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2021 Volume 37 Issue 1
2021, 37(1): 200405
doi: 10.3866/PKU.WHXB202004058
Abstract:
Lithium (Li) metal is considered a promising anode material for high energy density secondary Li metal batteries because it has the highest specific energy (3860 mAh·g-1) and lowest redox potential (-3.04 V compared to standard hydrogen electrodes. However, the development of high-performance Li metal batteries is challenging. Firstly, Li dendrites tend to grow on the surface of Li metal foil, leading to a limited anodic coulombic efficiency (CE), poor cyclability, and even explosion hazards when an internal cell short circuit occurs. Moreover, Li metal suffers from serious surface stability problems and is easily corroded by electrolytes during cycling, further resulting in low CE, thus shortening the life cycle. We have developed a Li-carbon nanotube (Li-CNT) composite microsphere via a facile molten impregnation method. The Li-CNT composite's CNT framework can suppress volume changes during the charge/discharge process and help stabilize the solid electrolyte interphase (SEI), which is typically mechanically fragile. As a result, Li-CNT shows a high specific capacity (2000 mAh·g-1) and can significantly suppress dendrite formation by reducing the current density, resulting in enhanced safety and cycling stability. However, the large specific surface area of the Li-CNT microspheres also enables increased reaction with the air and the electrolyte. A passivation layer is critical for the practical application of Li-CNT during the electrochemical cycling and manufacturing process. LiF is an important component of SEI in the liquid electrolyte system, and a uniform and dense LiF-rich SEI film can enable stable cycling. Moreover, LiF has been widely used as the preferred coating material to protect Li metal anodes through different methods. In this study, we improved the Li-CNT composite stability by constructing a uniform LiF-rich protecting layer on the surface through in situ polymerization of 4-fluorostyrene. The F functional group of 4-fluorostyrene, which is a lithiophilic group, reacts with the Li-CNT to produce a uniform LiF-rich layer on the surface of the Li-CNT via a facile and scalable liquid-phase reaction. The resulting passivation layer effectively suppresses the Li-CNT corrosion by the electrolyte and air, leading to better environmental and electrochemical stability. Consequently, after exposure to dry-air with a dew point -40 ℃ for 24 h, the specific capacity of the surface passivated Li-CNT is still as high as 1129 mAh·g-1, corresponding to a capacity retention of 52.85%. When the surface passivated Li-CNT is paired with a LiFePO4 cathode (the capacity ratio of cathode and anode is 1 : 6), a prolonged lifespan of over 280 cycles at 0.5C was reached, corresponding to a CE of 97.7%. The in situ polymerization passivation is simple and easy to be scale up; thus, it is a promising method for developing Li metal anodes towards the practical Li metal batteries. ![]()
Lithium (Li) metal is considered a promising anode material for high energy density secondary Li metal batteries because it has the highest specific energy (3860 mAh·g-1) and lowest redox potential (-3.04 V compared to standard hydrogen electrodes. However, the development of high-performance Li metal batteries is challenging. Firstly, Li dendrites tend to grow on the surface of Li metal foil, leading to a limited anodic coulombic efficiency (CE), poor cyclability, and even explosion hazards when an internal cell short circuit occurs. Moreover, Li metal suffers from serious surface stability problems and is easily corroded by electrolytes during cycling, further resulting in low CE, thus shortening the life cycle. We have developed a Li-carbon nanotube (Li-CNT) composite microsphere via a facile molten impregnation method. The Li-CNT composite's CNT framework can suppress volume changes during the charge/discharge process and help stabilize the solid electrolyte interphase (SEI), which is typically mechanically fragile. As a result, Li-CNT shows a high specific capacity (2000 mAh·g-1) and can significantly suppress dendrite formation by reducing the current density, resulting in enhanced safety and cycling stability. However, the large specific surface area of the Li-CNT microspheres also enables increased reaction with the air and the electrolyte. A passivation layer is critical for the practical application of Li-CNT during the electrochemical cycling and manufacturing process. LiF is an important component of SEI in the liquid electrolyte system, and a uniform and dense LiF-rich SEI film can enable stable cycling. Moreover, LiF has been widely used as the preferred coating material to protect Li metal anodes through different methods. In this study, we improved the Li-CNT composite stability by constructing a uniform LiF-rich protecting layer on the surface through in situ polymerization of 4-fluorostyrene. The F functional group of 4-fluorostyrene, which is a lithiophilic group, reacts with the Li-CNT to produce a uniform LiF-rich layer on the surface of the Li-CNT via a facile and scalable liquid-phase reaction. The resulting passivation layer effectively suppresses the Li-CNT corrosion by the electrolyte and air, leading to better environmental and electrochemical stability. Consequently, after exposure to dry-air with a dew point -40 ℃ for 24 h, the specific capacity of the surface passivated Li-CNT is still as high as 1129 mAh·g-1, corresponding to a capacity retention of 52.85%. When the surface passivated Li-CNT is paired with a LiFePO4 cathode (the capacity ratio of cathode and anode is 1 : 6), a prolonged lifespan of over 280 cycles at 0.5C was reached, corresponding to a CE of 97.7%. The in situ polymerization passivation is simple and easy to be scale up; thus, it is a promising method for developing Li metal anodes towards the practical Li metal batteries.
2021, 37(1): 200705
doi: 10.3866/PKU.WHXB202007058
Abstract:
With the booming growth market of electric vehicles and portable electronics, high-energy-density rechargeable lithium ion batteries are being extensively used to advance high-end devices. Lithium-ion batteries with graphite anodes approach the ceiling in energy density, but they cannot satisfy the current demand. Among the next-generation electrodes, lithium metal anodes are strong candidates because of their high theoretical capacity and the most negative electrochemical potential. However, lithium metal batteries have been abandoned because of their poor safety resulting from the growth of lithium dendrites during lithium deposition. Although several strategies have been proposed to suppress the generation of lithium dendrites as well as the side reactions between active lithium and the electrolyte, lithium metal anodes have not been practically applied so far. Various studies have been conducted on the factors influencing lithium deposition, with the aim of understanding the growth behavior of lithium dendrites. The electrolyte plays a crucial role in the performance of the working Li metal anode. In this study, a unique battery system is proposed to realize columnar lithium deposition, which is convenient for obtaining the length and diameter of lithium deposits. The influence of different electrolytes on lithium deposition was investigated by comparing the length-diameter (L/D) ratio of the lithium deposits in two kinds of electrolytes (1.0 mol·L-1 LiPF6-ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume) and 1.0 mol·L-1 LiPF6-5% (volume fraction) fluoroethylene carbonate (FEC)-EC/DEC (1 : 1 by volume)). The morphology of the lithium deposits was strongly affected by the electrolyte composition. In the electrolyte with the FEC additive, the diameter of columnar lithium increased from 0.3-0.6 μm to 0.7-1.3 μm, while the L/D ratio decreased from 12.5 to 5.6. The small L/D ratio can reduce the reactive area between the lithium metal anode and the electrolyte, which is beneficial for achieving high lithium utilization and a long lifespan. To probe the origin of this influence, the surface chemistry of the cycled lithium metal anode was investigated by X-ray photoelectron spectroscopy. The FEC additive can increase the proportion of lithium fluoride (LiF) in the solid electrolyte interphase, which is conducive for the rapid diffusion of lithium ions. As a result, fewer nucleation sites are formed, providing more space for the growth of lithium cores with a large diameter. Therefore, the addition of FEC leads to a decrease in the L/D ratio of columnar lithium. ![]()
With the booming growth market of electric vehicles and portable electronics, high-energy-density rechargeable lithium ion batteries are being extensively used to advance high-end devices. Lithium-ion batteries with graphite anodes approach the ceiling in energy density, but they cannot satisfy the current demand. Among the next-generation electrodes, lithium metal anodes are strong candidates because of their high theoretical capacity and the most negative electrochemical potential. However, lithium metal batteries have been abandoned because of their poor safety resulting from the growth of lithium dendrites during lithium deposition. Although several strategies have been proposed to suppress the generation of lithium dendrites as well as the side reactions between active lithium and the electrolyte, lithium metal anodes have not been practically applied so far. Various studies have been conducted on the factors influencing lithium deposition, with the aim of understanding the growth behavior of lithium dendrites. The electrolyte plays a crucial role in the performance of the working Li metal anode. In this study, a unique battery system is proposed to realize columnar lithium deposition, which is convenient for obtaining the length and diameter of lithium deposits. The influence of different electrolytes on lithium deposition was investigated by comparing the length-diameter (L/D) ratio of the lithium deposits in two kinds of electrolytes (1.0 mol·L-1 LiPF6-ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume) and 1.0 mol·L-1 LiPF6-5% (volume fraction) fluoroethylene carbonate (FEC)-EC/DEC (1 : 1 by volume)). The morphology of the lithium deposits was strongly affected by the electrolyte composition. In the electrolyte with the FEC additive, the diameter of columnar lithium increased from 0.3-0.6 μm to 0.7-1.3 μm, while the L/D ratio decreased from 12.5 to 5.6. The small L/D ratio can reduce the reactive area between the lithium metal anode and the electrolyte, which is beneficial for achieving high lithium utilization and a long lifespan. To probe the origin of this influence, the surface chemistry of the cycled lithium metal anode was investigated by X-ray photoelectron spectroscopy. The FEC additive can increase the proportion of lithium fluoride (LiF) in the solid electrolyte interphase, which is conducive for the rapid diffusion of lithium ions. As a result, fewer nucleation sites are formed, providing more space for the growth of lithium cores with a large diameter. Therefore, the addition of FEC leads to a decrease in the L/D ratio of columnar lithium.
2021, 37(1): 200709
doi: 10.3866/PKU.WHXB202007092
Abstract:
Li metal batteries (LMBs) have attracted worldwide attention in recent years with a focus on the extremely high theoretical energy density of 3580 Wh·kg-1 (Li-O2) and 2600 Wh·kg-1 (Li-S), which benefit from the highest specific capacity (3860 mAh·g-1) and the lowest negative potential (-3.04 V) of Li metal anodes. However, further development and practical applications are hindered by the formation of Li dendrites and a large volume expansion, which not only lowers the coulombic efficiency but also leads to many security risks, such as internal short circuits, fires, and even explosions. In this study, we selected a low-cost and commercial carbon fiber cloth (CC) as a 3D framework for accommodating Li metal and relieving the volume expansion during the Li plating/stripping process. In addition, lithiophilic SnS2 nano-sheet arrays were grown on the surface of carbon fiber cloth via a one-step method. The SnS2 arrays can be partially converted to Li-Sn alloy and Li2S components during the Li plating process. The as-formed Li-Sn alloy can provide reversible sites for further Li deposition and improve the electrochemical kinetics process. As a typical component of the solid electrolyte interface (SEI), Li2S can promote Li+ migration at SEI and ensure a homogeneous distribution of Li+-flux near the electrode surface, thereby reducing the overpotential of Li deposition and suppressing the formation and growth of Li dendrites. Meanwhile, the 3D carbon skeleton can also reduce the local current density of the electrode because of its high specific surface area to ensure uniform Li deposition. Benefiting from the design of the combination bulk and the surface, the composite SnS2@carbon fiber cloth (SnS2@CC) demonstrated excellent prospects for practical applications. Upon pairing with Li foils, the SnS2@CC electrode displayed stable cycling performance with improved coulombic efficiency (> 98%) over 100 cycles at 1.0 mA·cm-2/5.0 mAh·cm-2. After loading 10 mAh·cm-2 Li metal, the composite Li metal anode could run over 400 h with a low overpotential of 60 mV at a current density of 1.0 mA·cm-2, even when the current density was increased to 2.0 mA·cm-2; additionally, a low overpotential of 85 mV could also be maintained over 350 h, manifesting one of the most stable composite Li metal anodes to date. Moreover, when the composite Li metal anode was assembled with a LiFePO4 cathode, the full cells exhibited a high initial specific discharge capacity of 160.6 mAh·g-1 and high cycling stability. At a rate of 2.0C, the cell showed a high capacity retention of 80.6% after 500 cycles.We believe that the lithiophilic SnS2@CC composite electrode offers a simple and effective strategy to suppress dendritic Li growth and relieve the volume change during the charging/discharging process. ![]()
Li metal batteries (LMBs) have attracted worldwide attention in recent years with a focus on the extremely high theoretical energy density of 3580 Wh·kg-1 (Li-O2) and 2600 Wh·kg-1 (Li-S), which benefit from the highest specific capacity (3860 mAh·g-1) and the lowest negative potential (-3.04 V) of Li metal anodes. However, further development and practical applications are hindered by the formation of Li dendrites and a large volume expansion, which not only lowers the coulombic efficiency but also leads to many security risks, such as internal short circuits, fires, and even explosions. In this study, we selected a low-cost and commercial carbon fiber cloth (CC) as a 3D framework for accommodating Li metal and relieving the volume expansion during the Li plating/stripping process. In addition, lithiophilic SnS2 nano-sheet arrays were grown on the surface of carbon fiber cloth via a one-step method. The SnS2 arrays can be partially converted to Li-Sn alloy and Li2S components during the Li plating process. The as-formed Li-Sn alloy can provide reversible sites for further Li deposition and improve the electrochemical kinetics process. As a typical component of the solid electrolyte interface (SEI), Li2S can promote Li+ migration at SEI and ensure a homogeneous distribution of Li+-flux near the electrode surface, thereby reducing the overpotential of Li deposition and suppressing the formation and growth of Li dendrites. Meanwhile, the 3D carbon skeleton can also reduce the local current density of the electrode because of its high specific surface area to ensure uniform Li deposition. Benefiting from the design of the combination bulk and the surface, the composite SnS2@carbon fiber cloth (SnS2@CC) demonstrated excellent prospects for practical applications. Upon pairing with Li foils, the SnS2@CC electrode displayed stable cycling performance with improved coulombic efficiency (> 98%) over 100 cycles at 1.0 mA·cm-2/5.0 mAh·cm-2. After loading 10 mAh·cm-2 Li metal, the composite Li metal anode could run over 400 h with a low overpotential of 60 mV at a current density of 1.0 mA·cm-2, even when the current density was increased to 2.0 mA·cm-2; additionally, a low overpotential of 85 mV could also be maintained over 350 h, manifesting one of the most stable composite Li metal anodes to date. Moreover, when the composite Li metal anode was assembled with a LiFePO4 cathode, the full cells exhibited a high initial specific discharge capacity of 160.6 mAh·g-1 and high cycling stability. At a rate of 2.0C, the cell showed a high capacity retention of 80.6% after 500 cycles.We believe that the lithiophilic SnS2@CC composite electrode offers a simple and effective strategy to suppress dendritic Li growth and relieve the volume change during the charging/discharging process.
2021, 37(1): 200808
doi: 10.3866/PKU.WHXB202008081
Abstract:
Commercialization of high-energy rechargeable batteries can promote the rapid development of portable electronics and electric vehicles. Li metal batteries (LMBs) have attracted considerable attention owing to their high theoretical energy density. Li metal anodes (LMAs) used in LMBs suffer from the disadvantages of high reactivity, interface instability and dendrite growth, which impede the practical development of the LMBs. Coulombic efficiency (CE), which depends on the type of electrolyte used, is one of the key parameters for evaluating the reversibility of battery systems. Herein, we use atomic force microscopy (AFM) to study the initial plating stages and growth of the lithium metal in different electrolytes, such as 1 mol·L-1 lithium hexafluorophosphate (LiPF6)-ethylene carbonate/dimethyl carbonate (EC/DMC, 1 : 1, V/V), 1 mol·L-1 LiPF6-EC/DMC (1 : 1, V/V) + 5% (mass fraction, w) fluoroethylene carbonate (FEC), 1 mol·L-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-1, 3-dioxolane/dimethoxyethane (DOL/DME, 1 : 1, V/V) + 2% (w) lithium nitrate (LiNO3), and 4 mol·L-1 lithium bis(fluorosulfonyl)imide (LiFSI)-DME, and further investigate the correlation between the CE of LMA and Li plating morphology. There are two types of Li morphologies in these electrolytes: strip-like and particle-like morphology. Since the specific surface area of particle-like deposits is much smaller than that of strip-like deposits, the particle-like morphology facilitates higher CE. (1) In the conventional carbonate electrolyte (1 mol·L-1 LiPF6-EC/DMC), Li predominantly forms strip-like deposits with large specific surface area, consuming much active Li (due to the side reaction between Li and the electrolyte). The dendrite morphology of the Li deposits lead to the formation of dead Li during the stripping process, which results in low CE. (2) FEC, an effective additive often used in carbonate electrolyte, can induce the transformation of Li plating morphology from strip-like to particle-like morphology. Therefore, the CE in FEC-containing electrolytes has been significantly improved with stable electrode/electrolyte interphase and small specific surface area of deposited Li. (3) In ether electrolytes, which have better compatibility with LMAs than carbonate electrolytes, Li metal exhibits a particle-like morphology and achieves high CE. (4) In the highly concentrated electrolyte (4 mol·L-1 LiFSI-DME), Li metal grows into large particles without dendrite formation, which hampers the parasitic side reactions, and further enhances CE. ![]()
Commercialization of high-energy rechargeable batteries can promote the rapid development of portable electronics and electric vehicles. Li metal batteries (LMBs) have attracted considerable attention owing to their high theoretical energy density. Li metal anodes (LMAs) used in LMBs suffer from the disadvantages of high reactivity, interface instability and dendrite growth, which impede the practical development of the LMBs. Coulombic efficiency (CE), which depends on the type of electrolyte used, is one of the key parameters for evaluating the reversibility of battery systems. Herein, we use atomic force microscopy (AFM) to study the initial plating stages and growth of the lithium metal in different electrolytes, such as 1 mol·L-1 lithium hexafluorophosphate (LiPF6)-ethylene carbonate/dimethyl carbonate (EC/DMC, 1 : 1, V/V), 1 mol·L-1 LiPF6-EC/DMC (1 : 1, V/V) + 5% (mass fraction, w) fluoroethylene carbonate (FEC), 1 mol·L-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-1, 3-dioxolane/dimethoxyethane (DOL/DME, 1 : 1, V/V) + 2% (w) lithium nitrate (LiNO3), and 4 mol·L-1 lithium bis(fluorosulfonyl)imide (LiFSI)-DME, and further investigate the correlation between the CE of LMA and Li plating morphology. There are two types of Li morphologies in these electrolytes: strip-like and particle-like morphology. Since the specific surface area of particle-like deposits is much smaller than that of strip-like deposits, the particle-like morphology facilitates higher CE. (1) In the conventional carbonate electrolyte (1 mol·L-1 LiPF6-EC/DMC), Li predominantly forms strip-like deposits with large specific surface area, consuming much active Li (due to the side reaction between Li and the electrolyte). The dendrite morphology of the Li deposits lead to the formation of dead Li during the stripping process, which results in low CE. (2) FEC, an effective additive often used in carbonate electrolyte, can induce the transformation of Li plating morphology from strip-like to particle-like morphology. Therefore, the CE in FEC-containing electrolytes has been significantly improved with stable electrode/electrolyte interphase and small specific surface area of deposited Li. (3) In ether electrolytes, which have better compatibility with LMAs than carbonate electrolytes, Li metal exhibits a particle-like morphology and achieves high CE. (4) In the highly concentrated electrolyte (4 mol·L-1 LiFSI-DME), Li metal grows into large particles without dendrite formation, which hampers the parasitic side reactions, and further enhances CE.
2021, 37(1): 200902
doi: 10.3866/PKU.WHXB202009020
Abstract:
With the rapid development of electric vehicles and portable electronic devices, traditional lithium-ion batteries with graphite anodes cannot satisfy demands for increased energy density. Lithium metal, with a high theoretical specific capacity (3860 mAh·g-1), low density (0.534 g·cm-3), and the lowest potential (-3.040 V vs. standard hydrogen electrode), has received much attention as an ideal anode material for next-generation energy storage devices. However, the uncontrolled growth of lithium dendrites and low Coulombic efficiency caused by negative side reactions have severely hindered the development of lithium metal batteries. Here, we propose a strategy based on the synergistic effect between a porous copper foam and thiourea, which uses the "super-filling" effect of thiourea molecules to achieve the uniform deposition of lithium metal on the surface of the porous copper foam. The unique curvature enhance coverage mechanism of thiourea molecules can accelerate Li deposition rate in grooves and achieve "super-filling" growth. The porous copper foam was obtained through simple multi-step processing. Scanning electron microscopy images showed many small pores evenly distributed on the surface; these pores acted as nucleation sites for lithium deposition. With the effect of thiourea, lithium was preferentially deposited in the small pores and then filled to the top, and finally deposited uniformly on the surface of the porous copper foam. The morphologies of the different electrodes deposited with capacities of 1, 3, and 10 mAh·cm-2 demonstrated the synergistic effect between the porous copper foam and thiourea, which can inhibit the growth of lithium dendrites. Through this strategy, stable lithium plating/stripping over 500 h was achieved at a current density of 1 mA·cm-2 with a fixed capacity of 1 mAh·cm-2 while maintaining a voltage hysteresis below 20 mV. Meanwhile, greatly enhanced Coulombic efficiency and longer cycle life times were achieved: the Li||LiFePO4 full cell maintained 94% capacity after 300 cycles at 5C. Exploiting the synergy between the electrolyte and framework provides a novel approach for fabricating advanced lithium metal batteries. This work thus details a novel strategy for lithium anode protection that may also be extended to other metal anodes, thereby facilitating the development of next-generation energy storage devices. ![]()
With the rapid development of electric vehicles and portable electronic devices, traditional lithium-ion batteries with graphite anodes cannot satisfy demands for increased energy density. Lithium metal, with a high theoretical specific capacity (3860 mAh·g-1), low density (0.534 g·cm-3), and the lowest potential (-3.040 V vs. standard hydrogen electrode), has received much attention as an ideal anode material for next-generation energy storage devices. However, the uncontrolled growth of lithium dendrites and low Coulombic efficiency caused by negative side reactions have severely hindered the development of lithium metal batteries. Here, we propose a strategy based on the synergistic effect between a porous copper foam and thiourea, which uses the "super-filling" effect of thiourea molecules to achieve the uniform deposition of lithium metal on the surface of the porous copper foam. The unique curvature enhance coverage mechanism of thiourea molecules can accelerate Li deposition rate in grooves and achieve "super-filling" growth. The porous copper foam was obtained through simple multi-step processing. Scanning electron microscopy images showed many small pores evenly distributed on the surface; these pores acted as nucleation sites for lithium deposition. With the effect of thiourea, lithium was preferentially deposited in the small pores and then filled to the top, and finally deposited uniformly on the surface of the porous copper foam. The morphologies of the different electrodes deposited with capacities of 1, 3, and 10 mAh·cm-2 demonstrated the synergistic effect between the porous copper foam and thiourea, which can inhibit the growth of lithium dendrites. Through this strategy, stable lithium plating/stripping over 500 h was achieved at a current density of 1 mA·cm-2 with a fixed capacity of 1 mAh·cm-2 while maintaining a voltage hysteresis below 20 mV. Meanwhile, greatly enhanced Coulombic efficiency and longer cycle life times were achieved: the Li||LiFePO4 full cell maintained 94% capacity after 300 cycles at 5C. Exploiting the synergy between the electrolyte and framework provides a novel approach for fabricating advanced lithium metal batteries. This work thus details a novel strategy for lithium anode protection that may also be extended to other metal anodes, thereby facilitating the development of next-generation energy storage devices.
2021, 37(1): 200602
doi: 10.3866/PKU.WHXB202006021
Abstract:
The Li metal anode is considered the most promising anode for next-generation high energy density batteries owing to its high theoretical capacity and low electrode potential. The development of batteries with high energy density is essential to meet the growing demand for energy storage devices in the modern world. However, the Li metal anode has operational problems. The high activity of Li causes dendritic growth during the cycling process, which leads to the cracking of the SEI (solid-electrolyte interphase), increased side reactions, and formation of dead Li. Furthermore, if the growth of Li dendrites is left uncontrolled, it can penetrate the separator and create a short-circuit accompanied by thermal runaway. Additionally, the complete utilization of active Li is challenged by the infinite volume expansion of the Li anode. To improve the application scope of Li metal batteries, it is imperative to develop advanced strategies for inhibiting Li dendritic growth, enhancing the stability of the SEI, reducing the accumulation of dead Li, and buffering the volume expansion. Understanding the mechanisms and models of Li nucleation and growth provides insight into solving these problems. This review summarizes some of the important models of Li nucleation and growth such as the surface nucleating model, charge-induced model, SEI model, and deposition/dissolution model. These models aid comprehension of the Li nucleation and growth process under various conditions. This review also discusses the strategies explored in the literature for improving the electrodes (such as three-dimensional (3D) matrix), electrolyte, SEI, and separator to realize uniform deposition of Li and improved utilization of Li. The 3D matrix strategy for improving the electrode design explores various matrices including graphene-based, carbon fiber-based, porous metal-based, and powder-based for buffering the volume expansion and reducing the local current density. To improve the electrolyte, concentrated lithium salts and functional additives are employed to stabilize the SEI and inhibit dendritic growth by regulating the chemical composition of SEI and inducing the deposition of Li. With respect to improving the design of the SEI, strategies for the construction of inorganic or organic components with high ionic conductivity and stable structure are explored for even distribution of Li ions and to avoid SEI rupture. This can reduce electrolyte consumption and dead Li formation. The modification of the separator by functional nanocarbon layer can control the direction of dendritic growth, thereby preventing the penetration of dendrites into the separator and achieving a uniform Li deposition layer. Finally, all solid state Li metal batteries (ASSLMBs) are discussed that utilize ceramic and polymer electrolytes owing to high safety of the solid state electrolyte. Therefore, reducing the interfacial resistance and suppressing dendritic growth between the Li anode and the electrolyte is key for the practical applications of ASSLMBs. Overall, this review provides a summary and outlook for promoting the practical applications of Li metal batteries. ![]()
The Li metal anode is considered the most promising anode for next-generation high energy density batteries owing to its high theoretical capacity and low electrode potential. The development of batteries with high energy density is essential to meet the growing demand for energy storage devices in the modern world. However, the Li metal anode has operational problems. The high activity of Li causes dendritic growth during the cycling process, which leads to the cracking of the SEI (solid-electrolyte interphase), increased side reactions, and formation of dead Li. Furthermore, if the growth of Li dendrites is left uncontrolled, it can penetrate the separator and create a short-circuit accompanied by thermal runaway. Additionally, the complete utilization of active Li is challenged by the infinite volume expansion of the Li anode. To improve the application scope of Li metal batteries, it is imperative to develop advanced strategies for inhibiting Li dendritic growth, enhancing the stability of the SEI, reducing the accumulation of dead Li, and buffering the volume expansion. Understanding the mechanisms and models of Li nucleation and growth provides insight into solving these problems. This review summarizes some of the important models of Li nucleation and growth such as the surface nucleating model, charge-induced model, SEI model, and deposition/dissolution model. These models aid comprehension of the Li nucleation and growth process under various conditions. This review also discusses the strategies explored in the literature for improving the electrodes (such as three-dimensional (3D) matrix), electrolyte, SEI, and separator to realize uniform deposition of Li and improved utilization of Li. The 3D matrix strategy for improving the electrode design explores various matrices including graphene-based, carbon fiber-based, porous metal-based, and powder-based for buffering the volume expansion and reducing the local current density. To improve the electrolyte, concentrated lithium salts and functional additives are employed to stabilize the SEI and inhibit dendritic growth by regulating the chemical composition of SEI and inducing the deposition of Li. With respect to improving the design of the SEI, strategies for the construction of inorganic or organic components with high ionic conductivity and stable structure are explored for even distribution of Li ions and to avoid SEI rupture. This can reduce electrolyte consumption and dead Li formation. The modification of the separator by functional nanocarbon layer can control the direction of dendritic growth, thereby preventing the penetration of dendrites into the separator and achieving a uniform Li deposition layer. Finally, all solid state Li metal batteries (ASSLMBs) are discussed that utilize ceramic and polymer electrolytes owing to high safety of the solid state electrolyte. Therefore, reducing the interfacial resistance and suppressing dendritic growth between the Li anode and the electrolyte is key for the practical applications of ASSLMBs. Overall, this review provides a summary and outlook for promoting the practical applications of Li metal batteries.
2021, 37(1): 200704
doi: 10.3866/PKU.WHXB202007048
Abstract:
Lithium metal batteries (LMBs) are representative systems for high-energy-density batteries. The design of LMBs with high capacity and high cycle stability is imperative. However, the development of LMBs is hindered by typical interface-related problems such as lithium dendrite growth, incompatible separator interfaces, and unstable cathode interfaces because of the inhomogeneous ionic flux and composition distribution. The intrinsic instability significantly hinders electron/ion transfer at the interface, causing serious issues such as dendrite growth, volume changes, low coulombic efficiency, dead lithium, interface deterioration, capacity degradation, and loss of safety. Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with a stable highly porous structure, which can allow for highly efficient gas adsorption, separation and purification, catalysis, etc., in addition to facilitating their application in nanomedicine and other fields. In recent years, MOFs have attracted much attention in the field of LMBs as a possible solution to the typical interface problems abovementioned. The porous structure and open metal sites (OMs) of MOFs provide an excellent interface structure for uniform and high ionic conductivity. As additional bonus, the stable structure provides high mechanical strength with different functional groups and metal sites, resulting in significant versatility of functionality for interface stabilization. MOFs are usually synthesized by hydrothermal/solvothermal, microwave-assisted, electrochemical, and spray-drying methods. The excellent properties of MOFs have prompted researchers to pursue their rational design and modification. Much progress has been made in this direction, and exemplary investigations have been performed to solve the abovementioned interfacial problems encountered with LMBs. Consequently, metallic lithium deposition frameworks, artificial solid electrolyte interface films, electrolyte additives, separator materials, cathode materials for lithium-sulfur batteries, and lithium-air batteries have been developed. However, there is a long way to go before the commercialization of batteries based on MOF materials. In practical, more complex electrochemical reactions occur at the lithium-metal interface, and the operating conditions (temperature, over charging/discharging, external stress, etc.) vary widely. Moreover, MOFs as electrode materials have intrinsic drawbacks, including structural collapse, pore blockage, and low inherent conductivity during the cycles. Based on these interfacial challenges, in LMBs, it introduces the structural characterization and optimization of MOFs and the key chemical components that determines the MOFs of structure (central atom, organic ligand, etc.). Subsequently, we summarized the growth mechanism of lithium dendrites and discussed the applications of MOFs and their derivatives to battery cathodes, separators, anodes, and electrolytes.The manuscript contents would be a guide to solve the problem of unstable interfaces in LMBs by the use of MOFs. Furthermore, the prospects and rational design of MOF-based materials are discussed. ![]()
Lithium metal batteries (LMBs) are representative systems for high-energy-density batteries. The design of LMBs with high capacity and high cycle stability is imperative. However, the development of LMBs is hindered by typical interface-related problems such as lithium dendrite growth, incompatible separator interfaces, and unstable cathode interfaces because of the inhomogeneous ionic flux and composition distribution. The intrinsic instability significantly hinders electron/ion transfer at the interface, causing serious issues such as dendrite growth, volume changes, low coulombic efficiency, dead lithium, interface deterioration, capacity degradation, and loss of safety. Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with a stable highly porous structure, which can allow for highly efficient gas adsorption, separation and purification, catalysis, etc., in addition to facilitating their application in nanomedicine and other fields. In recent years, MOFs have attracted much attention in the field of LMBs as a possible solution to the typical interface problems abovementioned. The porous structure and open metal sites (OMs) of MOFs provide an excellent interface structure for uniform and high ionic conductivity. As additional bonus, the stable structure provides high mechanical strength with different functional groups and metal sites, resulting in significant versatility of functionality for interface stabilization. MOFs are usually synthesized by hydrothermal/solvothermal, microwave-assisted, electrochemical, and spray-drying methods. The excellent properties of MOFs have prompted researchers to pursue their rational design and modification. Much progress has been made in this direction, and exemplary investigations have been performed to solve the abovementioned interfacial problems encountered with LMBs. Consequently, metallic lithium deposition frameworks, artificial solid electrolyte interface films, electrolyte additives, separator materials, cathode materials for lithium-sulfur batteries, and lithium-air batteries have been developed. However, there is a long way to go before the commercialization of batteries based on MOF materials. In practical, more complex electrochemical reactions occur at the lithium-metal interface, and the operating conditions (temperature, over charging/discharging, external stress, etc.) vary widely. Moreover, MOFs as electrode materials have intrinsic drawbacks, including structural collapse, pore blockage, and low inherent conductivity during the cycles. Based on these interfacial challenges, in LMBs, it introduces the structural characterization and optimization of MOFs and the key chemical components that determines the MOFs of structure (central atom, organic ligand, etc.). Subsequently, we summarized the growth mechanism of lithium dendrites and discussed the applications of MOFs and their derivatives to battery cathodes, separators, anodes, and electrolytes.The manuscript contents would be a guide to solve the problem of unstable interfaces in LMBs by the use of MOFs. Furthermore, the prospects and rational design of MOF-based materials are discussed.
2021, 37(1): 200806
doi: 10.3866/PKU.WHXB202008065
Abstract:
Lithium ion batteries have been widely used in the fields of portable energy storage devices and electric vehicles due to their high energy density and high safety, and have a profound impact on modern society. However, the frequent occurrence of battery fire and explosion accidents has caused widespread concern of thermal runaway and thermal safety issues. Many reviews have reported the measures to mitigate thermal runaway of lithium ion batteries. Due to the use of graphite with a smaller capacity as the negative electrode material, the specific energy of lithium ion batteries has approached the theoretical limit, and there is an urgent need to develop more efficient electrode materials to meet the growing demand of the energy storage market. Lithium metal anode has smaller density, higher theoretical capacity and lower potential, which is the ideal anode material for the next generation of high energy density battery system. However, the high reactivity of lithium will cause uncontrollable lithium dendrite growth during the cycle, which may penetrate the separator and cause internal short circuit of the battery, and then cause thermal runaway, fire and even explosion. Therefore, the thermal runaway of lithium metal batteries are more complicated and serious, which hinders the commercial application of batteries. Aiming at the thermal runaway problem of lithium metal batteries, this article first introduces the causes of thermal runaway, which are mainly uncontrollable exothermic reactions caused by internal short circuits. The basic process of thermal runaway is divided into three stages. By analyzing the three characteristic temperatures and heating rate, it is proved that improving the thermal stability of electrolyte and seperator can alleviate thermal runaway. Then we investigated the influence of thermodynamics on the nucleation and growth of lithium dendrites, revealing the dual effects of temperature, and proving that a uniform thermal field is beneficial to obtain uniform lithium deposition and improve battery cycle performance and safety. Secondly, a variety of strategies to improve battery thermal safety are reviewed at the material level. In terms of liquid electrolytes, the development of non-flammable electrolyte systems includes the use of flame-retardant electrolytes and ionic liquid electrolytes with lower flammability. In addition, high-concentration electrolyte and local high-concentration electrolyte can change the solvation structure of lithium ions, and improve safety by reducing the number of free solvent molecules. In terms of separators, high thermal stability separators and thermal response separators with thermal shutdown function have been developed. The flame-retardant separators can release flame retardants to inhibit combustion. In addition, the new intelligent separators have dendrite detection, early warning and elimination functions, which effectively improve the safety and cycle life of the battery. In terms of solid electrolytes, thermally responsive polymer electrolytes have been developed to avoid thermal runaway through the strain function of polymer materials. Finally, further research on the thermal runaway of lithium metal batteries in the future is prospected. ![]()
Lithium ion batteries have been widely used in the fields of portable energy storage devices and electric vehicles due to their high energy density and high safety, and have a profound impact on modern society. However, the frequent occurrence of battery fire and explosion accidents has caused widespread concern of thermal runaway and thermal safety issues. Many reviews have reported the measures to mitigate thermal runaway of lithium ion batteries. Due to the use of graphite with a smaller capacity as the negative electrode material, the specific energy of lithium ion batteries has approached the theoretical limit, and there is an urgent need to develop more efficient electrode materials to meet the growing demand of the energy storage market. Lithium metal anode has smaller density, higher theoretical capacity and lower potential, which is the ideal anode material for the next generation of high energy density battery system. However, the high reactivity of lithium will cause uncontrollable lithium dendrite growth during the cycle, which may penetrate the separator and cause internal short circuit of the battery, and then cause thermal runaway, fire and even explosion. Therefore, the thermal runaway of lithium metal batteries are more complicated and serious, which hinders the commercial application of batteries. Aiming at the thermal runaway problem of lithium metal batteries, this article first introduces the causes of thermal runaway, which are mainly uncontrollable exothermic reactions caused by internal short circuits. The basic process of thermal runaway is divided into three stages. By analyzing the three characteristic temperatures and heating rate, it is proved that improving the thermal stability of electrolyte and seperator can alleviate thermal runaway. Then we investigated the influence of thermodynamics on the nucleation and growth of lithium dendrites, revealing the dual effects of temperature, and proving that a uniform thermal field is beneficial to obtain uniform lithium deposition and improve battery cycle performance and safety. Secondly, a variety of strategies to improve battery thermal safety are reviewed at the material level. In terms of liquid electrolytes, the development of non-flammable electrolyte systems includes the use of flame-retardant electrolytes and ionic liquid electrolytes with lower flammability. In addition, high-concentration electrolyte and local high-concentration electrolyte can change the solvation structure of lithium ions, and improve safety by reducing the number of free solvent molecules. In terms of separators, high thermal stability separators and thermal response separators with thermal shutdown function have been developed. The flame-retardant separators can release flame retardants to inhibit combustion. In addition, the new intelligent separators have dendrite detection, early warning and elimination functions, which effectively improve the safety and cycle life of the battery. In terms of solid electrolytes, thermally responsive polymer electrolytes have been developed to avoid thermal runaway through the strain function of polymer materials. Finally, further research on the thermal runaway of lithium metal batteries in the future is prospected.
2021, 37(1): 200809
doi: 10.3866/PKU.WHXB202008091
Abstract:
Conventional lithium-ion batteries with graphite anode have gradually ceased satisfying demand for the rapid development of modern electric commodities, such as portable electronic devices and electric vehicles. Therefore, metallic lithium is considered the ultimate alternative anode material for future high-energy-density lithium batteries because of its excellent properties, including the highest theoretical capacity and lowest potential of available materials as well as its low density. However, research on lithium metal anodes in traditional liquid batteries has encountered impediments. Numerous studies have shown that lithium dendrites, dead lithium, solid electrolyte interphase problems, and the correlating safety hazards are the main hindrances to the practical application of liquid-based lithium metal batteries. For solid-state batteries, the challenges of lithium metal anodes continue to grow. Studies on the mechanical, thermal, chemical, and electrochemical stability of solid-state electrolyte and lithium metal anode indicate that, unlike early recognition, solid-state lithium metal batteries remain far from commercialization. Unexpected issues like lithium growth along crystal boundaries, mixed-conductivity interphase generation, and interfacial contact losses have emerged that complicate the solid-state lithium metal battery. To achieve practically applicable lithium metal anodes, it is necessary to deepen our understanding of the basic scientific issues. This review systematically discusses the electrode behaviors of lithium metal and the corresponding electrode characterization techniques at multiple spatial scales. First, the basic science and technology issues of lithium metal anodes at different scales are reviewed. Lithium electrodeposition behaviors from the atomic to the macroscale are divided into ion transportation, deposition, nucleation, crystallization, expansion and growth. Various issues are also categorized among different characteristic scales. Second, advanced characterization techniques for all spatial scales are reviewed in light of recent works. Finally, the technical characteristics of various characterization techniques from the atomic to macroscale are analyzed. Features and possible directions of improvement of various characterization techniques used to examine lithium metal anodes in solid-state batteries are highlighted. In situ observation has become a common requirement for battery characterization as it can connect macroscale phenomena to microscale mechanisms. Meanwhile, non-damaging detection techniques have faced growing demand because of the urgent need to understand the complete actual reactions at the bulk and interfaces of solid-state electrolytes and lithium anodes. The combination of techniques for different scales should provide comprehensive information to characterize lithium metal anodes and identify reasonable mechanisms for their behaviors. ![]()
Conventional lithium-ion batteries with graphite anode have gradually ceased satisfying demand for the rapid development of modern electric commodities, such as portable electronic devices and electric vehicles. Therefore, metallic lithium is considered the ultimate alternative anode material for future high-energy-density lithium batteries because of its excellent properties, including the highest theoretical capacity and lowest potential of available materials as well as its low density. However, research on lithium metal anodes in traditional liquid batteries has encountered impediments. Numerous studies have shown that lithium dendrites, dead lithium, solid electrolyte interphase problems, and the correlating safety hazards are the main hindrances to the practical application of liquid-based lithium metal batteries. For solid-state batteries, the challenges of lithium metal anodes continue to grow. Studies on the mechanical, thermal, chemical, and electrochemical stability of solid-state electrolyte and lithium metal anode indicate that, unlike early recognition, solid-state lithium metal batteries remain far from commercialization. Unexpected issues like lithium growth along crystal boundaries, mixed-conductivity interphase generation, and interfacial contact losses have emerged that complicate the solid-state lithium metal battery. To achieve practically applicable lithium metal anodes, it is necessary to deepen our understanding of the basic scientific issues. This review systematically discusses the electrode behaviors of lithium metal and the corresponding electrode characterization techniques at multiple spatial scales. First, the basic science and technology issues of lithium metal anodes at different scales are reviewed. Lithium electrodeposition behaviors from the atomic to the macroscale are divided into ion transportation, deposition, nucleation, crystallization, expansion and growth. Various issues are also categorized among different characteristic scales. Second, advanced characterization techniques for all spatial scales are reviewed in light of recent works. Finally, the technical characteristics of various characterization techniques from the atomic to macroscale are analyzed. Features and possible directions of improvement of various characterization techniques used to examine lithium metal anodes in solid-state batteries are highlighted. In situ observation has become a common requirement for battery characterization as it can connect macroscale phenomena to microscale mechanisms. Meanwhile, non-damaging detection techniques have faced growing demand because of the urgent need to understand the complete actual reactions at the bulk and interfaces of solid-state electrolytes and lithium anodes. The combination of techniques for different scales should provide comprehensive information to characterize lithium metal anodes and identify reasonable mechanisms for their behaviors.
2021, 37(1): 200809
doi: 10.3866/PKU.WHXB202008094
Abstract:
The attention towards lithium (Li) metal anode (LMA) has been rekindled in recent years as it can augment the energy density of Li batteries due to its high theoretical specific capacity (3860 mAh·g-1) and low electrochemical potential (-3.04 V versus standard hydrogen electrode), especially when paired with Li-free cathodes such as Li-oxygen and Li-sulfur. However, severe interfacial instability and safety concerns on rechargeable LMA, associated with Li dendrite formation, continuous side reactions, and infinite volume changes, extremely hinder its commercialization. Numerous strategies have been employed to modify LMA for realizing a uniform distribution of the Li ion flux through interface and dendrite-free Li deposits during repeated Li plating/stripping, which leads to a better cycling performance; however, to the best of our knowledge, a clear understanding of the Li deposition/dissolution behavior and the nucleation growth mechanism of Li dendrites is still lacking, which is conducive to more efficient modification studies on LMA. Therefore, it is critical to achieve considerable progress in the development of advanced characterization techniques. However, the high reactivity of Li metal, which leads to complexity of products and diversity in morphology, causes many difficulties in the characterization of in situ spectroscopy. Recently, some promising characterization techniques have been introduced to further investigate the evolution of LMA during cycling, such as cryo-electron microscopy, solid-state nuclear magnetic resonance technology, and neutron depth profiling (NDP) technique. Because of its high-penetration characteristics, quantitative and nondestructive merits, and highly selective sensitivity to 6Li via the capture reaction with neutrons, the NDP technique shows a broad application prospect for obtaining real-time information of the electrochemical behavior of Li in Li metal batteries. The NDP results contain a wealth of information about time and space for Li. Accordingly, not only can the real-time distribution and migration of Li ions be detected, but also changes in the active sites of Li deposition/dissolution can be analyzed to understand the formation principle of Li dendrites and the failure mechanism of Li metal batteries. In addition, the NDP technique has shown its potential in the diagnosis and prediction of short circuit in Li metal batteries, which is confirmed through voltage curves. This review first briefly introduces the principle of the NDP technique and the methods for improving its space/time resolution; second, it summarizes the recent use of the NDP technique in the research on LMAs based on liquid or solid cell systems. Finally, it provides a prospect for the future development of NDP technique. ![]()
The attention towards lithium (Li) metal anode (LMA) has been rekindled in recent years as it can augment the energy density of Li batteries due to its high theoretical specific capacity (3860 mAh·g-1) and low electrochemical potential (-3.04 V versus standard hydrogen electrode), especially when paired with Li-free cathodes such as Li-oxygen and Li-sulfur. However, severe interfacial instability and safety concerns on rechargeable LMA, associated with Li dendrite formation, continuous side reactions, and infinite volume changes, extremely hinder its commercialization. Numerous strategies have been employed to modify LMA for realizing a uniform distribution of the Li ion flux through interface and dendrite-free Li deposits during repeated Li plating/stripping, which leads to a better cycling performance; however, to the best of our knowledge, a clear understanding of the Li deposition/dissolution behavior and the nucleation growth mechanism of Li dendrites is still lacking, which is conducive to more efficient modification studies on LMA. Therefore, it is critical to achieve considerable progress in the development of advanced characterization techniques. However, the high reactivity of Li metal, which leads to complexity of products and diversity in morphology, causes many difficulties in the characterization of in situ spectroscopy. Recently, some promising characterization techniques have been introduced to further investigate the evolution of LMA during cycling, such as cryo-electron microscopy, solid-state nuclear magnetic resonance technology, and neutron depth profiling (NDP) technique. Because of its high-penetration characteristics, quantitative and nondestructive merits, and highly selective sensitivity to 6Li via the capture reaction with neutrons, the NDP technique shows a broad application prospect for obtaining real-time information of the electrochemical behavior of Li in Li metal batteries. The NDP results contain a wealth of information about time and space for Li. Accordingly, not only can the real-time distribution and migration of Li ions be detected, but also changes in the active sites of Li deposition/dissolution can be analyzed to understand the formation principle of Li dendrites and the failure mechanism of Li metal batteries. In addition, the NDP technique has shown its potential in the diagnosis and prediction of short circuit in Li metal batteries, which is confirmed through voltage curves. This review first briefly introduces the principle of the NDP technique and the methods for improving its space/time resolution; second, it summarizes the recent use of the NDP technique in the research on LMAs based on liquid or solid cell systems. Finally, it provides a prospect for the future development of NDP technique.
2021, 37(1): 200901
doi: 10.3866/PKU.WHXB202009012
Abstract:
Li is highly attractive anode material for next-generation high-energy-density batteries, such as Li-air, Li-sulfur, and solid-state Li-based systems because of its exceedingly low electrode potential (-3.04 V vs the standard hydrogen electrode) and ultra-high theoretical capacity (3860 mAh-g-1). However, Li metal anodes and Li-based batteries are plagued by issues, including unstable solid electrolyte interface (SEI), dead Li formation, and uncontrollable dendritic growth. These limitations result in low cycling stability and could induce short circuits, thermal runaway, and safety hazards. In recent years, a variety of efficient strategies have been proposed to alleviate the challenges faced by Li anodes. For example, the design of Li-free anodes (with Li supplied from the lithiated cathode) or Li-composite anodes has attracted significant attention. Their population can be ascribed to the use of non-excessive Li metal that could be potentially safer and easier to produce. In Li-free and Li-composite anodes, the initial nucleation sites play a crucial role in influencing the subsequent Li electroplating behavior. Stable, homogenous Li electrodeposition is crucial for improving Coulomb efficiency and inhibiting dendrite formation. Moreover, it is also desirable to explore the nucleation and growth mechanism of Li metal on substrates or current collectors. Therefore, in this article, we aim to provide an overview of the mechanism of Li nucleation and strategies to enhance Li metal batteries via substrate modification. The mechanisms of Li nucleation are discussed in terms of nucleation-driven forces and the relation between nuclei size/distribution and overpotential/current density. Heterogeneous nucleation and Chazalviel space charge models are introduced to describe the deposition behaviors of Li in the initial nucleation stage. In the heterogeneous nucleation process, the formation of Li nuclei and its kinetics depend on the nucleation barrier, which correlates with the properties of substrates, such as their crystal structure, lattice matching, facets, and defects. The space charge model can be applied to low-concentration electrolytes or rapid Li deposition, where the decrease in ion concentration on the electrode surface induces a localized space charge and polarized electric field. This subsequently affects the microstructure and morphology of the deposited Li. After discussing the nucleation mechanism and substrate effect, strategies to stabilize nucleation and suppress dendrite are highlighted, such as three-dimensional frameworks, heterogeneous crystal nuclei, Li storage buffer layers, electric field effects, and lattice matching engineering. Information gained from the perspective of Li nucleation and the substrate effect might enlighten the development of strategies to upgrade metallic Li anodes for application in Li-based batteries. ![]()
Li is highly attractive anode material for next-generation high-energy-density batteries, such as Li-air, Li-sulfur, and solid-state Li-based systems because of its exceedingly low electrode potential (-3.04 V vs the standard hydrogen electrode) and ultra-high theoretical capacity (3860 mAh-g-1). However, Li metal anodes and Li-based batteries are plagued by issues, including unstable solid electrolyte interface (SEI), dead Li formation, and uncontrollable dendritic growth. These limitations result in low cycling stability and could induce short circuits, thermal runaway, and safety hazards. In recent years, a variety of efficient strategies have been proposed to alleviate the challenges faced by Li anodes. For example, the design of Li-free anodes (with Li supplied from the lithiated cathode) or Li-composite anodes has attracted significant attention. Their population can be ascribed to the use of non-excessive Li metal that could be potentially safer and easier to produce. In Li-free and Li-composite anodes, the initial nucleation sites play a crucial role in influencing the subsequent Li electroplating behavior. Stable, homogenous Li electrodeposition is crucial for improving Coulomb efficiency and inhibiting dendrite formation. Moreover, it is also desirable to explore the nucleation and growth mechanism of Li metal on substrates or current collectors. Therefore, in this article, we aim to provide an overview of the mechanism of Li nucleation and strategies to enhance Li metal batteries via substrate modification. The mechanisms of Li nucleation are discussed in terms of nucleation-driven forces and the relation between nuclei size/distribution and overpotential/current density. Heterogeneous nucleation and Chazalviel space charge models are introduced to describe the deposition behaviors of Li in the initial nucleation stage. In the heterogeneous nucleation process, the formation of Li nuclei and its kinetics depend on the nucleation barrier, which correlates with the properties of substrates, such as their crystal structure, lattice matching, facets, and defects. The space charge model can be applied to low-concentration electrolytes or rapid Li deposition, where the decrease in ion concentration on the electrode surface induces a localized space charge and polarized electric field. This subsequently affects the microstructure and morphology of the deposited Li. After discussing the nucleation mechanism and substrate effect, strategies to stabilize nucleation and suppress dendrite are highlighted, such as three-dimensional frameworks, heterogeneous crystal nuclei, Li storage buffer layers, electric field effects, and lattice matching engineering. Information gained from the perspective of Li nucleation and the substrate effect might enlighten the development of strategies to upgrade metallic Li anodes for application in Li-based batteries.
2021, 37(1): 200701
doi: 10.3866/PKU.WHXB202007016
Abstract:
In past decades, lithium-ion batteries (LIBs) were the dominant energy storage systems for powering portable electronic devices because of their reliable cyclability. However, further increase in the energy density of LIBs was met by a bottleneck when low-specific- capacity graphite was used at the anode. Li metal has long been regarded as the ideal anode material for building the next high-energy-density batteries due to its ultrahigh capacity of 3860 mAh·g-1, which is ten times higher than that of graphite. However, using Li metal as an anode in rechargeable batteries is challenging due to its high uncontrolled volume expansion and aggressive side reactions with liquid electrolytes. In this study, we demonstrate the effect of a three-dimensional (3D) framework with enriched fluorinated sites for Li metal protection. This framework is obtained via a facile integration of down-sized fluorinated graphite (CFx) particles into Li+ conducting channels. Thermogravimetry, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy results show that Li+ conducting channels rich in lithium fluoride (LiF) are formed in situ across the embedded CFx particles during the initial lithiation process, leading to fast Li+ transfer. Scanning electron microscopy results show that residual CFx particles could act as high-quality nucleation sites for uniform Li deposition inside the framework. These features could not be achieved with a 2D structure consisting of large CFx flakes, due to the limited Li+ transfer paths and low utilization ratio of CFx for conversion into LiF-based solid electrolyte interphase (SEI) layers. Consequently, better performance of Li metal anodes in a 3D framework with enriched fluorinated sites is demonstrated. Stable Li plating/stripping over 240 cycles is obtained at a current density of 0.5 mA·cm-2 for a fixed capacity of 1 mAh·cm-2 by maintaining a voltage hysteresis below 80 mV. Improved Li-LiFePO4 full cell performance with a practical negative/positive capacity ratio of 3 is also demonstrated. These results show the rational combination of well-developed 3D Li+ transfer channels and enriched fluorinated sites as an optimized interfacial design beyond the single use of a 2D fluorinated interface, giving new insight into the protection of Li metal anodes in high-energy-density batteries. ![]()
In past decades, lithium-ion batteries (LIBs) were the dominant energy storage systems for powering portable electronic devices because of their reliable cyclability. However, further increase in the energy density of LIBs was met by a bottleneck when low-specific- capacity graphite was used at the anode. Li metal has long been regarded as the ideal anode material for building the next high-energy-density batteries due to its ultrahigh capacity of 3860 mAh·g-1, which is ten times higher than that of graphite. However, using Li metal as an anode in rechargeable batteries is challenging due to its high uncontrolled volume expansion and aggressive side reactions with liquid electrolytes. In this study, we demonstrate the effect of a three-dimensional (3D) framework with enriched fluorinated sites for Li metal protection. This framework is obtained via a facile integration of down-sized fluorinated graphite (CFx) particles into Li+ conducting channels. Thermogravimetry, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy results show that Li+ conducting channels rich in lithium fluoride (LiF) are formed in situ across the embedded CFx particles during the initial lithiation process, leading to fast Li+ transfer. Scanning electron microscopy results show that residual CFx particles could act as high-quality nucleation sites for uniform Li deposition inside the framework. These features could not be achieved with a 2D structure consisting of large CFx flakes, due to the limited Li+ transfer paths and low utilization ratio of CFx for conversion into LiF-based solid electrolyte interphase (SEI) layers. Consequently, better performance of Li metal anodes in a 3D framework with enriched fluorinated sites is demonstrated. Stable Li plating/stripping over 240 cycles is obtained at a current density of 0.5 mA·cm-2 for a fixed capacity of 1 mAh·cm-2 by maintaining a voltage hysteresis below 80 mV. Improved Li-LiFePO4 full cell performance with a practical negative/positive capacity ratio of 3 is also demonstrated. These results show the rational combination of well-developed 3D Li+ transfer channels and enriched fluorinated sites as an optimized interfacial design beyond the single use of a 2D fluorinated interface, giving new insight into the protection of Li metal anodes in high-energy-density batteries.
2021, 37(1): 200900
doi: 10.3866/PKU.WHXB202009006
Abstract:
Solid-state batteries have garnered significant attention, owing to their high safety and improved energy density. Among various solid-state electrolytes (SSEs), garnet-type SSEs are promising for application in solid-state batteries, owing to their high ionic conductivities (10-4–10-3 S·cm-1) at room temperature and excellent stability against Li metal. However, the poor contact between the rigid ceramic and Li metal will result in high interfacial impedance and uneven lithium ion flux during cycling. Consequently, this will lead to rapid dendrite penetration along the grain boundary and eventual short circuit. Herein, inspired by the unique H+/Li+ exchange reaction of the garnet electrolyte, we propose a facile and efficient metal salt aqueous-solution-based strategy to construct an in situ lithiophilic ZnO layer on the garnet surface without employing any specific apparatus. A Zn(NO3)2 aqueous solution was selected to modify the garnet surface. Within one minute, LiOH spontaneously formed as a result of the H+/Li+ exchange reaction reacted with Zn(NO3)2 to produce homogeneous precipitates. After heat treatment, a lithiophilic ZnO layer was obtained. This was verified by the results of X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy analyses. Furthermore, combined with scanning electron microscopy (SEM) images and corresponding elemental mapping, it was proved that a thin in situ interlayer can be successfully deposited on the garnet surface using our strategy. Moreover, the deposited ZnO nanoparticles were uniformly and densely distributed on the garnet surface. In the presence of the introduced layer, the wettability of the garnet-type SSE with molten Li was greatly improved. The introduced ZnO nanoparticles reacted with molten Li to form a LiZn alloy, achieving a tight and continuous contact at the Li–garnet interface, thereby greatly reducing the interfacial impedance to ~10 Ω·cm2 . In the case of the untreated SSE in contact with the molten Li, the cross-sectional SEM image shows obvious gaps at the interface, indicating poor contact with Li. This resulted in a large interfacial resistance of up to 1350 Ω·cm2 . Moreover, the slow ion transport at the interface reduces the capacity of the battery, and the uneven Li ion flux shortens the life of the cell. With a modified layer, the formed LiZn alloy interphase acting as a mixed ionic and electronic conductive interlayer ensures a uniform Li ion flux at the interface and an appreciable electrochemical performance. Symmetric Li cells with modified garnet-type electrolytes can achieve long cycling stability for approximately 1000 h at a current density of 0.1 mA·cm-2 at room temperature (RT). The quasi solid-state batteries with LiNi0.5Co0.2Mn0.3O2 (NCM523) or LiFePO4 cathodes can cycle stably for over 100 cycles at RT. ![]()
Solid-state batteries have garnered significant attention, owing to their high safety and improved energy density. Among various solid-state electrolytes (SSEs), garnet-type SSEs are promising for application in solid-state batteries, owing to their high ionic conductivities (10-4–10-3 S·cm-1) at room temperature and excellent stability against Li metal. However, the poor contact between the rigid ceramic and Li metal will result in high interfacial impedance and uneven lithium ion flux during cycling. Consequently, this will lead to rapid dendrite penetration along the grain boundary and eventual short circuit. Herein, inspired by the unique H+/Li+ exchange reaction of the garnet electrolyte, we propose a facile and efficient metal salt aqueous-solution-based strategy to construct an in situ lithiophilic ZnO layer on the garnet surface without employing any specific apparatus. A Zn(NO3)2 aqueous solution was selected to modify the garnet surface. Within one minute, LiOH spontaneously formed as a result of the H+/Li+ exchange reaction reacted with Zn(NO3)2 to produce homogeneous precipitates. After heat treatment, a lithiophilic ZnO layer was obtained. This was verified by the results of X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy analyses. Furthermore, combined with scanning electron microscopy (SEM) images and corresponding elemental mapping, it was proved that a thin in situ interlayer can be successfully deposited on the garnet surface using our strategy. Moreover, the deposited ZnO nanoparticles were uniformly and densely distributed on the garnet surface. In the presence of the introduced layer, the wettability of the garnet-type SSE with molten Li was greatly improved. The introduced ZnO nanoparticles reacted with molten Li to form a LiZn alloy, achieving a tight and continuous contact at the Li–garnet interface, thereby greatly reducing the interfacial impedance to ~10 Ω·cm
2021, 37(1): 201106
doi: 10.3866/PKU.WHXB202011061
Abstract:
