mAh/cm2, and ultrastable cycling capability (84.5% of capacity retention after 1700 cycles). Moreover, full cells utilizing 3D-Li4Ti5O12@VSe2 as the anode and 3D-LiFePO4@AC/rGO as the cathode with 1.81 V potential yield a high specific capacity of 152.5 mAh/g at 0.1C, a high specific energy density of 276.025 Wh/kg, and a power density of 30.5 W/kg at 0.1C. This work paves the way to designing thick electrodes for high-performance LIBs.

Mechanistic understanding of aging behaviors of critical-material-free Li4Ti5O12//LiNi0.9Mn0.1O2 cells with fluorinated carbonate-based electrolytes for safe energy storage with ultra-long life span

Behind-the-meter storage (BTMS) systems—a viable method to minimize potential risk of blackout events and stabilize the grid—require a different type of cost-effective energy storage with excellent safety, ultra-long (>20 years) cycle life and reasonable energy density compared that of electric vehicles. To increase the energy density and reduce the cost of a long-term cyclable lithium-titanate-based cell, it is required to employ a critical-material-free high voltage cathode and an electrolyte with good electrochemical and transport properties. Here, the long-term electrochemical performance and behaviors of selected critical-material-free Li4Ti5O12 (LTO)//LiNi0.9Mn0.1O2 (LNMO) full cells for BTMS applications are evaluated and analyzed in the optimized voltage range of 1.4–2.7 V at 45 °C with different fluorinated carbonate-based electrolytes. The fluoroethylene carbonate (FEC)-based electrolyte cell shows the highest capacity retention of 57.9 % and Coulombic efficiency (CE) of 99.96 % after 1000 cycles, potentially attributed to a dense, homogenous and less resistive LiF-rich solid-electrolyte interphase (SEI) layer formed on the surface of LTO that may mitigate electrolyte decomposition and maintain relatively low cell impedance during cycling. The 3,3,3-fluoroethylmethyl carbonate (F-EMC)-based electrolyte cell, however, presents the worst performance with lower capacity and a sharp decrease of CE, due to unstable and non-uniform SEI formation and continuous oxidative electrolyte decomposition. This mechanistic understanding of cell aging behaviors and failure mechanisms with detailed analysis of surface chemistry and electrode morphology can guide design of new electrode chemistries and electrolyte formulations for the development of BTMS batteries.

Li4Ti5O12 Coated by Biomass-Derived Carbon Quantum Dots as Anode Material with Enhanced Electrochemical Performance for Lithium-Ion Batteries

Li4Ti5O12 (LTO) is a promising anode material for lithium-ion batteries (LIBs) due to its stable reversibility, high-rate cyclability, and high operational potential. On the other hand, it suffers from poor electronic conductivity and low capacitance. To overcome these disadvantages, modification of the LTO surface is frequently undertaken. Considering this idea, the production of a biomass-derived carbon-coated LTO material (LTO/C) and its application as an anode in LIBs is described in this work. The carbon precursor was obtained from commercial carrot juice, which was degraded using microwaves. According to the UV studies, the carbon precursor revealed similar properties to carbon quantum dots. Then, it was deposited on LTO synthetized through a sol-gel method. The LTO/C electrode exhibited a high specific capacity of 211 mAhg−1 at 0.1 C. Capacity retention equal to 53% of the initial value was found for the charge–discharge rate increase from 0.1 C to 20 C. The excellent electrochemical performance of LTO/C was caused by the carbon coating, which provided (i) short diffusion pathways for the Li+ ions into the LTO structure and (ii) enhanced electronic conductivity. The obtained results indicated that biomass-derived carbon quantum dot-coated LTO can be considered as a promising anode for LIBs.

Mesoporous/Microporous Li4Ti5O12/rutile-TiO2 as an anode material for lithium-ion batteries synthesized by the sol-gel method

In this work, we presented a modified sol-gel method to synthesize the nanocrystalline composite composed of Li4Ti5O12 (LTO) and rutile-TiO2, and utilized it as an efficient anode material for lithium-ion battery application. The detailed structural studies revealed that the spherical shaped rutile nanoparticles of ∼10–20 nm were homogeneously distributed on the surface of LTO and therefore the mesoporous/microporous nature of the two-phase Li4Ti5O12-TiO2 (LTO-T) was exhibited. According to a series of galvanostatic charge/discharge tests, the LTO-T electrode with the mesoporous/microporous microstructure demonstrated the improved electrochemical performance, especially at a high C rate. Interestingly, the LTO-T electrode achieved an excellent specific capacity of 175 mAh g−1 and 155 mAh g−1 even after 50 charge/discharge cycles at 1C and 5C, respectively.

Application of Poly-L-Lysine for Tailoring Graphene Oxide Mediated Contact Formation between Lithium Titanium Oxide LTO Surfaces for Batteries.

When producing stable electrodes, polymeric binders are highly functional materials that are effective in dispersing lithium-based oxides such as Li4Ti5O12 (LTO) and carbon-based materials and establishing the conductivity of the multiphase composites. Nowadays, binders such as polyvinylidene fluoride (PVDF) are used, requiring dedicated recycling strategies due to their low biodegradability and use of toxic solvents to dissolve it. Better structuring of the carbon layers and a low amount of binder could reduce the number of inactive materials in the electrode. In this study, we use computational and experimental methods to explore the use of the poly amino acid poly-L-lysine (PLL) as a novel biodegradable binder that is placed directly between nanostructured LTO and reduced graphene oxide. Density functional theory (DFT) calculations allowed us to determine that the (111) surface is the most stable LTO surface exposed to lysine. We performed Kubo–Greenwood electrical conductivity (KGEC) calculations to determine the electrical conductivity values for the hybrid LTO–lysine–rGO system. We found that the presence of the lysine-based binder at the interface increased the conductivity of the interface by four-fold relative to LTO–rGO in a lysine monolayer configuration, while two-stack lysine molecules resulted in 0.3-fold (in the plane orientation) and 0.26-fold (out of plane orientation) increases. These outcomes suggest that monolayers of lysine would specifically favor the conductivity. Experimentally, the assembly of graphene oxide on poly-L-lysine-TiO2 with sputter-deposited titania as a smooth and hydrophilic model substrate was investigated using a layer-by-layer (LBL) approach to realize the required composite morphology. Characterization techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), scanning electron microscopy (SEM) were used to characterize the formed layers. Our experimental results show that thin layers of rGO were assembled on the TiO2 using PLL. Furthermore, the PLL adsorbates decrease the work function difference between the rGO- and the non-rGO-coated surface and increased the specific discharge capacity of the LTO–rGO composite material. Further experimental studies are necessary to determine the influence of the PLL for aspects such as the solid electrolyte interface, dendrite formation, and crack formation.

Terminal sulfur atoms formation via defect engineering strategy to promote the conversion of lithium polysulfides

The defect engineering shows great potential in boosting the conversion of lithium polysulfides intermediates for high energy density lithium-sulfur batteries (LSBs), yet the catalytic mechanisms remain unclear. Herein, the oxygen-defective Li4Ti5O12-x hollow microspheres uniformly encapsulated by N-doped carbon layer ([email protected]) is delicately designed as an intrinsically polar inorganic sulfur host for the research on the catalytic mechanism. Theoretical simulations have demonstrated that the existence of oxygen deficiencies enhances the adsorption capability of spinel Li4Ti5O12 towards soluble lithium polysulfides. Some -S-S- bonds of the Li2S6 on the defective Li4Ti5O12 surface are fractured by the strong adsorption force, which allows the inert bridging sulfur atoms to be converted into the susceptible terminal sulfur atoms, and reduces the activation energy of the polysulfide conversion in some degree. In addition, with the N-doped carbon layer, secondary hollow microspheres architecture built with primary ultrathin nanosheets provide a large amount of void space and active sites for sulfur storage, adsorption and conversion. The as-designed sulfur host exhibits a remarkable rate capability of 547 mAh g−1 at 4 C (1 C=1675 mA g−1) and an outstanding long-term cyclability (519 mAh g−1 after 1000 cycles at 3 C). Besides, a high specific capacity of 832 mAh g−1 is delivered even after 100 cycles under a high sulfur mass loading of 3.2 mg cm−2, indicating its superior electrochemical performances. This work not only provides a strong proof for the application of oxygen defect in the adsorption and catalytic conversion of lithium polysulfides, but offers a promising avenue to achieve high performance LSBs with the material design concept of incorporating oxygen-deficient spinel structure with hierarchical hollow frameworks.

Direct Double Coating of Carbon and Nitrogen on Fluoride-Doped Li4Ti5O12 as an Anode for Lithium-Ion Batteries

Graphite as a commercial anode for lithium-ion batteries has significant safety concerns owing to lithium dendrite growth at low operating voltages. Li4Ti5O12 is a potential candidate to replace graphite as the next-generation anode of lithium-ion batteries. In this work, fluoride-doped Li4Ti5O12 was successfully synthesized with a direct double coating of carbon and nitrogen using a solid-state method followed by the pyrolysis process of polyaniline. X-ray diffraction (XRD) results show that the addition of fluoride is successfully doped to the spinel-type structure of Li4Ti5O12 without any impurities being detected. The carbon and nitrogen coating are distributed on the surface of Li4Ti5O12 particles, as shown in the Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy (SEM-EDS) image. The Transmission Electron Microscopy (TEM) image shows a thin layer of carbon coating on the Li4Ti5O12 surface. The fluoride-doped Li4Ti5O12 has the highest specific discharge capacity of 165.38 mAh g−1 at 0.5 C and capacity fading of 93.51% after 150 cycles compared to other samples, indicating improved electrochemical performance. This is attributed to the synergy between the appropriate amount of carbon and nitrogen coating, which induced a high mobility of electrons and larger crystallite size due to the insertion of fluoride to the spinel-type structure of Li4Ti5O12, enhancing lithium-ion transfer during the insertion/extraction process.

Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li+, Na+, and Mg2+ with PF6 - on the Li4Ti5O12 (111) Surface.

Spinel lithium titanate, Li4Ti5O12 (LTO), emerges as a “universal” electrode material for Li-ion batteries and hybrid Li/Na-, Li/Mg-, and Na/Mg-ion batteries functioning on the basis of intercalation. Given that LTO operates in a variety of electrolyte solutions, the main challenge is to understand the reactivity of the LTO surface toward single- and dual-cation electrolytes at the molecular level. This study first reports results on ion desolvation and electrolyte solvent/salt degradation on an LTO surface by means of periodic DFT calculations. The desolvation stages are modeled by the adsorption of mono- and binuclear complexes of Li+, Na+, and Mg2+ with a limited number of ethylene carbonate (EC) solvent molecules on the oxygen-terminated LTO (111) surface, taking into account the presence of a PF6– counterion. Alongside cation adsorption, several degradation reactions are discussed: surface-catalyzed dehydrogenation of EC molecules, simultaneous dehydrogenation and fluorination of EC, and Mg2+-induced decay of PF6– to PF5 and F–. Data analysis allows the rationalization of existing experimentally established phenomena such as gassing and fluoride deposition. Among the three investigated cations, Mg2+ is adsorbed most tightly and is predicted to form a thicker fluoride-containing film on the LTO surface. Gassing, characteristic for carbonate-based electrolytes with LTO electrodes, is foreseen to be suppressed in dual-cation batteries. The latter bears promise to outperform the single-ion ones in terms of durability and safety.

Behavior Investigation of Oxide Electrodes for Li Ion Batteries Viewing from Charge Compensation

Charge compensation resulted defect reactions have played important role on affecting electrical and structural properties of oxides. Depending upon the types of ions available, ionic defects and/or electronic defects are created. For instance, ionic defects tend to enhance the migration of cations/anions. Electronic defects including electrons or hole defects may enhance the electronic conductivity of oxides. For Li batteries, the lithiation of transition metal oxides may be viewed as substitution of Li for transition metal ions. As a result, the lithiation induces the oxidation of Ni, Co ions. Further lithiation may also cause the structural transformation from rock-salt oxide to layered oxides. During a charging/delithiation process at cathode, electron extraction and Li removal from the Li sublattice may be viewed as the creation of electronic/ionic defects. On the contrary, opposite defect reactions including electron injection (or reduction) and lithiation may be observed from the anode. For a high-capacity cathode, Li-rich layered oxides formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) show a unique charging plateau around 4.5V. The extraction of electrons is given by rearranged oxygen ions during 1st charging process. In the following discharging as seen in Fig. 1, reduction of Mn+4 from accepting e’ gave a reversible capacity as high as 250 mAh/g. Interestingly, a backward defect reaction was observed in Li4Ti5O12 (LTO) spinel anode during the charging/lithiation process. A self-improving behavior may be explained through accompanying defect reaction. During low-pO2 synthesis, lattice oxygen near surface of LTO tends to be removed with the introduction of Ti+3. As a result, the electron conduction and electrochemical properties of LTO were significantly improved. Finally, the structure transformation of cycled Li-rich oxides from Layered to Spinel structure may be illustrated due to the formation high concentration electronic defects.

Detection of a real heterogeneous catalyst with an inactive oxygen-covered surface: Au/Li4Ti5O12

In the investigation of the catalytic mechanism, O-covered transition-metal oxide surfaces with concealed transition-metal cations are occasionally employed as model catalyst support structures. These are the most extreme models for investigating the importance of O species on the support surface. There are few cases in which a catalytic metal is supported on O-covered surfaces, and its catalytic activity is confirmed. In this study, we confirmed that the most exposed surface of Li4Ti5O12 is the O-covered (Ti-sublayer) (111) surface, and we prepared and evaluated a real catalyst using Li4Ti5O12 as the support. Au nanoparticles were used as the catalytic metal, and the interface orientation was determined to be Au(111)//Li4Ti4O12(111). Furthermore, the interface comprising the inactive O-covered (Ti-sublayer) surfaces of transition-metal oxides exhibited catalytic activity. These are important for examining the catalytic mechanism using the surface O species of the support by theoretical calculations, thereby bridging the gap between theory and experiments.

Dual-cation electrolytes for low H2 gas generation in Li4Ti5O12//AC hybrid capacitor system

H2 gas generation with associated swelling using Li4Ti5O12 (LTO)-based hybrid capacitors upon high-voltage operation over 3.0 V has been a major challenge owing to catalytic reductive decomposition of solvent such as propylene carbonate (PC) at the Lewis-acid sites on the LTO surface. Herein, we report a method to suppress gassing behavior by adding spiro(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF4) to the lithium-based electrolyte [1 M lithium tetrafluoroborate (LiBF4) in PC]. The dual-cation (Li+/SBP+) electrolyte, which has previously been reported as the electrolyte composition used for rate-capability enhancement, reduced 35–50% volume of generated H2 compared to that for the single-cation (Li+) electrolyte under high-voltage floating conditions (3.5 V, 60 °C). This behavior was attributed to the LTO surface passivated by the Lewis-based layer via Hofmann elimination of SBP+. The concept of passivation of the LTO Lewis-acid site in the presence of SBP+ was further confirmed by applying alternative quaternary ammonium cations, such as tetraethylammonium (TEA+) and triethylmethylammonium (TEMA+), resulting in reduced H2 generation to the same level as that obtained using the SBP+-based dual-cation.

Effect of carbon and N-doped carbon nanomaterials on the electrochemical performance of lithium titanate-based composites

The composite nanomaterials based on hydrothermally synthesized Li4Ti5O12 and carbon nanotubes (CNTs) or carbon nanoflakes, including N-doped ones were prepared. The composites were investigated by X-ray powder diffraction, TEM, SEM, Raman spectroscopy, BET, dc-measurements, galvanostatic charge-discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. Carbon nanomaterial addition provides the formation of a highly conductive 3D network leading to a significant increase in the electrical conductivity and excellent electrochemical performance of the composites at high charge-discharge rates. At a current density of 6400 mA/g (~37C), the reversible discharge capacities of Li4Ti5O12 and its composites with CNTs and N-doped CNTs are 60, 92, and 96 mAh/g, respectively. They exhibit a remarkable long-term cycling stability: less than 0.034%, 0.013% and 0.005% loss per cycle over 500 cycles, respectively. The results obtained by Raman spectroscopy, BET, and electrochemical measurements suggest the interaction between polar fragments of the N-doped carbon nanomaterials and Li4Ti5O12 surface.

Surface Evolution of Lithium Titanate upon Electrochemical Cycling Using a Combination of Surface Specific Characterization Techniques

AbstractAmong the electrodes for Li‐ion batteries, Li4Ti5O12 (LTO) stands out as anode owing to its stability and safety, in part ascribed to its low surface reactivity. However, the overlayer formation on the LTO surface upon electrochemical cycling is reported in recent years; a rough surface layer of electrochemically inactive α‐Li2TiO3 on top of the LTO (111) surface is suggested on the grounds of scanning probe techniques and theoretical ab initio calculations which would negatively strike on the battery performance. Hence the investigation of the LTO surface evolution is key to achieve more stable and safer Li‐ion batteries. LTO (111) thin film electrodes are used as model system where a variety of surface specific characterization techniques are applied to unveil the surface behavior of LTO in Li‐ion batteries. In contrast with previous studies, with the help of high‐resolution transmission electron microscopy and synchrotron‐based surface X‐ray diffraction, α‐Li2TiO3 is found to be a surface preparation product. Of special importance is the use of high‐resolution electron backscatter diffraction to report an increase on the LTO surface strain upon electrochemical cycling which can have a critical effect in long cycling performance of LTO that is always considered a zero‐strain material.

Probing the in-depth distribution of organic/inorganic molecular species within the SEI of LTO/NMC and LTO/LMO batteries: A complementary ToF-SIMS and XPS study

Spinel Li4Ti5O12 (LTO) is an attractive candidate for negative electrode materials of Li-ion batteries because of its outstanding safety characteristics. In this paper, the influence of common high voltage cathodes, LiNi3/5Co1/5Mn1/5O2 (NMC) and LiMn2O4 (LMO), upon the electrochemical performances of LTO/LMO and LTO/NMC cells, in relation with the Solid Electrolyte Interphase (SEI) properties formed over the LTO surface, is studied. After cycling, the electrodes were analyzed by ToF-SIMS and XPS with two X-ray sources (Ag and Al) to investigate both the chemical composition and the in-depth distribution of specific SEI species at the electrode surface. Facing both counter-electrodes, LTO electrodes are covered by surface layers due to the degradation of electrolyte components inducing an irreversible capacity loss, more important for LTO/LMO cells. The chemical composition of both layers is similar: organic and inorganic species but the SEI formed on LTO electrode cycled facing LMO electrode is thicker and contains small amounts of manganese compounds from the positive electrode. Moreover, 3D mappings reconstructed from ToF-SIMS depth-profile experiments, display different in-depth spatial distributions of SEI species, which are in agreement with XPS results. Consequently, the impact of interactions between electrodes on the formation of surface films is discussed.

Disclosing the Complex Surface Reactivity of LiNi0.8Co0.15Al0.05O2 at High Potential By Combining XPEEM, XAS and XPS Measurements

The development of the next generation high-energy density Li-ion batteries requires a deep understanding on the interface between high voltage cathode and electrolyte. When cathode materials are cycled to high potential, especially above 4.3 V vs. Li+/Li, different reactions can take place at the cathode-electrolyte interface: electrolyte oxidation1, surface structural changes, dissolution of the transition metals (TMs)2, oxygen oxidation3 and release4, and binder degradation5. Moreover, all those parasitic reactions can also affect the counter electrode via a "cross-talk" process of organic/inorganic species and TMs. To elucidate these complex reaction mechanisms, a combination of different spectroscopy techniques is required in order to have full picture of the interface reactivity. Although X-ray photoelectron spectroscopy (XPS) provides valuable knowledge concerning the binder and electrolyte stability, information about the TMs oxidation states and the surface structure are more difficult to obtain. On the other hand, X-ray absorption spectroscopy (XAS) of TM L-edges, C K and O K-edges provides very important knowledge on the oxidation states and structural evolution during cycling. By combining the high lateral resolution and surface sensitivity of X-ray photoemission electron microscopy (XPEEM)6 with total electron yield (TEY) and total fluorescence yield (TFY), one can gain a better understanding of the evolution of both the surface and the bulk of the particles. In this contribution, we show the successful combination of XPS, XPEEM and XAS in TEY and TFY modes to elucidate the complex surface reactivity of LiNi0.8Co0.15Al0.05O2 (NCA) when it is cycled versus Li4Ti5O12 (LTO), especially at 4.9 V vs. Li+/Li in LP30 electrolyte. Our results show the instability at high potential of the organic binder contained in the electrodes and the LiPF6 salt in the electrolyte, while EC:DMC oxidation byproducts are not observed on neither the cathode nor the anode despite the high potential. Reduced Ni is detected on the NCA surface and, due to the limited oxygen release observed from the NCA material7, reversible oxygen oxidation is observed in the O K-edge spectra at the NCA surface, proving the participation of O together with Ni in the redox process of NCA at high potential (Figure 1a,b). The cubic-like NiO structure is also visible in the O K-edge caused by structural change. Despite the presence of Ni, Co and LiF on the LTO surface (Figure 1c), due to the instability of reduced TMs and the LiPF6/binder at the cathode-electrolyte interface, their influence on the electrochemical performances of LTO is limited. Instead, the overall fading of the NCA vs. LTO cell is mainly attributed to the structural degradation of the NCA particles.

Li4Ti5O12 coated with ultrathin aluminum-doped zinc oxide films as an anode material for lithium-ion batteries

Aluminum-doped zinc oxide (AZO) films were coated on the surface of Li4Ti5O12 (LTO) particles via atomic layer deposition (ALD) in a fluidized bed reactor. The electrochemical characterization results indicated that the coating of AZO significantly improved the electrochemical performance of LTO between 0.1 V and 3.0 V with a proper coating thickness. Among all AZO-coated LTO samples, 250-10AZO (nine cycles of ZnO ALD followed by one cycle of Al2O3 ALD at a deposition temperature of 250 °C) showed the best performance. After 250 cycles of charge/discharge at a 1 C rate (1 C = 175 mA g−1), the capacity decreased from 203 to 190 mAh g−1 with a capacity retention of ~94% at room temperature and from 224 to 216 mAh g−1 with a capacity retention of ~96% at 55 °C. The AZO coating layer with an appropriate thickness not only increased the capacity of LTO by enhancing conductivity, but also assisted the LTO electrodes to form a beneficial interface layer to protect the LTO from the continuous attack by harmful components in the electrolyte, especially under extreme conditions, such as high temperature and high current rates.

Influence of Defect Reaction on Electrical/Electrochemical Behaviors of Oxide Electrodes for Li Batteries

Functional oxides are known to show interesting electrical properties by introduction of desired defect structures. Electrode performance in Li batteries is also highly dependent on its defect reaction through synthesis and/or charging-discharging cycles. For examples, the reversible capacity of layered cathode such as LiCoO2 is limited by its highly defective layered structure when state of charge greater than 50%. On the other hand, capacity of spinel or olivine cathodes shows higher delithiation rate due to better structural stability. In this study, the behavior of Li-rich layered oxide cathode and Li4Ti5O12 (LTO) are investigated and illustrated based on their defect reactions. For examples, the as-received zero-stain anode oxide Li4Ti5O12 shows conductivity as low as 10-9 S/cm and then causes high interface polarization and low rate capability. With proper processing under low pO2, the electron conduction and electrochemical properties of LTO was significantly improved. Such property enhancement may be well illustrated by defect reaction under low pO2 environment. In other words, lattice oxygen near surface of LTO tends to be removed under very low pO2. Consequently, positively charged oxygen vacancy is formed and then charge-compensated by the creation of two negatively charged electrons. These electrons are eventually associated with tetravalent Ti ions. The fast electron pathway is established by the coexistence of Ti+3 and Ti+4 in Ti cation sublattice. Furthermore, Li-rich layer-structured cathode formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) during 1st charging process was found to exhibit oxygen oxidation, vacancy formation. In the following discharging, the manganese reduction (or activation) occurred. As a result, a reversible capacity as high as 250 mAh/g was observed (Fig. 1(a)). Such redox reactions may be illustrated by defect chemistry. In other words, during 1st charging, electrons and oxygen was extracted from lattice oxygen with the formation of oxygen vacancies. In the next discharging process, electrons was accepted by cathode oxides accompanied by valence changing of Mn+4 to Mn+3. It is known that typical electrochemical reactions are highly dependent up movement of electrons and ions. With understanding of defect reaction during materials processing and charging/discharging, advanced electrode materials with much improved electrochemical and/or stability may be developed.