- Nano Express
- Open Access
MoS2/C Multilayer Nanospheres as an Electrode Base for Lithium Power Sources
© Shyyko et al. 2016
- Received: 29 November 2015
- Accepted: 25 April 2016
- Published: 4 May 2016
Multilayer nanospheres with alternating 2H-MoS2 and C layers were studied as a cathode base for lithium power sources. Interesting hierarchical structure, synergetic effect, and the presence of defects as supplementary active sites, introduced by the additional annealing at 773 K in Ar atmosphere, have determined the conductivity, referred to symmetric hopping or random barrier model, and led to achieve the high values of specific capacity of 3700, 1390, and 790 A h kg−1 at currents 0.1, 0.3, and 0.5 C. Such unusual result was never reported before and could be explained by combining of the faradaic and non-faradaic accumulation processes within electrode material.
- Multilayer nanospheres
- Electrical conductivity
- Specific capacity
Since the batteries were introduced in 1980s as new high-energy density power sources, one of the most promising materials used as electrode materials for further commercialization was MoS2. Being a layered transition metal dichalcogenide compound, it possesses the ability to accommodate Li ions within interlayer spaces that are due to the noncovalent Van der Waals forces between S-Mo-S packages. Taking into account its low cost, owing to the natural abundance, the first MoS2-based commercial batteries were developed by Moli Energy Ltd., (British Columbia, Canada) in 1989 and have the specific energy values 100 W h kg−1 [1, 2]. In spite of few failures in operating such systems, the study of MoS2 as an electrode material still is continuing.
Theoretically, the capacity of MoS2 is about 167 A h kg−1, when in conversion reaction, only one mole of Li+ takes part per mole of MoS2, but reported typical capacity value even for bulk material is more than 600 A h kg−1 and became a reason to consider at least four Li+ ions intercalated per MoS2 unit. According to , it takes around six lithium ions during the first discharge cycle. However, the first discharge process capacity of the MoS2 quickly decreases by several times. Most of the intercalated Li+ ions remain localized in the crystal structure within the interlayer space between S-Mo-S packages after the first discharge in nanostructures despite that the Li+ diffusion path is significantly shortened in comparison to bulk material. Both bulk and exfoliated materials exhibit capacity reduction upon cycling; moreover, for exfoliated MoS2, this decrease can be more sharp [4, 5]. The initial discharge capacity of exfoliated MoS2 is typically more than 1000 A h kg−1, and capacity gain is caused by the Li2S formation and Mo metal reduction. MoS2 can be additionally exfoliated via lithium intercalation with a metastable phase formation: electron transfer from lithium during the intercalation causes the change in the electron density and the additional deformation of crystal structure, in particular, in Mo symmetry—from trigonal prismatic (2H) to octahedral. The increasing of MoS2 structural disorder leads to the possibility for more Li+ ions to reversibly penetrate into the expanded interlayer spaces. But still, the capacity of exfoliated MoS2 dramatically decreases with the increasing of cycle number.
The electrochemical performance of MoS2 as an electrode for lithium batteries was believed to be significantly influenced by morphology, structure, and particle size. In order to shorten the Li+ diffusion path for improving the performance, many research efforts have been directed to prepare nanostructured MoS2 for application as electrode material. The most popular approach to increase the capacity is to enlarge the interlayer distance and lower the barrier for Li+ intercalation. A good example is MoS2 nanoplates , consisting of disordered graphene-like layers, with a thickness of ∼30 nm and interlayer distance of 0.69 nm (for bulk, it is 0.62 nm), that showed reversible capacity of 700 A h kg−1 even at 50 C. Another way is to play on morphology effect; 3D MoS2 nanospheres  and flower-like structures  have been demonstrated with the reversible capacity of >850 A h kg−1 however at low current rates. In the same time, 1D nanoribbons and nanotubes possess 776 A h kg−1 [9, 10].
But the latest tendency is a combination of MoS2 with carbon materials (as nanotubes, carbon coating, and graphene). Being a good conductor and chemically stable substance, it contributes to the overall conductivity of the composite, facilitating the charge transfer within the material, and prevents the volume expansion and restacking of MoS2. The synergetic effect between these two materials improves the electrochemical performance of the composite in Li batteries. Indeed, a MoS2/GNS (graphene nanosheets) composite with a Mo to C mole ratio of 1:2 that delivered the highest specific capacity (1300 A·h·kg-1), although the specific capacity of samples with mole ratios of 1:1 and 1:4 (i.e., 1001 A·h·kg-1and 1132 A·h·kg-1, respectively) still exhibited high specific capacity and better cycling stability than pure MoS2 and GNS. One of the largest reported value of specific capacity is 1549 A·h·kg-1 for 2D MoS2 grown on the surface of 1D multiwall carbon nanotubes . At the same time, it should be taken into account the possibility of pseudocapacitive mechanism of charge storage with electron transfer and oxidation/reduction of Mo4+ . In some cases, the contribution of pseudocapacity is dominant and has significant impact on the electrochemical performance .
Here, we present hierarchically structured nanospheres with alternating layers of MoS2 and carbon as an electrode base for lithium power sources. Studied nanocomposite showed the very high specific capacity during Li+ intercalation and interesting conductivity features.
The relative element contents received from EDS
MoS2/C as-prepared, at.%
MoS2/C after annealing, at.%
15.8 ± 0.6
13.0 ± 0.6
27.0 ± 0.6
23.0 ± 0.5
46.8 ± 0.4
53.9 ± 0.5
Electrical conductivity σ as a function of frequency (0.01–100 kHz range) and temperature were measured by the method of impedance spectroscopy (Autolab, PGSTAT12, FRA-2 software). All samples were made in pellet form with the diameter of 1.7 × 10−2 m and thickness of 0.6 × 10−3 m under pressure of 34 MPa. Taking into account the nature of ultrafine material to avoid, the probable oxidation of the air at it was chosen to conduct the conductivity evaluation is in the narrow temperature range of 293–333 K with precision of ±1 K.
The galvanostatic and potentiodynamic measurements were conducted in two electrode cells with Li as a counter electrode and 1М LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate as an electrolyte. The working electrode consists of a test material (MoS2/C multilayered nanospheres), carbon black, and polyvinylidene difluoride (PVDF) in a weight ratio of 8:1:1 coated on Cu foil.
In the case of materials obtained after annealing at 773 K in a stream of argon, the σ(ω) dependences (Fig. 4b) were approximated just partially to the zone of inflection. It was found that parameter s for this material is weakly dependent on the annealing temperature, varying within the approximation error within 0.33–0.37. Thus, we can assume that in this case, we observe the displays of quantum mechanical tunneling of charge carriers . As it was already mentioned, the curves σ(ω) have a distinctive look, indicating a higher level of disorder. In this case, it becomes possible to use the symmetric hopping model (or random barrier model), whereby the charge transfer is a jump between close equilibrium positions in non-periodic potential . In this model, the probability of hopping between individual positions is considered as the same with the normal distribution of potential barrier height, which provides no explicit value of activation energy. Reducing the equilibrium conductivity with increasing frequency and temperature is explained as follows: energy comes into the system as a result of both thermal excitation and application of the external periodic potential. At lower temperatures, the hops occur at higher frequencies, and thermal excitation effect is small. Thus, the frequency growth causes an increase of hop probability over the barriers, the heights of whose are distributed by the Gauss function, which explains the smooth curve growth progress—a sharp increase (area corresponded to the vicinity of mode value of the barrier height)—a saturation. With increasing temperature, this situation persists on providing the probability growth of carrier scattering on phonons and saturation at relatively lower frequencies.
Spherical nanoparticles with alternating MoS2 and C layers synthesized by hydrothermal method were studied as an electrode base for Li power sources. It was determined that the obtained values of specific capacity (3700, 1390, and 790 A h kg−1 at currents 0.1, 0.3, and 0.5 C, respectively) are caused by synergetic effect of the following factors: (i) deformation, expanding, and breaches of MoS2 crystal structure as a result of carbon layers’ presence and thermal treatment; (ii) conductivity growth for MoS2/C nanocomposite comparatively to bulk materials; and (iii) combination both faradaic and pseudocapacitive non-faradaic mechanisms of charge accumulation. The conductivity character of the obtained MoS2/C composite is being changed after thermal treatment from typical for crystalline MoS2 to symmetric hopping or random barrier model. The conductivity saturation point, observed in the annealed material, is balancing between temperature and frequency of applied field and decreasing at higher temperatures. Without modifying the 2H structure of MoS2, the annealing has introduced a number of defects—the supplementary active sites—where the redox reactions occur. This together with spherical hollow structure of MoS2/C nanoparticles affected the results of galvanostatic and potentiodynamic studies.
The authors are very thankful to Michal Rawski, the specialist at the Analytical Laboratory of the Faculty of Chemistry, Maria Curie-Skłodowska University (Lublin, Poland), for the TEM and EDS investigation of the studied material and for invaluable support and help.
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