High Voltage Region Research Articles

A sustainable LiFePO4/graphite hybrid cathode capable of stepwise cation and anion storage

Dual ion battery (DIB) with graphite cathode is a renewable energy storage system but suffers from low capacity. Addition of lithium ion battery (LIB) cathode to graphite cathode not only facilitates capacity increment but also serves as proof-of-concept for LIB-DIB hybridization. In this study, a hybrid cathode, incorporating a physical mixture of high working potential graphite and nontoxic lithium iron phosphate (LiFePO4, LFP), is proposed and examined. The graphite interacts with anion and works as electrically conductive matrix. While additive LFP is active to cation, serving as capacity facilitator. Characterization results reveal that, during charging, the hybrid cathode starts with LFP delithiation in lower voltage region and finishes with anion intercalation in graphite at higher voltage region. During discharging, processes are reversed with stepwise anion deintercalation and cation insertion, without interfering individual constituent's proper functioning. The transfer of two polarity distinct ions inside the hybrid cathode maintains high independence, without any added kinetical difficulties. Such hybrid cathode is demonstrated to work stably with varied LFP-graphite proportions. The addition of 40 wt% LFP to graphite cathode shows doubled capacity and excellent long-term cyclability. And the hybrid cathode is also cyclable in a full cell configuration.

Tuning oxygen redox chemistry of P2-type manganese-based oxide cathode via dual Cu and Co substitution for sodium-ion batteries

High-voltage layered Mn-based oxide is promising high-capacity cathode via exploiting the oxygen redox chemistry. But such redox often suffer from irreversible oxygen loss, leading to severe voltage hysteresis, particle cracking and capacity decay. Hence, the complementary strategy of dual Cu and Co substitution is revealed to stabilizing the anion redox chemistry of P2-Na2/3(Mn-Ni-Cu-Co)O2 cathode under upper cut-off voltage. Dual substitution with Cu for Mn and Co for Ni forms stable Cu-O and Co-O octahedrons, which modulate the lattice structure with the shrinkage of TMO2 sheets and expansion of Na layer spacing to enhance the Na+ diffusion kinetics. Co substitution can raise high voltage region to tune the cut-off voltage up to 4.3 V and activate oxygen redox to weaken irreversible O2 loss. And Cu substitution can inhibit the O2 loss and suppress the voltage decay via the interaction between Cu 3d and O 2p orbitals. The P2-type Na0.67Mn0.6Ni0.2Cu0.1Co0.1O2 cathode induced by the complementary effect can boost long cycling life (82.07% capacity retention after 500 cycles) and remarkable rate capability under high upper cut-off voltage in half-cell. Furthermore, the full cells also exhibit outstanding rate capability with a high discharge capacity of 62.6 mAh g–1 at 20 C.

β-LiVP2O7 as a positive electrode material for Li-ion batteries

A novel polymorph of lithium vanadium pyrophosphate has been obtained using electrochemical Na+ for Li+ exchange in β-NaVP2O7. β-LiVP2O7 has a triclinic KAlP2O7-type structure with large tunnels along the [100] direction and demonstrates electrochemical capacity of 100 mAh g−1 within a high-voltage region of 4.0-4.2 V vs. Li/Li+ with excellent C-rate capability: increasing discharge current density from C/10 to 10C results in a capacity drop of only 7%. The (de)intercalation process is characterized by a solid solution-like behavior with the diffusion coefficient amounting to 10−12 – 10−11 cm2 s−1. A redox process at ~ 1.8 V vs. Li/Li+ leads to 200 mAh g−1 of the overall intercalation capacity within the extended voltage range that corresponds to “β-VP2O7 ↔ β-Li2VP2O7” two-electron process.

(Invited) Anionic Redox Chemistry in Layered Oxide Cathodes

Li-ion battery remains as the critical energy storage device and starts to invade the large electrical vehicle (EV) market. Extensive efforts have been devoted to developing advanced layered oxide cathodes, including both stoichiometric and Li-rich layered oxides, for Li-ion batteries of even better performance. For all these cathodes, there is a clear need for the high-voltage cycling to obtain an even higher practical capacity. However, the redox reactions become more complex when charged at high voltages. Beyond the conventional transition metal (TM) redox, anionic oxygen becomes active and participates in the electrochemical reaction in these cathodes. The anionic oxygen activity involves both the reversible oxygen redox and irreversible oxygen loss accompanied by other byproducts resulting from the undesirable side reactions at high voltages.In the past, my group has been focusing on understanding the redox chemistry, including both cationic and anionic redox reactions, in both stoichiometric and Li-rich layered oxide cathodes, especially towards the high-voltage cycling. We specifically investigated the electronic states of TM and O using combined electrochemical, soft X-ray absorption (XAS), resonant inelastic X-ray spectroscopy (RIXS) and in situ differential electrochemical mass spectrometry (DEMS) in the high voltage region to elucidate the explicit redox chemistry in various layered oxide cathodes. This presentation will highlight a summary on the different TM and O redox behaviors in these technologically relevant cathode materials for a close comparison. Of particular interest is the fundamental understanding of how the anionic oxygen participates in the electrochemistry of layered oxides with the presence of different Li and TM stoichiometry.

Enabling superior rate capability and reliable sodium ion batteries by employing galvanostatic-potentiostatic operation mode

Hard carbon (HC) is one of the widely used anodes for Sodium-ion batteries (SIBs) thanks to their extensive resources and excellent stability. However, the commercial HC is still confronted with inferior rate capability and serious sodium precipitation after assembling the full battery with cathode. Herein, an efficient and feasible galvanostatic-potentiostatic operation mode is introduced to solve above issues. The sodium storage mechanism has been clarified by a series of advanced characterization techniques. The results indicate that Na-ions firstly adsorb on the oxygen functional group (CO) at high voltage region and then adsorption and filling of sodium co-exist at low voltage region in galvanostatic step. Finally filling of sodium in micropores occurs in potentiostatic step. The results demonstrate that potentiostatic step can enable more Na-ions filling in the pores with reduced electrochemical polarization and can reduce the risk of sodium precipitation as well. As a result, a high capacity of ~300 mAh g−1 at the current range of 0.1 C–5 C can be achieved for a HC-based half-cell, and at a capacity retention of 5 C from 58% to 73% (relative to 1 C) has been observed for pouch cell after introducing potentiostatic step, confirming the feasibility of proposed operation mode.

Structural analytical and electrical conduction mechanisms of 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine nanostructure films

The 2,7,12,17-tetra-tert-butyl 5,10,15,20-tetraaza-21,23H-porphine (TTBTP) films were prepared using thermal evaporation technique. The X-ray diffraction (XRD) of the powder showed that TTBTP is polycrystalline with a tetragonal system. Also, the analysis of XRD pattern shows that the TTBTP film of thickness 55 nm has a crystallite size of 24.69 nm which tends to increase with the film thickness. By SEM of 177 nm thick TTBTP film, the topography of the surface is characterized by significant spherical granules. The mean diameter was estimated to be 250 nm for the spherical granules. It was observed that the conductivity increases with increasing the film thickness. Also, the TTBTP is a semiconductor film with thermally activated conduction mechanisms. The current–density–voltage (J–V) characteristics showed Ohmic drives in low voltage, whereas the space charge limited conductivity mechanism is familiar in the high voltage region. The carrier mobility values of TTBTP film are relatively the same for other organic molecules. The 50 nm TTBTP film has a carrier mobility of 1.48 × 10− 9 cm2 V− 1 s− 1, which increases with increasing the film thickness.

Suppressing irreversible phase transition and enhancing electrochemical performance of Ni-rich layered cathode LiNi0.9Co0.05Mn0.05O2 by fluorine substitution

Ni-rich layered oxide LiNixCoyMn1-x-yO2 (x ≥ 0.8) is the most promising cathodes for future high energy automotive lithium-ion batteries. However, its application is hindered by the undesirable cycle stability, mainly due to the irreversible structure change at high voltage. Herein, we demonstrate that F substitution with the appropriate amount (1 at%) is capable for improve the electrochemical performance of LiNi0.9Co0.05Mn0.05O2 cathode significantly. It is revealed that F substitution can reduce cation mixing, stabilize the crystal structure and improve Li transport kinetics. The resulted LiNi0.9Co0.05Mn0.05O1.99F0.01 cathode can deliver a high capacity of 194.4 mAh g−1 with capacity retention of 95.5% after 100 cycles at 2C and 165.2 mAh g−1 at 5C. In-situ synchrotron X-ray technique proves that F ions in the cathode materials can suppress the irreversible phase transition from H2 phase to H3 phase in high voltage region by preventing oxygen gliding in a-b planes, ensuring a long-term cycle stability.

Cycling mechanism of Li2MnO3: Li–CO2 batteries and commonality on oxygen redox in cathode materials

Summary Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.

Whole-Voltage-Range Oxygen Redox in P2-Layered Cathode Materials for Sodium-Ion Batteries.

Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na0.7 Mg0.2 [Fe0.2 Mn0.6 □0.2 ]O2 cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "□-O-□", "Na-O-□" and "Mg-O-□" to trigger oxygen-redox in the whole voltage range of charge-discharge. Time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.

Tuning Sodium Occupancy Sites in P2‐Layered Cathode Material for Enhancing Electrochemical Performance

AbstractDifferent sodium occupancy sites in P2‐layered cathode materials can reorganize Na‐ion distribution and modify the Na+/vacancy superstructure, which have a vital impact on the Na‐ion transport and Na storage behavior during charge and discharge processes, but have not been investigated specifically and are not yet well understood. Herein, the occupancy ratio of two different Na sites (sites below transition metal ions and sites below oxygen ions along the c direction) in P2‐Na0.67[Mn0.66Ni0.33]O2 cathode is tuned successfully by inducing Sb5+ ions with strong repulsion toward Na sites right below transition metals. It is found that the decrease of Na occupancy right below transition metal ions is beneficial to the electrochemical performance of P2‐layered cathode materials, regarding cycle stability and rate capability. In situ X‐ray absorption spectroscopy reveals that the reversible Mn3.3+/Mn4+ and Ni2+/Ni3+ redox couples provide charge compensation in different voltage regions of 1.8–2.3 and 2.3–4.2 V, respectively. The transmission X‐ray microscopy confirms the uniform redox reaction over the whole electrode particle. In addition, Sb substitution can suppress the “P2‐O2” phase transition in high voltage region by preventing oxygen gliding in a–b planes, thus ensuring robust structure stability during cycling.

A P2/P3 composite-layered cathode material with low-voltage decay for sodium-ion batteries

Average discharge voltage of the P-type-layered oxides cathode materials is one of the key factors that determine the sodium-ion batteries (SIBs) performance, especially for the high energy density performance. Inspired by the strategy of lithium and magnesium doping in the transition metal layer to promote the anionic oxygen redox and the synergistic effect of composite materials for SIBs, we develop a layered P2/P3-Na0.75Li0.2Mg0.05Al0.05Mn0.7O2 composite cathode material enabled by the anionic oxygen redox. The synthesized composite delivers a reversible capacity of 180 mAh g−1 between 2.0 and 4.5 V at 0.2 C and keep ≈ 3.1 V stable average voltage above 2.5 V the discharge cutoff voltage. With a combination of the electrochemical performance, hard and soft X-ray absorption spectroscopy, we demonstrate that the irreversible anionic oxygen activity in the high-voltage region results in the capacity fading and the irreversible cationic manganese activity in the low-voltage region is responsible for the voltage decay. These findings provide a simple and effective method for designing low-voltage decay and high-energy density Na-deficient layered oxide composite.

Structure and reaction mechanism of binary Ni-Al oxides as materials for lithium-ion battery anodes.

A nanometer sized solid solution of NiO and Al2O3 was synthesized by calcination of Ni-Al layered double hydroxides (LDHs). The crystal structure of the obtained compound was determined by XRD and XAFS analyses: Ni2+ and Al3+ ions are located at the metal ion site of the rock salt structure and a certain amount of cation vacancies are also introduced for charge compensation. The electrochemical properties of the Ni-Al binary metal oxide as an anode material for lithium ion batteries were examined by the constant current charge-discharge test. Ni-Al oxide showed higher charging capacity in comparison with pristine NiO. In particular, the capacity in the lower voltage region (below 1.5 V), the limited capacity in this region is the weak point of the conversion anode, was improved to 540 mA h g-1 that is about twice that of pristine NiO. This improvement in the capacity in the lower voltage region is concluded to be due to the redox activity of Al3+ ions during the charge-discharge on the basis of the results of electrochemical measurements and ex situ XAFS measurements at the Ni and Al edge. The reaction mechanism of this compound is investigated using ex situ XRD and XAFS methods. For the charge (reduction) in the higher voltage region (OCV-1.0 V), lithium ion intercalation into the cation vacancy sites and/or lithium ion adsorption on the surface of particles are proceeding. For the charge in the lower voltage region (1.0-0.03 V), conversion reaction occurs by the reduction of Ni2+ and Al3+ ions to metal particles with surface electrolyte interface (SEI) layer formation. For the discharge in the lower voltage region (0.03-1.5 V), only Al metal particles are oxidized to Al3+ ions and some intermediate complexes are formed. For the discharge in the higher voltage region (1.5-3.0 V), the lattice of the Ni-Al binary oxide solid solution is reconstructed with the oxidation of Ni to Ni2+.

No-Snapback LDMOS Using Adaptive RESURF and Hybrid Source for Ideal SOA

A simple modification to the lateral DMOS is demonstrated, enabling a significant extension to the electrical safe operating region. This approach uses a novel Hybrid Source to suppress the parasitic bipolar, prevent snapback and enable operation at high drain voltage & current regions that have traditionally been inaccessible due to triggering of the parasitic bipolar. Trigger currents exceeding 10x that of conventional PN source devices under grounded gate, very fast TLP conditions have been achieved. This improvement does not compromise the basic DC parameters, such as specific on-resistance or breakdown voltage. This paper covers the device architecture, formation of the Hybrid Source, electrical performance, TCAD simulation and discussion of the mechanisms behind this new device and the improvements it enables.

Relaxation Analysis of LixNi0.8Co0.1Mn0.1O2 after Lithium Extraction to High-Voltage Region (x ≤ 0.12)

After the lithium extraction from LiNi0.8Co0.1Mn0.1O2 (NCM-811) to high-voltage region, structural variation has been investigated during relaxation process. When the lithium ions are extracted to x ≤ 0.12 for Li x Ni0.8Co0.1Mn0.1O2, H3 phase is mainly observed in addition to small amount of H2 phase. For highly lithium extraction up to x = 0.09 or 0.06, excess amount of H3 phase is created at the charging which turns into H2 at the relaxation. On the other hand, no significant variation is observed for the sample of x = 0.12. In comparison with LiNiO2, less amount of transformation from H3 to H2 occurs during the relaxation for NCM-811. For all the samples, Ni interlayer distance of H3 phase decreases with relaxation time, which is presumably due to the increase in valence of Ni for diminishment of lithium concentration in the remaining H3 phase.

Charge Compensation Mechanism of Li2MnO3 Cathode in All-Solid-State Thin Film Battery Investigated By Using Operando HAXPES

Lithium battery has been used as a power source for not only small devices but also large equipment like an electric vehicle because of its potentially high energy density and high rate performance. All-solid-state battery is a promising battery form to achieve high energy density. Since solid-electrolyte has a wider potential window than a liquid one, high voltage cathode material can be implemented. Among novel cathode materials, we focus on a Li-rich manganese oxide Li2MnO3, which has a layered rock-salt structure with a honeycomb-ordered LiMn2 layer. Although having a high theoretical capacity of ~459 mAh g-1, the cathode delivers a smaller reversible capacity than the theoretical value. Recently, we have found that an all-solid-state battery with a Li2MnO3 thin film can deliver a reversibly capacity of 270 mAh g-1, and the rate and cyclic performance are extremely high [1]. However, the redox species of the Li2MnO3 during the electrochemical charge/discharge reactions are still unknown for this battery system. In this study, the charge compensation mechanism of the Li2MnO3 cathode in an all-solid-state thin film battery was investigated by using an operando hard X-ray photoelectron spectroscopy (HAXPES) method. Although X-ray photoelectron spectroscopy is a powerful method to investigate the electronic structure of the cathode, there is an issue that the probing depth is about a few nm when using Al or Mg Ka source. On the other hand, HAXPES has a high probing depth of several 10 nm, which makes it possible to detect a signal from the buried region of the battery. Solid-electrolyte supported thin film batteries were developed for HAXPES measurement as illustrated in Fig (a). Using solid-electrolyte as substrate, cathode, and anode layers can be separated on the front and the back of the substrate, respectively, which enables us to detect the HAXPES signal of the cathode layer. The battery composed of a polycrystalline Li2MnO3 cathode, an amorphous Li3PO4 buffer layer, a lithium anode, an Al current collector, and a LICGC glass-ceramics solid-electrolyte substrate (manufactured by Ohara Inc.) was prepared by using pulsed laser deposition, magnetron sputtering, and thermal or electron-beam vapor deposition methods. Charge-discharge experiments were performed galvanostatically at a 0.2 C rate between 2.0 and 5.0 V (vs. Li/Li+) at room temperature. After the cell voltage reached at 5.0 V, the voltage was kept for 30 minutes.An operando HAXPES measurement was conducted at BL28XU in SPring-8. The incident photon energy of 7940 eV was used. The operando O1s, Mn3s, and Li1s spectra were measured during the 1st and the 5th charge-discharge processes. The charge-discharge curves showed a plateau at 4.6 V in the 1st charge and slope-like curves from the 1st discharge. Since these characteristics are also identified in the case of the battery using the Li2MnO3 epitaxial thin film [1], the prepared battery is suitable to conduct a HAXPES measurement for the purpose. The figures (b, c) show O1s operando spectra during the 1st charge-discharge process. During the 1st charge, a new peak at 531.1 eV is generated from 3.7 V in addition to a lattice oxygen O2- at 529.8 eV. According to previous reports, the new peak can be assigned as higher oxidized lattice oxygen such as O- [2]. After 3.7 V, the area of the new peak increases while that of O2- decreases as the voltage increases. During the 1st discharge, the new peak gradually decreases and then disappears at 3.3 V. Therefore, the reversible oxygen redox could contribute the charge compensation for the battery reactions. The average valence state of Mn was analyzed from Mn3s operando spectra. Manganese is reduced from 4+ to 3.5+ during the 1st charge process then Mn is further reduced to 3.1+ from 3.3 V during 1st discharging process. Upon the 5th charge-discharge process, the Mn redox occurred reversibly below 3.5 V. These results indicate that the oxygen as well as Mn redox reactions occur reversibly in Li2MnO3 cathode. Furthermore, the oxygen redox occurs on the higher voltage region between ~3.3 and 5.0 V and the Mn redox occurs on the lower region between 2.0 and ~3.3 V. Therefore, it is evident that the large reversible capacity of Li2MnO3 thin film results from the oxygen redox activity. Realizing the stable oxygen redox reaction on batteries with a larger scale will be a key for practical application.[1] K. Hikima et al., Chem. Lett. 48, 192 (2019).[2] M. Sathiya et al., Nature Materials 12, 827 (2013). Figure 1

The Role of Niobium Substitution in Lithium Vanadyl Phosphate for Lithium-Ion Batteries

Li-ion intercalation compound enlisting multi-electron redox is a promising strategy to maximize the storage capacity of LIBs. Lithium vanadyl phosphate, LiVOPO4, is one such material that can intercalate up to two Li+ ions per vanadium ion through the change in vanadium valence from V5+ to V3+, which translates to a theoretical capacity of 305 mAh/g and an energy density above 900 Wh/kg. However, Li-ion intercalation in the high voltage region (V5+/V4+) is kinetically limited by the sluggish Li-ion diffusion, and thus impedes the use of LiVOPO4 in practical applications. The goal of this work is to explore experimentally if niobium substitution can enhance the electrochemical performance of LiVOPO4. Lab X-ray diffraction, electron microscopy-energy-dispersive X-ray analysis, and inductively coupled plasma atomic emission spectroscopy indicate niobium is substituted into the LiVOPO4 crystal structure. A qualitative difference in the charge-discharge voltage profile is observed between the pristine and the niobium-substituted samples, suggesting a more complex substitution mechanism than the direct V substitution with Nb. The effects of niobium substitution on the crystal structure and the electrochemical performance of LiVOPO4 will be discussed.

Understanding the Role of Electrochemically Inactive Substituent in Layered Oxide Cathode Materials for Advanced Sodium Ion Batteries

The importance of electrochemical energy storage systems for large-scale application has been growing increasingly over the last decades. Due to high cost, limited source and several technical barriers of the lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) are considered as one of promising candidates for large-scale energy storage systems based on their cost-effective, earth abundant and recyclable characteristics.[1] As promising cathode materials for SIBs, P2-type sodium iron manganese oxide with layered structures (Na x Mn y Fe1-y O2, 0.5 < x < 0.8) demonstrates similar high energy density analogous to the LiBs.[2] However, the capacity retention of the cell with layered structure cathode is not optimized especially when it is charged and discharged with wide voltage window to obtain higher capacity. The major reason for this is a phase transition induced by gliding of transition metal slabs, which causes irreversible structural changes gradually during cycling.[3]To address this phase transition issue, we introduce electrochemically inactive component (e.g., Mg2+) in the material. The introduction of the Mg2+ by substituting with existing transition metal(s) is able to provide improved capacity retention due to their enhanced structural stability. We systematically analyzed the capacity fading trends at high and low voltage regions, where each different transition metal dominantly contributes to the redox reactions, which provides mechanistic understanding of the structural degradation of P2-type NaxMnyFe1-yO2 and the effect of Mg2+ on transition metal redox behaviors. In addtion, by tracing the structural changes in the cathode, we demonstrated that the introduction of Mg2+ effectively abbreviates the unwanted phase transitions. The results can provide the insights of the phase transition mechanism and redox behaviors of layered type cathode materials and the strategies to stabilize the structure by employing electrochemically inactive substituent(s), thus enabling rational design of new cathode materials for the development of next generation SIBs.[1] M. D. Slater, D. Kim, E. Lee, C. S. Johnson, Adv. Funct. Mater. 23 (2013) 947-958.[2] C. Delmas, C. Fouassier, P. Hagenuller, Physica B+C 99 (1980) 81-85.[3] E. Talaie, V. Duffort, H. L. Smith, B. Fultz, L. F. Nazar, Energy Environ. Sci., 5 (2015) 2512. Acknowledgement J. Yang and S.-D. Han acknowledge funding support by the Laboratory Directed Research and Development (LDRD) program at National Renewable Energy Laboratory (NREL).The work done at Brookhaven National Laboratory was supported by Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE, through the Advanced Battery Materials Research (BMR) Program under contract DE-SC0012704.

Activation of anion redox in P3 structure cobalt-doped sodium manganese oxide via introduction of transition metal vacancies

Additional capacity delivered by oxygen redox activity may in principle represent a means of enhancing the electrochemical performance of layered sodium transition metal oxides. However, irreversible structural changes occurring during cycling typically cause significant capacity fade with limited reversibility of oxygen redox processes. Here, P3-structure Na0.67Co0.2Mn0.8O2 was synthesised under two different reaction conditions. Both materials exhibit very stable cycling performance in the voltage range 1.8–3.8 V where the redox couples of transition metals entirely dominate the electrochemical reaction. For the compound prepared under more oxidising conditions, anion redox activity is triggered in the wider voltage window 1.8–4.4 V in a reversible manner with exceptionally small voltage hysteresis (20 mV). The presence of vacancies in the transition metal layers is shown to play a critical role not only in generating unpaired O 2p states but also in stabilising the crystal structure in the high voltage region.

Improvement of AlN Material Quality by High-Temperature Annealing Toward Power Diodes

AlN devices have the potential to outperform current GaN devices, especially in the high-voltage region, thanks to AlN’s larger critical electric field and thermal conductivity. In order for the AlN-based power diodes to be realized and performance to be demonstrated, high-quality AlN crystals would be the prerequisite. In this article, significantly improved AlN quality has been demonstrated through high-temperature treatment of AlN thin layers at high temperatures in the range of 1670–1730 °C, followed by regrowth of AlN layers on top. The produced AlN layer shows much improved crystalline quality when compared to AlN layer without thermal treatment. The thermal treatment of AlN nucleation layers recrystallizes the AlN materials and results in a significant reduction of the full-width at half-maximum (FWHM) values of X-ray rocking curves of AlN (002) and (102) planes. The annealing of AlN nucleation also produced much pronounced atomic step features and improves the transmittance for the nucleation layers. Regrowth of AlN templates on such annealed nucleation improves AlN crystalline quality further, producing high-quality templates for power diode applications.

High‐efficiency design method of LLC resonant converter for PHEV battery chargers (based on time‐domain model)

High efficiency, high power density and wide output voltage are required for on-board charger (OBC) applications. LLC resonant converter has the advantage of achieving zero-voltage switching for variable frequency and different load conditions. Compared with conventional fundamental harmonic approximation method, operation-mode analysis based on the time domain model provides an accurate description of resonant current, voltage and DC gain. In this study, the efficiency-oriented modified method is proposed based on the operation-mode analysis. The converter inductance ratio, characteristic impedance and transformer turns ratio are optimised and both-side soft switching can be achieved in the high output voltage region. Conduction loss can be minimised by optimising resonant parameters. Finally, a high-efficiency design method is proposed and validated through experiments on a 3.3 kW prototype applied in OBC. The prototype of the LLC resonant converter has been tested for dynamic charging voltage (230–430 V) with a peak efficiency value of 98.5%.