Supporting Information for

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1 Supporting Information for Capacity Degradation Mechanism and Cycling Stability Enhancement of AlF 3 coated Nano-Rod Gradient Na[Ni 0.65 Co 0.08 Mn 0.27 ]O 2 Cathode for Sodium-Ion Batteries Ho-Hyun Sun, a Jang-Yeon Hwang, b Chong Seung Yoon*, c Adam Heller, a and C. Buddie Mullins* a,d a McKetta Department of Chemical Engineering, The University of Texas at Austin, Texas , United States b Department of Energy Engineering, Hanyang University, Seoul , Republic of Korea c Department of Materials Science and Engineering, Hanyang University, Seoul , Republic of Korea d Department of Chemistry, The University of Texas at Austin, Texas , United States Keywords: Ni-rich Layered Oxide Cathode, AlF3 Coating, Gradient Cathode, Na-ion Batteries, O3-type Cathode, Degradation Mechanism, HR-TEM 1

2 Ni Co Mn Precursor Na[Ni0.65Co0.08Mn0.27]O AlF3-coated Na[Ni0.65Co0.08Mn0.27]O Table S1. ICP results of transition metal content of precursor, bare nano-rod gradient Na[Ni0.65Co0.08Mn0.27]O2, and AlF3 coated nano-rod gradient Na[Ni0.65Co0.08Mn0.27]O2. 2

3 Figure S1. (a) Nano-rod gradient [Ni0.65Co0.08Mn0.27](OH)2 precursor particle SEM image and (b) close-up Nano-rod gradient Na[Ni0.65Co0.08Mn0.27]O2 particle SEM image. 3

4 Figure S2. Nano-rod gradient Na[Ni0.65Co0.08Mn0.27]O2 particle cross-section (a) SEM image and (b) corresponding aluminum energy dispersive X-ray spectroscopy. Note that in the EDX Al mapping image of (b) the particle is in the same position as in panel (a) and indicates the presence of Al in the coating on the particle. However, the reader must look very carefully. 4

5 Figure S3. (a) SEM image of constant concentration Na[Ni0.65Co0.08Mn0.27]O2 and (b) 1 st cycle charge-discharge curves and (c) cycle performance of constant concentration, and (d) rate capability performance of Na[Ni0.65Co0.08Mn0.27]O2 vs. bare nano-rod gradient Na[Ni0.65Co0.08Mn0.27]O2. 5

6 Figure S4. After-cycled half-cell XRD patterns of bare NRG65 and AlF3 coated NRG65. 6

7 Discharge capacity / mah g NRG65 AlF 3 coated NRG V, 0.5C, 55 o C Number of Cycle Figure S5. High temperature electrochemical cycling performance of bare NRG6 and AlF3 coated NRG65. 7

8 Figure S th cycle full-cell bare NRG65 and AlF3-NRG65 EIS measurement scheme. 8

9 Cathode Voltage Window 1 st Discharge Capacity Half-cell Cycling Performance Full-cell Performance Ref P2-type Al2O3 ALD coated Na2/3(Mn0.54Ni0.13Co0.13)O V 123 mah g -1 at 160 ma g %/100 th at 160 ma g -1 X S1 P2-type TiO2 ALD coated Na0.66(Mn0.54Co0.13Ni0.13)O V 106 mah g -1 at 160 ma g %/100 th at 160 ma g -1 X S2 P2-type Cu-doped/MgOcoated Na0.5Ni0.33Mn0.67O V 131 mah g -1 at 34 ma g %/100th at 45 ma g -1 X S1 P2-type Al2O3-coated Na2/3[Ni1/3Mn2/3]O V 160 mah g -1 at 73 %/300th at ma g -1 ma g -1 X S2 P2-type NaPO3-coated Na2/3[Ni1/3Mn2/3]O V 194 mah g -1 at 20 ma g %/50th at 20 ma g -1 Presodiated, 73 %/200th at 40 ma g -1 S3 O3-type Carbon-coated NaCrO V 116 mah g -1 at 93.6 %/40th at 5 5 ma g -1 ma g -1 X S4 O3-type Al2O3-coated Na[Ni0.6Co0.2Mn0.1]O V 151 mah g -1 at 15 ma g %/50th at 75 ma g %/300th at 75 ma g -1 S5 O3-type AlF3-coated NRG V 161 mah g -1 at 15 ma g %/50 th at 75 ma g %/200th at 75 ma g -1 This work Table S2. Table comparing the electrochemical performance of AlF3 coated NRG65 to other coated layered material electrochemical performance. 9

10 Figure S7. As-synthesized nano-sized AlF3 particle (a) SEM image and (b) XRD pattern. 10

11 References (1) Kaliyappan, K.; Liu, J.; Lushington, A.; Li, R.; Sun, X. Highly Stable Na2/3(Mn0.54Ni0.13Co0.13)O2 Cathode Modified by Atomic Layer Deposition for Sodium-Ion Batteries. ChemSusChem 2015, 8, (2) Kaliyappan, K.; Liu, J.; Lushington, A.; Li, R.; Sham, T.-K.; Sun, X. Enhanced Performance of P2-Na0.66(Mn0.54Co0.13Ni0.13)O2 Cathode for Sodium-Ion Batteries by Ultrathin Metal Oxide Coatings via Atomic Layer Deposition. Adv. Funct. Mater. 2017, 27, (3) Ramasamy, H.V.; Kaliyappan, K.; Thangavel, R.; Aravindan, V.; Kang, K.; Kim, D.U.; Park, Y.; Sun, X.; Lee, Y.-S. Cu-doped P2-Na0.5Ni0.33Mn0.67O2 Encapsulated with MgO as a Novel High Voltage Cathode with Enhanced Na-sotrage Properties. J. Mater. Chem. A. 2017, 5, (4) Liu, Y.; Fang, X.; Zhang, A.; Shen, C.; Liu, Q.; Enaya, H.; Zhou, C. Layered P2- Na2/3[Ni1/3Mn2/3]O2 as High-voltage Cathode for Sodium-ion Batteries: The Capacity Decay Mechanism and Al2O3 Surface Modification. Nano Energy 2016, 27, (5) Jo, J.H.; Choi, J.U.; Knarov, A.; Yahiro, H.; Yuan, S.; Shi, L.; Sun, Y.-K.; Myung, S.-T. Sodium-Ion Batteries: Building Effective Layered Cathode Materials with Long-Term Cycling by Modifying the Surface via Sodium Phosphate. Adv. Funct. Mater. 2018, 28, (6) Ding, J.-J.; Zhou, Y.-N.; Sun, Q.; Fu, Z.-W. Cycle Performance Improvement of NaCrO2 Cathode by Carbon Coating for Sodium Ion Batteries. Electrochemistry Communications 2012, 22, (7) Hwang, J.-Y.; Myung, S.-T.; Choi, J.U.; Yoon, C.S.; Yashiro, H.; Sun, Y.-K. Resolving the Degradation Pathways of the O3-type Layered Oxide Cathode Surface Through the Nano-scale Aluminum Oxide Coating for High-Energy Density Sodium-ion Batteries. J. Mater. Chem. A. 2017, 5,

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