Mn-rich Layered Cathode Research Articles

Effect of Mo6+ induced valence changes of transition metal ions in Mn-rich layered cathode materials on electrochemical performance

Layered cathode materials for lithium-ion batteries have been extensively investigated; however, comparatively scant attention has been paid to Mn-rich layered cathode materials. Herein, LiNi0.5Mn0.5O2 cathode materials are doped to investigate the effect of doping elements on the electrochemical properties of Mn-rich layered cathodes. XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), and SQUID (superconducting quantum interference device) results reveal that Mo doping affects the electrochemical properties of the cathode materials by changing the valence state of transition metal ions. Due to the Mo6+ ions doping, a large proportion of Mn3+ ions present in LiNi0.5Mn0.5O2 were converted to Mn4+ ions, which alleviated the Jahn-Teller effect and reduced the elongated Mn-O octahedra, thus increasing the Li layer spacing and facilitating Li+ diffusion. Therefore, 1 % Mo-doped cathode exhibits a significantly improved Li-ion diffusion coefficient and superior rate performance with a specific discharge capacity of 103 mAh·g−1 at 5C. Meanwhile, the cathode materials significantly improve cycling performance due to the reduction in the amount of Mn3+. The current study provides guidelines for the modification of Mn-rich layered cathodes in the future.

Enhancement of La–Ti modification on the conductivity and stability of Co-free Li- and Mn-rich layered cathode

Lithium (Li) - and manganese (Mn)-rich oxides (xLi2MnO3·(1−x) LiTMO2, TM = Ni, Co, Mn, etc.) are one of the most promising cathode materials because of their high theorical capacity, which is contributed by the redox of both transition metal cations and oxygen anion. However, anionic redox leads to transition-metal migration and release of lattice oxygen, which further triggers voltage decay and capacity fade. Here, we report a dual-doping and multiphase co-existence strategy to tackle the structure degradation of Li- and Mn-rich materials and enhance electrochemical kinetics. La–Ti dual cations doping strength the TM-O bonds and improve cycling stability, and the inducement of LaMnO3+δ (LaMO) phase provides higher conductivity. As a result, the unique architecture presents a favourable rate performance, improved capacity retention, and suppressed voltage decay. After 200 cycles at 1C (250 mA g−1), the La–Ti dual cations doped Li-and Mn-rich material shows 24 % higher capacity retention and 52 % lower voltage decay compared to the non-doped material. This study demonstrates the potential of dual transition-metal ion doping combined with doping-induced perovskite phase to enhance the performance of Li- and Mn-rich materials.

Nanocomposite Engineering of High-Capacity Partially Ordered Cathode Materials for Li-Ion Batteries

Understanding the local cation order in the crystal structure and its correlation with cycling performances has promoted the development of high-energy Mn-rich cathode materials for Li-ion batteries, such as Li- and Mn-rich layered cathodes (LMR, e.g., Li1.2Ni0.2Mn0.6O2) that are considered as nanocomposite layered materials with C2/m Li2MnO3-type medium-range order (MRO). Also, Li diffusivity in high-capacity Mn-based disordered rock-salt (DRX) cathodes (e.g., Li1.2Mn0.6Nb0.2O2) was found to be influenced by the short-range order (SRO) of cations, highlighting the criticality of engineering the local cation order in designing high-energy materials. In this presentation, we will discuss the nanocomposite, heterogeneous nature (like MRO found in LMR) of ultrahigh-capacity partially ordered cathodes (e.g., Li1.68Mn1.6O3.7F0.3) made of distinct domains of spinel-, DRX- and layered-like phases, which is different from conventional single-phase DRX cathodes. This multi-scale understanding of ordering informs engineering the nanocomposite material via Ti doping, altering the intra-particle characteristics to increase the content of the rock-salt phase and heterogeneity within a particle. This strategy greatly improves the reversibility of Mn- and O-redox processes to enhance the cycling stability of the partially ordered DRX cathodes (nearly ~30 % improvement of capacity retention). Overall, our work sheds light on the importance of nanocomposite engineering to develop ultrahigh-performance, low-cost Li-ion cathode materials.

Nanocomposite Engineering of a High-Capacity Partially Ordered Cathode for Li-Ion Batteries.

Understanding the local cation order in the crystal structure and its correlation with electrochemical performances has advanced the development of high-energy Mn-rich cathode materials for Li-ion batteries, notably Li- and Mn-rich layered cathodes (LMR, e.g., Li1.2 Ni0.13 Mn0.54 Co0.13 O2 ) that are considered as nanocomposite layered materials with C2/m Li2 MnO3 -type medium-range order (MRO). Moreover, the Li-transport rate in high-capacity Mn-based disordered rock-salt (DRX) cathodes (e.g., Li1.2 Mn0.4 Ti0.4 O2 ) is found to be influenced by the short-range order of cations, underlining the importance of engineering the local cation order in designing high-energy materials. Herein, the nanocomposite is revealed, withaheterogeneous nature (like MRO found in LMR) of ultrahigh-capacity partially ordered cathodes (e.g., Li1.68 Mn1.6 O3.7 F0.3 ) made of distinct domains of spinel-, DRX- and layered-like phases, contrary to conventional single-phase DRX cathodes. This multi-scale understanding of ordering informs engineering the nanocomposite material via Ti doping, altering the intra-particle characteristics to increase the content of the rock-salt phase and heterogeneity within a particle. This strategy markedly improves the reversibility of both Mn- and O-redox processes to enhance the cycling stability of the partially ordered DRX cathodes (nearly ≈30% improvement of capacity retention). This work sheds light on the importance of nanocomposite engineering to develop ultrahigh-performance, low-cost Li-ion cathode materials.

Impact of Ni Content on the Electrochemical Performance of the Co-Free, Li and Mn-Rich Layered Cathode Materials

Li and Mn-rich layered cathode (LLC) materials show great potential as the next generation cathode materials because of their high, practical and achievable specific capacity of ~250 mAh/g, thermal stability and lower raw material cost. However, LLC materials suffer from degradation of specific capacity, voltage fading due to phase transformation upon cycling and transition-metal dissolution, which presents a significant barrier for commercialization. Here, we report the effects of Ni content on the electrochemical performance, structural and thermal stability of a series of Co-free, LLC materials (Li1.2NixMn0.8-xO2, x = 0.12, 0.18, 0.24, 0.30 and 0.36) synthesized via a sol-gel method. Our study shows that the structure of the material as well as the electrochemical and thermal stability properties of the LLC materials are strongly dependent on the Ni or Mn content. An increase in the Ni to Mn ratio results in an increase in the average discharge voltage and capacity, as well as improved structural stability but decreased thermal stability.

Achieving structural stability and enhanced electrochemical performance through Nb-doping into Li- and Mn-rich layered cathode for lithium-ion batteries.

Although Li- and Mn-rich layered oxides are attractive cathode materials possessing high energy densities, they have not been commercialized owing to voltage decay, low rate capability, poor capacity retention, and high irreversible capacity in the first cycle. To circumvent these issues, we propose a Li1.2Ni0.13Co0.13Mn0.53Nb0.01O2 (Nb-LNCM) cathode material, wherein Nb doping strengthens the transition metal oxide (TM-O) bond and alleviates the anisotropic lattice distortion while stabilizing the layered structure. During long-term cycling, maintaining a wider LiO6 interslab thickness in Nb-LNCM creates a favorable Li+ diffusion path, which improves the rate capability. Moreover, Nb doping can decrease oxygen loss, suppress the phase transition from layered to spinel and rock-salt structures, and relieve structural degradation. Nb doping results in less capacity contributions of Mn and Co and more reversible Ni and O redox reactions compared to pristine Li1.2Ni0.133Co0.133Mn0.533O2 (LNCM), which significantly mitigates the voltage decay (Δ0.289 and Δ0.516 V for Nb-LNCM and LNCM, respectively) and ensures stable capacity retention (82.7 and 70.3% for Nb-LNCM and LNCM, respectively) during the initial 100 cycles. Our study demonstrates that Nb doping is an effective and practical strategy to enhance the structural and electrochemical integrity of Li- and Mn-rich layered oxides. This promotes the development of stable cathode materials for high-energy-density lithium-ion batteries.

Sulfuretted Li- and Mn-rich cathode material with epitaxial spinel stabilizer for ultra-long cycle Li-ion battery

The utilization of Li- and Mn-rich layered cathode materials is an effective route to break the energy density bottleneck of lithium-ion batteries for practical applications. However, these materials always suffer from severe structural degradation and oxygen loss during long-term cycling process. Herein, the integrated modification was realized by epitaxial spinel phase film grown on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 (LMR) and proper S elements doping to obtain outstanding electrochemical performance. The introduced spinel phase stabilizer can enhance Li ion conductivity and reduce the particle cracks and phase collapse during repeated cycling processes. The formation of TM-S bond configuration induced by S incorporation can effectively accelerate the lithium ions diffusion and suppress the undesired oxygen redox. Therefore, the LMRS@S cathode displays an excellent electrochemical performance: ultra-long-cycling stability with capacity retention of 82.1% after 600 cycles at 1 C, higher initial Coulomb efficiency (84.7% vs 76.3%) and excellent full cell performance (81.7% capacity retention after 140 cycles). The synergistic strategies with surface stabilizer and S-incorporation affords a promising way to design superior cycling performance LMR cathode material.

Delocalized Li@Mn6 superstructure units enable layer stability of high-performance Mn-rich cathode materials

<h2>Summary</h2> The search for cathode materials with high energy density, long-term cycling stability, and low cost is one of the most important challenges for current lithium-ion batteries. To address the structural instability in Mn-rich layered cathodes, we demonstrate herein through thorough experimental and theoretical studies that delocalizing Li@Mn₆ superstructure units within transition-metal layers is an effective strategy to enhance the layer stability of a Li-excess Mn-rich layered oxide (LMRO) cathode. The delocalized Li@Mn₆ superstructure units can not only increase the Mn valence to inhibit the adverse Jahn-Teller effect but also harness the anionic redox activity with suppressed O–Mn₀ species. Benefited from its stable layered structure, the LMRO cathode can retain a high capacity and energy density of 251 mA h g⁻¹ and 791 W h kg⁻¹, respectively, after 100 cycles with nearly 100% retention. This work provides a feasible route to develop the high-performance layered cathodes with stable anionic redox chemistry.

Relieving capacity decay and voltage fading of Li1.2Ni0.13Co0.13Mn0.54O2 by Mg2+ and PO43- dual doping

Li- and Mn-rich layered cathode materials (LMR) have been widely studied for their ultrahigh energy density. Unfortunately, the rapid potential decay and capacity degradation during cycling hinder their commercialization process. In this work, Mg2+ cation and PO43− polyanion dual elements coupling effect induced lattice modification strategy is developed to address these drawbacks for a typical lithium-rich material. The structural characteristic, chemical composition, valence state and elemental distribution of the materials are investigated by scanning electron microscopy, X-ray diffraction, inductively coupled plasma optical emission spectroscopy, focused ion beam and high-resolution transmission electron microscopy, respectively. Mg2+ and PO43− co-modified lithium-rich cathode material shows excellent electrochemical properties. It provides an improved initial coulombic efficiency of 87 % and ultrahigh initial discharge capacity of 307 mA h g-1 during a cut-off voltage of 2.0–4.8 V and a current density of 25 mA g-1. In conclusion, the Mg2+ and PO43− co-modified material exhibits not only significantly improved rate performance (150.7 mA h g−1 @ 5 C) but also markedly improved cycling stability (83 % @ 1 C @ 300 cycles).

Surface-to-Bulk Redox Coupling through Thermally Driven Li Redistribution in Li- and Mn-Rich Layered Cathode Materials.

Li- and Mn-rich (LMR) layered cathode materials have demonstrated impressive capacity and specific energy density thanks to their intertwined redox centers including transition metal cations and oxygen anions. Although tremendous efforts have been devoted to the investigation of the electrochemically driven redox evolution in LMR cathode at ambient temperature, their behavior under a mildly elevated temperature (up to ∼100 °C), with or without electrochemical driving force, remains largely unexplored. Here we show a systematic study of the thermally driven surface-to-bulk redox coupling effect in charged Li1.2Ni0.15Co0.1Mn0.55O2. We for the first time observed a charge transfer between the bulk oxygen anions and the surface transition metal cations under ∼100 °C, which is attributed to the thermally driven redistribution of Li ions. This finding highlights the nonequilibrium state and dynamic nature of the LMR material at deeply delithiated state upon a mild temperature perturbation.

Defects in Li-rich manganese-based layered oxide: A first-principles study

Lithium-rich manganese-based layered oxides are of great interest as cathode materials for lithium ion batteries due to their high energy density. The voltage decay and capacity fading during prolonged charge/discharge cycling are the key obstacles for their practical usage. In this work, using density functional theory, we investigated the origin of the Ni surface segregation by calculating the defect formation energies of antisite defects, including Ni cation substituting a Li cation [Formula: see text] and pairs of Ni cation swapping with Li cation ([Formula: see text]–[Formula: see text]) in [Formula: see text], [Formula: see text] and [Formula: see text] to represent the Mn-rich, Mn–Ni equivalence and Ni-rich conditions, respectively. Results show that [Formula: see text]–[Formula: see text] defect is of energy favorable in Li-rich and Mn-rich layered oxide cathode. Al-doping is beneficial for improving the structure stability and preventing the surface segregation, thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes. This work provides deep insights for the design of layered cathode materials with both high capacity and voltage stability during cycling.

Cation and anion Co-doping synergy to improve structural stability of Li- and Mn-rich layered cathode materials for lithium-ion batteries

Cobalt-free Li and Mn-rich layered cathode materials are promising for next generation lithium ion batteries due to their high specific capacity and low cost. However, these materials are facing serious issues, such as structural collapse due to volume shrinkage during Ni2+ oxidation to Ni4+ and oxygen vacancy generation over 4.5V. In this work, we report a strategy to address the issue of volume shrinkage caused by structural deformation via constructing a large-size rigid frame by simultaneously doping inactive and large radius atoms of Cd and S. This strategy significantly reduces the volume shrinkage when fully delithiated relative to the pristine material. The Cd- and S-doped cathode materials present a highly reversible structure, which were determined by using HAADF-STEM and in-situ XRD characterization. The doped Cd and S atoms act as supporting atoms and synergistically enlarge the layer spacing of crystalline from 0.455 to 0.483 nm. The in-situ XRD monitors the structural changes in real time during charging and discharging processes, further verifying structural stability during cycling. Due to the unique structural properites, an initial capacity of 268.5 mAh g−1 at 0.1 C along with a good rate capacity of 153.8 mAh g−1 at 5 C were achieved. Also, a capacity of 243.4 mAh g-1 at 0.1 C over 80 cycles indicate good cyclic stability of the co-doped Li- and Mn-rich layered cathode.

Understanding the influence of Mg doping for the stabilization of capacity and higher discharge voltage of Li- and Mn-rich cathodes for Li-ion batteries.

Although Li- and Mn-rich layered cathodes exhibit high specific capacity, the cathode materials of the general formula Li1+x[NiyMnzCow]O2 (x + y + z + w = 1) suffer from capacity fading and discharge-voltage decay during prolonged cycling, due to the layered-to-spinel transformation upon cycling to potentials higher than 4.5 V vs. Li. In this paper, we study the effect of Mg doping (by partial replacement of Mn ions) on the electrochemical performance of Li- and Mn-rich cathodes in terms of specific capacity, capacity retention and discharge voltage upon cycling. Mg-doped Li- and Mn-rich Li1.2Ni0.16Mn0.54Mg0.02Co0.08O2 and Li1.2Ni0.16Mn0.51Mg0.05Co0.08O2 cathode materials were synthesized by a self-combustion reaction (SCR), and their electrochemical performance in Li-ion batteries was tested. The replacement of a small amount of Mn ions by Mg ions in these materials results in a decrease in their specific capacity. The doping of a small amount of Mg (x = 0.02) resulted only in the stabilization of the capacity, whereas a greater amount (x = 0.05) resulted in improved capacity retention and discharge voltage upon cycling. Li1.2Ni0.16Mn0.51Mg0.05Co0.08O2 exhibits a low specific capacity of about 160 mA h g-1, which increases and then stabilizes at about 230 mA h g-1, and finally decreases to 210 mA h g-1 during 100 cycles. The substitution of Mg for Mn (x = 0.05) results in a higher discharge voltage than the other two cathode materials examined in this study. Structural analysis of the cycled electrodes suggests that Mg suppresses the activation of Li2MnO3 during the initial cycling, and hence, partially prevents layered-to-spinel transformation, resulting in a better electrochemical performance of the Mg-doped cathode material as compared to the undoped material.

Al Doping for Mitigating the Capacity Fading and Voltage Decay of Layered Li and Mn‐Rich Cathodes for Li‐Ion Batteries

Li and Mn‐rich layered cathodes, despite their high specific capacity, suffer from capacity fading and discharge voltage decay upon cycling. Both specific capacity and discharge voltage of Li and Mn‐rich cathodes are stabilized upon cycling by optimized Al doping. Doping Li and Mn‐rich cathode materials Li1.2Ni0.16Mn0.56Co0.08O2 by Al on the account of manganese (as reflected by their stoichiometry) results in a decrease in their specific capacity but increases pronouncedly their stability upon cycling. Li1.2Ni0.16Mn0.51Al0.05Co0.08O2 exhibits 96% capacity retention as compared to 68% capacity retention for Li1.2Ni0.16Mn0.56Co0.08O2 after 100 cycles. This doping also reduces the decrease in the average discharge voltage upon cycling, which is the longstanding fatal drawback of these Li and Mn‐rich cathode materials. The electrochemical impedance study indicates that doping by Al has a surface stabilization effect on these cathode materials. The structural analysis of cycled electrodes by Raman spectroscopy suggests that Al doping also has a bulk stabilizing effect on the layered LiMO2 phase resulting in the better electrochemical performance of Al doped cathode materials as compared to the undoped counterpart. Results from a prolonged systematic work on these cathode materials are presented and the best results that have ever been obtained are reported.

Effect of Lithium Bis(oxalato) Borate (LiBOB) As an Additive in Electrolyte for Enhanced Cycling Stability of Li-Rich Li1.2Ni0.16Mn0.56Co0.08O2 cathodes

In recent years, Li and Mn rich layered xLi2MnO3.(1-x)LiMO2 (M=Mn, Co and Ni) compounds are considered as attractive cathode materials for high energy density Li-ion batteries as these materials can provide capacities ≥ 250 mAh g-1 [1-3]. These are considered as structurally integrated materials of two phases; Li2MnO3 with monoclinic (C/2m) and other LiMO2 with rhombohedral phase. These integrated cathode materials are usually cycled to potentials higher than 4.5 V during charging in the 1st cycle in order to activate the initially inactive Li2MnO3 component. The activation process involves removal of some lithium and oxygen. Once the Li2MnO3 phase is activated, it can undergo lithiation during discharge and contributes to the overall capacity, resulting in a high specific capacity. However, these materials suffer from capacity fading upon extensive cycling. Surface coating with a chemically inert layer of MgO [4], Al2O3 [5], AlF3 [6] and AlPO4 [7] etc. have been used to mitigate the capacity fading upon cycling of these materials. The standard electrolyte solution EC/DMC-LiPF6 may undergo oxidation on the cathodes surface at potential higher than 4.5 V. An alternative route is the use of additives that were found to be useful for high voltage applications by reducing the possibility of detrimental anodic reactions via formation of protective surface layers on the cathodes. LiBOB has been studied as an important additive for improving the cycling stability of high voltage cathodes based on LiNi0.5Mn1.5O4[8, 9]. It is known that the oxidation of LiBOB on cathode surfaces at high potentials results in the formation of passivating surface films. Hence the presence of LiBOB as an additive to standard solutions may have a positive impact on the electrochemical performance of Li and Mn rich layered cathode materials which are usually cycled to potentials > 4.5V vs. Li. In the present study, Li1.2Mn0.56Ni0.16Co0.08O2 synthesized by self-combustion reaction (SCR) was studied as a cathode material for advanced Li ion batteries in standard electrolyte solutions with and without LiBOB at 30 and 45 ◦C. It exhibited an initial specific capacity around 270 mAhg-1, which stabilized at about 215 mAh g-1 during 50 cycles in standard electrolyte solutions. When these cathodes were tested with LiBOB as an additive in solution, a capacity retention of 98 % could be demonstrated during 50 cycles due to a unique stabilizing effect of this additive (Fig. 1). When cycled at 45 ◦C, the capacity retention of Li1.2Mn0.56Ni0.16Co0.08O2 cathodes in LiBOB containing electrolyte was found to be about 97 % after 50 cycles as compared to 78 % in standard electrolyte solutions. The presence of LiBOB in solutions leads to relatively low impedance of these cathodes. HRTEM measurements show that while the initially present Li2MnO3 phase completely disappears upon cycling (above 4.5V) in standard electrolyte solutions, it does not fully disappear upon cycling in solutions containing LiBOB and may help in stabilizing the capacity of these integrated cathode materials. These results will be discussed.

Structural and Chemical Evolution of Li- and Mn-Rich Layered Cathode Material

Lithium (Li)- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, the voltage fading mechanism in these materials as well as its relationships to fundamental structural changes is far from being sufficiently understood. Here we report the detailed phase transformation pathway in the LMR cathode (Li[Li0.2Ni0.2Mn0.6]O2) during cycling for samples prepared by the hydrothermal assisted (HA) method. It is found that the transformation pathway of the LMR cathode is closely correlated to its initial structure and preparation conditions. The results reveal that the LMR cathode prepared by the HA approach experiences a phase transformation from the layered structure (initial C2/m phase transforms to R3̅m phase after activation) to a LT-LiCoO2 type defect spinel-like structure (with the Fd3̅m space group) and then to a disordered rock-salt structure (with the Fm3̅m space group). The voltage fade can be well correlated with Li ion insertion into octahedral sites, rather than tetrahedral sites, in both defect spinel-like and disordered rock-salt structures. The reversible Li insertion/removal into/from the disordered rock-salt structure is ascribed to the Li excess environment that permits Li percolation in the disordered rock-salt structure despite the increased kinetic barrier. Meanwhile, because of the presence of a large quantity of oxygen vacancies, a significant decrease in the Mn valence is detected in the cycled particle, which is below that anticipated for a potentially damaging Jahn–Teller distortion (+3.5). Clarification of the phase transformation pathway, cation redistribution, oxygen vacancy and Mn valence change provides unique understanding of the voltage fade and capacity degradation mechanisms in the LMR cathode. The results also inspire us to further enhance the reversibility of the LMR cathode via improved surface structural stability.

Structural and Electrochemical Evidence of Layered to Spinel Phase Transformation of Li and Mn Rich Layered Cathode Materials of the Formulae xLi[Li1/3Mn2/3]O2.(1-x)LiMn1/3Ni1/3Co1/3O2 (x = 0.2, 0.4, 0.6) upon Cycling

Layered Li and Mn rich cathode materials of the xLi[Li1/3Mn2/3]O2.(1-x)LiMn1/3Ni1/3Co1/3O2 (x = 0.2, 0.4, 0.6) were synthesized by a self-combustion method, characterized by XRD, SEM, HRTEM and Raman spectroscopy and studied as positive electrode materials for Li-ion batteries. The cathode material with x = 0.6 exhibits an initial high discharge specific capacity of 270 mAh g−1 at C/10 rate in galvanostatic charge-discharge cycling, which decreases to 220 mAh g−1 after 50 cycles. It also exhibits a high rate capability as compared to other composites. Structural studies using the electron diffraction technique with TEM and spectral studies by Raman spectroscopy indicate continuous structural changes upon cycling that include formation of a spinel phase. The electrochemical impedance spectra recorded at various potentials present evidence of a substantial increase in the charge-transfer resistance at potentials higher than 4.4 V during charge and also at potentials lower than 3.8 V during discharge. The chemical diffusion coefficient of Li+ions in these materials was calculated to be around 10−13 cm2 s−1 and itreaches a minimum near 3.3 V from PITT studies. The minimum value of the diffusion coefficient of Li+around 3.3 V potential can be explained as resulting from the formation of spinel phase upon cycling.