A review of conduction phenomena in Li-ion batteries

Conduction has been one of the main barriers to further improvements in Li-ion batteries and is expected to remain so for the foreseeable future. In an effort to gain a better understanding of the conduction phenomena in Li-ion batteries and enable breakthrough technologies, a comprehensive survey of conduction phenomena in all components of a Li-ion cell incorporating theoretical, experimental, and simulation studies, is presented here. Included are a survey of the fundamentals of electrical and ionic conduction theories; a survey of the critical results, issues and challenges with respect to ionic and electronic conduction in the cathode, anode and electrolyte; a review of the relationship between electrical and ionic conduction for three cathode materials: LiCoO₂, LiMn₂O₄, LiFePO₄; a discussion of phase change in graphitic anodes and how it relates to diffusivity and conductivity; and the key conduction issues with organic liquid, solid-state and ionic liquid electrolytes.     

Characterizing capacity loss of lithium oxygen batteries by impedance spectroscopy

Polymer based carbon aerogels were prepared by synthesis of a resorcinol formaldehyde gel followed by pyrolysis at 1073 K under Ar and activation of the resultant carbon under CO₂ at different temperatures. The prepared carbon aerogels were used as active materials in the preparation of cathode electrodes for lithium oxygen cells and the electrochemical performance of the cells was evaluated by galvanostatic charge/discharge cycling and electrochemical impedance measurements. It was shown that the storage capacity and discharge voltage of a Li/O₂ cell strongly depend on the porous structure of the carbon used in cathode. EIS results also showed that the shape and value of the resistance in the impedance spectrum of a Li/O₂ cell are strongly affected by the porosity of carbon used in the cathode. Porosity changes due to the build up of discharge products hinder the oxygen and lithium ion transfer into the electrode, resulting in a gradual increase in the cell impedance with cycling. The discharge capacity and cycle life of the battery decrease significantly as its internal resistance increases with charge/discharge cycling.     

Understanding the characteristics of high-voltage additives in Li-ion batteries: Solvent effects

Calculations are made of the ionization potential (IP) and the oxidation potential (E_ox) values of 108 organic molecules that are potential electrolyte additives for the overcharge protection of lithium-ion batteries (LIBs). The calculated E_ox values are in close agreement with the experimental ones, where the root-mean-square deviation is 0.08 V and the maximum deviation is 0.15 V. The molecules exhibiting high E_ox (>4.5 V) show one of the following two features: (1) IP > 7.70 eV or (2) IP < 7.70 eV with a relatively large molecule size. Consideration of bulk solvent effects, in particular the electrostatic attraction between solute and solvent, is crucial in determining E_ox. Considering its accuracy and reliability, the density functional calculation is recommended as a useful tool for screening electrolyte additives for LIBs.     

High capacity and high rate capability of nanostructured CuFeO2 anode materials for lithium-ion batteries

Non-toxic, cheap, nanostructured ternary transition metal oxide CuFeO₂ was synthesised using a simple sol-gel method at different temperatures. The effects of the processing temperature on the particle size and electrochemical performance of the nanostructured CuFeO₂ were investigated. The electrochemical results show that the sample synthesised at 650 °C shows the best cycling performance, retaining a specific capacity of 475 mAh g⁻¹ beyond 100 cycles, with a capacity fading of less than 0.33% per cycle. The electrode also exhibits good rate capability in the range of 0.5C-4C. At the high rate of 4C, the reversible capacity of CuFeO₂ is around 170 mAh g⁻¹. It is believed that the ternary transition metal oxide CuFeO₂ is quite acceptable compared with other high performance nanostructured anode materials.     

Improved electrolytes for Li-ion batteries: Mixtures of ionic liquid and organic electrolyte with enhanced safety and electrochemical performance

Physical and electrochemical characteristics of Li-ion battery systems based on LiFePO₄ cathodes and graphite anodes with mixture electrolytes were investigated. The mixed electrolytes are based on an ionic liquid (IL), and organic solvents used in commercial batteries. We investigated a range of compositions to determine an optimum conductivity and non-flammability of the mixed electrolyte. This led us to examine mixtures of ILs with the organic electrolyte usually employed in commercial Li-ion batteries, i.e., ethylene carbonate (EC) and diethylene carbonate (DEC). The IL electrolyte consisted of (trifluoromethyl sulfonylimide) (TFSI) as anion and 1-ethyl-3-methyleimidazolium (EMI) as the cation. The physical and electrochemical properties of some of these mixtures showed an improvement characteristics compared to the constituents alone. The safety was improved with electrolyte mixtures; when IL content in the mixture is ≥40%, no flammability is observed. A stable SEI layer was obtained on the MCMB graphite anode in these mixed electrolytes, which is not obtained with IL containing the TFSI-anion. The high-rate capability of LiFePO₄ is similar in the organic electrolyte and the mixture with a composition of 1:1. The interface resistance of the LiFePO₄ cathode is stabilized when the IL is added to the electrolyte. A reversible capacity of 155 mAh g⁻¹ at C/12 is obtained with cells having at least some organic electrolyte compared to only 124 mAh g⁻¹ with pure IL. With increasing discharge rate, the capacity is maintained close to that in the organic solvent up to 2 C rate. At higher rates, the results with mixture electrolytes start to deviate from the pure organic electrolyte cell. The evaluation of the Li-ion cells; LiFePO₄//Li₄Ti₅O₁₂ with organic and, 40% mixture electrolytes showed good 1st CE at 98.7 and 93.0%, respectively. The power performance of both cell configurations is comparable up to 2 C rate. This study indicates that safety and electrochemical performance of the Li-ion battery can be improved by using mixed IL and organic solvents.     

Online estimation of model parameters and state-of-charge of LiFePO4 batteries in electric vehicles

The accurate estimation of internal parameters and state-of-charge (SoC) of battery, which greatly depends on proper models and corresponding high-efficiency, high-accuracy algorithms, is one of the critical issues for the battery management system. A model-based online estimation method of a LiFePO₄ battery is presented for application in electric vehicles (EVs) by using an adaptive extended Kalman filter (AEKF) algorithm. The Thevenin equivalent circuit model is selected to model the LiFePO₄ battery and its mathematics equations are deduced to some extent. Additionally, an implementation of the AEKF algorithm is elaborated and employed for the online parameters’ estimation of the LiFePO₄ battery model. To illustrate advantages of the online parameters’ estimation, a comparison analysis is performed on the terminal voltages between the online estimation and the offline calculation under the Hybrid pulse power characteristic (HPPC) test and the Urban Dynamometer Driving Schedule (UDDS) test. Furthermore, an efficient online SoC estimation approach based on the online estimation result of open-circuit voltage (OCV) is proposed. The experimental results show that the online SoC estimation based on OCV-SoC can efficiently limit the error below 0.041.     

Effect of propane sultone on elevated temperature performance of anode and cathode materials in lithium-ion batteries

A detailed investigation of the effect of the thermal stabilizing additive, propane sultone (PS), on the reactions of the electrolyte with the surface of the electrodes in lithium-ion cells has been conducted. Cells were constructed with meso-carbon micro-bead (MCMB) anode, LiNi_0.8Co_0.2O₂ cathode and 1.0 M LiPF₆ in 1:1:1 EC/DEC/DMC electrolyte with and without PS. After formation cycling, cells were stored at 75 °C for 15 days. Cells containing 2% PS had better capacity retention than cells without added PS after storage at 75 °C. The surfaces of the electrodes from cycled cells were analyzed via a combination of TGA, XPS and SEM. The addition of 2% PS results in the initial formation of S containing species on the anode consistent with the selective reduction of PS. However, modifications of the cathode surface in cells with added PS appear to be the source of capacity resilience after storage at 75 °C.     

Fractional‐order modeling and SOC estimation of lithium‐ion battery considering capacity loss

For state‐of‐charge (SOC) estimation, the resistance deterioration and continuous capacity loss can lead to erroneous estimation results. In this paper, an SOC estimator of lithium‐ion battery based on the fractional‐order model and adaptive dual Kalman filtering algorithm is proposed first. Then, to improve the accuracy of SOC estimation considering capacity loss, the particle filter algorithm is applied to update capacity online in real time. Then, an SOC estimation method is proposed considering battery capacity loss. The simulation results show that the accuracy of battery capacity prediction based on particle filter is high under the condition of capacity loss.     

Increased cycling efficiency and rate capability of copper-coated silicon anodes in lithium-ion batteries

Cycling efficiency and rate capability of porous copper-coated, amorphous silicon thin-film negative electrodes are compared to equivalent silicon thin-film electrodes in lithium-ion batteries. The presence of a copper layer coated on the active material plays a beneficial role in increasing the cycling efficiency and the rate capability of silicon thin-film electrodes. Between 3C and C/8 discharge rates, the available cell energy decreased by 8% and 18% for 40 nm copper-coated silicon and equivalent silicon thin-film electrodes, respectively. Copper-coated silicon thin-film electrodes also show higher cycling efficiency, resulting in lower capacity fade, than equivalent silicon thin-film electrodes. We believe that copper appears to act as a glue that binds the electrode together and prevents the electronic isolation of silicon particles, thereby decreasing capacity loss. Rate capability decreases significantly at higher copper coating thicknesses as the silicon active material is not accessed, suggesting that the thickness and porosity of the copper coating need to be optimized for enhanced capacity retention and rate capability in this system.     

Low‐temperature reversible capacity loss and aging mechanism in lithium‐ion batteries for different discharge profiles

In this paper, reversible capacity loss of lithium‐ion batteries that cycled with different discharge profiles (0.5, 1, and 2 C) is investigated at low temperature (?10°C). The results show that the capacity and power degradation is more severe under the condition of low discharge rate, not the widely accepted high discharge rate. To shed some light on the aging phenomena, noninvasive electrochemical methods, ie, incremental capacity and differential voltage analysis, are applied to identify and quantify the effects of different degradation modes (DMs). Apart from the resistance increase, the DMs include the loss of lithium inventory (LLI) and the loss of active material (LAM). Both LLI and LAM decay to a greater extent for the cell cycled with lower discharge rate, and the growth of LAM is higher than that of LLI. Further, the analysis of state of charge (SOC) window shows that the earlier cutoff of the high discharge rate can lead to less mechanical and thermal stress on cathode materials, thus a lower degradation rate. Another cause is that the lithium plating on the anode materials can be mitigated by increasing the charging temperature which results from preceding high rate discharging.     

Effect of electrolytes on the structure and evolution of the solid electrolyte interphase (SEI) in Li-ion batteries: A molecular dynamics study

We have studied the formation and growth of solid-electrolyte interphase (SEI) for the case of ethylene carbonate (EC), dimethyl carbonate (DMC) and mixtures of these electrolytes using molecular dynamics simulations. We have considered SEI growth on both Li metal surfaces and using a simulation framework that allows us to vary the Li surface density on the anode surface. Using our simulations we have obtained the detailed structure and distribution of different constituents in the SEI as a function of the distance from the anode surfaces. We find that SEI films formed in the presence of EC are rich in Li₂CO₃ and Li₂O, while LiOCH₃ is the primary constituent of DMC films. We find that dilithium ethylene dicarbonate, LiEDC, is formed in the presence of EC at low Li surface densities, but it quickly decomposes to inorganic salts during subsequent growth in Li rich environments. The surface films formed in our simulations have a multilayer structure with regions rich in inorganic and organic salts located near the anode surface and the electrolyte interface, respectively, in agreement with depth profiling experiments. Our computed formation potentials 1.0 V vs. Li/Li⁺ is also in excellent accord with experimental measurements. We have also calculated the elastic stiffness of the SEI films; we find that they are significantly stiffer than Li metal, but are somewhat more compliant compared to the graphite anode.     

Comparison of multiwalled carbon nanotubes and carbon black as percolative paths in aqueous-based natural graphite negative electrodes with high-rate capability for lithium-ion batteries

The effects of multiwalled carbon nanotubes (MWNTs) and carbon black (CB) as conducting additives on the rate capability of natural graphite negative electrodes in lithium-ion (Li-ion) batteries is investigated within concentration ranges where no degradation of anode capacity is observed. MWNT or CB solutions prepared with Nafion in an 80:20 volume mixture of water:1-propanol are incorporated into graphite precursor suspensions consisting of graphite particulates, carboxymethyl cellulose, and styrene butadiene rubber prepared in an aqueous medium. While negative electrodes with MWNTs demonstrate much better rate behaviour than those without MWNTs at a high C-rate, the rate capability of negative electrodes with MWNTs is not much different from that with CB. The reason for this similar behaviour is investigated with respect to the structural changes and aspect ratio of MWNTs, as well as the density difference between MWNTs and carbon black. Scanning electron microscopy images and Raman spectra for the dispersed MWNTs indicate that MWNTs are significantly damaged and shortened during dispersion, which reduces their electrical conductivity and increases their percolation threshold. This damage negatively affects the rate capability of graphite-nanotube composite electrodes.     

High stability and superior rate capability of three-dimensional hierarchical SnS2 microspheres as anode material in lithium ion batteries

3D hierarchical SnS₂ microspheres have been designed and fabricated via a one-pot biomolecule-assisted hydrothermal method. When used as anode material in rechargeable Li-ion batteries, the as-formed SnS₂ microspheres self-assembled by layered nanosheets, show high lithium storage capacity, long-term cycling stability and superior rate capability. After charge-discharge for 100 cycles, the remaining discharge capacities are kept as high as 570.3, 486.2, and 264 mAh g⁻¹ at 1C (0.65 A g⁻¹), 5C, and 10C rate, respectively. Such outstanding performance of these SnS₂ microspheres is ascribed to their unique 3D hierarchical structures. The new charge-discharge mechanism of 3D SnS₂ microsphere as anode in Li-ion battery is further revealed.     

Effect of electrolyte transport properties and variations in the morphological parameters on the variation of side reaction rate across the anode electrode and the aging of lithium ion batteries

In this work, for the first time, we model the variation of solid electrolyte interface (SEI) across the depth of anode electrode of lithium ion battery. It is anticipated that due to higher thickness of SEI layer at the electrode side connected to the separator, a more critical condition prevails there. The present work also investigates the effects of variations in the morphological parameters including porosity, interfacial surface area and active particle radius across anode electrode on the uniformity of side reaction. Moreover, the sensitivity of the side reaction uniformity to electrolyte parameters, such as diffusion and ionic conductivity, is studied. Results show that the ionic conductivity has a major role on the uniformity, and could reduce critical conditions in the part of electrode next to the separator. Moreover, simulation results show that increasing ionic conductivity could significantly prolong the lifetime of the battery. An increase in electrolyte diffusion improves side reaction uniformity. Results also show that positive gradients of morphological parameters across anode electrode, when parameters are changed independently, have considerable effects on uniformity of side reaction. This could be a criterion in choosing new morphologies for the part of anode electrode connected to separator.     

Preparation and electrochemical properties of Li[Ni1/3Co1/3Mn1−x/3Zrx/3]O2 cathode materials for Li-ion batteries

Layer-structured Zr doped Li[Ni_1/3Co_1/3Mn_1−x/3Zr_x/3]O₂ (0 ≤ x ≤ 0.05) were synthesized via slurry spray drying method. The powders were characterized by XRD, SEM and galvanostatic charge/discharge tests. The products remained single-phase within the range of 0 ≤ x ≤ 0.03. The charge and discharge cycling of the cells showed that Zr doping enhanced cycle life compared to the bare one, while did not cause the reduction of the discharge capacity of Li[Ni_1/3Co_1/3Mn_1/3]O₂. The unchanged peak shape in the differential capacity versus voltage curve suggested that the Zr had the effect to stabilize the structure during cycling. More interestingly, the rate capability was greatly improved. The sample with x = 0.01 presented a capacity of 160.2 mAh g⁻¹ at current density of 640 mA g⁻¹(4 C), corresponding to 92.4% of its capacity at 32 mA g⁻¹(0.2 C). The favorable performance of the doped sample could be attributed to its increased lattice parameter.     

The dual capacity of the NiSn alloy/MWCNT nanocomposite for sodium and hydrogen ions storage using porous Cu foam as a current collector

A nanostructured NiSn alloy/multi-walled carbon nanotube (MWCNT) composite was successfully synthesized for highly reversible sodium and hydrogen ions storage by using an electrochemical deposition process on porous Cu foam. The surface morphology of the resulting NiSn alloy/MWCNT nanocomposite was characterized using a field-emission scanning electron microscope, indicating the formation of sphere-like NiSn alloy nanoparticles with an average size of 190 nm. On the other hand, X-ray diffraction analysis, energy dispersive and Fourier transform infrared spectroscopies were employed to determine the crystalline structure, elemental surface and chemical composition of the nanocomposite electrode. The initial sodium discharge capacity of the electrode was maximized at ∼550 mAh g⁻¹ under the current density of 1000 mA g⁻¹, and a high hydrogen discharge capacity of 5200 mAh g⁻¹ was obtained at 1100 mA g⁻¹ after 20 cycles. A comprehensive comparison between the sodium and hydrogen ions capacities in this study and those of the literature for different materials and structures was also performed. Accordingly, the resulting nanocomposite electrode with dual capacity may offer promising applications in both sodium-ion battery and hydrogen storage.