Patent Application
20240145706
This application claims priority to Taiwan Patent Application No. 111141394, filed on Oct. 31, 2022. The entire content of the application is hereby incorporated by reference.
The present invention relates to a core-shell cathode and a lithium-sulfur battery using the same. More particularly, the present invention relates to a core-shell cathode structure which blocks passage of an active material from the cathode, and a lean-electrolyte lithium-sulfur battery using said core-shell cathode, which attains high sulfur loading, high sulfur content, excellent areal capacity and energy density, and stable long-term cyclability.
Nowadays, the development of energy-storage technology aims for high energy density and low production cost. Commercial lithium-ion battery cathodes offer high theoretical capacity of 200-250 mAh/g, and therefore dominate the energy-storage market for over 30 years. However, the mature insertion lithium-ion technology has now encountered challenges, such as the instability of the crystalline structure during cycling, the limited theoretical charge-storage capacity, and increasing high costs.
In order to break through the technological bottleneck in lithium-ion battery technology, various new types of batteries have developed and risen rapidly. Among them, high energy density lithium-sulfur batteries apply the electrochemical theory of conversion battery chemistry. During charge and discharge, per mole of active material, sulfur, involved in the phase transformations of the redox reaction involves two moles of electrons, and thus renders lithium-sulfur batteries having a high theoretical specific capacity of 1,672 mAh/g. Moreover, during phase transformations of charge and discharge, the intermediate product, liquid polysulfides, has strong reactivity with lithium, which can eliminate lithium dendrites and thus ensure the safety of batteries. In addition, sulfur element is abundant and inexpensive, and sulfur-based cathode material does not contain heavy metals. Therefore, sulfur is a low-polluting electrode material that is easily available and environmentally benign.
Despite the abovementioned advantages of lithium-sulfur batteries, during charge and discharge, the cathode active material undergoes multiphase transformations from solid phase to liquid phase and then to solid phase, which result in a huge volume change of 80% and a solid-liquid state electrochemical conversion of active material suffered by the electrode material. In addition, solid-state sulfur and its final reduction product after discharge, solid-state lithium sulfide, have high resistivity of 10−30 S/cm and 10−14 S/cm respectively, which causes only a small fraction of the active material simultaneously in contact with the cathode current collector and electrolyte to be electrochemically utilized. Also, the liquid-state polysulfides dissolve easily in the electrolyte, resulting in easy diffusion of the active material, thereby leaving the cathode and eventually causing capacity fade. If the active material diffuses to the Li-metal anode, it undergoes chemical reduction and form solid-state lithium sulfide precipitate on the surface of lithium metal. The Li-metal anode will thus lose its reactivity, thereby causing damage in capacity and charge-discharge reaction capacity. Due to these natures of sulfur cathode materials, current research on lithium-sulfur batteries often encounters challenges such as diffusion and loss of active material, and fast capacity fade.
Furthermore, if conventional electrode manufacturing process is utilized, which uses binders and conductive additives in an aluminum-foil current collector, the co-structure of electrode materials and current collector may gradually collapse after long-term cycling, resulting in a short cycle life. Also, addition of these inactive materials will render the lithium-sulfur batteries to encounter challenges from a material science perspective when attempting to satisfy high energy density requirement.
(1) Challenges in Increasing the Cathode Active Material Loading and Content
Conventional lithium-sulfur batteries have a sulfur loading of only 1-2 mg/cm2, and a sulfur content of less than 50 wt % due to large addition of electrical materials and composite materials. Moreover, sulfur and lithium sulfide, as the final products of lithium-sulfur batteries after charge and discharge reaction, both have high resistivity. Therefore, it is difficult to attain excellent theoretical capacity. In practice, it is also difficult to increase active material loading and simultaneously maintain desirable cycling performance.
(2) Complex Composite Cathode/Battery Materials
In order to promote the reactivity of cathode active material, additional conductive or catalytic inactive materials may be added through a complicated manufacturing process. Thus, it is difficult to increase cathode active content, causing the lithium-sulfur batteries to face challenges from a material science perspective when attempting to reach high energy density.
(3) Instability of Current Collector Structure
The active materials of lithium-sulfur battery have a significant volume change of up to 80% during charge and discharge reaction. If the conventional manufacturing process is applied, which uses binders to fix the active materials and conductive additives in aluminium-foil current collector, the co-structure of electrode materials and current collector may gradually collapse after long-term cycling, resulting in a short cycle life.
(4) Diffusion and Loss of Active Materials
During charge and discharge of lithium-sulfur battery, liquid-state polysulfides are formed, which are easily dissolved in liquid electrolytes and thereby escape from the electrodes. It causes irreversible capacity fade during repeated electrochemical reactions, thereby resulting in loss of active materials and rapid decline in electricity.
(5) Challenges in Lowering Electrolyte Content in a Battery
Lowering electrolyte content in a lithium-sulfur battery increases the concentration of polysulfides in the electrolyte, resulting in an increase in the viscosity of the electrolyte. Consequently, the ionic conductivity decreases, thereby leading to increased polarization of the battery. In addition, during the electrochemical reaction, active materials, lithium metal, and many battery components consume the electrolytes. Therefore, the decreased content of the electrolyte lowers the utilization of the active materials, and even leads to precipitations of active martials formed in an inert area, causing increase in impedance in the battery and reduction in capacity. It therefore affects stability of long-term cycling, coulombic efficiency of charge and discharge, and energy density. Also, the performance of the battery significantly declined during charge and discharge at a high rate, and thus loses its applicability. In view of this, addition of large amounts of electrolytes is applied in conventional lithium-sulfur batteries with an electrolyte-to-capacity ratio of over 20 μL/mg. Although high electrolyte content has an advantage of active materials utilization, thereby improving the battery performances in such as capacity, stability and coulombic efficiency during charge and discharge, excessive electrolyte content prevents the lithium-sulfur battery from attaining the required high energy density.
Hence, conventional lithium-sulfur batteries have problems with basic physical properties, for example, the key issues of the insulating nature of sulfur and polysulfide diffusion. Moreover, low sulfur loading, low sulfur content and excessive electrolytes all cause overestimation of the lithium-sulfur battery efficiency, and thus limit its commercial application.
In view of the abovementioned technical problems, the objective of the present invention is to provide a core-shell cathode, as well as synthesis of liquid polysulfides with high reactivity. Electrically conductive, porous carbon material is applied to form a shell, which is then assembled and tightly enclose the active material, sulfur, in a core, to improve reaction kinetics of the sulfur electrode, and eliminate diffusion of liquid polysulfides out from the cathode, thereby attaining high sulfur loading and high sulfur content, as well as excellent electrochemical efficiency and reversibility. Moreover, the present invention also provides a lithium-sulfur battery using said core-shell cathode, which simultaneously attains high energy density, high capacity retention and high cycling stability under lean-electrolyte condition.
In order to achieve the abovementioned objective, the present invention provides the following technical means.
In an aspect, the present invention provides a core-shell cathode, which is characterized by comprising:
The core-shell cathode of the present invention is assembled by stacking materials of two shapes, a solid shell and a hollow core, forming a three-dimensional structure with a central cavity. The active materials are enclosed in the central cavity of the core-shell cathode. The core-shell cathode, on a macro-scale, is required to enclose the active materials and function as a current collector as well.
In some embodiments, the shell comprises an electrically conductive, porous carbon material. In some embodiments, the electrically conductive, porous carbon material may be composed of highly conductive carbon fiber substrates or various functional carbon papers and carbon materials, for example, the electrically conductive, porous carbon material may comprise carbon nanofibers and carbon nanotubes, such as single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. In some embodiments, the electrically conductive, porous carbon material may include graphene and carbon powders. In some embodiments, the material of the shell includes various solid materials that function as current collectors.
The core-shell cathode of the present invention may function as a carrier for active materials as well as a current collector, thus eliminating the usage of aluminium-foil current collectors, such that the present invention attains high sulfur loading, high sulfur content, high electrochemical utilization, and high efficiency in charge and discharge.
The core-shell cathode of the present invention may effectively trap active materials within the cathode without addition of any binders, such that the present invention attains high sulfur loading, high sulfur content, and the electrochemical performance of cycling stability and rate capacity.
The sulfur loading described in the present invention is defined as total weight (mg) of the active material sulfur added in the battery. Due to electrochemical reaction properties and battery structure, sulfur element is only added in the cathode of a battery, so only the total weight of sulfur in the cathode is calculated. In some embodiments, the core-shell cathode of the present invention has a sulfur loading of at least 4 mg/cm2, preferably at least 5 mg/cm2, more preferably at least 8 mg/cm2, even more preferably at least 10 mg/cm2. In some embodiments, the core-shell cathode of the present invention has a sulfur loading of between 4 mg/cm2 and 30 mg/cm2, preferably between 5 mg/cm2 and 25 mg/cm2, more preferably between 8 mg/cm2 and 25 mg/cm2, more preferably between 8 mg/cm2 and 20 mg/cm2, even more preferably between 10 mg/cm2 and 20 mg/cm2. In some embodiments, the core-shell cathode of the present invention has a sulfur loading of 12 mg/cm2.
In some embodiments, the core-shell cathode of the present invention has a sulfur content of at least 50 wt %, preferably between 50 wt % and 95 wt %, more preferably at least 60 wt %, more preferably between 60 wt % and 85 wt %, even more preferably between 60 wt % and 70 wt %. In some embodiments, the core-shell cathode of the present invention has a sulfur content of 64 wt %.
In some embodiments, the shell of the core-shell cathode of the present invention comprises an electrically conductive, porous carbon material. In some embodiments, the electrically conductive, porous carbon material may be formed from a highly conductive carbon fiber substrate, or may be composed of various functional carbon papers and carbon materials, for example, the electrically conductive, porous carbon material may comprise carbon nanofibers and carbon nanotubes, such as single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. In some embodiments, the electrically conductive, porous carbon material may include graphene and carbon powders. In some embodiments, the material of the shell includes various solid materials that function as current collectors.
In some embodiments, the electrically conductive, porous carbon material has a specific surface area of preferably less than 200 m2/g, preferably less than 150 m2/g, more preferably less than 100 m2/g, more preferably between 60 m2/g and 90 m2/g, even more preferably between 75 m2/g and 85 m2/g. In some embodiments, the electrically conductive, porous carbon material has a specific surface area of approximately 80 m2/g. In some embodiments, the electrically conductive, porous carbon material has a total pore volume of between 0.3 cm3/g and 0.55 cm3/g, preferably between 0.3 cm3/g and 0.5 cm3/g, more preferably between 0.35 cm3/g and 0.45 cm3/g, even more preferably approximately 0.4 cm3/g. In some embodiments, the electrically conductive, porous carbon material has an average pore diameter of between 10 nm and 40 nm, preferably between 12 nm and 28 nm, more preferably approximately 20 nm.
In another aspect, the present invention provides a lithium-sulfur battery, which is characterized by comprising: a lithium anode, a separator, the core-shell cathode described herein, and an electrolyte.
The electrolyte-to-sulfur ratio (μL/mg) described in the present invention represents the electrolyte volume provided to active materials for solid-liquid state electrochemical conversion, which is highly correlated to energy density of a battery. The lower the electrolyte-to-sulfur ratio, the higher the energy density of a battery. Said electrolyte volume is defined as the total volume (μL) of electrolyte added in a battery, including the volume of all electrolytes added in the cathode and anode of a battery. So far, most of the research on lithium-sulfur batteries still applies an electrolyte-to-sulfur ratio of greater than 20 μL/mg, or even ignores this parameter. In some embodiments, the lithium-sulfur battery of the present invention has an electrolyte-to-sulfur ratio of less than 20 μL/mg, preferably between 3 μL/mg and 15 μL/mg, more preferably less than 10 μL/mg, more preferably between 3 μL/mg and 10 μL/mg, even more preferably between 3 μL/mg and 7 μL/mg.
In some embodiments, the lithium-sulfur battery of the present invention has an areal capacity of between 5 mAh/cm2 and 20 mAh/cm2, preferably between 5 mAh/cm2 and 12 mAh/cm2, more preferably between 8 mAh/cm2 and 12 mAh/cm2.
In some embodiments, the lithium-sulfur battery of the present invention has an energy density of at least 10 mWh/cm2, preferably at least 15 mWh/cm2, more preferably between 10 mWh/cm2 and 30 mWh/cm2, more preferably between 12 mWh/cm2 and 25 mWh/cm2, even more preferably at least 20 mWh/cm2. In some embodiments, the lithium-sulfur battery of the present invention has an energy density of between 18.1 mWh/cm2 and 20.5 mWh/cm2.
The electrolyte volume described in the present invention is defined as the total volume (μL) of electrolyte added in a battery, including the volume of all electrolytes added in the cathode and anode of a battery. The capacity described in the present invention is defined as the discharge capacity (mAh/g) exhibited by a battery discharged to cut-off voltage after it is fully charged in the previous charge-discharge cycle. The electrolyte-to-capacity ratio (μL/mAh) described in the present invention represents the performance of utilization of the active material sulfur for the electrolyte. This parameter cannot be directly controlled as the electrolyte-to-sulfur ratio, since electrolyte content significantly affects utilization of active materials, thereby influencing the battery capacity. Since electrolyte-to-capacity ratio is related to the weight of inactive materials in a battery and the battery capacity, for a given electrolyte-to-sulfur ratio, the lower the electrolyte-to-capacity ratio, the higher the utilization of active materials in the battery system, showing a higher specific energy. In some embodiments, the lithium-sulfur battery of the present invention has an electrolyte-to-capacity ratio of less than 10 μL/mAh, preferably less than 5 μL/mAh.
In some embodiments, the lithium-sulfur battery of the present invention, after cycling for 100 cycles at a c-rate between 0.10 C and 0.20 C, has a capacity retention of at least 70%, preferably at least 80%. In some embodiments, the lithium-sulfur battery of the present invention has a capacity retention of at least 70% and a coulombic efficiency of higher than 98% after 200 cycles.
In some embodiments, the core-shell cathode of the present invention may simultaneously attain a high sulfur loading of 12 mg/cm2 and a high sulfur content of 64 wt %; also, the lithium-sulfur battery with said core-shell cathode exhibits excellent areal capacity and energy density, respectively approaching 8.7 to 10.2 mAh/cm2, and 18.1 to 20.5 mWh/cm2, under lean-electrolyte condition, for example an electrolyte-to-sulfur ratio of only 3 to 7 μL/mg, and reversibility in long-term cycling of at least 100 cycles, successfully achieving high energy under lean-electrolyte condition for it to be commercially viable.
The macro-scale and micro-scale structural design of the core-shell cathode of the present invention may effectively utilize the electrolytes in the system and greatly reduce electrolyte loss, such that the lithium-sulfur battery of the present invention has a high energy density under lean-electrolyte condition, with outstanding rate capability, cycling stability, electrochemical efficiency, and reversibility.
The core-shell cathode of the present invention encloses active materials in a central cavity. The core-shell cathode of the present invention traps the active materials and functions as current collectors as well. Without any binders, the core-shell cathode of the present invention attains high sulfur loading and sulfur content, as well as high electrochemical utilization and charge-discharge efficiency, and effectively utilizes electrolytes in the system and reduces electrolyte loss.
The core-shell cathode of the present invention improves the loading and reactivity of the active materials. With high-concentration liquid polysulfides included in catholytes and serving as initial active material, the present invention increases the active material loading. This configuration is also the state with the highest infiltration level between active materials and electrolytes. Due to strong mobility of liquid polysulfide, it flows and diffuses easily when being filled in the core of the core-shell cathode, enabling it to evenly contact with the electrically conductive, porous carbon material of the shell. In addition, the electrochemical reaction in a battery occurs at the current collector/active material/electrolyte interface. Hence, a battery with the catholytes as initial reactant is capable of sufficiently utilizing the active materials filled therein under the condition of high active material loading.
The core-shell cathode of the present invention traps the solid-state and liquid-state active materials within the cathode. The core-shell cathode of the present invention utilizes its physical structure to enclose the active materials in the core with three-dimensional structure on a macro-scale. On a micro-scale, the core-shell cathode of the present invention takes advantage of the strong tortuosity of the electrically conductive, porous carbon material of the shell, for example, intricately arranged and inextricably intertwined carbon nanofibers and carbon nanotubes, such that the highly layered network constructs strongly tortuous channels, deterring the viscous polysulfides migrating out from the shell of the shell of the core-shell cathode. Accordingly, the core-shell cathode of the present invention traps the active materials in the core and within the micro-scale structure of the electrically conductive, porous carbon material the of the shell, deterring active material loss due to its diffusion during reaction.
The core-shell cathode of the present invention increases electrical conductivity of the electrode, such that the active material content and loading is improved. The material of the shell of the core-shell cathode comprises an electrically conductive, porous carbon material. In some embodiments, highly conductive carbon nanotubes and carbon nanofibers are used, and multiple fibers are intertwined to form a network-like conductive carbon skeleton structure. Hence, an effective high-conductivity network is formed, such that the lithium-sulfur battery with the core-shell cathode of the present invention exhibits a low cathode impedance of 60 to 100Ω before cycling.
The core-shell cathode of the present invention effectively improves ionic conductivity of the electrode, which greatly lowers the volume of the electrolytes in the system. In addition, the gaps within the strongly tortuous and inextricably intertwined carbon fiber skeletons of the shell are on the scale of giant pores, which may function as a channel for transporting the lithium ions in the electrolytes, and thus may effectively transmit the lithium ions to the active materials in the core. Moreover, the electrically conductive, porous carbon material of the shell only contains a small number of nano-scale pores, and has a low specific surface area, thereby preventing absorption loss of electrolytes and allowing efficient utilization of electrolytes by the system. Hence, the lithium-sulfur battery of the present invention exhibits stability in charge and discharge cycle with high coulombic efficiency under lean-electrolyte condition with a low electrolyte-to-sulfur ratio of 3 μL/mg to 7 μL/mg. In some embodiments, the lithium-sulfur battery of the present invention has a lithium-ion diffusion coefficients of 2.2×10−7 cm2/s to 6.3×10−8 cm2/s.
The core-shell cathode of the present invention attains high energy density, high capacity retention and high cycling stability. In some embodiments, the lithium-sulfur battery with the core-shell cathode of the present invention exhibits high stability in charge and discharge, stability in cycling 200 cycles at a constant rate with high capacity retention of at least 70%, a high coulombic efficiency of greater than 98%, a high areal capacity of 8.7 to 10.2 mAh/cm2, and an energy density of 18.1 to 20.5 mWh/cm2. These electrochemical performances outperform the critical areal capacity required for powering an electric vehicle of 2 to 4 mAh/cm2, and also surpass the conventional commercial oxide electrodes with an energy density of approximately 10.1 mWh/cm2.
Exemplary embodiments are illustrated in referenced figures. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
The present invention will be further exemplified by the following examples, which are not to be seen as limiting. The embodiments and description are used for illustrating the details and effect of the present invention.
Fabrication of a Core-Shell Cathode
The structure of a core-shell cathode may be comprised of a modular assembled shell made of highly conductive carbon fiber substrates and a core filled with active materials, polysulfide, within its inner cavity. The material of the shell may include highly conductive carbon fiber substrates or various functional carbon papers. For example, the material of shell 110 may be highly conductive carbon fiber substrates having a weight per unit area of 46 g/cm2. As shown in
The active material filled within the core 120 is formed from liquid polysulfides as the initial reactants. The polysulfide catholyte is a composite of lithium sulfide (99+%, Alfa Aesar) and sulfur (99.5%, Alfa Aesar), both served as precursors; then filled into the core 120 and thus the fabrication of a core-shell cathode is completed.
The synthesis method of catholyte containing liquid polysulfides:
The polysulfide catholyte (Li2S6, 1.5M) is a mixture of liquid-state polysulfides and organic electrolytes. The synthesis reaction, Li2S+5S→Li2S6, forms the long-chain polysulfide, Li2S6, as an intermediate in a multistep reaction while forming a sulfur cathode. Firstly, weigh two types of precursors, 0.276 g of lithium sulfide (Li2S) and 0.960 g of sulfur (S) in a mole ratio of 1:5; after that, weigh two types of electrolyte salts, 2.124 g of lithium salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, 1.85M) and 0.140 g of lithium nitrate (LiNO3, 0.5M) co-salt. Then, these four types of powder are thoroughly mixed in a 20 ml glass scintillation vial with a magnetic stirrer. Subsequently, two types of electrolyte solvents, 2 ml of 1,3-dioxacyclopentane (DOL, C3H6O2) and 2 ml of 1,2-dimethoxyethane (DME, C4H10O2), are added into the glass scintillation vial in a volume ratio of 1:1, and stir at a speed of 360 rpm at 84° C. for approximately 6 hours to form a gray-orange solution, and then stir at a speed of 270 rpm at 82° C. for 36 hours to form a 4 ml red-brown homogeneous solution without precipitate, which is the polysulfide catholyte. The entire manufacturing process is completed in a purified gas glove box filled with argon gas, during which the oxygen and moisture concentrations are constantly maintained under 1 ppm.
The choice of liquid polysulfides as the initial reactant was for the immediate thorough mixing of active materials and electrolytes during the fabrication of catholyte. In addition, liquid-state long-chain polysulfide is soluble in the electrolytes, and thus the reactivity and mobility of active materials are higher than the reactivity and mobility of the final product, solid-state sulfur or lithium sulfide, enabling sufficient utilization of active materials during electrochemical reaction. Moreover, the material structure characteristics of core-shell cathode may efficiently trap the active materials within the cathode to deter diffusion and loss. Thus, a lithium-sulfur battery with a core-shell cathode is equipped with the capacity of functioning high sulfur loading electrode.
The Assembly Method of Catholyte with Active Materials and Core-Shell Cathode
As shown in
Preparation of Liquid-State Electrolyte
Weigh two types of electrolyte salt, 5.310 g of lithium salt (LiTFSI, 1.85M) and 0.350 g of lithium nitrate co-salt (LiNO3, 0.5M), and then add the powders into a 20 ml glass scintillation vial. After that, add two types of electrolyte solvent, 5 ml of 1,3-dioxacyclopentane (C3H6O2) and 5 ml of 1,2-dimethoxyethane (C4H10O2) in a volume ratio of 1:1, into the glass scintillation vial, and stir with a magnetic stirrer at room temperature until the solute dissolves entirely. Hence, the preparation of liquid-state electrolyte is completed.
The Assembling Method of Lithium-Sulfur Battery with Core-Shell Cathode
The electrolyte-to-sulfur ratio (μL/mg) calculation formula of the present invention:
[(the volume of catholyte−sulfur loading)+addition of electrolyte]/sulfur loading=electrolyte-to-sulfur ratio (μL/mg).
Wherein, the volume of catholyte is the volume of the active materials filled within the battery cathode, which contains a specified volume of active materials and electrolytes; sulfur loading is the total weight (mg) of active materials added in the battery.
For example, 31 μL of polysulfide catholyte is filled into the cavity core 120 area surrounded by O-ring 112, and when preparing a core-shell cathode having a sulfur loading of 9 mg, the assembly parameter of the lithium-sulfur battery is fixed in electrolyte-to-sulfur ratio (μL/mg) of 3 μL/mg, 4 μL/mg, 5 μL/mg, 6 μL/mg or 7 μL/mg, which is equivalent to the addition of 5 μL, 14 μL, 23 μL, 32 μL or 41 μL of electrolyte in the battery.
In terms of the physiochemical property of materials, the microstructural, morphological and elemental analysis of the materials of core-shell cathode are carried out with the field-emission scanning electron microscope (FE-SEM, JEOL JSM-7001) and the energy-dispersive X-ray spectroscope (EDS) equipped therein.
The material of the shell in this example is formed from highly conductive carbon fiber substrates, wherein the pore structure of carbon nanofibers and carbon nanotubes are tested with the Specific Surface Area and Pore Size Distribution Analyzer (Autosorb iQ-MP-MP, Anton Paar), and analyzed with methods by Brunauer-Emmett-Teller (BET), Horvath-Kawazoe (HK), Density Function Theory (DFT), and Barrett-Joyner-Halenda (BJH) to calculate its specific surface area, pore size distribution, total pore volume, and average pore diameter.
In order to determine whether highly conductive carbon fiber substrate satisfies the needs of being the material of lithium-sulfur battery cathode, this example will analyze it from two physical aspects, microstructure and pore structure.
The result of examining the pore structure of carbon shell layer using the specific surface area and pore size distribution analyzer is shown in
To conclude the abovementioned analysis results of microstructure and pores, amorphous macropores come from the nano-sized space between the intertwined carbon fibers, while the average pore diameter is the average result of macropores and a small amount of micro/mesopores. By applying said highly conductive carbon fiber substrate without nanopore in a lithium-sulfur battery cathode, the problem of electrolyte being absorbed by pores so that it cannot be utilized in the electrochemical reaction is avoided, and therefore the electrolytes added in the cathode is utilizable with high efficiency. Hence, when active materials in the core area are effectively enclosed, the objective of lowering the usage of electrolyte is attained as well.
Therefore, in the following examples, a lean-electrolyte battery is set to have assembling parameters of a high sulfur loading of 12 mg/cm2, a high sulfur content of 64.1 wt %, and an electrolyte-to-sulfur ratio between 3 μL/mg and 7 μL/mg to improve the energy density of the system.
The Raman spectra of the core and the shell of a core-shell cathode before cycling in
Through Raman analysis, the micro-structure and macro-structure of highly tortuous carbon fiber network of a core-shell cathode are proven to be effective in restraining active materials from diffusing, and keeping its high conductivity during cycling.
As shown in
The core-shell cathode is disassembled after long-term cycling. Then, the core is exposed to a detectable level. As shown in
The analysis result of Site A (core-shell interface) shows that there is an obvious boundary between the shell on the left and the core on the right, and the carbon signals are mainly distributed among the shell on the left, while the sulfur signals are mainly distributed among the core on the right. At Site B (shell region), carbon fiber structure is highly visible and has a strong carbon signal, and only a few small sulfur particles precipitated and attached to the fiber surface with relatively weak sulfur signals. As for Site C (core region), a large number of sulfur particles precipitated and shows strong sulfur signals, and the carbon fiber structure is covered by a large number of sulfur particles with only weak carbon signals.
The abovementioned analysis results show that the microstructure and macro-design of a core-shell cathode may provide active materials to be effectively kept and evenly precipitated in the core after long-term cycling, which shows the excellent electrochemical reactivity of the shell. In addition, the diffusion of cathode active materials enclosed within the core-shell structure is effectively retrained. Although the diffusion of active materials to the shell is inevitable, said active materials are still effectively utilized within the inner structure of shell. Therefore, lithium-sulfur battery with core-shell cathode remains high capacity retention even after long-term cycling.
In terms of electrochemistry and properties of the battery, electrochemical impedance spectroscopy (Research-grade potentiostat, SP-150, Bio-Logic) is used to analyze the impact of electrolyte content in a battery system on impedance, with the alternating current frequency range of 1 MHz to 100 MHz, and at an alternating current amplitude of 5 mV. For the cycling performance of a battery with core-shell cathode, battery components are sealed in a button cell battery, and undergo a cycling test with a battery cycler (Programmable Battery Cycler, BCS model, Bio-Logic) at a constant rate of 0.10 C, 0.15 C, or 0.20 C in the voltage window of 1.7 V to 2.8 V. Electrochemical potential of the battery system and the reversibility of redox reaction are analyzed by cyclic voltammetry (galvanostat system, VMP3, Bio-Logic) at scanning rates from 0.01 mV/s to 0.05 mV/s in the voltage window of 1.7 V to 2.8 V.
To test the impact of different electrolyte content on battery impedance performance, a lean-electrolyte lithium-sulfur battery with a core-shell cathode undergoes electrochemical impedance analysis before and after cycling. As shown in
To further test and analyze the diffusion capacity of lithium ions, the cyclic voltammograms under an electrolyte-to-sulfur ratio of 4 μL/mg to 7 μL/mg are presented in
The abovementioned analyzing results illustrate the excellent electronic and ionic conductivity of a lithium-sulfur battery with the core-shell cathode of the present invention. Then, a long-term cycling test is conducted under different charge/discharge rates at 0.10 C, 0.15 C, and 0.20 C rates. The electrochemical reactivity is analyzed by charge/discharge voltage profiles of the lithium-sulfur battery under electrolyte-to-sulfur ratios between 3 μL/mg and 7 μL/mg as shown in
The abovementioned excellent electrochemical reactivity proves that the highly conductive carbon substrate shell of a core-shell cathode can effectively make up for the disadvantage of the low conductivity of solid-state active materials, and also effectively restrain the diffusion of liquid-state long-chain polysulfide easily dissolved in electrolytes to slow down the irreversible loss in active materials. Also, highly conductive carbon substrate shell without micropores does not absorb electrolytes so the electrolytes can be effectively utilized, and the spaces between intertwined fibers of said shell restrain the diffusion of active materials and simultaneously provide transmission channel for lithium ions. According to above excellent electrochemical properties, a core-shell cathode enables a high sulfur loading electrode and lean-electrolyte lithium-sulfur battery to approach a highly efficient electrochemical reaction and 100 cycles of stable and reversible charge/discharge cycle.
Electrochemical analysis proves that the core-shell cathode of the present invention has high electronic and ionic conductivity. In this example, the rate capacity of a battery is further tested at different charge-discharge rates of 0.10 C, 0.15 C, and 0.20 C.
After stable and reversible cycling for 50 cycles at different rates, discharge capacity retains at 686 to 723 mAh/g under an electrolyte-to-sulfur ratio of 7 μL/mg; discharge capacity retains at 619 to 675 mAh/g under an electrolyte-to-sulfur ratio of 6 μL/mg; discharge capacity retains at 666 to 722 mAh/g under an electrolyte-to-sulfur ratio of 5 μL/mg; discharge capacity retains at 713 to 782 mAh/g under an electrolyte-to-sulfur ratio of 4 μL/mg. After stable and reversible cycling for 10 cycles, discharge capacity retains at 655 mAh/g under an electrolyte-to-sulfur ratio of 3 μL/mg. By calculation, the battery performance does not show an obvious decrease due to lowering electrolyte-to-sulfur ratio, but all retain a capacity retention greater than 80%, thus proving the successful fabrication of lean-electrolyte lithium-sulfur battery in the present invention.
Furthermore, considering battery parameters of electrode with high sulfur content reaching a high sulfur loading of 12 mg/cm2 and a high sulfur content of 64.1 wt % at the same time, the density of liquid-state polysulfide dissolved in electrolytes thus increases with the decreases of electrolyte-to-sulfur ratio, which further raises the concentration gradient of polysulfide between cathode and anode and causes the increasing probability of retention capacity loss due to diffusion of active materials out from cathode. Due to the increased density of polysulfide in electrolytes, the viscosity of electrolyte increases at the same time and thus sabotages the stable diffusion of lithium ions, causing decreasing battery system reactivity and active material utilizing capability. Said disadvantage becomes even more severe during cycling under high electronic stream and high ionic stream. However, the properties of high electronic and ionic conductivity of the core-shell cathode of the present invention response to the challenge brought by high active material loading and low electrolyte-to-sulfur ratio in high energy density battery:intertwined fiber structure without micropores provides highly efficient utilizing capability of electrolytes, and also effectively traps active materials within cathode, which proves that lithium-sulfur battery with core-shell cathode still maintains its excellent reactivity, stability, and capacity retention when cycling in a lean-electrolytic environment.
The long-term cycling stability of the lean-electrolyte lithium-sulfur battery with a high sulfur loading core-shell cathode is shown in
As shown in
To specifically present and to verify the performance advantages and commercial viability of the lithium-sulfur battery with the core-shell cathode of the present invention, battery performance and engineering parameters are compared in the following experiments. In terms of battery performance, as shown in
Moreover, in terms of engineering parameters, a high sulfur loading of 12 mg/cm2, a high sulfur content of 64.1 wt %, and a low electrolyte-to-sulfur ratio of 3 to 7 μL/mg are set to reach simultaneously in this example, and a stable long-term cycling for more than 100 cycles is attained under such parameters. The prior art documents and reports are analyzed and organized in
The core-shell cathode of the present invention may be fabricated based on highly conductive carbon fiber substrates. With its highly tortuous and inextricably intertwined carbon fiber reticulated structure, high active material loading, high capacity retention, long-range charge transport, and low consumption in electrolytes are attained at the same time. Consequently, utilizing the contained large amount of active materials under lean-electrolyte conditions with high efficiency is viable, and thus attaining the stability and highly efficient cyclability of electrode with high sulfur loading in a lean-electrolyte battery. In terms of battery techniques of materials engineering, core-shell sulfur electrode attains a high sulfur loading of 12 mg/cm2 and a high sulfur content of 64.1 wt % simultaneously, achieving it in a lean-electrolyte lithium-sulfur battery by an electrolyte-to-sulfur ratio of merely 3 to 7 μL/mg. Accordingly, each parameter is far superior to the level prior art could reach.
In terms of battery performance of material science, the core-shell electrode high sulfur loading shows excellent rate capacity at 0.10 C, 0.15 C, and 0.20 C rates, charge-discharge reversibility of stable long-term cycling for 100 cycles, and excellent areal capacity and energy density, by up to 10.1 to 10.8 mAh/cm2 and 20.1 to 21.6 mWh/cm2 respectively. Each electrochemical property is also superior to the performances of the prior art, and outperforming the commercial requirements for powering electrical vehicles.
The present invention elevates battery techniques and electrochemical performance entirely, which proves that by applying a novel core-shell cathode structure design in a lithium-sulfur battery system, the material limitation of lithium-sulfur battery can be overcome with improvement of battery performances. Thus, the present invention successfully achieves the commercial goal of a lithium-sulfur battery with high energy density under lean-electrolyte conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 111141394 | Oct 2022 | TW | national |
