Residual alkali effect in LLZO-modified composite solid polymer electrolyte

Abstract

Solid polymer electrolytes (SPEs) are regarded as ideal solid electrolytes for next-generation lithium-ion batteries due to their high safety, but their low ionic conductivity limits practical applications. The incorporation of garnet-type Lithium Lanthanum Zirconium Oxide (LLZO)-based electrolytes with high Li⁺ conductivity is an effective strategy to enhance the performance of polyethylene oxide (PEO)-based SPEs. In this study, highly Li⁺-conductive Ga-doped LLZO (Li_6.4Ga_0.2La₃Zr₂O₁₂) was prepared by wet and dry methods, yielding samples denoted as wet-LLZO and dry-LLZO, with abundant and minimal surface residual alkali, respectively. These LLZO nanoparticles with different residual alkali were incorporated as fillers in composite solid polymer electrolytes (CSPEs). Studies demonstrate that the residual alkali, such as Li₂CO₃, on the LLZO surface effectively disrupts the ordered arrangement of PEO segments, significantly reducing their crystallinity and thus facilitating Li⁺ transport. Based on this, CSPE with 15 wt% wet-LLZO achieves a high ionic conductivity of 0.80 mS cm^-1 and a lithium-ion transference number of 0.36 at 60 °C. The assembled Li/LiFePO₄ all-solid-state battery maintains a discharge specific capacity of 142.5 mAh g^-1 and a Coulombic efficiency exceeding 99% after 300 cycles at a 1.0 C rate. This work reveals the role of residual surface alkali in the LLZO composite solid electrolyte, providing novel insights for designing high-performance solid-state batteries through filler surface engineering.

Keywords

Solid polymer electrolytes / garnet-type electrolytes / residual alkali / surface engineering

References

[1] Joshi A,Singh R,Ding Y.A comprehensive review of solid-state batteries.Appl Energy2025;386:125546

[2] Liu J,Zhou Z.Multifunctional polar polymer boosting PEO electrolytes toward high room temperature ionic conductivity, high-voltage stability, and excellent elongation.ACS Appl Mater Interfaces2025;17:7821-9

[3] Du A,Liu S.Breaking the trade-off between ionic conductivity and mechanical strength in solid polymer electrolytes for high-performance solid lithium batteries.Adv Energy Mater2024;14:2400808

[4] Qiao Q,Song C.Recent advances and remaining challenges of solid-state electrolytes for lithium batteries.Prog Mater Sci2026;156:101559

[5] He H,Wang X.Design strategies, characterization mechanisms, and applications of MOFs in polymer composite electrolytes for solid-state lithium metal batteries.Adv Funct Mater2025;35:2421670

[6] Jiang Y,Ma Z.Development of the PEO based solid polymer electrolytes for all-solid state lithium ion batteries.Polymers2018;10:1237 PMCID:PMC6401925

[7] Tan S,Ma Q,Guo Y.Recent advancements in polymer-based composite electrolytes for rechargeable lithium batteries.Electrochem Energ Rev2018;1:113-38

[8] Xu Q,Feng X.Monoclinic Li₂ZrO₃ with cationic vacancy-based ion transport channels enhanced composite polymer electrolytes for high-rate solid-state lithium metal batteries.Nano Energy2026;147:111571

[9] Polu AR,Kareem AA.Impact of tetracyanoethylene plasticizer on PEO based solid polymer electrolytes for improved ionic conductivity and solid-state lithium-ion battery performance.J Power Sources2025;625:235742

[10] Wang Z,Fu J,Guo X.Polymer-based electrolytes for high-voltage solid-state lithium batteries.Energy Mater2024;4:400050

[11] Fu Y,Gan Q.A review on the ionic conductivity and mechanical properties of composite polymer electrolytes (CPEs) for lithium batteries: insights from the perspective of polymer/filler composites.Mater Sci Eng R Rep2024;160:100815

[12] Liu S,Ba D.Filler-integrated composite polymer electrolyte for solid-state lithium batteries.Adv Mater2023;35:e2110423

[13] Wen W,Chen P.Enhancing Li-ion conduction and mechanical properties via addition of fluorine-containing metal-organic frameworks in all-solid-state cross-linked hyperbranched polymer electrolytes.Nano Res2022;15:8946-54

[14] Xiao Z,Song L,Wang C.Research progress of polymer-inorganic filler solid composite electrolyte for lithium-ion batteries.Ionics2022;28:15-26

[15] Ji K,Kim J.Role of functional nano-sized inorganic fillers in poly(ethylene) oxide-based polymer electrolytes.J Power Sources2003;117:124-30

[16] Ayalew KH,Mathe MK.Garnet-type LLZO electrolytes for solid-state lithium batteries: Interfaces, conductivity, in-situ processing, and industrial prospects.Chem Eng J2025;524:168098

[17] Wang L,Bao C,Lu Y.Interfacial engineering for high-performance garnet-based solid-state lithium batteries.SusMat2024;4:72-105

[18] Wang Y,Jiang K,Passerini S.Accelerating the development of LLZO in solid-state batteries toward commercialization: a comprehensive review.Small2024;20:e2402035

[19] Chu J,Wang J,Zhang X.Optimization strategies for key interfaces of LLZO-based solid-state lithium metal batteries.Mater Chem Front2024;8:2109-34

[20] Samson AJ,Bag S.A bird’s-eye view of Li-stuffed garnet-type Li₇La₃Zr₂O₁₂ ceramic electrolytes for advanced all-solid-state Li batteries.Energy Environ Sci2019;12:2957-75

[21] Löwe R,Zinkevich T.Structure-property relationship of polymerized ionic liquids for solid-state electrolyte membranes.Polymers2021;13:792 PMCID:PMC7961940

[22] Counihan MJ,Mirmira P.Improved interfacial li-ion transport in composite polymer electrolytes via surface modification of LLZO.Energy Mater2025;5:500032

[23] Seo J,Park HJ.Lithium-ion conduction pathways in LLZO-PEO composite solid electrolytes.ACS Appl Energy Mater2025;8:1518-25

[24] Go W,Tucker MC.Investigation of MgO additives on microstructure and properties of thin LLZO electrolytes for all-solid-state batteries.J Mater Chem A2025;13:8835-42

[25] Ye R,Imanishi N,Figgemeier E.A review on Li⁺/H⁺ exchange in garnet solid electrolytes: from instability against humidity to sustainable processing in water.ChemSusChem2021;14:4397-407

[26] Zhang N,Li L.Dynamical evolution of CO₂ and H₂O on garnet electrolyte elucidated by ambient pressure X-ray spectroscopies.Nat Commun2024;15:2777 PMCID:PMC10981710

[27] Dubey R,Cancellieri C.Building a better Li-garnet solid electrolyte/metallic li interface with antimony.Adv Energy Mater2021;11:2102086

[28] Liu M,Zhang X.Interfacial lithium-ion transportation in solid-state batteries: challenges and prospects.Nano Energy2025;136:110749

[29] Siniscalchi M,Tufnail J.Removal and reoccurrence of LLZTO surface contaminants under glovebox conditions.ACS Appl Mater Interfaces2024;16:27230-41 PMCID:PMC11145597

[30] Zhang H,Okur F.On high-temperature thermal cleaning of Li₇La₃Zr₂O₁₂ solid-state electrolytes.ACS Appl Energy Mater2023;6:6972-80

[31] Park G,Song M,Lee KJ.Recent progress on solvent-free electrode fabrication for lithium-based batteries.Chem Eng J2025;511:161888

[32] Muhammad T,Guerreiro A.Solid phase synthesis of molecularly imprinted polymers for analytical and life science applications.TrAC Trends Anal Chem2025;184:118134

[33] Li J,Liu K.Excellent stability of Ga-doped garnet electrolyte against Li metal anode via eliminating LiGaO₂ precipitates for advanced all-solid-state batteries.ACS Appl Mater Interfaces2023;15:7165-74

[34] Su J,Song Z.Overcoming the abnormal grain growth in Ga-doped Li₇La₃Zr₂O₁₂ to enhance the electrochemical stability against Li metal.Ceram Int2019;45:14991-6

[35] Aguesse F,Buannic L.Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal.ACS Appl Mater Interfaces2017;9:3808-16

[36] Yi E,Kieffer J.Key parameters governing the densification of cubic-Li₇La₃Zr₂O₁₂ Li⁺ conductors.J Power Sources2017;352:156-64

[37] Afyon S,Rupp JLM.A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries.J Mater Chem A2015;3:18636-48

[38] Yu X,Li Y.Weakening ionic coordination for high ionic conductivity composite solid electrolytes.ACS Energy Lett2024;9:2109-15

[39] Huang X,Xiu T.Sintering, micro-structure and Li+ conductivity of Li_7-xLa₃Zr_2-xNb_xO₁₂/MgO (x= 0.2-0.7) Li-garnet composite ceramics.Ceram Int2019;45:56-63

[40] Huang X,Niu Y.From protonation & Li-rich contamination to grain-boundary segregation: evaluations of solvent-free vs. wet routes on preparing Li₇La₃Zr₂O₁₂ solid electrolyte.J Energy Chem2022;73:223-39

[41] Yao KPC,Quinlan RA.Thermal stability of Li₂O₂ and Li₂O for Li-air batteries: in situ XRD and XPS studies.J Electrochem Soc2013;160:A824-31

[42] Huo H,Zhao N.In-situ formed Li₂CO₃-free garnet/Li interface by rapid acid treatment for dendrite-free solid-state batteries.Nano Energy2019;61:119-25

[43] Zhang H,Cancellieri C.Bilayer dense-porous Li₇La₃Zr₂O₁₂ membranes for high-performance Li-garnet solid-state batteries.Adv Sci2023;10:e2205821

[44] Sun C,Yin L.Fast lithium ion transport in solid polymer electrolytes from polysulfide-bridged copolymers.Nano Energy2020;75:104976

[45] Li W,Wang B.Nanoporous adsorption effect on alteration of the Li⁺ diffusion pathway by a highly ordered porous electrolyte additive for high-rate all-solid-state lithium metal batteries.ACS Appl Mater Interfaces2018;10:23874-82