A review of advanced and practica...
All electrodes, excluding 1.5 V systems such as LiTiOx anodes, are surface-film controlled (SFC) systems. At the anode side, all conventional electrolyte systems can be reduced in the presence of Li ions below 1.5 V, thus forming insoluble Li-ion salts that comprise a passivating surface layer of particles referred to as the solid electrolyte interphase (SEI).10 The cathode side is less trivial. Alkyl carbonates can be oxidized at potentials below 4 V.11 These reactions are inhibited on the passivated aluminium current collectors (Al CC) and on the composite cathodes. There is a rich surface chemistry on the cathode surface as well. In their lithiated state, nucleophilic oxygen anions in the surface layer of the cathode particles attack electrophilic RO(CO)OR solvents, forming different combinations of surface components (e.g. ROCO2Li, ROCO2M, ROLi, ROM etc.) depending on the electrolytes used.12 The polymerization of solvent molecules such as EC by cationic stimulation results in the formation of poly- carbonates.13 The dissolution of transition metal cations forms surface inactive LixMOy phases.14 Their precipitation on the anode side destroys the passivation of the negative electrodes.15 Red-ox reactions with solution species form inactive LiMOy with the transition metal M at a lower oxidation state.14 LiMOy compounds are spontaneously delithiated in air due to reactions with CO2.16 Acid–base reactions occur in the LiPF6 solutions (trace HF, water) that are commonly used in Li-ion batteries. Finally, LiCoO2 itself has a rich surface chemistry that influences its performance: 4LiCo IIIO2!Co IVO2þCo IICo2III O4þ2Li2O !4LiF 4HF þ 2H2O CoIII compounds oxidize alkyl carbonates CO2 is one of the products, CoIII / CoII / Co2+ dissolution.14 Interestingly, this process seems to be self-limiting, as the presence of Co2+ ions in solution itself stabilizes the LiCoO2 electrodes,17 However, Co metal in turn appears to deposit on the negative electrodes, destroying their passivation. Hence the performance of many types of electrodes depends on their surface chemistry. Unfortunately surface studies provide more ambiguous results than bulk studies, therefore there are still many open questions related to the surface chemistry of Li- ion battery systems. It is for these reasons that proper R&D of advanced materials for Li-ion batteries has to include bulk structural and perfor- mance studies, electrode–solution interactions, and possible reflections between the anode and cathode. These studies require the use of the most advanced electrochemical,18 structural (XRD, HR microscopy), spectroscopic and surface sensitive analytical techniques (SS NMR,19 FTIR,20 XPS,21 Raman,22 X-ray based spectroscopies23). This presentation provides a review of the forefront of the study of advanced materials—electrolyte systems, current collectors, anode materials, and finally advanced cathodes materials used in Li-ion batteries, with the emphasis on contributions from the authors’ group. Experimental Many of the materials reviewed were studied in this laboratory, therefore the experimental details have been provided as follows. The LiMO2 compounds studied were prepared via self-combus- tion reactions (SCRs).24 Li[MnNiCo]O2 and Li2MnO3$Li/ MnNiCo]O2 materials were produced in nano- and submicrometric particles both produced by SCR with different annealing stages (700 C for 1 hour in air, 900 C or 1000 C for 22 hours in air, respectively). LiMn1.5Ni0.5O4 spinel particles were also synthesized using SCR. Li4T5O12 nanoparticles were obtained from NEI Inc., USA. Graphitic material was obtained from Superior Graphite (USA), Timcal (Switzerland), and Conoco-Philips. LiMn0.8Fe0.2PO4 was obtained from HPL Switzerland. Standard electrolyte solutions (alkyl carbonates/ LiPF6), ready to use, were obtained from UBE, Japan. Ionic liquids were obtained from Merck KGaA (Germany and Toyo Gosie Ltd., (Japan)). The surface chemistry of the various electrodes was charac- terized by the following techniques: Fourier transform infrared (FTIR) spectroscopy using a Magna 860 Spectrometer from Nicolet Inc., placed in a homemade glove box purged with H2O and CO2 (Balson Inc. air purification system) and carried out in diffuse reflectance mode high-resolution transmission electron microscopy (HR-TEM) and scanning electron microscopy (SEM), using a JEOL-JEM-2011 (200kV) and JEOL-JSM-7000F electron microscopes, respectively, both equipped with an energy dispersive X-ray microanalysis system from Oxford Inc. X-ray photoelectron spectroscopy (XPS) using an HX Axis spectrom- eter from Kratos, Inc. (England) with monochromic Al Ka (1486.6eV) X-ray beam radiation solid state 7 Li magic angle spinning (MAS) NMR performed at 194.34 MHz on a Bruker Avance 500 MHz spectrometer in 3.2 mm rotors at spinning speeds of 18–22 kHz single pulse and rotor synchronized Hahn echo sequences were used, and the spectra were referenced to 1 M LiCl at 0 ppm MicroRaman spectroscopy with a spectrometer from Jobin-Yvon Inc., France. We also used Mossbauer € spec- troscopy for studying the stability of LiMPO4 compounds (conventional constant-acceleration spectrometer, room temperature, 50 mC: 57Co:Rh source, the absorbers were put in Perspex holders. In situ AFM measurements were carried out using the system described in ref. 25. The following electrochemical measurements were conducted. Composite electrodes were prepared by spreading slurries comprising the active mass, carbon powder and poly-vinylidene difluoride (PVdF) binder (ratio of 75% : 15% : 10% by weight, mixed into N-methyl pyrrolidone (NMP), and deposited onto aluminium foil current collectors, followed by drying in a vacuum oven. The average load was around 2.5 mg active mass per cm2. These electrodes were tested in two-electrode, coin-type cells (Model 2032 from NRC Canada) with Li foil serving as the counter electrode, and various electrolyte solutions. Computer- ized multi-channel battery analyzers from Maccor Inc. (USA) and Arbin Inc. were used for galvanostatic measurements (voltage vs. time/capacity, measured at constant currents). Results and discussion Our road map for materials development Fig. 1 indicates a suggested road map for the direction of Li-ion research. The axes are voltage and capacity, and a variety of electrode materials are marked therein according to their respective values. As is clear, the main limiting factor is the cathode material (in voltage and capacity). The electrode mate- rials currently used in today’s practical batteries allow for This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem. Published on 23 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04225K View Online
a nominal voltage of below 4 V. The lower limit of the electro- chemical window of the currently used electrolyte solutions (alkyl carbonates/LiPF6) is approximately 1.5 V vs. Li26 (see later discussion about the passivation phenomena that allow for the operation of lower voltage electrodes, such as Li and Li–graphite). The anodic limit of the electrochemical window of the alkyl carbonate/LiPF6 solutions has not been specifically determined but practical accepted values are between 4.2 and 5 V vs. Li26 (see further discussion). With some systems which will be discussed later, meta-stability up to 4.9 V can be achieved in these standard electrolyte solutions. Electrolyte solutions The anodic stability limits of electrolyte solutions for Li-ion batteries (and those of polar aprotic solutions in general) demand ongoing research in this subfield as well. It is hard to define the onset of oxidation reactions of nonaqueous electrolyte solutions because these strongly depend on the level of purity, the presence of contaminants, and the types of electrodes used. Alkyl carbonates are still the solutions of choice with little competition (except by ionic liquids, as discussed below) because of the high oxidation state of their central carbon (+4). Within this class of compounds EC and DMC have the highest anodic stability, due to their small alkyl groups. An additional benefit is that, as discussed above, all kinds of negative electrodes, Li, Li–graphite, Li–Si, etc., develop excellent passivation in these solutions at low potentials. The potentiodynamic behavior of polar aprotic solutions based on alkyl carbonates and inert electrodes (Pt, glassy carbon, Au) shows an impressive anodic stability and an irreversible cathodic wave whose onset is 1.5 vs. Li, which does not appear in consequent cycles due to passivation of the anode surface by the SEI. The onset of these oxidation reactions is not well defined (4/5 V vs. Li). An important discovery was the fact that in the presence of Li salts, EC, one of the most reactive alkyl carbonates (in terms of reduction), forms a variety of semi-organic Li-con- taining salts that serve as passivation agents on Li, Li–carbon, Li–Si, and inert metal electrodes polarized to low potentials. Fig. 2 and Scheme 1 indicates the most significant reduction schemes for EC, as elucidated through spectroscopic measure- ments (FTIR, XPS, NMR, Raman).27–29 It is important to note (as reflected in Scheme 1) that the nature of the Li salts present greatly affects the electrode surface chemistry. When the pres- ence of the salt does not induce the formation of acidic species in solutions (e.g., LiClO4, LiN(SO2CF3)2), alkyl carbonates are reduced to ROCO2Li and ROLi compounds, as presented in Fig. 2. In LiPF6 solutions acidic species are formed: LiPF6 decomposes thermally to LiF and PF5. The latter moiety is a Lewis acid which further reacts with any protic contaminants (e.g. unavoidably present traces of water) to form HF. The presence of such acidic species in solution strongly affects the surface chemistry in two ways. One way is that PF5 interacts with Fig. 1 The road map for R&D of new electrode materials, compared to today’s state-of-the-art. The y and x axes are voltage and specific capacity, respectively. Fig. 2 A schematic presentation of the CV behavior of inert (Pt) elec- trodes in various families of polar aprotic solvents with Li salts.26 J. Mater. Chem. This journal is ª The Royal Society of Chemistry 2011 Published on 23 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04225K View Online
the carbonyl group and channels the reduction process of EC to form ethylene di-alkoxide species along with more complicated alkoxy compounds such as binary and tertiary ethers, rather than Li-ethylene dicarbonates (see schemes in Fig. 2) the other way is that HF reacts with ROLi and ROCO2Li to form ROH, ROCO2H (which further decomposes to ROH and CO2), and surface LiF. Other species formed from the reduction of EC are Li-oxalate and moieties with Li–C and C–F bonds (see Scheme 1).27–31 Efforts have been made to enhance the formation of the passivation layer (on graphite electrodes in particular) in the presence of these solutions through the use of surface-active additives such as vinylene carbonate (VC) and lithium bi-oxalato borate (LiBOB).27 At this point there are hundreds of publica- tions and patents on various passivating agents, particularly for graphite electrodes their further discussion is beyond the scope of this paper. Readers may instead be referred to the excellent review by Xu32 on this subject. Ionic liquids (ILs) have excellent qualities that could render them very relevant for use in advanced Li-ion batteries, including high anodic stability, low volatility and low flammability. Their main drawbacks are their high viscosities, problems in wetting particle pores in composite structures, and low ionic conductivity at low temperatures. Recent years have seen increasing efforts to test ILs as solvents or additives in Li-ion battery systems.33 Fig. 3 shows the cyclic voltammetric response (Pt working electrodes) of imidazolium-, piperidinium-, and pyrrolidinium- based ILs with N(SO2CF3)2 anions containing LiN(SO2CF3)2 salt.34 This figure reflects the very wide electrochemical window and impressive anodic stability (5 V) of piperidium- and pyr- rolidium-based ILs. Imidazolium-based IL solutions have a much lower cathodic stability than the above cyclic quaternary ammonium cation-based IL solutions, as demonstrated in Fig. 3. The cyclic voltammograms of several common electrode mate- rials measured in IL-based solutions are also included in the figure. It is clearly demonstrated that the Li, Li–Si, LiCoO2, and LiMn1.5Ni0.5O4 electrodes behave reversibly in piperidium- and pyrrolidium-based ILs with N(SO2CF3)2 and LiN(SO2CF3)2 salts. This figure demonstrates the main advantage of the above IL systems: namely, the wide electrochemical window with exceptionally high anodic stability. It was demonstrated that aluminium electrodes are fully passivated in solutions based on derivatives of pyrrolidium with a N(SO2CF3)2 anion and LiN(SO2CF3)2.35 Hence, in contrast to alkyl carbonate-based solutions in which LiN(SO2CF3)2 has limited usefulness as a salt due to the poor passivation of aluminium in its solutions in the above IL-based systems, the use of N(SO2CF3)2 as the anion doesn’t limit their anodic stability at all. In fact it was possible to demonstrate prototype graphite/LiMn1.5Ni0.5O4 and Li/L- iMn1.5Ni0.5O4 cells operating even at 60 C in solutions Scheme 1 A reaction scheme for all possible reduction paths of EC that form passivating surface species (detected by FTIR, XPS, Raman, and SSNMR28–31,49). Fig. 3 Steady-state CV response of a Pt electrode in three IL solutions, as indicated. (See structure formulae presented therein.) The CV presentations include insets of steady-state CVs of four electrodes, as indicated: Li, Li–Si, LiCoO2, and LiMn1.5Ni0.5O4.34 This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem. Published on 23 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04225K View Online