Two- and three-electrode impedance spectroscopy of lithium-ion batteries

251

275

A comparative study of the electrode/electrolyte interface was carried out for lithium and sodium metal anodes in electrolytes consisting in 1 M LiPF 6 in EC 0.5 :DMC 0.5 (LP30) and 1 M NaPF 6 in both EC 0.5 :DMC 0.5 and EC 0.45 PC 0.45 DMC 0.1 . Symmetric Li/Li cells exhibited low polarization and smooth charge discharge curves with current densities of 0.1 and 1 mA/cm 2 . In contrast, large overpotentials were observed even at 0.1 mA/cm 2 for Na/Na cells. Such differences cannot be related to ionic conductivity of the electrolytes but are rather due to an enhanced interfacial resistance (R ct + R SEI ) as deduced from impedance measurements. The composition of the SEI layer was investigated by FTIR and found to be stable for Li electrodes but to evolve upon cycling for Na electrodes which is also in agreement with differences in surface morphology detected by SEM. A lower stability (partial solubility) of the SEI would also enable to understand the differences in the impedance of identical hard carbon (HC) electrodes in cells with either Li or Na counterelectrodes. These results cast some concerns on the reliability of the so termed half cell characterization and call for caution when interpreting the results of potential electrode materials for sodium ion batteries. The use of metallic anodes has attracted large attention in the battery field due to their high capacity and low operation potential. Nonetheless, almost all commercialized technologies are either primary cell (e.g. alkaline Zn/MnO 2 ), operating through chemical reaction mechanisms (e.g. Ni/Cd in which Cd oxidizes to Cd(OH) 2 , using liquid metal anodes (Na/S cells) or polymer 1 electrolytes, thus avoiding many of the problems of metal reversible plating/stripping in liquid electrolytes. Indeed, the formation of metal dendrites upon cycling was early identified as bottleneck for the development of safe Li metal anode batteries which lead to the commercialization of Li-ion batteries (LIB) to the expense of a severe penalty in energy density. Even so, lithium metal anodes are widely used as counter (or also reference) electrodes in the so termed half cell tests, performed mostly at laboratory scale within the battery research community. These are mostly intended to assess the electrochemical performance of a given compound as either positive or negative electrode material for LIB in a simple way, avoiding the assembly of full cells in which electrode balancing can severely affect performance. Their outcomes are typically the electrochemical capacity and operation potential values at different rates. The reliability of extrapolating such half cell testing results to potential performance in full cells relies on the stability of the Solid Electrolyte Interphase (SEI) typically formed on the surface of lithium metal anodes as a result of electrolyte degradation reactions. 2The development of a room temperature sodium (solid) metal anode battery technology, using conventional liquid electrolytes does not seem to have been seriously considered in the pa...

Section: Resultsmentioning

confidence: 89%

On the Comparative Stability of Li and Na Metal Anode Interfaces in Conventional Alkyl Carbonate Electrolytes

Iermakova

Dugas

Palacín

et al. 2015

251

275

“…For instance, Song et al 40 and Solchenbach et al 41 reported that the higher impedance value for Li-ion batteries resulted from the lithium anode. However, counter-arguments stating that silicon may be the source of the dominant impedance of a SiLi half-cell have been also reported in many papers.…”

Section: Resultsmentioning

confidence: 99%

In Situ DRIFTS Analysis of Solid Electrolyte Interphase of Si-Based Anode with and without Fluoroethylene Carbonate Additive

Yohannes

Lin

2017

The effect of fluoroethylene carbonate (FEC) additive has been studied in the formation of solid electrolyte interphase (SEI) over Si-based anode using in-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). SEI species were observed in the first lithiation cycle at an onset potential of 1.4 V in electrolyte containing 2 wt% vinylene carbonate (VC) + 10 wt% FEC and at 1.1 V in electrolyte without FEC additive. With blended VC and FEC, high carbon containing species including poly (FEC), poly (VC), and polycarbonates were identified, while poly (VC) and polycarbonates formed in the absence of FEC. The FEC additive also led to a higher content of organic phosphorous fluorides as compared to the electrolyte containing no FEC. Electrochemical analyses indicated that the combination of 2 wt% VC and 10 wt% FEC resulted in lower impedances and improved the stability of the Si-electrode through cycling as compared to that without FEC. DRIFTS provided evidence that similar SEI species formed after the initiation in the first cycle, and this formation was recorded for five cycles. With the increasing demand for battery systems with high energy density and high power for applications such as hybrid and electric vehicles, the development of Li-ion batteries has attracted much attention in recent years.1,2 This self-supporting energy storage system 3 works based on the principle of redox reactions that can reversibly convert chemical energy into electrical energy. In Li-ion battery operation, the negative electrodes function at low potentials close to the potential of metallic lithium, where electrolytes are not stable thermodynamically. As a result, the reduction of solvents and salts of the electrolyte will result in the formation of a surface film on the electrode. Typically, this surface film is composed of organic reduction products (closer to the electrolyte) and inorganic species (closer to the electrode). 4 This film, called the solid electrolyte interphase (SEI), is known to be an important feature of graphite anodes that can allow for reversible cycling and long-term stability due to surface passivation. 5,6 Numerous replacements for the graphite anode have been investigated, with silicon 7,8 being one of the most attractive for its high theoretical specific capacity of 4200 mAhg −1 , which is more than ten times larger than that of graphite (372 mAhg −1 ). 9 However, Si undergoes large volume changes during lithiation and delithiation cycling, which cause capacity fading.10 Many studies have tried to improve the capacity retention of Si anodes by using nano-structured silicon, [11][12][13] silicon composite electrodes, 14,15 carbon coated silicon, 16 thin film silicon, [17][18][19] and new binders. [20][21][22] The formation of a stable surface layer for Si-based anodes would have an impact on achieving a long cycle life. One way to address this issue is to use a new electrolyte and/or to add a small amount (< 10 wt%) of electrolyte additives. Different types of electrolyte additives, such as...

“…Here, a constant phase element was used to take into account the slight depression of the charge transfer semi-circle commonly associated to the existence of an SEI layer. 68,69 It is immediately obvious as the electrode charge transfer resistance increases upon cycling, starting from about 73 in the first cycle to around 150 in the 20 th cycle and to about 195 in the 40 th cycle (Figure 6b). Such a continuous increase in resistance is related to an ongoing electrolyte decomposition on the TiO 2 (B) nanoparticles surface, resulting in the continuous growth of the SEI layer.…”

Section: 42mentioning

confidence: 99%

Nanocrystalline TiO₂(B) as Anode Material for Sodium-Ion Batteries

Wu¹,

Bresser²,

Buchholz³

et al. 2014

110

High surface area, nanostructured, and phase-pure TiO 2 (B) noodles-like secondary particles were successfully synthesized by a facile one-pot synthesis, based on the hydrolysis of TiCl 3 using a mixture of ethylene glycol and water at moderate temperature. The primary nanoparticles have a uniform size and are about 15 nm in diameter as determined by TEM analysis and exhibit an increased exposure of the (010) facet as indicated by XRD analysis. Unlike the electrochemical reaction with lithium, the application as sodiumion electrode material reveals substantial differences, including the initial amorphization of the TiO 2 (B) particles, accompanied by a partial irreversibility of the sodium storage, presumably related to sodium trapping inside the active material particles and the absence of a stable solid electrolyte interphase, as indicated by galvanostatic cycling and electrochemical impedance spectroscopy, respectively. Besides, TiO 2 (B)-based electrodes show a stabilized reversible capacity of about 100 mAh g −1 and a very good C rate capability. While sodium-ion batteries were initially considered only as lowcost alternative for lithium-ion batteries with a particular focus on their application as stationary energy storage devices, 1-3 recent developments indicated that such devices might provide even similar energy densities in case suitable cathode and anode active materials are combined. [4][5][6] However, particularly regarding the anode side, the identification of long-term stable, environmentally friendly, and abundant active materials, providing high specific capacities and operating at a reasonably low potential, is still considered to be one of the major challenges for this technology. 4,6,7 47 Among these, the best results in terms of specific capacity, long-term cycling stability, and high rate capability were certainly reported for anatase TiO 2 . [40][41][42][43] However, the reversible sodium storage mechanism is obviously different from the classic (de-)insertion mechanism known for lithium, [48][49][50][51][52][53] as an initial reduction of TiO 2 to metallic titanium and an amorphous sodium titanate occurs. 42 TiO 2 (B) is a very well performing anode material for lithium-ion applications, 54-60 but so far -to the best of our knowledge -only one study reported its application as sodium-ion active material. The electrochemical performance, which might be best described by a rather rapid initial capacity fading and a low reversible capacity of about 50 mAh g −1 , is certainly not that promising, 47 although this might be also related to the cut-off potentials of 3.0 and 0.8 V vs. Na/Na + . Besides, the authors observed a rather huge expansion of the (001) 3+ and Ti 4+ at the nanotubes surface, while the general morphology of the tubes remained after sodiation. Consequently, a solid solution mechanism for the reversible sodium ion (de-)insertion comparable to the lithium ion storage mechanism was proposed.Herein, however, we show that TiO 2 (B) -similarly to anatase TiO 2 -presents a rath...