Layered P2-Na(x)[Ni(1/3)Mn(2/3)]O(2) (0 < x < 2/3) is investigated as a cathode material for Na-ion batteries. A combination of first principles computation, electrochemical and synchrotron characterizations is conducted to elucidate the working mechanism for the improved electrochemical properties. The reversible phase transformation from P2 to O2 is observed. New configurations of Na-ions and vacancy are found at x = 1/3 and 1/2, which correspond to the intermediate phases upon the electrochemical cycling process. The mobility of Na-ions is investigated using the galvanostatic intermittent titration technique (GITT) and the Na diffusion barriers are calculated by the Nudged Elastic Band (NEB) method. Both techniques prove that the mobility of Na-ions is faster than Li-ions in the O3 structure within the 1/3 < x < 2/3 concentration region. Excellent cycling properties and high rate capability can be obtained by limiting the oxygen framework shift during P2-O2 phase transformation, suggesting that this material can be a strong candidate as a sustainable low-cost Na-ion battery cathode.
Li-substituted
layered P2–Na0.80[Li0.12Ni0.22Mn0.66]O2 is investigated
as an advanced cathode material for Na-ion batteries. Both neutron
diffraction and nuclear magnetic resonance (NMR) spectroscopy are
used to elucidate the local structure, and they reveal that most of
the Li ions are located in transition metal (TM) sites, preferably
surrounded by Mn ions. To characterize structural changes occurring
upon electrochemical cycling, in situ synchrotron X-ray diffraction
is conducted. It is clearly demonstrated that no significant phase
transformation is observed up to 4.4 V charge for this material, unlike
Li-free P2-type Na cathodes. The presence of monovalent Li ions in
the TM layers allows more Na ions to reside in the prismatic sites,
stabilizing the overall charge balance of the compound. Consequently,
more Na ions remain in the compound upon charge, the P2 structure
is retained in the high voltage region, and the phase transformation
is delayed. Ex situ NMR is conducted on samples at different states
of charge/discharge to track Li-ion site occupation changes. Surprisingly,
Li is found to be mobile, some Li ions migrate from the TM layer to
the Na layer at high voltage, and yet this process is highly reversible.
Novel design principles for Na cathode materials are proposed on the
basis of an atomistic level understanding of the underlying electrochemical
processes. These principles enable us to devise an optimized, high
capacity, and structurally stable compound as a potential cathode
material for high-energy Na-ion batteries.
Significant progress has been achieved in the research on sodium intercalation compounds as positive electrode materials for Na-ion batteries. This paper presents an overview of the breakthroughs in the past decade for developing high energy and high power cathode materials. Two major classes, layered oxides and polyanion compounds, are covered. Their electrochemical performance and the related crystal structure, solid state physics and chemistry are summarized and compared.
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