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2021 Volume 37 Issue 2
2021, 37(2): 200500
doi: 10.3866/PKU.WHXB202005003
Abstract:
Solid-state Li metal batteries are considered promising next-generation energy storage systems due to its exceptional advantages in terms of safety and high energy density. The continuous process on the development of solid-state fast ionic electrolytes enables the solid-state battery to operate at room temperature. Among these, sulfide-based solid electrolytes have attracted significant attentions due to their extremely high ionic conductivity, excellent deformability, and mild low-temperature processability. However, the full demonstration of practical batteries remains challenging due to the slow lithium-ion transport kinetics at working solid-solid interfaces. The sluggish interfacial transport kinetics mainly result from the poor solid-solid contacts, resulting in poor battery performance. Especially for solid-state pouch cells, the high local current due to the poor contact is amplified by the high working current, leading to rapid failure. Constructing fast ion transport paths between the Li metal anode and solid electrolyte interface is key for the practical application of solid-state batteries. Here a simple protocol was developed to realize fast ionic transportation by wetting the solid electrolyte/Li metal anode interface with localized high salt concentration liquid electrolyte. First, 3.5 mmol lithium trifluoroalfonylimide (LiTFSI) was added into 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (HFE) and dimethoxyethane (DME) mixed solvent, and stirred to obtain uniformly dispersed localized high-concentration liquid electrolyte, denoted as HFE-DME LiTFSI. The fluidity of liquid electrolyte ensures sufficiently conformal contacts between lithium anode and liquid electrolyte, as well as solid-state electrolyte and liquid electrolyte. Thus, fast ion transportation channels were constructed between the solid electrolyte and Li metal anode by wetting HFE-DME LiTFSI at a concentration of 3.0 μL·cm-2. After liquid phase therapy, the interfacial resistance of solid-state Li|Li4Ti5O12 (LTO) pouch cell rapidly reduced from 4366 to 64 Ω·cm-2 and even lower than the cell that was pressed at 3 MPa in the assemble process (340 Ω·cm-2). This suggests that the ion transport kinetics are significantly improved by liquid phase therapy. Therefore, the solid-state Li metal pouch cell with dimensions of 30 mm × 30 mm showed excellent cycling performances with specific capacities of 107 and 96 mAh·g-1 at 0.1C and 0.5C, respectively. Furthermore, the solid-state Li-S pouch cell delivered capacities of 1100 and 932 mAh·g-1 at 0.01C and 0.02C, respectively. This study demonstrates the effectiveness of the novel liquid phase therapy to construct fast ionic transportation channels, which providing an effective strategy for the practical application of solid-state Li metal pouch cells. ![]()
Solid-state Li metal batteries are considered promising next-generation energy storage systems due to its exceptional advantages in terms of safety and high energy density. The continuous process on the development of solid-state fast ionic electrolytes enables the solid-state battery to operate at room temperature. Among these, sulfide-based solid electrolytes have attracted significant attentions due to their extremely high ionic conductivity, excellent deformability, and mild low-temperature processability. However, the full demonstration of practical batteries remains challenging due to the slow lithium-ion transport kinetics at working solid-solid interfaces. The sluggish interfacial transport kinetics mainly result from the poor solid-solid contacts, resulting in poor battery performance. Especially for solid-state pouch cells, the high local current due to the poor contact is amplified by the high working current, leading to rapid failure. Constructing fast ion transport paths between the Li metal anode and solid electrolyte interface is key for the practical application of solid-state batteries. Here a simple protocol was developed to realize fast ionic transportation by wetting the solid electrolyte/Li metal anode interface with localized high salt concentration liquid electrolyte. First, 3.5 mmol lithium trifluoroalfonylimide (LiTFSI) was added into 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (HFE) and dimethoxyethane (DME) mixed solvent, and stirred to obtain uniformly dispersed localized high-concentration liquid electrolyte, denoted as HFE-DME LiTFSI. The fluidity of liquid electrolyte ensures sufficiently conformal contacts between lithium anode and liquid electrolyte, as well as solid-state electrolyte and liquid electrolyte. Thus, fast ion transportation channels were constructed between the solid electrolyte and Li metal anode by wetting HFE-DME LiTFSI at a concentration of 3.0 μL·cm-2. After liquid phase therapy, the interfacial resistance of solid-state Li|Li4Ti5O12 (LTO) pouch cell rapidly reduced from 4366 to 64 Ω·cm-2 and even lower than the cell that was pressed at 3 MPa in the assemble process (340 Ω·cm-2). This suggests that the ion transport kinetics are significantly improved by liquid phase therapy. Therefore, the solid-state Li metal pouch cell with dimensions of 30 mm × 30 mm showed excellent cycling performances with specific capacities of 107 and 96 mAh·g-1 at 0.1C and 0.5C, respectively. Furthermore, the solid-state Li-S pouch cell delivered capacities of 1100 and 932 mAh·g-1 at 0.01C and 0.02C, respectively. This study demonstrates the effectiveness of the novel liquid phase therapy to construct fast ionic transportation channels, which providing an effective strategy for the practical application of solid-state Li metal pouch cells.
2021, 37(2): 200801
doi: 10.3866/PKU.WHXB202008013
Abstract:
Lithium is a promising anode material for next-generation high-energy-density rechargeable batteries owing to its high specific capacity, low density, and low electrochemical reduction potential. However, dendrite growth during cycling impedes its practical application and causes safety hazards. Extensive research has been conducted to obtain dendrite-free safe Li anodes with an extended cycle life by electrolyte or anode surface modification. In previous studies, the symmetrical Li/Li cell test was widely applied to evaluate the effect of various Li anode modification methods on the cycle stability and Li deposition overpotential. However, a general criterion has not yet been established to identify the short circuit in Li/Li cells. Some researchers have even made incorrect conclusions based on the Li/Li cycling data. The most common misjudgment is the ignorance of short circuit signals and mixing up of soft short circuit and normal potential decrease caused by electrode activation. In some studies, the fractal voltage signals were attributed to the unstable activation process of the symmetrical cell. Therefore, this study uses an in situ optical cell to demonstrate that a short circuit caused by the contact of dendrites from two opposite electrodes can cause a sudden drop in cell voltage to certain extent. According to the reversibility of the voltage, the short circuit induced by dendrite growth can be classified into unrecoverable hard short circuits and recoverable soft short circuits. Typical short circuit data were summarized and described to establish a rule to determine the different types of short circuits. The voltage profiles provide characteristic signals to distinguish between the soft short circuit, hard short circuit, and cell activation processes in symmetrical cells. Furthermore, this study provides a reference for identifying dendrite growth and cell short circuits, which is important for confirming the practical effect of different modification methods. ![]()
Lithium is a promising anode material for next-generation high-energy-density rechargeable batteries owing to its high specific capacity, low density, and low electrochemical reduction potential. However, dendrite growth during cycling impedes its practical application and causes safety hazards. Extensive research has been conducted to obtain dendrite-free safe Li anodes with an extended cycle life by electrolyte or anode surface modification. In previous studies, the symmetrical Li/Li cell test was widely applied to evaluate the effect of various Li anode modification methods on the cycle stability and Li deposition overpotential. However, a general criterion has not yet been established to identify the short circuit in Li/Li cells. Some researchers have even made incorrect conclusions based on the Li/Li cycling data. The most common misjudgment is the ignorance of short circuit signals and mixing up of soft short circuit and normal potential decrease caused by electrode activation. In some studies, the fractal voltage signals were attributed to the unstable activation process of the symmetrical cell. Therefore, this study uses an in situ optical cell to demonstrate that a short circuit caused by the contact of dendrites from two opposite electrodes can cause a sudden drop in cell voltage to certain extent. According to the reversibility of the voltage, the short circuit induced by dendrite growth can be classified into unrecoverable hard short circuits and recoverable soft short circuits. Typical short circuit data were summarized and described to establish a rule to determine the different types of short circuits. The voltage profiles provide characteristic signals to distinguish between the soft short circuit, hard short circuit, and cell activation processes in symmetrical cells. Furthermore, this study provides a reference for identifying dendrite growth and cell short circuits, which is important for confirming the practical effect of different modification methods.
2021, 37(2): 200809
doi: 10.3866/PKU.WHXB202008073
Abstract:
Lithium metal has the highest theoretical specific energy density (3860 mAh∙g−1) and the most negative redox potential (−3.04 V vs standard hydrogen electrode) among all alkali metals. These features have attracted the interest of battery researchers to put lithium metal into practical use in rechargeable batteries. However, lithium metal tends to deposit as dendritic or mossy morphology during the charging process, and such non-uniform deposition induces low Coulombic efficiency and poor cycling stability. In addition, dendritic metallic lithium can easily penetrate the separator, which causes internal short circuit and leads to severe safety issues. Thus it is important to control the electrodeposition process of lithium to inhibit the formation of Li dendrites. Surface modification of lithium is a widely adopted strategy that can induce uniform deposition of Li. In this paper, a LiC6 heterogeneous interfacial layer is decorated on the surface of lithium metal anode. It is prepared in a simple manner by mechanically rolling graphitized carbon nanospheres on a Li foil. The increase in surface area by this LiC6 layer can homogenize the current density on the surface of the lithium foil. Simultaneously, the electronegativity of LiC6 can also homogenize the lithium ion flux. The effect of heterogeneous interface on the electrochemical plating and stripping behavior of lithium in carbonate electrolyte is also studied. Morphological characterization and electrochemical performance tests reveal that the LiC6 heterogeneous interface can significantly improve the reversibility and uniformity of the electrochemical plating and stripping of Li, thereby inhibiting dendritic growth and maintaining the stability of the anode/electrolyte interface. Alternating current electrochemical impedance spectroscopy analysis determines that the solid electrolyte interface (SEI) impedance of bare lithium decreases from the initial 275 to 100 Ω as the deposition capacity increases, suggesting that severe rupture of the SEI is caused by the huge volume change after lithium deposition. On the contrary, the SEI impedance of the lithium foil with the LiC6 heterogeneous interface layer remains nearly constant (from the initial 26 to 25 Ω after electrodeposition) indicates that the LiC6 layer is able to inhibit dendrite growth and stabilize the interface. Thus, stable lithium plating/stripping over 300 h is achieved at a current density of 1 mA∙cm−2 and at a fixed capacity of 1 mAh∙cm−2 with a voltage hysteresis of less than 50 mV. The Li-LiFePO4 full cell test demonstrates that the cycling stability of the modified lithium metal anode is superior to that of the bare one. ![]()
Lithium metal has the highest theoretical specific energy density (3860 mAh∙g−1) and the most negative redox potential (−3.04 V vs standard hydrogen electrode) among all alkali metals. These features have attracted the interest of battery researchers to put lithium metal into practical use in rechargeable batteries. However, lithium metal tends to deposit as dendritic or mossy morphology during the charging process, and such non-uniform deposition induces low Coulombic efficiency and poor cycling stability. In addition, dendritic metallic lithium can easily penetrate the separator, which causes internal short circuit and leads to severe safety issues. Thus it is important to control the electrodeposition process of lithium to inhibit the formation of Li dendrites. Surface modification of lithium is a widely adopted strategy that can induce uniform deposition of Li. In this paper, a LiC6 heterogeneous interfacial layer is decorated on the surface of lithium metal anode. It is prepared in a simple manner by mechanically rolling graphitized carbon nanospheres on a Li foil. The increase in surface area by this LiC6 layer can homogenize the current density on the surface of the lithium foil. Simultaneously, the electronegativity of LiC6 can also homogenize the lithium ion flux. The effect of heterogeneous interface on the electrochemical plating and stripping behavior of lithium in carbonate electrolyte is also studied. Morphological characterization and electrochemical performance tests reveal that the LiC6 heterogeneous interface can significantly improve the reversibility and uniformity of the electrochemical plating and stripping of Li, thereby inhibiting dendritic growth and maintaining the stability of the anode/electrolyte interface. Alternating current electrochemical impedance spectroscopy analysis determines that the solid electrolyte interface (SEI) impedance of bare lithium decreases from the initial 275 to 100 Ω as the deposition capacity increases, suggesting that severe rupture of the SEI is caused by the huge volume change after lithium deposition. On the contrary, the SEI impedance of the lithium foil with the LiC6 heterogeneous interface layer remains nearly constant (from the initial 26 to 25 Ω after electrodeposition) indicates that the LiC6 layer is able to inhibit dendrite growth and stabilize the interface. Thus, stable lithium plating/stripping over 300 h is achieved at a current density of 1 mA∙cm−2 and at a fixed capacity of 1 mAh∙cm−2 with a voltage hysteresis of less than 50 mV. The Li-LiFePO4 full cell test demonstrates that the cycling stability of the modified lithium metal anode is superior to that of the bare one.
2021, 37(2): 200900
doi: 10.3866/PKU.WHXB202009001
Abstract:
As an ideal negative electrode material for next-generation high-energy-density batteries, lithium (Li) metal has received extensive attention from the global research community. However, the safety hazards and short cycle life caused by the growth of Li dendrites have seriously hampered the application of Li metal batteries. Based on electrochemical phenomena and theory, this paper discusses the mechanism of dendritic growth, dead Li formation, and full battery failure from the perspective of concentration polarization. During the electrodeposition process, the consumption of Li ions on the surface induces concentration polarization. After the initial deposition, a relatively loose dendrite layer appears on the Li metal surface; the electrolyte can penetrate this dendrite layer to reach the dense Li metal surface. When the grown dendrites penetrate the concentration polarization layer, the interface concentration battery is short-circuited. In this case, the concentration difference battery tends to release all stored power and reach a potential balance between the high- and low-concentration regions, which causes the deposition of Li ions over the dendrites to reduce the ion concentration in the surrounding electrolyte. Meanwhile, the dissolution of Li ions that occurs at the roots of the dendrites increases the local ion concentration. This process accelerates the formation of a dead Li layer. A similar electrochemical process often occurs in columnar Li, as reported in other studies. When columnar Li is cycled several times, each Li column degenerates into a matchstick shape with a large head and thin neck. Therefore, eliminating concentration polarization is necessary for the application of columnar Li. Furthermore, in this work, concentration polarization and dendrite suppression in state-of-the-art porous host electrodes are analyzed. The larger specific surface area of the porous electrode greatly reduces the local current density on the electrode surface, which can reduce the interface concentration polarization and thus prevent dendrite growth. In charge-discharge cycling, a constant-voltage charging or shelving step is often inserted in each cycle in order to eliminate the influence of concentration polarization. However, if a dendritic layer has been formed on the Li metal surface after charging, in addition to the self-diffusion of ions, the self-discharge process of the interface concentration battery causes the detachment of the dendrite layer, thus resulting in the above-mentioned dead Li. Therefore, a larger amount of deposited Li yields a thicker Li dendritic layer, thus accelerating the capacity decay and failure of the battery, especially to those with high-capacity, high-voltage positive electrodes. The conclusions obtained in this paper can provide a theoretical basis for researchers to further explore Li metal protection strategies. ![]()
As an ideal negative electrode material for next-generation high-energy-density batteries, lithium (Li) metal has received extensive attention from the global research community. However, the safety hazards and short cycle life caused by the growth of Li dendrites have seriously hampered the application of Li metal batteries. Based on electrochemical phenomena and theory, this paper discusses the mechanism of dendritic growth, dead Li formation, and full battery failure from the perspective of concentration polarization. During the electrodeposition process, the consumption of Li ions on the surface induces concentration polarization. After the initial deposition, a relatively loose dendrite layer appears on the Li metal surface; the electrolyte can penetrate this dendrite layer to reach the dense Li metal surface. When the grown dendrites penetrate the concentration polarization layer, the interface concentration battery is short-circuited. In this case, the concentration difference battery tends to release all stored power and reach a potential balance between the high- and low-concentration regions, which causes the deposition of Li ions over the dendrites to reduce the ion concentration in the surrounding electrolyte. Meanwhile, the dissolution of Li ions that occurs at the roots of the dendrites increases the local ion concentration. This process accelerates the formation of a dead Li layer. A similar electrochemical process often occurs in columnar Li, as reported in other studies. When columnar Li is cycled several times, each Li column degenerates into a matchstick shape with a large head and thin neck. Therefore, eliminating concentration polarization is necessary for the application of columnar Li. Furthermore, in this work, concentration polarization and dendrite suppression in state-of-the-art porous host electrodes are analyzed. The larger specific surface area of the porous electrode greatly reduces the local current density on the electrode surface, which can reduce the interface concentration polarization and thus prevent dendrite growth. In charge-discharge cycling, a constant-voltage charging or shelving step is often inserted in each cycle in order to eliminate the influence of concentration polarization. However, if a dendritic layer has been formed on the Li metal surface after charging, in addition to the self-diffusion of ions, the self-discharge process of the interface concentration battery causes the detachment of the dendrite layer, thus resulting in the above-mentioned dead Li. Therefore, a larger amount of deposited Li yields a thicker Li dendritic layer, thus accelerating the capacity decay and failure of the battery, especially to those with high-capacity, high-voltage positive electrodes. The conclusions obtained in this paper can provide a theoretical basis for researchers to further explore Li metal protection strategies.
2021, 37(2): 200501
doi: 10.3866/PKU.WHXB202005012
Abstract:
Although traditional graphite anodes ensure the cycling stability and safety of lithium-ion batteries, the inherent drawbacks, particularly low theoretical specific capacity (372 mAh·g-1) and Li-free character, of such anodes limit their applications in high energy density battery systems, especially in lithium-sulfur and lithium-air batteries. Lithium metal has been considered as one of the best next-generation anode materials due to its extremely high theoretical specific capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. the standard hydrogen electrode). The first generation of commercial rechargeable lithium metal batteries were developed by Moli Energy in the late 1980s and were not widely used due to several problems, including low coulombic efficiency, poor cycle stability, and safety hazards. These problems associated with the Li metal anode are mainly caused by lithium dendrite growth, electrode volume changes, and interface instability. During the charge and discharge processes, Li deposition is not uniform across the electrode surface. Due to the low surface energy and high migration energy of Li metal, dendrites are preferentially formed during Li deposition. These dendrites proceed to grow with successive battery cycling, penetrate the separator, and eventually reach the cathode, thereby causing short circuits and thermal runaway. Additionally, the growth of the lithium dendrite is inherently correlated with the reaction interface structure, and dendrite growth results in inhomogeneity of the SEI (solid electrolyte interface) which is inevitably formed on the Li metal surfaces. Moreover, the volume change of lithium metal anodes is of importance, particularly during battery cycling and Li stripping/deposition processes which make the SEI layers considerably unstable. SEI layers usually cannot withstand the mechanical deformation caused by volume changes; such layers continuously break and repair during cycling and consume large amounts of the electrolyte. Additionally, some Li dendrites could break and become wrapped by SEI layers to form electrically isolated "dead" Li, which results in the loss of active Li in the Li metal anode. All these factors are responsible for the failure of Li metal anodes. Herein, recent investigations on the failure mechanisms of lithium metal anodes are reviewed and summarized, including the formation of SEI layers on the surface of Li metal anodes, the behavior and mechanism of lithium dendrite growth, and the mechanism of "dead" lithium formation. Additionally, some advanced characterization techniques for investigating lithium metal anodes are introduced, including in situ tools, cryo-electron microscopy, neutron depth analysis technology, and solid state nuclear magnetic resonance technology. These techniques enable researchers to gain in-depth insights into the failure mechanisms of Li metal anodes. ![]()
Although traditional graphite anodes ensure the cycling stability and safety of lithium-ion batteries, the inherent drawbacks, particularly low theoretical specific capacity (372 mAh·g-1) and Li-free character, of such anodes limit their applications in high energy density battery systems, especially in lithium-sulfur and lithium-air batteries. Lithium metal has been considered as one of the best next-generation anode materials due to its extremely high theoretical specific capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. the standard hydrogen electrode). The first generation of commercial rechargeable lithium metal batteries were developed by Moli Energy in the late 1980s and were not widely used due to several problems, including low coulombic efficiency, poor cycle stability, and safety hazards. These problems associated with the Li metal anode are mainly caused by lithium dendrite growth, electrode volume changes, and interface instability. During the charge and discharge processes, Li deposition is not uniform across the electrode surface. Due to the low surface energy and high migration energy of Li metal, dendrites are preferentially formed during Li deposition. These dendrites proceed to grow with successive battery cycling, penetrate the separator, and eventually reach the cathode, thereby causing short circuits and thermal runaway. Additionally, the growth of the lithium dendrite is inherently correlated with the reaction interface structure, and dendrite growth results in inhomogeneity of the SEI (solid electrolyte interface) which is inevitably formed on the Li metal surfaces. Moreover, the volume change of lithium metal anodes is of importance, particularly during battery cycling and Li stripping/deposition processes which make the SEI layers considerably unstable. SEI layers usually cannot withstand the mechanical deformation caused by volume changes; such layers continuously break and repair during cycling and consume large amounts of the electrolyte. Additionally, some Li dendrites could break and become wrapped by SEI layers to form electrically isolated "dead" Li, which results in the loss of active Li in the Li metal anode. All these factors are responsible for the failure of Li metal anodes. Herein, recent investigations on the failure mechanisms of lithium metal anodes are reviewed and summarized, including the formation of SEI layers on the surface of Li metal anodes, the behavior and mechanism of lithium dendrite growth, and the mechanism of "dead" lithium formation. Additionally, some advanced characterization techniques for investigating lithium metal anodes are introduced, including in situ tools, cryo-electron microscopy, neutron depth analysis technology, and solid state nuclear magnetic resonance technology. These techniques enable researchers to gain in-depth insights into the failure mechanisms of Li metal anodes.
2021, 37(2): 200804
doi: 10.3866/PKU.WHXB202008044
Abstract:
Improvement in the energy density of conventional lithium-ion batteries (LIBs), based on the intercalation-extraction chemistry of graphite and transition metal layered oxides, has apparently lagged behind the advances in consumer electronics and electric vehicles. Secondary Li-metal batteries (LMBs), utilizing metallic Li as the anode material, have incomparable advantages in terms of energy density due to their high specific capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode) of Li metal. Irrespective of whether Li anodes are coupled with intercalation-type cathodes (e.g. LiFePO4, LiCoO2, LiNixCoyMnzO2, etc.) or conversion-type cathodes (S, O2), the energy density of LMBs is much higher than that of traditional LIBs, which should solve the range concern of electric vehicles. However, the intrinsically high reactivity between metallic Li and organic electrolytes could induce the formation of a solid electrolyte interface (SEI). The heterogeneous SEI, consisting of a flexible organic outer layer and a brittle inorganic inner layer, suffers from repeated rupture and regeneration due to infinite volume expansions associated with Li deposition and dissolution reactions. Meanwhile, Li is preferentially deposited on the "hot sites" and is stripped from the root of sediments, resulting in uncontrolled dendrite growth during charging and formation of electrochemically isolated Li ("dead" Li) during discharging. Thus, the Columbic efficiency of Li metal full cells is greatly limited by interfacial side effects and continuous loss of active Li, especially in conventional carbonate-based electrolyte, viz. 1 mol·L-1 LiPF6-EC/DEC (ethylene carbonate/diethyl carbonate), which impedes the large-scale employment of Li metal batteries. Recently, novel electrolytes with high or localized-high salt concentrations have attracted considerable attention because of their unique physiochemical properties and excellent electrochemical performance. In high-concentration electrolytes, the reduction in the population of free solvent molecules inhibits irreversible electrolyte decomposition at the electrode-electrolyte interface. In localized-high-concentration electrolytes, the introduction of a dilute reagent retains the desired solvation structure, while improving the physicochemical properties (conductivity and viscosity) of the electrolyte. Herein, we systemically review the latest progress in high-concentration and localized-high-concentration electrolytes for use in Li metal batteries. The solvation chemistry structure, physicochemical properties, and interfacial-stabilizing mechanisms are analyzed in detail, and special attention is devoted to their superior interfacial compatibility with Li metal anodes. Finally, we briefly clarify the current problems associated with the research of high-concentration and localized-high-concentration electrolytes from the viewpoints of basic scientific research and practical applications, and some possible solutions are provided to further pave the way to practical Li metal batteries. ![]()
Improvement in the energy density of conventional lithium-ion batteries (LIBs), based on the intercalation-extraction chemistry of graphite and transition metal layered oxides, has apparently lagged behind the advances in consumer electronics and electric vehicles. Secondary Li-metal batteries (LMBs), utilizing metallic Li as the anode material, have incomparable advantages in terms of energy density due to their high specific capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode) of Li metal. Irrespective of whether Li anodes are coupled with intercalation-type cathodes (e.g. LiFePO4, LiCoO2, LiNixCoyMnzO2, etc.) or conversion-type cathodes (S, O2), the energy density of LMBs is much higher than that of traditional LIBs, which should solve the range concern of electric vehicles. However, the intrinsically high reactivity between metallic Li and organic electrolytes could induce the formation of a solid electrolyte interface (SEI). The heterogeneous SEI, consisting of a flexible organic outer layer and a brittle inorganic inner layer, suffers from repeated rupture and regeneration due to infinite volume expansions associated with Li deposition and dissolution reactions. Meanwhile, Li is preferentially deposited on the "hot sites" and is stripped from the root of sediments, resulting in uncontrolled dendrite growth during charging and formation of electrochemically isolated Li ("dead" Li) during discharging. Thus, the Columbic efficiency of Li metal full cells is greatly limited by interfacial side effects and continuous loss of active Li, especially in conventional carbonate-based electrolyte, viz. 1 mol·L-1 LiPF6-EC/DEC (ethylene carbonate/diethyl carbonate), which impedes the large-scale employment of Li metal batteries. Recently, novel electrolytes with high or localized-high salt concentrations have attracted considerable attention because of their unique physiochemical properties and excellent electrochemical performance. In high-concentration electrolytes, the reduction in the population of free solvent molecules inhibits irreversible electrolyte decomposition at the electrode-electrolyte interface. In localized-high-concentration electrolytes, the introduction of a dilute reagent retains the desired solvation structure, while improving the physicochemical properties (conductivity and viscosity) of the electrolyte. Herein, we systemically review the latest progress in high-concentration and localized-high-concentration electrolytes for use in Li metal batteries. The solvation chemistry structure, physicochemical properties, and interfacial-stabilizing mechanisms are analyzed in detail, and special attention is devoted to their superior interfacial compatibility with Li metal anodes. Finally, we briefly clarify the current problems associated with the research of high-concentration and localized-high-concentration electrolytes from the viewpoints of basic scientific research and practical applications, and some possible solutions are provided to further pave the way to practical Li metal batteries.
2021, 37(2): 200807
doi: 10.3866/PKU.WHXB202008078
Abstract:
In the early 1990s, Sony launched the first commercial lithium ion battery (LIB), which achieved great success in energy storage systems. The current commercially used insertion anode, graphite, is approaching its capacity limit (~372 mAh·g-1), and is inadequate to satisfy the ever-increasing energy demand for power grids and large-scale energy storage systems. In order to address this challenge, lithium metal anodes have been the focus of considerable research effort in recent years, and are regarded as the most promising anode materials because of their extremely high theoretical capacity (3860 mAh·g-1), lowest electrode potential (-3.04 V vs. standard hydrogen electrode), and low density (0.534 g·cm-3). For example, the theoretical energy densities of lithium-sulfur batteries and lithium-air batteries are as high as 2567 and 3505 Wh·kg-1, respectively. However, the uncontrollable dendrite growth during cycling leads to low coulombic efficiency and puncture of the separator, causing a short circuit or even explosion of the battery, thereby seriously hindering the development of the lithium metal anode. Many solutions have been proposed to inhibit dendrite growth, including the use of electrolyte additives, solid electrolytes, and artificial protective films. During charging and discharging, the solid electrolyte interphase (SEI) plays an important role in lithium metal anodes. However, the infinite volume changes of the electrode during plating/stripping processes result in breakage of the SEI film, which continuously consumes the electrolyte and lithium metal. Designing an artificial interface on the surface of lithium metal anodes has been considered as a simple and efficient strategy to control lithium deposition behavior, and is achieved by precoating a protective layer on the surface of lithium metal. An ideal artificial protective film should possess high ionic conductivity, chemical stability, and excellent mechanical strength, in order to prevent side reactions between lithium metal and the electrolyte and realize dendrite-free lithium metal anodes with a long cycle life and high coulombic efficiencies. In this paper, the research progress on artificial protective films for lithium metal anodes in recent years is reviewed. Further, the structural characteristics and preparation methods of various protective films are introduced in detail, including polymer protective films, inorganic protective films, organic-inorganic composite protective films, and alloy protective films. The mechanisms of various protective films toward the suppression of dendrite growth are summarized. Existing challenges and future research directions are also proposed, which together provide a reference for promoting the use of lithium metal in high-energy batteries. ![]()
In the early 1990s, Sony launched the first commercial lithium ion battery (LIB), which achieved great success in energy storage systems. The current commercially used insertion anode, graphite, is approaching its capacity limit (~372 mAh·g-1), and is inadequate to satisfy the ever-increasing energy demand for power grids and large-scale energy storage systems. In order to address this challenge, lithium metal anodes have been the focus of considerable research effort in recent years, and are regarded as the most promising anode materials because of their extremely high theoretical capacity (3860 mAh·g-1), lowest electrode potential (-3.04 V vs. standard hydrogen electrode), and low density (0.534 g·cm-3). For example, the theoretical energy densities of lithium-sulfur batteries and lithium-air batteries are as high as 2567 and 3505 Wh·kg-1, respectively. However, the uncontrollable dendrite growth during cycling leads to low coulombic efficiency and puncture of the separator, causing a short circuit or even explosion of the battery, thereby seriously hindering the development of the lithium metal anode. Many solutions have been proposed to inhibit dendrite growth, including the use of electrolyte additives, solid electrolytes, and artificial protective films. During charging and discharging, the solid electrolyte interphase (SEI) plays an important role in lithium metal anodes. However, the infinite volume changes of the electrode during plating/stripping processes result in breakage of the SEI film, which continuously consumes the electrolyte and lithium metal. Designing an artificial interface on the surface of lithium metal anodes has been considered as a simple and efficient strategy to control lithium deposition behavior, and is achieved by precoating a protective layer on the surface of lithium metal. An ideal artificial protective film should possess high ionic conductivity, chemical stability, and excellent mechanical strength, in order to prevent side reactions between lithium metal and the electrolyte and realize dendrite-free lithium metal anodes with a long cycle life and high coulombic efficiencies. In this paper, the research progress on artificial protective films for lithium metal anodes in recent years is reviewed. Further, the structural characteristics and preparation methods of various protective films are introduced in detail, including polymer protective films, inorganic protective films, organic-inorganic composite protective films, and alloy protective films. The mechanisms of various protective films toward the suppression of dendrite growth are summarized. Existing challenges and future research directions are also proposed, which together provide a reference for promoting the use of lithium metal in high-energy batteries.
2021, 37(2): 200808
doi: 10.3866/PKU.WHXB202008089
Abstract:
Lithium-metal anode batteries have the potential to serve as next-generation, high energy density batteries with high specific capacity and low electrode potential. However, due to the high reactivity of lithium, complex interfacial reactions and uncontrollable dendrite growth obstruct their application. These lithium-metal anode interfacial reactions are often accompanied by the organic electrolyte spontaneously decomposing and combustible gas subsequently escaping, which is a safety concern. It also affects the form of the solid electrolyte interphase (SEI), which is important for stabilizing the interface between the Li-metal anode and electrolyte. Uncontrollable Li dendrite growth could penetrate the separator or electrolyte, creating the risk of a short circuit. Therefore, it is necessary to optimize the lithium nucleation and deposition processes. Solid state electrolytes (SSEs) have also attracted attention for improving the energy density and safety of Li-ion batteries; however, problems such as poor ionic conductivity still exist. Computational simulations, such as molecular dynamics (MD) simulations and first-principles calculations based on density function theory (DFT), can help elucidate reaction mechanisms, explore electrode materials, and optimize battery design. In this review, we summarize the theoretical perspective gained from computational simulation studies of lithium-metal anodes. This review is organized into four sections: interfacial reactions, SEIs, lithium nucleation, and SSEs. We first explore organic-electrolyte interfacial reaction mechanisms that were revealed through MD simulations and how electrolyte additives, electrolyte concentration, operating temperature affect them. For SEI, DFT can provide an in-depth understanding of the surface chemical reaction, surface morphology, electrochemical properties, and kinetic characteristics of SEI. We review the developments in SEI transmission mechanisms and SEI materials' properties alteration by lithium metal. We further explore artificial SEI design requirements and compare the performances of artificial SEIs, including double-layer, fluorine-, and sulfur-SEIs. Lithium dendrite growth as a result of lithium nucleation and deposition is then discussed, focusing on computational studies that evaluated how doped graphene, 3D carbon fibers, porous metals, and other matrix materials regulated these processes and inhibited dendrite growth. Computational simulations evaluating transport phenomena and interface reactions between SSEs and lithium-metal anodes are then explored, followed by ideas for further design optimization. Finally, potential research directions and perspectives in this field are proposed and discussed. ![]()
Lithium-metal anode batteries have the potential to serve as next-generation, high energy density batteries with high specific capacity and low electrode potential. However, due to the high reactivity of lithium, complex interfacial reactions and uncontrollable dendrite growth obstruct their application. These lithium-metal anode interfacial reactions are often accompanied by the organic electrolyte spontaneously decomposing and combustible gas subsequently escaping, which is a safety concern. It also affects the form of the solid electrolyte interphase (SEI), which is important for stabilizing the interface between the Li-metal anode and electrolyte. Uncontrollable Li dendrite growth could penetrate the separator or electrolyte, creating the risk of a short circuit. Therefore, it is necessary to optimize the lithium nucleation and deposition processes. Solid state electrolytes (SSEs) have also attracted attention for improving the energy density and safety of Li-ion batteries; however, problems such as poor ionic conductivity still exist. Computational simulations, such as molecular dynamics (MD) simulations and first-principles calculations based on density function theory (DFT), can help elucidate reaction mechanisms, explore electrode materials, and optimize battery design. In this review, we summarize the theoretical perspective gained from computational simulation studies of lithium-metal anodes. This review is organized into four sections: interfacial reactions, SEIs, lithium nucleation, and SSEs. We first explore organic-electrolyte interfacial reaction mechanisms that were revealed through MD simulations and how electrolyte additives, electrolyte concentration, operating temperature affect them. For SEI, DFT can provide an in-depth understanding of the surface chemical reaction, surface morphology, electrochemical properties, and kinetic characteristics of SEI. We review the developments in SEI transmission mechanisms and SEI materials' properties alteration by lithium metal. We further explore artificial SEI design requirements and compare the performances of artificial SEIs, including double-layer, fluorine-, and sulfur-SEIs. Lithium dendrite growth as a result of lithium nucleation and deposition is then discussed, focusing on computational studies that evaluated how doped graphene, 3D carbon fibers, porous metals, and other matrix materials regulated these processes and inhibited dendrite growth. Computational simulations evaluating transport phenomena and interface reactions between SSEs and lithium-metal anodes are then explored, followed by ideas for further design optimization. Finally, potential research directions and perspectives in this field are proposed and discussed.
2021, 37(2): 200809
doi: 10.3866/PKU.WHXB202008092
Abstract:
The ever-increasing demand for high-energy Li-ion batteries has acted as a powerful stimulus for the development of Li metal as an anode material. Li metal has long been regarded as a "Holy Grail" in Li-ion batteries due to its high discharge capacity (3860 mAh·g-1) and low electric potential (-3.04 V). However, the formation of unstable solid electrolyte interphases (SEIs) and Li dendrites, as well as the resultant safety issues initiated by catastrophic dendrite growth, have greatly impeded further application. High ion conductivity, surface electron insulation, and favorable mechanical strength are essential properties for an ideal SEI film, which can allow for uniform Li deposition, providing a fast transfer path for Li ions and suppressing Li dendrite growth. Therefore, designing a functional SEI layer is an effective strategy to solve the problems encountered with Li metal anodes. So far, a variety of inorganic, organic, and inorganic/organic hybird SEI layers have been designed and fabricated. Inorganic SEIs are characterized by mechanical strength and ion conductivity; organic SEIs are flexible and have electron insulation properties. Inorganic/organic composite SEIs show favorable ion conductivity derived from the inorganic components, electron insulation properties originating from the organic components, and mechanical strength benefiting from the reinforcing effect between the inorganic and organic components. Oxides, metal sulfides, lithium nitride (Li3N) and its derivates, lithium halide (LiX, X = F, Cl), two-dimensional (2D) layered structure materials, lithium phosphate and "Janus" composite are the representative examples of inorganic SEIs. The design principle of various SEI layers is based on the inhibition of Li dendrite formation and growth. Therefore, it is a prerequisite to better understand the relevant intrinsic mechanisms. Despite past investigations, further studies are still required to fully elucidate the related mechanisms by providing more broadly accepted evidence combined with theoretical calculations and offer reliable guidance for the design of multifunctional SEI layers, boosting the performance of Li metal anodes. In this review, on the basis of the mechanisms underlying Li dendrite formation and growth, strategies for constructing various functional SEI films, highlights in structure and property of the films, and their effects on the performance of Li metal anodes are summarized. Moreover, some challenges encountered with the practical applications of Li metal anodes and the future direction for the development of Li metal anodes are addressed. This review can reveal possible strategies for the commercialization of high-energy, safe and stable Li-ion batteries. ![]()
The ever-increasing demand for high-energy Li-ion batteries has acted as a powerful stimulus for the development of Li metal as an anode material. Li metal has long been regarded as a "Holy Grail" in Li-ion batteries due to its high discharge capacity (3860 mAh·g-1) and low electric potential (-3.04 V). However, the formation of unstable solid electrolyte interphases (SEIs) and Li dendrites, as well as the resultant safety issues initiated by catastrophic dendrite growth, have greatly impeded further application. High ion conductivity, surface electron insulation, and favorable mechanical strength are essential properties for an ideal SEI film, which can allow for uniform Li deposition, providing a fast transfer path for Li ions and suppressing Li dendrite growth. Therefore, designing a functional SEI layer is an effective strategy to solve the problems encountered with Li metal anodes. So far, a variety of inorganic, organic, and inorganic/organic hybird SEI layers have been designed and fabricated. Inorganic SEIs are characterized by mechanical strength and ion conductivity; organic SEIs are flexible and have electron insulation properties. Inorganic/organic composite SEIs show favorable ion conductivity derived from the inorganic components, electron insulation properties originating from the organic components, and mechanical strength benefiting from the reinforcing effect between the inorganic and organic components. Oxides, metal sulfides, lithium nitride (Li3N) and its derivates, lithium halide (LiX, X = F, Cl), two-dimensional (2D) layered structure materials, lithium phosphate and "Janus" composite are the representative examples of inorganic SEIs. The design principle of various SEI layers is based on the inhibition of Li dendrite formation and growth. Therefore, it is a prerequisite to better understand the relevant intrinsic mechanisms. Despite past investigations, further studies are still required to fully elucidate the related mechanisms by providing more broadly accepted evidence combined with theoretical calculations and offer reliable guidance for the design of multifunctional SEI layers, boosting the performance of Li metal anodes. In this review, on the basis of the mechanisms underlying Li dendrite formation and growth, strategies for constructing various functional SEI films, highlights in structure and property of the films, and their effects on the performance of Li metal anodes are summarized. Moreover, some challenges encountered with the practical applications of Li metal anodes and the future direction for the development of Li metal anodes are addressed. This review can reveal possible strategies for the commercialization of high-energy, safe and stable Li-ion batteries.
2021, 37(2): 200901
doi: 10.3866/PKU.WHXB202009011
Abstract:
Lithium (Li) metal is considered as the most promising anode material for high-energy-density batteries owing to its ultra-high theoretical capacity (3860 mAh·g-1) and the lowest negative electrochemical potential (-3.040 V versus standard hydrogen electrode). However, the unstable solid electrolyte interphase (SEI) layers, uncontrollable dendrite growth, and huge volume changes during the plating/stripping processes significantly limit the practical applications of Li metal anodes. Since the unstable SEI layers can promote the nucleation and growth of Li dendrites, they play a crucial role in the decay process of Li metal anodes. The fracture and regeneration of SEI layers continuously consume electrolytes and Li metal anodes during plating/ stripping processes, and the accumulation of SEI layers can increase the interface impedance. Therefore, building artificial interphase layers is one of the most effective strategies to construct a stable SEI, reduce dendrite growth, accommodate large volume changes, and thus obtain excellent cycling performance. In this review, artificial interphase layers have been summarized into three parts based on the conductive properties of interphase, including artificial SEI layers (electronically insulating while ionically conducting), mixed ionic and electronic conductor interphase layers, and nanostructured interphase passivation layers (both ionically and electronically insulating). Artificial SEI layers with high ionic conductivity and low electronic conductivity can be classified into inorganic, organic, and organic/inorganic complex SEI according to the composition of artificial SEI layers. The artificial inorganic SEI layers with a high Young's modulus can suppress the dendrite growth. The artificial organic SEI layers with flexible features can accommodate large interface fluctuations and improve the interphase wettability. The artificial organic/inorganic complex SEI layers with a rigid-flexible structure can restrain dendrite growth and buffer volume change. The mixed ionic and electronic conductor layers possess high ionic conductivity and high Young's modulus, which are beneficial for enhancing the interphase stability and reducing dendrite growth. The artificial alloy mixed conductor layers can improve the Li diffusion coefficient and reduce Li nucleation overpotential, guiding uniform Li plating/stripping. Furthermore, the artificial mixed conductor layers comprising inorganic and organic matter have commendable flexibility and excellent interface compatibility, thereby enhancing the interphase stability and reducing dendrite growth. The nanostructured interphase passivation layers with high chemical stability can deliver Li ion through a confined electrolyte in a uniform porous structure, thereby achieving homogeneous Li plating/stripping. In addition, the structure-effective relationship of artificial interphase layers has been analyzed, and methods for improving the performance of artificial interphase layers, such as physical and chemical stability, ion transportation, interface strength and flexibility, and interfacial compatibility, have been discussed in this review. Finally, we present the main challenge and perspectives of artificial interphase layers. ![]()
Lithium (Li) metal is considered as the most promising anode material for high-energy-density batteries owing to its ultra-high theoretical capacity (3860 mAh·g-1) and the lowest negative electrochemical potential (-3.040 V versus standard hydrogen electrode). However, the unstable solid electrolyte interphase (SEI) layers, uncontrollable dendrite growth, and huge volume changes during the plating/stripping processes significantly limit the practical applications of Li metal anodes. Since the unstable SEI layers can promote the nucleation and growth of Li dendrites, they play a crucial role in the decay process of Li metal anodes. The fracture and regeneration of SEI layers continuously consume electrolytes and Li metal anodes during plating/ stripping processes, and the accumulation of SEI layers can increase the interface impedance. Therefore, building artificial interphase layers is one of the most effective strategies to construct a stable SEI, reduce dendrite growth, accommodate large volume changes, and thus obtain excellent cycling performance. In this review, artificial interphase layers have been summarized into three parts based on the conductive properties of interphase, including artificial SEI layers (electronically insulating while ionically conducting), mixed ionic and electronic conductor interphase layers, and nanostructured interphase passivation layers (both ionically and electronically insulating). Artificial SEI layers with high ionic conductivity and low electronic conductivity can be classified into inorganic, organic, and organic/inorganic complex SEI according to the composition of artificial SEI layers. The artificial inorganic SEI layers with a high Young's modulus can suppress the dendrite growth. The artificial organic SEI layers with flexible features can accommodate large interface fluctuations and improve the interphase wettability. The artificial organic/inorganic complex SEI layers with a rigid-flexible structure can restrain dendrite growth and buffer volume change. The mixed ionic and electronic conductor layers possess high ionic conductivity and high Young's modulus, which are beneficial for enhancing the interphase stability and reducing dendrite growth. The artificial alloy mixed conductor layers can improve the Li diffusion coefficient and reduce Li nucleation overpotential, guiding uniform Li plating/stripping. Furthermore, the artificial mixed conductor layers comprising inorganic and organic matter have commendable flexibility and excellent interface compatibility, thereby enhancing the interphase stability and reducing dendrite growth. The nanostructured interphase passivation layers with high chemical stability can deliver Li ion through a confined electrolyte in a uniform porous structure, thereby achieving homogeneous Li plating/stripping. In addition, the structure-effective relationship of artificial interphase layers has been analyzed, and methods for improving the performance of artificial interphase layers, such as physical and chemical stability, ion transportation, interface strength and flexibility, and interfacial compatibility, have been discussed in this review. Finally, we present the main challenge and perspectives of artificial interphase layers.
2021, 37(2): 200808
doi: 10.3866/PKU.WHXB202008082
Abstract:
The emerging market for consumer electronics and electric vehicles has stimulated intensive research on lithium metal batteries (LMBs) with high energy densities and large cycle lifetimes. A metallic Li anode has a high theoretical specific capacity of 3860 mAh·g-1 and lowest redox potential of -3.04 V (vs. the standard hydrogen electrode) and is generally considered an ideal electrode for next-generation high-energy-density LMBs. However, their deployment in practical batteries is severely hindered by the formation of unsafe dendrites and fast capacity decay due to the uncontrollable formation of fragile solid electrolyte interfaces (SEIs). Herein, we describe the stable cycling of carbon paper (Cp)-supported Li-Sn alloy anodes in carbonate electrolytes modified with 1 mol·L-1 bis(2, 2, 2-trifluorotoluene) carbonate (DTFEC). The molten Li-Sn alloy with 8% (mass fraction) Sn was synthesized through thermal treatment at 400 ℃ in an atmosphere of Ar. The Li-Sn-alloy-coated carbon paper (SnLi/Cp) was obtained after the molten alloy was conformally loaded onto the surface of a carbon paper under the action of capillarity. The as-synthesized interconnected SnLi/Cp composite was characterized by X-ray diffraction, energy-dispersive spectrometry, and scanning electron microscopy. The porous SnLi/Cp composite consisted of only Li and Sn5Li22 phases supported by the mechanically strong carbon paper with a good conductivity; no impurity was observed in the XRD results. The synergy of the DTFEC additive and alloying with Sn provided composite anodes with significantly improved rate capability and remarkable stability owing to the formation of a dense fluorinated SEI layer with high mechanical strength and ion penetration. Moreover, with the porous SnLi alloy covered by a fluorinated protection layer, lithium avoids the intrinsic issues of uncontrollable volume expansion and dendrite growth, which restrict the practical application of Li metal, exhibiting a stabilized over-potential of only 90 mV after 100 cycles at 8 mA·cm-2. Notably, stable cycling with a 12 μL lean electrolyte was also observed at 5 mA·cm-2. Overall, the prototype full cell assembled with the SnLi/Cp anode and NMC811 cathode exhibited a high Coulombic efficiency (98.1%) and remarkable cycling stability for 300 cycles at 1C (1.5 mA·cm-2). The rate capability was evaluated at various rates of 0.5C to 5C. Compared to pure Li, the SnLi/Cp anode in the full cell exhibited a higher capacity, particularly at a high rate (~126 mAh·g-1 at 5C). Our approach provides integrated Li metal electrodes with effectively improved cycle stabilities and is very attractive for practical high-energy-density lithium batteries. ![]()
The emerging market for consumer electronics and electric vehicles has stimulated intensive research on lithium metal batteries (LMBs) with high energy densities and large cycle lifetimes. A metallic Li anode has a high theoretical specific capacity of 3860 mAh·g-1 and lowest redox potential of -3.04 V (vs. the standard hydrogen electrode) and is generally considered an ideal electrode for next-generation high-energy-density LMBs. However, their deployment in practical batteries is severely hindered by the formation of unsafe dendrites and fast capacity decay due to the uncontrollable formation of fragile solid electrolyte interfaces (SEIs). Herein, we describe the stable cycling of carbon paper (Cp)-supported Li-Sn alloy anodes in carbonate electrolytes modified with 1 mol·L-1 bis(2, 2, 2-trifluorotoluene) carbonate (DTFEC). The molten Li-Sn alloy with 8% (mass fraction) Sn was synthesized through thermal treatment at 400 ℃ in an atmosphere of Ar. The Li-Sn-alloy-coated carbon paper (SnLi/Cp) was obtained after the molten alloy was conformally loaded onto the surface of a carbon paper under the action of capillarity. The as-synthesized interconnected SnLi/Cp composite was characterized by X-ray diffraction, energy-dispersive spectrometry, and scanning electron microscopy. The porous SnLi/Cp composite consisted of only Li and Sn5Li22 phases supported by the mechanically strong carbon paper with a good conductivity; no impurity was observed in the XRD results. The synergy of the DTFEC additive and alloying with Sn provided composite anodes with significantly improved rate capability and remarkable stability owing to the formation of a dense fluorinated SEI layer with high mechanical strength and ion penetration. Moreover, with the porous SnLi alloy covered by a fluorinated protection layer, lithium avoids the intrinsic issues of uncontrollable volume expansion and dendrite growth, which restrict the practical application of Li metal, exhibiting a stabilized over-potential of only 90 mV after 100 cycles at 8 mA·cm-2. Notably, stable cycling with a 12 μL lean electrolyte was also observed at 5 mA·cm-2. Overall, the prototype full cell assembled with the SnLi/Cp anode and NMC811 cathode exhibited a high Coulombic efficiency (98.1%) and remarkable cycling stability for 300 cycles at 1C (1.5 mA·cm-2). The rate capability was evaluated at various rates of 0.5C to 5C. Compared to pure Li, the SnLi/Cp anode in the full cell exhibited a higher capacity, particularly at a high rate (~126 mAh·g-1 at 5C). Our approach provides integrated Li metal electrodes with effectively improved cycle stabilities and is very attractive for practical high-energy-density lithium batteries.
2021, 37(2): 200808
doi: 10.3866/PKU.WHXB202008088
Abstract:
Lithium (Li) metal anodes are critical components for next-generation high-energy density batteries, owing to their high theoretical specific capacity (3800 mAh·g-1) and low voltage (-3.040 V versus the standard hydrogen electrode). However, their applications are hindered by dendrite growth, which potentially induces inner short circuit and leads to safety issues. Employing three-dimensional (3D) current collectors is an effective strategy to suppress dendrite growth by decreasing the local current density. However, many of the reported 3D current collectors have a lithiophobic surface, which leads to non-uniform Li+ ion deposition. Thus, a complicated modification process is required to increase the lithiophilic property of the current collectors. In addition, they have a large weight or volume, which greatly lowers the energy density of the entire anode. In this work, we report a lightweight 3D carbon current collector with a lithiophilic surface by employing the direct carbonization of low-cost bacterial cellulose (BC) biomass. The current collector is composed of electrically conductive, robust, and interconnected carbon nanofiber networks, which provide sufficient void space to accommodate a large amount of Li and buffer the volume changes during Li plating and stripping. More importantly, homogeneously distributed oxygen-containing functional groups on the nanofiber surface are retained by controlling the carbonization temperature. These functional groups serve as uniform nucleation sites and help realize uniform and dendrite-free Li deposition. Notably, the areal mass density of the 3D carbon current collector was only 0.32 mg·cm-2 and its mass ratio in the whole anode was 28.8%, with a capacity of 3 mAh·cm-2. This 3D carbon current collector facilitates the stable working of the half cells for 150 cycles under a high current density of 3 mA·cm-2 or a high capacity of 4 mAh·cm-2. Symmetric cells exhibit a steady cycling life as long as 600 h under a current density of 1 mA·cm-2 and a capacity of 1 mAh·cm-2. Moreover, appreciable cycling performance was realized in the full cells when the anodes were paired with LiNi0.8Co0.15Al0.05 cathodes. Furthermore, the low-cost raw materials and the simple preparation method promise significant potential for the future applications of the proposed 3D current collectors. ![]()
Lithium (Li) metal anodes are critical components for next-generation high-energy density batteries, owing to their high theoretical specific capacity (3800 mAh·g-1) and low voltage (-3.040 V versus the standard hydrogen electrode). However, their applications are hindered by dendrite growth, which potentially induces inner short circuit and leads to safety issues. Employing three-dimensional (3D) current collectors is an effective strategy to suppress dendrite growth by decreasing the local current density. However, many of the reported 3D current collectors have a lithiophobic surface, which leads to non-uniform Li+ ion deposition. Thus, a complicated modification process is required to increase the lithiophilic property of the current collectors. In addition, they have a large weight or volume, which greatly lowers the energy density of the entire anode. In this work, we report a lightweight 3D carbon current collector with a lithiophilic surface by employing the direct carbonization of low-cost bacterial cellulose (BC) biomass. The current collector is composed of electrically conductive, robust, and interconnected carbon nanofiber networks, which provide sufficient void space to accommodate a large amount of Li and buffer the volume changes during Li plating and stripping. More importantly, homogeneously distributed oxygen-containing functional groups on the nanofiber surface are retained by controlling the carbonization temperature. These functional groups serve as uniform nucleation sites and help realize uniform and dendrite-free Li deposition. Notably, the areal mass density of the 3D carbon current collector was only 0.32 mg·cm-2 and its mass ratio in the whole anode was 28.8%, with a capacity of 3 mAh·cm-2. This 3D carbon current collector facilitates the stable working of the half cells for 150 cycles under a high current density of 3 mA·cm-2 or a high capacity of 4 mAh·cm-2. Symmetric cells exhibit a steady cycling life as long as 600 h under a current density of 1 mA·cm-2 and a capacity of 1 mAh·cm-2. Moreover, appreciable cycling performance was realized in the full cells when the anodes were paired with LiNi0.8Co0.15Al0.05 cathodes. Furthermore, the low-cost raw materials and the simple preparation method promise significant potential for the future applications of the proposed 3D current collectors.
2021, 37(2): 200809
doi: 10.3866/PKU.WHXB202008090
Abstract:
The applications of lithium-ion batteries have been limited because their energy density can no longer meet the requirements of an emerging energy society. Lithium metal batteries (LMBs) are being considered as potential candidate for next-generation energy storage systems owing to the high theoretical specific capacity and low electrochemical potential of lithium metal. However, the commercialization of LMB is limited due to several challenges, such as uncontrollable formation of dendrites, unstable solid electrolyte interface, and infinite anode volume change, which can lead to grievous catastrophe. In this study, several typical mechanisms of lithium dendrite formation and growth are summarized. The results suggest that a smaller current density, greater Li+ transference number, higher mechanical strength of the electrolyte, and a more homogeneous distribution of Li+ on the substrate are conducive to the uniform deposition morphology of lithium metal. In view of these results, combined with the researches on LMBs conducted in recent years, composite anodes can be summarized into three level from internal to external. (ⅰ) Internal composite of lithium metal anode: the scaffolds composited with lithium metal are classified as non-conductive (NC), electron-conductive (EC), ion-conductive (IC), and mixed ion and electron-conductive (MIEC) scaffolds. Composited with NC scaffolds, the tip effect can be weakened through the interaction between polar functional groups and Li+. The composite of lithium metal and EC scaffolds can effectively reduce the local current density, while IC scaffolds can increase the ion flux. However, the performance of LMBs may be hindered by the insulation of electrons or Li+ at high rates. In comparison, MIEC scaffolds can provide fast ion/electron transfer channels for the deposition or dissolution of lithium metal, which is beneficial for the electrochemical performance of LMBs even at high rates. (ⅱ) Internal composite of LMB: Compared with liquid electrolytes, solid-state electrolytes (SSEs) and quasi-solid-state electrolytes are much safer. However, their interfacial contact with lithium metal anodes has been seriously criticized. Lithium metal anodes can be composited with SSEs or quasi-solid-state electrolytes to optimize the interface contact performance and reduce the interface resistance, thereby promoting the development of solid-state batteries. (ⅲ) Composite of internal environment and external operating conditions: Composited with external physical fields, such as electric fields, magnetic fields, and temperature fields, the distribution of Li+ can be homogeneous and the initial nucleation process can be regulated. Overall, this review summarizes several composite anodes that have greatly optimized the performance of LMBs and highlights the potential of multi-level composites for applications in lithium metal anodes. ![]()
The applications of lithium-ion batteries have been limited because their energy density can no longer meet the requirements of an emerging energy society. Lithium metal batteries (LMBs) are being considered as potential candidate for next-generation energy storage systems owing to the high theoretical specific capacity and low electrochemical potential of lithium metal. However, the commercialization of LMB is limited due to several challenges, such as uncontrollable formation of dendrites, unstable solid electrolyte interface, and infinite anode volume change, which can lead to grievous catastrophe. In this study, several typical mechanisms of lithium dendrite formation and growth are summarized. The results suggest that a smaller current density, greater Li+ transference number, higher mechanical strength of the electrolyte, and a more homogeneous distribution of Li+ on the substrate are conducive to the uniform deposition morphology of lithium metal. In view of these results, combined with the researches on LMBs conducted in recent years, composite anodes can be summarized into three level from internal to external. (ⅰ) Internal composite of lithium metal anode: the scaffolds composited with lithium metal are classified as non-conductive (NC), electron-conductive (EC), ion-conductive (IC), and mixed ion and electron-conductive (MIEC) scaffolds. Composited with NC scaffolds, the tip effect can be weakened through the interaction between polar functional groups and Li+. The composite of lithium metal and EC scaffolds can effectively reduce the local current density, while IC scaffolds can increase the ion flux. However, the performance of LMBs may be hindered by the insulation of electrons or Li+ at high rates. In comparison, MIEC scaffolds can provide fast ion/electron transfer channels for the deposition or dissolution of lithium metal, which is beneficial for the electrochemical performance of LMBs even at high rates. (ⅱ) Internal composite of LMB: Compared with liquid electrolytes, solid-state electrolytes (SSEs) and quasi-solid-state electrolytes are much safer. However, their interfacial contact with lithium metal anodes has been seriously criticized. Lithium metal anodes can be composited with SSEs or quasi-solid-state electrolytes to optimize the interface contact performance and reduce the interface resistance, thereby promoting the development of solid-state batteries. (ⅲ) Composite of internal environment and external operating conditions: Composited with external physical fields, such as electric fields, magnetic fields, and temperature fields, the distribution of Li+ can be homogeneous and the initial nucleation process can be regulated. Overall, this review summarizes several composite anodes that have greatly optimized the performance of LMBs and highlights the potential of multi-level composites for applications in lithium metal anodes.
