High-voltage Li-ion Batteries Research Articles

2 V-Class Aqueous Multi-Ion Batteries Realized By Superconcentrated Na/K Electrolytes

Aqueous superconcentrated electrolyte solutions have recently attracted attention for aqueous high-voltage Li-ion batteries due to their wider potential window than that of conventionally concentrated aqueous ones. According to literatures, 21 mol kg-1 LiN(SO2CF3)2 (LiTFSA)/H2O and 27.8 mol kg-1 Li(TFSA)0.7[N(SO2C2F5)2 (BETI)]0.3/H2O electrolytes show potential window wider than 2 V and 3.5 V, respectively (1, 2). As well as the Li electrolytes, superconcentrated aqueous Na and K electrolytes have been recently studied. Thanks to weak Lewis acidity of Na+ and K+ ions, aqueous Na and K solutions show higher ionic conductivity than Li counterparts in principle. Indeed, 17 mol kg-1 NaClO4/H2O solution exhibits a very high ionic conductivity of 108 mS cm-1; however, potential window wider than 2 V is still challenging in aqueous Na-ion batteries (3). In this study, we have developed superconcentrated mixed cation electrolytes demonstrating wide potential window and high ionic conductivity based on NaN(SO2F)2 (NaFSA), KFSA, and H2O system to realize 2 V-class aqueous Na/K multi-ion batteries. Ionic conductivities of the electrolytes were measured at 25 °C. NaTi2(PO4)3 carbon composite (NTP/C) (4) and Na3V2(PO4)3 (NVP) (5) were used as negative electrode materials, and K2Mn[Fe(CN)6] (6) was as a positive electrode material. Al and Ti foils were used as a current collector for the negative and positive electrodes, respectively. Figure 1a shows water content of saturated aqueous K x Na1-x FSA solutions. The concentration of the saturated NaFSA and KFSA solutions as endmembers is 20 mol kg-1 and 31 mol kg-1, respectively. Higher concentration is realized at eutectic or close to eutectic composition of NaFSA - KFSA: 35 mol kg-1 for Na0.55K0.45FSA/H2O and 33 mol kg-1 for Na0.45K0.55FSA/H2O. Both Na/K mixed electrolytes show ionic conductivity of 20–25 mS cm-1 which is much higher than 3 mS cm-1 of 27.8 mol kg-1 Li(TFSA)0.7(BETI)0.3 (2). Figure 1b shows the LSV curves of Al and Ti foil for a cathodic and anodic scan, respectively, in the superconcentrated Na, K, and mixed electrolytes. 20 mol kg-1 NaFSA/H2O and 31 mol kg-1 KFSA/H2O as endmembers show potential window of 3.2 V and 3.4 V, respectively. As expected, the Na/K mixed electrolytes demonstrate a wider potential window of 3.5 V. NTP//K2Mn[Fe(CN)6] and NVP//K2Mn[Fe(CN)6] cells were fabricated using 33 mol kg-1 K0.55Na0.45FSA/H2O electrolyte. The NTP//K2Mn[Fe(CN)6] cell shows reversible charge/discharge curves with mainly two discharge voltage plateaus located at 1.7 V and 1.3 V and delivers reversible capacity of ca. 130 mAh g (positive electrode)-1 and 70 mAh g (negative electrode)-1 with excellent capacity retention over 30 cycles as shown in Fig. 1c. On the other hand, the NVP//K2Mn[Fe(CN)6] cell demonstrate 2 V-class operation as shown in Fig. 1d. Electrochemical performance and charge/discharge mechanism of the electrode materials in terms of insertion/extraction of Na/K multi-ions will be discussed in the presentation. References L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science, 350, 938 (2015).Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama and A. Yamada, Nat. Energy, 1, 16129 (2016).K. Nakamoto, R. Sakamoto, Y. Sawada, M. Ito and S. Okada, Small Methods, 1800220 (2018).S. I. Park, I. Gocheva, S. Okada and J. Yamaki, J. Electrochem. Soc., 158, A1067 (2011).K. Saravanan, C. W. Mason, A. Rudola, K. H. Wong and P. Balaya, Adv. Energy Mater., 3, 444 (2013).T. Hosaka, K. Kubota, H. Kojima and S. Komaba, Chem. Commun., 54, 8387 (2018). Figure 1

Overcoming the High-Voltage Limitations of Li-Ion Batteries Using a Titanium Nitride Current Collector

The major obstacle to commercialization of high-voltage Li-ion batteries is the lack of oxidatively stable and inexpensive current collectors that can operate at potentials of up to 5 V vs Li+/Li. In this work, we present titanium nitride as a compelling cathode current collector for high-voltage Li-ion batteries exhibiting higher oxidative stability in LiPF6 and lithium bis(fluorosulfonyl)imide electrolytes than aluminum or stainless steel current collectors. Its high oxidative stability has been assessed with a high-voltage LiMn1.5Ni0.5O4 cathode. TiN/LiMn1.5Ni0.5O4 half cells demonstrated a high Coulombic efficiency of 98.5% at a low C-rate of 0.2 C after 100 cycles.

Electrolyte Design for Self-Forming and Self-Healing SEI Layer for Li Metal Anodes

The driving range and cost of electric vehicles is severely limited by the energy density of the state of art Li-ion batteries. The energy density of Li-ion batteries can be improved by replacing graphite anodes with lithium metal anode. The US Department of Energy targets for lithium metal batteries are cost < 100 $/kWh, gravimetric energy density > 500 Wh/kg, volumetric density > 800 Wh/L and have a charging time of 15 min. Lithium metal anodes suffer from dendrite/mossy formation and low cycling efficiency. It has been shown that forming stable SEI layers on top of lithium metal anodes can stop dendrite formation.[i],[ii] Electrolyte components play a crucial role in formation of stable SEI and also the SEI morphology, as the SEI components primarily comprise of decomposition products of the solvent, the salt and the additives in the electrolyte. In this work, we will discuss a new approach of forming a stable SEI layer with sufficient ionic conductivity and mechanical strength to suppress dendrites . The battery under cycling conditions will unavoidably form cracks in the SEI layer. Our approach is to provide a persistent source for self-healing of these cracks formed in the self-formed SEI during battery cycling. This is achieved by choosing appropriate solvents, salt anions and additives in the battery, which will form the same SEI layer during a forming step and during cycling of the battery. Using the appropriate solvent, salt and additives in the desired proportion leads to a spontaneously formed SEI layer comprised of specific components such as LiF, LiOH, Li2O, Li2CO3, Li2SO3, Li2S. The design principles associated with the selection of the electrolyte additives and salt components will be discussed. The effectiveness of the different SEI components will also be compared. We will quantify the amount of electrolyte components required for self-healing during entire cycle life. An important consideration is that the dissolved species in the electrolyte should also be stable to against high voltage Li-ion battery cathodes. Thus the high voltage stability of the electrolytes will also be shown. We will also explore the ion conduction pathways in SEI through the interfaces formed by the different SEI components. We will also discuss formation of stable SEI in the context of thin lithium foils (< 20 mm) and anode free batteries. Lastly the effect of the current collector in the context of anode free batteries will be discussed. [i] Xin-Bing Cheng, Rui Zhang, Chen-Zi Zhao, and Qiang Zhang. Toward safe lithium metal anode in rechargeable batteries: a review. Chemical reviews, 117(15):10403–10473, 2017. [ii] Dingchang Lin, Yayuan Liu, and Yi Cui. Reviving the lithium metal anode for high-energy batteries. Nature nanotechnology, 12(3):194, 2017.

Functional additives assisted ester-carbonate electrolyte enables wide temperature operation of a high-voltage (5 V-Class) Li-ion battery

Nowadays, wide temperature range and high-energy Li-ion batteries are increasingly required. However, it is difficult to get a compromise between subzero temperature performance and high temperature performance. Herein, methyl acetate co-solvent with a freezing point of −98.1 °C (50% by volume) is blended with carbonates to obtain a high-conductivity LiPF6-based electrolyte, which contains tris(trimethylsilyl) phosphite and 1,3-propanediolcyclic sulfate binary functional additives to significantly enhance cycling performance of a challenging high-voltage (5 V-class) battery system using a LiNi0.5Mn1.5O4 cathode and a graphitic mesocarbon microbead anode, unprecedentedly ranging from −60 °C to 50 °C. High reactivity between methyl acetate co-solvent and graphitic mesocarbon microbead anode is innovatively proposed to be associated with graphitic mesocarbon microbead anode catalytic formation of methoxy free radical. More importantly, high reactivity between methyl acetate co-solvent and graphitic mesocarbon microbead anode can be greatly suppressed by species derived from functional additives. This paper highlights the crucial rule of both high Li+ conductivity and favorable graphite anode interface layer for achieving high performance wide temperature range Li-ion batteries.

Effectively enhanced structural stability and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials via poly-(3,4-ethylenedioxythiophene)-in situ coated for high voltage Li-ion batteries.

Spinel LiNi0.5Mn1.5O4 shows promise as a potential candidate for Li-ion batteries due to its high energy density and high rate performance. However, LiNi0.5Mn1.5O4 (LNMO) spinel oxides usually deliver poor cycle life because of the increasing impedance and gradually dissolving Mn resulting in the destruction of crystal structure. Here, a conductive polymer poly-(3,4-ethylenedioxythiophene) (PEDOT) surface modified strategy is introduced to settle the above challenges. The main purpose is to construct a uniform and dense shell film on the surface of LiNi0.5Mn1.5O4 (Industrial Grade), which is prepared by a simple chemical in situ oxidative polymerization method. The Mn dissolving from the lattice during the long-term cycling is well inhibited as the polymer shell protects LiNi0.5Mn1.5O4 from direct exposure to the highly active electrolyte. As expected, the 3 wt% poly-(3,4-ethylenedioxythiophene) coated sample reveals long cycle life with acceptable capacity of 114.5 mA h g−1 and high capacity retention of 91.6% after 200 cycles, compared to 70.9 mA h g−1 and 56.5%, respectively, for the bare LiNi0.5Mn1.5O4 sample. Furthermore, the coated sample demonstrates a higher capacity of 110 mA h g−1 and 63 mA h g−1 at 5C and 10C rate respectively. The improved performance is believed to be attributed to the formation of high conductivity and stable interface structure between electrolyte and LNMO, which is beneficial to suppress the destruction of crystalline structure due to the Mn dissolution and undesired side-reaction between electrolyte and LiNi0.5Mn1.5O4 in long cycle, and improve simultaneously the conductivity and interface stability of LiNi0.5Mn1.5O4 for high voltage lithium-ion batteries.

LiVPO4F@C particles anchored on boron-doped graphene sheets with outstanding Li+ storage performance for high-voltage Li-ion battery

LiVPO4F has been suggested as an excellent electrode for Li-ion battery due to its various advantages. However, the LiVPO4F-based materials reported so far showed poor rate property and required further improvement. Herein, we describe a simple method to synthesize the boron-doped graphene-decorated LiVPO4F@C (BG-LiVPO4F@C) composite for the first time. This designed nanostructure can provide short diffusion distance for Li+ and offer enhanced electronic conductivity for the transport of electron, leading to superior Li+ storage performance. The obtained electrode delivers high capacity of 125.2 mAh g−1 at 10C. Besides, it exhibits excellent cyclic stability with a high capacity retention of about 92.5% after 300 cycles at 20C. Thus, the as-synthesized BG-LiVPO4F@C cathode is a promising candidate for Li-ion battery.

Controlling the Electrochemical Properties of Spinel Intercalation Compounds

Spinel intercalation compounds are well-known to facilitate high rate and high voltage Li-ion batteries. Little is known, however, about spinel based electrodes for Na- and Mg-ion batteries. Here we systematically study six spinel chemistries with first-principles statistical mechanics approaches to determine how the electrochemical properties of spinel compounds are affected by (i) host ionicity, (ii) guest cation radius, and (iii) guest cation oxidation state. As model systems, we consider spinel CoO2 and TiS2 as hosts and Li, Na, and Mg as guest cations. Electrochemical properties of spinel compounds are strongly dependent on guest cation site preferences. Our study shows that large guest cations prefer octahedral coordination, while the electrostatic interactions in ionic hosts favor tetrahedrally coordinated guest cations. The insights of this study suggest simple design principles with which new spinel intercalation compounds can be developed, with our predictions indicating that transition metal oxide spinel compounds containing Co, Ni, and/or Mn could enable high rate Na-ion batteries.

Novel cell designs and methods for characterizing lithium protective membranes for lithium metal batteries

Battery systems employing a lithium metal negative electrode are attractive due to their high theoretical specific energy. However, it is necessary to protect the lithium electrode from crossover species such as polysulfides in Li-S systems, redox mediators in Li-O2 cells, or dissolved cathode species or electrolyte oxidation products in high voltage Li-ion batteries. One approach to tackling these issues is the use of a lithium-ion selective conducting membrane to protect the lithium metal electrode. Such a membrane allows passage of lithium ions, but is impermeable to other species. As well as ability to block crossover species, key requirements of any material used in this way are high Li+ conductivity and high Li+ transport number (together with low interfacial impedance, wide electrochemical stability window, good chemical, thermal and mechanical stability, low cost and ease of processing/device integration). This paper discusses a novel cell design and method developed to characterize lithium-protective membranes in lithium metal batteries, specifically their ability to block crossover species.

A Monolithic High-Voltage Li-Ion Battery Charger With Sharp Mode Transition and Partial Current Control Technique

A high-voltage (HV) lithium-ion battery charger control chip with sharp mode transition and partial current control technique is proposed in this paper. The proposed sharp mode transition eliminates the transition region between constant current (CC) and constant voltage (CV) stages in traditional CC–CV chargers. This technique reduces the charging time and simplifies the compensator design. Furthermore, by adopting the HV partial current control technique, the charging current at the CC stage can be regulated only by sensing part of inductor current. The charger is designed and fabricated using Taiwan Semiconductor Manufacturing Company 0.25- $\mu \text{m}$ 1P3M bipolar-CMOS-DMOS HV process. With a 25-V input voltage, the output voltage (i.e., battery voltage) may range from 6 to 22 V and the maximum charging current is 2.5 A. The peak efficiency reaches 97%, occurring at 1-A charging current.

Towards high-voltage Li-ion batteries: Reversible cycling of graphite anodes and Li-ion batteries in adiponitrile-based electrolytes

Due to their low vapor pressure and their promising electrochemical and thermal stability, NC- (CH2)n-CN dinitriles are proposed as an electrolyte solvent for Li-ion batteries. Adiponitrile (ADN) has substantial advantages, especially for applications requiring high potential cathodes, because it has high electrochemical/thermal stability (up to 6 V vs. Li/Li+,> 120 °C). However, to obtain very high voltage batteries, ADN electrolytes must also passivate the anode of the battery. In this work, reversible cycling of graphite in adiponitrile was successfully achieved by adding a few percent of fluoroethylene carbonate allowing the realization of Graphite/NMC Li-ion battery. The battery of specific capacity of 135 mAhh.g−1 showed a cycling stability for more than 40 cycles. The composition of the solid electrolyte interphase (SEI) was determined as a function of the FEC concentration as well as the state of charge of the graphite anode using hard X-ray photoelectron spectroscopy (HAXPES) and XPS. With FEC, the SEI layer is thinner and depends on the SOC of the anode, but does not depend on the FEC concentration. SEM characterizations clearly showed that the surface of the anode is completely covered by the SEI layer, regardless of the concentration of FEC. Indeed, 2% of FEC is sufficient to suppress the reduction of adiponitrile which is explained by a specific adsorption of FEC on the graphite anode.

Solvent-salt synergy offers a safe pathway towards next generation high voltage Li-ion batteries

Solvent-salt synergy offers a safe pathway towards next generation high voltage Li-ion batteries

A carbonate-free, sulfone-based electrolyte for high-voltage Li-ion batteries

Abstract Practical implementation of next-generation Li-ion battery chemistries is to a large extent obstructed by the absence of an electrolyte that is capable of simultaneously supporting reversible electrochemical reactions at two extreme electrochemical potentials—above 4.5 V at the positive electrode and near 0 V vs. Li at the negative electrode. Electrolytes based on carbonate esters have been reliable in satisfying state-of-the-art Li-ion battery (LIB) chemistries below 4.4 V), releasing CO2. In this work, we explore a carbonate-free electrolyte system based on a single sulfone solvent, in which a newly discovered synergy between solvent and salt simultaneously addresses the interfacial requirements of both graphitic anode and high-voltage cathode (LiNi0.5Mn1.5O4 (LNMO)). Experimental measurements, quantum chemistry (QC) calculations, and molecular dynamics simulations reveal the system’s fast ion conduction, stability over a wide temperature range, and non-flammability. At the anode, a LiF-rich interphase generated by early-onset reduction of the salt anion effectively suppresses solvent co-intercalation and subsequent graphite exfoliation, enabling unprecedented and highly reversible graphite cycling in a pure sulfone system. Under oxidative conditions, QC calculations predict that high salt concentration promotes complex/aggregate formation which slow the decomposition of sulfolane and leads to polymerizable rather than gaseous products—a fundamental improvement over carbonate solvents. These predictions are corroborated by X-ray photoelectron spectroscopy (XPS), cryogenic-transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) experiments, which revealed the presence of a thin, conformal, sulfur-based cathode electrolyte interphase (CEI). Together, the functional interphases (SEI/CEI) generated by this electrolyte system supported long term operation of a high-voltage (4.85 V) LNMO/graphite full cell, which retained ∼70% of its original first-cycle discharge capacity after the 1000th cycle. Based on these results, this new carbonate-free electrolyte system, supported by the mechanistic understanding of its behavior, presents a promising new direction toward unlocking the potential of next generation Li-ion battery electrodes.

Synthesis and characterization of LiFe0.5Mn0.3Co0.2PO4/C composite material for high-voltage Li-ion battery application

A LiFe0.5Mn0.3Co0.2PO4/C composite cathode material was prepared using a solid-state ball-milling method. The flower-like Co3O4 precursor was prepared by hydrothermal method and used to improve the electrochemical properties of composite material. The galvanostatic charge–discharge profile is performed in the potential range of 2–5 V by using different electrolyte compositions with/without 1 wt% trimethyl boroxane (TMB) additive at various C rates. The highest discharge capacities of the composite material were 150.42 mAh g−1 at 0.1C and 120 mAh g−1 at 1C in LiPF6+1 wt%TMB in EC:EMC (1:2, v/v). In addition, the excellent cycle-life was observed at 0.1C and 1C rate for 30 and 100 cycles with the charge retention of 97.7% and 73%, respectively. These appreciable results were obtained due to the carbon coating layer and highly active composite material. The thickness of cathode electrolyte interphase layer on composite electrode is ca. 3 nm which was measured by secondary ion mass spectroscopy. And also, we found that the B element on interphase layer that acts as F-scavenger to reduce the amount of LiF formation over cathode interphase layer. As a result, it can markedly reduce the charge transfer resistance and improve the electrochemical performance for long-term cycling.

First-Principles Study of Spinel MgTiS2 as a Cathode Material

Spinel intercalation hosts are well known to to facilitate high rate capability and high voltage Li-ion batteries. A recent experimental study has shown that Mg can reversibly intercalate in spinel TiS2, demonstrating the viability of Li intercalation host crystal structures for Mg-ion batteries [Sun, X.; Energy Environ. Sci. 2016, 9, 2273−2277]. We report on a first-principles statistical mechanics study of Mg insertion into spinel TiS2, accounting for occupancy on both octahedrally and tetrahedrally coordinated interstitial sites. In contrast to Li-containing spinels, we predict mixed octahedral and tetrahedral site occupancy at nondilute Mg concentrations consistent with the recent experimental study of Sun et al. The onset of mixed occupancy is correlated with an increase in the spinel volume upon Mg insertion, which is more pronounced in MgxTiS2 than in its Li counterpart. The results in this study suggest that the degree of mixed occupancy could be controlled through the volume of the host with addi...

A simple, fast and accurate in-situ method to measure the rate of transport of redox species through membranes for lithium batteries

Lithium ion conducting membranes are important to protect the lithium metal electrode and act as a barrier to crossover species such as polysulphides in Li-S systems, redox mediators in Li-O2 cells or dissolved cathode species or electrolyte oxidation products in high voltage Li-ion batteries. We present an in-situ method for measuring permeability of membranes to crossover redox species. The method employs a ‘Swagelok’ cell design equipped with a glassy carbon working electrode, in which redox species are placed initially in the counter electrode compartment only. Permeability through the membrane, which separates working and counter electrodes, is determined using a square wave voltammetry technique that allows the concentration of crossover redox species to be evaluated over time with very high precision. We test the method using a model and well-behaved electrochemical system to demonstrate its sensitivity, reproducibility and reliability relative to alternative approaches. This new method offers advantages in terms of small electrolyte volume, and simple, fast, quantitative and in-situ measurement.

Enhanced electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode at high cutoff voltage by modifying electrode/electrolyte interface with lithium metasilicate

Developing high-voltage Li ion batteries (LIBs) is an important trend to meet the requirement of high energy density battery. However, high voltage will cause a series of problems harming the cycle performance of LIBs at the same time. This work is to investigate the effect of inorganic substance Li2SiO3 on the electrochemical performance of LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode at high cutoff voltage of 4.6V. XRD result shows that the structure of NCM622 cathode material is not affected by mixing Li2SiO3. However, XPS and EIS tests indicate that Li2SiO3 has an evident influence on suppressing the decomposition of LiPF6 and carbonate solvents at high voltage, reducing interfacial solid film impedance and modifying electrode/electrolyte interface. In addition, Li2SiO3 retards the transition metal dissolution by consuming HF. Therefore, it enhances the electrochemical properties of the NCM622 cathode significantly. The highest discharge capacity increases to 191.7mAhg-1 by mixing Li2SiO3, compared with the value of 180mAhg-1 in the case of NCM622 cathode. The NCM622 electrode mixed with Li2SiO3 also exhibits a better capacity retention of 73.4% after 200 cycles and a high rate capability at 20C with the value of 89mAhg-1, in contrast with 62.2% and 31mAhg-1 attained in the NCM622 cathode.

Impact of Different Graphite Anodes on the Performance of Lithium-Ion Cells with Ni-Rich NMC Cathodes

The development of nickel-rich NMC cathodes (LiNixMnyCo1-x-yO2 with x > 0.5) is among the most promising routes to deliver lithium-ion cells with energy densities exceeding 250 Wh/kg. Achieving this ambitious target will require cycling cells to higher voltage (>4.35 V) and developing new strategies to mitigate undesirable side-reactions like electrolyte oxidation and cathode surface reconstruction. Since the cathode has long been considered the limiting factor in developing high-energy, high-voltage cells, the impact of the graphite anode has been largely ignored. In this contribution, we show that the choice of graphite significantly impacts both the rate capability and long-term cycling stability of cells with Ni-rich NMC cathodes. Six commercially-available natural and synthetic graphites were tested in full cells with NMC 811 cathodes (LiNi0.8Mn0.1Co0.1O2). The variations in cell performance were correlated with the chemical, morphological, and mechanical properties of the different graphite anodes. Surface chemistry was systematically investigated using X-ray photoelectron spectroscopy (XPS), while electron microscopy and mercury porosimetry were used to understand how electrode structure impacts the electrochemical performance. Peel tests were used to identify the importance of mechanical adhesion of different graphites to the current collector. In-depth electrochemical studies including impedance spectroscopy further shed light on the nature of capacity fading. Our study illustrates the important role of the graphite anode in developing high-energy, high-voltage Li-ion batteries. This work serves as a guide to select the appropriate graphite to optimize performance of Ni-rich NMC cells. Acknowledgement: This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Deputy Director: David Howell) Applied Battery Research subprogram (Program Manager: Peter Faguy).

Analyzing High-Voltage Graphite/Lnmo Cells with a Novel Impedance Procedure Utilizing a Micro-Reference Electrode

Graphite/LiNi0.5Mn1.5O4 full cells show severe capacity fading, especially at elevated temperatures, frequently related to: (i) electrolyte oxidation, (ii) transition metal dissolution, and (iii) damage of the Solid Electrolyte Interphase (SEI) on the graphite anode [1]. All these effects can lead to an increase in the full-cell impedance. However, to understand the degradation mechanism of high-voltage Li-Ion batteries, anode and cathode impedances have to be investigated separately. To deconvolute the contribution of anode and cathode from the full-cell impedance, micro-reference electrodes can be used [2, 3]. This type of reference electrodes allows to record the impedance of anode and cathode during cycling without the need of full-cell disassembly to determine anode and cathode impedance in symmetric cells [4]. Commonly the semi-circles in a Nyquist plot are interpreted as combination of a charge transfer resistance (R CT) and a surface film resistance (R SF), e.g., the solid electrolyte interphase (SEI) on graphite anodes [5]. However, the contributions of the ionic resistance (R Ionic) in the pores of the electrode and also the contact resistance (RContact ) between the active material/current collector interface are often neglected, which can lead to misinterpretations in the impedance analysis [6]. In this study, we use a novel impedance approach utilizing a gold-wire reference electrode (GWRE) [2], which allows for the recording the half-cell impedances in its deconvolution into contributions from: (i) contact resistance (R Contact), (ii) ionic resistance in the pores (R Ionic), and (iii) charge transfer resistance (RCT ). This is illustrated for a cathode without surface film formation. Therefore, graphite/LiNi0.5Mn1.5O4 (LNMO) cells are assembled, which are equipped with a gold-wire reference electrode (GWRE) [2] and contain 1 M LiPF6 in EC/EMC (3/7 by volume) as electrolyte. The impedance is measured at two distinct points during cycling: (i) at 50% state of charge (SOC) and (ii) at 100% SOC. Both impedance spectra are analyzed using Matlab employing a transmission line model. Figure 1 exemplarily presents the results of the impedance analysis for more than 80 cycles of a LNMO electrode in a full-cell. We can show that the R CT has only minor contribution to the half-cell impedance of the LNMO cathode and that R Contact and R Ionic are the dominating resistances, which are slightly increasing during cycling. We explain the higher ionic resistance in the pores with electrolyte oxidation products, causing pore clogging. Furthermore, we will show results for the negative electrode (Graphite). Therefore, the anode is analyzed analogously and these results are compared with Graphite/LFP cells, where the upper cut-off potential is lower by 0.8 V (compared to a graphite/LNMO cells). Hence, electrolyte oxidation products should not affect cathode or anode. Figure 1: Fitted resistances from the Nyquist plots at 50% and 100% SOC. The Impedance is recorded at 40°C from 100 kHz to 1 Hz with a perturbation of 15 mV. The Loading of the anode was 6.6 mgGraphite/cm2 (0.95 cm2 area) and 13 mgLNMO/cm2 (0.95 cm2 area) for the cathode. A Swagelok T-cell with 60 ml 1M LiPF6 in EC/EMC (3/7 by volume) and two glass fiber separators with a gold-wire reference electrode (GWRE) was used.

Synthesis and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 via an original wet-chemical route for high voltage Li-ion batteries

The LNCM oxide precursor (lithiated transition metal oxide precursor) with α-NaFeO2 structure and imperfect crystallinity is successfully synthesized via an original wet-chemical process. The layered LiNi1/3Co1/3Mn1/3O2 cathode material for Li-ion batteries (LIBs) with good crystallinity, satisfied cation ordering and homogeneous morphology is prepared from the LNCM oxide precursor following by heat treatment. The initial capacities of the obtained material are 143.0, 154.9 and 164.6mAh·g−1 at 1C rate (150mA·g−1) in different voltage ranges of 3.0–4.3, 3.0–4.4 and 3.0–4.5V, respectively. After 100 cycles it retains 91.2, 91.0 and 90.8% of the initial capacities. It shows that the original wet-chemical process followed by heat treatment is an ideal route to prepare the LiNi1/3Co1/3Mn1/3O2 material with outstanding electrochemical properties for high voltage LIBs.

Computational screening of phosphite derivatives as high-performance additives in high-voltage Li-ion batteries

We presented a computational screening protocol for the efficient development of cathode-electrolyte interphase (CEI)-forming additive materialsviathe first-principles calculations.