Understanding Cation-Disordered Cathode Materials Based on Percolation Theory and Ligand Field Theory

Transcription

1 2016 ECS Prime meeting (10/5/2016, 8:40 9:00 am) Understanding Cation-Disordered Cathode Materials Based on Percolation Theory and gand Field Theory Jinhyuk Lee, Dong-Hwa Seo, Alexander Urban, Gerbrand Ceder UC Berkeley & LBNL These slides can be downloaded at 1

2 High E-density cathodes from well-ordered close-packed -TM oxides. TM Oxygen FCC framework TM Cho et al., Chem. Mater. (2000) Layered - CoO 2 in Samsung phones Spinel - Mn 2 O 4 in 1 st gen. Nissan Leaf Layered CoO 2 (> 600 Wh/kg) 2

3 Voltage (V) Negligible attention to cation-disordered oxides (i.e. disordered rocksalt), as they tend to cycle poorly with limited diffusion. Random /TM Oxygen FCC framework negligible capacity (1) Disordered -Co-O Obrovac et al., Solid State Ionics (1998) (2) VO 2 : Disorder during cycle Disordered rocksalt -TM oxides Capacity (mah/g) Zhang et al., JPS (2007) 3

4 diffusion can be facile in cation-disordered materials if with excess. (e.g. 1.2 TM 0.8 O 2 ) > 260 mah/g Lee et al., Science (2014) Mo Cr 0.3 O 2 ~220 mah/g Kitajou et al., Electrochemistry (2016) 1.2 Mn 0.4 Ti 0.4 O 2 ~300 mah/g > 300 mah/g (1) Yabuuchi et al., PNAS (2015) (2) Wang et al., Electrochem. Commun. (2015) 1.3 Mn 0.4 Nb 0.3 O 2 Chen et al., AEM (2015) 2 VO 2 F 4

5 diffusion in general rocksalt type materials (e.g. in layered, disordered, spinel-like, γ-feo 2 -like etc.) o-t-o hopping in -TM oxides (layered-, disordered-rocksalt etc.) Three possible local geometries J. Lee, A. Urban, X., D. Su, G. Hautier, G. Ceder, Science (2014) 5

6 ex) diffusion in stoichiometric layered oxides 1-TM channel (TM3+, 4+, etc) Must stay Large!! Ex. Layered CoO 2 1-TM channels can support diffusion in the layered structure (i.e. active), as long as the size of an intermediate tetrahedral site stays large enough for the activated ion to relax away the strong electrostatic repulsion from the face-sharing high valent TM ion. J. Lee, A. Urban, X., D. Su, G. Hautier, G. Ceder, Science (2014) 6

7 In cation-disordered materials, only 0-TM channels can support diffusion. Inactive Channels exist but too high barriers. active J. Lee, A. Urban, X., D. Su, G. Hautier, G. Ceder, Science (2014) 7

8 Small tetrahedron size in disordered materials prevents an activated + ion to relax away strong repulsion from high-valent TM species in 2-TM and 1-TM channels. The activated + ion in 0-TM channel do not have TM high Disordered valent TM species to face share with. Therefore, 0-TM channels can support diffusion even in the Layered disordered materials with small tetrahedron Layered size. Tetrahedron height (Å) (Tetrahedron size) Disordered J. Lee, A. Urban, X., D. Su, G. Hautier, G. Ceder, Science (2014) 8

9 For macroscopic diffusion in disordered materials, 0-TM channels must be percolating in the structure. 0-TM channels need to percolate. Active in disordered materials 9

10 0-TM channels percolate in layered and disordered structures as soon as their composition becomes highly lithium excess (x> 1.09 in x TM 2-x O 2 ) disordered excess 0-TM percolating layered Composition (1) J. Lee et al., Science (2014), (2) A. Urban, J. Lee, Ceder, Adv. Energy Mater. (2014) 10

11 Is 0-TM percolation enough for designing high-capacity cation-disordered materials? No, 0-TM percolation => diffusion excess Stay active in disordered materials Fast diffusion by 0-TM percolation A good electrode requires enough electrons to be extracted or inserted upon cycling, => We need to understand the redox mechanism. 11

12 Controversial -excess strategy: Improves diffusion after sacrificing TM-redox -excess decreases TM contents and increases the average oxidation state of TM species, reducing TM redox capacity. Example: ( 1/3 Mn 2/3 )O 2 2 Mn 4+ O 3 => e - + Mn 6+ O 3 (Mn 4+ /Mn 6+?) : 0-TM percolation (O), TM-redox (X). 12

13 Can Reversible O-redox O-redox reversibly resolve occur this in -excess controversy materials by to making give extra electron capacity capacity at a unbound reasonably to low TM voltage? redox. (1) Reversible O-redox (O2-/O1-) Shin et al. Chem. Mater. (2016) 2 Mn 4+ O 2-3 => e - + Mn 4+ O (O 2- /O - ) If oxygen redox can reversible occur, we are no longer bound to TM redox capacity for electron capacity. Moreover, oxygen redox typically delivers high voltage. O redox is important in -excess materials. 13

14 (bonding) O states (antibonding) M states Oxygen Fortunately, electrons both that -excess form and highly cation-disorder covalent bonding with promote TM species oxygen will redox be which too stable is difficult to participate to occur in O-redox, in stoichiometric which is layered the case materials in the stoichiometric layered materials. TM layer layer E M M a 1g O * e g * a 1g b M t 1u * t 2g t 1u b Local O-coordination In stoichiometric e b g layered -M oxides e.g. CoO2 Band structure of stoichiometric layered -M oxides M d/s/p Too stable for electron extraction (antibonding) M-states As M-O covalency increases (e.g. Ni, Co, Ru etc.) (bonding) O-states O 2p 14

15 How does excess and cation disorder promote oxygen redox? -excess layered O-coordination (1) O 2p orbitals along the O M or M-O-M configurations O M + O -excess disordered three O M (as in CoO 2 ) O-coordination (2) O 2p orbitals along O configurations O M + O D.-H. Seo, J. Lee et al., Nature Chem. (2016) [ equal contribution] 15

16 gand field theory: Lack of hybridization makes O 2p electrons along O configurations unstable. -excess layered E + t 1u * + Oxygen electrons in the labile -O- state can be easily -excess disordered removed to give extra capacity in -excess layered or cationdisordered materials. a 1g * e g * a b 1g e b g O (labile; t 2g lack of hybridization) t 1u b Stable bonding O 2p states -excess Layered/disordered -M oxides D.-H. Seo, J. Lee et al., Nature Chem. (2016) [ equal contribution] 16

17 DFT calculations show that indeed the labile O 2p electrons from O states contribute to extra capacity in -excess materials. Layered 1.17 Ni 0.25 Mn 0.58 O 2 Layered 2 Ru 0.5 Sn 0.5 O 3 Disordered 1.17 Ni 0.33 Ti 0.42 Mo 0.08 O 2 Disordered 1.25 Mn 0.5 Nb 0.25 O 2 O-oxidation from -O- states leads to extra capacity, therefore we don t need to worry about limited TM redox in -excess disordered materials However, there is one more thing to consider. D.-H. Seo, J. Lee et al., Nature Chem. (2016) [ equal contribution] -O- direction Spin density on oxygen oxygen hole 17

18 Too much O redox => O loss with densification => destroys 0-TM percolation. (1) Reversible O redox [ 2 MnO 3 => ( 1/3 Mn 2/3 )O 2 ] 2 Mn 4+ O 2-3 => e - + [2V ]Mn 4+ O :Mn = 2:1 (-excess) Let s lower some energies! Let s lower even more! [2V ]Mn 4+ O 2-2[V O ] O 2 (g) Oxygen loss with O vacancies [V ]Mn 4+ O 2-2 cation densification Now, we are in trouble. :Mn = 1:1 (no -excess) 0-TM perc. gone 18

19 Disordered -excess cathodes that lose oxygen show larger polarization upon cycling. High-capacity cation-disordered -excess Ni-Ti-Mo oxides J. Lee et al., Energy. Environ. Sci. (2015) 19

20 Consistent with percolation theory, the reversible capacity dramatically improves with excess. 1 st cycle Ni 1/2 Ti 1/2 O 2 (105 mah/g) 1.0 excess Ni 1/3 Ti 1/3 Mo 2/15 O 2 (225 mah/g) J. Lee et al., Energy. Environ. Sci. (2015) 20

21 While delivering high capacity, 1.2 Ni 1/3 Ti 1/3 Mo 2/15 O 2 still shows large polarization (difference in c/dc profiles). 1.2 Ni 1/3 Ti 1/3 Mo 2/15 O 2 GITT upon discharge J. Lee et al., Energy. Environ. Sci. (2015) 21

22 Large polarization in 1.2 Ni 1/3 Ti 1/3 Mo 2/15 O 2 appears when charging above ~4.3 V V If charging cut off is set below 4.3 V, polarization become much less. J. Lee et al., Energy. Environ. Sci. (2015) 22

23 Above 4.3 V, O loss occurs from 1.2 Ni 1/3 Ti 1/3 Mo 2/15 O 2, destroying 0-TM percolation at the surface. Surface EELS data on Ti L-edges and O K-edge Substantial reduction of peak intensity ratio between Oxygen : Titanium 1.2TM0.8O2 (20 % -excess) => 0.7TM1.3O2 (30 % TM-excess) From these results, we argue that preventing oxygen loss will be the key to preserve good cycling performances of disordered -excess materials. J. Lee et al., Energy. Environ. Sci. (2015) 23

24 Conclusions 1. If we want facile diffusion in cation-disordered materials, we need to introduce excess for 0-TM percolation. 2. However, this -excess often reduces TM redox capacity, thus O-redox is further necessary for high electron capacity. And fortunately, O-redox from -O- states can resolve this. 3. We might want to avoid using too much oxygen redox because it can trigger O loss with densification which reduces the -excess level and therefore destroys the 0-TM percolation for facile diffusion. 24

25 Acknowledgement Special thanks to: Dr. Seo, Dr. Urban, and Prof. Gerbrand Ceder Thank you very much for your attention. References [1] Lee, Urban,, Su, Hautier, Ceder, Science 343 (2014) [2] Seo, Lee, Urban, Malik, Kang, Ceder, Nature Chem. 8 (2016) [ equal contribution] [3] Lee, Seo, Balasubramanian, Twu,, Ceder, Energy Environ. Sci. 8 (2015) [4] Urban, Lee, Ceder, Adv. Energy Mater. 4 (2014) These slides can be downloaded at 25