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The LiCoO2 cathode undergoes undesirable electrochemical performance when cycled with a high cut-off voltage (≥4.5 V versus Li/Li+). The unstable interface with poor kinetics is one of the main contributors to the performance failure. Hence, a hybrid Li-ion conductor (Li1.5Al0.5Ge1.5P3O12) and electron conductor (Al-doped ZnO) coating layer was built on the LiCoO2 surface. Characterization studies prove that a thick and conductive layer is homogeneously covered on LiCoO2 particles. The coating layer can not only enhance the interfacial ionic and electronic transport kinetics but also act as a protective layer to suppress the side reactions between the cathode and electrolyte. The modified LiCoO2 (HC-LCO) achieves an excellent cycling stability (77.1% capacity retention after 350 cycles at 1C) and rate capability (139.8 mAh g-1 at 10C) at 3.0-4.6 V. Investigations show that the protective layer can inhibit the particle cracks and Co dissolution and stabilize the cathode electrolyte interface (CEI). Furthermore, the irreversible phase transformation is still observed on the HC-LCO surface, indicating the phase transformation of the LiCoO2 surface may not be the main factor for fast performance failure. This work provides new insight of interfacial design for cathodes operating with a high cut-off voltage.

The pre-edges of oxygen-K X-ray absorption spectra have been ubiquitous in transition metal (TM) oxide studies in various fields, especially on the fervent topic of oxygen redox states in battery electrodes. However, critical debates remain on the use of the O-K pre-edge variations upon electrochemical cycling as evidences of oxygen redox reactions, which has been a popular practice in the battery field. This study presents an investigation of the O-K pre-edge of 55 oxides covering all 3d TMs with different elements, structures and electrochemical states through combined experimental and theoretical analyses. It is shown unambiguously that the O-K pre-edge variation in battery cathodes is dominated by changing TM-d states. Furthermore, the pre-edge enables a unique opportunity to project the lowest unoccupied TM-d states onto one common energy window, leading to a summary map of the relative energy positions of the low-lying TM states, with higher TM oxidation states at lower energies, corresponding to higher electrochemical potentials. The results naturally clarify some unusual redox reactions, such as Cr3+/6+. This work provides a critical clarification on O-K pre-edge interpretation and more importantly, a benchmark database of O-K pre-edge for characterizing redox reactions in batteries and other energy materials.

While the theoretical capacity of LiCoO2 is as high as 274 mA h g−1, its practical specific capacity is only about 140 mA h g−1 when it was first applied in lithium-ion batteries. Raising the charg...

With the growing demand for high specific volume energy-density, surface modifications are devoted to increasing the electrochemical performances of LiCoO2 at high operating voltage. In this work, LiCoO2 materials with a phosphate-rich coating layer are developed using a facile spray-drying method. The stable phosphate-rich coating layer acts as protection and effectively suppresses the impedance growth in LiCoO2 cells. The phosphate-rich layer coated LiCoO2 samples show long cycle-life and good rate performance when cycled with a high cut-off voltage of 4.5 V at room temperature and 45 °C.

Although Ni-rich cathode materials have made a great success in the field of electric vehicles due to their high capacity and low cost, many efforts are still focused on further increasing their capacity through increasing the Ni content. However, the increased Ni content usually leads to a more serious surface structure reconstruction and hence higher resistance during electrochemical cycling, which has become a major issue for Ni-rich cathodes. In order to explore a balance between surface structure stability and high energy density, a model Ni-rich material with transition metal ion gradient is prepared using a mechanical fusion and co-lithiation method, which is easy for large-scale fabrication. The gradient structure sample contains an inter bulk of LiNi0.90Co0.05Mn0.05O2 and a thin gradient outer layer with lower nickel content. The gradient sample shows a superior cycling property, rate retention, and improved safety performance. Systematic study suggests that the possible reasons for the improved electrochemical and mechanical performance of the gradient nickel-rich materials are the high stability of lower-Ni surface and the decreased surface tensile stress. This work provides an easy up-scaled method to build a gradient structure and a further in-depth understanding on its structure stabilization mechanisms, which are essential for developing high energy-density lithium-ion batteries.

Binders play a significant role in the electrochemical performance of electrodes in batteries, especially for high-capacity conversion/alloying-type electrodes. However, the effects of binders on the electrochemical performance of the conversion/alloying-type anodes in sodium ion batteries are not widely investigated. In this work, we use SnS 2 as a model anode and comparatively investigate the performance of six different types of binders in SnS 2 electrodes of sodium ion batteries by half-cell testing. The binders are sodium carboxymethyl cellulose (CMC-Na), sodium polyacrylate (PAA-Na), CMC-Na-PAA-Na (1:1, wt%, denoted as PAA-CMC), sodium alginate (ALG-Na), PVDF, PTFE. The PAA-CMC binder electrodes exhibit outstanding cycling and rate performance, delivering a reversible capacity of 400 mAh g −1 at the current density of 100 mA g −1 within 70 cycles. Our results indicate that the binder with a large fraction of carboxylate and hydroxyl groups, which lead to stronger hydrogen bonds and/or covalent chemical bonds with the carbon black and active materials, is advantageous for the electrochemical performances of SnS 2 electrodes. The synergistic interactions among the binder and the surface of both the active materials of SnS 2 and the conductive additive of ketjen black have been also schematically proposed in this study.

Although various cathode materials have been explored to improve the energy density of lithium-ion batteries, LiCoO2 is still the first choice for 3C consumer electronics due to the high tap density and high volumetric energy density. However, only 0.5 mol of lithium ions can be extracted from LiCoO2 to avoid side reactions and irreversible structure change, which typically occur at high voltage (>4.2 V). To improve the electrochemical performances of the LiCoO2 cathode material at high cut-off voltage and elevated temperature for higher energy density, an in situ formed spinel interfacial coating layer of LiCoxMn2–xO4 is achieved by the reaction of the surface region of the LiCoO2 host. The capacity retention of the modified LiCoO2 cycled at a high voltage of 4.5 V is significantly increased from 15.5 to 82.0% after 300 cycles at room temperature, due to the stable spinel interfacial inhibiting interfacial reactions between LiCoO2 and the electrolyte as confirmed by impedance spectroscopy. We further dem...

Li2MnO3-based Li-rich cathode materials can offer high capacities for lithium-ion batteries but suffer from poor rate capability, low initial coulombic efficiency, and voltage fade upon extended cycling. Significant attention has been focused on the replacement of Li2MnO3 with other materials featuring Li2MO3 (M = Ru, Mo, etc.) components; however, the cycle performance of these materials is inferior. Herein, coating with Al2O3 is proposed to improve the electrochemical performance of the Li2RuO3-based Li-rich cathode material Li1.2Ni0.2Mn0.2Ru0.4O2. Rietveld refinement data indicate Ru-Ru dimer formation in these samples. The Al2O3 coating can enhance the initial coulombic efficiency, cycling stability, rate performance, and mitigate the voltage decay. In situ X-ray diffraction (XRD) demonstrates that the compound remains a layered structure during the first cycle. The formation of Ru-Ru dimers is suggested to be responsible for the unusual lattice parameter changes during the initial charge.

Room-temperature sodium-ion batteries offer an alternative solution for large-scale energy storage system because of the huge abundant, and low cost of sodium resources. An O3-type sodium-excess layered material Na1.2Ni0.2Mn0.2Ru0.4O2 (0.4Na2RuO3·0.4NaNi0.5Mn0.5O2) is prepared as a cathode material for sodium-ion batteries, which reveals an enhanced cycling stability, high coulombic efficiency, and superior dynamics properties. In-situ X-ray diffraction analysis is conducted to get insight into the sodium storage mechanism. Keywords: Sodium-ion batteries, O3-type cathode materials, In-situ X-ray diffraction, Structure evolution

As a promising additive for film forming on anodes, fluoroethylene carbonate (FEC) has been applied widely in commercial lithium ion batteries (LIBs). However, the influence of FEC in high-voltage Li+ energy storage systems are still under disputing for the negative effects on cathodes. Considering a FEC molecule would be electro-oxidized to a radical onium ion, we investigate hexamethylene diisocyanate (HDI) as the additive which can react with the intermediate to form an amide-based polymer via thermodynamic processes. The polymer would be stable at high potential. In the FEC-containing electrolyte with HDI added, a surface layer is formed on the LiNi0.5Mn1.5O4 cathode, which would protect the compounds from decomposing and improve the performances of the batteries. The capacity retention of Li/LiNi0.5Mn1.5O4 cells cycled between 3.5–4.9 V (vs. Li+/Li) is significantly increased from 59.1% to 76.1% in 200 cycles, and the rate capability is also improved compared with the ones without HDI. The electrolyte system also shows a good compatibility with graphite, presenting a promising prospect for the practical application of high-voltage LIBs.

Lithium-excess cation disordered rock-salt materials have received much attention because of their high-capacity as a candidate for cathodes for lithium-ion batteries. The ultra-high specific capacity comes from the coordinated charge compensation of both transition metal and lattice oxygen. However, the oxygen redox at high voltage usually leads to irreversible oxygen release, thereby degrading the structure stability and electrochemical performance. Lithium-excess Li1.14Ni0.57+0.5 x Ti0.19 – 0.5 x Mo0.10O2 – x F x (x = 0, 0.05, 0.10, 0.15, and 0.20) with different amounts of fluorine substitution were synthesized. Among them, Li1.14Ni0.620Ti0.140Mo0.10O1.85F0.15 exhibits a lower capacity decline, better rate performance, and lower structure damage. The effects of fluorine substitution on the electrochemical property and structural stability were systematic studied by x-ray photoelectron spectroscopy and in situ XRD etc. Results show that fluorine substitution reduces the average valence of the anion, allowing a larger proportion of low-valent redox active transition metals, increasing the transition metal redox capacity, inhibiting irreversible oxygen release and side reaction. Fluorine substitution further improves the structural stability and suppresses lattice deformation of the material.

The superior tap density (4.1 g cm−3) and high theoretical specific capacity (274 mA h·g−1) of LiCoO2 make it still the first choice of cathode for batteries in next-generation portable electronic devices. However, the structural instability in the deeply delithiated state severely restricts the reversible capacity of LiCoO2 for practical applications. Owing to the recently popular Ni and Mn elements in ternary cathodes, we explored the effects of Ni–Mn co-doping and each individual dopant on the electrochemical behavior and structural evolution of LiCoO2 at a high upper cut-off voltage of 4.6 V by the means of in-situ XRD and GITT measurements. LiMn0.05Co0.95O2 shows the best cycle stability at 4.6 V with a capacity retention of 55% after 100 cycles. The doping suppresses various two-phase transitions, especially, the O3/H1-3 phase transition at ∼4.55 V, leading to better cycling stability of LiCoO2 at 4.6 V. Due to the significant stabilizing effect on the structure, Mn seems to be an ideal choice for the modification of high-voltage LiCoO2 in the future.

The lithium-rich Li[Li0.2Ni0.13Mn0.54Co0.13]O2 nanoplates were synthesized using a molten-salt method. The nanoplates showed an initial reversible discharge capacity of 233 mA&middot, h&middot, g&minus, 1, with a fast capacity decay. The morphology and micro-structural change, after different cycles, were studied by a scanning electron microscope (SEM) and transmission electron microscopy (TEM) to understand the mechanism of the capacity decay. Our results showed that the cracks generated from both the particle surface and the inner, and increased with long-term cycling at 0.1 C rate (C = 250 mA&middot, 1), together with the layered to spinel and rock-salt phase transitions. These results show that the cracks and phase transitions could be responsible for the capacity decay. The results will help us to understand capacity decay mechanisms, and to guide our future work to improve the electrochemical performance of lithium-rich cathode materials.

Cr-based cathode materials for Li-ion batteries have attracted significant attentions due to the feature of multiple electron transfer. The origin of the poor electrochemical inactivity of LiCrO2 has not been clarified for decades. Here an irreversible phase transformation from the layered to the rock-salt structure is observed at atomic scale in partially electrochemical delithiated LiCrO2: Cr ions migrate from Cr layers into Li layers in the surface regions. The Cr ions at Li layers in the surface regions could block extraction of lithium from the interior regions. Density functional theory (DFT) calculations confirm that Cr ions in Li layers can stabilize the structure in the Li-poor area, but the diffusion energy barrier of Li ions will be greatly increased. It is proposed accordingly that the surface phase transformation and the blocking of diffusion channel are the main origin for the poor electrochemical reactivity of LiCrO2. Such a surface blocking phenomenon may be a common origin for inactivity of some cathode materials, in which cation mixing become significant after initial delithiation. In addition, Cr ions in LiCrO2 are oxidized only from Cr3+ to Cr4+ during electrochemical delithiation, instead of Cr6+ as usually expected, based on synchrotron X-ray absorption spectra (XAS) studies.

Layered germanium phosphide, which combines the advantages of both germanium and phosphorus, is believed to be a potential anode for sodium ion battery. Here, the sodium storage mechanism and electrochemical performance of layered germanium phosphide have been deeply investigated by advanced in-situ transmission electron microscopy technique combining half-cell testing. Dynamic reaction process reveals that individual layered germanium phosphide nanoflake undergoes total area expansion of 248% without any detectable fracture or cracking in sodiation. In contrast, germanium phosphide experiences multi-step reactions, i.e. intercalation and alloying, accompanied by sequentially forming NaxGeP (0

Carbon-coated Li2MoO4 hexagonal hollow nanotubes were fabricated via a facile sol–gel method involving the solution synthesis of Li2MoO4 with subsequent annealing under an inert atmosphere to decompose the organic carbon source. To the best of our knowledge, this is the first report on the synthesis of Li2MoO4 nanotubes. More significantly, we have found that Li2MoO4 can be used as an anode material for lithium-ion batteries (LIBs). When evaluated as an anode material, the carbon-coated Li2MoO4 hollow nanotubes show an excellent electrochemical performance with a high reversible capacity (∼550 mA h g−1) after 23 cycles, good rate capability and cycling stability. Meanwhile, carbon-free Li2MoO4 sample, fabricated via a solid state reaction, was also prepared for comparison. The Li storage mechanism has been investigated in-detail by advanced XPS, in situ XRD and HRTEM.

Layered and spinel materials have been used successfully as intercalation-type cathode active materials in commercial Li-ion batteries. The physical and chemical properties, electrochemical reactions, structure evolution mechanisms, stability and safety issues have been widely investigated. Based on comprehensive fundamental researches, since 1980s, their electrochemical performances are improved continuous after various modifications, such as doping, surface coating, forming solid solution and composite, controlling morphology, size and crystallinity. Here, basic features of layered and spinel materials are summarized.

Lithium ion batteries are important electrochemical energy storage devices for consumer electronics and the most promising candidates for electrical/hybrid vehicles. The surface chemistry influences the performance of the batteries significantly. In this short review, the evolution of the surface structure of the cathode materials at different states of the pristine, storage and electrochemical reactions are summarized. The main methods for the surface modification are also introduced.

Li-rich layered materials are important cathode compounds used in commercial lithium ion batteries, which, however, suffers from some drawbacks including the so-called voltage fade upon electrochemical cycling. This study employs novel transmission X-ray microscopy to investigate the electrochemical reaction induced morphological and chemical changes in the Li-rich Li2Ru0.5Mn0.5O3 cathode particles at the meso to nano scale. Combined X-ray spectroscopy, diffraction and microscopy experiments are performed to systematically study this cathode material's evolution upon cycling as well as to establish a comprehensive understanding of the structural origin of capacity fade through 2D and 3D fine length scale morphology and heterogeneity change of this material. This work suggests that atomic manipulation (e.g. doping, substitution etc.) or nano engineering (e.g. nano-sizing, heterogeneous structure) are important strategies to mitigate the internal strain and defects induced by extensive lithium insertion/extraction. It also shows that maintaining the structural integrity is the key in designing and synthesizing lithium-rich layered materials with better cycle stability.