Irreversible capacity loss of graphite electrode in lithium-ion batteries

The irreversible capacity loss of the carbon electrode in lithium-ion batteries at the first cycle is caused mostly by surface film growth. We inspected an unknown irreversible capacity loss (UICL) of the natural graphite electrodes. The charge/discharge behavior of graphite and meso-phase carbon microbeads heat-treated at 2800°C (MCMB28) as the materials of the carbon anode in the lithium-ion battery were compared. It was found that the capacity loss of the natural graphite electrode in the first cycle is caused not only by surface film growth, but also by irreversible lithium-ion intercalation on the new formed surface at the potential range of lithium intercalation, while the capacity loss of the MCMB28 electrode is mainly originated from surface film growth. The reason for the difference of their irreversible capacity losses of these two kinds of carbon material was explained in relation to their structural characteristics.     

Carbon-coated graphite for anode of lithium ion rechargeable batteries: Carbon coating conditions and precursors

Carbon coating of natural graphite particles was performed by mechanical mixing of natural graphite with different carbon precursors in a scale of about 100 g. Anode performance in lithium ion rechargeable batteries was studied on the resultant carbon-coated graphite. Carbon formed on graphite particles had amorphous structure and low density. By carbon coating, a decrease in irreversible capacity of the first charge/discharge cycle in an electrolyte solution of EC/PC = 3/1 was observed, without noticeable change in discharge capacity. Carbon derived from different precursors did not give any marked difference in anode performance of carbon-coated graphite. Optimum conditions for carbon coating were determined as the coating of 4-13 mass% at 700-1000 °C. The present mechanical mixing of natural graphite and carbon precursor in powder is concluded to be a simple but sufficient process to produce carbon-coated graphite for anode material in lithium ion rechargeable batteries. As carbon precursor, PVA was shown to be one of the appreciable carbon precursors.     

Electrochemical characterization of thermally oxidized natural graphite anodes in lithium-ion batteries

Natural graphite, which is used as an anode material in lithium-ion batteries, is thermally treated to improve its cycleability and reduce irreversible reactions with the electrolyte. Natural graphite is treated in air at 550 °C. The weight loss increases when the thermal oxidation time is increased. The BET surface area of the graphite decreases with increasing weight loss. The cycleability and efficiency of the thermally oxidized natural graphite improves significantly. Thermal oxidation decreases the irreversible capacity for side-reactions with the electrolyte on the first cycle. By contrast, it does not change the reversible capacity and rate capability. The improvement in the cycleability after thermal oxidation may be due to the removal of imperfect sites on the graphite.     

Lithium-ion batteries based on carbon–silicon–graphite composite anodes

The paper is devoted to the development of lithium-ion battery grade negative electrode active materials with higher reversible capacity than that offered by conventional graphite. The authors report on results of their experiments as related to the electrochemical performance of silicon-based materials for lithium-ion batteries. A commercial grade of spherically shaped natural graphite (FormulaBT™ SLA1025) was modified in a number of different ways with nano-sized silicon. The reversible capacity of SLA1025 modified by 9.2 wt% of the nano-sized amorphous silicon was seen to be as high as 590 mAh g⁻¹. The irreversible capacity loss with this compound was 20%. Lithium-ion batteries using such material were observed to display sharp capacity decay during prolonged cycling. In contrast, the reversible capacity of another experimental grade, the SLA1025 modified by 7.9 wt% of the carbon-coated Si was as high as 604 mAh g⁻¹. The irreversible capacity loss with this material was as low as 8.1%. This grade, also, was seen to display much better cycling performance than the baseline natural graphite.     

Active lithium replenishment to extend the life of a cell employing carbon and iron phosphate electrodes

We describe and implement a method of extending the life of a LiFePO₄/graphite lithium ion battery by replenishing the lost active lithium during cell operation and concomitant capacity fade. The approach may prove helpful in terms of increasing lithium ion cell life. After the cell had lost 30% of its capacity, analysis showed that the cell had not experienced significant impedance increase or cathode capacity loss, and the anode had lost about 5% of its storage capacity. The analysis confirmed that the loss of active lithium greatly outpaced the loss of capacity for either electrode and is responsible for cell capacity decay. The cathode was then discharged against an external lithium electrode to increase the amount of active lithium within the cell. About half of the lost capacity was recovered, and the cell cycled for 1500 more cycles. Active lithium replenishment from a reserve electrode may be an effective method of extending the life of lithium ion batteries.     

Study of polypyrrole graphite composite as anode material for secondary lithium-ion batteries

Pyrrole was polymerized onto commercial SFG10 graphite by in situ polymerization technique. Polymerization decreases the initial irreversible capacity loss of the graphite anode. The decrease in the irreversible capacity loss is due to the reduction in the thickness of the solid electrolyte interface (SEI) layer formed. PPy/C (7.8%) gives the optimum performance based on the irreversible capacity loss and the discharge capacity of the composite. The composite material has been studied for specific discharge capacity, coulombic efficiency, rate capability and cycle life using a variety of electrochemical methods. The composite SFG10 graphite possess good reversibility, higher coulombic efficiency, good rate capability and better cycle life than the bare SFG10 graphite.     

Electrochemical intercalation of lithium into different varieties of carbon in solid polymer electrolyte

The electrochemical intercalation of lithium into different varieties of carbon was carried out using a solid polymer electrolyte. The faradaic efficiency obtained in the first cycle is relatively low compared with those obtained with a liquid electrolyte. The specific capacity exhibited by graphite materials is greater than that displayed with coke. A discharge capacity of 290 mAh/g after 15 cycles is encountered with a synthetic graphite, for which the variation in impedance before and after cycling was less significant. For natural graphite, a decrease in discharge capacity was found to be associated with an increase in impedance. This probably involves an irreversible reduction of the electrolyte which is more likely to occur on natural graphite presenting a greater BET surface area. Analysis by electron energy loss spectroscopy shows that the graphite surface was covered with carbonates.     

Comparative studies of MCMB and CC composite as anodes for lithium-ion battery systems

Mesocarbon microbead (MCMB 2528) and CC composite have been investigated as anodes for lithium-ion batteries using half-cells with lithium counter electrode and three electrode cell systems containing LiCoO₂ cathode and lithium reference electrodes in 1 M LiPF₆ electrolyte (EC/DMC 1:1 v/v). The test results show that the practical capacity of CC composite anode is 50% higher than that of MCMB-based anode (based on total anode weight). The irreversible capacity loss of CC composite is significantly lower than that of MCMB carbon. Lithium-ion cells made with CC composite anode can accept repeated overdischarge without performance deterioration. The extra capacity of CC composite can be utilized to improve energy density and safety issues related to overcharge of lithium-ion cells. Differential scanning calorimetry (DSC) results indicates that the thermal stability of fully charged CC composite anode (lithiated anode) is much better than that of fully charged MCMB anode.     

A new SiO/C anode composition for lithium-ion battery

A new anode composition comprising SiO and graphite(C) is prepared through a high-energy ball milling process. During the first cycle, the anode delivers high discharge and charge capacity values of 1556 and 693 mAh g⁻¹, respectively. The electrode shows a reversible charge capacity value of 688 mAh g⁻¹ at the 30th cycle with 99% Coulombic efficiency. X-ray diffraction analysis reveals that ball milling does not produce any new compound, but only causes a reduction in particle size. The irreversible and reversible capacities appear to be interdependent.     

Modified natural graphite as anode material for lithium ion batteries

A concentrated nitric acid solution was used as an oxidant to modify the electrochemical performance of natural graphite as anode material for lithium ion batteries. Results of X-ray photoelectron spectroscopy, electron paramagnetic resonance, thermogravimmetry, differential thermal analysis, high resolution electron microscopy, and measurement of the reversible capacity suggest that the surface structure of natural graphite was changed, a fresh dense layer of oxides was formed. Some structural imperfections were removed, and the stability of the graphite structure increased. These changes impede decomposition of electrolyte solvent molecules, co-intercalation of solvated lithium ions and movement of graphene planes along the a-axis direction. Concomitantly, more micropores were introduced, and thus, lithium intercalation and deintercalation were favored and more sites were provided for lithium storage. Consequently, the reversible capacity and the cycling behavior of the modified natural graphite were much improved by the oxidation. Obviously, the liquid–solid oxidation is advantageous in controlling the uniformity of the products.     

Improved electrochemical performance of modified natural graphite anode for lithium secondary batteries

Modified natural graphite is synthesized by surface coating and graphitizing process on the base of spherical natural graphite. The modified natural graphite is examined discharge capacity and coulombic efficiency for the initial charge–discharge cycle. Modification process results in marked improvement in electrochemical performance for a larger discharge capacity and better coulombic efficiency. The mechanism of the enhancement are investigated by means of X-ray powder diffraction, scan electron microscopy, and physical parameters examination. The proportion of rhombohedral crystal structure was reduced by the heat treatment process. The modified natural graphite exhibits 40 mAh g⁻¹ reduction in the first irreversible capacity while the reversible capacity increased by 16 mAh g⁻¹ in comparison with pristine graphite electrode. Also, it has an excellent capacity retention of ∼94% after 100 cycles and ∼87% after 300 cycles.     

Temperature effect on the graphite exfoliation in propylene carbonate based electrolytes

Graphite exfoliation at a low potential has long been an issue for lithium-ion cells using a propylene carbonate (PC) based electrolyte. Two different mechanisms have been proposed in literature to explain this structural degradation. In this study, the initial lithium intercalation temperature is found to have a great impact on the extent of the graphite exfoliation. At an elevated temperature, the exfoliation can be largely suppressed and the irreversible capacity loss is reduced substantially. After the initial cycling at 50 °C, the graphite anode can be cycled in a PC-based electrolyte at room temperature without the exfoliation problem. It is also discovered that such a graphite anode gives rise to a specific capacity of over 372 mAh g⁻¹ at 50 °C and a room temperature capacity higher than that of a graphite anode with the initial lithium intercalation at room temperature. This finding sheds a new light on the exfoliation mechanism. It may lead to a simple cycling procedure that allows us to make rechargeable lithium-ion batteries with better safety and higher capacity.     

Carbon-coated graphite for anode of lithium ion rechargeable batteries: Graphite substrates for carbon coating

Anode performance in lithium ion rechargeable batteries (LIBs) was studied on four kinds of graphite powders, including synthetic graphite. Carbon-coated synthetic graphite gave a smaller irreversible capacity of about 20 mAh g⁻¹ and a better cyclic performance in an electrolyte solution of EC/DMC than natural graphite, though its discharge capacity of about 300 mAh g⁻¹ is a little smaller than natural graphite. Even in a PC-containing solution as EC/PC = 3/1, carbon-coated synthetic graphite had almost the same anode performance as in the solution without PC. Carbon coating of above 5 mass% on graphite particles was found to be effective to improve the anode performance at a low temperature of −5 °C, high retention in discharge capacity of about 90% being obtained. On both natural and synthetic graphite powders, carbon coating by the amount of 3–10 mass% at a temperature of 700–1000 °C was found to be optimum for the improvement of anode performance in LIBs, to have a lower irreversible capacity and higher retention in discharge capacity at −5 °C than without carbon coating.     

High performance Si/C@CNF composite anode for solid-polymer lithium-ion batteries

The electrochemical performance of a composite of nano-Si powder and a pyrolytic carbon of polyvinyl chloride (PVC) with carbon nanofiber (CNF) was examined as an anode for solid-polymer lithium-ion batteries. Nano-Si powder was firstly coated with carbon by pyrolysis of PVC and then mixed with CNF (referred to as Si/C@CNF) using a rotation mixer. The composite exhibited good cycling performance, but suffered from a large irreversible capacity loss of which the retention was less than 60%. In order to reduce the loss, a thin lithium sheet was attached to the Si/C@CNF electrode surface as a reducing agent. The irreversible capacity of the first cycle was lowered to as much as 0 mAh g⁻¹ and after the third cycle, the lithium insertion and extraction efficiency was almost 100%. A reversible capacity of more than 1000 mAh g⁻¹ was still maintained after 40 cycles.     

Graphite materials prepared by HTT of unburned carbon from coal combustion fly ashes: Performance as anodes in lithium-ion batteries

The behaviour as the potential negative electrode in lithium-ion batteries of graphite-like materials that were prepared by high temperature treatment of unburned carbon concentrates from coal combustion fly ashes was investigated by galvanostatic cycling. Emphasis was placed on the relation between the structural/morphological and electrochemical characteristics of the materials. In addition, since good electrode capacity retention on cycling is an important requirement for the manufacturing of the lithium-ion batteries, the reversible capacity provided by the materials prepared on prolonged cycling (50 cycles) was studied and the results were compared with those of petroleum-based graphite which is commercialized as anodic material for lithium-ion batteries. The graphite-like materials prepared lead to battery reversible capacities up to ∼310 mA hg⁻¹ after 50 cycles, these values were similar to those of the reference graphite. Moreover, they showed a remarkable stable capacity along cycling and low irreversible capacity. Apparently, both the high degree of crystallinity and the irregular particle shape with no flakes appear to contribute to the good anodic performance in lithium-ion batteries of these materials, thus making feasible their utilization to this end.     

Study of the charging process of a LiCoO2-based Li-ion battery

A three-electrode Li-ion cell with metallic lithium as the reference electrode was designed to study the charging process of Li-ion cells. The cell was connected to three independent testing channels, of which two channels shared the same lithium reference to measure the potentials of anode and cathode, respectively. A graphite/LiCoO₂ cell with a C/A ratio, i.e., the reversible capacity ratio of the cathode to anode, of 0.985 was assembled and cycled using a normal constant-current/constant-voltage (CC/CV) charging procedure, during which the potentials of the anode and cathode were recorded. The results showed that lithium plating occurred under most of the charging conditions, especially at high currents and at low temperatures. Even in the region of CC charging, the potential of the graphite might drop below 0 V versus Li⁺/Li. As a result, lithium plating and re-intercalating of the plated lithium into the graphite coexist, which resulted in a low charging capacity. When the current exceeded a certain level (0.4C in the present case), increasing the current could not shorten the charging time significantly, instead it aggravated lithium plating and prolonged the CV charging time. In addition, we found that lowering the battery temperature significantly aggravated lithium plating. At −20 °C, for example, the CC charging became impossible and lithium plating accompanied the entire charging process. For an improved charging performance, an optimized C/A ratio of 0.85–0.90 is proposed for the graphite/LiCoO₂ Li-ion cell. A high C/A ratio results in lithium plating onto the anode, while a low ratio results in overcharge of the cathode.     

Composite materials of silver and natural graphite as anode with low sensibility to humidity

Sensitivity of anode materials to humidity is an important factor for the performance of lithium ion batteries. Here it is demonstrated for the first time that the sensitivity of composite anode materials of silver and natural graphite can be strikingly lowered. The composites are prepared by depositing silver ions onto the surface of natural graphite. After the following heat-treatment, silver ions turn into metallic silver and carbide Ag_xC by covering and/or removing active sites that absorb water very easily. Under high humidity condition (about 1000 ppm H₂O), the composite materials absorb strikingly less water resulting in still good electrochemical performance. In comparison, natural graphite without this treatment shows fast fade in capacity under high humidity even though it is good in cycling under low humidity (<100 ppm H₂O). Silver is a good matrix for lithium storage, and is assumed to contribute to reversible capacity since it enhances with the amount of deposited silver. This method can effectively lower the sensitivity of anode materials to humidity, and is promising in manufacturing lithium ion batteries under less critical conditions.     

Mitigation of the irreversible capacity and electrolyte decomposition in a LiNi0.5Mn1.5O4/nano-TiO2 Li-ion battery

Nanosized titanium oxides can achieve large reversible specific capacity (above 200 mAh g⁻¹) and good rate capabilities, but suffer irreversible capacity losses in the first cycle. Moreover, due to the intrinsic safe operating potential (1.5 V), the use of titanium oxide requires to couple it with high-potential cathodes, such as lithium nickel manganese spinel (LNMO) in order to increase the energy density of the final cell. However the use of the 4.7 V vs. Li⁺/Li⁰ LNMO cathode material requires to tackle the continuous electrolyte decomposition upon cycling. Coupling these two electrodes to make a lithium ion battery is thus highly appealing but also highly difficult because the cell balancing must account not only for the charge reversibly exchanged by each electrode but also for the irreversible charge losses. In this paper a LNMO-nano TiO₂ Li-ion cell with liquid electrolyte is presented: two innovative approaches on both the cathode and the anode sides were developed in order to mitigate the electrolyte decomposition upon cycling. In particular the LNMO surface was coated with ZnO in order to minimize the surface reactivity, and the TiO₂ nanoparticles where activated by incorporating nano-lithium in the electrode formulation to compensate for the irreversible capacity loss in the first cycle. With these strategies we were able to assemble balanced Li-ion coin cells thus avoiding the use of electrolyte additives and more hazardous and expensive ex-situ SEI preforming chemical or electrochemical procedures.     

Effect of carbon coating on thermal stability of natural graphite spheres used as anode materials in lithium-ion batteries

To investigate the effect of non-graphitic carbon coatings on the thermal stability of spherical natural graphite at elevated temperature, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements are performed. Data from DSC studies show that the thermal stability of the surface modified natural graphite electrode is improved. The surface modification results in a decrease in the BET surface specific area. An improvement in coulombic efficiency and a reduction in irreversible capacity are also observed for the carbon-coated natural graphite. X-ray diffraction analysis confirms that carbon coating alleviates the release of intercalated lithium from natural graphite at an elevated temperature and acts as a protective layer against electrolyte attack.     

Improvement of cyclic behavior of a ball-milled SiO and carbon nanofiber composite anode for lithium-ion batteries

The cyclic performance of a composite SiO and carbon nanofiber (CNF) anode was examined for lithium-ion batteries. SiO powder of several micrometers was pulverized using high energy mechanical milling. The SiO was ball-milled for 12 h with CNF to produce a composite electrode material that exhibited excellent cycling performance. A reversible capacity of approximately 700 mAh g⁻¹ was observed after 200 cycles. The excellent cyclic performance was discussed with respect to the change of the valence state of Si by ball-milling. A large irreversible capacity at the first cycle for the SiO/CNF composite electrode was reduced to 2% by chemically pre-charging with a lithium film attached to the rim of the electrode.