Gradient Cathode Research Articles

Concentrated Gradient Cathode Materials with LiNi0.9Mn0.05Co0.05O2 Core and LiNi0.33Mn0.33Co0.33O2 Surface Composition

Layered nickel-rich cathode material, LiNi0.9Co0.05Mn0.05O2, exhibits high specific capacity of approximately 220 mAh/g-oxide and shows attracted significant interest for use in plug-in hybrid electric vehicles owing to its high capacity and low cost. However, significant challenges remain to improve capacity retention during cycling and thermal-abuse tolerance of this material. To improve the electrochemical performance and stability of this kind nickel-rich LiNixMnyCozO2 (NMC, x ≥ 0.6, x+y+z=1) cathode material for the usage in practical batteries, concentrated gradient cathode materials have been investigated. This family of materials shows the gradual decrease of nickel concentration and increase of manganese concentration from the center towards the outer layer of the particle [1-2]. Even though concentrated gradient cathode material is one of promising approaches to mitigate the capacity fade during cycling and thermal runaway of layered nickel-rich cathode materials, its working mechanism has not been fully understood yet. In this study, we explore two types of concentrated gradient cathode materials, core-shell and core-gradient, with nickel-rich LiNi0.9Mn0.05Co0.05O2 core and LiNi0.33Mn0.33Co0.33O2 surface composition which have LiNi0.8Mn0.1Co0.1O2 as overall composition. The precursors were synthesized using hydroxide co-precipitation method using a 20L batch reactor and then dried, mixed with lithium source and calcined. We report the characterized physical properties of the prepared core-shell and core-gradient materials. The detailed electrochemical performance comparison of the prepared materials with commercial NMC811 and NMC333 will be discussed. Acknowledgement Funding for this work from the Office of Vehicle Technologies of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. References Y.-K. Sun, S.-T. Myung, M.H. Kim, J. Prakash and K. Amine, J. Am. Chem. Soc. 127(38), 13411 (2005).U.-H. Kim, E.-J. Lee, C. S. Yoon, S.-T. Myung and Y.-K. Sun, Adv. Energy Mater. 6, 1601417 (2016)

Gradient design of Pt/C ratio and Nafion content in cathode catalyst layer of PEMFCs

In order to increase the utilization of Pt, reduce mass transfer loss and improve the performance of polymer electrolyte membrane fuel cells (PEMFCs) under low humidity and high current densities, the cathode catalyst layers with two layers of different Pt/C ratio and Nafion content are fabricated and evaluated. Polarization curves (IVs), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are employed to characterize and compare the effects of Pt/C ratio and Nafion content gradient on the performance of PEMFCs under different humidification conditions. The results indicate that the performance of the membrane electrode assembly (MEA) can be significantly improved via allocating more Nafion and Pt/C in the sublayer near the membrane in cathode catalyst layer. The MEA with optimal gradient cathode catalyst layer results in improved catalysts utilization compared to MEA with single cathode catalyst layer, 0.403 g kWrated−1 and 0.711 g kWrated−1 under 80 RH% and 20 RH%, respectively. The areal power density of the optimal MEA is 28.4% and 135.7% higher than the conventional single-layer catalyst layer MEA under high and low humidity, respectively.

Extending the Battery Life Using an Al-Doped Li[Ni0.76Co0.09Mn0.15]O2 Cathode with Concentration Gradients for Lithium Ion Batteries

The cycling stability of a Ni-enriched compositionally graded Li[Ni0.76Co0.09Mn0.15]O2 cathode doped with Al (1 and 2 mol %) was explicitly demonstrated by cycling the cathodes in a full cell against a graphite anode up to 1000 cycles. Without Al doping, the pristine gradient cathode retained 88% of the initial discharge capacity, whereas the 2 mol % Al-doped gradient cathode retained 95% of its original capacity. Meanwhile, Li[Ni0.82Co0.14Al0.04]O2 (NCA), representing a typical cathode for commercialized electric vehicles, retained only 80% of the initial capacity. It was shown that Al doping together with the unique morphology of the compositionally graded cathode was able to suppress the microcracking and helped to preserve the mechanical integrity of the cathode particles, whereas the benchmark NCA cathode sustained continuous capacity loss during cycling and was completely pulverized. The remarkable long-term cyclability of the Al-doped gradient cathodes was attributed to the enhanced structural and ...

Strategies to mitigate Pt dissolution in low Pt loading proton exchange membrane fuel cell: I. A gradient Pt particle size design

The catalyst durability at low Pt loading remains a barrier for industrial commercialization of the proton exchange membrane fuel cell. Degradation of low loaded Pt catalyst not only reduced the electrochemical surface area but also revealed a Pt depletion zone adjacent to the cathode/membrane interface where about 80% of the Pt was lost due to dissolution and migration into the membrane. We hypothesized that the Pt degradation can be mitigated using a gradient cathode design without sacrificing the initial fuel cell performance. With a focus on the mitigation, the first of two companion papers focuses on the Type I cathode with larger Pt particles (5nm average size) near the cathode/membrane interface. In the second of these two papers, the Type II cathode with higher Pt loading (60wt.% Pt on carbon) near the cathode membrane interface will be investigated. The catalyst coated membranes with gradient cathode were fabricated by reactive spray deposition technology with low Pt loading of 0.05mgcm−2 on the anode and 0.1mgcm−2 on the cathode. The DOE defined accelerated stress test was performed by imposing a triangular wave potential cycling from 0.6V to 1.0V for 30,000 cycles at 50mVs−1 scan rate. Results of accelerated stress test showed that the loss of electrochemical surface area and PEMFC performance were reduced for the Type I cathode compared to the control cathodes that consisted of 2nm averaged Pt particle size only. Microscopy analysis suggested two possible mechanisms for this durability improvement: 1) the reduction of the amount of Pt lost due to Pt dissolution, and 2) the preservation of Pt particles at the cathode/membrane interface. Comparison with the control cathode showed that the loss of Pt near the cathode/membrane interfaced were reduced from 80% to 40% and the overall Pt loss of the cathode were reduced from 30% to 15%.

Strategies to mitigate Pt dissolution in low Pt loading proton exchange membrane fuel cell: II. A gradient Pt loading design

In the second companion paper on low Pt-loaded gradient cathode, the degradation of Type II cathode with higher Pt loading (60wt%) near the cathode/membrane interface was studied and compared to a control cathode with uniform Pt loading (40wt%). All catalyst coated membranes were deposited through reactive spray deposition technology with average Pt particle size of 2nm and Pt loadings of 0.05mgcm−2 and 0.1mgcm−2 on anode and cathode, respectively. The Type II cathode showed improved fuel cell performance at beginning-of-test (BOT) due to higher Pt utilization benefited from high Pt loading close to the membrane. Although the loss of electrochemical surface area for the Type II cathode was ∼80%, similar to that of the control cathode, the end-of-test performance (EOT) was improved by 50–100mV, which can be attributed to the reduction of Pt loss in the Pt depletion zone (from 80% to 60%) as well as from 30% to 18% for the entire cathode. A comparison between Type I and Type II cathode was made in terms of Pt retention and particle size distribution. Type II cathode turned out to be more effective than Type I cathode as a result of being able to maintain a higher percentage of smaller particles (<5nm), even though the Pt loss% was a slightly higher than the Type I cathode. Therefore, when evaluating the end-of-test performance, two crucial parameters need to be considered: (1) the retention the Pt particles in the depletion zone within 0∼2μm from the cathode/membrane interface and (2) the percentage of small Pt particles in the cathode at EOT. The usual performance indicator, the electrochemical surface area, ceased to be an effective parameter when the Pt/C catalyst has been degraded.

The Rod-Shaped Crystallographic Texture of FCG(Full Concentration Gradient) Cathode Material with Long-Term Cycling Stability for Xev Application

Al is introduced into a compositionally graded cathode with average composition of Li[Ni0.61Co0.12Mn0.27]O2 (FCG61) whose Ni and Mn concentrations are designed to vary continuously within the cathode particle. The Al-substituted full concentration gradient (Al-FCG61) cathode is tested for 3000 cycles in a full-cell, mainly to gauge its viability for daily charge/discharge cycles during the service life of electric vehicles (≈10 years). The Al-substitution enables the Al-FCG61 cathode to maintain 84% of its initial capacity even after 3000 cycles. It is demonstrated that the Al-substitution strengthens the grain boundaries, substantiated by the mechanical strength data, thereby delaying the nucleation of microcracks at the phase boundaries which is shown to be the main reason for the cathode failure during long-term cycling. It also shows that the Al-substitution decreases the cation mixing and suppresses the deleterious formation of the secondary phase that likely initiates the microcracks. Unlike an NCA cathode, whose depth of discharge (DOD) must be limited to 60% for long-term cycling, the proposed Al-FCG61 cathode is cycled at 100% DOD for 3000 cycles to fully utilize its available capacity for maximum energy density and subsequent reduction in cost of the battery.

Modelling Analysis of Platinum Dissolution in Polymer Electrolyte Membrane Fuel Cells with Gradient Cathode Catalyst Layers

Over the past decades, significant effort has been dedicated to the development of polymer electrolyte membrane fuel cells (PEMFC) with improved durability. To improve the durability of PEM fuel cells, the scientific focus has been to understand the factors that determine a PEM fuel cell’s lifetime, so that the lifetime can be increased without losing performance or increasing cost. Specifically, attention has been devoted to the stability of low Platinum cathode catalyst layers. Degradation of Platinum nanoparticles over carbon support is critical in consequence of several mechanisms, among which Platinum dissolution. As proposed in the literature, a possible strategy to mitigate Platinum dissolution consists in the development of catalyst layers with gradient properties [1]. In the present work, a theoretical analysis of Platinum dissolution is applied to the interpretation of degradation data recorded on catalyst-coated membranes (CCMs) fabricated by reactive spray deposition technique with 25 cm2 active area and low Pt loading 0.10 mg cm-2 for the cathode and 0.05 mg cm-2 for anode. The standard protocol defined by U.S. Department of Energy is used to subject the cells to accelerated stress test (AST) with triangle sweep cycles between 0.6 V and 1.0 V at 50 mVs−1 scan rate under inert atmosphere. The effect of aging on cell performance was monitored by measuring the electrocatalyst active surface (ECSA), hydrogen crossover, polarization curves and Electrochemical Impedance Spectroscopy (EIS) after 5000, 10,000, and 30,000 cycles. In order to get insight into the degradation process of cathode catalyst layer under accelerated durability testing, a 1D transient model referenced to the literature [2] has been calibrated and validated on a set of experimental data. The model solves the mass conservation equation for Platinum dissolved in the ionomer phase and a simple model for Platinum dissolution is proposed. The validity of the proposed model is discussed versus the experimental data that include in situ measurements collected on samples with different Platinum nanoparticle size (2nm, 3nm, 5nm) and ex situ data obtained from TEM observations. The evolution of catalyst active surface as predicted by the simulations reveals good consistency between model predictions and measurements, as reported in Fig. 1. It is concluded that the model captures the effect of particle size on nanoparticle stability and is thus used as a tool to analyze PEMFC durability in accelerated degradation tests. The model predicts the formation of a catalyst depleted zone, in consequence of the Platinum flux from the catalyst layer to the polymer membrane, in agreement with TEM observations. The model is later applied to the analysis of samples with gradient properties. Two sets of gradient catalyst layers have been analysed: materials with gradient in catalyst loading across the catalyst layer thickness; materials with gradient in Platinum particle size across the catalyst layer. In both cases the model can predict the trend of measured ECSA. The model predicts that the gradient materials show improved durability if they limit Platinum depletion in the region close to the polymer membrane. According to the model predictions, gradient samples with improved durability show higher content of Platinum catalyst retained in the region next to the membrane. Consistency with TEM observations obtained on gradient samples is discussed and some discrepancies are highlighted as a starting point to improve the model. Figure 1

Long term stability of porosity gradient composite cathode controlled by electro-static slurry spray deposition

Proton conducting fuel cells (PCFCs) have attracted much attention due to the lower activation energy of proton transport at intermediate temperatures. The cathode of PCFCs has been studied for enhancement of cell performance. Unlike solid oxide fuel cells (SOFCs), the product of chemical reaction, H2O, is generated in the cathode region. Generally, as the structure of the PCFC' cathode is denser than the anode of SOFC, the vapor is not easily emitted outside of the cathode, and this remaining vapor in the isolated cathode structure has a negative effect on cell performance. In this work, a gradient porous La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)/BaCe0.7Zr0.15Y0.15O3−δ (BCZY) composite cathode was developed for fuel cells based on the BCZY electrolyte using an electrostatic spray slurry deposition method. The cathode porosity and deposited powder size gradually increased in the direction away from the electrolyte-electrode. The results indicate that the structure of the porosity gradient cathode might enhance the long-term stability in proton conducting fuel cells.

(Invited) Progress in High-Capacity Gradient Cathode Materials for Rechargeable Lithium Batteries

High-energy-density rechargeable batteries are needed to fulfill various demands such as portable electronic devices, energy storage systems, and (hybrid) electric vehicles. As a result, high-energy electrode materials enabling a long cycle life and reliable safety need to be developed. To ensure these requirements, new material chemistries can be derived from hybridization of at least two compounds in a secondary particle with varying chemical composition and primary particle morphologies having a core−shell structure and spherical cathode-active materials with rod-shaped primary particle. To this end, several layer gradient cathode materials were developed to ensure high capacity, reliability, and safety.

Compositionally Graded Cathode Material with Long‐Term Cycling Stability for Electric Vehicles Application

Al is introduced into a compositionally graded cathode with average composition of Li[Ni0.61Co0.12Mn0.27]O2 (FCG61) whose Ni and Mn concentrations are designed to vary continuously within the cathode particle. The Al‐substituted full concentration gradient (Al‐FCG61) cathode is tested for 3000 cycles in a full‐cell, mainly to gauge its viability for daily charge/discharge cycles during the service life of electric vehicles (≈10 years). The Al‐substitution enables the Al‐FCG61 cathode to maintain 84% of its initial capacity even after 3000 cycles. It is demonstrated that the Al‐substitution strengthens the grain boundaries, substantiated by the mechanical strength data, thereby delaying the nucleation of microcracks at the phase boundaries which is shown to be the main reason for the cathode failure during long‐term cycling. It also shows that the Al‐substitution decreases the cation mixing and suppresses the deleterious formation of the secondary phase that likely initiates the microcracks. Unlike an NCA cathode, whose depth of discharge (DOD) must be limited to 60% for long‐term cycling, the proposed Al‐FCG61 cathode is cycled at 100% DOD for 3000 cycles to fully utilize its available capacity for maximum energy density and subsequent reduction in cost of the battery.

Thermal Poling of Soda‐Lime Silica Glass with Nonblocking Electrodes—Part 1: Effects of Sodium Ion Migration and Water Ingress on Glass Surface Structure

It is generally well known that not only the sodium itself, but also the non‐bridging oxygen (NBO) sites associated with sodium ions are largely responsible for the surface reactivity of soda‐lime‐silica (SLS) glass. Thermal poling can modify the distribution of sodium in the subsurface region. In this work, a commercial SLS float glass was thermally poled using nonblocking electrodes in air. The Na+−depleted anode surface and the Na+−gradient cathode surface were characterized using a variety of methods to find the compositional, structural and morphological effects of thermal poling. Of particular significance is the use of nondestructive vibrational spectroscopy methods, which can lead to new and improved understanding of water interactions with sodium and its sites in the glass. It was found that during thermal poling, the Na+−depleted glass network on the anode side undergoes condensation reactions of NBO sites accompanied by the increase in concentrations of silanol (SiOH) groups and molecular water species. In contrast, silanol and water species do not increase and the silicate network change is negligible in the Na+−gradient cathode side. Vibrational sum frequency generation (SFG) spectroscopy analysis revealed the difference in distributions of hydrous species in the Na+−depleted and Na+−gradient surfaces. The structural information of the thermally‐poled surfaces provides critical insights needed to understand the mechanical and mechanochemical properties of the Na+−concentration modified SLS glass surfaces reported in the Part 2 companion paper.

Synthesis of full concentration gradient cathode studied by high energy X-ray diffraction

Nickel-rich metal oxides have been widely pursued as promising cathode materials for high energy-density lithium-ion batteries. Nickel-rich lithium transition metal oxides can deliver a high specific capacity during cycling, but can react with non-aqueous electrolytes. In this work, we have employed a full concentration gradient (FCG) design to provide a nickel-rich core to deliver high capacity and a manganese-rich outer layer to provide enhanced stability and cycle life. In situ high-energy X-ray diffraction was utilized to study the structural evolution of oxides during the solid-state synthesis of FCG lithium transition metal oxide with a nominal composition of LiNi0.6Mn0.2Co0.2O2. We found that both the pre-heating step and the sintering temperature were critical in controlling phase separation of the transition metal oxides and minimizing the content of Li2CO3 and NiO, both of which deteriorate the electrochemical performance of the final material. The insights revealed in this work can also be utilized for the design of other nickel-rich high energy-density cathode materials.

High Energy X-Ray Used to Investigate the Synthesis of Full Concentration Gradient Cathode

Nickel-rich lithium transition metal oxides can deliver a high specific capacity during cycling, but there is a concern about the highly reaction between the cathode and the non-aqueous electrolytes. To maintain high specific capacity and improve cycling performance , we have employed solid state reaction to synthesize a full concentration gradient (FCG) cathode, which has a nominal composition of LiNi0.6Mn0.2Co0.2. It is designed to provide a nickel-rich core to deliver high capacity and a manganese-rich outer layer to provide enhanced stability and cycle life. The solid-state reaction to form a high performance nickel-rich cathode material is a complicated multiple-step reaction. Therefore, in situ high-energy X-ray diffraction was utilized to study the structural evolution during the solid-state synthesis of FCG cathode. We found that both the pre-heating step and the sintering temperature were critical in controlling phase separation of the transition metal oxides and minimizing the content of Li2CO3 and NiO, both of which deteriorate the electrochemical performance of the final material. The insights revealed in this talk can also be utilized for the design of other nickel-rich high energy-density cathode materials.

Advanced Performances of Full Concentration Gradient Li[Ni0.65Co0.08Mn0.27]O2 Cathode Materials for Lithium Ion Batteries

Full concentration gradient (FCG) cathode material which consists of Ni-rich core Li[Ni0.89Co0.01Mn0.10]O2 and Ni-less outer layer Li[Ni0.61Co0.09Mn0.30]O2 was synthesized by co-precipitation method.1,2 A gradually changing concentration gradients for Ni, Co, and Mn extended from the core to the surface within a single cathode particle with an average composition of Li[Ni0.65Co0.08Mn0.27]O2. We have evaluated electrochemical and thermal properties of the FCG cathode material and compared it with the conventional cathode (CC) material, Li[Ni0.65Co0.08Mn0.27]O2 without the concentration gradient. The BET surface area measurement demonstrated the presence of high-density primary particles in the FCG cathode material. The FCG cathode exhibited higher discharge capacity, excellent cycle retention and superior thermal stability compared to the CC Li[Ni0.65Co0.08Mn0.27]O2 cathode material.1

Diffusion in the Layered Li-Ni-Mn-Co Oxide Based Core-Shell/Gradient Materials during Sintering Simulated with Pellets in Contact

Core-shell structured positive electrode materials based on layered Li-Ni-Mn-Co oxide could be the next generation of positive electrode materials for high energy density lithium-ion batteries. This is because a high energy core material with poor stability against the electrolyte can be protected by a thin layer of a stable shell material. In our previous report1,2, Li and Mn-rich materials were used as the protecting shell, and Ni-rich materials were used as the core. It was shown that the Mn-rich shell can effectively protect the Ni-rich core from reactions with the electrolyte while the Ni-rich core renders a high and stable average voltage.2However, it was also found that diffusion of the cations between the core and shell phases occurs during sintering. Figure 1 shows energy dispersive X-ray spectroscopy (EDS) results of the core-shell precursor and lithiated samples. It is demonstrated that a Mn-rich shell was still maintained whereas the Co, which was only in the shell in the precursor, was approximately homogeneous throughout the particles after sintering at 900oC compared to the prepared hydroxide precursors. This diffusion phenomena in core-shell/gradient materials has not been previously examined in detail, to our knowledge, despite the large amount of work on the nickel rich NMC (LiNixMnyCozO2) (without excess lithium) gradient cathode material, which has been published and cited.3–5In this work, a series of experiments to measure the interdiffusivity of the cations were designed. Pellets of core material and shell material were placed in contact and heated to induce cation diffusion over long, measurable distances. Based on the analyzed composition profiles, interdiffusion constants can be extracted. Li1.09(Ni0.8Mn0.2)0.81O2, Li1.0 5(Co0.9Mn0.1)0.8 5O2, LiNi0.8Co0.2O2 and LiCoO2 powders were produced at 900oC in oxygen for 10 h, while Li2MnO3 was made in air. Powder a was first compressed to 15 MPa for 5 minutes, then powder b was added to the mould on top of the pellet a. The powders were compressed again to 50 MPa for 10 mins to ensure a good contact at the interface. The pellets were heated to 900oC for 10 h and then cooled slowly. The heated pellets were purposely fractured in to smaller pieces, which were subsequently embedded into Crystal BondTM, and eventually polished to a mirror surface finish for scanning electron microscopy (SEM) and EDS experiments. Figure 2 shows the back scattered SEM image of the cross-section of a pellet that had LiCoO2 powder on the left side and LiNi0.8Co0.2O2 on the right side. Small pores are observed at the interface. Figure 3a shows the Ni and Mn atomic concentration profile against position for the Li1.09(Ni0.8Mn0.2)0.81O2/Li2MnO3 pellet measured using EDS line scans. The symbols show the original data points. The concentration of Ni decreased from 80% to about 0% in about 2 µm while the concentration of Mn increased from 20% to about 100% over the same distance. Figure 3b shows the Ni and Co atomic concentration profiles versus position for the LiCoO2/LiNi0.8Co0.2O2 pellet measured using EDS. Figure 3b shows that the atomic concentration of Co decreased from 100% to about 20% in about 20 µm, which is about 10 times larger than that for the Ni and Mn shown in Figure 3a, indicating the interdiffusion between Ni3+ and Co3+ is much larger than that between Ni3+ and Mn4+. This result agrees well with the trends observed in the core-shell samples shown in Figure 1. Interdiffusion constants of Ni, Co and Mn in various couples as a function of temperature will be extracted and will be reported in the presentation.

Advanced Concentration Gradient Cathode Material with Two‐Slope for High‐Energy and Safe Lithium Batteries

Li[Ni0.65Co0.13Mn0.22]O2 cathode with two‐sloped full concentration gradient (TSFCG), maximizing the Ni content in the inner part of the particle and the Mn content near the particle surface, is synthesized via a specially designed batch‐type reactor. The cathode delivers a discharge capacity of 200 mAh g−1 (4.3 V cutoff) with excellent capacity retention of 88% after 1500 cycles in a full‐cell configuration. Overall electrochemical performance of the TSFCG cathode is benchmarked against conventional cathode (CC) with same composition and commercially available Li[Ni0.8Co0.15Al0.05]O2 (NCA). The TSFCG cathode exhibits the best cycling stability, rate capability, and thermal stability of the three electrodes. Transmission electron microscopy analysis of the cycled TSFCG, CC, and NCA cathodes shows that the TSFCG electrode maintains both its mechanical and structural integrity whereas the NCA electrode nearly pulverizes due to the strain during cycling.

Improved Performances of Li[Ni0.65Co0.08Mn0.27]O2 Cathode Material with Full Concentration Gradient for Li-Ion Batteries

Full concentration gradient (FCG) cathode material having nickel-rich core Li[Ni0.89Co0.01Mn0.10]O2 and nickel-deficient outer layer Li[Ni0.61Co0.09Mn0.30]O2 was synthesized via co-precipitation method. A smoothly varying concentration gradients for Ni, Co, and Mn extended from the core to the surface within a single cathode particle with an average composition of Li[Ni0.65Co0.08Mn0.27]O2. Electrochemical and thermal properties of the FCG cathode were evaluated and compared to the conventional cathode (CC) material, Li[Ni0.65Co0.08Mn0.27]O2 without the concentration gradient. The BET surface area measurement demonstrated the presence of densely agglomerated primary particles in the FCG cathode. Owing to the unique spatial distribution of the cations and particle morphology, the FCG cathode delivered higher discharge capacity, superior cycle stability and excellent thermal stability compared to the CC Li[Ni0.65Co0.08Mn0.27]O2 .

A gradient porous cathode for non-aqueous lithium-air batteries leading to a high capacity

In conventional porous cathodes of non-aqueous lithium-air batteries, the higher discharge reaction rate in the oxygen richer region will result in a gradient distribution of the solid product, Li2O2, decreasing from the air side to the separator side. Such a distribution of the solid product means that although the pores toward the separator side remain open, the pores at the air side will be blocked first with an increase in the discharge capacity, terminating the discharge process and resulting in a low discharge capacity. In this work, we design and fabricate a cathode structure with a stepwise gradient pore distribution (pore size reducing from 500 to 300 and 100nm) by mixing carbon powder and nanotubes. The gradient porous cathode enables the capacity of a non-aqueous lithium-air battery discharging at 0.1mA/cm2 to be 19.2% higher than that by a uniform porous cathode (~100nm in pore size) and 82.3% higher than that by a uniform porous cathode (~500nm in pore size). The SEM image analysis suggests that the increased discharge capacity can be mainly attributed to the fact that the gradient cathode can not only increase oxygen transport pathways but also retain a sufficiently large specific surface area.

Electrochemical performance of continuously gradient composite cathode fabricated by electro-static slurry spray deposition

The cobalt-based cathode, LSCF, BSCF and SSC, have high thermal expansion coefficients, which cause poor thermal compatibility with electrolytes and limited cell performance. In this work, a continuously gradient Sm0.5Sr0.5CoO3−δ (SSC) – BaCe0.7Zr0.2Y0.1O3−δ (BCZY) composite cathode was developed for BCZY electrolyte based fuel cells by electrostatic spray slurry deposition (ESSD). In the deposited cathode the content of BCZY gradually decreased and that of SSC gradually increased in the direction away from the electrolyte–cathode interface. The single cell, consisting of a BCZY–NiO anode substrate, a BCZY electrolyte and a compositionally gradient SSC-BCZY cathode layer, was assembled and tested from 550 to 700 °C under humidified hydrogen atmosphere (∼3% H2O) and static air as an oxidant. The cross-sectional morphology of the cathode was examined by energy dispersive X-ray (EDX) analysis. The comparison of electrochemical performances between various cathode configurations was conducted to demonstrate the effect of gradient cathode. The power density and impedance spectra indicate that the optimized cathode structure greatly improves the polarization resistance and ohmic resistance. The results suggest that the migration of protons to triple phase boundaries (TPBs) and the surface diffusion of O− might be enhanced in the gradient SSC–BCZY composite cathodes.

Synthesis and electrochemical performance of LiNi0.7Co0.15Mn0.15O2 as gradient cathode material for lithium batteries

LiNi0.7Co0.15Mn0.15O2 as a cathode material for lithium batteries was synthesized by mixing hydroxide co-precipitated precursors with 5% excess LiOH·H2O. Its structural and electrochemical properties were investigated using X-ray diffractometry, scanning electron microscopy, galvanostatic charge–discharge test, and electrochemical impedance spectroscopy. The results indicated that well-ordering layered LiNi0.7Co0.15Mn0.15O2 cathode materials were successfully prepared in air at 750, 800, and 850°C with α-NaFeO2 typical crystal. The results of charge–discharge test demonstrated that the gradient cathode material sintered at 850°C exhibited the best electrochemical performance with the initial discharge capacity of 164mAhg−1 at 0.2C and lower electrochemical impedance. Nickel has low price. LiNiO2 cathode materials have high specific capacity, their theoretical capacity is 274mAhg−1 and with low self-discharge rate. So the Ni, Co, Mn ternary layer-structural compounds with high Ni content are showing to be promising cathode materials for lithium batteries. The techniques and research results in this paper are utilizable for the study of this kind of lithium battery materials.