Supplementary Information

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1 Supplementary Information Unraveling the Voltage Fade Mechanism in High-Energy Density Lithium-ion Batteries: Origin of Tetrahedral Cations for Spinel Conversion Table S1: Cell capacity, and lithium content, of cycled HE5050 oxides at different SOCs during second and 26th charge-discharge cycle State of Charge Cell Capacity (mah/g) Lithium Content (mole) Second cycle a 4.1 V charged V charged V discharged V discharged V discharged th cycle b 3.5 V charged V charged V charged V discharged V discharged V discharged a Initial lithium content mol, assuming 55 mah/g loss in first cycle, which corresponds to Li. b Initial lithium content is 0.93 mol, assuming 15% (30 mah/g) of capacity loss between second and 26th cycles. Figure S1: XRD patterns from pristine (a) and cycled HE5050 oxides (b-f) at different cell voltages during the second cycle. The cation-ordering peak intensity (see peak indices in red color) was lower in all the patterns from cycled oxides as compared to the pristine oxide. 1

2 Figure S2: XRD patterns from pristine (a) and cycled HE5050 oxides (b-f) at different cell voltages during the 26 th cycle. The cation-ordering peaks (see peak indices in red color) were not observed in XRD patterns from the cycled oxides. Figure S3: The (003) peak shift observed at cell voltages during second and 26th cycles. Lattice expansion along (003) is observed at 3.2 V-discharged after the 26th cycle. Figure S4: Neutron diffraction patterns from pristine HE5050 oxide (a) and cycled HE5050 oxide at different voltages during the second cycle. b:4.1v charge, c:4.5v charge, d: 4.1V discharge, e: 3.5V discharge, f: 3.2V discharge.

3 Figure S5: Neutron diffraction patterns from pristine HE5050 oxide (a) and cycled HE5050 oxide at different cell voltages SOCs during the 26 th cycle. b:3.2v charge, c:4.1v charge, d: 4.5V charge, e: 4.1V discharge, f: 3.5V discharge, g: 3.2V discharge. Lattice parameter(s) of trigonal and monoclinic unit cells Figure S6: Refined lattice parameters of trigonal (left) and monoclinic (right)unit cells in the pristine and cycled HE5050 oxides in the second cycle (a-b) and 26 th cycle (c-d). 3

4 During second cycle: From figure S6, c-lattice parameter in the trigonal unit cell increases and that the a-lattice parameter decreases as compared to pristine HE5050 oxide. The increase in the c-lattice parameter is due to extraction of Li Li ions from the octahedral 3b site of the lithium layer. The decrease in the a-lattice parameter is generally referred to as a decrease in the metal-metal bond distance, which might be due to formation of M ions with higher oxidation states (e.g., Ni 3+ /Ni 4+ /Co +4 ) (for charge compensation ) and with lower ionic radii than Ni 2+ /Co 3+. Interestingly, the c-lattice parameter of the monoclinic unit cell also increases, and the a- and b- lattice parameters decrease. A closer look reveals that the a and b lattices change in a very similar fashion indicating that unit cell expansion might occur due to the enlargement of the host (trigonal) unit cell. The lattice expansion in this phase might be due to the elongation of the host unit cell, without involvement lithium ions from the Li 2 MnO 3 -like component. At a 4.5 V charged state, the c-lattice parameters in both the trigonal and monoclinic phases decrease slightly and the a lattice parameter decreases in both the phases. The decrease in c-lattice parameter might be due to the change in oxidation states of oxygen anion and/or different structural rearrangements in the HE5050 oxide unit cells. At the 4.1 V discharged state, the lattice parameters increase for the both trigonal and monoclinic unit cells. On further discharging, at 3.5 V, the c-lattice parameters (for the monoclinic and trigonal phases) decrease, and similar behavior is observed on further discharging to 3.2 V. Increase in the a-lattice parameter for the trigonal phase was observed during discharge, and continued to decrease at 3.2 V, indicating the reduction of metal ions to form ions with higher ionic radii (Ni 2+ /Ni 3+ /Co 3+ ). In case of the monoclinic phase, the a- and b-lattice parameters do not increase significantly; they remain almost constant at 3.5 V and increase on further discharging to 3.2 V. During 26 th cycle: The major observations, which are different from the second cycle, are: i) increased value of the c- and a-lattice parameters in the trigonal unit cell when discharged voltage is less than 3.5V. ii) c-lattice parameter in the monoclinic unit cell does not change when discharge voltage is less than 3.5 V.

5 Crystallographic parameters from Rietveld refinement on neutron diffraction patterns HE V charged During 2 nd cycle Overall Composition: Li 0.63 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.13±0.03) Agreement parameters: R WP = 1.78% R p =3.08 χ 2 =1.35 Lattice constants: a=b= (12) (10), V= (7), Z=6 Phase percentage : 68%(δ=0.02 ) (3) (3) Ni (4) Mn (4) 6c Li (4) Mn (2) (4) (3) Ni (4) Li (3) 6c O (20) (6) (6) Lattice constants: a=4.9264(4), b= (4), c= (3), β = (32), V= (9), Z= 4, Phase percentage: 32% (δ=0.01 ) Site Atom X y Z SOF Uiso 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (4) (9) (4) (4) 8j O (3) (3) (6) (4) (4) 5

6 HE V charged Overall Composition: Li 0.41 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.12±0.03) Agreement parameters: R WP = 1.59% R p =1.89 χ 2 =3.1 Lattice constants: a=b= (12) c= (10), V= (7), Z= (3) (3) Ni (4) Mn (6) 6c Li (3) Mn (4) (2) (4) 0.045(3) Ni (4) Li (3) 6c O (8) (2) (6) Lattice constants: a=4.9168(7), b= (5), c= (5), β = (32), V= (9), Z= 4 2c Li (2) (2) 4h Li (3) (2) 2b Li (4) 4g Mn (4) 4i O (5) (9) (4) (4) 8j O (6) (8) (6) (3) (8)

7 HE V discharged Overall Composition: Li 0.37 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.15±0.03) Agreement parameters: R WP = 5.05% R p = 8.09 χ 2 =2.50 Lattice constants: a=b= c=14.45(10), V= (7), Z= (3) Ni (5) (3) Mn (3) 6c Li (4) (3) Mn (4) (4) (3) Ni (4) Li (3) 6c O (8) (2) (6) Composition: Li 2 MnO 3 Lattice constants: a= (27), b= (5), c= (21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (4) (9) (2) (4) 8j O (6) (8) (6) (3) (8) 7

8 HE V discharged Overall Composition: Li 0.64 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.14±0.03) Agreement parameters: R WP = 4.5% R p = 6.09 χ 2 =3.06 Lattice constants: a=b= 2.844(12) c=14.425(10), V= (7), Z= (3) (3) Ni (4) Mn (3) 6c Li (4) Mn (4) (4) (3) Ni (4) Li (6) 6c O (20) 0.75(6) (6) Composition: Li 2 MnO 3 Lattice constants: a=4.92, b= (5), c= 5.079(21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (10) (9) (5) (4) 8j O (7) (8) (6) (6) (8)

9 HE V discharged* * Spinel phase was also included in the refinement however, no improvements were observed. Overall Composition: Li 0.86 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.15±0.03) Agreement parameters: R WP = 3.05% R p = 7.01 χ 2 =1.91 Lattice constants: a=b= 2.852(12) c= (10), V= (7), Z=6 Phase percentage : 67% (δ=0.01 ) (5) (3) Ni (4) Mn (4) 6c Li (3) Mn (5) (4) (3) Ni 0.334(4) Li (3) (6) 6c O (8) (3) Lattice constants: a=4.9392(27), b=8.548 (5), c= 5.039(21), β = (32), V= (9), Z= 4, Phase percentage: 33% (δ=0.01 ) 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (4) (9) (4) (4) 8j O (4) (5) (6) (4) (8) 9

10 During 26 th cycle HE V charged Trigonal Monoclinic Spinel Overall Composition: Li 0.80 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.20±0.03) Agreement parameters: R WP = 3.95% R p = 6.09 χ 2 =3.22 Lattice constants: a=b= (12) c= (10), V= (7), Z=6 Phase percentage :70% (δ=0.01 ) Site Atom x Y Z SOF Uiso (4) (3) Ni (4) Mn (3) 6c Li (4) Mn (3) (4) (3) Ni (4) Li (3) 6c O (4) (4) (6) Lattice constants: a=4.9554(27), b= (5), c= (21), β = (32), V= (9), Z= 4, Phase percentage: 18% (δ=0.01 ) Site Atom X y Z SOF Uiso 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (3) (4) (3) (4) 8j O (5) (5) (3) (3) (8) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Phase percentage :12% Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (4) 32e O (3)

11 HE V charged Overall Composition: Li 0.49 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.21±0.03) Agreement parameters: R WP = 3.00% R p = 5.56 χ 2 =1.98 Lattice constants: a=b= (12) c= (10), V= (7), Z= (3) Ni Mn c Li (3) Mn (4) (3) Ni (4) Li (4) 6c O (8) (4) (6) Lattice constants: a= (27), b= (5), c= 5.15(21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (3) (3) (4) (4) 8j O (4) (4) (4) (4) (4) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (3) 32e O (4) 11

12 HE V charged Overall Composition: Li 0.30 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.19±0.03) Agreement parameters: R WP = 2.08% R p = 3.21 χ 2 =1.97 Lattice constants: a=b= (12) c= (10), V= (7), Z= (3) (3) Ni (3) Mn 0.045(4) 6c Li (5) (3) Mn (3) (4) (3) Ni (4) Li (4) 6c O (4) (4) (4) Lattice constants: a= (27), b= (5), c= (21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (5) (5) (5) (4) 8j O (4) (4) (4) (4) (8) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (4) 32e O (2)

13 HE V discharged Overall Composition: Li 0.32 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.19±0.03) Agreement parameters: R WP = 2.05% R p = 6.09 χ 2 =3.08 Phase: LiMO 2 (M=co, Mn, Ni) Lattice constants: a=b= (12) c= (10), V= (7), Z= (3) (3) Ni (3) Mn (4) 6c Li 0.096(3) (3) Mn 0.06(3) (4) (3) Ni (4) Li (3) 6c O (5) (3) (6) Lattice constants: a= (27), b=8.524 (5), c= (21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (4) (4) (4) (4) 8j O (7) (5) (6) (3) (8) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (3) 32e O (2) 13

14 HE V discharged Overall Composition: Li 0.47 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.20±0.03) Agreement parameters: R WP = 2.11% R p = 2.27 χ 2 =2.59 Lattice constants: a=b= (12) c= (10), V= (7), Z= (3) Ni (3) (3) Mn (5) 6c Li (4) (3) Mn 0.03(4) (4) (3) Ni (4) Li (3) 6c O (8) (4) (6) Lattice constants: a=4.9284(27), b=8.541 (5), c= (21), β = (32), V= (9), Z= 4 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (5) (9) (3) (4) 8j O (4) (6) (6) (8) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (4) 32e O (3)

15 HE V discharged Overall Composition: Li 0.73 Co 0.1 Mn 0.55 Ni 0.15 O 2-δ (δ= 0.20±0.03) Agreement parameters: R WP = 2.00% R p =1.90 χ 2 =2.00 Lattice constants: a=b= (12) c= (10), V= (7), Z=3 Phase percentage : 68% (δ=0.01 ) (5) Ni (3) (3) Mn (3) 6c Li (3) (3) Mn (4) (4) (3) Ni (4) Li 0.082(4) 6c O (8) (5) (6) Lattice constants: a=4.952(27), b= (5), c= (21), β = (32), V= (9), Z= 4, Phase percentage: 17% (δ=0.01 ) 2c Li (2) 4h Li (2) 2b Li (4) 4g Mn (4) 4i O (6) (9) (2) (4) 8j O (7) (7) (6) (4) (8) Phase 3 Crystal System: Cubic spinel SG: Fd3 m Phase: LiMn 2 O 4 Lattice constants: a= b=c= 8.01Å α=γ=β=90, Z=8, V= 513.8(9) Phase percentage :15% Site Atom X Y Z SOF Uiso 8a Li (4) 16d Mn (5) 32e O (3) 15