Three-electrode Test Cells Research Articles

Effect of Pore Size Distribution on Energy Storage of Nanoporous Carbon Materials in Neat and Dilute Ionic Liquid Electrolytes.

This study investigates three carbide-derived carbon (CDC) materials (TiC, NbC, and Mo2C) characterized by uni-, bi-, and tri-modal pore sizes, respectively, for energy storage in both neat and acetonitrile-diluted 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A distribution of micro- and mesopores was studied through low-temperature N2 and CO2 adsorption. To elucidate the relationships between porosity and the electrochemical properties of carbon materials, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy measurements were conducted using three-electrode test cells. The ultramicroporous TiC-derived carbon is characterized by a high packing density of 0.85 g cm-3, resulting in superior cathodic and anodic capacitances for both neat ionic liquid (IL) and a 1.9 M IL/acetonitrile electrolyte (93.6 and 75.8 F cm-3, respectively, in the dilute IL). However, the bi-modal pore-sized microporous NbC-derived carbon, with slightly lower cathodic and anodic capacitances (i.e., 85.0 and 73.7 F cm-3 in the dilute IL, respectively), has a lower pore resistance, making it more suitable for real-world applications. A symmetric two-electrode capacitor incorporating microporous CDC-NbC electrodes revealed an acceptable cycle life. After 10,000 cycles, the cell retained approximately 75% of its original capacitance, while the equivalent series resistance (ESR) only increased by 13%.

Novel Symmetric Cell Design for Analyzing All-Solid-State Battery Electrode

All-solid-state lithium batteries (ASSBs) with sulfide-based solid electrolytes (SSEs) have drawn much attention recently for the next-generation batteries. But there are some obstacles to overcome for their practical use. The SSE|cathode interface with poor contact and (electro)chemical instability is the subject of special interest, since it critically aggravates the core performance of battery, including cycling stability and rate capability. In addition to the interfacial property, it has been widely known that Li+ transport characteristics inside the cathode is another key factor in terms of fast and efficient operation of all the lithium battery systems, including the ASSBs. Thus, it goes without saying that the accurate characterization of the interfacial and bulk properties of the cathode is essential to develop the ASSBs.There are limited numbers of test cells to analyze the interface and bulk properties of the battery. First, a conventional half-cell includes Li metal as a counter electrode (CE), which is assumed to have a negligible resistance compared to that of the working electrode (i.e., cathode). It has been reported, however, that the effect of Li CE on total electrochemical signal cannot be disregarded. Moreover, the relative contribution of Li CE to total signal is increasing with the use of the state-of-the-art cathode, whose resistance is getting dramatically lower with technological progress. That is, it is getting difficult to obtain a reliable cathode response by using the two-electrode half-cell configuration. Second, a three-electrode test cell provides the signal of each electrode separately and has been extensively used for the electrochemical analysis. When one uses it for the ASSBs, however, the signals can be erroneous due to the structural distortion caused by the reference electrode (RE) insertion and its own chemical instability, which is especially severe in ASSBs with SSEs. Finally, an alternative cell design to avoid the above problems is a symmetric cell including two identical electrodes facing each other. A general symmetric cell is fabricated by charging/discharging two unit-cell, disassembling them, and re-assembling the identical electrodes to face each other. Unfortunately, a series of such procedure, esp. disassembling/re-assembling, is not applicable to the ASSB systems.In this study, a novel non-destructive potential-controllable symmetric cell (PCSC) is proposed for ASSBs. It features a porous temporary CE (TCE) in parallel between two symmetric electrodes. TCE works as an acceptor or provider of Li to charge or discharge, thereby controls the electrode potential. Symmetric electrodes are then evaluated electrochemically after disconnecting the TCE. As a case study, we investigated and comparatively analyze the electrochemical characteristics of an ASSB cathode, LiNbO3-coated LiNi0.8Co0.1Mn0.1O2, obtained from the conventional test cells and PCSC. Figure presents the impedance spectra of test cells. The two-electrode half-cell impedance is noticeably affected by Li CE over a wide frequency range and the three-electrode cell impedance suffers from severe noise probably caused by RE in the low frequency region. This indicates that both test cells are not suitable for interpreting reliably the interfacial and bulk properties of the cathode. In contrast, the impedance spectra from the PCSC have similar trend to that of three-electrode cell, but provides much more stable impedance spectra, allowing more accurate estimation of interface and bulk characters. In this presentation, the results obtained from DC techniques, such as galvanostatic method, are also comparatively analyzed with conventional test cells and our PCSC. Moreover, the reliability of the estimated values from the PCSC will be discussed by the comparison with those of the previous works. Figure 1

In-Depth Analysis of the Substantial Effect of Fast Formation on Lithium-Ion Cell Characteristics

The formation of lithium-ion batteries is a crucial process step in battery production. Although it covers only the first few charging and discharging cycles, it impacts both the long-term degradation as well as the performance characteristics of the final cell. Commonly, formation times are on the order of multiple days due to relatively low C-rates. Although this prevents safety-critical lithium plating, research has shown that neither the slowest nor the fastest formation result into the best possible cell characteristics. [1]To allow for a knowledge-based design of the cell formation process, detailed cell diagnostics including the characterization of the formed solid electrolyte interphase (SEI) are indispensable. However, the preparative effort for most experimental methods for SEI characterization like SEM or XPS is significant and will also require cell disassembly. An in-operando cell diagnosis can be realized with physicochemical modeling based on non-destructive dynamic electrochemical measurements. However, the dynamics of the SEI are commonly either modeled in a simplified way or the models are not designed for the simulation of various measurement types. To overcome these limitations, we extended the classic battery model from Doyle et al. [2] with a detailed SEI modeling. This finally allows to describe C-Rate and EIS data with the same parameter set (see Fig. 1a,1b), providing detailed insights into performance-limiting processes and their changes along cell aging. [3]On the experimental side, we performed a broad formation study at different temperatures with different currents and current profiles, using small-scale three-electrode test cells. Fig. 1c) shows the discharge capacity for different formation procedures. Clearly, the performance significantly depends on the chosen formation conditions. The model-based cell diagnosis helps to shed light onto this interrelation. Surprisingly, we found that the bulk and interfacial properties of the SEI are not the root cause for the substantial differences in the cell’s fast charge/discharge capability. In fact, the effective transport properties in the anode electrolyte phase are driving the performance differences. Furthermore, the cathode reaction kinetics are affected by the chosen cell formation protocol. Ultimately, our experimental formation study in combination with the model-based cell diagnosis highlights that the cell formation process is not only about a stable SEI but also about minimizing the impact of reaction products from the SEI formation on the bulk electrolyte phase.

MnO2 nanosheets synthesized on nitrogen-doped vertically aligned carbon nanotubes as a supercapacitor electrode material

In the present study, nitrogen-doped vertically aligned carbon nanotubes (N-VACNTs) were synthesized on carbon cloth (CC) using the floating catalyst chemical vapor deposition (FCCVD) method, and MnO2 nanosheets were grown on them using the hydrothermal method to produce an N-VACNT@MnO2 nanocomposite structure. In this structure, a synergistic effect was obtained by combining N-VACNTs, which allow high electrical conductivity and fast ion transfer. In the galvanostatic charge-discharge (GCD) measurements performed separately in a three-electrode test cell using 1 M of Na2SO4 electrolyte, the specific capacitance of the N-VACNTs was found to be 53.7 F g−1 at a current density of 0.5 A g−1, and the specific capacitance of the N-VACNT@MnO2 electrode structure was found to be 497 F g−1. In addition, after 10,000 charge-discharge cycles at a current density of 10 A g−1, the N-VACNT electrode retained 99 % of its initial specific capacitance, while the N-VACNT@MnO2 electrode exhibited outstanding cyclic stability with 76 % of its initial specific capacitance. Moreover, in the as-fabricated asymmetric supercapacitor (ASC) using N-VACNTs and N-VACNTs@MnO2, the negative and positive electrodes achieved a stable voltage window of 0–1.7 V, with a specific capacitance of 59.4 F g−1 at a current density of 1 A g−1. Additionally, this ASC showed good cycling stability retaining 84 % of its specific capacitance at the end of 10,000 cycles. Therefore, this ASC structure with an energy density of 24.7 Wh kg−1 and a power density of 15.7 kW kg−1 are promising electrode materials for energy storage.

Fabrication of Fe3O4-incorporated MnO2 nanoflowers as electrodes for enhanced asymmetric supercapacitor performance.

Manganese dioxide (MnO2) is considered a promising aspirant for energy storage materials on account of its higher theoretical capacitance along with low capital cost. However, its performance is generally limited by its poor lifespan and intrinsic conductivity. In this study, MnO2-incorporated iron oxide (Fe3O4) nanoflowers were synthesized through a facile hydrothermal route and their electrochemical performance was probed. The surface composition and morphology of the as-synthesized samples were confirmed using X-ray diffraction, X-ray photoemission spectroscopy, and field emission scanning microscopy. The nanoflower-like structure and synergistic effect between the two oxides of the as-prepared MnO2@Fe3O4 nanocomposite electrode result in desirable surface area and intrinsic conductivity. Owing to its higher surface area and electrical conductivity, the MnO2@Fe3O4 nanoflower-like nanocomposite exhibits an enhanced specific capacitance (Cs) of 1651 F g-1 (1 A g-1) in a three-electrode test cell, which is two-fold higher than those of pristine Fe3O4 and MnO2. In addition, an asymmetric supercapacitor (ASC) was readily constructed by sandwiching a cellulose membrane (separator) between MnO2@Fe3O4 (cathode) and activated carbon (anode). Significantly, the ASC displayed a high Cs of 131 F g-1 (1 A g-1) along with a pretty high cycling stability of 96% over 5000 cycles at 15 A g-1 and a high energy density of 46.6 Wh kg-1 at 0.8 kW kg-1. These results demonstrate the significant potential of the MnO2@Fe3O4 nanoflower ASC device for state-of-the-art futuristic advanced energy storage applications.

Fast-charging capability of lithium-ion cells: Influence of electrode aging and electrolyte consumption

Fast charging of electric vehicles is becoming more and more important for achieving customer acceptance of electromobility. During fast charging, the maximum charging rate of the lithium-ion cells used in the traction batteries of electric vehicles has to be controlled properly to avoid the deposition of metallic lithium on the surface of the negative electrode, known as lithium plating. However, cycle life tests have shown that after a few hundred cycles at a moderate aging rate, the fast-charging capability of lithium-ion cells decreases and a sudden, rapid loss of capacity is observable. Therefore, to achieve a long service life and, concurrently, short charging times, it is crucial to analyze the non-plating critical charging rate depending on the mode of battery degradation.In this paper, we separately investigate the influence of the main aging factors, such as electrode aging and electrolyte consumption, on the fast-charging capability of large-format automotive lithium-ion pouch cells. The pouch cells are cycled to different states of health and a post-mortem local degradation analysis is performed. Using three-electrode test cells, the non-plating critical charging curves are determined for the aged electrode material and for test cells assembled with different amounts of electrolyte. Our findings reveal that for the investigated lithium-ion pouch cells the fast-charging capability is reduced over cycling not by the aging of the electrodes but by the consumption of electrolyte.

Local degradation and differential voltage analysis of aged lithium-ion pouch cells

Abstract Knowledge of the underlying degradation mechanisms in a lithium-ion cell is crucial for the development process and control of automotive lithium-ion battery packs. In this paper, a local post-mortem degradation analysis is performed on two differently cycled large-format pouch cells to improve the understanding of the underlying degradation mechanisms. For each cell, anode and cathode sheets are extracted and electrode coins are cut out from the sheets in an equally spaced, five-by-five pattern to assemble three-electrode test cells. Subsequently, the local capacities and differential voltage curves of the test cells are determined showing an inhomogeneous loss of lithium inventory for one cell and a capacity degradation of the cathode for the other cell. In a parallel connection of the test cells, the influence of the degradation mechanisms on the pouch cell’s differential voltage curve is shown and an approach is validated for calculating the resulting differential voltage curve by superimposing the single measured curves. In addition, the local discharge resistance is evaluated, revealing for both aged cells that the measured resistance increase at pouch cell level is the result of an increased resistance of the cathode and a decreased anode resistance.

Fast charging of an electric vehicle lithium-ion battery at the limit of the lithium deposition process

When charging a lithium-ion cell, the deposition of metallic lithium on the negative electrode surface, known as lithium plating, must be avoided. In this paper, the charging process of a commercial high energy lithium-ion pouch cell is investigated. Three-electrode test cells are assembled using electrode materials from the high energy lithium-ion pouch cell together with lithium metal as reference electrode to acquire the potential at the negative electrode-electrolyte interface. During charging, the cells’ current is controlled in a way that the negative electrode potential is maintained constantly slightly above 0 V vs. Li/Li+.The resulting current map depending on temperature and state of charge is used to control the charging process of the pouch cell. Following this new charging procedure, a state of charge of 80% is reached in 15 min at 25°C ambient temperature. Different cycle life tests are performed to examine iteratively an approach to how the charging current has to be reduced over the lifetime in order to avoid accelerated aging. To prove the practicability, the method is tested at the battery pack level.

Cell optimisation of supercapacitors using a quasi-reference electrode and potentiostatic analysis

A new criterion and potentiostatic method is demonstrated to determine the electrochemically stable potential window (ESPW) of supercapacitors, enabling the calculation of the optimal electrode mass ratio for two different electrode-electrolyte pairings. The criterion uses the decay of electrode current in a three-electrode test cell during potentiostatic ageing at progressively higher voltages to determine the ESPW. To perform these measurements a quasi-reference electrode (QRE) was incorporated within a CR2032 coin cell with a small and simple modification. This enabled three-electrode measurements to be performed inside the coin cell using organic electrolytes whilst outside a glovebox. The voltage-optimised mass ratio (m+/m−) for activated carbon electrodes in 1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN) and propylene carbonate (PC) electrolytes was shown to be 2.38 and 0.61 respectively. The validity of the technique was verified by full-cell potentiostatic ageing experiments on mass-balanced and mass-symmetric supercapacitors. For both electrolytes the maximum operating voltage, energy storage capacity and cycle lifetime were improved significantly with mass balancing. The maximum voltage increased from 2.0 V to 2.9 V for ACN and from 2.3 V to 2.7 V for PC, whilst the energy density increased by ∼90% for ACN and by ∼40% for PC.

Cycling Porous Silicon with Lithium Iron Phosphate for High Performance Li-Ion Batteries

Silicon has drawn significant attention as an anode material for lithium ion batteries due to it significant capacity over carbon anode materials. Much of the literature as focused on cycling silicon with lithium-metal as the counter-electrode, which serves as an infinite source of lithium ions. We have recently shown that porous silicon is able to cycle at 1000 mAh/g at 1C for 600+ cycles against a lithium-metal counter-electrode. Here we will present the practical applicability and cycling performance of porous silicon with commercially available lithium-iron phosphate (LFP) cathodes. Having excess anode capacity is the standard widely used in industry with graphite-based lithium ion batteries to prevent lithium plating issue. However, we have observed that our porous silicon exhibits longer cycle life when the cathode capacity is in excess. This is likely due to the large consumption of lithium-ions during the formation of solid-electrolyte interface (SEI) layer. Our experiments will show that two different cycle performance based on either the anode or cathode in excess for the purpose of assembling a full-cell battery. Furthermore, a three-electrode test cell is used to monitor the I-V curves for each electrode independently to determine if lithium plating or other electrode degradation mechanisms occur.

Facile synthesis and characterization of MnO2 nanomaterials as supercapacitor electrode materials

MnO2 nanomaterials are synthesized via calcinations in air at various temperatures. Amorphous MnO2 masses appear between 100 and 300 °C and nanorods form above 400 °C. Transmission and scanning electron microscopy are used to observe the geometries of each material, with further structural analyses conducted using X-ray photoelectron spectroscopy, X-ray diffraction, and BET method. The electrochemical properties are investigated through galvanostatic charge/discharge cycling, electrochemical impedance spectra, and cyclic voltammetry within a three-electrode test cell filled with 1 mol L−1 Na2SO4 solution. The slightly asymmetric galvanostatic cycling curves suggest that the reversibility of the Faradaic reactions are imperfect, requiring a larger time to charge than discharge. The specific capacitances of each sample are calculated and trends are identified, proving that the samples synthesized at higher temperatures exhibit poorer electrochemical behaviors. The highest calculated specific capacitance is 175 F g−1 by the sample calcinated at 400 °C. However, the lower temperature samples exhibit more favorable geometric properties and higher overall average specific capacitances. For future research, it is suggested that surface modifications such as a carbon coating could be used in conjunction with the MnO2 nanorods to reach the electrochemical properties required by contemporary industrial applications.

Cellulosic Binder-Assisted Formation of Graphene-Paper Electrode with Flat Surface and Porous Internal Structure

We have reported the fabrication of flexible graphene-paper electrode (GPE) with a flat surface, whose internal structure has been formed with gradient porous build-up (from the surface to the 2-hydroxyethyl cellulose (HC)-coated paper). HC solution was used as a binder to form the gradient porous graphene layer, enabling it to create an anchoring force between the porous graphene layer and the filter paper. The morphology of GPE was investigated using a scanning electron microscope, and the surface resistance of the GPE as a function of graphene content was determined using four-probe method. The electrochemical performance of the GPE was evaluated using a three-electrode test cell by cyclic voltammetry. The gravimetric capacitance of GPE was found to be 120 F per gram of graphene, and the capacitance retention was within ca. 96% for over 500 cycles. This could be attributed to both the low surface resistance resulting from the flat surface and the high electrochemical activity caused by the gradient porous structure. This unique structure not only offers an enhanced conductivity and good electrical contact between the electrode and electrolyte but also helps GPE to maintain good cyclic stability, proving its potential for use in various rechargeable and portable energy-storage devices.

FEM modelling of a coaxial three-electrode test cell for electrochemical impedance spectroscopy in lithium ion batteries

Electrochemical impedance spectroscopy for lithium ion batteries has recently gained increasing attention due to its ability of non-invasive evaluation of important electrochemical parameters. Commonly used three-electrode test cells, however, proved unreliable due to asymmetric current line distributions, causing severe distortions of impedance spectra. Finite element method (FEM) simulations can visualize these current lines at different frequencies and simulate impedance spectra at given geometries. By applying FEM simulations to a recently developed coaxial impedance test cell, limiting conditions for reliable impedance measurements could be identified. Using a reference electrode in coaxial position yields sufficiently reliable results as long as the electrode misalignment is small compared to the electrolyte thickness and edge effects are prevented.

Novel NaClO4 and NaPF6 Based Non-Aqueous Electrolytes for Electrical Double Layer Capacitor Application

Novel non-aqueous electrolytes – 1 M NaClO4 and 1 M NaPF6 solutions in ethylene carbonate/dimethyl carbonate solvent mixture – were studied for the application in electrical double layer capacitors (EDLCs) with carbide-derived carbon electrodes. Electrochemical characteristics – region of ideal polarizability, values of series capacitance, high-frequency resistance, time constants, etc. were analysed by applying cyclic voltammetry, impedance spectroscopy and constant current charge/discharge methods both in two- and three-electrode test cells. Based on calculated specific energy and power values and other data obtained, Na-salts based EDLC test cells have similar electrochemical behavior compared to LiPF6 and LiClO4 based systems (studied earlier). Na intercalation/chemical bounding to electrode surface was verified by XPS analysis. Despite the moderate cycle life of Na-salts based EDLCs, these electrolytes are an interesting option for high power/energy devices.

The importance of cell geometry for electrochemical impedance spectroscopy in three-electrode lithium ion battery test cells

The influence of geometric and electrochemical asymmetries on the impedance spectra recorded in three-electrode test cells for lithium ion batteries was investigated. These asymmetries lead to distortions such as e.g. scaling effects which appear in common Swagelok cells. Moving the reference electrode to a coaxial position in combination with a precise alignment of the electrode stack optimized the geometry of current lines, leading to reliable impedance spectra up to frequencies of 50kHz regardless of the electrode configuration.

NaClO4 and NaPF6 as potential non-aqueous electrolyte salts for electrical double layer capacitor application

1M NaClO4 and 1M NaPF6 electrolyte solutions in ethylene carbonate–dimethyl carbonate solvent mixture (1:1 by volume) were studied for the possible application in electrical double layer capacitors (EDLCs) with carbide-derived carbon electrodes (synthesized from Mo2C). Two- and three-electrode test cells were electrochemically characterized by using cyclic voltammetry, constant current charge/discharge and electrochemical impedance spectroscopy methods. Region of ideal polarizability, values of series capacitance, high-frequency resistance, time constants, etc. have been established. Based on the specific energy and power calculations, and other characteristics, it was found that the Na-salts based EDLC test cells have similar electrochemical behavior compared to LiPF6 and LiClO4 based systems that have been studied earlier. The studied Na-salts based electrolytes are potential candidates for EDLCs, however, future improvement of the cyclability is inevitable to achieve long-lasting high performance.

Preparation and electrochemical characterization of LiNi0.8Co0.2O2 cathode material by a modified sol–gel method

LiNi0.8Co0.2O2 cathode powders for lithium-ion batteries were prepared by a modified sol–gel method with citric acid as chelating agent and a small amount of hydroxypropyl cellulose as dispersant agent. The structure and morphology of LiNi0.8Co0.2O2 powders calcined at various temperatures for 4 h in air were characterized by means of powder X-ray diffraction analyzer, scanning electron microscope, thermogravimetric analyzer and differential thermal analyzer, and Brunauer–Emmett–Teller specific surface area analyzer. The results show that LiNi0.8Co0.2O2 powders calcined at 800 °C exhibit the best layered structure ordering and appear to have monodispersed particulates surface. In addition, the electrochemical properties of LiNi0.8Co0.2O2 powders as cathode material were investigated by the charge–discharge and cyclic voltammetry studies in a three-electrode test cell. The initial charge–discharge studies indicate that LiNi0.8Co0.2O2 cathode material obtained from the powders calcined at 800 °C shows the largest charge capacity of 231 mAh g−1 and the largest discharge capacity of 191 mAh g−1. And, the cyclic voltammetry studies indicate that Li+ insertion and extraction in LiNi0.8Co0.2O2 powders is reversible except for the first cycle.

A structural influence on the electrical double-layer characteristics of Al4C3-derived carbon

Several carbon materials were produced by reacting aluminum carbide with chlorine gas at different temperatures (400–900 °C). Chlorination temperature and porosity values showed the inversely related trends whereby the graphitization degree rises with the chlorination temperature. Electrochemical measurements performed in three-electrode test cells with 1.0-M Et3MeNBF4 electrolyte revealed that the changes in porosity parameters and the degree of graphitization are in good correlation with specific capacitance values. Capacitance depends on the structure of carbon and varies in studied chlorination range from 109 to 60 F g−1 and from 114 to 64 F g−1 for the negatively and positively charged electrode materials, respectively. An exceptionally low capacitance was observed for the material produced at 700 °C that was explained by the multiwall carbon nanobarrels and the highly ordered curved graphitic flakes, which have low specific surface and possess the relatively low specific surface-related capacitance.

Composite cathode La 0.15Bi 0.85O 1.5-Ag for intermediate-temperature solid oxide fuel cells

Composites consisting of silver and lanthanum stabilized bismuth oxide (La 0.15Bi 0.85O 1.5) were investigated as cathodes for intermediate-temperature solid oxide fuel cells with doped ceria as electrolyte. No stable phases were formed via reaction between La 0.15Bi 0.85O 1.5 and Ag. The microstructure of the interfaces between composite cathodes and Ce 0.8Sm 0.2O 1.5 electrolytes was studied by scanning electron microscopy after sintering at various temperatures. Impedance spectroscopy measurements revealed that the performance of cathode fired at 700 °C was the best. When the optimum fraction of Ag was 50 vol.%, polarization resistance values for the LSB-Ag50 cathode were as low as 0.14 Ω cm 2 at 700 °C and 0.18 Ω cm 2 at 650 °C. The steady-state polarization investigations on LSB and LSB-Ag50 cathodes were performed using typical three-electrode test cells in air. The results showed that the LSB-Ag50 composite cathode exhibited a lower overpotential and higher exchange current density than LSB, which indicated the electrochemical performance of LSB-Ag50 for the oxygen reduction reaction was superior to the LSB.

Electrochemical characteristics of LiNi 1− xCo xO 2 as positive electrode materials for lithium secondary batteries

Physical properties and electrochemical behavior of LiNi 1− x Co x O 2 (0.1≤ x≤0.9) were investigated for use as positive electrode material in lithium secondary batteries with long cycle life. LiNi 1− x Co x O 2 are characterized by an α-NaFeO 2 type structure. In a three-electrode test cell, LiNi 0.7Co 0.3O 2 showed a large discharge capacity of 159 Ah kg −1, and good cycle performance, with a discharge capacity deterioration ratio of 0.73% per cycle. In a 2 Wh-class cell, LiNi 0.7Co 0.3O 2 exhibited a discharge capacity of 520 mAh, and good cyclability, with a discharge capacity deterioration ratio of 0.026% per cycle. LiNi 0.7Co 0.3O 2 is a promising positive electrode material, having the largest discharge capacity and the best cycle performance in the series of LiNi 1− x Co x O 2 compounds.