An effective solid-electrolyte interphase for stable solid-state batteries

Interface chemistry inevitably manipulates Li-dendrite and interfacial resistance, which pose serious challenges in solid-state Li-metal batteries. In the November issue of Chem, Wang and co-workers designed a thin, polymeric, fluoride-salt-concentrated interlayer as a precursor and template to form a stable, polymeric, LiF-rich solid-electrolyte interphase, thereby overcoming the above-mentioned issues. Interface chemistry inevitably manipulates Li-dendrite and interfacial resistance, which pose serious challenges in solid-state Li-metal batteries. In the November issue of Chem, Wang and co-workers designed a thin, polymeric, fluoride-salt-concentrated interlayer as a precursor and template to form a stable, polymeric, LiF-rich solid-electrolyte interphase, thereby overcoming the above-mentioned issues. Main textSolid-state Li-metal batteries (SSLBs) with high-capacity (3,860 mAh g−1) Li metal and nonflammable solid electrolytes are expected to satisfy the urgent need for next-generation energy upgrades by considerably improving energy density and safety relative to the most-advanced commercial Li-ion batteries available nowadays.1Zhao Q. Stalin S. Zhao C.-Z. Archer L.A. Designing solid-state electrolytes for safe, energy-dense batteries.Nat. Rev. Mater. 2020; 5: 229-252Google Scholar As an extremely important component of SSLBs, solid-state electrolytes have been extensively investigated to enhance their Li-ion conducting capability; examples include Li9.54Si1.74P1.44S11.7Cl0.3 sulfides and dual-salt polymers with high ion conductivity close to that of liquid electrolytes.2Kato Y. Hori S. Saito T. Suzuki K. Hirayama M. Mitsui A. Yonemura M. Iba H. Kanno R. High-power all-solid-state batteries using sulfide superionic conductors.Nat. Energy. 2016; 1: 16030Google Scholar,3Li S. Chen Y.-M. Liang W. Shao Y. Liu K. Nikolov Z. Zhu Y. A Superionic Conductive, Electrochemically Stable Dual-Salt Polymer Electrolyte.Joule. 2018; 2: 1838-1856Google Scholar However, most solid-state electrolytes are chemically unstable when in contact with Li metal and tend to form an uncontrollable solid-electrolyte interphase (SEI) layer during the charging/discharging process, which triggers a decrease in Li-ion conductivity and an increase in interfacial resistance.4Chen Y. Wang Z. Li X. Yao X. Wang C. Li Y. Xue W. Yu D. Kim S.Y. Yang F. et al.Li metal deposition and stripping in a solid-state battery via Coble creep.Nature. 2020; 578: 251-255Google Scholar The inhomogeneous chemical composition along the Li/SEI interface also restrains uniform Li-ion flux and amplifies the local current density, ultimately inducing the growth of irregular Li dendrite that eventually evolves into dead Li. These defects substantially restrict the power density and cycle life of SSLBs.Although composite polymer electrolyte (CPE) presents superiority in terms of flexibility, ionic conductivity, and ease of mass fabrication in comparison with fragile inorganic electrolytes and low Li-ion conductive polymer electrolytes, CPE can only be recycled at a low critical current density (CCD) of 0.2−0.5 mA cm−2 due to the obstinate interfacial issues of SSLBs. Several principles have been explored to address these challenges, and high levels of success have been achieved for polymeric SSLBs. Using external pressure to adjust Li-dendrite growth is a good option given its versatility and convenience, but it lowers the cell’s total energy density due to the additional pressure devices.5Kasemchainan J. Zekoll S. Spencer Jolly D. Ning Z. Hartley G.O. Marrow J. Bruce P.G. Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells.Nat. Mater. 2019; 18: 1105-1111Google Scholar Optimizing SEI, which directly performs action on Li metal, is considered to be particularly promising.6Liang J.Y. Zhang X.D. Zhang Y. Huang L.B. Yan M. Shen Z.Z. Wen R. Tang J. Wang F. Shi J.L. et al.Cooperative Shielding of Bi-Electrodes via In Situ Amorphous Electrode-Electrolyte Interphases for Practical High-Energy Lithium-Metal Batteries.J. Am. Chem. Soc. 2021; 143: 16768-16776Google Scholar By introducing Mg(ClO4)2 additive in CPE, an in situ-formed Li2MgCl4/LiF interfacial layer successfully improves the mobility of Li ion and promotes Li deposition with CCD of up to 2 mA cm−2. However, the CCD and stability of CPE are still not high enough to meet the practical requirements of SSLBs, such as those from electric-powered road vehicles. Thus, effective strategies for optimizing the CPE interface are urgently needed to achieve competitive SSLBs with high stability and safety.With regard to the widely adopted and extensively studied liquid-electrolyte system, diverse SEI remedies have been proposed to promote battery performance, such as highly concentrated electrolytes (HCEs), interfacial catalysis, and polymer-participated SEI.7Wan Y. Song K. Chen W. Qin C. Zhang X. Zhang J. Dai H. Hu Z. Yan P. Liu C. et al.Ultra-High Initial Coulombic Efficiency Induced by Interface Engineering Enables Rapid, Stable Sodium Storage.Angew. Chem. Int. Ed. Engl. 2021; 60: 11481-11486Google Scholar, 8Chen L. Song K. Shi J. Zhang J. Mi L. Chen W. Liu C. Shen C. PAANa-induced ductile SEI of bare micro-sized FeS enables high sodium-ion storage performance.Sci. China Mater. 2020; 64: 105-114Google Scholar, 9Cheng X.B. Zhang R. Zhao C.Z. Zhang Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.Chem. Rev. 2017; 117: 10403-10473Google Scholar However, applying these effective approaches to SSLBs is difficult. For example, the challenge in interfacial catalysis from electrode slides via chemical bonding or a specific metal is further enhanced due to the difficult-to-process Li metal surface. Moreover, the high viscosity, low wettability, and high cost of HCEs limit their application to SSLBs.Recently, in Chem, Wang and co-workers rationally designed cross-linked polymer interlayers of HCEs formed in situ on CPE to obtain a polymer-inorganic (LiF)-rich SEI with uniform mechanical strength and rapid Li-ion transfer capability along the Li/SEI interface, and it greatly reduced interfacial resistance and achieved a high CCD (4.5 mA cm−2).10Deng T. Cao L. He X. Li A.-M. Li D. Xu J. Liu S. Bai P. Jin T. Ma L. et al.In situ formation of polymer-inorganic solid-electrolyte interphase for stable polymeric solid-state lithium-metal batteries.Chem. 2021; 7: 3052-3068Google Scholar (Figures 1A and 1B ) This work is enlightening and shows that by careful design, excellent ideas in liquid electrolytes can be effectively applied to solid-state electrolytes.The authors skillfully fabricated an in situ polymerized HCE (PHCE) thin layer on a rigid but flexible PVDF-based CPE film through ultraviolet (UV) curing process. During cycling, the PHCE layer functions as a customized template and precursor with excellent ion conductivity, flexibility, and concentrated fluoride salt features inherited from HCE to form an effective SEI with an inorganic (LiF)-rich inner layer and polymeric Li salts as the outer layer. Such polymer-inorganic SEI resolves the challenges in dendrite growth and continuous increase of interfacial resistance, thus helping realize stable Li platting/stripping and suppress Li dendrites and dead Li.The designed CPE-PHCE solid-state electrolyte shows rapid Li-ion conductivity (1.2 × 10−4 S cm−1), high Li-ion transfer number (0.67), and extended oxidation stability (relative to Li/Li+) >5.0 V due to the coating of the PHCE layer. Reduced interfacial resistance between the high-energy Co-free LiNiO2 cathode (LNO) and the CPE-PHCE electrolyte is achieved (Figure 1C). Consequently, the assembled SSLBs attain 81% capacity after 200 cycles (Coulombic efficiency >99.5%) and are more stable than solid-state Li|CPE|LNO cells and Li|LiPF6-EC/DMC|LNO cells with conventional liquid electrolytes.The elaborate design of the PHCE layer for critical polymer-inorganic SEI is described as follows: (1) the authors adopted a simple UV in situ polymerization strategy to compactly bridge the two layers of CPE-PHCE; (2) the crosslinked low-molecular-weight poly(ethylene glycol)methyl ether methacrylate (PEGMA) generally presents a small polymer crystal region and accessible flexibility; (3) benefiting from the low molarity, viscosity, and the less stable S-F bond compared with the C-F bond, lithium bis(fluorosulfonyl)imide (LiFSI) instead of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is selected as the Li salt of HCEs in PHCE (the former can easily form LiF-rich SEI); and (4) the confined fluoroethylene carbonate (FEC) in the thin PHCE layer contributes to the formation of SEI without spoiling the ion conductivity of the CPE layer.In summary, a well-designed polymer-inorganic SEI based on in situ UV-polymerized high-concentration salt film on PVDF-based CPE is developed to enhance Li metal/electrolyte interfacial stability. The LiF-rich SEI improves the interfacial ionic conductivity of CPE-PHCE, and its high mechanical strength helps suppress the Li-dendrite growth and uniform Li plating/stripping. This SEI design strategy via UV polymerization of HCEs is potentially applicable to other types of solid-state and quasi-solid-state batteries or to artificial SEI engineering in liquid-state batteries. Meanwhile, other pertinent and targeted experiment layouts and characterizations are expected to reveal the decomposition mechanism of the PHCE layer, which could guide in the formation of accurately decorated SEI with sophisticated design to prolong cycle life to thousands of cycles for SSLBs’ practical application. Main textSolid-state Li-metal batteries (SSLBs) with high-capacity (3,860 mAh g−1) Li metal and nonflammable solid electrolytes are expected to satisfy the urgent need for next-generation energy upgrades by considerably improving energy density and safety relative to the most-advanced commercial Li-ion batteries available nowadays.1Zhao Q. Stalin S. Zhao C.-Z. Archer L.A. Designing solid-state electrolytes for safe, energy-dense batteries.Nat. Rev. Mater. 2020; 5: 229-252Google Scholar As an extremely important component of SSLBs, solid-state electrolytes have been extensively investigated to enhance their Li-ion conducting capability; examples include Li9.54Si1.74P1.44S11.7Cl0.3 sulfides and dual-salt polymers with high ion conductivity close to that of liquid electrolytes.2Kato Y. Hori S. Saito T. Suzuki K. Hirayama M. Mitsui A. Yonemura M. Iba H. Kanno R. High-power all-solid-state batteries using sulfide superionic conductors.Nat. Energy. 2016; 1: 16030Google Scholar,3Li S. Chen Y.-M. Liang W. Shao Y. Liu K. Nikolov Z. Zhu Y. A Superionic Conductive, Electrochemically Stable Dual-Salt Polymer Electrolyte.Joule. 2018; 2: 1838-1856Google Scholar However, most solid-state electrolytes are chemically unstable when in contact with Li metal and tend to form an uncontrollable solid-electrolyte interphase (SEI) layer during the charging/discharging process, which triggers a decrease in Li-ion conductivity and an increase in interfacial resistance.4Chen Y. Wang Z. Li X. Yao X. Wang C. Li Y. Xue W. Yu D. Kim S.Y. Yang F. et al.Li metal deposition and stripping in a solid-state battery via Coble creep.Nature. 2020; 578: 251-255Google Scholar The inhomogeneous chemical composition along the Li/SEI interface also restrains uniform Li-ion flux and amplifies the local current density, ultimately inducing the growth of irregular Li dendrite that eventually evolves into dead Li. These defects substantially restrict the power density and cycle life of SSLBs.Although composite polymer electrolyte (CPE) presents superiority in terms of flexibility, ionic conductivity, and ease of mass fabrication in comparison with fragile inorganic electrolytes and low Li-ion conductive polymer electrolytes, CPE can only be recycled at a low critical current density (CCD) of 0.2−0.5 mA cm−2 due to the obstinate interfacial issues of SSLBs. Several principles have been explored to address these challenges, and high levels of success have been achieved for polymeric SSLBs. Using external pressure to adjust Li-dendrite growth is a good option given its versatility and convenience, but it lowers the cell’s total energy density due to the additional pressure devices.5Kasemchainan J. Zekoll S. Spencer Jolly D. Ning Z. Hartley G.O. Marrow J. Bruce P.G. Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells.Nat. Mater. 2019; 18: 1105-1111Google Scholar Optimizing SEI, which directly performs action on Li metal, is considered to be particularly promising.6Liang J.Y. Zhang X.D. Zhang Y. Huang L.B. Yan M. Shen Z.Z. Wen R. Tang J. Wang F. Shi J.L. et al.Cooperative Shielding of Bi-Electrodes via In Situ Amorphous Electrode-Electrolyte Interphases for Practical High-Energy Lithium-Metal Batteries.J. Am. Chem. Soc. 2021; 143: 16768-16776Google Scholar By introducing Mg(ClO4)2 additive in CPE, an in situ-formed Li2MgCl4/LiF interfacial layer successfully improves the mobility of Li ion and promotes Li deposition with CCD of up to 2 mA cm−2. However, the CCD and stability of CPE are still not high enough to meet the practical requirements of SSLBs, such as those from electric-powered road vehicles. Thus, effective strategies for optimizing the CPE interface are urgently needed to achieve competitive SSLBs with high stability and safety.With regard to the widely adopted and extensively studied liquid-electrolyte system, diverse SEI remedies have been proposed to promote battery performance, such as highly concentrated electrolytes (HCEs), interfacial catalysis, and polymer-participated SEI.7Wan Y. Song K. Chen W. Qin C. Zhang X. Zhang J. Dai H. Hu Z. Yan P. Liu C. et al.Ultra-High Initial Coulombic Efficiency Induced by Interface Engineering Enables Rapid, Stable Sodium Storage.Angew. Chem. Int. Ed. Engl. 2021; 60: 11481-11486Google Scholar, 8Chen L. Song K. Shi J. Zhang J. Mi L. Chen W. Liu C. Shen C. PAANa-induced ductile SEI of bare micro-sized FeS enables high sodium-ion storage performance.Sci. China Mater. 2020; 64: 105-114Google Scholar, 9Cheng X.B. Zhang R. Zhao C.Z. Zhang Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.Chem. Rev. 2017; 117: 10403-10473Google Scholar However, applying these effective approaches to SSLBs is difficult. For example, the challenge in interfacial catalysis from electrode slides via chemical bonding or a specific metal is further enhanced due to the difficult-to-process Li metal surface. Moreover, the high viscosity, low wettability, and high cost of HCEs limit their application to SSLBs.Recently, in Chem, Wang and co-workers rationally designed cross-linked polymer interlayers of HCEs formed in situ on CPE to obtain a polymer-inorganic (LiF)-rich SEI with uniform mechanical strength and rapid Li-ion transfer capability along the Li/SEI interface, and it greatly reduced interfacial resistance and achieved a high CCD (4.5 mA cm−2).10Deng T. Cao L. He X. Li A.-M. Li D. Xu J. Liu S. Bai P. Jin T. Ma L. et al.In situ formation of polymer-inorganic solid-electrolyte interphase for stable polymeric solid-state lithium-metal batteries.Chem. 2021; 7: 3052-3068Google Scholar (Figures 1A and 1B ) This work is enlightening and shows that by careful design, excellent ideas in liquid electrolytes can be effectively applied to solid-state electrolytes.The authors skillfully fabricated an in situ polymerized HCE (PHCE) thin layer on a rigid but flexible PVDF-based CPE film through ultraviolet (UV) curing process. During cycling, the PHCE layer functions as a customized template and precursor with excellent ion conductivity, flexibility, and concentrated fluoride salt features inherited from HCE to form an effective SEI with an inorganic (LiF)-rich inner layer and polymeric Li salts as the outer layer. Such polymer-inorganic SEI resolves the challenges in dendrite growth and continuous increase of interfacial resistance, thus helping realize stable Li platting/stripping and suppress Li dendrites and dead Li.The designed CPE-PHCE solid-state electrolyte shows rapid Li-ion conductivity (1.2 × 10−4 S cm−1), high Li-ion transfer number (0.67), and extended oxidation stability (relative to Li/Li+) >5.0 V due to the coating of the PHCE layer. Reduced interfacial resistance between the high-energy Co-free LiNiO2 cathode (LNO) and the CPE-PHCE electrolyte is achieved (Figure 1C). Consequently, the assembled SSLBs attain 81% capacity after 200 cycles (Coulombic efficiency >99.5%) and are more stable than solid-state Li|CPE|LNO cells and Li|LiPF6-EC/DMC|LNO cells with conventional liquid electrolytes.The elaborate design of the PHCE layer for critical polymer-inorganic SEI is described as follows: (1) the authors adopted a simple UV in situ polymerization strategy to compactly bridge the two layers of CPE-PHCE; (2) the crosslinked low-molecular-weight poly(ethylene glycol)methyl ether methacrylate (PEGMA) generally presents a small polymer crystal region and accessible flexibility; (3) benefiting from the low molarity, viscosity, and the less stable S-F bond compared with the C-F bond, lithium bis(fluorosulfonyl)imide (LiFSI) instead of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is selected as the Li salt of HCEs in PHCE (the former can easily form LiF-rich SEI); and (4) the confined fluoroethylene carbonate (FEC) in the thin PHCE layer contributes to the formation of SEI without spoiling the ion conductivity of the CPE layer.In summary, a well-designed polymer-inorganic SEI based on in situ UV-polymerized high-concentration salt film on PVDF-based CPE is developed to enhance Li metal/electrolyte interfacial stability. The LiF-rich SEI improves the interfacial ionic conductivity of CPE-PHCE, and its high mechanical strength helps suppress the Li-dendrite growth and uniform Li plating/stripping. This SEI design strategy via UV polymerization of HCEs is potentially applicable to other types of solid-state and quasi-solid-state batteries or to artificial SEI engineering in liquid-state batteries. Meanwhile, other pertinent and targeted experiment layouts and characterizations are expected to reveal the decomposition mechanism of the PHCE layer, which could guide in the formation of accurately decorated SEI with sophisticated design to prolong cycle life to thousands of cycles for SSLBs’ practical application. Solid-state Li-metal batteries (SSLBs) with high-capacity (3,860 mAh g−1) Li metal and nonflammable solid electrolytes are expected to satisfy the urgent need for next-generation energy upgrades by considerably improving energy density and safety relative to the most-advanced commercial Li-ion batteries available nowadays.1Zhao Q. Stalin S. Zhao C.-Z. Archer L.A. Designing solid-state electrolytes for safe, energy-dense batteries.Nat. Rev. Mater. 2020; 5: 229-252Google Scholar As an extremely important component of SSLBs, solid-state electrolytes have been extensively investigated to enhance their Li-ion conducting capability; examples include Li9.54Si1.74P1.44S11.7Cl0.3 sulfides and dual-salt polymers with high ion conductivity close to that of liquid electrolytes.2Kato Y. Hori S. Saito T. Suzuki K. Hirayama M. Mitsui A. Yonemura M. Iba H. Kanno R. High-power all-solid-state batteries using sulfide superionic conductors.Nat. Energy. 2016; 1: 16030Google Scholar,3Li S. Chen Y.-M. Liang W. Shao Y. Liu K. Nikolov Z. Zhu Y. A Superionic Conductive, Electrochemically Stable Dual-Salt Polymer Electrolyte.Joule. 2018; 2: 1838-1856Google Scholar However, most solid-state electrolytes are chemically unstable when in contact with Li metal and tend to form an uncontrollable solid-electrolyte interphase (SEI) layer during the charging/discharging process, which triggers a decrease in Li-ion conductivity and an increase in interfacial resistance.4Chen Y. Wang Z. Li X. Yao X. Wang C. Li Y. Xue W. Yu D. Kim S.Y. Yang F. et al.Li metal deposition and stripping in a solid-state battery via Coble creep.Nature. 2020; 578: 251-255Google Scholar The inhomogeneous chemical composition along the Li/SEI interface also restrains uniform Li-ion flux and amplifies the local current density, ultimately inducing the growth of irregular Li dendrite that eventually evolves into dead Li. These defects substantially restrict the power density and cycle life of SSLBs. Although composite polymer electrolyte (CPE) presents superiority in terms of flexibility, ionic conductivity, and ease of mass fabrication in comparison with fragile inorganic electrolytes and low Li-ion conductive polymer electrolytes, CPE can only be recycled at a low critical current density (CCD) of 0.2−0.5 mA cm−2 due to the obstinate interfacial issues of SSLBs. Several principles have been explored to address these challenges, and high levels of success have been achieved for polymeric SSLBs. Using external pressure to adjust Li-dendrite growth is a good option given its versatility and convenience, but it lowers the cell’s total energy density due to the additional pressure devices.5Kasemchainan J. Zekoll S. Spencer Jolly D. Ning Z. Hartley G.O. Marrow J. Bruce P.G. Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells.Nat. Mater. 2019; 18: 1105-1111Google Scholar Optimizing SEI, which directly performs action on Li metal, is considered to be particularly promising.6Liang J.Y. Zhang X.D. Zhang Y. Huang L.B. Yan M. Shen Z.Z. Wen R. Tang J. Wang F. Shi J.L. et al.Cooperative Shielding of Bi-Electrodes via In Situ Amorphous Electrode-Electrolyte Interphases for Practical High-Energy Lithium-Metal Batteries.J. Am. Chem. Soc. 2021; 143: 16768-16776Google Scholar By introducing Mg(ClO4)2 additive in CPE, an in situ-formed Li2MgCl4/LiF interfacial layer successfully improves the mobility of Li ion and promotes Li deposition with CCD of up to 2 mA cm−2. However, the CCD and stability of CPE are still not high enough to meet the practical requirements of SSLBs, such as those from electric-powered road vehicles. Thus, effective strategies for optimizing the CPE interface are urgently needed to achieve competitive SSLBs with high stability and safety. With regard to the widely adopted and extensively studied liquid-electrolyte system, diverse SEI remedies have been proposed to promote battery performance, such as highly concentrated electrolytes (HCEs), interfacial catalysis, and polymer-participated SEI.7Wan Y. Song K. Chen W. Qin C. Zhang X. Zhang J. Dai H. Hu Z. Yan P. Liu C. et al.Ultra-High Initial Coulombic Efficiency Induced by Interface Engineering Enables Rapid, Stable Sodium Storage.Angew. Chem. Int. Ed. Engl. 2021; 60: 11481-11486Google Scholar, 8Chen L. Song K. Shi J. Zhang J. Mi L. Chen W. Liu C. Shen C. PAANa-induced ductile SEI of bare micro-sized FeS enables high sodium-ion storage performance.Sci. China Mater. 2020; 64: 105-114Google Scholar, 9Cheng X.B. Zhang R. Zhao C.Z. Zhang Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.Chem. Rev. 2017; 117: 10403-10473Google Scholar However, applying these effective approaches to SSLBs is difficult. For example, the challenge in interfacial catalysis from electrode slides via chemical bonding or a specific metal is further enhanced due to the difficult-to-process Li metal surface. Moreover, the high viscosity, low wettability, and high cost of HCEs limit their application to SSLBs. Recently, in Chem, Wang and co-workers rationally designed cross-linked polymer interlayers of HCEs formed in situ on CPE to obtain a polymer-inorganic (LiF)-rich SEI with uniform mechanical strength and rapid Li-ion transfer capability along the Li/SEI interface, and it greatly reduced interfacial resistance and achieved a high CCD (4.5 mA cm−2).10Deng T. Cao L. He X. Li A.-M. Li D. Xu J. Liu S. Bai P. Jin T. Ma L. et al.In situ formation of polymer-inorganic solid-electrolyte interphase for stable polymeric solid-state lithium-metal batteries.Chem. 2021; 7: 3052-3068Google Scholar (Figures 1A and 1B ) This work is enlightening and shows that by careful design, excellent ideas in liquid electrolytes can be effectively applied to solid-state electrolytes. The authors skillfully fabricated an in situ polymerized HCE (PHCE) thin layer on a rigid but flexible PVDF-based CPE film through ultraviolet (UV) curing process. During cycling, the PHCE layer functions as a customized template and precursor with excellent ion conductivity, flexibility, and concentrated fluoride salt features inherited from HCE to form an effective SEI with an inorganic (LiF)-rich inner layer and polymeric Li salts as the outer layer. Such polymer-inorganic SEI resolves the challenges in dendrite growth and continuous increase of interfacial resistance, thus helping realize stable Li platting/stripping and suppress Li dendrites and dead Li. The designed CPE-PHCE solid-state electrolyte shows rapid Li-ion conductivity (1.2 × 10−4 S cm−1), high Li-ion transfer number (0.67), and extended oxidation stability (relative to Li/Li+) >5.0 V due to the coating of the PHCE layer. Reduced interfacial resistance between the high-energy Co-free LiNiO2 cathode (LNO) and the CPE-PHCE electrolyte is achieved (Figure 1C). Consequently, the assembled SSLBs attain 81% capacity after 200 cycles (Coulombic efficiency >99.5%) and are more stable than solid-state Li|CPE|LNO cells and Li|LiPF6-EC/DMC|LNO cells with conventional liquid electrolytes. The elaborate design of the PHCE layer for critical polymer-inorganic SEI is described as follows: (1) the authors adopted a simple UV in situ polymerization strategy to compactly bridge the two layers of CPE-PHCE; (2) the crosslinked low-molecular-weight poly(ethylene glycol)methyl ether methacrylate (PEGMA) generally presents a small polymer crystal region and accessible flexibility; (3) benefiting from the low molarity, viscosity, and the less stable S-F bond compared with the C-F bond, lithium bis(fluorosulfonyl)imide (LiFSI) instead of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is selected as the Li salt of HCEs in PHCE (the former can easily form LiF-rich SEI); and (4) the confined fluoroethylene carbonate (FEC) in the thin PHCE layer contributes to the formation of SEI without spoiling the ion conductivity of the CPE layer. In summary, a well-designed polymer-inorganic SEI based on in situ UV-polymerized high-concentration salt film on PVDF-based CPE is developed to enhance Li metal/electrolyte interfacial stability. The LiF-rich SEI improves the interfacial ionic conductivity of CPE-PHCE, and its high mechanical strength helps suppress the Li-dendrite growth and uniform Li plating/stripping. This SEI design strategy via UV polymerization of HCEs is potentially applicable to other types of solid-state and quasi-solid-state batteries or to artificial SEI engineering in liquid-state batteries. Meanwhile, other pertinent and targeted experiment layouts and characterizations are expected to reveal the decomposition mechanism of the PHCE layer, which could guide in the formation of accurately decorated SEI with sophisticated design to prolong cycle life to thousands of cycles for SSLBs’ practical application. This work is supported by National Nature Science Foundation of China ( U1804129 , 21771164 ), Zhongyuan Youth Talent Support Program of Henan Province , and the Project of Zhengzhou University . In situ formation of polymer-inorganic solid-electrolyte interphase for stable polymeric solid-state lithium-metal batteriesDeng et al.ChemJuly 12, 2021In BriefPolymeric highly concentrated electrolyte on composite polymer electrolytes (PHCE-CPEs) enable both Li anode and high-energy Co-free LiNiO2 cathode to achieve high Coulombic efficiency of >99%, representing a new solution for polymeric solid-state batteries (SSBs). The design of PHCE-CPEs resolves the challenges from surface contact and dendrite penetration, while the principles can also be applied to other SSBs. These findings should therefore be of general interest to a broad audience working on batteries, material science and engineering, and electrochemistry. Full-Text PDF