Li-rich Phase Research Articles

Ab initio identification of the Li-rich phase in LiFePO4.

A recent discovery of anionic redox activity in Li-rich layered compounds opens a new direction for the design of high-capacity cathode materials for lithium-ion batteries. Here using extensive ab initio calculations, the thermodynamic existence of the Li-rich phase in LiFePO4 to form Li1+xFe1-xPO4 with x not exceeding 12.5% has been proved. Anionic redox activity and structural stability during delithiation are further investigated. Interestingly, it is found that Li1+xFe1-xPO4 cannot be delithiated completely and thus cannot achieve extra capacity by anionic redox activity, because the local oxygen-ion redox will cause the fracture of the rigid framework formed by phosphate tetrahedral polyanions. Although an extra capacity cannot be realized, the excess Li-ions at Fe sites can enhance the Li-ion diffusivity along the adjacent [010] channel and contribute to the shift from 1D to 2D/3D diffusion. This study provides a fresh perspective on olivine-type LiFePO4 and offers some important clues on designing Li-rich cathode materials with high energy density.

Effect of Cobalt and Nickel Contents on the Performance of Lithium Rich Materials Synthesized in Glycerol Solvent

Lithium-rich cathode materials in the form of Li1.2Mn0.51Ni0.145+xCo0.145-xO2 (x = 0 (LR2), 0.0725 (LR1)) have been successfully synthesized by a sol-gel method using glycerol as solvent. These materials were characterized by X-ray diffractions (XRD), scanning electron microscopy (SEM), and electrochemical measurements. XRD patterns showed that both materials have typical diffraction peaks of Li-rich material with a well-crystallized structure, but the LR1 sample with less Co has a well-distinguished Li2MnO3 peak between 20°–25°. SEM images demonstrate that LR1 also has a more uniform particle distribution. Electrochemical measurements revealed that LR2 with more Co has a higher initial discharge capacity of 223 mAh/g than LR1 of 185 mAh/g at 0.1 C between 2 and 4.8 V. However, LR1 was observed to have an increasingly improved capacity over several dozens of cycles. The specific discharge capacity of LR1 increased from 185 to 213 mAh/g after 20 cycles. This is ascribed to a gradual activation of the Li-rich phase from the surface of the particles. It is shown that the smaller Co/Mn ratio (1:7.134) of LR1 plays an important role in its continuous activation. Its capacity retention is 98% at 100 cycles at 1C rate, better than that of LR2 and most previously reported lithium-rich materials.

Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li-Sn Compounds at Ambient and Moderately High Pressure.

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li-Sn batteries. It can, however, be overcome with the help of void space engineering by using a LixSn phase as the prelithiated anode, where an optimal value for x is desired. Currently, Li4.25Sn is known as the most lithiated Li-Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (x > 4.25) Li-Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li-Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li-Sn system, in general, but also in improving our understanding of the reaction mechanism in Li-Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li-Sn compounds, our study reveals the existence of five unreported stoichiometries (Li8Sn3, Li3Sn1, Li4Sn1, Li5Sn1, and Li7Sn1) and their associated crystal structures at ambient and high pressure. Although Li8Sn3 has been identified as one of the most stable Li-Sn compound in the entire pressure range (1 atm-20 GPa) with R3̅m symmetry, the Li-rich compounds like Li3Sn1-P2/m, Li4Sn1-R3̅m, Li5Sn1-C2/m, and Li7Sn1-C2/m are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li5Sn1 and Li7Sn1 opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li17Sn4.

Enhanced electrochemical performance of lithium rich layered cathode materials by Ca2+ substitution

The poor cycling stability and large voltage decay in Li-rich cathode materials is related to the layer to spinel structural transformation. It is understood that the ease of structural transformation is correlated to the amount of oxygen gas released during the first charge above 4.5V. So one of the effective strategy to improve the electrochemical properties is by suppressing oxygen evolution through stabilizing oxygen radical intermediates by tuning metal-oxygen bond characteristics such as covalency, bond energy, iconicity, etc. through cation substitutions in Li rich phases. In this work we report that small amount of Ca substitution in Li layers of Li rich phases, Li1.2-2xCaxCo0.13Ni0.13Mn0.54O2 (x=0.005) improves the electrochemical cycling stability as well as the rate capability. With x=0.005 calcium substitution, the initial coulombic efficiency increased from 70% (for the pristine) to 83% and the capacity retention is improved from 71% to 87% after 100 cycles. Similarly Ca doping improves the rate capability especially at higher rates. The improved electrochemical performance of the Ca doped Li-rich cathode can be attributed to the fine-tuning of the crystal-chemical aspects manifested through enhanced structural stability and increased interlayer distance.

Li-Removal Mechanism and Its Effect on Oxygen Stability Influencing the Electrochemical Performance of Li1.17Ni0.17Mn0.67O2: Experimental and First-Principles Analysis

The high capacity of Li-rich layered cathode materials is always accompanied by the removal of oxygen from the crystal structure. These oxygen vacancies alter the structural stability, which subsequently deteriorates the electrochemical performance. The electronic origin of oxygen stability with partial delithiation has not been extensively studied so far in the presence of multiple d-orbital elements. Current work presents the experimental and density functional theory based study of the Li-rich phase, Li1.17Ni0.17Mn0.67O2. This study reveals the lithium removal mechanism and its influence on the oxygen stability. Further, the study suggests how lithium removal from different lithium sites, i.e., 2b, 2c, and 4h Wyckoff positions, influence the partial intercalation potential. On higher degree of delithiation, electrochemical potential increases and oxygen binding energy decreases. Thus, the oxygen stability reduces in the compound. At this stage, the material becomes metallic with zero band gap, which fa...

Interface Instability during Ion Intercalation Process

Misfit stress generated by lattice mismatch between the lithiated and delithiated phases upon ion (de)intercalation plays a key role in cycling-induced mechanical degradation and capacity fading of battery electrodes. As a general phenomenon, misfit stress also leads to interface instability. A well-known example is the Stranski-Krastanov mode or Asaro-Tiller-Grinfeld instability [1] of epitaxial thin film growth (Figure 1(a)), where the misfit strain between thin film and substrate promotes the undulation of thin film and the transition to the island growth behavior. In this work, we provide the first theoretical analysis to predict that an analogous phenomenon can also occur during the ion (de)intercalation process, in which the misfit stress triggers the morphological instability of the moving phase boundary and results in non-uniform (de)intercalation, Figure 1(a). Such a phenomenon is generally applicable to battery compounds that undergo phase transformations with volume change upon ion insertion/extraction. It could have important implications for battery performance and degradation, such as increasing the kinetic resistance to (de)interclation and enhancing stress concentration that damages electrodes and accelerates capacity fading. Our analysis offers useful insights on potential practical strategy to suppress the onset of stress-induced interface instability during electrode charge/discharge. In the theoretical treatment, we consider a general intercalation system in two dimensions in which ion intercalation proceeds through a first-order phase transformation with isotropic misfit strain and elasticity. The phase boundary between the Li-rich and Li-poor phases is assumed to maintain (partial) coherency. We consider interface-limited phase boundary migration, i.e. the interface velocity is proportional to the Li chemical potential difference between the Li-rich and Li-poor phases at the interface, which includes contribution from coherency stress. Linear stability analysis is carried out to determine the stability of a small periodic perturbation applied to the originally flat interface, Figure 1(b). The analysis results in the following main findings: 1) The phase boundary is unstable against perturbations with wave vector below a critical value kc, or equivalently, when perturbation wavelength is larger than the critical wave length λc = 2*π/kc. As shown in Figure 1(c), there exists a fastest growing wave vector km, at which the perturbation amplitude has the maximum growth rate. Both kc and km are proportional to A/(E*e0 2), where A is interfacial energy, E is the Young’s modulus and e0 is the misfit strain. With typical values of these properties for intercalation compounds (A ~ 0.1 J/m^2, E ~ 100 GPa, e0~ 0.01), the critical and fastest growing wave lengths of interface perturbation are in the range of 10 – 100 nm. Interface stability is hence expected in nano- and micro-sized particles. 2) Interface is most susceptible to instability development at the beginning stage of the (de)intercalation process. As shown in Figure 1(d), the fastest growing wave vector of interface perturbation depends on the depth of the interface from the particle surface, z0. While km and kcare the largest when interface is near the electrode surface, they decay with interface moving into the interior of the particle. 3) Interface instability is suppressed in the interior of intercalation compounds below a critical particle size. Figure 1(d) illustrates that the fastest growing wave vector drops to 0, i.e. no perturbation with finite wave length can grow, when the system dimension in the main intercalation direction is smaller than a critical thickness L*, which is on the order of 100 nm. Therefore, nanoscaling electrodes not only benefits intercalation kinetics but also improves interface stability. Complementing the linear stability analysis, we also performed phase-field simulation of the phase boundary evolution during delithiation of LiFePO4 cathode. Consistent with the theoretical prediction, interface instability is observed in the simulation (Figure 1(e)), which reveals the details of the interface morphological evolution at later stage. Notably, similar interface morphology is observed in a recent operando hard x-ray microscopy experiment [2]. Based on the findings from this study, we identify two useful approaches to stabilize phase boundary during ion (de)intercalation: i) Reduce one dimension of the electrode particles to the sub-micron regime, i.e. nanoplate morphology is desirable. ii) Apply large current pulses at the beginning of discharge/charge to have interface rapidly traverse the near-surface region where instability is easiest to develop.

Phase Transformations During Li-Insertion into V2O5 at Elevated Temperature

Recent interest in developing cathode materials for an elevated temperature operation of Li-ion batteries has motivated researchers to explore the possibility of using layered V2O5 as a potential candidate because of its high capacity and cyclic stability. Despite a wide lithiation voltage window of V2O5 (between 1.0 V and 4.0 V), compositional fluctuations, metal dissolution, and so on contribute to capacity loss at high temperatures. A first discharge of V2O5 to voltages below 2.0 V has been observed to be associated with a series of phase transformations at both room temperature and high temperature and has been characterized here. From structural characterization of harvested electrodes post–first discharge, a new Li-rich phase was observed to be formed at 120°C and the composition was estimated.

Mitigating the Surface Degradation and Voltage Decay of Li1.2Ni0.13Mn0.54Co0.13O2 Cathode Material through Surface Modification Using Li2ZrO3.

In the quest to tackle the issue of surface degradation and voltage decay associated with Li-rich phases, Li-ion conductive Li2ZrO3 (LZO) is coated on Li1.2Ni0.13Mn0.54Co0.13O2 (LNMC) by a simple wet chemical process. The LZO phase coated on LNMC, with a thickness of about 10 nm, provides a structural integrity and facilitates the ion pathways throughout the charge–discharge process, which results in significant improvement of the electrochemical performances. The surface-modified cathode material exhibits a reversible capacity of 225 mA h g–1 (at C/5 rate) and retains 85% of the initial capacity after 100 cycles. Whereas, the uncoated pristine sample shows a capacity of 234 mA h g–1 and retains only 57% of the initial capacity under identical conditions. Electrochemical impedance spectroscopy reveals that the LZO coating plays a vital role in stabilizing the interface between the electrode and electrolyte during cycling; thus, it alleviates material degradation and voltage fading and ameliorates the electrochemical performance.

Investigation of the Li solubility in the intermediate phase Li17Sn4 relevant to understanding lithiation mechanisms in Sn-based anode materials

A comprehensive study of the Li-rich phase boundary of the intermediate Li17Sn4 compound is given based on thermochemical and electrochemical investigations. From the results a partial phase diagram showing the homogeneity range of this phase could be constructed. Using these results, the defect mechanism for the compound Li17Sn4 could also be clarified. Galvanostatic coulometry with potential limitation (GCPL) was performed using 2032 coin cell equipment. This allowed for the determination of the amount of Li insertion on special crystallographic sites at 25 °C, which is 0.41 ± 0.04 at.%. The respective Li loss mechanisms (solid electrolyte interface formation, Li plating, Li trapping) are discussed. Irreversible capacity according to Li trapping could be excluded due to the limited voltage window of the experiments. Moreover, differential scanning calorimetry (DSC) was carried out on two individual Tian-Calvet type calorimeters. The Li-rich boundary of the Li17Sn4 compound was determined at 81.31 ± 0.04 (C-80) and 81.28 ± 0.05 at.% Li (M-HTC) at 427 °C. In addition, the latter results were compared to the change of lattice parameters of alloys with compositions Li17+xSn4.

In situ observation of macroscopic phase separation in cobalt hexacyanoferrate film

Lithium-ion secondary batteries (LIBs) store electric energy via Li+ deintercalation from cathode materials. The Li+ deintercalation frequently drives a first-order phase transition of the cathode material as a result of the Li-ordering or Li-concentration effect and causes a phase separation (PS) into the Li-rich and Li-poor phases. Here, we performed an in situ microscopic investigation of the PS dynamics in thin films of cobalt hexacyanoferrate, LixCo[Fe(CN)6]0.9, against Li+ deintercalation. The thick film (d = 1.5 μm) shows a characteristic macroscopic PS of several tens of μm into the green (Li1.6Co[Fe(CN)6]0.9) and black (Li.6Co[Fe(CN)6]0.9) phases in the x range of 1.0 < x < 1.6. Reflecting the substrate strain, the thin film (d = 0.5 μm) shows no trace of the PS in the entire x region. Our observation suggests that the macroscopic PS plays a significant role in the charge/discharge dynamics of the cathode.

Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy

Understanding the reaction pathway and kinetics of solid-state phase transformation is critical in designing advanced electrode materials with better performance and stability. Despite the first-order phase transition with a large lattice mismatch between the involved phases, spinel LiMn1.5Ni0.5O4 is capable of fast rate even at large particle size, presenting an enigma yet to be understood. The present study uses advanced two-dimensional and three-dimensional nano-tomography on a series of well-formed LixMn1.5Ni0.5O4 (0≤x≤1) crystals to visualize the mesoscale phase distribution, as a function of Li content at the sub-particle level. Inhomogeneity along with the coexistence of Li-rich and Li-poor phases are broadly observed on partially delithiated crystals, providing direct evidence for a concurrent nucleation and growth process instead of a shrinking-core or a particle-by-particle process. Superior kinetics of (100) facets at the vertices of truncated octahedral particles promote preferential delithiation, whereas the observation of strain-induced cracking suggests mechanical degradation in the material.

The Synthesis and Thermoelectric Properties of p-Type Li1−x NbO2-Based Compounds

We investigated the thermoelectric (TE) properties of a p-type oxide material (Li1−xNbO2, with x = 0–0.6). The composition was synthesized via a solid-state reaction method under a reducing atmosphere. The charge transport properties were determined through the electrical conductivity and Seebeck coefficient measurements. The electrical conductivity was non-monotonically varied with x value and showed metallic behavior with increased temperature and above 650 K temperature independent behavior dominated by extrinsic conduction. On the other hand, the Seebeck coefficient was increased with an increase in the temperature, and decreased gradually with an increase in the Li vacancy concentration by both synthesis and gradual phase transition to a Li-rich Li3NbO4 phase with temperature and appeared as an n-type TE at x = 0.6 under high temperatures, which was attributed to an Nb substitution into the Li site. The thermal conductivity was monotonically reduced with the increase in temperature due to the cation disorder defects and second phases. The Li vacancy induced Li1−xNbO2-based compounds under low oxygen partial pressure show promise as a candidate p-type material for thermoelectric applications, particularly for co-processing with n-type oxide thermoelectric materials fabricated under conditions of low oxygen partial pressure.

Differences between the Kinetically Preferred States of LiFePO4 during Charging and Discharging Observed Using In Situ X-ray Diffraction Measurements

In-situ X-ray diffraction measurements were performed on LiFePO4 during charging and discharging using synchrotron radiation to elucidate the origin of the hysteresis in the kinetic mechanisms of the charging and discharging processes. The experimental results clearly showed that fewer intermediate phases were formed during the charging process than the discharging process, in keeping with previous studies. Further, during the discharging process, a Li-containing Li-lean phase and a Li-defective Li-rich phase with large lattice spacing were generated kinetically in the initial stage. Based on these results, we propose a new model for the hysteresis between the charging and discharging processes.

Transition from Deceleration to Acceleration of Lithiation Front Movement in Hollow Phase Transformation Electrodes

In phase transformation electrodes under lithiation, growth of the Li-rich phase from the Li-poor phase usually proceeds with a moving phase boundary. Their high- rate capability is determined by how fast the phase interface can move. Unfortunately recent studies show that the stress-mediated migration velocity of the phase interface is significantly slowed down. We employ a non-linear stress-kinetics coupled model to address the effect of inner surface in hollow structures on the phase interface movement. We demonstrate that the inner surface can trigger a transition from deceleration to acceleration of the phase interface movement. The plastic deformation at the phase interface dissipates extra energy required to drive the inward migration of the phase interface and leads to deceleration. Another plastic zone arises near the inner surface and expands outward as two-phase lithiation proceeds. The coalescence of these two plastic zones significantly decreases the resistance of the phase interface movement and leads to acceleration. Furthermore, we show that tuning the size of hollow spheres regulates the phase interface migration toward high packing density and superior rate capability.

Effects of excess Li on the structure and electrochemical performance of Li1+zMnTiO4+δ cathode for Li-ion batteries

Spinel-based LiMn2O4 is the most attractive cathode for Li-ion battery due to high voltage, low cost, and non-toxicity. The cycle life of the spinel cathodes could be improved by replacing Mn4+ with Ti4+ leading to the formation of new spinel cathode, LiMnTiO4. However, its application is restricted due to the associated loss in the specific capacity. In this work, spinel-layered Li1+zMnTiO4+δ (z=0, 0.5, and 1.0; δ is the value to reflect the composite character of the material) cathodes were fabricated to achieve long cycle life, without compromising on the specific capacity. Cathodes with excess Li (z=0.5 and 1.0) formed a spinel-layered composite structure with notation (1-a)LiMn2-xTixO4.aLi2MnyTi1-yO3 [y=0.5–((1/a−1)(1−x))]. These cathodes exhibited an enhanced specific capacity of ∼218mAhg−1 (20% higher), with a capacity retention of 94% after 60 cycles. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction, galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique to understand the mechanisms underlying the enhanced capacity and cycle stability. The effect of the Li-rich layered phase on the electrochemical performance of the Li1+zMnTiO4+δ cathodes was also investigated.

Difference of Relaxation Behavior Between LiNiO2 and Li(Ni, Co, Al)O2 after Lithium Extraction

Introduction LiNiO2 (LNO) is one of the famous cathode materials of layered rock salt structure having larger discharge capacity in addition to lower toxicity and lower cost in comparison with LiCoO2. For the utilization of higher voltage region as the stable charge-discharge capacity, it is important to investigate the static and kinetic properties of this cathode material. We have investigated the structural variation of electrode materials after the termination of lithium insertion and/or extraction to reveal the structural transient from kinetic to equilibrium states clearly. We named this technique as “Relaxation analysis” and conducted this method to various types of cathode or anode materials, e.g. γ-Fe2O3 [1,2], LiMn2O4[3], LiFePO4[4], LiCoO2[5], or graphite [6], to clarify the lithium insertion and extraction process. We have recently carried out the relaxation analysis on LNO after charging to relatively higher voltage region (x ≤ 0.12 in the form of LixNiO2) [7], in which two phases with similar structures (Li-rich and Li-lean phases with the space group ) coexist [8,9]. It has been revealed that excess amount of Li-lean phase, formed during the lithium extraction process, separates into Li-rich phase and Li-lean phase with decreased lithium concentration. Partial substitution of Co and Al instead of Ni (Li(Ni, Co, Al)O2; NCA) is known to provide more stable charge-discharge cycle performance [10] and it is also expected that relaxation analysis would help to understand this improved property. In the present study, relaxation analysis is applied to NCA to compare the structural relaxation with the previously studied LNO. Experimental Working electrode was prepared by mixing Li(Ni0.933Co0.031Al0.036)O2 powder (Sumitomo Metal Mining Co., Ltd.) with Acetylene Black (AB) as a supplemental conductor and PVdF powder as an adhesive agent with the weight ratio of 80:10:10. Lithium foil and 1 mol∙dm-3 LiPF6 in EC/DMC (2:1 v/v%, Kishida chemical Co., Ltd) were employed as the counter electrode and the electrolyte, respectively to construct a two electrode cell (Hohsen Co.). Lithium ion was electrochemically extracted from NCA at a constant current of 0.01C rate to achieve x = 0.12, 0.09 and 0.06 of Lix(Ni0.933Co0.031Al0.036)O2. After the termination of Li extraction, we immediately removed the working electrode from the cell in a glove box to avoid the local cell reaction between the electrode material and the current collector. XRD data were collected from 15 ° to 75° in 2θ for CuKα radiaton (RINT-TTR, Rigaku Co., Japan) at various relaxation time and were served for Rietveld structure analysis using RIEVEC code [11]. Results and discussion Fig.1 represents the measured X-ray diffraction pattern for x = 0.09 of Lix(Ni0.933Co0.031Al0.036)O2, which is obtained 48 hours after the termination of lithium extraction. The Rietveld refined pattern assuming the two phases (Li-rich and lean phases) is also drawn. The measured and the calculated patterns agree well with sufficiently small R wp value. The calculated mole fractions of Li-lean phase are plotted in Fig. 2. For all samples, the molar ratio of Li-lean phase decreases with relaxation time, indicating that Li-lean phase partly transforms into the Li-rich phase for all the compositions. Such a behavior is essentially the similar to the previously reported LNO. The relaxation time variation of the lattice parameter c is represented in Fig. 3. While LNO possesses a large difference in c -length between Li-rich and lean phases, NCA has small difference, especially in x = 0.12. In addition, the relaxation time variations of the c -parameter are very small in NCA for all compositions. It is supposed that small difference in c-length between the Li-rich and lean phases as well as small variation of cduring the relaxation provide the better charge/discharge cycle performance in NCA even at high voltage region.

Electrochemical and Crystalstructural Analysis of Intermediate Phase Behavior in Co-Substituted LiFePO4 with Reduced Lattice Mismatch

INTRODUCTION Olivine-type LiFePO4 is one of the most promising cathode material for lithium-ion batteries as it exhibits high rate performance. Under high rate cycling, we revealed the metastable phase formation of Li0.6FePO4(LxFP) which acts as a buffer layer between Li-rich phase (LFP) and Li-poor phase(FP) .1 To improve the rate performances of LiFePO4, controlling the lattice strain between two phases was one of the idea. Recently, it is reported that the relative volume change of the LiFePO4 was controlled by the substitution of the cation in the LiFePO4. From the DFT calculation, the relative volume change between two endmembers of Li(Fe1-xZrx)(P1-2xSi2x)O4 (Z2S) was smaller than 3%, and show the better cycle stability.2 However, more detailed reaction mechanisms has not been clarified yet. Recently, the LiFePO4 doped with different cations were also reported. For example, vanadium doped LiFePO4 showed the better rate properties and the temperature of phase transition from two phase to solid-solution was more decreased than undoped LiFePO4. The stability of LxFP was expected one of the key factor for the improvement of the rate properties of the LiFePO4. Here we investigated the relationship between the rate property of Z2S and the appearance of LxFP by using intermediate temperature cell. EXPERIMENTAL LiFePO4 (Undoped) and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4 (Z2S) were synthesized in the same manner as reported one.2 The temperature-controlled XRD under Ar atmosphere was performed for the both Li0.66FePO4 powders.(denoted Undoped Li0.66FePO4, and Z2S Li0.66FePO4) The Li0.66FePO4 was prepared by the mixture of the pristine powders and chemically delithiated powders. The temperature was changed from 25°C to 300°C by 5ºC min-1, and XRD measurements was performed after keeping for 10 hours at several temperatures. The 2θrange was 29.0º ~ 31.5º. The working electrodes for the electrochemical tests were prepared by mixing 80% active material, 10% carbon black, and 10% polyimmide binder with 1-methyl-2-pyrrolidinone solvent and coating on the aluminum current collector. A binary molten salt electrolyte based on MN(SO2CF3)2(M=Li,Cs) was used as the electrolyte. The temperature range of galvanostatic charge-discharge was performed between 150°C and 170°C, rate performance was measured in 170°C. RESULTS AND DISCCUSION The temperature controlled XRD patterns of Undoped Li0.66FePO4 corresponded with the reported phase diagrams.4 The peaks of LxFP 020 was appeared above 200ºC. On the other hand, Z2S Li0.66FePO4 tends to decrease the solid solution formation temperature. The peaks of LxFP 020 was appeared above 150ºC. Z2S sample stabilized the LxFP phase at lower temperature than undoped LiFePO4. We also performed galvanostatic charge-discharge measurements at 170ºC by using intermediate temperature ionic liquids. The charge curves of Z2S have the two plateau regions despite the curves of undoped LiFePO4 have the single plateau. This phenomena corresponded with the results of the temperature controlled XRD. At a high rate of 30C, the capacity of Z2S was higher than undoped LiFePO4. The difference of the obtained capacity between Z2S and undoped LiFePO4 at high rate was larger than the room temperature conditions. The LxFP phase stabilized by cation substitutions and appearance of LxFP phase leads the high rate performance of the LiFePO4electrodes. REFERENCES 1. Y. Orikasa, T. Maeda, Y. Koyama, H. Murayama, K. Fukuda, H. Tanida, H. Arai, E. Matsubara, Y. Uchimoto and Z. Ogumi, J. Am. Chem. Soc., 135, 5497 (2013). 2. M. Nishijima, T. Ootani, Y. Kamimura, T. Sueki, S. Esaki, S. Murai, K. Fujita, K. Tanaka, K. Ohira, Y. Koyama and I. Tanaka, Nat. Commun., 5(2014). 3. F. Omenya, N. A. Chernova, Q. Wang, R. Zhang, and M. S. Whittingham, Chem. Mater., 25, 2691, (2013). 4. Dodd, J. L., Yazami, R., Fultz, B., Electrochem. Solid State lett., 9 (3), A151 (2006).

Stabilization of Intermediate Phase in LiFePO4 Leading to High-Rate Performance

INTRODUCTION LiFePO4 is a commercially successful cathode material due to low costs and high-power energy density. The electrode reaction proceeds divided into two phases, Li-rich phase and Li-poor phase, with a large volume change. We reported the intrinsic high-rate nature that the intermediate phase, which is metastable at room temperature, emerges to moderate the lattice mismatch and gets closer to the single-phase reaction1. Here, we show how the stabilization of the intermediate phase can improve the rate capability using time-resolved in-situ XRD measurements. We also derive general non-equilibrium criteria for two-phase reaction in Li-intercalation compounds using the LiFePO4 system as a specific example. EXPERIMENTAL Undoped LiFePO4 and Li(Fe0.95Zr0.05)(P0.9Si0.1)O4, denoted UL and Z2S respectively, were synthesized in the same manner as reported2. The partially delithiated materials which is Li concentration of x = 0.66 in LixFePO4 were prepared by the chemical reduction. The ex situ X-ray diffraction (XRD) patterns were obtained at various Li concentration and elevated temperature. The electrochemical tests were performed using a liquid electrolyte at room temperature and a molten salt electrolyte at elevated temperature. The time-resolved in situ XRD measurements were performed at BL28XU, SPring-8 with a wavelength of 0.619862(2) Å using a 1D detector, MYTHEN 1K. The data were collected in the 2θ range of 10° to 13° with an exposure time of 1 s. RESULTS AND DISCCUSION The ex situ XRD patterns of Li0.66FePO4 show that the intermediate phase explicitly emerge at 150°C in Z2S whereas 200°C in UL, which indicate the stabilization of the intermediate phase by the reduced lattice volume change. This result is also supported the electrochemical tests at elevated temperature whether or not the two-step voltage plateau could be observed. The ex situ XRD patterns of various Li concentration and the open-circuit voltage measurements show a typical two-phase characteristics in both cathodes. There is no significant difference in phase transition behavior based on an equilibrium system because the intermediate phase is metastable at room temperature. We performed time-resolved in-situ XRD measurements to access the non-equilibrium phase transition under a large overpotential. At a high rate of 10C, the peak shift mainly occurs in Z2S compared with UL. This suggests the quasi-single phase reaction proceeds via the intermediate phase. The reaction kinetics was then evaluated. At higher rates, the differences of both capacities are lager, which indicates the enhancement of the rate capability in Z2S. The stabilization of the intermediate phase by the reduction of the lattice volume change leads to the ability of high-rate cycling. This insight involves new concepts in non-equilibrium thermodynamics, which may apply other two-phase electrode materials. REFERENCES 1. Y. Orikasa, T. Maeda, Y. Koyama, H. Murayama, K. Fukuda, H. Tanida, H. Arai, E. Matsubara, Y. Uchimoto and Z. Ogumi, J. Am. Chem. Soc., 135, 5497 (2013). 2. M. Nishijima, T. Ootani, Y. Kamimura, T. Sueki, S. Esaki, S. Murai, K. Fujita, K. Tanaka, K. Ohira, Y. Koyama and I. Tanaka, Nat. Commun., 5 (2014). 3. A. Van der Ven, K. Garikipati, S. Kim and M. Wagemaker, J. Electrochem. Soc., 156, A949 (2009).

Enhancing Surface Diffusion in Battery Electrodes Using Solvent Adsorbates

LiFePO4 is one of the most well-studied positive electrode materials. A well-established doctrine is that Li only diffuses within one-dimensional channels, with negligible diffusivities in the other directions, and almost no movement of Li between channels. However, existing experimental measurements and density functional theory calculations have generally been conducted for bulk LiFePO4, and have largely neglected surface diffusion at the solid/liquid interface. In our work, we directly study the ambipolar Li diffusion along these nominally slow directions perpendicular to the fast conduction channels. Because ambipolar transport of Li is controlled by the slow species, it is difficult to quantify using standard electrical measurements, which generally probe the fast species. To overcome this challenge, we measure the time for a solid solution Li0.5FePO4 electrode to separate into Li-rich and Li-poor phases. Because phase separation necessitates the movement of Li within a particle between the fast conduction channels, this technique directly provides the ambipolar transport of Li. Our results show that phase separation is extremely slow under dry, inert conditions. However, when placed in an organic solvent containing ethylene carbonate but no electrolyte salt, the rate of phase separation is enhanced by more than two order of magnitude. Because the ethylene carbonate only interacts with the surface of the LiFePO4 particles, the enhancement in Li transport demonstrate that surface diffusion is many orders of magnitude faster than bulk diffusion. Additionally, the rate of surface diffusion can be tuned by environmental adsorbates. We confirm our findings through density functional theory calculations, which show a significant reduction in migration enthalpy in the presence of a solvent adsorbate. Our results show that surfaces diffusion enhances the conductivity of Li along the nominally slow directions by several orders of magnitude, and thus reduces the anisotropy of Li diffusion in LiFePO4. Additionally, by demonstrating that the conductivity can be tuned using environmental adsorbates, we offer a novel approach towards enhancing the conductivity of electrode materials. By choosing the correct adsorbates to activate surface diffusion, nominally insulating materials can be conductive at the solid/liquid interface. This introduces a new paradigm in interfacial engineering of electrodes, and could significantly expand the class of insulating materials which are suitable as electrodes in Li-ion batteries.

The strain effect on lithium ion migration in Li-Si alloys: A first-principles study

Abstract First-principles calculations were performed to investigate the strain effect on the lithium ion migration in Li 1 Si 1 , Li 2 Si 1 and Li 7 Si 2 alloy. The three kind of Li-Si phase respectively correspond the lithium concentration change from low to high in lithiation process. The calculation focuses on migration pathways and energy barriers of the lithium ion migration in different strain by using the climbing-image nudged elastic band method. The result show that the tensile strain can cause the lithium ion migration energy barrier decrease, while the compressive strain can lead to the energy barrier increase in three Li-Si alloys. Under the considered strain range, the effect of compression strain stage is greater in the Li-poor phase (Li 1 Si 1 , Li 2 Si 1 ) than in Li-rich phase (Li 7 Si 2 ), while the effect of tensile strain stage is bigger in the Li-rich phase than in Li-poor phase.