李文亮1, 李春光2, 孙昊1
1.东北师范大学 化学学院, 吉林 长春 130024; 2.中国能建捷硕海阳电力有限公司, 越南 河内 118000
引用格式:
李文亮, 李春光, 孙昊. 锂离子在固态聚合物电解质中传输机制[J]. 中国粉体技术, 2025, 31(4): 1-15.
LI Wenliang, LI Chunguang, SUN Hao. Lithium-ion transport mechanisms in solid polymer electrolytes[J]. China Powder Science and Technology, 2025, 31(4): 1-15.
DOI:10.13732/j.issn.1008-5548.2025.04.004
收稿日期: 2025-03-27, 修回日期: 2025-05-29,上线日期: 2025-06-10。
基金项目:国家自然科学基金项目, 编号: 22279014; 吉林省发展与改革委员会基金项目, 编号: 2024C018-3 。
第一作者简介: 李文亮(1983—),男,副教授,博士,博士生导师,吉林省优秀青年基金获得者,研究方向为电池材料设计与模拟。E-mail:liwl926@nenu.edu.cn。
通信作者简介: 李春光(1984—),男,工程师,硕士。研究方向为混合储能系统。E-mail:lichunguang@nepdi.net。
摘要: 【目的】 为了设计新型高性能固态聚合物电解质(solid polymer electrolytes,SPEs),深入理解锂离子在SPEs中的传输机制是核心科学问题,可以在根本上实现高性能SPEs的理性设计。【研究现状】 分析阿伦尼乌斯模型、 VTF模型和WLF模型等经典离子传输理论模型的特征及其适用范围; 重点阐述近年来利用先进光谱技术(如红外光谱、 太赫兹光谱等)在实时观测锂离子传输过程中配位环境动态变化方面取得的实验进展;综述分子动力学(molecular dynamics,MD)模拟中的经典分子动力学模拟、 粗粒化分子动力学模拟、 从头算分子动力学及机器学习分子动力学模拟在该领域的研究现状与发展趋势: 经典MD模拟因其在计算效率与精度间的良好平衡,仍是当前研究锂离子传输机制的主要手段。【结论与展望】先进模拟技术与实验光谱技术实现协同发展,认为这种模拟和实验的结合在解决复杂界面问题等挑战性课题中将发挥越来越重要的作用,为理性设计SPEs提供坚实依据。
关键词: 固态聚合物电解质; 传输机制; 经验模型; 分子动力学模拟; 光谱表征
Abstract
Significance The transport mechanisms of lithium ions in solid polymer electrolytes (SPEs) are critical for determining the performance of next-generation batteries. The mechanisms encompass multiple intricate processes, including ion coordination, intra- and inter-chainhopping, and segmental motions of polymer chains. These processes collectively influence essential parameters such as ionic conductivity and lithium-ion mobility, which are pivotal for the development of high-performance SPEs. However, a fundamental understanding of these mechanisms remains a key challenge,and the rational design of advanced SPEs is required. This paper comprehensively reviews the research progress in this field, covering theoretical models, experimental spectroscopic characterization, and computational simulations, highlighting future research directions and opportunities.
Progress Historically, the investigation of ion transport in SPEs has employed empirical and semi-empirical models to describe the temperature- and composition-dependent ionic conductivity. Among these, the Arrhenius model has been widely employed to characterize thermally activated ion transport, particularly in crystalline or glassy electrolytes. However, it often fails to capture the complex behavior of polymer systems, where segmental motion dominates. The Vogel-Tammann-Fulcher (VTF) model addresses this limitation by incorporating free volume and glass transition temperature, making it more appropriate for amorphous polymers. The William-Landel-Ferry (WLF) equation further refines this approach by providing a more nuanced description of temperature-dependent polymer dynamics. However, these models have inherent limitations and need experimental data to achieve a more precise prediction of ion transport behavior in SPEs.Recent advancements in spectroscopic techniques have revolutionized our understanding of the dynamic processes underlying lithium ion transport in SPEs. Infrared (IR) spectroscopy, for instance, has been instrumental in probing the coordination environment of lithium ions and their interactions with polymer chains. Terahertz (THz) spectroscopy offers a distinctive perspective on the low-frequency dynamics of ions and polymer segments, revealing details about ion hopping and collective motion. These techniques, often combined with time-resolved measurements, have enabled the direct observation of real-time ion coordination states and transport processes. Such experimental breakthroughs are invaluable for validating theoretical models and guiding the design of novel SPE materials.Molecular dynamics (MD) simulations have emerged as an essential tool for studying ion transport in SPEs at the atomic and molecular levels. Classical MD simulations, utilizing empirical force fields, are widely used due to their balance between computational efficiency and accuracy. These simulations have significantly advanced our understanding of ion coordination, polymer segmental motion, and ion hopping. However, the simulation accuracy is often constrained by the quality of the force fields, particularly for complex polymer systems. Recent methodological advancements have promoted the development of coarse-grained MD techniques, where computational costs are substantially reduced by simplifying the representation of polymer chains while preserving critical physical features. Additionally, machine learning-based MD simulations have emerged as an advantageous alternative, potentially achieving quantum-level accuracy at a significantly lower computational cost. These advanced simulation methods are particularly promising for studying complex interfacial phenomena in composite electrolytes and electrode-electrolyte systems.Despite significant progress, several challenges persist. A prominent challenge lies in the development of accurate and transferable force fields for MD simulations, especially for multi-component systems and interfaces. Another challenge is the integration of experimental and computational approaches to provide a more holistic understanding of ion transport mechanisms. For example, combining spectroscopic data with MD simulations can bridge the gap between macroscopic properties and microscopic processes. Additionally, the development of new electrolyte materials, such as hybrid organic-inorganic electrolytes and gel-based systems, presents new opportunities and challenges for both experimental and computational studies. Future research should also focus on translating these insights into practical battery systems, with particular attention to electrode compatibility, cycling stability, and safety.
Conclusions and Prospects Understanding the transport mechanisms of lithium ions in SPEs is a complex scientific challenge that demands integrated theoretical, experimental, and computational studies. Although significant progress has been made, many challenges persist, particularly in relation to complex materials and interfaces. Advancements in spectroscopic techniques, MD simulations, and machine learning methods are promising in addressing these issues. A deeper understanding of ion transport in SPEs could pave the way for the development of next-generation batteries with improved performance, safety, and sustainability. This review highlights the importance of interdisciplinary collaboration and innovative methodologies in advancing this critical research field.
Keywords: solid polymer electrolyte; transport mechanism; empirical model; molecular dynamics simulation; spectral charact-erization
参考文献(References)
[1]LIU Y, FU F, TENG H, et al. Dual-phase elastomeric electrolyte with a latitude-longitude interwoven structure for high-energy solid-state lithium metal batteries[J]. Advanced Energy Materials, 2024, 14(38): 2402040.
[2]WANG B, WANG Z, QU C Q. High concentration LiI realizes stable and efficient utilization of sulfur cathode[J]. Journal of Molecular Science, 2023, 39(2): 118-126.
[3]WANG Z, YANG G D, XIA N, et al.Carbon dot modified Zno nanoarrays to construct 3D current collectors for lithium metalanodes[J]. Journal of Molecular Science, 2023, 39(5): 446-452.
[4]ZHANG Y Q, CHEN Q, LI D, et al.AgTFSI Pretreated Li anode in LiI-mediated Li-O2battery: enabling lithiophilicsolid electrolyte interphase generation to suppress the redox shuttling[J]. CCS Chemistry, 2024, 6(10): 2400-2410.
[5]ZENG L Y, ZHOU T, XU X J, et al. General construction of lithiophilic 3D skeleton for dendrite-free lithium metal anode via a versatile MOF-derived route[J]. Science China Materials, 2022, 65(2): 337-348.
[6]XIONG W T, LIN Y, PAN X M. Theoretical study on the structure and properties of five-membered cyclic carbonate-based electrolytes[J]. Journal of Molecular Science, 2024, 40(3): 275-282.
[7]TIKEKAR M D, CHOUDHURY S, TU Z, et al. Design principles for electrolytes and interfaces for stable lithium-metal batteries[J]. Nature Energy, 2016, 1: 1-7.
[8]FU K K, GONG Y, LIU B, et al. Toward garnet electrolyte-based Li metal batteries: an ultrathin, highly effective, artificial solid-state electrolyte/metallic Li interface[J]. Science Advances, 2017, 3(4): 1601659.
[9]LU G L, MENG G, LIU Q, et al. Advanced strategies for solid electrolyte interface design with MOF materials[J]. AdvancedPowder Materials, 2024, 3(1): 100154.
[10]GERASIMOV M, SOTO F A, WAGNER J, et al. Species distribution during solid electrolyte interphase formation on lithium using MD/DFT-parameterized kinetic Monte Carlo simulations[J]. Journal of Physical Chemistry C, 2023, 127(10):4872-4886.
[11]DAI R H, ZHAO Y H, REN J, et al.Theoretical study on the structure and performance of polypropylene carlbonate and butanedinitrile blended polymer electrolyte[J]. Journal of Molecular Science, 2023, 39(6): 548-558.
[12]MACKANIC D G, MICHAELS W, LEE M, et al. Crosslinked poly(tetrahydrofuran) as a loosely coordinating polymer ele-ctrolyte[J]. Advanced Energy Materials, 2018, 8(25): 1800703.
[13]LU J J, SHENG B F, CHEN M F, et al. Localized high concentration polymer electrolyte enabling room temperature solid-state lithium metal batteries with stable LiF-rich interphases[J]. Energy Storage Materials, 2024, 71: 103570.
[14]LI J X, GUAN D H, WANG X X, et al. Highly stable organic molecular porous solid electrolyte with one‐dimensional ion migration channel for solid‐state lithium-oxygen battery[J]. Advanced Materials, 2024, 36(23): 2312661.
[15]REN Y, ZOU Z Y, ZHAO Q, et al. Brief overview of microscopic physical image of ion transport in electrolytes[J]. Acta PhysicaSinica, 2020, 69(22): 226601.
[16]SALEEM A, IQBAL R, MAJEED M K, et al. Boosting lithium-ion conductivity of polymer electrolyte by selective introduction of covalent organic frameworks for safe lithium metal batteries[J]. Nano Energy, 2024, 128: 109848.
[17]MAI W Q, CAO Q Y, ZHENG M T, et al. A fast ionic transport copolymeric network for stable quasi-solid lithium metal battery[J]. Journal of Energy Chemistry, 2023, 87: 491-500.
[18]WANG T H, CHEN B T, LIU C, et al. Build a high‐performance all‐solid‐state lithium battery through introducing competitive coordination induction effect in polymer‐based electrolyte[J]. AngewandteChemie International Edition, 2024, 63(16): e202400960.
[19]LOU X C, ZHONG J, CHENG D P, et al. Solvent-free quasi-solid polymer electrolyte with a high dielectric constant for stable lithium metal anodes[J]. Chemical Engineering Journal, 2023, 468: 143681.
[20]LU X, WANG Y M, XU X Y, et al. Polymer‐based solid‐state electrolytes for high‐energy‐density lithium‐ion batteries- review[J]. Advanced Energy Materials, 2023, 13(38): 2301746.
[21]ZENG X, LIU X, ZHU H, et al. Advanced crosslinked solid polymer electrolytes: molecular architecture, strategies, and future perspectives[J]. Advanced Energy Materials, 2024, 14(46): 2402671.
[22]LIU Y, ZENG Q H, LI Z F, et al. Recent development in topological polymer electrolytes for rechargeable lithium batteries[J]. Advanced Science, 2023, 10(15): 2206978.
[23]WANG X X, SONG L N, ZHENG L J, et al. Polymers with intrinsic microporosity as solid ion conductors for solid‐state lithium batteries[J]. AngewandteChemie International Edition, 2023, 62(37): e202308837.
[24]SAHU A K, VARADWAJ K S K, NAYAK S K, et al. Single-ion conducting polymer electrolyte: a promising electrolyte formulation to extend the lifespans of LMBs[J]. Nano Energy, 2024, 122: 109261.
[25]XU H, LI W Y, HUANG L, et al. Zwitterion-doped self-supporting single-ion conducting polymer electrolyte membrane for dendrite-free lithium metal secondary batteries[J]. Science China Materials, 2023, 66(10): 3799-3809.
[26]ZHOU X Y, FU J L, LI Z, et al. Research progress on solid polymer electrolytes[J]. Chinese Science Bulletin, 2021, 67(9): 842-859.
[27]GERDROODBAR A E, ALIHEMMATI H, SAFAVI-MIRMAHALEH SA, et al. A review on ion transport pathways and coordination chemistry between ions and electrolytes in energy storage devices[J]. Journal Energy Storage, 2023, 74: 109311.
[28]BASKARAN R, SELVASEKARAPANDIAN S, KUWATA N, et al. Structure, thermal and transport properties of PVAc-LiClO4 solid polymer electrolytes[J]. Journal of Physical Chemistry Solids, 2007, 68(3): 407-412.
[29]BENEDICT T J, BANUMATHI S, VELUCHAMY A, et al. Characterization of plasticized solid polymer electrolyte by XRDand AC impedance methods[J]. Journal Power Sources, 1998, 75(1): 171-174.
[30]MICHAEL M S, JACOB M M E, PRABAHARAN S R S, et al. Enhanced lithium ion transport in PEO-based solid polymer electrolytes employing a novel class of plasticizers[J]. Solid State Ionics, 1997, 98(3): 167-174.
[31]TSUNEMI K, OHNO H, TSUCHIDA E. A mechanism of ionic conduction of poly (vinylidene fluoride)-lithium perchlorate hybrid films[J]. Electrochimica Acta, 1983, 28(6): 833-837.
[32]CARVALHO L M, GUÉGAN P, CHERADAME H, et al. Variation of the mesh size of PEO-based networks filled with TFSILi: from an Arrhenius to WLF type conductivity behavior[J]. European Polymer Journal, 2000, 36(2): 401-409.
[33]MERTENS I J A, WÜBBENHORST M, OOSTERBAAN W D, et al. Novel polymer electrolytes based on amorphous poly(ether-ester)s containing 1,4,7-trioxanonyl main chain units. Ionic conductivity versus polymer chain mobility[J]. Macromolecules, 1999, 32(10): 3314-3324.
[34]BOBADE R S, PAKADE S V, YAWALE S P. Electrical investigation of polythiophene-poly(vinyl acetate) composite filmsvia VTF and impedance spectroscopy[J]. Journal of Non-Crystalline Solids, 2009, 355(48/49): 2410-2414.
[35]VAN HEUMEN J D, STEVENS J R. The role of lithium salts in the conductivity and phase morphology of a thermoplastic polyurethane[J]. Macromolecules, 1995, 28(12): 4268-4277.
[36]AZIZ S B, WOO T J, KADIR M F Z, et al. A conceptual review on polymer electrolytes and ion transport models[J]. Journal of Science: Advanced Materials and Devices, 2018, 3(1): 1-17.
[37]MUNSHI M Z A.Handbook of solid state batteries and capacitors[M]. Singapore:World Scientific, 1995: 740.
[38]SIVA KUMAR J, SUBRAHMANYAM A R, JAIPAL REDDY M, et al. Preparation and study of properties of polymer electrolyte system (PEO+NaClO3)[J]. Materials Letters, 2006, 60(28): 3346-3349.
[39]JAIPAL REDDY M, SREEKANTH T, SUBBA RAO U V. Study of the plasticizer effect on a (PEO+NaYF4) polymer electrolyte and its use in an electrochemical cell[J]. Solid State Ionics, 1999, 126(1): 55-63.
[40]KUO P L, LIANG W J, CHEN T Y. Solid polymer electrolytes V: microstructure and ionic conductivity of epoxide-crossli-nked polyether networks doped with LiClO4[J]. Polymer, 2003, 44(10): 2957-2964.
[41]ZHENG Y, YAO Y Z, OU J H, et al. A review of composite solid-state electrolytes for lithium batteries: fundamentals, key materials and advanced structures[J]. Chemical Society Reviews, 2020, 49(23): 8790-8839.
[42]AZIZ S B, ABIDIN Z H Z, AROF A K. Effect of silver nanoparticles on the DC conductivity in chitosan-silver triflate polymer electrolyte[J]. Physica B: Condensed Matter, 2010, 405(21): 4429-4433.
[43]DOOLITTLE A K. Studies in newtonian flow. III. The dependence of the viscosity of liquids on molecular weight and free space (in homologous series)[J]. Journal of Applied Physics, 1952, 23(2): 236-239.
[44]WILLIAMS M L, LANDEL R F, FERRY J D, The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids[J].Journal of the American Chemical Society,1955, 77(14), 3701-3707.
[45]郑强, 林宇, 叶一兰, 等. 《高分子物理》教学中WLF方程的系数求解与分析[J]. 高分子通报, 2010, 43(6): 99-105.
ZHENG Q, LIN Y, YE Y L, et al. Thesolutionand analysis on parameters of WLF equationin teaching the course polymerphysics[J]. Chinese Polymer Bulletin, 2010, 43(6): 99-105.
[46]GOLODNITSKY D, STRAUSS E, PELED E, et al. Review—on order and disorder in polymer electrolytes[J]. Journal of the Electrochemical Society,2015, 162(14): A2551-A2566.
[47]RAMESH S, YUEN T F, SHEN C J. Conductivity and FTIR studies on PEO-LiX [X:CF3SO3-, SO42-] polymer electrolytes[J]. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2008, 69(2): 670-675.
[48]WEIDELT J, NAIR J R, DIDDENS D, et al. Fundamental picture of the conduction mechanism in solid-state polymer ele-ctrolytes revealed by terahertz spectroscopy[J]. Journal of Physical Chemistry C, 2024, 128(16): 6868-6876.
[49]VIGIL D L, FERKO B T, SAUMER A, et al. Partial solvation of lithium ions enhances conductivity in a nanophase-separated polymer electrolyte[J]. Chemistry of Materials, 2024, 36(19): 9970-9979.
[50] FANG C, CHAKRABORTY S, LI Y H, et al. Ion solvation cage structure in polymer electrolytes determined by combining X-ray scattering and simulations[J]. ACS Macro Letters, 2023, 12(9): 1244-1250.
[51]WANG H, SONG J, ZHANG K, et al. A strongly complexed solid polymer electrolyte enables a stable solid state high-vol-tage lithium metal battery[J]. Energy and Environmental Science, 2022, 15(12): 5149-5158.
[52]YAO N, CHEN X, FU Z H, et al. Applying classical, ab initio, and machine-learning molecular dynamics simulations to the liquid electrolyte for rechargeable batteries[J]. Chemical Reviews, 2022, 122(12): 10970-11021.
[53]TYANOVA S, TEMU T, SINITCYN P, et al. The perseus computational platform for comprehensive analysis of (prote)omics data[J]. Nature Methods, 2016, 13(9): 731-740.
[54]ROY R, AL-HASHIMI H M. AlphaFold3 takes a step toward decoding molecular behavior and biological computation[J]. Nature Structural & Molecular Biology, 2024, 31(7): 997-1000.
[55]XIE T, FRANCE-LANORD A, WANG Y, et al. Accelerating amorphous polymer electrolyte screening by learning to red-uce errors in molecular dynamics simulated properties[J]. Nature Communications, 2022, 13(1): 3415.
[56]SCHAUSER N S, KLIEGLE G A, COOKE P, et al. Database creation, visualization, and statistical learning for polymer Li+-electrolyte design[J]. Chemistry of Materials, 2021, 33(13): 4863-4876.
[57]WANG K, SHI H, LI T, et al. Computational and data-driven modelling of solid polymer electrolytes[J]. Digital Discovery, 2023, 2(6): 1660-1682.
[58]BROOKS D J, MERINOV B V, GODDARD W A, et al. Atomistic description of ionic diffusion in PEO-LiTFSI: effect of temperature, molecular weight, and ionic concentration[J]. Macromolecules, 2018, 51(21): 8987-8995.
[59]GUDLA H, ZHANG C, BRANDELL D. Effects of solvent polarity on Li-ion diffusion in polymer electrolytes: an all-atom molecular dynamics study with charge scaling[J]. Journal of Physical Chemistry B, 2020, 124(37): 8124-8131.
[60]NGUYEN M T, ABBAS U L,QI Q, et al.Distinct effects of zwitterionic molecules on ionic solvation in (ethylene oxide)10: a molecular dynamics simulation study[J]. Physical Chemistry Chemical Physics., 2023, 25(11): 8180-8189.
[61]JIANG W B, WANG D H, LI W L, et al. High Li+-transference number in loose coordinated poly(tetrahydrofuran): amoleculardynamics study[J]. Macromolecules, 2024.57(18): 8682-8689.
[62]PAN J, CHEN C, LI Y, et al. Constructing ionic highway in alkaline polymer electrolytes[J]. Energy and Environmental Science, 2014, 7(1): 354-360.
[63]ZHANG X Z, SHI R, LU Z Y, et al. Chemically specific systematic coarse-grained polymer model with both consistently structural and dynamical properties[J]. JACS Au, 2024, 4(3): 1018-1030.
[64]ZHANG L, ZHANG X Z, LYU J T, et al. Surface-cross-linked protein-like single-chain nanoparticle globules unexpectedly stabilized with a low cross-linking degree[J]. Macromolecules, 2024, 57(3): 858-868.
[65]SHI R, QIAN H J, LU Z Y. Coarse-grained molecular dynamics simulation of polymers: structures and dynamics[J]. Wiley Interdisciplinary Reviews-Computational Molecular Science, 2023, 13(6): e1683.
[66]PERIOLE X, HUBER T, MARRINK S J, et al. G protein-coupled receptors self-assemble in dynamics simulations of model bilayers[J]. Journal of the American Chemical Society, 2007, 129(33): 10126-10132.
[67]PADDING J T, BRIELS W J. Systematic coarse-graining of the dynamics of entangled polymer melts: the road from chemistry to rheology[J]. Journal of Physics-Condensed Matter, 2011, 23(23): 233101.
[68]SWAMINATHAN J, CHUNG H W, WARSINGER D M, et al. Energy efficiency of permeate gap and novel conductive gapmembrane distillation[J]. Journal of Membrane Science, 2016, 502: 171-178.
[69]RAJAHMUNDRY G K, PATRA T K. Understanding ion distribution and diffusion in solid polymer electrolytes[J]. Langmuir, 2024, 40(36): 18942-18949.
[70]ANDERSSON E K W, WU L T, BERTOLI L, et al. Initial SEI formation in LiBOB-, LiDFOB- and LiBF4-containing PEOelectrolytes[J]. Journal of Materials Chemistry A, 2024, 12(15): 9184-9199.
[71]EBADI M, MARCHIORI C, MINDEMARK J, et al. Assessing structure and stability of polymer/lithium-metal interfaces from first-principles calculations[J]. Journal of Materials Chemistry A, 2019, 7(14): 8394-8404.
[72]BEHLER J, PARRINELLO M. Generalized neural-network representation of high-dimensional potential-energy surfaces[J]. Physical Review Letters, 2007, 98(14): 146401.
[73]BEHLER J. Constructing high-dimensional neural network potentials: a tutorial review[J]. International Journal of Quantum Chemistry, 2015, 115(16): 1032-1050.
[74]ZHANG Y L, HU C, JIANG B. Embedded atom neural network potentials: efficient and accurate machine learning with a physically inspired representation[J]. Journal of Physical Chemistry Letters, 2019, 10(17): 4962-4967.
[75]ZHANG L F, HAN J Q, WANG H, et al. Deep potential molecular dynamics: ascalable model with the accuracy of quantum mechanics[J]. Physical Review Letters, 2018, 120(14): 143001.
[76]CHMIELA S, SAUCEDA H E, MÜLLER K R, et al. Towards exact molecular dynamics simulations with machine-learned force fields[J]. Nature Communications, 2018, 9(1): 3887.
[77]YANG Y, MA L, DING M M. A machine learning method for accelerating calculation of hydrodynamic interactions in polymer solutions[J]. Journal of Molecular Science, 2024, 40(1): 51-56.
[78]LONG T, LI J, WANG C, et al. Polymers simulation using machine learning interatomic potentials[J]. Polymer, 2024, 308: 127416.
[79]HONG S J, CHUN H, LEE J, et al. First-principles-based machine-learning molecular dynamics for crystalline polymers with van der Waals interactions[J]. Journal of Physical Chemistry Letters, 2021, 12: 6000-6006.
[80]HAJIBABAEI A, HA M, POURASAD S, et al. Machine learning of first-principles force-fields for alkane and polyene hydrocarbons[J]. Journal of Physical Chemistry A, 2021, 125(42): 9414-9420.
[81]FANG C, YU X, CHAKRABORTY S, et al. Molecular origin of high cation transference in mixtures of poly(pentyl malonate) and lithium salt[J]. ACS Macro Letters, 2023, 12(5): 612-618.