Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li + Intercalation Storage (original) (raw)
Related papers
Advanced Energy Materials, 2015
rock-salt structures, with Li + /TM evenly sharing cation sites and close-packed anion arrays, may be a reasonable alternative for effi cient Li + storage. After Li + extraction, the TM may remain at the cation sublattice sites and uphold well the disordered rocksalt framework. A recent work based on ab initio computations revealed that Li + transport can be facile in a disordered cubic rock-salt oxide (Li 1.211 Mo 0.467 Cr 0.3 O 2 ) with Li-excess. [ 8 ] It was found that such framework is stable with a maximum of 45% of the cation sites vacant. Herein, we demonstrate that a new dilithium disordered rock-salt Li 2 VO 2 F intercalation material can deliver up to about 1.8 Li + capacity per TM (420 mAh g −1 ) at ≈2.5 V (1000 Wh kg −1 ) with only minor lattice volume change (≈3%). Such material with mixed O 2− /F − anion environment has been synthesized by a simple ball-milling method. The two Li + storage with V 3+ /V 5+ redox reactions (Li 2 VO 2 F ↔ 2 Li + + 2e − + VO 2 F) leads to an attractively high theoretical capacity of 462 mAh g −1 . Moreover, Li 2 VO 2 F shows good capacity retention and minor increase in polarization upon fast charging/discharging or upon low-temperature operation.
Lithiation-driven structural transition of VO 2 F into disordered rock-salt Li x VO 2 F
We synthesize a new vanadium oxyfluoride VO 2 F (rhombohedral, R 3c) through a simple one-step ballmilling route and demonstrate its promising lithium storage properties with a high theoretical capacity of 526 mA h g À1 . Similar to V 2 O 5 , VO 2 F transfers into an active disordered rock-salt (Fm 3m) phase after initial cycling against the lithium anode, as confirmed by diffraction and spectroscopic experiments. The newly formed nanosized Li x VO 2 F remains its crystal structure over further cycling between 4.1 and 1.3 V.
Li + intercalation in isostructural Li 2 VO 3 and Li 2 VO 2 F with O 2À and mixed O 2À /F À anions
Mixed-anion materials for Li-ion batteries have been attracting attention in view of their tunable electrochemical properties. Herein, we compare two isostructural (Fm % 3m) model intercalation materials Li 2 VO 3 and Li 2 VO 2 F with O 2À and mixed O 2À /F À anions, respectively. Synchrotron X-ray diffraction and pair distribution function data confirm large structural similarity over long-range and at the atomic scale for these materials. However, they show distinct electrochemical properties and kinetic behaviour arising from the different anion environments and the consequent difference in cationic electrostatic repulsion. In comparison with Li 2 VO 3 with an active V 4+/5+ redox reaction, the material Li 2 VO 2 F with oxofluoro anions and the partial activity of V 3+/5+ redox reaction favor higher theoretical capacity (460 mA h g À1 vs. 230 mA h g À1), higher voltage (2.5 V vs. 2.2 V), lower polarization (0.1 V vs. 0.3 V) and faster Li + chemical diffusion (B10 À9 cm 2 s À1 vs. B10 À11 cm 2 s À1). This work not only provides insights into the understanding of anion chemistry, but also suggests the rational design of new mixed-anion battery materials.
Pure and Applied Chemistry, 2000
Results on the local cation ordering in layered lithium-nickel/cobalt oxides and metal-substituted lithium-manganese spinels are presented. It is shown that electron spin resonance of Ni 3+ and Mn 4+ and magnetic susceptibility measurements are powerful tools to monitor the short-range cation ordering in these compounds, which is not accessible by diffraction techniques. Thus, owing to the different strength of the 90°and 180°N i 3+ -O-Ni 3+/2+ exchange interactions, the distribution of Ni 3+ /Ni 2+ between the lithium and nickel layers in Li 1-x Ni 1+x O 2 with 0 < x < 0.4 can be determined. For layered LiNi 1-y Co y O 2 and spinel LiMn 2-x Co x O 4 solid solutions, analysis of the temperature-independent EPR line width in terms of dipole-dipole and exchange interactions has been used to examine the local Ni 3+ /Co 3+ and Mn 4+ /Co 3+ ordering. The results obtained are correlated with the electrochemical intercalation of lithium in these compounds. the effect of the synthesis conditions on the local cation distribution in LiNi/CoO 2 and LiMn 2-x M x O 4 (M = Co, Mg) are presented. Since magnetic properties are sensitive to the cation distribution, magnetic measurements (electron spin resonance and magnetic susceptibility measurements) were used to monitor the short-range cation ordering in these compounds, which is not possible by means of diffraction techniques.
J. Mater. Chem. A, 2015
Operando X-ray diffraction and X-ray absorption spectroscopic (XAS) measurements were carried out on layered Li[Li 1/18 Co 1/6 Ni 1/3 Mn 4/9 ]O 2 to investigate the structural changes during the first charging and discharging. The XRD results showed a phase transformation from rhombohedral to monoclinic during the charging process. X-ray absorption near-edge spectroscopy (XANES) measurements showed that the major charge compensation takes place at the Ni metal site as occurring through a two-step process, i.e. Ni 2+ / Ni 3+ / Ni 4+ , while the cobalt ions (Co 3+ ) and manganese ions (Mn 4+ ) remain unchanged, during Li-ion insertion/de-insertion. Extended X-ray absorption fine structure (EXAFS) results at the Ni edge showed a significant distortion of the Ni-O and Mn-O shells, while no significant distortion was observed at the Co-O shell during charging and discharging. From the structural analysis results, the cobalt doped Li[Li 1/18 Co 1/6 Ni 1/3 Mn 4/9 ]O 2 cathode was shown to undergo a partial increase of oxygen ions at the Mn ion environment due to the oxygen ion migration from the bulk to the surface of the electrode structure during charging. It indicated that the LiMn 2 O 3 domain plays an important role in the oxygen-activation plateau. The mechanism also showed that the partial amount of Co in a transition metal slab of excess lithium layered oxide materials can stably maintain the lithium ions in the transition metal inter slabs and the surface electronic structure of oxygen ions is reversible during electrochemical cycling between 4.6 V and 2.5 V.
Chemical and Structural Evolution during the Synthesis of Layered Li(Ni,Co,Mn)O2 Oxides
Chemistry of Materials, 2020
The discovery of Li-containing transition-metal (TM) oxides has attracted broad interest and triggered intensive studies on these oxides as cathodes for lithium-ion batteries over decades. Unfortunately, a clear picture of how Li/TM/O ions are transported and electrons are transferred during synthesis of these compounds is still missing, especially when cubic close-packed (ccp) anion sublattices are involved, as it is the case for spinel, layered, or rock-salt systems. In the present study, a series of layered Li(Ni,Co,Mn)O 2 oxides was chosen as target materials to elucidate the underlying formation mechanism of these compounds during high-temperature lithiation reaction. The consistent experimental results demonstrate that, as lithium ions are inserted from surface to bulk, some transition metal cations located within the bulk of crystallites are able to diffuse to the near-surface region. They create cation vacancies for the inserted lithium ions, the mass transport behavior of these elements is driven by chemical potential gradient. Concurrently, oxygen anions from lithium oxides and/or ambient oxygen are adsorbed and incorporated into the ccp oxygen lattice on the surface structure, connecting the relocated transition metal cations and the incorporated lithium ions by forming ionic bonds. This process is concomitant with crystal growth, surface reorganization caused by phase transformation, occurrence and disappearance of pores.
Energy & Environmental Science, 2012
High pressure-high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO 2 and Li[Ni x Li 1/3À2x/3 Mn 2/3Àx/3 ]O 2 where x ¼ 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO 2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO 2 remain almost identical to the pristine sample. The high pressure treatment of LiNi 0.5 Mn 0.5 O 2 (x ¼ 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li 1/6 Ni 1/4 Mn 7/12 ]O 2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li 1/6 Ni 1/4 Mn 7/12 ]O 2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li 1/6 Ni 1/4 Mn 7/12 ]O 2 deliver capacities upon cycling near 200 mA h g À1 , even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li 1/6 Ni 1/4 Mn 7/12 ]O 2 ; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi 0.5 Mn 0.5 O 2 . In that sense, the Li excess materials are superior to LiNi 0.5 Mn 0.5 O 2 , whose electrochemical characteristics are very sensitive to structural modifications.
Advanced Energy Materials
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201602888\. multi-electron Li-ion) [10] may lead to advances in battery performance, there is significant room for improvement in state-of-the-art Li-ion batteries. The system-level usable specific energy density of Li-ion batteries used in current electric vehicles represents less than a quarter of the theoretical capacity of the electrochemically active cathode material. [11] While part of this system-level performance gap is due to the weight of the anode and dead weight of electrochemically inactive battery components (e.g., binder, electrolyte, packaging), a significant fraction of the gap represents unused capacity in the active material. For example, the canonical cathode material LiCoO 2 can cycle reversibly with only ≈50% of its theoretical capacity, and LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) electrodes with only ≈70%. [12,13] This active-material performance gap is due to two factors. First, Li is unable to fully intercalate during discharge. Second, battery operation is constrained so as to avoid extracting all of the lithium from the cathode during charge. This chastity is typically implemented as a ≈4.3 V cutoff voltage during charging. [14] The purpose of this review is to summarize the challenges and progress in bridging the gap in theoretical and practical capacities of layered lithium oxides, especially the tradeoff between cycle life and extent of Li extraction during charge. This review starts with a discussion of the fundamental mechanisms and challenges for intercalation in layered materials, and then discusses how these play out differently for specific cathode compositions. The baseline Li x MO 2 (M = Co, Ni, or Mn) materials here set the stage for the advanced NCA [15,16] (Li x Ni y Co z Al 1−y−z O 2) and NMC [17-20] (Li x Ni y Mn z Co 1−y−z O 2) alloys. The last section reviews proposed strategies to address these challenges, including extensions to layered lithium oxides, such as Na-ion intercalation and cation-disordered materials. There are many additional aspects to Li-ion battery materials not addressed in this review, but more details can be found in prior literature. Recent articles provide a birds-eye view of research on Li-ion battery materials, [13,21-26] as well as experimental [27,28] and computational [29-31] methods for studying them. Both liquid [32-34] and solid [35-39] Li-ion electrolytes have been reviewed extensively. A number of reviews focus on specific classes of Li-ion materials, such as layered oxides, [12,27,40] Li-excess layered oxides, [41-43] phosphates, [10,44] high-voltage spinel oxides, [45,46] and Li metal anodes. [47] For details about the
Fictitious phase separation in Li layered oxides driven by electro-autocatalysis
Nature Materials
nderstanding phase diagrams, whether it be for equilibrium (for example, temperature-composition) or kinetics (for example, time-temperature-transformation), is fundamental in materials science. Careful attention to rate and path dependence is crucial for distinguishing equilibrium and kinetic effects in phase behaviour, and battery materials are no exception to this basic prescription. The dynamics associated with the many-particle (ensemble) structure in battery electrodes 1,2 (for example, inter-and intra-particle phase separation) only make such rate and path dependencies ever more critical. In phase-separating LiFePO 4 , for example, it has been recognized that the reaction rate determines both the emergence of a thermodynamically forbidden solid solution 3-5 as well as the transition from particle-by-particle behaviour to concurrent intercalation 1,6. Recently, rate-dependent pathways have also been suggested in Li 4 Ti 5 O 12 (refs. 7,8). All these non-equilibrium phenomena contribute to the excellent rate capability of these materials. Meanwhile, in the so-called solid-solution layered oxides, studies on phase evolution have not been as comprehensive since this material class is deemed a single phase. Included are compounds such as Li(Ni,Mn,Co)O 2 (NMC) and Li(Ni,Co,Al)O 2 (NCA), typically viewed as having extensive single-phase composition ranges down to a lithium fraction of at least 0.5. This standard view is based on monotonic Nernst potential profiles and X-ray diffraction (XRD) data on equilibrated samples 9-14. Contradicting the standard view, phase separation at more than half lithium filling has also been reported in numerous operando XRD studies 15-22. This anomaly has been observed during the first charge, but not during the following discharge. At the rates used in these studies, the effect did not repeat on the second cycle, leading to the prevailing view that the anomaly is a 'first-cycle effect' 17-19. Surface passivation by Li 2 CO 3 has been suggested as one cause 19. More recently, apparent phase separation has also been reported in the second cycle, attributed to sluggish lithium diffusion near fully lithiated compositions 22. Other authors maintain that the observed phases are equilibrium phases 15,16,21 , designating them as H1 and H2 phases analogous to LiNiO 2 (ref. 23). However, rate and path dependencies have not been comprehensively addressed for any NMC or NCA composition, despite their widespread use 24. Partly responsible are the restricted designs of operando experiments based on available instrument time, limiting the range of rates and cycles. Likewise, the lack of particle-resolved composition mapping across an ensemble makes it difficult to assess nanoscale variations that arise from reaction and transport limitations. For these reasons, and despite the success of porous electrode theory 25-27 , a quantitative and predictive model explaining the rate and path dependencies in layered oxides has not been developed. Here, we report that the apparent phase separation persists in later cycles, even in LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111) and