Layered Transition Metal Research Articles

Electrolyte Deprotonation Stimulates Phase Transition of Ni-Rich Cathodes in Na-Ion Batteries

Sodium-ion batteries (SIBs) are considered as a promising alternative to lithium-ion batteries (LIBs), Since they are composed of naturally abundant elements, cheaper and larger scale battery modules can be assembled. However, due to their similar electrochemical mechanisms, their practical utilization is hindered by high reactivity of conventional carbonate electrolytes with electrode materials leading to the capacity fading over repeated charging cycles. To overcome this obstacle, localized high concentration electrolytes (LHCEs) are used as an effective approach to maintain localized solvation structure of active electrolyte salts by using hydrofluoroether as diluents. Earlier studies are mainly focusing on the interactions between electrolytes with anode materials. Little was known about the electrochemical reactions of LHCEs on SIB cathode. In this work, we examined the reactivities of carbonate electrolytes and LHCEs on NaNiO2 surfaces. Ab initio molecular dynamics simulations showed that carbonate electrolytes involve more dehydrogenation reactions than LHCEs. The proton-transfer reactions between electrolyte molecules and cathode surface oxygens cause the formation of hydroxyl (-OH) groups, which leads to reduction of neighboring surface nickel atoms from Ni3+ to Ni2+. As cathode surface gets desodiated, more proton-transfer reactions were observed and Ni migrations from transition metal layer to Na layer were also identified. Consequently, it results in the oxygen loss of NaNiO2 which accelerates the layered-rock salt phase transition of bulk cathodes.

Na2[Mn3Vac0.1Ti0.4]O7: A new layered negative electrode material for aqueous Na-ion batteries

Layered oxides have been investigated as electrode materials for Na-ion battery owing to its abundant species and easy preparation. Most of the layered oxides are sensitive to water and can’t be applied to aqueous Na-ion battery. Layered anode Na2[Mn3Vac0.5]O7 with vacancies distributed uniformly in the transition-metal layer is moisture-stable, and the working potential is suitable for aqueous system. However, no effort has been devoted on the development aqueous Na-ion battery based on layered Na2[Mn3Vac0.5]O7. Moreover, the cycling and rate performance are poor. In this paper, we first proposed to change the local crystal structure by doping the vacancy with Ti4+ ions. Na2[Mn3Vac0.1Ti0.4]O7 was screened out with enhanced rate capability and cycling stability. As demonstrated by in-situ X-ray diffraction patterns and electrochemical impedance spectroscopy, the enhanced performance is ascribed to the improved kinetics after Ti doping at the vacancy, which facilitates fast transportation of electron and Na+ ion. Soft X-ray absorption results suggest that Mn4+/Mn3+ redox is involved in the charge transfer during the cycling process. Base on this, aqueous Na-ion cell was successfully fabricated using layered Na2[Mn3Vac0.1Ti0.4]O7 as anode for the first time, which exhibited excellent rate and cycle performance.

Stabilizing Li-Rich Layered Cathode Materials Using a LiCoMnO4 Spinel Nanolayer for Li-Ion Batteries.

Lithium–rich cathodes have excess lithium in the transition metal layer and exhibit an extremely high specific capacity and good energy density. However, they still have some disadvantages. Here, we propose LiCoMnO4, a new nanolayer coating material with a spinel structure, to modify the surface of lithium cathode oxide (Li7/6Mn1/2Ni1/6Co1/6O2) with a layered structure. The designed cathode with nanolayer spinel coating delivers an excellent reversible capacity, outstanding rate capability, and superior cycling ability whilst exhibiting discharge capacities of 300, 275, 220, and 166 mAh g−1 at rates of 0.1 C at 2.0−4.8 V formation and 0.1, 1, and 5 C, respectively, between 2.0 and 4.6 V. The cycling ability and voltage fading at a high operational voltage of 4.9 V were also investigated, with results showing that the nanolayer spinel coating can depress the surface of the lithium cathode oxide layer, leading to phase transformation that enhances the electrochemical performance.

Oxycarbide MXenes and MAX phases identification using monoatomic layer-by-layer analysis with ultralow-energy secondary-ion mass spectrometry.

The MXene family of two-dimensional transition metal carbides and nitrides already includes ~50 members with distinct numbers of atomic layers, stoichiometric compositions and solid solutions, in-plane or out-of-plane ordering of atoms, and a variety of surface terminations. MXenes have shown properties that make them attractive for applications ranging from energy storage to electronics and medicine. Although this compositional variability allows fine-tuning of the MXene properties, it also creates challenges during the analysis of MXenes because of the presence of multiple light elements (for example, H, C, N, O, and F) in close proximity. Here, we show depth profiling of single particles of MXenes and their parent MAX phases with atomic resolution using ultralow-energy secondary-ion mass spectrometry. We directly detect oxygen in the carbon sublattice, thereby demonstrating the existence of oxycarbide MXenes. We also determine the composition of adjacent surface termination layers and show their interaction with each other. Analysis of the metal sublattice shows that Mo2TiAlC2 MAX exhibits perfect out-of-plane ordering, whereas Cr2TiAlC2 MAX exhibits some intermixing between Cr and Ti in the inner transition metal layer. Our results showcase the capabilities of the developed secondary-ion mass spectrometry technique to probe the composition of layered and two-dimensional materials with monoatomic-layer precision.

Intergranular Diffusion‐Assisted Liquid‐Phase Chemical Vapor Deposition for Wafer‐Scale Synthesis of Patternable 2D Semiconductors

Abstract2D semiconductors have attracted considerable interest in the quest to overcome some of the challenges associated with 3D bulk semiconductors. The application of 2D semiconductors in transistor‐based electronic devices requires a reliable patterning technology with thickness controllability for continued transistor scaling. In this study, a facile synthesis approach is developed that allows direct patterning of transition metal dichalcogenides (TMDs) with thickness controllability at the wafer scale through intergranular diffusion‐assisted liquid‐phase chemical vapor deposition using a sacrificial metal layer. By depositing a liquid‐phase transition metal precursor onto the pre‐patterned polycrystalline Ni/SiO2 substrate, a directly patterned transition metal layer can be formed on SiO2 via intergranular diffusion through the Ni grain boundaries, enabling the growth of patternable TMDs with a controllable thickness. The as‐synthesized directly patterned WS2 transistor exhibits typical n‐type transport behavior with a stable photoswitching performance. The proposed patterning technique can make the application of 2D semiconductors in advanced electronic devices more viable.

Improved electrochemical performance of B doped O'3-NaMnO2 for Na-ion battery

O'3-NaMnO2 possesses a high theoretical capacity due to its high initial sodium content as a cathode material in Na-ion batteries. However, complex phase transition from O'3 to P3, even to O1 phase caused by the gliding of layered structure occurred during Na extraction, leads to the collapse of the layered structure, and causes loss of battery capacity. To improve the structural stability of layered O'3-NaMnO2 cathode, the electrochemically inert B3+ with good affinity for oxygen was selected to substitute the Mn3+ in O'3-NaMnO2 and a series of O'3-NaBxMn1-xO2 (x=0, 0.05, 0.1, 0.15, 0.25) oxides were obtained by optimized preparation methods. The average effective radius of the transition-metal layer is reduced after B doping, making the structure more stable. At the same time, B inhibits the Jahn-Teller distortion caused by Mn3+. Among them, NaB0.1Mn0.9O2 shows improved cycle and rate performance. In-situ XRD and EIS results reveal that the enhanced electrochemical performance is resulted from the higher structure reversibility than O'3-NaMnO2.

Cation configuration in transition-metal layered oxides

<h2>Summary</h2> The cation configuration of transition-metal (TM) layered oxides used as cathode materials for batteries, which significantly affects their intrinsic performance, is extremely important for understanding their electrochemical behavior and remained unclear until now. Herein, the local cation configuration of a series of Mn-based Li-rich layered oxides (Mn-LLOs) was revealed thoroughly by statistical methods, and the influence of the cation configuration on the cationic/anionic redox was also elucidated. Using neutron and synchrotron X-ray pair distribution function combined with Monte Carlo simulation, the existence of Li₂MnO₃ rather than Li₂MnO₃-like crystal domain in all Mn-LLOs is revealed strongly, showing each Li atom in the TM layer is surrounded by six Mn atoms with no Ni atom to form a LiMn₆ unit, and the reversibility of oxygen redox is closely related with those local structure environments. This work provides insights for the cation configuration investigation of oxide materials and also sheds light on the understanding of the local structure-activity relationship.

An Undoped Tri‐Phase Coexistent Cathode Material for Sodium‐Ion Batteries

AbstractThe layered transition‐metal‐oxide materials are one type of promising cathode materials for sodium‐ion batteries, which typically include P2, P3, and O3 phases, and each has its own advantages and challenges. Taking into account the complementarity of each single‐phase structure, constructing the composite structures is an efficient pathway to stabilize the structure and improve the electrochemical performance. Herein, three composite cathode materials owning the phase structures of P3/P2, P2/O3, and P3/P2/O3 are prepared by changing the calcinating conditions without introducing any doping element. Among them, the composite of P3/P2/O3 shows the best capacity retention of 80 mAh g–1 after 200 cycles and the highest rate performance of 100 mAh g–1 at a current density of 750 mA g–1. The improved electrochemical performance can be attributed to the staggered arrangement of different phase structures and the gradient Na‐extraction/intercalation voltages of different phases. The slip of the transition metal layer is subjected to the constraint of the adjacent phase structure, thus inhibiting the phase transformation for capacity fading. This work provides an easy way for the preparation of composite cathode structures and brings a clear case for understanding the advantage of composite structures on electrochemical performance.

Complementary dual-doping of LiNi0.8Co0.1Mn0.1O2 cathode enhances ion-diffusion and stability for Li-ion batteries

The Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) layered cathodes endow Li-ion batteries (LIBs) with high energy density. However, they usually suffer from limited ion-diffusion and structural instability during cycling. Although doping strategy can effectively alleviate these issues, the coupling effects of multi-element doping and the corresponding performance enhancement mechanism have been yet unclear. Here, we report a Zr/Ti dual-doped NCM811 cathode material (ZT-NCM811), in which Zr-ion is doped into both transition metal (TM) layers and lithium layers and Ti-ion is only distributed in TM layers. The dual-doping can effectively enhance crystal structure stability via inhibiting the lattice collapse along c-axis and decreasing the Li/Ni disorder. Meantime, the lattice oxygen escape is also greatly reduced due to the presence of stronger Zr-O and Ti-O bonds, further mitigating the crystal surface parasitic reactions with electrolyte. The resultant ZT-NCM811 exhibits high specific capacity of 124 mAh/g at even 10 C, much higher than undoped and single-doped NCM811, and a retention of 98.8% at 1 C after 100 cycles. The assembled ZT-NCM811/graphite full cell also delivers superior battery performances and durability.

Enhanced electrochemical performance of O3-type NaNi0.5Mn0.3Co0.2O2 cathodes for sodium-ion batteries via Al-doping

O3-type NaNi0.5Mn0.3Co0.2O2 based positive electrode materials are very promising for sodium-ion batteries. However, the irreversible phase transition due to structural deformation leads to sluggish kinetics, rapid capacity fade, and low C-rate performance, limiting its wide practical applications. The partial substitution of Co3+ (0.545 Å) by Al3+ (0.535 Å) ions in the transition-metal layer in NaNi0.5Mn0.3Co0.2-xAlxO2 (0.01 ≤ x ≤ 0.02) is an effective strategy to address the issue of structural deformation and thus to improve the electrochemical performance of NaNi0.5Mn0.3Co0.2O2 cathode. Solution combustion synthesis of NaNi0.5Mn0.3Co0.2-xAlxO2 (x = 0.01, 0.02) shows O3-type structure of NaNi0.5Mn0.3Co0.2−xAlxO2 material with the space group of R3¯m. The composition with an overall x = 0.02 Al doping delivers an initial 120 mAh g−1 capacity at a 0.1 C rate. It retains 90 % capacity even after 200 cycles than the other stoichiometric aluminum substitution, x = 0.01 (77 %). Moreover, the NaNi0.5Mn0.3Co0.18Al0.02O2 shows a good capacity of ~ 83 mAh g−1 even at a high C-rate of 5 C, almost 70 % of the initial capacity at the 0.1 C rate. The Al-substitute NaNi0.5Mn0.3Co0.2−xAlxO2 cathode's electrochemical performance is attributed to the enhancement in the structural stability of the sodium layered transition metal oxide after the partial substitution of Co3+ by Al3+ ion. Finally, the practical sodium-ion full cells are realized using a hard carbon-based anode and NaNi0.5Mn0.3Co0.18Al0.02O2 cathode, showing 91 % capacity retention at the end of 100th cycles with an OCV of 3.5 V.

Synthesis and characterization of Nickel-rich layered LiNi1-xMnxO2 (x=0.02,0.05) cathodes for lithium-ion batteries

In this paper, LiNi0.98Mn0.02O2 and LiNi0.95Mn0.05O2 binary materials were synthesized by chemical co-precipitation method. The phase characterization and electrochemical performance analysis of Ni-Mn binary materials were carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electrochemical tests, and the action of Mn element on the material was studied. XRD analysis shows that LiNi0.95Mn0.05O2 material has the better layered structure (1.47 of the (003)/(104) peak intensity ratio), and the lower Li-Ni mixing ratio (1.98%). The initial discharge capacity of LiNi0.95Mn0.05O2 material at 1 C is 189.6 mAh/g, and the capacity retention rate is 89.93% after 100 cycles at 1 C, which is much higher than LiNiO2 of 177.4 mAh/g and 60.26%. In addition, the initial discharge specific capacity of LiNi0.95Mn0.05O2 can reach 205.8 mAh/g at 0.5 C under the high temperature of 50 °C, and the capacity retention rate is 78.7% at 0.5 C for 100 cycles, while the discharge specific capacity of LiNiO2 is 191.1 mAh/g at 0.5 C and the capacity retention rate is only 63.9%. Excellent electrochemical properties of Ni-Mn materials than LiNiO2 at room and high temperature is because Mn4+ without electrochemical activity is stabilized in the transition metal layer during the charge and discharge process of the materials, alleviating the structural collapse caused by the Li/Ni mixing. By comparing the cross-section of the materials before and after 100 cycles at 1 C, LiNi0.95Mn0.05O2 has fewer microcracks due to the high reversible phase transformation of H2-H3 according to the analysis of the dQ/dV versus voltage curves. Meanwhile, the stable layered structure and lithium-ion diffusion channel also make LiNi0.95Mn0.05O2 has the better lithium-ion conductivity.

Layered Oxide Cathodes Promoted by Crystal Regulation Strategies for Potassium-Ion Batteries.

Layered oxide cathodes have demonstrated great potential for potassium-ion batteries (PIBs) on account of high reversible capacity, appropriate diffusion paths, and low cost. However, their electrochemical performance in PIBs is generally worse than that in lithium-ion batteries due to large structural changes and deformations during charging and discharging. To improve their potassium storage performance, a series of strategies have been developed in recent studies. In this review, we summarize the latest advancements in layered oxide cathodes for PIBs through different crystal regulation strategies, including transition metal layer doping, potassium content optimization, oxygen partial substitution, functional morphology construction and air stability improvement. Meanwhile, the relationship between the electrochemical properties and structural evolution of these modified cathodes is also investigated. In addition, the challenges and prospects of these layered oxide cathodes in PIBs are analyzed in detail, providing constructive insights for future applications of PIBs.

Controllable carrier concentration of two-dimensional TMDs by forming transition-metal suboxide layer for photoelectric devices

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have the potential to drive future innovation in the electronics industry. However, the controllability of charge concentration remains challenging due to the atomically thin channel, which perturbs the charge transport characteristics of nanodevices. Here, we demonstrate a strategy that uses the transition metal suboxide layer to modulate the photoelectrical characteristics of TMD channels. The carrier concentration in an n-type MoS2 channel is reduced from 2.05 × 1012 to 6.15 × 1010 cm−3, while it increases from 1.71 × 1010 to 2.76 × 1012 cm−3 for p-type WSe2 channels. Remarkably, the channel mobility remains unchanged or even slightly improves when the carrier concentration is appropriately tuned. Also, the homogenous channel is modulated into a photovoltaic homojunction with a tenfold enhancement of photoelectrical detectivity and response speed. The controllable strategy provides a simple design principle to realize high-performance 2D semiconductor-based optoelectronic and logic devices.

Simple and Scalable Ball Milling Synthesis of MoS2/Graphite Heterostructures with High Rate Capability for Li-Ion Batteries

With the constant rise of consumer demands for energy storage devices, improvements in current Li-ion battery (LIB) technology are paramount. At present, graphite is the most common commercial anode material, with a theoretical capacity of 372 mA h g-1. However, it is inevitable that graphite alone will no longer suffice to meet large capacity and high power requirements.Transition metal dichalcogenides (TMDs) are isostructural analogues of graphite where a transition metal layer is sandwiched between two chalcogen layers by strong intraplanar bonds while each unit is stacked together through weak van der Waal’s interplanar forces. MoS2 is a TMD which has also been considered as a potential candidate for LIB anodes as its theoretical capacity (670 mA h g-1) is almost double that of graphite. Unfortunately, the anode’s practical performance is very poor due to large volume expansion upon charging/discharging and formation of undesirable polysulphides.Combining MoS2 and graphite could generate an alternating layer heterostructure that exploits the desirable properties of each material. Most syntheses of MoS2/Graphite heterostructures in the literature either require extensive reaction conditions such as long periods of ultrasonication, or produce very low yields such as chemical vapour deposition. Additionally, ultrasonication often introduces many undesirable defects, due to the solvent used and harsh conditions. Alternatively, dry ball milling is a scalable and economically viable technique for producing MoS2/Graphite heterostructures. The process uses mechanical forces exerted by balls to strip layered materials through their weaker van der Waal’s bonding. To date, the only MoS2/Graphite composite synthesised with this method (though under Ar) for Li-ion batteries, was ball milled for 40 hrs and exhibited a practical capacity of ~700 mA h g-1 at 100 mA g-1 current density for 100 cycles.(1)This work uses simple dry ball milling under atmospheric conditions for 12 hrs to synthesise a MoS2/Graphite composite which exhibits ~350 mA h g-1 at a high current density of 1 A g-1. Additionally, this work shows that dry ball milling MoS2 alone for 12 hrs produces a phase transformation to a conductive 1T MoS2 product. Raman spectroscopy, XRD, XPS, SEM and impedance spectroscopy techniques were used to characterise the materials. By combining MoS2 and graphite with this simple and scalable method, we provide a viable solution to meet rising consumer demands of improved energy storage devices.(1) Zhao H, Zeng H, Wu Y, Zhang S, Li B, Huang Y. Facile scalable synthesis and superior lithium storage performance of ball-milled MoS2-graphite nanocomposites. J Mater Chem A. 2015;3(19):10466–70.

Enhanced cyclic performance of O2-type Mn-based layered oxide via Al doping for lithium-ion battery

The layered lithium nickel manganese oxide Li y Ni x Mn 1−x O 2 cathode materials with O2 structure exhibit good cyclic stability owing to their special arrangement of oxygen layers. However, the O2-type oxides generally present limited discharge capacity because of Li + ions defects in the lithium layer (as in Li y Ni x Mn 1−x O 2 , y < 1). Herein, an O2-type layered lithium nickel manganese oxide LiNi 0.33 Mn 0.65 Al 0.02 O 2 with high capacity and good capacity retention is obtained by introducing lithium and aluminum into the transition metal layer. The results of structural characterization and performance tests show that the optimal amount of Li + and Al 3+ in the transition metal layer effectively inhibits the disordered migration of transition metal ions and increases the electrochemical stability. The LiNi 0.33 Mn 0.65 Al 0.02 O 2 cathode delivers an initial discharge capacity of 153 mAh g −1 and the capacity only attenuates 13% after 100 cycles at the current density of 200 mA g −1 , which are higher than that of undoped oxide LiNi 0.33 Mn 0.67 O 2 . • O2-type Mn-based oxides with Li and Al in transition metal layer were synthesized. • Al doping amount has important effect on the performance of LiNi 0.33 Mn 0.67−x Al x O 2 . • LiNi 0.33 Mn 0.65 Al 0.02 O 2 has high stability and high initial capacity at 200 mA g −1 .

Controlling Iron Versus Oxygen Redox in the Layered Cathode Na0.67Fe0.5Mn0.5O2: Mitigating Voltage and Capacity Fade by Mg Substitution

AbstractLayered oxides for Na‐ion batteries containing Fe have attracted strong interest mainly due to their low cost. However, full oxidation of Fe3+ to Fe4+ is rarely seen before O‐redox sets in and is typically accompanied by voltage and capacity fade on cycling. On charging P2‐Na0.67[Fe0.5Mn0.5]O2, Fe3+ is oxidized to only ≈Fe3.3+ before the onset of O‐redox. O‐redox occurs when the Na content is sufficiently low (Na ≈0.3) to permit the transition from P‐type to O‐type stacking, thus enabling Fe3+ migration to the Na layer. Fe3+ migration generates cation vacancies in the transition metal layer, forming □‐O‐□ configurations, which trigger the onset of O‐redox. In contrast, doping this material with Mg2+ to form P2‐Na0.67[Fe0.25Mn0.6Mg0.15]O2 allows full oxidation of Fe3+ to Fe4+ before the Na content is low enough to favor O‐type stacking. During O‐redox, Mg2+ is displaced into the Na layers instead of Fe. Mg substitution enables greater reversibility of the Fe3+/Fe4+ redox couple and significantly suppresses Fe migration, which is responsible for the voltage and capacity fade observed for P2‐Na0.67Fe0.5Mn0.5O2.

Triggering and Stabilizing Oxygen Redox Chemistry in Layered Li[Na1/3Ru2/3]O2 Enabled by Stable Li–O–Na Configuration

Li-rich layered oxides are promising cathode candidates for high-energy-density Li-ion batteries because of the combination of cationic and anionic redox activities. However, severe lattice oxygen loss inevitably induces irreversible Li migration in both transition metal (TM) and Li layers, which degrades the stability of the Li–O–Li configuration, resulting in serious structure distortion and capacity decay. Herein, a Li–O–Na configuration was first introduced in Li-based layered oxides by Na substitution within TM layers, which not only obtained a discharge capacity of 232 mAh g–1 boosted by oxygen redox reactions but also achieved superior structural and electrochemical stability upon cycling (capacity retention of 83.2% after 1000 cycles). Moreover, comprehensive analyses of anionic/cationic redox activities clarify the underlying charge compensation mechanisms upon cycling. Altogether, these findings demonstrated that the Li–O–Na configuration can be regarded as an effective structure to achieve stable oxygen redox chemistry within Li-based cathode materials.

Cation Mixing and Capacity Loss in Li||Ni0.6Mn0.2Co0.2O2 Cells: Experimental Investigation and Application of the Multi-Site, Multi-Reaction Model

We clarify the degradation phenomena in a pouch cell that contains an insertion electrode (LizNi0.6Mn0.2Co0.2O2 or lithated NMC622) and a Li counter electrode. Greater than 500 cycles have been achieved in these cells employing 4 mAh/cm2 for both the initial Li metal negative and the NMC622 positive, and we find that cation mixing within the NMC622 is prevalent. That is, transition metals (Ni, and to a lesser extent, Mn and Co) in the transition-metal layer of NMC622 irreversibly exchange places with Li in the Li layer of NMC622, corresponding to a loss of Li sites and a concomitant loss of Coulombic capacity. The use of 1) a perturbation procedure of a recent publication employing the multi-site, multi-reaction model for the porous positive electrode and 2) a procedure to average the degradation phenomena over each cycle, which is shown to be consistent with slow degradation, simplifies the analysis of the experimental data and enables straightforward parameter regression. The resulting agreement between the model calculations and the experimental data is quite good, with the differences being similar to experiment-to-experiment variation.

The enhancement of cyclability of Ni‐rich LiNi 0 . 9 Co 0 . 05−x Mn 0 . 05 Zn x cathode materials by the substitution of Zn for Co

In the development of LiNixCoyMnzO2 (NCM) cathode active materials in lithium-ion batteries (LIBs) for electric vehicles, it is widely accepted that Ni-rich and Co-free NCM is the best candidate for the promotion of electric vehicle deployment. Inspired by the fact that Zn is an element that can be incorporated in regenerated NCM in the LIB recycling, Zn is doped into NCM from an impurity level in the leachate from spent LIBs to a level where Co is completely replaced by Zn. LiNi0.9Co0.05Mn0.05O2 (NCM955), LiNi0.9Co0.0495Mn0.05Zn0.0005O2 (NCMZ-0.05), LiNi0.9Co0.045Mn0.05Zn0.005O2 (NCMZ-0.5), and LiNi0.9Mn0.05Zn0.05O2 (NMZ955) are synthesized by a co-precipitation method. It is confirmed that Zn, which has a similar radius to Li+ and Ni2+, is incorporated into both Li and transition metal layers. An optimum cation mixing on the Zn-doped NCM surface and the expanded NCM lattice due to Zn doping lead to improved cyclability and rate capability. For example, the optimized NCMZ-0.5 exhibits enhanced capacity retention of 91.7% compared to NCM955 (69.8%) after 80 cycles. The cyclability improvement of Zn-doped NCM is ascribed to better reversibility as well as lower overvoltage than NCM955. In particular, NMZ955 displays outstanding capacity retention in cycling tests with a wide voltage range, which implies that Co-free NMZ has a competitive edge in cyclability compared to NCM. This work not only elucidates the positive effect of Zn doping in regenerated NCM, but also further provides a practical approach to designing a Co-free Ni-rich cathode for higher energy density.

Improving the stability of P2-type NaMn2/3Ni1/3O2 via phasic intergrowth induced by Li-ion substitution

Layered transition metal oxides are attractive cathode materials for sodium-ion batteries but are largely hindered by the low capacity and poor cycling stability. Herein, Na is substituted by Li to enhance the capacity and stability of Na1-xLixMn2/3Ni1/3O2. It is found that Li substitution can provide robust structure benefiting from contracted adjacent transition metal layers owing to strong bonding property of O–Li–O and incremental inactive Mn4+. So it follows that noxious Mn3+ Jahn-Teller distortion and slabs slipping are mitigated. Also, the presence of “Li pillar” disturbs the Na+/vacancies and transition metal ordering during Na extraction, preventing phase transformation during deep desodiation process. Consequently, the Na1-xLixMn2/3Ni1/3O2 demonstrates high capacity, remarkable rate capability, and long-term cycling life. Among them, Na0.8Li0.2Mn2/3Ni1/3O2 cathode delivers a discharge capacity of 115.3 mA h/g, with a capacity retention of 86.0% after 100 cycles. This simple strategy to stabilize the layered structure oxides is inspiring to design high-performance cathode materials for sodium-ion batteries.