The energy density of lithium batteries is currently restricted by the cathode material, delivering ~ 160-190 mAh g^-1.¹ New high-energy-density cathode materials are much sought after to meet the increasing requirements of consumer electronics, electric vehicles and grid energy storage. The so called lithium-rich layered oxide materials (e.g. Li_1-x[Li_0.2Mn_0.6Ni_0.2]O₂), exceed the conventional limit of charge storage as far more lithium (x ≈ 1, corresponding to 310 mAh g^-1) can be extracted than can be accounted for through transition metal oxidation (x ≈ 0.4). The source of this “extra” capacity has been explained previously by a number of models including oxygen loss, electrolyte decomposition and anion redox.

 Recently, several groups have identified that this “extra” capacity is resulting from a reversible oxygen redox process^2-7. In this work, we examine in detail the charging process in Li[Li_0.2Ni_0.2Mn_0.6]O₂ with direct experimental evidence of a dominant O redox process, accompanied by a minor contribution from oxygen loss, when the material is charged beyond 4.4 V. The formation of hole states around the oxygen during charging was probed using soft x-ray absorption spectroscopy (SAXS), as shown in Figure 1. Additionally, recent results that underpin the oxygen redox process in a number of closely related Li-rich compositions will be discussed, as well as the requirements necessary to form holes on the oxygen in these compounds. The conclusions of this work, and that of other researchers working to understand anion redox, provide us with a guidance which can be used to discover future high energy density cathode materials.

References

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2. Luo, K.; Roberts, M. R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y. S.; Edstrom, K.; Guo, J. H.; Chadwick, A. V.; Duda, L. C.; Bruce, P. G. Nat Chem 2016, 8, (7), 684-691.

3. Luo, K.; Roberts, M. R.; Guerrini, N.; Tapia-Ruiz, N.; Hao, R.; Massel, F.; Pickup, D. M.; Ramos, S.; Liu, Y. S.; Guo, J.; Chadwick, A. V.; Duda, L. C.; Bruce, P. G. J Am Chem Soc 2016, 138, (35), 11211-8.

4. Koga, H.; Croguennec, L.; Menetrier, M.; Mannessiez, P.; Weill, F.; Delmas, C. J Power Sources 2013, 236, 250-258.

5. Koga, H.; Croguennec, L.; Menetrier, M.; Mannessiez, P.; Weill, F.; Delmas, C.; Belin, S. J Phys Chem C 2014, 118, (11), 5700-5709.

6. Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C. P.; Vezin, H.; Sougrati, M. T.; Doublet, M. L.; Foix, D.; Gonbeau, D.; Walker, W.; Prakash, A. S.; Ben Hassine, M.; Dupont, L.; Tarascon, J. M. Nat Mater 2013, 12, (9), 827-835.

7. Seo, D. H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. Nat Chem 2016, 8, (7), 692-7.