Journal of Advanced Ceramics

, Volume 8, Issue 1, pp 1–18 | Cite as

Recent progress in Ti-based nanocomposite anodes for lithium ion batteries

  • Shitong Wang
  • Yong Yang
  • Yanhao Dong
  • Zhongtai Zhang
  • Zilong TangEmail author
Open Access


Studying on the anode materials with high energy densities for next-generation lithium-ion batteries (LIBs) is the key for the wide application for electrochemical energy storage devices. Ti-based compounds as promising anode materials are known for their outstanding high-rate capacity and cycling stability as well as improved safety over graphite. However, Ti-based materials still suffer from the low capacity, thus largely limiting their commercialized application. Here, we present an overview of the recent development of Ti-based anode materials in LIBs, and special emphasis is placed on capacity enhancement by rational design of hybrid nanocomposites with conversion-/ alloying-type anodes. This review is expected to provide a guidance for designing novel Ti-based materials for energy storage and conversion.


lithium-ion batteries (LIBs) anode titania lithium titanate 



This work was financially supported by the National Natural Science Foundation of China (Nos. 51472137 and 51772163).


  1. [1]
    Choi J W, Aurbach D. Promise and reality of post-lithium-ion batteries with high energy densities. Nat Rev Mater 2016, 1: 16013.CrossRefGoogle Scholar
  2. [2]
    Chen Z, Belharouak I, Sun Y-K, et al. Titanium-based anode materials for safe lithium-ion batteries. Adv Funct Mater 2013, 23: 959–969.CrossRefGoogle Scholar
  3. [3]
    Yang Z, Choi D, Kerisit S, et al. Nanostructures and lithium electrochemical reactivity of lithium titanites and titanium oxides: A review. J Power Sources 2009, 192: 588–598.CrossRefGoogle Scholar
  4. [4]
    Wang S, Quan W, Zhu Z, et al. Lithium titanate hydrates with superfast and stable cycling in lithium ion batteries. Nat Commun 2017, 8: 627.CrossRefGoogle Scholar
  5. [5]
    Zhao B, Ran R, Liu M, et al. A comprehensive review of Li4Ti5O12-based electrodes for lithium-ion batteries: The latest advancements and future perspectives. Mat Sci Eng R 2015, 98: 1–71.CrossRefGoogle Scholar
  6. [6]
    Yi T-F, Yang S-Y, Xie Y. Recent advances of Li4Ti5O12 as a promising next generation anode material for high power lithium-ion batteries. J Mater Chem A 2015, 3: 5750–5777.CrossRefGoogle Scholar
  7. [7]
    Zhu G-N, Wang Y-G, Xia Y-Y. Ti-based compounds as anode materials for Li-ion batteries. Energy Environ Sci 2012, 5: 6652–6667.CrossRefGoogle Scholar
  8. [8]
    Yuan T, Tan Z, Ma C, et al. Challenges of spinel Li4Ti5O12 for lithium-ion battery industrial applications. Adv Energy Mater 2017, 7: 1601625.CrossRefGoogle Scholar
  9. [9]
    Reddy MV, Subba Rao GV, Chowdari BVR. Metal oxides and oxysalts as anode materials for Li ion batteries. Chem Rev 2013, 113: 5364–5457.CrossRefGoogle Scholar
  10. [10]
    Ji L, Lin Z, Alcoutlabi M, et al. Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries. Energy Environ Sci 2011, 4: 2682–2699.CrossRefGoogle Scholar
  11. [11]
    Obrovac MN, Chevrier VL. Alloy negative electrodes for Li-ion batteries. Chem Rev 2014, 114: 11444–11502.CrossRefGoogle Scholar
  12. [12]
    Aravindan V, Lee Y-S, Madhavi S. Research progress on negative electrodes for practical Li-ion batteries: Beyond carbonaceous anodes. Adv Energy Mater 2015, 5: 1402225.CrossRefGoogle Scholar
  13. [13]
    Zuo X, Zhu J, Müller-Buschbaum P, et al. Silicon based lithium-ion battery anodes: A chronicle perspective review. Nano Energy 2017, 31: 113–143.CrossRefGoogle Scholar
  14. [14]
    Zhang W-J. A review of the electrochemical performance of alloy anodes for lithium-ion batteries. J Power Sources 2011, 196: 13–24.CrossRefGoogle Scholar
  15. [15]
    Hong Z, Wei M. Layered titanate nanostructures and their derivatives as negative electrode materials for lithium-ion batteries. J Mater Chem A 2013, 1: 4403–4414.CrossRefGoogle Scholar
  16. [16]
    Chiu H-C, Lu X, Zhou J, et al. Capacity fade mechanism of Li4Ti5O12 nanosheet anode. Adv Energy Mater 2017, 7: 1601825.CrossRefGoogle Scholar
  17. [17]
    Zhou W, Liu H, Boughton RI, et al. One-dimensional single-crystalline Ti–O based nanostructures: Properties, synthesis, modifications and applications. J Mater Chem 2010, 20: 5993–6008.CrossRefGoogle Scholar
  18. [18]
    Deng D, Kim MG, Lee JY, et al. Green energy storage materials: Nanostructured TiO2 and Sn-based anodes for lithium-ion batteries. Energy Environ Sci 2009, 2: 818–837.CrossRefGoogle Scholar
  19. [19]
    Hua X, Liu Z, Fischer MG, et al. Lithiation thermodynamics and kinetics of the TiO2(B) nanoparticles. J Am Chem Soc 2017, 139: 13330–13341.CrossRefGoogle Scholar
  20. [20]
    Li J, Wan W, Zhou H, et al. Hydrothermal synthesis of TiO2(B) nanowires with ultrahigh surface area and their fast charging and discharging properties in Li-ion batteries. Chem Commun 2011, 47: 3439–3441.CrossRefGoogle Scholar
  21. [21]
    Liu H, Bi Z, Sun X-G, et al. Mesoporous TiO2-B microspheres with superior rate performance for lithium ion batteries. Adv Mater 2011, 23: 3450–3454.CrossRefGoogle Scholar
  22. [22]
    Tang Y, Zhang Y, Deng J, et al. Mechanical force-driven growth of elongated bending TiO2-based nanotubular materials for ultrafast rechargeable lithium ion batteries. Adv Mater 2014, 26: 6111–6118.CrossRefGoogle Scholar
  23. [23]
    Tang Y, Zhang Y, Deng J, et al. Unravelling the correlation between the aspect ratio of nanotubular structures and their electrochemical performance to achieve high-rate and long-life lithium-ion batteries. Angew Chem Int Edit 2014, 53: 13488–13492.CrossRefGoogle Scholar
  24. [24]
    Liu S, Wang Z, Yu C, et al. A flexible TiO2(B)-based battery electrode with superior power rate and ultralong cycle life. Adv Mater 2013, 25: 3462–3467.CrossRefGoogle Scholar
  25. [25]
    Ren G, Hoque MNF, Liu J, et al. Perpendicular edge oriented graphene foam supporting orthogonal TiO2(B) nanosheets as freestanding electrode for lithium ion battery. Nano Energy 2016, 21: 162–171.CrossRefGoogle Scholar
  26. [26]
    Guan BY, Yu L, Li J, et al. A universal cooperative assembly-directed method for coating of mesoporous TiO2 nanoshells with enhanced lithium storage properties. Sci Adv 2016, 2: e1501554.Google Scholar
  27. [27]
    Wei H, Rodriguez EF, Hollenkamp AF, et al. High reversible pseudocapacity in mesoporous yolk–shell anatase TiO2/TiO2(B) microspheres used as anodes for Li-ion batteries. Adv Funct Mater 2017, 27: 1703270.CrossRefGoogle Scholar
  28. [28]
    Naldoni A, Allieta M, Santangelo S, et al. Effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles. J Am Chem Soc 2012, 134: 7600–7603.CrossRefGoogle Scholar
  29. [29]
    Xia T, Zhang W, Li W, et al. Hydrogenated surface disorder enhances lithium ion battery performance. Nano Energy 2013, 2: 826–835.CrossRefGoogle Scholar
  30. [30]
    Liu H, Li W, Shen D, et al. Graphitic carbon conformal coating of mesoporous TiO2 hollow spheres for high-performance lithium ion battery anodes. J Am Chem Soc 2015, 137: 13161–13166.CrossRefGoogle Scholar
  31. [31]
    Zhao L, Wang S, Pan F, et al. Thermal convection induced TiO2 microclews as superior electrode materials for lithium-ion batteries. J Mater Chem A 2018, 6: 11688–11693.CrossRefGoogle Scholar
  32. [32]
    Lee GH, Lee JW, Choi JIL, et al. Ultrafast discharge/charge rate and robust cycle life for high-performance energy storage using ultrafine nanocrystals on the binder-free porous graphene foam. Adv Funct Mater 2016, 26: 5139–5148.CrossRefGoogle Scholar
  33. [33]
    Wang H, Jia G, Guo Y, et al. Atomic layer deposition of amorphous TiO2 on carbon nanotube networks and their superior Li and Na ion storage properties. Adv Mater Interfaces 2016, 3: 1600375.CrossRefGoogle Scholar
  34. [34]
    Sun L, Kong W, Wu H, et al. Mesoporous Li4Ti5O12 nanoclusters anchored on super-aligned carbon nanotubes as high performance electrodes for lithium ion batteries. Nanoscale 2016, 8: 617–625.CrossRefGoogle Scholar
  35. [35]
    Mao S, Huang X, Chang J, et al. One-step, continuous synthesis of a spherical Li4Ti5O12/graphene composite as an ultra-long cycle life lithium-ion battery anode. NPG Asia Materials 2015, 7: e224–e224.Google Scholar
  36. [36]
    Borghols WJH, Wagemaker M, Lafont U, et al. Size effects in the Li4+xTi5O12 spinel. J Am Chem Soc 2009, 131: 17786–17792.CrossRefGoogle Scholar
  37. [37]
    Ohzuku T, Ueda A, Yamamoto N. Zero-strain insertion material of Li[Li1/3Ti5/3]O4 for rechargeable lithium cells. J Electrochem Soc 1995, 142: 1431–1435.CrossRefGoogle Scholar
  38. [38]
    Haetge J, Hartmann P, Brezesinski K, et al. Ordered large-pore mesoporous Li4Ti5O12 spinel thin film electrodes with nanocrystalline framework for high rate rechargeable lithium batteries: Relationships among charge storage, electrical conductivity, and nanoscale structure. Chem Mater 2011, 23: 4384–4393.CrossRefGoogle Scholar
  39. [39]
    He Y, Muhetaer A, Li J, et al. Ultrathin Li4Ti5O12 nanosheet based hierarchical microspheres for high-rate and long-cycle life Li-ion batteries. Adv Energy Mater 2017, 7: 1700950.CrossRefGoogle Scholar
  40. [40]
    Wang C, Wang S, He Y-B, et al. Combining fast Li-ion battery cycling with large volumetric energy density: Grain boundary induced high electronic and ionic conductivity in Li4Ti5O12 spheres of densely packed nanocrystallites. Chem Mater 2015, 27: 5647–5656.CrossRefGoogle Scholar
  41. [41]
    Ge H, Cui L, Sun Z, et al. Unique Li4Ti5O12/TiO2 multilayer arrays with advanced surface lithium storage capability. J Mater Chem A 2018, 6: 22053–22061.CrossRefGoogle Scholar
  42. [42]
    Wang S, Yang Y, Quan W, et al. Ti3+-free three-phase Li4Ti5O12/TiO2 for high-rate lithium ion batteries: Capacity and conductivity enhancement by phase boundaries. Nano Energy 2017, 32: 294–301.CrossRefGoogle Scholar
  43. [43]
    Shen L, Zhang X, Uchaker E, et al. Li4Ti5O12 nanoparticles embedded in a mesoporous carbon matrix as a superior anode material for high rate lithium ion batteries. Adv Energy Mater 2012, 2: 691–698.CrossRefGoogle Scholar
  44. [44]
    Yao Z, Xia X, Zhou C, et al. Smart construction of integrated CNTs/Li4Ti5O12 core/shell arrays with superior high-rate performance for application in lithium-ion batteries. Adv Sci 2018, 5: 1700786.CrossRefGoogle Scholar
  45. [45]
    Tang Y, Zhang Y, Rui X, et al. Conductive inks based on a lithium titanate nanotube gel for high-rate lithium-ion batteries with customized configuration. Adv Mater 2016, 28: 1567–1576.CrossRefGoogle Scholar
  46. [46]
    Wang Y-Q, Gu L, Guo Y-G, et al. Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery. J Am Chem Soc 2012, 134: 7874–7879.CrossRefGoogle Scholar
  47. [47]
    Zhou C, Xia X, Wang Y, et al. Pine-needle-like Cu–Co skeleton composited with Li4Ti5O12 forming core–branch arrays for high-rate lithium ion storage. Small 2018, 14: 1704339.CrossRefGoogle Scholar
  48. [48]
    Goriparti S, Miele E, De Angelis F, et al. Review on recent progress of nanostructured anode materials for Li-ion batteries. J Power Sources 2014, 257: 421–443.CrossRefGoogle Scholar
  49. [49]
    Mei J, Liao T, Kou L, et al. Two-dimensional metal oxide nanomaterials for next-generation rechargeable batteries. Adv Mater 2017, 29: 1700176.CrossRefGoogle Scholar
  50. [50]
    Liu J, Liu X-W. Two-dimensional nanoarchitectures for lithium storage. Adv Mater 2012, 24: 4097–4111.CrossRefGoogle Scholar
  51. [51]
    Devan RS, Patil RA, Lin J-H, et al. One-dimensional metal-oxide nanostructures: Recent developments in synthesis, characterization, and applications. Adv Funct Mater 2012, 22: 3326–3370.CrossRefGoogle Scholar
  52. [52]
    Wang Z, Zhou L, Lou XW. Metal oxide hollow nanostructures for lithium-ion batteries. Adv Mater 2012, 24: 1903–1911.CrossRefGoogle Scholar
  53. [53]
    Mohana Reddy AL, Gowda SR, Shaijumon MM, et al. Hybrid nanostructures for energy storage applications. Adv Mater 2012, 24: 5045–5064.CrossRefGoogle Scholar
  54. [54]
    Kim MG, Cho J. Reversible and high-capacity nanostructured electrode materials for Li-ion batteries. Adv Funct Mater 2009, 19: 1497–1514.CrossRefGoogle Scholar
  55. [55]
    Jiang J, Li Y, Liu J, et al. Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage. Adv Mater 2012, 24: 5166–5180.CrossRefGoogle Scholar
  56. [56]
    Palacin MR. Recent advances in rechargeable battery materials: A chemist’s perspective. Chem Soc Rev 2009, 38: 2565–2575.CrossRefGoogle Scholar
  57. [57]
    Wang S, Zhang Z, Yang Y, et al. Efficient lithium-ion storage by hierarchical core–shell TiO2 nanowires decorated with MoO2 quantum dots encapsulated in carbon nanosheets. ACS Appl Mater Interfaces 2017, 9: 23741–23747.CrossRefGoogle Scholar
  58. [58]
    Luo J, Xia X, Luo Y, et al. Rationally designed hierarchical TiO2@Fe2O3 hollow nanostructures for improved lithium ion storage. Adv Energy Mater 2013, 3: 737–743.CrossRefGoogle Scholar
  59. [59]
    Wang H, Ma D, Huang X, et al. General and controllable synthesis strategy of metal oxide/TiO2 hierarchical heterostructures with improved lithium-ion battery performance. Sci Rep 2012, 2: 701.CrossRefGoogle Scholar
  60. [60]
    Liao J-Y, Luna BD, Manthiram A. TiO2-B nanowire arrays coated with layered MoS2 nanosheets for lithium and sodium storage. J Mater Chem A 2016, 4: 801–806.CrossRefGoogle Scholar
  61. [61]
    Yang Y, Wang S, Luo M, et al. Li4Ti5O12–TiO2/MoO2 nanoclusters-embedded into carbon nanosheets core/shell porous superstructures boost lithium ion storage. J Mater Chem A 2017, 5: 12096–12102.CrossRefGoogle Scholar
  62. [62]
    Xu G, Yang L, Wei X, et al. MoS2-quantum-dot-interspersed Li4Ti5O12 nanosheets with enhanced performance for Li-and Na-ion batteries. Adv Funct Mater 2016, 26: 3349–3358.CrossRefGoogle Scholar
  63. [63]
    Wang L, Sasaki T. Titanium oxide nanosheets: Graphene analogues with versatile functionalities. Chem Rev 2014, 114: 9455–9486.CrossRefGoogle Scholar
  64. [64]
    Luo W, Hu X, Sun Y, et al. Surface modification of electrospun TiO2 nanofibers via layer-by-layer self-assembly for high-performance lithium-ion batteries. J Mater Chem 2012, 22: 4910–4915.CrossRefGoogle Scholar
  65. [65]
    Liao J-Y, Higgins D, Lui G, et al. Multifunctional TiO2–C/MnO2 core–double-shell nanowire arrays as high-performance 3D electrodes for lithium ion batteries. Nano Lett 2013, 13: 5467–5473.CrossRefGoogle Scholar
  66. [66]
    Li X, Chen Y, Yao H, et al. Core/shell TiO2–MnO2/MnO2 heterostructure anodes for high-performance lithium-ion batteries. RSC Adv 2014, 4: 39906–39911.CrossRefGoogle Scholar
  67. [67]
    Li S, Wang M, Luo Y, et al. Bio-inspired hierarchical nanofibrous Fe3O4–TiO2–carbon composite as a high-performance anode material for lithium-ion batteries. ACS Appl Mater Interfaces 2016, 8: 17343–17351.CrossRefGoogle Scholar
  68. [68]
    Pan L, Zhu X-D, Xie X-M, et al. Smart hybridization of TiO2 nanorods and Fe3O4 nanoparticles with pristine graphene nanosheets: Hierarchically nanoengineered ternary heterostructures for high-rate lithium storage. Adv Funct Mater 2015, 25: 3341–3350.CrossRefGoogle Scholar
  69. [69]
    Yang J, Wu Q, Yang X, et al. Chestnut-like TiO2@α-Fe2O3 core–shell nanostructures with abundant interfaces for efficient and ultralong life lithium-ion storage. ACS Appl Mater Interfaces 2017, 9: 354–361.CrossRefGoogle Scholar
  70. [70]
    Huang G, Zhang F, Du X, et al. Core–shell NiFe2O4@TiO2 nanorods: An anode material with enhanced electrochemical performance for lithium-ion batteries. Chem Eur J 2014, 20: 11214–11219.CrossRefGoogle Scholar
  71. [71]
    Wang X, Xiang Q, Liu B, et al. TiO2 modified FeS nanostructures with enhanced electrochemical performance for lithium-ion batteries. Sci Rep 2013, 3: 2007.CrossRefGoogle Scholar
  72. [72]
    Zhang C, Shao D, Gao Q, et al. Electrochemical lithium storage of Li4Ti5O12/NiO nanocomposites for high-performance lithium-ion battery anodes. J Solid State Electrochem 2015, 19: 1859–1866.CrossRefGoogle Scholar
  73. [73]
    Hong J-E, Oh R-G, Ryu K-S. Li4Ti5O12/Co3O4 composite for improved performance in lithium-ion batteries. J Electrochem Soc 2015, 162: A1978–A1983.Google Scholar
  74. [74]
    Chen M, Li W, Shen X, et al. Fabrication of core–shell α-Fe2O3@Li4Ti5O12 composite and its application in the lithium ion batteries. ACS Appl Mater Interfaces 2014, 6: 4514–4523.CrossRefGoogle Scholar
  75. [75]
    Li Z, Zhao H, Lv P, et al. Watermelon-like structured SiOx–TiO2@C nanocomposite as a high-performance lithium-ion battery anode. Adv Funct Mater 2018, 28: 1605711.CrossRefGoogle Scholar
  76. [76]
    Chen JS, Lou XW. SnO2-based nanomaterials: Synthesis and application in lithium-ion batteries. Small 2013, 9: 1877–1893.CrossRefGoogle Scholar
  77. [77]
    Xie H, Chen M, Wu L. Hierarchical TiO2/SnO2 hollow spheres coated with graphitized carbon for high-performance electrochemical Li-ion storage. Small 2017, 13: 1604283.CrossRefGoogle Scholar
  78. [78]
    Chen J, Yang L, Zhang Z, et al. Mesoporous TiO2–Sn@C core–shell microspheres for Li-ion batteries. Chem Commun 2013, 49: 2792–2794.CrossRefGoogle Scholar
  79. [79]
    Liao J-Y, Manthiram A. Mesoporous TiO2–Sn/C core–shell nanowire arrays as high-performance 3D anodes for Li-ion batteries. Adv Energy Mater 2014, 4: 1400403.CrossRefGoogle Scholar
  80. [80]
    Cai R, Yu X, Liu X, et al. Li4Ti5O12/Sn composite anodes for lithium-ion batteries: Synthesis and electrochemical performance. J Power Sources 2010, 195: 8244–8250.CrossRefGoogle Scholar
  81. [81]
    Han SY, Kim IY, Lee SH, et al. Electrochemically active nanocomposites of Li4Ti5O12 2D nanosheets and SnO2 0D nanocrystals with improved electrode performance. Electrochim Acta 2012, 74: 59–64.CrossRefGoogle Scholar
  82. [82]
    Wang S, Yang Y, Jiang C, et al. Nitrogen-doped carbon coated Li4Ti5O12–TiO2/Sn nanowires and their enhanced electrochemical properties for lithium ion batteries. J Mater Chem A 2016, 4: 12714–12719.CrossRefGoogle Scholar
  83. [83]
    Yang Y, Wang S, Lin S, et al. Rational design of hierarchical TiO2/epitaxially aligned MoS2–carbon coupled interface nanosheets core/shell architecture for ultrastable sodium ion and lithium-sulfur batteries. Small 2018, 2: 1800119.CrossRefGoogle Scholar
  84. [84]
    Ji G, Ma Y, Ding B, et al. Improving the performance of high capacity Li-ion anode materials by lithium titanate surface coating. Chem Mater 2012, 24: 3329–3334.CrossRefGoogle Scholar
  85. [85]
    Cheong JY, Kim C, Jang JS, et al. Rational design of Sn-based multicomponent anodes for high performance lithium-ion batteries: SnO2@TiO2@reduced graphene oxide nanotubes. RSC Adv 2016, 6: 2920–2925.CrossRefGoogle Scholar
  86. [86]
    Lin Y-M, Nagarale RK, Klavetter KC, et al. SnO2 and TiO2-supported-SnO2 lithium battery anodes with improved electrochemical performance. J Mater Chem 2012, 22: 11134–11139.CrossRefGoogle Scholar
  87. [87]
    Jeun J-H, Park K-Y, Kim D-H, et al. SnO2@TiO2 double-shell nanotubes for a lithium ion battery anode with excellent high rate cyclability. Nanoscale 2013, 5: 8480–8483.CrossRefGoogle Scholar
  88. [88]
    Yang Z, Du G, Guo Z, et al. Encapsulation of TiO2(B) nanowire cores into SnO2/carbon nanoparticle shells and their high performance in lithium storage. Nanoscale 2011, 3: 4440–4447.CrossRefGoogle Scholar
  89. [89]
    Sivashanmugam A, Gopukumar S, Thirunakaran R, et al. Novel Li4Ti5O12/Sn nano-composites as anode material for lithium iontteries. Mater Res Bull 2011, 46: 492–500.CrossRefGoogle Scholar

Copyright information

© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit

Authors and Affiliations

  • Shitong Wang
    • 1
    • 2
  • Yong Yang
    • 3
  • Yanhao Dong
    • 1
  • Zhongtai Zhang
    • 2
  • Zilong Tang
    • 2
    Email author
  1. 1.Department of Nuclear Science and EngineeringMassachusetts Institute of TechnologyCambridgeUSA
  2. 2.State Key Lab of New Ceramics and Fine Processing, School of Materials Science and EngineeringTsinghua UniversityBeijingChina
  3. 3.Department of Materials Science & Engineering, College of EngineeringPeking UniversityBeijingChina

Personalised recommendations