Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy

• This thesis presents experimental investigations of the electronic structure of emerging and novel cathode materials used in lithium- and sodium-ion batteries. The investigated materials include a range of oxide materials containing the elements nickel and manganese. Central goals are to find fundamental explanations for favorable, respectively, unfavorable electrochemical cycling behavior and to arrive at a better understanding of the roles that the different elemental constituents of the compounds play. The experiments are based on the application of X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) in the soft X-ray region and have been performed at synchrotron radiation facilities such as The Advanced Light Source (USA), The Swiss Light Source (Switzerland) and SPring-8 (Japan). XAS and RIXS of spinel LiNi0.44Mn1.56O4 at the O K-edge as well as the Ni and Mn L-edges were measured for two different crystal structures, namely, transition-metal-ordered and -disordered, respectively. The results show that both Ni and O contribute strongly as redox centers for the charge compensation during electrochemical cycling. The Ni L-RIXS spectra show evidence of a more stable Ni--O bond in the disordered material. In the layered manganese oxide materials Li[Li0.2Ni0.2Mn0.6]O2, Na0.67[Mg0.28Mn0.72]O2, and Na0.78[Li0.25Mn0.75]O2, as well as the disordered Li1.9Mn0.95O2.05F0.95 one observes that reversible O redox leads to two distinct features in O K-RIXS. Both features resonate in a narrow incident energy range suggesting that localized O hole states are formed, one close to the elastic peak and the other as a strong emission peak at an energy loss of about 8 eV. These features appear reversibly on the voltage plateau of the charge-discharge curve and can be used to identify a certain type of O redox reactions.The work also includes investigations that compare two different compositions of the structurally related material Li2MnO3 grown epitaxially as thin films. Evidence is found for anionic activity during the initial cycle that is of a different kind than the above as no evidence for localized O holes is found. Instead, excess Li in the transition metal layer is shown to lead to a more rapid loss of covalency in the Mn--O bonds.In short, this work presents some of the first explorations into the role of different types of anionic redox centers in cathodes, by means of XAS and RIXS thereby also demonstrating the utility and power of synchrotron based techniques for gaining atomic-level understanding of battery electrode materials.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

Nyckelord

soft X-ray spectroscopy

X-ray absorption spectroscopy (XAS)

resonant inelastic X-ray scattering (RIXS)

lithium-ion battery (LIB)

sodium-ion battery (SIB)

anionic redox

cathode materials

layered manganese oxide

spinel LNMO

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vet (ämneskategori)

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