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The electrochemical activity of LiNiO2 at the initial cycle and factors affecting its activity were understood. Even though LiNiO2 can achieve almost theoretical charge capacity, it cannot deliver the theoretical discharge capacity that would result in low 1st Coulombic efficiency (CE). For different upper cut-off voltages at 4.3 and 4.1 V, the 1st CE barely increases. Given that the H2-H3 phase transition occurs at ∼4.2 V, the low 1st CE is not caused by this phase transition but is a result of the additional 3.5 V discharge reaction, which is kinetically limited and thereby not activated even at a reasonable current density. We found out that the several phase transitions during charge/discharge in LiNiO2 barely affect the 3.5 V reaction. Under galvanostatic intermittent titration technique (GITT) conditions, LiNiO2 can achieve ∼250 mAh/g of discharge capacity and 100% CE even with the 4.3 V cut-off voltage by fully activating the 3.5 V reaction. Using neutron diffraction and 6Li nuclear magnetic resonance (NMR) measurements, the sluggish kinetics of the 3.5 V reaction can be ascribed to difficult insertion of Li at the end of the discharge because this reaction can be accompanied by the rearrangement of cations or local structure change in the structure. To achieve high discharge capacity in LiNiO2 with the 4.3 V cut-off voltage, this 3.5 V sluggish reaction should be improved. The finding and understanding underlying the mechanism of the electrochemical activity will stimulate further research on high-capacity Ni-rich layered materials for high-performance Li-ion batteries.

Out of practicality, ambient air rather than oxygen is preferred as a fuel in electrochemical systems, but CO2 and H2O present in air cause severe irreversible reactions, such as the formation of carbonates and hydroxides, which typically degrades performance. Herein, we report on a Na-air battery enabled by a reversible carbonate Na2CO3·xH2O (x = 0 or 1) reaction in Nasicon solid electrolyte (Na3Zr2Si2PO12) that delivers a much higher discharge potential of 3.4 V than other metal-air batteries leading to high energy density and achieves > 86% energy efficiency at 0.1 mA cm− 2 over 100 cycles. This cell design takes advantage of moisture in ambient air which the deliquescence of NaOH forms an in-situ catholyte that catalyzes the activation of reversible electrochemical reaction of Na2CO3·xH2O. Our result demonstrates that free ambient air can be used reversibly as a fuel when enabled by reversible electrochemical reaction of carbonates with a solid electrolyte.

Ni-rich layered electrode materials have attracted great attention as a promising cathode candidate for high-energy-density lithium-ion batteries because of their high capacity and relatively low cost. However, they have been suffering from severe capacity fading for cycles, which can originate from several factors such as the phase transition at the end of charge and disintegration of the particles. Herein, a simple and novel sublimation-induced gas-reacting (SIGR) process has been developed by using elemental sulfur to conformally coat Ni-rich layered materials. The sublimated gas-phase S can react with detrimental residual Li compounds on the surface of the particles. As a result, the reacted layer of LixSyOz phases forms on the outside of the secondary particles and simultaneously in the boundaries between primary particles inside the secondary particles. Compared to other reported surface modification processes, the SIGR-treated Ni-rich materials show substantially increased capacity retention and superior voltage retention by protecting the surface from the electrolyte and mitigating disintegration of the secondary particles. The SIGR process is a simple and scalable solid-state reaction at low temperature to improve the cycling stability of high-capacity Ni-rich electrode materials.

A fast charging/discharging capability in electrode materials is one of the essential properties of Li-ion batteries. A well-known approach for improving the kinetics of electrode materials is minimization of the particle size to the nanoscale, especially for phase separating materials such as LiFePO4 and Li4Ti5O12. Here, we report on ultrafast kinetics in a phase separating material with submicron particles enabled by the formation of an intermediate phase during discharge even without minimizing the particle size. The intermediate phase during (dis)charging acts as a structural buffer between the Li-rich and Li-poor phases, leading to reduced mechanical stress/strain on the interface in a particle, which can suppress structural defects, and a substantial reduction in the phase transformation barrier, which can make the electrochemical reaction of particles in the electrode homogenous. As a result, the resulting phase separating material can deliver ∼70 mA h g−1 at a 200C discharge rate (18 s for full discharge) with a 1C charge rate, and prolonged cycling stability for 1000 cycles even though it has a wide particle size distribution from 50 nm to 600 nm with an average of ∼300 nm. This finding will provide a new avenue for achieving fast electrochemical kinetics with a high volumetric energy density in electrode materials for high performance Li-ion batteries.

Here, we, for the first time, report on the simultaneous enhancement in cubic phase stability and Li-ion conductivity of garnet-type solid electrolytes (SEs) by adding excess Li/Al. The excess Al/Li creates very large grains of up to 170 μm via the segregation of Al at the grain boundaries and enables preferential Al occupation at 96h sites over 24d sites, a behavior contrary to previous observations. The resulting SE shows improved Li-ion conductivity due to the large grain size and less blocking Li pathway caused by different preferential Al occupation. Surprisingly, it is observed that the cubic phase of the garnet-type SE is transformed to the tetragonal phase on the surface and in the bulk under the applied voltage, and the preferential Al occupation enables its cubic phase stability. Under battery operating conditions, the LLZO SE with excess Li/Al can maintain high ionic conductivity due to the cubic phase stability and large grain size. We clearly demonstrate that the cubic phase stability and ionic conductivity of LLZO can be simultaneously improved by excess Li/Al without any post-treatments. The findings and understanding will provide new insights into practical use of the garnet-type SEs for advanced all solid-state batteries.

Polyanion compounds have relatively low energy density compared to plain oxides as cathode materials. We for the first time report on a Li-rich fluorophosphate compound, Li5V2PO4F8, that can have h...

We tried to understand the reversible capacity of SiO during the first cycle and its effect on the poor initial Coulombic efficiency (ICE). Several SiO samples that have slightly different microstr...

Summary Cation disordering in electrode materials for Li-ion batteries is deemed as an undesired structural feature for achieving high electrochemical activity owing to sluggish Li diffusion in the structure. Therefore, extensive efforts have been made to develop electrode materials having a well cation-ordered structure. We report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with ∼ 60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

Tavorite LiVPO4F has great potential as a cathode material due to its high energy density, superior thermal stability and fast kinetics. It has been controversial whether the redox reaction pathway of LiVPO4F is symmetric or not, but the origin of such symmetric/asymmetric redox reaction pathways has not been clearly understood. By combining ab initio calculations with experiments, we found that oxygen defects on fluorine sites, OF−, can be a key factor that affects the symmetry of the redox pathway of LiVPO4F1−δOδ (δ < 1). The computational results indicate that (1) the thermodynamically metastable ‘triclinic’ polymorph of VPO4F1−δOδ, which maintains the structural symmetry of LiVPO4F, can be kinetically stabilized considering its marginally higher energy than the stable ‘monoclinic’ phase; and (2) the OF− defects slightly destabilize the intermediate phase (x = 0.667) (Li0.667VPO4F versus Li0.667VPO4F0.917O0.083), inferring that the presence of OF− defects can induce the asymmetric redox pathway of LiVPO4F1−δOδ. We also experimentally found that the concentration of OF− defects is critically affected by the synthesis process of LiVPO4F, and VPO4F with lower OF− concentration can support the possible presence of the metastable (triclinic) phase, leading to the redox pathway being prone to being symmetric. Therefore, controlling oxygen defects by a synthesis processes can affect electrochemical performance via different redox reaction pathways of LiVPO4F. This understanding provides further possibilities for improving the electrochemical performance.

The electrochemical activity of LiNiO

We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs tunneling time (≈50) and singlet-triplet splitting vs thermal energy (≈20) allows energy-selective tunneling-based spin-to-charge conversion with a readout visibility of ≈92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs. Further, we discuss the generality of the method for use in other materials, including silicon.

To meet the growing demand for global electrical energy storage, high‐energy‐density electrode materials are required for Li‐ion batteries. To overcome the limit of the theoretical energy density in conventional electrode materials based solely on the transition metal redox reaction, the oxygen redox reaction in electrode materials has become an essential component because it can further increase the energy density by providing additional available electrons. However, the increase in the contribution of the oxygen redox reaction in a material is still limited due to the lack of understanding its controlled parameters. Here, it is first proposed that Li‐transition metals (TMs) inter‐diffusion between the phases in Li‐rich materials can be a key parameter for controlling the oxygen redox reaction in Li‐rich materials. The resulting Li‐rich materials can achieve fully exploited oxygen redox reaction and thereby can deliver the highest reversible capacity leading to the highest energy density, ≈1100 Wh kg−1 among Co‐free Li‐rich materials. The strategy of controlling Li/transition metals (TMs) inter‐diffusion between the phases in Li‐rich materials will provide feasible way for further achieving high‐energy‐density electrode materials via enhancing the oxygen redox reaction for high‐performance Li‐ion batteries., A new key parameter for fully exploiting the oxygen redox reaction by Li/transition metals (TMs) inter‐diffusion between the two phases in Co‐free Li‐rich materials is first proposed. The resulting Li‐rich materials can achieve fully exploited oxygen redox reaction and thereby can deliver the highest reversible capacity leading to the highest energy density, ≈1100 Wh kg−1 among Co‐free Li‐rich materials.

Rhizobium radiobacter species complex (biovar 1 Agrobacterium) has been known for the crown-gall and hairy-root disease causative pathogens. Strains within this complex were also found as endophytes from many plant species with no symptom. The aim of this study is to reveal the endophyte variation of this complex and how these endophytic strains differ from pathogenic strains. In this study we devised a simple but effective screening method by exploiting the high resolution power of mass spectrometry. We screened endophyte isolates from young wheat and barley plants, which are resistant to the diseases, and identified seven isolates from wheat as members in the R. radiobacter species complex. Through further analyses, we assigned five strains to the genomovar (genomic group) G1 and two strains to G7 in the species. Notably, these two genomovar groups harbor many known pathogenic strains. In fact, the two G7 endophyte strains showed pathogenicity on tobacco, as well as the virulence prerequisites including a 200-kbp Ri plasmid. All of the five G1 strains possessed a 500-kbp plasmid, which is present in well-known crown-gall pathogens. These data strongly suggest a role of healthy wheat plants as a reservoir for pathogenic strains of this species. IMPORTANCE Crown gall and hairy root diseases exhibit very wide host-plant ranges that cover gymnosperm and dicot plants. Rhizobium radiobacter species complex is the representative species that harbor causative agents of the two diseases. Recently, a number of endophyte isolates from many plant species have been assigned to this species complex. We isolated seven endophyte strains belonging to the species complex from wheat plants, and revealed their genomovar affiliations and plasmid profile. The significance of this study is the finding of the genomovar correlation between the endophytes and the known pathogens, the presence of a virulence ability in two of the seven endophyte strains, and the high ratio of the pathogenic strains in the endophyte strains. This study therefore provides convincing evidences that could unravel the mechanism to maintain pathogenic agents of this species and to deliver them to susceptible plants sporadically.

Demands for Li-ion batteries (LIBs) with high energy density and superior safety have been increasingly grown. To meet strong demand, all-Solid-State Batteries (ASSB) based on oxide-based solid electrolytes (SEs) have been considered as a promising energy storage platform. However, the realization of ASSB and its practical utilization in applications are very challenging because of high interfacial resistance between particles in a composite electrode and between electrodes and the use of Li metal as an anode. In this paper, we overview recent research progresses on oxide-based SEs for the ASSB with respect to the use of Li metal. Especially, we focus on research progresses on garnet-type solid electrolytes (Li7La3Zr2O12) because they have high ionic conductivity, good chemical stability with Li metal, and wide electrochemical potential window. This review will be also discussed about Li dendritic behavior in the oxide-based SEs and its relationship with critical current density (CCD). We will close this review with remarks on the prospects of ASSB.

Molten sodium (Na) anode high temperature batteries, such as Na-NiCl2 and Na-S, draw attentions to be used in stationary electricity storage applications. Recent efforts are exerted to lower their operating temperatures down to below 200 °C in order to adopt ultra-low cost cell production, establish easier maintenance, pursue enhanced safety, and more. One of main challenges in lowering the operation temperature is radical decrease in ionic conductivity of their solid electrolytes. Na3Zr2Si2PO12 (Nasicon) is considered as a solid electrolyte for the lower temperature operation. Here we report Na ionic conductivity of Nasicon at 150 °C increases by adding Ge element. The ionic conductivity of Ge-added sample (Na3[Zr2-δGeδ]Si2PO12, δ = 0.1, 0.2) is measured as high as 1.4 × 10−2 S cm-1 at 150 °C which is about two times higher than those of the bare Nasicon. The phase transition temperature of the Ge-added samples is lowered, thereby the volume fraction of the rhombohedral phase, which is stable at higher temperatures and exhibits higher Na ion conductivities, increases. This finding provides a useful guideline to further increase the ionic conductivity of Nasicon solid electrolytes, which can advance materialization of lower temperature operating Na batteries.

We report a new triplite-type iron fluoro-sulfate compound, a cation-disordered NaFeSO4F that has redox potential of ∼3.7 V versus Na+/Na0 and can have 138 mA·h/g of theoretical capacity. This compound shows practical energy density (∼430 W·h/kg) comparable to that of several Li-ion battery positive electrode materials such as LiMn2O4 (430 W·h/kg). Therefore, triplite NaFeSO4F is a candidate positive electrode material which can meet the requirements for high energy density Na-ion batteries. Furthermore, this study contributes to expanding the spectrum of Na electrode materials to cation-disordered polyanion compounds and opens up new opportunities for the design of electrode materials for advanced Na-ion batteries.

Chemical reactions at the solid electrolyte (SE) and Li metal interface form an interphase before electrochemical reactions occur. This study investigates the effects of the chemically formed interphase between Li metal and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) on cell failures under various experimental conditions. LAGP forms a black interphase by chemically reacting with Li metal. The interphase comprises a stoichiometrically changed LAGP and Li-related oxides and behaves as a mixed ionic and electronic conductor with the electronic conductivity dominating. Thus, upon application of an electrical current to Li metal anode, most of the Li ions can be reduced at the SE side surface of the interphase rather than the Li metal side, causing a local volumetric increase that triggers cracks in the SE. This crack formation process continues the pulverization of SE, leading to a gradual increase in cell resistance. Under cell operating conditions, electrochemical reactions with the chemically formed interphase can lead t...

Polyanion compounds as an electrode material in lithium-ion battery are of great interest due to superior structural and thermal stability, but they have relatively low energy density. Here, we report high energy density electrode material, LiVPO4O1–xFx (x ≈ 0.25) with tavorite structure that is first synthesized by single-step solid-state reaction and that can use more than 1.6 electrons during charging/discharging process. The doping of F instead of O into LiVPO4O was confirmed with a structural solid-solution measured by neutron powder diffraction and with a mixed valence state of V measured by X-ray absorption near edge spectroscopy. The resulting material has distinct and beneficial electrochemical properties compared to both LiVPO4F and LiVPO4O. LiVPO4O0.75F0.25 has higher average operating voltage and better electrochemical activity than LiVPO4O and shows higher operating voltage (∼2.4 V) than LiVPO4F (∼1.8 V). As a result, LiVPO4O0.75F0.25 can achieve high specific capacity of 260 mA h/g in the vo...

In this study, triplite 3.9V-LiFeSO4F in single step solid-state reaction with various precursors was synthesized by understanding the reaction pathway that it forms. The loss of fluorine during a heat treatment can be a main problem because it can cause the formation of non-fluorine compounds such as Li2Fe(SO4)2 phase, which are undesirable impurity phases. An additional fluorine source was used to compensate the loss of fluorine and led to the formation of highly-pure triplite LiFeSO4F. Furthermore, additional supplementation with sucrose as a carbon source produced highly-pure LiFeSO4F nano-particles, coated with a carbon layer. This resulting material can achieve superior rate capability up to 20C rate (3 min discharge), ∼73 mAhg−1 and have excellent capacity retention for 350 cycles at 1C charge/1C discharge rate even though the structure of triplite LiFeSO4F does not have an apparent long Li diffusion pathway due to a disordered Li/Fe distribution. These results demonstrate that triplite LiFeSO4F is a promising cathode material with respect to energy density and scalable synthesis capability. Furthermore, this solid-state reaction with additional fluorine source will open novel ways to further engineer the material properties of LiFeSO4F or other fluorine containing compounds.

International audience; The impending quest for high energy density and high power density electrode materials for lithium-ion batteries has been intensified to meet strongly growing demand for powering electric vehicles. Conventional layered oxides such as Co-rich LiCoO2 and Ni-rich Li(NixMnyCoz)O2 that rely on only transition metal redox reaction have been faced with growing constraints on the resources and the availability due to soaring price on cobalt. Therefore, Mn-rich electrode materials excluding cobalt would be desirable with respect to available resources and low cost. Here, we report on the strategy of achieving both high energy density and high power density in Mn-rich electrode materials by controlling of the atoms solubility between phases in a composite. The resulting Mn-rich material that is composed of defective spinel phase and partially cation-disordered layered phase can achieve the highest

The Na superionic conductor (aka Nasicon, Na1+xZr2SixP3–xO12, where 0 ≤ x ≤ 3) is one of the promising solid electrolyte materials used in advanced molten Na-based secondary batteries that typically operate at high temperature (over ∼270 °C). Nasicon provides a 3D diffusion network allowing the transport of the active Na-ion species (i.e., ionic conductor) while blocking the conduction of electrons (i.e., electronic insulator) between the anode and cathode compartments of cells. In this work, the standard Nasicon (Na3Zr2Si2PO12, bare sample) and 10 at% Na-excess Nasicon (Na3.3Zr2Si2PO12, Na-excess sample) solid electrolytes were synthesized using a solid-state sintering technique to elucidate the Na diffusion mechanism (i.e., grain diffusion or grain boundary diffusion) and the impacts of adding excess Na at relatively low and high temperatures. The structural, thermal, and ionic transport characterizations were conducted using various experimental tools including X-ray diffraction (XRD), differential sca...

To the best of our knowledge, this is first report about composite of perovskite materials and reduced graphene oxide as anode material for high performance lithium ion battery (LIB). Here, we report outstanding specific capacities and cyclic stabilities of perovskite BaSnO 3 nanoparticles (NPs), decorated on 2D graphene sheets. Electrochemical measurements show that the composite BaSnO 3 /rGO20 (20 wt.%rGO) delivered a highest capacity of 1200 mAh g −1 at a current rate of 0.5C (650 mA g −1 ), which is comparatively much higher than individual pristine BaSnO 3 and graphene anodes. The BaSnO 3 /rGO composite anode exhibits a very high rate capability even at the 10C discharge rate (1059 mAh g −1 ) and outstanding long-term stability. Notably, the composite anode material maintains its discharge capacity even after 100 cycles.

We use confocal μ-Raman spectroscopy to characterize the local microstructure of SiO x particles that are composed of amorphous and crystalline phase and to understand its effects on electrochemical reactions. Particles of bare SiO x (x ∼ 1) consist of an inhomogeneous mixture of a- Si and c- Si; some particles mainly have a- Si phase but others consist mainly of micro-sized crystalline Si phase. In contrast, particles of C-coated SiO x are composed of uniform nano-sized crystalline Si embedded in a- SiO 2 matrix with small amounts of a- Si. The two samples had different electrochemical properties due to this difference in microstructure. Initial coulombic efficiency (ICE) and capacity retention were higher in the C-coated SiO x than in the bare SiO x . The findings demonstrate that confocal μ-Raman spectroscopy is very useful tool for characterizing the microstructure of SiO x and the understanding of local microstructures of SiO x is very important for understanding electrochemical reactions.

In this study, electrochemical behaviors of Li2S such as a large potential barrier at the beginning of the 1st charging process and a continuous increase in potential to ∼4 V during the rest of this process were understood through X-ray photoelectron spectroscopy measurements and electrochemical evaluations for a full utilization of Li2S. The large potential barrier to the 1st charge in Li2S can be caused by the presence of insulating oxidized products (Li2SO3 or Li2SO4-like structures) on the surface; simple surface etching can remove them and thereby reduce the potential barrier. Even though the potential barrier was substantially reduced, the electrochemical activity of Li2S might not be improved due to the continuous increase in potential. This increase in potential was related to the polarization caused by the Li2S-conversion reaction; the polarization can affect the utilization of Li2S in subsequent cycles. We speculate that the increase in potential is related to the decomposition of oxidized products such as Li2CO3-like or Li2O-like structures on the surface of the Li2S particles. These findings indicate that the full utilization of Li2S can be achieved by controlling their surface characteristics, especially the surface oxidation products.

FeF3 is a promising cathode material for lithium ion batteries but its poor electronic conductivity makes it non-practical. Here, we significantly improve the electrochemical activity of FeF3 by reducing the strong ionic character of Fe–F with the replacement of some F with N atoms. N-doped nanocrystalline FeF3/C achieves the best electrochemical performance among FeF3 compounds reported to date: ∼95 mA h g−1 at 21.1C discharge rate for 250 cycles. The results illustrate that the poor electronic conductivity of metal fluorides can be controlled by doping and this enables FeF3 or metal fluorides to be practically utilized in possible applications including energy conversion and storage.

A scalable solid-state reaction is presented to synthesize an FeF3 cathode material by using PTFE as a source of both fluorine and carbon. The method yields nanocrystalline FeF3/C showing excellent electrochemical performance even without any conducting additive. This method can be utilized for engineering MFs' properties and developing other fluorine compounds.

Understanding of the fundamental mechanisms causing significant enhancement of Li-ionic conductivity by Al 3+ doping to a solid LiGe 2 (PO 4 ) 3 (LGP) electrolyte is pursued using first principles density functional theory (DFT) calculations combined with experimental measurements. Our results indicate that partial substitution Al 3+ for Ge 4+ in LiGe 2 (PO 4 ) 3 (LGP) with aliovalent (Li 1+x Al x Ge 2−x (PO 4 ) 3 , LAGP) improves the Li-ionic conductivity about four-orders of the magnitude. To unveil the atomic origin we calculate plausible diffusion paths of Li in LGP and LAGP materials using DFT calculations and a nudged elastic band method, and discover that LAGP had additional transport paths for Li with activation barriers as low as only 34% of the LGP. Notably, these new atomic channels manifest subtle electrostatic environments facilitating cooperative motions of at least two Li atoms. Ab-initio molecular dynamics predict Li-ionic conductivity for the LAGP system, which is amazingly agreed experimental measurement on in-house made samples. Consequently, we suggest that the excess amounts of Li caused by the aliovalent Al 3+ doping to LGP lead to not only enhancing Li concentration but also opening new conducting paths with substantially decreases activation energies and thus high ionic conductivity of LAGP solid-state electrolyte.

Surface carbon coating to improve the inherent poor electrical conductivity of lithium iron phosphate (LiFePO4, LFP) has been considered as most efficient strategy. Here, we also report one of the conventional methods for LFP but exhibiting a specific capacity beyond the theoretical value, ultrahigh rate performance, and excellent long-term cyclability: the specific capacity is 171.9 mAh/g (70 μm-thick electrode with ∼10 mg/cm(2) loading mass) at 0.1 C (17 mA/g) and retains 143.7 mAh/g at 10 C (1.7 A/g) and 95.8% of initial capacity at 10 C after 1000 cycles. It was found that the interior conformal N-C coating enhances the intrinsic conductivity of LFP nanorods (LFP NR) and the exterior reduced graphene oxide coating acts as an electrically conducting secondary network to electrically connect the entire electrode. The great electron transport mutually promoted with shorten Li diffusion length on (010) facet exposed LFP NR represents the highest specific capacity value recorded to date at 10 C and ultralong-term cyclability. This conformal carbon coating approach can be a promising strategy for the commercialization of LFP cathode in lithium ion batteries.

To understand the delithiation behavior of particles of different sizes, we developed a centrifugation method to separate particles in a chemically-delithiated Li0.49FePO4 with respect to their size. Small particles were obtained at high centrifuge speed whereas large particles were obtained at low centrifuge speed. The proportions of FePO4 and LiFePO4 in the separated particles were investigated to understand their delithiation behavior. Small particles were almost fully-transformed to FePO4 whereas large particles were still mostly LiFePO4. The inhomogeneous delithiation of particles could be also observed in the electrode. The electrode showed a voltage overshoot in the middle of the two-phase region during charging. Considering that the voltage overshoot is related to nucleation, this observation indicates that some particles in the middle of two-phase region are still in the LiFePO4 phase that must be nucleated. The inhomogeneous delithiation behavior of LiFePO4 particles of different sizes may be an intrinsic property irrespective of the delithiation method. To improve the utilization of active materials, the inhomogeneous delithiation of particles should be minimized by engineering the configuration of the electrode.

Fe-based polyanion materials such as LiFePO4 or LiFeBO3 are safe and can achieve high energy density, so they have been considered as cathode materials for large-scale lithium ion batteries. Among these compounds, LiFeSO4F achieves the highest voltage (3.9 V) among Fe2+/Fe3+ redox couples and therefore can have higher energy density than LiFePO4. However, full utilization of 3.9 V LiFeSO4F is severely limited by its non-scalable synthesis process and poor electrochemical activity. Here, we report a method to synthesize 3.9 V LiFeSO4F by a scalable solid-state reaction within 1 h by understanding the thermodynamic stability of 3.9 V LiFeSO4F. The resulting material shows the best electrochemical performance reported to date. The process yields nanosized particles that achieve almost full capacity, 140 mA h g−1, which is 93% of the theoretical capacity and that retain excellent capacity, ∼75 mA h g−1 at 1 C for 150 cycles and high rate capability even at 10 C (6 min). This scalable solid-state reaction for 3.9 V LiFeSO4F makes it a plausible replacement for LiFePO4 in the next generation of lithium ion batteries.

To understand how a phase transformation pathway affects the electrochemical properties of LiFePO4 nanoparticles (NPs), a sample with the solid-solution phase (SS sample) was prepared by thermal treatment of a sample that contained two phases, LiFePO4 and FePO4, (TP). The SS sample had NPs that experience the solid-solution phase and then may undergo solid-solution phase transformation, and the TP sample had NPs that do not experience and then can undergo typical two-phase transformation. And their electrochemical characteristics were compared under various conditions. The thermodynamic properties of the two samples were evaluated using a galvanostatic intermittent titration technique (GITT) and a potentiostatic intermittent titration technique (PITT), and electrochemical kinetic properties were evaluated by applying current. The two samples had quite similar thermodynamic properties such as OCV and diffusion coefficient, but quite different kinetic properties such as polarization, current decay, voltage flatness and underpotential behavior even though they had a similar particle size and size distribution. The SS sample showed lower polarization, faster current decay at a constant voltage and less-significant underpotential behavior than did the TP sample. Furthermore, during charging/discharging, the voltage profile was a slope for the SS sample but flat for the TP sample even though the OCV of the two samples did not show any significant difference. These facile electrochemical characteristics can be related to nucleation indicating that the SS sample can have less significant nucleation than the TP sample. These different electrochemical properties are caused by different phase transformation pathways rather than the particle size that is the typical cause for those different kinetic properties. The phase transformation pathway of LiFePO4 strongly affects the electrochemical kinetic properties, not the thermodynamic ones. We reveal that undergoing the solid-solution pathway can be kinetically better off leading to a fast electrochemical response.

We highlight that the off-stoichiometric compositional variation is a simply effective method to improve the power density of LiFe0.6Mn0.4PO4. This strategy does not require a supplementary separate coating and is likely applicable to other compositions given the feasibility of the method.

We determined how Li doping affects the Ni/Mn ordering in high-voltage spinel LiNi0.5Mn1.5O4(LNMO) by using neutron diffraction, TEM image, electrochemical measurements, and NMR data. The doped Li occupies empty octahedral interstitials (16c site) before the ordering transition, and can move to normal octahedral sites (16d (4b) site) after the transition. This movement strongly affects the Ni/Mn ordering transition because Li at 16c sites blocks the ordering transition pathway and Li at 16d (4b) sites affects electrostatic interactions with transition metals. As a result, Li doping increases in the Ni/Mn disordering without the effect of Mn3+ ions even though the Li-doped LNMO undergoes order-disorder transition at 700 °C. Li doping can control the amount of Ni/Mn disordering in the spinel without the negative effect of Mn3+ ions on the electrochemical property.

Use of compounds that contain fluorine (F) as electrode materials in lithium ion batteries has been considered, but synthesizing single-phase samples of these compounds is a difficult task. Here, it is demonstrated that a simple scalable single-step solid-state process with additional fluorine source can obtain highly pure LiVPO4F. The resulting material with submicron particles achieves very high rate capability approximate to 100 mAh g(-1) at 60 C-rate (1-min discharge) and even at 200 C-rate (18 s discharge). It retains superior capacity, approximate to 120 mAh g(-1) at 10 C charge/10 C discharge rate (6-min) for 500 cycles with >95% retention efficiency. Furthermore, LiVPO4F shows low polarization even at high rates leading to higher operating potential >3.45 V (approximate to 3.6 V at 60 C-rate), so it achieves high energy density. It is demonstrated for the first time that highly pure LiVPO4F can achieve high power capability comparable to LiFePO4 and much higher energy density (approximate to 521 Wh g(-1) at 20 C-rate) than LiFePO4 even without nanostructured particles. LiVPO4F can be a real substitute of LiFePO4.