Lithium- and oxygen-driven structural evolution in Co-free Li-Mn-rich oxides as cathodes for lithium ion batteries

Nowadays, the lithium-rich layered 3d-transition-metal (TM) oxides (LLOs) are regarded as one of the most attractive cathode materials for next-generation lithium-ion batteries (LIBs), because they can exhibit a discharge capacity (approaching 300 mAh g-1 at 0.1 C) much higher than that of the currently used materials (~ 180 mAh g-1 at 0.1 C). However, the formation mechanism of LLOs has not been fully understood on a fundamental level yet. One of the most interesting and still open questions is about lithium and oxygen atoms are incorporated into the precursor’s crystal structure during the synthesis of final LLO products. A systematic exploration of this subject was conducted in this PhD work.
In order to obtain the desired LLOs with high performance, it is necessary to design a proper morphology for the precursor which acts as an important source for the high-temperature lithiation reaction. A powerful hydroxide coprecipitation process was employed to synthesize the three dimensional (3D) nanoflower-structured precursor for LLOs.
A combination of thermogravimetric (TG), differential scanning calorimetric (DSC) and in situ high-temperature synchrotron radiation diffraction (SRD) experiments was utilized to investigate the thermally induced structural evolution. ... mehrThe results show that the precursor composed of layered TMOOH (C2/m) and tetragonal TM3O4 (I41/amd) transforms into a single cubic spinel TM3O4 phase (Fd-3m) with successive oxygen loss during thermal treatment. On the contrary, oxygen would be inserted into the host structure during synthesis of LLOs starting from a mixture of the precursor and lithium source (Li2CO3).
It is the original formation mechanism of LLOs that inspires to develop a promising and practical method, a coprecipitation route followed by a microwave heating process, for controllable synthesis of the layered monoclinic Li[Li0.2Ni0.2Mn0.6]O2 cathode materials with high rate performance (i.e. a specific discharge capacity of 171 mAh g-1 at 10 C). An increased oxidation of transition metal cations during high-temperature lithium insertion process reveals that oxygen atoms are continuously inserted into the host structure to keep charge neutrality and provide the open coordination sites for incorporated lithium ions and relocated TM ions during preparation of LLOs. The high-temperature lithiation reaction is accompanied by phase transition, atomic rearrangement, and surface reconstruction.
Despite the fact that the synthesized monoclinic Li[Li0.2Ni0.2Mn0.6]O2 cathode could deliver a good rate performance, it suffers from a serious voltage decay during electrochemical cycling. Therefore, in situ high-resolution SRD was carried out in order to understand the structural evolution of the Li-excess layered electrode during electrochemical cycling and to figure out what are the important factors responsible for the degradation of LLOs. The in situ SRD results suggest that the nanodomain formation of a layered phase and a spinel-like phase after charging to high voltages (above 4.5 V) is the main contributing factor for the structural instability. The fatigue crack in the electrode material after prolonged cycling is directly observed by high-resolution transmission electron microscopy (HRTEM), which is ascribed to the volume variation induced by anisotropic lattice strain during the delithiation/lithiation process.
To better understand the relationship between the formation mechanism and degradation mechanism of LLOs, a series of LixNi0.2Mn0.6Oy oxides with a large variety of provided lithium contents (0.00 ≤x≤ 1.52) was prepared via a thermal treatment. The consistent results demonstrate that the structural properties of LixNi0.2Mn0.6Oy oxides are strongly dependent on the chemical composition with respect to lithium and oxygen. The Li-excess layered Li[Li0.2Ni0.2Mn0.6]O2 oxide is only stable when a considerable amount of lithium and oxygen is available during synthesis, while at a lower concentration of them, the spinel/rock-salt-type phase is thermally stable. These findings offer new insights into the nature of fatigue processes in LLOs.
Due to fact that the competition between thermally-induced oxygen loss and lithium-insertion-induced oxygen uptake occurs, the high-temperature reaction of the precursor and lithium source gets much more complicated. Lastly, in situ high-temperature SRD technique was utilized to explore the chemically (i.e. Li & O) induced structural evolution for the pure spinel oxide (Fd-3m), so as to provide a guarantee of reliable lithiation reaction mechanism with oxygen uptake. The original formation mechanism of Li-containing oxides indicates that the Li-excess layered oxides can be formed as a result of lithium and oxygen insertion into the spinel (Fd3 ̅m) and/or Li-containing rock-salt-type phase (Fm-3m) during synthesis at air atmosphere. If a small amount of lithium is provided, lithium atoms have a tendency to be located on tetrahedral positions, forming the Li-containing spinel oxides. As more lithium ions are gradually incorporated into the spinel matrix, lithium atoms tend to preferentially occupy the octahedral sites forming Li-containing rock-salt-type phase and/or Li-rich layered phase. Because the oxygen anion cubic close-packed lattice is involved during phase transformation, oxygen atoms are supposed to be inserted only into the oxygen lattice at the surface, associated with crystal growth and/or recrystallization.
These discoveries not only contribute to a comprehensive understanding of the correlation between preparation, structure and performance for next-generation LIBs, but also provide new insights into the interaction of oxygen with lithium in Li-containing oxides during synthesis and electrochemical cycling.