Aliovalent Doping Research Articles

Computational Investigation on the Role of Aliovalent Cation Dopants to the Na+ Ion Superionic Conductivity of Na3SbS4 Solid Electrolytes By Density Functional Theory Methods

Na+ ion batteries are regarded as one of the most promising alternative to Li+ ion batteries especially from the viewpoint of applications wherein the form factor is not a major issue, such as in large-scale smart grids and stationary back up power sources. Similar to all-solid-state Li+ ion batteries (Li-ASSBs), all-solid-state Na+ ion batteries (Na-ASSBs) with a solid electrolyte are considered to be highly promising for realizing high safety. Amongst the solid electrolyte types, sulfide-based ones are especially known to demonstrate high ionic conductivity owing to the high polarizability of sulfur anions. In addition, sulfide-based solid electrolyte materials can be prepared under mild processing conditions for crystallization. They are also highly elastic, enabling for a good interfacial contact with the electrodes and other components.Recently, a sulfide-type Na+ superionic conductor, Na2.88Sb0.88W0.12S4, was reported with an unprecedented conductivity on the order of 10-2 S/cm.1 Although it appears trivial that the W6+ aliovalent dopant cation directly modifies the Na vacancy concentration and thus the ionic conductivity, it is still unclear why the it reached a one-order-of-magnitude enhancement. This in contrast to other reported aliovalent doping strategies which used other dopant elements but failed to obtain such conductivity improvement. In this work, we employed density functional theory methods (e.g., geometry optimization, molecular dynamics) in order to study the role of aliovalent cation dopants (e.g., W6+, Mo6+) in Na3SbS4 for realizing superionic ionic conductivity.2 It was determined that the transport process in Na3SbS4 is governed by a concerted migration behavior of Na+ ions. The modulation of Na vacancy concentration by cation doping was confirmed to play a role, but it cannot solely account for the observed order of magnitude rise in conductivity. To find the crucial factors, the pathway channels and phonon density of states were investigated together with the effects of doping to the jump frequency (conductivity prefactor) and activation energy (conductivity Boltzmann factor). In particular, correlations were analyzed between conductivity and the sizes of Na Wyckoff site cage and site-to-site jump bottleneck in the pathway channel. Our results provide new useful insights towards the rational design of solid electrolytes in general in ASSBs.This work was supported in part by MEXT as Elements Strategy Initiative (Grant Number JPMXP0112101003), the Program for Promoting Researches on the Supercomputer Fugaku (Fugaku Battery & Fuel Cell Project, Grant Number JPMXP1020200301), and the Materials Processing Science project (“Materealize”, Grant Number JPMXP0219207397). The work was also supported by JSPS KAKENHI (Grant Numbers JP19H05815 and JP19H05816). References A. Hayashi, N. Masuzawa, S. Yubuchi, F. Tsuji, C. Hotehama, A. Sakuda, M. Tatsumisago, Nat. Commun. 2019, 10, 5266. R. Jalem, A. Hayashi, F. Tsuji, A. Sakuda, Y. Tateyama, Chem. Mater. 2020, 32, 8373-8381.

Intrinsic vacancy suppression and band convergence to enhance thermoelectric performance of (Ge, Bi, Sb)Te crystals

GeTe-based alloys have drawn increasing attention because of the excellent thermoelectric (TE) performance and potential applications in mid-temperature range. Although reducing carrier concentration by aliovalent doping plays an important role in maximizing the TE performance of GeTe alloys, modulating the intrinsic Ge vacancies is also indispensable. In this work, the intrinsic Ge vacancies of GeTe crystals were suppressed by the synthesis process of temperature gradient cooling technique (TGCT) and Bi-Sb co-doping. The later aliovalent doping also derives a facilitated band convergence, enhanced density-of-state effective mass, and maintained carrier mobility with optimized carrier concentration, along with enhanced phonon scattering from the fluctuations of mass and strain. Benefiting from enhanced electrical properties and reduced thermal conductivity, Ge0.91Bi0.06Sb0.03Te achieved a maximum ZT of 1.9 at 753 K, together with a comparable average ZT of 1.14 at 300–773 K. The results reveal the potential of the TGCT fabrication and Bi-Sb co-doping on enhancing TE performance of GeTe-based alloys with a strengthened suppression of Ge vacancies.

Tunable Band-Edge Potentials and Charge Storage in Colloidal Tin-Doped Indium Oxide (ITO) Nanocrystals.

Degenerately doped metal-oxide nanocrystals (NCs) show localized surface plasmon resonances (LSPRs) that are tunable via their tunable excess charge-carrier densities. Modulation of excess charge carriers has also been used to control magnetism in colloidal doped metal-oxide NCs. The addition of excess delocalized conduction-band (CB) electrons can be achieved through aliovalent doping or by postsynthetic techniques such as electrochemistry or photodoping. Here, we examine the influence of charge-compensating aliovalent dopants on the potentials of excess CB electrons in free-standing colloidal degenerately doped oxide NCs, both experimentally and through modeling. Taking Sn4+:In2O3 (ITO) NCs as a model system, we use spectroelectrochemical techniques to examine differences between aliovalent doping and photodoping. We demonstrate that whereas photodoping introduces excess CB electrons by raising the Fermi level relative to the CB edge, aliovalent impurity substitution introduces excess CB electrons by stabilizing the CB edge relative to an externally defined Fermi level. Significant differences are thus observed electrochemically between spectroscopically similar delocalized CB electrons compensated by aliovalent dopants and those compensated by surface cations (e.g., protons) during photodoping. Theoretical modeling illustrates the very different potentials that arise from charge compensation via aliovalent substitution and surface charge compensation. Spectroelectrochemical titrations allow the ITO NC band-edge stabilization as a function of Sn4+ doping to be quantified. Extremely large capacitances are observed in both In2O3 and ITO NCs, making these NCs attractive for reversible charge-storage applications.

Importance of Mixed-Conducting Materials for High-Temperature Fuel Cells Applications

In many electrochemical devices, mixed ionic/electronic conduction have received great attention. For instance, oxygen transport membrane (OTM) based on (Ba,Sr)(Co,Fe)O3 perovskite exhibit unique hole/oxygen ion mixed conduction behavior for oxygen separation application.Accordingly, electrodes/electrolytes exhibiting both electronic and/or multiple ionic conducting properties have been used for high temperature fuel cell application. For example, mixed conducting electrodes based on composite electrode (e.g. LSM/SDC, LSM/YSZ) show improved SOFC performance in comparison to conventional electrode. Such unique conduction behavior may be rationalized based on defect chemistry consideration. Thus, the materials processing and microstructure design may become key issues to optimize their mixed conduction properties. For instance, better hole and oxygen conducing perovskite may be obtained when desired aliovalent dopant and oxidizing atmosphere are used.Thus, the objective of this work is to conduct (1) fabrication/design of MIEC through defect-favored processing, (2) microstructure development for better gas transport, (3) characterization of conduction/interfacial kinetics for better electrodes/electrolytes performance. Desired analytical equipment such as SEM, XRD, electrochemical impedance spectroscopy (EIS) and cell testing are performed to conduct microstructural, structural and impedance analyses. The electrical conduction behavior of composite electrode/electrolytes will be discussed based on defect chemistry consideration and individual microstructure and morphology including pore size, pore distribution and interface area.

Strong in-plane anisotropy in the electronic properties of doped transition metal dichalcogenides exhibited in W1−xNbxS2

In this work, we study the electronic properties of monolayer transition metal dichalcogenide materials subjected to aliovalent doping, using Nb-doped WS2 as an exemplar. Scanning transmission electron microscopy imaging of the as-grown samples reveals an anisotropic Nb dopant distribution, prompting our investigation of anisotropy in electronic properties. Through electronic structure calculations on supercells representative of observed structures, we confirm that local Nb-atom distributions are consistent with energetic considerations, although kinetic processes occurring during sample growth must be invoked to explain the overall symmetry-breaking. We perform effective bandstructure and conductivity calculations on realistic models of the material that demonstrate that a high level of anisotropy can be expected in electronic properties including conductivity and mobility.

Review on the synthesis and doping strategies in enhancing the Na ion conductivity of Na3Zr2Si2PO12 (NASICON) based solid electrolytes

Na3Zr2Si2PO12 based solid electrolytes are well known materials because of their potential applications in different solid-state ionic devices. This review article presents the structural and transport results of Na3Zr2Si2PO12 based solid electrolytes prepared by different dry and wet methods known in the literature. The advantages and limitations of different synthesis methodologies to achieve the phase pure, high dense Na3Zr2Si2PO12 based materials are presented. The influence of the valence state, ionic radii of aliovalent dopants on the ionic conductivity of Na3Zr2Si2PO12 have been discussed with respect to their crystal structure. The role of sintering temperature, sintering durations, and sintering agents on the phase purity, density, and conductivity of Na3Zr2Si2PO12 based solid electrolytes are reviewed. Besides Na3Zr2Si2PO12 based polycrystalline solid electrolytes, the transport results of Na3Zr2Si2PO12 based grain boundary free glasses and flexible polymer electrolytes are also discussed. The applications of Na3Zr2Si2PO12 based solid electrolytes in different electrochemical devices are outlined.

Light-Activated Chemoresistive and Plasmonic-Resonant Optical Sensors for NO2 and H2 Sensing Based on ZnO Doped Nanoparticles

Light-Activated Chemoresistive and Plasmonic-Resonant Optical Sensors for NO2 and H2 Sensing Based on ZnO Doped Nanoparticles

ZnO thin films containing aliovalent ions for NO2 gas sensor activated by visible light

In this paper we report on the combined visible-light (red λ = 630 nm; green λ = 570 nm; purple-blue λ = 430 nm) and mild thermal-activation modes (25 °C–100 °C) towards ppb-level NO2 detection of ZnO nanocrystalline film containing trivalent (Al-Ga) and tetravalent (Si-Ge) ions. Both trivalent and tetravalent doping trigger visible light fast response/recovery behavior as respect to dark conditions. Remarkably Purple-blue light (λ = 430 nm) combined with thermal activation at 75 °C OT results in best performances in terms of variation of sensor resistance as compared to that in dry air, topping maximum gas relative response RR= RGas/RAir ≈ 20 for the 5% Si doped ZnO at 400 ppb NO2. Aliovalent doping of ZnO is obtained using a colloidal heat-up synthesis yielding highly transparent in the visible range degenerately doped n-type ZnO semiconductors films which were characterized by SEM, XRD, XPS techniques to assess morphology, crystal phases and concentration of surface oxygen vacancies, respectively. Optical characterization of the doped films showed a distinctive localized surface plasmon resonance (LSPR) peak in the near infrared region, deriving from the free charge carrier. Aluminum doping is the most effective to narrow the band gap of the doped ZnO films down to 3.22 eV. The influence of the relative humidity (RH) on the gas sensing performances is investigated and for 40% RH a slight decrease of the NO2 relative response was observed. A consistent model explaining the gas sensing mechanism under dark and light conditions and the influence of RH is presented.

Aliovalent Doping Engineering for A- and B-Sites with Multiple Regulatory Mechanisms: A Strategy to Improve Energy Storage Properties of Sr0.7Bi0.2TiO3-Based Lead-Free Relaxor Ferroelectric Ceramics.

Sr0.7Bi0.2TiO3 (SBT) is a promising pulse energy storage material due to minor hysteresis, but its low maximum polarization (Pmax) is bad for energy storage. K+-Bi3+ defect pairs were introduced into the A-site of SBT to obtain Sr0.35Bi0.35K0.25TiO3 (SBKT) with larger Pmax. Through first-principles calculations, we determined that the introduction of defect pairs destroys the paraelectric order phase and increases local polarization, resulting in more and larger polar nanoregion (PNR) formation. On this basis, doping NaNbO3 (NN) in A- and B-sites of SBKT increases the cationic disorder and ferroelectric destabilization, further destroying the long-range order structure and forming more PNRs with smaller sizes. This enhances relaxation and decreases remnant polarization, and the broadened dielectric peak enables 0.85SBKT-0.15NN to meet the X7R specification. Furthermore, the decreased grain size and oxygen vacancy, increased thermal conductivity, and weakened local electric field (simulated by COMSOL) increase the dielectric breakdown strength (BDS). As a result, 0.95SBKT-0.05NN exhibits a high energy storage density (W) of 2.45 J/cm3 with a high efficiency of 93.1%, a high pulsed discharge energy density of 2.1 J/cm3, and a high power density of 54.1 MW/cm3 at 220 kV/cm. The energy storage properties show excellent stability of temperature (-55 to 150 °C), frequency (10-500 Hz), and cycling (105 cycles). Notably, for the pulse charge-discharge properties, 0.95SBKT-0.05NN shows great fatigue resistance during 105 cycles under 25 and 150 °C, accompanied by excellent thermal stability. Moreover, the BDS and Pmax of 0.95SBKT-0.05NN sintered in O2 further enhance. A higher W of 2.92 J/cm3 with a high efficiency of 89% at 250 kV/cm is achieved. Therefore, 0.95SBKT-0.05NN shows great application potential for pulse energy storage. In this work, we provide a novel strategy and systematic in-depth study for improving the energy storage properties of SBT.

Unusual Role of Point Defects in Perovskite Nickelate Electrocatalysts.

Low-cost transition-metal oxide is regarded as a promising electrocatalyst family for an oxygen evolution reaction (OER). The classic design principle for an oxide electrocatalyst believes that point defect engineering, such as oxygen vacancies (VO..) or heteroatom doping, offers the opportunities to manipulate the electronic structure of material toward optimal OER activity. Oppositely, in this work, we discover a counterintuitive phenomenon that both VO.. and an aliovalent dopant (i.e., proton (H+)) in perovskite nickelate (i.e., NdNiO3 (NNO)) have a considerably detrimental effect on intrinsic OER performance. Detailed characterizations unveil that the introduction of these point defects leads to a decrease in the oxidative state of Ni and weakens Ni-O orbital hybridization, which triggers the local electron-electron correlation and a more insulating state. Evidenced by first-principles calculation using the density functional theory (DFT) method, the OER on nickelate electrocatalysts follows the lattice oxygen mechanism (LOM). The incorporation of point defect increases the energy barrier of transformation from OO*(VO) to OH*(VO) intermediates, which is regarded as the rate-determining step (RDS). This work offers a new and significant perspective of the role that lattice defects play in the OER process.

Isovalent vs. aliovalent transition metal doping of zinc oxide lithium-ion battery anodes — in-depth investigation by ex situ and operando X-ray absorption spectroscopy

The precise determination of de-/lithiation mechanisms in alternative lithium-ion battery electrode materials is crucial for their potential future success, but quite challenging — e.g., due to the occurrence of multiple crystalline and (frequently) amorphous phases. Herein, we report an in-depth ex situ/operando characterization of (carbon-coated) Fe- and Co-doped zinc oxide anodes via X-ray absorption spectroscopy to probe the oxidation state and local structural environment of the different metals upon de-/lithiation. The results provide fundamental insights into the mechanism of the conversion and alloying reaction taking place for these two active materials. In addition, this comparative investigation allows for an evaluation of the impact of isovalent (Co2+) and aliovalent (Fe3+) doping on the lithiation mechanism, having an impact on the initial lithiation kinetics, while both dopants generally enable a greatly increased re-oxidation of zinc compared to pure zinc oxide and, thus, a more reversible conversion reaction.

Effects of aliovalent dopants in LaMnO3: Magnetic, structural and transport properties

In this paper, we have investigated the modification in structural, magnetic, transport, and magneto-transport properties of La0.95X0.05MnO3 polycrystalline samples prepared by the citrate combustion route with doping of different valence ions (X = Ce, Dy, Er, Sr, Na and La) as well as erbium-doped La1-xErxMnO3 samples (x = 0.0, 0.05, 0.1, and 0.3). X-ray diffraction patterns measured at room temperature confirmed the single-phase perovskite hexagonal structure with R-3c space group in all of these samples, excluding the 30% Er substituted sample. The grain sizes estimated from the SEM images were found to be in a range of 1.8–3 µm for all of the sintered samples (at 1300 °C/3h), and below 100 nm for the calcined powders (at 600 °C/3h). Curie temperatures of undoped and doped LaMnO3 powder samples were observed in the range of 230–255 K, significantly higher than the reported value (~140 K) for undoped samples. The maximum saturation magnetization (at 10 K) was obtained for the undoped sample (92 emu/g) and the largest coercivity (60 Oe) was found for the 5% Er-doped LaMnO3 sample. Temperature dependence of resistivity was measured in the absence and presence of the magnetic field (5 T), and the curves suggest a metal–insulator transition. The 5% Er-doped sample exhibited insulating behaviour while the higher Er concentration (10%) doping caused the metal–insulator transition. The rich and tuneable magnetic, transport and magneto-transport properties make these samples promising candidates for biosensors, memory devices, and other technological applications.

Examining the Effect of Dopant Ionic Radius on Plasmonic M:ZnO Nanocrystals (M = Al3+, Ga3+, In3+)

Understanding the role of dopant deactivation on plasmon frequency and extinction is important for the rational design of plasmonic semiconductor nanocrystals (PSNCs). Aliovalent dopants do not alw...

Revealing and excluding the root cause of the electronic conductivity in Mg-ion MgSc2Se4 solid electrolyte

MgSc2Se4 is a promising magnesium-ion solid electrolyte possessing a high room temperature Mg+2 conductivity (~ 10−4 S/cm). MgSc2Se4 also has demonstrated considerably high electronic conductivity (10−5÷10−8 S/cm). Such high electronic conductivity substantially restricts the applications of the material in all solid-state Mg-ion batteries. The efforts to lower the electronic conductivity include the enrichment of the material with selenium aliovalent doping, albeit without any tangible success. This suggests that the electronic conductivity may follow a model, which is different from a common model of charge transfer by electrons in a conduction band of the wide band gap semiconductor MgSc2Se4. Here, we provide an evidence that the electronic transport in the Mg-Sc-Se ceramic is facilitated via the Berthelot-type conductivity mechanism, suggesting an electronic transport through a low-conducting matrix stuffed with nano-scaled free electron-containing regions. These inclusions are well-distributed across the Mg-Sc-Se, and the electronic transport takes place via electron tunneling between these conductive regions, through low-conducting matrix spacers. It is shown that these conductive zones comprised of elemental metallic Sc and ScSe mix being a metallic-type electronic conductor. The nature of these conductive inclusions and the synthetic parameters controlling the appearance of such conductive areas are discussed, as well as routes to mitigate their formation.

Multiple Roles of Unconventional Heteroatom Dopants in Chalcogenide Thermoelectrics: The Influence of Nb on Transport and Defects in Bi2Te3.

Improvements in the thermoelectric performance of n-type Bi2Te3 materials to more closely match their p-type counterparts are critical to promote the continued development of bismuth telluride thermoelectric devices. Here the unconventional heteroatom dopant, niobium, has been employed as a donor in Bi2Te3. Nb substitutes for Bi in the rhombohedral Bi2Te3 structure and exhibits multiple roles in its modulation of electrical transport and defect-induced phonon scattering. The carrier concentration is significantly increased as electrons are afforded by aliovalent doping and formation of vacancies on the Te sites. In addition, incorporation of Nb in the pseudoternary Bi2-xNbxTe3-δ system increases the effective mass, m*, which is consistent with cases of "conventional" elemental doping in Bi2Te3. Lastly, inclusion of Nb induces both point and extended defects (tellurium vacancies and dislocations, respectively), enhancing phonon scattering and reducing the thermal conductivity. As a result, an optimum zT of 0.94 was achieved in n-type Bi0.92Nb0.08Te3 at 505 K, which is dramatically higher than an equivalent undoped Bi2Te3 sample. This study suggests not only that is Nb an exciting and novel electron dopant for the Bi2Te3 system but also that unconventional dopants might be utilized with similar effects in other chalcogenide thermoelectrics.

Heterovalent Substitution in Mixed Halide Perovskite Quantum Dots for Improved and Stable Photovoltaic Performance

One of the strategies to improve the device performance of metal-halide perovskite solar cells is to alter the electronic and optical properties of the perovskite lattice by metal ion doping. In this work, aliovalent doping of silver at the lead site of CsPbBr1.5I1.5 quantum dots (QDs) shows striking prospects in improving the photovoltaic (PV) device performance. Lattice doping could be achieved only up to ∼3.5 atom % Ag+ with respect to Pb2+ which has significant impact on the electronic and optical properties of the QDs. The band gap could be reduced from 2.15 eV for pristine QDs to 2.12 eV with 3.5 atom % Ag-doping, beyond which midband gap states are created along with the formation of a secondary AgI phase with higher Ag+ substitution. Density functional theory (DFT) shows enhanced optical absorption coefficient and p-type character of the Ag-doped QDs. With the QDs having 3.5 atom % Ag, a significant ∼20% enhancement in photoconversion efficiency (PCE) is observed up to 9.67 ± 0.12% due to the reduction of surface and intrinsic defects, decrease in nonradiative recombination leading to an increase in carrier lifetime, and increase in charge transfer. Although in general all-inorganic perovskite (AIPSK) QDs are attractive because of their higher crystallinity, better optical property, and easy solution processability, the solution stability of AIPSK QDs is not the same as that in PV devices. After 3.5 atom % Ag-doping, our PV devices show extremely promising ambient stability for 575 h (24 days) with less than 5% drop in PCE in comparison to 12% drop for pristine QD devices.

Synergizing aliovalent doping and interface in heterostructured NiV nitride@oxyhydroxide core-shell nanosheet arrays enables efficient oxygen evolution

Abstract An earth-abundant and highly efficient oxygen evolution reaction (OER) electrocatalyst has long been the holy grail in the entire energy conversion chain. Despite the considerable efforts in advancing non-precious-metal candidates by multiscale structural engineering, an adequate structural integration remains a significant challenge in achieving an efficient OER, largely bottlenecked by a low population of active sites and limited synergistic effect. Herein, we propose a synergistic strategy of effectively combining aliovalent doping and interface in the NiV nitride@oxyhydroxide (NiVN@OOH) heterostructured nanosheet arrays, successfully developed by in-situ electrochemical surface reconfiguration (ESR) from the core-shell nanostructured Ni3N@Ni3VN aiming for enabling OER kinetics. The thus-optimized NiVN@OOH with abundant core-shell interfaces, vertically aligned nanosheet arrays and purposely-chosen V-doping, demonstrates superior OER activity with an ultralow overpotential of 233 mV at the current density of 50 mA cmgeo−2, 64-fold rise in catalytic current density at 1.47 V vs. reversible hydrogen electrode (RHE) and 37-fold increase in turn-over frequency at an overpotential of 240 mV, over those of Ni3N@OOH, together with a robust long-term stability in 1 M KOH. Our DFT calculations further reveal that the synergistic effects of the aliovalent V-doping and interface engineering have boosted the intrinsic OER activity on adjacent oxygen active sites. The discovery in the present work provides a new paradigm of multiscale-controlled synergy for much enhanced electrocatalysis.

Engineering defect clusters in distorted NaMgF3 perovskite and their important roles in tuning the emission characteristics of Eu3+ dopant ion.

An attempt has been made to explore various new defect clusters in distorted NaMgF3 perovskite and their important role in tuning optical properties. We have tried to tailor the defect clusters and to understand the impact on the luminescence of the lanthanide, for example the Eu3+ ion. Defect engineering has been carried out by doping aliovalent dopant ions to create a charge imbalance in the matrix, which in turn led to the creation of various mono-, di- and new cluster vacancies. Such vacancies have been characterized by Electron Para-magnetic Resonance (EPR), Positron Annihilation Lifetime Spectroscopy (PALS) and Photoluminescence (PL) studies. The PALS data of both undoped and Eu3+ doped compounds confirmed that in addition to Mg mono vacancies, cluster vacancies with different configurations comprising Mg, Na and F atom vacancies also exist in the matrix. The PL study revealed that depending on the surrounding defect structure, three different types of Eu3+ components can be created. The position of the Eu3+ ion with respect to these cluster vacancies determines the respective emission profiles and the decay kinetics. It has been found that when Li+ ions are co-doped with Eu3+, there is a sudden change in the decay kinetics and the emission profiles. The PALS study revealed that Li+ co-doping modified the configuration of the vacancy clusters, which in turn changes the emission characteristics. The EPR study confirmed the presence of different types of F-centers (F, F2, etc.) which are responsible for the host emission. Overall, this new study will be very helpful for a detailed understanding of the defect structures, in particular the cluster vacancies in distorted NaMgF3 perovskite, which have a direct or indirect impact on many physical properties.

(Battery Division Technology Award Address) Exploring Stable Manganese-Based Oxides As Cathodes for Na-Ion Batteries

Sodium ion batteries (SIBs) promise the potential for large-scale grid storage, due to the high abundance and wide distribution of Na sources as compared to their lithium counterparts.1-3 Among various sodium storage cathodes, layered sodium transition metal oxides (NaxMnO2) have gained significant attention owing to their great variety of compositions, ease of scalable preparation and high reversible specific capacity. In the last several years, we have utilized different strategies such as alivovalent doping, ALD coating and controlled thermal treatments to synthesize several series of P2-type Mn-based cathodes materials and obtained promising results for future development[2-5]. For example, we propose an effective and synergetic strategy in combination of liquid N2 quenching and aliovalent doping to get new layered cathode materials [5]. As evidenced by X-ray diffraction, neutron diffraction and solid-state 23Na nuclear magnetic resonance techniques, the results reveal the significance of manipulating transition metal ions vacancies and valences to the electrochemical performance of P2-type Mn-based materials. Our results demonstrate that such a new structure significantly enhances the deliverable capacity, Na+ mobility and electronic conductivity of the materials. Furthermore, the effects of aliovalent doping elements and cooling approach on the long-range structure, local environment and electrochemical performance are comprehensively studied by comparing 15 sets of Na0.67MxMn1-xO2 materials (M = Al, Fe, Mg, Li ). The optimized Na0.67Al0.1Fe0.05Mn0.85O2 material exhibits a remarkably high initial capacity of 202 mAh g-1 among ever reported P2-type layered oxides within 2-4 V, a stable capacity retention of 81% after 600 cycles and outstanding rate capability of the specific capacity up to 122 mAh g-1 at 1200 mA g-1. In addition, some work about the air-stability of these oxides materials will be also presented.

Fast Lithium Ion Conductivity in the Solid Solution Li8+X Al X Ge1-X P4 (0≤X≤1) By Aliovalent Substitution

All solid-state batteries are considered to extend the limits of lithium ion batteries in terms of power density, cycle life stability and device safety. One key component for this technology is the solid electrolyte. Heavy research on oxide and sulfide based lithium ion conductors has brought out some highly optimized systems, which show ionic conductivities up to 25 mScm-1.[1] Recently lithium phosphides have been introduced as a new class of lithium ion conductors with ionic conductivities of 1.1 mScm-1 for Li14SiP6 [2] and 3 mScm-1 for Li9AlP4. [3] Due to the increased negative charge of the phosphide anion compared to conventionally used chalcogenide anions, the lithium content and therefore the charge carrier concentration is significantly higher. Since aliovalent doping can lead in sulfides to an ionic conductivity an order of magnitude higher than the parent undoped compounds, we aim here for the stepwise aliovalent substitution of the four-valent tetrel-ion in lithium phosphido-tetrelates with three-valent aluminum, to further increase the lithium concentration as well as the volume of the unit cell.[5][6] In this work we show the effect of the substitution of germanium by aluminum on the ionic conductivity in the solid solution Li8+x Al x Ge1-x P4 (0≤x≤1). β-Li8GeP4,[4] Li9AlP4 and the solid solution Li8+x Al x Ge1-x P4 crystallize in the cubic space group P-43n with the phosphorus atoms building a distorted fcc-lattice. The solid solution strictly follows Vegard’s law showing a linear dependency of the lattice parameter on the substitution grade over the whole investigated range (figure 1). Ge and Al atoms occupy 1/8 of the tetrahedral voids. Remaining tetrahedral voids and some octahedral voids are partially occupied by lithium. The cubic structure makes the system a convenient model for investigations of substitution effects, as symmetry restrictions prohibit extent distortion. Lithium ions are mobile and migrate between face sharing octahedral and tetrahedral positions as well as edge sharing tetrahedral voids. Impedance spectroscopy measurements were conducted in a custom-made cell setup for powder samples, where powders are pelletized in situ and contacted by steel electrodes. High pressure is applied over six screws, tightened with defined torque. The temperature was controlled by a specially designed heating block connected to a heating circulator, which allows to do all measurements in argon atmosphere. The ionic conductivity in Li8+x Al x Ge1-x P4 increases with the amount x of aluminum by nearly one order of magnitude. This increase does not proceed linearly, revealing a more complex correlation (figure 1). Figure 1: Left: Vegard plot for the solid solution Li8+x Al x Ge1-x P4 (0≤x≤1) created on powder X-ray diffraction data. The black line acts as a guide to the eye, emphasizing the linear dependency of the lattice parameter a on the grade of substitution x. The upper left corner shows the crystal structure of Li8+x Al x Ge1-x P4. Right: Temperature dependency of the ionic conductivity for different compositions of Li8+x Al x Ge1-x P4.