High-voltage Positive Electrode Research Articles

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.

Understanding the electrode – electrolyte interphase of high voltage positive electrode Na4Co3(PO4)2P2O7 for rechargeable sodium-ion batteries

Sodium-ion batteries (SIBs) have been postulated as a potential solution for large-scale stationary applications and light electromobility. Among positive electrode materials for SIBs, Na4Co3(PO4)2P2O7 attracted significant attention due to its high voltage and good specific capacity even at very high current densities. However, details of the formed electrode – electrolyte interphase (EEI) are still uncertain, being this of extreme importance considering that the high operating voltage of this electrode material is around the stability edge of most of the conventional electrolytes which, in some cases, already display side reactions above 3.0 V vs. Na/Na+. In this work, the EEI of Na4Co3(PO4)2P2O7 is analyzed in half-cell configuration using 1 M NaPF6 in EC:DEC as electrolyte. Conventional and high energy X-ray photoelectron spectroscopy (XPS) has been used so as to understand the stability and the chemical composition of the EEI. The results reveal that a bilayer EEI is formed at full Na+ extracted state of charge (SOC - 4.7 V vs. Na/Na+), with semi-organic-rich species found in the subsurface region close to the electrode while more organic species are formed in the outermost surface region close to the electrolyte. Meanwhile, after full Na+ insertion (SOC - 3.0 V vs. Na/Na+) an additional outermost inorganic overlayer is formed which is composed of sodium carbonate and sodium fluorophosphate. Additionally, this inorganic-rich EEI is dissolving upon oxidation / charge process - affecting the outermost ~10 nm of the EEI. Despite this dynamic behavior of the EEI, the Na4Co3(PO4)2P2O7 positive electrode delivers excellent cyclability (94% capacity retention after 100 cycles at 0.2C), proving that it can be a good candidate as positive electrode material for SIBs.

Solid Polymer Electrolytes in Lithium Batteries: Challenges and Opportunities for High Specific Energy

Besides enhanced safety aspects, solid polymer electrolytes (SPEs) offer the opportunity to increase the specific energy of secondary batteries by enabling lithium metal as negative electrode in all-solid-state lithium batteries (ASSLB). Therefore, the SPE needs to provide sufficient mechanical strength to prevent short circuits caused by lithium dendrite penetration especially favoured at high current densities.[1] In order to obtain the maximum specific energy of ASSLBs, lithium metal negative electrodes are combined with high voltage positive electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622).[2]In the frame of this work, a novel SPE is applied to NMC622 based ASSLB cells and studied regarding energy limits on battery cell and pack level. In a first step, the active mass loading is systematically increased revealing the specific energy limit on battery cell level. Additionally, the chosen system is applied to a bipolar (serial) stacking aiming towards further energy improvements on battery pack level.[3] In sum, this work reveals remaining challenges but still points towards opportunities enabling the aimed energy increase of ASSLBs.[1] G. Homann, L. Stolz, J. Nair, I. C. Laskovic, M. Winter and J. Kasnatscheew, Sci Rep, 10, 4390 (2020).[2] J. Kasnatscheew, M. Evertz, R. Kloepsch, B. Streipert, R. Wagner, I. Cekic Laskovic and M. Winter, Energy Technology, 5, 1670-1679 (2017).[3] G. Homann, P. Meister, L. Stolz, J. P. Brinkmann, J. Kulisch, T. Adermann, M. Winter and J. Kasnatscheew, ACS Appl. Energy Mater., 3, 4, 3162–3168 (2020). Figure 1

Reversible electrochemical potassium deintercalation from >4 V positive electrode material K6(VO)2(V2O3)2(PO4)4(P2O7)

Reversible electrochemical deintercalation of potassium from the K6(VO)2(V2O3)2(PO4)4(P2O7) phosphate-pyrophosphate-oxide of potassium and vanadium (IV,V) obtained through a ceramic route has been demonstrated. The material has been tested as a high voltage positive electrode (cathode) for K-ion batteries, demonstrating 53% of the theoretical capacity of 88 mAh/g at the current density C/50 in the 1.8–4.4 V vs K/K+ potential range. The crystal structures of K6(VO)2(V2O3)2(PO4)4(P2O7) in the pristine, charged and discharged states were refined by Rietveld method from powder X-ray diffraction data using a structure model with stochastic orientation of the pyrophosphate groups (space group Pnma, a = 6.9891(1)Å, b = 13.3735(2)Å, c = 14.2495(2)Å in the pristine state). In spite of deintercalation of bulky K+ cations, the cell volume change upon charge amounts to 0.9% rendering K6(VO)2(V2O3)2(PO4)4(P2O7) a “low strain” cathode. The potassium (de)intercalation occurs equally from two distinct crystallographic sites through a single ~4.2 V sloped plateau featuring solid-solution-like behavior, which was demonstrated by operando powder X-ray diffraction. The evaluation of diffusion barriers from the first principles has been conducted by the nudged elastic band method for two ordered arrangements of the pyrophosphate groups derived from the refined disordered structure. The diffusion barriers along crystallographic direction b for both investigated ordered variants were found to be below 0.3 eV.

New Insights in All-Solid-State Ceramic Batteries: Dendrite Growth through Pressure and Density Effects

Despite some progress performed, state-of-the-art lithium ion batteries still require improvements in energy and power to extend the range of electric vehicles and reduce charging time. In this domain, all-solid-state batteries are viable alternatives to conventional batteries employing organic electrolytes because of their benefits, i.e., high power density, high energy density, long-life operation and safety. These advantages stem from the great features of inorganic solid electrolytes, which are single ion conductor, so a high lithium ion transport number, and no-liquid nature. In particular, the sulfide-based solid electrolytes possess favorable mechanical properties, allowing all-solid-state batteries to be easily prepared via simple mixing and cold-pressing processes, facilitating the scale-up.Sulfide-based solid-electrolytes can potentially be employed in conjunction with a lithium metal negative electrode and 5V-class high voltage positive electrode material. Indeed, lithium metal is believed to be the most promising negative electrode due to its specific large capacity (3862 mAh.g-1), low volumetric density (0.534 g.cm-3 at 20°C) and the lowest electrochemical potential (-3.03V vs ENH). Nevertheless, like most metal, lithium metal is morphologically dynamic. Its surface morphology is modified, since during the electrochemical cycling, a part of lithium migrates to the other electrode to react and is then plating on its surface heterogeneously, leading to a volume change and sometimes dendrite growth with potential internal short circuit and life-threatening accidents. Ceramic solid electrolytes have been considered to be the ideal solution to prevent dendrite growth because of their high shear modulus and high lithium transference number. In the same time, the chemical nature and composition of ceramic solid electrolyte can affect the dendrite growth by the interfacial chemical and electrochemical stability with lithium metal forming solid electrolyte interphase as a passivating layer. Since the lithium dendrites have to grow through this layer, its composition should play an important role in the dendrite formation. Moreover, it is known that the stack can be easily deformed because lithium dendrite growth with a high shear modulus, indicating that the solid electrolyte and its interface with lithium metal should be sufficiently strong to endure the pressure originating from lithium dendrite growth. The oxide–based ceramic solid electrolytes can be shaped by sintering at high temperature, leading to a grain and grain-boundary microstructure with some porosity, facilitating the lithium dendrite growth through grain boundaries. Due to the low density and plasticity of sulfide based inorganic solid electrolyte, the dendrite growth through the particle-particle contact can be reduced but still present.Different parameters can influence the lithium dendrite growth and the critical current density in ceramic all-solid-state configuration, such as the solid electrolyte chemical composition, particle size, and the compactness of ceramic solid electrolyte. The pressure effect on the electrochemical performances of sulfide electrolytes was investigated. The pressure affects resistive grain boundaries, contact between lithium metal and solid electrolyte and lithium plating. As, the kinetics of reaction is derived from thermodynamic parameters, the temperature can affect the plating/stripping phenomena. These different parameters and the relationship between them will be presented and explained through complete studies based on sulfide solid electrolytes with the combination of various chemical and electrochemical techniques.

Poly(Ethylene Oxide)-based Electrolyte for Solid-State-Lithium-Batteries with High Voltage Positive Electrodes: Evaluating the Role of Electrolyte Oxidation in Rapid Cell Failure

Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) typically reveal a sudden failure in Li metal cells particularly with high energy density/voltage positive electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622), which is visible in an arbitrary, time – and voltage independent, “voltage noise” during charge. A relation with SPE oxidation was evaluated, for validity reasons on different active materials in potentiodynamic and galvanostatic experiments. The results indicate an exponential current increase and a potential plateau at 4.6 V vs. Li|Li+, respectively, demonstrating that the main oxidation onset of the SPE is above the used working potential of NMC622 being < 4.3 V vs. Li|Li+. Obviously, the SPE│NMC622 interface is unlikely to be the primary source of the observed sudden failure indicated by the “voltage noise”. Instead, our experiments indicate that the Li | SPE interface, and in particular, Li dendrite formation and penetration through the SPE membrane is the main source. This could be simply proven by increasing the SPE membrane thickness or by exchanging the Li metal negative electrode by graphite, which both revealed “voltage noise”-free operation. The effect of membrane thickness is also valid with LiFePO4 electrodes. In summary, it is the cell set-up (PEO thickness, negative electrode), which is crucial for the voltage-noise associated failure, and counterintuitively not a high potential of the positive electrode.

Electrochemical investigations of high-voltage Na4Ni3(PO4)2P2O7 cathode for sodium-ion batteries

The electrochemical properties of carbon and reduced graphene-coated Na4Ni3(PO4)2P2O7 materials have been evaluated as high-voltage positive electrodes for sodium-ion batteries. Na4Ni3(PO4)2P2O7 exhibits the highest Ni3+/Ni2+ redox potential of 4.8 V vs. Na+/Na with a theoretical capacity of 127 mAh g−1. Here, we report on the synthesis and characterizations of Na4Ni3(PO4)2P2O7-reduced graphene oxide and Na4Ni3(PO4)2P2O7-carbon composites. The high-voltage dimethyl carbonate–based electrolyte has been chosen to explore the electrochemical properties of Na4Ni3(PO4)2P2O7 as a cathode. Carbon-coated Na4Ni3(PO4)2P2O7 composite electrode delivers a stable discharge capacity of 51 mAh g−1 at 0.1 C rate for 40 cycles which corresponds to a reversible intercalation/de-intercalation of 1.3 sodium ions. The structural deformation has been observed during the charge–discharge process beyond the removal of 1.3 Na+ ions and has been confirmed by in situ PXRD measurements. The present results provide a guideline to improve the performances of the high-voltage Na4Ni3(PO4)2P2O7 material for the next generation sodium-ion batteries.

Deconvoluting Li-Ion Battery Electrode Crosstalk with Electrochemical Generator-Collector Measurements

Accelerated capacity fade in high-voltage Li-ion batteries is attributed to crosstalk between the positive and negative electrodes. During low potential operation of the negative electrode and high potential operation of the positive electrode, the nonaqueous electrolyte is reduced and oxidized, respectively, forming soluble, insoluble, and gaseous byproducts. Additionally, during electrolyte oxidation transition metals dissolve from active material of the cathode. After cycling, soluble species from the positive electrode are detectable at the negative electrode and are suspected to play an important role in disrupting the formation, growth, and performance of the solid-electrolyte interphase (SEI), leading to unmitigated capacity fade. In full-cell studies the effects of electrolyte decomposition and metal dissolution on battery lifetime and coulombic efficiency are well-documented, however the precise nature of crosstalk species and the mechanism of capacity fade remain to be understood. Here we present two applications of our generator-collector approach to understanding battery crosstalk. First, we employ rotating-ring disk electrode (RRDE) voltammetry to study the effects of metal dissolution from LNMO (LiNi0.5Mn1.5O4) on SEI performance. By cycling an LNMO-coated glassy carbon disk to high operating potentials, we stimulate metal dissolution and collect these soluble species on the downstream glassy carbon ring electrode, which is cycling to low potentials to form and grow an SEI. The effects of metal contamination on SEI passivation are characterized via a redox mediator method and quantified using finite element simulations. We find that metal dissolution has a more profound impact on electronic than morphological SEI properties. Interpreting these results with physics-based models of SEI passivation imply that the inorganic layer is critical to passivation, while organic layers are inert at best. In the second application, we demonstrate an improved method for performing generator-collector measurements on composite electrodes via the development of a microfluidic electrochemical cell. Our design incorporates commercial high-voltage positive electrodes, a graphite negative electrode, and commercial Li-ion battery electrolyte. Forced convection from electrolyte flow isolates the upstream electrode from crosstalk species generated at the downstream electrode, and deconvolutes the role of electrolyte oxidation and reduction on capacity fade. We study the effects of flow rate and flow direction on charge/discharge behavior for each electrode, and correlate coulombic efficiencies to the extent of crosstalk. These electrochemical generator-collector studies provide a greater understanding of the mechanism behind transition metal travel from the positive to negative electrode in a full-cell, as well as the importance of electrolyte decomposition in this transport mechanism.

High-Voltage Sulfolane Plasticized UV-Curable Gel Polymer Electrolyte.

A high-voltage electrolyte can match high-voltage positive electrode material to fully exert its capacity. In this research, a sulfolane plasticized polymer electrolyte was prepared by in situ photocuring. First, the effect of the sulfolane content on the ionic conductivity of the gel polymer electrolyte was investigated. Results showed that the ionic conductivity variation trend was in good agreement with the exponential function model for curve fitting. Second, the activation energy was calculated from the results of the variable temperature conductivity tests. The activation energy was inversely proportional to the sulfolane content. For the sulfolane content of 80 wt. % in gel polymer electrolyte (GPE)-80 (19.5 kJ/mol), the activation energy was close to conventional liquid electrolyte (9.5 kJ/mol), and the conductivity and electrochemical window were 0.64 mS/cm and 5.86 V, respectively. The battery cycle performance test showed that the initial specific discharge capacities of GPE-80 and liquid electrolyte were 176.8 and 148.3 mAh/g, respectively. After 80 cycles, the discharge capacities of GPE-80 and liquid electrolyte were 115.8 and 41.1 mAh/g, and the capacity retention rates were 65.5% and 27.7%, respectively; indicating that GPE-80 has a better specific discharge capacity and cycling performance than the liquid electrolyte. SEM images indicated that GPE-80 can suppress the growth of lithium dendrites. The EDS test showed that GPE-80 can inhibit the dissolution of metal ions in the cathode material.

Sodium and Lithium Metal Nitridophosphates AxMy(PO3)3N as high-voltage positive electrode materials for Sodium-ion and Lithium-Ion batteries

Lithium-ion batteries have become the dominant technology from portable electronics to mid-scale applications owing to their high-energy density and light weight. However, risks on cost fluctuations and resources scarcity have induced an intense search for alternatives. Sodium-ion batteries are considered to be effective energy storage devices, as well as promising candidates that may well replace Lithium-ion technology for stationary applications.1,2 Current research into cathode materials encompasses a wide range of different chemistries, amongst which polyanionic compounds stand out as attractive candidates due to their stable 3D framework.3 Furthermore, according to theoretical calculations, N-substituted polyanionic compounds should enable the use of more than one redox couple of transition metals due to the lowering their working voltage.4 On the example of Na3Ti(PO3)3N and Na2Fe2(PO3)3N,5,6 we first focused on sodium metal nitridophosphates, with the general formula NaxMy(PO3)3N, which crystallize in a cubic structure 7 where sodium cations occupy three independent crystallographic sites (see Figure 1a).8,9 This CUBICON structure is favorable to both electrochemical and ionic properties, resulting in facile ion migration at room temperature and a stable and reversible electrochemistry. In particular, our research on sodium vanadium nitridophospate, Na3V(PO3)3N, has uncovered the remarkably high working voltage of this compound, which together with Na7V3(P2O7)4 is the sodium-based cathode with the highest operation voltage for the VIII+/VIV+ redox couple (4.0V vs. Na+/Na0, Figure 1b).10,11 Na3V(PO3)3N can be as well successfully applied in a hybrid Lithium-ion configuration. The reaction mechanism of this material vs. both Li and Na and the possibility to reach the VIV+/VV+ redox couple will be discussed. Aiming to lower the cost and improve the electrochemistry we have further explored the family of nitridophosphates. Screening several compositions by the replacement of V with other transition metals we have discovered a new family of compounds which will be also shown.12 References 1 V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-González, and T. Rojo, Energy Environ. Sci. 5, 5884 (2012). 2 V. Palomares, M. Casas-Cabanas, E. Castillo-Martínez, M.H. Han, and T. Rojo, Energy Environ. Sci. 6, 2312 (2013). 3 C. Masquelier and L. Croguennec, Chem. Rev. 113, 6552 (2013). 4 M. Armand and M.E.A. y de Dompablo, J. Mater. Chem. 21, 10026 (2011). 5 J. Liu, X. Yu, E. Hu, K.-W. Nam, X.-Q. Yang, and P.G. Khalifah, Chem. Mater. 25, 3929 (2013). 6 J. Liu, D. Chang, P. Whitfield, Y. Janssen, X. Yu, Y. Zhou, J. Bai, J. Ko, K.-W. Nam, L. Wu, Y. Zhu, M. Feygenson, G. Amatucci, A. Van der Ven, X.-Q. Yang, and P. Khalifah, Chem. Mater. 26, 3295 (2014). 7 W. Feldmann, Z. Chem. 27, 182 (1987). 8 I.V. Zatovsky, T.V. Vorobjova, K.V. Domasevitch, I.V. Ogorodnyk, and N.S. Slobodyanik, Acta Crystallographica Section E Structure Reports Online 62, i32 (2006). 9 M. Kim and S.-J. Kim, Acta Crystallographica Section E Structure Reports Online 69, i34 (2013). 10 M. Reynaud, A. Wizner, N. A. Katcho, L.C. Loaiza, M. Galceran, J. Carrasco, T. Rojo, M. Armand, and M. Casas-Cabanas, Electrochemistry Communications 84, 14 (2017). 11 C. Deng, S. Zhang, and B. Zhao, Energy Storage Materials 4, 71 (2016). 12 A. Wizner, M. Reynaud, J.X. Lian, F. Bonilla, J.-M. Lopez del Amo, J. Carrasco, and M. Casas-Cabanas, submitted. Figure 1

Na4Co3(PO4)2P2O7 through Correlative Operando X-ray Diffraction and Electrochemical Impedance Spectroscopy

Na4Co3(PO4)2P2O7, a high-voltage positive Na-ion electrode material, is thoroughly analyzed by coupling operando X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) measurement...

Improved electrochemical performance of LiCoPO4 using eco-friendly aqueous binders

The electrochemical performance of LiCoPO4 (LCP) as a high-voltage positive electrode for lithium-ion batteries is significantly improved by using the aqueous binder sodium carboxymethyl cellulose (CMC). The CMC not only provides a uniform electrode surface as shown by scanning electron microscopy and elemental mapping, but also suppresses the degradation of LiCoPO4 by scavenging HF in the electrolyte solution as demonstrated by FT-IR. In comparison with other water-soluble binders such as sodium alginate (ALG) and polyacrylic acid sodium salt (PAA), the homogeneous distribution of CMC within the electrodes accompanied by high accessibility of carboxylate groups in CMC are shown to be crucial factors to achieve enhanced performance with an excellent capacity retention of 94% after 20 cycles at a rate of C/10.

Effects of Cr Doping on Physical and Electrochemical Properties of Li2MnSiO4 Cathode Materials for Li-Ion Batteries

INTRODUCTION Lithium metal othosilicate (Li2 MSiO4, where M = Fe, Mn, Co or Ni) has currently attracted a good deal of interest as a high voltage positive electrode material due to its high theoretical charge and discharge capacities of 330 mAh g-1 and high voltage up to 5.1 V. Such high capacity arises from the ability to extract two lithium ions per one transition metal. However, this class of material has been reported to exhibit low conductivity and poor cycling performance due to structural instabilities with the transition from orthorhombic to monoclinic phase during charge and discharge.[1] Mn-site doping with transition metal ions (Ni2+, Cr2+, Ti4+, V5+) or the aliovalent ions (Al3+, Ga3+) can induce defects in the lattice and stabilize overall structure. Since the structural instability is generally caused by the collapse of MnO4 coordination structure to MnO6, works have been done to retain the MO4 structure.[2] A study by Cheng, et al. reported that substituting Cr on Mn site partly helps retain M-O coordination of MnO4.[3] In this study, our research focuses on stabilizing an othosilicate-based positive lithium-ion battery electrode material, specifically, lithium manganese silicate (Li2MnSiO4) via partial substitution of Mn with Cr up to 15 mol%. To understand the effect of Cr doping on structural evolution during the charge/discharge process, the in situ XRD is also conducted to understand the correlation between the electrode material’s structural and electrochemical behavior. EXPERIMENTAL Li2Mn1-x Cr x SiO4 (x = 0 - 0.15) were prepared by with an acetic acid-assisted sol–gel method. The dried materials were fired at 800°C for 6 hours in Ar atmosphere. Structural properties of the synthesized samples were studied by X-ray diffraction (Rigaku TTRAXII) using Cu-Kα (λ=1.5418 Å) radiation. Particle morphologies and size of the resulting compounds were observed using a field emission scanning electron microscope (FE-SEM, Hitachi SU8030). Electrochemical characterizations were performed by in Swagelok® cells. Active material, conductive carbon black and polyvinylidene fluoride (PVDF) were mixed in the ratio of 87:5:8 by weight. Galvanostatic charge-discharge cycling was done between 1.5-4.8 V at 0.05C using a MACCOR Series 4000 at room temperature. RESULTS AND DISCUSSIONS Figure 1 shows XRD patterns of the synthesized Li2Mn1-x Cr x SiO4 where Cr = 0 - 0.05. Main diffraction peaks could be indexed as an orthorhombic unit cell with a space group Pmnb. Appearance of the evidence of small amount of monoclinic phase, MnO and Li2SiO3. There is no obvious evidence of a phase containing Cr. With an increasing the Cr doping ratio, the unit cell volume slightly increases from 339.52 Å3 to 339.79 Å3 as expected due to larger ionic radii of Cr2+ than Mn2+ indicating the substitution of Cr into the structure of Li2MnSiO4. Electrochemical performance in relations to structural stability of Li2Mn1-x Cr x SiO4 will be discussed in the meeting. REFERENCES [1] R. Dominko, M. Bele, A. Kokalj, M. Gaberscek, J. Jamnik, Journal of Power Sources, 2007, 174, 457-461. [2] V.V. Politaev, A.A. Petrenko, V.B. Nalbandyan, B.S. Medvedev, E.S. Shvetsova, The Journal of Solid State Chemistry, 2007, 180, 1045–1050. [3] H.M. Cheng, S.X. Zhao, X. Wu, J.W. Zhao, L. Wei, Applied Surface Science, 2018, 433, 1067-1074. Fig. 1 Powder X-ray diffraction patterns of Li2Mn1-x Cr x SiO4 (x = 0, 0.025 and 0.05). Figure 1

On the Electrochemistry of Lipon Breakdown

Solid electrolytes are considered a necessary step for Li-ion ions to surpass the practical limit of 800Wh/L. Next to safety aspects, solid-state electrolytes could enable more compact batteries thus increasing their volumetric storage capacity. A main driver, however, is the potential for solid-electrolytes to increase the electrochemical window to accommodate high voltage positive electrodes (5V cathodes) and stability against metallic lithium as negative electrode. As such, solid-state batteries could enable high energy density through increased cell voltage. Numerous Li-ion conducting solids are known today, however most do not have the broad electrochemical window and stability against lithium yet. Nitrogen doped lithium phosphate glass or LiPON is a well known solid electrolyte material attributed with excellent electrochemical stability above 5V and stable against metallic lithium [1]. However, due to its limited ionic conductivity (~10-6 S/cm) it is only applicable in thin-film batteries for use for medical implants and various sensor systems. It has also been evaluated as solid-electrolyte coatings on electrode particles as buffer layer between active material and the bulk electrolyte. In a thin-film stack, it is an ideal model system for electrochemical study. In this paper, we report on the electrochemistry of RF-sputtered LiPON thin films and studied the out plating of lithium until break down. The LiPON thin-films had ionic conductivities ranging between 1x10-7 and 1x10-6 S/cm [2]. The decomposition potential was calculated from basic thermodynamic quantities and was linked to current-voltage (I-V) measurements with great consistency. The reported I-V measurements were conducted on metal-electrolyte-metal or MEM structures. The decomposition of LiPON was shown to occur at a potential of 4.8 V and proceeds in a diffusion limited way. It could be shown that LiPON breakdown is rather determined by the time needed for the decomposition reactions to complete, rather than being solely driven by the electric field. From the diffusion limited Li-ion current, the Li+ diffusion coefficient was extracted and found to be 2x10-11 cm2/s. This value is in excellent agreement with literature reports [3]. Ultimately hard breakdown was shown by TOF-SIMS imaging experiments and occurs through the formation of metallic lithium filaments. In the present work, we have for the first time formulated a breakdown mechanism for LiPON layers based on experimental measurements and thermodynamic considerations. The presented work provides a detailed evaluation of a solid electrolyte's breakdown and stability using I-V measurements. As such these measurements allow determination of ion transport phenomena as well as of the electrochemical stability window. It can also provide insight in the breakdown mechanism of RRAM devices.

Impedance Analysis of Graphite/Lnmo Cells with a Micro-Reference Electrode: Role of the Graphite Anode

Lithium-Ion Batteries containing a high-voltage positive electrode as LiNi0.5Mn1.5O4 (LNMO) have a high operating voltage and are therefore considered for high energy batteries [1]. However, it’s high potential (4.75 V vs. Li/Li+) causes some severe side reactions, especially at elevated temperatures, such as electrolyte oxidation and transition metal dissolution [2]. A further failure mechanism of graphite/LNMO cells is the increase of the cell impedance during cycling [3]. In a previous study of our group [4] we introduced a novel impedance procedure where we measured the LNMO impedance at two distinct states of charge: at 4.4 V cell voltage (10% SOC, referred to as non-blocking conditions) and at 4.9 V cell voltage (100% SOC, referred to as blocking conditions). By fitting the two obtained spectra at the same time, we were able to deconvolute the cathode impedance (R Cathode) into contributions of the charge transfer resistance (R CT), the pore resistance (R Pore) and the contact resistance between current collector and cathode coating (R Anode). In the present study, we apply the same approach by measuring the anode impedance at 4.4 V cell voltage (10% SOC, non-blocking condition) and 3.0 V cell voltage (0% SOC, blocking condition for the anode) This was examined by assembling graphite/LNMO or graphite/LFP Swagelok T-cells equipped with a gold-wire reference electrode (GWRE) [5] and using 1 M LiPF6 in EC/EMC as electrolyte. The cycling procedure was carried out at 40 °C. The GWRE allows the measurement of the half-cell impedances (anode vs. RE and cathode vs. RE) during cycling, so that the impedance evolution of each electrode can be monitored independently. First, we will show that blocking conditions – a 45° transmission line which represents 1/3 of the pore resistance – can be achieved for the graphite anode cycled versus an LFP cathode. Interestingly this transmission line is also observed for the blocking spectra of a graphite anode cycled versus an LNMO cathode, however upon extended charge/discharge cycling at elevated temperatures (40°C), the transmission line turns into a semi-circle in the impedance spectra (Nyquist representation). This means that after switching the temperature from 25°C (after formation) to 40°C (during charge/discharge cycling), a novel interface resistance evolves, which only appears for graphite anodes cycled versus an LNMO cathode. We attribute this semi-circle evolving in the blocking spectra to occur at the separator/graphite anode interface: (i) temperature dependent impedance measurements suggest an activation energy of ~20 kJ/mol which lies in the range of typical values for ionic resistances [6] and, (ii) estimating the contributing surface area by the capacitance, a very small surface area of ~ 1cm² is obtained that fits to the interface separator/anode. Lastly, we support this hypothesis by adding defined amounts of a manganese (II)-salt into graphite/LFP cells, which in the absence of manganese show a blocking electrode behavior (transmission line) for the anode impedance upon cycling at 40°C. When the manganese salt is added to cell after formation, a semi-circle evolves in the blocking spectra. This leads us to the conclusion that manganese is reduced at the top of the graphite anode and leads to an enhanced SEI formation, which can be detected by our novel impedance procedure. By fitting the obtained anode impedance spectra at blocking and non-blocking conditions, we can deconvolute the anode impedance (R Anode) into contributions of a charge transfer resistance (R CT), the high frequency resistance (R HFR) and the evolving interface resistance between anode and separator (R Contact, ionic), as shown in Figure 1. Figure 1: Fitting results of the anode impedance in blocking conditions (3V cell voltage) and non-blocking conditions (4.4V cell voltage). The Impedance is recorded at 40°C from 100 kHz to 0.1 Hz with a perturbation 15 mV.

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-Ion Batteries.

P2-Na2/3Ni1/3Mn2/3O2 (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na+/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.

The Investigation of Electrolyte Oxidation and Film Deposition Characteristics at High Potentials in a Carbonate-Based Electrolyte Using Pt Electrode

High voltage positive electrodes for lithium ion batteries have suffered from continuous oxidation of the electrolyte during cycling, which largely offsets the benefits of high energy and power densities. In this work, the electrolyte oxidation and concomitant film deposition/dissolution behaviors were investigated on Pt electrode by using linear sweep voltammetry (LSV), electrochemical quartz crystal microbalance (EQCM), and X-ray photoelectron spectroscopy (XPS). Two characteristics were identified. First, film deposition is relatively unfavorable at higher potentials (>4.7 V vs. Li/Li+) because the oxidation products are mostly gaseous or soluble species. Second, the concentration of inorganic species decreases in the surface film as the potential increases, which is likely dissolved by HF or polar species. The dominance of gaseous or soluble products and the partial dissolution of the surface film, are two characteristics which hamper passivation of the electrode surface, leading to severe electrolyte oxidation at the high potentials.

Sodium vanadium nitridophosphate Na3V(PO3)3N as a high-voltage positive electrode material for Na-ion and Li-ion batteries

We report the preparation and electrochemical properties of Na3V(PO3)3N made by ammonolysis. Na3V(PO3)3N is reversibly oxidized to Na2V(PO3)3N at high voltage (4.0V vs. Na+/Na0 and 4.1V vs. Li+/Li0) with an unusually small difference in the insertion/extraction voltage between both alkali metal reference electrodes. In both cases, the voltage hysteresis is extremely small (~0.035V vs. sodium and ~0.065V vs. lithium), which suggests facile migration of alkali cations within the structure. Further oxidation to NaV(PO3)3N is predicted to occur beyond the voltage stability window of the electrolytes.

Identifying Contact Resistances in High-Voltage Cathodes By Impedance Spectroscopy

Lithium-Ion Batteries containing a high-voltage positive electrode as LiNi0.5Mn1.5O4 (LNMO) have a high operating voltage and are therefore considered for high energy batteries [1]. However, it’s high potential (4.75 V vs. Li/Li+) causes some severe side reactions, especially at elevated temperatures, such as electrolyte oxidation and transition metal dissolution [2]. A further failure mechanism of graphite/LNMO cells is the increase of the cell impedance during cycling [1]. In the literature, the impedance spectrum (in Nyquist representation) of the LNMO electrode (measured versus a reference electrode) is commonly described by a high-frequency semi-circle and one at lower frequencies [3]. The impedance spectra are interpreted as a surface film resistance (R SF ) which is attributed to the semi-circle at high frequencies and a charge transfer resistance (R CT ) for the semi-circle at low frequencies [3, 4]. The aim of this study is to examine the true origin of the semi-circle at high frequencies and its implications for the failure mechanism of high-voltage cells. This was examined by assembling graphite/LNMO and graphite/LFP Swagelok T-cells equipped with a gold-wire reference electrode (GWRE) [5] and using 1 M LiPF6 in EC/EMC (3/7 by volume) as electrolyte. The cycling procedure was carried out at 40 °C. The GWRE allows the measurement of the half-cell impedances (anode vs. RE and cathode vs. RE) during cycling, so that the impedance evolution of each electrode can be monitored independently. By means of temperature dependent impedance measurements (0°C, 10°C and 40°C) we will show that the semi-circle at high frequencies (100 kHz – 100 Hz) corresponds to a resistance at the interface between the LNMO electrode and the current collector (contact resistance, R Contact ) rather than to a surface film resistance (R SF ). This is based on the finding that the semi-circle at high frequencies shows negligible temperature dependence, whereas the semi-circle at low frequencies (100 Hz – 1 Hz) strongly depends on temperature. A low temperature dependence is typically reported for a contact resistance in Li-Ion batteries [6]. Furthermore, we will prove that the high-frequency semi-circle corresponds to a contact resistance by: (i) compressing the T-cell after 50 cycles, and (ii) coating the LNMO electrode on a glassy carbon support. In the first case (i), the high frequency semi-circle is no longer observable, and in case (ii), the increase of R Contact is lower by a factor of ten compared to a LNMO electrode coated on an aluminum substrate. As a next step, the potential dependence of the increasing R Contact will be investigated. In Figure 1, graphite/LFP cells are charged to different upper-cut-off potentials (4.0 V, 4.5 V and 5.0 V cell voltage). It can clearly be seen that between 4.5 and 5.0 V the LFP impedance shows an additional semi-circle at high frequencies (R Contact ). This leads us to the conclusion that the increase of R Contact is a potential dependent rather than a material dependent property and thus might also play a role in all high-voltage materials (e.g., in overlithiated NCM’s with upper-cut off potentials of »4.7 V vs. Li/Li+). Lastly, we will show XPS spectra of cycled current collectors and identify the origin of R Contact . Figure 1: (a) Two formation cycles (C/10) are carried out in a graphite/LFP (on carbon-coated aluminum foil) cell with a GWRE (two glassfiber separators, 60 µL of 1 M LiPF6 in EC/EMC, 3/7). Subsequently, the cell is charged to 4.5 V and 5.0 V cell voltage at C/2. (b) The Impedance is recorded at 50% SOC (indicated by black symbols in a) after each upper potential step (at 40°C from 100 kHz to 0.1 Hz with a perturbation of 0.5 mA).

Cycling Performance of Lithium Iron Sulphate in the Presence of Binders and Carbon Additives

Fe-based high voltage positive electrode material Li2Fe(SO4)2 has shown consistent cycling only in experimental Swagelok cell configurations.[1, 2] In order for it to be practically viable, knowledge of its interaction with commercially used additives in the coating process is lacking. Thus, this work seeks to study the cycling performance of Li2Fe(SO4)2in a half cell configuration with different types of binders and carbon based additives as a step towards commercial utilisation of the material. Flourinated binder polvinylidene fluoride (PVDF) and non-fluorinated binder polyacrylonitrile (PAN) were both evaluated and shown to perform similarly. The long term cycling at low C-rates was found to be consistent for the systems with and without binder. However, at high C-rates a significant fall in capacity of nearly 60% was observed for the system with binders in comparison to the system without binder. Li2Fe(SO4)2 being polyanionic has poor electronic conductivity and requires carbon coating to enhance performance. This study investigates the effects of amorphous Super-P Carbon, Multi Walled Carbon Nanotubes and Single Walled Carbon Nanotubes when ball-milled with the electrode material. The carbon-coated material was then subjected to galvanostatic charge discharge tests over multiple cycles and at different charging rates. This study on the effect of binders and carbon additives lays the foundation for future work on an all-iron lithium battery and is a step forward in commercialising sulphate based cathode materials. [1] M. Reynaud, M. Ati, B. C. Melot, M. T. Sougrati, G. Rousse, J.-N. Chotard, et al., "Li2Fe(SO4)2 as a 3.83 V positive electrode material," Electrochemistry Communications, vol. 21, pp. 77-80, 2012. [2] L. Lander, M. Reynaud, G. Rousse, M. T. Sougrati, C. Laberty-Robert, R. J. Messinger, et al., "Synthesis and electrochemical performance of the orthorhombic Li2Fe (SO4) 2 polymorph for Li-ion batteries," Chemistry of Materials, vol. 26, pp. 4178-4189, 2014. Figure 1