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. The 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

Probing thermally-induced structural evolution during the synthesis of layered Li-, Na-, or K-containing 3d transition-metal oxides

Layered alkali-containing 3d transition-metal oxides are of the utmost importance in the use of electrode materials for advanced energy storage applications such as Li-, Na-, or K-ion batteries. A significant challenge in the field of materials chemistry is understanding the dynamics of the chemical reactions between alkali-free precursors and alkali species during the synthesis of these compounds. In this study, in situ high-resolution synchrotron-based X-ray diffraction was applied to reveal the Li/Na/K-ion insertion-induced structural transformation mechanism during high-temperature solid-state reaction. The in situ diffraction results demonstrate that the chemical reaction pathway strongly depends on the alkali-free precursor type, which is a structural matrix enabling phase transitions. Quantitative phase analysis identifies for the first time the decomposition of lithium sources as the most critical factor for the formation of metastable intermediates or impurities during the entire process of Li-rich layered Li[Li0.2Ni0.2Mn0.6]O2 formation. Since the alkali ions have different ionic radii, Na/K ions tend to be located on prismatic sites in the defective layered structure (Na2/3-x[Ni0.25Mn0.75]O2 or K2/3-x[Ni0.25Mn0.75]O2) during calcination, whereas the Li ions prefer to be localized on the tetrahedral and/or octahedral sites, forming O-type structures

Investigation of Structural and Electronic Changes Induced by Postsynthesis Thermal Treatment of LiNiO2_{2}

Postsynthesis thermal treatments at various temperatures in air have been applied to LiNiO2, and the induced structural and electronic changes have been uncovered. Except for the familiar decomposition process at higher temperatures, a series of transformations also take place under mild conditions. To identify such subtle changes, ex situ and in situ synchrotron radiation diffraction, ex situ7Li nuclear magnetic resonance spectroscopy, and ex situ measurements of magnetic properties have been performed. We show that the reaction between LiNiO2 and CO2 starts already at a temperature of 200 °C, forming Li1–zNi1+zO2 layers. If the thickness of this layer is well adjusted, the electrochemical performance of LiNO2 can be improved. A cation off-stoichiometry of [Li0.90Ni0.10]NiO2 is identified at 600 °C even before the decomposition occurs. We also investigate the interplay of the reaction between LiNiO2 and CO2 with the decomposition at 700 °C. The changes in the Ni oxidation state and local Li environments are also monitored during the whole process

Electrochemical Investigation of Calcium Substituted Monoclinic Li3_3 V2_2(PO4_4)3_3 Negative Electrode Materials for Sodium‐ and Potassium‐Ion Batteries

Herein, the electrochemical properties and reaction mechanism of Li$_{3‒2x}$Ca$_x$V$_2$(PO$_4$)$_3$/C (x = 0, 0.5, 1, and 1.5) as negative electrode materials for sodium-ion/potassium-ion batteries (SIBs/PIBs) are investigated. All samples undergo a mixed contribution of diffusion-controlled and pseudocapacitive-type processes in SIBs and PIBs via Trasatti Differentiation Method, while the latter increases with Ca content increase. Among them, Li$_3$V$_2$(PO$_4$)$_3$/C exhibits the highest reversible capacity in SIBs and PIBs, while Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C shows the best rate performance with a capacity retention of 46% at 20 C in SIBs and 47% at 10 C in PIBs. This study demonstrates that the specific capacity of this type of material in SIBs and PIBs does not increase with the Ca-content as previously observed in lithium-ion system, but the stability and performance at a high C-rate can be improved by replacing Li$^+$ with Ca$^{2+}$. This indicates that the insertion of different monovalent cations (Na$^+$/K$^+$) can strongly influence the redox reaction and structure evolution of the host materials, due to the larger ion size of Na$^+$ and K$^+$ and their different kinetic properties with respect to Li$^+$. Furthermore, the working mechanism of both LVP/C and Ca$_{1.5}$V$_2$(PO$_4$)$_3$/C in SIBs are elucidated via in operando synchrotron diffraction and in operando X-ray absorption spectroscopy

Synergy of cations in high entropy oxide lithium ion battery anode

High entropy oxides (HEOs) with chemically disordered multi-cation structure attract intensive interest as negative electrode materials for battery applications. The outstanding electrochemical performance has been attributed to the high-entropy stabilization and the so-called ‘cocktail effect’. However, the configurational entropy of the HEO, which is thermodynamically only metastable at room-temperature, is insufficient to drive the structural reversibility during conversion-type battery reaction, and the ‘cocktail effect’ has not been explained thus far. This work unveils the multi-cations synergy of the HEO Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O at atomic and nanoscale during electrochemical reaction and explains the ‘cocktail effect’. The more electronegative elements form an electrochemically inert 3-dimensional metallic nano-network enabling electron transport. The electrochemical inactive cation stabilizes an oxide nanophase, which is semi-coherent with the metallic phase and accommodates Li$^+$ ions. This self-assembled nanostructure enables stable cycling of micron-sized particles, which bypasses the need for nanoscale pre-modification required for conventional metal oxides in battery applications. This demonstrates elemental diversity is the key for optimizing multi-cation electrode materials

Structural Origin of Suppressed Voltage Decay in Single‐Crystalline Li‐Rich Layered Li[Li0.2_{0.2}Ni0.2_{0.2}Mn0.6_{0.6}]O2_{2} Cathodes

Lithium- and manganese-rich layered oxides (LMLOs, ≥ 250 mAh g$^{−1}$) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo-mechanical breakdown. Herein, a Co-free single-crystalline LMLO, that is, Li[Li$_{0.2}$Ni$_{0.2}$Mn$_{0.6}$]O$_{2}$ (LLNMO-SC), is prepared via a Li$^+$/Na$^+$ ion-exchange reaction. In situ synchrotron-based X-ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro-strain are observed in LLNMO-SC compared to its polycrystalline counterpart (LLNMO-PC) during the charge–discharge process. Specifically, the as-synthesized LLNMO-SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li-ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long-term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO-SC decreases by only ≈0.2 V after 100 cycles at 28 mA g$^{-1}$ between 2.0 and 4.8 V, which is much lower than that of LLNMO-PC (≈0.5 V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs)

Quasi-Solid-State Ion-Conducting Arrays Composite Electrolytes with Fast Ion Transport Vertical-Aligned Interfaces for All-Weather Practical Lithium-Metal Batteries

The rapid improvement in the gel polymer electrolytes (GPEs) with high ionic conductivity brought it closer to practical applications in solid-state Li-metal batteries. The combination of solvent and polymer enables quasi-liquid fast ion transport in the GPEs. However, different ion transport capacity between solvent and polymer will cause local nonuniform Li$^+$ distribution, leading to severe dendrite growth. In addition, the poor thermal stability of the solvent also limits the operating-temperature window of the electrolytes. Optimizing the ion transport environment and enhancing the thermal stability are two major challenges that hinder the application of GPEs. Here, a strategy by introducing ion-conducting arrays (ICA) is created by vertical-aligned montmorillonite into GPE. Rapid ion transport on the ICA was demonstrated by $^6$Li solid-state nuclear magnetic resonance and synchrotron X-ray diffraction, combined with computer simulations to visualize the transport process. Compared with conventional randomly dispersed fillers, ICA provides continuous interfaces to regulate the ion transport environment and enhances the tolerance of GPEs to extreme temperatures. Therefore, GPE/ICA exhibits high room-temperature ionic conductivity (1.08 mS cm$^{−1}$) and long-term stable Li deposition/stripping cycles (> 1000 h). As a final proof, Li||GPE/ICA||LiFePO$_4$ cells exhibit excellent cycle performance at wide temperature range (from 0 to 60 °C), which shows a promising path toward all-weather practical solid-state batteries

SnCN₂: A Carbodiimide with an Innovative Approach for Energy Storage Systems and Phosphors in Modern LED Technology

The carbodiimide SnCN$_{2}$ was prepared at low temperatures (400 °C–550 °C) by using a patented urea precursor route. The crystal structure of SnCN$_{2}$ was determined from single‐crystal data in space group C2/c (no. 15) with a=9.1547(5), b=5.0209(3), c=6.0903(3) Å, β=117.672(3), V=247.92 Å$^{3}$ and Z=4. As carbodiimide compounds display remarkably high thermal and chemical resistivity, SnCN$_{2}$ has been doped with Eu and Tb to test it for its application in future phosphor‐converted LEDs. This doping of SnCN$_{2}$ proved that a color tuning of the carbodiimide host with different activator ions and the combination of the latter ones is possible. Additionally, as the search for novel high‐performing electrode materials is essential for current battery technologies, this carbodiimide has been investigated concerning its use in lithium‐ion batteries. To further elucidate its application possibilities in materials science, several characterization steps and physical measurements (XRD, in situ XANES, Sn Mössbauer spectroscopy, thermal expansion, IR spectroscopy, Mott‐Schottky analysis) were carried out. The electronic structure of the n‐type semiconductor SnCN$_{2}$ has been probed using X‐ray absorption spectroscopy and density functional theory (DFT) computations

A method to prolong lithium-ion battery life during the full life cycle

Extended lifetime of lithium-ion batteries decreases economic costs and environmental burdens in achieving sustainable development. Cycle life tests are conducted on 18650-type commercial batteries, exhibiting nonlinear and inconsistent degradation. The accelerated fade dispersion is proposed to be triggered by the evolution of an additional potential of the anode during cycling as measured vs. Li$^+$/Li. A method to prolong the battery cycle lifetime is proposed, in which the lower cutoff voltage is raised to 3 V when the battery reaches a capacity degradation threshold. The results demonstrate a 38.1% increase in throughput at 70% of their beginning of life (BoL) capacity. The method is applied to two other types of lithium-ion batteries. A cycle lifetime extension of 16.7% and 33.7% is achieved at 70% of their BoL capacity, respectively. The proposed method enables lithium-ion batteries to provide long service time, cost savings, and environmental relief while facilitating suitable second-use applications