Oxidation processes on conducting carbon additives for lithium-ion batteries
- First Online:
- Cite this article as:
- La Mantia, F., Huggins, R.A. & Cui, Y. J Appl Electrochem (2013) 43: 1. doi:10.1007/s10800-012-0499-9
- 1.3k Views
The oxidation processes at the interface between different types of typical carbon additives for lithium-ion batteries and carbonates electrolyte above 5 V versus Li/Li+ were investigated. Depending on the nature and surface area of the carbon additive, the irreversible capacity during galvanostatic cycling between 2.75 and 5.25 V versus Li/Li+ could be as high as 700 mAh g−1 (of carbon). In the potential region below 5 V versus Li/Li+, high surface carbon additives also showed irreversible plateaus at about 4.1–4.2 and 4.6 V versus Li/Li+. These plateaus disappeared after thermal treatments at or above 150 °C in inert gas. The influence of the irreversible capacity of carbon additives on the overall performances of positive electrodes was discussed.
KeywordsLithium-ion batteriesConductive additivesHigh voltage positive electrodeElectrolyte stability windowSolid electrolyte interphase
Lithium-ion batteries are now used in a wide range of applications, particularly in connection with portable electronic devices. The challenge of using these systems in larger applications such as for personal or public transportation requires improvements in energy and power density, and also cycle life. One of the major limitations to the improvement of the energy and power density in today’s Li-ion batteries concerns the positive electrode material: the active materials used in most applications are the layer-structured LiMO2 materials, with M = Co, Ni, Mn, or a mixture of them, or the olivine LiFePO4. With the exception of overlithiated phases, all these compounds have very similar theoretical values of energy, if used with the same negative electrode (graphite or Li4Ti5O12) [1–5].
Current efforts to increase the specific energy of Li-ion batteries are mainly focused on the investigation of lithiated compounds with high reaction potentials, above 4.6 V versus Li/Li+ [6–10]. This is far beyond the stability window of the organic electrolytes [11, 12], which are then thermodynamically driven to be oxidized; stabilizing kinetically the active materials, for example through the formation of a more stable solid electrolyte interphase (SEI) [13, 14], becomes a challenge. Although the typical positive electrode contains a considerable volumetric amount of conductive carbon additives (such as Super-P, acetylene black, and graphite), the low percentage mass load (<8 wt%) of these compounds in the total electrode induces to consider such additives as inert or passive, which means they take a negligible part to the specific reversible and irreversible charge of the electrode. Graphite was also investigated as positive electrode, due to the possibility to intercalate anions from the electrolyte in its layered structure at high potentials (around 4.7 V vs. Li/Li+ [15–17]), leading to a specific reversible charge of circa 140 mAh g−1.
Considering the higher surface of the carbons additives in contact with the electrolyte, their relatively high conductivity and their higher affinity towards the carbonates, in this study we want to investigate the reversible and irreversible-specific charge and the charge/discharge profiles for both low specific surface (graphite) and high specific surface (Super-P, acetylene black, and carbon nanofibers), in the attempt to understand their properties in the potential window between 2.75 and 5.25 V versus Li/Li+, and their influence on the overall electrochemical performances of positive electrodes.
2 Experimental aspects
2.1 Scanning electron microscopy
SEM measurements were performed on a FEI XL30 Sirion microscope on uncoated powder samples. Images were recorded at 5 kV with a secondary electron beam.
2.2 BET measurements
Carbon powders weighing 150–250 mg were used to get the isotherms using Micromeritics porosity analyzer (ASAP 2020). Before actual adsorption measurements, the samples were degassed at 350 °C and high vacuum for ~8 h. The BET area was obtained by analyzing the data in the range of 0.05 ≤ P/P0 ≤ 0.3.
2.3 Electrochemical measurements
Four different carbon additives were investigated: graphite (Fluka, product no. 78391); acetylene black (Alfa Aesar, product no. 45527); Super-P (TIMCAL); and carbon nanofibers (Aldrich, product no. 719781). The electrochemical cycling experiments were performed at room temperature in coffee-bag type cells with a two-electrode half-cell configuration, and metallic lithium serving as the counter electrode. The electrolyte was a 1 M solution of LiPF6 in a 1:1 by weight mixture of EC and DMC (Ferro). Galvanostatic cycling was performed with a Versatile Multichannel Potentiostat, VMP3 (Bio-Logic SA) between the cut-off potentials of 2.75 and 5.25 V versus Li/Li+. No potentiostatic step was applied.
Pure carbon working electrodes were prepared using the “doctor-blading” technique, starting from an N-methyl pyrrolidone (NMP) (Aldrich, product no. 328634) based slurry with the carbon powders and polyvinylidene fluoride (PVdF) as binder (Fluka, MW = 530,000). The weight percent of the carbon powders depended on the type of carbon: 80 % for graphite-based electrodes, and 40 % for the other carbon powder-based electrodes. The slurry was doctor bladed onto aluminum current collectors and then dried at 90 °C in air overnight. The samples were transferred in a glove box with a high purity argon atmosphere (O2 < 1 ppm, H2O < 1 ppm). The thermally treated samples were heated on a hot plate at 100, 150, or 200 °C inside the glove box for 3 h. All the pure carbon working electrodes were cycled at a specific current of 20 mA g−1.
Alumina–carbon working electrodes were also prepared using the doctor-blading technique, starting from an NMP-based slurry with Al2O3 (J.T. Baker Chemical Co.) (81 wt%), Super-P (TIMCAL) (9 wt%), and polymeric binder (PVdF) (10 wt%). These electrodes were used to simulate the performances of a real composite electrode, with Al2O3 keeping the carbon particles apart, as the active material would do. Being Al2O3 inert, these electrodes allowed the study of the carbon additives in a real geometry. These samples were also transferred into the glove box and thermally treated on a hot plate at 150 °C for 3 h. The alumina–carbon working electrodes were cycled at specific currents of 150 and 100 mA g−1 of Super-P, simulating 16.7 and 11.1 mA g−1 of Al2O3 specific currents, respectively.
3 Results and discussion
Characteristics and irreversible charge for the different carbon additives investigated. Qirr and qirr are the specific irreversible charge and the density of irreversible charge with respect to the BET area, respectively
BET (m2 g−1)
Qirr (mAh g−1)
qirr (mC cm−2)
Pure carbon working electrodes were tested in the high potential region, above 5 V versus Li/Li+, to investigate their properties under a highly oxidative environment. The charge/discharge profile of the carbon additive is important for understanding the irreversible processes due to side reactions occurring on its surface. The most probable side reaction in this potential range is the oxidation of the electrolyte, and formation of a SEI. The latter occurs upon both the active material and the carbon additive. The very first cycles were investigated in this study, for that is when the irreversible capacity is normally higher.
In Table 1, the specific irreversible charge and the density of irreversible charge (with respect to the BET area) are reported for the different carbon additives. We want to stress that the differences in the density of irreversible charge, being not correlated to geometrical effects, are due to the nature of the carbon additives. These differences could be caused by the nature of the surface groups, which depend on the synthesis condition of the carbon additive. The best result is obtained with Super-P, while acetylene black and graphite have similar density of irreversible charge.
The strong dependence of the specific irreversible charge on the current rate and on the cycle number suggests the formation of a SEI-like protective layer on the surface of the carbon additive, probably composed of the products of oxidation of the electrolyte. This SEI-like layer strongly reduces the oxidation rate after the first cycle, but does not block it completely. If the cycling is interrupted for 1 day and then started again (not shown here), the value of the specific irreversible charge is similar to the one of the second and later cycles, suggesting that this SEI-like layer is effectively stable, or at least has a stable component. The SEI layer formed on the surface of common active materials for positive electrodes was investigated by many authors [14, 20, 21]. We believe that the similar compounds produced by the oxidation of the electrolyte, such as Li2CO3, LiF, and polycarbonates, should be present on the SEI layer of the carbon additives. It is noteworthy to stress that, for the carbon additives, it is not necessary that the SEI layer is ionically conductive.
Even if the high current rate results show that the specific irreversible charge can be very low, relative to the reversible charge of a possible active material, the conditions under which the carbon additive interacts with the electrolyte strongly depends on the active material itself. Indeed, the longer the charge/discharge plateau of the active material and the higher its potential, the more time the carbon additive will spend in the high potential region. As consequence, the specific irreversible charge will increase. To properly evaluate experiments on real electrodes, containing both an active material and a carbon additive, one needs to know the detailed electrochemical behavior of the carbon additive, and subtract it from the observed results.
Experiments on different types of carbon additives have shown that they can be a significant source of specific irreversible charge, in some cases higher than 700 mAh g−1. The value of the irreversible-specific charge is dependent on the nature of the carbon additive and thermal treatments. There is generally a strong reduction of the specific irreversible charge after the first cycle. Experiments at different current rates have suggested that the main mechanism occurring during the charge of the carbon additives is the formation of a SEI-like layer, probably composed of products of the oxidation of the electrolyte. However, further post-mortem investigations are necessary to confirm this explanation and will be the aim of future work.
The study was partially supported by the Global Climate and Energy Project at Stanford and King Abdullah University of Science and Technology (KAUST) under the award No. KUS-l1-001-12. We thank Heather Deshazer and Dr. Jang Wook Choi for experimental assistance.